Lysosome-Sarcoplasmic Reticulum Junctions: A Trigger Zone for Calcium Waves in Vascular Smooth Muscle

Abstract

We investigate a hypothesis for the genesis of nicotinic acid adenine dinucleotide phosphate (NAADP)-mediated Ca2+ waves in vascular smooth muscle. Agonist-stimulated waves of elevated cytoplasmic calcium concentration regulate blood vessel tone and vasomotion in vascular smooth muscle. In pulmonary artery smooth muscle cells, the calcium mobilizing messenger NAADP triggers bursts of Ca2+ release from lysosomes by activating the Two Pore Segment Channel subtype 2 (TPC2). These Ca2+ transients initiate a propagating wave by Ca2+-induced Ca2+ release (CICR) from the SR via ryanodine receptors (RyRs). Results from deconvolution and confocal microscopy studies, including immunofluorescence, suggest that lysosome clusters may selectively couple to RyR subtype 3 (RyR3) in regions where lysosomes and proximal SR are separated by a narrow cleft possibly <100 nm and certainly beyond the resolution of light microscopy. These results naturally lead to the hypothesis that lysosome-SR (L-SR) junctions may form a cytoplasmic trigger zone for the observed Ca2+ bursts and subsequent cell-wide Ca2+ waves. The present study combines prior optical microscopy observations with a thorough ultrastructural characterization of the L-SR junctions as input data for a quantitative model of the junction to test the above hypothesis. With this model, we simulate the Ca2+ bursts that may be generated within L-SR junctions to determine whether or not these bursts give rise to a sufficient increase in junctional [Ca2+] to breach the threshold for RyR3 activation by CICR and thus initiation of a propagating Ca2+ wave, and the degree to which this depends on the contribution to SR luminal Ca2+ load of concomitant SR Ca2+ uptake by sarco/endoplasmic reticulum Ca2+ ATPase (SERCA).