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.

Reference:
Wu LS, Cheng WC, Shen CK. Targeted depletion of TDP-43 expression in the spinal cord motor neurons leads to the development of ALS-like phenotype in mice. J Biol Chem. 2012 Jun 20. Abstract