Takuya Iyoda

Preferential involvement of Na+/Ca2+ exchanger type-1 in the brain damage caused by transient focal cerebral ischemia in mice

The Na(+)/Ca(2+) exchanger (NCX), an ion-transporter located in the plasma membrane of neuronal cells, contributes to intracellular Ca(2+) homeostasis. Within the brain, three isoforms (NCX1, NCX2, and NCX3) are widely distributed. However, it is not clear to what extent these isoforms are involved in ischemic brain damage in mammals. We therefore used genetically altered mice and isoform-selective NCX inhibitors in a model of transient focal ischemia to investigate the role of each NCX isoform in ischemic brain damage. NCX isoform-mutant mice (NCX1(+/-), NCX2(+/-), and NCX3(+/-)) and wild-type mice were subjected to 90min of middle cerebral artery occlusion (MCAO) followed by 24h of reperfusion. One of three NCX inhibitors [SN-6, KB-R7943, or SEA0400 (3 or 10mgkg(-1), i.p.)] was administered to ddY mice at 30min before more prolonged (4-h) MCAO followed by 24h of reperfusion. After transient MCAO reperfusion, the cerebral infarcts in NCX1(+/-) mice, but not those in NCX2(+/-) or NCX3(+/-) mice, were significantly smaller than those in wild-type mice. SN-6 and SEA0400, which are more selective for the NCX1 isoform, significantly reduced the infarct volume at 10mg/kg. In contrast, KB-R7943, which is more selective for NCX3, did not. These results suggest that the NCX1 isoform may act preferentially (vs. the NCX2 and NCX3 isoforms) to exacerbate the cerebral damage caused by ischemic insult in mice, and that NCX1-selective inhibitors warrant investigation as a potential therapeutic agents for stroke.

Pretreatment of donor islets with the Na(+) /Ca(2+) exchanger inhibitor improves the efficiency of islet transplantation.

Pancreatic islet transplantation is an attractive therapy for the treatment of insulin‐dependent diabetes mellitus. However, the low efficiency of this procedure necessitating sequential transplantations of islets with the use of 2–3 donors for a single recipient, mainly due to the early loss of transplanted islets, hampers its clinical application. Previously, we have shown in mice that a large amount of HMGB1 is released from islets soon after their transplantation and that this triggers innate immune rejection with activation of DC, NKT cells and neutrophils to produce IFN‐γ, ultimately leading to the early loss of transplanted islets. Thus, HMGB1 release plays an initial pivotal role in this process; however, its mechanism remains unclear. Here we demonstrate that release of HMGB1 from transplanted islets is due to hypoxic damage resulting from Ca2+ influx into β cells through the Na+/Ca2+ exchanger (NCX). Moreover, the hypoxia‐induced β cell damage was prevented by pretreatment with an NCX‐specific inhibitor prior to transplantation, resulting in protection and long‐term survival of transplanted mouse and human islets when grafted into mice. These findings suggest a novel strategy with potentially great impact to improve the efficiency of islet transplantation in clinical settings by targeting donor islets rather than recipients.

Caveolin-3 regulates the Volume-Regulated Anion Channel in Mouse Ventricular Cells

Caveolae are small invaginated microdomains located with a variety of signal transduction molecules on the plasma membrane. Recent reports showed that the knockout mice of a muscle-specific protein caveolin-3 (Cav-3 KO), a principal component of the caveolae in heart, displayed an enlargement of ventricular cells. Volume-regulated outwardly rectifying anion channel (VRAC) is activated by membrane stretch, and play a significant role in cell volume regulation in cardiac cells. However, it is unknown the properties of VRAC in the enlarged cardiac cells from Cav-3 KO mice. In this study, we examined that VRAC current and the cell volume regulation in freshly isolated single ventricular cells from Cav-3 KO mice (Hagiwara et al. 2000). Whole-cell current recording showed that the density of VRAC current induced by extracellular hypotonic solution (HYPO) is markedly reduced in the cells from Cav-3 KO mice, compared to that from wild-type (WT) mice. Video-image analysis revealed that the degree of HYPO-induced cell swelling in Cav-3 KO mice is significantly bigger than that in WT mice, and the regulatory volume decrease, which was seen in WT cells after osmotic swelling, is almost lost in cells from Cav-3 KO mice. This result is in parallel with the VRAC inhibition. In contrast, acidic extracellular pH-activated chloride current and extracellular UTP-activated CFTR current were affected less by the deficiency of caveoline-3. The attenuated VRAC current was restored by intracellular application of a VRAC modulator, phosphatidylinositol 3,4,5-trisphosphate (PIP3). These findings suggested that the attenuation of cardiac VRAC current is due to the PIP3 depletion in Cav-3 KO mice.