Sequestration of CAPNS1 into Polyglycine Aggregates in a Cellular Model of NOTCH2NLC Repeat Expansion

Neuronal Intranuclear Inclusion Disease (NIID) is caused by GGC repeat expansions in the 5′ untranslated region of the notch 2 N-terminal like C (NOTCH2NLC) gene. An upstream open reading frame within the mutant transcript produces the NOTCH2NLC upstream open reading frame-derived polyglycine protein (uN2CpolyG) containing expanded polyglycine (polyG), which forms intranuclear inclusions. Although uN2CpolyG is thought to play a critical role in disease pathogenesis, the mechanisms underlying its toxicity and inclusion formation remain incompletely understood. In this study, we first expressed a pure GGC repeat encoding polyG in Neuro2a cells and identified aggregate-associated proteins by mass spectrometry. We then confirmed the formation of intracellular aggregates using both transient expression and drug-inducible expression systems for uN2CpolyG. Among the proteins identified by mass spectrometry, Calpain small subunit 1 (Capns1), the regulatory subunit of calpain, was found to be sequestered into uN2CpolyG aggregates. Notably, the N-terminus of Capns1 contains a glycine-rich sequence, which mediated its co-aggregation with uN2CpolyG. Furthermore, knockdown of Capns1 appeared to reduce the accumulation of uN2CpolyG aggregates. Collectively, these findings identify Capns1 as a potential modifier of NIID pathology.