Parkinson's disease (PD) is a neurodegenerative disorder characterized by loss of dopaminergic neurons in the Substantia Nigra and the formation of ubiquitin- and alpha-synuclein (aSYN)-positive cytoplasmic inclusions called Lewy bodies (LBs). Although most PD cases are sporadic, families with genetic mutations have been found. Mutations in PARK7/DJ-1 have been associated with autosomal recessive early-onset PD, while missense mutations or duplications of aSYN (PARK1, PARK4) have been linked to dominant forms of the disease. In this study, we identify the E3 ubiquitin ligase tumor necrosis factor-receptor associated factor 6 (TRAF6) as a common player in genetic and sporadic cases. TRAF6 binds misfolded mutant DJ-1 and aSYN. Both proteins are substrates of TRAF6 ligase activity in vivo. Interestingly, rather than conventional K63 assembly, TRAF6 promotes atypical ubiquitin linkage formation to both PD targets that share K6-, K27- and K29- mediated ubiquitination. Importantly, TRAF6 stimulates the accumulation of insoluble and polyubiquitinated mutant DJ-1 into cytoplasmic aggregates. In human post-mortem brains of PD patients, TRAF6 protein colocalizes with aSYN in LBs. These results reveal a novel role for TRAF6 and for atypical ubiquitination in PD pathogenesis.

Huntington disease (HD) is a neurodegenerative disorder caused by an expansion of polyglutamines in the first exon of huntingtin (HTT), which confers aggregation-promoting properties to amino-terminal fragments of the protein (N-HTT). Mutant N-HTT aggregates are enriched for ubiquitin and contain ubiquitin E3 ligases, thus suggesting a role for ubiquitination in aggregate formation. Here, we report that tumor necrosis factor receptor-associated factor 6 (TRAF6) binds to WT and polyQ-expanded N-HTT in vitro as well as to endogenous full-length proteins in mouse and human brain in vivo. Endogenous TRAF6 is recruited to cellular inclusions formed by mutant N-HTT. Transient overexpression of TRAF6 promotes WT and mutant N-HTT atypical ubiquitination with Lys6, Lys27, and Lys29 linkage formation. Both interaction and ubiquitination seem to be independent from polyQ length. In cultured cells, TRAF6 enhances mutant N-HTT aggregate formation, whereas it has no effect on WT N-HTT protein localization. Mutant N-HTT inclusions are enriched for ubiquitin staining only when TRAF6 and Lys6, Lys27, and Lys29 ubiquitin mutants are expressed. Finally, we show that TRAF6 is up-regulated in post-mortem brains from HD patients where it is found in the insoluble fraction. These results suggest that TRAF6 atypical ubiquitination warrants investigation in HD pathogenesis.

Hemoglobin is the oxygen carrier in blood erythrocytes. Oxygen coordination is mediated by α2β2 tetrameric structure via binding of the ligand to the heme iron atom. This structure is essential for hemoglobin function in the blood. In the last few years, expression of hemoglobin has been found in atypical sites, including the brain. Transcripts for α and β chains of hemoglobin as well as hemoglobin immunoreactivity have been shown in mesencephalic A9 dopaminergic neurons, whose selective degeneration leads to Parkinson's disease. To gain further insights into the roles of hemoglobin in the brain, we examined its quaternary structure in dopaminergic neurons in vitro and in vivo. Our results indicate that (i) in mouse dopaminergic cell line stably over-expressing α and β chains, hemoglobin exists as an α2β2 tetramer; (ii) similarly to the over-expressed protein, endogenous hemoglobin forms a tetramer of 64kDa; (iii) hemoglobin also forms high molecular weight insoluble aggregates; and (iv) endogenous hemoglobin retains its tetrameric structure in mouse mesencephalon in vivo. In conclusion, these results suggest that neuronal hemoglobin may be endowed with some of the biochemical activities and biological function associated to its role in erythroid cells. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

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Erythropoietin receptor (EpoR) regulates erythrocytes differentiation in blood. In the brain, EpoR has been shown to protect several neuronal cell types from cell death, including the A9 dopaminergic neurons (DA) of the Substantia Nigra (SN). These cells form the nigrostriatal pathway and are devoted to the control of postural reflexes and voluntary movements. Selective degeneration of A9 DA neurons leads to Parkinson's disease. By the use of nanoCAGE, a technology that allows the identification of Transcription Start Sites (TSSs) at a genome-wide level, we have described the promoter-level expression atlas of mouse A9 DA neurons purified with Laser Capture Microdissection (LCM). Here, we identify mRNA variants of the Erythropoietin Receptor (DA-EpoR) transcribed from alternative TSSs. Experimental validation and full-length cDNA cloning is integrated with gene expression analysis in the FANTOM5 database. In DA neurons, the EpoR gene encodes for a N-terminal truncated receptor. Based on STAT5 phosphorylation assays, we show that the new variant of N-terminally truncated EpoR acts as decoy when co-expressed with the full-length form. A similar isoform is also found in human. This work highlights new complexities in the regulation of Erythropoietin (EPO) signaling in the brain.