VEGF signalling causes stalls in brain capillaries and reduces cerebral blood flow in Alzheimer’s mice

Abstract

Increased incidence of stalled capillary blood flow caused by adhesion of leucocytes to the brain microvascular endothelium leads to a 17% reduction of cerebral blood flow (CBF) and exacerbates short-term memory loss in multiple mouse models of Alzheimer’s disease. Here, we report that Vascular Endothelial Growth Factor (VEGF) signaling at the luminal side of the brain microvasculature plays an integral role in the capillary stalling phenomenon of the APP/PS1 mouse model. Administration of the anti-mouse VEGF-A164 antibody, an isoform that inhibits blood brain barrier (BBB) hyperpermeability, reduced the number of stalled capillaries within an hour of injection, leading to an immediate increase in average capillary blood flow but not capillary diameter. VEGF-A inhibition also reduced the overall eNOS protein concentrations, increased occludin levels, and decreased the penetration of circulating Evans Blue dye across the BBB into the brain parenchyma, suggesting increased BBB integrity. Capillaries prone to neutrophil adhesion after anti-VEGF-A treatment also had lower occludin concentrations than flowing capillaries. Taken together, our findings demonstrate that VEGF-A signaling in APP/PS1 mice contributes to aberrant eNOS/occludin- associated BBB permeability, increases the incidence of capillary stalls, and leads to reductions in CBF. Reducing leucocyte adhesion by inhibiting luminal VEGF signaling may provide a novel and well-tolerated strategy for improving brain microvascular blood flow in Alzheimer’s disease. patients.

Behavioral and Neural Activity-Dependent Recanalization of Plugged Capillaries in the Brain of Adult and Aged Mice

“…Although they do not recapitulate the components an embolus, microspheres offer many experimental benefits such as the precise timing of induction (which was critical in the present study) and the ability for high throughput testing of therapeutics. For example, the microsphere model previously identified Vascular Endothelial Growth Factor (VEGF) as a crucial factor in regulating capillary obstruction clearance (Reeson et al, 2018), which has now been successfully applied to improving capillary blood flow in a mouse model of Alzheimer's disease (Ali et al, 2022). Of course, naturally occurring capillary obstructions arise from many different types of blocks or plugs, from cellular debris to a variety of circulating cells (Rapp et al, 2008;Santisakultarm et al, 2014;Cruz Hernández et al, 2019).…”

“…Specifically the capillary bed, which comprises >90% of vascular length and primarily distributes blood flow throughout the brain, is prone to spontaneous obstructions by cells or circulating debris (Blinder et al, 2013;Santisakultarm et al, 2014;Gould et al, 2017;Reeson et al, 2018;Erdener and Dalkara, 2019;Bracko et al, 2020;Schager and Brown, 2020). These obstructions can be ephemeral or persistent, and are far more abundant in disease states (Santisakultarm et al, 2014;Cruz Hernández et al, 2019;Bracko et al, 2020;Ali et al, 2022). Given the fact that persistently obstructed capillaries are eventually pruned (Reeson et al, 2018), any intervention that promotes capillary recanalization could conceivably preserve blood supply and buttress the vasculature's ability to support cognitive functions.…”

VEGF Paradoxically Reduces Cerebral Blood Flow in Alzheimer’s Disease Mice

“…Specifically, expression of the VEGF-A-associated tight junction protein occludin was downregulated in occluded capillaries. 19 The capillary stalling hypothesis suggests pathological VEGF-A/occludin- associated blood-brain barrier hyperpermeability activates local inflammatory markers in endothelial cells, recruiting leukocytes to the site of injury, increasing the incidence of stalled capillaries, and ultimately leading to cerebral blood flow reductions ( Figure 1D ). We targeted this pathway using an anti-VEGF-A antibody.…”

“…Increased VEGF-A levels leads to increased eNOS activity, downregulation of occludin, impairment of the blood-brain barrier, activation of local inflammatory markers, recruitment of leukocytes, stalling of capillary flow, and reduced cerebral blood flow. Images taken from Ali et al 19 …”

“… 31 In our study mice were administered at most 6 anti-VEGF-A injections over the course of 2 weeks. 19 Given the intraperitoneal mode of injection and size of the antibody used in our experiments, we largely saw effects on the vascular system rather than the brain parenchyma itself. Indeed, increased exposure to anti-VEGF-A injections could place patients at greater risk for impaired neuroprotection.…”