Enhanced Mitophagy Activity in Prion-Infected Cultured Cells and Prion-Infected Experimental Mice via a Pink1/Parkin-Dependent Mitophagy Pathway

Mitophagy is an important process for removing damaged mitochondria in cells, the dysfunction of which has been directly linked to an increasing number of neurodegenerative disorders. However, the details of mitophagy in prion diseases still need to be deeply explored. In this study, we identified more autophagosomes and large swelling mitochondria structures in the prion-infected cultured cell line SMB-S15 by transmission electron microscopy, accompanying the molecular evidence of activated autophagic flux. Western blots illustrated that the levels of Pink1 and Parkin, particularly in the mitochondrial fraction, were increased in SMB-S15 cells, whereas the levels of mitochondrial membrane proteins TIMM44, TOMM20, and TIMM23 were decreased. The amount of whole polyubiquitinated proteins decreased, but that of phosphor-polyubiquitinated proteins increased in SMB-S15 cells. The level of MFN2 in SMB-S15 cells were down-regulated, but its polyubiquitinated form was up-regulated. Knockdown of the expressions of Pink1 and Parkin by the individual SiRNAs in SMB-S15 cells reduced autophagic activity but did not seem to influence the expressions of TOMM20 and TIMM23. Moreover, we also demonstrated that the brain levels of Pink1 and Parkin in the mice infected with scrapie strains 139A and ME7 were remarkably increased at the terminal stage of the disease by Western blot and immunohistochemical (IHC) assays. Immunofluorescent assays revealed that Pink1 signals widely colocalized with GAFP-, Iba1-, and NeuN-positive cells in the brains of scrapie-infected mice. IHC assays with serial sections of the brain tissues infected with agents 139A and ME7 showed more Pink1- and Parkin-positive cells located at the areas with more PrP^Sc deposit. These results suggest an activated mitophagy in prion-infected cells and prion-infected experimental mice, probably via an enhanced Pink-Parkin pathway.

Post-translational modification of Parkin and its research progress in cancer

Clinical practice has shown that Parkin is the major causative gene found in an autosomal recessive juvenile parkinsonism (AR-JP) via Parkin mutations and that the Parkin protein is the core expression product of the Parkin gene, which itself belongs to an E3 ubiquitin ligase. Since the discovery of the Parkin gene in the late 1990s, researchers in many countries have begun extensive research on this gene and found that in addition to AR-JP, the Parkin gene is associated with many diseases, including type 2 diabetes, leprosy, Alzheimer’s, autism, and cancer. Recent studies have found that the loss or dysfunction of Parkin has a certain relationship with tumorigenesis. In general, the Parkin gene, a well-established tumor suppressor, is deficient and mutated in a variety of malignancies. Parkin overexpression inhibits tumor cell growth and promotes apoptosis. However, the functions of Parkin in tumorigenesis and its regulatory mechanisms are still not fully understood. This article describes the structure, functions, and post-translational modifications of Parkin, and summarizes the recent advances in the tumor suppressive function of Parkin and its underlying mechanisms.

Alternative splicing generates different parkin protein isoforms: evidences in human, rat, and mouse brain

Parkinson protein 2, E3 ubiquitin protein ligase (PARK2) gene mutations are the most frequent causes of autosomal recessive early onset Parkinson's disease and juvenile Parkinson disease. Parkin deficiency has also been linked to other human pathologies, for example, sporadic Parkinson disease, Alzheimer disease, autism, and cancer. PARK2 primary transcript undergoes an extensive alternative splicing, which enhances transcriptomic diversification. To date several PARK2 splice variants have been identified; however, the expression and distribution of parkin isoforms have not been deeply investigated yet. Here, the currently known PARK2 gene transcripts and relative predicted encoded proteins in human, rat, and mouse are reviewed. By analyzing the literature, we highlight the existing data showing the presence of multiple parkin isoforms in the brain. Their expression emerges from conflicting results regarding the electrophoretic mobility of the protein, but it is also assumed from discrepant observations on the cellular and tissue distribution of parkin. Although the characterization of each predicted isoforms is complex, since they often diverge only for few amino acids, analysis of their expression patterns in the brain might account for the different pathogenetic effects linked to PARK2 gene mutations.

Genetic mouse models for Parkinson's disease display severe pathology in glial cell mitochondria

We recently described mitochondrial pathology in neurons of transgenic mice with genes associated with Parkinson's disease (PD). Now we describe severe mitochondrial damage in glial cells of the mesencephalon in mice carrying a targeted deletion of parkin (PaKO) or overexpressing doubly mutated human alpha-synuclein (asyn). The number of mitochondria with altered morphology in glial cells is cell type-dependent, but always higher than in neurons. Interestingly, mitochondrial damage also occurs in mesencephalic glia of mice carrying mutated asyn controlled by the tyrosine hydroxylase promoter. Such mice do not show glial expression of the transgene, but show expression in neighboring neurons. However, we found strong overexpression of endogenous asyn in mesencephalic astrocytes from these mice. Cortical astrocytes neither display enhanced asyn expression nor mitochondrial damage. Cultivated mesencephalic astrocytes from newborn transgenic mice display various functional defects along with the morphological damage of mitochondria. First, the mitochondrial Ca(2+)-storage capacity is reduced in asyn transgenic mesencephalic astrocytes, but not in astrocytes from PaKO. Second, the expression of the mitochondrial protein PTEN-induced putative kinase is constitutively increased in asyn transgenic mice, while in PaKO it reacts to oxidative stress by overexpressing this protein along with other mitochondria-related proteins. Third, the neurotrophic effects exerted by control astrocytes, stimulating cortical neurons from healthy mice to develop longer processes and larger neuronal areas, are lacking in co-cultures with transgenic mesencephalic astrocytes. In summary, glial mitochondria from transgenic mice display morphological and functional alterations. Such transgenic astrocytes fail to influence neuronal differentiation, reflecting an important role that glia may play in PD pathogenesis.

Distribution of PINK1 and LRRK2 in rat and mouse brain

Mutations in two kinases, PTEN induced kinase 1 (PINK1) and leucine-rich repeat kinase 2 (LRRK2), have been shown to segregate with familial forms of Parkinson's disease. Although these two genes are expected to be involved in molecular mechanisms relevant to Parkinson's disease, their precise anatomical localization in mammalian brain is unknown. We have mapped the expression of PINK1 and LRRK2 mRNA in the rat and mouse brain via in situ hybridization histochemistry using riboprobes. We found that both genes are broadly expressed throughout the brain with similar neuroanatomical distribution in mouse compared to rat. PINK1 mRNA abundance was rather uniform throughout the different brain regions with expression in cortex, striatum, thalamus, brainstem and cerebellum. LRRK2, on the other hand, showed strong regional differences in expression levels with highest levels seen in the striatum, cortex and hippocampus. Weak LRRK2 expression was seen in the hypothalamus, olfactory bulb and substantia nigra. We confirmed these distributions for both genes using quantitative RT-PCR and for LRRK2 by western immunoblot. As their broad expression patterns contrast with localized neuropathology in Parkinson's disease, the pathogenicity of clinical mutant forms of PINK1 and LRRK2 may be mediated by nigrostriatal-specific mechanisms.