The postsynaptic scaffolding protein SAPAP3 shapes mitochondrial activity: the case of Huntington’s disease

Abstract

Abstract Postsynaptic scaffolding protein SAP90/PSD95-associated protein 3 (SAPAP3) modulates cortico-striatal signalling and regulates the maintenance of synaptic structure. Notably, SAPAP3 defects have been reported in several human psychiatric disorders that share pathophysiological features with Huntington’s disease (HD), a neurodegenerative disorder characterized by the expression of mutant huntingtin (mHTT) and marked dysfunction of cortico-striatal synapses and mitochondria. However, the role of SAPAP3 in mitochondrial function and HD pathophysiology remains unexplored. SAPAP3 was extracted from striatal synaptoneurosomes and analyzed by SWATH-MS proteomics to identify potential interactors, revealing SAPAP3 association with several mitochondrial proteins, particularly Mic60. These data were further complemented with proximity ligation assay (PLA) and immunoprecipitation experiments, supporting a direct or indirect interaction between SAPAP3 and Mic60. SAPAP3 knockout (SAPAP3-KO) mouse striatum exhibited reduced Mic60 protein levels, decreased mitochondrial respiratory chain activity and Ca 2+ uptake, as well as increased mitochondrial roundness and circularity, accompanied by decreased perimeter and area, consistent with mitochondrial fragmentation. Analysis of SAPAP3 protein levels in postmortem caudate tissue from HD patients, striatal tissue and isolated mitochondria from YAC128 mice, as well as mitochondria in primary striatal YAC128 neurons and ST Hdh Q111 cells, showed consistently reduced SAPAP3 levels. HD cells and WT cells subjected to SAPAP3 silencing showed alterations in mitochondrial morphology, movement and function. In contrast, HD striatal cells transfected with ovSAPAP3 constructs to restore SAPAP3 levels showed a rescue of mitochondrial morphology, motility, function and Ca 2+ handling, together with decreased levels of reactive oxygen species (ROS). These findings uncover a novel role for postsynaptic scaffolding protein SAPAP3 in preserving mitochondrial health, particularly mitochondrial dynamics and function, under neuropathological conditions characterized by mitochondrial abnormalities such as HD.