Immunohistochemical Characterization of Phosphorylated Ubiquitin in the Mouse Hippocampus

Mitochondrial autophagy (mitophagy) is an essential and evolutionarily conserved process that maintains mitochondrial integrity via the removal of damaged or superfluous mitochondria in eukaryotic cells. Phosphatase and tensin homolog (PTEN)-induced putative kinase 1 (PINK1) and Parkin promote mitophagy and function in a common signaling pathway. PINK1-mediated ubiquitin phosphorylation at Serine 65 (Ser65-pUb) is a key event in the efficient execution of PINK1/Parkin-dependent mitophagy. However, few studies have used immunohistochemistry to analyze Ser65-pUb in the mouse. Here, we examined the immunohistochemical characteristics of Ser65-pUb in the mouse hippocampus. Some hippocampal cells were Ser65-pUb positive, whereas the remaining cells expressed no or low levels of Ser65-pUb. PINK1 deficiency resulted in a decrease in the density of Ser65-pUb-positive cells, consistent with a previous hypothesis based on in vitro research. Interestingly, Ser65-pUb-positive cells were detected in hippocampi lacking PINK1 expression. The CA3 pyramidal cell layer and the dentate gyrus (DG) granule cell layer exhibited significant reductions in the density of Ser65-pUb-positive cells in PINK1-deficient mice. Moreover, Ser65-pUb immunoreactivity colocalized predominantly with neuronal markers. These findings suggest that Ser65-pUb may serve as a biomarker of in situ PINK1 signaling in the mouse hippocampus; however, the results should be interpreted with caution, as PINK1 deficiency downregulated Ser65-pUb only partially.