labial

DEVELOPMENTAL BIOLOGY

Embryonic

The lab gene product accumulates in a complex pattern in both embryonic and imaginal tissue. During embryogenesis, lab is expressed in the endodermally derived cells of the midgut, in ectodermally derived cells of the procephalon and dorsal ridge, and in a small subset of progenitor sensory cells. Imaginal expression is restricted to a narrow region of the peripodial membrane of the eye-antennal disc (Chouinard, 1991).

Fos-related antigen is predominantly (if not exclusively) nuclear in all cell types observed. Fra protein accumulates in all endodermal cells in the second gut lobe. This contrasts with labial, which is expressed only in a subset of the endodermal cells in this lobe; lab expression is not detectable in the most posterior cells within this lobe; throughout the lobe, lab-expressing cells are interspersed with cells not expressing lab (Reise, 1997). Fra staining is strongest in the central region of the lobe, fading slightly towards both constrictions (Reise, 1997), whereas Lab staining shows a striking anteroposterior gradient of expression, with highest levels most posterior (Immergluck, 1990).

labial is first expressed in the procephalon [Images], even before it is morphologically recognizeable. This occurs during stage 9. There is a highlighting of the boundary between procephalon and gnathocephalon (future mandibular, maxillary and labial lobes). At the same time, there is expression in the invaginating posterior midgut. labial expression is confined to the ectodermal and endodermal germ layers. There is no detectable expression of labial in the labial lobes. This suggests that the action of labial inhibits labial lobe development in the procephalon (Diederich, 1989).

Expression of labial in the midgut coincides with copper cells. Labial has a role not only in determination and differentiation of copper cells, but also in the maintenance of their differentiated state. The function of copper cells may be to adsorb metal ions from the gut lumin (Hoppler, 1994).

It has been hypothesized that the hypopharyngeal lobe is the ventral aspect of the intercalary segment, in which only the dorsal component expresses lab during stages 10 and 11. The anterior expression boundary of most homeotic genes differs in the ventral ectoderm compared to the dorsolateral ectoderm, so the expression of lab in only the dorsolateral intercalary segment would not be unusual. However, knot ectodermal expression, which appears to mark the hypopharyngeal lobe, is separated by hh-expressing cells from the intercalary anterior compartment in which lab is expressed (the short stripe of hh-expressing cells extends from the ventral margin of the ectoderm where it meets the endoderm in the stomatodeal opening to the dorsal limit of kn ectodermal expression). Furthermore, while the ventral portion of the stripe of kn expression gives rise to the hypopharyngeal lobe, the dorsolateral portion does not appear to give rise to any ectodermal derivatives, but instead to cephalic mesoderm. Thus, the hypopharyngeal lobe is not thought to represent the ventral component of the intercalary segment, but rather is a separate domain posterior to it. Because the anterior compartment cells of the hypopharyngeal lobe primordium are not separated by intervening hh- or en-expressing cells from the anterior mandibular segment, the hypopharyngeal lobe should be considered nominally part of the mandibular segment, although it appears to be defined independently at blastoderm stages, as judged by kn activation. This is in keeping with the mandibular defects found in kn mutants that eliminate the hypopharyngeal lobe (Seecoomar, 2000 and references therein).

Ample evidence indicates, however, that in other insects the hypopharyngeal lobe is part of the intercalary segment. Unlike Drosophila, lab is expressed in the hypopharyngeal lobe in the flea and other insects. In all insects, en-expressing cells marking the posterior compartment of the intercalary segment, form on the posterior margin of the lab-expressing cells. Perhaps this difference in lab expression indicates that what is called the 'hypopharyngeal lobes' in Drosophila are not homologous to the hypopharyngeal lobes of other insects, but rather reflect a novel proturberance in the mandibular segment. Alternatively, it is possible that the Drosophila 'hypopharyngeal lobes' are indeed homologous to the hypopharyngeal lobes of other species, but there has been a concerted shift in Drosophila relative to the head morphology of both lab expression and posterior compartment of the intercalary segment, such that the hypopharyngeal lobe no longer resides in the intercalary segment. Because the hypopharyngeal primordium (via activation of kn) appears to be set aside independently at blastoderm stage, much earlier than the segmentation of the cephalic segments, such a shift in segmentation vis-a-vis the hypopharyngeal lobe seems plausible. Further analysis of the expression of the homeotic genes (such as lab, Dfd and cnc) and the upstream patterning genes (such as kn, btd and other gap/pair-rule genes) in a number of closely and distantly related insects will be necessary to resolve these hypotheses (Seecoomar, 2000 and references therein).

The Drosophila brain develops from the procephalic neurogenic region of the ectoderm. About 100 neural precursor cells (neuroblasts) delaminate from this region on either side in a reproducible spatiotemporal pattern. Neuroblast maps have been prepared from different stages of the early embryo (stages 9, 10 and 11, when the entire population of neuroblasts has formed), in which about 40 molecular markers representing the expression patterns of 34 different genes are linked to individual neuroblasts. In particular, a detailed description is presented of the spatiotemporal patterns of expression in the procephalic neuroectoderm and in the neuroblast layer of the gap genes empty spiracles, hunchback, huckebein, sloppy paired 1 and tailless; the homeotic gene labial; the early eye genes dachshund, eyeless and twin of eyeless; and several other marker genes (including castor, pdm1, fasciclin 2, klumpfuss, ladybird, runt and unplugged). Based on the combination of genes expressed, each brain neuroblast acquires a unique identity, and it is possible to follow the fate of individual neuroblasts through early neurogenesis. Furthermore, despite the highly derived patterns of expression in the procephalic segments, the co-expression of specific molecular markers discloses the existence of serially homologous neuroblasts in neuromeres of the ventral nerve cord and the brain. Taking into consideration that all brain neuroblasts are now assigned to particular neuromeres and individually identified by their unique gene expression, and that the genes found to be expressed are likely candidates for controlling the development of the respective neuroblasts, these data provide a basic framework for studying the mechanisms leading to pattern and cell diversity in the Drosophila brain, and for addressing those mechanisms that make the brain different from the truncal CNS (Urbach, 2003).

The expression was analyzed of the homeotic genes proboscipedia (pb) and labial (lab), both members of the Antennapedia complex and known to be expressed in the head ectoderm and in the brain after mid-embryogenesis. Antibody staining against Pb reveals that at stage 11, the protein is restricted to internal cells of the mandibular segment (presumably mesodermal cells) and to dorsal ectoderm of the maxillary and labial appendages. No Pb protein was detected in brain NBs (Urbach, 2003).

lab has been described as being expressed in the posterior tritocerebrum at stage 14. Using an antibody, the expression of Lab protein during early neurogenesis was investigated. From stage 9 onwards, Lab is detected in the ectoderm of the intercalary segment, and presumably in a small part of the posteroventral antennal segment. At that stage, the only NB expressing Lab protein is Dv2. Double labelling against En reveals that at stage 11 Lab is expressed throughout the ectoderm of the intercalary segment. The Lab domain overlaps posteriorly with the en intercalary stripe (en is), indicating that posterior borders of lab expression and of the intercalary segment coincide. The character of the anterior border of the lab domain is less clear. Dorsally, it runs along the posterior border of the en antennal stripe (en as); ventrally, however, it reaches the anterior border of the en as. This suggests, that the anterior border of the lab domain is segmental in the dorsal region and parasegmental in the ventral region. Interestingly, also for scr and dfd, which are other members of the ANT-C, it has been reported that they initiate expression in a jagged stripe resolving into a pattern that is dorsally segmental and ventrally parasegmental. All NBs arising from the Lab-positive neuroectoderm express lab, among them all tritocerebral NBs and two ventral NBs, which are attributed to the deutocerebrum (Dv2 and Dv4) because they are located on the same anteroposterior level as the en-expressing Dv8 and Dd5 (Urbach, 2003).

ventral veins lacking is required for specification of the tritocerebrum in embryonic brain development of Drosophila; vvl expression in the tritocerebrum is dependent on lab activity

The homeotic or Hox genes encode a network of conserved transcription factors which provide axial positional information and control segment morphology in development and evolution. During embryonic brain development of Drosophila, the Hox gene labial (lab) is essential for tritocerebral neuromere specification; lab loss of function results in tritocerebral cells that fail to adopt a neuronal identity, causing axonal pathfinding defects. Evidence is presented that the POU-homeodomain DNA-binding protein ventral veins lacking (vvl) acts genetically downstream of lab in the specification of the tritocerebral neuromere. In the embryonic brain, vvl expression is seen in all brain neuromeres, including the tritocerebral lab domain. Lab mutant analysis shows that vvl expression in the tritocerebrum is dependent on lab activity. Loss-of-function analysis focussed on the tritocerebrum reveals that inactivation of vvl results in patterning defects which are comparable to the brain phenotype caused by null mutation of lab. In the absence of vvl, mutant tritocerebral cells are generated and positioned correctly, but these cells fail to express neuronal markers indicating defects in neuronal differentiation. Moreover, longitudinal axon pathways in the tritocerebrum are severely reduced or absent and the tritocerebral commissure is missing in the vvl mutant brain. Genetic rescue experiments show that vvl is able to partially replace lab in the specification of the tritocerebral neuromere. These results indicate that vvl acts downstream of the Hox gene lab and regulates specific aspects of neuronal differentiation within the tritocerebral neuromere during embryonic brain development of Drosophila (Meier, 2006).

vvl is expressed in the embryonic brain from the extended germ band stage onwards. For an analysis of the protein distribution pattern of vvl in the embryonic brain, immunocytochemical experiments with a polyclonal antibody against the Vvl protein were carried out in combination with an anti-HRP antibody; anti-HRP immunoreactivity reveals the entire neural lineage of the developing CNS excluding the glial lineage. At late stage 11, vvl expression is first detected in few neuroblasts of the developing brain anlage, and by stage 13 became abundant in neuroblasts and their progeny within each brain neuromere. By stage 15, when neural progeny are generated and axonal projections are formed, Vvl protein is observed in specific cell clusters within all brain neuromeres (Meier, 2006).

Expression of vvl in the tritocerebrum suggests a possible overlap with the expression of the Hox gene lab. To investigate this, double-immunocytochemical experiments were carried out either on transgenic flies expressing a lab-lacZ reporter construct in which antibodies against Vvl were used together with anti-βgal antibodies, or on wildtype embryos in which antibodies against Vvl were used together with anti-Lab antibodies. These experiments revealed that the majority of cells expressing vvl in the tritocerebrum are located within the lab expression domain. Together with the observation that vvl appears to be differentially regulated by lab, the co-expression of lab and vvl in the tritocerebrum suggests that vvl activity might be lab-dependent in this neuromere. To study this further, whether mutational inactivation of lab affects vvl expression in the tritocerebrum was investigated (Meier, 2006).

Mutational inactivation of lab results in regionalized axonal patterning defects which are due to both cell-autonomous and cell-nonautonomous effects. Thus, in the absence of lab, mutant cells are generated and positioned correctly in the brain, but these cells do not extend axons. Additionally, extending axons of neighboring wildtype neurons stop at the mutant domains or project ectopically, resulting in the disruption of the longitudinal connectives and a lack of the tritocerebral commissure. To characterize vvl expression in a lab-/- background, double-immunocytochemical experiments were carried out on homozygous lab null mutant embryos using antibodies against Vvl and HRP. These experiments revealed that vvl immunoreactivity is lacking in the tritocerebral lab mutant domain, in addition to the expected lack of anti-HRP immunoreactivity despite the persistence of cells in this region. This suggests that vvl expression in the posterior tritocerebrum is affected by loss of lab function during late stages of embryonic brain development, indicating that vvl expression in the tritocerebrum is lab-dependent (Meier, 2006).

To assess the functional role of vvl in tritocerebral neuromere formation, vvl null mutants were analyzed using immunocytochemical markers including anti-HRP, anti-ELAV, anti-REPO, and anti FASII, which label general neuronal (or glial) domains and tracts in the developing embryonic brain. In vvl loss-of-function mutants, a pronounced brain phenotype is observed in the late stage embryonic brain. Immunolabelling with neuron-specific anti-HRP and anti-FASII antibodies identified a gap separating the deutocerebral brain region from the neuromeres of the more posterior subesophageal ganglion. This dramatic phenotype is associated with severe axonal patterning defects in the embryonic brain. The longitudinal connectives that normally run from the deutocerebral and tritocerebral neuromeres to the subesophageal ganglion are severely reduced or missing and the tritocerebral commissure, which interconnects the brain hemispheres at the level of the tritocerebrum, is completely absent. Analysis of FASII immunoreactivity revealed that descending and ascending axons, which in the wildtype normally project through the tritocerebrum in well formed fascicles, fail to project through this domain in vvl mutants. Moreover, a loss of anti-ELAV immunolabelling was observed in the tritocerebral domain, whereas glia-specific anti-REPO immunoreactivity revealed that glial cells are present in the vvl mutant but fail to be correctly localized in the affected region. In addition to the observed defects in the tritocerebral brain region, marked axonal patterning defects in the protocerebrum are also seen in vvl mutant embryos. Moreover the organization of the subesophageal ganglion and the VNC is affected in the vvl mutant (Meier, 2006).

At the gross histological level, the vvl mutant brain phenotype is, in part, reminiscent of the mutant brain phenotype observed for lab. Since lab and vvl show overlapping expression in the tritocerebral neuromere, and vvl expression is lacking in lab mutants, these findings suggested that either lab expression itself or lab expressing tritocerebral cells are affected in vvl mutant embryos. To investigate this anti-Lab immunolabelling was carried out in late vvl loss-of-function mutant brains. Surprisingly, despite the expected lack of expression of neuronal differentiation markers, anti-LAB immunolabelling was detected in a wildtype-like pattern in the vvl mutant tritocerebral domain. This suggests that the expression of lab os not affected in the absence of vvl during late stages of embryonic brain development. Moreover, the lab expressing cells in the vvl mutant generally have the same relative position in the brain as does the normal lab expressing cells in the wildtype. Thus, despite the severe axonal patterning defects observed in this domain, mutant cells are generated and appear to be properly positioned in the developing tritocerebrum of the vvl null mutant. This, in turn, suggests that the pattern of proliferation in the tritocerebrum is initiated correctly in the absence of the vvl gene product, but that the cells that normally express vvl might become incorrectly specified in the vvl mutant leading to axogenesis defects. Moreover, the lack of anti-ELAV and anti-HRP immunolabelling together with the observed severe fasciculation defects in the vvl mutant tritocerebrum strongly suggest that mutational inactivation of vvl affects neuronal differentiation in the developing tritocerebrum (Meier, 2006).

These data imply that vvl might be involved in the specification of tritocerebral neuronal identity—either by acting directly or indirectly downstream of tritocerebral lab activity. To further assess a possible lab-dependent vvl activity in the developing tritocerebrum, the potential of vvl to rescue the lab mutant brain phenotype was determined using the Gal4-UAS system. For this, a transgenic fly line carrying a Gal4 transcriptional activator under the control of the lab promoter together with CNS-specific upstream enhancer elements of the lab gene was used. By crossing this lab::Gal4 line to different UAS-responders it was possible to express the responder constructs in a pattern that corresponded to that of the endogenous lab gene. Using this approach, it was shown that the lab mutant brain phenotype can be rescued by transgenic expression of the Lab protein in a lab null mutant background. To determine whether vvl might also be able, at least in part, to rescue the lab mutant brain defects, a transgenic UAS::vvl line was used in which the vvl coding sequence was placed under UAS control (Meier, 2006).

As a control, it was first determined whether lab::Gal4 driven misexpression of vvl in a lab+background has any effects on the development and specification of the tritocerebral Lab domain. In none of these experiments were morphological abnormalities detected in the tritocerebrum or in any other part of the embryonic brain. The UAS::vvl responder was then expressed under the control of the lab::Gal4 driver in the lab mutant domain. Remarkably, the Vvl protein is able to rescue specific lab mutant brain defects. Thus, the longitudinal pathways are restored and cells in the mutant domain show wildtype-like anti-HRP immunolabelling. Moreover, FASII immunoreactivity revealed that descending and ascending axons from other parts of the brain again project through the tritocerebral lab mutant domain. The vvl responder achieved a rescue efficiency (97.3%) which is comparable to the rescue efficiency of Lab, which was taken as 100%. In contrast, lab::Gal4-specific expression of vvl in the lab mutant domain does not rescue tritocerebral commissure formation, nor correct axonal projection of the frontal connective. This suggests that lab::Gal4 driven misexpression of vvl is sufficient to restore both neuronal marker gene expression like HRP and correct axonal patterning of longitudinal connectives in the lab mutant tritocerebrum. These findings together with the vvl mutant brain phenotype indicate that vvl acts genetically downstream of lab in the specification of the triocerebral neuromere (Meier, 2006).

Taken together, these findings demonstrate that vvl function is required for the specification of the developing tritocerebrum. The vvl gene is important for correct axon guidance and fasciculation of longitudinal connectives in the tritocerebral neuromere. In the absence of vvl, longitudinal and commissural axon pathways are severely affected. Comparable findings have been reported for the role of vvl in VNC development, where vvl mutant embryos exhibit aberrantly localized midline glia and axonal defects in that commissures are often fused and the longitudinal connectives are severly reduced or even disrupted. These findings suggest that vvl acts genetically downstream of the Hox gene lab in the control of regionalized neuronal identity and tritocerebral brain neuromere specification. In accordance with this notion is the successive timing of lab and vvl expression in the tritocerebral neuromere. From stage nine onwards, lab expression commences in the intercalary segment and by early stage 11, lab is detected in all neuroblasts of the developing tritocerebrum. Accordingly, by late stage 11, vvl expression is first seen in the tritocerebral neuromere and thus succeeds initial lab expression. Interestingly, this time of initial vvl expression exactly coincides with the temporal requirement of lab for tritocerebral neuronal fate specification. Moreover, the results demonstrate that tritocerebral vvl expression is lab-dependent. In addition, in vvl mutants, lab is expressed normally in the tritocerebrum and yet cells in the affected tritocerebral domain phenocopy the lab mutant brain and do not express molecular markers characteristic of neuronal cells. Furthermore, the vvl gene can mediate neuronal specification and longitudinal connective formation in the absence of the Hox gene lab if expressed under appropriate spatiotemporal control. This indicates that vvl is required for the specification of neuronal identity in the tritocerebral lab domain and sufficient to provide a permissive substrate for the migration of axons originating from outside this region. However, vvl cannot rescue tritocerebral commissure formation in the lab mutant brain. This indicates that lab exerts at least some of its effects on tritocerebral development through other subordinate genes than vvl (Meier, 2006).

vnd is required for tritocerebral neuromere formation during embryonic brain development: vnd is required for the formation of a ventral subset of lab-expressing neuroblasts in the developing tritocerebrum

In Drosophila, evolutionarily conserved transcription factors are required for the specification of neural lineages along the anteroposterior and dorsoventral axes, such as Hox genes for anteroposterior and columnar genes for dorsoventral patterning. This report analyses the role of the columnar patterning gene ventral nervous system defective (vnd) in embryonic brain development. Expression of vnd is observed in specific subsets of cells in all brain neuromeres. Loss-of-function analysis focussed on the tritocerebrum shows that inactivation of vnd results in regionalized axonal patterning defects, which are comparable with the brain phenotype caused by mutation of the Hox gene labial (lab). However, in contrast to lab activity in specifying tritocerebral neuronal identity, vnd is required for the formation and specification of tritocerebral neural lineages. Thus, in early vnd mutant embryos, the Tv1-Tv5 neuroblasts, which normally express lab, do not form. Later in embryogenesis, vnd mutants show an extensive loss of lab-expressing cells because of increased apoptotic activity, resulting in a gap-like brain phenotype that is characterized by an almost complete absence of the tritocerebral neuromere. Correspondingly, genetic block of apoptosis in vnd mutant embryos partially restores tritocerebral cells as well as axon tracts. Taken together, these results indicate that vnd is required for the genesis and proper identity specification of tritocerebral neural lineages during embryonic brain development of Drosophila (Sprecher, 2006; full text of article).

During an initial phase of embryonic neurogenesis, vnd expression is seen in the neurectoderm and delaminating neuroblasts in the ventral domains of the protocerebral, deutcerebral and tritocerebral brain neuromeres. During later stages of embryogenesis, vnd expression is also seen in specific cell clusters within these brain neuromeres. Thus, at late stage 12, a large expression domain is seen in the protocerebral neuromere and two smaller expression domains are observed in the deutocerebrum and in the tritocerebrum. Although the tritocerebral and deuterocerebral vnd expression clusters are in close proximity to each other, they do not overlap. Towards the end of embryogenesis, at embryonic stage 15, expression of vnd is still visible in these three neuromeric domains. Throughout embryonic neurogenesis, vnd expression is found in neuroblasts, ganglion mother cells and neurons as judged by immunolabelling with anti-PROS and neuron-specific anti-ELAV antibodies. Immunolabelling with glia-specific anti-REPO antibody indicates that none of the glia cells of the embryonic brain express vnd. vnd-expressing cells are also seen in the neuromeres of the s ganglion and the VNC, as well as in peripheral sense organs (Sprecher, 2006).

Mutational inactivation of vnd results in a pronounced brain phenotype in the late stage embryonic brain. Immunolabelling with neuron-specific anti-HRP and anti-ELAV antibodies identifies a large gap separating the anterior deutocerebral brain region from the neuromeres of the posterior s ganglion. Evaluation of the penetrance of the vnd-null mutant phenotype reveals that in 36% of the mutant embryos this gap is completely devoid of Elav- and HRP-immunoreactive cells, while in the majority of vnd mutant embryos (63%) a thin strand of ELAV- and HRP-immunoreactive cells remains and interconnects the protocerebrum and the s ganglion. This cell loss is associated with axonal patterning defects in the embryonic brain. The longitudinal connectives that normally run from the protocerebrum to the s ganglion are missing or strongly reduced and the tritocerebral commissure is completely absent. Glia-specific anti-REPO immunoreactivity reveals that glial cells are present in the mutant but fail to be correctly localized in the affected region, most probably owing to the absence of neuronal tissue (Sprecher, 2006).

To delineate the region affected in vnd mutants in more detail, the expression of engrailed (en), which in the wild-type embryonic brain is located in several small clusters of cells that demarcate the posterior boundary of the brain neuromeres, was studied. The b1 en-stripe (or en head spot) delimits the posterior protocerebrum (several en cells are also seen more anteriorly in the protocerebrum as the secondary head spot), the b2 en-stripe (or en antennal stripe) delimits the posterior deutocerebrum, and the b3 en-stripe (or en intercalary stripe) delimits the posterior tritocerebrum. In late vnd mutant brain (embryonic stage 13 onwards), only the b1 en-stripe and the secondary head spot are visible; neither the b2 en-stripe nor the b3 en-stripe can be identified. This supports the observation that major parts of the embryonic tritocerebrum and parts of the deutocerebrum are lacking in the vnd mutant. In addition to the cell loss defect in the tritocerebral/deutocerebral brain region, a less marked reduction in overall size of the protocerebrum is also seen in vnd mutant embryos. Moreover the organization of the s ganglion and the VNC is affected in the vnd mutant. These latter two phenomena were not studied further (Sprecher, 2006).

At the gross histological level, the vnd mutant brain phenotype described above is, in part, reminiscent of the mutant brain phenotype observed for the Hox gene labial (lab). In lab-null mutants, tritocerebral cells are generated and positioned correctly; however, these cells fail to differentiate into neurons and marked axogenesis defects occur, including the disruption of longitudinal connectives and lack of the tritocerebral commissure. As lab and vnd also show overlapping expression in a subset of tritocerebral neuroblasts, these findings suggest that lab-expressing tritocerebral neuroblasts are affected in vnd mutant embryos. To investigate this, focus was placed on the developing tritocerebrum, and specifically on the lab expression domain of this neuromere, and whether loss of vnd function affects formation of lab-expressing neuroblasts was determined (Sprecher, 2006).

During the early phase of brain neurogenesis, the lab-expressing neuroectodermal domain gives rise to 15 neuroblasts, which include all of the tritocerebral neuroblasts and two deutocerebral neuroblasts. By stage 11, all of these neuroblasts are present and express lab; they include a ventral group of tritocerebral neuroblasts, Tv1-Tv5, a more dorsal group of tritocerebral neuroblasts, Td1-Td8, and two deutocerebral neuroblasts, Dv2 and Dv4. In the wild type, the most ventral part of the neuroectodermal domain, from which the tritocerebral neuroblasts Tv1-Tv5 and the two deutocerebral neuroblasts originate, dynamically co-expresses lab and vnd between stages 8 and 11 (Sprecher, 2006).

In vnd mutants this ventral-most part of the lab-expressing domain appears to be reduced in size and accordingly, the number of lab-expressing neuroblasts that derive from this brain region is diminished. Generally only four to six large rounded cells are observed that co-express lab and the neuroblast-specific marker Deadpan (this may be a slight underestimate as a few enlarged rounded cells in sub-ectodermal position lacking Deadpan expression are sometimes observed in this region). Based on the expression of molecular markers indicative of dorsal neuroblasts [e.g. ladybird early, empty spiracles, wingless, this reduction in lab-expressing neuroblasts appears to affect preferentially ventral neuroblasts of the tritocerebrum and adjacent part of the deutocerebrum. These data imply that vnd is required for the formation of a ventral subset of lab-expressing neuroblasts in the developing tritocerebrum (Sprecher, 2006).

Although the reduction in tritocerebral neuroblast number seen in vnd mutants can account for some of the tritocerebral defects, this mechanism alone is unlikely to be the exclusive cause for the massive cell loss phenotype observed in the late embryonic vnd mutant brain. This is because a dorsal subset of the lab-expressing tritocerebral neuroblasts, as well as large number of lab-expressing neural progeny are generated in the tritocerebrum of stage 11 vnd mutant brains. Hence, in addition to defective neuroblast formation, other phenomena must be responsible for the gap-like phenotype observed in vnd mutant brains, implying that vnd is required also later in embryogenesis - either by acting directly on lab expression in tritocerebral cells or through a lab-independent requirement (Sprecher, 2006).

To investigate this, whether vnd and lab show overlapping expression during later stages of tritocerebral neuromere formation was determined. Immunocytochemical analysis indicates that a partial overlap of vnd and lab expression persists in the differentiating tritocerebrum throughout embryogenesis and is prominent in the ventral region (according to neuraxis) of this neuromere. lab expression was analyzed in late vnd loss-of-function mutant brains. Owing to extensive cell loss in the vnd mutant tritocerebrum, this analysis was limited to the remaining strand of cells that interconnects the protocerebrum and the remaining part of the deutocerebrum with the s ganglion. Despite the extensive cell loss seen in vnd mutant brains, remaining cells of the interconnecting strand do show lab expression (Sprecher, 2006).

Whether expression of vnd occurs in the lab mutant tritocerebrum was investigated by studying lab loss-of-function mutants. For this, advantage was taken of the fact that in lab-null mutants, cells in the tritocerebral mutant domain are generated and can be visualized by a 7.31 lab-lacZ reporter construct. Surprisingly, despite the lack of expression of neuronal differentiation markers in cells of the lab mutant domain, vnd is expressed normally and shows partial overlap with tritocerebral lab mutant cells, as visualized by the lab-specific reporter construct. This indicates that expression of vnd is not affected by the absence of lab during late stages of embryonic brain development (Sprecher, 2006).

These results indicate that the homeotic gene lab, which is part of the anteroposterior patterning system, and the columnar gene vnd, which is involved in dorsoventral patterning, act in an integrated manner but independently in the formation and specification of the tritocerebral neuromere. Although vnd and lab show overlapping expression in tritocerebral neuroblasts and subsequently in neural cells of the posterior tritocerebrum, expression of vnd appears unaffected in lab mutant cells. Conversely, vnd does not act on lab expression; the complete absence of lab expression in vnd mutants (with the exception of a rare thin strand of neuronal cells) reflects a secondary defect because of the absence of cells that normally express lab. This independent genetic activity of vnd and lab is further supported by the fact that blocking apoptosis restores tritocerebral lab expression in vnd-null mutant embryos (Sprecher, 2006).

Thus, although the lab and vnd mutant brain phenotypes result in comparable axonal patterning defects (loss of the tritocerebral commissure and perturbation of the longitudinal connectives that normally run through this neuromere), their mode of action within the developing tritocerebrum is discriminable. The results suggest that vnd is required for the specification of neural lineages within the developing tritocerebral neuromere, whereas the Hox gene lab appears to be independently required for the specification of neuronal identity within the same territory during later stages. This indicates that the activity of the columnar gene vnd is integrated into pattern formation along the anteroposterior neuraxis by ensuring proper formation and development of tritocerebral neural lineages that subsequently become further specified by the activity of the Hox gene lab (Sprecher, 2006).

The Drosophila columnar gene vnd belongs to the highly conserved Nkx2 class of transcription factors that have been found in various animals, including mammals. Notably, the vnd/Nkx2 family of genes is exceptionally well conserved, both in terms of expression and function. Thus, the vertebrate homologues of vnd are expressed in the neural plate, or tube, in topologically similar positions as is vnd in the Drosophila ventral neuroectoderm and in the absence of vnd/Nkx2 genes, ventral-most cells in the spinal cord and the Drosophila VNC are missing or transformed. Moreover, this evolutionary conservation in expression and function of vnd/Nkx2 genes appears to apply to some extent to brain development. A comparison of the anteroposterior order of vnd/Nkx2 gene expression in the early embryonic brains of Drosophila and mouse reveals remarkable similarities. In terms of function, genetic knockouts in mice have shown that Nkx2 genes appear to play a crucial role in patterning and neuronal specification during embryonic development of the telencephalon and hindbrain. Nkx2.1 mutant mice display numerous brain patterning defects: the entire pituitary is missing; the number of cortical interneurons is halved; there is a complete absence of TrkA-expressing cells in the developing telencephalon; and the ventral-most aspect of the telencephalon (the medial ganglionic eminence) becomes trans-fated to that of the adjacent more dorsolateral ganglionic eminence. Thus, comparable with the role of vnd during Drosophila brain development, Nkx2.1 is involved in pattern formation and in cell fate determination during embryonic brain development in mice (Sprecher, 2006).

In addition, recent studies have shown that Nkx2.2 is involved in neural lineage specification in the developing hindbrain. In particular, the sequential generation of visceral motoneurons and serotonergic neurons from a common pool of neural progenitors located in the ventral hindbrain crucially depend on the integrated activities of Nkx2.2- and Hox1/2-class homeodomain proteins. An important function of these proteins is to coordinate the spatial and temporal activation of the homeodomain protein Phox2b, which in turn acts as a binary switch in the selection of motor neuron or serotonergic neuronal fate. De-repressive activity of Nkx2.2 at or in vicinity of Pbx/Hox-binding sites proximal to the Phox2b enhancer enhances transcriptional activation of Phox2b by Hox1 and Pbx factors. These data suggest that comparable with the integrated activity of vnd and lab in Drosophila brain neuromere specification, integrated activity of the Nkx2.2 and Hox1/2 proteins is involved in the specification of segmental neural lineages. Thus, integration of anteroposterior and dorsoventral patterning systems by homeodomain transcription factors of the Hox and vnd/Nkx2 genes might represent an ancestral feature of insect and mammalian brain development (Sprecher, 2006).

Larval

lab and Dfd occupy adjacent non-overlapping expression domains in the eye-antennal disc. During embryogenesis, lab and Dfd exhibit limited overlapping expression in areas that have consequences for imaginal development. The head of Drosophila is characterized by an extreme morphological difference between the larval and adult stages. Since homeotic transformations produced by the lab, Dfd, and proboscipedia (pb) loci are manifested only in the adult, it has been suggested that distinct regulatory paradigms evolved for homeotic gene function in the development of the larval versus adult head (Diederich, 1991).

Effects of Mutation or Deletion

labial mutants revealed a failure of head involution and the loss or disruption of several head structures, including the salivary glands and the larval chewing apparatus parts, H-piece and ventral arm of the cephalopharyngeal apparatus (considered to be part of the mandibular segment). Ventrally, a deletion and/or disruption of tissue occurred in the maxillary palp and vibrissae regions. Dorsally, the posterior head appeared to be transformed to a thoracic-like identity. Mutations in lab, like those in Deformed and proboscipedia, reveal a homoeotic phenotype only in the adult stage of the life cycle (Merrill, 1989).

Despite this report, labial mutant embryos are thought to make normal salivary glands. Salivary gland determination is regulated by Sex combs reduced (Panzer, 1992).

Mutations in Drosophila alpha spectrin cause larval lethality and defects in cell shape and adhesion. An examination was made of the effects on development and function of the larval midgut for two lethal alpha spectrin alleles (alpha-specrg41 and alpha-specrg35). Homozygous null alpha-specrg41-mutant larvae exhibit a striking defect in middle midgut acidification. Domains of acidification and alkalinization in the gut were identified by feeding yeast, containing pH-sensitive dyes, to wild-type instar larve. In contrast, many homozygous alpha-specrg35 mutants are capable of acidification, indicating partial function of the truncated alpha-specrg35 product. Acidification is also blocked by a mutation in the labial gene, which is required for differentiation of cuprophilic cells in the midgut, suggesting that these cells secrete acid. Two isoforms of spectrin (alphabeta and alphabetaH) are segregated within the basolateral and apical domains of cuprophilic cells, respectively. The most conspicuous defect in cuprophilic cells from labial and alpha spectrin mutants is in morphogenesis of the invaginated apical domain, although basolateral defects may also contribute to the acidification phenotype. Acid secretion in vertebrate systems is thought to involve the polarized activities of apical proton pumps and basolateral anion exchangers, both of which interact with spectrin. It is proposed that the alpha-specrg41 mutation in Drosophila interferes with the polarized activities of homologous molecules that drive acid secretion in cuprophilic cells. Three possible explanations for the acid secretion defect in alpha-spectrin mutant larvae are proposed: (1) Spectrin may serve a support role in the apical domain of cuprophilic cells. Loss of alphabetaH spectrin function may lead to a collapse of the apical domain, thereby compromising all apical membrane activities, including proton pumps. (2) The alphabetaH isoform of spectrin may interact directly with proton pumps in the apical domain. (3) Spectrin may interact with and stabilize a basolateral activity, such as an anion-exchange protein, that is indirectly required for apical proton secretion (Dubreuil, 1998).

Drosophila embryos lacking the homeotic gene labial show two types of defects in brain development: (1) cells in the brain lab domain do not express neuronal markers or extend axons, and (2) axons originating from outside the lab domain stop at this region or project ectopically. A severe disruption of neuronal patterning and axon scaffolding is the net result. It is not clear how the absence of Lab can result in both neuronal fate defects and axon pathfinding defects. Lab was expressed ectopically in short pulses in lab loss-of-function embryos, and this gives almost complete rescue; for example, the tritocerebral commissure is restored. Rescue only occurs when Lab is provided at the time when cells in the brain are adopting a neuronal fate. Lab expression later, when the first axons are seen in the lab domain, does not give rescue. It is concluded that Lab expression helps to establish neuronal identity in the lab domain, and these neurons act as a permissive substrate for axon extension. However, Lab itself is not required at the time of axon pathfinding through this region (Page, 2000).

During embryonic development of the Drosophila brain, labial is required for the regionalized specification of the tritocerebral neuromere; in the absence of labial, the cells in this brain region do not acquire a neuronal identity and major axonal pathfinding deficits result. labial is expressed in the posterior half of the tritocerebral neuromere. In lab loss-of-function mutants, regionalized axonal patterning defects occur in the lab domain that are due to both cell-autonomous effects and non cell-autonomous effects. Thus, in the absence of lab, mutant cells are generated and positioned correctly in the brain, but these cells do not extend axons. Moreover, extending axons from other neighboring wild-type neurons stop at the mutant domains or project ectopically. As a result, dramatic defects in commissural and longitudinal axon pathways occur; the tritocerebral commissure, which links the two tritocerebral hemiganglia, is absent and the longitudinal pathways between the supraesophageal and subesophageal ganglia are reduced or absent. Immunocytochemical analysis demonstrates that cells in the mutant domain do not express any of the numerous neuronal markers such as Elav that positionally equivalent cells express in the wild type, indicating a complete lack of neuronal identity in the lab mutant brain domain. These data indicate that lab is involved in the specification of tritocerebral neuronal identity in the Drosophila brain (Hirth, 2001).

Genetic rescue experiments were used to investigate the functional equivalence of the Drosophila Hox gene products in the specification of the tritocerebral neuromere. Using the Gal4-UAS system, it was first demonstrated that the labial mutant brain phenotype can be rescued by targeted expression of the Labial protein under the control of CNS-specific labial regulatory elements. Under the control of these CNS-specific regulatory elements, all other Drosophila Hox gene products, except Abdominal-B, are able to efficiently replace Labial in the specification of the tritocerebral neuromere. A correlation between the rescue efficiency of the Hox proteins and the chromosomal arrangement of their encoding loci was observed. These results indicate that, despite considerably diverged sequences, most Hox proteins are functionally equivalent in their ability to replace Labial in the specification of neuronal identity. This suggests that in embryonic brain development, differences in Hox gene action rely mainly on cis-acting regulatory elements and not on Hox protein specificity. This surprising functional equivalence contrasts with the general notion, which is derived from experiments on the specification of other body parts in Drosophila, that Hox proteins assign different identities along the anteroposterior body axis by acting as specific selectors of different, alternative developmental pathways (Hirth, 2001).

The fact that the expression of different Hox genes in the lab mutant domain does not cause homeotic transformation of tritocerebral identity, suggests that Hox proteins act as 'mediators' rather than as 'selectors' within the developmental pathway that specifies segmental neuronal identity in the Drosophila brain. Recent experiments using both loss- and gain-of-function mutations suggest that this also applies to the specification of other structures along the anteroposterior body axis of Drosophila. For example, in haltere development, abd-A and to some extent Abd-B can substitute for Ubx gene action. Moreover, a comparable lack of Hox gene specificity has been observed in gonad development (Hirth, 2001 and references therein).

Finally, the high degree of functional interchangeability of Lab and all of the other Drosophila Hox proteins, with the exception of Abd-B, is consistent with evolutionary studies that propose a common origin of all of the Hox genes from a single ancestral progenitor and an early singularity of Abd-B-like genes in the ancestral Hox gene cluster. Given the striking evolutionary conservation of structure, expression and brain-specific function of lab and its mammalian Hox1 orthologs, it will now be important to determine whether functional equivalence among non-paralogous Hox gene products is also valid for vertebrate hindbrain development (Hirth, 2001).

Cell-autonomous and non-cell-autonomous function of Hox genes specify segmental neuroblast identity in the gnathal region of the embryonic CNS in Drosophila

In thoracic and abdominal segments of Drosophila, the expression pattern of Bithorax-Complex Hox genes is known to specify the segmental identity of neuroblasts (NB) prior to their delamination from the neuroectoderm. This study identified and characterized a set of serially homologous NB-lineages in the gnathal segments and used one of them (NB6-4 lineage) as a model to investigate the mechanism conferring segment-specific identities to gnathal NBs. It was shown that NB6-4 is primarily determined by the cell-autonomous function of the Hox gene Deformed (Dfd). Interestingly, however, it also requires a non-cell-autonomous function of labial and Antennapedia that are expressed in adjacent anterior or posterior compartments. The secreted molecule Amalgam (Ama) was identified as a downstream target of the Antennapedia-Complex Hox genes labial, Dfd, Sex combs reduced and Antennapedia. In conjunction with its receptor Neurotactin (Nrt) and the effector kinase Abelson tyrosine kinase (Abl), Ama is necessary in parallel to the cell-autonomous Dad pathway for the correct specification of the maxillary identity of NB6-4. Both pathways repress CyclinE (CycE) and loss of function of either of these pathways leads to a partial transformation (40%), whereas simultaneous mutation of both pathways leads to a complete transformation (100%) of NB6-4 segmental identity. Finally, the study provides genetic evidences, that the Ama-Nrt-Abl-pathway regulates CycE expression by altering the function of the Hippo effector Yorkie in embryonic NBs. The disclosure of a non-cell-autonomous influence of Hox genes on neural stem cells provides new insight into the process of segmental patterning in the developing CNS (Becker, 2016).

The Drosophila head consists of seven segments (4 pregnathal and 3 gnathal) all of which contribute neuromeres to the CNS. The brain is formed by approximately 100 NBs per hemisphere, which have been individually identified and assigned to specific pregnathal segments [The anterior pregnathal region (procephalon) is composed of the labral, ocular, antennal, intercalary segments, see Segment polarity and DV patterning gene expression reveals segmental organization of the Drosophila brain]. As judged from comparison of the combinatorial codes of marker gene expression only few brain NBs appear to be serially homologous to NBs in the thoracic/abdominal ventral nerve cord, reflecting the highly derived character of the brain neuromeres. The connecting tissue between brain and the thoracic VNC consists of three neuromeres formed by the gnathal head segments named mandibular (mad), maxillary (max) and labial (lab) segment, but the number and identity of the neural stem cells and their lineage composition in these segments is still unknown. Compared to the thoracic ground state the segmental sets of gnathal NBs might be reduced to different degrees, but are thought to be less derived compared to the brain NBs. Therefore, to fully understand segmental specification during central nervous system development, it is important to identify the neuroblasts and their lineages in these interconnecting segments (Becker, 2016).

Assuming that most NBs in the gnathal segments still share similarities to thoracic and abdominal NBs, this study sought serially homologous NB-lineages, which are suitable for genetic analyses. Using the molecular marker eagle (eg), which specifically labels four NB-lineages in thoracic/abdominal hemisegments this study identified three serial homologs (NB3-3, NB6-4 and NB7-3) in the gnathal region. To investigate the mechanisms conferring segmental identities, focus was placed on one of them, the NB6-4 lineage, which shows the most significant segment-specific modifications. The analysis reveals a primary role of the Antennapedia-Complex (Antp-C) Hox gene Deformed (Dfd) in cell-autonomously specifying the maxillary fate of NB6-4 (NB6-4max). Surprisingly, an additional, non-cell-autonomous function was uncovered of the Antp-C Hox genes labial (lab, expressed anterior to Dfd) and Antennapedia (Antp, expressed posterior to Dfd) in specifying NB6-4max. In a mini-screen for downstream effectors the secreted protein Amalgam (Ama) was identified as being positively regulated by lab, Dfd and Antp and negatively regulated by the Antp-C Hox gene Sex combs reduced (Scr). Loss of function of Ama and its receptor Neurotactin (Nrt) as well as the downstream effector kinase Abelson tyrosine kinase (Abl) lead to a transformation of NB6-4max similar to Dfd single mutants. Thus, in parallel to the cell-autonomous role of Dfd, a non-cell-autonomous function of Hox genes lab and Antp, mediated via the Ama-Nrt-Abl pathway, is necessary to specify NB6-4max identity. Disruption of either of these pathways leads to a partial misspecification of NB6-4max (approx. 40%), whereas simultaneous disruption of both pathways leads to a complete transformation (approx. 100%) of NB6-4max to a labial/thoracic identity. It was further shown that both pathways regulate the expression of the cell cycle gene CyclinE, which is necessary and sufficient to generate labial/thoracic NB6-4 identity. Whereas Dfd seems to directly repress CyclinE transcription (similar to AbdA/AbdB in the trunk), indications are provided that the Ama-Nrt-Abl pathway prevents CyclinE expression by altering the activity of the Hippo/Salvador/Warts pathway effector Yorkie (Yki) (Becker, 2016).

Along the anterior-posterior axis the CNS consists of segmental units (neuromeres) the composition of which is adapted to the functional requirements of the respective body parts. In Drosophila the CNS comprises 10 abdominal, three thoracic, three gnathal and four pregnathal (brain) neuromeres that are generated by stereotyped populations of neural stem cells (neuroblasts, NBs). The pattern of NBs in thoracic segments resembles the ground state while NB patterns in the other segments are derived to various degrees. Within each segment individual NBs are specified by positional information in the neuroectoderm. NBs delaminating from corresponding positions in different segments express similar sets of molecular markers, generate similar lineages, and are called serial homologs. However, for thoracic and abdominal neuromeres it has been shown that the composition of a number of serially homologous NB-lineages shows segment-specific differences. In the more derived gnathal and pregnathal head segments embryonic NB-lineages and the mechanisms of their segmental specification have not been analyzed so far (Becker, 2016).

Using the well-established molecular marker Eagle (Eg) which labels four embryonic NB-lineages (NB2-4, NB3-3, NB6-4, NB7-3) in all thoracic and most of the abdominal segments this study identified serially homologous lineages of NB3-3, NB6-4 and NB7-3 in gnathal segments. The embryonic NB7-3 lineage shows segmental differences as it comprises increasing cell numbers from mandibular (2 cells), maxillary (3 cells) to labial (3-5 cells) segments, while cell numbers are decreasing from T1-T2 (4 cells), T3-A7 (3 cells) to A8 (2-3 cells). Reduced cell numbers in the mandibular and maxillary NB7-3 lineages depend on Dfd and Scr function, respectively . While NB7-3 appeared in all three gnathal segments, NB3-3 and NB6-4 was only found in labial and maxillary segments, and NB2-4 was not found in any of them. Preliminary data suggest that the missing NBs are not generated in these segments, instead of being eliminated by apoptosis. For the terminal abdominal neuromeres (A9, A10) it has recently been shown that the formation of a set of NBs (including NB7-3) is inhibited by the Hox gene Abdominal-B. Similarly, in Dfd mutants the formation was observed of a NB with NB6-4 characteristics in mandibular segments (10%), in which it is never found in wild type (Becker, 2016).

Similar to the thoracic and abdominal segments NB6-4 showed dramatic differences between maxillary and labial segments. NB6-4max produces glial cells only (like abdominal NB6-4), whereas the labial homolog produces neurons in addition to glial cells (like thoracic NB6-4). The number of glial cells produced by the glioblasts NB6-4max (4 cells) and abdominal NB6-4 (2 cells) and by the neuroglioblasts NB6-4lab (3 glia) and thoracic NB6-4 (3 glia) is segment-specific(Becker, 2016).

Thus segment-specific differences among serially homologous lineages may concern types and/or numbers of specific progeny cells and may result from differential specification of NBs and their progeny, differential proliferation and/or differential cell death of particular progeny cells. It has been shown that the segment-specific modification of serially homologous lineages is under the control of Hox genes and that during neurogenesis Hox genes act on different levels, i.e. they act in a context-specific manner at different developmental stages and in different cells. In the thoracic/abdominal region segmental identity is conferred to NBs early in the neuroectoderm by cell-autonomous function of Hox genes of the Bithorax-Complex. This study used the NB6-4 lineage to clarify mechanisms of segmental specification in the gnathal segments (Becker, 2016).

In segments of the trunk, the action of Hox genes strictly follows the rule of the posterior prevalence concept: More posterior expressed Hox genes repress anterior Hox genes and thereby determine the segmental identities. In the gnathal segments this phenomenon was not observed on the level of the nervous system. Removing Hox genes of the Antp-C had no or only minor impact on the expression domain of other Antp-C Hox genes. Similar results were also obtained in a study that analyzed cross-regulation of Hox genes upon ectopic expression (Becker, 2016).

Moreover, it seems that at least in the case of the differences monitored between labial and maxillary segments Hox gene function has to be added to realize the more anterior fate. Antennapedia has no impact on NB6-4 identity in the labial segment, but specification of the maxillary NB6-4 requires the function of Deformed and Sex combs reduced. These two Hox genes are not repressed or activated by Antp. Also, cross-regulation between Dfd and Scr seems to be unlikely or is very weak since only mild effects were observed on the protein level and on the phenotypic penetrance. In principle Scr can repress Dfd, but it was suggested that this occurs only when products are in sufficient amounts. In NB6-4 Dfd and Scr are co-expressed, but Scr levels appear to be insufficient to repress Dfd. Dfd seems to be the major Hox gene that cell-autonomously confers the maxillary NB6-4 fate, since the loss of Dfd showed the highest transformation rate and, more importantly, ectopic expression of Dfd in thoracic segments leads to a robust transformation towards maxillary fate. Scr does not act redundantly since in double mutants Dfd/Scr no synergistic effect was observed. It might have a fine-tuning effect, as it was shown that Scr influences Ama by repressing its transcription, whereas all other Antp-C Hox genes seem to activate Ama. However, since only minor changes were found in cell identities and numbers in Scr LoF background, the role of Scr in NB6-4max stays enigmatic (Becker, 2016).

Surprisingly cell-autonomous Hox gene function was not the only mechanism that confers segmental identity in NB6-4max. Loss of Dfd showed an effect in approx. 43% of all segments. Moreover, mutations of the adjacently expressed Hox genes labial and Antennapedia in combination with Dfd LoF showed a dramatic increase in the transformation rate of NB6-4max. Their expression patterns on the mRNA and protein level were carefully studied in wild type and Hox mutant background. In no case were these genes found to be expressed in NB6-4max or in the neuroectodermal region from which NB6-4max delaminates. This indicates that labial and Antennapedia influence NB6-4max fate in a non-cell-autonomous manner. That Hox genes can act non-cell-autonomously on stem cells was recently shown in the male germ-line, were AbdB influences centrosome orientation and the proliferation rate through regulation of the ligand Boss in the Sevenless-pathway. In this study Antp-C Hox genes controled the expression of the secreted molecule Amalgam, which spreads to adjacent segments and ensures segmental specification of NB6-4max in a parallel mechanism to the cell-autonomous function of Dfd. Thus, this study provides first evidence for parallel non-cell-autonomous and cell-autonomous functions of Antp-C genes during neural stem cell specification in the developing CNS (Becker, 2016).

Abelson kinase (Abl) was shown to be required for proper development of the Drosophila embryonic nervous system. In neurons Abl interacts with proteins like Robo or Chickadee and influences the actin cytoskeleton in the growth cone to regulate axonogenesis and pathfinding. In this system it was also demonstrated that Ama and Nrt are dominant modifiers of the Abl phenotype. It is proposed that the interaction of secreted Ama and the membrane-bound Nrt regulates Abl function in NBs. This leads to the correct segmental specification of NB6-4max. Antp-C Hox genes lab, Antp and Dfd regulate the expression of Ama and in mutants for theses Hox genes expression of Ama is severely reduced, which leads to the transformation of NB6-4max due to missing Abl function and de-repression of the cell cycle gene CyclinE. That Abl can influence the expression of CyclinE was also demonstrated in a modifier-screen in the Drosophila eye, but the mechanism remained unclear. Genetic analysis now suggests that in NBs this might occur via the regulation of the highly conserved Hippo-Salvador-Warts pathway and its downstream transcriptional co-activator Yki, which is known to regulate CyclinE expression. The Hippo-Salvador-Warts pathway controls organ growth and cell proliferation in Drosophila and vertebrates but so far has not been implicated in embryonic NB development. This study observed Yki cytoplasmic localization in wild type NB6-4max prior to division suggesting the active Hippo pathway. Nuclear localization of Yki could not be detected in Abl mutants, the loss of Yki activity in the Abl mutant background leads to a significant reduction in the strength of the Abl single mutant phenotype showing their genetic interaction and therefore supporting the proposed model in which Abl influences Yki activity. Moreover, expression of constitutive active Yki also lead to the transformation of NB6-4max and phenotypes that were similar to those observed in Abl mutants. Attempts were made to assess how Abl might influence Yki activity. Work in vertebrates suggests that this could be at least on two levels: first, c-Abl was shown to directly phosphorylate and activate the vertebrate MST1 and MST2 (Hpo homologue) and the Drosophila Hpo on a conserved residue (Y81) and second, c-Abl can also phosphorylate YAP1, which changes its function to become pro-apoptotic. This analysis suggests that in NBs Abl might regulate Hpo, since changes were found in the stability of Salvador, which is used as a Hpo activity readout, but a parallel direct regulation of Yki could not be ruled out, since it was recently shown that other pathways like the AMPK/LKB1 pathway can directly influence Yki activity. Since severe over-proliferation was observed in Abl or lab/Dfd mutants, that have an impaired Ama-Nrt-Abl pathway, or upon overexpression of Yki^CA, future studies need to elucidate whether and how the proto-oncogene Abl kinase and Hox genes act on growth and proliferation or even tumor initiation through regulation of the Hippo/Salvador/Warts pathway (Becker, 2016).

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date revised: 10 April 2013