Investigation of the early stages of human D-crystallin aggregation process

Cataract, a major cause of visual impairment worldwide, is a common disease of the eye lens related to protein aggregation. Several factors including the exposure of ultraviolet irradiation and possibly acidic condition may induce the unfolding and subsequent aggregation of the crystallin proteins leading to crystalline lens opacification. Human D-crystallin (HDC), a 173 residue monomeric protein, abundant in the nucleus of the human eye lens, has been shown to aggregate and form amyloid fibrils under acidic conditions and that this aggregation route is thought to be a potential initiation pathway for the onset of age-related nuclear cataract. However, the underlying mechanism of fibril formation remains elusive. This report is aimed at examining the structural changes and possible amyloid fibril formation pathway of HDC using molecular dynamics and molecular docking simulations. Our findings demonstrated that incubation of HDC under the acidic condition redistributes the protein surface charges and affects the protein interaction with its surrounding solvent environment. This brings about a twist motion in the overall tertiary structure that gives rise to newly formed anti-parallel -strands in the C-terminal flexible loop regions. The change in protein structural conformation also involves an alteration in specific salt-bridge interactions. Altogether, these findings revealed a plausible mechanism for amyloid fibril formation of HDC that is important to the early stages of HDC aggregation involved in cataractogenesis.