Sheryl A. Scott

Hoxc10 and Hoxd10 regulate mouse columnar, divisional and motor pool identity of lumbar motoneurons

A central question in neural development is how the broad diversity of neurons is generated in the vertebrate CNS. We have investigated the function of Hoxc10 and Hoxd10 in mouse lumbar motoneuron development. We show that Hoxc10 and Hoxd10 are initially expressed in most newly generated lumbar motoneurons, but subsequently become restricted to the lateral division of the lateral motor column (lLMC). Disruption of Hoxc10 and Hoxd10 caused severe hindlimb locomotor defects. Motoneurons in rostral lumbar segments were found to adopt the phenotype of thoracic motoneurons. More caudally the lLMC and dorsal-projecting axons were missing, yet most hindlimb muscles were innervated. The loss of the lLMC was not due to decreased production of motoneuron precursors or increased apoptosis. Instead, presumptive lLMC neurons failed to migrate to their normal position, and did not differentiate into other motoneurons or interneurons. Together, these results show that Hoxc10 and Hoxd10 play key roles in establishing lumbar motoneuron columnar, divisional and motor pool identity.

Shh influences cell number and the distribution of neuronal subtypes in dorsal root ganglia

The molecular mechanisms responsible for specifying the dorsal-ventral pattern of neuronal identities in dorsal root ganglia (DRG) are unclear. Here we demonstrate that Sonic hedgehog (Shh) contributes to patterning early DRG cells. In vitro, Shh increases both proliferation and programmed cell death (PCD). Increasing Shh in vivo enhances PCD in dorsal DRG, while inducing greater proliferation ventrally. In such animals, markers characteristic of ventral sensory neurons are expanded to more dorsal positions. Conversely, reducing Shh function results in decreased proliferation of progenitors in the ventral region and decreased expression of the ventral marker trkC. Later arising trkA(+) afferents make significant pathfinding errors in animals with reduced Shh function, suggesting that accurate navigation of later arising growth cones requires either Shh itself or early arising, Shh-dependent afferents. These results indicate that Shh can regulate both cell number and the distribution of cell types in DRG, thereby playing an important role in the specification, patterning and pathfinding of sensory neurons.

Onset of ETS expression is not accelerated by premature exposure to signals from limb mesenchyme

The ETS transcription factors ER81 and PEA3 are expressed in discrete populations of sensory and motor neurons and regulate late events in neuronal development and limb innervation. Although initiation of ETS expression requires limb‐derived signals, we show here that precocious axon growth into transplanted older donor limbs, which prematurely exposes neurons to limb‐derived signals, does not accelerate the onset of expression of Er81 or Pea3. Similarly, neither MN‐cadherin, which is reportedly regulated by ER81, nor T‐cadherin is expressed precociously in neurons innervating older donor limbs. Thus, neurons must attain a particular level of differentiation to respond to inducing signals from limb. We also show that signals emanating from limb mesenchyme are sufficient to initiate Er81 and Pea3 expression in sensory and motor neurons in the absence of myogenic cells in Spd mutant mice and that induction of ETS expression is unlikely to directly involve retinoid signaling from limb mesenchyme. Developmental Dynamics 236:2109–2117, 2007.

An Early Broad Competence of Motoneurons to Express ER81 Is Later Sculpted by the Periphery

The ETS transcription factor ER81 is expressed in sensory neurons and motoneurons that innervate the adductor and femorotibialis muscles in chick hindlimb and is essential for the development of monosynaptic connections between these two populations of neurons. Neurons need a signal(s) from limb bud mesoderm to initiate ER81 expression. It is not known whether the mature expression pattern arises because adductor and femorotibialis motoneurons are uniquely competent to respond to peripheral signals and express ER81, or whether all motoneurons are competent to express ER81, but normally only adductor and femorotibialis motoneurons are exposed to the requisite activating signal. To investigate these possibilities, we examined ER81 expression in motoneurons that encountered limb tissue surgically mismatched with their target identity at stages after motor pool identities are established. We found that ER81 expression was not invariably linked to motor pool identity or target innervation and was more malleable in later-born femorotibialis motoneurons than in earlier-born adductor motoneurons. We also found that ER81 expression is regulated differently in sensory neurons and motoneurons. Most striking was the observation that motoneurons caudal to the normal adductor and femorotibialis pools could express ER81 when exposed to the appropriate peripheral signals, although this competence did not extend through the entire lumbosacral (LS) region. Thus, it appears that a prepattern of competence to express ER81 is established in early LS motoneurons, most likely in concert with their target identity, and that the expression domains of motoneurons are subsequently refined by peripheral signals at later stages.

Lack of specificity in the interactions of cranial sensory and motoneuron axons in vitro

During embryogenesis sensory innervation is established quite precisely, but the mechanisms responsible are poorly understood. Whereas sensory neurons that supply muscle appear to require nearby motor axons to reach their target muscles, sensory neurons that supply skin do not. We have investigated the specificity with which sensory axons interact with motor axons, using the avian trigeminal sensory system, where prospective cutaneous and muscle afferents are anatomically separate. To test whether muscle afferents selectively associate with the appropriate motor axons, we co-cultured muscle afferents from the trigeminal mesencephalic nucleus with appropriate trigeminal motoneurons from rhombomeres 2/3 and with inappropriate facial motoneurons from rhombomeres 4/5. To test whether prospective cutaneous and muscle afferents can be distinguished by their interactions with motor axons, we cocultured cutaneous neurons from trigeminal ganglia with trigeminal motoneurons. Dye labeling and time-lapse videomicroscopy revealed no obvious differences between the interactions of muscle afferents with appropriate and inappropriate motor axons or between the interactions of cutaneous and muscle afferents with motor axons. Sensory axons intermixed freely with and crossed over motor axons without fasciculating, regardless of the combination of sensory and motor axons examined. These results suggest that outgrowing sensory neurons may not yet have distinct identities, raising the possibility that sensory innervation patterns are determined more by spatial or temporal constraints on axon growth than by active pathway or target selection. In contrast, motor axons often retracted upon contacting sensory afferents, indicating that there are marked differences between sensory and motor growth cones at the stages studied.