Chaperone nanobodies protect gelsolin against MT1-MMP degradation and alleviate amyloid burden in the gelsolin amyloidosis mouse model.

Gelsolin amyloidosis is an autosomal dominant incurable disease caused by a point mutation in the GSN gene (G654A/T), specifically affecting secreted plasma gelsolin. Incorrect folding of the mutant (D187N/Y) second gelsolin domain leads to a pathological proteolytic cascade. D187N/Y gelsolin is first cleaved by furin in the trans-Golgi network, generating a 68 kDa fragment (C68). Upon secretion, C68 is cleaved by MT1-MMP-like proteases in the extracellular matrix, releasing 8 kDa and 5 kDa amyloidogenic peptides which aggregate in multiple tissues and cause disease-associated symptoms. We developed nanobodies that recognize the C68 fragment, but not native wild type gelsolin, and used these as molecular chaperones to mitigate gelsolin amyloid buildup in a mouse model that recapitulates the proteolytic cascade. We identified gelsolin nanobodies that potently reduce C68 proteolysis by MT1-MMP in vitro. Converting these nanobodies into an albumin-binding format drastically increased their serum half-life in mice, rendering them suitable for intraperitoneal injection. A 12-week treatment schedule of heterozygote D187N gelsolin transgenic mice with recombinant bispecific gelsolin-albumin nanobody significantly decreased gelsolin buildup in the endomysium and concomitantly improved muscle contractile properties. These findings demonstrate that nanobodies may be of considerable value in the treatment of gelsolin amyloidosis and related diseases.