FER-1 regulates Ca2+ -mediated membrane fusion during C. elegans spermatogenesis

A novel function for the presenilin family member spe-4: inhibition of spermatid activation in Caenorhabditis elegans

Background

Sperm cells must regulate the timing and location of activation to maximize the likelihood of fertilization. Sperm from most species, including the nematode Caenorhabditis elegans, activate upon encountering an external signal. Activation for C. elegans sperm occurs as spermatids undergo spermiogenesis, a profound cellular reorganization that produces a pseudopod. Spermiogenesis is initiated by an activation signal that is transduced through a series of gene products. It is now clear that an inhibitory pathway also operates in spermatids, preventing their premature progression to spermatozoa and resulting in fine-scale control over the timing of activation. Here, we describe the involvement of a newly assigned member of the inhibitory pathway: spe-4, a homolog of the human presenilin gene PS1. The spe-4(hc196) allele investigated here was isolated as a suppressor of sterility of mutations in the spermiogenesis signal transduction gene spe-27.

Results

Through mapping, complementation tests, DNA sequencing, and transformation rescue, we determined that allele hc196 is a mutation in the spe-4 gene. Our data show that spe-4(hc196) is a bypass suppressor that eliminates the need for the spermiogenesis signal transduction. On its own, spe-4(hc196) has a recessive, temperature sensitive spermatogenesis-defective phenotype, with mutants exhibiting (i) defective spermatocytes, (ii) defective spermatids, (iii) premature spermatid activation, and (iv) spermatozoa defective in fertilization, in addition to a small number of functional sperm which appear normal microscopically.

Conclusion

A fraction of the sperm from spe-4(hc196) mutant males progress directly to functional spermatozoa without the need for an activation signal, suggesting that spe-4 plays a role in preventing spermatid activation. Another fraction of spermatozoa from spe-4(hc196) mutants are defective in fertilization. Therefore, prematurely activated spermatozoa may have several defects: we show that they may be defective in fertilization, and earlier work showed that they obstruct sperm transfer from males at mating. hc196 is a hypomorphic allele of spe-4, and its newly-discovered role inhibiting spermiogenesis may involve known proteolytic and/or calcium regulatory aspects of presenilin function, or it may involve yet-to-be discovered functions.

Dysferlin domain-containing proteins, Pex30p and Pex31p, localized to two compartments, control the number and size of oleate-induced peroxisomes in Pichia pastoris

Yarrowia lipolytica Pex23p and Saccharomyces cerevisiae Pex30p, Pex31p, and Pex32p comprise a family of dysferlin domain-containing peroxins. We show that the deletion of their Pichia pastoris homologues, PEX30 and PEX31, does not affect the function or division of methanol-induced peroxisomes but results in fewer and enlarged, functional, oleate-induced peroxisomes. Synthesis of Pex30p is constitutive, whereas that of Pex31p is oleate-induced but at a much lower level relative to Pex30p. Pex30p interacts with Pex31p and is required for its stability. At steady state, both Pex30p and Pex31p exhibit a dual localization to the endoplasmic reticulum (ER) and peroxisomes. However, Pex30p is localized mostly to the ER, whereas Pex31p is predominantly on peroxisomes. Consistent with ER-to-peroxisome trafficking of these proteins, Pex30p accumulates on peroxisomes upon overexpression of Pex31p. Additionally, Pex31p colocalizes with Pex30p at the ER in pex19Delta cells and can be chased from the ER to peroxisomes in a Pex19p-dependent manner. The dysferlin domains of Pex30p and Pex31p, which are dispensable for their interaction, stability, and subcellular localization, are essential for normal peroxisome number and size. The growth environment-specific role of these peroxins, their dual localization, and the function of their dysferlin domains provide novel insights into peroxisome morphogenesis.

Dysferlin deficiency enhances monocyte phagocytosis: a model for the inflammatory onset of limb-girdle muscular dystrophy 2B

Dysferlin deficiency causes limb-girdle muscular dystrophy type 2B (LGMD2B; proximal weakness) and Miyoshi myopathy (distal weakness). Muscle inflammation is often present in dysferlin deficiency, and patients are frequently misdiagnosed as having polymyositis. Because monocytes normally express dysferlin, we hypothesized that monocyte/macrophage dysfunction in dysferlin-deficient patients might contribute to disease onset and progression. We therefore examined phagocytic activity, in the presence and absence of cytokines, in freshly isolated peripheral blood monocytes from LGMD2B patients and in the SJL dysferlin-deficient mouse model. Dysferlin-deficient monocytes showed increased phagocytic activity compared with control cells. siRNA-mediated inhibition of dysferlin expression in the J774 macrophage cell line resulted in significantly enhanced phagocytosis, both at baseline and in response to tumor necrosis factor-alpha. Immunohistochemical analysis revealed positive staining for several mononuclear cell activation markers in LGMD2B human muscle and SJL mouse muscle. SJL muscle showed strong up-regulation of endocytic proteins CIMPR, clathrin, and adaptin-alpha, and LGMD2B muscle exhibited decreased expression of decay accelerating factor, which was not dysferlin-specific. We further showed that expression levels of small Rho family GTPases RhoA, Rac1, and Cdc 42 were increased in dysferlin-deficient murine immune cells compared with control cells. Therefore, we hypothesize that mild myofiber damage in dysferlin-deficient muscle stimulates an inflammatory cascade that may initiate, exacerbate, and possibly perpetuate the underlying myofiber-specific dystrophic process.

Membrane traffic and muscle: lessons from human disease

Like all mammalian tissues, skeletal muscle is dependent on membrane traffic for proper development and homeostasis. This fact is underscored by the observation that several human diseases of the skeletal muscle are caused by mutations in gene products of the membrane trafficking machinery. An examination of these diseases and the proteins that underlie them is instructive both in terms of determining disease pathogenesis and of understanding the normal aspects of muscle biology regulated by membrane traffic. This review highlights our current understanding of the trafficking genes responsible for human myopathies.

Core proteins of the secretory machinery

Members of the Rab, SM- and SNARE-protein families play key roles in all intracellular membrane trafficking steps. While SM- and SNARE-proteins become directly involved in the fusion reaction at a late stage, Rabs and their effectors mediate upstream steps such as vesicle budding, delivery, tethering, and transport. Exocytosis of synaptic vesicles and regulated secretory granules are among the best-studied fusion events and involve the Rab3 isoforms Rab3A-D, the SM protein munc18-1, and the SNAREs syntaxin 1A, SNAP-25, and synaptobrevin 2. According to the current view, syntaxin 1A and SNAP-25 at the presynaptic membrane form a complex with synaptic vesicle-associated synaptobrevin 2. As complex formation proceeds, the opposed membranes are pulled tightly together, enforcing the fusion reaction. Munc18-1 is essential for regulated exocytosis and interacts with syntaxin 1A alone or with SNARE complexes, suggesting a role for munc18-1 in controlling the SNARE-assembly reaction. Compared to other intracellular fusion steps, special adaptations evolved in the synapse to allow for the tight regulation and high membrane turnover rates required for synaptic transmission. Synaptic vesicle fusion is triggered by the intracellular second messenger calcium, with members of the synaptotagmin protein family being prime candidates for linking calcium influx to fusion in the fast phase of exocytosis. To compensate for the massive incorporation of synaptic vesicles into the plasma membrane during exocytosis, special adaptations to endocytic mechanisms have evolved at the synapse to allow for efficient vesicle recycling.

In silico functional and structural characterisation of ferlin proteins by mapping disease-causing mutations and evolutionary information onto three-dimensional models of their C2 domains

Ferlins are C2 domain proteins involved in membrane fusion events, including membrane repair and synaptic exocytosis, and their deficiency can result in muscular dystrophy and deafness. We have undertaken a structural study of their C2 domains by sequence comparison and homology modelling to understand the function of these poorly characterised proteins and to predict the molecular impact of disease-causing mutations. We observe that non-conservative mutations affecting buried residues tend to result in detrimental phenotypes, likely because of decreased protein stability, whereas most variants with replacements in surface residues do not. The few cases of exposed residues altered in variants known to cause diseases are found in conserved areas of functional importance, including essential calcium-binding regions, as deduced by analogy to other characterised C2 domains. Furthermore, we report distinct features of some C2 domains in the two known ferlin subfamilies that correlates with the presence or absence of the DysF domains. Taken altogether, our results highlight potential targets for further experimental analyses to understand the function of ferlin proteins. We believe our modelling data will aid the diagnosis of diseases associated with ferlin mutations and the development of therapeutic strategies.

Myoferlin regulates vascular endothelial growth factor receptor-2 stability and function

Myoferlin and dysferlin are members of the ferlin family of membrane proteins. Recent studies have shown that mutation or genetic disruption of myoferlin or dysferlin promotes muscular dystrophy-related phenotypes in mice, which are the result of impaired plasma membrane integrity. However, no biological functions have been ascribed to myoferlin in non-muscle tissues. Herein, using a proteomic analysis of endothelial cell (EC) caveolae/lipid raft microdomains we identified myoferlin in these domains and show that myoferlin is highly expressed in ECs and vascular tissues. The loss of myoferlin results in lack of proliferation, migration, and nitric oxide (NO) release in response to vascular endothelial growth factor (VEGF). Western blotting and surface biotinylation experiments show that loss of myoferlin reduces the expression level and autophosphorylation of VEGF receptor-2 (VEGFR-2) in native ECs. In a reconstituted cell system, transfection of myoferlin increases VEGFR-2 membrane expression and autophosphorylation in response to VEGF. In vivo, VEGFR-2 levels and VEGF-induced permeability are impaired in myoferlin-deficient mice. Mechanistically, myoferlin forms a complex with dynamin-2 and VEGFR-2, which prevents CBL-dependent VEGFR-2 polyubiquitination and proteasomal degradation. These data are the first to report novel biological activities for myoferlin and reveal the role of membrane integrity to VEGF signaling.

Dysferlin and muscle membrane repair

The ability to repair membrane damage is conserved across eukaryotic cells and is necessary for the cells to survive a variety of physiological and pathological membrane disruptions. Membrane repair is mediated by rapid Ca(2+)-triggered exocytosis of various intracellular vesicles, such as lysosomes and enlargeosomes, which lead to the formation of a membrane patch that reseals the membrane lesion. Recent findings suggest a crucial role for dysferlin in this repair process in muscle, possibly as a Ca(2+) sensor that triggers vesicle fusion. The importance of membrane repair is highlighted by the genetic disease, dysferlinopathy, in which the primary defect is the loss of Ca(2+)-regulated membrane repair due to dysferlin deficiency. Future research on dysferlin and its interacting partners will enhance the understanding of this important process and provide novel avenues to potential therapies.

Dysferlin in Membrane Trafficking and Patch Repair

The muscular dystrophies are a heterogeneous group of inherited disorders, defined by progressive muscle weakness and atrophy. Following the discovery of dystrophin, remarkable progress has been made in defining the molecular properties of proteins involved in the various dystrophies. This has underlined the importance of the dystrophin-associated protein complex as a cell membrane scaffold, providing structural stability to muscle cells (McNeil PL, Khakee R. Disruptions of muscle fiber plasma membranes. Role in exercise-induced damage. Am J Pathol 1992;140:1097-1109). While the dystrophies linked to loss of function of dystrophin and its associated proteins are caused by diminished membrane integrity, it is now believed that a new class of dystrophies arises because of a diminished capacity for rapid muscle membrane repair after injury. Dysferlin is the first identified member of a putative muscle-specific repair complex that permits rapid resealing of membranes disrupted by mechanical stress. Membrane resealing is a function conserved by most cells and is mediated by a mechanism closely resembling regulated, Ca2+-dependent exocytosis. A primary role for dysferlin in this pathway, as a Ca2+-regulated fusogen, has been suggested, and a number of candidate partner proteins have been identified. This review outlines the current understanding of the role of dysferlin in membrane repair and the evolving picture of dysferlin-related signaling pathways in muscle cell physiology and pathology.

Complex regulation and multiple developmental functions of misfire, the Drosophila melanogaster ferlin gene

Background

Ferlins are membrane proteins with multiple C2 domains and proposed functions in Ca²⁺ mediated membrane-membrane interactions in animals. Caenorhabditis elegans has two ferlin genes, one of which is required for sperm function. Mammals have several ferlin genes and mutations in the human dysferlin (DYSF) and otoferlin (OTOF) genes result in muscular dystrophy and hearing loss, respectively. Drosophila melanogaster has a single ferlin gene called misfire (mfr). A previous study showed that a mfr mutation caused male sterility because of defects in fertilization. Here we analyze the expression and structure of the mfr gene and the consequences of multiple mutations to better understand the developmental function of ferlins.

Results

We show that mfr is expressed in the testis and ovaries of adult flies, has tissue-specific promoters, and expresses alternatively spliced transcripts that are predicted to encode distinct protein isoforms. Studies of 11 male sterile mutations indicate that a predicted Mfr testis isoform with five C2 domains and a transmembrane (TM) domain is required for sperm plasma membrane breakdown (PMBD) and completion of sperm activation during fertilization. We demonstrate that Mfr is not required for localization of Sneaky, another membrane protein necessary for PMBD. The mfr mutations vary in their effects in females, with a subset disrupting egg patterning and causing a maternal effect delay in early embryonic development. Locations of these mutations indicate that a short Mfr protein isoform carries out ferlin activities during oogenesis.

Conclusion

The mfr gene exhibits complex transcriptional and post-transcriptional regulation and functions in three developmental processes: sperm activation, egg patterning, and early embryogenesis. These functions are in part due to the production of protein isoforms that vary in the number of C2 domains. These findings help establish D. melanogaster as model system for understanding ferlin function and dysfunction in animals, including humans.

From T-tubule to sarcolemma: damage-induced dysferlin translocation in early myogenesis

The dysferlin gene is mutated in limb-girdle muscular dystrophy type 2B, Miyoshi myopathy, and distal anterior compartment myopathy. In mature skeletal muscle, dysferlin is located predominantly at the sarcolemma, where it plays a role in membrane fusion and repair. To investigate the role of dysferlin during early muscle differentiation, its localization was studied at high resolution in a muscle cell line. This demonstrated that dysferlin is not expressed at the plasmalemma of myotubes but mostly localizes to the T-tubule network. However, dysferlin translocated to the site of injury and toward the plasma membrane in a Ca2+-dependent fashion in response to a newly designed in vitro wounding assay. This reaction was specific to the full-length protein, as heterologously expressed deletion mutants of distinct C2 domains of dysferlin did not show this response. These results shed light on the dynamics of muscle membrane repair and are highly indicative of a specific role of dysferlin in this process in early myogenesis.