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The receptor tyrosine kinase PDGFRβ is essential for pericyte migration to the endothelium. In mice lacking one allele of PDGFRβ (PDGFRβ+/−), previous reports have described an age-dependent loss of pericytes in the brain, leading to cerebrovascular dysfunction and subsequent neurodegeneration reminiscent of that seen in Alzheimer’s disease and vascular dementia. We examined 12–20-month-old PDGFRβ+/− mice to better understand how pericyte loss affects brain microvascular structure and perfusion in vivo. We observed a mild reduction of cortical pericyte number in PDGFRβ+/− mice (27% fewer cell bodies) compared to controls, but no decrease in pericyte coverage of the endothelium. This mild degree of pericyte loss caused no discernable change in cortical microvascular density, length, basal diameter or reactivity to hypercapnia. Yet, it was associated with an increase in basal blood cell velocity, primarily in pre-capillary arterioles. Taken together, our results suggest that mild pericyte loss can lead to aberrant cerebral blood flow despite a lack of apparent effect on microvascular structure and reactivity.

Background Multi-photon imaging of the cerebrovasculature provides rich data on the dynamics of cortical arterioles, capillaries, and venules. Vascular diameter is the major determinant of blood flow resistance, and is the most commonly quantified metric in studies of the cerebrovasculature. However, there is a lack of accessible and easy-to-use methods to quantify vascular diameter in imaging data. Methods We created VasoMetrics, a macro written in ImageJ/Fiji for spatiotemporal analysis of microvascular diameter. The key feature of VasoMetrics is rapid analysis of many evenly spaced cross-sectional lines along the vessel of interest, permitting the extraction of numerous diameter measurements from individual vessels. Here we demonstrated the utility of VasoMetrics by analyzing in vivo multi-photon imaging stacks and movies collected from lightly sedated mice, as well as data from optical coherence tomography angiography (OCTA) of human retina. Results Compared to the standard approach, which is to measure cross-sectional diameters at arbitrary points along a vessel, VasoMetrics accurately reported spatiotemporal features of vessel diameter, reduced measurement bias and time spent analyzing data, and improved the reproducibility of diameter measurements between users. VasoMetrics revealed the dynamics in pial arteriole diameters during vasomotion at rest, as well as changes in capillary diameter before and after pericyte ablation. Retinal arteriole diameter was quantified from a human retinal angiogram, providing proof-of-principle that VasoMetrics can be applied to contrast-enhanced clinical imaging of microvasculature. Conclusions VasoMetrics is a robust macro for spatiotemporal analysis of microvascular diameter in imaging applications.

The majority of the brain’s vasculature is comprised of intricate capillary networks lined by capillary pericytes. However, it remains unclear whether capillary pericytes contribute to blood flow control. Using two-photon microscopy to observe and manipulate single capillary pericytes in vivo, we find their optogenetic stimulation decreases lumen diameter and blood flow, but with slower kinetics than mural cells of upstream pial and pre-capillary arterioles. This slow, optogenetically-induced vasoconstriction was inhibited by the clinically-used vasodilator fasudil, a Rho kinase inhibitor that blocks contractile machinery. Capillary pericytes were also slower to constrict back to baseline following hypercapnia-induced dilation, and relax towards baseline following optogenetically-induced vasoconstriction. In a complementary approach, optical ablation of single capillary pericytes led to sustained local dilation and a doubling of blood cell flux in capillaries lacking pericyte contact. Altogether these data indicate that capillary pericytes contribute to basal blood flow resistance and slow modulation of blood flow throughout the capillary bed.

Supplemental material, JCB900543 Supplemental Figures for Mild pericyte deficiency is associated with aberrant brain microvascular flow in aged PDGFRβ+/− mice by Ashley N Watson, Andree-Anne Berthiaume, Anna V Faino, Konnor P McDowell, Narayan R Bhat, David A Hartmann and Andy Y Shih in Journal of Cerebral Blood Flow & Metabolism

The majority of the brain’s vasculature is comprised of intricate capillary networks lined by capillary pericytes. However, it remains unclear whether capillary pericytes influence blood flow. Using two-photon microscopy to observe and manipulate brain capillary pericytes in vivo, we find that their optogenetic stimulation decreases lumen diameter and blood flow, but with slower kinetics than similar stimulation of mural cells on upstream pial and pre-capillary arterioles. This slow vasoconstriction was inhibited by the clinically-used vasodilator fasudil, a Rho-kinase inhibitor that blocks contractile machinery. Capillary pericytes were also slower to constrict back to baseline following hypercapnia-induced dilation, and slower to dilate towards baseline following optogenetically-induced vasoconstriction. Optical ablation of single capillary pericytes led to sustained local dilation and a doubling of blood cell flux selectively in capillaries lacking pericyte contact. These data indicate that capillary pericytes contribute to basal blood flow resistance and slow modulation of blood flow throughout the brain.

Summary: Direct contact and communication between pericytes and endothelial cells is critical for maintenance of cerebrovascular stability and blood-brain barrier function. Capillary pericytes have thin processes that reach hundreds of micrometers along the capillary bed. The processes of adjacent pericytes come in close proximity but do not overlap, yielding a cellular chain with discrete territories occupied by individual pericytes. Little is known about whether this pericyte chain is structurally dynamic in the adult brain. Using in vivo two-photon imaging in adult mouse cortex, we show that while pericyte somata were immobile, the tips of their processes underwent extensions and/or retractions over days. The selective ablation of single pericytes provoked exuberant extension of processes from neighboring pericytes to contact uncovered regions of the endothelium. Uncovered capillary regions had normal barrier function but were dilated until pericyte contact was regained. Pericyte structural plasticity may be critical for cerebrovascular health and warrants detailed investigation. : Pericyte-endothelial contact is important for many aspects of cerebrovascular health. Berthiaume et al. use longitudinal two-photon imaging to show that the processes of brain capillary pericytes are structurally plastic in vivo. Their processes can grow hundreds of micrometers to ensure contact with exposed endothelium following ablation of a single pericyte. Keywords: capillary, pericyte, endothelium, blood-brain barrier, blood flow, plasticity, two-photon imaging, Alzheimer’s disease, dementia, stroke