Electro-Calcium uncoupling precedes neurodegeneration in Alzheimers disease

Abstract

Alzheimers disease (AD) is categorized as a neurodegenerative disease, but there is a growing recognition of the vascular components in AD pathophysiology. Reduction in cerebral perfusion is routinely observed in AD patients and preclinical models prior to overt clinical symptoms. However, there is a limited mechanistic understanding of the early neurovascular deficits in AD, and how these may ultimately contribute to pathology. Here, we investigated the mechanisms of early neurovascular dysfunction in AD by using 3-month-old 5xFAD mice, a familial mouse model of AD. Functional hyperemia--the increase in cerebral blood flow (CBF) in response to neuronal activity--is driven by inward rectifier K+ (Kir2.1)-mediated hyperpolarizing (electrical) signals and Ca2+-dependent nitric oxide production within the capillary endothelial cells (cECs). Electrical and Ca2+ signals are tightly coupled through cECs membrane potential, referred to as Electro-Calcium (E-Ca) coupling. We hypothesize that E-Ca uncoupling contributes to impaired functional hyperemia in 5xFAD mice and that these neurovascular deficits precede the neurodegeneration and cognitive decline. At three months of age, 5xFAD mice did not exhibit any impairment in spatial learning and memory, or neuronal density. However, whisker stimulation-induced functional hyperemia was significantly reduced in 5xFAD mice compared to controls. Functional hyperemia exhibited a bimodal response in controls--consisting of fast and slow phases--with the slow phase being significantly reduced in 5xFAD mice. To identify mechanisms underlying these deficits, we measured cortical neuronal and endothelial Ca2+ activity using in-vivo imaging. Neuronal Ca2+ activity was comparable between controls and 5xFAD mice, while cECs Ca2+ activity was significantly reduced in 5xFAD mice. Moreover, Kir2.1 channel blocker, barium (100 M) significantly suppressed cECs Ca2+ activity in controls, but not in 5xFAD mice, consistent with crippled E-Ca coupling. Despite these vascular functional impairments, capillary density was preserved in 5xFAD mice. TRPV4 channels are one of the major Ca2+ entry pathways in cECs and potentiate E-Ca coupling. cECs TRPV4 current density was significantly reduced in 5xFAD mice while Kir2.1 current density was unchanged, indicating that impaired TRPV4 function underlies the E-Ca uncoupling. In summary, early E-Ca uncoupling leads to impaired functional hyperemia in 5xFAD mice and may contribute to later neuronal and cognitive decline.