Focal adhesion kinase-like

DEVELOPMENTAL BIOLOGY

Expression of Fak56D protein was examined throughout development. Total lysates from embryonic, first instar, third instar, early pupa, mid/late pupa, and adult developmental stages were analyzed on SDS-PAGE followed by immunoblotting for Fak56D. Levels of Fak56D expression are high during embryonic development, decrease early in larval development, and then are high again at later larval and pupal stages, indicating that Fak56D protein expression is indeed regulated during development. Interestingly, Fak56D appears as a doublet at all stages, although during the later stages of development the upper band is prevalent. Currently, the nature of this doublet is unknown; it may reflect the phosphorylation status of Fak56D, although alternative splicing could also be responsible, because this has previously been reported for both FAK and Pyk2. A 4.5-kb Fak56D RNA was detected by Northern blot analysis, and the levels of this RNA parallel the levels of Fak56D protein during development (Palmer, 1999).

Immunostaining of Drosophila embryos with affinity-purified anti-Fak56D antibodies shows strong Fak56D expression in the central nervous system. Immunostaining of primary cell cultures plated on laminin and tiggrin show that anti-Fak56D antibodies also strongly stain neurite networks. These data are consistent with in situ hybridization analysis of Fak56D RNA during development, which indicates that Fak56D is widely expressed during embryogenesis with a high level of expression within the developing nervous system. Additionally, in situ hybridization analysis of Drosophila embryos by the Berkeley Drosophila Genome Project using the CK1820 EST has revealed expression of Fak56D in the central nervous system, embryonic brain, epidermis, nerve cord, and visceral mesoderm, correlating with the in situ hybridization and immunostaining data (Palmer, 1999).

In stage 16 embryos, strong expression of the Fak56D mRNA is detected in the central nervous system and the junction of muscle and epidermis. Immunohistochemical analysis also showed expression of the Fak56D protein in the muscle attachment. A cross-sectioned view of the immunostained image suggests that Dfak56 is expressed predominantly in the epidermal cells, but less or not at all in the muscular cells (Fujimoto, 1999).

The subcellular localization of the Fak56D protein was examined using the Drosophila neuronal cell line BG2-c6. The BG2-c6 cells express the FAK56D mRNA. They also express integrins, and formation of integrin clusters is observed in cells with several focal adhesion proteins, including p21-activated kinase and alpha-actinin. Immunofluorescent staining of the BG2-c6 cells with anti-Fak56D antibodies produces a focal adhesion-like pattern. The nucleus is also stained with these antibodies. In some populations, the edges of the cells are also stained. Staining with anti-phosphotyrosine antibody produces a pattern that overlaps with that of anti-Fak56d antibodies, except for the nucleus. Staining with anti-phosphotyrosine and anti-betaPS integrin antibodies shows colocalization of tyrosine-phosphorylated proteins with integrin. These data suggest that Fak56D is a component of focal adhesions in Drosophila (Fujimoto, 1999).

To determine when Fak56D is expressed during development, a developmental Northern blot was carried out with a probe from the 5' end of the Fak56D cDNA: three bands of 4.7, 5.3, and 6.5 kb were detected. The 4.7- and 5.3-kb transcripts are expressed during all developmental stages examined, but most notably during embryogenesis. During larval and pupal stages, expression is reduced. Expression is high in ovaries. The abundance of these messages in both 0- to 2-hr embryos and ovaries suggests a maternal contribution. The 6.5-kb message is present at comparatively low levels during development but is up-regulated at 8- to 14-hr of embryogenesis. Identical results were obtained with a Fak56D 3'UTR probe (Fox, 1999).

In situ hybridization to Drosophila embryos was performed by using a biotinylated anti-sense RNA probe. At early stages of development, Fak56D is expressed throughout the embryo. After cellularization, there is an abrupt, yet transient, drop in the level of mRNA in the blastoderm. Expression levels increase during gastrulation, with highest levels in the mesoderm. At stage 13, Fak56D mRNA decreases in some cells, giving rise to a pattern of segmental stripes, and enriched expression in the gut and central nervous system becomes apparent. By stage 14, the expression pattern is once again uniform, with the exception of elevated expression in the brain (Fox, 1999).

Guinea pig antibodies were raised against the C-terminal domain of Fak56D and embryos were stained. Consistent with the mRNA expression pattern, Fak56D protein is uniformly distributed during early embryogenesis, although there is no drop in protein levels after cellularization of the blastoderm. Expression increases in the central nervous system and gut during stage 13, and these heightened levels persist through stage 15 and beyond. Variation in protein concentration along the length of the body wall first appears at stage 15. Examination of the body wall from stage 16 embryos at higher magnification shows that the antiserum does not detect accumulation of Fak56D protein in muscle attachment sites, although there is a striking decrease in signal in ectodermal cells located at the segmental boundaries (Fox, 1999).

Given the presence of Fak56D mRNA in ovaries, developing egg chambers were stained with Fak56D antiserum. Fak56D is abundant in the germ cells at early stages of oogenesis, but decreases significantly by stage 6. A similar, but less dramatic, decrease occurs in the somatic follicle cells, but they continue to express Fak56D at later stages. Strikingly, the level of Fak56D does not drop in follicle cells at the anterior end of stage 6 egg chambers. An enlarged view of the anterior tip of the stage 6 egg chamber shows clearly the difference in Fak56D levels between follicle cells at the anterior tip and those more posterior. This elevated level of Fak56D protein persists through early stage 9, when a group of follicle cells, known as the border cells, begin to migrate between the nurse cells toward the oocyte. The border cells originate from the anterior tip of the egg chamber and thus contain high levels of Fak56D protein. Fak56D is prominent in the border cells throughout their migration. Notably, Fak56D does not localize to ring canals in which elevated levels of phosphotyrosine and F-actin have been reported. It does, however, seem to accumulate in the basal region of the posterior follicle cells (Fox, 1999).

The Drosophila gene taiman encodes a steroid hormone receptor coactivator related to AIB1. Mutations in tai cause defects in the migration of specific follicle cells, the border cells, in the Drosophila ovary. Drosophila E-cadherin (Shotgun) is required for border cell migration. To determine whether the tai migration defect might be due to reduction in Shotgun expression, egg chambers containing tai mutant clones were stained with antibodies against Shotgun. In all wild-type stages examined, Shotgun accumulates in the central, nonmigratory polar cells, as well as in the junctions between individual border cells. Shotgun colocalizes with cortical F-actin in these locations. Prior to migration, when the border cells are still part of the follicular epithelium, Shotgun also accumulates at the junctions between border cells and nurse cells. However, once the border cells leave the follicular epithelium and invade the neighboring germline cell cluster, much less Shotgun staining is evident at the junctions between the nurse cells and border cells, relative to the level between border cells or in the polar cells. When migration is complete, Shotgun accumulates again in the junctions between the border cells and the oocyte (Bai, 2000).

In tai mutant clusters, Shotgun staining is abnormally elevated at the border cell/nurse cell junctions. In contrast, in slbo mutants, Shotgun expression fails to rise at the time of migration and Shotgun immunoreactivity is only detected at high levels within the polar cells. Armadillo (Arm) colocalizes with Shotgun in wild-type and mutant border cells. The abnormal accumulation of Shotgun and Arm in tai mutants does not appear to result from increased transcription of Shotgun because overexpression of Shotgun in border cells causes neither a migration defect nor specific accumulation of cadherin staining at the border cell/nurse cell junctions. Nor does the abnormal accumulation of Shotgun and Arm appear to be simply a consequence of the migration failure. In addition to slbo, Shotgun and Arm expression were examined in border cells that fail to migrate due to mutations in the jing locus: no defect in either expression or localization of adhesion complexes was observed. Nor are defects in either Shotgun or Arm expression or localization found in border cells that fail to migrate due to expression of dominant-negative Rac (Bai, 2000).

The accumulation of Shotgun at the border cell/nurse cell boundary suggests that the role of tai in border cell migration might be to stimulate turnover of adhesion complexes during migration in order to allow forward movement. One protein believed to play a role in turnover of adhesion complexes is Focal adhesion kinase. Drosophila FAK (Fak56D) is highly enriched in the border cells during their migration, but not in the polar cells (Bai, 2000).

To determine whether Fak56D expression or localization is affected by mutations that disrupt border cell migration, wild-type and slbo mutant egg chambers were stained and the staining was compared to that of egg chambers containing tai mosaic clones. Fak56D expression is significantly reduced in slbo mutant border cells. Furthermore, the level of reduction correlates with the degree of inhibition of migration. That is, in some slbo egg chambers, border cell migration fails completely and the cells remain at the anterior tip. In such egg chambers, Fak56D expression is undetectable. In a minority of slbo mutant chambers, the cells migrate a little. In these egg chambers, Fak56D expression is reduced compared to wild type, but is detectable. In tai mutant border cells, Fak56D expression is present; however, its distribution is altered relative to wild type. Rather than being evenly distributed throughout the cytoplasm, Fak56D appears to accumulate at the would-be leading edge of the cluster. Some border cell clusters that are mutant for tai exhibit partial migration and in these clusters, the abnormal distribution of Fak56D is only slightly affected such that little Fak56D accumulation can be detected at the most posterior position within the cluster. Thus, the severity of the migration defect in tai mutants correlates with the severity of the defect in Fak56D localization (Bai, 2000).

Effects of Mutation

The mammalian focal adhesion kinase (FAK) family of non-receptor protein-tyrosine kinases has been implicated in controlling a multitude of cellular responses to the engagement of cell-surface integrins and G-protein-coupled receptors. The high level of sequence conservation between the mammalian proteins and the Drosophila homologue of FAK, Fak56, suggests that Fak56 would have similar functions. However, Drosophila Fak56 is shown to be not essential for integrin functions in adhesion, migration or signaling in vivo. Furthermore, animals lacking Fak56 are viable and fertile, demonstrating that Fak56 is not essential for other developmental or physiological functions. Despite this, overexpressed Fak56 is a potent inhibitor of integrins binding to the extracellular matrix, suggesting that Fak56 may play a subtle role in the negative regulation of integrin adhesion (Grabbe, 2004).

While the conclusion that FAK family PTKs are not required in Drosophila, and are thus are not essential players in integrin function in vivo, data from C. elegans indicate that FAK is also non-essential in the nematode. There appears to be only one FAK family member in the genome of this species, thus eliminating the possibility of redundancy explaining these results. While this conclusion has not been formally reported for C. elegans, two separate findings support this hypothesis: (1) RNAi towards FAK (known as kin-32 in the nematode) causes no obvious phenotypes, and (2) a large deletion in the kin-32 open reading frame is viable (Grabbe, 2004).

In Drosophila, mutants for many of the proteins known to localize at the muscle attachment site cause lethality, in many cases due to muscle detachment, i.e. the integrins themselves, ILK, Talin, Tiggrin (although 1% of flies eclose), Laminin, alpha-actinin, and PINCH. Other molecules, such as the Drosophila Tensin homologue (also known as Blistery), are not lethal, but do display phenotypes indicative of failure of adhesion, such as wing blistering. However, it is interesting to note that another cytoskeletal protein -- Vinculin -- has also been shown to be non-essential in Drosophila, in contrast with previous results in the mouse, where animals mutant for the vinculin gene display a lethal phenotype due to heart and brain defects during embryogenesis. The non-essential nature of Fak56 in Drosophila perhaps explains why extensive attempts to target Fak56 through various methods have thus far been unsuccessful, since the general assumption has been that such mutants would be lethal. Moreover, this may also be the reason why, despite extensive genetic screening by several groups with the purpose of identifying molecules involved in integrin-mediated signaling, no mutations have ever been identified in Fak56, although the effectiveness of these screens has been demonstrated by the fact that they have independently identified several common loci in addition to existing PS integrin genes (Grabbe, 2004).

While Fak56 protein appears to be ubiquitously expressed, phospho-Fak56 is strongly localized at muscle attachment sites. This implies that Fak56 is not only localized, but also activated at these locations, since phosphorylation of the FAK^Y397 site (which is conserved in Fak56), is considered to reflect FAK activation in vivo. The anti-phospho-FAK^Y397 antibodies seem to be specific for phosphorylated Drosophila Fak56, based on two criteria: (1) loss of immunoreactivity in the Fak56 mutants, and (2) overexpressed wild-type Fak56 is recognized by the antiphospho-FAK^Y397 antibodies, whereas overexpressed Fak56^Y430F mutant protein is not. A role for Fak56, albeit an accessory one, at muscle attachment sites, is endorsed by the finding that phosphorylated Fak56 is absent from muscle attachment sites in integrin mutants. Thus, while not required for integrin function, Fak56 appears to be involved through an as yet undefined mechanism in integrin-mediated events in Drosophila embryogenesis. In spite of the absence of the Fak56 PTK at muscle attachment sites in Fak56 mutant embryos, the levels of phosphotyrosine observed are indistinguishable from wild-type, suggesting that another PTK(s) is activated at these sites. This is interesting in light of a recent report that Src kinases can be activated by direct interaction with integrin ß-subunit tails. Such a model implies that Src family kinases can be activated by integrins independently of FAK and could explain the unexpected lack of phenotypes in the Fak56 mutant. Interestingly, it has recently been shown that v-Src transformation of Ptk2^-/- fibroblasts rescues the motility defects observed in Ptk2^-/- fibroblasts, which is in keeping with these results (Grabbe, 2004).

Consistent with an accessory involvement of FAK in integrin-mediated adhesion, the Fak56-induced wing blister phenotype is sensitized in an integrin mutant background, thus indicating a genetic interaction between Fak56 and integrins in this context. Employing the UAS-GAL4 system to drive Fak56 specifically in muscles it is clear that overexpression of Fak56 disrupts muscle attachment, thus overexpression of Fak56 causes much more serious defects than its absence. This is consistent with Fak56 either functioning as an adaptor by displacing more critical proteins from the integrin cytoplasmic tail required for the extracellular binding to ECM ligands, or causing a dissociation of the integrin-containing complex by excessive phosphorylation. The evidence in mammalian systems suggests that FAK plays a critical role in cell migration, which is a complex, highly regulated process that involves the continuous formation and disassembly of adhesions. Indeed, recently it has elegantly been shown that FAK is important for adhesion turnover at the cell front, a process central to migration. The migration of (1) the endoderm, (2) border cells during oogenesis, (3) germ cells and (4) the trachea, has been examined in Fak56 mutant animals, and no defects were found in any process in the absence of Fak56. However, such a role of FAK in the disassembly, rather than in the actual assembly of these structures, is in agreement with findings that Fak56 overexpression results in detached muscles where the alphaPS2 integrin is still localized at the muscle ends, as though dissociated from the ECM. This then raises the question of why Fak56 is normally found in the developing muscles. It is speculated that Fak56 contributes to keeping the strong adhesive junctions dynamic so that they can be remodeled during the formation of the attachments and the subsequent growth of the muscles. Perhaps a defect in this process would be more apparent under more strenuous growth conditions than those found in the laboratory (Grabbe, 2004).

One important point to consider is the wealth of data from mammalian cell experiments indicating a critical role for FAK family PTKs in integrin function. Such a function is corroborated by the phenotype of the FAK knockout mouse, which dies early in embryogenesis. Indeed, Ptk2^-/- fibroblasts derived from FAK mutant embryos, exhibit a rounded morphology, and have an increased number of focal contact sites and decreased rates of cell migration. A recent RNAi-based screen of the Drosophila genome to find novel genes affecting cell morphology identified Fak56 as a molecule affecting cell spreading. In addition, overexpression of Fak56 in the fly using the GAL4-UAS system also produces a wealth of integrin-related phenotypes. Overexpressed Fak56 does localize to muscle attachment sites under such conditions, and it is entirely possible that this creates a neomorphic phenotype, i.e. such that the presence of excess Fak56 protein -- or indeed Fak56 activity -- binds up multiple factors and acts in a dominant negative fashion to produce these defects. Thus, experiments in mammalian cells employing overexpression of FAK to analyze integrin function may not be an ideal model for the in vivo scenario. Furthermore, a radical explanation for the strong defects observed in Ptk2^-/- fibroblasts, which would be consistent with the current findings, is if the primary defect of removing FAK is the observed elevated expression and activity of the related kinase Pyk2, and the Src family PTKs. If overexpressed Pyk2 causes similar dominant negative effects on other components of integrin adhesion, as is seen when Fak56 is overexpressed, then this could contribute to the severe phenotype observed in the FAK mutant mice. Finally, it is also possible that the functions of the FAK family PTKs are not totally conserved between Drosophila and mice; this suggests that FAK has assumed a more critical role during evolution of vertebrates (Grabbe, 2004).

Disruption of the single Drosophila Fak56 gene is the first example of an animal completely lacking FAK PTK function. However, it is surprising that a genetic null for a protein such as FAK, that has always been thought to be a critical link between integrins and the actin cytoskeleton, exhibits a viable phenotype. Since this also appears to be true for the C. elegans FAK family PTK, it must be assumed that while FAK may play an accessory role in modulating integrin functions in vivo, it is by no means essential for integrin-mediated adhesion or signaling in the fruitfly. It is obvious that further experiments will be required to define and understand the exact role of the Fak56 PTK in such processes in vivo (Grabbe, 2004).

Focal adhesion kinase controls morphogenesis of the Drosophila optic stalk

Photoreceptor cell axons (R axons) innervate optic ganglia in the Drosophila brain through the tubular optic stalk. This structure consists of surface glia (SG) and forms independently of R axon projection. A screen for genes involved in optic stalk formation identified Fak56D, encoding a Drosophila homolog of mammalian focal adhesion kinase (FAK). FAK is a main component of the focal adhesion signaling that regulates various cellular events, including cell migration and morphology. Fak56D mutation causes severe disruption of the optic stalk structure. These phenotypes were completely rescued by Fak56D transgene expression in the SG cells but not in photoreceptor cells. Moreover, Fak56D genetically interacts with myospheroid, which encodes an integrin ß subunit. In addition, CdGAPr is also required for optic stalk formation and genetically interacts with Fak56D. CdGAPr encodes a GTPase-activating domain that is homologous to that of mammalian CdGAP, which functions in focal adhesion signaling. Hence the optic stalk is a simple monolayered structure that can serve as an ideal system for studying glial cell morphogenesis and the developmental role(s) of focal adhesion signaling (Murakami, 2007).

The Drosophila visual system consists of the compound eyes and the optic ganglia. The compound eye consists of approximately 750 ommatidial units, each of which contains eight photoreceptor cells. During the development of the visual system, photoreceptor cells send their axons (R axons) from the eye primordium (eye disc) to their targets in the optic lobe of the brain through a structure called the optic stalk. All R axons from an eye disc come together and make a single thick bundle in the optic stalk. The optic stalk also contains two kinds of glial cells, the wrapping glial (WG) cells and surface glial (SG) cells. The WG cells are intermingled with R axons and extend long processes that wrap around several R axons, so that the entire R axon bundle is subdivided into groups of smaller bundles. The WG processes form an inner sheath to segregate these bundles from each other during the pupal stage. By contrast, SG cells form an outer sheath that surrounds the entire bundle of R axons. It is clear that the optic stalk glia play roles in R axon innervation and ensheathment; however, the precise structure and development of the optic stalk remain largely unknown (Murakami, 2007).

SG cells in the optic stalk have characteristic bipolar morphology and form a single-cell monolayer covered by a basement membrane (BM). Optic stalk expansion occurs before R axon innervation. Moreover, even in those mutants with no R axon innervation, SG cells form a single-cell-layer tube that develops normally. Therefore, it is concludes that SG cells autonomously form the optic stalk. This study sought to elucidate the molecular mechanisms underlying optic stalk formation. In screening for mutants that exhibit defects in R axon innervation or for genes that are specifically expressed in the optic stalk, the Fak56D and CdGAPr genes were identified as encoding important components required for optic stalk formation. Fak56D is a Drosophila homolog of mammalian focal adhesion kinase (FAK; also known as Ptk2), which is known to be a main regulatory component of focal contacts. Focal contacts are large integrin complexes that anchor the cytoskeleton of cells to the extracellular matrix. While focal contacts generate traction forces during migration, their disassembly is also crucial to the control of cell migration. FAK is involved in cell migration through disassembly of focal contacts, or through regulation of cytoskeletal rearrangement via Rho GTPases. In Drosophila, several studies indicate that Fak56D also acts in focal contacts in a similar way to that of mammalian FAK. Fak56D is tyrosine-phosphorylated and localized to focal contacts upon the plating of embryonic cells onto extracellular matrix components. However, as no Fak56D loss-of-function phenotype has been reported, the in vivo function of Fak56D remains elusive (Murakami, 2007 and references therein).

In Fak56D mutant animals, SG cells were not arranged into a tubular structure, although cell proliferation and differentiation were normal. Clone labeling analysis suggested that SG cell distribution along the anteroposterior (AP) axis was defective in Fak56D mutants. This study also identified CdGAPr as a possible functional partner of Fak56D. CdGAPr has a GTPase activating domain that is homologous to that of mammalian CdGAP (Sagnier, 2000). It has been shown that mammalian CdGAP regulates cell cytoskeletal rearrangement through Rac or Cdc42; however, in vivo function of neither CdGAP nor CdGAPr has been described. This study found that loss-of-function alleles of CdGAPr exhibited a Fak56D-like phenotype. Moreover, a strong genetic interaction was observed between Fak56D and CdGAPr, indicating that these genes act together to regulate SG cell behavior. Recently, mammalian CdGAP was shown to localize to focal contacts, and to be required for regulation of cell morphology and motility (LaLonde, 2006). The current results provide strong evidence that optic stalk shape is controlled by mechanisms acting autonomously in SG cells, in which Fak56D and CdGAPr play crucial roles (Murakami, 2007).

SG cells are regularly arranged and form the optic stalk, a monolayered tube that surrounds the entire R axon bundle. During larval development, the optic stalk becomes larger as SG cells proliferate, and this seems to adjust the size of the stalk so that it can wrap around the R axon bundle. The proliferation and morphogenesis of SG cells could depend on signals from the incoming R axons, because various cellular events of glial cells, such as differentiation, migration and proliferation, have been shown to require interaction with neuronal axons. For example, in the developing rodent optic nerve, Sonic hedgehog (Shh) from retinal ganglion cells (RGCs) promotes the proliferation of astrocytes. Moreover, initial formation of the optic stalk during embryogenesis was shown to be dependent on the axons from larval photoreceptor cells. However, the current data demonstrate that SG cells in Drosophila form the optic stalk independently of R axons. (1) SG cells proliferate before R axon innervation, which leads to the expansion of the optic stalk. (2) In so¹ mutants the optic stalk develops normally despite complete lack of R axon innervation. In addition, specific expression of a Fak56D transgene in SG but not photoreceptor cells rescued a defect in optic stalk morphology, hence indicating the presence of an intrinsic mechanism in SG cells to regulate optic stalk morphogenesis. Some kinds of glial cells are reported to develop independently of axon innervation in a similar way to SG cells. For instance, during Drosophila visual system development, retinal basal glia (RBG) are derived from the optic stalk and migrate into the eye disc. This process was shown to be independent of R axons. In these cases, glial cells must behave by unknown mechanisms independent of axon cues. Since the current results demonstrated that SG cell development is highly independent of R axons, this system can provide an excellent model to elucidate mechanisms underlying axon-independent glial development and behavior (Murakami, 2007).

This study has elucidated cellular mechanisms underlying optic stalk morphogenesis in Drosophila. Optic stalk morphology and development are precisely regulated, and the optic stalk expands throughout the larval stages via cell proliferation. SG cells are distributed along the AP axis during optic stalk formation. Since cells in a clone are often associated with each other, SG cells may migrate along the nearby SG cells that have extended processes along the AP axis. Such homotypic migration is reported for neuronal precursors in the adult mammalian subventricular zone (SVZ). Neuronal precursors migrate to the olfactory bulb in chains, by sliding along each other without the assistance of other cell types. Such directed distribution of SG cells is likely to be necessary for keeping the optic stalk thin and long. The mechanism regulating tube morphogenesis is largely unknown. The finding that SG cells undergo cell arrangement during optic stalk expansion provides an interesting insight into the morphogenesis of a tubular structure (Murakami, 2007).

It is known that the optic stalk disappears during the pupal stage and that the ommatidia are set much closer to the optic lobe. This study found that SG cells begin to locate at the surface of the optic lobe during early pupal stages. This is similar to what was observed in Fak56D mutants during larval stages. It seems that the optic stalk is degenerating during the pupal stage through relocation of SG cells to the optic lobe, and this process might be regulated by FAK activity (Murakami, 2007).

In an attempt to identify the molecular mechanisms underlying optic stalk formation, it was found that the optic stalk was disrupted in Fak56D mutants. Expression of a Fak56D transgene in SG cells, but not photoreceptor cells, rescued the defect. This indicates that Fak56D is autonomously required in SG cells. Fak56D is apparently required during the expansion of the optic stalk. Since differences were detected in number of SG cells between wild type and Fak56D mutants, it is unlikely that Fak56D regulates the proliferation of SG cells. SG cells are mis-localized on the surface of the optic lobe in Fak56D mutants instead of forming a tubular structure. Visualization of newly divided cells by clonal labeling suggests that SG cells tend to migrate along the AP axis, and this could lead to formation of a longitudinal tube structure. In Fak56D mutants SG cells partially lose tendency to migrate along the AP axis; this could lead to enlargement of the diameter as opposed to the length. Ectopic localization of SG cells on the optic lobe observed in Fak56D mutants is likely to be the result of the optic stalk widening. Mammalian FAK plays a central role in cell migration; hence in Drosophila Fak56D may regulate optic stalk formation via regulation of cell migration (Murakami, 2007).

Fak56D might also be required for adhesion between SG cells. siRNA-mediated mammalian FAK knockdown results in loss of N-cadherin-based cell-cell adhesion in Hela cells. In addition, it has been shown that overexpression of a kinase-defective mutant of FAK in cultured cells blocks the hyperosmolarity-induced E-cadherin accumulation at the cell periphery. Since SG cells are closely attached to each other, adhesion between SG cells may be important for keeping optic stalk tubular structure. SG cells are also attached to the BM throughout optic stalk formation. Because Fak56D is implied to regulate integrin adhesion, it is possible that Fak56D is required for SG cells to form proper adhesion to the BM (Murakami, 2007).

In addition to Fak56D, CdGAPr was identified as a regulator of optic stalk morphology. Fak56D and CdGAPr exhibit a strong genetic interaction. Since both mammalian FAK and CdGAPr are known to act at focal contacts, this raised the possibility that focal contacts are important for optic stalk morphogenesis. Main components of focal contacts, such as integrins or Paxillin, are conserved between vertebrates and invertebrates. In Drosophila, Fak56D is implicated in integrin-involving molecular mechanisms. This study found a genetic interaction between Fak56D and myospheroid (mys), which encodes a ß-subunit of integrin, ßPS. This suggests that Fak56D and CdGAPr act together with integrins in focal contacts to regulate SG cell behavior. Because the interaction between Fak56D^CG1 and mys¹ (a null allele) is much weaker than the interaction between Fak56D^CG1 and CdGAPr hypomorphic alleles, it is possible that another ß subunit, ßnu, compensates for the loss of ßPS. ßPS and ßnu have shown to act together to regulate midgut cell migration. It is also possible that Fak56D function is regulated via other receptors, such as G-protein coupled receptors (GPCRs). Mammalian Pyk2, another FAK family member, is known to be activated via GPCRs (Murakami, 2007).

CdGAPr encodes a GAP domain that regulates the activity of Rho-family GTPases. The mammalian CdGAP regulates Rac and Cdc42, which are known regulators of the actin cytoskeleton, and are required for cell morphology and motility. This raises the possibility that FAK regulates cytoskeletal rearrangement via activation of CdGAP, although exact functional interaction between the two proteins remains elusive. In fact, the mammalian FAK interacts directly with guanine nucleotide exchange factors or GTPase-activating proteins in the regulation of cytoskeleton. More biochemical studies are required for elucidating the molecular mechanisms that underlie cell behaviors regulated by focal adhesion signaling (Murakami, 2007).

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date revised: 25 September 2007

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