Disabled

DEVELOPMENTAL BIOLOGY

Embryonic

The embryonic expression pattern of Dab mRNA, examined by in situ hybridization to whole-mount embryos, shows uniform expression from blastoderm through gastrulation. At the end of germ-band extension, higher levels of Dab mRNA are detected in the mesoderm and in the CNS. Polyclonal rabbit antibody to Dab detects a similar distribution. In the cellular blastoderm, immunohistochemical staining for Dab protein is detected in the cytoplasm and not the nucleus. During gastrulation, Dab protein is broadly distributed, but is higher in the CNS. Later in development, the signal in the CNS is concentrated in axon bundles. Dab protein is also detected in PNS cell clusters and the body wall musculature (Gertler, 1993).

Effects of Mutation or Deletion

During Drosophila embryogenesis, Abl is localized in the axons of the central nervous system (CNS). Mutations in Abl have no detectable effect on the morphology of the embryonic CNS; the mutant animals survive to the pupal and adult stages. However, in the absence of Abl function, heterozygous mutations or deletions of Disabled (Dab) exert dominant effects, disrupting axonal organization and shifting the lethal phase of the animals to embryonic and early larval stages. Embryos that are homozygous mutant for both Abl and Dab fail to develop any axon bundles in the CNS, although the peripheral nervous system and the larval cuticle appear normal. The genetic interaction between these two genes begins to define a process in which both the Abl tyrosine kinase and Enabled participate in establishing axonal connections in the embryonic CNS of Drosophila (Gertler, 1989).

In the absence of the Drosophila Abl protein-tyrosine kinase (PTK), loss-of-function mutations in either Disabled or prospero have dominant phenotypic effects on embryonic development. Molecular and genetic characterizations indicate that the products of these genes interact with the Abl PTK by different mechanisms. The interaction between Abl and prospero, which encodes a nuclear protein required for correct axonal outgrowth, is likely to be indirect. In contrast, the product of Disabled may be a substrate for the Abl PTK. The Disabled protein is colocalized with Abl in axons; its predicted amino acid sequence contains 10 motifs similar to the major autophosphorylation site of Abl, and the protein is recognized by antibodies to phosphotyrosine (Gertler, 1993).

Mutations in the failed axon connections (fax) gene have been identified as dominant genetic enhancers of the Abl mutant phenotype. These mutations in fax all result in defective or absent protein product. In a genetic background with wild-type Abl function, the fax loss-of-function alleles are homozygous viable, demonstrating that fax is not an essential gene unless the animal is also mutant for Abl. The fax gene encodes a novel 47-kD protein expressed in a developmental pattern similar to that of Abl in the embryonic mesoderm and axons of the central nervous system. The conditional, extragenic noncomplementation between fax and another Abl modifier gene, Disabled, reveals that the two proteins are likely to function together in a process downstream or parallel to the Abl protein tyrosine kinase (Hill, 1995).

Enabled signaling pathway regulates Golgi architecture in Drosophila photoreceptor neurons

The golgi apparatus is optimized separately in different tissues for efficient protein trafficking, little is known of how cell signaling shapes this organelle. This study finds that the Abl tyrosine kinase signaling pathway controls the architecture of the golgi complex in Drosophila photoreceptor (PR) neurons. The Abl effector, Enabled (Ena), selectively labels the cis-golgi in developing PRs. Overexpression or loss-of-function of Ena increases the number of cis and trans-golgi cisternae per cell, and Ena overexpression also redistributes golgi to the most basal portion of the cell soma. Loss of Abl, or of its upstream regulator, the adaptor protein Disabled, lead to the same alterations of golgi as does overexpression of Ena. The increase in golgi number in Abl mutants arises in part from increased frequency of golgi fission events and a decrease in fusions, as revealed by live imaging. Finally, it was demonstrated that the effects of Abl signaling on golgi are mediated via regulation of the actin cytoskeleton. Together, these data reveal a direct link between cell signaling and golgi architecture. Moreover, they raise the possibility that some of the effects of Abl signaling may arise, in part, from alterations of protein trafficking and secretion (Kannan, 2014).

The Abl tyrosine kinase signaling pathway controls golgi morphology and localization in Drosophila photoreceptors through its regulation of the actin cytoskeleton. Ena, the main effector of Abl in morphogenesis, is associated with the cis-golgi compartment, and it regulates golgi localization and dynamics under the control of Abl and its interacting adaptor protein, Dab. Reducing the levels of Abl or Dab, or overexpressing Ena, led to similar defects in golgi fragmentation state and subcellular distribution. During golgi biogenesis, Abl increases the frequency of fusion of golgi cisternae, and decreases fission events. Abl evidently controls golgi organization through its regulation of actin structure, as the effect of Abl signaling on golgi could be blocked by modulating actin structure genetically or pharmacologically. Collectively, these data reveal an unexpected link between a fundamental tyrosine kinase signaling pathway in neuronal cells and the structure of the golgi compartment (Kannan, 2014).

The data reported here suggest that the Abl signaling pathway controls golgi morphology and localization through its control of actin structure. This is consistent with previous reports that altering the levels of actin modulators perturbs the structure and function of the golgi apparatus. A variety of proteins that modulate actin dynamics have been localized to golgi. Ultra-structural studies established the association of actin filaments with golgi membranes and the association of β and γ actin with the golgi. In cultured cell models, including neurons, actin depolymerization leads to golgi compactness, fragmentation and altered subcellular distribution. It is noted, moreover, that the reported golgi-associated signaling proteins include several that have been linked to Abl signaling, including the Abl target Abi, the Abi binding partner WAVE, and various effectors of Rac GTPase including ADF/cofilin, WASH and Arp2/3. Thus, for example, Abi and WAVE have been implicated in actin dependent golgi stack reorganization and in scission of the golgi at cell division to allow faithful inheritance of golgi complex to daughter cells in Drosophila S2 cell cycles (Kondylis, 2007). These data reinforce the importance of actin-regulating signaling pathways for controlling golgi biogenesis (Kannan, 2014).

Two lines of evidence suggest that the increase observed in golgi number in Abl pathway mutants is due primarily to net fragmentation of pre-existing golgi cisternae and not to de novo synthesis of golgi. First, live imaging of golgi dynamics in neurons of the Drosophila eye disc reveals that the steady-state number of golgi cisternae reflects an ongoing balance of fusion and fission events, much as observed previously in yeast. Quantification of these events in wildtype vs Abl mutant tissue demonstrated directly that loss of Abl significantly increased the frequency of fission events, and reduced the frequency of fusions. Second, the absolute volume of cis-golgi in Abl mutant photoreceptors was not substantially greater than that in wildtype, as judged by direct measurement of the volume of GM130- immunoreactive material in deconvoluted image stacks of photoreceptor clusters. While a small apparent increase was observed in golgi volume in the mutants (~55%, based on pixel counts), it is noted that golgi cisternae are small on the length scale of the point spread function of visible light, such that the fluorescent signal from a single cisterna extends into the surrounding cytoplasm. The increase in apparent golgi volume is therefore within the range expected due simply to fluorescence 'spillover' from the three-fold greater number of separate golgi cisternae in the mutants (Kannan, 2014).

It is striking that both increase and decrease of Ena led to net fragmentation of golgi. Why might this be? It is known that both fission and fusion of membranes requires actin dynamics: at scission, polymerization provides force for separating membranes, while in fusion, actin polymerization is essential for bringing membranes together and for supplying membrane vesicles, among other things. As a result, altering actin dynamics is apt to change the probabilities of multiple aspects of both fission and fusion events, making it impossible to predict a priori how the balance will be altered by a given manipulation, just as either increase or decrease of Ena can inhibit cell or axon motility, depending on the details of the experiment, due to the non-linear nature of actin dynamics. Indeed, this study also observed net golgi fragmentation when actin was stabilized with jasplakinolide, just as was done from depolymerization with cytochalasin or latrunculin. More direct experiments will be necessary to fully understand this dynamic, however. deficits selectively disrupt dendritic morphogenesis but not axogenesis, and perhaps consistent with this, Abl/Ena function is essential for dendrite arborization in these cells but has not been reported to affect their axon patterning. Finally, in some contexts, neuronal development requires local translation of guidance molecules in the growth cone rather than translation in the cell soma. It is likely that the need for actin dynamics to target different subcellular compartments in different cell types will be reflected in different patterns of Abl/Ena protein localization (Kannan, 2014).

This study reports the role of Abl/Ena-dependent regulation of actin structure on overall golgi structure and localization but there may be more subtle effects on golgi function as well. For example, recent evidence supports a role for actin-dependent regulation of the specificity of protein sorting in the golgi complex. Preferential sorting of cargos is achieved by nucleation of distinct actin filaments at the golgi complex. In Hela cells, for example, Arp2/3 mediated nucleation of actin branches at cis-golgi regulates retrograde trafficking of the acid hydroxylase receptor CI-MPR, while Formin family mediated nucleation of linear actin filaments at golgi regulates selective trafficking of the lysosomal enzyme cathepsin D. Similarly, the actin-severing protein ADF/cofilin, the mammalian ortholog of Drosophila twinstar, sculpts an actin-based sorting domain at the trans-golgi network for selective cargo sorting. It will be important to investigate whether the effects of Abl/Ena on golgi morphology have functional consequences on bulk secretion or protein sorting (Kannan, 2014).

Protein trafficking and membrane addition in neurons need to be coordinated with the growth requirements of the axonal and dendritic plasma membranes, but the mechanisms that do so have been obscure. Abl pathway proteins associate with many of the ubiquitous guidance receptors that direct axon growth and guidance throughout phylogeny, including Netrin, Roundabout, the receptor tyrosine phosphatase DLAR, Notch and others. The data therefore suggest a potential link between the regulatory machinery that senses guidance information and the secretory machinery that helps execute those patterning choices. Indeed, preliminary experiments suggest that some of the axonal defects of Abl pathway mutants may arise from alterations in golgi function. Beyond this, Abl signaling is essential in neuronal migration, epithelial polarity and integrity, cell adhesion, hematopoiesis and oncogenesis, among other processes The data reported in this study now compel a reexamination of the many functions of Abl to ascertain whether some of these effects arise, at least in part, from regulation of secretory function (Kannan, 2014).

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date revised: 5 February 2015

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