TRPA1 Channel: New Kid in the ‘Neurovascular Coupling’ Town

Abstract

The just-in time delivery of oxygen and nutrients to active brain regions to support function (functional hyperemia; FH) is mediated by not yet fully understood mechanisms collectively referred to as ‘neurovascular coupling’ (NVC). In a recent publication (eLife 2021) Thakore et al. provide profound mechanistic insight how the capillary endothelial transient receptor potential ankyrin 1 (TRPA1) channel contribute to blood flow control and functional hyperemia in the brain.

In their latest study published in eLife, Thakore and colleagues [1] suggests that brain capillary transient receptor potential ankyrin 1 (TRPA1) channels serve as sensors of increased neural activity thereby adjusting local blood flow in concert with neuronal demand. Specifically, they support previous established neuron-to-vasculature regulatory mechanisms [2] and present novel evidence that TRPA1 channels also act as transducers of neuronal signals.

Cerebral blood flow (CBF) is finely controlled to match the changing neuronal metabolic needs. This on-demand process of efficient blood supply to areas of greater demand, a phenomenon termed functional hyperemia (FH), is finely tuned by a network of various cell types within the brain through mechanisms collectively termed ‘neurovascular coupling’ (NVC). Although functional hyperemia is firmly established concept [3], a consensus view of the cellular and molecular underlying mechanisms has remained elusive, largely owing to the inherent complexity of these hemodynamic processes and limitations in existing research strategies. Our recent work has demonstrated that brain capillaries, the narrowest of all blood vessels which lie in close proximity to all neurons [4], are exquisitely capable of sensing elevations in local external [K⁺]—a byproduct of neuronal activity—via capillary endothelial cell (cEC) inward-rectifier K⁺ (Kir2.1) channels, and convert this into a rapid retrograde electrical (hyperpolarizing) signal that propagates upstream along the vascular network, evoking vasodilation of feeding arterioles and thereby increasing local perfusion [2]. However, the integrative nature of NVC signaling events raises the exciting possibility of the coexistence of multiple and overlapping sensors in brain capillary cell components: cECs and pericytes. Although recent work has begun to characterize the key biophysical properties of capillary cells [2,5,6], their repertoire of functional ion channel species is as yet undefined.

TRPA1, the only mammalial ankyrin TRP subfamily member, functions as a large-conductance Ca²⁺ permeable non-selective cation channel [7]. Previous work has revealed the existence of functional TRPA1 channels in the endothelium of cerebral arteries/arterioles by optically recording elementary endothelial Ca²⁺ signals representing Ca²⁺ influx through single TRPA1 channels detected as ‘TRPA1 sparklets’ and thus, suggesting a key role for these channels in regulating CBF in health and disease states [8, 9]. In this compelling new study, electrophysiological recordings from native capillary ECs provided robust evidence for a TRPA1 current—induced by the endogenous agonist 4-HNE, blocked by the selective inhibitor HC-030031, and absent in cECs isolated from endothelial cell-specific TRPA1-knockout mice. Thakore et al. argue that TRPA1 channels act as optimal sensors of neuronal activity—that may respond to reactive oxygen species (ROS) released from astrocytes—by initiating a retrograde Ca²⁺ signal that travels to upstream arterioles in a remarkably slower velocity than that triggered by activation of Kir channels [2], thus implying a different signaling pathway that complements rapid and direct effects of external K⁺ on capillaries. Intriguingly, the data revealed that the velocity of vasodilation from the post-arteriole transitional zone—that is fully covered by pericytes—to the upstream parenchymal arteriole exhibited comparable kinetics to that elicited by a brief puff of elevated [K⁺] and thus, suggesting a mechanism that requires the activity of Kir2.1 channels. The authors showed that this is indeed the case.

Using our recently described ex vivo capillary-parenchymal arteriole preparation (CaPa; [2]) and in vivo two photon imaging, Thakore et al. provided a molecular mechanism for this cEC TRPA1-mediated short-range intercellular Ca²⁺ signal propagation phase involving the activation of cEC Pannexin 1 (Panx1) channels. This results in the subsequent release of ATP which in turn, binds and stimulates Ca²⁺-permeable ionotropic purinergic (P₂X) receptor channels on adjacent cells to enhance intracellular Ca²⁺ and propagate Ca²⁺ signals that will be next converted into a fast-conducting phase in pericyte-covered transitional segments to ultimately cause dilation of upstream parenchymal arterioles. The authors further revealed that the later mechanism results from the activation of small- (SK) and intermediate-conductance (IK) Ca²⁺-activated K⁺ channels residing in endothelial cells within the post-arteriole transition segment. This rapid-phase is thus generated in these transitional segments—including them into the ‘brain vascular sensory network’—and shares a common signal-propagation mechanism with Kir2.1 channels. Subsequent in vivo laser Doppler flowmetry experiments revealed that TRPA1 channels are required to sustain somatosensory stimulus-(whisker stimulation [WS]; 5 s) evoked cortical increases in relative CBF as the hyperemic response was blunted by TRPA1 inhibition/genetic ablation. Furthermore, the authors showed no effect of TRPA1 blockade under shorter WS (1–2 s), suggesting that these channels are only functioning as instigators of functional hyperemia during extended periods of neuronal activation.

This work provides a biphasic model of conducted vasodilation involving TRPA1 as a novel capillary-based initiator of FH that complements the rapid cEC-Kir2.1 propagating mechanism. FH is vital for brain health, and NVC deficiencies contribute to many neurological/cerebrovascular disorders including Alzheimer’s disease and small vessel diseases of the brain, primary causes of dementia and cognitive decline. However, many questions remain to fully understand the precise interactions among every single piece of the complex neurovascular puzzle, a needed answer to develop potential therapeutic opportunities for cerebrovascular dysfunction.

Figure 1.

Biphasic model of conducting vasodilation following activation of capillary endothelial TRPA1 channels proposed by Thakore et al. A. Schematic diagram of a brain section illustrating the microvascular preparations used for this study: obtained from the subcortical region supplied by the middle cerebral artery (MCA). B. Magnification of inset depicted in A. The authors propose that activation of TRPA1 channels in capillary endothelial cells (cECs) generates an electrical signal (Ca²⁺ wave) that slowly propagates through the capillary network, and it subsequently converts into a faster propagation hyperpolarizing signal when reaches the post–arteriole (pericyte–covered) transitional segment. C. Detailed molecular mechanisms of the slow and rapid phases of the proposed mechanism that ultimately leads to vasodilation of feeding arterioles and increase in regional cerebral blood flow (VCCC: voltage–gated Ca²⁺ channel; Panx1: Pannexin 1 channel; P₂X: Ca²⁺–permeable ionotropic purinergic receptor; SK: small–conductance Ca²⁺–activated K⁺ channel; IK: intermediate–conductance Ca²⁺–activated K⁺ channel; Kir2.1: inward-rectifier K⁺ channel; ROS: reactive oxygen species).