Polarized expression of Ca2+ pumps in pancreatic and salivary gland cells. Role in initiation and propagation of [Ca2+]i waves.

Min Goo Lee,Weizhong Zeng,Xin Xu,Frank Wuytack,Shmuel Muallem,Julie Diaz,Tuan H Kuo,Luc Racymaekers

doi:10.1074/jbc.272.25.15771

Abstract

The present study was aimed at localization of plasma membrane (PMCA) and intracellular (SERCA) Ca2+ pumps and characterizing their role in initiation and propagation of Ca2+ waves. Specific and polarized expression of Ca2+ pumps was observed in all epithelial cells examined. Immunolocalization revealed expression of PMCA in both the basolateral and luminal membranes of all cell types. SERCA2a appeared to be expressed in the luminal pole, whereas SERCA2b was expressed in the basal pole and the nuclear envelope of pancreatic acini. Interestingly, SERCA2b was found in the luminal pole of submandibular salivary gland acinar and duct cells. These cells expressed SERCA3 in the basal pole. To examine the significance of the polarized expression of SERCA and perhaps PMCA pumps in secretory cells, we compared the effect of inhibition of SERCA pumps with thapsigargine and partial Ca2+ release with ionomycin on Ca2+ release evoked by agonists and Ca2+ uptake induced by antagonists. Despite their polarized expression, Ca2+ uptake by SERCA pumps and Ca2+ efflux by PMCA resulted in uniform reduction in [Ca2+]i. Surprisingly, inhibition of the SERCA pumps, but not Ca2+ release by ionomycin, eliminated the distinct initiation sites and propagated Ca2+ waves, leading to a uniform increase in [Ca2+]i. In addition, inhibition of SERCA pumps reduced the rate of Ca2+ release from internal stores. The implication of these findings to rates of Ca2+ diffusion in the cytosol, compartmentalization of Ca2+ signaling complexes, and mechanism of Ca2+ wave propagation are discussed.

Highlights

The present study was aimed at localization of plasma membrane (PMCA) and intracellular (SERCA) Ca2؉ pumps and characterizing their role in initiation and propagation of Ca2؉ waves
SERCA2a appeared to be expressed in the luminal pole, whereas SERCA2b was expressed in the basal pole and the nuclear envelope of pancreatic acini
To examine the significance of the polarized expression of SERCA and perhaps PMCA pumps in secretory cells, we compared the effect of inhibition of SERCA pumps with thapsigargine and partial Ca2؉ release with ionomycin on Ca2؉ release evoked by agonists and Ca2؉ uptake induced by antagonists

Summary

Introduction

The present study was aimed at localization of plasma membrane (PMCA) and intracellular (SERCA) Ca2؉ pumps and characterizing their role in initiation and propagation of Ca2؉ waves. SERCA2b was found in the luminal pole of submandibular salivary gland acinar and duct cells These cells expressed SERCA3 in the basal pole. In recent years it has become clear that [Ca2ϩ]i changes occur in the form of elemental events emanating in specialized microdomains and propagating as Ca2ϩ waves (see reviews in Ref. 2) Such behavior was described in pancreatic acinar cells [3,4,5,6] in which the [Ca2ϩ]i wave is initiated in the luminal pole (LP). A recent study in which external [Ca2ϩ] was measured suggested that Ca2ϩ pumping across the luminal membrane dominates Ca2ϩ efflux in stimulated acinar cells [11] This predicts that the luminal membrane expresses high levels of PMCA. Whether the cells express other SERCA isoforms and the localization of the SERCA pumps in pancreatic acini or SMG cells is not known

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Conclusion