Sildenafil prevents the up-regulation of transient receptor potential canonical channels in the development of cardiomyocyte hypertrophy

TRPC1 channels underlie stretch-modulated sarcoplasmic reticulum calcium leak in cardiomyocytes

Transient receptor potential canonical 1 (TRPC1) channels are Ca2+-permeable ion channels expressed in cardiomyocytes. An involvement of TRPC1 channels in cardiac diseases is widely established. However, the physiological role of TRPC1 channels and the mechanisms through which they contribute to disease development are still under investigation. Our prior work suggested that TRPC1 forms Ca2+ leak channels located in the sarcoplasmic reticulum (SR) membrane. Prior studies suggested that TRPC1 channels in the cell membrane are mechanosensitive, but this was not yet investigated in cardiomyocytes or for SR localized TRPC1 channels. We applied adenoviral transfection to overexpress or suppress TRPC1 expression in neonatal rat ventricular myocytes (NRVMs). Transfections were evaluated with RT-qPCR, western blot, and fluorescent imaging. Single-molecule localization microscopy revealed high colocalization of exogenously expressed TRPC1 and the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA2). To test our hypothesis that TRPC1 channels contribute to mechanosensitive Ca2+ SR leak, we directly measured SR Ca2+ concentration ([Ca2+]SR) using adenoviral transfection with a novel ratiometric genetically encoded SR-targeting Ca2+ sensor. We performed fluorescence imaging to quantitatively assess [Ca2+]SR and leak through TRPC1 channels of NRVMs cultured on stretchable silicone membranes. [Ca2+]SR was increased in cells with suppressed TRPC1 expression vs. control and Transient receptor potential canonical 1-overexpressing cells. We also detected a significant reduction in [Ca2+]SR in cells with Transient receptor potential canonical 1 overexpression when 10% uniaxial stretch was applied. These findings indicate that TRPC1 channels underlie the mechanosensitive modulation of [Ca2+]SR. Our findings are critical for understanding the physiological role of TRPC1 channels and support the development of pharmacological therapies for cardiac diseases.

Modulation of Calcium Transients in Cardiomyocytes by Transient Receptor Potential Canonical 6 Channels

Transient receptor potential canonical 6 (TRPC6) channels are non-selective cation channels that are thought to underlie mechano-modulation of calcium signaling in cardiomyocytes. TRPC6 channels are involved in development of cardiac hypertrophy and related calcineurin-nuclear factor of activated T cells (NFAT) signaling. However, the exact location and roles of TRPC6 channels remain ill-defined in cardiomyocytes. We used an expression system based on neonatal rat ventricular myocytes (NRVMs) to investigate the location of TRPC6 channels and their role in calcium signaling. NRVMs isolated from 1- to 2-day-old animals were cultured and infected with an adenoviral vector to express enhanced-green fluorescent protein (eGFP) or TRPC6-eGFP. After 3 days, NRVMs were fixed, immunolabeled, and imaged with confocal and super-resolution microscopy to determine TRPC6 localization. Cytosolic calcium transients at 0.5 and 1 Hz pacing rates were recorded in NRVMs using indo-1, a ratio-metric calcium dye. Confocal and super-resolution microscopy suggested that TRPC6-eGFP localized to the sarcolemma. NRVMs infected with TRPC6-eGFP exhibited higher diastolic and systolic cytosolic calcium concentration as well as increased sarcoplasmic reticulum (SR) calcium load compared to eGFP infected cells. We applied a computer model comprising sarcolemmal TRPC6 current to explain our experimental findings. Altogether, our studies indicate that TRPC6 channels play a role in sarcolemmal and intracellular calcium signaling in cardiomyocytes. Our findings support the hypothesis that upregulation or activation of TRPC6 channels, e.g., in disease, leads to sustained elevation of the cytosolic calcium concentration, which is thought to activate calcineurin-NFAT signaling and cardiac hypertrophic remodeling. Also, our findings support the hypothesis that mechanosensitivity of TRPC6 channels modulates cytosolic calcium transients and SR calcium load.

TRPC Channels in Cardiac Plasticity

The heart flexibly changes its structure in response to changing environments and oxygen/nutrition demands of the body. Increased and decreased mechanical loading induces hypertrophy and atrophy of cardiomyocytes, respectively. In physiological conditions, these structural changes of the heart are reversible. However, chronic stresses such as hypertension or cancer cachexia cause irreversible remodeling of the heart, leading to heart failure. Accumulating evidence indicates that calcium dyshomeostasis and aberrant reactive oxygen species production cause pathological heart remodeling. Canonical transient receptor potential (TRPC) is a nonselective cation channel subfamily whose multimodal activation or modulation of channel activity play important roles in a plethora of cellular physiology. Roles of TRPC channels in cardiac physiology have been reported in pathological cardiac remodeling. In this review, we summarize recent findings regarding the importance of TRPC channels in flexible cardiac remodeling (i.e., cardiac plasticity) in response to environmental stresses and discuss questions that should be addressed in the near future.

Specific Upregulation of TRPC1 and TRPC5 Channels by Mineralocorticoid Pathway in Adult Rat Ventricular Cardiomyocytes

Whereas cardiac TRPC (transient receptor potential canonical) channels and the associated store-operated Ca²⁺ entry (SOCE) are abnormally elevated during cardiac hypertrophy and heart failure, the mechanism of this upregulation is not fully elucidated but might be related to the activation of the mineralocorticoid pathway. Using a combination of biochemical, Ca²⁺ imaging, and electrophysiological techniques, we determined the effect of 24-h aldosterone treatment on the TRPCs/Orai-dependent SOCE in adult rat ventricular cardiomyocytes (ARVMs). The 24-h aldosterone treatment (from 100 nM to 1 µM) enhanced depletion-induced Ca²⁺ entry in ARVMs, as assessed by a faster reduction of Fura-2 fluorescence decay upon the addition of Mn²⁺ and increased Fluo-4/AM fluorescence following Ca²⁺ store depletion. These effects were prevented by co-treatment with a specific mineralocorticoid receptor (MR) antagonist, RU-28318, and they are associated with the enhanced depletion-induced N-[4-[3,5-Bis(trifluoromethyl)-1H-pyrazol-1-yl]phenyl]-4-methyl-1,2,3-thiadiazole-5-carboxamide (BTP2)-sensitive macroscopic current recorded by patch-clamp experiments. Molecular screening by qRT-PCR and Western blot showed a specific upregulation of TRPC1, TRPC5, and STIM1 expression at the messenger RNA (mRNA) and protein levels upon 24-h aldosterone treatment of ARVMs, corroborated by immunostaining. Our study provides evidence that the mineralocorticoid pathway specifically promotes TRPC1/TRPC5-mediated SOCE in adult rat cardiomyocytes.

Physiological and pathophysiological role of transient receptor potential canonical channels in cardiac myocytes

Transient receptor potential canonical (TRPC) channels constitute a family of seven Ca²⁺ permeable ion channels, named TRPC1 to 7. These channels are abundantly expressed in the mammalian heart, yet mechanisms underlying activation of TRPC channels and their precise role in cardiac physiology remain poorly understood. In this review, we perused original literature regarding TRPC channels in cardiomyocytes. We first reviewed studies on TRPC channel assembly and sub-cellular localization across multiple species and cell types. Our review indicates that TRPC localization in cardiac cells is still a topic of controversy. We then examined common molecular biology tools used to infer on location and physiological roles of TRPC channels in the heart. We subsequently reviewed pharmacological tools used to modulate TRPC activity in both cardiac and non-cardiac cells. Suggested physiological roles in the heart include modulation of heart rate and sensing of mechanical strain. We examined studies on the contribution of TRPC to cardiac pathophysiology, mainly hypertrophic signaling. Several TRPC channels, particularly TRPC1, 3 and 6 were proposed to play a crucial role in hypertrophic signaling. Finally, we discussed gaps in our understanding of the location and physiological role of TRPC channels in cardiomyocytes. Closing these gaps will be crucial to gain a full understanding of the role of TRPC channels in cardiac pathophysiology and to further explore these channels as targets for treatments for cardiac diseases, in particular, hypertrophy.

The parathyroid hormone family member TIP39 interacts with sarco/endoplasmic reticulum Ca2+ - ATPase activity by influencing calcium homoeostasis

Darier disease (DD) is a genetic skin disease that is associated with mutations in the ATP2A2 gene encoding the type 2 sarco/endoplasmic reticulum (ER) Ca²⁺ - ATPase (SERCA2). Mutations of this gene result in alterations of calcium homoeostasis, abnormal epidermal adhesion and dyskeratosis. Silencing of ATP2A2 in monolayer cell culture of keratinocytes reduces desmoplakin expression at the borders of cells and impacts cell adhesion. Here, we report establishment of a three-dimensional (3D) epidermal model of DD and use this model to evaluate peptide therapy with tuberoinfundibular peptide of 39 residues (TIP39) to normalize calcium transport. Gene silencing of ATP2A2 in keratinocytes grown in a 3D model resulted in dyskeratosis, partial parakeratosis and suprabasal clefts that resembled the histological changes seen in skin biopsies from patients with DD. TIP39, a peptide recently identified as a regulator of keratinocyte calcium transport, was then applied to this ATP2A2-silenced 3D epidermal model. In normal keratinocytes, TIP39 increased [Ca²⁺ ]_i through the inositol trisphosphate (IP3) receptor pathway and stimulated differentiation. In monolayer ATP2A2-silenced keratinocytes, although TIP39 increased cytosolic calcium from the ER, the response was incomplete compared with its control. TIP39 was observed to reduce intercellular clefts of the gene-silenced epidermal model but did not significantly upregulate keratinocyte differentiation genes such as keratin 10 and filaggrin. These findings indicate that TIP39 is a modulator of ER calcium signalling and may be used as a potential strategy for improving aspects of DD.

The phosphatidylinositol(4,5)bisphosphate-binding sequence of transient receptor potential channel canonical 4α is critical for its contribution to cardiomyocyte hypertrophy

Cardiomyocyte hypertrophy requires a source of Ca(2+) distinct from the Ca(2+) that regulates contraction. The canonical transient receptor potential channel (TrpC) family, a family of cation channels regulated by activation of phospholipase C (PLC), has been implicated in this response. Cardiomyocyte hypertrophy downstream of Gq-coupled receptors is mediated specifically by PLCβ1b that is scaffolded onto a SH3 and ankyrin repeat protein 3 (Shank3) complex at the sarcolemma. TrpC4 exists as two splice variants (TrpC4α and TrpC4β) that differ only in an 84-residue sequence that binds to phosphatidylinositol(4,5)bisphosphate (PIP2), the substrate of PLCβ1b. In neonatal rat cardiomyocytes, TrpC4α, but not TrpC4β, coimmunoprecipitated with both PLCβ1b and Shank3. Heightened PLCβ1b expression caused TrpC4α, but not TrpC4β, translocation to the sarcolemma, where it colocalized with PLCβ1b. When overexpressed in cardiomyocytes, TrpC4α, but not TrpC4β, increased cell area (893 ± 18 to 1497 ± 29 mm(2), P < 0.01) and marker gene expression (atrial natriuretic peptide increased by 409 ± 32%, and modulatory calcineurin inhibitory protein 1 by 315 ± 28%, P < 0.01). Dominant-negative TrpC4 reduced hypertrophy initiated by PLCβ1b, or PLCβ1b-coupled receptor activation, by 72 ± 8% and 39 ± 5 %, respectively. We conclude that TrpC4α is selectively involved in mechanisms downstream of PLCβ1b culminating in cardiomyocyte hypertrophy, and that the hypertrophic response is dependent on the TrpC4α splice variant-specific sequence that binds to PIP2.

TRPC6: physiological function and pathophysiological relevance

TRPC6 is a non-selective cation channel 6 times more permeable to Ca(2+) than to Na(+). Channel homotetramers heterologously expressed have a characteristic doubly rectifying current-voltage relationship and are directly activated by the second messenger diacylglycerol (DAG). TRPC6 proteins are also regulated by specific tyrosine or serine phosphorylation and phosphoinositides. Given its specific expression pattern, TRPC6 is likely to play a number of physiological roles which are confirmed by the analysis of a Trpc6 (-/-) mouse model. In smooth muscle Na(+) influx through TRPC6 channels and activation of voltage-gated Ca(2+) channels by membrane depolarisation is the driving force for contraction. Permeability of pulmonary endothelial cells depends on TRPC6 and induces ischaemia-reperfusion oedema formation in the lungs. TRPC6 was also identified as an essential component of the slit diaphragm architecture of kidney podocytes and plays an important role in the protection of neurons after cerebral ischaemia. Other functions especially in immune and blood cells remain elusive. Recently identified TRPC6 blockers may be helpful for therapeutic approaches in diseases with highly activated TRPC6 channel activity.