The Role of FBXO7 in Mitochondrial Biology and Parkinson's Disease

Parkinson’s disease is a progressive neurodegenerative disorder of the central nervous system, manifesting with both motor and non-motor symptoms. Autosomal recessive mutations in the FBXO7 gene have been identified to cause a rapidly progressing early-onset form of PD. Canonically, FBXO7 functions as a substrate-recruiting subunit of the SCF-type E3 ubiquitin ligase. However, it also has a variety of other atypical functions, such as cell cycle regulation, proteasome regulation, and mitophagy. The overall aim of this research was to characterise the functional role of FBXO7 in various in vitro and in vivo PD models. The models examined included FBXO7 shRNA knockdown SH-SY5Y cell lines, FBXO7 CRISPR knockout SH-SY5Y cell lines, primary patient fibroblasts with a FBXO7 mutation, and MEFs and tissues from a Fbxo7 KO mouse. My analysis of fibroblasts from a patient without FBXO7 expression revealed several interesting phenotypes. Briefly, the patient fibroblasts proliferated slower due to increased apoptosis and lower CDK6 and cyclin D1 expression, which led to fewer cells progressing through the G1 phase of the cell cycle. My experiments showed that these cells also had mitochondrial respiration defects, exhibiting lower basal respiration, ATP production, maximal respiration and spare capacity, in addition to complex I, III and IV deficiencies. Patient fibroblasts also had significantly lower levels of 12S and 16S ribosomal mRNA transcripts, which are necessary for the translation of mitochondrially encoded subunits of complexes I, III, and IV. Similar phenotypes were also observed in MEFs from a Fbxo7 KO mouse model, indicating conservation between human and mouse FBXO7 in regulating mitochondria, cell death and proliferation. In a tissue-specific KO mouse model of PD, where FBXO7 expression was ablated in the dopaminergic neurons, I analysed proteins regulated by FBXO7 which might be responsible for cell loss in the substantia nigra. I discovered that RPL23, a regulator of MDM2, was ubiquitinated by SCFFbxo7 using K48 chain linkages, promoting its degradation by the proteasome. This suggests that misregulation of the MDM2:p53 axis may underlie the cell loss observed in this conditional Fbxo7 KO mouse model. In conclusion, these results elaborate on the role of FBXO7 in mitochondrial biology, and identify a new ubiquitination substrate of FBXO7 in a mouse model of PD. It is hoped that by elucidating the potential pathogenic mechanisms of FBXO7 in rare familial forms of the disease, it will be possible to translate findings to the more prevalent sporadic forms of Parkinson’s disease as well.Department of Pathology PhD Studentshi

Loss of FBXO7 results in a Parkinson’s-like dopaminergic degeneration via an RPL23-MDM2-TP53 pathway

The field of Parkinson’s disease research has been impeded by the absence of animal models that clearly phenocopy the features of this neurodegenerative condition. Mutations in FBXO7/PARK15 are associated with both sporadic Parkinson’s disease and a severe form of autosomal recessive early-onset Parkinsonism. Here we report that conditional deletion of Fbxo7 in the midbrain dopamine neurons results in an early reduction in striatal dopamine levels, together with a slow, progressive loss of midbrain dopamine neurons and onset of locomotor defects. Unexpectedly, a later compensatory response led to a near-full restoration of dopaminergic fibre innervation in the striatum, but nigral cell loss was irreversible. Mechanistically, there was increased expression in the dopamine neurons of FBXO7-interacting protein, RPL23, which is a sensor of ribosomal stress that inhibits MDM2, the negative regulator of p53. A corresponding activated p53 transcriptional signature biased towards pro-apoptotic genes was also observed. These data suggest the neuroprotective role of FBXO7 involves its suppression of the RPL23-MDM2-p53 axis that promotes cell death in dopaminergic midbrain neurons.Biotechnology and Biological Sciences Research Council (BB/J007846/1), DDPDgenes, Parkinson's UK and the CurePD Trust, and Wellcome Trust-MRC funded Cambridge Stem Cell Institute and an NIHR award of a Biomedical Research Centre for Addenbrooke’s Hospital/University of Cambridge

VCP mutations in familial and sporadic amyotrophic lateral sclerosis

Mutations in the valosin-containing protein (VCP) gene were recently reported to be the cause of 1%-2% of familial amyotrophic lateral sclerosis (ALS) cases. VCP mutations are known to cause inclusion body myopathy (IBM) with Paget's disease (PDB) and frontotemporal dementia (FTD). The presence of VCP mutations in patients with sporadic ALS, sporadic ALS-FTD, and progressive muscular atrophy (PMA), a known clinical mimic of inclusion body myopathy, is not known. To determine the identity and frequency of VCP mutations we screened a cohort of 93 familial ALS, 754 sporadic ALS, 58 sporadic ALS-FTD, and 264 progressive muscular atrophy patients for mutations in the VCP gene. Two nonsynonymous mutations were detected; 1 known mutation (p.R159H) in a patient with familial ALS with several family members suffering from FTD, and 1 mutation (p.I114V) in a patient with sporadic ALS. Conservation analysis and protein prediction software indicate the p.I114V mutation to be a rare benign polymorphism. VCP mutations are a rare cause of familial ALS. The role of VCP mutations in sporadic ALS, if present, appears limited. (C) 2012 Elsevier Inc. All rights reserve