HTRA2 and Parkinson disease: Interestingly, in SN DA neurons from adult PARK-gene PD-model mice (Pink1 and HtrA2/Omi KO) impaired intracellular Ca2+ signalling had also been described, that lead to a functional reduction of SK channel activation, accompanied by a more irregular pacemaker and a higher tendency for burst activity32 — further linking altered Ca2+ homeostasis and impaired related ion channel function in SN DA neurons to their altered activity, high vulnerability to degeneration, and to PD.