SMPD1 and Parkinson disease: Remarkably, the link between endolysosomal impairment and synaptic dysfunction within PD derives also from these approaches, which have recently confirmed the causal role of several “lysosomal” and “synaptic” genetic hits (i.e., ARSA, CTSB, GALC, IDUA, RAB29, RAB7L1, SH3GL2, SMPD1, STX1B, TMEM175, VAMP42, and ZSWIM7) [333,343,345,346,347,348,349].