Differences in developmental cell death between somatic and autonomic motor neurons of rat spinal cord

ADAPTIVE ROLES OF PROGRAMMED CELL DEATH DURING NERVOUS SYSTEM DEVELOPMENT

The programmed cell death (PCD) of developing cells is considered an essential adaptive process that evolved to serve diverse roles. We review the putative adaptive functions of PCD in the animal kingdom with a major focus on PCD in the developing nervous system. Considerable evidence is consistent with the role of PCD in events ranging from neurulation and synaptogenesis to the elimination of adult-generated CNS cells. The remarkable recent progress in our understanding of the genetic regulation of PCD has made it possible to perturb (inhibit) PCD and determine the possible repercussions for nervous system development and function. Although still in their infancy, these studies have so far revealed few striking behavioral or functional phenotypes.

Loss of leukemia inhibitory factor receptor beta or cardiotrophin-1 causes similar deficits in preganglionic sympathetic neurons and adrenal medulla

Leukemia inhibitory factor (LIF) receptor beta (LIFRbeta) is a receptor for a variety of neurotrophic cytokines, including LIF, ciliary neurotrophic factor (CNTF), and cardiotrophin-1 (CT-1). These cytokines play an essential role for the survival and maintenance of developing and postnatal somatic motoneurons. CNTF may also serve the maintenance of autonomic, preganglionic sympathetic neurons (PSNs) in the spinal cord, as suggested by its capacity to prevent their death after destruction of one of their major targets, the adrenal medulla. Although somatic motoneurons and PSNs share a common embryonic origin, they are distinct in several respects, including responses to lesions. We have studied PSNs in mice with targeted deletions of the LIFRbeta or CT-1 genes, respectively. We show that LIF, CNTF, and CT-1 are synthesized in embryonic adrenal gland and spinal cord and that PSNs express LIFRbeta. In embryonic day 18.5 LIFRbeta (-/-) and CT-1 (-/-) mice, PSNs were reduced by approximately 20%. PSNs projecting to the adrenal medulla were more severely affected (-55%). Although LIFRbeta (-/-) mice revealed normal numbers of adrenal chromaffin cells and axons terminating on chromaffin cells, levels of adrenaline and numbers of adrenaline-synthesizing cells were significantly reduced. We conclude that activation of LIFRbeta is required for normal development of PSNs and one of their prominent targets, the adrenal medulla. Thus, both somatic motoneurons and PSNs in the spinal cord depend on LIFRbeta signaling for their development and maintenance, although PSNs seem to be overall less affected than somatic motoneurons by LIFRbeta deprivation.

Rapid functional recovery after spinal cord injury in young rats

Responses to traumatic injury in the immature spinal cord may be different from those in adults. We modified an adult model of weight-drop injury to characterize the histopathology and functional recovery after spinal cord injury (SCI) in rat pups at postnatal day 14-15. A 10-g weight was dropped from 2.5 or 5.0 cm at T8-T9. Hindlimb function was evaluated at 24 h and 1, 2, 3, and 4 weeks after injury using the Combined Behavioral Score that estimates overall hind limb sensorimotor function, and the BBB scale for open field locomotion. Histopathology was examined at 15 min, 24 h, and 4 weeks after SCI. The initial hemorrhagic lesion was similar to that seen in adults, but the time course of secondary loss of ventral horn motor neurons was extended. By 4 weeks, only a partial rim of white matter surrounding a central cavity was seen. The 5.0 cm injury group exhibited significantly less recovery of function at 4 weeks than the 2.5 cm group. In the latter, the degree of hindlimb deficit at 4 weeks was similar to that previously described for adults with 10 g x 2.5 cm SCI. However, pups in both injury groups exhibited a significantly faster rate of recovery than adults. Recovery was maximal by 1 week after SCI in pups as compared to 3-4 weeks in adults. The more rapid functional recovery observed in the pups suggests that this new model may be useful for studying mechanisms of functional plasticity after SCI.

Preganglionic inputs to the stellate ganglion of the cat during postnatal ontogenesis

The localization of preganglionic neurons forming, connections with the stellate ganglion was studied using a method based on retrograde axonal transport of horseradish peroxidase in cats during postnatal ontogenesis from birth to six months of life. The number of preganglionic neurons and spinal cord segments connected to the ganglion increased from birth to 10 days of age, and then decreased. The proportion of preganglionic neurons in different spinal cord nuclei changed during life and became comparable with that in adults by age six months. This was accompanied by an increase in the proportion of cells in the nucleus intermediolateralis thoracolumbaris pars principalis, with a decrease in the number of neurons in the nucleus intermediolateralis thoracolumbaris parts funicularis and in the ventral horns of the spinal cord.

Quantitative measurement of glutamate receptor subunit protein expression in the postnatal rat spinal cord

Glutamate is the major excitatory neurotransmitter in the CNS and its effects on neurons are dependent on the type and composition of glutamate receptors with which it interacts. In this study, the protein expression levels of several ionotropic glutamate receptor subunits (N-methyl-D-aspartate (NMDA) subunits NR1, NR2A, NR2B, and alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor subunits GluR1, GluR2, GluR4) were quantified in particulate preparations from rat spinal cord at various ages after birth. We found that all six subunits showed high expression in the early postnatal period, followed by a subsequent decline as the rats matured to adults. The levels of two subunits (NR2A and GluR4) were found to initially increase during the first postnatal week prior to the decline to adult levels. The high levels of expression observed of these subunits in the early postnatal period may have implications for mechanisms of neural injury and cell death in the immature nervous system that involve cation influx through ionotropic glutamate receptors.

Growth and neurotrophic factors regulating development and maintenance of sympathetic preganglionic neurons

The functional anatomy of sympathetic preganglionic neurons is described at molecular, cellular, and system levels. Preganglionic sympathetic neurons located in the intermediolateral column of the spinal cord connect the central nervous system with peripheral sympathetic ganglia and chromaffin cells inside and outside the adrenal gland. Current knowledge is reviewed of the development of these neurons, which share their origin with progenitor cells, giving rise to somatic motoneurons in the ventral horn. Their connectivities, transmitters involved, and growth factor receptors are described. Finally, we review the distribution and functions of trophic molecules that may have relevance for development and maintenance of preganglionic sympathetic neurons.

Development of cholinergic terminals around rat spinal motor neurons and their potential relationship to developmental cell death

Neuron death seems to be regulated mainly by postsynaptic target cells. In chicks, nicotinic antagonists such as alpha-bungarotoxin (alphaBT) can prevent normal cell death of somatic motor neurons (SMNs). For this effect, however, alphaBT could be acting at peripheral neuromuscular junctions and/or central cholinergic synapses. To investigate this issue, we first studied the development of cholinergic terminals in the rat spinal cord by using vesicular acetylcholine transporter immunocytochemistry. Labeled terminals were seen in the ventral horn as early as embryonic day 15 (E15), the beginning of the cell death period. Thus, central cholinergic synapses form at the correct time and place to be able to influence SMN death. We next added alphaBT to organotypic, spinal slice cultures made at E15. After 5 days in vitro, the number of SMNs in treated cultures was substantially greater than in control cultures, indicating that alphaBT can reduce SMN cell death in rats as it does in chicks. Moreover, peripheral target removal led to extensive loss of SMNs, and such a loss occurred even in the presence of alphaBT, indicating the necessity of peripheral target for the alphaBT effect. Finally, to determine whether central cholinergic terminals also may be involved in SMN death, we delayed the alphaBT treatment until after central cholinergic terminals had disappeared from the slice cultures. The increased number of surviving SMNs, even in the absence of central terminals, argued that alphaBT acts at peripheral, not central, cholinergic synapses to rescue SMNs from developmental cell death.

Ultrastructural deficiency in autonomic innervation in cremasteric muscle of boys with undescended testis

BACKGROUND/PURPOSE

The cremaster muscles (CM) associated with undescended testis reveal neurogenic alterations that mainly affect type 2 fibers. The ultrastructure of CM has been evaluated to define if further evidence to explain the alterations could be identified.

METHODS

CM of 8 boys with inguinal hernia and 8 boys with undescended testis at similar ages were biopsied. Samples were processed for electron microscopic evaluations. Semithin and thin sections were examined under an electron microscope.

RESULTS

The CM associated with inguinal hernia showed normal ultrastructure. However, some alterations were encountered in CM associated with undescended testis. Unmyelinated fibers were diminished in number, and myelinated fibers were outnumbering the unmyelinated fibers. Marked disorientation of myofibers, redundant sarcolemma, empty sleeves of basal lamina, disarray of myofibrils, densely packed myofilaments, Z disk streaming, dilated sarcoplasmic reticulum, and dense-irregularly shaped mitochondria were repeatedly encountered. Satellite cells appeared inactive. Most of the fibers were contracted.

CONCLUSIONS

The decrease in number of unmyelinated fibers appears to represent a decrease in autonomic nerve fibers. The alterations within muscle fibers may reflect a deficiency in autonomic innervation. Autonomic nervous system is highly responsive to circulating androgens. Factors decreasing the vulnerability of autonomic nervous system against androgenic effects may result in a CM with neurogenic alterations, thus inhibiting testicular descent. J Pediatr Surg 36:573-578.

Manipulation of intracellular calcium has no effect on rate of migration of rat autonomic motor neurons in organotypic slice cultures

Migration of neurons is a key step in the formation of the central nervous system, and an increase in internal Ca(2+) concentration has been shown to increase the rate of migration of granule cells along radial glial processes in slices of postnatal cerebellum. In embryonic spinal cord, the non-radial migration of autonomic motor neurons from the ventral horn dorsally into the region of the intermediolateral nucleus differs from that of granule cells, so it is possible that the role of Ca(2+) may also differ in the migration of these two types of neurons. To investigate this possibility, we made organotypic slice cultures of thoracic spinal cord from rat embryos. In control slices after about one day in vitro, diaphorase-positive autonomic motor neurons had migrated 100 microm at a rate of 3.6 microm/h. In experimental slice cultures, we added pharmacological reagents that are known to either increase or decrease internal Ca(2+) levels, including some reagents used successfully in the aforementioned granule cell studies. None of the nine reagents had a significant effect on migration speed of autonomic motor neurons in slice cultures. Our results suggest that autonomic motor neuron migration is not regulated by internal Ca(2+) levels, and hence this mechanism may not be used universally by all types of neurons.

Patterns of programmed cell death in populations of developing spinal motoneurons in chicken, mouse, and rat

During embryonic development, approximately one-half of the spinal motoneurons initially generated are lost during a wave of programmed cell death (PCD). Classical studies in this system laid the basis of much work on the role and control of neuronal cell death during development. However, we have little information concerning the timing of cell death in motoneuron pools at different rostrocaudal levels, especially in rodents. We developed a novel protocol for whole-mount TUNEL labeling that allows apoptotic nuclei to be visualized in whole-mount preparations of embryonic spinal cord; double labeling with antibodies to Islet 1/2 showed that nearly all TUNEL-positive cells were motoneurons. In chicken and mouse embryos, the density of TUNEL-positive nuclei was specifically increased following target ablation. The pattern of naturally occurring motoneuron PCD was studied in spinal cords from different species and ages: chick (E4.5-E9.0), mouse (E11.5-E15.5), and rat (E13.5-E16. 5). In all species, motoneuron PCD is first apparent at cervical levels and last at sacral levels. However, motoneuron PCD does not follow a strict rostrocaudal sequence. Following cervical motoneuron PCD, TUNEL profiles are first observed at lumbar levels in chick but at thoracic levels in rat. At a given rostrocaudal level, medial motoneurons tend to die before lateral populations, but here too there are exceptions. Motoneuron cell death is thus regulated in a highly stereotyped manner during development of vertebrate spinal cord. Our technique will provide a basis for the monitoring even localized changes in this pattern.

Alterations in AMPA receptor subunit expression after experimental spinal cord contusion injury

The AMPA-preferring subtype of ionotropic glutamate receptors (GluRs) is a hetero-oligomeric ion channel assembled from various combinations of four subunits: GluR1, GluR2, GluR3, and GluR4. Antagonists of these receptors can mitigate the effects of experimental spinal cord injury (SCI), indicating that these receptors play a significant role in pathophysiology after spinal trauma. We tested the hypothesis that SCI alters expression of AMPA receptors using a standardized thoracic weight-drop model of rat contusive spinal cord injury. AMPA receptor subunit expression was measured at 24 hr and at 1 month after SCI with quantitative Western blot analysis and in situ hybridization. GluR2 protein levels were preferentially reduced near the injury site 24 hr after SCI. This reduction persisted at 1 month. At a cellular level, a significant decrease in both GluR2 and GluR4 mRNA was found in spared ventral motor neurons adjacent to the injury site and distal to it, with other AMPA subunit mRNAs maintained at control levels. In contrast, only GluR1 mRNA was decreased in the sympathetic preganglionic neurons of the intermediolateral horn. These results suggest population-specific and long-lasting changes in neuronal AMPA receptor composition, which may alter response to glutamate after SCI. These alterations may contribute not only to acute neuropathological consequences of injury, but they may also be partially responsible for the altered functional state of preserved tissue seen chronically after SCI.

Naturally occurring somatic motoneuron death in a teleost angelfish, Pterophyllum scalare

Naturally occurring somatic motoneuron death in a teleost angelfish, Pterophyllum scalare, was investigated histochemically and electron microscopically. The number of motor axons in the ventral root, which corresponds to the motoneuron number in spinal hemisegment, was rapidly increased beyond the adult value within 3 days after hatching, and then decreased to reach the adult value within a few weeks. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) histochemistry, which detects fragmented nuclear DNA characteristic to apoptotic cells, showed that the apoptotic cells are located in the motor column of the cord in the larvae at specific developmental stages. Electron microscopic observations of the spinal cells further confirmed the motoneuron apoptosis. The present data suggest that the massive death of somatic motoneurons at certain ontogenic stages which has been known to occur in higher vertebrates also takes place in fish.

Peripheral and central target requirements for survival of embryonic rat dorsal root ganglion neurons in slice cultures

Developmental cell death in the nervous system usually is controlled by the availability of target-derived trophic factors. It is well established that dorsal root ganglia (DRG) neurons require the presence of their peripheral target for survival, but because of their central projections, it is possible that the spinal cord also may be required. Before examining this possibility in rat embryos, we first used terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) to determine that thoracic DRG cell death occurred from embryonic day 15 (E15) to E18. To determine the target requirements of DRG neurons, we used organotypic slice cultures of E15 thoracic trunk segments. After peripheral target removal, essentially all DRG neurons disappeared within 5 d. In contrast, after removal of the spinal cord, approximately half of the DRG neurons survived for at least 8 d. Hence, some E15 DRG neurons could survive without the spinal cord. However, those DRG neurons that died after spinal cord ablation apparently required trophic factors from both central and peripheral targets, because the presence of only one of these tissues was not adequate by itself to support this cell group. Addition of neurotrophin-3 (NT-3) to the culture medium rescued some DRG neurons after CNS removal, suggesting a possible role for NT-3 in vivo. In other experiments, cultures were established from older (E16) embryos, and essentially all neurons survived after spinal cord ablation, even without added factors. These and other experiments indicated that approximately 65% of DRG neurons are transiently dependent on the CNS early in development.