INDUCIBLE MOTOR NEURON DEATH IN MICE AS A GENETIC MODEL OF MOTOR NEURON DISEASE

Motor neurons (MN) naturally die as a result of aging and disease in mammals. The motor disease Spinal Muscular Atrophy with Respiratory Distress (SMARD) shows dramatic loss of up to 90% of MNs. We hypothesize that cell death affects preferentially MNs but not sensory neurons (SNs), resulting in mismatched populations of the two. Therefore, the connections between MNs and their cognate SNs within the spinal cord may become imprecise after cell death.  As a consequence, we hypothesize that sensory feedback from individual skeletal muscles may be relayed to inappropriate SNs and compromise accurate movement, thus explaining the striking movement imprecision in aged or diseased individuals.

To simulate progressive MN loss in SMARD, two transgenic mouse lines, Olig2-creER and Isl2-DTA were bred to obtain embryos that, following Cre-directed genetic recombination at the Isl2-DTA locus, conditionally express diphtheria toxin specifically in MNs, leading to MN death whenever tamoxifen is introduced. In the experiment, pregnant Isl2-DTA females mated with Olig2-creER males are injected with two different doses of tamoxifen to induce Cre-mediated recombination. One dose produces 40% MN ablation and two doses produce %70 MN ablation. First, we quantify MN and SN populations in experimental embryos detecting combinations of transcription factors, which define MN classes, each of which innervates distinct groups of muscles, and dorsal root ganglia SNs. These experiments elucidate the optimal conditions for progressive MN ablation and thus enable our future studies of changed MN-SN connectivity.