The major focus of the Rao Lab is to understand molecular cues that control the radial growth of axons and death of motor neurons. Radial growth of axons is studied in mice by ES cell mediated homologous recombination. Motor neurons disease is studied in mouse models of amyotrophic lateral sclerosis (ALS) in transgenic mice for neurofilaments and superoxide dismutase 1 (SOD1).

During development, axons undergo two major changes. First, the long, thin axons make growth cones and establish stable synapses. Second, axonal volume increases 100 fold, and large myelinated axons accumulate large numbers of neurofilaments (NFs). NFs are 10 nm filaments composed of NF-H (200 kd), NF-M (150 kd) and NF-L (68 kd) subunits. Genetic analysis has shown a direct correlation between the number of filaments and axonal volume. Our transgenic mouse models have indicated that subunit ratios play a critical role in controlling radial growth of axons. Recent gene deletion analysis on individual subunits indicate that NF-L is important for filament formation; NF-M is important for filament assembly and control of filament number. In order to identify the domains of NF subunits that are responsible for controlling the radial growth of axons, a systematic domain deletion approach is being used. We have successfully produced mice for carboxyl terminal deletions of NF-M and NF-H, and analyses of these mice are in progress.

Approximately 2% of human ALS patients have mutations in the gene encoding for SOD1. A hallmark of ALS is neurofilament accumulation in cell bodies and proximal axons. Over 50 mutations in SOD1 have been identified in human ALS patients. A large number of mouse models exist for ALS. The role of each NF subunit in disease onset and progression of motor neuron death is unknown. In order to address this question, NF-H deleted and over-expressing mice are bred with SOD1 over-expressing mice and efforts are underway to characterize these mice. Results will indicate the role of NF-H in motor neuron-mediated cell death.