Calpain-3 Is Not a Sodium Dependent Protease and Simply Requires Calcium for Activation

Calpain-3 (CAPN3) is a muscle-specific member of the calpain family of Ca²⁺-dependent proteases. It has been reported that CAPN3 can also be autolytically activated by Na⁺ ions in the absence of Ca²⁺, although this was only shown under non-physiological ionic conditions. Here we confirm that CAPN3 does undergo autolysis in the presence of high [Na⁺], but this only occurred if all K⁺ normally present in a muscle cell was absent, and it did not occur even in 36 mM Na⁺, higher than what would ever be reached in exercising muscle if normal [K⁺] was present. CAPN3 in human muscle homogenates was autolytically activated by Ca²⁺, with ~50% CAPN3 autolysing in 60 min in the presence of 2 µM Ca²⁺. In comparison, autolytic activation of CAPN1 required about 5-fold higher [Ca²⁺] in the same conditions and tissue. After it was autolysed, CAPN3 unbound from its tight binding on titin and became diffusible, but only if the autolysis led to complete removal of the IS1 inhibitory peptide within CAPN3, reducing the C-terminal fragment to 55 kDa. Contrary to a previous report, activation of CAPN3, either by raised [Ca²⁺] or Na⁺ treatment, did not cause proteolysis of the skeletal muscle Ca²⁺ release channel-ryanodine receptor, RyR1, in physiological ionic conditions. Treatment of human muscle homogenates with high [Ca²⁺] caused autolytic activation of CAPN1, accompanied by proteolysis of some titin and complete proteolysis of junctophilin (JP1, full length ~95 kDa), generating an equimolar amount of a diffusible ~75 kDa N-terminal JP1 fragment, but without any proteolysis of RyR1.