Alsin/Rac1 signaling controls survival and growth of spinal motoneurons

Abstract Objective Recessive mutations in alsin, a guanine‐nucleotide exchange factor for the GTPases Rab5 and Rac1, cause juvenile amyotrophic lateral sclerosis (ALS2) and related motoneuron disorders. Alsin function in motoneurons remained unclear because alsin knock‐out mice do not develop overt signs of motoneuron degeneration. Methods To generate an alsin loss‐of‐function model in an ALS‐relevant cell type, we developed a new small interfering RNA electroporation technique that allows efficient knock down of alsin in embryonic rat spinal motoneurons. Results After small interfering RNA–mediated alsin knockdown, cultured motoneurons displayed a reduced apparent size of EEA1‐labeled early endosomes and an increased intracellular accumulation of transferrin and L1CAM. Alsin knockdown induced cell death in 32 to 48% of motoneurons and significantly inhibited axon growth in the surviving neurons. Both cellular phenotypes were mimicked by expression of a dominant‐negative Rac1 mutant and were completely blocked by expression of a constitutively active Rac1 mutant. Expression of dominant‐negative or constitutively active forms of Rab5 had no such effects. Interpretation Our data demonstrate that alsin controls the growth and survival of motoneurons in a Rac1‐dependant manner. The strategy reported here illustrates how small interfering RNA electroporation can be used to generate cellular models of neurodegenerative disease involving a loss‐of‐function mechanism. Ann Neurol 2006;60:105–117