Myosin VI is required for asymmetric segregation of cellular components during C. elegans spermatogenesis

The two C. elegans class VI myosins, SPE-15/HUM-3 and HUM-8, share similar motor properties, but have distinct developmental and tissue expression patterns

Myosins of class VI move toward the minus-end of actin filaments and play vital roles in cellular processes such as endocytosis, autophagy, protein secretion, and the regulation of actin filament dynamics. In contrast to the majority of metazoan organisms examined to date which contain a single MYO6 gene, C. elegans, possesses two MYO6 homologues, SPE-15/HUM-3 and HUM-8. Through a combination of in vitro biochemical/biophysical analysis and cellular assays, we confirmed that both SPE-15/HUM-3 and HUM-8 exhibit reverse directionality, velocities, and ATPase activity similar to human MYO6. Our characterization also revealed that unlike SPE-15/HUM-3, HUM-8 is expressed as two distinct splice isoforms, one with an additional unique 14 amino acid insert in the cargo-binding domain. While lipid and adaptor binding sites are conserved in SPE-15/HUM-3 and HUM-8, this conservation does not enable recruitment to endosomes in mammalian cells. Finally, we performed super-resolution confocal imaging on transgenic worms expressing either mNeonGreen SPE-15/HUM-3 or wrmScarlet HUM-8. Our results show a clear distinction in tissue distribution between SPE-15/HUM-3 and HUM-8. While SPE-15/HUM-3 exhibited specific expression in the gonads and neuronal tissue in the head, HUM-8 was exclusively localized in the intestinal epithelium. Overall, these findings align with the established tissue distributions and localizations of human MYO6.

Myosin VI stabilizes intercellular junctions in the testis through the LHR and MAPK signalling pathway during spermatogenesis in Eriocheir sinensis

The myosin motor protein myosin VI plays an essential role in mammalian spermatogenesis, however, the effects of myosin VI on male reproduction in Crustacea remain obscure. We identified the macromolecule es-Myosin VI in Eriocheir sinensis, and studied it by multiple methods. It co-localized with F-actin and was highly expressed in the testis. We interfered es-Myosin VI using dsRNA in vivo, an apparent decrease in spermatozoa count was detected. We also found that the MAPK signalling pathway was changed, subsequently causing disruption of intercellular junctions and damage to the functional hemolymph-testis barrier. We observed that luteinizing hormone receptor es-LHR was located within seminiferous tubules, which was different from the expression in mammals. Es-LHR could bind with es-Myosin VI in testis of E. sinensis, its localization was significantly altered when es-Myosin VI was deleted. Moreover, we obtained consistent results for the MAPK signalling pathway and spermatogenesis defects between the es-LHR and es-Myosin VI knockdown groups. In summary, our research demonstrated that knockdown of es-Myosin VI disturbed the intercellular junction and HTB function via the MAPK signalling pathway by changing the localization of es-LHR in the testis of E. sinensis, which was the potential reason for its negative impact on spermatogenesis.

Unconventional Myosins from Caenorhabditis elegans as a Probe to Study Human Orthologues

Unconventional myosins are a superfamily of actin-based motor proteins that perform a number of roles in fundamental cellular processes, including (but not limited to) intracellular trafficking, cell motility, endocytosis, exocytosis and cytokinesis. 40 myosins genes have been identified in humans, which belong to different 12 classes based on their domain structure and organisation. These genes are widely expressed in different tissues, and mutations leading to loss of function are associated with a wide variety of pathologies while over-expression often results in cancer. Caenorhabditis elegans (C. elegans) is a small, free-living, non-parasitic nematode. ~38% of the genome of C. elegans has predicted orthologues in the human genome, making it a valuable tool to study the function of human counterparts and human diseases. To date, 8 unconventional myosin genes have been identified in the nematode, from 6 different classes with high homology to human paralogues. The hum-1 and hum-5 (heavy chain of an unconventional myosin) genes encode myosin of class I, hum-2 of class V, hum-3 and hum-8 of class VI, hum-6 of class VII and hum-7 of class IX. The hum-4 gene encodes a high molecular mass myosin (307 kDa) that is one of the most highly divergent myosins and is a member of class XII. Mutations in many of the human orthologues are lethal, indicating their essential properties. However, a functional characterisation for many of these genes in C. elegans has not yet been performed. This article reviews the current knowledge of unconventional myosin genes in C. elegans and explores the potential use of the nematode to study the function and regulation of myosin motors to provide valuable insights into their role in diseases.

Lagging X chromatids specify the orientation of asymmetric organelle partitioning in XX spermatocytes of Auanema rhodensis

The unequal partitioning of molecules and organelles during cell division results in daughter cells with different fates. An extreme example is female meiosis, in which consecutive asymmetric cell divisions give rise to 1 large oocyte and 2 small polar bodies with DNA and minimal cytoplasm. Here, we test the hypothesis that during an asymmetric cell division during spermatogenesis of the nematode Auanema rhodensis, the late segregating X chromatids orient the asymmetric partitioning of cytoplasmic components. In previous studies, the secondary spermatocytes of wild-type XO males were found to divide asymmetrically to generate functional spermatids that inherit components necessary for sperm viability and DNA-containing residual bodies that inherit components to be discarded. Here we extend that analysis to 2 novel contexts. First, the isolation and analysis of a strain of mutant XX pseudomales revealed that such animals have highly variable patterns of X-chromatid segregation. The pattern of late segregating X chromatids nevertheless predicted the orientation of organelle partitioning. Second, while wild-type XX hermaphrodites were known to produce both 1X and 2X sperm, here, we show that spermatocytes within specific spermatogonial clusters exhibit 2 different patterns of X-chromatid segregation that correlate with distinct patterns of organelle partitioning. Together this analysis suggests that A. rhodensis has coopted lagging X chromosomes during anaphase II as a mechanism for determining the orientation of organelle partitioning.

Sperm-specific glycogen synthase kinase 3 is required for sperm motility and the post-fertilization signal for female meiosis II in Caenorhabditis elegans

In most sexually reproducing animals, sperm entry provides the signal to initiate the final stages of female meiosis. In Caenorhabditis elegans, this signal is required for completion of female anaphase I and entry into meiosis II (MII). memi-1/2/3 (meiosis-to-mitosis) encode maternal components that facilitate this process; memi-1/2/3(RNAi) results in a skipped-MII phenotype. Previously, we used a gain-of-function mutation, memi-1(sb41), to identify genetic suppressors that represent candidates for the sperm-delivered signal. Herein, we characterize two suppressors of memi-1(sb41): gskl-1 and gskl-2. Both genes encode functionally redundant sperm glycogen synthase kinase, type 3 (GSK3) protein kinases. Loss of both genes causes defects in male spermatogenesis, sperm pseudopod treadmilling and paternal-effect embryonic lethality. The two kinases locate within the pseudopod of activated sperm, suggesting that they directly or indirectly regulate the sperm cytoskeletal polymer major sperm protein (MSP). The GSK3 genes genetically interact with another memi-1(sb41) suppressor, gsp-4, which encodes a sperm-specific PP1 phosphatase, previously proposed to regulate MSP dynamics. Moreover, gskl-2 gsp-4; gskl-1 triple mutants often skip female MII, similar to memi-1/2/3(RNAi). The GSK3 kinases and PP1 phosphatases perform similar sperm-related functions and work together for post-fertilization functions in the oocyte that involve MEMI.

Mitochondria-actin cytoskeleton crosstalk in cell migration

Mitochondria perform diverse functions in the cell and their roles during processes such as cell survival, differentiation, and migration are increasingly being appreciated. Mitochondrial and actin cytoskeletal networks not only interact with each other, but this multifaceted interaction shapes their functional dynamics. The interrelation between mitochondria and the actin cytoskeleton extends far beyond the requirement of mitochondrial ATP generation to power actin dynamics, and impinges upon several major aspects of cellular physiology. Being situated at the hub of cell signaling pathways, mitochondrial function can alter the activity of actin regulatory proteins and therefore modulate the processes downstream of actin dynamics such as cellular migration. As we will discuss, this regulation is highly nuanced and operates at multiple levels allowing mitochondria to occupy a strategic position in the regulation of migration, as well as pathological events that rely on aberrant cell motility such as cancer metastasis. In this review, we summarize the crosstalk that exists between mitochondria and actin regulatory proteins, and further emphasize on how this interaction holds importance in cell migration in normal as well as dysregulated scenarios as in cancer.

Membrane-associated cytoplasmic granules carrying the Argonaute protein WAGO-3 enable paternal epigenetic inheritance in Caenorhabditis elegans

Epigenetic inheritance describes the transmission of gene regulatory information across generations without altering DNA sequences, enabling offspring to adapt to environmental conditions. Small RNAs have been implicated in this, through both the oocyte and the sperm. However, as much of the cellular content is extruded during spermatogenesis, it is unclear whether cytoplasmic small RNAs can contribute to epigenetic inheritance through sperm. Here we identify a sperm-specific germ granule, termed the paternal epigenetic inheritance (PEI) granule, that mediates paternal epigenetic inheritance by retaining the cytoplasmic Argonaute protein WAGO-3 during spermatogenesis in Caenorhabditis elegans. We identify the PEI granule proteins PEI-1 and PEI-2, which have distinct functions in this process: granule formation, Argonaute selectivity and subcellular localization. We show that PEI granule segregation is coupled to the transport of sperm-specific secretory vesicles through PEI-2 in an S-palmitoylation-dependent manner. PEI-like proteins are found in humans, suggesting that the identified mechanism may be conserved.

Diverse functions of myosin VI in spermiogenesis

Spermiogenesis is the final stage of spermatogenesis, a differentiation process during which unpolarized spermatids undergo excessive remodeling that results in the formation of sperm. The actin cytoskeleton and associated actin-binding proteins play crucial roles during this process regulating organelle or vesicle delivery/segregation and forming unique testicular structures involved in spermatid remodeling. In addition, several myosin motor proteins including MYO6 generate force and movement during sperm differentiation. MYO6 is highly unusual as it moves towards the minus end of actin filaments in the opposite direction to other myosin motors. This specialized feature of MYO6 may explain the many proposed functions of this myosin in a wide array of cellular processes in animal cells, including endocytosis, secretion, stabilization of the Golgi complex, and regulation of actin dynamics. These diverse roles of MYO6 are mediated by a range of specialized cargo-adaptor proteins that link this myosin to distinct cellular compartments and processes. During sperm development in a number of different organisms, MYO6 carries out pivotal functions. In Drosophila, the MYO6 ortholog regulates actin reorganization during spermatid individualization and male KO flies are sterile. In C. elegans, the MYO6 ortholog mediates asymmetric segregation of cytosolic material and spermatid budding through cytokinesis, whereas in mice, this myosin regulates assembly of highly specialized actin-rich structures and formation of membrane compartments to allow the formation of fully differentiated sperm. In this review, we will present an overview and compare the diverse function of MYO6 in the specialized adaptations of spermiogenesis in flies, worms, and mammals.

Depletion of gipc-1 and gipc-2 causes infertility in Caenorhabditis elegans by reducing sperm motility

The G-protein signaling pathway plays a key role in multiple cellular processes and is well conserved in eukaryotes. Although GIPC (G-protein α subunit interacting protein (GAIP)-interacting protein, C terminus) has been studied in several model organisms, little is known about its role in Caenorhabditis elegans. In the present study, we investigated the roles of gipc-1 and gipc-2 in C. elegans. We observed that they were exclusively expressed in sperm throughout the development and that gipc-1; gipc-2 double mutants were infertile. Further examination of sperm development in gipc-1; gipc-2 mutants revealed defective sperm activation and abnormal pseudopod extension that resulted in reduced sperm motility. Moreover, major sperm protein (MSP) was abnormally segregated between spermatids and residual bodies in gipc-1; gipc-2 mutants. Our findings indicate that gipc-1 and gipc-2 are required for the proper pseudopod extension of sperm during the terminal differentiation of spermatids. During this process, the segregation of MSP into spermatids is important for ensuring normal sperm motility during fertilization.

Knockout of serine-rich single-pass membrane protein 1 (Ssmem1) causes globozoospermia and sterility in male mice†

Globozoospermia (sperm with an abnormally round head shape) and asthenozoospermia (defective sperm motility) are known causes of male infertility in human patients. Despite many studies, the molecular details of the globozoospermia etiology are still poorly understood. Serine-rich single-pass membrane protein 1 (Ssmem1) is a conserved testis-specific gene in mammals. In this study, we generated Ssmem1 knockout (KO) mice using the CRISPR/Cas9 system, demonstrated that Ssmem1 is essential for male fertility in mice, and found that SSMEM1 protein is expressed during spermatogenesis but not in mature sperm. The sterility of the Ssmem1 KO (null) mice is associated with globozoospermia and loss of sperm motility. To decipher the mechanism causing the phenotype, we analyzed testes with transmission electron microscopy and discovered that Ssmem1-disrupted spermatids have abnormal localization of Golgi at steps eight and nine of spermatid development. Immunofluorescence analysis with anti-Golgin-97 to label the trans-Golgi network, also showed delayed movement of the Golgi to the spermatid posterior region, which causes failure of sperm head shaping, disorganization of the cell organelles, and entrapped tails in the cytoplasmic droplet. In summary, SSMEM1 is crucial for intracellular Golgi movement to ensure proper spatiotemporal formation of the sperm head that is required for fertilization. These studies and the pathway in which SSMEM1 functions have implications for human male infertility and identifying potential targets for nonhormonal contraception.

Distinct roles of two myosins in C. elegans spermatid differentiation

During spermatogenesis, interconnected haploid spermatids segregate undesired cellular contents into residual bodies (RBs) before detaching from RBs. It is unclear how this differentiation process is controlled to produce individual spermatids or motile spermatozoa. Here, we developed a live imaging system to visualize and investigate this process in C. elegans. We found that non-muscle myosin 2 (NMY-2)/myosin II drives incomplete cytokinesis to generate connected haploid spermatids, which are then polarized to segregate undesired cellular contents into RBs under the control of myosin II and myosin VI. NMY-2/myosin II extends from the pseudo-cleavage furrow formed between two haploid spermatids to the spermatid poles, thus promoting RB expansion. In the meantime, defective spermatogenesis 15 (SPE-15)/myosin VI migrates from spermatids towards the expanding RB to promote spermatid budding. Loss of myosin II or myosin VI causes distinct cytoplasm segregation defects, while loss of both myosins completely blocks RB formation. We found that the final separation of spermatids from RBs is achieved through myosin VI-mediated cytokinesis, while myosin II is dispensable at this step. SPE-15/myosin VI and F-actin form a detergent-resistant actomyosin VI ring that undergoes continuous contraction to promote membrane constriction between spermatid and RB. We further identified that RGS-GAIP-interacting protein C terminus (GIPC)-1 and GIPC-2 cooperate with myosin VI to regulate contractile ring formation and spermatid release. Our study reveals distinct roles of myosin II and myosin VI in spermatid differentiation and uncovers a novel myosin VI-mediated cytokinesis process that controls spermatid release.

Myosins as fundamental components during tumorigenesis: diverse and indispensable

Myosin is a kind of actin-based motor protein. As the crucial functions of myosin during tumorigenesis have become increasingly apparent, the profile of myosin in the field of cancer research has also been growing. Eighteen distinct classes of myosins have been discovered in the past twenty years and constitute a diverse superfamily. Various myosins share similar structures. They all convert energy from ATP hydrolysis to exert mechanical stress upon interactions with microfilaments. Ongoing research is increasingly suggesting that at least seven kinds of myosins participate in the formation and development of cancer. Myosins play essential roles in cytokinesis failure, chromosomal and centrosomal amplification, multipolar spindle formation and DNA microsatellite instability. These are all prerequisites of tumor formation. Subsequently, myosins activate various processes of tumor invasion and metastasis development including cell migration, adhesion, protrusion formation, loss of cell polarity and suppression of apoptosis. In this review, we summarize the current understanding of the roles of myosins during tumorigenesis and discuss the factors and mechanisms which may regulate myosins in tumor progression. Furthermore, we put forward a completely new concept of “chromomyosin” to demonstrate the pivotal functions of myosins during karyokinesis and how this acts to optimize the functions of the members of the myosin superfamily.

Cytoskeletal variations in an asymmetric cell division support diversity in nematode sperm size and sex ratios

Asymmetric partitioning is an essential component of many developmental processes. As spermatogenesis concludes, sperm are streamlined by discarding unnecessary cellular components into cellular wastebags called residual bodies (RBs). During nematode spermatogenesis, this asymmetric partitioning event occurs shortly after anaphase II, and both microtubules and actin partition into a central RB. Here, we use fluorescence and transmission electron microscopy to elucidate and compare the intermediate steps of RB formation in Caenorhabditis elegans, Rhabditis sp. SB347 (recently named Auanema rhodensis) and related nematodes. In all cases, intact microtubules reorganize and move from centrosomal to non-centrosomal sites at the RB-sperm boundary whereas actin reorganizes through cortical ring expansion and clearance from the poles. However, in species with tiny spermatocytes, these cytoskeletal changes are restricted to one pole. Consequently, partitioning yields one functional sperm with the X-bearing chromosome complement and an RB with the other chromosome set. Unipolar partitioning may not require an unpaired X, as it also occurs in XX spermatocytes. Instead, constraints related to spermatocyte downsizing may have contributed to the evolution of a sperm cell equivalent to female polar bodies.

Hydrogen peroxide produced by superoxide dismutase SOD-2 activates sperm in Caenorhabditis elegans

Superoxide dismutase (SOD) is a ubiquitous antioxidant enzyme that catalytically converts the superoxide radical to hydrogen peroxide (H₂O₂). In mammals, high SOD activity is detectable in sperm and seminal plasma, and loss of SOD activity has been correlated with male infertility; however, the underlying mechanisms of sperm infertility remain to be clarified. Here we report that the deletion of two major SOD genes in Caenorhabditis elegans, sod-1 and sod-2, causes sperm activation defects, leading to a significant reduction in brood size. By examining the reactivity to the sperm activation signals Pronase and triethanolamine, we found that sod-1;sod-2 double mutant sperm cells display defects in pseudopod extension. Neither the content nor oxidative modification of major sperm protein, an essential cytoskeletal component for crawling movement, were significantly affected in sod-1;sod-2 mutant sperm. Surprisingly, H₂O₂, the dismutation product of SOD, could activate sod-1;sod-2 mutant sperm treated with Pronase. Moreover, the H₂O₂ scavenger ebselen completely inhibited pseudopod extension in wild-type sperm treated with Pronase, and H₂O₂ could directly induce pseudopod extension in wild-type sperm. Analysis of Pronase-triggered sperm activation in sod-1 and sod-2 single mutants revealed that sod-2 is required for pseudopod extension. These results suggest that SOD-2 plays an important role in the sperm activation of C. elegans by producing H₂O₂ as an activator of pseudopod extension.

The Adverse Effects of Triptolide on the Reproductive System of Caenorhabditis elegans: Oogenesis Impairment and Decreased Oocyte Quality

Previous studies have revealed that Triptolide damages female reproductive capacity, but the mechanism is unclear. In this study, we used Caenorhabditis elegans to investigate the effects of Triptolide on the germline and explore its possible mechanisms. Our data show that exposure for 4 h to 50 and 100 mg/L Triptolide reduced C. elegans fertility, led to depletion and inactivation of spermatids with the changes in the expression levels of related genes, and increased the number of unfertilized oocytes through damaging chromosomes and DNA damage repair mechanisms. After 24 and 48 h of the 4 h exposure to 50 and 100 mg/L Triptolide, we observed shrink in distal tip cells, an increase in the number of apoptotic cells, a decrease in the number of mitotic germ cells and oocytes in diakinesis stage, and chromatin aggregates in -1 oocytes. Moreover, expression patterns of the genes associated with mitotic germ cell proliferation, apoptosis, and oocyte quality were altered after Triptolide exposure. Therefore, Triptolide may damage fertility of nematodes by hampering the development of oocytes at different developmental stages. Alterations in the expression patterns of genes involved in oocyte development may explain the corresponding changes in oocyte development in nematodes exposed to Triptolide.

Molecular evolution of the multiple calmodulin-like cal genes in C. elegans and in nematodes

Calmodulin (CaM) is a major EF hand containing intracellular calcium receptor in animals and plants; however, eukaryotes also express a number of related CaM-like proteins. We have previously characterized an embryonic phenotype of the single Caenorhabditis elegans CaM gene cmd-1, reported no visible RNAi phenotype for the four related cal-1 to cal-4 genes and started tissue-specific expression analyses of these proteins. In the present study, we analyzed evolutionary aspects of the previously reported CAL-1 to CAL-4 proteins, along with the four new CAL-5 to CAL-8 sequences retrieved from the worm database. Phylogenetic analyses suggest that all C. elegans CAL proteins arose from a CaM ancestor through repeated gene duplications, fusions and sequence divergence. The same holds, also, for the variable N-terminal extensions of the CAL-1 to CAL-4 proteins, which have evolved from the CaM-like core domain. We found 97 CAL homologs in different nematode clades and also detected two CAL-7-related sequences outside the nematodes. Moreover, the C. elegans-specific cal-6 gene, representing the most CaM-related sequence found in nematodes so far, harbours many deletions, insertions and sequence substitutions and is predicted, therefore, to be non-functional. These analyses provide an insight into a complex and dynamic origin of the multiple CAL genes in C. elegans and in nematodes and represent also a basis for further functional studies of these CaM-related sequences in nematodes.

Sperm Affects Head Sensory Neuron in Temperature Tolerance of Caenorhabditis elegans

Tolerance to environmental temperature change is essential for the survival and proliferation of animals. The process is controlled by various body tissues, but the orchestration of activity within the tissue network has not been elucidated in detail. Here, we show that sperm affects the activity of temperature-sensing neurons (ASJ) that control cold tolerance in Caenorhabditis elegans. Genetic impairment of sperm caused abnormal cold tolerance, which was unexpectedly restored by impairment of temperature signaling in ASJ neurons. Calcium imaging revealed that ASJ neuronal activity in response to temperature was decreased in sperm mutant gsp-4 with impaired protein phosphatase 1 and rescued by expressing gsp-4 in sperm. Genetic analysis revealed a feedback network in which ASJ neuronal activity regulates the intestine through insulin and a steroid hormone, which then affects sperm and, in turn, controls ASJ neuronal activity. Thus, we propose that feedback between sperm and a sensory neuron mediating temperature tolerance.

Positively charged residues within the MYO19 MyMOMA domain are essential for proper localization of MYO19 to the mitochondrial outer membrane

Myosins are well characterized molecular motors essential for intracellular transport. MYO19 copurifies with mitochondria, and can be released from mitochondrial membranes by high pH buffer, suggesting that positively-charged residues participate in interactions between MYO19 and mitochondria. The MYO19-specific mitochondria outer membrane association (MyMOMA) domain contains approximately 150 amino acids with a pI approximately 9 and is sufficient for localization to the mitochondrial outer membrane. The minimal sequence and specific residues involved in mitochondrial binding have not been identified. To address this, we generated GFP-MyMOMA truncations, establishing the boundaries for truncations based on sequence homology. We identified an 83-amino acid minimal binding region enriched with basic residues (pI ∼ 10.5). We sequentially replaced basic residues in this region with alanine, identifying residues R882 and K883 as essential for mitochondrial localization. Constructs containing the RK882-883AA mutation primarily localized with the endoplasmic reticulum (ER). To determine if ER-associated mutant MyMOMA domain and mitochondria-associated wild type MyMOMA display differences in kinetics of membrane interaction, we paired FRAP analysis with permeabilization activated reduction in fluorescence (PARF) analysis. Mitochondria-bound and ER-bound MYO19 constructs displayed slow dissociation from their target membrane when assayed by PARF; both constructs displayed exchange within their respective organelle networks. However, ER-bound mutant MYO19 displayed more rapid exchange within the ER network than did mitochondria-bound MYO19. Taken together these data indicate that the MyMOMA domain contains strong membrane-binding activity, and membrane targeting is mediated by a specific, basic region of the MYO19 tail with slow dissociation kinetics appropriate for its role(s) in mitochondrial network dynamics. © 2016 Wiley Periodicals, Inc.

Myosin superfamily: The multi-functional and irreplaceable factors in spermatogenesis and testicular tumors

Spermatogenesis is a fundamental process in sexual development and reproduction, in which the diploid spermatogonia transform into haploid mature spermatozoa. This process is under the regulation of multiple factors and pathway. Myosin has been implicated in various aspects during spermatogenesis. Myosins constitute a diverse superfamily of actin-based molecular motors that translocate along microfilament in an ATP-dependent manner, and six kinds of myosins have been proved that function during spermatogenesis. In mitosis and meiosis, myosins play an important role in spindle assembly and positioning, karyokinesis and cytokinesis. During spermiogenesis, myosins participate in acrosomal formation, nuclear morphogenesis, mitochondrial translocation and spermatid individualization. In this review, we summarize current understanding of the functions of myosin in spermatogenesis and some reproductive system diseases such as testicular tumors and prostate cancer, and discuss the roles of possible upstream molecules which regulate myosin in these processes.

Effects of Microcystin-LR Exposure on Spermiogenesis in Nematode Caenorhabditis elegans

Little is known about the effect on spermiogenesis induced by microcystin-leucine arginine (MC-LR), even though such data are very important to better elucidate reproductive health. In the current work, with the aid of nematode Caenorhabditis elegans (C. elegans) as an animal model, we investigated the defects on spermiogenesis induced by MC-LR. Our results showed that MC-LR exposure induced sperm morphology abnormality and caused severe defects of sperm activation, trans-activation, sperm behavior and competition. Additionally, the expression levels of spe-15 were significantly decreased in C. elegans exposed to MC-LR lower than 16.0 μg/L, while the expression levels of spe-10 and fer-1 could be significantly lowered in C. elegans even exposed to 1.0 μg/L of MC-LR. Therefore, the present study reveals that MC-LR can induce adverse effects on spermiogenesis, and those defects of sperm functions may be induced by the decreases of spe-10, spe-15 and fer-1 gene expressions in C. elegans.

Mitochondrial dynamics and inheritance during cell division, development and disease

During cell division, it is critical to properly partition functional sets of organelles to each daughter cell. The partitioning of mitochondria shares some common features with that of other organelles, particularly in the use of interactions with cytoskeletal elements to facilitate delivery to the daughter cells. However, mitochondria have unique features - including their own genome and a maternal mode of germline transmission - that place additional demands on this process. Consequently, mechanisms have evolved to regulate mitochondrial segregation during cell division, oogenesis, fertilization and tissue development, as well as to ensure the integrity of these organelles and their DNA, including fusion-fission dynamics, organelle transport, mitophagy and genetic selection of functional genomes. Defects in these processes can lead to cell and tissue pathologies.

The regulation of spermatogenesis and sperm function in nematodes

In the nematode C. elegans, both males and self-fertile hermaphrodites produce sperm. As a result, researchers have been able to use a broad range of genetic and genomic techniques to dissect all aspects of sperm development and function. Their results show that the early stages of spermatogenesis are controlled by transcriptional and translational processes, but later stages are dominated by protein kinases and phosphatases. Once spermatids are produced, they participate in many interactions with other cells - signals from the somatic gonad determine when sperm activate and begin to crawl, signals from the female reproductive tissues guide the sperm, and signals from sperm stimulate oocytes to mature and be ovulated. The sperm also show strong competitive interactions with other sperm and oocytes. Some of the molecules that mediate these processes have conserved functions in animal sperm, others are conserved proteins that have been adapted for new roles in nematode sperm, and some are novel proteins that provide insights into evolutionary change. The advent of new techniques should keep this system on the cutting edge of research in cellular and reproductive biology.

Chlorpyrifos exposure reduces reproductive capacity owing to a damaging effect on gametogenesis in the nematode Caenorhabditis elegans

Previous studies have revealed that chlorpyrifos exposure adversely affects the reproductive capacity of male rodents. The present study investigated the reproductive toxicity of chlorpyrifos exposure and possible related mechanisms using the nematode Caenorhabditis elegans. L4 nematode larvae were exposed to chlorpyrifos at concentrations of 0.003, 0.03, 0.3 and 3.0 mg l(-1) for different durations. In addition to decreased brood size, reduced spermatid size, increased percentage of abnormal spermatids, suppressed spermatid activation and motility of sperm, damaged oocyte morphology, increased numbers of apoptotic cells and unfertilized oocytes were observed in nematodes exposed to various concentrations of chlorpyrifos. Moreover, expression patterns of the genes spe-10, spe-15, fer-1, prg-1, glp-1, mlh-1, cyb-3, ced-3, ced-4 and ced-9 (which are associated with spermatid size, spermatid activation and morphology, oocyte morphology, oocyte function, and apoptosis) were altered after chlorpyrifos exposure. Therefore, chlorpyrifos exposure may adversely affect fertility in nematodes by influencing both spermatogenesis and oogenesis. Alterations in the expression patterns of genes involved in gametogenesis may explain the corresponding changes in gametogenesis in nematodes exposed to chlorpyrifos. Hence, the model organism Caenorhabditis elegans is recommended for assessment of reproductive toxicity relating to gametogenesis.

Sperm development and motility are regulated by PP1 phosphatases in Caenorhabditis elegans

Sperm from different species have evolved distinctive motility structures, including tubulin-based flagella in mammals and major sperm protein (MSP)-based pseudopods in nematodes. Despite such divergence, we show that sperm-specific PP1 phosphatases, which are required for male fertility in mouse, function in multiple processes in the development and motility of Caenorhabditis elegans amoeboid sperm. We used live-imaging analysis to show the PP1 phosphatases GSP-3 and GSP-4 (GSP-3/4) are required to partition chromosomes during sperm meiosis. Postmeiosis, tracking fluorescently labeled sperm revealed that both male and hermaphrodite sperm lacking GSP-3/4 are immotile. Genetic and in vitro activation assays show lack of GSP-3/4 causes defects in pseudopod development and the rate of pseudopodial treadmilling. Further, GSP-3/4 are required for the localization dynamics of MSP. GSP-3/4 shift localization in concert with MSP from fibrous bodies that sequester MSP at the base of the pseudopod, where directed MSP disassembly facilitates pseudopod contraction. Consistent with a role for GSP-3/4 as a spatial regulator of MSP disassembly, MSP is mislocalized in sperm lacking GSP-3/4. Although a requirement for PP1 phosphatases in nematode and mammalian sperm suggests evolutionary conservation, we show PP1s have independently evolved sperm-specific paralogs in separate lineages. Thus PP1 phosphatases are highly adaptable and employed across a broad range of sexually reproducing species to regulate male fertility.

Mitochondria: the next (neurode)generation

Adult-onset neurodegenerative disorders are disabling and often fatal diseases of the nervous system whose underlying mechanisms of cell death remain unknown. Defects in mitochondrial respiration had previously been proposed to contribute to the occurrence of many, if not all, of the most common neurodegenerative disorders. However, the discovery of genes mutated in hereditary forms of these enigmatic diseases has additionally suggested defects in mitochondrial dynamics. Such disturbances can lead to changes in mitochondrial trafficking, in interorganellar communication, and in mitochondrial quality control. These new mechanisms by which mitochondria may also be linked to neurodegeneration will likely have far-reaching implications for our understanding of the pathophysiology and treatment of adult-onset neurodegenerative disorders.

Asymmetric spermatocyte division as a mechanism for controlling sex ratios

Although Mendel's first law predicts that crosses between XY (or XO) males and XX females should yield equal numbers of males and females, individuals in a wide variety of metazoans transmit their sex chromosomes unequally and produce broods with highly skewed sex ratios. Here, we report two modifications to the cellular programme of spermatogenesis, which, in combination, help to explain why males of the free-living nematode species Rhabditis sp. SB347 sire <5% male progeny. First, the spermatogenesis programme involves a modified meiosis in which chromatids of the unpaired X chromosome separate prematurely, in meiosis I. Second, during anaphase II, cellular components essential for sperm motility are partitioned almost exclusively to the X-bearing sperm. Our studies reveal a novel cellular mechanism for the differential transmission of X-bearing sperm and suggest Rhabditis sp. SB347 as a useful model for studying sex chromosome drive and the evolution of new mating systems.

Spermatogenesis-defective (spe) mutants of the nematode Caenorhabditis elegans provide clues to solve the puzzle of male germline functions during reproduction

In most species, each sex produces gametes, usually either sperm or oocytes, from its germline during gametogenesis. The sperm and oocyte subsequently fuse together during fertilization to create the next generation. This review focuses on spermatogenesis and the roles of sperm during fertilization in the nematode Caenorhabditis elegans, where suitable mutants are readily obtained. So far, 186 mutants defective in the C. elegans male germline functions have been isolated, and many of these mutations are alleles for one of the approximately 60 spermatogenesis-defective (spe) genes. Many cloned spe genes are expressed specifically in the male germline, where they play roles during spermatogenesis (spermatid production), spermiogenesis (spermatid activation into spermatozoa), and/or fertilization. Moreover, several spe genes are orthologs of mammalian genes, suggesting that the reproductive processes of the C. elegans and the mammalian male germlines might share common pathways at the molecular level.

A toolkit for rapid gene mapping in the nematode Caenorhabditis briggsae

BACKGROUND

The nematode C. briggsae serves as a useful model organism for comparative analysis of developmental and behavioral processes. The amenability of C. briggsae to genetic manipulations and the availability of its genome sequence have prompted researchers to study evolutionary changes in gene function and signaling pathways. These studies rely on the availability of forward genetic tools such as mutants and mapping markers.

RESULTS

We have computationally identified more than 30,000 polymorphisms (SNPs and indels) in C. briggsae strains AF16 and HK104. These include 1,363 SNPs that change restriction enzyme recognition sites (snip-SNPs) and 638 indels that range between 7 bp and 2 kb. We established bulk segregant and single animal-based PCR assay conditions and used these to test 107 polymorphisms. A total of 75 polymorphisms, consisting of 14 snip-SNPs and 61 indels, were experimentally confirmed with an overall success rate of 83%. The utility of polymorphisms in genetic studies was demonstrated by successful mapping of 12 mutations, including 5 that were localized to sub-chromosomal regions. Our mapping experiments have also revealed one case of a misassembled contig on chromosome 3.

CONCLUSIONS

We report a comprehensive set of polymorphisms in C. briggsae wild-type strains and demonstrate their use in mapping mutations. We also show that molecular markers can be useful tools to improve the C. briggsae genome sequence assembly. Our polymorphism resource promises to accelerate genetic and functional studies of C. briggsae genes.

Worms from venus and mars: proteomics profiling of sexual differences in Caenorhabditis elegans using in vivo 15N isotope labeling

Hermaphrodites of the nematode Caenorhabditis elegans produce both sperm and oocytes in the same germline. To investigate the process underlying spermatogenesis and oogenesis separately, we used a quantitative proteomics approach applied to two mutant worm lines (fem-3(q20) and fem-1(hc17)) developing only male and female germlines, respectively. We used stable isotopic labeling of whole animals by feeding them either (14)N or (15)N labeled Escherichia coli. This way, we could confidently identify and quantify 1040 proteins in two independent experiments. Of these, approximately 400 proteins showed significant differential expression between female-like and male-like animals. As expected, proteins linked to oogenesis were found to be highly upregulated in the feminized worms, whereas proteins involved in spermatogenesis were found to be highly upregulated in the masculinized worms. This was complemented by many proteins strongly enriched in either mutant. Although the function of the majority of these proteins is unknown, their expression profile indicates that they have an as yet unrecognized role in the development and/or function of the female- and male germline in C. elegans. We show that members of several protein complexes as well as functionally similar proteins show comparable abundance ratios, indicating coregulation of protein expression. Additional analysis comparing our protein data to a previously published microarray data set shows that mRNA and protein expression are poorly correlating. We provide one of the first examples of a large-scale quantitative proteomics experiment in C. elegans and show the potential and feasibility of an approach enabling system-wide accurate quantitative proteomics experiments in this model organism.

The ESCRT-III protein CeVPS-32 is enriched in domains distinct from CeVPS-27 and CeVPS-23 at the endosomal membrane of epithelial cells

BACKGROUND INFORMATION

Within the endocytic pathway, the ESCRT (endosomal sorting complex required for transport) machinery is essential for the biogenesis of MVBs (multivesicular bodies). In yeast, ESCRTs are recruited at the endosomal membrane and are involved in cargo sorting into intralumenal vesicles of the MVBs.

RESULTS

In the present study, we characterize the ESCRT-III protein CeVPS-32 (Caenorhabditis elegans vacuolar protein sorting 32) and its interactions with CeVPS-27, CeVPS-23 and CeVPS-4. In contrast with other CevpsE (class E vps) genes, depletion of Cevps-32 is embryonic lethal with severe defects in the remodelling of epithelial cell shape during organogenesis. Furthermore, Cevps-32 animals display an accumulation of enlarged early endosomes in epithelial cells and an accumulation of autophagosomes. The CeVPS-32 protein is enriched in epithelial tissues and in residual bodies during spermatid maturation. We show that CeVPS-32 and CeVPS-27/Hrs (hepatocyte-growth-factor-regulated tyrosine kinase substrate) are enriched in distinct subdomains at the endosomal membrane. CeVPS-27-positive subdomains are also enriched for the ESCRT-I protein CeVPS-23/TSG101 (tumour susceptibility gene 101). The formation of CeVPS-27 subdomains is not affected by the depletion of CeVPS-23, CeVPS-32 or the ATPase CeVPS-4.

CONCLUSION

Our results suggest that the formation of membrane subdomains is essential for the maturation of endosomes.

Involvement of myosin VI immunoanalog in pinocytosis and phagocytosis in Amoeba proteus

Recently, we found a 130-kDa myosin VI immunoanalog in amoeba, which bound to actin in an ATP-sensitive manner and in migrating amoebae colocalized to filamentous actin and dynamin II-containing vesicular structures. To further characterize this protein, we assessed its involvement in amoeba pinocytosis and phagocytosis. Confocal immunofluorescence microscopy and electron microscopy of immunogold-stained cells revealed that, in pinocytotic and phagocytotic amoebae, the myosin VI immunoanalog was visible throughout the cells, including pinocytotic channels and pinocytotic vesicles as well as phagosomes and emerging phagocytic cups. Blocking endogenous protein with anti-porcine myosin VI antibody (introduced into cells by means of microinjection) caused severe defects in pinocytosis and phagocytosis. In comparison with control cells, the treated amoebae formed ~75% less pinocytotic channels and phagocytosed ~65% less Tetrahymena cells. These data indicate that the myosin VI immunoanalog has an important role in pinocytosis and phagocytosis in Amoeba proteus (Pal.).

Environmentally-modulated changes in fluorescence distribution in cells with oscillatory genetic network dynamics

We investigated the distribution of green fluorescent protein (GFP) expression levels in a population of E. coli cells expressing an artificial genetic regulatory network, known as the "repressilator". This network originally constructed by Elowitz and Leibler in 2000 consists of three cyclically-inhibiting promoter-repressor pairs. It is because of this architecture that the network has been known to oscillate at the single-cell level under certain conditions. A series of shake flask experiments were performed and analyzed using flow cytometry to test how cell populations carrying this system could be controlled extracellularly using the inducers anhydrotetracycline (aTc) and isopropyl-beta-d-thiogalactopyranoside (IPTG). With variation of [aTc], it exhibits a novel bi-threshold behavior, such that the entire culture reaches one of three steady states at a quasi-time-invariant "reference state." Also, there is significant hysteresis. Transiently, the middle state shows damping oscillations, while the low and high states show a stable steady state. The addition of IPTG serves to fine-tune the characteristics of the aTc-only expression, lowering the average and coefficient of variation (CV) of the distributions, and possibly perturbing the network to a different state. However, in modeling this system, the multiplicity and bi-threshold behavior are not theoretically possible according to the designed interactions. In order to explain this discrepancy, we hypothesize that one or more of the repressors have a significant nonspecific interaction with a promoter that does not contain its operator site. The new modeling results incorporating these extra interactions qualitatively match our experimental findings. After constructing plasmids to test these hypotheses, we discover that at least four of these interactions exist, which can create the low and high states and multiplicity seen experimentally. This genetic architecture has flexibility in its behavior that has not been demonstrated before, and the combination of experiment and modeling enlightened our understanding of the molecular interactions driving the network's behavior, leading us to discover the significance of nonspecific interactions.

A Myo6 mutation destroys coordination between the myosin heads, revealing new functions of myosin VI in the stereocilia of mammalian inner ear hair cells

Myosin VI, found in organisms from Caenorhabditis elegans to humans, is essential for auditory and vestibular function in mammals, since genetic mutations lead to hearing impairment and vestibular dysfunction in both humans and mice. Here, we show that a missense mutation in this molecular motor in an ENU-generated mouse model, Tailchaser, disrupts myosin VI function. Structural changes in the Tailchaser hair bundles include mislocalization of the kinocilia and branching of stereocilia. Transfection of GFP-labeled myosin VI into epithelial cells and delivery of endocytic vesicles to the early endosome revealed that the mutant phenotype displays disrupted motor function. The actin-activated ATPase rates measured for the D179Y mutation are decreased, and indicate loss of coordination of the myosin VI heads or ‘gating’ in the dimer form. Proper coordination is required for walking processively along, or anchoring to, actin filaments, and is apparently destroyed by the proximity of the mutation to the nucleotide-binding pocket. This loss of myosin VI function may not allow myosin VI to transport its cargoes appropriately at the base and within the stereocilia, or to anchor the membrane of stereocilia to actin filaments via its cargos, both of which lead to structural changes in the stereocilia of myosin VI–impaired hair cells, and ultimately leading to deafness.

Human deafness is extremely heterogeneous, with mutations in over 50 genes known to be associated with this common form of sensory loss. Among them, mutations in five myosins are associated with human hereditary hearing impairment, demonstrating that this family of proteins is essential for the proper function of the inner ear. Myosins, motor proteins found in eukaryotic cells, are responsible for actin-based motility. Composed of a motor domain and a tail, the former binds filamentous actin and uses ATP hydrolysis to generate force and move along the filaments, while the latter binds to cargos in the cell. Myosin VI is unique among myosins due to its movement along actin towards the minus or pointed end, rather than the positive or barbed end. Mutations in this myosin are associated with human deafness. Much of our information regarding myosin VI comes from studies in cell culture or mouse mutants with mutations leading to deafness. Here, we describe a deaf mouse mutant, Tailchaser, with a mutation in myosin VI. Our data describe new functions for myosin VI in the hair cells of the inner ear, showing how alterations in this motor can lead to a human sensory disorder.

E1 ubiquitin-activating enzyme UBA-1 plays multiple roles throughout C. elegans development

Poly-ubiquitination of target proteins typically marks them for destruction via the proteasome and provides an essential mechanism for the dynamic control of protein levels. The E1 ubiquitin-activating enzyme lies at the apex of the ubiquitination cascade, and its activity is necessary for all subsequent steps in the reaction. We have isolated a temperature-sensitive mutation in the Caenorhabditis elegans uba-1 gene, which encodes the sole E1 enzyme in this organism. Manipulation of UBA-1 activity at different developmental stages reveals a variety of functions for ubiquitination, including novel roles in sperm fertility, control of body size, and sex-specific development. Levels of ubiquitin conjugates are substantially reduced in the mutant, consistent with reduced E1 activity. The uba-1 mutation causes delays in meiotic progression in the early embryo, a process that is known to be regulated by ubiquitin-mediated proteolysis. The uba-1 mutation also demonstrates synthetic lethal interactions with alleles of the anaphase-promoting complex, an E3 ubiquitin ligase. The uba-1 mutation provides a sensitized genetic background for identifying new in vivo functions for downstream components of the ubiquitin enzyme cascade, and it is one of the first conditional mutations reported for the essential E1 enzyme in a metazoan animal model.

A C. elegans Piwi, PRG-1, regulates 21U-RNAs during spermatogenesis

BACKGROUND

Epigenetic regulation by diverse classes of small RNAs is mediated by the highly conserved Argonaute/Piwi family of proteins. Although Argonautes are broadly expressed, the Piwi subfamily primarily functions in the germ line. Piwi proteins are associated with germline-specific ribonucleoprotein (RNP) granules in Drosophila, zebrafish, and mouse. Depending on the species and on the specific family member, Piwis play important roles in spermatogenesis and/or in maintaining germ cell and stem cell totipotency. Piwis bind to a newly discovered class of small RNAs, called piRNAs. C. elegans contains a large set of Argonaute/Piwi-related proteins, including two closely related to piwi called prg-1 and prg-2. The function of prg-1 and prg-2 and whether piRNAs exist in C. elegans is unknown.

RESULTS

Here, we demonstrate that the Piwi-like protein PRG-1 is localized to P granules in germ cells entering spermatogenesis and is required for successful spermatogenesis. Loss of prg-1 causes a marked reduction in expression of a subset of mRNAs expressed during spermatogenesis, and prg-1 mutant sperm exhibit extensive defects in activation and fertilization. Moreover, prg-1 activity is required for the presence of the small RNAs called 21U-RNAs.

CONCLUSIONS

Our data suggest that PRG-1 promotes expression, processing, or stability of 21U-RNAs, which, in turn or in concert with PRG-1, promote proper expression of spermatogenesis transcripts.

The class V myosin motor protein, Myo2, plays a major role in mitochondrial motility in Saccharomyces cerevisiae

The actin cytoskeleton is essential for polarized, bud-directed movement of cellular membranes in Saccharomyces cerevisiae and thus ensures accurate inheritance of organelles during cell division. Also, mitochondrial distribution and inheritance depend on the actin cytoskeleton, though the precise molecular mechanisms are unknown. Here, we establish the class V myosin motor protein, Myo2, as an important mediator of mitochondrial motility in budding yeast. We found that mutants with abnormal expression levels of Myo2 or its associated light chain, Mlc1, exhibit aberrant mitochondrial morphology and loss of mitochondrial DNA. Specific mutations in the globular tail of Myo2 lead to aggregation of mitochondria in the mother cell. Isolated mitochondria lacking functional Myo2 are severely impaired in their capacity to bind to actin filaments in vitro. Time-resolved fluorescence microscopy revealed a block of bud-directed anterograde mitochondrial movement in cargo binding–defective myo2 mutant cells. We conclude that Myo2 plays an important and direct role for mitochondrial motility and inheritance in budding yeast.

What can myosin VI do in cells?

The recently solved structure of the myosin VI motor demonstrates that the unique insert at the end of the motor is responsible for the reversal of the normal myosin directionality. A second class-specific insert near the nucleotide-binding pocket contributes to myosin VI's unique kinetic tuning, allowing it to function either as an actin-based transporter or as an anchoring protein. Recent biochemical and biophysical studies have shown that the native molecule can form dimers upon clustering, and cell biological studies have demonstrated that it clearly does play both transport and anchoring roles in cells. These mechanistic insights allow us to speculate on how unusual aspects of myosin VI structure and function allow it to fill unique niches in cells.

Myosin VI altered at threonine 406 stabilizes actin filaments in vivo

Myosin VI is a minus-end directed actin-based molecular motor implicated in uncoated endocytic vesicle transport. Recent kinetic studies have shown that myosin VI displays altered ADP release kinetics under different load conditions allowing myosin VI to serve alternately as a transporter or as an actin tether. We theorized that one potential regulatory event to modulate between these kinetic choices is phosphorylation at a conserved site, threonine 406 (T406) in the myosin VI motor domain. Alterations mimicking the phosphorylated (T406E) and dephosphorylated state (T406A) were introduced into a GFP-myosin VI fusion (GFP-M6). Live cell imaging revealed that GFP-M6(T406E) expression changed the path myosin VI took in its transport of uncoated endocytic vesicles. Rather than routing vesicles inwards as seen in GFP-M6 and GFP-M6(T406A) expressing cells, GFP-M6(T406E) moved vesicles into clusters at distinct peripheral sites. GFP-M6(T406E) expression also increased the density of the actin cytoskeleton. Filaments were enriched at the vesicle cluster sites. This was not due to a gross redistribution of the actin polymerization machinery. Instead the filament density correlated to the fixed positioning of GFP-M6(T406E)-associated vesicles on F-actin, leading to inhibition of actin depolymerization. Our study suggests that phosphorylation at T406 changes the nature of myosin VI's interaction with actin in vivo.

Androcam is a tissue-specific light chain for myosin VI in the Drosophila testis

Myosin VI, a ubiquitously expressed unconventional myosin, has roles in a broad array of biological processes. Unusual for this motor family, myosin VI moves toward the minus (pointed) end of actin filaments. Myosin VI has two light chain binding sites that can both bind calmodulin (CaM). However unconventional myosins could use tissue-specific light chains to modify their activity. In the Drosophila testis, myosin VI is important for maintenance of moving actin structures, called actin cones, which mediate spermatid individualization. A CaM-related protein, Androcam (Acam), is abundantly expressed in the testis and like myosin VI, accumulates on these cones. We have investigated the possibility that Acam is a testis-specific light chain of Drosophila myosin VI. We find that Acam and myosin VI precisely colocalize at the leading edge of the actin cones and that myosin VI is necessary for this Acam localization. Further, myosin VI and Acam co-immunoprecipitate from the testis and interact in yeast two-hybrid assays. Finally Acam binds with high affinity to peptide versions of both myosin VI light chain binding sites. In contrast, although Drosophila CaM also shows high affinity interactions with these peptides, we cannot detect a CaM/myosin VI interaction in the testis. We conclude that Acam and not CaM acts as a myosin VI light chain in the Drosophila testis and hypothesize that it may alter the regulation of myosin VI in this tissue.

Myosin VI is a mediator of the p53-dependent cell survival pathway

Myosin VI is an unconventional motor protein, and its mutation is responsible for the familiar conditions sensorineural deafness and hypertrophic cardiomyopathy. Myosin VI is found to play a key role in the protein trafficking and homeostasis of the Golgi complex. However, very little is known about how myosin VI is regulated and whether myosin VI has a function in the DNA damage response. Here, we found that myosin VI is regulated by DNA damage in a p53-dependent manner and possesses a novel function in the p53-dependent prosurvival pathway. Specifically, we show that myosin VI is induced by p53 and DNA damage in a p53-dependent manner. We found that p53 directly binds to, and activates, the promoter of the myosin VI gene. We also show that the intracellular localization of myosin VI is substantially altered by p53 and DNA damage in a p53-dependent manner such that the pool of myosin VI in endocytic vesicles, membrane ruffles, and cytosol migrates to the Golgi complex, perinuclear membrane, and nucleus. Furthermore, we show that knockdown of myosin VI attenuates activation of p53 and impairs Golgi complex integrity, which makes myosin VI-deficient cells susceptible to apoptosis upon DNA damage. Taken together, we found a novel function for p53 in the maintenance of Golgi complex integrity and for myosin VI in the p53-dependent prosurvival pathway.

Myosin VI stabilizes an actin network during Drosophila spermatid individualization

Here, we demonstrate a new function of myosin VI using observations of Drosophila spermatid individualization in vivo. We find that myosin VI stabilizes a branched actin network in actin structures (cones) that mediate the separation of the syncytial spermatids. In a myosin VI mutant, the cones do not accumulate F-actin during cone movement, whereas overexpression of myosin VI leads to bigger cones with more F-actin. Myosin subfragment 1-fragment decoration demonstrated that the actin cone is made up of two regions: a dense meshwork at the front and parallel bundles at the rear. The majority of the actin filaments were oriented with their pointed ends facing in the direction of cone movement. Our data also demonstrate that myosin VI binds to the cone front using its motor domain. Fluorescence recovery after photobleach experiments using green fluorescent protein-myosin VI revealed that myosin VI remains bound to F-actin for minutes, suggesting its role is tethering, rather than transporting cargo. We hypothesize that myosin VI protects the actin cone structure either by cross-linking actin filaments or anchoring regulatory molecules at the cone front. These observations uncover a novel mechanism mediated by myosin VI for stabilizing long-lived actin structures in cells.

Myosin VI: cellular functions and motor properties

Myosin motor proteins use the energy derived from ATP hydrolysis to move cargo along actin tracks. Myosin VI, unlike almost all other myosins, moves toward the minus end of actin filaments and functions in a variety of intracellular processes such as vesicular membrane traffic, cell migration, and mitosis. These diverse roles of myosin VI are mediated by interaction with a number of different binding partners present in multi-protein complexes. Myosin VI can work in vitro as a processive dimeric motor and as a nonprocessive monomeric motor, each with a large working stroke. The possibility that both monomeric and dimeric forms of myosin VI operate in the cell may represent an important regulatory mechanism for controlling the multiple steps in transport pathways where nonprocessive and processive motors are required.

Myosin VI: a structural role in actin organization important for protein and organelle localization and trafficking

Myosin VI is a member of a superfamily of actin-based motors with at least 18 different sub-types or classes. Myosins are best known as proteins that use ATP-hydrolysis-mediated conformational changes to move along actin filaments. Because of this property, some myosins, including myosins I, V, and VI, are thought to be transporters of vesicle or protein cargoes. Myosin VI has been implicated in many seemingly different processes through functional studies in flies, worms and mammals. In several cases, its role is not easily explained by transport along actin. In addition, some of the biochemical and biophysical properties of myosin VI suggest other mechanisms of action. In this review, we summarize recent data that suggest diverse functions for myosin VI and offer an explanation for how myosin VI may function similarly in all of them. We hypothesize that the main function of myosin VI is to bind tightly to actin, stabilizing actin cytoskeletal structures and linking actin structures to membranes and protein complexes.