Intracellular Ca2+ signalling: unexpected new roles for the usual suspect

Abstract

Cytosolic Ca²⁺ signals are organized in complex spatial and temporal patterns that underlie their unique ability to regulate multiple cellular functions. Changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) are finely tuned by the concerted interaction of membrane receptors and ion channels that introduce Ca²⁺ into the cytosol, Ca²⁺-dependent sensors and effectors that translate the elevation in [Ca²⁺]_i into a biological output, and Ca²⁺-clearing mechanisms that return the [Ca²⁺]_i to pre-stimulation levels and prevent cytotoxic Ca²⁺ overload. The assortment of the Ca²⁺ handling machinery varies among different cell types to generate intracellular Ca²⁺ signals that are selectively tailored to subserve specific functions. The advent of novel high-speed, 2D and 3D time-lapse imaging techniques, single-wavelength and genetic Ca²⁺ indicators, as well as the development of novel genetic engineering tools to manipulate single cells and whole animals, has shed novel light on the regulation of cellular activity by the Ca²⁺ handling machinery. A symposium organized within the framework of the 72nd Annual Meeting of the Italian Society of Physiology, held in Bari on 14-16th September 2022, has recently addressed many of the unexpected mechanisms whereby intracellular Ca²⁺ signalling regulates cellular fate in healthy and disease states. Herein, we present a report of this symposium, in which the following emerging topics were discussed: 1) Regulation of water reabsorption in the kidney by lysosomal Ca²⁺ release through Transient Receptor Potential Mucolipin 1 (TRPML1); 2) Endoplasmic reticulum-to-mitochondria Ca²⁺ transfer in Alzheimer's disease-related astroglial dysfunction; 3) The non-canonical role of TRP Melastatin 8 (TRPM8) as a Rap1A inhibitor in the definition of some cancer hallmarks; and 4) Non-genetic optical stimulation of Ca²⁺ signals in the cardiovascular system.

Keywords: Ca2+ signalling; TRP channels; lysosomal Ca2+; mitochondria-ER contact sites; non-canonical signalling; optical stimulation.

FIGURE 1

Schematic diagram showing the effect of TRPML1 activation on AQP2-mediated water reabsorption in mouse renal collecting duct cells. ML-SA1 triggers TRPML1-dependent local Ca²⁺ events that are sustained by the endoplasmic reticulum (ER) Ca²⁺ content. Activation of the Ca²⁺/calcineurin/NFAT pathway determines depolymerization of the actin cytoskeleton, thus leading to accumulation of AQP2 at the apical plasma membrane and enhancing water membrane permeability. The putative role of lysosomal Ca²⁺ signaling events as switch for changes in AQP2 expression level through the modulation of the transcriptional activity of NFAT needs further investigation (question mark). Created with BioRender.com (agreement number: FY259UYCKW).

FIGURE 2

Proposed scheme relationships between AD-related mutations, mitochondrial-ER interaction, mitochondrial and ER Ca²⁺ signaling, and cellular dysfunctions in astrocytes. Altered ER-mitochondrial interaction impairs ER-mitochondrial Ca²⁺ transfer, resulting in mitochondrial bioenergetic deficit and increased production of ROS, induction of a low-grade chronic ER stress and derangement of proteins synthesis and degradation. Cellular dysproteostasis results in an impaired secretion of factors including adhesion molecules, components of extracellular matrix, pro-neurogenic and neuroprotective molecules. Altogether, this impairs homeostatic and signaling activity of AD astrocytes eventually leading to impairment of synaptic functions, blood-brain barrier integrity and to development of neurodegeneration.

FIGURE 3

Schematic representation of TRPM8 subcellular localization and activity in cancer cells. TRPM8 Full length isoform localizes at the plasma membrane and is subjected to androgen regulation. Smaller isoforms typically localize in the ER and mediate Ca²⁺ release in the cytosol or Ca²⁺ transfer in the mitochondria. TRPM8 also act independently from its channel activity as an inhibitor of the small GTPase Rap1A thus inhibiting cell adhesion and migration. Created with BioRender.com (agreement number: AT259UYHZ3).

FIGURE 4

Geneless light stimulation of Ca²⁺ signals in cardiac cells. (A) Polymer-mediated optical excitation induces a robust enhancement of proliferation and bidimensional tube formation in ECFCs seeded on top of P3HT thin films (λ = 520 nm). ECFC modulation in ECFCs requires TRPV1 activation on the plasma membrane, which in turn mediates extracellular Ca²⁺ entry to engage a NF-kB-dependent gene expression program. (B) Ziapin2 internalizes into the plasma membrane of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). Upon photoexcitation (λ = 470 nm) the molecule isomerizes, changing hiPSC-CMs membrane capacitance. This triggers action potential generation and consequently modulates the “excitation-contraction coupling” process at a whole extent opening a new way towards hybrid soft robotics and heart disease therapies. Adapted from (Vurro et al., 2023a). Created with BioRender.com (agreement number: RO259UYLP6).