Lesions and Loss of Smooth Muscle Cells in Brain Underlies Small Vessel Disease - In response to the FOA, PAR-22-026, we propose an integrated and technologically/conceptually innovative approach to advance our mechanistic understanding of how dysfunction of small brain vessels can have long-term impacts on the brain parenchyma and cause cognitive impairment. Cerebral small vessel disease (cSVD) accounts for up to 25% of ischemic strokes and more than 90% of spontaneous intracerebral hemorrhages (ICHs), and as such is a major driver of dementia. Recent studies have shown that there are at least four types of mural cells defining four major microvascular zones: smooth muscle cells (SMCs) on arterioles, contractile pericytes (PCs) on the post-arteriole region of the capillary bed (i.e., transition zone), non-contractile PCs on distal capillaries and venular PCs on venules. Loss of arteriolar SMCs in human post-mortem brains is a feature shared by both multifactorial and inherited cSVD, whether associated with ischemic strokes or ICHs. We have provided compelling evidence that loss of SMCs on arterioles coupled with enhanced function of contractile PCs on the transition zone mutually reinforce each other to cause ICHs, and recently discovered that arteriolar SMCs and contractile PCs in the transition zone are lost early in the brain and retina of clinically relevant mouse models of ischemic cSVDs with gain- or loss-of-function mutations in the NOTCH3 receptor. On the basis of these and other observations, we propose that loss of SMCs specifically in brain arterioles is a common factor underlying the development of cSVDs and that changes in the properties/density of contractile PCs in the post-arteriole transition zone modify disease presentation. To test this, we will explore the causal relationship between the loss of SMCs/PCs and cSVD- related brain pathologies and cognitive symptoms (Aim 1) and determine how loss of SMCs/contractile PCs compromises integrative vascular functions (Aim 2). To this end, we will employ existing and novel mouse models with conditional inactivation of Notch3 in specific mural cell populations and deploy a powerful array of new techniques, including 1) a novel coordinate-based object analysis methodology capable of simultaneously quantifying all imageable parameters, including small vessel pathology, in massive 4D (3D over time) imaging datasets from high-resolution sections of the entire brain; 2) a novel pressurized retina preparation in conjunction with labeled red blood cells and microspheres for quantitatively assessing the impact of small vessel pathology on intravascular pressure and flow across different microvascular segments; and 3) ultrafast functional ultrasound imaging, an emerging technology that can non-invasively sample cerebral blood volume changes in vivo in a complete coronal section of a mouse brain with a high spatiotemporal resolution, for elucidating how small brain vessel pathologies affect cerebral blood flow regulation in deep parts of the brain. We expect that the proposed studies will contribute to a better understanding of the mechanistic basis of deep brain lesions and cognitive impairment in cSVDs and establish arteriolar SMCs and contractile PCs as critical new targets.
