Effects of diminished SMN on segmental spinal cord innervation of motor neurons

Reduced abundance of the Survival Motor Neuron protein (SMN) causes the disease Spinal Muscular Atrophy (SMA). Precisely how reduction in cellular SMN levels leads to defects in the motor system is an area of active research. The level of weakness and motor dysfunction in SMA is out of proportion to the number of motor neurons lost and neuromuscular junction (nmj) degradation. This has lead to a number of investigations showing that abnormalities in the circuitry that talks to motor neurons is an important contributor to motor system deficits. This has been well established in a Drosophila melanogaster model of SMA as well as work in my own lab using a Caenorhabditis elegans (C.elegans) model of SMA. In work described in the Preliminary Studies section of this proposal, evidence is provided that reduction in gamma aminobutyric acid (GABA)-mediated neurotransmission is impaired in the worm model of disease. Enhancing GABAergic neurotransmission improves several aspects of locomotion. These observations lead to the hypothesis that defects in GABAergic neurotransmission within the segmental spinal cord circuitry contributes to impaired activation of motor neurons and thus motor function. In specific aim #1, we will determine if there is a reduction of GABAergic innervation of motor neurons in the SMA mice. We will use a trans-synaptic viruses engineered to express a fluorescent reporter to label neurons making monosynaptic contact with motor neurons. Neurochemical phenotype of this pre-motor neurons will be assigned and quantitative analysis undertaken. A specific subtype of interneurons with monosynaptic input to motor neurons is called the “V1 interneuron”. This is a very diverse subtype of pre-motor neurons and plays a critical role in motor neuron activation. In Specific aim #2 we will determine if the innervation of motor neurons by a specific class of V1 interneurons is lacking in the SMA mice. This will be accomplished by combining monosynaptic labeling of pre-motor neurons with immunohistological identification of V1 interneuron subtypes. Together these studies will lead to mechanistic insights into motor system dysfunction in mouse models of SMA.