Optogenetic stimulation of pericytes lacking alpha smooth muscle actin produces a decrease in capillary blood flow in the living mouse brain

Abstract

Impaired cerebral blood flow contributes to the pathogenesis of numerous neurological diseases, ranging from dementia to traumatic brain injury. It is critical that we understand the cell types that control cerebral blood flow, so that we can target them to correct blood flow impairments and improve the trajectory of these diseases. Although it is accepted that vascular smooth muscle cells encircling arterioles can control blood flow, it is debated whether pericytes, which collectively adorn over 90% of the brain's vascular length, can also influence blood flow. To clarify the ability of pericytes to influence blood flow, we stimulated channelrhodopsin (ChR2) in individual pericytes using two photon illumination, as done previously (Hill, et al. Neuron 2015). Although we used mice that express ChR2 in both smooth muscle cells and pericytes, the spatial specificity of two photon excitation permitted selective optogenetic stimulation of individual pericytes, thus avoiding incidental activation of nearby vascular smooth muscle cells that also express ChR2. Simultaneously, with the same two photon laser used for excitation of ChR2 in pericytes, we measured capillary diameter and red blood cell velocity (RBCV) using intravenous dextrans. We found that pericyte excitation for 60 seconds produced a ~20% reduction in diameter and RBCV, indicating that pericytes can indeed influence blood flow. Surprisingly, the magnitude and kinetics of the decreases in diameter and RBCV were not different between pericytes with and without expression of alpha smooth muscle actin, a protein believed to be required for vascular contraction and blood flow control. Importantly, identical stimulation parameters did not produce hemodynamic changes in control mice with cytosolic YFP or membrane-bound GFP in smooth muscle cells and pericytes, indicating that our observed reductions in blood flow in ChR2 mice were not an artifactual result of light illumination, animal movement, or tissue damage. Further, the observed decrease in velocity and diameter in response to pericyte ChR2 stimulation was inhibited when we applied fasudil, a Rho kinase inhibitor and clinically-used vasodilator, to an intact dura (dose of 10 mM). Our results to date suggest that pericytes, even those without alpha smooth muscle actin, have the capacity to modulate blood flow. These findings will advance our understanding of vascular biology, and could be relevant for treating disease processes associated with deficits in capillary blood flow, such as dementia and traumatic brain injury. Support or Funding Information A.Y.S. thanks his support: NS085402, NS096997, NS097775, P20GM12345, NSF 1539034, AHA 14GRNT20480366, Alzheimer's Association, and CCAD. D.A.H. thanks his support: UL1 TR001450, TL1 TR001451, and F30NS096868. We optogenetically stimulated individual pericytes in vivo using two photon microscopy (A), and observed a significant decrease in the diameter and red blood cell flow in the underlying capillary (B). Surprisingly, similar responses were seen between pericytes with and without expression of alpha smooth muscle actin, an important protein in canonical vascular contraction mechanisms. Pericytes can therefore modulate blood flow using non-canonical mechanisms that can be targeted for therapies. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.