HTRA2 and neurodegenerative disease: Mechanisms relating to ER stress, mitochondrial functions and endosomal transport have also been extensively reported in neurodegenerative diseases including PD, with PARK17 playing a role in the retrotransfer of proteins from endosomes in the pre-lysosomal compartment network to the trans-Golgi network, and PARK9 and ATP13A2 coding for endo-/lysosomal-related proteins, HTRA2 (PARK13) being crucial to maintaining normal mitochondrial function and ERS-coupled apoptotic cell death being implicated in neurodegeneration [17, 65].