Expression of the 53 kD forked protein rescues F-actin bundle formation and mutant bristle phenotypes in Drosophila

Actin Bundles Dynamics and Architecture

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties-both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

Dusky works upstream of Four-jointed and Forked in wing morphogenesis in Tribolium castaneum

Dusky (dy) is required for cytoskeletal reorganization during wing morphogenesis in Drosophila melanogaster, but which genes participate together with dy for wing morphogenesis has remained unclear. In Tribolium castaneum, dy is highly expressed at the late embryonic stage. Tissue-specific expression analysis indicated high expression levels of dy in the epidermis, head and fat body of late-stage larvae. RNA interference (RNAi) targeting dy significantly decreased adult wing size and caused improper folding of the elytra. Meanwhile, dy knockdown reduced the transcription of four-jointed (fj) and forked (f). Our results show that fj RNAi reduces adult wing size and that silencing f results in abnormal wing folding in T. castaneum. Interestingly, knocking down fj and f simultaneously phenocopies dy RNAi, suggesting that dy probably acts upstream of fj and f to regulate wing morphogenesis in T. castaneum.

Tau promotes neurodegeneration via DRP1 mislocalization in vivo

Mitochondrial abnormalities have been documented in Alzheimer's disease and related neurodegenerative disorders, but the causal relationship between mitochondrial changes and neurodegeneration, and the specific mechanisms promoting mitochondrial dysfunction, are unclear. Here, we find that expression of human tau results in elongation of mitochondria in both Drosophila and mouse neurons. Elongation is accompanied by mitochondrial dysfunction and cell cycle-mediated cell death, which can be rescued in vivo by genetically restoring the proper balance of mitochondrial fission and fusion. We have previously demonstrated that stabilization of actin by tau is critical for neurotoxicity of the protein. Here, we demonstrate a conserved role for actin and myosin in regulating mitochondrial fission and show that excess actin stabilization inhibits association of the fission protein DRP1 with mitochondria, leading to mitochondrial elongation and subsequent neurotoxicity. Our results thus identify actin-mediated disruption of mitochondrial dynamics as a direct mechanism of tau toxicity in neurons in vivo.

Effectors of tridimensional cell morphogenesis and their evolution

One of the most challenging problems in biology resides in unraveling the molecular mechanisms, hardwired in the genome, that define and regulate the multiscale tridimensional organization of organs, tissues and individual cells. While works in cultured cells have revealed the importance of cytoskeletal networks for cell architecture, in vivo models are now required to explore how such a variety in cell shape is produced during development, in interaction with neighboring cells and tissues. The genetic analysis of epidermis development in Drosophila has provided an unbiased way to identify mechanisms remodeling the shape of epidermal cells, to form apical trichomes during terminal differentiation. Since hearing in vertebrates relies on apical cell extensions in sensory cells of the cochlea, called stereocilia, the mapping of human genes causing hereditary deafness has independently identified several factors required for this peculiar tridimensional organization. In this review, we summarized recent results obtained toward the identification of genes involved in these localized changes in cell shape and discuss their evolution throughout developmental processes and species.

The Drosophila javelin gene encodes a novel actin-associated protein required for actin assembly in the bristle

The Drosophila melanogaster bristle is a highly polarized cell that builds specialized cytoskeletal structures. Whereas actin is required for increasing bristle length, microtubules are essential for bristle axial growth. To identify new proteins involved in cytoskeleton organization during bristle development, we focused on identifying and characterizing the javelin (jv) locus. We found that in a jv mutant, the bristle tip is swollen and abnormal organization of bristle grooves is seen over the entire bristle. Using confocal and electron microscopy, we found that in jv mutant bristles, actin bundles do not form properly due to a loss of actin filaments within the bundle. We show that jv is an allele of the predicted CG32397 gene that encodes a protein with no homologs outside insects. Expression of the Jv protein fused to a green fluorescent protein (GFP) shows that the protein is colocalized with actin bundles in the bristle. Moreover, expression of Jv-GFP within the germ line led to the formation of ectopic actin bundles that surround the nucleus of nurse cells. Thus, we report that Jv is a novel actin-associated protein required for actin assembly during Drosophila bristle development.

A Drosophila homolog of the polyglutamine disease gene SCA2 is a dosage-sensitive regulator of actin filament formation

Spinocerebellar ataxia type 2 (SCA2) is a neurodegenerative disorder caused by the expansion of a CAG repeat encoding a polyglutamine tract in ataxin-2, the SCA2 gene product. The normal cellular function of ataxin-2 and the mechanism by which polyglutamine expansion of ataxin-2 causes neurodegeneration remain unknown. In this study we have used genetic and molecular approaches to investigate the function of a Drosophila homolog of the SCA2 gene (Datx2). Like human ataxin-2, Datx2 is found throughout development in a variety of tissue types and localizes to the cytoplasm. Mutations that reduce Datx2 activity or transgenic overexpression of Datx2 result in female sterility, aberrant sensory bristle morphology, loss or degeneration of tissues, and lethality. These phenotypes appear to result from actin filament formation defects occurring downstream of actin synthesis. Further studies demonstrate that Datx2 does not assemble with actin filaments, suggesting that the role of Datx2 in actin filament formation is indirect. These results indicate that Datx2 is a dosage-sensitive regulator of actin filament formation. Given that loss of cytoskeleton-dependent dendritic structure defines an early event in SCA2 pathogenesis, our findings suggest the possibility that dysregulation of actin cytoskeletal structure resulting from altered ataxin-2 activity is responsible for neurodegeneration in SCA2.