Sequestration of chaperones and proteasome into Lafora bodies and proteasomal dysfunction induced by Lafora disease-associated mutations of malin

Abstract

Lafora disease (LD) is an autosomal recessive progressive myoclonic epilepsy characterized by the presence of intracellular polyglucosan inclusions commonly known as Lafora bodies in many tissues, including the brain, liver and skin. The disease is caused by mutations in either EPM2A gene, encoding the protein phosphatase, laforin, or EPM2B gene, encoding the ubiquitin ligase, malin. But how mutations in these two genes cause disease pathogenesis is poorly understood. In this study, we show that the Lafora bodies in the axillary skin and brain stain positively for the ubiquitin, the 20S proteasome and the molecular chaperones Hsp70/Hsc70. Interestingly, mutant malins that are misfolded also frequently colocalizes with Lafora bodies in the skin biopsy sample of the respective LD patient. The expression of disease-causing mutations of malin in Cos-7 cells results in the formation of the profuse cytoplasmic aggregates that colocalize with the Hsp70/Hsc70 chaperones and the 20S proteasome. The mutant malin expressing cells also exhibit proteasomal dysfunction and cell death. Overexpression of Hsp70 decreases the frequency of the mutant malin aggregation and protects from mutant malin-induced cell death. These findings suggest that Lafora bodies consist of abnormal proteins, including mutant malin, targeted by the chaperones or the proteasome for their refolding or clearance, and failure of these quality control systems could lead to LD pathogenesis. Our data also indicate that the Hsp70 chaperone could be a potential therapeutic target of LD.

Malin and laforin are essential components of a protein complex that protects cells from thermal stress

“…Nonetheless, our semi-quantitative RT-PCR assay and the Hsp70 promoter reporter assay suggest that loss of laforin or malin does not alter the level of Hsp70-encoding transcripts when the cells are not under stress; therefore, the increased level of Hsp70 protein is probably due to its inefficient clearance from the cellular pool. In this regard, it is interesting that the loss of laforin or malin results in reduced proteasomal activity (Vernia et al, 2009;Rao et al, 2010a). Taken together, the results from the present study suggest that loss of any one of the components of the malinlaforin-CHIP complex affects the ability of the cell to achieve the stress response and might also result in the accumulation of unwanted proteins.…”

Emerging role of autophagy in pediatric neurodegenerative and neurometabolic diseases

“…60 In contrast, another recent study revealed no changes in LC3-II or other autophagy proteins in brain lysates of another laforin-deficient mouse model, arguing that autophagosome formation and function in the brain of these mice are not critically abnormal. 55 Nevertheless, the recruitment of crucial proteins of the proteasomal, endosomal, and autophagy pathway to LBs, as observed in laforin-deficient mice 55 and brain biopsy tissue of a LD patient, 61 seems to indicate that secondary changes in protein degradation pathways are important contributors to neuronal pathology.…”

Malin Regulates Wnt Signaling Pathway through Degradation of Dishevelled2

“…This is further supported by some recent studies indicating the role of either laforin or malin in cell survival pathways (19,20). Emerging evidence now points toward the defect in intracellular protein degradation pathways in LD pathogenesis (21)(22)(23)(24). Because malin is an ubiquitin ligase, its loss of function could result in improper clearance and accumulation of its target substrates, leading to the pathogenesis of LD.…”