Blood flow into the brain is dynamically regulated to satisfy the changing metabolic requirements of neurons, but how this is accomplished has remained unclear. Here, we demonstrate a central role for capillary endothelial cells in sensing neural activity and communicating it to upstream arterioles in the form of an electrical vasodilatory signal. We further demonstrate that this signal is initiated by extracellular potassium (K+)—a byproduct of neural activity—which activates capillary endothelial cell inward-rectifier K+ (KIR2.1) channels to produce a rapidly propagating retrograde hyperpolarization that causes upstream arteriolar dilation, increasing blood flow into the capillary bed. Our results establish brain capillaries as an active sensory web that converts changes in external K+ into rapid, ‘inside-out’ electrical signaling to direct blood flow to active brain regions.

Maintaining constant blood flow in the face of fluctuations in blood pressure is a critical autoregulatory feature of cerebral arteries. An increase in pressure within the artery lumen causes the vessel to constrict through depolarization and contraction of the encircling smooth muscle cells. This pressure-sensing mechanism involves activation of two types of transient receptor potential (TRP) channels: TRPC6 and TRPM4. We provide evidence that the activation of the γ1 isoform of phospholipase C (PLCγ1) is critical for pressure sensing in cerebral arteries. Inositol 1,4,5-trisphosphate (IP3), generated by PLCγ1 in response to pressure, sensitized IP3 receptors (IP3Rs) to Ca2+ influx mediated by the mechanosensitive TRPC6 channel, synergistically increasing IP3R-mediated Ca2+ release to activate TRPM4 currents, leading to smooth muscle depolarization and constriction of isolated cerebral arteries. Proximity ligation assays demonstrated colocalization of PLCγ1 and TRPC6 with TRPM4, suggesting the presence of a force-sensitive, local signaling network comprising PLCγ1, TRPC6, TRPM4, and IP3Rs. Src tyrosine kinase activity was necessary for stretch-induced TRPM4 activation and myogenic constriction, consistent with the ability of Src to activate PLCγ isoforms. We conclude that contraction of cerebral artery smooth muscle cells requires the integration of pressure-sensing signaling pathways and their convergence on IP3Rs, which mediate localized Ca2+-dependent depolarization through the activation of TRPM4.

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), caused by dominant mutations in the NOTCH3 receptor in vascular smooth muscle, is a genetic paradigm of small vessel disease (SVD) of the brain. Recent studies using transgenic (Tg)Notch3 R169C mice, a genetic model of CADASIL, revealed functional defects in cerebral (pial) arteries on the surface of the brain at an early stage of disease progression. Here, using parenchymal arterioles (PAs) from within the brain, we determined the molecular mechanism underlying the early functional deficits associated with this Notch3 mutation. At physiological pressure (40 mmHg), smooth muscle membrane potential depolarization and constriction to pressure (myogenic tone) were blunted in PAs from TgNotch3 R169C mice. This effect was associated with an ∼60% increase in the number of voltage-gated potassium (K V ) channels, which oppose pressure-induced depolarization. Inhibition of K V 1 channels with 4-aminopyridine (4-AP) or treatment with the epidermal growth factor receptor agonist heparin-binding EGF (HB-EGF), which promotes K V 1 channel endocytosis, reduced K V current density and restored myogenic responses in PAs from TgNotch3 R169C mice, whereas pharmacological inhibition of other major vasodilatory influences had no effect. K V 1 currents and myogenic responses were similarly altered in pial arteries from TgNotch3 R169C mice, but not in mesenteric arteries. Interestingly, HB-EGF had no effect on mesenteric arteries, suggesting a possible mechanistic basis for the exclusive cerebrovascular manifestation of CADASIL. Collectively, our results indicate that increasing the number of K V 1 channels in cerebral smooth muscle produces a mutant vascular phenotype akin to a channelopathy in a genetic model of SVD.

Brain capillaries play a critical role in sensing neural activity and translating it into dynamic changes in cerebral blood flow to serve the metabolic needs of the brain. The molecular cornerstone of this mechanism is the capillary endothelial cell inward rectifier K (Kir2.1) channel, which is activated by neuronal activity-dependent increases in external K concentration, producing a propagating hyperpolarizing electrical signal that dilates upstream arterioles. Here, we identify a key regulator of this process, demonstrating that phosphatidylinositol 4,5-bisphosphate (PIP) is an intrinsic modulator of capillary Kir2.1-mediated signaling. We further show that PIP depletion through activation of G protein-coupled receptors (GPCRs) cripples capillary-to-arteriole signal transduction in vitro and in vivo, highlighting the potential regulatory linkage between GPCR-dependent and electrical neurovascular-coupling mechanisms. These results collectively show that PIP sets the gain of capillary-initiated electrical signaling by modulating Kir2.1 channels. Endothelial PIP levels would therefore shape the extent of retrograde signaling and modulate cerebral blood flow.

Rationale Acidosis is a powerful vasodilator signal in the brain circulation. However, the mechanisms by which this response occurs are not well understood, particularly in the cerebral microcirculation. One important mechanism to dilate cerebral (pial) arteries is by activation of large-conductance, calcium-sensitive potassium (BKCa) channels by local Ca2+ signals (Ca2+ sparks) through ryanodine receptors (RyRs). However, the role of this pathway in the brain microcirculation is not known. Objective The objectives of this study were to determine the mechanism by which acidosis dilates brain parenchymal arterioles (PAs) and to elucidate the roles of RyRs and BKCa channels in this response. Methods and Results Internal diameter and vascular smooth muscle cell (VSMC) Ca2+ signals were measured in isolated pressurized murine PAs, using imaging techniques. In physiological pH (7.4), VSMCs exhibited primarily RyR-dependent Ca2+ waves. Reducing external pH from 7.4 to 7.0 in both normocapnic and hypercapnic conditions decreased Ca2+ wave activity, and dramatically increased Ca2+ spark activity. Acidic pH caused a dilation of PAs which was inhibited by about 60% by BKCa channel or RyR blockers, in a non-additive manner. Similarly, dilator responses to acidosis were reduced by nearly 60% in arterioles from BKCa channel knockout mice. Dilations induced by acidic pH were unaltered by inhibitors of KATP channels or nitric oxide synthase. Conclusions These results support the novel concept that acidification, by converting Ca2+ waves to sparks, leads to the activation of BKCa channels to induce dilation of cerebral parenchymal arterioles.