Ubiquitin protein ligases and neurodegeneration
Mutations in components of the ubiquitin proteasome system (UPS) are responsible for several familial forms of common neurodegenerative diseases. The UPS is the major route by which abnormal, short-lived and misfolded proteins are broken down in eukaryotic cells. However, as we age the efficiency of the UPS (in particular the proteasome) decreases.
Taken together, these observations strongly suggest that accumulation of aberrant (ubiquitylated) proteins inside neuronal cells plays a fundamental role in neurodegeneration.
Indeed, protein inclusions including the Lewy bodies and the neurofibrillary tangles of Parkinson’s and Alzheimer’s diseases, respectively, have been observed in the surviving brain cells of patients. Such inclusions can be replicated in cell culture systems which allow us to study their properties in order to understand their role in disease progression.
I am particularly interested in the role of the ubiquitin protein ligases (E3s) in neurodegenerative disorders. E3s allow the specificity of ubiquitin-mediated protein degradation, controlling when and where a particular protein is targeted for proteasomal degradation. Parkin is one such ligase. Mutations to PARKIN cause early onset autosomal recessive Parkinsonism (AR-JP). We have discovered that when proteasomal function is impaired (ie, mimicking what occurs as we age) Parkin accumulates to form Lewy body-like inclusions. We have generated these models in both neuronal and non-neuronal cell lines transiently or stably expressing the Parkin protein.
These models are allowing us to investigate many aspects of how these inclusions form, how they are removed from the cell and what may exacerbate or prevent them from forming. Parkin is thought to have other non-UPS related cellular roles. These cellular models are also being utilised to investigate these alternative functions of Parkin.
Figure 1 Over-expression of neurodegenerative disease associated proteins often causes aggresome formation in vitro. Disease associated mutant proteins readily aggregate (a), wild-type proteins usually require additional stresses for inclusions formation (b-d). Classical microtubule dependent aggresomes (a & c) and ribbon aggresomes (b & d). PolyQ associated with HD, Presenilin-1 with AD, Parkin and UCH-L1 with PD. Arrows indicate the inclusions.
Figure 2. SH-SY6Y cell lines were generated using the SY-TR host cell line (kindly provided by Dr Buee, INSERM France). Parkin is found throughout the cell and forms inclusions on addition of 5 uM MG132 for 16h as indicated by arrows.
Ardley HC and Robinson PA. E3 Ubiquitin Ligases. Essays in Biochemistry. (2005) 41: 15-30.
Garside H, Waters C, Berry A, Rice L, Ardley HC, White A, Robinson PA and Ray D. UBCH7 interacts with the glucocorticoid receptor and mediates receptor autoregulation. Journal of Endocrinology (2006) 190: 621-629.
Hung C-C, Davison EJ, Robinson PA and Ardley HC. The aggravating role of the ubiquitin-proteasome system in neurodegeneration. Biochem. Soc. Trans (2006) 34: 743-5.