Can the Heat Shock Response Rescue Muscles and Motor Neurons?

In many degenerative diseases, misfolded proteins accumulate and stress out cells. Could revving up cellular chaperones, which wrangle damaged proteins, provide some relief? Two recent papers report evidence in support of this.

In the March 23 Science Translational Medicine, researchers led by Linda Greensmith and Michael Hanna at University College London and Richard Barohn and Mazen Dimachkie at the University of Kansas Medical Center, Kansas City, describe benefits from the experimental drug arimoclomol. This hydroxylamine compound switches on the heat shock response, a protective pathway normally activated during cellular stress to remove misfolded proteins. In cellular and mouse models of the degenerative muscle disease inclusion body myositis, arimoclomol ameliorated classic IBM pathology, including accumulation of the amyloid precursor protein (APP) and mislocalization of TDP-43. It also improved muscle function. The compound recently completed Phase 1 safety testing in people with the condition, and is headed for a Phase 2 trial.

In the March 1 Brain, researchers led by Christopher Shaw at King’s College London reported that activation of heat shock proteins cleared TDP-43 deposits in cell cultures. TDP-43 accumulates in motor neurons of people with amyotrophic lateral sclerosis and in cortical neurons in some forms of dementia. Shaw and colleagues found lowered levels of heat shock proteins in the spinal cords of people with ALS, suggesting this protective response is perturbed in the disorder and might make a therapeutic target. Arimoclomol is currently in a Phase 2/3 trial for amyotrophic lateral sclerosis, as well.

“Augmenting this endogenous cytoprotective pathway may be an effective approach for slowing disease progression,” Greensmith told Alzforum. She is exploring the potential of this approach for several motor neuron and muscle diseases.

Numerous studies have demonstrated that particular heat shock proteins can protect against protein aggregation in disorders such as Huntington’s, ALS, and Parkinson’s (see Feb 2010 newsNovoselov et al., 2013Aug 2014 news). Arimoclomol takes the strategy a step further, because it prolongs the activity of heat shock factor 1 (HSF-1), the master regulator that turns on the whole pathway. Because arimoclomol acts only in cells where HSF-1 is already present, it is unlikely to have side effects on healthy, unstressed cells, Greensmith believes. She previously reported that the compound made SOD1 mouse models of ALS live longer (see Kieran et al., 2004; Apr 2005 conference newsKalmar et al., 2008).

Greensmith wondered if arimoclomol might be more effective in a simpler protein misfolding disorder. She turned to inclusion body myositis (IBM), a disease of unknown cause in which muscles become inflamed and degenerate. Muscle cells accumulate deposits of misfolded proteins, including TDP-43, APP, phosphorylated tau, and heat shock proteins. People with IBM eventually lose the ability to walk, swallow, and breathe. No treatments exist. Previous clinical trials have tested anti-inflammatory strategies without success, but have not tackled protein misfolding.

The researchers first had to develop cell culture models of the disease. Co-first authors Mhoriam Ahmed, Pedro Machado, Adrian Miller, and Charlotte Spicer overexpressed human APP in primary rat myocytes. The muscle cells developed inclusion bodies reminiscent of those seen in IBM, and their death rate quintupled. Treatment with arimoclomol prevented these deposits and restored cell survival to normal. The compound also improved other measures of cell health, such as intracellular calcium homeostasis.

No mouse model for IBM exists, but in 2010 Paul Taylor and colleagues at St. Jude Children’s Research Hospital, Memphis, generated a mouse that expresses human mutant valosin-containing protein (VCP) (see Custer et al., 2010). In people, this gene causes inclusion body myopathy associated with Paget’s disease of bone and frontotemporal dementia. Their muscles develop inclusion bodies and degenerate, just as they do in inclusion body myositis. Greensmith and colleagues treated the VCP mice with arimoclomol from four to 14 months of age. Treated mice maintained their grip strength and muscle force at 14 months. Their muscles were less inflamed and degenerated than those of untreated littermates.

Based on these findings, the authors tested arimoclomol in a Phase 1 trial of 24 people with IBM, at the University of Kansas Medical Center. Sixteen people received the drug, the rest placebo, for four months. Adverse events were similar in both groups, and the drug was well-tolerated. The trial was not powered to detect efficacy, but the researchers saw a trend toward better maintenance of muscle strength in the treatment group. They have obtained approval and funding for a 12-month Phase 2 trial that will begin enrolling 150 IBM patients later this year, Greensmith said.

Since heat shock proteins can clean up many types of misfolded protein, the strategy could have promise for other degenerative diseases as well. The Danish startup company Orphazyme, based in Copenhagen, recently acquired the rights to arimoclomol and plans to test its efficacy for the childhood lysosomal storage disorder Niemann-Pick Disease in a Phase 2/3 trial. Meanwhile, the Phase 2/3 trial of arimoclomol for ALS will wrap up this year. Arimoclomol was originally developed by a Hungarian company, Biorex, and then sold to CytRx, Los Angeles, which provided it for the ALS trial.

The findings from Shaw and colleagues add to the basic science evidence that the heat shock response could help in ALS. First author Han-Jou Chen expressed a constitutively active form of HSF-1 in cells that overexpress TDP-43. This strategy slashed aggregate load by three-quarters and helped cells survive. Conversely, a dominant-negative form of HSF-1 tripled the load of insoluble TDP-43.

The authors wondered which specific heat shock proteins might be responsible for the improvement. They tested several candidates in these cultures, and found that DNAJB2a lowered insoluble TDP-43 levels nearly as much as HSF-1 did. DNAJB2a, a member of the HSP40 family, acts as a co-chaperone with HSP70 proteins. When the authors removed DNAJB2a’s ability to bind HSP70, it lost its protective power, showing that this interaction was critical for clearing TDP-43 deposits. DNAJB2a can also direct proteins to the proteasome for degradation, but removing this ability had no effect on TDP-43. In addition, treatment with this heat shock protein did not change the total level of TDP-43. Together, the data suggest that DNAJB2a and HSP70 help re-fold aggregated TDP-43 and make it soluble again, but do not degrade it, the authors noted.

Given this ability to clear TDP-43 deposits, does the heat shock response play a role in ALS, where the protein accumulates? The authors examined spinal cord lysates from mouse models of ALS as well as from people with the condition. In both types of tissue, they found lower levels of DNAJB2a and HSP70 than in controls. Restoring the activity of these heat shock proteins might be a viable therapeutic strategy, they suggest. Mutations in DNAJB2a have been linked to motor neuropathy and Charcot-Marie-Tooth disease, highlighting the importance of this protein for motor function (see Blumen et al., 2012; Gess et al., 2014).

“The data suggest again that DNAJ proteins could be excellent targets for combating neurodegenerative protein aggregation diseases,” Harm Kampinga at the University of Groningen, The Netherlands, wrote to Alzforum. Kampinga noted that DNAJ proteins have also been found to squelch protein aggregation and ameliorate toxicity in diseases where polyQ proteins accumulate.

Chen told Alzforum that she will next validate the findings in ALS mouse models by expressing the constitutively active HSF-1 in motor neurons. She also plans to screen for additional activators of HSF-1. Little is known about how to control DNAJB2a, so it is not yet possible to target that protein specifically. “Although we’ve known about the heat shock response for a long time, we still do not understand its regulation,” Chen noted. “That will be important to figure out, because protein folding plays such a crucial role in neurodegenerative disease.” Activators of the heat shock response might be useful for many such diseases, Chen believes.

1. Ahmed M, Machado PM, Miller A, Spicer C, Herbelin L, He J, Noel J, Wang Y, McVey AL, Pasnoor M, Gallagher P, Statland J, Lu CH, Kalmar B, Brady S, Sethi H, Samandouras G, Parton M, Holton JL, Weston A, Collinson L, Taylor JP, Schiavo G, Hanna MG, Barohn RJ, Dimachkie MM, Greensmith L. Targeting protein homeostasis in sporadic inclusion body myositis. Sci Transl Med. 2016 Mar 23;8(331):331ra41. [PubMed].

2. Chen HJ, Mitchell JC, Novoselov S, Miller J, Nishimura AL, Scotter EL, Vance CA, Cheetham ME, Shaw CE. The heat shock response plays an important role in TDP-43 clearance: evidence for dysfunction in amyotrophic lateral sclerosis. Brain. 2016 Mar 1. [Pubmed]. Epub ahead of print.

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