ventral nervous system defective
Mutation of NK-2 homeobox genes The mouse inner ear develops from the otic vesicle, a
one-cell-thick epithelium, which eventually transforms into highly complex structures
including the sensory organs for balance (vestibulum) and hearing (cochlea). Several
mouse inner ear mutations with hearing and balance defects have been described, but
most the underlying genes have not been identified; for example, the genes
controlling the development of the vestibular organs. Inactivation
of the homeobox gene, Nkx5-1, was accomplished by homologous recombination in mice. This gene is
expressed in vestibular structures throughout inner ear development. Mice carrying the
Nkx5-1 null mutation exhibit behavioral abnormalities that resemble the typical
hyperactivity and circling movements of the shaker/waltzer type mutants. The balance
defect correlates with severe malformations of the vestibular organ in Nkx5-1(-/-)
mutants, which fail to develop the semicircular canals. Nkx5-1 is the first ear-specific
molecule identified to play a crucial role in the formation of the mammalian vestibular
system (Hadrys, 1998).
The telencephalon is organized into distinct longitudinal
domains: the cerebral cortex and the basal ganglia. The
basal ganglia primarily consists of a dorsal region
(striatum) and a ventral region (pallidum). Within the
telencephalon, the anlage of the pallidum expresses the
Nkx2.1 homeobox gene.
Nkx2.1 expression is first detectable in the basal telencephalon
of the mouse at approximately the 11 somite stage. As the basal telencephalon develops, Nkx2.1 is
maintained in regions that form morphologically distinct
structures such as the medial ganglionic eminence (MGE), as
well as parts of the septum, anterior entopeduncular area
and preoptic area (POA). Focus was placed on the role
of Nkx2.1 in the development of the MGE, a proliferative zone
that gives rise to pallidal components of the basal ganglia.
At E10.5 in the mouse, the MGE is a neuroepithelial
protrusion from the wall of the anterobasal telencephalon into
the lateral ventricle (LV). Nkx2.1
RNA is expressed uniformly throughout the MGE
neuroepithelium. Between E11.5 and E12.5, a second
morphologically distinct basal ganglia anlage, the lateral
ganglionic eminence (LGE), emerges between the MGE and
the cortex. At these stages the LGE lacks Nkx2.1
expression. By E12.5, the MGE is composed of three
molecularly distinct cell layers. The ventricular zone (VZ),
which is adjacent to the telencephalic ventricle, is composed of
undifferentiated, rapidly dividing cells. VZ cells contribute to
the subventricular zone (SVZ), a secondary proliferating
population of cells.
Postmitotic cells migrate from the proliferative zones to
generate the mantle zone. Nkx2.1 is expressed in all three
layers of the MGE. At E14.5 and later stages,
Nkx2.1 expression is prominent in the developing globus
pallidus (GP). As development continues, Nkx2.1
expression can be detected in several other ventral
telencephalic structures including the bed nucleus of the stria
terminalis (BNST), parts of the septum, the ventral pallidum
and parts of the amygdala.
A mouse deficient in Nkx2.1
function does not form pallidal structures; it lacks basal
forebrain TrkA-positive neurons (probable cholinergic
neurons) and shows reduced numbers of cortical cells that
express GABA, DLX2 and calbindin. These cells all normally migrate from
the pallidum through the striatum and into the cortex. Evidence is presented that these phenotypes result from a
ventral-to-dorsal transformation of the pallidal
primordium into a striatal-like anlage (Sussel, 1999).
Sonic hedgehog (SHH) secretion from the axial mesendoderm
is required for patterning of the anteromedial neural plate,
including the hypothalamus and basal telencephalon. In addition, SHH can induce markers of the basal
telencephalon, such as Nkx2.1. Shh begins to be expressed in the
VZ of the ventral-most regions of the basal telencephalon
(preoptic and anterior entopeduncular areas; POA, AEP)
between the 10-12 somite stage, at approximately the same time
as Nkx2.1 begins to be expressed in the basal telencephalon. Subsequently, Shh expression spreads
into the SVZ and mantle of the MGE.
Because Shh is expressed early in MGE development and
has a potential role in telencephalic patterning, its expression was assessed in the developing MGE of the Nkx2.1
mutant. Surprisingly, at E10.5 and E11.5, Shh expression
is undetectable in the mutant basal telencephalon and
hypothalamus, aside from trace levels of Shh in
the rostral midline at E10.5. Shh expression in
the midbrain and more posterior regions of the central nervous
system appeared normal at all ages examined. To determine whether Nkx2.1 expression is required
for induction or maintenance of Shh expression in the
forebrain, E8.75-E9.5 Nkx2.1 mutant embryos were examined by
whole-mount in situ hybridization. Even at these stages, which
ordinarily show the expression of Shh in the forebrain, Shh transcripts in the hypothalamus and basal
telencephalon are not detectable, while Shh expression in the
anterior mesendoderm is normal (Sussel, 1999).
Nkx2.1 is related to the Drosophila gene ventral
nervous system defective (vnd). vnd encodes the NK2
homeodomain protein that is expressed in the ventral part
of central nervous system. The CNS in fly embryos lacking vnd has a
ventral-to-dorsal transformation, analogous to the
phenotype in the Nkx2.1 mutants. There are several Nkx genes
expressed in the ventral CNS of vertebrates, including Nkx2.2
and Nkx6.1 (Drosophila homolog: Nkx6). Mutations of Nkx2.2 and
Nkx6.1 also have ventral-to-dorsal transformations. In both of these cases, Shh expression is
unaffected, suggesting that Nkx2.2 and Nkx6.1, like vnd, have
primary roles in ventral specification. It is uncertain that
Nkx2.1 alone is necessary for fate specification of the MGE,
because Shh expression, which is essential for ventral
specification, is also reduced in the
Nkx2.1 knockout mice. However, the described functions of
vnd, Nkx2.2 and Nkx6.1 support the hypothesis that Nkx2.1 has
a primary role in regional specification, like its homologs.
Furthermore, in all regions of the CNS, the Nkx genes are
expressed before Shh, supporting the model
that Nkx2.1 is upstream of Shh in the basal telencephalon.
Finally, in Gli2 mutant mice, Shh is not expressed in the floor
plate, yet Nkx2.2 is expressed and motor neurons form. Thus, while Shh expression in the axial
mesendoderm is essential for ventral specification of the CNS, Shh expression in neural tissue may not
have a major role in regionalization (Sussel, 1999 and references).
In the absence of Nkx2.1, the transformed MGE does not produce normal
ventral telencephalic cell types. Evidence is accumulating that
dorsoventral patterning of the vertebrate CNS is regulated by
common mechanisms along the entire anteroposterior axis. For instance, the Nkx
genes, which are induced by SHH in all ventral CNS regions, are required for ventral specification
in the forebrain hindbrain and
spinal cord. While there are
a variety of ventral CNS cell types, all axial levels produce
cholinergic neurons (e.g. basal forebrain cholingeric neurons
and motor neurons in the midbrain, hindbrain and spinal
cord). Nkx2.1 mutants lack TrkA-expressing cells in their
telencephalons and therefore presumably lack
cholinergic neurons.
In addition to the loss of TrkA/cholinergic cells, Nkx2.1
mutants lack cells that migrate from the MGE through the LGE
and into the entire cerebral cortex. Thus, the Nkx2.1 mutants,
like the Dlx1/Dlx2 double mutants,
have a reduction of GABAergic neurons in the cerebral cortex.
However, whereas the Dlx1/Dlx2 double mutants have
abnormal differentiation/migration of late born cells from both
the LGE and the MGE, the Nkx2.1 mutants lack an MGE, but
have a LGE. This has allowed an initial dissection of the relative
contributions of the MGE and the LGE to telencephalic
tangentially migrating cells. For instance, whereas the
Dlx1/Dlx2 mutants lack GABAergic interneurons in the
olfactory bulb,
the olfactory bulb in the Nkx2.1 mutants appears normal. This shows that the MGE is not required to make
interneuron precursors that follow the rostral migratory stream
from the basal ganglia to the olfactory bulb. Likewise there are differences between the
Dlx1/Dlx2 and the Nkx2.1 mutants in the distribution of
interneuron loss in the paleocortex and neocortex.
These results suggest that there are several distinct basal
telencephalon sources that produce cells that migrate to the
cortex. It is hypothesized that the Nkx2.1-positive ventral
telencephalon (including the MGE, POA and AEP) produces
cells expressing acetylcholine, calbindin and GABA, all of which
migrate dorsally into the LGE. At least some of the calbindin-positive
and GABAergic cells continue to migrate and populate
the paleocortex, neocortex and hippocampus. It is suggested that
the LGE produces GABAergic cells that migrate rostrally into
the olfactory bulb and perhaps the cortex, and there may be a
separate dorsal migration of GABAergic cells from the LGE
to the cortex (Sussel, 1999 and references).
NKX2.1, a vertebrate homolog of Drosophila NK2, more properly termed Ventral nervous system defective, is a homeodomain transcriptional factor expressed in thyroid, lung, and parts of the brain. Septation of the anterior foregut along the dorsoventral axis into distinct tracheal and
esophageal structures is blocked in mouse embryos carrying a homozygous targeted disruption of the
Nkx2.1 locus. This is consistent with the loss of Nkx2.1 expression, which defines the dorsoventral
boundary within the anterior foregut in wild-type E9 embryos. Failure in septation between the trachea
and the esophagus in Nkx2.1(-/-) mice leads to the formation of a common lumen that connects the
pharynx to the stomach, serving both as trachea and as esophagus, similar in phenotype to a human
pathologic condition termed tracheoesophageal fistula. The main-stem bronchi bifurcate from this
common structure and connect to profoundly hypoplastic lungs. The mutant lungs fail to undergo
normal branching embryogenesis: the lung defect consists of highly dilated sacs that are not capable of sustaining
normal gas exchange functions, and leads to immediate postnatal death. In situ hybridization suggests
reduced Bmp-4 expression in the mutant lung epithelium, providing a possible mechanistic clue for
impaired branching. Functional deletion of Nkx2.1 blocks pulmonary-specific epithelial cell
differentiation marked by the absence of pulmonary surfactant protein gene expression. Altered
expression of temporally regulated genes (such as Vegf) demonstrates that the lung in Nkx2.1(-/-)
mutant embryos is arrested at an early pseudoglandular (E11-E15) stage. These results demonstrate a
critical role for Nkx2.1 in morphogenesis of the anterior foregut and the lung as well as in
differentiation of pulmonary epithelial cells (Minoo, 1999).
NKX2.3 of mouse is related to both Drosophila homeobox protein Vnd and to the Drosophila protein Scarecrow. Targeted disruption of the transcription factor NKX2.3 gene in mice results in anatomical defects of intestine and secondary lymphoid
organs. Spleen and Peyer's patches of NKX2.3-deficient mice are considerably reduced in size and lack the
ordered tissue architecture. T and B cells are misplaced within the spleen and mesenteric lymph nodes and fail to segregate into the
appropriate T and B cell areas. Furthermore, splenic marginal zones, characterized by specific B cells and various types of
macrophage-derived cells around the marginal sinus, are absent in mutants. Homozygous NKX2.3 mutants lack the mucosal addressin
cell adhesion molecule-1 (MAdCAM-1) that is normally expressed in mucosa-associated lymphoid tissue (MALT) and spleen. NKX2.3 can activate MAdCAM-1 transcription directly, suggesting that MAdCAM-1 is at least partly responsible for the migration and
homing defects of lymphocytes and macrophages in mutants. Therefore, expression of MAdCAM-1 seems to be required for building functional structures in spleen
and MALT, a prerequisite for unimpaired migration and segregation of B and T cells to and within these organs (Pabst, 2000).
The homeodomain-containing transcription factor Nkx2.9 is expressed in the ventralmost neural progenitor domain of the neural tube together with the related protein Nkx2.2 during early mouse embryogenesis. Cells within this region give rise to V3 interneurons and visceral motoneurons in spinal cord and hindbrain, respectively. To investigate the role of the Nkx2.9 gene, Nkx2.9 deficiency was generated by targeted gene disruption. Homozygous mutant animals lacking Nkx2.9 are viable and fertile with no apparent morphological or behavioral phenotype. The distribution of neuronal progenitor cells and differentiated neurons in spinal cord appears unaffected in Nkx2.9-deficient animals. This finding is in contrast to Nkx2.2-null mutants, which have been shown to exhibit ventral to dorsal transformation of neuronal cell fates in spinal cord. These results suggest that specification of V3 interneurons in the posterior CNS does not require Nkx2.9, most probably because of functional redundancy with the co-expressed Nkx2.2 protein. In hindbrain, however, absence of Nkx2.9 results in a significantly altered morphology of the spinal accessory nerve (XIth), which appears considerably shorter and thinner than in wild-type animals. Consistent with this phenotype, immature branchial motoneurons of the spinal accessory nerve, which normally migrate from a ventromedial to a dorsolateral position within the neural tube, are markedly reduced in Nkx2.9-deficient embryos at E10.5, while ventromedial motor column cells were increased in numbers. In addition, the vagal and glossopharyngeal nerves appear abnormal in approximately 50% of mutant embryos, which may be related to the observed reduction of Phox2b expression in the nucleus ambiguus of adult mutant mice. From these observations, it is concluded that Nkx2.9 has a specific function in the hindbrain as a determinant of the branchial motoneuron precursor cells for the spinal accessory nerve and possibly other nerves of the branchial-motor column. Like other Nkx genes expressed in the CNS, Nkx2.9 seems to be involved in converting positional information into cell fate decisions (Pabst, 2003).
Thus, expression of Nkx2.9 in brain parallels that of Nkx2.2
and continues until at least E11 of embryogenesis, in contrast to its early
repression in spinal cord. In line with the prolonged presence of Nkx2.9 in
brain domains and supporting the idea of redundant activities, Nkx2.2-deficient mice exhibit no obvious phenotypic switch in motoneuron identity within the hindbrain. In the reverse situation presented here by the Nkx2.9 knockout mouse, e.g. lack of Nkx2.9 but continuing presence of Nkx2.2, the phenotypic rescue is at least incomplete. In hindbrain neuronal progenitors of the p3 domain that
co-express Nkx2.2 and Nkx2.9 give rise to branchiovisceral
motoneurons. These cells can be identified by the expression of the
transcription factor Phox2b. In E10.5 mutant embryos, Phox2b expression appears
essentially normal in hindbrain at different axial levels, indicating that the
formation of branchiovisceral motoneurons is not generally dependent on Nkx2.9
function but rather affects a subset or only some of these cells.
Significantly, the population of presumptive branchial motoneurons of the
spinal accessory nucleus, which are characterized by expressing Isl1 alone and
their dorsolateral position in neural tube, is markedly reduced in the mutant
mouse. Moreover, the population of somatic motoneurons of the median motor
column, which co-express Isl1, Isl2 and Lim3, is increased. This result
somewhat resembles the cell fate switch observed in the spinal cord of
Nkx2.2 mutants with respect to the increase of somatic motoneurons at
the expense of another neuronal subpopulation originating in the
Nkx2.2/Nkx2.9 domain. Whether this reflects aberrant
specification of neuronal identity in response to graded Shh signaling, as
is the case in Nkx2.2 mutants, cannot be decided easily here, because
the precise local relationship of the premigratory spinal accessory nucleus progenitors and the median motor column precursors is not clear. Consistent with the reduction of branchial motoneurons in the dorsolateral position of the neural tube at the level of C4-C3 a severely abnormal spinal accessory nerve was found in all
mutant mice. The partial phenotype of the glossopharyngeal and the vagal
nerves in ~50% of mutant embryos may be related to the finding that, in
the absence of Nkx2.9, Phox2b expression in cells of the nucleus ambiguus is
drastically reduced, although the number of cells appears largely unaltered.
Since Nkx2.9 is only expressed in the progenitor domain and Phox2b-positive
progenitors are present in normal numbers in the neuroepithelium of mutant
embryos (E10.5), but not in the mature nucleus ambiguus, Nkx2.9 seems to have a
function in maintaining the phenotypic trait of branchial motoneurons rather
than specifying them. Another unknown transcription factor is possible
involved acting downstream of Nkx2.9. Whether the alterations in the nucleus
ambiguus also contribute to the mutant phenotype of the spinal accessory nerve
appears disputable, since the existence of projections from the nucleus ambiguus
has been recently questioned, at least in humans. The
three nerves affected in the mutant belong to the branchial motor column,
suggesting that Nkx2.9 in the hindbrain has a unique role in formation of
branchial motoneurons, a function that can not be fully substituted for by
Nkx2.2. Clearly, visceral motoneurons are not affected by the Nkx2.9
mutation. It is also interesting to note
that the more rostrally located branchial motor nerves, such as the facialis
and the trigeminus nerve, appear quite normal in mutants, suggesting a
differential requirement for Nkx2.9 along the rostrocaudal axis. Whether the
fractional loss of branchial motoneurons in hindbrain of Nkx2.9
mutants is due to partial rescue by redundant Nkx2.2 function or,
alternatively, reflects the total loss of a neuronal subpopulation whose fate
is entirely dependent on Nkx2.9, cannot be decided unequivocally by the
available data. The latter possibility, however, seems less likely given the
reduced size but not the complete absence of the affected nerves. Taken
together, these data provide evidence that Nkx2.9 is a crucial transcription
factor for the determination and/or differentiation of at least a subset of
branchial motoneurons during hindbrain development. Its early role in
establishing the p3-domain in spinal cord remains to be determined in
Nkx2.2/Nkx2.9 double mutants (Pabst, 2003).
Regional patterning of the mammalian telencephalon requires the function of three homeodomain-containing transcription factors, Pax6, Gsh2 and Nkx2.1. These factors are required for the development of the dorsal, lateral and medial domains of the telencephalon, respectively. Pax6 and Gsh2 have been shown to cross-repress one another in the formation of the border between dorsal and lateral region of the telencephalon. This study examines whether similar interactions are responsible for the establishment of other boundaries of telencephalic gene expression. Surprisingly, despite the fact that, at specific times in development, both Pax6 and Gsh2 maintain a complementary pattern of expression with Nkx2.1, in neither case are these boundaries maintained through a similar cross-repressive mechanism. Rather, as revealed by analysis of double-mutant mice, Nkx2.1 and Gsh2 act cooperatively in many aspects to pattern the ventral telencephalon. By contrast, as indicated by both loss- and gain-of-function analysis, Gsh2 expression in the medial ganglionic eminence after E10.5 may negatively regulate Nkx2.1 dependent specification of oligodendrocytes. Taken together with previous studies, a hierarchy of gene expression for producing interneurons and oligodendrocytes is becoming apparent. Initiating the generation of these cell types in ventral regions are extrinsic cues, including Shh. These cues result in the expression of homeodomain genes, including Nkx2.1 and Gsh2, that ensure the expression of pan-ventral transcription factors, such as Dlx1/2, Mash1 and Olig2, in the MGE and LGE. These genes, in turn, may act as key effectors in the generation of specific ventral cell types, such as interneurons, and distinct populations of oligodendrocytes (Corbin, 2003).
In vitro studies have suggested that members of the GATA and Nkx
transcription factor families physically interact, and synergistically
activate pulmonary epithelial- and cardiac-gene promoters. However, the
relevance of this synergy has not been demonstrated in vivo. This study shows that
Gata6-Titf1 (Gata6-Nkx2.1) double heterozygous
(G6-Nkx DH) embryos and mice have severe defects in pulmonary
epithelial differentiation and distal airway development, as well as reduced
phospholipid production. The defects in G6-Nkx DH embryos and mice
are similar to those observed in human neonates with respiratory distress
syndromes, including bronchopulmonary dysplasia, and differential gene
expression analysis reveals essential developmental pathways requiring
synergistic regulation by both Gata6 and Titf1 (Nkx2.1). These studies
indicate that Gata6 and Nkx2.1 act in a synergistic manner to direct pulmonary
epithelial differentiation and development in vivo, providing direct evidence
that interactions between these two transcription factor families are crucial
for the development of the tissues in which they are co-expressed (Zhang, 2007).
The Sox2 transcription factor is expressed early in the stem cells of the blastocyst inner cell mass and, later, in neural stem cells. Previous work has identified a Sox2 5'-regulatory region directing transgene expression to the inner cell mass and, later, to neural stem cells and precursors of the forebrain. This study identified a core enhancer element able to specify transgene expression in forebrain neural precursors of mouse embryos, and the same core element was shown to efficiently activate transcription in inner cell mass-derived embryonic stem (ES) cells. Mutation of POU factor binding sites, able to recognize the neural factors Brn1 and Brn2, shows that these sites contribute to transgene activity in neural cells. The same sites are also essential for activity in ES cells, where they bind different members of the POU family, including Oct4, as shown by gel shift assays and chromatin immunoprecipitation with anti-Oct4 antibodies. These findings indicate a role for the same POU binding motifs in Sox2 transgene regulation in both ES and neural precursor cells. Oct4 might play a role in the regulation of Sox2 in ES (inner cell mass) cells and, possibly, at the transition between inner cell mass and neural cells, before recruitment of neural POU factors such as Brn1 and Brn2 (Catena, 2004).
Human embryonic stem cells (hESCs) offer a platform to bridge what has been learned from animal studies to human biology. Using oligodendrocyte differentiation as a model system, this study shows that sonic hedgehog (SHH)-dependent sequential activation of the transcription factors OLIG2, NKX2.2 and SOX10 is required for sequential specification of ventral spinal OLIG2-expressing progenitors, pre-oligodendrocyte precursor cells (pre-OPCs) and OPCs from hESC-derived neuroepithelia, indicating that a conserved transcriptional network underlies OPC specification in human as in other vertebrates. However, the transition from pre-OPCs to OPCs is protracted. FGF2, which promotes mouse OPC generation, inhibits the transition of pre-OPCs to OPCs by repressing SHH-dependent co-expression of OLIG2 and NKX2.2. Thus, despite the conservation of a similar transcriptional network across vertebrates, human stem/progenitor cells may respond differently to those of other vertebrates to certain extrinsic factors (Hu, 2009).
Inaccessibility to human embryo experimentation calls for an alternative, in vitro model to study human cells or tissues directly. In recent years, directed neural differentiation from human embryonic stem cells (hESCs) has allowed a re-examination of the fundamental principles of early neural development learned from vertebrate studies. The present study revealed that human oligodendrocyte development involves a transcriptional network of nearly identical sequence to that observed in vertebrate models. Following expression of OLIG2 and genesis of motoneurons, the human OLIG2 progenitors become pre-OPCs by co-expressing NKX2.2, and finally differentiate into OPCs by activation of SOX10 and PDGFRα. Blocking OLIG2 expression inhibits OPC production, confirming the requirement of OLIG2 for human OPC specification. The vast majority of human OLIG2 progenitors are generated in a SHH-dependent manner, because cultures without exogenous SHH, or those in which endogenous SHH signaling has been blocked with cyclopamine, have few OLIG2 progenitors and OPCs. It was also found that SHH is not only crucial for efficiently inducing pre-OPCs, but it is also required for the transition from pre-OPCs to OPCs, for which endogenous SHH signaling is sufficient. Thus, the SHH-dependent signaling network underlying vertebrate OPC development is conserved in humans (Hu, 2009).
This study also reveals unique aspects of human OPC generation. In human OPC differentiation cultures, NKX2.2 is the first OPC-related transcription factor that is co-expressed with OLIG2 at the fifth week, preceding the expression of PDGFRα. This expression pattern resembles that in the chick and in the mouse hindbrain, but differs from that in the mouse spinal cord, where NKX2.2 expression is induced after PDGFRα+ OPCs are formed. This might be partly related to the hindbrain/cervical spinal identity of the human progenitors patterned by RA, although species differences cannot be excluded. A protracted transition period was found from pre-OPCs at the fifth week to human OPCs at the fourteenth week. In chick and mouse brainstem, the OLIG2+ NKX2.2+ progenitors quickly express PDGFRα and become migrating OPCs. OLIG2 progenitors differentiated from mESCs also co-express PDGFRα and NG2 shortly following motoneuron differentiation. It is possible that the serum-free culture conditions are not optimal for acquisition of the OPC identity at an earlier time. The addition of FGF2, EGF, SHH or noggin, however, did not accelerate the transition of pre-OPCs to OPCs. Similarly, removal of PDGF, NT3 and/or IGF1 did not alter the time course of OPC generation either. Alternatively, this protracted transition might be intrinsically controlled. The long OPC-specification process (3 months) appears to match the earliest appearance of OPCs in human embryos at the end of the first trimester. One potential explanation for the late appearance of OPCs is a need for expanding neurogenic progenitors, as large numbers of neurons are needed for the evolutionarily enlarged human brain and spinal cord (Hu, 2009).
FGF2 is a mitogen for rodent and human neural stem/progenitor cells and enhances the generation of OPCs in rodents. In the present study, it is interesting that following the induction of OLIG2, FGF2 nearly completely blocked motoneuron differentiation and preferentially increased the proportion of OLIG2 progenitors. The inhibition of motoneuron differentiation is not attributable to the mitogenic effect of FGF2 preventing the OLIG2 progenitors from exiting the cell cycle. Nor does FGF2 preferentially promote the survival of the OLIG2 progenitors. It is likely that FGF2 enhances the transition from the neurogenic OLIG2 progenitors to pre-OPCs. Although fine dissection of the mechanism is not possible with the available system, this finding provides, as it stands, a simple way of switching neurogenesis to gliogenesis from a pool of progenitors (Hu, 2009).
Continued use of FGF2 inhibited the generation of OPCs from hESCs. This is reminiscent of observations in the past decade that human neural stem/progenitor cells, following expansion with FGF2, or FGF2 and EGF, rarely produce oligodendrocytes in vitro. Even when the human neural progenitor cultures are enriched for OPCs, the OPCs quickly disappear after culturing in the presence of FGF2. The present finding that co-expression of OLIG2 and NKX2.2 is suppressed by FGF2 in the pre-OPCs explains the phenomenon. The inhibitory effect appears specific to FGF2, as EGF did not affect the co-expression of OLIG2 and NKX2.2 and subsequent generation of OPCs. FGF2 induces OPC generation from mouse neural stem/progenitor cells by activating endogenous SHH signaling or by as yet unknown SHH-independent pathways. In the human cell differentiation system, FGF2 inhibits endogenous SHH expression and significantly increases the level of GLI2 and GLI3, which are downstream repressors of SHH signaling, thus disrupting co-expression of OLIG2 and NKX2.2 in pre-OPCs. This results in the loss of pre-OPCs and the conversion to progenitors expressing OLIG2 (at a low level) or NKX2.2, which generates astrocytes or neurons. Nevertheless, the possibility was not excluded that FGF does this through other oligodendroglial transcription factors, such as SOX10 and ASCL1, or by selectively promoting the survival/expansion of neuronal progenitors in the long-term cultures (Hu, 2009).
The present study has developed an effective strategy for reproducibly directing hESCs to an enriched population of OPCs with an unequivocal oligodendrocyte identity and myelination potential. In the previous reports of OPC differentiation from hESCs, SHH was not applied and the expression of OLIG2 in neural progenitors was not examined by single-cell-resolution immunocytochemistry, but instead by PCR on bulk cultures. Based on these findings, it is believed that the OPCs described in those reports are differentiated from OLIG2 progenitors spontaneously induced by endogenous or alternatively activated SHH signaling. The near pure population of 'OPCs' generated from hESCs without application of SHH cannot be explained by the current model, nor has the result been replicated by in other reports. The identity of the OPCs in that report has not been unequivocally verified either (Hu, 2009).
The SHH-dependent transcriptional network underlying human OPC specification and the time course of OPC generation, consistent with that in human embryo development, revealed in the present study suggest that the hESC differentiation system is a useful tool for understanding the biology of human cells. The divergent responses to common growth factors such as FGF2 between human and other vertebrate cells might be related to the in vitro system, but may also reflect the nature of normal human development. Similar to the present finding, the maintenance of human and mouse ESCs depends on the same transcriptional network. However, the common extrinsic factor, BMP, maintains the self-renewal of mESCs but induces cell differentiation of hESCs. This seemingly 'trivial' deviation has slowed down the translation of findings from mESCs to the establishment of hESCs. Similarly, over the past decade, many laboratories have stumbled in trying to replicate the finding in rodents, so as to differentiate human neural stem/progenitors to OPCs. Thus, the confirmation of conserved principles and the revelation of 'nuances' using the hESC differentiation system might bear significant consequences (Hu, 2009).
Previous work has demonstrated that the character of mouse cortical interneuron subtypes can be directly related to their embryonic temporal and spatial origins. The relationship between embryonic origin and the character of mature interneurons is likely reflected by the developmental expression of genes that direct cell fate. However, a thorough understanding of the early genetic events that specify subtype identity has been hampered by the perinatal lethality resulting from the loss of genes implicated in the determination of cortical interneurons. This study employed a conditional loss-of-function approach to demonstrate that the transcription factor Nkx2-1 is required for the proper specification of specific interneuron subtypes. Removal of this gene at distinct neurogenic time points results in a switch in the subtypes of neurons observed at more mature ages. The strategy reveals a causal link between the embryonic genetic specification by Nkx2-1 in progenitors and the functional attributes of their neuronal progeny in the mature nervous system (Hutt, 2008).
In the zebrafish spinal cord, two classes of neurons develop from the lateral floor plate: Kolmer-Agduhr' (KA') and V3 interneurons. The differentiation of the correct number of KA' cells depends on the activity of the homeobox transcription factor Nkx2.9. This factor acts in concert with Nkx2.2a and Nkx2.2b. These factors are also required for the expression of the zinc-finger transcription factor Gata2 in the lateral floor plate. In turn, Gata2 is necessary for expression of the basic helix-loop-helix transcription factor Tal2 that acts upstream of the GABA-synthesizing enzyme glutamic acid decarboxylase 67 gene (gad67) in KA' cells. Expression of the transcription factor Sim1, which marks the V3 interneurons in the lateral floor plate, depends also on the three Nkx2 factors. sim1 expression does not require, however, gata2 and tal2. KA' cells of the lateral floor plate and the KA' cells located more dorsally in the spinal cord share expression of transcription factors. The functional connections between the different regulatory genes, however, differ in the two GABAergic cell types: although gata2 and tal2 are expressed in KA' cells, they are dispensable for gad67 expression in these cells. Instead, olig2 and gata3 are required for the differentiation of gad67-expressing KA' cells. This suggests that the layout of regulatory networks is crucially dependent on the lineage that differs between KA' and KA' cells (Yang, 2010).
NK-2 homeobox genes and oligodendrocyte development The role of Sonic hedgehog (Shh) in
promoting the generation of oligodendrocytes in the mouse
telencephalon is addressed in this study. In the forebrain, expression of
the early oligodendrocyte markers Olig2, plp/dm20 and
PDGFRalpha corresponds to regions of Shh expression. To
directly test if Shh can induce the development of
oligodendrocytes within the telencephalon,
retroviral vectors were used to ectopically express Shh within the
mouse embryonic telencephalon. Infections
with Shh-expressing retrovirus at embryonic day 9.5 result
in ectopic Olig2 and PDGFRalpha expression by mid-embryogenesis.
By postnatal day 21, cells expressing ectopic Shh overwhelmingly adopt an oligodendrocyte identity. To determine if the loss of telencephalic Shh
correspondingly results in the loss of oligodendrocyte
production, Nkx2.1 mutant mice were studied, in which
telencephalic expression of Shh is selectively lost. In
accordance with Shh playing a role in oligodendrogenesis,
within the medial ganglionic eminence of Nkx2.1 mutants,
the early expression of PDGFRalpha is absent and the level of
Olig2 expression is diminished in this region. In addition,
in these same mutants, expression of both Shh and
plp/dm20 is lost in the hypothalamus. Notably, in the
prospective amygdala region where Shh expression persists
in the Nkx2.1 mutant, the presence of plp/dm20 is
unperturbed. Further supporting the idea that Shh is
required for the in vivo establishment of early
oligodendrocyte populations, expression of PDGFRalpha can
be partially rescued by virally mediated expression of Shh
in the Nkx2.1 mutant telencephalon. Interestingly, despite
the apparent requirement for Shh for oligodendrocyte
specification in vivo, all regions of either wild-type or
Nkx2.1 mutant telencephalon are competent to produce
oligodendrocytes in vitro. Furthermore, analysis of CNS
tissue from Shh null animals definitively shows that, in
vitro, Shh is not required for the generation of
oligodendrocytes. It is proposed that oligodendrocyte
specification is negatively regulated in vivo and that Shh
generates oligodendrocytes by overcoming this inhibition.
Furthermore, it appears that a Shh-independent pathway
for generating oligodendrocytes exists (Nery, 2001).
Olig2, a basic helix-loop-helix (bHLH) transcription factor, is expressed in a restricted domain of the spinal cord ventricular zone that sequentially generates motoneurons and oligodendrocytes. Just prior to oligo-dendrocyte precursor formation, the domains of Olig2 and Nkx2.2 expression switch from being mutually exclusive to overlapping, and Neurogenins1 and 2 are extinguished within this region. Coexpression of Olig2 with Nkx2.2 in the spinal cord promotes ectopic and precocious oligodendrocyte differentiation. Both proteins function as transcriptional repressors in this assay. This effect is blocked by forced expression of Neurogenin1. By contrast, misexpression of Olig2 alone derepresses Neurogenins and promotes motoneuron differentiation. Olig2 therefore functions sequentially in motoneuron and oligodendrocyte fate specification. This dual action is enabled by spatio-temporal changes in the expression domains of other transcription factors with which Olig2 functionally interacts (Zhou, 2001).
If the domain of Olig gene expression indeed determines the site of origin of oligodendrocytes, then the timing of Olig gene expression within the spinal cord might determine the time at which such precursors are specified. Paradoxically, however, Olig2 is also expressed in the pMN domain at earlier stages, when motoneurons are being generated. Moreover, misexpression of Olig2 promotes motoneuron fate specification. These observations suggest either that Olig genes function in motoneuron and not oligodendrocyte fate determination, or alternatively that they play a role in both processes. To address this question, a chicken homolog of Olig2 has been identified, and its expression and function in relation to that of other transcription factors involved in neuronal differentiation in the ventral spinal cord has been examined (Zhou, 2001).
At E3.0, the time that motoneurons are being generated, the ventral boundary of the Olig2 expression domain is adjacent to, and nonoverlapping with, that of the underlying Nkx2.2 domain. However, just before oligodendrocyte precursors start to be produced (E6-E7), these two boundaries become overlapping, and numerous Olig2+, Nkx2.2+ cells are seen migrating away from the ventricular zone in a pattern characteristic of oligodendrocyte precursors. Combined misexpression of both Olig2 and Nkx2.2, but not of either gene alone, causes ectopic and precocious oligodendrocyte differentiation in the spinal cord. The normal generation of oligodendrocytes is also preceded by an extinction of the proneural bHLH factors Ngn1 and Ngn2 from the Olig2+ domain. Forced expression of Ngn1 blocks both ectopic oligodendrocyte differentiation induced by coexpression of Olig2+Nkx2.2 as well as normal oligodendrocyte differentiation. Taken together, these data suggest that Olig2 plays sequential roles in both motoneuron and oligodendrocyte fate specification. This dual action is enabled by spatio-temporal changes in the expression domains of other transcription factors expressed in the ventral spinal cord, with which Olig2 functionally interacts (Zhou, 2001).
In the vertebrate spinal cord, oligodendrocytes (OL) arise from
the ventral part of the neuroepithelium, a region also
known to generate somatic motoneurons. The emergence of
oligodendrocytes, like that of motoneurons, depends on an
inductive signal mediated by Sonic hedgehog. The precise timing of oligodendrocyte progenitor specification in the cervico-brachial spinal cord of the chick
embryo has been determined. Ventral neuroepithelial explants,
isolated at various development stages, are unable to
generate oligodendrocytes in culture until E5 but become
able to do so in an autonomous way from E5.5. This
indicates that the induction of oligodendrocyte precursors
is a late event that occurs between E5 and E5.5, precisely
at the time when the ventral neuroepithelium stops
producing somatic motoneurons. Analysis of the spatial
restriction of oligodendrocyte progenitors, evidenced by
their expression of O4 or PDGFRalpha, indicate that they
always lie within the most ventral Nkx2.2-expressing
domain of the neuroepithelium, and not in the adjacent
domain characterized by Pax6 expression from which
somatic motoneurons emerge. Shh is necessary between E5 and E5.5 to specify oligodendrocyte precursors but is no longer required beyond this stage
to maintain ongoing oligodendrocyte production.
Furthermore, Shh is sufficient to induce oligodendrocyte
formation from ventral neuroepithelial explants dissected
at E5. Newly induced oligodendrocytes express Nkx2.2
but not Pax6, correlating with the in vivo observation.
Altogether, these results show that, in the chick spinal
cord, oligodendrocytes originate from Nkx2.2-expressing
progenitors (Soula, 2001).
It is now well established that somatic MNs derive from the
ventral-most part of the Pax6+ region of the neuroepithelium
in the chick spinal cord and not from the
Nkx2.2+ domain. Recent evidence indicates that Nkx2.2
represses somatic MN genesis and restricts their emergence to
the ventral-most part of the Pax6-expressing domain. In addition, a mechanism of
mutual repression between the two factors leads to the
formation of a sharp boundary between their two domains of
expression. Double-staining experiments confirm that at
E5.5 the last MNR2+ somatic MNs to be produced migrate
from a level located dorsal to the OL/Nkx2.2 domain.
Therefore, in chick spinal cord, OL and somatic MN precursors
do not share the same transcription factors and do not originate
from the same site in the neuroepithelium. The production of OLPs from a region different from the
domain of somatic MN origin does not exclude the existence
of common progenitor for OLs and other populations of
ventral neurons. Indeed, in the spinal cord, the Nkx2.2+
neuroepithelial domain generates at least one population of
uncharacterized V3 neurons, which in the mouse and chick embryos express the bHLH transcription factor Sim1. Similarly, in the hindbrain, OLPs could share a common progenitor with visceral MNs that are also generated from Nkx2.2-expressing neuroepithelial cells (Soula, 2001).
The relationship of Olig2+ and Nkx2.2+ oligodendrocyte progenitors (OLPs) has been investigated by comparing the expression of Olig2 and Nkx2.2 in embryonic chicken and mouse spinal cords before and during the stages of oligodendrogenesis. At the stages of neurogenesis, Olig2 and Nkx2.2 are expressed in adjacent non-overlapping domains of ventral neuroepithelium. During oligodendrogenesis stages, these two domains generate distinct populations of OLPs. From the Olig2+ motoneuron precursor domain (pMN) arise the Olig2+/Pdgfra+ OLPs, whereas the Nkx2.2+ p3 domain gives rise to Nkx2.2+ OLPs. Despite their distinct origins, both populations of OLPs eventually appear to co-express Olig2 and Nkx2.2 in the same cells. However, there is a species difference in the timing of acquiring Nkx2.2 expression by the Olig2+/Pdgfra+ OLPs. The co-expression of Nkx2.2 and Olig2 in OLPs is tightly associated with myelin gene expression in the normal and PDGFA-/- embryos, suggesting a cooperative role for these transcription factors in the control of oligodendrocyte differentiation. In support of this suggestion, inhibition of expression of these two transcription factors in culture by antisense oligonucleotides has an additive inhibitory effect on OLP differentiation and proteolipid protein (PLP) gene expression (Fu, 2002).
Development of oligodendrocytes, myelin-forming glia in the central nervous system (CNS), proceeds on a protracted schedule. Specification of oligodendrocyte progenitors (OLPs) begins early in development, whereas their terminal differentiation occurs at late embryonic and postnatal periods. How these distinct steps are controlled remains unclear. The helix-loop-helix (HLH) transcription factor Ascl1 plays an important role in early generation of OLPs in the developing spinal cord. Ascl1 is also involved in terminal differentiation of oligodendrocytes late in development. Ascl1-/- mutant mice showed a deficiency in differentiation of myelin-expressing oligodendrocytes at birth. In vitro culture studies demonstrate that the induction and maintenance of co-expression of Olig2 and Nkx2-2 in OLPs, and thyroid hormone-responsive induction of myelin proteins are impaired in Ascl1-/-. Gain-of-function studies further showed that Ascl1 collaborates with Olig2 and Nkx2-2 in promoting differentiation of OLPs into oligodendrocytes in vitro. Overexpression of Ascl1, Olig2 and Nkx2-2 alone stimulates the specification of OLPs, but the combinatorial action of Ascl1 and Olig2 or Nkx2-2 is required for further promoting their differentiation into oligodendrocytes. Thus, Ascl1 regulates multiple aspects of oligodendrocyte development in the spinal cord (Sugimori, 2008).
Astrocytes constitute the most abundant cell type in the central nervous system (CNS) and play diverse functional roles, but the ontogenetic origins of this phenotypic diversity are poorly understood. This study investigated whether positional identity, a fundamental organizing principle governing the generation of neuronal subtype diversity, is also relevant to astrocyte diversification. Three positionally distinct subtypes of white-matter astrocytes (WMA) were identified in the spinal cord, that can be distinguished by the combinatorial expression of Reelin and Slit1. These astrocyte subtypes derive from progenitor domains expressing the homeodomain transcription factors Pax6 and Nkx6.1, respectively. Loss- and gain-of-function experiments indicate that the positional identity of these astrocyte subtypes is controlled by Pax6 and Nkx6.1 in a combinatorial manner. Thus, positional identity is an organizing principle underlying astrocyte, as well as neuronal, subtype diversification and is controlled by a homeodomain transcriptional code whose elements are reutilized following the specification of neuronal identity earlier in development (Hochstim, 2008).
Reelin and Slit1 were not expressed by all astrocytes, but rather by positionally distinct subsets in the ventral white matter. Reelin was expressed in the dorsolateral and ventrolateral white matter, but not in astrocytes close to the ventral midline. Slit1, conversely, was expressed in astrocytes in the ventromedial and ventrolateral white matter, but not in the dorsolateral white matter. Double labeling for Reelin and Slit1 revealed the existence of three adjacent domains of WMAs: a dorso-lateral domain of Reelin+, Slit1− cells; a ventro-lateral domain of Reelin+, Slit1+ cells; and a ventro-medial domain of Slit1+, Reelin− cells. For convenience, these subpopulations henceforth are referred to as ventral astrocyte subtypes 1, 2, and 3 (VA1, VA2, and VA3, respectively. Quantification indicated that each of these three subpopulations is present in roughly equal numbers (Hochstim, 2008).
In principle, VA1-VA3 phenotypes could be established after astrocyte precursors migrate to the WM, under the influence of local environmental cues, or could be specified by positional mechanisms prior to emigration from the VZ. As a first step toward addressing this question, it was asked whether Reelin and Slit1 were expressed by positionally distinct subsets of astrocyte precursors within the neuroepithelium. Examination of spinal cord sections at E13.5, a stage when most astrocyte precursors have been specified in the ventral VZ, revealed that Reelin and Slit1 are expressed in cells within the germinal layer. Triple labeling for Reelin, Slit1-GFP, and NFIA indicated that Reelin and Slit1 are expressed by NFIA+ glial precursors and that the domains of their expression partially overlap. This partial overlap subdivides the ventral-most VZ into three domains: a dorsal-most Reelin+, Slit1− domain; a more ventral Reelin+, Slit1+ domain; and a ventro-medial Reelin−, Slit1+ domain. The spatial organization of these progenitor domains, which are referred to as pA1, pA2, and pA3, respectively, therefore mirrors that of the VA1, VA2, and VA3 domains in the WM (Hochstim, 2008).
The homeodomain transcription factor Nkx2-1 plays key roles in the developing telencephalon, where it regulates the identity of progenitor cells in the medial ganglionic eminence (MGE) and mediates the specification of several classes of GABAergic and cholinergic neurons. This study investigated the postmitotic function of Nkx2-1 in the migration of interneurons originating in the MGE. Experimental manipulations and mouse genetics show that downregulation of Nkx2-1 expression in postmitotic cells is necessary for the migration of interneurons to the cortex, whereas maintenance of Nkx2-1 expression is required for interneuron migration to the striatum. Nkx2-1 exerts this role in the migration of MGE-derived interneurons by directly regulating the expression of a guidance receptor, Neuropilin-2, which enables interneurons to invade the developing striatum. These results demonstrate a role for the cell-fate determinant Nkx2-1 in regulating neuronal migration by direct transcriptional regulation of guidance receptors in postmitotic cells (Nóbrega-Pereira, 2008).
The upper layers (II-IV) are the most prominent distinguishing feature of mammalian neocortex compared with avian or reptilian dorsal cortex, and are vastly expanded in primates. Although the time-dependent embryonic generation of upper-layer cells is genetically instructed within their parental progenitors, mechanisms governing cell-intrinsic fate transitions remain obscure. POU-homeodomain transcription factors Pou3f3 and Pou3f2 (Brn1 and Brn2) are known to label postmitotic upper-layer cells, and are redundantly required for their production. This study found that the onset of Pou3f3/2 expression actually occurs in ventricular zone (VZ) progenitors, and that Pou3f3/2 subsequently label neural progeny switching from deep-layer Ctip2(+) identity to Satb2(+) upper-layer fate as they migrate to proper superficial positions. By using an Engrailed dominant-negative repressor, this study showed that sustained neurogenesis after the deep- to upper-layer transition requires the proneural action of Pou3fs in VZ progenitors. Conversely, single-gene overexpression of any Pou3f in early neural progenitors is sufficient to specify the precocious birth of Satb2(+) daughter neurons that extend axons to the contralateral hemisphere, as well as exhibit robust pia-directed migration that is characteristic of upper-layer cells. Finally, this study demonstrated that Pou3fs influence multiple stages of neurogenesis by suppressing Notch effector Hes5, and promoting the expression of proneural transcription factors Tbr2 and Tbr1 (Dominguez, 2012).
Otx2 plays essential roles in rostral brain development, and its counteraction with Gbx2 has been suggested to determine the midbrain-hindbrain boundary (MHB) in vertebrates. The FM enhancer has been identified that is conserved among vertebrates and drives Otx2 transcription in forebrain/midbrain from the early somite stage. This study found that the POU homeodomain of class III POU factors (Brn1, Brn2, Brn4, and Oct6) associates with noncanonical target sequence TAATTA in the FM enhancer. MicroRNA-mediated knockdown of Brn2 and Oct6 diminished the FM enhancer activity in anterior neural progenitor cells (NPCs) differentiated from P19 cells. The class III POU factors associate with the FM enhancer in forebrain and midbrain but not in hindbrain. It was also demonstrated that the Gbx2 homeodomain recognizes the same target TAATTA in the FM enhancer, and Gbx2 associates with the FM enhancer in hindbrain. Gbx2 misexpression in the anterior NPCs represses the FM enhancer activity and inhibits Brn2 association with the enhancer, whereas Gbx2 knockdown caused ectopic Brn2 association in the posterior NPCs. These results suggest that class III POU factors and Gbx2 share the same target site, TAATTA, in the FM enhancer and that their region-specific binding restricts Otx2 expression at the MHB (Inoue, 2012).
This study aimed to identify cis-regulatory elements that control gene expression in progenitors of the cerebral cortex. A list of 975 putative enhancers were retrieved from a ChIP-Seq experiment performed in NS5 mouse stem cells with antibodies to Sox2, Brn2/Pou3f2, or Brn1/Pou3f3. Through a selection pipeline including gene ontology and expression pattern, the number of candidate enhancer sequences was reduced to 20. Ex vivo electroporation of green fluorescent pProtein (GFP) reporter constructs in the telencephalon of mouse embryos showed that 35% of the 20 selected candidate sequences displayed enhancer activity in the developing cortex at E13.5. In silico transcription factor binding site (TFBS) searches and mutagenesis experiments showed that enhancer activity is related to the presence of Sox/Pou TFBS pairs in the sequence. Comparative genomic analyses showed that enhancer activity is not related to the evolutionary conservation of the sequence. Finally, the combination of in utero electroporation of GFP reporter constructs with immunostaining for Tbr2 (basal progenitor marker) and phospho-histoneH3 (mitotic activity marker) demonstrated that each enhancer is specifically active in precise subpopulations of progenitors in the cortical germinal zone, highlighting the heterogeneity of these progenitors in terms of cis-regulation (Retaux, 2013).
The progenitor zones of the embryonic mouse ventral telencephalon give rise to GABAergic and cholinergic neurons. Two LIM-homeodomain (LIM-HD) transcription factors, Lhx6 and Lhx8, that are downstream of Nkx2.1, are critical for the development of telencephalic GABAergic and cholinergic neurons. This study investigated the role of Ldb1, a nuclear protein that binds directly to all LIM-HD factors, in the development of these ventral telencephalon derived neurons. Ldb1 is expressed in the Nkx2.1 cell lineage during embryonic development and in mature neurons. Conditional deletion of Ldb1 causes defects in the expression of a series of genes in the ventral telencephalon and severe impairment in the tangential migration of cortical interneurons from the ventral telencephalon. Similar to the phenotypes observed in Lhx6 or Lhx8 mutant mice, the Ldb1 conditional mutants show a reduction in the number of both GABAergic and cholinergic neurons in the telencephalon. Furthermore, defects were shown in the development of the parvalbumin-positive neurons in the globus pallidus and striatum of the Ldb1 mutants. These results provide evidence that Ldb1 plays an essential role as a transcription co-regulator of Lhx6 and Lhx8 in the control of mammalian telencephalon development (Zhao, 2014).
NK-2 homeobox genes - thyroid and pancreas expression and function The cDNA for TTF-1 (the same as Nkx2.1), a thyroid nuclear factor that binds to the promoter of thyroid specific genes, has been cloned. The protein
encoded by the cDNA shows binding properties indistinguishable from those of TTF-1 present in nuclear extracts of differentiated rat
thyroid cells. The DNA binding domain of TTF-1 is a novel mammalian homeodomain that shows considerable sequence homology to
the Drosophila NK-2 homeodomain. TTF-1 mRNA and corresponding binding activity are detected in thyroid and lung. The
chromosomal localization of the TTF-1 gene has been determined in humans and mice and corresponds to chromosomes 14 and 12,
respectively, demonstrating that the TTF-1 gene is not located within previously described clusters of homeobox-containing genes (Guazzi, 1990).
A cDNA clone encoding a thyroid-specific enhancer-binding protein (T/EBP also known as Nkx2.1) was isolated from a rat thyroid-derived FRTL-5 cell
lambda gt 11 expression library. Nucleotide and deduced amino acid sequences of the cDNA
reveals that T/EBP is identical to the previously reported thyroid-specific transcription factor 1 (TTF-1), which binds to the promoter
of the rat thyroglobulin gene and controls its thyroid-specific expression. Expression of the T/EBP cDNA under control of the human
cytomegalovirus major immediate-early gene promoter confers thyroid-specific enhancer activity as high as 26-fold to
nonpermissive human hepatoma HepG2 cells when cotransfected with a vector containing 6.3 kbp of upstream sequence of the human
thyroid peroxidase gene connected to a luciferase reporter gene. T/EBP was further expressed in HepG2 cells by using the vaccinia
virus expression system. The expressed protein was partially purified by using sequence-specific affinity column chromatography and
was further shown to specifically bind to the enhancer-derived double-stranded oligonucleotide.
These results clearly indicate that the binding of T/EBP (TTF-1) to the specific cis-acting enhancer element is largely responsible for
thyroid-specific enhancer activity (Mizuno, 1991).
The thyroid-specific enhancer-binding protein (T/ebp or Nkx2.1) gene has been disrupted by homologous recombination in embryonic stem cells to
generate mice lacking T/EBP expression. Heterozygous animals develop normally, whereas mice homozygous for the disrupted gene
are born dead and lack the lung parenchyma. Instead, they have a rudimentary bronchial tree associated with an abnormal
epithelium in their pleural cavities. Furthermore, the homozygous mice have no thyroid gland but have a normal parathyroid. In addition,
extensive defects are found in the brain of the homozygous mice, especially in the ventral region of the forebrain. The entire pituitary,
including the anterior, intermediate, and posterior pituitary, is also missing. In situ hybridization shows that the T/ebp gene is
expressed in the normal thyroid, lung bronchial epithelium, and specific areas of the forebrain during early embryogenesis. These results
establish that the expression of T/EBP, a transcription factor known to control thyroid-specific gene transcription, is also essential for
organogenesis of the thyroid, lung, ventral forebrain, and pituitary (Kimura, 1996).
Most insulin-producing beta-cells in the fetal mouse pancreas
arise during the secondary transition, a wave of
differentiation starting at embryonic day 13. Disruption of homeobox gene Nkx6.1 in mice leads to
loss of beta-cell precursors and blocks beta-cell neogenesis
specifically during the secondary transition. In contrast,
islet development in Nkx6.1/Nkx2.2 double mutant embryos
is identical to Nkx2.2 single mutant islet development: beta-cell precursors survive but fail to differentiate into beta-cells
throughout development. Together, these experiments
reveal two independently controlled pathways for beta-cell
differentiation, and place Nkx6.1 downstream of Nkx2.2 in
the major pathway of beta-cell differentiation (Sander, 2000).
Mice lacking the homeodomain transcription factor Nkx2.2 have diabetes due to arrested differentiation of pancreatic beta cells.
Nkx2.2 is the member of the vertebrate homeodomain
transcription factor gene family that is most homologous to the
Drosophila NK2/ventral nervous system defective (vnd) gene.
Nkx2.2 was originally identified as a gene that is expressed in
ventral regions of the developing vertebrate CNS. In addition to Nkx2.2, five other family members have
been identified in mice: Nkx2.1, Nkx2.2, Nkx2.3 and Nkx2.4
are closely related, while
Nkx2.5 and Nkx2.6 represent more divergent members of the
family. NK2 family members have now
been shown to be key regulators of development and
differentiation in several tissues: Nkx2.1 is necessary for lung,
thyroid and ventral forebrain development and Nkx2.5 is
required for proper heart formation. Therefore, it is possible that Nkx2.2 may play a
similar role in the development of the pancreas. The endocrine pancreas is organized into clusters of cells called islets of Langerhans comprizing four well-defined cell types: alpha, beta, delta and PP cells. While recent genetic studies indicate that islet development depends on the function of an integrated network of transcription factors, the specific roles of these factors in early cell-type specification and differentiation remain elusive. Within the pancreas, Nkx2.2 is expressed in alpha, beta and PP cells, but not in delta cells. Mice homozygous for a null mutation of Nkx2.2 develop severe hyperglycemia and die shortly after birth. Immunohistochemical analysis reveals that the mutant embryos lack insulin-producing beta cells and have fewer glucagon-producing alpha cells and PP cells. Remarkably, in the mutants there remains a large population of islet cells that do not produce any of the four endocrine hormones. These cells express some beta cell markers, such as islet amyloid polypeptide and Pdx1 (a homeodomain
transcription factor that is an important factor in the proliferation and differentiation of the pancreatic buds
to form a mature pancreas), but lack other definitive beta cell markers including glucose transporter 2 and Nkx6.1. It is proposed that Nkx2.2 is required for the final differentiation of pancreatic beta cells, and in its absence, beta cells are trapped in an incompletely differentiated state (Sussel, 1998).
Genetic studies are beginning to outline the hierarchy of
transcription factors involved in beta cell development. For
example, in Nkx2.2 mutant embryos, early expression of the beta
cell specific transcription factor Nkx6.1 is unaffected. However,
between E12.5 and E18.5, when there are major changes taking
place within the pancreas (differentiation of exocrine tissue; beta
cell proliferation; delta and PP cell formation) Nkx6.1 expression
disappears. The data are consistent with a model where Nkx2.2
is required for the maintenance of Nkx6.1 expression as beta cells
differentiate, and that continued expression of Nkx6.1 is
necessary for complete beta cell differentiation.
The genetic relationship between Nkx2.2 and Pdx1 is more
complicated. Early in development, Pdx1 is expressed in all
cells of the pancreatic bud, and is required for expansion of the
bud.
However, later in embryonic development, Pdx1 becomes
progressively restricted to beta cells (and some delta cells); and at
approximately E14.5, Pdx1 becomes upregulated in beta cells suggesting it plays a role in beta cell
differentiation. In the Nkx2.2 mutant, early expression of Pdx1
is not affected. The later beta cell restriction of Pdx1 expression
also occurs, but is quantitatively reduced in comparison to
wild-type beta cells. Therefore, Nkx2.2 may be
required for inducing high level expression of Pdx1 in beta cells,
and the up-regulation of Pdx1 may be a necessary step in the
final differentiation of the beta cell.
In contrast to Nkx6.1 or Pdx1 expression, the expression of
Isl1, Pax6 and Brn4 during embryogenesis does not appear to
require Nkx2.2. These genes may therefore either lie upstream
or in different pathways relative to the Nkx genes. Since Isl1 is expressed in islet cells soon after they exit the cell
cycle, normal expression of Isl1 in the
Nkx2.2 mutant suggests that all the islet cells are able to
normally exit a proliferating state and proceed with a program
of differentiation. This result supports the hypothesis that the
immature beta cells are able to initiate beta cell development and it
is subsequent steps of terminal differentiation that are blocked (Sussel, 1998 and references).
The homeodomain protein Nkx2.2 (Nkx2-2) is a key regulator of pancreatic islet cell specification in mice; Nkx2.2 is essential for the differentiation of all insulin-producing β-cells and of the majority of glucagon-producing alpha-cells, and, in its absence, these cell types are converted to a ghrelin cell fate. To understand the molecular functions of Nkx2.2 that regulate these early cell-fate decisions during pancreatic islet development, Nkx2.2-dominant-derivative transgenic mice were created. In the absence of endogenous Nkx2.2, the Nkx2.2-Engrailed-repressor derivative is sufficient to fully rescue glucagon-producing alpha-cells and to partially rescue insulin-producing β-cells. Interestingly, the insulin-positive cells that do form in the rescued mice do not express the mature β-cell markers MafA or Glut2 (Slc2a2), suggesting that additional activator functions of Nkx2.2 are required for β-cell maturation. To explore the mechanism by which Nkx2.2 functions as a repressor in the islet, the pancreatic expression was assessed of the Groucho co-repressors, Grg1, Grg2, Grg3 and Grg4 (Tle1-Tle4), which have been shown to interact with and modulate Nkx2.2 function. Grg3 is highly expressed in the embryonic pancreas in a pattern similar to Nkx2.2. Furthermore, Grg3 physically interacts with Nkx2.2 through its TN domain. These studies suggest that Nkx2.2 functions predominantly as a transcriptional repressor during specification of endocrine cell types in the pancreas (Doyle, 2007).
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