Myosin VI regulates actin structure specialization through conserved cargo-binding domain sites

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.

PFTAIRE Kinase L63 Interactor 1A (Pif1A Protein) Is Required for Actin Cone Movement during Spermatid Individualization in Drosophila melanogaster

A useful model for determining the mechanisms by which actin and actin binding proteins control cellular architecture is the Drosophila melanogaster process of spermatogenesis. During the final step of spermatogenesis, 64 syncytial spermatids individualized as stable actin cones move synchronously down the axonemes and remodel the membranes. To identify new genes involved in spermatid individualization, we screened a collection of Drosophila male-sterile mutants and found that, in the line Z3-5009, actin cones formed near to the spermatid nuclei but failed to move, resulting in failed spermatid individualization. However, we show by phalloidin actin staining, electron microscopy and immunocytochemical localization of several actin binding proteins that the early cones had normal structure. We sequenced the genome of the Z3-5009 line and identified mutations in the PFTAIRE kinase L63 interactor 1A (Pif1A) gene. Quantitative real-time PCR showed that Pif1A transcript abundance was decreased in the mutant, and a transgene expressing Pif1A fused to green fluorescent protein (GFP) was able to fully rescue spermatid individualization and male fertility. Pif1A-GFP localized to the front of actin cones before initiation of movement. We propose that Pif1A plays a pivotal role in directing actin cone movement.

Myosin VI maintains the actin-dependent organization of the tubulobulbar complexes required for endocytosis during mouse spermiogenesis†‡

Myosin VI (MYO6) is an actin-based motor that has been implicated in a wide range of cellular processes, including endocytosis and the regulation of actin dynamics. MYO6 is crucial for actin/membrane remodeling during the final step of Drosophila spermatogenesis, and MYO6-deficient males are sterile. This protein also localizes to actin-rich structures involved in mouse spermiogenesis. Although loss of MYO6 in Snell's waltzer knock-out (KO) mice causes several defects and shows reduced male fertility, no studies have been published to address the role of MYO6 in sperm development in mouse. Here we demonstrate that MYO6 and some of its binding partners are present at highly specialized actin-based structures, the apical tubulobulbar complexes (TBCs), which mediate endocytosis of the intercellular junctions at the Sertoli cell-spermatid interface, an essential process for sperm release. Using electron and light microscopy and biochemical approaches, we show that MYO6, GIPC1 and TOM1/L2 form a complex in testis and localize predominantly to an early endocytic APPL1-positive compartment of the TBCs that is distinct from EEA1-positive early endosomes. These proteins also associate with the TBC actin-free bulbular region. Finally, our studies using testis from Snell's waltzer males show that loss of MYO6 causes disruption of the actin cytoskeleton and disorganization of the TBCs and leads to defects in the distribution of the MYO6-positive early APPL1-endosomes. Taken together, we report here for the first time that lack of MYO6 in mouse testis reduces male fertility and disrupts spatial organization of the TBC-related endocytic compartment during the late phase of spermiogenesis.

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.

Loss of myosin VI expression affects acrosome/acroplaxome complex morphology during mouse spermiogenesis†

During spermiogenesis in mammals, actin filaments and a variety of actin-binding proteins are involved in the formation and function of highly specialized testis-specific structures. Actin-based motor proteins, such as myosin Va and VIIa, play a key role in this complex process of spermatid transformation into mature sperm. We have previously demonstrated that myosin VI (MYO6) is also expressed in mouse testes. It is present in actin-rich structures important for spermatid development, including one of the earliest events in spermiogenesis—acrosome formation. Here, we demonstrate using immunofluorescence, cytochemical, and ultrastructural approaches that MYO6 is involved in maintaining the structural integrity of these specialized actin-rich structures during acrosome biogenesis in mouse. We show that MYO6 together with its binding partner TOM1/L2 is present at/around the spermatid Golgi complex and the nascent acrosome. Depletion of MYO6 in Snell’s waltzer mice causes structural disruptions of the Golgi complex and affects the acrosomal granule positioning within the developing acrosome. In summary, our results suggest that MYO6 plays an anchoring role during the acrosome biogenesis mainly by tethering of different cargo/membranes to highly specialized actin-related structures.

Transcriptome Sequencing Reveals the Traits of Spermatogenesis and Testicular Development in Large Yellow Croaker (Larimichthys crocea)

Larimichthys crocea is an economically important marine fish in China. To date, the molecular mechanisms underlying testicular development and spermatogenesis in L. crocea have not been thoroughly elucidated. In this study, we conducted a comparative transcriptome analysis between testes (TES) and pooled multiple tissues (PMT) (liver, spleen, heart, and kidney) from six male individuals. More than 54 million clean reads were yielded from TES and PMT libraries. After mapping to the draft genome of L. crocea, we acquired 25,787 genes from the transcriptome dataset. Expression analyses identified a total of 3853 differentially expressed genes (DEGs), including 2194 testes-biased genes (highly expressed in the TES) and 1659 somatic-biased genes (highly expressed in the PMT). The dataset was further annotated by blasting with multi-databases. Functional genes and enrichment pathways involved in spermatogenesis and testicular development were analyzed, such as the neuroactive ligand-receptor interaction pathway, gonadotropin-releasing hormone (GnRH) and mitogen-activated protein kinase (MAPK) signaling pathways, cell cycle pathway, and dynein, kinesin, myosin, actin, heat shock protein (hsp), synaptonemal complex protein 2 (sycp2), doublesex- and mab-3-related transcription factor 1 (dmrt1), spermatogenesis-associated genes (spata), DEAD-Box Helicases (ddx), tudor domain-containing protein (tdrd), and piwi genes. The candidate genes identified by this study lay the foundation for further studies into the molecular mechanisms underlying testicular development and spermatogenesis in L. crocea.

GIPC proteins negatively modulate Plexind1 signaling during vascular development

Semaphorins (SEMAs) and their Plexin (PLXN) receptors are central regulators of metazoan cellular communication. SEMA-PLXND1 signaling plays important roles in cardiovascular, nervous, and immune system development, and cancer biology. However, little is known about the molecular mechanisms that modulate SEMA-PLXND1 signaling. As PLXND1 associates with GIPC family endocytic adaptors, we evaluated the requirement for the molecular determinants of their association and PLXND1's vascular role. Zebrafish that endogenously express a Plxnd1 receptor with a predicted impairment in GIPC binding exhibit low penetrance angiogenesis deficits and antiangiogenic drug hypersensitivity. Moreover, gipc mutant fish show angiogenic impairments that are ameliorated by reducing Plxnd1 signaling. Finally, GIPC depletion potentiates SEMA-PLXND1 signaling in cultured endothelial cells. These findings expand the vascular roles of GIPCs beyond those of the Vascular Endothelial Growth Factor (VEGF)-dependent, proangiogenic GIPC1-Neuropilin 1 complex, recasting GIPCs as negative modulators of antiangiogenic PLXND1 signaling and suggest that PLXND1 trafficking shapes vascular development.

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.

Disruption of INOS, a Gene Encoding myo-Inositol Phosphate Synthase, Causes Male Sterility in Drosophila melanogaster

Inositol is a precursor for the phospholipid membrane component phosphatidylinositol (PI), involved in signal transduction pathways, endoplasmic reticulum stress, and osmoregulation. Alterations of inositol metabolism have been implicated in human reproductive issues, the therapeutic effects of drugs used to treat epilepsy and bipolar disorder, spinal cord defects, and diseases including diabetes and Alzheimer’s. The sole known inositol synthetic enzyme is myo-inositol synthase (MIPS), and the homolog in Drosophilia melanogaster is encoded by the Inos gene. Three identical deletion strains (inos^ΔDF/CyO) were constructed, confirmed by PCR and sequencing, and homozygotes (inos^ΔDF/inos^ΔDF) were shown to lack the transcript encoding the MIPS enzyme. Without inositol, homozygous inos^ΔDF deletion fertilized eggs develop only to the first-instar larval stage. When transferred as pupae to food without inositol, however, inos^ΔDF homozygotes die significantly sooner than wild-type flies. Even with dietary inositol the homozygous inos^ΔDF males are sterile. An inos allele, with a P-element inserted into the first intron, fails to complement this male sterile phenotype. An additional copy of the Inos gene inserted into another chromosome rescues all the phenotypes. These genetic and phenotypic analyses establish D. melanogaster as an excellent model organism in which to examine the role of inositol synthesis in development and reproduction.

Drosophila LKB1 is required for the assembly of the polarized actin structure that allows spermatid individualization

In mammals, a testis-specific isoform of the protein kinase LKB1 is required for spermiogenesis, but its exact function and specificity are not known. Human LKB1 rescues the functions of Drosophila Lkb1 essential for viability, but these males are sterile, revealing a new function for this genes in fly. We also identified a testis-specific transcript generated by an alternative promoter and that only differs by a longer 5'UTR. We show that dLKB1 is required in the germline for the formation of the actin cone, the polarized structure that allows spermatid individualization and cytoplasm excess extrusion during spermiogenesis. Three of the nine LKB1 classical targets in the Drosophila genome (AMPK, NUAK and KP78b) are required for proper spermiogenesis, but later than dLKB1. dLkb1 mutant phenotype is reminiscent of that of myosin V mutants, and both proteins show a dynamic localization profile before actin cone formation. Together, these data highlight a new dLKB1 function and suggest that dLKB1 posttranscriptional regulation in testis and involvement in spermatid morphogenesis are evolutionarily conserved features.

Expression and localization of myosin VI in developing mouse spermatids

Myosin VI (MVI) is a versatile actin-based motor protein that has been implicated in a variety of different cellular processes, including endo- and exocytic vesicle trafficking, Golgi morphology, and actin structure stabilization. A role for MVI in crucial actin-based processes involved in sperm maturation was demonstrated in Drosophila. Because of the prominence and importance of actin structures in mammalian spermiogenesis, we investigated whether MVI was associated with actin-mediated maturation events in mammals. Both immunofluorescence and ultrastructural analyses using immunogold labeling showed that MVI was strongly linked with key structures involved in sperm development and maturation. During the early stage of spermiogenesis, MVI is associated with the Golgi and with coated and uncoated vesicles, which fuse to form the acrosome. Later, as the acrosome spreads to form a cap covering the sperm nucleus, MVI is localized to the acroplaxome, an actin-rich structure that anchors the acrosome to the nucleus. Finally, during the elongation/maturation phase, MVI is associated with the actin-rich structures involved in nuclear shaping: the acroplaxome, manchette, and Sertoli cell actin hoops. Since this is the first report of MVI expression and localization during mouse spermiogenesis and MVI partners in developing sperm have not yet been identified, we discuss some probable roles for MVI in this process. During early stages, MVI is hypothesized to play a role in Golgi morphology and function as well as in actin dynamics regulation important for attachment of developing acrosome to the nuclear envelope. Next, the protein might also play anchoring roles to help generate forces needed for spermatid head elongation. Moreover, association of MVI with actin that accumulates in the Sertoli cell ectoplasmic specialization and other actin structures in surrounding cells suggests additional MVI functions in spermatid movement across the seminiferous epithelium and in sperm release.

miR-8 modulates cytoskeletal regulators to influence cell survival and epithelial organization in Drosophila wings

The miR-200 microRNA family plays important tumor suppressive roles. The sole Drosophila miR-200 ortholog, miR-8 plays conserved roles in Wingless, Notch and Insulin signaling - pathways linked to tumorigenesis, yet homozygous null animals are viable and often appear morphologically normal. We observed that wing tissues mosaic for miR-8 levels by genetic loss or gain of function exhibited patterns of cell death consistent with a role for miR-8 in modulating cell survival in vivo. Here we show that miR-8 levels impact several actin cytoskeletal regulators that can affect cell survival and epithelial organization. We show that loss of miR-8 can confer resistance to apoptosis independent of an epithelial to mesenchymal transition while the persistence of cells expressing high levels of miR-8 in the wing epithelium leads to increased JNK signaling, aberrant expression of extracellular matrix remodeling proteins and disruption of proper wing epithelial organization. Altogether our results suggest that very low as well as very high levels of miR-8 can contribute to hallmarks associated with cancer, suggesting approaches to increase miR-200 microRNAs in cancer treatment should be moderate.

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.

Dock7: a GEF for Rho-family GTPases and a novel myosin VI-binding partner in neuronal PC12 cells

Myosin VI (MVI), the only known myosin that walks towards the minus end of actin filaments, is involved in several processes such as endocytosis, cell migration, and cytokinesis. It may act as a transporting motor or a protein engaged in actin cytoskeleton remodelling via its binding partners, interacting with its C-terminal globular tail domain. By means of pull-down technique and mass spectrometry, we identified Dock7 (dedicator of cytokinesis 7) as a potential novel MVI-binding partner in neurosecretory PC12 cells. Dock7, expressed mainly in neuronal cells, is a guanine nucleotide exchange factor (GEF) for small GTPases, Rac1 and Cdc42, which are the major regulators of actin cytoskeleton. MVI-Dock7 interaction was further confirmed by co-immunoprecipitation of endogenous MVI complexed with Dock7. In addition, MVI and Dock7 colocalized in interphase and dividing cells. We conclude that in PC12 cells MVI-Dock7 complexes may function at different cellular locations during the entire cell cycle. Of note, MVI and Dock7 colocalized in primary culture hippocampal neurons also, predominantly in the outgrowths. We hypothesize that this newly identified interaction between MVI and Dock7 may help explain a mechanism for MVI-dependent regulation of actin cytoskeleton organization.

Myosin VI regulates actin dynamics and melanosome biogenesis

Myosin VI has been implicated in various steps of organelle dynamics. However, the molecular mechanism by which this myosin contributes to membrane traffic is poorly understood. Here, we report that myosin VI is associated with a lysosome-related organelle, the melanosome. Using an actin-based motility assay and video microscopy, we observed that myosin VI does not contribute to melanosome movements. Myosin VI expression regulates instead the organization of actin networks in the cytoplasm. Using a cell-free assay, we showed that myosin VI recruited actin at the surface of isolated melanosomes. Myosin VI is involved in the endocytic-recycling pathway, and this pathway contributes to the transport of a melanogenic enzyme to maturing melanosomes. We showed that depletion of myosin VI accumulated a melanogenic enzyme in enlarged melanosomes and increased their melanin content. We confirmed the requirement of myosin VI to regulate melanosome biogenesis by analysing the morphology of melanosomes in choroid cells from of the Snell's waltzer mice that do not express myosin VI. Together, our results provide new evidence that myosin VI regulates the organization of actin dynamics at the surface of a specialized organelle and unravel a novel function of this myosin in regulating the biogenesis of this organelle.