Calcium triggers the dissociation of myosin-Va from ribosomes in ribonucleoprotein complexes.

The sorting of RNAs to specific regions of the cell for local translation represents an important mechanism directing protein distribution and cell compartmentalization. While significant progress has been made in understanding the mechanisms underlying the transport and localization of mRNAs, the mechanisms governing ribosome mobilization are less well understood. Ribosomes present in the cytoplasm of multiple cell types can form ribonucleoprotein complexes that also contain myosin-Va (Myo5a), a processive, actin-dependent molecular motor. Here, we report that Myo5a can be disassociated from ribosomes when ribonucleoprotein complexes are exposed to calcium, both in vitro and in vivo. We suggest that Myo5a may act as a molecular switch able to anchor or release ribosomes from the actin cytoskeleton in response to intracellular signaling.

Myosin Va associates with mRNA in ribonucleoprotein particles present in myelinated peripheral axons and in the central nervous system.

Sorting of specific mRNAs to particular cellular locations and regulation of their translation is an essential mechanism underlying cell polarization. The transport of RNAs by kinesins and dyneins has been clearly established in several cell models, including neurons in culture. A similar role appears to exist in higher eukaryotes for the myosins. Myosin Va (Myo5a) has been described as a component of ribonucleoprotein particles (RNPs) in the adult rat nervous system and associated to ZBP1 and ribosomes in ribosomal periaxoplasmic plaques (PARPs), making it a likely candidate for mediating some aspects of RNA transport in neurons. To test this hypothesis, we have characterized RNPs containing Myo5a in adult brains of rats and mice. Microarray analysis of RNAs co-immunoprecipitated with Myo5a indicates that this motor may associate with a specific subpopulation of neuronal mRNAs. We found mRNAs encoding α-synuclein and several proteins with functions in translation in these RNPs. Immunofluorescence analyses of RNPs showed apparent co-localization of Myo5a with ribosomes, mRNA and RNA-binding proteins in discrete structures present both in axons of neurons in culture and in myelinated fibers of medullary roots. Our data suggest that PARPs include RNPs bearing the mRNA coding for Myo5a and are equipped with kinesin and Myo5a molecular motors. In conclusion, we suggest that Myo5a is involved in mRNA trafficking both in the central and peripheral nervous systems.

Role of myosin Va in neuritogenesis of chick dorsal root ganglia nociceptive neurons.

Myosin-Va, widely distributed throughout the developing nervous system, is involved in the transport of vesicles and other intracellular components with its globular tail domain (GTD) implicated in cargo recognition/interaction. Inactivation of myosin-Va in dorsal root ganglia (DRG) neurons of chick embryos, in vitro, decreases the rate of filopodial extension. MYO5A mutant mice have severe neurological defects. We have found that the overexpression of GTD in DRG cultures reduces the number of neurons with long neurites (above fourfold cell body length) and increased the number of neurons with short or no neurites. However, if transfection occurred after the onset of neuritogenesis, this was not seen. In embryo, we characterized the expression pattern of myosin-Va during neuritogenesis of TrkA-positive cells at different stages of chick DRG development. Myosin-Va expression was detected starting from HH25. At this stage, it was present in cells both with and without neurites. The presence of myosin-Va in DRG neurites persisted throughout the last stage analysed (HH34). The data suggest that Myosin Va can participate in embryonic DRG neuritogenesis.

Glia to axon RNA transfer.

The existence of RNA in axons has been a matter of dispute for decades. Evidence for RNA and ribosomes has now accumulated to a point at which it is difficult to question, much of the disputes turned to the origin of these axonal RNAs. In this review, we focus on studies addressing the origin of axonal RNAs and ribosomes. The neuronal soma as the source of most axonal RNAs has been demonstrated and is indisputable. However, the surrounding glial cells may be a supplemental source of axonal RNAs, a matter scarcely investigated in the literature. Here, we review the few papers that have demonstrated that glial-to-axon RNA transfer is not only feasible, but likely. We describe this process in both invertebrate axons and vertebrate axons. Schwann cell to axon ribosomes transfer was conclusively demonstrated (Court et al. [2008]: J. Neurosci 28:11024-11029; Court et al. [2011]: Glia 59:1529-1539). However, mRNA transfer still remains to be demonstrated in a conclusive way. The intercellular transport of mRNA has interesting implications, particularly with respect to the integration of glial and axonal function. This evolving field is likely to impact our understanding of the cell biology of the axon in both normal and pathological conditions. Most importantly, if the synthesis of proteins in the axon can be controlled by interacting glia, the possibilities for clinical interventions in injury and neurodegeneration are greatly increased.

Estudo da participação das proteínas Paxilina e Miosina-va na infectividade do Vírus Linfotrópico de Células T Humanas do tipo 1 (HTLV-1) / Study participation of Paxillin proteins and myosin-va in infectivity of the virus lymphotropic Human T cells type 1 (HTLV-1)

As ORFs I e IV do genoma do HTLV-1 codificam, respectivamente, as proteínas p12/p8 (acessória) e Tax (regulatória). p12/p8, de 99 aminoácidos, pode ser clivada em sua extremidade amino terminal gerando a proteína p8. A primeira clivagem proteolítica de p12 remove o sinal de retenção ao RE, enquanto a segunda clivagem, gera o produto de 8kDa, referido como p8. p12 localiza-se no sistema de endomembranes, residindo em RE e aparato de Golgi, enquanto p8 dirige-se para a membrana plasmática, onde é recrutada para a sinapse imunológica, através da ligação com o receptor de células T (TCR), além de participar da sinapse virológica e da formação de conduítes. A proteína Tax, por outro lado, atua como transativador transcricional do HTLV-1, sendo referida também na indução da expressão de diversos genes celulares, aumentando a proliferação e a migração das células infectadas. Na via de transporte de vesículas secretórias, vesículas produzidas como pós-Golgi são transportadas ao longo do citoesqueleto por motores celulares. A Miosina-Va, um motor não convencional, transporta diversos cargos, incluindo vesículas secretórias, vesículas sinápticas e de retículo endoplasmático. Outra proteína relacionada ao citoesqueleto é a Paxilina, que atua como molécula adaptadora nas adesões focais e cuja expressão está aumentada em indivíduos TSP-HAM...

HTLV-1 ORFs I and IV encode respectively p12/p8 (accessory protein) and Tax (regulatory protein). The 99 amino acid p12 protein can be proteolytically cleaved at the amino terminus to generate the p8 protein. The first proteolytic cleavage removes the ER retention/retrieval signal at the amino terminus of p12, while the second cleavage generates the p8 protein. The p12 protein localizes to cellular endomembranes, within the ER and Golgi apparatus, while p8 traffics to lipid rafts at the cell surface and is recruited to the immunological synapse upon T-cell receptor (TCR) ligation, virological synapse and conduits. Tax on the other hand acts as viral transactivator and induces expression of many cellular genes, increasing proliferation and migration of infected cells. In secretory vesicle transport, vesicles produced as post-Golgi are moved along the cytoskeleton by motor proteins. The unconventional myosin motor, Myosin-Va, moves several cargoes including secretory vesicles, synaptic vesicles, and the endoplasmic reticulum. Another cytoskeleton associated protein is Paxillin, an adapter on focal adhesions which expression is increased in TSP-HAM patients...

Myosin Va is developmentally regulated and expressed in the human cerebellum from birth to old age

Myosin Va functions as a processive, actin-based motor molecule highly enriched in the nervous system, which transports and/or tethers organelles, vesicles, and mRNA and protein translation machinery. Mutation of myosin Va leads to Griscelli disease that is associated with severe neurological deficits and a short life span. Despite playing a critical role in development, the expression of myosin Va in the central nervous system throughout the human life span has not been reported. To address this issue, the cerebellar expression of myosin Va from newborns to elderly humans was studied by immunohistochemistry using an affinity-purified anti-myosin Va antibody. Myosin Va was expressed at all ages from the 10th postnatal day to the 98th year of life, in molecular, Purkinje and granular cerebellar layers. Cerebellar myosin Va expression did not differ essentially in localization or intensity from childhood to old age, except during the postnatal developmental period. Structures resembling granules and climbing fibers in Purkinje cells were deeply stained. In dentate neurons, long processes were deeply stained by anti-myosin Va, as were punctate nuclear structures. During the first postnatal year, myosin Va was differentially expressed in the external granular layer (EGL). In the EGL, proliferating prospective granule cells were not stained by anti-myosin Va antibody. In contrast, premigratory granule cells in the EGL stained moderately. Granule cells exhibiting a migratory profile in the molecular layer were also moderately stained. In conclusion, neuronal myosin Va is developmentally regulated, and appears to be required for cerebellar function from early postnatal life to senescence.

Differential patterns of myosin Va expression during the ontogenesis of the rat hippocampus

Myosin Va is an actin-based, processive molecular motor protein highly enriched in the nervous tissue of vertebrates. It has been associated with processes of cellular motility, which include organelle transport and neurite outgrowth. The in vivo expression of myosin Va protein in the developing nervous system of mammals has not yet been reported. We describe here the immunolocalization of myosin Va in the developing rat hippocampus. Coronal sections of the embryonic and postnatal rat hippocampus were probed with an affinity-purified, polyclonal anti-myosin Va antibody. Myosin Va was localized in the cytoplasm of granule cells in the dentate gyrus and of pyramidal cells in Ammon's horn formation. Myosin Va expression changed during development, being higher in differentiating rather than already differentiated granule and pyramidal cells. Some of these cells presented a typical migratory profile, while others resembled neurons that were in the process of differentiation. Myosin Va was also transiently expressed in fibers present in the fimbria. Myosin Va was not detected in germinative matrices of the hippocampus proper or of the dentate gyrus. In conclusion, myosin Va expression in both granule and pyramidal cells showed both position and time dependency during hippocampal development, indicating that this motor protein is under developmental regulation.