Tubulin synthesis in the regenerating rat superior cervical ganglion: a biphasic response

gp130 cytokines are positive signals triggering changes in gene expression and axon outgrowth in peripheral neurons following injury

Adult peripheral neurons, in contrast to adult central neurons, are capable of regeneration after axonal damage. Much attention has focused on the changes that accompany this regeneration in two places, the distal nerve segment (where phagocytosis of axonal debris, changes in the surface properties of Schwann cells, and induction of growth factors and cytokines occur) and the neuronal cell body (where dramatic changes in cell morphology and gene expression occur). The changes in the axotomized cell body are often referred to as the "cell body response." The focus of the current review is a family of cytokines, the glycoprotein 130 (gp130) cytokines, which produce their actions through a common gp130 signaling receptor and which function as injury signals for axotomized peripheral neurons, triggering changes in gene expression and in neurite outgrowth. These cytokines play important roles in the responses of sympathetic, sensory, and motor neurons to injury. The best studied of these cytokines in this context are leukemia inhibitory factor (LIF) and interleukin (IL)-6, but experiments with conditional gp130 knockout animals suggest that other members of this family, not yet determined, are also involved. The primary gp130 signaling pathway shown to be involved is the activation of Janus kinase (JAK) and the transcription factors Signal Transducers and Activators of Transcription (STAT), though other downstream pathways such as mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) may also play a role. gp130 signaling may involve paracrine, retrograde, and autocrine actions of these cytokines. Recent studies suggest that manipulation of this cytokine system can also stimulate regeneration by injured central neurons.

Differential Regulation of Calcitonin Gene-related Peptide (CGRP) in Regenerating Rat Facial Nucleus and Dorsal Root Ganglion

The content of calcitonin gene-related peptide (CGRP) and CGRP-mRNA were determined in axotomized rat facial motor nucleus and sensory fifth lumbar dorsal root ganglion (L5 DRG) using radioimmunoassay and Northern blot analysis. After facial nerve transection CGRP levels in the facial nucleus showed a biphasic, approximately five-fold increase. A first peak occurred at postoperative day 3 and, after a transient decrease to normal levels at day 9, another increase was observed reaching a peak around the time of reinnervation (postoperative day 21). CGRP-mRNA showed a similar, biphasic increase. The first peak in CGRP mRNA preceded the peptide peak by 2 days, the second peak was approximately day 21. In contrast, a decrease in CGRP levels is seen in L5 DRG after sciatic nerve section, reaching minimal levels of 45% of control during the second postoperative week. CGRP-mRNA in axotomized DRG also decreases preceding the decrease in peptide levels. No recovery to normal levels is seen for either peptide or mRNA levels in regenerating DRG up to 45 days after injury. Thus, axotomy leads to a differential regulation of both CGRP and CGRP-mRNA in regenerating facial motor nucleus and sensory L5 DRG. This difference may be due to different regulating factors present in both the respective target tissues and the CNS regions and could reflect different functions of CGRP in regenerating motor and sensory neurons.

Changes in cytoskeletal protein synthesis following axon injury and during axon regeneration

Injury to the axons of facial motoneurons stimulates increases in the synthesis of actin, tubulins, and GAP-43, and decreases in the synthesis of neurofilament proteins: mRNA levels change correspondingly. In contrast to this robust response of peripheral neurons to axotomy, injured central nervous system neurons show either an attenuated response that is subsequently aborted (rubrospinal neurons) or overall decreases in cytoskeletal protein mRNA expression (corticospinal and retinal ganglion neurons). There is evidence that these changes in synthesis are regulated by a variety of factors, including loss of endoneurially or target-derived trophic factors, positive signals arising from the site of injury, changes in the intraaxonal turnover of proteins, and substitution of target-derived trophic support by factors produced by glial cells. It is concluded that there is, as yet, no coherent explanation for the upregulation or downregulation of any of the cytoskeletal proteins following axotomy or during regeneration. In considering the relevance of these changes in cytoskeletal protein synthesis to regeneration, it is emphasized that they are unlikely to be involved in the initial outgrowth of the injured axons, both because transit times between cell body and injury site are too long, and because sprouting can occur in isolated axons. Injury-induced acceleration of the axonal transport of tubulin and actin in the proximal axon is likely to be more important in providing the cytoskeletal protein required for initial axonal outgrowth. Subsequently, the increased synthesis and transport velocity for actin and tubulin increase the delivery of these proteins to support the increased volume of the maturing regenerating axons. Reduction in neurofilament synthesis and changes in neurofilament phosphorylation may permit the increased transport velocity of the other cytoskeletal proteins. There is little direct evidence that alterations in cytoskeletal protein synthesis are necessary for successful regeneration, nor are they sufficient in the absence of a supportive environment. Nevertheless, the correlation that exists between a robust cell body response and successful regeneration suggests that an understanding of the regulation of cytoskeletal protein synthesis following axon injury must be a part of any successful strategy to improve the regenerative capacity of the central nervous system.

Increases in mRNA for cytoskeletal proteins in the denervated neuropil of the dentate gyrus: an in situ hybridization study using riboprobes for beta-actin and beta-tubulin

Following destruction of entorhinal cortical (EC) input to the dentate gyrus there is an increase in protein synthesis within the denervated dendritic laminae. The present study utilized in situ hybridization to determine whether there were increases in the messenger RNAs (mRNAs) for beta-actin and beta-tubulin within the denervated neuropil of the dentate gyrus during the time of increased protein synthesis. Animals were prepared for in situ hybridization 2-21 days after a unilateral lesion of the EC. Brain sections were hybridized with either 3H or 35S-labeled riboprobes prepared from chick beta-actin and chick beta-tubulin mRNA. Analysis of light microscopic autoradiograms revealed increases in the mRNAs for beta-actin and beta-tubulin within the denervated neuropil between 6 and 8 days postlesion when compared to the intact dentate gyrus of the contralateral side. Labeling over the granule cell body layer was comparable on the two sides for both probes. Increases in both mRNAs were also observed in the scar tissue at the lesion site. These results suggest that local protein synthesis within the denervated neuropil of the dentate gyrus involves, in part, an increase in the production of actin and tubulin.

Phosphorylation of superior cervical ganglion proteins during regeneration

The incorporation of radioactive phosphate into proteins of both normal and regenerating ganglia of the sympathetic nervous system of the rat is reported. The incorporation reactions were carried out in vitro by incubating homogenates of excised ganglia with [gamma-32P]ATP under various conditions. It was found that incorporation of phosphate into proteins of regenerating ganglia in the molecular mass range 10,000-100,000 daltons increased up to 40% over incorporation into proteins from control ganglia during the first 3 days following injury and returned to control levels after 14 days. Analysis of the proteins by two-dimensional electrophoresis revealed that only few, i.e., less than 20, became radioactively labelled in homogenates of superior cervical ganglia in the presence of Ca2+, and even fewer in the presence of cyclic AMP. Furthermore, all these proteins fell within a narrow pI range of 4-6. The growth-associated protein, variously designated GAP-43, B-50, F-1, and pp46, has an enhanced level of expression and phosphorylation in regenerating ganglia compared with controls at day 3. Injury also caused consistently higher levels of incorporation into two other proteins with molecular masses at positions 55,000 and 85,000 and pI values of 5.1 and 4.5, respectively; the former protein most probably is beta-tubulin. The fact that both proteins are found in the 15,000 g pellet after the tissue has been solubilized in 0.5% nonionic detergent indicates that they may indeed by components of filament assemblies. Thus, the results suggest that protein phosphorylation is a mechanism involved in cytoskeletal function in regenerating nerve.

Reciprocal regulation of tachykinin- and vasoactive intestinal peptide-gene expression in rat sensory neurones following cut and crush injury

The relative abundance of preprotachykinin- (PPT), actin- and vasoactive intestinal peptide- (VIP) mRNA's was measured in L5 dorsal root ganglia of rats after resecting or crushing the sciatic nerve. PPT-mRNA levels fell to 40% of control values 3, 6 and 9 days following nerve resection. Crushing produced a lesser fall at 3 and 6 days with a partial recovery at 9 days. Following resection actin-mRNA levels transiently rose to twice control values and had returned to normal by day 9. VIP-mRNA was not detectable in control ganglia but increasing amounts of VIP-mRNA were present 3, 6 and 9 days after nerve injury. The results are discussed in terms of the control mechanisms operating.

Effect of cutting or crushing the rat sciatic nerve on synthesis of substance P by isolated L5 dorsal root ganglia

Following cutting or crushing the rat sciatic nerve, synthesis of substance P by L5 dorsal root ganglia in vitro was reduced respectively to 20 and 40 per cent of control values. By day 64 the ability to synthesise substance P had been fully restored in crushed neurones but in cut neurones remained at a low level. We conclude that substance P synthesis is a more sensitive index of the effect of nerve injury than is substance P content and further that regenerating axons are able to support substance P synthesis before they reach their target tissue.