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 2a 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 2a trial of 24 people with IBM, at the University of Kansas Medical Center and the MRC Centre for Neuromuscular Diseases at University College London. 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 multicenter Phase 2b/3 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). 25274842

“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.—Madolyn Bowman Rogers

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  1. It is clear that there is a correlation between persistent higher order assembly of RNA-binding proteins and disease (including ALS, FTD and IBM). The most prominent protein is TDP-43, but similar behavior is observed for FUS, hnRNPA1, hnRNPA2B1, TIA-1, and hnRNPDL, and likely others. A normal feature of all of these RNA-binding proteins is assembly into dynamic structures to form ribonucleoprotein bodies (e.g., RNA granules) but these structures become less dynamic—more persistently assembled—in disease states. The assembly and disassembly of these structures is normally subject to several types of regulation—chaperones contribute importantly to this (we showed this for DNAJB6 in a paper published last month—see Li et al., 2016). I think the point of overlap between Chen et al. and Ahmed et al. is the idea that the chaperone system can be amplified to reverse abnormally persistent higher order assemblies of RNA-binding proteins, and that this is accompanied by therapeutic benefit.

    References:

    . Genetic interaction of hnRNPA2B1 and DNAJB6 in a Drosophila model of multisystem proteinopathy. Hum Mol Genet. 2016 Mar 1;25(5):936-50. Epub 2016 Jan 6 PubMed.

  2. DNAJ-Yeah!

    Hsp70 co-chaperones of the Hsp40 (DNAJ) family prevent formation of toxic aggregates

    Protein aggregates hallmark nearly all age-related neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), several polyglutamine (PolyQ) disorders such as Huntington’s disease (HD) and different forms of spinocerebellar ataxias (SCA1,2,3,6,7), as well as amyotrophic lateral sclerosis (ALS). This has been taken as evidence to suggest that a collapse in cellular protein homeostasis might be a central theme underlying these diseases and that boosting rate-limiting components of the cellular protein quality-control system might be a potential strategy to counteract protein aggregates or their toxic consequences (Balch et al., 2008). 

    In a recent paper in Brain from the groups of Christopher Shaw and Michael Cheetham, this paradigm was investigated for the Tar DNA binding protein 43 (TDP-43) that is a major constituent of aggregates that hallmark ALS and for which multiple mutations have been shown to be causative for heritable forms of the disease. Hereto, they first modulated a central regulator of the cellular protein quality-control system, heat shock transcription factor-1 (HSF-1). This transcription factor, amongst others, can elevate the expression of a set of proteins called heat shock proteins (HSPs) that play a central role in regulating protein quality control (Akerfelt et al., 2010). By binding to protein substrates, HSPs assist in both protein folding and protein degradation. In line with their expectations, HSF-1 activation was found to reduce, and HSF-1 inactivation to enhance, TDP-43 aggregation in several cell models, including rat primary cortical neurons.

    Since HSF-1 activates a broad spectrum of different HSPs at the same time, they next tested whether the upregulation of a single member of HSPs would suffice to suppress TDP-43 aggregation. Surprisingly, upregulation of several main targets of HSF-1 like Hsp70 (HSPA1A), Hsc70 (HSPA8), Hsp40 (DNAJB1) or Hsp27 (HSPB1) did not lead to a suppression of TDP-43 aggregation. However, the sole upregulation of a few members of the DNAJ (Hsp40-like) family of proteins, including DNAJB2a, DNAJB4, DNAJB6a, DNAJB6b, and DNAJB8, were as effective as activating HSF-1 in suppressing TDP-43.

    DNAJ proteins are considered to be co-chaperones of HSP70 machines: They are thought to bind protein substrates and “deliver” them for further client processing (Kampinga and Craig, 2010). For DNAJB2a, which the authors chose to further investigate, it was shown that the ability to interact with Hsp70 was crucial for DNAJB2a-mediated suppression of TDP-43 aggregation. However, a specific C-terminal domain of DNAJB2a that contains two ubiquitin interacting motifs (UIMs) that distinguish it from most other DNAJ family members, was found to be dispensable for aggregation suppression. These UIMs in DNAJB2a previously had been shown to be important for proteasomal targeting of substrates, such as mutants of superoxide dismutase-1 (SOD-1), which also cause ALS (Novoselov et al., 2013). In turn, the authors showed that DNAJB2a-mediated prevention of TDP-43 aggregation was unaffected by proteasomal inhibition. In addition, the fact that other co-chaperones, such asDNAJB4, DNAJB6a, DNAJB6b, and DNAJB8 (which lack such UIMs), were effective in suppressing TDP43 aggregation suggests that mechanisms other than promoting ubiquitin-directed protein degradation must be responsible for regulating TDP43 protein homeostasis.  The authors speculate that DNAJB2a might assist in TDP43 folding.

    Irrespective of what the mechanism might be, the data suggest again that DNAJ proteins could be excellent targets for combatting neurodegenerative protein aggregation diseases. Besides inhibiting aggregation of TDP43 and mutant SOD-1 (both causing ALS), DNAJ proteins have been shown to suppress aggregation and toxicity in models of PolyQ diseases. Strikingly, also for PolyQ diseases DNAJB2a, DNAJB6a, DNAJB6b, and DNAJB8 were amongst the most effective ones (Hageman et al., 2010). These proteins are closely related, evolutionarily, but yet have distinct structural features. What the communalities are that make them so effective in several of these aggregation diseases remains to be established. 

    How one may manipulate the levels or activity of these DNAJBs for therapeutic approaches remains unclear. Whilst HSF-1 activators have been developed, their potential for chronic treatment may be limited because along with having multiple targets they may come with unwanted side effects, and because activation of HSF-1 declines with age (Akerfelt et al., 2010). Moreover, it cannot yet be concluded from the work by Shaw et al. whether the HSF-1-mediated protection can really be attributed to DNAJB2a, as this would require HSF-1-activation experiments under conditions of blocking DNAJB2a induction. In fact, HSF-1 mediated protection could depend on mechanisms that are still distinct from direct DNAJB2b protection. Fortunately, however, all the DNAJB proteins identified as TDP43 suppressors are only weakly HSF-1-regulated, so options for their upregulation independent of HSF-1 activation might be found.

    References:

    . Adapting proteostasis for disease intervention. Science. 2008 Feb 15;319(5865):916-9. PubMed.

    . Heat shock factors: integrators of cell stress, development and lifespan. Nat Rev Mol Cell Biol. 2010 Aug;11(8):545-55. Epub 2010 Jul 14 PubMed.

    . The HSP70 chaperone machinery: J proteins as drivers of functional specificity. Nat Rev Mol Cell Biol. 2010 Aug;11(8):579-92. PubMed.

    . Molecular chaperone mediated late-stage neuroprotection in the SOD1(G93A) mouse model of amyotrophic lateral sclerosis. PLoS One. 2013;8(8):e73944. Epub 2013 Aug 30 PubMed.

    . A DNAJB chaperone subfamily with HDAC-dependent activities suppresses toxic protein aggregation. Mol Cell. 2010 Feb 12;37(3):355-69. PubMed.

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References

News Citations

  1. Chaperones Join HDACs on Road to Neutralizing Poly-Q Toxicity
  2. Yeast Chaperone Melts Protein Aggregates
  3. Dublin: Warm Sun, Hot Science Fire up Meeting

Paper Citations

  1. . Molecular chaperone mediated late-stage neuroprotection in the SOD1(G93A) mouse model of amyotrophic lateral sclerosis. PLoS One. 2013;8(8):e73944. Epub 2013 Aug 30 PubMed.
  2. . Treatment with arimoclomol, a coinducer of heat shock proteins, delays disease progression in ALS mice. Nat Med. 2004 Apr;10(4):402-5. PubMed.
  3. . .
  4. . Transgenic mice expressing mutant forms VCP/p97 recapitulate the full spectrum of IBMPFD including degeneration in muscle, brain and bone. Hum Mol Genet. 2010 May 1;19(9):1741-55. Epub 2010 Feb 10 PubMed.
  5. . A rare recessive distal hereditary motor neuropathy with HSJ1 chaperone mutation. Ann Neurol. 2012 Apr;71(4):509-19. PubMed.
  6. . HSJ1-related hereditary neuropathies: novel mutations and extended clinical spectrum. Neurology. 2014 Nov 4;83(19):1726-32. Epub 2014 Oct 1 PubMed.

External Citations

  1. Phase 2a trial 
  2. Phase 2/3 trial
  3. Phase 2/3 trial

Further Reading

Primary Papers

  1. . Targeting protein homeostasis in sporadic inclusion body myositis. Sci Transl Med. 2016 Mar 23;8(331):331ra41. PubMed.
  2. . Targeting protein homeostasis in sporadic inclusion body myositis. Sci Transl Med. 2016 Mar 23;8(331):331ra41. PubMed.