Depending on the stress, histone deacetylase inhibitors act as heat shock protein co-inducers in motor neurons and potentiate arimoclomol, exerting neuroprotection through multiple mechanisms in ALS models.

Upregulation of heat shock proteins (HSPs) is an approach to treatment of neurodegenerative disorders with impaired proteostasis. Many neurons, including motor neurons affected in amyotrophic lateral sclerosis (ALS), are relatively resistant to stress-induced upregulation of HSPs. This study demonstrated that histone deacetylase (HDAC) inhibitors enable the heat shock response in cultured spinal motor neurons, in a stress-dependent manner, and can improve the efficacy of HSP-inducing drugs in murine spinal cord cultures subjected to thermal or proteotoxic stress. The effect of particular HDAC inhibitors differed with the stress paradigm. The HDAC6 (class IIb) inhibitor, tubastatin A, acted as a co-inducer of Hsp70 (HSPA1A) expression with heat shock, but not with proteotoxic stress induced by expression of mutant SOD1 linked to familial ALS. Certain HDAC class I inhibitors (the pan inhibitor, SAHA, or the HDAC1/3 inhibitor, RGFP109) were HSP co-inducers comparable to the hydroxyamine arimoclomol in response to proteotoxic stress, but not thermal stress. Regardless, stress-induced Hsp70 expression could be enhanced by combining an HDAC inhibitor with either arimoclomol or with an HSP90 inhibitor that constitutively induced HSPs. HDAC inhibition failed to induce Hsp70 in motor neurons expressing ALS-linked mutant FUS, in which the heat shock response was suppressed; yet SAHA, RGFP109, and arimoclomol did reduce loss of nuclear FUS, a disease hallmark, and HDAC inhibition rescued the DNA repair response in iPSC-derived motor neurons carrying the FUSP525Lmutation, pointing to multiple mechanisms of neuroprotection by both HDAC inhibiting drugs and arimoclomol.

A novel small molecule HSP90 inhibitor, NXD30001, differentially induces heat shock proteins in nervous tissue in culture and in vivo.

Heat shock proteins (HSPs) are attractive therapeutic targets for neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), characterized by aberrant formation of protein aggregates. Although motor neurons have a high threshold for activation of HSP genes, HSP90 inhibitors are effective inducers. This study evaluated NXD30001, a novel, small molecule HSP90 inhibitor based on the radicicol backbone, for its ability to induce neuronal HSPs and for efficacy in an experimental model of ALS based on mutations in superoxide-dismutase 1 (SOD1). In motor neurons of dissociated murine spinal cord cultures, NXD30001-induced expression of HSP70/HSPA1 (iHSP70) and its co-chaperone HSP40/DNAJ through activation of HSF1 and exhibited a protective profile against SOD1(G93A) similar to geldanamycin, but with less toxicity. Treatment prevented protein aggregation, mitochondrial fragmentation, and motor neuron death, important features of mutant SOD1 toxicity, but did not effectively prevent aberrant intracellular Ca(2+) accumulation. NXD30001 distributed to brain and spinal cord of wild-type and SOD1(G93A) transgenic mice following intraperitoneal injection; however, unlike in culture, in vivo levels of SOD1 were not reduced. NXD30001-induced expression of iHSP70 in skeletal and cardiac muscle and, to a lesser extent, in kidney, but not in liver, spinal cord, or brain, with either single or repeated administration. NXD30001 is a very useful experimental tool in culture, but these data point to the complex nature of HSP gene regulation in vivo and the necessity for early evaluation of the efficacy of novel HSP inducers in target tissues in vivo.

Acute exercise remodels mitochondrial membrane interactions in mouse skeletal muscle.

A unique property of mitochondria in mammalian cells is their ability to physically interact and undergo dynamic events of fusion/fission that remodel their morphology and possibly their function. In cultured cells, metabolic perturbations similar to those incurred during exercise influence mitochondrial fusion and fission processes, but it is unknown whether exercise acutely alters mitochondrial morphology and/or membrane interactions in vivo. To study this question, we subjected mice to a 3-h voluntarily exercise intervention following their normal physical activity patterns, and quantified mitochondrial morphology and membrane interactions in the soleus using a quantitative electron microscopy approach. A single exercise bout effectively decreased blood glucose (P < 0.05) and intramyocellular lipid content (P < 0.01), indicating increased muscle metabolic demand. The number of mitochondria spanning Z-lines and proportion of electron-dense contact sites (EDCS) between adjacent mitochondrial membranes were increased immediately after exercise among both subsarcolemmal (+116%, P < 0.05) and intermyofibrillar mitochondria (+191%, P < 0.001), indicating increased physical interactions. Mitochondrial morphology, and abundance of the mitochondrial pro-fusion proteins Mfn2 and OPA1 were unchanged. Collectively, these results support the notion that mitochondrial membrane dynamics are actively remodelled in skeletal muscle, which may be regulated by contractile activity and the metabolic state. Future studies are required to understand the implications of mitochondrial dynamics in skeletal muscle physiology during exercise and inactivity.