Disabled

EVOLUTIONARY HOMOLOGS part 1/2 | part 2/2

Lipoprotein receptors and a Disabled family cytoplasmic adaptor protein regulate EGL-17/FGF export in C. elegans

Growth factors and morphogens need to be secreted to act on distant cells during development and in response to injury. Evidence is presented that efficient export of a fibroblast growth factor (FGF), EGL-17, from the C. elegans developing vulva requires the lipoprotein receptor-related proteins Ce-LRP-1 and Ce-LRP-2 and a cytoplasmic adaptor protein, Ce-DAB-1 (Disabled). Lipoprotein receptors are transmembrane proteins best known for their roles in endocytosis. Ce-LRP-1 and Ce-LRP-2 possess a conserved intraluminal domain that can bind to EGL-17, as well as a cytosolic FXNPXY motif that can bind to Ce-DAB-1. Ce-DAB-1 contains signals that confer subcellular localization to Golgi-proximal vesicles. These results suggest a model in which Ce-DAB-1 coordinates selection of receptors and cargo, including EGL-17, for transport through the secretory pathway (Kamikura, 2003).

Dab family adaptor proteins interact functionally with lipoprotein receptors in both nematodes and mammals, even though the biological processes they mediate vary greatly. Ce-DAB-1 regulates secretion, Dab2 regulates endocytosis in the kidney, and Dab1 relays extracellular signals during brain development, each via lipoprotein receptors. Although the role of Ce-DAB-1 in signaling is unclear, the high degree of functional conservation across species suggests that vertebrate Dab family members or other PTB-containing proteins may participate in regulated traffic of lipoprotein receptors and associated cargoes to the cell surface. Indeed, it is possible that an early embryonic requirement for Dab2 might be a consequence of altered protein traffic in polarized epithelial cells of the embryo (Kamikura, 2003).

Regulated trafficking of the MSP/Eph receptor during oocyte meiotic maturation in C. elegans

In C. elegans, a sperm-sensing mechanism regulates oocyte meiotic maturation and ovulation, tightly coordinating sperm availability and embryo production; sperm release the major sperm protein (MSP) signal to trigger meiotic resumption. Meiotic arrest depends on the parallel function of the oocyte VAB-1 MSP/Eph receptor and somatic G protein signaling. MSP promotes meiotic maturation by antagonizing Eph receptor signaling and counteracting inhibitory inputs from the gonadal sheath cells. This study presents evidence suggesting that in the absence of the MSP ligand, the VAB-1 Eph receptor inhibits meiotic maturation while either in or in transit to the endocytic-recycling compartment. VAB-1::GFP localization to the RAB-11-positive endocytic-recycling compartment is independent of ephrins but is antagonized by MSP signaling. Two negative regulators of oocyte meiotic maturation, DAB-1/Disabled and RAN-1, interact with the VAB-1 receptor and are required for its accumulation in the endocytic-recycling compartment in the absence of MSP or sperm (hereafter referred to as MSP/sperm). Inactivation of the endosomal recycling regulators rme-1 or rab-11.1 causes a vab-1-dependent reduction in the meiotic-maturation rate in the presence of MSP/sperm. Further, Gαs signaling in the gonadal sheath cells, which is required for meiotic maturation in the presence of MSP/sperm, affects VAB-1::GFP trafficking in oocytes. It is concluded that regulated endocytic trafficking of the VAB-1 MSP/Eph receptor contributes to the control of oocyte meiotic maturation in C. elegans. Eph receptor trafficking in other systems may be influenced by the conserved proteins DAB-1/Disabled and RAN-1 and by crosstalk with G protein signaling in neighboring cells (Cheng, 2008).

Cloning and effects of mutation of vertebrate Disabled homologs

A mouse homolog of Drosophila Disabled (Dab), mDab1, is an adaptor molecule that functions in neural development. mDab1 is expressed in certain neuronal and hematopoietic cells, and is localized to the growing nerves of embryonic mice. mDab1 expression is observed in the head in neural tracts corresponding to the developing cranial nerves, such as the oculomotor and trochlear nerves. In the body, mDab1 expression is apparent in the spinal accessory nerve and dorsal root ganglia. At embryonic day 13, mDab1 expression is observed in sensory nerves that innervate the vibrissae, and in the extremities of developing bone. All nerves identified at these times by neurofilament antibody also express mDab1. During embryogenesis, mDab1 is tyrosine phosphorylated when the nervous system is undergoing dramatic expansion. However, once nerve tracts are established, mDab1 lacks detectable phosphotyrosine. Tyrosine-phosphorylated mDab1 associates with the SH2 domains of Src, Fyn and Abl. An interaction between mDab1 and Src is observed when embryonal carcinoma cells undergo differentiation into neuronal cell types. mDab1 can also form complexes with cellular phosphotyrosyl proteins through a domain that is related to the phosphotyrosine binding (PTB) domains of the Shc family of adaptor proteins. The mDab1 PTB domain binds to phosphotyrosine-containing proteins of 200, 120, and 40 kDa from extracts of embryonic mouse heads. The properties of mDab1 and genetic analysis of Dab in Drosophila suggest that these molecules function in key signal transduction pathways involved in the formation of neural networks (Howell, 1997a).

DOC-2 is a human gene originally identified as a 767-bp cDNA fragment isolated from normal ovarian epithelial cells by differential display against ovarian carcinoma cells. The complete cDNA sequence of the 3.2-kb DOC-2 transcript has been determined and the gene has been localized to chromosome 5. A 12.5-kb genomic fragment at the 5'-end of DOC-2 has also been sequenced, revealing the intron-exon structure of the first eight exons (788 bases) of the DOC-2 gene. Translation of the DOC-2 cDNA predicts a hydrophobic protein of 770 amino acid residues with a molecular weight of 82.5 kDa. Comparison of the DNA and amino acid sequences of DOC-2 to publicly accessible sequence databases reveals 83% identify to p96, a murine protein of similar size, thought to be a mitogen-responsive phosphoprotein. In addition, about 45% identity was observed between the first 140 N-terminal residues of DOC-2 and the Caenorhabditas elegans M110.5 and Drosophila melanogaster Dab genes (Albertsen, 1996).

Using RNA fingerprinting (RAP) strategy and Northern blot analysis, a differentially expressed sequence DOC-2 has been identified that is detectable in all normal human ovarian surface epithelial (HOSE) cell cultures but not in ovarian cancer cell lines and tissues. Subsequent cloning of DOC-2 from a cDNA library generated from the HOSE cells was carried out using the 3' and 5' RACE approach. A 3268 base pair full length cDNA of DOC-2 was isolated and sequenced. The predicted protein has a length of 770 amino acids. Homology search of all NCBI sequences indicates that the amino acid sequence of DOC-2 shares 93% homology with the mouse p96/mDab2 phosphoprotein and has a phosphotyrosine interacting domain (PID) and multiple SH3 binding motifs. The DOC-2 gene is located on chromosome 5p13. The 105 kDa DOC-2 protein is down-regulated in all the carcinoma cell lines. In-situ immunohistochemistry performed on normal ovaries, and benign, borderline and invasive ovarian tumor tissues shows down regulation of DOC-2 protein particularly in serous ovarian tumor tissues. When DOC-2 is transfected into the ovarian carcinoma cell line SKOV3, the stable transfectants show significantly reduced growth rate and the ability to form tumors in nude mice. These data suggest that down-regulation of DOC-2 may play an important role in ovarian carcinogenesis (Mok, 1998).

Disabled-2 (Dab2) functions in the mitogenic signal transduction pathway, and is frequently activated by homozygous gene deletion in tumors, suggesting that Dab2 is a candidate tumor suppressor. Dab2 is expressed in a variety of tissues; the level of expression is particularly high in ovary and breast. Dab2 expression has also been detected in immortalized breast and ovarian epithelial cells. However, in more than a dozen established tumor cell lines derived from breast and ovarian epithelial tumors examined by Western blotting, Dab2 expression is undetectable in 90% of these cell lines. Histological staining of human ovarian tissues with specific anti-Dab2 antibodies indicates that Dab2 is highly expressed in the surface epithelial layer. In an immunohistological study of 26 ovarian carcinomas, 22 (85%) of the tumors were found to lose the expression of Dab2 in the tumor cells, which are epithelial in origin. Loss of Dab2 expression is not correlated with tumor grade, suggesting that Dab2 is lost in an early stage of tumorigenicity. Indeed, loss of Dab2 correlates closely with morphological transformation of the surface epithelial cells. Additionally, loss of Dab2 protein occurs in hyperproliferative, but histologically benign ovarian epithelium, suggesting that loss of Dab2 occurs in pre-malignant lesions. Thus, this study indicates that the loss of Dab2 expression is correlated with tumorigenicity of the cells disregarding the grade of the tumors, and loss of Dab2 expression is an early event in ovarian malignancies (Fazili, 1999).

The signal transduction adapter protein Disabled-2 (Dab2) is one of the two mammalian orthologs of Drosophila Disabled. The brain-specific Disabled-1 (Dab1) functions in positional organization of brain cells during development. Dab2 is widely distributed and is highly expressed in many epithelial cell types. The dab2 gene was interrupted by in-frame insertion of ß-galactosidase (LacZ) in embryonic stem cells, and transgenic mice were produced. Dab2 expression is first observed in the primitive endoderm at E4.5, immediately following implantation. The homozygous Dab2-deficient mutant is embryonic lethal (earlier than E6.5) due to defective cell positioning and structure formation of the visceral endoderm. In E5.5 dab2 (-/-) conceptus, visceral endoderm-like cells are present in the deformed primitive egg cylinder; however, the visceral endoderm cells are not organized -- the cells of the epiblast have not expanded, and the proamniotic cavity fails to form. Disorganization of the visceral endodermal layer is evident, since cells with positive visceral endoderm markers are scattered throughout the dab2 (-/-) conceptus. Only degenerated remains are observed at E6.5 for dab2 (-/-) embryos, and by E7.5, the defective embryos have been completely reabsorbed. In blastocyst in vitro culture, initially cells with characteristics of endoderm, trophectoderm, and inner cell mass were observed in the outgrowth of the hatched dab2 (-/-) blastocysts. However, the dab2 (-/-) endodermal cells are much more dispersed and disorganized than those from wild-type blastocysts, the inner cell mass fails to expand, and the outgrowth degenerates by day 7. Thus, Dab2 is required for visceral endodermal cell organization during early mouse development. The absence of an organized visceral endoderm in Dab2-deficient conceptus leads to the growth failure of the inner cell mass. It is suggested that Dab2 functions in a signal pathway to regulate endodermal cell organization using endocytosis of ligands from the blastocoel cavity as a positioning cue (Yang, 2002).

Disabled protein interactions

Disabled-2 (Dab2), a mammalian structural homolog of Drosophila Disabled (Dab), is a mitogen-responsive phosphoprotein. It has been speculated that Dab2 is a negative regulator of growth since its expression is lost in ovarian carcinomas. Dab2 contains a C-terminal proline-rich domain with sequences similar to those found in Sos, a guanine nucleotide exchange factor for Ras. The proline-rich sequences of Sos mediate the interaction of Sos with Grb2, an adaptor protein which couples tyrosine kinase receptors to Sos. The possibility that Dab2 interacts with Grb2 has been investigated. In experiments of co-immunoprecipitation from BAC1.2F5 macrophage cell lysates, significant quantities of Grb2 are associated with both Sos and Dab2, although Dab2 and Sos are not present in the same complex. Transfection of Dab2 into a Dab2-negative cell line (293 cells) decreases the amount of Grb2 associated with Sos, suggesting that Dab2 competes with Sos for binding to Grb2. Proline-rich peptides corresponding to Dab2 (aa661-669) and to Sos (aa1146-1161) inhibit the binding of Dab2 to Grb2, but are less effective in disrupting the Grb2-Sos complex. The expressed proline-rich domain of Dab2 (aa600-730) binds Grb2, but other regions of Dab2 fail to bind Grb2. Both of the individual SH3 domains of Grb2 bind to Sos (N-terminal SH3 domain binds to a much greater extent than C-terminal SH3 domain), but binding to Dab2 requires the intact Grb2, suggesting cooperative binding using both SH3 domains of Grb2. These data indicate that Dab2 binds to the SH3 domains of Grb2 via Dab2's C-terminal proline-rich sequences. Dab2 may modulate growth factor/Ras pathways by competing with Sos for binding to Grb2 (Xu, 1998).

Disruption of the disabled-1 gene (Dab1) results in aberrant migration of neurons during development and disorganization of laminar structures throughout the brain. Dab1 is thought to function as an adapter molecule in signal transduction processes. It contains a protein-interaction (PI) domain similar to the phosphotyrosine-binding domain of the Shc oncoprotein; it is phosphorylated by the Src protein tyrosine kinase, and it binds to SH2 domains in a phosphotyrosine-dependent manner. To investigate the function of Dab1, binding proteins were sought using the yeast two-hybrid system. The PI domain of Dab1 interacts with the amyloid precursor-like protein 1 (APLP1), a member of the family of proteins including APP. The association of Dab1 with APLP1 was confirmed in biochemical assays, and the site of interaction was localized to a cytoplasmic region of APLP1 containing the amino acid sequence motif Asn-Pro-x-Tyr (NPxY). NPxY motifs are involved in clathrin-mediated endocytosis, and they have been shown to bind to PI domains present in several proteins. This region of APLP1 is conserved among all members of the amyloid precursor family of proteins. Indeed, it was found that Dab1 also interacts with amyloid precursor protein (APP) and APLP2 in biochemical association experiments. In transiently transfected cells, Dab1 and APLP1 colocalized in membrane ruffles and vesicular structures. Cotransfection assays in cultured cells indicate that APP family members increase serine phosphorylation of Dab1. Dab1 and APLP1 are expressed in similar cell populations in developing and adult brain tissue. These results suggest that Dab1 may function, at least in part, through association with APLP1 in the brain (Homayouni, 1999).

Dab1-deficient mice show abnormalities in neuronal migration and positioning of neurons in the brain. The observation that members of the APP family of proteins interact with Dab1 suggests APP proteins may play a role in neuronal migration during brain development. However, targeted disruption of APP, APLP1, and APLP2 genes in mice does not result in altered lamination in the brain. The lack of a major phenotype in these mice may be attributable, in part, to compensation or functional redundancy among closely related APP family members. Importantly, mice in which two of the genes have been disrupted, for example APP and APLP2, or APLP1 and APLP2, die before birth. Thus, it seems that the overlapping function of APP family members is required for normal development. Several studies have suggested developmental roles for APP family genes. For example, their level of gene expression is regulated during development of the nervous system. Also, induction of neuronal differentiation in cultured cells increases expression of all three family members. Furthermore, both APP and APLP2 are present in elongating axons. Other studies have shown that APP is expressed on radial fibers, which are present transiently in the developing cortex and provide a substrate for neuronal migration. Thus, there is some circumstantial evidence supporting an interaction between the Reelin-Dab1 pathway and APP family proteins. Mutations in APP have been linked to autosomal dominant familial Alzheimer's disease, the most common form of late-onset dementia. Alzheimer's disease is characterized pathologically by the appearance of neuritic plaques containing Abeta peptide derived from APP and neurofibrillary tangles containing hyperphosphorylated tau protein. Thus far, no direct link has been established between the appearance of amyloid plaques and tau phosphorylation. One of the kinases responsible for the phosphorylation of tau is Cdk5, and Cdk5 immunoreactivity increases in neurons that exhibit early-stage neurofibrillary tangles. It is intriguing that disruption of either Cdk5 or its activating subunit p35 in mice causes a neuronal migration defect similar to that seen in mice lacking Reelin or Dab1. These findings, in combination with the data presented here, suggest that Cdk5-p35 and Dab1 may provide a link between APP and tau metabolism in the adult brain. Numerous functions have been suggested for APP. It has been implicated in differentiation, attachment, survival, and outgrowth of neurons. Different regions of the extracellular domain of APP have been shown to inhibit proteases and to modulate synaptic activity. However, the normal function of APP and the consequences of the interaction of Dab1 with APP family proteins in the adult brain are unclear at present. The results presented here suggest that Dab1 may influence processes involving the APP family of proteins that are important in the developing, as well as the adult, brain (Homayouni, 1999 and references).

The gene products of Disabled are important for nervous system development in Drosophila and mammals. During neuronal positioning in mice, the Dab1 protein is thought to function downstream of the extracellular protein Reelin. The structures of Dab proteins suggest that they mediate protein-protein or protein-membrane docking functions. The amino-terminal phosphotyrosine-binding (PTB) domain of Dab1 binds to the transmembrane glycoproteins of the amyloid precursor protein (APP) and low-density lipoprotein receptor families and the cytoplasmic signaling protein Ship. Dab1 associates with the APP cytoplasmic domain in transfected cells and is coexpressed with APP in hippocampal neurons. Screening of a set of altered peptide sequences has shown that the sequence GYXNPXY present in APP family members is an optimal binding sequence, with approximately 0.5 µM affinity. Unlike other PTB domains, the Dab1 PTB does not bind to tyrosine-phosphorylated peptide ligands. The PTB domain also binds specifically to phospholipid bilayers containing phosphatidylinositol 4P (PtdIns4P) or PtdIns4,5P2 in a manner that does not interfere with protein binding. It is proposed that the PTB domain permits Dab1 to bind specifically to transmembrane proteins containing an NPXY internalization signal (Howell, 1999b).

The function of Dab1 binding to LDL receptor-related protein (LRP)-alpha-2 macroglobulin receptor, APP, and their relatives could be to regulate trafficking or processing. The internalization signals of LRP and APP contain the NPXY sequence, which is bound by the Dab1 PTB domain. Nonetheless, two observations make it unlikely that Dab1 functions in the internalization process per se. (1) Dab1 is absent from many cell types that successfully internalize LDL receptors or APP, and (2) the LDL receptor is thought to be clustered into coated pits by direct binding to clathrin. However, Dab1 may compete for internalization signals. By altering protein sorting into coated pits, Dab1 may affect membrane flow from the surface to intracellular membrane systems and hence influence membrane recycling, which is important for cell movement. APP is best known for its increased cleavage and the accumulation of a degradation product, beta amyloid, in Alzheimer's disease. Interestingly, the NPXY motif appears to be important for proteolysis of APP to produce beta amyloid. Overexpression of brain proteins X11 and FE65 affects APP processing in an opposing manner. While X11 overexpression leads to an increased half-life for APP, possibly by slowing the sorting of this protein into the endosomal compartment, overexpression of FE65 leads to increased translocation of APP to the cell surface and increased production of the proteolytic fragments. It remains to be determined what effect Dab1 might have on these processes (Howell, 1999b and references).

It is also possible that Dab1 PTB domain signaling is mediated by proteins other than APP and LDL receptor family proteins. The high-affinity binding sequence found in APP family members, GYXNPXY, is found in approximately 10 other eukaryotic proteins listed in current (November 1998) databases, including the APP orthologs from Drosophila and C. elegans. The high degree of conservation of this motif, 100% over the seven residues, suggests that selective pressures, possibly exerted by binding partners, act on it. The binding studies do not exclude the possibility that the Dab1 PTB domain has additional ligands with distinct sequences. Five cDNA clones were isolated that encode apparent Dab1 PTB domain binding partners that lack NPXY motifs in the interacting regions. They also lack other common sequence patterns. They may, therefore, bind to the PTB domain through novel interacting sequences (Howell, 1999b).

PTB domains and PH domains are similar in structure. Many PH domains bind with 10^-5 to 10^-7 M affinity to phosphoinositides with characteristic stereospecificity. Making use of a triple-charge mutation of Shc that prevents lipid binding without preventing phosphopeptide binding evidence has shown that lipid binding is important for Shc function. A model has been suggested in which weak interaction between the Shc PTB domain and membrane phospholipids is a prerequisite for recruitment of Shc to activated growth factor receptors and Shc is released from phospholipids as it binds to the receptor. Similarly, the Shc PTB domain with affinities in the 10^-4 to 10^-5 M range binds to PtdIns4P, PtdIns4,5P2, and PtdIns3,4,5P3. Making use of a triple-charge mutation of Shc that prevents lipid binding without preventing phosphopeptide binding, the Dab1 PTB domain binds to PtdIns4P and PtdIns4,5P₂ but less to PtdIns or PtdIns3,4,5P₃. Since PtdIns4,5P₂ is more abundant than PtdIns3,4,5P₃, it is likely to be the major lipid bound to the Dab1 PTB domain in the cell. Unlike the Shc PTB domain, the Dab1 PTB domain can bind simultaneously to synthetic peptides and phosphoinositides. Thus, binding to membrane phospholipids could reinforce binding to PhiXNPXY motifs in the cytoplasmic domains of transmembrane proteins. Unlike Shc, recruitment of Dab1 to a protein ligand would therefore not occur at the expense of binding to phospholipids (Howell, 1999 and references).

The Dab1 protein acts within embryonic neurons and is required for appropriate neuronal placement within the brain. Recent findings suggest that it functions downstream of the extracellular matrix protein Reelin, which may define targets for the migrating neurons. The current findings show that the Dab1 PTB domain binds with high affinity to unphosphorylated targets and that binding is dramatically reduced by tyrosine phosphorylation. One of the targets identified in this study, Ship, is regulated by phosphorylation, while APP and LRP are not known to be tyrosine phosphorylated. However, Reelin increases tyrosine phosphorylation of Dab1 itself, and Dab1 PTB domain function may be regulated as a consequence of this. Exposure of binding surfaces or changes in subcellular distribution could alter Dab1 PTB domain activity. It will be interesting to determine the Dab1 PTB domain functions required for Reelin signaling and how the PTB domain ligands are involved in neuronal placement (Howell, 1999b and references).

gp600/megalin, an endocytic receptor, belongs to the low-density lipoprotein receptor family. It is most abundant in the renal proximal tubular cells, where it is implicated in the reabsorption of a number of molecules filtered through the glomerulus. The cytoplasmic tail (CT) of gp600/megalin contains a number of sequence similarities, which indicate that gp600/megalin might be involved in signal transduction. To find intracellular proteins that would interact with the gp600/megalin CT, a human kidney cDNA library was screened by using the yeast two-hybrid system. The phosphotyrosine interaction domain (PID) of the Disabled protein 2 (Dab2), a mammalian structural analog of Drosophila Disabled, was found to bind to the gp600/megalin CT in this system. The interaction between these two proteins was confirmed by a binding assay in vitro and by the co-immunoprecipitation of both proteins from renal cell lysates. The gp600/megalin CT contains three PsiXNPXY motifs (in which Psi represents a hydrophobic residue) that are potentially able to interact with PID. Analysis of the CT deletion and point-mutation variants of gp600/megalin by the two-hybrid system reveals that the third PsiXNPXY motif is most probably involved in this interaction. Dab2 is a mitogen-responsive phosphoprotein thought to be an adaptor molecule involved in signal transduction, and a suggested negative regulator of cell growth. Dab2 is the first intracellular ligand identified for gp600/megalin; gp600/megalin is the first known transmembrane receptor that interacts with the cytosolic protein Dab2. It is speculated that their interaction might involve gp600/megalin in signal transduction pathways or might mediate the intracellular trafficking of this receptor (Oleinikov, 2000).

Using a genetic complementation approach disabled-2 (Dab2), a structural homolog of the Dab1 adaptor molecule, has been identified as a critical link between the transforming growth factor ß (TGFß) receptors and the Smad family of proteins. Expression of wild-type Dab2 in a TGFß-signaling mutant restores TGFß-mediated Smad2 phosphorylation, Smad translocation to the nucleus and Smad-dependent transcriptional responses. TGFß stimulation triggers a transient increase in association of Dab2 with Smad2 and Smad3, which is mediated by a direct interaction between the N-terminal phosphotyrosine binding domain of Dab2 and the MH2 domain of Smad2. Dab2 associates with both the type I and type II TGFß receptors in vivo, suggesting that Dab2 is part of a multiprotein signaling complex. Together, these data indicate that Dab2 is an essential component of the TGFß signaling pathway, aiding in transmission of TGFß signaling from the TGFß receptors to the Smad family of transcriptional activators (Hocevar, 2001).

The ability of Dab2 to rescue TGFß-induced Smad2 phosphorylation and Smad-dependent transcriptional responses in the mutant 903 cell line points to a critical role for Dab2 in mediating Smad-dependent responses. Additionally, the PTB domain of Dab2 binds directly to the MH2 domain of Smad2 in vitro, and the association of Dab2 with Smad2 and Smad3 occurs in a ligand- and time-dependent manner. Other receptor systems, namely EGF and HGF, have recently been shown to activate Smad2-dependent transcription, while transfection of MEKK1, the upstream activator of the c-Jun N-terminal kinase (JNK) pathway, has been shown to activate Smad2, resulting in its increased association with Smad4 and subsequent nuclear accumulation. These results thus suggest functional crosstalk between signaling pathways, which is also supported by the ability of the Smads to bind to and augment the activity of transcription factors, such as c-Jun and c-Fos, which are known targets of MAPK kinase signaling pathways. The C-terminal region of Dab2 contains proline-rich PXXP sequences that have been shown to bind to SH3-containing signaling proteins. Recently, Dab2 has been shown to interact with Grb2 through this region. Dab2 may also play a role in activation of other signaling pathways through recruitment of SH3-containing signaling molecules to its C-terminal PRD, which may explain why Dab2 can modulate basal as well as TGFß-mediated fibronectin levels in the 903 mutant cell line, a response that is dependent on the JNK pathway and independent of Smad4 expression. The requirement for the C-terminal domain of Dab2 for efficient TGFß signaling is demonstrated by the inability of constructs that lack this domain to complement the mutant cells. It may be that Dab2 serves as a bridge to link the Smad and the JNK signaling pathways, a relationship that has recently been postulated to be required for TGFß-mediated transcriptional responses. Identification of the binding partners for the C-terminal region of Dab2 may thus help to clarify the role of other signaling pathways in TGFß signaling (Hocevar, 2001).

Loss of responsiveness to the growth-inhibitory effects of TGFß is commonly observed in human carcinomas, indicating that inactivation of the TGFß signaling pathway is a common target that may permit cancer initiation and progression. This may occur as a result of mutations in either the type I or type II TGFß receptors or a mutation in a component of the signaling pathway. Dab2 was initially identified as a transcript that was down-regulated in ovarian carcinoma, but was present in normal ovarian epithelial cells. Subsequently, Dab2 expression has been demonstrated to be down-regulated in breast and prostate carcinoma as well. Loss of expression, which is seen early in tumor progression, is not due to loss or gross chromosomal rearrangements of the gene. Re-introduction of Dab2 in ovarian, prostate and choriocarcinoma cell lines results in a decreased growth rate, while Dab2-transfected SKOV3 ovarian carcinoma cells form tumors 50% smaller compared with parental cells when injected into nude mice, demonstrating that Dab2 acts as a tumor suppressor gene. Whether the re-introduction of Dab2 in these cell lines mediates restoration of TGFß signaling is unknown at this time. Dab2 inactivation by mutation or down-regulation, an early event in cancer progression, may thus represent a new mechanism by which cancer cells acquire resistance to the growth-inhibitory effects of TGFß. Further investigation of the role of Dab2 in TGFß signaling may therefore provide important new insight as to how loss of TGFß signaling leads to cancer initiation and progression (Hocevar, 2001).

Myosin VI (see Drosophila Jaguar) is a molecular motor that moves processively along actin filaments and is believed to play a role in cargo movement in cells. DOC-2/DAB2, a signaling molecule inhibiting the Ras cascade, binds to myosin VI at the globular tail domain. DOC-2/DAB2 binds stoichiometrically to myosin VI with one molecule per one myosin VI heavy chain. The C-terminal 122 amino acid residues of DOC-2/DAB2, containing the Grb2 binding site has been found to be critical for the binding to myosin VI. Actin gliding assay revealed that the binding of DOC-2/DAB2 to myosin VI can support the actin filament gliding by myosin VI, suggesting that it can function as a myosin VI anchoring molecule. The C-terminal domain but not the N-terminal domain of DOC-2/DAB2 functions as a myosin VI anchoring site. The present findings suggest that myosin VI plays a role in transporting DOC-2/DAB2, a Ras cascade signaling molecule, thus involved in Ras signaling pathways (Inoue, 2002).

Myosin VI, an actin-based motor protein, and Disabled 2 (Dab2), a molecule involved in endocytosis and cell signaling, have been found to bind together using yeast and mammalian two-hybrid screens. In polarized epithelial cells, myosin VI is known to be associated with apical clathrin-coated vesicles and is believed to move them towards the minus end of actin filaments, away from the plasma membrane and into the cell. Dab2 belongs to a group of signal transduction proteins that bind in vitro to the FXNPXY sequence found in the cytosolic tails of members of the low-density lipoprotein receptor family. The central region of Dab2, containing two DPF motifs, binds to the clathrin adaptor protein AP-2, whereas a C-terminal region contains the binding site for myosin VI. This site is conserved in Dab1, the neuronal counterpart of Dab2. The interaction between Dab2 and myosin VI was confirmed by in vitro binding assays and coimmunoprecipitation and by their colocalization in clathrin-coated pits/vesicles concentrated at the apical domain of polarized cells. These results suggest that the myosin VI-Dab2 interaction may be one link between the actin cytoskeleton and receptors undergoing endocytosis (Morris, 2002b).

The adaptor molecule Disabled-2 (Dab2) has been shown to link cell surface receptors to downstream signaling pathways. Using a small-pool cDNA screening strategy, the N-terminal domain of Dab2 has been shown to interact with Dishevelled-3 (Dvl-3), a signaling mediator of the Wnt pathway. Ectopic expression of Dab2 in NIH-3T3 mouse fibroblasts attenuates canonical Wnt/ß-catenin-mediated signaling, including accumulation of ß-catenin, activation of ß-catenin/T-cell-specific factor/lymphoid enhancer-binding factor 1-dependent reporter constructs, and endogenous cyclin D1 induction. Wnt stimulation leads to a time-dependent dissociation of endogenous Dab2-Dvl-3 and Dvl-3-axin interactions in NIH-3T3 cells, while Dab2 overexpression leads to maintenance of Dab2-Dvl-3 association and subsequent loss of Dvl-3-axin interactions. In addition, Dab2 can associate with axin in vitro and stabilize axin expression in vivo. Mouse embryo fibroblasts that lack Dab2 exhibit constitutive Wnt signaling as evidenced by increased levels of nuclear ß-catenin and cyclin D1 protein levels. Based on these results, it is proposed that Dab2 functions as a negative regulator of canonical Wnt signaling by stabilizing the ß-catenin degradation complex, which may contribute to its proposed role as a tumor suppressor (Hocevar, 2003).

Disabled and endocytosis

Clathrin-coated pits at the cell surface select material for transportation into the cell interior. A major mode of cargo selection at the bud site is via the µ2 subunit of the AP-2 adaptor complex, which recognizes tyrosine-based internalization signals. Other internalization motifs and signals, including phosphorylation and ubiquitylation, also tag certain proteins for incorporation into a coated vesicle, but the mechanism of selection is unclear. Disabled-2 (Dab2) recognizes the FXNPXY internalization motif in LDL-receptor family members via an N-terminal phosphotyrosine-binding (PTB) domain. Here, it is shown that in addition to binding AP-2, Dab2 also binds directly to phosphoinositides and to clathrin, assembling triskelia into regular polyhedral coats. The FXNPXY motif and phosphoinositides contact different regions of the PTB domain, but can stably anchor Dab2 to the membrane surface, while the distal AP-2 and clathrin-binding determinants regulate clathrin lattice assembly. It is proposed that Dab2 is a typical member of a growing family of cargo-specific adaptor proteins, including ß-arrestin, AP180, epsin, HIP1 and numb, which regulate clathrin-coat assembly at the plasma membrane by synchronizing cargo selection and lattice polymerization events (Sanjay, 1999).

Disabled-2 (Dab2) is a widely expressed relative of Disabled-1, a neuron-specific signal-transduction protein that binds to and receives signals from members of the low-density lipoprotein receptor (LDLR) family. Members of the LDLR family internalize through clathrin-coated pits and vesicles to endosomes, from where they return to the cell surface through the secretory pathway. Dab2 phosphotyrosine-binding domain binds peptides containing the sequence FXN-PXY. This core sequence is found in the intracellular domains of LDLR family members and is important for receptor internalization. Dab2 transiently colocalizes with the LDLR in clathrin-coated pits, but is absent from endosomes and lysosomes. Dab2 is alternatively spliced and its localization depends on a region of the protein that contains two DPF motifs that are present in the p96 Dab2 protein and absent in the p67 splice variant. This region is sufficient to confer Dab2 binding to the alpha-adaptin subunit of the clathrin adaptor protein, AP-2. Overexpression of p96 but not of p67 Dab2 disrupts the localization of AP-2. These findings suggest that in addition to previously reported signal-transduction functions, Dab2 could also act as an adaptor protein that may regulate protein trafficking (Morris, 2001).

Clathrin-coated pits at the cell surface select material for transportation into the cell interior. A major mode of cargo selection at the bud site is via the µ2 subunit of the AP-2 adaptor complex, which recognizes tyrosine-based internalization signals. Other internalization motifs and signals, including phosphorylation and ubiquitylation, also tag certain proteins for incorporation into a coated vesicle, but the mechanism of selection is unclear. Disabled-2 (Dab2) recognizes the FXNPXY internalization motif in LDL-receptor family members via an N-terminal phosphotyrosine-binding (PTB) domain. In addition to binding AP-2, Dab2 also binds directly to phosphoinositides and to clathrin, assembling triskelia into regular polyhedral coats. The FXNPXY motif and phosphoinositides contact different regions of the PTB domain, but can stably anchor Dab2 to the membrane surface, while the distal AP-2 and clathrin-binding determinants regulate clathrin lattice assembly. It is proposed that Dab2 is a typical member of a growing family of cargo-specific adaptor proteins, including ß-arrestin, AP180, epsin, HIP1 and Numb, which regulate clathrin-coat assembly at the plasma membrane by synchronizing cargo selection and lattice polymerization events (Mishra, 2002).

The epidermal growth factor, transferrin and LDL receptors all utilize the same basic endocytic elements to enter the cell. All three receptors can be found in common endosomes and, if clathrin-mediated endocytosis is grossly perturbed, as in stonin 2 overexpression, which prevents proper recruitment of AP-2 to the plasma membrane, transferrin, EGF and LDL internalization are all impaired. Yet these three transmembrane proteins each appear to use different mechanisms for selective entry into clathrin-coated buds. Four-fold overexpression of the LDL receptor in HeLa cells does not inhibit the rate of transferrin uptake and cells from hypercholesterolemic patients with no mutations in the LDL receptor, apolipoprotein (apo) B or AP-2 µ2-subunit fail to internalize LDL while transferrin endocytosis proceeds normally. This suggests that LDL receptor uptake might depend on alternate endocytic machinery (Mishra, 2002).

A large body of data links the NPXY internalization sequence present in LDL receptor family members to PTB domain recognition. The PTB domains of Dab1 and the rodent CED-6 ortholog, GULP, bind directly to NPXY sequence-bearing LDL receptor family members, and to amyloid precursor proteins, albeit with differing selectivity. Intact Dab2 associates directly with megalin and the isolated Dab2 PTB domain binds to LDL receptor family FXNPXY sequences. Compelling genetics ties Dab1, with ~65% identity to the Dab2 PTB domain, to the very low density lipoprotein (VLDL) receptor, apoE receptor-2 (apoER2), and the extracellular ligand reelin in the process of neural migration and spatial positioning in the brain. Genetic disruption of both apoER2 and VLDL receptors in mice precisely phenocopies the mutant strains reeler (reelin defective) and scrambler (Dab1 defective). The results indicate that a vectorial pathway of reelin-->VLDL receptor/apoER2-->Dab1 is vital for correct central nervous system formation (Mishra, 2002).

Several types of mutation in the LDL receptor including Class 4, internalization defective due to alterations in the NPXY internalization sequence, cause familial hypercholesterolemia. Naturally occurring mutation of the major ligand, apoB, results in a similar, but milder, clinical phenotype termed familial defective apoB-100. However, unlike the Dab1 signaling pathway paradigm, no mutations in Dab2 have been linked to hypercholesterolemia. This suggests that other PTB domain proteins might compensate or substitute for Dab2 loss, and thus Dab2 would not be singularly responsible for LDL receptor incorporation into clathrin-coated vesicles. This notion is also probable because Dab2 is not expressed ubiquitously, and many ovarian and breast carcinomas fail to express Dab2 at all. siRNA knockdown of Dab2 levels by >90% in HeLa cells does not inhibit diI-LDL uptake. Proteins structurally related to Dab2 are, in fact, already known; Numb is a PTB domain-containing protein that, like Dab2, binds to AP-2 and also exhibits several NPF triplets in the distal segment of the protein. More interesting is the recent identification of the gene mutated in patients with a rare form of non-LDL receptor-dependent hypercholesterolemia. Several different natural mutations in the encoded protein, ARH (autosomal recessive hypercholesterolemia), phenocopy LDL receptor-mutant familial hypercholesterolemia. Primary sequence alignments of the ARH PTB domain show that Numb, CED-6/GULP and Dab1/2 are the most closely related members of the PTB domain-containing superfamily, predicting that ARH will probably bind preferentially to non-phosphorylated NPXY motifs. It thus appears that several related PTB-type endocytic adaptors might function to cluster the LDL receptor and/or other NPXY-bearing proteins into clathrin-coated buds, possibly in tissue-specific combinations (Mishra, 2002).

The phenotype of Dab2 nullizygous mice also supports a role for Dab2 in endocytosis. Targeted disruption of the Dab2 gene is lethal, but if expression is disrupted only in the embryo, the animals are viable and survive to adulthood. These mice have a prominent kidney sorting dysfunction that mimics, in a milder form, megalin gene disruption. The mice exhibit proteinuria and secrete several vitamin-binding proteins normally efficiently reabsorbed in the kidney proximal tubule by internalization of the scavenger receptor megalin within clathrin-coated vesicles. Proximal tubules of Dab2-/- mice, like those of megalin-deficient animals, display decreased numbers of apical clathrin-coated pits and a generally diminished subapical endosome compartment. This links Dab2 to megalin endocytosis in the kidney in vivo. An additional link between Dab2 and clathrin-mediated endocytosis comes from the recent identification of myosin VI as a Dab2 interaction partner. Myosin VI is a component of some clathrin-coated vesicles and Dab2 probably plays an important role in targeting this motor to the vesicle (Mishra, 2002).

Overexpression of Dab1 in CHO cells results in ~2-fold more LDL receptor at the cell surface without affecting the rate of uptake. The increased residency of the LDL receptor at the plasma membrane could be due to Dab1 PTB-mediated interference with receptor incorporation into endocytic coated buds, since Dab1 does not interface with the clathrin-coat machinery. This is in full accord with the demonstration that the PTBx2 potently, but selectively, prevents LDL uptake in COS cells. In addition to two NPXY motifs, LRP also contains a YXXØ sequence within the cytosolic domain that confers more rapid endocytosis than either the LDL, megalin, VLDL or apoER2 receptor cytosolic domain. This suggests that NPXY-driven internalization might be augmented by also engaging the AP-2 complex at the bud site (Mishra, 2002).

Since overexpression of a single Dab2 PTB domain has negligible effect on LDL uptake, it is believed that Dab2 normally encounters FXNPXY-bearing cargo in the context of an assembling coated bud. The PtdIns(4,5)P₂-, AP-2- and clathrin-binding properties will facilitate Dab2 placement at the bud site, as do analogous determinants in AP180, epsin and HIP1. Although able to assemble clathrin cages in vitro, it is not contended that Dab2 plays an indispensable role in clathrin lattice assembly. Rather, like epsin and AP180, it potentiates AP-2-driven coat formation, perhaps also determining vesicle size. Each of these proteins is likely to be found within a single forming bud. This distinguishes this group of proteins from ß-arrestin, which, despite also containing PtdIns(4,5)P₂-, AP-2- and clathrin-binding determinants, appears to accompany heptahelical receptors into existing coated buds (Mishra, 2002).

If the overall domain organization and binding properties of epsin, AP180, HIP1 and Dab2 predict a common function as dedicated endocytic sorting adaptors, what cargo might the other proteins be selecting? AP180 could actively cluster synaptobrevin/VAMP. A potential cargo for HIP1 is huntingtin, which interacts with the N-terminal segment of HIP1 and has been localized to clathrin-coated structures and endosomes. Epsin might be part of the long-sought-after machinery that recognizes ubiquitin as an endocytic signal. Importantly, the identification of AP180, epsin, HIP1 and Dab2 as putative adaptors, each dedicated to selection of certain cargo to the bud site, clarifies an understanding of clathrin-coat assembly; it is believed that these proteins can now be viewed as a general class of functionally similar cargo sorters (Mishra, 2002).

Like epsin, Dab2 also contains multiple NPF triplets, the EH domain ligand. Mammalian EH domain-containing proteins include eps15, intersectin, POB-1 and EHD1. Dab2 is, therefore, capable of establishing a complex web of protein-protein interactions at the clathrin bud site. These additional functional sequences argue that, like Numb, Dab2 probably accompanies the clathrin-coated vesicle during budding and fission. These multifaceted connections may be a critical aspect of ensuring the fidelity of cargo incorporation into assembling coated buds (Mishra, 2002).

Finally, Dab2 may participate in more sorting events than only those involving LDL receptor family members. The protein was initially identified as a prominent phosphoprotein accompanying colony-stimulating factor-1 (CSF-1) receptor activation. The CSF-1 receptor utilizes ubiquitin as an endocytic , but the NPF triplets in Dab2 could recruit eps15, which, like epsin, can bind ubiquitin directly. Alternatively, the observed phosphorylation of Dab2 might possibly prevent incorporation into coated buds, favoring epsin/eps15 inclusion and possibly enhancing CSF-1 receptor internalization. Dab2 is also implicated in transforming growth factor ß receptor activation and downstream Smad activation. Direct association of the Dab2 PTB domain with Smad2/3 has been reported. Intriguingly, Smad2 and Smad3 activation by liganded TGF-ß receptors occurs only post-endocytic internalization, raising the possibility that Dab2, acting at the coated vesicle, coordinates aspects of TGF-ß signaling (Mishra, 2002).

Signaling downstream of Dab

During brain development, many neurons migrate long distances before settling and differentiating. These migrations are coordinated to ensure normal development. The secreted protein Reelin controls the locations of many types of neurons, and its absence causes the classic 'Reeler' phenotype. Reelin action requires tyrosine phosphorylation of the intracellular protein Dab1 by Src-family kinases. However, little is known about signaling pathways downstream of Dab1. Several proteins have been identified in embryonic brain extract that bind to tyrosine-phosphorylated, but not non-phosphorylated, Dab1. Of these, the Crk-family proteins (CrkL, CrkI, and CrkII ), bind significant quantities of Dab1 when embryonic cortical neurons are exposed to Reelin. CrkL binding to Dab1 involves two tyrosine phosphorylation sites, Y220 and 232, that are critical for proper positioning of migrating cortical plate neurons. CrkL also binds C3G, an exchange factor (GEF) for the small GTPase Rap1 that is activated in other systems by tyrosine phosphorylation. Reelin stimulates tyrosine phosphorylation of C3G and activates Rap1. C3G and Rap1 regulate adhesion of fibroblasts and other cell types. Regulation of Crk/CrkL, C3G, and Rap1 by Reelin may be involved in coordinating neuron migrations during brain development (Ballif, 2003).

Disabled transcriptional regulation

Mice lacking transcription factor interferon consensus sequence binding protein (ICSBP) develop a syndrome similar to human chronic myeloid leukemia and are immunodeficient. In order to define the molecular mechanisms responsible for the cellular defects of ICSBP-/- mice, bone marrow-derived macrophages (BMM) were used to identify genes deregulated in the absence of ICSBP. Disabled-2 (Dab2), a signal phosphoprotein, is transcriptionally up-regulated and accumulates in the cytoskeleton/membrane fraction of ICSBP^-/- BMM. Dab2 is a novel IFN-gamma-response gene. Both ICSBP and the Ets-transcription factor PU.1 bind to the Dab2 promoter, whereby ICSBP represses PU.1-induced Dab2 promoter transactivation in vitro. Notably, repression of Dab2 expression by ICSBP is also found in myeloid progenitors. Overexpression of Dab2 leads to accelerated cell adhesion and spreading, accompanied by enhanced actin fiber formation. Furthermore, cell adhesion induces transient Dab2 phosphorylation and its translocation to the cytoskeletal/membrane fraction. These results identify a novel role of Dab2 as an inducer of cell adhesion and spreading, and strongly suggest that the up-regulation of Dab2 contributes to the hematopoietic defect seen in ICSBP^-/- mice (Rosenbauer, 2001).

Formation of highly organized neocortical structure depends on the production and correct placement of the appropriate number and types of neurons. POU homeodomain proteins Brn-1 and Brn-2 are coexpressed in the developing neocortex, both in the late precursor cells and in the migrating neurons. Double disruption of both Brn-1 and Brn-2 genes in mice leads to abnormal formation of the neocortex with dramatically reduced production of layer IV-II neurons and defective migration of neurons unable to express mDab1. These data indicate that Brn-1 and Brn-2 share roles in the production and positioning of neocortical neuron development (Sugitani, 2002).

To investigate the molecular mechanisms underlying neuronal migration defects in Brn-1/Brn-2 mutant cortex, an RT-PCR analysis was performed on various genes involved in neuronal migration. mDab1, VLDLR/ApoER2, and alpha3-integrin have been shown to function in positioning cortical neurons by mediating Reelin signal transduction. CDK5, p35 (one of the CDK5 activator subunits), Lis1 (Pafah1b1), and Doublecortin are also thought to affect neuronal migration in the developing cortex. Among all these tested genes, only mdab1 expression was clearly affected in the Brn-1/Brn-2 double mutant cortex at E16.5. Therefore, the spatial distribution of the mdab1 mRNA in the cortex of Brn-1/Brn-2 mutant embryos and wild-type littermates was examined by RNA in situ hybridization. In the wild-type cortex at E16.5, mdab1 mRNA is expressed throughout the cortical wall, except for the MZ and SP. High levels of mdab1 mRNA are detected in the upper regions of the IZ and in the CP. In the Brn-1/Brn-2-deficient cortex at E16.5, mdab1 mRNA expression is significantly reduced throughout the cortical wall and, in particular, is undetectable in the upper region of the IZ just beneath the chondroitin sulfate proteoglycans (CSPG)-positive SP, in which p35-highly expressing late-born neurons are abnormally congested. Therefore, the slight reduction in p35 mRNA levels in the E16.5 mutant cortex detected by RT-PCR analysis might be caused by decreased numbers of p35-expressing neurons produced from E14.5 onward. Furthermore, quantitative RT-PCR analysis showed that mdab1 expression is reduced also in Brn-1/Brn-2 double heterozygotes, which show no histological defects in their neocortex. RNA in situ hybridization also showed that precipitously graded reduction of mdab1 mRNA levels correlates well with Brn-1/Brn-2 gene dosages. These results imply that Brn-1 and Brn-2 act genetically upstream to activate mDab1-dependent positioning processes in cortical neurons. The early-born neurons lacking Brn-1 and Brn-2, however, migrate and split the preplate into the MZ and SP properly; such mobility is not seen in the mdab1 mutant cortex. In yotari and scrambler, mutant mice carrying loss-of-function mutations in the mdab1 gene, cortical neurons fail to split the preplate to form the CP between the MZ and SP. The maintenance of integrity of preplate splitting in Brn-1/Brn-2 mutant E16.5 cortex could be caused by the redundant function of another class III POU factor, Brn-4, that also shares high homology in its primary structure with Brn-1 and Brn-2. In wild-type as well as double-mutant cortex, Brn-4 expression is also detected in the migrating neurons at ~E15.5, but is reduced after then. In Brn-1/Brn-2 mutant cortex, mDab1 expression is detected until E15.5 but is hardly detectable at E16.5. Therefore, Brn-4, like Brn-1 and Brn-2, might also be able to activate mDab1-dependent processes in the positioning of early-born neurons (Sugitani, 2002).

Several lines of evidence suggest that mDab1 functions downstream of Reelin in a signaling pathway that controls cell positioning in the developing cortex. However, it is not yet clear how these molecules dictate the spatial position of cortical neurons, including subplate neurons. Interestingly, in the Brn-1/Brn-2-deficient cortex, mDab1 expression is severely reduced only at a late stage, when most of the E14.5-born neurons migrate through the IZ, but do not reach the MZ, remaining congested just beneath the SP. Therefore, these results imply that mDab1 may be necessary for CP neurons to migrate through the SP. Alternatively, Brn-1 and Brn-2 could also regulate expression of other molecules that may be essential in this process. However, the hypoplasticity of the Brn-1/Brn-2-deficient cortex cannot be explained by an inability to express mdab1, because reduced cell proliferation has not been reported in mdab1 mutant cortex, and loss of RORß-expressing or mSorLA-expressing neurons was not observed in yotari. tailless and pax6 expression, which are known to be essential for proper generation of cortical neurons, were examined. However, no changes were found in their expression in Brn-1/Brn-2 mutant cortex (Sugitani, 2002).

Dab roles in development

The Disabled-2 (Dab2) gene has been proposed to act as a tumor suppressor. Cell culture studies have implicated Dab2 in signal transduction by mitogens, TGFß and endocytosis of lipoprotein receptors. To identify in vivo functions of Dab2, targeted mutations were made in the mouse. In the absence of Dab2, embryos arrest prior to gastrulation with a phenotype reminiscent of that caused by deletion of some TGFß signal transduction molecules involved in Nodal signaling. Dab2 is expressed in the extra-embryonic visceral endoderm but not in the epiblast. Dab2 could be conditionally deleted from the embryo without affecting normal development, showing that Dab2 is required in the visceral endoderm but dispensable in the embryo proper. Conditionally mutant Dab2-/- mice are overtly normal, but have reduced clathrin-coated pits in kidney proximal tubule cells and excrete specific plasma proteins in the urine, consistent with reduced transport by a lipoprotein receptor, megalin/gp330, in the proximal tubule. This evidence indicates that Dab2 is pleiotropic and regulates both visceral endoderm function and lipoprotein receptor trafficking in vivo (Morris, 2002a).

Dab1, Reelin and brain development

Continued: Disabled Evolutionary homologs part 2/2

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