Cerebral Vascular Muscle

Introduction

Cerebral blood flow (CBF) is controlled predominantly by the level of arterial pressure (perfusion pressure) and the diameter of resistance vessels in brain. The moment-to-moment regulation of cerebral arterial and arteriolar diameter, and thus CBF, is primarily the function of vascular muscle. Vascular muscle receives, integrates, and responds to mechanical forces as well as signals from endothelium, neurons, astrocytes, and other cell types. Intrinsic and extrinsic factors regulate the amount of tone that specific segments of the vasculature generate. These factors include contractile forces such as myogenic tone and myogenic reactivity along with molecules and pathways that produce vasodilation including nitric oxide (NO), reactive oxygen species (ROS), and activation of potassium ion channels. As such, a defect in vascular muscle may disrupt normal regulation of CBF and can have dire consequences for the brain. For example, defects in vascular muscle impact CBF and cognitive function in cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), the most common genetic cause of small vessel disease known. In this chapter, we summarize recent advances regarding the regulation of vascular tone in cerebral arteries and the microcirculation. As part of this overview and as proof of principle, we briefly discuss our current understanding of the clinical features and pathobiology of CADASIL.

Myogenic Tone: Role of TRP Channels

One inherent feature of vascular muscle is its ability to dynamically respond to changes in intraluminal or transmural pressure. Myogenic responses involve constriction of arteries and arterioles when intraluminal pressures rises and dilation as pressure drops. This characteristic of smooth muscle in resistance vessels is a major contributor to what is known as autoregulation, where CBF remains relatively stable over a wide range of perfusion pressures . However, despite extensive study, the mechanism by which changes in pressure is sensed by vascular muscle and translated into a change in vessel diameter is not entirely understood. Key aspects remain unclear including the identity of the mechanosensor as well as the precise signaling cascade that links these events. G-protein-coupled receptors, ion channels, cytoskeletal elements, and extracellular matrix components have all been suggested to have mechanosensor properties . Which of these protein(s) are of greatest importance is still debated. However, it is also possible that the mechanosensor and signal transduction pathway mediating the myogenic response varies between vascular beds, along the vascular tree, as well as in health versus disease.

In an attempt to better define these mechanisms, studies have begun to unravel the molecular details by which increased intravascular pressure translates to vasoconstriction ( Fig. 7.1 ). Transient receptor potential (TRP) channels are a family of nonselective cation channels that have recently become a focus of effort in regard to regulation of vascular function. There are 28 identified members of the TRP family grouped into six subfamilies based on sequence homology. The six TRP subfamilies are designated canonical (TRPC), vanilloid (TRPV), melestatin (TRPM), ankyrin (TRPA), mucolipin (TRPML), and polycystin (TRPP) . As of 2016, seven have been identified in cerebral vascular muscle (TRPC1, TRPC3, TRPC5, TRPC6, TRPV4, TRPM4, and TRPP2). The majority of these channels have been implicated in the regulation of vasoconstriction, particularly myogenic tone, with the exception of TRPV4, which has been shown to mediate vasodilation (see the following paragraphs). Three TRP channels are thought to contribute to myogenic vasoconstriction, TRPC6, TRPM4, and TRPP2 . The other described roles of TRP channels in regulating vascular muscle function are summarized in the Table 7.1 .

A 2015 study proposed a pressure-sensing signaling system that is dependent on TRPC6 and TRPM4. TRPC6 channel activation, which may occur via a phospholipase Cγ1 (PLCγ1)-dependent pathway or by direct mechanical activation, results in calcium influx and triggers further calcium release from the sarcoplasmic reticulum. The localized calcium event (calcium spark) activates TRPM4 channels resulting in membrane depolarization, the opening of voltage-dependent calcium channels (VDCCs) and vasoconstriction . In addition, but separate from TRPC6 and TRPM4 channel activation, TRPP2 channels have also been implicated in the generation of myogenic tone. Knockdown of TRPP2 channels significantly attenuated myogenic tone in isolated cerebral arteries .

Ultimately, the increase in intracellular calcium activates calcium/calmodulin and myosin light chain kinase, resulting in increased phosphorylation of myosin light chain and cell contraction. Calcium sensitization is also an important component of the myogenic response and involves activation of Rho kinase (ROCK). Inhibitors of ROCK dilate pressurized cerebral arteries and arterioles with myogenic tone. Two isoforms of ROCK are expressed in vascular cells (ROCK1 and ROCK2). We found recently that a selective ROCK2 inhibitor dilates pressurized cerebral parenchymal arterioles to the same extent as a nonselective inhibitor of both ROCK isoforms , suggesting ROCK2 has the greater importance in brain arterioles in relation to myogenic tone.

In addition to activating TRPM4 channels, calcium sparks activate large-conductance calcium-activated K (BK) channels. Opening of BK channels hyperpolarizes the cell, resulting in vasodilation. Thus, modulation of the myogenic response by BK channels prevents excessive vasoconstriction in response to rises in blood pressure or other stimuli.

Apart from TRP channels, the calcium-activated chloride channel, TMEM16A, has been implicated in the generation of myogenic tone. Activation of chloride channels causes chloride efflux, membrane depolarization, activation of VDCC, and contraction of vascular muscle. Inhibition of TMEM16A with specific antibodies or knockdown with siRNA significantly reduces development of myogenic tone in cerebral arteries.

As discussed in the preceding paragraphs cerebral arteries and particularly cerebral arterioles display a robust myogenic response. However, there are uncertainties surrounding the identity of the cellular proteins and pathways that mediate these responses. Further investigation is needed to determine which ion channels and/or other molecules are of most importance in both health and disease.