Motor Neuron Size-Dependent Differences in mRNA Expression of Tropomyosin-Related Kinase Receptor B in Adult Rats

Area of Science:

• Neuroscience
• Molecular Biology
• Motor Control

Background:

• Motor neuron survival and plasticity rely on trophic factors, notably tropomyosin receptor kinase B (TrkB) signaling, which is crucial throughout life.
• Motor unit properties, including size and function, vary significantly, suggesting differential trophic factor influences.

Purpose of the Study:

• To investigate if tropomyosin receptor kinase B (TrkB) messenger RNA (mRNA) expression in rat phrenic motor neurons (PhMNs) correlates with motor unit size.
• To determine if larger PhMNs, indicative of fast-twitch, fatigable motor units, exhibit differential TrkB expression compared to smaller PhMNs.

Main Methods:

• Individual PhMNs from adult male Sprague-Dawley rats were retrogradely labeled.
• Somal surface area was measured to classify PhMNs by size (motor unit type).
• Fluorescence in situ hybridization (RNAscope) was used to quantify full-length TrkB mRNA transcripts in individual PhMNs.

Main Results:

• Significant variation in PhMN somal surface area was observed.
• Larger PhMNs (upper size tertile) expressed approximately threefold more TrkB mRNA transcripts per neuron compared to smaller PhMNs (lower tertile).
• TrkB mRNA density in cytoplasmic and nuclear compartments did not significantly correlate with PhMN size.

Conclusions:

• Larger motor neurons, associated with higher force production, exhibit greater TrkB mRNA levels.
• Differential TrkB expression may contribute to the functional adaptations and resilience of motor units involved in expulsive motor behaviors.
• TrkB signaling represents a potential mechanism for maintaining motor unit integrity and function across the lifespan.