TDP-43 mediates degeneration in a novel Drosophila model of disease caused by mutations in VCP/p97.

Inclusion body myopathy associated with Paget's disease of bone and frontotemporal dementia (IBMPFD) is a dominantly inherited degenerative disorder caused by mutations in the valosin-containing protein (VCP7) gene. VCP (p97 in mouse, TER94 in Drosophila melanogaster, and CDC48 in Saccharomyces cerevisiae) is a highly conserved AAA(+) (ATPases associated with multiple cellular activities) ATPase that regulates a wide array of cellular processes. The mechanism of IBMPFD pathogenesis is unknown. To elucidate the pathogenic mechanism, we developed and characterized a Drosophila model of IBMPFD (mutant-VCP-related degeneration). Based on genetic screening of this model, we identified three RNA-binding proteins that dominantly suppressed degeneration; one of these was TBPH, the Drosophila homolog of TAR (trans-activating response region) DNA-binding protein 43 (TDP-43). Here we demonstrate that VCP and TDP-43 interact genetically and that disease-causing mutations in VCP lead to redistribution of TDP-43 to the cytoplasm in vitro and in vivo, replicating the major pathology observed in IBMPFD and other TDP-43 proteinopathies. We also demonstrate that TDP-43 redistribution from the nucleus to the cytoplasm is sufficient to induce cytotoxicity. Furthermore, we determined that a pathogenic mutation in TDP-43 promotes redistribution to the cytoplasm and enhances the genetic interaction with VCP. Together, our results show that degeneration associated with VCP mutations is mediated in part by toxic gain of function of TDP-43 in the cytoplasm. We suggest that these findings are likely relevant to the pathogenic mechanism of a broad array of TDP-43 proteinopathies, including frontotemporal lobar degeneration and amyotrophic lateral sclerosis.

Zebrafish brd2a and brd2b are paralogous members of the bromodomain-ET (BET) family of transcriptional coregulators that show structural and expression divergence.

BACKGROUND: Brd2 belongs to the bromodomain-extraterminal domain (BET) family of transcriptional co-regulators, and functions as a pivotal histone-directed recruitment scaffold in chromatin modification complexes affecting signal-dependent transcription. Brd2 facilitates expression of genes promoting proliferation and is implicated in apoptosis and in egg maturation and meiotic competence in mammals; it is also a susceptibility gene for juvenile myoclonic epilepsy (JME) in humans. The brd2 ortholog in Drosophila is a maternal effect, embryonic lethal gene that regulates several homeotic loci, including Ultrabithorax. Despite its importance, there are few systematic studies of Brd2 developmental expression in any organism. To help elucidate both conserved and novel gene functions, we cloned and characterized expression of brd2 cDNAs in zebrafish, a vertebrate system useful for genetic analysis of development and disease, and for study of the evolution of gene families and functional diversity in chordates. RESULTS: We identify cDNAs representing two paralogous brd2 loci in zebrafish, brd2a on chromosome 19 and brd2b on chromosome 16. By sequence similarity, syntenic and phylogenetic analyses, we present evidence for structural divergence of brd2 after gene duplication in fishes. brd2 paralogs show potential for modular domain combinations, and exhibit distinct RNA expression patterns throughout development. RNA in situ hybridizations in oocytes and embryos implicate brd2a and brd2b as maternal effect genes involved in egg polarity and egg to embryo transition, and as zygotic genes important for development of the vertebrate nervous system and for morphogenesis and differentiation of the digestive tract. Patterns of brd2 developmental expression in zebrafish are consistent with its proposed role in Homeobox gene regulation. CONCLUSION: Expression profiles of zebrafish brd2 paralogs support a role in vertebrate developmental patterning and morphogenesis. Our study uncovers both maternal and zygotic contributions of brd2, the analysis of which may provide insight into the earliest events in vertebrate development, and the etiology of some forms of epilepsy, for which zebrafish is an important model. Knockdowns of brd2 paralogs in zebrafish may now test proposed function and interaction with homeotic loci in vertebrates, and help reveal the extent to which functional novelty or partitioning has occurred after gene duplication.

The cadherins fat and dachsous regulate dorsal/ventral signaling in the Drosophila eye.

The Drosophila eye is a polarized epithelium in which ommatidia of opposing chirality fall on opposite sides of the eye's midline, the equator. The equator is established in at least two steps: photoreceptors R3 and R4 adopt their fates, and then ommatidia rotate clockwise or counterclockwise in accordance with the identity of these photoreceptors. We report the role of two cadherins, Fat (Ft) and Dachsous (Ds), in conveying the polarizing signal from the D/V midline in the Drosophila eye. In eyes lacking Ft, the midline is abolished. In ft and ds mutant clones, wild-type tissue rescues genetically mutant tissue at the clonal borders, giving rise to ectopic equators. These ectopic equators distort a mosaic analysis of these genes and led to the possible misinterpretation that ft and ds are required to specify the R3 and R4 cell fates, respectively. Our interpretation of these data supports a significantly different model in which ft and ds are not necessarily required for fate determination. Rather, they are involved in long-range signaling during the formation of the equator, as defined by the presence of an organized arrangement of dorsal and ventral chiral ommatidial forms.

Screen for genetic modifiers of stbm reveals that photoreceptor fate and rotation can be genetically uncoupled in the Drosophila eye.

BACKGROUND: Polarity of the Drosophila compound eye arises primarily as a consequence of two events that are tightly linked in time and space: fate specification of two photoreceptor cells, R3 and R4, and the subsequent directional movement of the unit eyes of the compound eye, or ommatidia. While it is thought that these fates dictate the direction of ommatidial rotation, the phenotype of mutants in the genes that set up this polarity led to the hypothesis that these two events could be uncoupled. METHODOLOGY/PRINCIPAL FINDINGS: To definitively demonstrate these events are genetically separable, we conducted a dominant modifier screen to determine if genes, when misexpressed, could selectively enhance subclasses of mutant ommatidia in which the direction of rotation does not follow the R3/R4 cell fates, yet not affect the number of ommatidia in which rotation follows the R3/R4 cell fates. We identified a subset of P element lines that exhibit this selective enhancement. We also identified lines that behave in the opposite manner: They enhance the number of ommatidia that rotate in the right direction, but do not alter the number of ommatidia that rotate incorrectly with respect to the R3/R4 fates. CONCLUSIONS/SIGNIFICANCE: These results indicate that fate and direction of rotation can be genetically separated, and that there are genes that act between R3/R4 fate specification and direction of ommatidial rotation. These data affirm what has been a long-standing assumption about the genetic control of ommatidial polarity.

Valosin-containing protein and the pathogenesis of frontotemporal dementia associated with inclusion body myopathy.

Frontotemporal dementia with inclusion body myopathy and Paget's disease of bone (IBMPFD) is a rare, autosomal dominant disorder caused by mutations in the gene valosin-containing protein (VCP). The CNS pathology is characterized by a novel pattern of ubiquitin pathology distinct from sporadic and familial frontotemporal lobar degeneration with ubiquitin-positive inclusions without VCP mutations. Yet, the ubiquitin-positive inclusions in IBMPFD also stain for TAR DNA binding protein, a feature that links this rare disease with the pathology associated with the majority of sporadic FTD as well as disease resulting from different genetic alterations. VCP, a member of the AAA-ATPase gene family, associates with a plethora of protein adaptors to perform a variety of cellular processes including Golgi assembly/disassembly and regulation of the ubiquitin-proteasome system. However, the mechanism whereby mutations in VCP lead to CNS, muscle, and bone disease is largely unknown. In this report, we review current literature on IBMPFD, focusing on the pathology of the disease and the biology of VCP with respect to IBMPFD.