Reduction of neuronal activity mediated by blood-vessel regression in the adult brain

Introduction

Microvessels are composed of two closely interacting cell types, namely endothelial cells and pericytes¹, that are embedded within the endfeet of astrocytes². Neurons, astrocytic endfeet, pericytes, and endothelial cells as well as the vascular basal laminar layers form the neurovascular unit that coordinates neuronal function^{3, 4, 5–6}. Dysfunction or dysregulation of the neurovascular unit is associated with many diseases including stroke and Alzheimer’s disease⁷.

The density of brain vasculature and neurons declines significantly during normal aging (10–30%), and this decrease reaches 40–60% for Alzheimer’s patients^{8, 9–10}. However, little is known about how blood-vessel dynamics change with age and how this affects the activity of the adult mammalian brain^{11, 12–13}. Vessel regression was reported nearly two centuries ago¹⁰ and has been studied in the vasculature of embryonic and postnatal rodents^{14, 15, 16–17} and zebrafish^18,19. Several molecules (e.g., Wnt, Angiopoietin, Nrarp, VEGF, etc.) are found to be involved in normal vessel regression in developing organs^{20, 21, 22, 23–24}. However, the mechanisms underlying vessel regression are largely unknown, as are the fates of the various cellular components of the neurovascular unit. In this work, using genetic methods to specifically label endothelial cells, pericytes, and glial cells with different fluorescent proteins, we performed longitudinal in vivo imaging of functional microcirculation for up to 6 months, enabling a comprehensive understanding of vessel regression as the brain develops and ages and how vessel regression-related changes in the microcirculation contribute to neuronal activity in the adult brain.

Results

Alteration of the microcirculation in the adult mouse brain

To evaluate the plasticity of the blood vasculature in the adult brain, we measured functional blood flow after administering FITC-dextran (500 kDa) to normal mice via the tail vein (Fig. 1a). Blood flow rate was assessed by counting blood cells passing through the vessel. With this strategy, we monitored blood circulation within the same region of the cerebral cortex weekly for 3–6 months (Fig. 1b). Surprisingly, we found that 1.7% of the microvessels became non-functional (i.e., no FITC signal) across the entire field within a 5-week window (Fig. 1c). Interestingly, blood flow to ~75% of the occluded microvessels, which were observed at day 1, was restored within...