TRPV4 channels stimulate Ca ^{2 +} -induced Ca ²⁺ release in astrocytic endfeet and amplify neurovascular coupling responses

Abstract

In the CNS, astrocytes are sensory and regulatory hubs that play important roles in cerebral homeostatic processes, including matching local cerebral blood flow to neuronal metabolism (neurovascular coupling). These cells possess a highly branched network of processes that project from the soma to neuronal synapses as well as to arterioles and capillaries, where they terminate in "endfeet" that encase the blood vessels. Ca 2+ signaling within the endfoot mediates neurovascular coupling; thus, these functional microdomains control vascular tone and local perfusion in the brain. Transient receptor potential vanilloid 4 (TRPV4) channels-nonselective cation channels with considerable Ca 2+ conductance-have been identified in astrocytes, but their function is largely unknown. We sought to characterize the influence of TRPV4 channels on Ca 2+ dynamics in the astrocytic endfoot microdomain and assess their role in neurovascular coupling. We identified local TRPV4-mediated Ca 2+ oscillations in endfeet and further found that TRPV4 Ca 2+ signals are amplified and propagated by Ca 2+ -induced Ca 2+ release from inositol trisphosphate receptors (IP 3 Rs). Moreover, TRPV4-mediated Ca 2+ influx contributes to the endfoot Ca 2+ response to neuronal activation, enhancing the accompanying vasodilation. Our results identify a dynamic synergy between TRPV4 channels and IP 3 Rs in astrocyte endfeet and demonstrate that TRPV4 channels are engaged in and contribute to neurovascular coupling.calcium | parenchymal arteriole A strocytes are glial cells in the brain that are essential for the structural and functional integrity of the central nervous system. Astrocytes maintain cerebral homeostasis by acting as "switchboards," receiving and integrating communication from the surrounding microenvironment and translating that information into physiological and homeostatic responses. Numerous astrocytic projections make contact with neighboring synapses, while other projections terminate in "endfeet" that spread out and wrap around parenchymal arterioles and capillaries within the brain (1, 2). This structural orientation allows astrocytes to monitor synaptic activity in neuronal networks and mediate communication between neurons and the cerebral microcirculation.Calcium (Ca 2+ ) signaling is critical for astrocyte function. Transient increases in intracellular Ca 2+ concentration ([Ca 2+ ] i ) mediated by inositol 1,4,5-trisphosphate (IP 3 ) receptor Ca 2+ release channels (IP 3 Rs) in endoplasmic reticulum (ER) membranes drive the release of chemical transmitters like glutamate, adenosine triphosphate (ATP), and D-Serine that modulate synaptic transmission and neuronal excitability (3, 4). IP 3 Rdependent Ca 2+ signaling in astrocytes is also critical for neurovascular coupling (NVC), the process by which local cerebral blood flow (CBF) is matched to neuronal metabolism (5, 6). As neuronal activity increases, synaptically released glutamate binds to metabotropic glutamate receptors (mGluRs) on perisynaptic astrocytic projections, stimula...

Astrocyte Contributions to Flow/Pressure-Evoked Parenchymal Arteriole Vasoconstriction

“…Moreover, results showing blunted PA myogenic tone and decreased ability of PAs to sustain increases in tone during a hemodynamic challenge upon chelation of astrocytic Ca 2ϩ suggest that astrocytes are active regulators of basal cerebrovascular tone and can also modulate the magnitude and duration of flow/pressureevoked vasoconstrictions. Consistent with the notion that TRPV4 channels serve as the hemodynamic sensor and transducer and corroborating previous findings (Benfenati et al, 2007(Benfenati et al, , 2011Dunn et al, 2013), we find TRPV4 channels to be strategically expressed on the adventitial side of PAs, where they colocalize with astrocytic endfeet and processes. Conversely, and in contrast to pial arterioles (Earley et al, 2005;Marrelli et al, 2007;Zhang et al, 2013), we show that TRPV4 channel expression is not evident in the endothelium of PAs, further supporting a role for astrocytic TRPV4 channels in hemodynamic-induced changes in vascular tone.…”

“…Increased EC shear stress results in the release of arachidonic acid (AA) and downstream eicosanoids, such as epoxyeicosatrienoic acids (EETs). While AA does not directly activate TRVP4 channels (it requires metabolism; Watanabe et al, 2003), EETs are a downstream putative endogenous TRPV4 channel agonist (Watanabe et al, 2003;Dunn et al, 2013). In support of this idea we previously showed that EETs mediated biphasic PA vascular responses and increased Ca 2ϩ activity in astrocytes (Blanco et al, 2008).…”

“…TPRV4 channels can be activated by osmotic, physical (cell swelling, warmth, mechanical), and chemical (endocannabinoids, arachidonic acid metabolites, 4-␣-phorbol esters) stimuli (Vriens et al, 2004;Plant and Strotmann, 2007), making them attractive candidates to be influenced by hemodynamic variables (e.g., pressure/flow) and contributors to Ca 2ϩ -dependent gliovascular signals. TRPV4-dependent astrocyte Ca 2ϩ elevations during NVC were recently reported (Dunn et al, 2013). Thus, we tested whether astrocytic TRPV4 channels contribute to hemodynamic-evoked changes in PA tone.…”

Rapid stimulus-evoked astrocyte Ca 2+ elevations and hemodynamic responses in mouse somatosensory cortex in vivo

“…This association was further supported by the observation of fast astrocytic Ca 2+ responses in end-feet before dilation of an adjacent vessel. The rise in astrocytic Ca 2+ reported here is similar to what has been reported for slice preparations associated with local vasodilatation (14,43). We suggest that stimulation-induced Ca 2+ increase may provide the rapid neurovascular coupling needed to adapt metabolic activity to fluctuations in neuronal network activity.…”

Neuronal networks and mediators of cortical neurovascular coupling responses in normal and altered brain states

“…R. Soc. B 371: 20150350 epoxyeicosatrienoic acids (EETs) then dilate blood vessels, possibly through activation of the vanilloid transient receptor potential 4 (TRPV4) channels [125]. As revealed by the activity maps, GABA, probably released from activated VIP-containing GABA interneurons, was also required for the full expression of the NVC response to whisker stimulation [49] (figure 3).…”