Vertebrate dystrophin is known to interact via its cysteine-rich and C-terminal regions with α- and probably β-dystrobrevin, as well as with the syntrophins (α1, β1, β2, γ1 and γ2) and the transmembrane β-dystroglycan subunit. The sarcoglycans (α, β, γ and δ) are also found spanning the sarcolemma in close proximity to dystroglycan and sarcospan. The high degree of conservation of the dystrophin C-terminus between fly and human suggested the likely presence in fly of homologues of the proteins known to interact with this region. Partial cDNAs have been cloned suggestive of coding sequences for a dystrobrevin (DmDYB), dystroglycan (DmDG) and two syntrophins (DmSYN-1; DmSYN-2). Similarly, putative genes for three sarcoglycans have been found (DmSCG-αepsilon; DmSCG-β; DmSCG-γδ) (Greener 2000).
In vertebrate skeletal muscle, α-, β-, γ- and δ-sarcoglycans form a heterotetrameric complex, and are of particular interest due to their involvement in human limb-girdle muscular dystrophies. The highly homologous δ- and γ-sarcoglycans in turn show significant homology to β-sarcoglycan. α-Sarcoglycan differs considerably from β, γ and δ, and in smooth muscle, α-sarcoglycan is substituted by the closely related epsilon-sarcoglycan. The sarcoglycan family of proteins is simplified in D. melanogaster. The fly genome encodes a single orthologue of vertebrate α- and epsilon-sarcoglycans (DmSCG-αepsilon), a β-sarcoglycan (DmSCG-β) and a single orthologue of γ- and δ-sarcoglycans (DmSCG-γδ). Given the stoichiometry of the mammalian sarcoglycan complex, it is suggested that DmSCG-γδ exists as a homodimer in association with DmSCG-αepsilon and DmSCG-β. The DmSCG-γδ protein is the most conserved of Drosophila sarcoglycans (35% identical, 56% similar to human γ- and δ-sarcoglycans), sharing a short N-terminal region, a highly conserved transmembrane domain (typical of type II membrane topology) and four extracellular cysteine residues with its human orthologues. Similarly, DmSCG-β appears to share the same type II characteristics as its human counterpart, although its sequence is less conserved (19% identical, 35% similar). The homology between DmSCG-αepsilon and human α/epsilon-sarcoglycans is also quite low (18% identical, 36% similar), but the N-terminal regions share a predicted signal sequence, typical of type I transmembrane proteins (Greener 2000).
Vertebrate dystroglycan is a dystrophin-associated protein expressed as a large propeptide, which is later cleaved into α- and β-subunits. α-Dystroglycan retains its interactions with the transmembranal β-subunit, and interacts with components of the extracellular matrix. A single orthologous sequence has been identified in D. melanogaster that is moderately conserved throughout the length of the protein (31% identity, 48% similarity), with a predicted transmembrane domain and recognisable polyproline motif at the C-terminus. This motif is a characteristic signal for WW domain-binding and has recently been implicated in the phosphotyrosine-regulated interaction with the utrophin WW domain. The region corresponding to the C-terminal (membrane-proximal) one-third of the extracellular α-dystroglycan subunit is also more highly conserved, and this may provide further evidence for interactions with biglycan protein (Greener 2000).
Consistent with findings in C. elegans, the fly genome contains two expressed homologues of the five vertebrate syntrophins (DmSYN-1 and DmSYN-2). These cytoplasmic proteins are known to interact via their PDZ domain with the intracellular signalling molecule nNOS and voltage-gated sodium channels, as well as with the ‘syntrophin-binding’ region of the dystrophins and dystrobrevins. Both DmSYN-1 and DmSYN-2 retain the characteristic syntrophin domain structure: a PH domain interrupted by a PDZ domain is followed by a second PH domain, with the proteins ending C-terminal with a ‘syntrophin-unique’ (SU) region. DmSYN-1 is more similar to human α1-, β1- and β2-syntrophins (40% identical), and DmSYN-2 more similar to the recently characterised human γ1- and γ2-syntrophins (35% identical) (Greener 2000).
The characterisation of a Drosophila homologue of dystrobrevin completes the fly complement of known cytoplasmic DC components. Dystrobrevin is thought to have arisen very early in evolution through an initial duplication of the original dystrophin ancestor. The fly orthologue, DmDYB, shows continuous similarity to its two vertebrate counterparts, including four EF-hands and a ZZ domain, a syntrophin-binding region and a high degree of conservation towards its N-terminus, a region postulated to interact with the vertebrate sarcoglycans-sarcospan complex. The putative syntrophin-binding region contains a single highly conserved motif shared between vertebrates and invertebrates (DEEHRLIARYAARLA). As with the dystrophins, the second coiled-coil region is more similar between phyla and includes six conserved leucine-heptad repeats (Greener 2000).
During this investigation, it was not possible to identify a sarcospan homologue, despite its association with sarcoglycans in vertebrate muscle. It may be that sarcospan is a vertebrate-specific protein, or that low selective pressure has allowed the fly and human sequences to diverge beyond recognition (indeed, null mutations of mouse sarcospan cause no observable phenotype (Greener 2000).
Thus, characterisation of the fly DC shows that flies possess essentially the same DC components as vertebrates, implying a role of fundamental importance. Furthermore, regions and domains known to mediate the interactions between members of the complex are highly conserved between human and fly, suggesting that the gross structure of the complex is identical. Similarities between the human and fly proteins follow the expected phylogeny, and the ramified topology of the vertebrate branches presumably relates to significant adaptations in vertebrate evolution. Divergence of α- and epsilon-sarcoglycan may have coincided with the divergence of vertebrate smooth and striated muscle, and the evolving nervous system may have demanded newly adapted forms of syntrophin, dystrobrevin and dystrophin (DRP2). In this latter regard, it is interesting to note that the tree topologies of the dystrophin/dystrobrevin family and the syntrophin family are identical. It is not known whether this reflects coordinate specialisation of function or merely a need for more elaborately controlled spatial or temporal expression patterns. The sarcoglycans appear to have diverged rather unequally, with DmSCG-γδ and human γ/δ-sarcoglycans maintaining more similarity. This may suggest a more critical role for γ/δ-sarcoglycans in the functioning of its heteromeric complex. Surprisingly, the fly dystroglycan orthologue is very loosely conserved, despite its essential mammalian role. Serving as a link between the extracellular matrix proteins and the cytoplasmic DC, dystroglycan has only one recognisable binding motif. Its polyproline C-terminus is known to link both utrophin and dystrophin to the cell membrane; it is unclear as to what extent the primary sequence is important for direct extracellular interactions (Greener 2000).
The smaller number of syntrophins and dystrophin-related proteins represents the most noticeable reduction of complexity in the invertebrate DC. The vertebrate dystrophins and syntrophins appear to have diverse but unclear roles in the central and peripheral nervous systems, and in skeletal, cardiac and smooth muscle. Many of these roles may have been acquired through the elaboration of gene families, and it is not clear which are ancestral and which are derived. Synaptic localisation seems to be a recurring theme, with both dystrophin itself and DRP2 being associated with synapses in the brain, and utrophin being localised to the neuromuscular junction (NMJ). Given the possible association between DRP2 and central cholinergic transmission, the cholinergic nature of the NMJ (with a suggested role of dystroglycan in localising acetylcholinesterase and the perturbation of cholinergic signalling in the dystrophin-deficient worm, it is conceivable that a role in cholinergic transmission is ancestral in the dystrophin family, and that the sarcolemmal association of vertebrate dystrophins is a recent adaptation. The further identification of Drosophila proteins orthologous to the muscle-specific sarcoglycans reflects an important aspect of the conservation of the DC that will hopefully shed light on relationships between ancestral and acquired muscular functions (Greener 2000).
With completion of the Drosophila genome project, the clones reported in this study are likely to represent the entire Drosophila repertoire of proteins orthologous to the known vertebrate DC. Their existence implies that the fly (and presumably most metazoans) has the potential to form a complex almost identical to the well-characterised human skeletal muscle DC. It is suggested that the reduced heterogeneity of the DC components in this experimentally amenable organism makes it an ideal model for resolving the fundamental ancestral role of the DC (Greener 2000).
Perturbation in the Dystroglycan (Dg)-Dystrophin (Dys) complex results in muscular dystrophies and brain abnormalities in human. Drosophila is an excellent genetically tractable model to study muscular dystrophies and neuronal abnormalities caused by defects in this complex. Using a fluorescence polarization assay, a high conservation in Dg-Dys interaction between human and Drosophila is demonstrated. Genetic and RNAi-induced perturbations of Dg and Dys in Drosophila cause cell polarity and muscular dystrophy phenotypes: decreased mobility, age-dependent muscle degeneration and defective photoreceptor path-finding. Dg and Dys are required in targeting glial cells and neurons for correct neuronal migration. Importantly, Dg interacts with insulin receptor and Nck/Dock SH2/SH3-adaptor molecule in photoreceptor path-finding. This is the first demonstration of a genetic interaction between Dg and InR (Shcherbata, 2007).
The Dg-Dys binding interface is highly conserved in humans and Drosophila. Both proteins are required for oocyte cellular polarity and interact in this process. Futhermore, mutants of both Dg and Dys genes show symptoms observed in muscular dystrophy. Reduction of Dg and Dys proteins results in age-dependent mobility defects. Eliminating Dg and Dys specifically in mesoderm derived tissues reveals that these proteins are required for muscle maintenance in adult flies: age-dependent muscle degeneration was observed in mutant tissues. Dg-Dys complex is also required for neuron path-finding and has both cell autonomous and non-cell autonomous functions for this process. Further, in neuronal path-finding process Dg interacts with InR and an SH2/SH3-domain adapter molecule Nck/Dock (Shcherbata, 2007).
Animal models have been used efficiently in muscular dystrophy studies. Some of the models are naturally occurring mutations (mdx-mouse, muscular dystrophy dog, cat and hamster), others have been generated by gene targeting. However, the regulation and the control of Dg-Dys complex are not understood, and no successful therapeutics exist yet for muscular dystrophies. Recently developed models for muscular dystrophy exist in C. elegans and zebrafish. In C. elegans Dys mutant, the transporter snf-6 that normally participates in eliminating acetylcholine from the cholinergic synapses, is not properly localized, resulting in an increased acetylcholine concentration at the neuromuscular junction and muscle wasting (Kim, 2004). The function of Dys in neuromuscular junctions has been addressed in Drosophila. These results bring up the possibility that muscular dystrophies in humans might also at least partly be attributed to the altered kinetics of acetylcholine transmission through neuromuscular junctions (Shcherbata, 2007).
Drosophila acts as a remarkably good model for age-dependent progression of muscular dystrophy. Dg and Dys reduction in Drosophila show age-dependent muscle degeneration and lack of climbing ability. It is tempting to speculate that the common denominator between different defects observed in Dg-Dys mutants in Drosophila and C. elegans is defective cellular polarity. The defects observed in C. elegans could be due to a defect in polarization of a cell, which will generate a neuromuscular junction that leads to miss-targeted snf-6. Similarly, Drosophila Dg-Dys complex is required for cellular polarity in the oocyte. In addition, neural defects observed are plausibly due to polarity defects in the growing axon (Shcherbata, 2007).
Similar to neuronal defects observed in human muscular dystrophy patients, neuronal defects were also found in Drosophila Dg and Dys mutant brains. In vertebrate brains, Dg affects neuronal migration (Montanaro, 2003; Qu, 2004) possibly through interaction of neurons with their glial guides. The neuronal migration and process outgrowth have been shown to require supportive input from glial cells and involve the formation of adhesion junctions along the length of the soma. Also, the outgrowth of the leading process involves rapid extension and contraction over the length of the glial fiber. Disruption of the cytoskeletal organization within the neuron, either of actin filaments, has been shown to inhibit glial-mediated neuronal migration. The glial function in this process is less well studied (Shcherbata, 2007).
Drosophila photoreceptor path-finding provides an excellent system for genetic dissection of neuronal outgrowth and target recognition. During the formation of the nervous system, newly born neurons send out axons to find their targets. Each axon is led by a growth cone that responds to extracellular axon guidance cues and chooses between different extracellular substrates upon which to migrate. Recent work has also identified a variety of intracellular signaling pathways by which these cues induce cytoskeletal rearrangements, but the proteins connecting signals from cell surface receptors to actin cytoskeleton have not been clearly determined. Dg is a good candidate for linking receptor signaling to the remodeling of the actin cytoskeleton and thereby polarizing the growth cone. Perturbation of Dg-Dys complex causes phenotypes that resemble Nck/Dock-Pak-Trio axon path-finding phenotypes, suggesting that Dg may be one of the key players in Nck/Dock signaling pathway for axon guidance and target recognition in Drosophila (Shcherbata, 2007).
Interestingly, Insulin receptor-tyrosine kinase (InR) mutants also show similar phenotypes to those of Nck/Dock signaling in photoreceptor axon path-finding and these two proteins show genetic and biochemical interactions. These data have led to speculations of mammalian InR acting in conjunction with Nck/Dock pathway in learning, memory and eating behavior. The current data now add Dg-Dys complex to this pathway; similar to what is seen in the case of Dg and Dys photoreceptor mutants, InR mutants show no obvious defects in patterning of the photoreceptors. However, the guidance of photoreceptor cell axons from the retina to the brain is aberrant. Furthermore, genetic and biochemical evidence suggests that InR function in axon guidance involves the Dock-Pak pathway rather than the PI3K-Akt/PKB pathway. Independently, biochemical interaction between Nck/Dock and Dg has been reported supporting the hypothesis that InR, Dg and Nck/Dock interaction regulates Dg-Dys complex. Furthermore, Dg interacts genetically with InR and Dock in photoreceptor axon path-finding. Since Dys interacts with Dg but not with InR and Dock, it is tempting to speculate that Dg can selectively interact with either Dys or InR and Dock. One possibility is that the tyrosine kinase activity of InR could regulate the Dg-Dys interaction by tyrosine phosphorylation in the Dg-Dys binding interphase. This tyrosine phosphorylation could prohibit the Dg-Dys interaction and thereby result in rearrangements in the actin cytoskeleton. Alternatively, other components observed in Dg-Dys complex might be involved in this regulation. However, it is also possible that potential polarity defects in the Dg mutant axons result in defective InR membrane localization. Interestingly, in another cell type, the Drosophila oocyte, InR, Dg and Dys also show similar phenotypes. In addition, insulin-like growth factors (IGF) and InR are important in maintaining muscle mass in vertebrates. Further connection of InR to Dg-Dys complex comes from experiments showing that muscle specific expression of IGF counters muscle decline in mdx-mice. The work presented in this study is the first demonstration of genetic interaction between Dg and InR. Future biochemical studies should unravel the molecular mechanism of this interaction (Shcherbata, 2007).
Dg-Dys complex is required both in neural and in targeting glial cells for correct neuronal axon path-finding in Drosophila brain. These data reveal that Dg-Dys complex also has a non-cell autonomous effect on axon path-finding and suggest that Dg-Dys-controlled ECM both from neuron and glial cells regulate neuronal axon path-finding. Further experiments are required to reveal whether long-range Laminin fibers are involved in this process, as has been shown in epithelial planar polarity, or whether glial processes are observed in close proximity to the neural growth cone. Interestingly, similar phenotypes are observed with Integrin mutants, suggesting that, as in planar polarity, Integrin and Dg-Dys complex might act in concert to regulate the process of ECM organization that will regulate the cytoskeleton of the cells involved (Shcherbata, 2007).
Taken together, the phenotypes caused by Drosophila Dg and Dys mutations are remarkably similar to phenotypes observed in human muscular dystrophy patients, and therefore suggest that functional dissection of Dg-Dys complex in Drosophila should provide new insights into the origin and potential treatment of these fatal neuromuscular diseases. As a proof of principle, using Drosophila as a model, InR has now been determined to be a signaling pathway that genetically interacts with Dg. Future studies are directed to unravel the molecular mechanism of Dg and InR-Dock interactions in invertebrates as well as vertebrates (Shcherbata, 2007).
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