Well before symptoms of familial amyotrophic lateral sclerosis surface, motor neurons struggle to move cargo along their axons. A paper in the October 11 Nature Communications offers a new model system to study the defect, and suggests a possible fix. Researchers led by Catherine Verfaillie and Ludo Van Den Bosch, VIB-KU Leuven, Belgium, generated motor neurons derived from patient stem cells carrying mutations in the fused in sarcoma, a.k.a. FUS, gene. Among several previously described defects, the researchers spotted sluggish mitochondria and lazy endoplasmic reticulum vesicles in axons, as well as fewer contacts between the two organelles. The deficits were reversed by treating the cells with inhibitors of histone deacetylase 6, which removes acetyl groups from the microtubule tracks on which the organelles travel.

  • Axonal transport fails in motor neurons derived from FUS-ALS patients.
  • Inhibiting HDAC6 increases α-tubulin acetylation and rescues transport.

“The very important aspect of this study is that it used human cells, so we have a representation of patient neurons in culture. We’ve had many promising results from animal models in this field which have not translated to humans,” noted Majid Hafezparast at the University of Sussex in Brighton, U.K. Fen-Biao Gao, University of Massachusetts, Worcester, found the results convincing. He commended the use of multiple patient lines with different mutations. “This is a well-performed study,” he wrote. “Importantly, they generated one pair of isogenic lines with one corrected point mutation to confirm their key findings” (see full comment below).

Axonal transport defects appear in mice carrying mutations that cause amyotrophic lateral sclerosis (ALS), even before birth (Williamson and Cleveland, 1999; Kieran et al., 2015). The defects affect mice carrying ALS mutations in different genes, including SOD1, VAPB, TDP-43, and FUS (Baldwin et al., 2016; Chen et al., 2016). Researchers are beginning to shed light on the cause, but data in human cells are scarce (De Vos and Hafezparast, 2017). 

Traffic Unjammed. Mitochondria in motor neurons derived from an ALS patient carrying the P525L FUS mutation crawl along axons (top), but speed up after treatment with the HDAC6 inhibitor ACY-738 (bottom). [Courtesy of Ludo Van Den Bosch.]

First author Wenting Guo and colleagues generated motor neuron lines from induced pluripotent stem cells derived from skin fibroblasts of three ALS patients carrying the R521H FUS mutation and one patient carrying P525L FUS. Cells from two of the latter’s family members, who did not carry the mutation, served as controls. Seventy to 95 percent of the cells expressed several well-known motor neuron markers and sported very long, motor neuron-like axons. The patient cells also developed typical ALS phenotypes. FUS migrated out of the nucleus into neurites, and the cells fired fewer action potentials than neurons derived from the control volunteers. 

The researchers then labelled mitochondria and endoplasmic reticulum (ER) with fluorescent dyes to track their movements. Three weeks after motor neurons formed, the transport of both organelles started failing and continued to worsen over time. “It got to a point where nothing was moving anymore—it was a complete traffic jam,” said Van Den Bosch. “Still, the mitochondria look quite normal under the electron microscope,” he noted, suggesting these FUS mutations disrupt transport rather than overall mitochondrial health. The mutant cells also harbored fewer axonal ER vesicles, and they associated less with mitochondria than did ER vesicles in control cells, suggesting a possible disruption of mitochondrial-associated ER-membranes (MAMs). Among other things, MAMs appear to facilitate intracellular trafficking of both organelles (Krols et al., 2016). 

To ensure their observations were due to the FUS mutations, and not to some other variant or an artifact of the cell culture procedure, the researchers corrected the R521H mutation using CRISPR-Cas9. As expected, the disruptions disappeared. The researchers were also able to recreate the transport deficits in neurons derived from human embryonic stem cells engineered to overexpress one of the two FUS mutants. Patrik Verstreken, whose group studies HDAC6 in the same department as Van Den Bosch, wrote that “while this work does not provide an explanation of why FUS affects motor neurons, the work does show the defects are at least present in motor neurons derived from patient cells, a discovery in and of itself very exciting.” However, he noted that it remains an open question if FUS deficits are specific to motor neurons since the author did not examine other types of neurons.

Because Van Den Bosch’s group had previously discovered that blocking histone deacetylase 6 (HDAC6), a.k.a. α-tubulin deacetylase, improved axonal traffic in a mouse model of Charcot-Marie-Tooth disease type 2, they wondered if these inhibitors might clear the traffic jam in FUS-ALS motor neurons (d’Ydewalle et al., 2011). Acetylation of α-tubulin, an HDAC6 substrate, promotes the binding of motor proteins to microtubules (Reed et al., 2006). Indeed, treating the cells overnight with 1μM of Tubastatin A or ACY-738, both HDAC6 inhibitors, rescued the axonal transport defect (see video above). Antisense oligonucleotides that knocked down HDAC6 expression by 50 percent had similar effects. “The inhibitors completely reversed the traffic jam for both mitochondria and ER,” said Van Den Bosch. They also corrected the drop in ER-mitochondrial association. As expected, both drugs increased the acetylation levels of α-tubulin. Neither of these drugs is currently being tested in clinical trials for ALS, but Acetylon Pharmaceuticals, Inc., is testing HDAC6 inhibitors for a variety of diseases. Co-author Matthew Jarpe works at the company.

How do these deficits in axonal transport relate to FUS’s other known ways of causing trouble in ALS? “I don’t know, to be honest,” said Van Den Bosch. Normally located in the nucleus, the RNA-binding protein accumulates in the cytosol in ALS, and forms RNA-protein granules that may facilitate accumulation of toxic protein aggregates, both potentially caused by a disruption to transport across the nuclear membrane (e.g., May 2016 newsOct 2016 news). In this study, however, the authors did not see FUS aggregates. They speculate that, because aggregates co-localize with stress granule markers, stress might be a necessary factor for aggregates to form and they did not stress the cultures. Van Den Bosch said that in future studies he plans to repeat his experiments in stressed cells. “In a worst-case scenario, what we are seeing is something quite downstream [in the disease process],” he said. “If something is wrong with nucleocytoplasmic transport, or with stress granules, this burdens the cells such that other things may go wrong, including axonal transport.”

He noted that unraveling the cascade will be complicated, especially considering the many potential interactions. For example, FUS has been reported to regulate HDAC6 expression (Kim et al., 2010). 

Regardless of underlying mechanisms, HDAC6 inhibitors could be helpful, said Van Den Bosch. “I don’t think we’ll stop or reverse ALS by blocking HDAC6, but it could extend the life of motor neurons,” he said. HDAC6 inhibitors are currently being tested in cancer clinical trials and Van Den Bosch plans to continue his lab’s studies of HDAC6 inhibitors in mice. Besides Charcot-Marie-Tooth, other diseases may benefit from lowering HDAC6 activity, including Huntington’s, Parkinson’s, and Alzheimer’s (Dompierre et al., 2007; Godena et al., 2014Govindarajan et al., 2013). And because HDAC6 knockout mice seem to fare well, the strategy might have few serious side effects. “It is interesting that HDAC6 is emerging as a broadly applicable tool to promote axonal transport,” noted Verstreken.  

Hafezparast agreed that HDAC inhibitors appear to be good targets, but cautioned that HDAC6 appears to play a role in the clearance of aggregated proteins, including mutant SOD1 (Xia et al., 2014; Kawaguchi et al., 2003).—Marina Chicurel


  1. This is a well-performed study in part because the authors generated multiple iPSC lines from four ALS patients harboring different FUS mutations. More importantly, they generated one pair of isogenic lines with one corrected point mutation to confirm their key findings. Thus, their results are convincing and not due to clonal variations between different iPSC lines. Another important finding is that the observed axonal transport defects can be rescued by HDAC6 inhibition. Defects in this very important cellular pathway have been implicated in ALS pathogenesis before. Thus, their study suggests a novel therapeutic approach. It will be interesting in the future to test whether this therapeutic approach works or not in animal models of ALS.

  2. The issue of axonal transport in the context of ALS is relevant given the extreme polarization of motor neurons. Even subtle defects in transport would affect the delivery of cargo to (e.g., structural synaptic components, mitochondria, etc.) and transport of cargo from synapses (e.g., autophagosomes) that are frequently seen disrupted in ALS. Creating motor neurons from FUS mutation carriers, the Van Den Bosch group at VIB and KU Leuven not only shows FUS-related pathology in the cytoplasm but also axonal trafficking defects. An important question in many neurodegenerative diseases is that of cell type specificity. While this work does not provide an explanation of why FUS affects motor neurons, the work does show the defects are at least present in motor neurons derived from patient cells, a discovery in and of itself very exciting. However, if this defect is specific to motor neurons remains an open question, as in this work other types of neurons were not tested.

    The authors go a long way to control their experimental set-up: Using rescue experiments as well as expression of FUS in human embryonic stem cells, they show axonal transport defects are dependent on the presence of the FUS mutation. What is exciting is that HDAC6 inhibition restores the transport defects. The reason for this is that deacetylation of microtubuli is inhibited and acetylated microtubuli are a better substrate for motor-driven transport. The ability of HDAC6-inhibition to rescue ALS-related phenotypes is similar to reports of this group on the use of HDAC6 inhibitors in other disease conditions and also in the context of Drosophila models of ALS, where we found that loss of HDAC6 is protective (Miskiewicz et al., 2014). Hence, it is interesting that HDAC6 is emerging as a broadly applicable tool to promote axonal transport.

    Open questions include if transport defects are truly pathogenic in the context of disease. At least in the context of human motor neurons with FUS mutations, the transport defects do occur, but what is still unclear is if they are causal to aspects of the pathology. Another open question is if acetylation of other HDAC6 targets (besides tubulin) contributes to the rescue of FUS-related defects. HDAC6 is a cytoplasmic deacetylase and other targets exist as well. It will be interesting (and important in the context of therapy) to assess if they contribute to disease modification as well. 


    . HDAC6 is a Bruchpilot deacetylase that facilitates neurotransmitter release. Cell Rep. 2014 Jul 10;8(1):94-102. Epub 2014 Jun 26 PubMed.

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News Citations

  1. Stress Granule Protein Entwines and Misfolds Tau
  2. ALS Research ‘Gels’ as Studies Tie Disparate Genetic Factors Together

Paper Citations

  1. . Slowing of axonal transport is a very early event in the toxicity of ALS-linked SOD1 mutants to motor neurons. Nat Neurosci. 1999 Jan;2(1):50-6. PubMed.
  2. . A mutation in dynein rescues axonal transport defects and extends the life span of ALS mice. J Cell Biol. 2005 May 23;169(4):561-7. PubMed.
  3. . Axonal transport defects are a common phenotype in Drosophila models of ALS. Hum Mol Genet. 2016 Jun 15;25(12):2378-2392. Epub 2016 Apr 7 PubMed.
  4. . PINK1 and Parkin are genetic modifiers for FUS-induced neurodegeneration. Hum Mol Genet. 2016 Dec 1;25(23):5059-5068. PubMed.
  5. . Neurobiology of axonal transport defects in motor neuron diseases: Opportunities for translational research?. Neurobiol Dis. 2017 Sep;105:283-299. Epub 2017 Feb 22 PubMed.
  6. . Mitochondria-associated membranes as hubs for neurodegeneration. Acta Neuropathol. 2016 Apr;131(4):505-23. Epub 2016 Jan 7 PubMed.
  7. . HDAC6 inhibitors reverse axonal loss in a mouse model of mutant HSPB1-induced Charcot-Marie-Tooth disease. Nat Med. 2011 Jul 24;17(8):968-74. PubMed.
  8. . Microtubule acetylation promotes kinesin-1 binding and transport. Curr Biol. 2006 Nov 7;16(21):2166-72. PubMed.
  9. . Amyotrophic lateral sclerosis-associated proteins TDP-43 and FUS/TLS function in a common biochemical complex to co-regulate HDAC6 mRNA. J Biol Chem. 2010 Oct 29;285(44):34097-105. PubMed.
  10. . Histone deacetylase 6 inhibition compensates for the transport deficit in Huntington's disease by increasing tubulin acetylation. J Neurosci. 2007 Mar 28;27(13):3571-83. PubMed.
  11. . Increasing microtubule acetylation rescues axonal transport and locomotor deficits caused by LRRK2 Roc-COR domain mutations. Nat Commun. 2014 Oct 15;5:5245. PubMed.
  12. . Reducing HDAC6 ameliorates cognitive deficits in a mouse model for Alzheimer's disease. EMBO Mol Med. 2013 Jan;5(1):52-63. PubMed.
  13. . Loss of TDP-43 Inhibits Amyotrophic Lateral Sclerosis-Linked Mutant SOD1 Aggresome Formation in an HDAC6-Dependent Manner. J Alzheimers Dis. 2014 Dec 23; PubMed.
  14. . The deacetylase HDAC6 regulates aggresome formation and cell viability in response to misfolded protein stress. Cell. 2003 Dec 12;115(6):727-38. PubMed.

Further Reading


  1. . Histone deacetylases and their inhibitors in cancer, neurological diseases and immune disorders. Nat Rev Drug Discov. 2014 Sep;13(9):673-91. Epub 2014 Aug 18 PubMed.

Primary Papers

  1. . HDAC6 inhibition reverses axonal transport defects in motor neurons derived from FUS-ALS patients. Nat Commun. 2017 Oct 11;8(1):861. PubMed.