Characterization of a novel human putative mitochondrial transporter homologous to the yeast mitochondrial RNA splicing proteins 3 and 4

We report here a novel human gene, hMRS3/4, encoding a putative mitochondrial transporter structurally and functionally homologous to the yeast mitochondrial RNA splicing proteins 3 and 4. These proteins belong to the family of mitochondrial carrier proteins (MCF) and are likely to function as solute carriers. hMRS3/4 spans approximately 10 kb of genomic DNA on chromosome 10q24 and consists of four exons that encode a 364-aa protein with six transmembrane domains. A putative splice variant, encoding a 177-aa protein with three transmembrane domains, was also identified. hMRS3/4 has a well-conserved signature sequence of MCF and is targeted into the mitochondria. When expressed in yeast, hMRS3/4 efficiently restores the mitochondrial functions in mrs3(o)mrs4(o) knock-out mutants. Ubiquitous expression in human tissues and a well-conserved structure and function suggest an important role for hMRS3/4 in human cells.     

Removal of Ca2+ channel beta3 subunit enhances Ca2+ oscillation frequency and insulin exocytosis

An oscillatory increase in pancreatic beta cell cytoplasmic free Ca2+ concentration, [Ca2+]i, is a key feature in glucose-induced insulin release. The role of the voltage-gated Ca2+ channel beta3 subunit in the molecular regulation of these [Ca2+]i oscillations has now been clarified by using beta3 subunit-deficient beta cells. beta3 knockout mice showed a more efficient glucose homeostasis compared to wild-type mice due to increased glucose-stimulated insulin secretion. This resulted from an increased glucose-induced [Ca2+]i oscillation frequency in beta cells lacking the beta3 subunit, an effect accounted for by enhanced formation of inositol 1,4,5-trisphosphate (InsP3) and increased Ca2+ mobilization from intracellular stores. Hence, the beta3 subunit negatively modulated InsP3-induced Ca2+ release, which is not paralleled by any effect on the voltage-gated L type Ca2+ channel. Since the increase in insulin release was manifested only at high glucose concentrations, blocking the beta3 subunit in the beta cell may constitute the basis for a novel diabetes therapy.     

Phosphorylated inositol compounds in beta -cell stimulus-response coupling

Pancreatic beta-cell function is essential for the regulation of glucose homeostasis in humans, and its impairment leads to the development of type 2 diabetes. Inputs from glucose and cell surface receptors act together to initiate the beta-cell stimulus-response coupling that ultimately leads to the release of insulin. Phosphorylated inositol compounds have recently emerged as key players at all levels of the stimulus-secretion coupling process. In this current review, we seek to highlight recent advances in beta-cell phosphoinositide research by dividing our examination into two sections. The first involves the events that lead to insulin secretion. This includes both new roles for inositol polyphosphates, particularly inositol hexakisphosphate, and both conventional and 3-phosphorylated inositol lipids. In the second section, we deal with the more novel concept of the autocrine role of insulin. Here, released insulin initiates signal transduction cascades, principally through the activity of phosphatidylinositol 3-kinase. This new round of signal transduction has been established to activate key beta-cell genes, particularly the insulin gene itself. More controversially, this insulin feedback has also been suggested to either terminate or enhance insulin secretion events.     

Outer Hair Cell Lateral Wall Structure Constrains the Mobility of Plasma Membrane Proteins

Nature’s fastest motors are the cochlear outer hair cells (OHCs). These sensory cells use a membrane protein, Slc26a5 (prestin), to generate mechanical force at high frequencies, which is essential for explaining the exquisite hearing sensitivity of mammalian ears. Previous studies suggest that Slc26a5 continuously diffuses within the membrane, but how can a freely moving motor protein effectively convey forces critical for hearing? To provide direct evidence in OHCs for freely moving Slc26a5 molecules, we created a knockin mouse where Slc26a5 is fused with YFP. These mice and four other strains expressing fluorescently labeled membrane proteins were used to examine their lateral diffusion in the OHC lateral wall. All five proteins showed minimal diffusion, but did move after pharmacological disruption of membrane-associated structures with a cholesterol-depleting agent and salicylate. Thus, our results demonstrate that OHC lateral wall structure constrains the mobility of plasma membrane proteins and that the integrity of such membrane-associated structures are critical for Slc26a5’s active and structural roles. The structural constraint of membrane proteins may exemplify convergent evolution of cellular motors across species. Our findings also suggest a possible mechanism for disorders of cholesterol metabolism with hearing loss such as Niemann-Pick Type C diseases.     

Monomeric G-protein, Rhes, is not an imidazoline-regulated protein in pancreatic beta-cells

The monomeric G-protein, Rhes, is a candidate imidazoline-regulated molecule involved in mediating the insulin secretory response to efaroxan [S.L. Chan, L.K. Monks, H. Gao, P. Deaville, N.G. Morgan, Identification of the monomeric G-protein, Rhes, as an efaroxan-regulated protein in the pancreatic beta-cell, Br. J. Pharmacol. 136 (1) (2002) 31-36]. This suggestion was based on observations regarding changes in Rhes mRNA expression in rat islets and pancreatic beta-cells after prolonged culture with efaroxan, leading to desensitization of the insulin response to the compound. To verify this report, we have evaluated the effects of the imidazoline compounds efaroxan and BL11282 on Rhes mRNA expression in isolated rat pancreatic islets maintained in conditions identical to those used by Chan et al. The results demonstrate that desensitization of the insulin response to efaroxan, or to another imidazoline, BL11282, does not change Rhes mRNA expression levels. Transfection of MIN6 cells with plasmids containing Rhes or Rhes-antisense also does not alter efaroxan- or BL11282-induced insulin secretion. Together, these data do not support the hypothesis that Rhes is an imidazoline-regulated protein.