TDP-43 has been linked to RNA processing and trafficking, but its exact job is unclear (Freibaum et al., 2010). The protein normally hangs out in the nucleus, but often piles up in the cytoplasm in disease models (see ARF related news story on Barmada et al., 2010). Researchers theorize that TDP-43 mutations, found in some instances of human disease, can cause both a loss of the protein’s normal nuclear function and a toxic gain of function in the cytoplasm (see ARF related news story on Zhang et al., 2009). The current work supports the possibility that loss of TDP-43 causes disease, but does not preclude a gain of function, Kraemer said.

A loss-of-function hypothesis makes sense, the study authors reasoned, if TDP-43 is normally needed in neurons. To test this idea, Kraemer and colleagues engineered mice carrying a knockout allele. Heterozygotes were healthy, but homozygotes died in utero, showing that TDP-43 is essential in development. When the researchers analyzed TDP-43 protein in heterozygous mice, they found nearly wild-type amounts, suggesting cells regulate TDP-43 expression to maintain normal levels.

The work mirrors that in two other recent papers. Researchers in Texas and Taiwan also found that the TDP-43 knockout is lethal, but heterozygotes are okay (Sephton et al., 2010; Wu et al., 2010). Those researchers found no defects in their young heterozygous animals. Kraemer followed his mice longer, testing them for motor defects at one year or older. “It is only as they age that you start to see motor defects,” he said. The heterozygotes had a weaker forelimb grip, and were unable to hang upside-down on a wire grid as long as wild-type control mice. “It is a little bit of an odd phenotype,” Kraemer said, because although the muscle deficiency arose with age, it was not progressive like neurodegenerative disease.

The researchers also looked for pathological evidence of neurodegeneration and muscle atrophy in the aging heterozygotes. “There is nothing obvious,” Kraemer said. “We are not sure what is wrong with these mice at the molecular level.” Given that the heterozygotes have near-normal TDP-43 protein levels, perhaps subtle non-structural abnormalities, in metabolism, for example, might account for the muscle weakness, the authors suggest.

The logical next step is a conditional knockout that could survive to adulthood before scientists take away its TDP-43. Several labs are in the early stages of such a project, wrote Virginia Lee of the University of Pennsylvania and a co-author on the paper, in an e-mail to ARF.—Amber Dance.

Reference:
Kraemer BC, Schuck T, Wheeler JM, Robinson LC, Trojanowski JQ, Lee VM, Schellenberg GD. Loss of murine TDP-43 disrupts motor function and plays an essential role in embryogenesis. Acta Neuropathol. 2010 Mar 3. Abstract