Vascular calcium signalling and ageing

Abstract

Changes in cellular Ca²⁺ levels have major influences on vascular function and blood pressure regulation. Vascular smooth muscle cells (SMCs) and endothelial cells (ECs) orchestrate vascular activity in distinct ways, often involving highly specific fluctuations in Ca²⁺ signalling. Ageing is a major risk factor for cardiovascular diseases, but the impact of ageing per se on vascular Ca²⁺ signalling has received insufficient attention. We reviewed the literature for age-related changes in Ca²⁺ signalling in relation to vascular structure and function. Vascular tone dysregulation in several vascular beds has been linked to abnormal expression or activity of SMC voltage-gated Ca²⁺ channels, Ca²⁺ -activated K⁺ channels or TRPC6 channels. Some of these effects were linked to altered caveolae density, microRNA expression or 20-HETE abundance. Intracellular store Ca²⁺ handling was suppressed in ageing mainly via reduced expression of intracellular Ca²⁺ release channels, and Ca²⁺ reuptake or efflux pumps. An increase in mitochondrial Ca²⁺ uptake, leading to oxidative stress, could also play a role in SMC hypercontractility and structural remodelling in ageing. In ECs, ageing entailed diverse effects on spontaneous and evoked Ca²⁺ transients, as well as structural changes at the EC-SMC interface. The concerted effects of altered Ca²⁺ signalling on myogenic tone, endothelium-dependent vasodilatation, and vascular structure are likely to contribute to blood pressure dysregulation and blood flow distribution deficits in critical organs. With the increase in the world's ageing population, future studies should be directed at solving specific ageing-induced Ca²⁺ signalling deficits to combat the imminent accelerated vascular ageing and increased risk of cardiovascular diseases.

Keywords: ageing; blood flow dysregulation; calcium signalling; endothelium; hypertension; vascular dysfunction; vascular smooth muscle.

Figure 1.. Properties of small arteries and arterioles in relation to ageing

A, general vessel structure with a single endothelial cell layer (a.k.a. intima) towards the lumen and a smooth muscle layer (a.k.a. media) consisting of one cell layer (in arterioles) to several layers of smooth muscle cells in small arteries. Small arteries (<500 μm lumen diameter) and arterioles (<100 μm lumen diameter) are generally termed resistance vessels. For clarity, we have not shown an outer adventitia layer with peripheral nerves and fibroblasts. B, the concept of vascular tone is illustrated by a vessel with 50% reduction in active diameter compared with the fully relaxed state (i.e. 50% tone). C, schematic showing the possible effects of ageing in resistance vessels, and their likely causal relationship with age-related diseases.

Figure 2.. The prominent molecular players of endothelial cell Ca²⁺ handling and the impact of ageing

Top: The primary Ca²⁺ influx pathways in endothelial cells (ECs) include: TRP channels such as TRPV4 and TRPA1; the mechanosensitive Piezo1 channel; the ionotropic purinergic receptor P2X; and the Na⁺/Ca²⁺ exchanger (NCX) in the reverse mode. Ca²⁺ release from the endoplasmic reticulum (ER) is facilitated by IP₃R and can activate a Ca²⁺-sensitive target (K_Ca channels). Ca²⁺ influx by TRPV4 channel has also been linked to downstream activation of Ca²⁺-activated K⁺ channels. The efflux of Ca²⁺ across the plasma membrane and the reuptake into the ER are facilitated by the active transporters PMCA and SERCA, respectively. Bottom: EC Ca²⁺ handling and morphology are altered during ageing. Inset box summarizes key findings in studies that investigated the impact of ageing on Ca²⁺ transients (see text for details). A putative change (shown by ‘?’) should be further investigated.

Figure 3.. Targets of endothelial cell Ca²⁺ signals

Ca²⁺ is a crucial second messenger in endothelial cells (ECs) that affects several downstream signalling pathways. Cation influx depolarizes the membrane potential of ECs. Activation of the endothelial NO synthase (eNOS) leads to the generation of the potent vasodilator NO that dilates the neighbouring smooth muscle cells (SMCs). Microdomain Ca²⁺ signals (either through Ca²⁺ influx or release) can activate K_Ca channels leading to EC hyperpolarization that is transmitted to SMCs via gap junctions which ultimately induces vasodilation. Several protein kinases in ECs are Ca²⁺-activated such as PKC and CaM Kinase.

Figure 4.. Vascular smooth muscle model for Ca²⁺ signalling in young (top) vs. middle-aged or old (bottom) subjects

Normal Ca²⁺ signalling components in young vascular SMCs comprise voltage-gated Ca²⁺ channels (Ca_V1.2 L-type; Ca_V3.1, and Ca_V3.2 T-types), Transient Receptor Potential C6 channels (TRPC6), large-conductance Ca²⁺-activated K⁺ channels (BK_Ca), ryanodine receptor (RyR) and inositol-triphosphate (IP₃) receptor release channels, Ca²⁺-ATPase reuptake pump (SERCA) in the sarcoplasmic reticulum (SR) membrane, plasma membrane Ca²⁺-ATPase (PMCA) and Gα_q/11-coupled receptors (GPCR). Caveolae are shown as membrane invaginations in a limited area of the plasma membrane, for clarity. Mitochondria can release reactive oxygen species (ROS) under basal conditions. In middle-aged or old vascular smooth muscle cells (SMCs) the following mechanisms are changed, or a putative change (shown by ‘?’) should be investigated in further studies: the number of and area covered by caveolae are severely reduced, Ca²⁺ influx via Ca_V3.2 channels therefore causes less activation of RyR-mediated Ca²⁺ release and BK_Ca activation. Proximity of the SR to BK_Ca channels may be impaired via reduced function of junctophilin (JP) and microtubular networks (MTs). The SR is in general found at a deeper location in the cell, thereby preventing Ca_V3.2 channels from activating RyRs. Ca_V1.2 L-type channel expression is negatively affected by miR-155 and miR-328 expression. Putative regulation of TRPC6, T-type channels and BK_Ca channels by miRs should be investigated. Increased myogenic tone and intracellular [Ca²⁺] at relatively lower intravascular pressures (40–80 mm Hg) might be due to increased Ca_V3.1 activity (Björling et al. 2013). Loss of TRPC6 function in aged hypertensive animals is attributed to decreased Cyp4a activity and 20-HETE production. Ca²⁺ loading of mitochondria may lead to increased ROS release, causing increased activity of inflammatory mediators (INFL) (such as IL-1β, IL-6, and TNFα), growth factors (GFs) (such as IGF-1 and TGF-β1), and matrix metalloproteinases (MMPs) (such as MMP2 and MMP9). Reduced Ca²⁺ efflux and Ca²⁺ reuptake into the SR may be caused by crippled expression and/or activity of the key transporters involved.

Figure 5.. The impact of ageing on vascular Ca²⁺ signalling

Ageing impairs vascular function and myogenic tone. These changes are attributed partly to altered Ca²⁺ handling in vascular smooth muscle cells and endothelial cells. See text for further explanations and additional mechanisms. ‘?’ indicates future directions and unanswered questions.