Generating spinal motor neuron diversity: a long quest for neuronal identity

Abstract

Understanding how thousands of different neuronal types are generated in the CNS constitutes a major challenge for developmental neurobiologists and is a prerequisite before considering cell or gene therapies of nervous lesions or pathologies. During embryonic development, spinal motor neurons (MNs) segregate into distinct subpopulations that display specific characteristics and properties including molecular identity, migration pattern, allocation to specific motor columns, and innervation of defined target. Because of the facility to correlate these different characteristics, the diversification of spinal MNs has become the model of choice for studying the molecular and cellular mechanisms underlying the generation of multiple neuronal populations in the developing CNS. Therefore, how spinal motor neuron subpopulations are produced during development has been extensively studied during the last two decades. In this review article, we will provide a comprehensive overview of the genetic and molecular mechanisms that contribute to the diversification of spinal MNs.

Fig. 1

Topographic organization and molecular markers of motor columns along the anteroposterior axis of the developing spinal cord. a Schematic diagram illustrating the three-dimensional distribution of the motor columns. Graded levels of the morphogens FGFs, RA, and GDF11 determine spatial expression of Hox paralog groups that shape the spinal cord into a brachial, a thoracic, and a lumbar portion. At thoracic levels, MNs are distributed in a medial motor column (MMC) that innervates the axial muscles, a hypaxial motor column (HMC) connected to the intercostal muscles and a column of preganglionic MNs (PGC) that project to the sympathetic ganglia chain, which innervates visceral organs. At brachial and lumbar levels, MNs gather in an MMC column, a restricted HMC column, and a lateral motor column (LMC) that innervates locomotor muscles. MNs of the LMC are divided into a medial (LMCm) and a lateral (LMCl) complement that innervate ventral or dorsal limb muscles, respectively. b Location of each motor column and their molecular markers on schematic transverse sections in embryonic spinal cord at brachial/lumbar or at thoracic levels. Hox6 are determinants of the brachial MNs and Hox10 are required for lumbar MN fate, while Hoxc9 establishes thoracic MN identity. FGFs fibroblast growth factors, RA retinoic acid, GDF11 growth differentiation factor 11

Fig. 2

MN fate specification and consolidation. A combination of extrinsic signals including Shh, RA, and FGFs cooperate with intrinsic mechanisms that involve Olig2, Neurogenin-2 (Neurog2), and Lhx3 to promote the specification of newly born MN. GDE2 produced by the newly born MN acts in an on-cell autonomous manner to promote MN progenitor differentiation. Lhx3, Isl1, NLI, LMO4, and Hb9 act together to consolidate MN identity. Isl1 may additionally contribute to the survival of newly born MNs. FGFs fibroblast growth factors, RA retinoic acid, Shh Sonic Hedgehog, NLI nuclear Lim-domain interactor, GDE2 glycerophosphodiester phosphodiesterase 2

Fig. 3

MN diversification at thoracic levels of the spinal cord. Hoxc9 establishes thoracic MN identity. The differentiation into somatic or visceral MN is finely tuned by the combined action of OC, Foxp1, Hb9, and Isl proteins. In some newly born MNs, OC factors and a mutual stimulatory loop between Hb9 and Isl1 triggers high Isl protein levels and somatic MN differentiation. In other newly born MNs, OC stimulate Foxp1 expression, which represses Hb9 and thereby weakens the mutual stimulatory loop between Hb9 and Isl1 and reduces Isl levels, which favors visceral MN differentiation. OC also promote Sip1 expression, which supports the generation of PGC cells. Among somatic MNs, Lhx3 and activation of Wnt signaling pathway promote the differentiation of MMC neurons. The HMC fate may constitute the default differentiation fate. nNOS neuronal nitric oxide synthase

Fig. 4

MN diversification at limb levels. Exogenous RA from the paraxial mesoderm induces the generation of LMC neurons with LMCm identity. These cells rapidly initiate Raldh2 expression and RA synthesis, which promotes the generation of a later LMC population with LMCl identity. Hox proteins, possibly generic Hox5-8 paralogs, are necessary for high Foxp1 levels in the brachial LMC. Cross-repressive interactions between Isl1 and Lhx1, respectively, expressed by LMCm and LMCl neurons, stabilize this segregation and determine settling position and axonal projection of MNs in each division. Upon the influence of intrinsic factors, including Hox proteins and their cofactors, and of target-derived signals, subdivision of LMC neurons proceeds further and results in the formation of motor pools that individually innervate single target muscles. RALDH2 retinaldehyde dehydrogenase 2, RA retinoic acid