Calcium regulation in mouse mesencephalic neurons—Differential roles of Na+/Ca2+ exchanger, mitochondria and endoplasmic reticulum

Abstract

Midbrain dopaminergic (DA) neurons are the key to finely tune the voluntary movement, habit and motivation. The progressive and selective degeneration of these neurons is a pathological hallmark of Parkinson’s disease (PD). The susceptibility of DA neurons in the SNpc may result from differences in how Ca2+ is handled. However, very little information is available about the mechanisms involved in the regulation of intracellular Ca2+ concentration ([Ca2+]i) in DA neurons. In this study, the relative contributions of various Na+/Ca2+ exchangers and their interplay with internal Ca2+ stores, endoplasmic reticulum (ER) and the mitochondria, in the regulation of the [Ca2+]i of mouse mesencephalic neurons were characterized. Both the K+-dependent Na+/Ca2+ exchanger (NCKX) and the K+-independent Na+/Ca2+ exchanger (NCX) can be detected and are functional in DA and non-DA neurons. NCX accounts for the larger component of Na+/Ca2+ exchange activity. Single-cell RT-PCR analysis showed each individual neuron expressed a distinct set of the Na+/Ca2+ exchangers. Furthermore, the Na+/Ca2+ exchangers play prominent roles in removing [Ca2+]i induced by glutamate but not [Ca2+]i induced by depolarization. The mitochondria serve as a major Ca2+ sink and are functionally located close to NCX. In contrast, the ER is functionally located close to NCKX and acts primarily as a Ca2+ source with marginal effects. This study reveals that the Na+/Ca2+ exchangers, the ER and the mitochondria, which cooperate interactively, act similarly when regulating [Ca2+]i in mesencephalic DA and non-DA neurons. The heterogeneous expression of multiple types of Na+/Ca2+ exchangers and the quantitative differences found in [Ca2+]i regulation, together with other risk factors specific to DA neurons such as dopamine oxidation resulting in oxidative stress, may drive these cells to undergo selective degeneration.