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Store-operated Ca(2+) entry was investigated in isolated mouse sinoatrial nodes (SAN) dissected from right atria and loaded with Ca(2+) indicators. Incubation of the SAN in Ca(2+)-free solution caused a substantial decrease in resting intracellular Ca(2+) concentration ([Ca(2+)](i)) and stopped pacemaker activity. Reintroduction of Ca(2+) in the presence of cyclopiazonic acid (CPA), a sarcoplasmic reticulum Ca(2+) pump inhibitor, led to sustained elevation of [Ca(2+)](i), a characteristic of store-operated Ca(2+) channel (SOCC) activity. Two SOCC antagonists, Gd(3+) and SKF-96365, inhibited 72+/-8% and 65+/-8% of this Ca(2+) influx, respectively. SKF-96365 also reduced the spontaneous pacemaker rate to 27+/-4% of control in the presence of CPA. Because members of the transient receptor potential canonical (TRPC) gene family may encode SOCCs, we used RT-PCR to examine mRNA expression of the 7 known mammalian TRPC isoforms. Transcripts for TRPC1, 2, 3, 4, 6, and 7, but not TRPC5, were detected. Immunohistochemistry using anti-TRPC1, 3, 4, and 6 antibodies revealed positive labeling in the SAN region and single pacemaker cells. These results indicate that mouse SAN exhibits store-operated Ca(2+) activity which may be attributable to TRPC expression, and suggest that SOCCs may be involved in regulating pacemaker firing rate.

During heart development the second heart field (SHF) provides progenitor cells for most cardiomyocytes and expresses the homeodomain factor Nkx2-5. We now show that feedback repression of Bmp2/Smad1 signaling by Nkx2-5 critically regulates SHF proliferation and outflow tract (OFT) morphology. In the cardiac fields of Nkx2-5 mutants, genes controlling cardiac specification (including Bmp2) and maintenance of the progenitor state were upregulated, leading initially to progenitor overspecification, but subsequently to failed SHF proliferation and OFT truncation. In Smad1 mutants, SHF proliferation and deployment to the OFT were increased, while Smad1 deletion in Nkx2-5 mutants rescued SHF proliferation and OFT development. In Nkx2-5 hypomorphic mice, which recapitulate human congenital heart disease (CHD), OFT anomalies were also rescued by Smad1 deletion. Our findings demonstrate that Nkx2-5 orchestrates the transition between periods of cardiac induction, progenitor proliferation, and OFT morphogenesis via a Smad1-dependent negative feedback loop, which may be a frequent molecular target in CHD.

Overexpression of IGF-1 in C2C12 myoblasts causes hypertrophy when myoblasts fuse to form myotubes, a response that requires elevated intracellular calcium. We show that myoblasts contain a store-operated Ca2+ channel (SOCC) whose activity is enhanced with IGF-1 overexpression. A membrane protein, CD20, can cause Ca2+ entry, which is increased by IGF-1. We therefore tested whether CD20 mediates the SOCC activity in myoblasts. An antibody to the extracellular loop of CD20 detected a protein in myoblasts and this antibody also inhibited Ca2+ entry through SOCC. Overexpression of CD20 in myoblasts increased SOCC activity. However, we could not detect mRNA for CD20 in myoblasts and an antibody to the intracellular C-terminus of CD20 was unable to detect CD20 in these cells. These studies demonstrate that CD20 is a novel SOCC or modulates SOCC activity. However, the SOCC activity observed in C2C12 myoblasts is mediated not by CD20, but by a CD20-like protein. Activation of this SOCC may contribute to IGF-1-induced hypertrophy in these cells., (Copyright 2003 Wiley-Liss, Inc.)

Many factors have been shown to be involved in the development of hyperplasic lesions of vessels, but the role of extracellular nucleotides remains largely unknown. The presence of P2Y and P2X nucleotide receptors on arterial endothelial and smooth muscle cells suggests a potential role for nucleotides in the vessel pathophysiology. Although the role of P2X in physiology of vessels is well documented, that of P2Y is not completely understood. We recently demonstrated that extracellular nucleotides, and particularly UTP, induced migration of cultured arterial smooth muscle cells (ASMCs). This migration is dependent on osteopontin expression and involves the Rho and mitogen-activated protein (MAP) kinase pathways. An important question is to determine the specific role of the different P2Y receptors of rat ASMCs in the UTP-induced migration process. Therefore, we first quantified mRNA levels of P2Y(2), P2Y(4), and P2Y(6) nucleotide receptors in cultured rat ASMCs by a competitive RT-PCR approach and demonstrated that P2Y(2) is the most highly expressed among these receptors potentially involved in the UTP-mediated response. In addition to UTP, UDP also induced ASMC migration even when UTP regeneration was inhibited, suggesting the involvement of UDP receptor P2Y(6). Moreover, suramin, a specific antagonist of rat P2Y(2) receptor, acted as an inhibitor of UTP-induced migration. Taken together, these results suggest a prominent role for the UTP receptor, P2Y(2), and for the UDP receptor, P2Y(6), in UTP-induced rat ASMC migration.

Migration and proliferation of arterial smooth muscle cells (SMCs) play a prominent role in the development of atherosclerotic plaques and restenosis lesions. Most of the growth-regulatory molecules potentially involved in these pathological conditions also demonstrate chemotactic properties. Extracellular purine and pyrimidine nucleotides have been shown to induce cell cycle progression and to elicit growth of cultured vascular SMCs. Moreover, the P2Y(2) ATP/UTP receptor was overexpressed in intimal thickening, suggesting a role of these nucleotides in vascular remodeling. Using the Transwell system migration assay, we demonstrate that extracellular ATP, UTP, and UDP exhibit a concentration-dependent chemotactic effect on cultured rat aortic SMCs. UTP, the most powerful nucleotide inducer of migration, elicited significant responses from 10 nmol/L. In parallel, UTP increased osteopontin expression dose-dependently. The blockade of osteopontin or its integrin receptors alpha(v)beta(3)/beta(5) by specific antibodies or antagonists inhibited UTP-induced migration. Moreover, the blockade of ERK-1/ERK-2 MAP kinase or rho protein pathways led to the inhibition of both UTP-induced osteopontin increase and migration, demonstrating the central role of osteopontin in this process. Taken together, these results suggest that extracellular nucleotides, and particularly UTP, can induce arterial SMC migration via the action of osteopontin.

Although mineral deposits have long been described to be a prominent feature of atherosclerosis, the mechanisms of arterial calcification are not well understood. However, accumulation of the non-collagenous matrix bone-associated proteins, osteopontin, osteocalcin, and osteonectin, has been demonstrated in atheromatous plaques. The aim of this study was to evaluate the role of these proteins in arterial calcification and, more precisely, during the initiation of this process. A model of rapid aortic calcification was developed in rabbits by an oversized balloon angioplasty. Calcification was followed using von Kossa staining and osteopontin, osteocalcin, and osteonectin were identified using immunohistochemistry. The aortic injury was rapidly followed by calcified deposits that appeared in the media as soon as 2 days after injury and then accumulated in zipper-like structures. Osteonectin was not detected in calcified deposits at any time after injury. In contrast, osteopontin and osteocalcin were detected in 8- and 14-day calcified structures, respectively, but not in the very early 2-day mineral deposits. These results suggest that these matrix proteins, osteopontin, osteocalcin, and osteonectin, are not involved in the initiation step of the aortic calcification process and that the former two might play a role in the regulation of arterial calcification.