‘Quantal’ Ca2+release reassessed - a clue to oscillation and synchronization

Tumor-specific cholinergic CD4+ T lymphocytes guide immunosurveillance of hepatocellular carcinoma

Cholinergic nerves are involved in tumor progression and dissemination. In contrast to other visceral tissues, cholinergic innervation in the hepatic parenchyma is poorly detected. It remains unclear whether there is any form of cholinergic regulation of liver cancer. Here, we show that cholinergic T cells curtail the development of liver cancer by supporting antitumor immune responses. In a mouse multihit model of hepatocellular carcinoma (HCC), we observed activation of the adaptive immune response and induction of two populations of CD4+ T cells expressing choline acetyltransferase (ChAT), including regulatory T cells and dysfunctional PD-1+ T cells. Tumor antigens drove the clonal expansion of these cholinergic T cells in HCC. Genetic ablation of Chat in T cells led to an increased prevalence of preneoplastic cells and exacerbated liver cancer due to compromised antitumor immunity. Mechanistically, the cholinergic activity intrinsic in T cells constrained Ca2+-NFAT signaling induced by T cell antigen receptor engagement. Without this cholinergic modulation, hyperactivated CD25+ T regulatory cells and dysregulated PD-1+ T cells impaired HCC immunosurveillance. Our results unveil a previously unappreciated role for cholinergic T cells in liver cancer immunobiology.

Quantal Ca2+ release mediated by very few IP3 receptors that rapidly inactivate allows graded responses to IP3

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are intracellular Ca²⁺ channels that link extracellular stimuli to Ca²⁺ signals. Ca²⁺ release from intracellular stores is "quantal": low IP₃ concentrations rapidly release a fraction of the stores. Ca²⁺ release then slows or terminates without compromising responses to further IP₃ additions. The mechanisms are unresolved. Here, we synthesize a high-affinity partial agonist of IP₃Rs and use it to demonstrate that quantal responses do not require heterogenous Ca²⁺ stores. IP₃Rs respond incrementally to IP₃ and close after the initial response to low IP₃ concentrations. Comparing functional responses with IP₃ binding shows that only a tiny fraction of a cell's IP₃Rs mediate incremental Ca²⁺ release; inactivation does not therefore affect most IP₃Rs. We conclude, and test by simulations, that Ca²⁺ signals evoked by IP₃ pulses arise from rapid activation and then inactivation of very few IP₃Rs. This allows IP₃Rs to behave as increment detectors mediating graded Ca²⁺ release.

Endogenous TRPV1 stimulation leads to the activation of the inositol phospholipid pathway necessary for sustained Ca2+ oscillations

Sensory neuron subpopulations as well as breast and prostate cancer cells express functional transient receptor potential vanilloid type 1 (TRPV1) ion channels; however little is known how TRPV1 activation leads to biological responses. Agonist-induced activation of TRPV1 resulted in specific spatiotemporal patterns of cytoplasmic Ca²⁺ signals in breast and prostate cancer-derived cells. Capsaicin (CAPS; 50μM) evoked intracellular Ca²⁺ oscillations and/or intercellular Ca²⁺ waves in all cell lines. As evidenced in prostate cancer Du 145 cells, oscillations were largely dependent on the expression of functional TRPV1 channels in the plasma membrane, phospholipase C activation and on the presence of extracellular Ca²⁺ ions. Concomitant oscillations of the mitochondrial matrix Ca²⁺ concentration resulted in mitochondria energization evidenced by increased ATP production. CAPS-induced Ca²⁺ oscillations also occurred in a subset of sensory neurons, yet already at lower CAPS concentrations (1μM). Stimulation of ectopically expressed TRPV1 channels in CAPS-insensitive NIH-3T3 cells didn't provoke CAPS-triggered Ca²⁺ oscillations; rather it resulted in low-magnitude, long-lasting elevations of the cytosolic Ca²⁺ concentration. This indicates that sole TRPV1 activation is not sufficient to generate Ca²⁺ oscillations. Instead the initial TRPV1-mediated signal leads to the activation of the inositol phospholipid pathway. This in turn suffices to generate a biologically relevant frequency-modulated Ca²⁺ signal.

Ion channel activities in neural stem cells of the neuroepithelium

During the embryonic development of the central nervous system, neuroepithelial cells act as neural stem cells. They undergo interkinetic nuclear movements along their apico-basal axis during the cell cycle. The neuroepithelial cell shows robust increases in the nucleoplasmic [Ca²⁺] in response to G protein-coupled receptor activation in S-phase, during which the nucleus is located in the basal region of the neuroepithelial cell. This response is caused by Ca²⁺ release from intracellular Ca²⁺ stores, which are comprised of the endoplasmic reticulum and the nuclear envelope. The Ca²⁺ release leads to the activation of Ca²⁺ entry from the extracellular space, which is called capacitative, or store-operated Ca²⁺ entry. These movements of Ca²⁺ are essential for DNA synthesis during S-phase. Spontaneous Ca²⁺ oscillations also occur synchronously across the cells. This synchronization is mediated by voltage fluctuations in the membrane potential of the nuclear envelope due to Ca²⁺ release and the counter movement of K⁺ ions; the voltage fluctuation induces alternating current (AC), which is transmitted via capacitative electrical coupling to the neighboring cells. The membrane potential across the plasma membrane is stabilized through gap junction coupling by lowering the input resistance. Thus, stored Ca²⁺ ions are a key player in the maintenance of the cellular activity of neuroepithelial cells.

Synchronization of Ca2+ oscillations: a capacitative (AC) electrical coupling model in neuroepithelium

Increases in intracellular [Ca(2+)] occur synchronously between cells in the neuroepithelium. If neuroepithelial cells were capable of generating action potentials synchronized by gap junctions (direct current electrical coupling), the influx of Ca(2+) through voltage-activated Ca(2+) channels would lead to a synchronous increase in intracellular [Ca(2+)]. However, no action potential is generated in neuroepithelial cells, and the [Ca(2+)] increase is instead produced by the release of Ca(2+) from intracellular Ca(2+) stores. Recently, synchronous fluctuations in the membrane potential of Ca(2+) stores were recorded using an organelle-specific voltage-sensitive dye. On the basis of these recordings, a capacitative [alternating current (AC)] electrical coupling model for the synchronization of voltage fluctuations of Ca(2+) store potential was proposed [Yamashita M (2006) FEBS Lett580, 4979-4983; Yamashita M (2008) FEBS J275, 4022-4032]. Ca(2+) efflux from the Ca(2+) store and K(+) counterinflux into the store cause alternating voltage changes across the store membrane, and the voltage fluctuation induces ACs. In cases where the store membrane is closely apposed to the plasma membrane and the cells are tightly packed, which is true of neuroepithelial cells, the voltage fluctuation of the store membrane is synchronized between the cells by the AC currents through the series capacitance of these membranes. This article provides a short review of the model and its relationship to the structural organization of the Ca(2+) store. This is followed by a discussion of how the mode of synchronization of [Ca(2+)] increase may change during central nervous system development and new molecular insights into the synchronicity of [Ca(2+)] increase.