Department of Neurology, Center for Motor Neuron Biology and Disease, Columbia University, New York, New York 10032, USA.
Neural Dev. 2009 Dec 2;4:42. doi: 10.1186/1749-8104-4-42.
Gamma motor neurons (gamma-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 alpha-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 gamma-MNs. Recently, it was reported that early embryonic gamma-MN precursors are dependent on GDNF. Using this knowledge we characterized genetic strategies to label developing gamma-MNs based on GDNF receptor expression, showed their strict dependence for survival on muscle spindle-derived GDNF and generated an animal model in which gamma-MNs are selectively lost.
In mice heterozygous for both the Hb9::GFP transgene and a tau-lacZ-labeled (TLZ) allele of the GDNF receptor Gfralpha1, we demonstrated that small motor neurons with high Gfralpha1-TLZ expression and lacking Hb9::GFP display structural and synaptic features of gamma-MNs and are selectively lost in mutants lacking target muscle spindles. Loss of muscle spindles also results in the downregulation of Gfralpha1 expression in some large diameter MNs, suggesting that spindle-derived factors may also influence populations of alpha-MNs with beta-skeletofusimotor collaterals. These molecular markers can be used to identify gamma-MNs from birth to the adult and to distinguish gamma- from beta-motor axons in the periphery. We also found that postnatal gamma-MNs are also distinguished by low expression of the neuronal nuclear protein (NeuN). With these markers of gamma-MN identity, we show after conditional elimination of GDNF from muscle spindles that the survival of gamma-MNs is selectively dependent on spindle-derived GDNF during the first 2 weeks of postnatal development.
Neonatal gamma-MNs display a unique molecular profile characterized by the differential expression of a series of markers - Gfralpha1, 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 gamma-MNs with preservation of the spindle and its sensory innervation. This provides a mouse model with which to explore the specific role of gamma-fusimotor activity in motor behaviors.
γ 运动神经元（γ-MNs）选择性地支配肌梭内纤维，并调节其对拉伸的敏感性。它们构成了一个独特的亚群，在形态、生理和连接上与α-MNs 不同，α-MNs 支配肌梭外纤维并产生力。控制功能不同的梭内神经元分化的机制尚不清楚。由于缺乏专门区分和操纵γ-MNs 的分子标记，这个问题的进展受到了限制。最近，据报道，早期胚胎γ-MN 前体依赖 GDNF。利用这一知识，我们描述了基于 GDNF 受体表达来标记发育中的 γ-MN 的遗传策略，表明它们的生存严格依赖于肌梭衍生的 GDNF，并生成了一种可选择性丢失 γ-MN 的动物模型。
在 Hb9::GFP 转基因和 GDNF 受体 Gfralpha1 的 tau-lacZ 标记（TLZ）等位基因杂合的小鼠中，我们证明了高 Gfralpha1-TLZ 表达且缺乏 Hb9::GFP 的小运动神经元具有 γ-MN 的结构和突触特征，并在缺乏靶肌梭的突变体中选择性丢失。肌梭的缺失也导致一些大直径 MN 中 Gfralpha1 表达的下调，这表明梭内衍生的因子也可能影响具有β-骨骼肌梭侧支的α-MN 群体。这些分子标记可用于从出生到成年识别 γ-MN，并区分周围的 γ-和β-运动轴突。我们还发现，出生后 γ-MN 也以神经元核蛋白（NeuN）的低表达为特征。有了这些 γ-MN 身份的标记物，我们在条件性消除肌梭中的 GDNF 后发现，γ-MN 的存活在出生后发育的前 2 周内选择性地依赖于肌梭衍生的 GDNF。
新生 γ-MN 表现出独特的分子特征，其特征是一系列标记物的差异表达 - Gfralpha1、Hb9::GFP 和 NeuN - 以及对肌梭衍生的 GDNF 的选择性依赖。从肌梭中删除 GDNF 的表达会导致 γ-MN 的选择性消除，而保留梭及其感觉传入神经。这提供了一个小鼠模型，可以用来探索γ-梭内活动在运动行为中的特定作用。
