Quantitative cytochemistry of RNA in axotomized feline rubral neurons

Chromatolysis: Do injured axons regenerate poorly when ribonucleases attack rough endoplasmic reticulum, ribosomes and RNA?

After axonal injury, chromatolysis (fragmentation of Nissl substance) can occur in the soma. Electron microscopy shows that chromatolysis involves fission of the rough endoplasmic reticulum. In CNS neurons (which do not regenerate axons back to their original targets) or in motor neurons or dorsal root ganglion neurons denied axon regeneration (e.g., by transection and ligation), chromatolysis is often accompanied by degranulation (loss of ribosomes from rough endoplasmic reticulum), disaggregation of polyribosomes and degradation of monoribosomes into dust‐like particles. Ribosomes and rough endoplasmic reticulum may also be degraded in autophagic vacuoles by ribophagy and reticulophagy, respectively. In other words, chromatolysis is disruption of parts of the protein synthesis infrastructure. Whereas some neurons may show transient or no chromatolysis, severely injured neurons can remain chromatolytic and never again synthesize normal levels of protein; some may atrophy or die. Ribonuclease(s) might cause the following features of chromatolysis: fragmentation and degranulation of rough endoplasmic reticulum, disaggregation of polyribosomes and degradation of monoribosomes. For example, ribonucleases in the EndoU/PP11 family can modify rough endoplasmic reticulum; many ribonucleases can degrade mRNA causing polyribosomes to unchain and disperse, and they can disassemble monoribosomes; Ribonuclease 5 can control rRNA synthesis and degrade tRNA; Ribonuclease T2 can degrade ribosomes, endoplasmic reticulum and RNA within autophagic vacuoles; and Ribonuclease IRE1α acts as a stress sensor within the endoplasmic reticulum. Regeneration might be improved after axonal injury by protecting the protein synthesis machinery from catabolism; targeting ribonucleases using inhibitors can enhance neurite outgrowth and could be a profitable strategy in vivo. © 2018 Wiley Periodicals, Inc. Develop Neurobiol, 2018

Delayed treatment with chondroitinase ABC reverses chronic atrophy of rubrospinal neurons following spinal cord injury

Degradation of extracellular matrix chondroitin sulphate proteoglycans (CSPGs) using Chondroitinase ABC (ChABC) is a promising strategy for the treatment of spinal cord injury, with potent effects on promoting functional recovery and anatomical repair in spinal injured animals. We have previously demonstrated that ChABC treatment prevents atrophy of corticospinal projection neurons following spinal injury in adult YFP-H mice. Here, we investigate whether ChABC-mediated repair of the cell body extends to rubrospinal projection neurons (RSNs), whether neuroprotective effects can be sustained long-term and importantly, whether delayed treatment with ChABC can reverse chronic atrophy. Adult YFP-H mice underwent unilateral rubrospinal tract transection and were treated with ChABC or a control enzyme, delivered either acutely post-injury or after a one month delay. Eight weeks following injury and control treatment, RSNs in the injured red nucleus, identified by YFP label and NeuN immunoreactivity, showed severe atrophy, with ~40% loss of mean cell area compared to uninjured neurons in the contralateral red nucleus. Both acute and delayed treatment with ChABC promoted a significant rescue of injured RSNs, restoring cell area to ~80% and ~70%, respectively, of that in uninjured neurons. Thus, we demonstrate for the first time that CSPG degradation in the injured spinal cord not only promotes sustained rescue of cell atrophy when delivered acutely but can also reverse chronic atrophy in descending projection neurons. Thus, modulation of the extracellular matrix can mediate neuroprotective effects both early and late after spinal cord injury.

Axonally transported peripheral signals regulate alpha-internexin expression in regenerating motoneurons

The class IV neuronal intermediate filament (IF) family proteins includes the neurofilament (NF) triplet proteins NF-L, NF-M, and NF-H and also the more recently characterized alpha-internexin-NF66. It is well established that NF-L, -M, and -H protein and mRNA are downregulated after peripheral nerve injury. We examined alpha-internexin protein expression after three facial nerve lesion paradigms: crush, transection, and resection. Alpha-internexin immunoreactivity was absent in the perikarya of uninjured facial motoneurons but increased dramatically in all three injury paradigms, with maximum immunoreactivity observed at 7 d after injury. Twenty-eight days after nerve crush or transection, there was a dramatic decrease in the number of alpha-internexin-positive cells. In contrast, alpha-internexin remained elevated 28 d after nerve resection, an injury that hinders regeneration and target reinnervation. In situ hybridization studies showed an increase in alpha-internexin mRNA expression in the facial nucleus at 7 and 14 d after injury. Retrograde transport of fluorogold from the whisker pads to the facial nucleus was seen only in motoneurons that lacked alpha-internexin immunoreactivity, supporting the idea that target reinnervation and inhibitory signals from the periphery regulate the expression of alpha-internexin. Blockage of axonal transport through local colchicine application induced strong immunoreactivity in motoneurons. Alpha-internexin expression was also examined after central axotomy of rubrospinal neurons, which constitutively show alpha-internexin immunoreactivity. After rubrospinal tractotomy, alpha-internexin immunoreactivity transiently increased by 7 d after injury but returned to control levels by 14 d. We conclude that alpha-internexin upregulation in injured motoneurons suggests a role for this IF protein in neuronal regeneration.

Neuroprotective effects of progesterone on damage elicited by acute global cerebral ischemia in neurons of the caudate nucleus

BACKGROUND

In addition to the hippocampus, the dorsolateral caudate nucleus (CN) and the pars reticularis of the substantia nigra (SNr) are among the most vulnerable brain areas to ischemia. A possible association of the neuronal injury in these two subcortical nuclei has been proposed, the primary damage affecting the CN GABAergic neurons innervating the SNr, and secondarily the SNr neurons as a result of an imbalance of GABAergic and glutamatergic input to the SNr. Progesterone (P(4)) exerts a GABAergic action on the central nervous system (CNS) and is known to protect neurons in the cat hippocampus from the damaging effect of acute global cerebral ischemia (AGCI). The effects of AGCI on the neuronal populations of the CN and SNr, in addition to the possible neuroprotective effects of P(4), were assessed in cats in the present study.

METHODS

Ovariectomized adult cats were treated subcutaneously (s.c.) with either P(4) (10 mg/kg/day) or corn oil during the 7 days before and 7 days after being subjected to a period of AGCI by 15 min of cardiorespiratory arrest followed by 4 min of reanimation. After 14 days of survival, animals were sacrificed and their brains perfused in situ with phosphate-buffered 10% formaldehyde for histologic examination.

RESULTS

ACGI resulted in an intense glial reaction in the CN and a significant loss (43%) of medium-sized neurons of the CN, but no difference was found in the densities of SNr neurons between controls and ischemic oil- and P(4)-treated cats. Progesterone treatment completely prevented CN neuronal loss.

CONCLUSIONS

The overall results point to the higher vulnerability of CN neurons to ischemia as compared to neurons in the SNr and show the protective effects of P(4) upon CN neuronal damage after ischemia.

Response of abducens internuclear neurons to axotomy in the adult cat

The highly specific projection of abducens internuclear neurons on the medial rectus motoneurons of the oculomotor nucleus constitutes an optimal model for investigating the effects of axotomy in the central nervous system. We have analyzed the morphological changes induced by this lesion on both the cell bodies and the transected axons of abducens internuclear neurons in the adult cat. Axotomy was performed by the transection of the medial longitudinal fascicle. Cell counts of Nissl-stained material and calretinin-immunostained abducens internuclear neurons revealed no cell death by 3 months postaxotomy. Ultrastructural examination of these cells at 6, 14, 24, and 90 days postaxotomy showed normal cytological features. However, the surface membrane of axotomized neurons appeared contacted by very few synaptic boutons compared to controls. This change was quantified by measuring the percentage of synaptic coverage of the cell bodies and the linear density of boutons. Both parameters decreased significantly after axotomy, with the lowest values at 90 days postlesion ( approximately 70% reduction). We also explored axonal regrowth and the possibility of reinnervation of a new target by means of anterograde labeling with biocytin. At all time intervals analyzed, labeled axons were observed to be interrupted at the caudal limit of the lesion; in no case did they cross the scar tissue to reach the distal part of the tract. Nonetheless, a conspicuous axonal sprouting was present at the caudal aspect of the lesion site. Structures suggestive of axonal growth were found, such as large terminal clubs, from which short filopodium-like branches frequently emerged. Similar findings were obtained after parvalbumin and calretinin immunostaining. At the electron microscopy level, biocytin-labeled boutons originating from the sprouts appeared surrounded by either extracellular space, which was extremely dilated at the lesion site, or by glial processes. The great majority of labeled boutons examined were, thus, devoid of neuronal contact, indicating absence of reinnervation of a new target. Altogether, these data indicate that abducens internuclear neurons survive axotomy in the adult cat and show some form of axonal regrowth, even in the absence of target connection.

Influence of the postsynaptic target on the functional properties of neurons in the adult mammalian central nervous system

In this review we have attempted to summarize present knowledge concerning the regulatory role of target cells on the expression and maintenance of the neuronal phenotype during adulthood. It is well known that in early developmental stages the survival of neurons is maintained by specific neurotrophic factors derived from their target tissues. Neuronal survival is not the only phenotype that is regulated by target-derived neurotrophic factors since the expression of electrophysiological and cytochemical properties of neurons is also affected. However, a good deal of evidence indicates that the survival of neurons becomes less dependent on their targets in the adult stage. The question is to what extent are target cells still required for the maintenance of the pre-existing or programmed state of the neuron; i.e., what is the functional significance of target-derived factors during maturity? Studies addressing this question comprise a variety of neuronal systems and technical approaches and they indicate that trophic interactions, although less apparent, persist in maturity and are most easily revealed by experimental manipulation. In this respect, research has been directed to analyzing the consequences of disconnecting a group of neurons from their target-by either axotomy or selective target removal using different neurotoxins-and followed (or not) by the implant of a novel target, usually a piece of embryonic tissue. Numerous alterations have been described as taking place in neurons following axotomy, affecting their morphology, physiology and metabolism. All these neuronal properties return to normal values when regeneration is successful and reinnervation of the target is achieved. Nevertheless, most of the changes persist if reinnervation is prevented by any procedure. Although axotomy may represent, besides target disconnection, a cellular lesion, alternative approaches (e.g., blockade of either the axoplasmic transport or the conduction of action potentials) have been used yielding similar results. Moreover, in the adult mammalian central nervous system, neurotoxins have been used to eliminate a particular target selectively and to study the consequences on the intact but target-deprived presynaptic neurons. Target depletion performed by excitotoxic lesions is not followed by retrograde cell death, but targetless neurons exhibit several modifications such as reduction in soma size and in the staining intensity for neurotransmitter-synthesizing enzymes. Recently, the oculomotor system has been used as an experimental model for evaluating the functional effects of target removal on the premotor abducens internuclear neurons whose motoneuronal target is destroyed following the injection of toxic ricin into the extraocular medial rectus muscle. The functional characteristics of these abducens neurons recorded under alert conditions simultaneously with eye movements show noticeable changes after target loss, such as a general reduction in firing frequency and a loss of the discharge signals related to eye position and velocity. Nevertheless, the firing pattern of these targetless abducens internuclear neurons recovers in parallel with the establishment of synaptic contacts on a presumptive new target: the small oculomotor internuclear neurons located in proximity to the disappeared target motoneurons. The possibility that a new target may restore neuronal properties towards a normal state has been observed in other systems after axotomy and is also evident from experiments of transplantation of immature neurons into the lesioned central nervous system of adult mammals. It can be concluded that although target-derived factors may not control neuronal survival in the adult nervous system, they are required for the maintenance of the functional state of neurons, regulating numerous aspects of neuronal structure, chemistry and electro-physiology.(ABSTRUCT TRUNCATED)

The response of rubrospinal neurons to axotomy at different stages of development in the North American opossum

Rubral axons can grow around a lesion of their pathway in the thoracic spinal cord of developing opossums and a critical period exists for that plasticity. The critical period probably begins when rubral axons first grow into the thoracic cord, and it extends until approximately postnatal day 30. We previously noted that most rubrospinal neurons die after transection of their axon during the critical period, suggesting that plasticity results primarily from growth of axons not damaged by the lesion (Xu and Martin, J. Comp. Neurol. 279, 368-381, 1989). That observation led us to study the response of rubrospinal neurons to axotomy in more detail and at additional stages of development, using a prelabeling paradigm. We first injected fast blue (FB) into the caudal thoracic or rostral lumbar spinal cord in animals ranging from estimated postnatal day 9 to 50 and, about 4 days later, lesioned the rubrospinal tract several segments rostral to the injection. Approximately 30 days later, the animals were killed so that the red nucleus could be searched for labeled neurons. During the critical period for plasticity, rubrospinal neurons showed signs of degeneration 1 week after their axon was cut. When animals were killed 2-3 weeks after lesioning, there was an obvious decrease in axotomized neurons within the red nucleus, and by 4 weeks, more than 75% of them had degenerated. The marked susceptibility of rubrospinal neurons to axotomy during the critical period for plasticity is consistent with the hypothesis that developmental plasticity of the rubrospinal tract results primarily from growth of axons that were not damaged by the lesion. Our results also suggest that survival of axotomized rubrospinal neurons increases with age.

The response of rubrospinal neurons to axotomy in the adult opossum, Didelphis virginiana

To provide endpoints for developmental studies of rubrospinal plasticity in the North American opossum, we have attempted to determine the degree to which rubrospinal neurons survive axotomy in the adult animal. Bilateral or unilateral injections of the long-lasting fluorescent marker fast blue were made into the T-10 or the T-11 segment of the spinal cord to label rubrospinal neurons, and 7 days later, the rubrospinal tract was cut unilaterally four segments rostral to the injection(s). In cases with unilateral injections, the lesion was made ipsilateral to the injection. The animals were allowed to survive for 30-60 days before being sacrificed and perfused so that sections through the red nuclei could be examined for labeled neurons. The results show that most axotomized neurons survived the lesion, suggesting that lesion-dependent cell death is not a major factor in the failure of the rubrospinal tract to regenerate in the adult animal.

RNA content of normal and axotomized retinal ganglion cells of rat and goldfish

The responses of rat and goldfish retinal ganglion cells to axotomy were examined by a quantitative cytochemical method for RNA and by morphometric measurement 1-60 (rat) and 3-90 (goldfish) days after interruption of one optic nerve or tract intracranially. Unoperated control animals were studied also. The RNA content of axotomized neurons of rat fell 7-60 days postoperatively. Additionally, atrophy of the axotomized somas occurred. Over time, neuronal atrophy approximately paralleled the loss of RNA, and mean cell area and RNA content were reduced by about 25% 60 days after axotomy. Incorporation of 3H-uridine by axotomized neurons declined also. Axotomized retinal ganglion cells of goldfish behaved differently from those of the rat and showed increases in RNA content, most conspicuously 14-60 days postoperatively. Enlargement of axotomized fish neurons occurred but was less proportionately than concomitant increases in RNA content. The nonaxotomized ganglion cells of goldfish displayed statistically significant increases in size and RNA content 14-49 days after unilateral optic nerve or tract lesions. In contrast, alterations in rat retinal ganglion cells contralateral to interruption of one optic nerve were of limited and questionable significance. The contrasting reactions to axotomy by the retinal ganglion cells of these two vertebrates, one of which regenerates optic axons and one of which does not, may support the proposition that the somal response to axon injury has an important bearing upon the success or failure of CNS regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal chromatin changes in layer V pyramidal cells of somatomotor cortex after pyramidal tract lesions as demonstrated by [3H]actinomycin D binding

Changes in chromatin structure of pyramidal tract neurons after medullary pyramidal tract lesions were examined autoradiographically utilizing [3H]actinomycin D (Act D) binding to nuclei in frozen sections of brain. After a right pyramidal tract lesion, the binding of Act D to nuclei of axotomized pyramidal neurons of somatomotor cortex layer V increased sharply at 1 and 5 days postoperation, compared with pyramidal cells of the left side or hippocampal control cells of the left hemisphere. At 3, 7, 9, and 11 days the axotomized cells showed significantly decreased binding compared with controls. The unoperated pyramidal cells showed a significantly decreased Act D binding at 2 h and 9 days postoperation compared with the ipsilateral hippocampal control cells. The data suggested that intrinsic neurons of the central nervous system had a response pattern of chromatin changes to axotomy that was basically similar to that of peripheral neurons (sensory ganglion cells). However, the response was compressed into the 1st week postoperation with only a brief reaction which might be correlated to axonal regeneration. This reaction was followed by a prolonged depression of Act D nuclear binding which may be associated with cellular atrophy.

Enzyme changes in the rat facial nucleus following a conditioning lesion

The enzymatic changes in the facial nucleus of the rat occurring after single nerve transection were compared with those after double lesion. In a first operation the left facial nerve was transected and 2 weeks later, both the left and the right facial nerves were axotomized. The double or "conditioning" lesion produced a complex pattern of changes that differed from those after a single lesion. Three enzymes were investigated both biochemically and histochemically. Acetylcholinesterase is representative of the group of transmitter-related enzymes which in general showed a decrease after a single lesion. The hexose monophosphate shunt enzymes, represented here by glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, are known to increase in the perikaryon. 5'-Nucleotidase is a marker enzyme for the perineuronal satellite glia which also increase in number during chromatolysis. The following results were obtained: (i) In comparison with the single-lesion side the conditioning-lesion side exhibited less activity of the pentose phosphate shunt enzymes on days 7 and 12 after the second operation. On the conditioning-lesion side the amount of enzyme per perikaryon was higher on days 1 and 3, approximately the same on day 7, and less on day 12 compared with the single-lesion side. (ii) The conditioning-lesion side displayed a more pronounced decrease of acetylcholinesterase. (iii) 5'-Nucleotidase increased again after a second axotomy and reached the same level of activity as after a single lesion. These data suggest that a conditioning lesion does not simply amplify the ongoing axonal reaction of the cells in a linear fashion, but that it leads to a complex response. The data are in favor of a shorter initial delay prior to the axonal outgrowth which occurs after a conditioning lesion. However, our data could not explain an enhancement of axonal outgrowth velocity after the second operation.

Morphometric measurements and RNA content of axotomized feline cervical motoneurons

Microspectrophotometric estimates of RNA content and morphometric measurements of cytoplasmic, nuclear and nucleolar areas were made on 30 to 60 motoneurons (somal areas greater than 1000 microns2) ipsilateral and contralateral to brachial plexotomy performed unilaterally on adult cats 2-90 days before sacrifice. Nerve cells of unoperated animals were also assayed. Somal and cytoplasmic areas of axotomized motoneurons were larger than those of the corresponding, contralateral motor nerve cells 4, 6 and 75 days postoperatively. Because of between animal variability, it could not be determined, however, whether this difference was due to an increase in the area of the axotomized motoneurons or to a decrease in the area of the contralateral nerve cells. Nucleolar sizes did not change. In contrast, nuclei of axotomized motoneurons showed a temporary but unequivocal areal decrease. The cytoplasmic RNA content of axotomized motoneurons fell 14-28 days postoperatively but rose thereafter, being increased slightly but significantly 75-90 days after operation. At no postoperative interval, however, did the nucleolar RNA content of the axotomized cells deviate unequivocally from the unoperated or zero day condition. The following points may be emphasized: 1. these results differ from similar measurements of axotomized motoneurons of rodents and lagomorphs; 2. the data do not provide certain evidence of change in either morphometric parameters or RNA content of motoneurons on the side contralateral to surgery, although the possibility of a decrease in the size of these uninjured neurons should be considered; 3. morphometric and RNA measurements on axotomized peripheral (extrinsic) neurons of spinal anterior horn of cat contrast with similar measurements on axotomized central (intrinsic) neurons of cat red nucleus.

Polysaccharide and cytoplasmic changes in motoneurons during "chromatolysis" in the opossum spinal cord

Following axotomy, motoneurons of the opossum spinal cord display an early "axon reaction" or "chromatolysis" characterized by a redistribution of ribosomes accounting for a widespread basophilia and an apparent reduction in the size of two distinct varieties of Nissl bodies. This alteration is accompanied by zones of increased extracellular glycocalyx demonstrable in light and electron microscopy. In addition, large intracellular periodic acid-Schiff-positive vacuolated zones in the neuron periphery possess numerous free ribosomes, glycogen, lipids, and huge vacuolated sacs containing a flocculent matrix material similar to that found within the sacs of granular endoplasmic reticulum. "Artifacts" in the neuronal periphery associated with chromatolysis seen in light microscopy are probably related to polysaccharide alterations and redistribution of granular endoplasmic reticulum.

Axon reaction in dorsal motor vagal and hypoglossal neurons of the adult rat. Light microscopy and RNA-cytochemistry

Qualitative light microscopical observations, morphometric measurements, and cytophotometric values for nucleolar and cytoplasmic RNA were compared in axotomized rat dorsal motor vagal and hypoglossal neurons. These data were correlated with consective cell counts and examination of the peripheral nerves. Vagal neurons showed an early prominent chromatolysis, later accompanied by increased cytoplasmic basophilia. Morphometric data showed a transient slight cytoplasmic enlargement but no nucleolar hypertrophy. Nucleolar RNA was unchanged, but cytoplasmic RNA was elevated 7 to 84 days postoperatively. Cell counts demonstrated a final cell loss of about 70%. Hypoglossal neurons showed a moderate chromatolysis. Nucleolar and cytoplasmic areas were enlarged for a short period. Nucleolar and cytoplasmic RNA were elevated about 3 to 14 days and about 3 to 28 days postoperatively, respectively. Cell counts demonstrated a loss of 25% at the longest postoperative survival period. The results indicate that axotomized adult mammalian extrinsic neurons--even those destined to die--accumulate RNA. This response contrasts with axon reaction in many axotomized mammalian intrinsic neurons which appear to undergo depletion of RNA.

Fine structural changes in nerve cell bodies of the adult rabbit dorsal motor vagal nucleus during axon reaction

Counts of neuronal nucleoli were made in the dorsal motor vagal nucleus (DMV) of the adult rabbit 10, 18, 70 and 90 days following unilateral cervical vagotomy. The structural characteristics of nerve cell bodies in the DMV were studied electron microscopically 2--90 days after cervical vagotomy. The nucleolar counts indicated that 20% of the large DMV neurones had disappeared ipsilateral to the operation 10 days postoperatively (p.o.), 65% 18 days p.o. and 70% 70 and 90 days p.o. No loss of small neurones was found. Large neurones ipsilateral to the operation showed nuclear displacement, infoldings of the nuclear membrane and disappearance of granular endoplasmic reticulum beginning 4 days p.o. and being prominent 6--18 days p.o. At the peak of the response, 10--18 days p.o., reacting neurones showed nucleolar condensation and vacuolation, the appearance of intranuclear electron-dense particles, extensive accumulation of intracytoplasmic lipid droplets, increased numbers of microtubules and neurofilaments, focal mitochondrial aggregates, and widespread mitochondrial degeneration. Ten to 21 days p.o. degenerating neurones were observed. After 30 days p.o. survival a partial recovery of surviving large DMV neurones seemed to have taken place. The findings are interpreted as indications of distubed protein metabolism, oxidative metabolism and intraneuronal transport in the axotomized DMV neurones. The unique response of these neurones compared to previously studied peripherally projecting neurones is emphasized.