Evidence that phosphorylated ubiquitin signaling is involved in the etiology of Parkinson’s disease

Abstract

The ubiquitin (Ub) kinase PINK1 and the E3 Ub ligase Parkin, two gene products associated with young-onset Parkinson's disease (PD), participate in mitochondrial quality control. The phosphorylation of mitochondrial polyUb by PINK1, which is activated in a mitochondrial membrane potential (ΔΨm)-dependent manner, facilitates the mitochondrial translocation and concomitant enzymatic activation of Parkin, leading to the clearance of phospho-polyUb-tagged mitochondria via mitophagy. Thus, Ub phosphorylation is a key event in PINK1-Parkin-mediated mitophagy. Here, we examined the role of phospho-Ub signaling in the pathogenesis of PD using fly PD models, human brain tissue and dopaminergic neurons derived from induced pluripotent stem cells (iPSCs) containing Parkin or PINK1 mutations, as well as normal controls. We report that phospho-Ub signaling is highly conserved between humans and Drosophila, and that phospho-Ub signaling and the relocation of axonal mitochondria upon ΔΨm reduction are indeed compromised in human dopaminergic neurons containing Parkin or PINK1 mutations. Moreover, phospho-Ub signaling is prominent in tyrosine hydroxylase-positive neurons compared with tyrosine hydroxylase-negative neurons, suggesting that PINK1-Parkin signaling is more required for dopaminergic neurons. These results shed light on the particular vulnerability of dopaminergic neurons to mitochondrial stress.

Immunohistochemical Characterization of Phosphorylated Ubiquitin in the Mouse Hippocampus

“…Although we showed that PINK1 deficiency resulted in a significant reduction in the density of Ser65-pUb-positve cells, several Ser65-pUb-positive cells were detected in the hippocampus of PINK1 −/− mice. Notably, PINK1-independent Ser65-pUb signals were observed in regions adjacent to the PPM2 cluster in Drosophila lacking PINK1 [27]. These findings suggest that one or more uncharacterized kinases other than PINK1 produce Ser65-pUb.…”

“…Ubiquitin phosphorylation at Serine 65 is a key step in the efficient execution of PINK1/Parkin-dependent mitophagy [8]. The physiological and pathological importance of ubiquitin phosphorylation has been suggested [27][28][29]. However, immunohistochemical data on ubiquitin phosphorylation are scarce in mouse models.…”

“…or Parkin exhibit mitochondrial, neurological, and behavioral abnormalities that closely resemble the symptoms of human patients with PD [23][24][25][26]. Ser65-pUb expression is also observed in dopaminergic neurons in Drosophila, suggesting that PINK1/Parkin signaling functions faithfully [27]. A study using human postmortem brain specimens showed that Ser65-pUb is widely distributed throughout the brain, including the substantia nigra and hippocampus, and that 6 Ser65-pUb is upregulated in an age-dependent manner and in PD brains [28,29].…”

“…A study using human postmortem brain specimens showed that Ser65-pUb is widely distributed throughout the brain, including the substantia nigra and hippocampus, and that 6 Ser65-pUb is upregulated in an age-dependent manner and in PD brains [28,29]. In addition, Ser65-pUb generation is impaired in iPSCs derived from PD patients harboring PINK1 and Parkin mutations [27,29]. In contrast to fly models, PINK1 and Parkin knockout mice failed to exhibit PD pathologies, despite the presence of pronounced mitochondrial dysfunction (reviewed in [30]).…”

“…Here, we investigated immunohistochemically Ser65-pUb in mouse hippocampi, because Ser65-pUb is emerging as a possible biomarker of PINK1 signaling activity in vivo [27,29,33].…”