Reelin structure and interactions: relevance to Alzheimer’s disease

Abstract

Alzheimer’s Disease (AD) is a progressive, neurodegenerative disorder that currently affects around 6.2 million people in the US. Currently, there are no effective treatments to forestall or reverse disease progression. If left untreated, the number of affected individuals is projected to rise to 13.8 million by 2060 [1]. The first case report describes a cognitively protected individual with an apolipoprotein E3 Christchurch (ApoE3ch) mutation in a family with autosomal dominant Alzheimer’s Disease (ADAD). In the report, it was suggested the potential mechanism of protection is driven by decreased interaction of ApoE3ch with HSPGs. Recently, a second case of a cognitively protected individual in a family carrying a presenilin-1 (PSEN1) E280A mutation resulting in autosomal dominant AD has been studied (submitted). Here, I show the structure and function of a key C-terminal mutation (H3447R) in Reelin found in this individual. Reelin H3447R modulates heparan sulfate proteoglycans (HSPG) protein-protein interactions. Using in silico approaches, I show that the H3447R mutation is oriented in the same direction as other basic amino acids (AAs) in the heparin binding motif of Reelin. Nuclear magnetic resonance (NMR) measurements indicate that the previously uncharacterized 30 AAs C-terminal region (CTR) has an alpha-helical structure and the region containing the heparin binding domain is largely flexible. Kinetic data also suggest that the H3447R variant has ~2x increased interaction with heparin and ~10x increased interaction with neuropilin-1, a potential mediator of HSPG interaction. HPLC data further proves that the presence of basic AAs in the 3447 position has stronger heparin interactions in comparison to basic amino acids; thermodynamic measurements show that H3447R may be favored for heparin interaction in vivo. Taken together, these data indicate that H3447R has increased interaction with heparin, and I identify the key binding site of this interaction. Interactions with HSPGs are known to increase the local concentrations of ligands in proximity to downstream lipoprotein receptors, thus driving binding interactions [2, 3]. However, the biological mechanism behind Reelin and HSPGs interaction was previously unknown. Elucidating the biomolecular mechanism of Reelin protection could enable future therapeutics against autosomal dominant AD.