Biomedical subjects

B Sampo

Publications and source records attributed to B Sampo.

3 recordsLinked to original sources

PubMed2000

Direct interaction between synaptotagmin and the intracellular loop I-II of neuronal voltage-sensitive sodium channels.

Synaptotagmin, a synaptic vesicle protein involved in Ca(2+)-regulated exocytosis, displayed direct high affinity interaction with neuronal sodium channels. Monoclonal antibodies directed against synaptotagmins I and II adsorbed in a concentration-dependent and -specific manner [(3)H]saxitoxin prelabeled sodium channels extracted with detergent from nerve endings. Conversely, co-immunoprecipitation of synaptotagmin was achieved by antibodies against sodium channel subunits. Consistent with the co-immunoprecipitation assays, solubilized [(3)H]saxitoxin-prelabeled sodium channels were trapped on immobilized maltose binding protein (MBP)-synaptotagmin I. In vitro recombinant protein assays were employed to identify the interaction site of synaptotagmin I, which was located on the cytoplasmic loop between domains I and II of the sodium channel alphaIIA subunit. The co-immunoprecipitated synaptotagmin-sodium channel complexes were found to be Ca(2+)-dependent; this effect was mimicked by Ba(2+) and Sr(2+) but not Mg(2+). Finally the complex was shown to be distinct from the synaptotagmin-SNARE protein complex that can selectively interact with presynaptic calcium channels (N and P/Q types). Thus, our findings demonstrate an unexpected and direct interaction between sodium channels and synaptotagmin. The Ca(2+)-regulated association between sodium channels and a protein implicated in vesicular fusion may have intriguing consequences for the establishment and regulation of neuronal excitability.

Animals↗

PubMed1998

Multiple pathways regulate the expression of genes encoding sodium channel subunits in developing neurons.

In primary cultures of fetal neurons, activation of sodium channels with either alpha-scorpion toxin or veratridine caused a rapid and persistent decrease of mRNAs encoding beta2 and different sodium channel alpha mRNAs. In contrast, beta1 subunit mRNA was up-regulated by sodium channel activation. This phenomenon was calcium-independent. The effects of activating toxins on mRNAs of different sodium channel subunits were mimicked by membrane depolarization. An important aspect of this study was the demonstration that cAMP also caused rapid reduction of alphaI, alphaII and alphaIII mRNA levels whereas beta1 subunit mRNA was up regulated and beta2 subunit mRNA was not affected. Sodium channel activation by veratridine was shown to increase cAMP immunoreactivity in cultured neurons, but alphaII mRNA down-regulation induced by activating toxins was not reversed by protein kinase A antagonists, indicating that this phenomenon is not protein kinase A dependent. The effects of cAMP and membrane depolarisation were antagonized by the PKA inhibitor H89. These results are indicative of the existence of multiple and independent regulatory pathways modulating the expression of sodium channel genes in the developing central nervous system.

Animals↗

PubMed1997

Down-regulation of voltage-dependent sodium channels coincides with a low expression of alphabeta1 subunit complexes.

The association between the beta1 subunit and the alpha subunit of the sodium channel from rat brain was studied in hippocampus during postnatal development and in cultures of fetal rat forebrain neurons and cerebellar granule cells, using an anti-beta1 antipeptide antibody to specifically immunoprecipitate alphabeta1 complexes labeled with [3H]saxitoxin. In the hippocampus, the increase in beta1 RNA expression during development was accompanied by an increase in immunoprecipitated alphabeta1 complexes. Most of the alphabeta1 complexes were constituted during the first 3 postnatal weeks, with the steepest rise between postnatal days 5 and 12. In cultured fetal neurons, the amount of beta1 RNA and of alphabeta1 complexes was approximately 3-4% of that found in the adult, whereas it reached 60-70% in cultured cerebellar granule cells. We had previously described a neurotoxin-induced internalization of sodium channels which occurred in immature neurons but not in adult tissue. Internalization decreased during development in neurotoxin-treated hippocampal slices, and resistance of plasma membrane sodium channels to internalization followed the same time course than the appearance of alphabeta1 complexes. Similarly, neurotoxin activation resulted in sodium channel internalization in fetal neurons, while cerebellar granule cells, which express high levels of beta1 RNA and of alphabeta1 complexes, did not internalize their [3H]saxitoxin receptors in that same conditions. These data suggested that the association of the beta1 subunit with the alpha subunit could provide a suitable marker for the stabilization and anchoring of sodium channels in discrete membrane domains which occur during neuronal development.

Aging↗