InteractiveFly: GeneBrief

visceral mesodermal armadillo-repeats: Biological Overview | References


Gene name - visceral mesodermal armadillo-repeats

Synonyms -

Cytological map position - 42E1-42E1

Function - signaling

Keywords - guanine nucleotide exchange factor for Miro - involved in mitochondrial fusion - functional targets of miR-277 that modulates rCGG repeat-mediated neurodegeneration - expressed embryonically in the midgut and hindgut visceral mesoderm, as well as in the CNS and PNS - mutation causes shortened rhabdomere length in the eye

Symbol - vimar

FlyBase ID: FBgn0022960

Genetic map position - chr2R:6,975,285-6,985,712

NCBI classification - Armadillo repeat protein

Cellular location - cytoplasmic



NCBI links: EntrezGene, Nucleotide, Protein
BIOLOGICAL OVERVIEW

As fundamental processes in mitochondrial dynamics, mitochondrial fusion, fission and transport are regulated by several core components, including Miro. As an atypical Rho-like small GTPase with high molecular mass, the exchange of GDP/GTP in Miro may require assistance from a guanine nucleotide exchange factor (GEF). However, the GEF for Miro has not been identified. While studying mitochondrial morphology in Drosophila, it was incidentally observed that the loss of vimar, a gene encoding an atypical GEF, enhanced mitochondrial fission under normal physiological conditions. Because Vimar could co-immunoprecipitate with Miro in vitro, it was speculated that Vimar might be the GEF of Miro. In support of this hypothesis, a loss-of-function (LOF) vimar mutant rescued mitochondrial enlargement induced by a gain-of-function (GOF) Miro transgene; whereas a GOF vimar transgene enhanced Miro function. In addition, vimar lost its effect under the expression of a constitutively GTP-bound or GDP-bound Miro mutant background. These results indicate a genetic dependence of vimar on Miro. Moreover, mitochondrial fission was found to play a functional role in high-calcium induced necrosis, and a LOF vimar mutant rescued the mitochondrial fission defect and cell death. This result can also be explained by vimar's function through Miro, because Miro's effect on mitochondrial morphology is altered upon binding with calcium. In addition, a PINK1 mutant, which induced mitochondrial enlargement and had been considered as a Drosophila model of Parkinson's disease (PD), caused fly muscle defects, and the loss of vimar could rescue these defects. Furthermore, it was found that the mammalian homolog of Vimar, RAP1GDS1, played a similar role in regulating mitochondrial morphology, suggesting a functional conservation of this GEF member. The Miro/Vimar complex may be a promising drug target for diseases in which mitochondrial fission and fusion are dysfunctional (Ding, 2016).

Mitochondrial fission, fusion and transport play important roles for the function of this organelle. The balance between fusion and fission controls mitochondrial morphology, which is mediated by series of large dynamin-related GTPases. Among these GTPases, mitofusin1/mitofusin2 (MFN1/MFN2) and optic atrophy protein1 (OPA1) are the core components that are responsible for mitochondrial fusion, whereas dynamin-related protein 1 (Drp1) is the core component that is responsible for mitochondrial fission. In addition to these GTPases in dynamin-related family, mitochondrial Rho (Miro), an atypical member of the Rho small GTPase family, has a well-known function of transporting the mitochondria along microtubules. Miro also regulates mitochondrial morphology via inhibition of fission under physiological Ca2+ conditions, although the mechanism is not that clear. Large GTPases such as dynamin-like GTPase family members hydrolyze GTP and exchange GTP and GDP without the assistance from other regulators. However, members of the small GTPase family often require other proteins to help release their tightly bound GDP or enhance their low GTPase activities. These proteins are referred to as guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), respectively. To date, most small GTPases require unique GEFs or GAPs (Ding, 2016).

An understanding of the regulation of mitochondrial dynamics may help address many human diseases. For instance, mutations in OPA1 or MFN2 result in dominant optic atrophy or Charcot-Marie-Tooth neuropathy type 2A. Abnormal mitochondrial fission also promotes aging and cell death. In necroptosis, the formation of the necrosome promotes mitochondrial fission through dephosphorylation of Drp1. In neuronal excitotoxicity, calcium ions are overloaded, resulting in reduced levels of the MFN2 protein, which enhances mitochondrial fission and leads to neuronal necrosis In addition, other components such as Miro may participate in this process. Miro has two EF hand motifs that bind calcium; thus, Miro can couple calcium increase with reduced mitochondrial motility to meet the locally increased energy demands. Interestingly, Miro also promotes fission in the presence of excess calcium, which is distinct from its inhibitory role in fission under normal calcium concentrations. It is unclear whether Miro plays a functional role in neuronal necrosis (Ding, 2016).

The mitochondrial morphology represents a transient balance between mitochondrial fusion and fission. Using a systematic genetic screen in yeast covering approximately 88% of genes, 117 genes that regulate mitochondrial morphology were identified. Similarly, a screen of 719 genes that are predicted to encode mitochondrial proteins in worms demonstrated that more than 80% of these genes regulate mitochondrial morphology. Although many genes may regulate mitochondrial morphology, their relationships to the core mitochondrial fusion and fission components are unclear (Ding, 2016).

In studying mitochondrial morphology, it was accidently discovered that the loss of vimar (visceral mesodermal armadillo-repeats), which encodes an atypical GEF, promoted mitochondrial fission in Drosophila flight muscle cells. Furthermore, it was found that vimar was capable of interacting with Miro in vitro. Genetically, vimar required normal GDP- or GTP-bound activity of Miro to affect mitochondrial morphology, suggesting vimar is likely the Miro GEF. In addition, it was found that the Miro/Vimar complex suppressed mitochondrial fission during necrosis and mitochondrial fusion in PINK1 mutant model of Parkinson's disease (PD), making vimar a potential drug target (Ding, 2016).

Mitochondrial function can be assessed by the enzymatic activity of citrate synthase (CS), the first enzyme in the Krebs cycle that converts acetyl-CoA and oxaloacetate to citrate. In cultured Drosophila S2 cells, vimar knock down by RNAi resulted in reduced CS activity, indicating that vimar may positively regulate mitochondrial function. Because mitochondrial fission has generally been associated with reduced mitochondrial respiration, the decreased CS activity may be a result of mitochondrial fission. Consistent with this notion, the results demonstrated that the LOF of vimar promoted mitochondrial fission. In addition, a GOF vimar transgene had a minimal effect on mitochondrial morphology, indicating that vimar activity might be saturated under normal physiological conditions (Ding, 2016).

Because Vimar has been predicted to be a GEF, it was hypothesized that Vimar may regulate mitochondrial morphology by affecting a small GTPase, which requires a GEF to help with the GTP/GDP exchange process. Interestingly, Miro is one such small GTPase that is known to play important roles in mitochondrial fission and transport. It is proposed that Vimar and Miro may function as a complex. First, a fraction of the Vimar protein was localized to the mitochondria, possibly indicating a functional role on mitochondria. Interestingly, the mitochondrial localization of Vimar seems not dependent on Miro, because LOF Miro did not affect the mitochondrial fraction of Vimar. This indicates that Vimar may directly bind with mitochondria or through other scaffolding proteins. Second, Vimar and Miro could physically interact with each other, at least in vitro. Their interaction seems not affected by the GTPase activity of Miro, because the constitutively GDP- or GTP-bound Miro mutants did not affect their interactions. Third, vimar genetically interacted with Miro. This included the result demonstrating that the LOF vimar mutant reduced the effect of Miro on mitochondrial fission inhibition and the GOF vimar transgene had the opposite effect. Moreover, in the constitutive GFP-bound or GDP-bound Miro mutants, the effect of the GOF or LOF vimar was abolished. Therefore, Vimar requires the normal GDP/GTP binding activity of Miro to function. It is also known that Miro1 overexpression increase mitochondrial size partially by suppression of the Drp1 function. Consistently, increased mitochondrial fission in the LOF of Miro or vimar was abolished by loss of Drp1, suggesting the Miro/vimar complex depends on Drp1 to regulate mitochondrial morphology (Ding, 2016).

Familial PD caused by mutations in PINK1 or Parkin results in a series of mitochondrial dysfunctions, particularly the failure to eliminate damaged mitochondria through mitophagy. In these PINK1 or Parkin mutants, the key proteins involved in mitochondrial fusion and fission, such as Marf/Mitofusin and Miro, accumulate. In the PINK1 mutant flies, the flight muscle is damaged, resulting in wing posture defects. Similarly, it was observed that Miro overexpression in the flight muscle resulted in a strong wing posture defect. This result may explain the wing posture defect in the PINK1 mutant, in which the levels of the Miro protein are increased. This study demonstrated that the LOF of vimar could rescue the wing defect in the PINK1 mutant, consistent with the hypothesis that vimar functions through Miro (Ding, 2016).

When the intracellular calcium level is high, Miro switches from promoting mitochondrial fission inhibition to enhancing mitochondrial fission. The mechanism for this switch is unclear, although alterations of Drp1 function could be one possibility. Interestingly, Gem1, the yeast homolog of Miro GTPase, has been reported to function as a negative regulator for ER-mitochondria contacts, where Drp1 aggregates and cleaves mitochondria into smaller units. This may serve as the mechanism for Miro to regulate mitochondrial morphology via Drp1. In addition to affect mitochondrial fission, Miro also regulates mitochondrial transport in a calcium dependent manner. For mitochondrial transport, Miro forms protein complexes with Milton, a kinesin adaptor, and with motor proteins, such as kinesin and dynein. In high calcium conditions, Miro alters its binding patterns and results in reduced transport activity. Based on these reports, it is proposed that the Miro/Vimar complex acts together to affect mitochondrial morphology: at normal condition, Miro/Vimar inhibits fission via Drp1; at high calcium state, Ca2+ bound Miro switches its function to promote fission. Indeed, Vimar responds to the calcium change in the same way as Miro. In addition, the data demonstrated that knocking down RAP1GDS1 and Miro1 increased mitochondrial fission and could rescue calcium overload induced necrosis, similar to the loss of Vimar or Miro in Drosophila. These data support the hypothesis that RAP1GDS1 is the mammalian homolog of Vimar, supporting a previous prediction (Ding, 2016).

Mitochondrial fission plays important role in apoptosis by promoting mitochondrial outer-membrane permeabilization (MOMP) to release cytochrome c from the mitochondria. The use of the Drp1 inhibitor mdivi to block fission has been shown to be an effective treatment for stroke, and the function of mitochondrial fission on necrotic cell death has been well documented. The uncertainty lies in the lack of genetic evidence and downstream mechanism of mitochondrial fission in necrosis. The current data demonstrated that mitochondrial fragmentation occurs in necrotic neurons, and the LOF Drp1 and vimar mutants both suppressed neuronal necrosis (Ding, 2016).

Much evidence suggests that the mitochondrial fusion and fission defects are directly linked to many human diseases, and strategies that target the Miro/vimar complex may affect a broad spectrum of diseases. For instance, mutations in the fragile X mental retardation 1 (FMR1) gene, which result from expansion of trinucleotide repeat in the 5' untranslated region, often cause enhanced mitochondrial fission and mental retardation syndrome. Likewise, aberrant mitochondrial fusion was observed in a Drosophila Alzheimer's disease model induced by the ectopic expression of a human tau mutant (tauR406W). In this case, the tau mutant may promote excessive actin stabilization to decrease Drp1 recruitment to the mitochondria, which results in excessive mitochondrial fusion and neurodegeneration. Due to the dual function of the Miro/Vimar complex in high-Ca2+ induced necrosis and PINK1 mutant induced PD, a drug to target this complex may benefit both disease states. As a modulator, it may be safer to target Vimar/ RAP1GDS1 (Ding, 2016).

MicroRNA-277 modulates the neurodegeneration caused by Fragile X premutation rCGG repeats

Fragile X-associated tremor/ataxia syndrome (FXTAS), a late-onset neurodegenerative disorder, has been recognized in older male fragile X premutation carriers and is uncoupled from fragile X syndrome. Using a Drosophila model of FXTAS, it has been shown that transcribed premutation repeats alone are sufficient to cause neurodegeneration. miRNAs are sequence-specific regulators of post-transcriptional gene expression. To determine the role of miRNAs in rCGG repeat-mediated neurodegeneration, miRNA expression was profiled, and selective miRNAs were identified, including mir-277 stem loop, that are altered specifically in Drosophila brains expressing rCGG repeats. Their genetic interactions with rCGG repeats were tested, and it was found that miR-277 can modulate rCGG repeat-mediated neurodegeneration. Furthermore, Drep-2 and Vimar were identified as functional targets of miR-277 that could modulate rCGG repeat-mediated neurodegeneration. Finally, it was found that hnRNP A2/B1, an rCGG repeat-binding protein, can directly regulate the expression of miR-277. These results suggest that sequestration of specific rCGG repeat-binding proteins could lead to aberrant expression of selective miRNAs, which may modulate the pathogenesis of FXTAS by post-transcriptionally regulating the expression of specific mRNAs involved in FXTAS (Tan, 2012).

Fragile X syndrome (FXS), the most common form of inherited mental retardation, is caused by expansion of the rCGG trinucleotide repeat in the 5' untranslated region (5' UTR) of the fragile X mental retardation 1 (FMR1) gene, which leads to silencing of its transcript and the loss of the encoded fragile X mental retardation protein (FMRP). Most affected individuals have more than 200 rCGG repeats, referred to as full mutation alleles. Fragile X syndrome carriers have FMR1 alleles, called premutations, with an intermediate number of rCGG repeats between patients (>200 repeats) and normal individuals (<60 repeats). Recently, the discovery was made that male and, to a lesser degree, female premutation carriers are at greater risk of developing an age-dependent progressive intention tremor and ataxia syndrome, which is uncoupled from fragile X syndrome and known as fragile X-associated tremor/ataxia syndrome (FXTAS). This is combined with cognitive decline associated with the accumulation of ubiquitin-positive intranuclear inclusions broadly distributed throughout the brain in neurons, astrocytes, and in the spinal column (Tan, 2012).

At the molecular level, the premutation is different from either the normal or full mutation alleles. Based on the observation of significantly elevated levels of rCGG-containing FMR1 mRNA, along with either no detectable change in FMRP or slightly reduced FMRP levels in premutation carriers, an RNA-mediated gain-of-function toxicity model has been proposed for FXTAS. Several lines of evidence in mouse and Drosophila models further support the notion that transcription of the CGG repeats leads to this RNA-mediated neurodegenerative disease. The hypothesis is that specific RNA-binding proteins may be sequestered by overproduced rCGG repeats in FXTAS and become functionally limited, thereby contributing to the pathogenesis of this disorder. There are three RNA-binding proteins found to modulate rCGG-mediated neuronal toxicity: Pur α, hnRNP A2/B1, and CUGBP1, which bind rCGG repeats either directly (Pur α and hnRNP A2/B1) or indirectly (CUGBP1, through the interaction with hnRNP A2/B1) (Tan, 2012).

MicroRNAs (miRNAs) are small, noncoding RNAs that regulate gene expression at the post-transcriptional level by targeting mRNAs, leading to translational inhibition, cleavage of the target mRNAs or mRNA decapping/deadenylation. Mounting evidence suggests that miRNAs play essential functions in multiple biological pathways and diseases, from developmental timing, fate determination, apoptosis, and metabolism to immune response and tumorigenesis. Recent studies have shown that miRNAs are highly expressed in the central nervous system (CNS), and some miRNAs have been implicated in neurogenesis and brain development (Tan, 2012).

Interest in the functions of miRNAs in the CNS has recently expanded to encompass their roles in neurodegeneration. Investigators have begun to reveal the influence of miRNAs on both neuronal survival and the accumulation of toxic proteins that are associated with neurodegeneration, and are uncovering clues as to how these toxic proteins can influence miRNA expression. For example, miR-133b is found to regulate the maturation and function of midbrain dopaminergic neurons (DNs) within a negative feedback circuit that includes the homeodomain transcription factor Pitx3 in Parkinson's disease. In addition, reduced miR-29a/b-1-mediated suppression of BACE1 protein expression contributes to Aβ accumulation and Alzheimer's disease pathology. Moreover, the miRNA bantam is found to be a potent modulator of poly-Q- and tau-associated degeneration in Drosophila. Other specific miRNAs have also been linked to other neurodegenerative disorders, such as spinocerebellar ataxia type 1 (SCA1) and Huntington's disease (HD). Therefore, miRNA-mediated gene regulation could be a novel mechanism, adding a new dimension to the pathogenesis of neurodegenerative disorders (Tan, 2012).

This study shows that fragile X premutation rCGG repeats can alter the expression of specific miRNAs, including miR-277, in a FXTAS Drosophila model. miR-277 modulates rCGG-mediated neurodegeneration. Furthermore, Drep-2, which is associated with the chromatin condensation and DNA fragmentation events of apoptosis, and Vimar, a modulator of mitochondrial function, were identified two of the mRNA targets regulated by miR-277. Functionally, Drep-2 and Vimar could modulate the rCGG-mediated neurodegeneration, as well. Finally, hnRNP A2/B1, an rCGG repeat-binding protein, can directly regulate the expression of miR-277. These data suggest that hnRNP A2/B1 could be involved in the transcriptional regulation of selective miRNAs, and fragile X premutation rCGG repeats could alter the expression of specific miRNAs, potentially contributing to the molecular pathogenesis of FXTAS (Tan, 2012).

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder that afflicts fragile X syndrome premutation carriers, with earlier studies pointing to FXTAS as an RNA-mediated neurodegenerative disease. Several lines of evidence suggest that rCGG premutation repeats may sequester specific RNA-binding proteins, namely Pur α, hnRNP A2/B1, and CUGBP1, and reduce their ability to perform their normal cellular functions, thereby contributing significantly to the pathology of this disorder. The miRNA pathway has been implicated in the regulation of neuronal development and neurogenesis. A growing body of evidence has now revealed the role of the miRNA pathway in the molecular pathogenesis of neurodegenerative disorders. This study demonstrates that specific miRNAs can contribute to fragile X rCGG repeat-mediated neurodegeneration by post-transcriptionally regulating target mRNAs that are involved in FXTAS. miR-277 plays a significant role in modulating rCGG repeat-mediated neurodegeneration. Overexpression of miR-277 enhances rCGG repeat-induced neuronal toxicity, whereas blocking miR-277 activity could suppress rCGG repeat-mediated neurodegeneration. Furthermore, Drep-2 and Vimar were identified as the functional miR-277 targets that could modulate rCGG repeat-induced neurodegeneration. Finally, hnRNP A2/B1, an rCGG repeat-binding protein, can directly regulate the expression of miR-277. These biochemical and genetic studies demonstrate a novel miRNA-mediated mechanism involving miR-277, Drep-2, and Vimar in the regulation of neuronal survival in FXTAS (Tan, 2012).

Several lines of evidence from studies in mouse and Drosophila models strongly support FXTAS as an RNA-mediated neurodegenerative disorder caused by excessive rCGG repeats. The current working model is that specific RNA-binding proteins could be sequestered by overproduced rCGG repeats in FXTAS and become functionally limited, thereby contributing to the pathogenesis of this disorder. Three RNA-binding proteins are known to modulate rCGG-mediated neuronal toxicity: Pur α, hnRNP A2/B1, and CUGBP1, which bind rCGG repeats either directly (Pur α and hnRNP A2/B1) or indirectly (CUGBP1, through the interaction with hnRNP A2/B1); how the depletion of these RNA-binding proteins could alter RNA metabolism and contribute to FXTAS pathogenesis has thus become the focus in the quest to understand the molecular pathogenesis of this disorder. Nevertheless, the data presented in this study suggest that the depletion of hnRNP A2/B1 could also directly impact the transcriptional regulation of specific loci, such as miR-277. It is known that hnRNPs can interact with HP1 to bind to genomic DNA and modulate heterochromatin formation. The results indicate that hnRNP A2/B1 could participate in the transcriptional regulation of miR-277; however, it remains to be determined whether other loci could be directly regulated by hnRNP A2/B1, as well. Identifying those loci will be important to better understand how the depletion of rCGG repeat-binding proteins could lead to neuronal apoptosis (Tan, 2012).

In recent years, several classes of small regulatory RNAs have been identified in a range of tissues and in many species. In particular, miRNAs have been linked to a host of human diseases. Some evidence suggests the involvement of miRNAs in the emergence or progression of neurodegenerative diseases. For example, accumulation of nuclear aggregates that are toxic to neurons have been linked to many neurodegenerative diseases, and miRNAs are known to modulate the accumulation of the toxic proteins by regulating either their mRNAs or the mRNAs of proteins that affect their expression. Moreover, miRNAs might contribute to the pathogenesis of neurodegenerative disease downstream of the accumulation of toxic proteins by altering the expression of other proteins that promote or inhibit cell survival. The current genetic modifier screen revealed that miR-277 could modulate rCGG repeat-mediated neurodegeneration. By combining genetic screen and reporter assays, Drep-2 and Vimar were identified as the functional targets of miR-277 that could modulate rCGG-mediated neurodegeneration. The closest ortholog of miR-277 in human is miR-597 based on the seed sequence. It would be interesting to further examine the role of miR-597 in FXTAS using mammalian model systems (Tan, 2012).

Drep-2 is associated with the chromatin condensation and DNA fragmentation events of apoptosis. Drep-2 is one of four Drosophila DFF (DNA fragmentation factor)-related proteins. While Drep-1 is a Drosophila homolog of DFF45 that can inhibit CIDE-A mediated apoptosis. Drep-2 has been shown to interact with Drep-1 and to regulate its anti-apoptotic activity. Vimar is a Ral GTPase-binding protein that has been shown to regulate mitochondrial function via an increase in citrate synthase activity . In the presence of fragile X premutation rCGG repeats, overexpression of miR-277 will suppress the expression of both Drep-2 and Vimar, thereby altering anti-apoptotic activity as well as mitochondrial functions, which have been linked to neuronal cell death associated with neurodegenerative disorders in general. Interestingly, a significant reduction of Drep-2 mRNA was seen in the flies expressing rCGG repeats, while Vimar mRNA levels remained similar to control flies. This observed difference may be due to the fact that miRNA could be involved in different modes of action, including mRNA cleavage, translational inhibition and mRNA decapping/deadenylation its target mRNAs (Tan, 2012).

In summary, this study provides both biochemical and genetic evidence to support a role for miRNA and its selective mRNA targets in rCGG-mediated neurodegeneration. The results suggest that sequestration of specific rCGG repeat-binding proteins can lead to aberrant expression of selective miRNAs that could modulate the pathogenesis of FXTAS by post-transcriptionally regulating the expression of specific mRNAs involved in this disorder. Identification of these miRNAs and their targets could reveal potential new targets for therapeutic interventions to treat FXTAS, as well as other neurodegenerative disorders (Tan, 2012).

New Dystrophin/Dystroglycan interactors control neuron behavior in Drosophila eye

The Dystrophin Glycoprotein Complex (DGC) is a large multi-component complex that is well known for its function in muscle tissue. When the main components of the DGC, Dystrophin (Dys) and Dystroglycan (Dg) are affected cognitive impairment and mental retardation in addition to muscle degeneration can occur. Genetic screens have been performed using a Drosophila model for muscular dystrophy in order to find novel DGC interactors aiming to elucidate the signaling role(s) in which the complex is involved. Since the function of the DGC in the brain and nervous system has not been fully defined, this study has analyzed the DGC modifiers' function in the developing Drosophila brain and eye. Given that disruption of Dys and Dg leads to improper photoreceptor axon projections into the lamina and eye neuron elongation defects during development, the function of previously screened components and their genetic interaction with the DGC in this tissue were determined. This study first found that mutations in chif, CG34400, Nrk, Lis1, capt and Cam cause improper axon path-finding and loss of SP2353, Grh, Nrk, capt, CG34400, vimar, Lis1 and Cam cause shortened rhabdomere lengths. It was determined that Nrk, mbl, capt and Cam genetically interact with Dys and/or Dg in these processes. It is notable that most of the neuronal DGC interacting components encountered are involved in regulation of actin dynamics. These data indicate possible DGC involvement in the process of cytoskeletal remodeling in neurons. The identification of new components that interact with the DGC not only helps to dissect the mechanism of axon guidance and eye neuron differentiation but also provides a great opportunity for understanding the signaling mechanisms by which the cell surface receptor Dg communicates via Dys with the actin cytoskeleton (Marrone, 2011).

The roles that Dys and Dg play in disease have been apparent for some time since their disruption or misregulation has been closely linked to various MDs. Dg depletion results in congenital muscular dystrophy-like brain malformations associated with layering defects and aberrant neuron migration. These defects arise due to extracellular matrix protein affinity problems that influence neuronal communication and result in learning and memory defects. Similar to brain layer formation, the migration of R1-R6 growth cones into the lamina occurs in a similar manner where glia cells that migrate from progenitor regions into the lamina provide a termination cue to innervating axons. In Drosophila Dys and Dg are expressed in the CNS, PNS and visual system and both proteins are required for proper photoreceptor axon guidance and rhabdomere elongation. This work has identified novel components implicated in the process of eye-neuron development. Moreover, it was found that Nrk, Mbl, Cam and Capt genetically interact with Dys and/or Dg in visual system establishment (Marrone, 2011).

The proteins Mbl, Capt, Cam, Robo, Lis1 and Nrk have been shown previously to be associated with the nervous system, and this study has additionally found that mutations in chif, SP2353, CG34400 and vimar cause abnormal photoreceptor axon pathfinding and/or differentiation phenotypes. Lis1 has been shown to bind microtubules in the growth cone, and the human Lis1 homologue is important for neuronal migration and when mutated causes Lissencephaly, a severe neuronal migration defect characterized by a smooth cerebral surface, mental retardation and seizures. This study has found that Lis1RNAi/GMR-Gal4 mutants have abnormally formed lamina plexuses, shortened rhabdomeres, and retinal vacuoles. Chif has been shown to regulate gene expression during egg shell development and is related to a DNA replication protein in yeast. The human ortholog for SP2353 (AGRN) is involved in congenital MD development. Drosophila SP2353 is a novel agrin-like protein that contains Laminin G domains, which makes it a potential new extracellular binding partner for Dg. CG34400 encodes for a protein homologues to human DFNB31 (Deafness, autosomal recessive 31) that causes congenital hearing impairment in DFNB31 deficient people and mouse whirlin, that causes deafness in the whirler mouse. Hearing loss has been as well demonstrated in association with various forms of muscular dystrophy. Vimar has been shown to regulate mitochondrial function via an increase in citrate synthase activity (Marrone, 2011).

Mbl is a Drosophila homologue of the human gene MBNL1. Mutations of this gene cause myotonic dystrophy and are associated with the RNA toxicity of CUG expansion diseases protein. This study shows that Mbl deficiency results in similar phenotypes to Dys and Dg loss of function, and to specifically interact with Dys in axon projections which is in accord with the Dys specific interaction seen in muscle. Dys has multiple isoforms, and the variability of DMD patients to have mental impairment has been linked in part to small Dys isoform mutations, which leads to speculation that Dys is a target for Mbl mediated splicing (Marrone, 2011).

Interestingly, Mbl isoforms have been demonstrated to regulate splicing of α-actinin, which belongs to the spectrin gene superfamily that also includes dystrophins. α-actinin and Capt, the Drosophila homologue of Cyclase-associated protein (CAP) are actin-binding proteins in the growth cone. Capt was first identified in yeast and is highly conserved throughout eukaryotic evolution. The main known function of Capt is to act in the process of actin recycling by working in conjunction with Actin Depolymerization Factor (ADF a.k.a. Cofilin) to help displace Cofilin from G-actin during depolymerization. It has already been reported that ADF/Cofilin has a role in retinal elongation. The actin cytoskeleton is a major internal structure that defines the morphology of neurons, and Capt has already been shown to be required to maintain PNS neuronal dendrite homeostasis in Drosophila via kinesin-mediated transport. Additionally, Capt has been found to lead to excessive actin filament polymerization in the eye disc and to cause premature differentiation of photoreceptors. The rate of axon projection is much slower than the rate of microtubule polymerization during axonal growth, implying that depolymerization/polymerization of actin is important during pathfinding. This study has also shown that Capt interacts with Dys and is necessary for proper projection of photoreceptor axons in the developing brain, and when absent, eyes develop with abnormal rhabdomeres. Furthermore, captRNAi mutants exhibit overgrowth of photoreceptor axons, and it is believed that a possible explanation for this is improper turnover of actin (Marrone, 2011).

Importantly, proteins that can be regulated by Ca2+ to organize actin filament bundles and to promote filament turnover include α-actinin and (ADF)/Cofilin, respectively. Cam functions as an intracellular Ca2+ sensor, and when Ca2+-Cam was selectively disrupted in a subset of neurons in Drosophila embryos, stalls in axon extension and errors in growth cone guidance resulted. Actin turnover is highly regulated by Ca2+ levels, and many proteins are Ca2+-mediated to regulate motility and axon guidance. The results and those from prior studies suggest that Cam is a major functional player of Ca2+ regulation in growth cones. Since it was shown here that mutations in Cam and capt have similar phenotypes in photoreceptor axon pathfinding and rhabdomere development, it is postulated that actin dynamics is the link between these two proteins and the phenotypes described here. Due to the importance of Cam for actin dynamics, its interaction with both Dg and Dys suggests that the DGC coordinates the actin cytoskeleton in the developing eye (Marrone, 2011).

The last gene identified in this work is Nrk. Recently various kinases, channels and other enzymes have been shown to associate with the DGC, although only a few of these interactions have been confirmed in vivo. Since Nrk is a component found to interact with Dys in photoreceptor axon pathfinding, it is most likely that it functions as a receptor to sense guidance cues rather than as a molecule affecting actin cytoskeletal rearrangement. The data here hint that Dg and Nrk could be two receptors integral to transferring signals important for neuronal layering (Marrone, 2011).

It is concluded that dynamic rearrangement of the actin cytoskeleton is crucial for the central and peripheral nervous system establishment, which depends on proper neuron migration and differentiation. This process requires not only the cell autonomous regulation of neuron motility, but also the interaction between the migrating cell and its underlying substrate. This interaction is often dependent on the signaling transduced via the ECM. The DGC and other factors are believed to be mediators of actin dynamics in growing axons and during neuronal cell morphogenesis, and this study found components that interact with Dys and/or Dg in both of these activities (see The DGC coordinates actin cytoskeleton remodeling). Additionally, disruption in gene expression of these components results in the same phenotypes seen with Dys and Dg mutants in the developing and adult eye. The data lead to the conclusion that the DGC is involved in signaling to cause cytoskeletal rearrangement and actin turnover in growth cones. Since many cases of muscular dystrophies are associated with mental retardation, it is believed that it is important to understand the role of the DGC in axon migration because understanding of this process could aid in finding an adequate therapy for this aspect of the disease's physiology. Since the human brain continues to develop well after gestation, and evidence shows that nerves maintain plasticity throughout an individual's lifespan, therapies could be devised that reverse these defects after birth (Marrone, 2011).

bagpipe-dependent expression of vimar, a novel armadillo-repeats gene, in Drosophila visceral mesoderm

tinman and bagpipe, two homeobox-containing genes, play important roles during the specification of the midgut visceral musculature from the mesoderm during Drosophila embryogenesis. Expression of tinman in the dorsal mesoderm activates the expression of the bagpipe gene in segmental subsets of those cells, which then become determined to form the midgut visceral mesoderm. Understanding how the bagpipe gene affects this specification requires the isolation and characterization of its downstream target genes. Using an enhancer trap line that expresses its marker in the midgut visceral mesoderm, a novel gene (vimar) has been cloned and characterized that is expressed embryonically in the midgut and hindgut visceral mesoderm, as well as in the CNS and PNS. The expression of this gene in the midgut visceral mesoderm initiates shortly after bagpipe expression and depends on bagpipe function. Maternal and zygotic transcripts are produced from this gene by alternative polyadenylation, and encode the same 634-amino acid protein. The Vimar protein contains 15 tandem copies of the Armadillo repeat, a protein interaction domain, and is similar to mammalian Smg guanine dissociation stimulator (GDS) protein, which stimulates the activity of a number of different p21 small G-proteins, including Rap1, K-Ras, RhoA and Rac1. The Smg GDS protein is composed almost entirely of 11 tandem copies of an Armadillo repeat. No detectable abnormalities with respect to the visceral mesoderm and gut constrictions are observed in vimar mutants. These results, together with the observed postembryonic lethality of vimar mutations, indicate that vimar is one of the bagpipe target genes that are required for normal development and differentiation of the midgut visceral mesoderm (Lo, 1998).


REFERENCES

Search PubMed for articles about Drosophila Vimar

Ding, L., Lei, Y., Han, Y., Li, Y., Ji, X. and Liu, L. (2016). Vimar is a novel regulator of mitochondrial fission through Miro. PLoS Genet 12: e1006359. PubMed ID: 27716788

Lo, P. C. H. and Frasch, M. (1998). bagpipe-dependent expression of vimar, a novel armadillo-repeats gene, in Drosophila visceral mesoderm. Mech. Dev. 72(1-2): 65-75. PubMed ID: 9533953

Marrone, A. K., Kucherenko, M. M., Rishko, V. M. and Shcherbata, H. R. (2011). New Dystrophin/Dystroglycan interactors control neuron behavior in Drosophila eye. BMC Neurosci. 12: 93. PubMed Citation: 21943192

Tan, H., Poidevin, M., Li, H., Chen, D. and Jin, P. (2012). MicroRNA-277 modulates the neurodegeneration caused by Fragile X premutation rCGG repeats. PLoS Genet 8: e1002681. Pubmed: 22570635


Biological Overview

date revised: 3 August 2018

Home page: The Interactive Fly © 2011 Thomas Brody, Ph.D.