Targeting Fyn Kinase in Alzheimer's Disease

Abstract

The past decade has brought tremendous progress in unraveling the pathophysiology of Alzheimer's disease (AD). While increasingly sophisticated immunotherapy targeting soluble and aggregated brain amyloid-beta (Aβ) continues to dominate clinical research in AD, a deeper understanding of Aβ physiology has led to the recognition of distinct neuronal signaling pathways linking Aβ to synaptotoxicity and neurodegeneration and to new targets for therapeutic intervention. Identifying specific signaling pathways involving Aβ has allowed for the development of more precise therapeutic interventions targeting the most relevant molecular mechanisms leading to AD. In this review, I highlight the discovery of cellular prion protein as a high-affinity receptor for Aβ oligomers, and the downstream signaling pathway elucidated to date, converging on nonreceptor tyrosine kinase Fyn. I discuss preclinical studies targeting Fyn as a therapeutic intervention in AD and our recent experience with the safety, tolerability, and cerebrospinal fluid penetration of the Src family kinase inhibitor saracatinib in patients with AD. Fyn is an attractive target for AD therapeutics, not only based on its activation by Aβ via cellular prion protein but also due to its known interaction with tau, uniquely linking the two key pathologies in AD. Fyn is also a challenging target, with broad expression throughout the body and significant homology with other members of the Src family kinases, which may lead to unintended off-target effects. A phase 2a proof-of-concept clinical trial in patients with AD is currently under way, providing critical first data on the potential effectiveness of targeting Fyn in AD.

Keywords: Alzheimer’s disease; Amyloid-beta; Cellular prion protein; Fyn; Saracatinib.

Figure 1. Aβ oligomers (Aβo) bind Cellular Prion Protein (PrPC) to activate Fyn kinase at Tyr416

Extracellular binding initiates a pathologic intracellular signaling cascade, leading to acute changes in NMDA receptor trafficking, and persistent activation of Pyk2 and eEF2. Fyn plays a central role in the proposed signaling cascade, and can be inhibited in vivo by the orally available Src family kinase inhibitor saracatinib.