It was not straightforward, but researchers have managed to knock out the essential, ubiquitous TDP-43 gene in mouse motor neurons. While the scientists, from the Academia Sinica in Taipei, Taiwan, expected the deletion would damage motor neurons, they were impressed that the phenotype resembled amyotrophic lateral sclerosis (ALS), senior author Che-Kun James Shen told Alzforum. The work was published in the June 20 Journal of Biological Chemistry.

To better understand the natural role of the ALS gene in motor neurons, first author Lien-Szu Wu used Cre recombinase to delete TDP-43 specifically from that cell type. Getting rid of TDP-43 has been challenging because of its indispensable nature for cell health; early attempts simply resulted in unviable embryos (see ARF related news story; Wu et al., 2010; Sephton et al., 2010), and there are few other examples of tissue-specific TDP-43 knockouts (see ARF related news story; Chiang et al., 2010). In addition, TDP-43 regulates its own expression—take some away, and the cell will just produce more. Wu and Shen’s trick was to start with a mouse already missing a copy of TDP-43, so the inefficient Cre enzyme only had to delete one. In that manner, “it is easier to get tissue-specific loss of function,” Shen said. They observed that approximately 60 percent of motor neurons turned off TDP-43.

“Surprisingly, the mouse became an ALS mouse,” Shen said. As in human ALS, symptoms were more likely to occur in males: 80 percent of male mice, but only a quarter of females, developed the motor phenotype. Like other ALS models, these animals rapidly dropped off a rotating rod, starting at about 15 weeks of age. Not only did the mice suffer weight loss and muscle weakness, but they also lost motor neurons. Three-quarters of cells expressing the motor neuron marker choline acetyltransferase disappeared from the ventral horn, compared to control animals lacking cre. Of those neurons that remained, some 60 percent stained positive for ubiquitinated proteins, in keeping with the ALS-like phenotype. Shen anticipated some ALS-like traits, but not the extent of cellular and molecular pathology the mice exhibited, he said.

Zuoshang Xu of the University of Massachusetts Medical School in Worcester was more cautious about the phenotype. “The symptoms are hardly typical of ALS,” he wrote in an e-mail to Alzforum. “The mice are weak, but the authors did not provide convincing evidence that they develop paralysis and die of ALS.”

Shen agreed that his mice are not exactly like ALS, in that ubiquitinated proteins are not bound up in inclusions with TDP-43, as in the human disease. He countered that the animals do eventually become paralyzed, adding, “No mouse model could exhibit all characteristics of any human disease.” Because the mouse lacks the typical inclusions, he suggested it is a better system to probe the role of TDP-43 than to test out ALS treatments.

Based on his mice, Shen suspects that loss of TDP-43 function is an early event in ALS. Others have shown that pathogenic TDP-43 typically vacates its nuclear location and is clipped in the cytoplasm, producing a 25 kilodalton fragment (see ARF related news story on Barmada et al., 2010; ARF related news story on Zhang et al., 2009). This fragment, Shen proposes, then enacts a toxic gain of function: It seeds the inclusions that ensnare more TDP-43 as well as other ubiquitinated proteins.—Amber Dance

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References

News Citations

  1. Research Brief: TDP-43 Knockout Lethal, Hets Have Motor Symptoms
  2. DC: Myriad Mice Mimic ALS, FTLD, Loss of TDP-43 Function
  3. TDP-43: Modified and On the Move
  4. Toxic TDP-43 Truncates Point to Gain-of-Function Role in Disease

Paper Citations

  1. . TDP-43, a neuro-pathosignature factor, is essential for early mouse embryogenesis. Genesis. 2010 Jan;48(1):56-62. PubMed.
  2. . TDP-43 is a developmentally regulated protein essential for early embryonic development. J Biol Chem. 2010 Feb 26;285(9):6826-34. PubMed.
  3. . Deletion of TDP-43 down-regulates Tbc1d1, a gene linked to obesity, and alters body fat metabolism. Proc Natl Acad Sci U S A. 2010 Sep 14;107(37):16320-4. PubMed.
  4. . Cytoplasmic mislocalization of TDP-43 is toxic to neurons and enhanced by a mutation associated with familial amyotrophic lateral sclerosis. J Neurosci. 2010 Jan 13;30(2):639-49. PubMed.
  5. . Aberrant cleavage of TDP-43 enhances aggregation and cellular toxicity. Proc Natl Acad Sci U S A. 2009 May 5;106(18):7607-12. PubMed.

Further Reading

Papers

  1. . Understanding the role of TDP-43 and FUS/TLS in ALS and beyond. Curr Opin Neurobiol. 2011 Dec;21(6):904-19. PubMed.
  2. . TDP-43 functions and pathogenic mechanisms implicated in TDP-43 proteinopathies. Trends Mol Med. 2011 Nov;17(11):659-67. PubMed.
  3. . TDP-43 expression in mouse models of amyotrophic lateral sclerosis and spinal muscular atrophy. BMC Neurosci. 2008;9:104. PubMed.
  4. . Progressive motor weakness in transgenic mice expressing human TDP-43. Neurobiol Dis. 2010 Nov;40(2):404-14. PubMed.
  5. . Mislocalization of TDP-43 in the G93A mutant SOD1 transgenic mouse model of ALS. Neurosci Lett. 2009 Jul 17;458(2):70-4. PubMed.
  6. . Current hypotheses for the underlying biology of amyotrophic lateral sclerosis. Ann Neurol. 2009 Jan;65 Suppl 1:S3-9. PubMed.