Activation patterns of embryonic chick lumbosacral motoneurones following large spinal cord reversals

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.

Differential expression of a transcription regulatory factor, the LIM domain only 4 protein Lmo4, in muscle sensory neurons

In the stretch-reflex system, proprioceptive sensory neurons make selective synaptic connections with different subsets of motoneurons, according to the peripheral muscles they supply. To examine the molecular mechanisms that may influence the selection of these synaptic targets, we constructed single-cell cDNA libraries from sensory neurons that innervate antagonist muscles. Differential screening of these libraries identified a transcription regulatory co-factor of the LIM homeodomain proteins, the LIM domain only 4 protein Lmo4, expressed in most adductor but few sartorius sensory neurons. Differential patterns of Lmo4 expression were also seen in sensory neurons supplying three other muscles. A subset of motoneurons also expresses Lmo4 but the pattern of expression is not specific for motor pools. Differential expression of Lmo4 occurs early, as neurons develop their characteristic LIM homeodomain protein expression patterns. Moreover, ablation of limb buds does not block Lmo4 expression, suggesting that an intrinsic program controls the early differential expression of Lmo4. LIM homeodomain proteins are known to regulate several aspects of sensory and motor neuronal development. Our results suggest that Lmo4 may participate in this differentiation by regulating the transcriptional activity of LIM homeodomain proteins.

The acquisition of motoneuron subtype identity and motor circuit formation

Experiments in chick embryos using classical transplantation techniques introduced by Viktor Hamburger are reviewed; these demonstrated that chick-limb innervating motoneurons become specified by extrinsic signals prior to axon outgrowth and that they selectively grow to appropriate muscles by actively responding to guidance cues within the limb. More recent experiments reveal that fast/slow and flexor/extensor subclasses of motoneurons are distinct by E4-5 and that they exhibit patterned spontaneous activity while still growing to their targets. These observations are then related to the combinatorial code of LIM transcription factor expression, which has been hypothesized to specify motoneuron subtypes.

Development of specific connectivity between premotor neurons and motoneurons in the brain stem and spinal cord

Astounding progress has been made during the past decade in understanding the general principles governing the development of the nervous system. An area of prime physiological interest that is being elucidated is how the neural circuitry that governs movement is established. The concerted application of molecular biological, anatomical, and electrophysiological techniques to this problem is yielding gratifying insight into how motoneuron, interneuron, and sensory neuron identities are determined, how these different neuron types establish specific axonal projections, and how they recognize and synapse upon each other in patterns that enable the nervous system to exercise precise control over skeletal musculature. This review is an attempt to convey to the physiologist some of the exciting discoveries that have been made, within a context that is intended to link molecular mechanism to behavioral realization. The focus is restricted to the development of monosynaptic connections onto skeletal motoneurons. Principal topics include the inductive mechanisms that pattern the placement and differentiation of motoneurons, Ia sensory afferents, and premotor interneurons; the molecular guidance mechanisms that pattern the projection of premotor axons in the brain stem and spinal cord; and the precision with which initial synaptic connections onto motoneurons are established, with emphasis on the relative roles played by cellular recognition versus electrical activity. It is hoped that this review will provide a guide to understanding both the existing literature and the advances that await this rapidly developing topic.

Selective fasciculation and divergent pathfinding decisions of embryonic chick motor axons projecting to fast and slow muscle regions

Proper motor function requires the precise matching of motoneuron and muscle fiber properties. The lack of distinguishing markers for early motoneurons has made it difficult to determine whether this matching is established by selective innervation during development or later via motoneuron-muscle fiber interactions. To examine whether chick motoneurons selectively innervate regions of their target containing either fast or slow muscle fibers, we backlabeled neurons from each of these regions with lipophilic dyes. We found that motor axons projecting to fast and slow muscle regions sorted into separate but adjacent fascicles proximally in the limb, long before they reached the muscle. More distally, these fascicles made divergent pathfinding decisions to course directly to the appropriate muscle fiber region. In contrast, axons projecting to different areas of an all-fast muscle did not fasciculate separately and became more intermingled as they coursed through the limb. Selective fasciculation of fast- and slow-projecting motoneurons was similar both before and after motoneuron cell death, suggesting that motoneurons specifically recognized and fasciculated with axons growing to muscle regions containing the appropriate muscle fiber type. Taken together, these results strongly support the hypothesis that "fast" and "slow" motoneurons are molecularly distinct before target innervation and that they use these differences to selectively fasciculate, pathfind to, and branch within the correct muscle fiber region from the outset of neuromuscular development.

Selective fasciculation and divergent pathfinding decisions of embryonic chick motor axons projecting to fast and slow muscle regions

Proper motor function requires the precise matching of motoneuron and muscle fiber properties. The lack of distinguishing markers for early motoneurons has made it difficult to determine whether this matching is established by selective innervation during development or later via motoneuron-muscle fiber interactions. To examine whether chick motoneurons selectively innervate regions of their target containing either fast or slow muscle fibers, we backlabeled neurons from each of these regions with lipophilic dyes. We found that motor axons projecting to fast and slow muscle regions sorted into separate but adjacent fascicles proximally in the limb, long before they reached the muscle. More distally, these fascicles made divergent pathfinding decisions to course directly to the appropriate muscle fiber region. In contrast, axons projecting to different areas of an all-fast muscle did not fasciculate separately and became more intermingled as they coursed through the limb. Selective fasciculation of fast- and slow-projecting motoneurons was similar both before and after motoneuron cell death, suggesting that motoneurons specifically recognized and fasciculated with axons growing to muscle regions containing the appropriate muscle fiber type. Taken together, these results strongly support the hypothesis that "fast" and "slow" motoneurons are molecularly distinct before target innervation and that they use these differences to selectively fasciculate, pathfind to, and branch within the correct muscle fiber region from the outset of neuromuscular development.

Selective innervation of fast and slow muscle regions during early chick neuromuscular development

The electrical properties of adult motoneurons are well matched to the contractile properties of the fast or slow muscle fibers that they innervate. How this precise matching occurs developmentally is not known. To investigate whether motoneurons exhibit selectivity in innervating discrete muscle regions, containing either fast or slow muscle fibers during early neuromuscular development, we caused embryonic chick hindlimb muscles to become innervated by segmentally inappropriate motoneurons. We used the in vitro spinal cord-hindlimb preparation to identify electrophysiologically the pools of foreign motoneurons innervating the posterior iliotibialis (pITIB), an all-fast muscle, and the iliofibularis (IFIB), a partitioned muscle containing discrete fast and slow regions. The results showed that the pITIB and the fast region of the IFIB were exclusively innervated by motoneurons that normally supply fast muscles. In contrast, the slow region of the IFIB was always innervated by motoneuron pools that normally supply slow muscles. Some experimental IFIB muscles lacked a fast region and were innervated solely by "slow" motoneurons. In addition, the intramuscular nerve branching patterns were always appropriate to the fast-slow nature of the muscle (region) innervated. The selective innervation was found early in the motoneuron death period, and we found no evidence that motoneurons grew into appropriate muscle regions, but failed to form functional contacts. Together, these results support the hypothesis that different classes of motoneurons exhibit molecular differences that allow them to project selectively to, and innervate, muscle fibers of the appropriate type during early neuromuscular development.

Selective innervation of fast and slow muscle regions during early chick neuromuscular development

The electrical properties of adult motoneurons are well matched to the contractile properties of the fast or slow muscle fibers that they innervate. How this precise matching occurs developmentally is not known. To investigate whether motoneurons exhibit selectivity in innervating discrete muscle regions, containing either fast or slow muscle fibers during early neuromuscular development, we caused embryonic chick hindlimb muscles to become innervated by segmentally inappropriate motoneurons. We used the in vitro spinal cord-hindlimb preparation to identify electrophysiologically the pools of foreign motoneurons innervating the posterior iliotibialis (pITIB), an all-fast muscle, and the iliofibularis (IFIB), a partitioned muscle containing discrete fast and slow regions. The results showed that the pITIB and the fast region of the IFIB were exclusively innervated by motoneurons that normally supply fast muscles. In contrast, the slow region of the IFIB was always innervated by motoneuron pools that normally supply slow muscles. Some experimental IFIB muscles lacked a fast region and were innervated solely by "slow" motoneurons. In addition, the intramuscular nerve branching patterns were always appropriate to the fast-slow nature of the muscle (region) innervated. The selective innervation was found early in the motoneuron death period, and we found no evidence that motoneurons grew into appropriate muscle regions, but failed to form functional contacts. Together, these results support the hypothesis that different classes of motoneurons exhibit molecular differences that allow them to project selectively to, and innervate, muscle fibers of the appropriate type during early neuromuscular development.

Brachially innervated ectopic hindlimbs in the chick embryo. II. The role of supraspinal input in the loss of limb motility

Brachially innervated grafted hindlimbs display a progressive loss of motility as development proceeds. However, the virtually immobile grafted hindlimbs of E20 embryos exhibited strong, synchronous contractions of gastrocnemius and tibialis muscles upon intraperitoneal injection of strychnine nitrate (20 micrograms). This result indicated that the marked behavioral deficit was not due to an inability of the motoneurons that innervate the immobile grafted hindlimbs to initiate and propagate action potentials, but was probably the result of an effective loss of motoneuron excitation. To examine the hypothesis that interaction with the supraspinal nervous system is involved in the reduction of grafted hindlimb activity, the normal forelimb and grafted hindlimb movements of chronic spinal embryos were examined. The normal forelimbs of chronic spinal embryos exhibited the same number of movements as normal embryos at all stages examined. Thus the deficit in grafted hindlimb motility is not comparable to the behavior of the normal forelimb in chronic spinal embryos and is, therefore, unlikely to be due to a lack of excitation from the supraspinal nervous system. The possibility of an inhibitory influence via supraspinal projections was examined in chronic spinal embryos that had brachially innervated grafted hindlimbs. After E12, the grafted hindlimbs of chronic spinal embryos displayed significantly fewer movements than the normal forelimbs of chronic spinal embryos but significantly more movements than the brachial hindlimb of embryos with intact spinal cords. By E18, however, both spinal and nonspinal brachial hindlimbs, were equally dysfunctional.(ABSTRACT TRUNCATED AT 250 WORDS)

Development and survival of thoracic motoneurons and hindlimb musculature following transplantation of the thoracic neural tube to the lumbar region in the chick embryo: functional aspects

Following heterotopic transplantation of the thoracic neural tube to the lumbar region on embryonic day (E) 2, the transplanted cord differentiates normally and establishes neuroanatomical connections with the host central nervous system and hindlimb muscles. Beginning on about E12, however, the neuromuscular system begins to undergo regressive changes resulting in motoneuron degeneration and muscle atrophy (O'Brien and Oppenheim, 1990). In the present paper, we have examined the development of neuromuscular function in thoracic transplant embryos from E6 to the time of hatching on E20-21. The onset of hindlimb movements and reflexes occurred at the same time (E6-E8) in both control and thoracic transplant embryos. Further, both the nature (pattern) and frequency of these movements appeared normal in the thoracic transplants up to E10-E12, after which there was a gradual and marked reduction in the frequency, and an alteration in the pattern, of both spontaneous and reflex-evoked hindlimb movements. After E16 normal movements were virtually absent in many of the thoracic transplant cases. By contrast, movements of the head, trunk and wings were normal in these embryos throughout the observation period. Hindlimbs innervated partly by the thoracic transplant and partly by remaining host lumbar cord did not exhibit the regressive changes in function after E10 that occurred in hindlimbs innervated exclusively by the thoracic transplant. EMG recordings from specific hindlimb muscles innervated solely by thoracic motoneurons demonstrated that the activation pattern of both flexors and extensors was similar to the repetitive pattern observed in normal thoracically innervated intercostal muscles (i.e., extensor-like). Muscles did not show distinguishable EMG burst patterns with inhibitory periods as do control lumbar innervated muscles. We conclude that the development of the pattern generating circuitry in the transplanted thoracic cord was similar to normal thoracic cord and thus appeared to be uninfluenced by having contacted the foreign hindlimb muscle targets early in development. Activity blockade with curare from E6 to E14 suppressed the loss of motoneurons that occurs in the thoracic transplant after E10. Thus, the abnormal thoracic-like activation pattern of thoracically innervated hindlimbs may be a critical signal in the initiation of the neuromuscular regression that occurs after E10 in these preparations. Finally, although the innervation and formation of neuromuscular endplates in thoracic transplants appeared normal up to E12, by E14 both the intramuscular nerves and the endplates exhibited signs of degeneration and regression. Thoracic motoneurons are initially able to innervate and functionally activate hindlimb muscles in a manner similar to that of thoracically innervated intercostal muscles.(ABSTRACT TRUNCATED AT 400 WORDS)

The regulation of intramuscular nerve branching during normal development and following activity blockade

In vertebrates, approximately 50% of the lumbosacral motoneurons die during a short period of development that coincides with synaptogenesis in the limb. Although it has been postulated that these motoneurons die because they fail to obtain adequate trophic support from the muscles, it is not clear how this factor is supplied. The mechanism by which activity blockade prevents motoneurons cell death is also unknown. In order to begin to understand the nature of these proposed trophic interactions, we have examined the temporal sequence of axonal invasion and ramification within two muscles of the chick hindlimb, the predominantly slow iliofibularis and the fast posterior iliotibialis, during the cell death period. We found striking differences in intramuscular nerve ingrowth and branching between fast and slow muscle. We also observed differences in the molecular composition of fast and slow myotubes that may contribute to the nerve pattern differences. In addition, we observed a progressive increase in the degree of intramuscular nerve fasciculation as well as a precise temporal sequence of nerve branching. The earliest detectable response to chronic curarization was a dramatic decrease in the degree of intramuscular nerve fasciculation. Activity blockade also greatly enhanced nerve branching within the muscles from the time that nerve branches normally formed, and, additionally, interfered with the normal cessation of axon growth. Our results support the idea that nerve endings are the sites of trophic uptake. Furthermore, although our results do not allow us to exclude other activity-dependent influences on motoneuron survival, they suggest the following testable hypotheses: (1) the normal regulation of motoneuron survival may result from the precise control of intramuscular nerve branching, (2) activity blockade may increase motoneuron survival by enhancing intramuscular nerve branching, and (3) anything which affects this complex process of nerve branching may also alter motoneuron survival.