Brain-derived neurotrophic factor (BDNF), a diffusible protein produced by both motoneurons and skeletal muscle, is a bidirectional signaling molecule important for maintaining normal functioning of the motor unit. To date, the specific role that muscle-synthesized BDNF plays in the maintenance and health of motoneurons and skeletal muscle fibers is unknown. To determine the contributions that muscle-derived BDNF provides to the neuromuscular unit, we created transgenic mice missing the BDNF gene only in skeletal muscle fibers. Behavioral studies indicate that these mice suffer from an adult-onset loss of neuromuscular functioning. Mice with missing muscle-derived BDNF also exhibit myopathology, skeletal muscle fiber loss, and reduce dendritic BDNF protein. Overall, initial assessment of transgenic mice suggests that the phenotype has characteristics similar to established models of neuromuscular diseases (NMDs) such as amyotrophic lateral sclerosis. RESEARCH QUESTIONS: To evaluate whether these transgenic mice are appropriate models for neuromuscular disease research, several questions are addressed: (1) What is the timeline for the appearance of markers of progressive myopathy? (2) Do mice exhibit markers of progressive neuropathy, and if so, what is the timeline? (3) Does early treatment of skeletal muscle with exogenous BDNF alleviate later myopathology and neuropathy in transgenic mice? METHODS: We will characterize the pathophysiological phenotype of transgenic mice to evaluate them as a potential new model for use in NMD research. For this, we will use histological and immunocytochemical techniques to evaluate the number, health, and morphology of skeletal muscles at multiple ages before and after onset of behavioral neuromuscular deficits. We will also determine if loss of muscle-derived BDNF leads to progressive neuropathy by evaluating the number, health, and morphology of motoneuron cell bodies, dendrites, synapses and axons at multiple ages before and after onset of neuromuscular deficits. Finally, we will evaluate whether long-term treatment of limb muscles with exogenous BDNF alleviates potential myopathology and neuropathology in adult transgenic mice. Evaluation of muscle and spinal tissue will determine whether BDNF treatment reduces muscle fiber loss and/or potential neuropathy in motoneurons. OUTCOMES: Characterization of the disease pathology and progression in these transgenic mice is critical to evaluate them as a potential model of neuromuscular disease. In addition, results of these studies may provide a novel target for treatment of neuromuscular disorders and establish the importance of muscle- synthesized neurotrophins as a critical factor in the health and maintenance of the entire motor unit.
The neuroprotective properties of brain-derived neurotrophic factor (BDNF) have long been known, but the possibility that a loss of muscle-derived BDNF may trigger neuropathology and myopathology associated with neuromuscular diseases has not been explored. This project will act as a foundation for research in this unexplored area and enhance biomedical science research opportunities for undergraduate and graduate students at Northern Michigan University. !