Hemoglobin is present as a canonical α2β2 tetramer in dopaminergic neurons.

Abstract 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 64 kDa; (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

Hemoglobin is present as a canonical α2β2 tetramer in dopaminergic neurons

AbstractHemoglobin 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

Neuronal haemoglobin induces loss of dopaminergic neurons in mouse Substantia nigra, cognitive deficits and cleavage of endogenous α-synuclein

Parkinson's disease (PD) presents the selective loss of A9 dopaminergic (DA) neurons of Substantia Nigra pars compacta (SNpc) and the presence of intracellular aggregates called Lewy bodies. alpha-synuclein (alpha-syn) species truncated at the carboxy-terminal (C-terminal) accumulate in pathological inclusions and promote alpha-syn aggregation and toxicity. Haemoglobin (Hb) is the major oxygen carrier protein in erythrocytes. In addition, Hb is expressed in A9 DA neurons where it influences mitochondrial activity. Hb overexpression increases cells' vulnerability in a neurochemical model of PD in vitro and forms cytoplasmic and nucleolar aggregates upon short-term overexpression in mouse SNpc. In this study, alpha and beta-globin chains were co-expressed in DA cells of SNpc in vivo upon stereotaxic injections of an Adeno-Associated Virus isotype 9 (AAV9) and in DA iMN9D cells in vitro. Long-term Hb over-expression in SNpc induced the loss of about 50% of DA neurons, mild motor impairments, and deficits in recognition and spatial working memory. Hb triggered the formation of endogenous alpha-syn C-terminal truncated species. Similar alpha-syn fragments were found in vitro in DA iMN9D cells over-expressing alpha and beta- globins when treated with pre-formed alpha-syn fibrils. Our study positions Hb as a relevant player in PD pathogenesis for its ability to trigger DA cells' loss in vivo and the formation of C-terminal alpha-syn fragments

Inhibition of APE1-endonuclease activity affects cell metabolism in colon cancer cells via a p53-dependent pathway

The pathogenesis of colorectal cancer (CRC) involves different mechanisms, such as genomic and microsatellite instabilities. Recently, a contribution of the base excision repair (BER) pathway in CRC pathology has been emerged. In this context, the involvement of APE1 in the BER pathway and in the transcriptional regulation of genes implicated in tumor progression strongly correlates with chemoresistance in CRC and in more aggressive cancers. In addition, the APE1 interactome is emerging as an important player in tumor progression, as demonstrated by its interaction with Nucleophosmin (NPM1). For these reasons, APE1 is becoming a promising target in cancer therapy and a powerful prognostic and predictive factor in several cancer types. Thus, specific APE1 inhibitors have been developed targeting: i) the endonuclease activity; ii) the redox function and iii) the APE1-NPM1 interaction. Furthermore, mutated p53 is a common feature of advanced CRC. The relationship between APE1 inhibition and p53 is still completely unknown. Here, we demonstrated that the inhibition of the endonuclease activity of APE1 triggers p53-mediated effects on cell metabolism in HCT-116 colon cancer cell line. In particular, the inhibition of the endonuclease activity, but not of the redox function or of the interaction with NPM1, promotes p53 activation in parallel to sensitization of p53-expressing HCT-116 cell line to genotoxic treatment. Moreover, the endonuclease inhibitor affects mitochondrial activity in a p53-dependent manner. Finally, we demonstrated that 3D organoids derived from CRC patients are susceptible to APE1-endonuclease inhibition in a p53-status correlated manner, recapitulating data obtained with HCT-116 isogenic cell lines. These findings suggest the importance of further studies aimed at testing the possibility to target the endonuclease activity of APE1 in CRC

Parkinson's Disease DJ-1 L166P Alters rRNA Biogenesis by Exclusion of TTRAP from the Nucleolus and Sequestration into Cytoplasmic Aggregates via TRAF6

Mutations in PARK7/DJ-1 gene are associated to autosomal recessive early onset forms of Parkinson's disease (PD). Although large gene deletions have been linked to a loss-of-function phenotype, the pathogenic mechanism of missense mutations is less clear. The L166P mutation causes misfolding of DJ-1 protein and its degradation. L166P protein may also accumulate into insoluble cytoplasmic aggregates with a mechanism facilitated by the E3 ligase TNF receptor associated factor 6 (TRAF6). Upon proteasome impairment L166P activates the JNK/p38 MAPK apoptotic pathway by its interaction with TRAF and TNF Receptor Associated Protein (TTRAP). When proteasome activity is blocked in the presence of wild-type DJ-1, TTRAP forms aggregates that are localized to the cytoplasm or associated to nucleolar cavities, where it is required for a correct rRNA biogenesis. In this study we show that in post-mortem brains of sporadic PD patients TTRAP is associated to the nucleolus and to Lewy Bodies, cytoplasmic aggregates considered the hallmark of the disease. In SH-SY5Y neuroblastoma cells, misfolded mutant DJ-1 L166P alters rRNA biogenesis inhibiting TTRAP localization to the nucleolus and enhancing its recruitment into cytoplasmic aggregates with a mechanism that depends in part on TRAF6 activity. This work suggests that TTRAP plays a role in the molecular mechanisms of both sporadic and familial PD. Furthermore, it unveils the existence of an interplay between cytoplasmic and nucleolar aggregates that impacts rRNA biogenesis and involves TRAF6

TRAF6 promotes atypical ubiquitination of mutant DJ-1 and alpha-synuclein and is localized to Lewy bodies in sporadic Parkinson's disease brains

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

Parkinson"s disease DJ-1 L166P alters rRNA biogenesis by exclusion of TTRAP from the nucleolus and sequestration into cytoplasmic aggregates via TRAF6

Mutations in PARK7/DJ-1 gene are associated to autosomal recessive early onset forms of Parkinson"s disease (PD). Although large gene deletions have been linked to a loss-of-function phenotype, the pathogenic mechanism of missense mutations is less clear. The L166P mutation causes misfolding of DJ-1 protein and its degradation. L166P protein may also accumulate into insoluble cytoplasmic aggregates with a mechanism facilitated by the E3 ligase TNF receptor associated factor 6 (TRAF6). Upon proteasome impairment L166P activates the JNK/p38 MAPK apoptotic pathway by its interaction with TRAF and TNF Receptor Associated Protein (TTRAP). When proteasome activity is blocked in the presence of wild-type DJ-1, TTRAP forms aggregates that are localized to the cytoplasm or associated to nucleolar cavities, where it is required for a correct rRNA biogenesis. In this study we show that in post-mortem brains of sporadic PD patients TTRAP is associated to the nucleolus and to Lewy Bodies, cytoplasmic aggregates considered the hallmark of the disease. In SH-SY5Y neuroblastoma cells, misfolded mutant DJ-1 L166P alters rRNA biogenesis inhibiting TTRAP localization to the nucleolus and enhancing its recruitment into cytoplasmic aggregates with a mechanism that depends in part on TRAF6 activity. This work suggests that TTRAP plays a role in the molecular mechanisms of both sporadic and familial PD. Furthermore, it unveils the existence of an interplay between cytoplasmic and nucleolar aggregates that impacts rRNA biogenesis and involves TRAF

Processing of rRNA is altered in L166P overexpressing cells.

<p>(A) Schematic diagram showing pre-rRNA structure and processing steps. Positioning of A0, 1 and 4 cleavage sites on rRNA processing intermediates is shown. ETS, external transcribed spacer; ITS, internal transcribed spacer. (B) Analysis of steady-state levels of rRNA precursor and processing intermediates. SH-SY5Y cells stably expressing wt DJ-1 (W), L166P (L) and empty vector (c) were treated with Bars, 5 µM MG132 for 16 h, or left untreated. Total RNA was extracted and levels of pre-rRNA and processing intermediates were analyzed by qPCR with primers targeting A0 , 1 and 4 cleavage sites. Standard deviations are calculated on four replicas from two independent experiments. *, p<0.05. (C) Analysis of rRNA processing by metabolic labeling. L166P (L) and control (c) cells were treated as in A. After treatment, cells were pulse-labeled with ³²P-orthophosphate for 1 h and chased with cold medium for 3 h. RNA was extracted and equal quantities were separated on denaturing agarose gel. Nascent rRNA was visualized by autoradiography after gel drying (upper panel). rRNA processing intermediates and mature forms are indicated on the right. Loading was verified with ethidium bromide staining (lower panel).</p

Mutant DJ-1 L166P inhibits TTRAP nucleolar localization after proteasome inhibition.

<p>(A) TTRAP nucleolar localization is inhibited in L166P expressing cells. SH-SY5Y cells stably transfected with empty vector (c), FLAG-DJ-1 wt (W) or L166P (L) were treated with 5 µM MG132 for 16 h. Triple immunofluorescence was performed with anti-NPM (blue), anti-TTRAP (green) and anti-FLAG (red) antibodies. Images are representatives of three independent experiments. Data have been confirmed on two independent clones for each cell line. Bars, 10 µm. (B) Quantification of TTRAP nucleolar localization. Cells as in A. At least 200 cells from two independent experiments were counted and scored for TTRAP in the nucleolus (*, p<0.05). (C) Localization of TTRAP is unaffected in cells depleted of endogenous DJ-1. SH-SY5Y cells stably expressing an inducible short-hairpin RNA targeting DJ-1 (siDJ-1, clones A and B) or a scramble shRNA control (scramble, clones a and b) were treated with doxycycline for 10 days to induce silencing of DJ-1 expression. Then cells were treated with 5 µM MG132 for 16 h. Immunofluorescence was performed with anti-TTRAP (green) and anti-NCL (red) antibodies. Nuclei were visualized with DAPI (blue). Images are representatives of two independent experiments. Bars, 10 µm. (D) Quantification of TTRAP nucleolar localization in siDJ-1 cells. Data were collected as in B. (NS, not-significant). (E) DJ-1 is efficiently depleted in siDJ-1 cells. Total protein lysates were prepared from cells as in C. Levels of endogenous DJ-1 were measured by western blot with anti-DJ-1 antibody. Beta-actin was detected as loading control. (F) TTRAP nucleolar localization is altered by mutant huntingtin. SH-SY5Y cells were transfected with huntingtin N-terminal fragment fused to GFP with WT (Gln²¹, Q21) or mutated (Gln¹⁵⁰, Q150) polyglutamine expansion. Endogenous TTRAP was visualized by indirect immunofluorescence (red). N-HTT was visible by GFP autofluorescence. Nuclei were visualized with DAPI (blue). Bars, 10 µm. (F) Quantification of TTRAP nucleolar localization in N-HTT expressing cells. Data were collected as in B on GFP-positive cells (*, p<0.05; NS, not-significant).</p