Gamma motor neurons express distinct genetic markers at birth and require muscle spindle-derived GDNF for postnatal survival

Abstract

BackgroundGamma motor neurons (γ-MNs) selectively innervate muscle spindle intrafusal fibers and regulate their sensitivity to stretch. They constitute a distinct subpopulation that differs in morphology, physiology and connectivity from α-MNs, which innervate extrafusal muscle fibers and exert force. The mechanisms that control the differentiation of functionally distinct fusimotor neurons are unknown. Progress on this question has been limited by the absence of molecular markers to specifically distinguish and manipulate γ-MNs. Recently, it was reported that early embryonic γ-MN precursors are dependent on GDNF. Using this knowledge we characterized genetic strategies to label developing γ-MNs based on GDNF receptor expression, showed their strict dependence for survival on muscle spindle-derived GDNF and generated an animal model in which γ-MNs are selectively lost.ResultsIn mice heterozygous for both the Hb9::GFP transgene and a tau-lacZ-labeled (TLZ) allele of the GDNF receptor Gfrα1, we demonstrated that small motor neurons with high Gfrα1-TLZ expression and lacking Hb9::GFP display structural and synaptic features of γ-MNs and are selectively lost in mutants lacking target muscle spindles. Loss of muscle spindles also results in the downregulation of Gfrα1 expression in some large diameter MNs, suggesting that spindle-derived factors may also influence populations of α-MNs with β-skeletofusimotor collaterals. These molecular markers can be used to identify γ-MNs from birth to the adult and to distinguish γ- from β-motor axons in the periphery. We also found that postnatal γ-MNs are also distinguished by low expression of the neuronal nuclear protein (NeuN). With these markers of γ-MN identity, we show after conditional elimination of GDNF from muscle spindles that the survival of γ-MNs is selectively dependent on spindle-derived GDNF during the first 2 weeks of postnatal development.ConclusionNeonatal γ-MNs display a unique molecular profile characterized by the differential expression of a series of markers - Gfrα1, Hb9::GFP and NeuN - and the selective dependence on muscle spindle-derived GDNF. Deletion of GDNF expression from muscle spindles results in the selective elimination of γ-MNs with preservation of the spindle and its sensory innervation. This provides a mouse model with which to explore the specific role of γ-fusimotor activity in motor behaviors.

Targeted Depletion of TDP-43 Expression in the Spinal Cord Motor Neurons Leads to the Development of Amyotrophic Lateral Sclerosis-like Phenotypes in Mice

“…In the mammalian spinal cords, ␣ and ␥ motor neurons could be distinguished both by the cell body sizes, for which the ␥ motor neurons were significantly smaller than the ␣ motor neurons (43), and by the expression of specific molecular markers, e.g. NeuN for the ␣ motor neurons (44,45). As seen in Fig.…”

Pool-Specific Regulation of Motor Neuron Survival by Neurotrophic Support

“…This finding suggests that other factors are required to sustain survival of this population. One possibility is that these pools are dependent on another trophic factor such as GDNF, which plays multiple roles in their development (Henderson et al, 1994;Springer et al, 1995;Haase et al, 2002;Gould et al, 2008;Shneider et al, 2009). However, current evidence suggests that GDNF exerts its survival activity principally on gamma MNs, making this hypothesis less probable.…”

Fiber Types in Mammalian Skeletal Muscles

“…TrkC-or NT3-deficient mice show a complete absence of muscle spindles in limb muscles but not in head muscles, like the masseter, where other neurotrophins appear to compensate for the loss of NT3 (215). Intrafusal fibers also produce another neurotrophin, glial cell linederived neurotrophic factor (GDNF), which is essential for the postnatal survival of ␥-motor neurons by signaling through a receptor complex, common to most GDNF family ligands (GFLs), comprising GFL-specific co-receptors (GFR␣) and the RET tyrosine kinase (279,731). GDNF, like NT3, is preferentially expressed by intrafusal fibers in E18.5 muscle.…”