Regulatory mechanisms driving motor neuron functional diversification

Precise neural control of muscle tone, reflexes, and voluntary movements relies on circuits operating through the fusimotor neurons. Fusimotor neurons or gamma motor neurons (γ-MNs) control motor behavior by tuning the sensitivity of the muscle spindle (MS) stretch receptors. However, the molecular mechanisms that endow γ-MNs with unique functional properties that are different from the force-generating alpha motor (α-MNs) are largely unknown. Therefore, the role of estrogen-related receptors Err2 and Err3 in specifying γ-MN biophysical properties was determined using chick and mouse models. I hypothesized that Err2 and Err3 are functionally redundant transcription factors that regulate MN functional diversification by promoting γ-MN biophysical properties. To test this hypothesis, quantitative immunodetection studies were performed on Err2 and Err3 expression during early postnatal development in mice. Forced expression of Err2 and Err3 was achieved long-term in chick MNs through a Tol2 transposon-mediated gene transfer strategy, which was followed by whole cell patch-clamp recordings to determine their effects on MN biophysical properties. Furthermore, conditional knockout mice lacking Err2 and Err3 (Err2/3cKO) in γ-MNs were generated. Err2/3cKO MNs were recorded using whole cell patch-clamp and their motor behaviors were analyzed, including gait and precision movements. Lastly, RNA screening studies, reporter gene studies and whole cell patch-clamp recordings in chick were used to identify Err2/3 target genes. Based on data from these studies, I concluded that Err2 and Err3 are important for functional diversification of MNs into γ- and α-MNs and the acquisition of γ-MN biophysical properties that are required for accurate motor behaviors in mice.2019-04-2