Deafness Causing Neuroplastin Missense Variants Fail to Promote Plasma Membrane Ca-ATPase Levels and Ca Transient Regulation in Brain Neurons

Hearing, the ability to sense sounds, and the processing of auditory information are important for perception of the world. Mice lacking expression of neuroplastin (Np), a type-1 transmembrane glycoprotein, display deafness, multiple cognitive deficiencies, and reduced expression of plasma membrane calcium (Ca) ATPases (PMCAs) in cochlear hair cells and brain neurons. In this study, we transferred the deafness causing missense mutations pitch (C315S) and audio-1 (I122N) into human Np (hNp) constructs and investigated their effects at the molecular and cellular levels. Computational molecular dynamics show that loss of the disulfide bridge in hNp causes structural destabilization of immunoglobulin-like domain (Ig) III and that the novel asparagine in hNp results in steric constraints and an additional N-glycosylation site in IgII. Additional N-glycosylation of hNp was confirmed by PNGaseF treatment. In comparison to hNp, transfection of hNp and hNp into HEK293T cells resulted in normal mRNA levels but reduced the Np protein levels and their cell surface expression due to proteasomal/lysosomal degradation. Furthermore, hNp and hNp failed to promote exogenous PMCA levels in HEK293T cells. In hippocampal neurons, expression of additional hNp or hNp was less efficient than hNp to elevate endogenous PMCA levels and to accelerate the restoration of basal Ca levels after electrically evoked Ca transients. We propose that mutations leading to pathological Np variants, as exemplified here by the deafness causing Np mutants, can affect Np-dependent Ca regulatory mechanisms and may potentially cause intellectual and cognitive deficits in humans.