TAR DNA binding protein-43 has risen from relative obscurity just a few years ago to become a superstar in studies of frontotemporal lobar degeneration and amyotrophic lateral sclerosis. In 2006, scientists reported that TDP-43 inclusions are a hallmark of both diseases (Neumann et al., 2006). Last year, researchers discovered multiple TARDBP mutations in people with familial ALS (see ARF related news story, Gitcho et al., 2008, and Sreedharan et al., 2008), and since then, rapid-fire new data about the genetics and pathology of TDP-43 proteinopathy have kept the protein at the forefront of research.

Key to future TDP-43 studies will be rodent models that overexpress the protein. Scientists from the Louisiana State University Health Sciences Center in Shreveport reported the first such model in a paper published online February 17 in Molecular Therapy. First author Jason Tatom, principal investigator Ronald Klein, and colleagues injected an adeno-associated virus vector bearing the wild-type human TARDBP gene into the substantia nigra of rats. The animals then expressed approximately three times the normal amount of TDP-43 in the nigra and developed some characteristics of human TDP-43 disease. Primarily a nuclear protein, TDP-43 moved into the cytoplasm in approximately 1 percent of transduced cells. The excess TDP-43 was toxic to dopaminergic neurons.

“I thought it was a good first effort at modeling TDP-43 pathology in an animal,” said Brian Kraemer of the University of Washington in Seattle, who was not involved in the study. “The fact that they saw toxicity with wild-type TDP-43 is encouraging,” he said, because the majority of patients who exhibit TDP-43 inclusions do not have mutations in the gene.

“Having an AAV-based TDP-43 rodent model is not a substitution for a rodent model with stable germline transmission,” Samir Kumar-Singh of VIB—University of Antwerp, Belgium, also not involved in the study, wrote in an e-mail to ARF (see full comment below). “But until these models are developed, expression of TDP-43 by AAV-mediated somatic cell transfer approaches will continue to shed light on TDP-43-mediated disease mechanisms.”

At Washington University in St. Louis, Missouri, scientists are working on modeling a specific subset of TDP-43 diseases, that is, frontotemporal lobar degeneration due to a mutation in the valosin-containing protein (VCP) gene. Mutations in this gene are rare; principal investigator Nigel Cairns estimates they exist in approximately 20 American families. The mutations cause FTLD with inclusion body myopathy and Paget disease of bone. VCP has a number of cellular functions. It acts as a co-chaperone for the membrane fusion machinery, mediates endoplasmic reticulum-associated degradation of proteins, and plays a role in cell survival. Cairns, first author Michael Gitcho, and colleagues surveyed how VCP mutations affected various cellular processes. Their work appeared online February 23 in the Journal of Biological Chemistry. Expressing VCP mutants in neuroblastoma cells altered TDP-43 localization, sending it into the cytoplasm, as happens in human disease. The mutations also decreased proteasome activity, and ultimately killed cells.

On the human genetics front, the number of known TARDBP mutations continues to climb; last month, two Italian groups announced new ones in people with ALS. Writing February 17 in Human Mutation online, Lucia Corrado of the University of Eastern Piedmont in Novara, Italy, and colleagues reported nine new mutations. Two of those mutations were also discovered by Roberto Del Bo of the University of Milan and colleagues, whose own results were released online February 19 by the European Journal of Neurology.

TDP-43 offers researchers plenty to think about, and some suggest it is the central feature in a single disorder whose spectrum spans both FTLD and ALS (Geser et al., 2009 and see ARF related news story). Others are holding their applause as they wait for more evidence. “Are we at the point where we want to say that TDP-43 is the β amyloid of frontotemporal dementia?” asked Conrad Weihl, also of Washington University, who was not involved with the current research. “I’m worried that people are jumping to that conclusion,” although, he admitted, “It may well be correct.”—Amber Dance.

References:
Tatom JB, Wang DB, Dayton RD, Skalli O, Hutton ML, Dickson DW, Klein RL. Mimicking aspects of frontotemporal lobal degeneration and Lou Gehrig’s disease in rats via TDP-43 expression. Mol Ther 2009 Feb 17 [Epub ahead of print]. Abstract

Gitcho MA, Strider J, Carter D, Taylor-Reinwald L, Forman MS, Goate AM, Cairns AM. VCP mutations causing frontotemporal lobar degeneration disrupt localization of TDP-43 and induce cell death. J Biol Chem 2009 Feb 23 [Epub ahead of print]. Abstract

Corrado L, Ratti A, Gellera C, Buratti E, Castellotti B, Carlomagno Y, Ticozzi N, Mazzini L, Testa L, Taroni F, Baralle FE, Silani V, D’Alfonso S. High frequency of TARDBP gene mutations in Italian patients with amyotrophic lateral sclerosis. Hum Mutat 2009 Feb 17 [Epub ahead of print]. Abstract

Del Bo R, Ghezzi S, Corti S, Pandolfo M, Ranieri M, Santoro D, Ghione I, Prelle A, Orsetti V, Mancuso M, Sorarù G, Briani C, Angelini C, Siciliano G, Bresolin N, Comi GP. TARDBP(TDP-43) sequence analysis in patients with familial and sporadic ALS: identification of two novel mutations. Eur J Neurol 2009 Feb 19 [Epub ahead of print]. Abstract

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Comments on News and Primary Papers

  1. The issue of whether overexpression of wild-type TDP-43 in rodent brain could be neurotoxic is neatly brought out in this paper from the group of Ronald Klein with senior authors Dennis Dickson and Mike Hutton. Using an adeno-associated virus type 9 (AAV9) vector for human TDP-43 expression by stereotactic injection into the rat substantia nigra (SN), Tatom and colleagues show that overexpression of human wild-type TDP-43 on its own can kill dopaminergic neurons in rats in a dose-dependent manner (Tatom et al., 2009).

    This approach is surely welcome at a time when many laboratories are struggling to get a decent TDP-43 expression in transgenic germlines. The reason why this is problematic is made apparent by this paper, where (roughly estimated) threefold wild-type TDP-43 overexpression almost completely wipes out the targeted neurons accompanied by neurodegeneration-related astro- and microgliosis. TDP-43 was selectively expressed in neurons as AAV9 has a natural neurotropism, perhaps due to the virus capsid (Bartlett et al., 1998); and while SN is chosen for sake of convenience allowing a rapid estimation of neuronal loss and behavior deficit, it is not without relevance. TDP-43 is naturally expressed in this brain region, and TDP-43 pathology is observed in the nigrostriatal pathway in a variety of neurodegenerative diseases.

    The descriptive neuropathology is fascinating to read. The transgenic protein predominantly homes to the neuronal nuclei, but in approximately 1 percent of the neurons, diffuse cytoplasmic TDP-43 accumulations are also observed. Occasionally, granular textures are observed that are indicative of pre-inclusion lesions as expected at 4 weeks of disease duration. One of the pathological hallmarks of diseased neurons in ALS and FTLD-TDP patients (FTLD-U in the old terminology) is redistribution of TDP-43 from its normal nuclear localization to the cytoplasm, where it is phosphorylated and ubiquitinated and forms insoluble aggregates.

    Interestingly, ubiquitin labeling was also observed in neuronal cytoplasm. It is not clear what proportion of neurons were labeled or whether it co-localized with TDP-43, but given that it was not observed in the contralateral, uninjected side, it is very likely that it is the transgenic TDP-43 protein that is ubiquitinated. It will be interesting to study whether TDP-43 is phosphorylated and/or cleaved into the disease-characteristic ~25-kDa C-terminal fragments. This would further strengthen the justification of trying to develop a rodent model of TDP-43 proteinopathy.

    At present the mechanism for TDP-43 dose-related neurodegeneration is unclear. Even so, considering the various important physiological functions of TDP-43 and a tight control on its expression levels, these data are not surprising. Of relevance to the majority of ALS and FTLD-TDP patients is that it is the wild-type TDP-43 protein that is accumulating and causing neurotoxicity. Given the speed at which these studies could be accomplished, it would be very interesting to study whether a similar overexpression of TDP-43 C-terminus recapitulates key features of TDP-43 proteinopathy in rat brain, as has been shown recently in cell cultures (Igaz et al., 2009).

    Lastly, having an AAV-based TDP-43 rodent model is not a substitute for a rodent model with stable germline transmission achieved by either a constitutive or inducible expression system. That amongst all would allow us to understand which neurons are more vulnerable to TDP-43 gene dosage. But until these models are developed, expression of TDP-43 by AAV-mediated somatic cell transfer approaches will continue to shed light on TDP-43 mediated disease mechanisms.

    View all comments by Samir Kumar-Singh

References

News Citations

  1. Gene Mutations Place TDP-43 on Front Burner of ALS Research
  2. Genomewide Screen for SNPs Linked to Sporadic ALS Finds…Nothing Yet

Paper Citations

  1. . Ubiquitinated TDP-43 in frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Science. 2006 Oct 6;314(5796):130-3. PubMed.
  2. . TDP-43 A315T mutation in familial motor neuron disease. Ann Neurol. 2008 Apr;63(4):535-8. PubMed.
  3. . TDP-43 mutations in familial and sporadic amyotrophic lateral sclerosis. Science. 2008 Mar 21;319(5870):1668-72. PubMed.
  4. . Clinical and pathological continuum of multisystem TDP-43 proteinopathies. Arch Neurol. 2009 Feb;66(2):180-9. PubMed.
  5. . Mimicking aspects of frontotemporal lobar degeneration and Lou Gehrig's disease in rats via TDP-43 overexpression. Mol Ther. 2009 Apr;17(4):607-13. PubMed.
  6. . VCP mutations causing frontotemporal lobar degeneration disrupt localization of TDP-43 and induce cell death. J Biol Chem. 2009 May 1;284(18):12384-98. PubMed.
  7. . High frequency of TARDBP gene mutations in Italian patients with amyotrophic lateral sclerosis. Hum Mutat. 2009 Apr;30(4):688-94. PubMed.
  8. . TARDBP (TDP-43) sequence analysis in patients with familial and sporadic ALS: identification of two novel mutations. Eur J Neurol. 2009 Jun;16(6):727-32. PubMed.

Further Reading

Papers

  1. . Nuclear TAR DNA binding protein 43 expression in spinal cord neurons correlates with the clinical course in amyotrophic lateral sclerosis. J Neuropathol Exp Neurol. 2009 Jan;68(1):37-47. PubMed.
  2. . Mimicking aspects of frontotemporal lobar degeneration and Lou Gehrig's disease in rats via TDP-43 overexpression. Mol Ther. 2009 Apr;17(4):607-13. PubMed.
  3. . High frequency of TARDBP gene mutations in Italian patients with amyotrophic lateral sclerosis. Hum Mutat. 2009 Apr;30(4):688-94. PubMed.
  4. . Current hypotheses for the underlying biology of amyotrophic lateral sclerosis. Ann Neurol. 2009 Jan;65 Suppl 1:S3-9. PubMed.
  5. . The role of transactive response DNA-binding protein-43 in amyotrophic lateral sclerosis and frontotemporal dementia. Curr Opin Neurol. 2008 Dec;21(6):693-700. PubMed.
  6. . TDP-43: an emerging new player in neurodegenerative diseases. Trends Mol Med. 2008 Nov;14(11):479-85. PubMed.
  7. . The syndromes of frontotemporal dysfunction in amyotrophic lateral sclerosis. Amyotroph Lateral Scler. 2008 Dec;9(6):323-38. PubMed.
  8. . VCP mutations causing frontotemporal lobar degeneration disrupt localization of TDP-43 and induce cell death. J Biol Chem. 2009 May 1;284(18):12384-98. PubMed.
  9. . TARDBP (TDP-43) sequence analysis in patients with familial and sporadic ALS: identification of two novel mutations. Eur J Neurol. 2009 Jun;16(6):727-32. PubMed.

Primary Papers

  1. . Mimicking aspects of frontotemporal lobar degeneration and Lou Gehrig's disease in rats via TDP-43 overexpression. Mol Ther. 2009 Apr;17(4):607-13. PubMed.
  2. . High frequency of TARDBP gene mutations in Italian patients with amyotrophic lateral sclerosis. Hum Mutat. 2009 Apr;30(4):688-94. PubMed.
  3. . VCP mutations causing frontotemporal lobar degeneration disrupt localization of TDP-43 and induce cell death. J Biol Chem. 2009 May 1;284(18):12384-98. PubMed.
  4. . TARDBP (TDP-43) sequence analysis in patients with familial and sporadic ALS: identification of two novel mutations. Eur J Neurol. 2009 Jun;16(6):727-32. PubMed.