Mitochondrial deficits and abnormal mitochondrial retrograde axonal transport play a role in the pathogenesis of mutant Hsp27-induced Charcot Marie Tooth Disease

Abstract

Mutations in the small heat shock protein Hsp27, encoded by the HSPB1 gene, have been shown to cause Charcot Marie Tooth Disease type 2 (CMT-2) or distal hereditary motor neuropathy (dHMN). Protein aggregation and axonal transport deficits have been implicated in the disease. In this study, we conducted analysis of bidirectional movements of mitochondria in primary motor neuron axons expressing wild type and mutant Hsp27. We found significantly slower retrograde transport of mitochondria in Ser135Phe, Pro39Leu and Arg140Gly mutant Hsp27 expressing motor neurons than in wild type Hsp27 neurons, although anterograde movement velocities remained normal. Retrograde transport of other important cargoes, such as the p75 neurotrophic factor receptor was minimally altered in mutant Hsp27 neurons, implicating that axonal transport deficits primarily affect mitochondria and the axonal transport machinery itself is less affected. Investigation of mitochondrial function revealed a decrease in mitochondrial membrane potential in mutant Hsp27 expressing motor axons, as well as a reduction in mitochondrial complex 1 activity, increased vulnerability of mitochondria to mitochondrial stressors, leading to elevated superoxide release and reduced mitochondrial glutathione (GSH) levels, although cytosolic GSH remained normal. This mitochondrial redox imbalance in mutant Hsp27 motor neurons is likely to cause low level of oxidative stress, which in turn will contribute to, and indeed may be the underlying cause of the deficits in mitochondrial axonal transport. Together, these findings suggest that the mitochondrial abnormalities in mutant Hsp27-induced neuropathies may be a primary cause of pathology, leading to further deficits in the mitochondrial axonal transport and onset of disease.

Neuromuscular Diseases Due to Chaperone Mutations: A Review and Some New Results

“…In the experiments of Kalmar and colleagues on cultured motor neurons, the expression of HSPB1 mutants, most remarkably p.S135F, decreased mitochondrial membrane potential in neurites and impaired complex I activity [360]. This was accompanied with decreased glutathione levels and increased oxidative stress [360]. As the retrograde transport of p75NTD was affected to a lower extent, a primary impairment in mitochondrial function, rather than a cytoskeletal defect, was suggested to underlie the altered mitochondrial transport in this model [360].…”

“…This was accompanied with decreased glutathione levels and increased oxidative stress [360]. As the retrograde transport of p75NTD was affected to a lower extent, a primary impairment in mitochondrial function, rather than a cytoskeletal defect, was suggested to underlie the altered mitochondrial transport in this model [360]. One mechanism for this was identified by Schwartz et al, who showed that an abnormal interaction of mutant HSPB1 with ceramide synthase led to decreased mitochondrial ceramide content and impaired respiration [362].…”

“…Cytoskeletal defects, tubulin hypoacetylation in particular, may lead to neurodegeneration by disturbing the axonal transport of proteins and organelles. Indeed, a decreased proportion of moving mitochondria and/or lower mitochondrial velocity have been reported in neurons derived from the HSPB1 p.S135F mouse model [352] or from patient-derived induced pluripotent stem cells (iPSCs) [359] or transduced with mutant proteins [360]. The defects are relevant for the pathogenesis and seem to be at least partly due to MT hypoacetylation, as HDAC6 inhibitors can correct the mitochondrial transport in vitro [352,359,361] and reverse the neuropathic phenotype of the HSPB1 p.S135F mouse model in vivo [352].…”

“…The role of mitochondrial defects in the downstream pathomehcanism of HSPB1 mutations gained additional support from recent studies [360,362]. In the experiments of Kalmar and colleagues on cultured motor neurons, the expression of HSPB1 mutants, most remarkably p.S135F, decreased mitochondrial membrane potential in neurites and impaired complex I activity [360]. This was accompanied with decreased glutathione levels and increased oxidative stress [360].…”

Data-Independent Acquisition-Based Quantitative Proteomic Analysis of m.3243A>G MELAS Reveals Novel Potential Pathogenesis and Therapeutic Targets

“…The most key proteins were HSPB1 and CRYAB, which were small heat shock proteins (sHsp) that combined misfolded proteins to prevent them from denaturation, inhibited cellular apoptosis and regulated the intracellular redox state as molecular chaperones 15,16 . In mutant HSPB1 expressing motor axons, the researchers observed the decreased mitochondrial Complex I activity and increased mitochondrial vulnerability, resulted in increased superoxide release and decreased mitochondrial glutathione levels 17 . The CRYAB could protect cells from hypoxia and maintain mitochondrial integrity 18,19 and also had other roles in vascular biology 20 .…”