TRPV4 Channel Cooperativity in the Resistance Vasculature

Main Text

The cardiovascular system is composed of a muscular pump and a distribution network of arteries, veins, and capillaries. Within this integrated system, it is the responsibility of resistance arteries to regulate tissue perfusion. To effectively control the magnitude and distribution of organ blood flow, resistance arteries respond to a range of mechanical, neural, and metabolic stimuli. These stimuli alter arterial tone through cell-specific receptors whose signaling is intimately tied to the mobilization of Ca²⁺.

The study of Ca²⁺ dynamics in resistance vasculature is divisible into three distinct phases that parallel the advancement of fluorescence microscopy. It was ∼25 years ago that investigators first used ratiometric dyes to ascertain summative Ca²⁺ responses in arterial preparations. Sustained rises in smooth muscle [Ca²⁺] were tied to constriction, as a result of the ability of Ca²⁺ to mobilize myosin light chain kinase and inhibit myosin light chain phosphatase through its targeting subunit, MYPT1 (5). In striking contrast, global increases in endothelial [Ca²⁺] mediate vasodilation through downstream targets including nitric oxide synthase, and the small (SK) or intermediate (IK) conductance Ca²⁺-activated K⁺ channels (2,3). With improvements to Ca²⁺ indicator dyes and the use of confocal imaging, discrete Ca²⁺ signaling became evident, with the first discernable events in smooth muscle being Ca²⁺ sparks and Ca²⁺ waves (4). Both events originate from the sarcoplasmic reticulum, with sparks enabling dilation via the activation of large conductance Ca²⁺-activated K⁺ channels and waves linked to constriction by a mechanism not yet fully defined. Ca²⁺ waves have also been observed in endothelial cells along with so-called pulsars; both events activate IK channels and moderate smooth muscle hyperpolarization through the transfer of charge across myoendothelial gap junctions (6,10).

With the recent advances in rapid confocal imaging, along with total internal reflection fluorescence microscopy, investigators have started to dissect vascular Ca²⁺ events at the single-channel level. The group of Amberg et al. (1) was among the first to measure Ca²⁺ sparklets in smooth muscle, submembraneous events reflective of L-type Ca²⁺ channel activation. Similar events in 2012 were described in endothelial cells with the opening of TRPV4 channels, a member of the vanilloid subfamily of transient receptor potential (TRP) channels (8). Irrespective of the underlying channel, it is clear that Ca²⁺ sparklets are quantal events that arise when channels cluster display cooperative behavior. Why vascular channels exhibit cooperativity is not readily apparent, as it is a question difficult to address through biological experimentation.

It is in this context that the article by Parikh et al. (11), published in this issue of the Biophysical Journal, is timely. Using an established continuum model composed of an endothelial and smooth muscle cell, a cluster of TRPV4 channels, along with a variable number of IK channels, was incorporated into a microprojection, electrically linking the two cell types. Using distinct stochastic descriptions for TRPV4 channel flux, Parikh et al. (11) systematically described the consequence of cooperative behavior. They first observed that opening a single TRPV4 channel elicited a local Ca²⁺ increase with limited spatial spread; as such, this event only activated a small number of low-affinity IK channels. Enabling cooperativity produced larger Ca²⁺ sparklets whose properties mirrored experimental observations (8). These quantal events activated a broader pool of IK channels, eliciting endothelial-dependent hyperpolarizations whose magnitude rose with event frequency. To physiologists, the functional implications are clear. Channel cooperativity is essential for robust endothelial-dependent control of arterial tone and blood flow control.

Computational modeling can do more than highlight key quantitative relationships. It can illuminate areas of conceptual deficiency and aid in the design of biological experiments. In this regard, findings in this article highlight that a single cluster of TRPV4 channels is not sufficient, even with an unrealistic distribution of IK channels to elicit the substantive hyperpolarizations observed in a resistance artery (8). Such findings suggest that studies might be underestimating the number of TRPV4 clusters in a given endothelial cell. Alternatively, one could propose that another K⁺ conductance has yet to be fully implicated in the hyperpolarization process; logical candidates include SK and inward rectifying K⁺ channels (7).

In closing, the work of Parikh et al. (11) is opportune for a field where questions of clustered channel behaviors and their role in arterial tone development are forefront. The concepts developed herein can be used as a blueprint for other endothelial TRPs (e.g., TRPA1) and Ca²⁺ channel clusters in vascular smooth muscle (1,9). This article also shows that experimentation and computational modeling are natural partners and that deeper integrated insights develop when these tools are used together in a synergistic manner.