Role of sarcoplasmic/endoplasmic-reticulum Ca2+-ATPases in mediating Ca2+ waves and local Ca2+-release microdomains in cultured glia

Differential modulation of the phases of a Ca2+ spike by the store Ca2+-ATPase in human umbilical vein endothelial cells

1. Histamine-stimulated cytosolic free Ca2+ ([Ca2+]i) oscillations in human umbilical vein endothelial cells (HUVECs) comprise repetitive spikes generated by pulsatile release from stores. We have investigated the roles of the store Ca2+-ATPases in regulating both the upstroke and downstroke of a Ca2+ spike. 2. The sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) inhibitor cyclopiazonic acid (CPA) dramatically affected oscillations whereas inhibition of the plasma membrane Ca2+-ATPase (PMCA) with La3+ had little effect. This and other evidence suggested that the downstroke of a spike is predominantly mediated by SERCA. 3. Artificial [Ca2+]i spiking generated by repetitive pulsatile application of 0.3 microM histamine in Ca2+-free medium did not cause net loss of Ca2+ from the cell whereas repetitive pulsatile application of 1 and 10 microM histamine did, with the higher concentration being more effective. We conclude that there is an inverse relationship between stimulus intensity and relative SERCA activity. 4. For a Ca2+ transient, the initiation of release was suppressed by SERCA during either the lag phase or the interspike period (ISP) since: (i) the ISP was shortened by low CPA concentrations, (ii) higher concentrations of CPA stimulated an explosive Ca2+ release when applied during the ISP but not when applied in the absence of agonist, and (iii) CPA synchronized the initial Ca2+ response to a low histamine dose (even recruiting silent, histamine-unresponsive cells). 5. Two aspects of the regenerative upstroke of a spike were differently affected by SERCA inhibition: Ca2+ wave velocity was entirely unaffected by CPA whereas the local rate of rise was increased. 6. The [Ca2+]i at which a Ca2+ spike terminated depended on SERCA since CPA dose dependently enhanced the peak [Ca2+]i. 7. We conclude that SERCA plays a powerful and dynamic role in regulating [Ca2+]i oscillations in HUVECs. SERCA differentially modulates the phases of Ca2+ release in addition to bringing about the falling phase of a Ca2+ spike.

Calcium signalling in glial cells

Calcium signals are the universal way of glial responses to the various types of stimulation. Glial cells express numerous receptors and ion channels linked to the generation of complex cytoplasmic calcium responses. The glial calcium signals are able to propagate within glial cells and to create a spreading intercellular Ca2+ wave which allow information exchange within the glial networks. These propagating Ca2+ waves are primarily mediated by intracellular excitable media formed by intracellular calcium storage organelles. The glial calcium signals could be evoked by neuronal activity and vice versa they may initiate electrical and Ca2+ responses in adjacent neurones. Thus glial calcium signals could integrate glial and neuronal compartments being therefore involved in the information processing in the brain.

Images of Ca2+ flux in astrocytes: evidence for spatially distinct sites of Ca2+ release and uptake

In this study, we have developed a mathematical method to derive the Ca2+ fluxes underlying agonist-evoked Ca2+ waves in cultured rat cortical astrocytes. Astrocytes were stimulated with norepinephrine (100 nM) to evoke Ca2+ waves, which were recorded by measuring Fluo-3 fluorescence changes with high spatial and temporal resolution. Normalized fluorescence (delta F/F) was analyzed in discrete cellular spaces in a series of successive slices along the length of the cell. From these data, Ca2+ flux was then calculated using a one dimensional reaction-diffusion equation which utilizes the temporal and spatial derivatives of the fluorescence data and the diffusion coefficient of Ca2+ in the cytosol. This method identified distinct sites of positive flux (Ca2+ release into the cytosol) and of negative flux (Ca2+ removal from cytosol) and showed that in astrocytes, sites of Ca2+ release from stores regularly alternate with sites of Ca2+ removal from the cytosol. Cross correlation analysis of the two distribution patterns gave positive correlation at 2 microns out of phase and a negative correlation in phase. Thapsigargin-induced Ca2+ waves were analyzed to determine if the negative flux was due to Ca2+ uptake via thapsigargin-sensitive Ca2+ pumps. Negative flux sites were still found under these conditions, suggesting that multiple mechanisms of Ca2+ removal from the cytosol may contribute to negative flux sites. This method of calculation of flux may serve as a means to describe the distribution of functional ion channels and pumps participating in cellular Ca2+ signalling.

Mitochondrial Ca2+ uptake and release influence metabotropic and ionotropic cytosolic Ca2+ responses in rat oligodendrocyte progenitors

1. Many physiologically important activities of oligodendrocyte progenitor cells (O-2A cells), including proliferation, migration and differentiation, are regulated by cytosolic Ca2+ signals. However, little is known concerning the mechanisms of Ca2+ signalling in this cell type. We have studied the interactions between Ca2+ entry, Ca2+ release from endoplasmic reticulum and Ca2+ regulation by mitochondria in influencing cytosolic Ca2+ responses in O-2A cells. 2. Methacholine (MCh; 100 microM) activated Ca2+ waves that propagated from several initiation sites along O-2A processes. 3. During a Ca2+ wave evoked by MCh, mitochondrial membrane potential was often either depolarized (21 % of mitochondria) or hyperpolarized (20 % of mitochondria), as measured by changes in the fluorescence of 5,5',6,6'-tetrachloro-1,1',3, 3'-tetraethylbenzimidazole carbocyanine iodide (JC-1). 4. Stimulation with kainate (100 microM) evoked a slowly rising, sustained cytosolic Ca2+ elevation in O-2A cells. This also, in some cases, resulted in either a depolarization (15 % of mitochondria) or hyperpolarization (12 % of mitochondria) of mitochondrial membrane potential. 5. Simultaneous measurement of cytosolic (fluo-3 AM) and mitochondrial (rhod-2 AM) Ca2+ responses revealed that Ca2+ elevations in the cytosol evoked by either MCh or kainate were translated into long-lasting Ca2+ elevations in subpopulations of mitochondria. In some mitochondria, Ca2+ signals appeared to activate Ca2+ release into the cytosol. 6. Inhibition of the mitochondrial Na+-Ca2+ exchanger by CGP-37157 (25 microM) decreased kainate Ca2+ response amplitude and increased the rate of return of the response to basal Ca2+ levels. 7. Thus, both ionotropic and metabotropic stimulation evoke changes in mitochondrial membrane potential and Ca2+ levels in O-2A cells. Ca2+ uptake into some mitochondria is activated by Ca2+ entry into cells or release from stores. Mitochondrial Ca2+ release appears to play a key role in shaping kainate-evoked Ca2+ responses.

Role of mitochondrial Ca2+ regulation in neuronal and glial cell signalling

It is becoming increasingly clear that mitochondrial Ca2+ uptake from and release into the cytosol has important consequences for neuronal and glial activity. Ca2+ regulates mitochondrial metabolism, and mitochondrial Ca2+ uptake and release modulate physiological and pathophysiological cytosolic responses. In glial cells, inositol 1,4,5-trisphosphate-dependent Ca2+ responses are faithfully translated into elevations in mitochondrial Ca2+ levels, which modifies cytosolic Ca2+ wave propagation and may activate mitochondrial enzymes. The location of mitochondria within neurones may partially determine their role in Ca2+ signalling. Neuronal death due to NMDA-evoked Ca2+ entry can be delayed by an inhibitor of the mitochondrial permeability transition pore, and mitochondrial dysfunction is being increasingly implicated in a number of neurodegenerative conditions. These findings are illustrative of an emerging realization by neuroscientists of the importance of mitochondrial Ca2+ regulation as a modulator of cellular energetics, endoplasmic reticulum Ca2+ release and neurotoxicity.