PICALM is highly associated with late-onset AD based on genome-wide association studies. PICALM facilitates clathrin-mediated endocytosis and regulates the internalization and intracellular trafficking of cell surface receptors. Its N-terminus contains an epsin NH2-terminal homology domain for phosphatidylinositol-4,5-bisphosphate binding, which allows PICALM to sense membrane curvatures and regulate the size of clathrin-coated vesicles. These functions are central to its role in amyloid beta transvascular clearance across the blood-brain barrier and tau through autophagy. PICALM is highly expressed in brain endothelial cells and neurons.
We showed that diminished PICALM levels at the BBB in mice accelerated amyloid pathology and behavioral deficits, which can be ameliorated by endothelial re-expression of Picalm using an adeno-associated virus (AAV). In neurons, PICALM regulates internalization and intracellular trafficking of both amyloid precursor protein and the APP-cleaving enzyme gamma-secretase, thereby modulating amyloid beta production. Importantly, PICALM directly protects cultured neurons against amyloid beta toxicity. We confirmed these findings in vivo in Picalm +/-; APP Swe mice that show impaired amyloid beta transvascular clearance across the BBB, accelerated amyloid beta pathology and behavioral deficits compared with their littermate controls expressing normal levels of PICALM in brain endothelium and neurons.
Since PICALM protects against development of amyloid beta pathology and cognitive impairment in mice, we hypothesized that gene therapy and/or an FDA-approved drug hit that enhances PICALM expression will increase amyloid beta clearance across the BBB, reduce amyloid beta neurotoxicity and improve cognitive functions in Alzheimer’s disease mouse model. In studies previously supported by Cure Alzheimer’s Fund, we have validated that AAV-PHP.B mediated global gene delivery to the brain is superior compared with AAV2-BR1 viral vectors-mediated endothelial gene delivery. We also have performed high-throughput screening that led to 17 hits from an FDA-approved drug library that are capable of upregulating PICALM expression in vitro. We confirmed four hits from secondary screening in endothelial cells and further selected one final candidate (T-65) for in vivo efficacy testing in Picalm +/-;5xFAD mice. Our in vivo data preliminarily demonstrated that administration of T-65 ameliorated amyloid beta pathology and improved cognitive functions in Picalm +/-;5xFAD mice, suggesting that T-65 is a potential drug for AD. Therefore, we propose to continue examining the potential of using AAV-PHP.B-Picalm as a gene therapy for AD, and to fully validate the efficacy of T-65 in a mouse model of AD, under the following two specific aims:
AIM 1: To determine whether gene therapy with AAV-PHP.B-Picalm in Picalm +/-; 5xFAD mice will increase Picalm expression, reduce amyloid beta pathology and improve cognitive functions.
Hypothesis: AAV-PHP.B-Picalm systemic administration will increase Picalm levels in both brain endothelium and neurons, which will enhance amyloid beta transvascular clearance across the BBB and alleviate direct amyloid beta toxic effects on neurons, therefore reducing AD-like pathologies including amyloid burden and neuronal loss, as well as improving hippocampal-dependent learning and memory in Picalm +/-;5xFAD mice model, when compared with AAV-PHP.B-GFP treated mice.
AIM 2: To determine whether an FDA-approved drug T-65 can upregulate Picalm expression in vivo, increase amyloid beta clearance, reduce amyloid beta pathology and improve behavioral deficits in AD mouse models.
Hypothesis: Our top drug hit T-65 (which we show upregulates PICALM expression in the mouse brain endothelium) will enhance amyloid beta transvascular clearance via BBB and improve hippocampal-dependent learning and memory in Picalm +/-;5xFAD transgenic mice, but not in Picalm lox/lox; Cdh5 -Cre; 5xFAD mice (with both alleles of Picalm gene deleted from brain endothelium), when compared with vehicle-treated littermate controls.