Changes in rapid transport of phospholipids in the rat sciatic nerve during axonal regeneration

Seeding nerve sutures with minced nerve-graft (MINE-G): a simple method to improve nerve regeneration in rats

BACKGROUND

The aim of this study was to assess the effect of seeding the distal nerve suture with nerve fragments in rats.

METHODS

On 20 rats, a 15 mm sciatic nerve defect was reconstructed with a nerve autograft. In the Study Group (10 rats), a minced 1 mm nerve segment was seeded around the nerve suture. In the Control Group (10 rats), a nerve graft alone was used. At 4 and 12 weeks, a walking track analysis with open field test (WTA), hystomorphometry (number of myelinated fibers (n), fiber density (FD) and fiber area (FA) and soleus and gastrocnemius muscle weight ratios (MWR) were evaluated. The Student t-test was used for statistical analysis.

RESULTS

At 4 and 12 weeks the Study Group had a significantly higher n and FD (p = .043 and .033). The SMWR was significantly higher in the Study Group at 12 weeks (p = .0207).

CONCLUSIONS

Seeding the distal nerve suture with nerve fragments increases the number of myelinated fibers, the FD and the SMWR. The technique seems promising and deserves further investigation to clarify the mechanisms involved and its functional effects.

Changes in mRNA for choline transporter-like protein following facial nerve transection

When the axon of a motoneuron is transected, axonal regrowth occurs to reconnect it to the correct target. During the regeneration period, a large amount of new membrane synthesis is required for the axons to extend. Choline is an important metabolite in all cells because of the major contribution of phosphatidylcholine and sphingomyelin to the production of membranes. Therefore, choline uptake is necessary for axonal elongation. We cloned rat choline transporter-like protein 1 (rCTL1) as an upregulated gene in the axotomized facial motor nucleus by differential display polymerase chain reaction using adult rat facial nerve axotomy model. rCTL1 belongs to the choline transporter-like protein family, which takes up choline. We investigated the changes in rCTL1 mRNA levels in the facial motor nucleus of adult rats following axotomy by in situ hybridization. In the facial motoneurons signals of rCTL1 mRNA were rarely expressed, were transiently increased following axotomy and gradually returned to the control level. These results suggest that rCTL1 is involved in activated choline uptake for membrane synthesis in motoneurons following nerve transection.

The synthesis and transport of lipids for axonal growth and nerve regeneration

Neurons are unique polarized cells in which the growing axon is often located up to a meter or more from the cell body. Consequently, the intracellular movement of membrane lipids and proteins between cell bodies and axons poses a special challenge. The mechanisms of lipid transport within neurons are, for the most part, unknown although lipid transport via vesicles and via cholesterol- and sphingolipid-rich 'rafts' are considered likely mechanisms. Very active anterograde and retrograde transport of lipid-containing vesicles occurs between the cell body and distal axons. However, it is becoming clear that the axon need not obtain all of its membrane constituents from the cell body. For example, the synthesis of phosphatidylcholine, the major membrane phospholipid, occurs in axons, and its synthesis at this location is required for axonal elongation. In contrast, cholesterol synthesis appears to occur only in cell bodies, and cholesterol is efficiently delivered from cell bodies to axons by anterograde transport. Cholesterol that is required for axonal growth can also be exogenously supplied from lipoproteins to axons of cultured neurons. Several studies have suggested a role for apolipoprotein E in lipid delivery for growth and regeneration of axons after a nerve injury. Alternatively, or in addition, apolipoprotein E has been proposed to be a ligand for receptors that mediate signal transduction cascades. Lipids are also transported from axons to myelin, although the importance of this process for myelination is not clear.

Junction between parent and daughter axons in regenerating myelinated nerve: properties of structure and rapid axonal transport

The primary aim of this work was to investigate the properties of rapid axonal transport in regenerating myelinated axons in the sciatic nerve of Xenopus laevis, with particular attention to events at the junction between the proximal, intact axon (the "parent") and the distal, newly formed axon (the "daughter"). Morphological studies indicated that all myelinated axons initiated regeneration and that at least 80% of these axons regenerated at a rate of 1 mm/day or greater (20 degrees C). The ultrastructure of the junctional region was examined at regeneration times between 3 days and 20 weeks. The main qualitative change in the junctional axoplasm over this period was in its content of particulate organelles. At times up to 2 weeks regeneration, the junction contained abnormal numbers of 50 nm diameter vesicles and 10 nm granules. Between 2 and 5 weeks the junction showed in addition a peripheral rim of large membrane-bounded organelles around a central core of microtubules and neurofilaments. At longer times the numbers of large membrane-bounded organelles diminished and all junctions contained prominent accumulations of 10 nm granules. The rate of rapid axonal transport of protein was similar in parent and daughter axons. Compared to the parent axons, a 2-5 times greater amount of protein was deposited to a stationary phase in daughter axons. Specimens of nerve that were subjected to mechanical stress during the removal of the perineurium showed a large accumulation of rapidly transported protein in the region of the crush at regeneration times up to 40 days; some of the accumulated protein was subsequently transported retrogradely. Video microscopy of isolated axons supplied evidence that the transport deficit in mechanically stressed nerve was a partial block of anterograde vesicle transport, plus a reversal of anterograde transport, at the junction of parent with daughter axons. No structural changes were detected in mechanically stressed nerve. The results show that the junction between parent and daughter myelinated axons is a region with distinct morphology at which the dynamics of anterograde axonal transport may change dramatically.

Optic nerve regeneration in adult fish and apolipoprotein A-I

Fish optic nerves, unlike mammalian optic nerves, are endowed with a high capacity to regenerate. Injury to fish optic nerves causes pronounced changes in the composition of pulse-labeled substances derived from the surrounding non-neuronal cells. The most prominent of these injury-induced changes is in a 28-kilodalton (kDa) polypeptide whose level increases after injury, as revealed by one-dimensional gel electrophoresis and autoradiography. The present study identified as apolipoprotein A-I (apo-A-I) a polypeptide of 28 kDa in media conditioned by regenerating fish optic nerves. The level of this polypeptide increased after injury by approximately 35%. Apo-A-I was isolated by gel-permeation chromatography from delipidated high-density lipoproteins (HDL) that had been obtained from carp plasma by sequential ultracentrifugation. Further identification of the purified protein as apo-A-I was based on its molecular mass (28 kDa) as determined by gel electrophoresis, amino acid composition, and microheterogeneity studies. The isolated protein was further analyzed by immunoblots of two-dimensional gels and was found to contain six isoforms. Western blot analysis using antibodies directed against the isolated plasma protein showed that the 28-kDa polypeptide in the preparation of soluble substances derived from the fish optic nerves (conditioned media, CM) cross-reacted immunologically with the isolated fish plasma apo-A-I. Immunoblots of two-dimensional gels revealed the presence of three apo-A-I isoforms in the CM of regenerating fish optic nerves (pIs: 6.49, 6.64, and 6.73). At least some of the apo-A-I found in the CM is derived from the nerve, as was shown by pulse labeling with [35S]methionine, followed by immunoprecipitation. The apo-A-I immunoactive polypeptides in the CM of the fish optic nerve were found in high molecular-weight, putative HDL-like particles. Immunocytochemical staining revealed that apo-A-I immunoreactive sites were present in the fish optic nerves. Higher labeling was found in injured nerves (between the site of injury and the brain) than in non-injured nerves. The accumulation of apo-A-I in nerves that are capable of regenerating may be similar to that of apo-E in sciatic nerves of mammals (a regenerative system); in contrast, although its synthesis is increased, apo-A-I does not accumulate in avian optic nerves nor does apo-E in rat optic nerves (two nonregenerative systems).

Incorporation of [32P]phosphate into nucleotides of the dorsal root ganglia of regenerating rat sciatic nerve

[32P]Phosphate incorporation into nucleotides of the dorsal root ganglia (DRG) was studied after a crush lesion of the rat sciatic nerve. DRG were labelled during a 2-h, in vitro incubation in a balanced salt solution containing [32P]orthophosphate, 1, 2, 4 and 8 days after the crush lesion. Nucleotides were analyzed by HPLC on an ion-exchange column. An increased incorporation of 32P was found in DRG of the injured nerve for all the studied time periods. This increase was unevenly distributed among the nucleotides. UTP, CTP and ADP showed the largest and most persistent increases in labelling. The specific activity of 4 analyzed nucleotides (ATP, ADP, UTP and CDP) remained constant in DRG from crushed nerves. Thus, the observed increase in 32P-labelling could not solely be due to an increased uptake of label but must also reflect an enhanced metabolism of nucleotides in regenerating DRG. The finding that alterations of nucleotide metabolism could be observed within one day after the crush lesion suggests that this response can be used as a valuable tool for studies of the initial events of regeneration.

Molecular and cellular aspects of nerve regeneration

Injury of an axon leads to at least four independent events, summarized in Figure 1: first, deprivation of the nerve cell body from target-derived or mediated substances, which leads to a derepressed or a permissive state; second, disruption of anterograde transport, with a resultant accumulation of anterogradely transported molecules; third, environmental response with possible consequent changes in constituents of the extracellular matrix and substances secreted from the surrounding cells; and fourth, appearance of growth inhibitors and modified protease activity. It seems that the first three of these events are obligatory, but not sufficient, i.e., they lead to a growth state only if the cell body is able to respond to the injury-induced signals from the environment (a and b). The regenerative state is characterized by alterations in protein synthesis and axonal transport and by sprouting activity. The subsequent elongation of the growing fibers depends on a continuous supply of appropriate growth factors. These factors are presumably anchored to the appropriate extracellular matrix that serves as a substratum for elongating fibers. It should be mentioned that the proliferating nonneuronal cells have a conducive effect on regeneration by forming a scaffold for the growing fibers. Accordingly, the lack of regeneration may stem from a deficiency in the ability of glial cells to provide the appropriate soluble components or from insufficient formation of extracellular matrix. In this respect, one may consider regeneration of an injured axon as a process which involves regeneration of both the nonneuronal cells and the supported axons. The regeneration of glial cells may fulfill the rules which are applied to regeneration of any other proliferating tissue. Furthermore, the processes of regeneration in the axon and the glial cells are mutually dependent. Perhaps the triggering factors provided by the nonneuronal cells affect the nonneuronal cells themselves by modulating their postlesion gliosis and thereby inducing their appropriate activation. In such a case, regeneration of nonneuronal cells may resemble an autocrine type of regulation that exists also during ontogeny. The growth regulation is shifted back to the paracrine type upon neuronal maturation or cessation of axonal growth. When the elongating fibers reach the vicinity of the target organ, they are under the influence of the target-derived factors, which guide the fibers and eventually cease their elongation.

Nerve repair and axonal transport: outgrowth delay and regeneration rate after transection and repair of rabbit hypoglossal nerve

The axonal transport and distribution of the fast phase of [3H]leucine-labeled proteins were used to monitor the outgrowth delay and regeneration rate in rabbit hypoglossal nerves 5-21 days after crush or transection. The transected nerves were repaired with mesothelial chambers or epineurial sutures. Radiolabeled proteins were transported into regenerating axons in the distal nerve segment after an initial delay of 2.5 days for crushed nerves and after a delay (initial and scar delays) of 4.8 and 5.7 days for sutured and mesothelial chamber-reconnected nerves, respectively. Regeneration rate was 3.5 mm/day after a crush and 2 mm/day after a transection with either type of repair. Total radioactivity was greater in both crushed and repaired nerves than in their contralateral controls. Transported radioactivity accumulated at the site of the lesions. This accumulation was greater and persisted longer in repaired nerves than in crushed ones. The difference in regenerative response after different types of trauma with respect to changes in axonal transport is emphasized.

Technique for injection into the sciatic nerve of the mouse for quantitative in vivo metabolic studies

In this paper we describe a technique for intraneural injections, applicable to mouse peripheral nerves, which, compared with previous techniques, reduces trauma to the nerves and increases the level and reduces the variability of label recovery. Our technique employs glass needles (tip diameter, 50 micron) linked to a peristaltic pump by polyethylene tubing to inject small volumes (in the microliter range) of radiolabeled substrate solutions into mouse sciatic nerves, and allows the recovery of 20.9 +/- 1.9% (mean +/- standard deviation) and 30.5 +/- 4.8% of the injected radioactivity for 2 microliter [3H]acetate and 0.5 microliter of [3H]stearate, respectively.

Nerve repair and axonal transport. Distribution of axonally transported proteins during maturation period in regenerating rabbit hypoglossal nerve

The distribution of fast migrating [3H]leucine-labelled proteins was studied in transected and repaired rabbit hypoglossal nerves. The nerves were repaired 90 days earlier with mesothelial chamber or epineurial suture technique. Fast migrating radiolabelled proteins were transported into the distal nerve segment and neurophysiological recordings from the tongue as well as the presence of myelinated axons in the distal nerve segment verified successful regeneration. The total amount of radioactivity was increased in repaired nerves as compared to contralateral nerves. In both groups there was a significant accumulation of radiolabelled proteins at the site of lesion. Nerves repaired with mesothelial chambers showed significantly more radioactivity in the distal nerve segment as compared to sutured nerves. The present study indicates long-standing effects on axonal transport system after both types of nerve repair. It is our opinion that axonal transport studies are a valuable complement when evaluating experimental nerve repair.

Retrograde axonal transport of phospholipid in rat sciatic nerve

Retrograde axonal transport of phospholipid was studied in rat sciatic motoneuron axons by placing collection crushes on the nerve at intervals after injection of [methyl-3H]choline into the lumbosacral spinal cord, and allowing labelled material undergoing anterograde or retrograde movement to accumulate adjacent to the collection crushes. Control experiments showed that the accumulations of label were not a result of local uptake of circulating precursor. The majority of the 3H label was associated with phosphatidylcholine. Accumulation of label at the distal collection crush, representing retrograde transport, was observed subsequent to the anterograde transport of phospholipid. In comparison with a previous study on retrograde transport of protein, the following points were noted: (1) onset of retrograde transport occurred at approximately the same time after precursor injection (10-20 h) for both protein and phospholipid; (2) retrograde transport of lipids was more prolonged: maximum retrograde transport occurred later for phospholipid (approximately 30 h) than for protein (15-20 h), and declined to half-maximum between 49 and 99 h, compared to a corresponding value of 24-28 h for protein; (3) the proportion of total anterograde-transported activity subsequently undergoing retrograde transport was less in the case of phospholipid, at least over the time interval studied (up to 99 h after precursor injection). The similar times of onset of retrograde transport of phospholipid and protein support the concept of retrograde transport as a recycling mechanism returning to the cell body membrane fragments that were earlier transported into the axon. Coordinated retrograde transport of labelled protein and phospholipid components of the recycled membranes would be predicted.(ABSTRACT TRUNCATED AT 250 WORDS)

Ganglioside changes in the regenerating goldfish optic system: comparison with glycoproteins and phospholipids

Axonally transported radioactivity in sialoglycoconjugates, labeled by intraocular injection of [3H]N-acetylmannosamine, increased significantly during regeneration of goldfish optic axons at 30 degrees C. Ganglioside radioactivity showed the largest increase--approximately eightfold--in the optic nerve tract at 8 days after optic nerve crush while sialoglycoprotein radioactivity increased fourfold under the same conditions. As regeneration proceeded the magnitude of the increase in the nerve tract diminished for both glycoconjugates. In the optic tectum, however, transported radioactivities remained approximately twofold higher than controls between 15 and 25 days postcrush. The zwitterionic fraction of glycerophospholipids, labeled by intraocular injection of [14C]glycerol, also showed large increases during regeneration, but the acidic glycerophospholipids showed only modest increases. Thus while membrane components in general were elevated during the early stages of regeneration, the most pronounced increases occurred in gangliosides and certain glycerophospholipids. The significance of these changes in the regeneration process remain to be determined.

Myelination of regenerating sciatic nerve of the rat: lipid components and synthesis of myelin lipids

Changes of lipid, free fatty acid, protein, DNA, and RNA content in proximal and distal segments of regenerating sciatic nerve, from 14 to 120 days after crush, were determined. During the early stage of Wallerian degeneration, a marked decrease of phospholipid, cerebroside and sulfatide content and, in contrast, a marked increase of protein, DNA, RNA, and free fatty acid content, in the distal segment of crushed nerve compared to control, was observed. A gradual increase of phospholipid, cerebroside, and sulfatide levels, approaching normal values, and a gradual slope in the increase of protein, DNA, RNA, and free fatty acid levels over the ensuing time periods of regeneration was seen. Total cholesterol content was relatively constant during regeneration, slightly increasing at day 120. The activity of 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) of myelin fraction purified from distal segment of regenerating sciatic nerve showed a significant increase in the 30-120 day regenerating period. A marked increase of the incorporation of [2-3H]glycerol and of [Me-14C]choline into myelin lipids of distal segment of regenerating nerve, was found. Labeling of myelin lipids with [3H]oleic acid (injected intravenously seven days before crush) support the evidence that a similar pattern of degeneration exists between two different types of trauma, i.e. nerve crush or cut. The findings suggest that, in the distal segment of crushed nerve, the lipid content as well as the myelin lipid synthesis increase as the regeneration period proceeds.

Axonal transport of glycerophospholipids in regenerating sciatic nerve of the rat during aging

The effect of age upon the axoplasmic transport of glycerophospholipids has been studied using as a model the regenerating sciatic nerve of young (2-month-old), young adult (6-month-old), middle-aged (16-month-old), and aged (20-month-old) male rats. The right sciatic nerve was crushed 0.5 mm down the incisura ischiadica. Four and nine days after the lesion, a mixture of [2-3H] glycerol and [methyl-14C] choline was bilaterally injected into the spinal cord, at a level of the L4-L5 vertebrae. The animals were killed 18 hr after the isotope injection. Proximal and distal portions of crushed nerve and of contralateral sham-operated ones were dissected and consecutive 5-mm segments were subjected to lipid extraction and analysis. The findings of the present study are summarized as follows: (1) The accumulation of labeled lipid material axonally transported four days after nerve injury was mainly located at the crush site in young, young adult, middle-aged, and aged rats. The accumulation of both 3H-glycerolipids and 14C-choline phospholipids in postcrush segments was markedly higher for young and young adult than for aged rats, four and nine days after crush; (2) the average rate of axonal regeneration, determined between days 4 and 9 following crush injury was 3.6 and 4.2 mm/day for 2-month-old and 6-month-old rats, respectively; it decreased to the value of 2.5 mm/day for 16-20-month-old rats.