Epithelial Ca2+ entry channels: transcellular Ca2+ transport and beyond

The recently discovered apical calcium channels CaT1 (TRPV6) and ECaC (TRPV5) belong to a family of six members called the 'TRPV family'. Unlike the other four members which are nonselective cation channels functioning as heat or osmolarity sensors in the body, CaT1 and ECaC are remarkably calcium-selective channels which serve as apical calcium entry mechanisms in absorptive and secretory tissues. CaT1 is highly expressed in the proximal intestine, placenta and exocrine tissues, whereas ECaC expression is most prominent in the distal convoluted and connecting tubules of the kidney. CaT1 in the intestine is highly responsive to 1,25-dihydroxyvitamin D₃ and shows both fast and slow calcium-dependent feedback inhibition to prevent calcium overload. In contrast, ECaC only shows slow inactivation kinetics and appears to be mostly regulated by the calcium load in the kidney. Outside the calcium-transporting epithelia, CaT1 is highly expressed in exocrine tissues such as pancreas, prostate and salivary gland. In these tissues it probably mediates re-uptake of calcium following its release by secretory vesicles. CaT1 also contributes to store-operated calcium entry in Jurkat T-lymphocytes and prostate cancer LNCaP cells, possibly in conjunction with other cellular components which link CaT1 activity to the filling state of the calcium stores. Finally, CaT1 expression is upregulated in prostate cancer and other cancers of epithelial origin, highlighting its potential as a target for cancer therapy.     

Modulation of Ca2+ release and Ca2+ oscillations in HeLa cells and fibroblasts by mitochondrial Ca2+ uniporter stimulation

The recent availability of activators of the mitochondrial Ca²⁺ uniporter allows direct testing of the influence of mitochondrial Ca²⁺ uptake on the overall Ca²⁺ homeostasis of the cell. We show here that activation of mitochondrial Ca²⁺ uptake by 4,4',4"-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) or kaempferol stimulates histamine-induced Ca²⁺ release from the endoplasmic reticulum (ER) and that this effect is enhanced if the mitochondrial Na⁺–Ca²⁺ exchanger is simultaneously inhibited with CGP37157. This suggests that both Ca²⁺ uptake and release from mitochondria control the ability of local Ca²⁺ microdomains to produce feedback inhibition of inositol 1,4,5-trisphosphate receptors (InsP₃Rs). In addition, the ability of mitochondria to control Ca²⁺ release from the ER allows them to modulate cytosolic Ca²⁺ oscillations. In histamine stimulated HeLa cells and human fibroblasts, both PPT and kaempferol initially stimulated and later inhibited oscillations, although kaempferol usually induced a more prolonged period of stimulation. Both compounds were also able to induce the generation of Ca²⁺ oscillations in previously silent fibroblasts. Our data suggest that cytosolic Ca²⁺ oscillations are exquisitely sensitive to the rates of mitochondrial Ca²⁺ uptake and release, which precisely control the size of the local Ca²⁺ microdomains around InsP₃Rs and thus the ability to produce feedback activation or inhibition of Ca²⁺ release.     

Increased Ca2+ influx through Na+/Ca2+ exchanger during long-term facilitation at crayfish neuromuscular junctions

Intense motor neuron activity induces a long-term facilitation (LTF) of synaptic transmission at crayfish neuromuscular junctions (NMJs) that is accompanied by an increase in the accumulation of presynaptic Ca²⁺ ions during a test train of action potentials. It is natural to assume that the increased Ca²⁺ influx during action potentials is directly responsible for the increased transmitter release in LTF, especially as the magnitudes of LTF and increased Ca²⁺ influx are positively correlated. However, our results indicate that the elevated Ca²⁺ entry occurs through the reverse mode operation of presynaptic Na⁺/Ca²⁺ exchangers that are activated by an LTF-inducing tetanus. Inhibition of Na⁺/Ca²⁺ exchange blocks this additional Ca²⁺ influx without affecting LTF, showing that LTF is not a consequence of the regulation of these transporters and is not directly related to the increase in [Ca²⁺]_i reached during a train of action potentials. Their correlation is probably due to both being induced independently by the strong [Ca²⁺]_i elevation accompanying LTF-inducing stimuli. Our results reveal a new form of regulation of neuronal Na⁺/Ca²⁺ exchange that does not directly alter the strength of synaptic transmission.