Comparison of posttranslational protein modification by amino acid addition after crush injury to sciatic and optic nerves of rats

Regulation of Mitochondrial Respiratory Chain Complex Levels, Organization, and Function by Arginyltransferase 1

Arginyltransferase 1 (ATE1) is an evolutionary-conserved eukaryotic protein that localizes to the cytosol and nucleus. It is the only known enzyme in metazoans and fungi that catalyzes posttranslational arginylation. Lack of arginylation has been linked to an array of human disorders, including cancer, by altering the response to stress and the regulation of metabolism and apoptosis. Although mitochondria play relevant roles in these processes in health and disease, a causal relationship between ATE1 activity and mitochondrial biology has yet to be established. Here, we report a phylogenetic analysis that traces the roots of ATE1 to alpha-proteobacteria, the mitochondrion microbial ancestor. We then demonstrate that a small fraction of ATE1 localizes within mitochondria. Furthermore, the absence of ATE1 influences the levels, organization, and function of respiratory chain complexes in mouse cells. Specifically, ATE1-KO mouse embryonic fibroblasts have increased levels of respiratory supercomplexes I+III₂+IV_n. However, they have decreased mitochondrial respiration owing to severely lowered complex II levels, which leads to accumulation of succinate and downstream metabolic effects. Taken together, our findings establish a novel pathway for mitochondrial function regulation that might explain ATE1-dependent effects in various disease conditions, including cancer and aging, in which metabolic shifts are part of the pathogenic or deleterious underlying mechanism.

Posttranslational Arginylation Enzyme Arginyltransferase1 Shows Genetic Interactions With Specific Cellular Pathways in vivo

Arginyltransferase1 (ATE1) is a conserved enzyme in eukaryotes mediating posttranslational arginylation, the addition of an extra arginine to an existing protein. In mammals, the dysregulations of the ATE1 gene (ate1) is shown to be involved in cardiovascular abnormalities, cancer, and aging-related diseases. Although biochemical evidence suggested that arginylation may be involved in stress response and/or protein degradation, the physiological role of ATE1 in vivo has never been systematically determined. This gap of knowledge leads to difficulties for interpreting the involvements of ATE1 in diseases pathogenesis. Since ate1 is highly conserved between human and the unicellular organism Schizosaccharomyces pombe (S. pombe), we take advantage of the gene-knockout library of S. pombe, to investigate the genetic interactions between ate1 and other genes in a systematic and unbiased manner. By this approach, we found that ate1 has a surprisingly small and focused impact size. Among the 3659 tested genes, which covers nearly 75% of the genome of S. pombe, less than 5% of them displayed significant genetic interactions with ate1. Furthermore, these ate1-interacting partners can be grouped into a few discrete clustered categories based on their functions or their physical interactions. These categories include translation/transcription regulation, biosynthesis/metabolism of biomolecules (including histidine), cell morphology and cellular dynamics, response to oxidative or metabolic stress, ribosomal structure and function, and mitochondrial function. Unexpectedly, inconsistent to popular belief, very few genes in the global ubiquitination or degradation pathways showed interactions with ate1. Our results suggested that ATE1 specifically regulates a handful of cellular processes in vivo, which will provide critical mechanistic leads for studying the involvements of ATE1 in normal physiologies as well as in diseased conditions.

Posttranslational arginylation enzyme Ate1 affects DNA mutagenesis by regulating stress response

Arginyltransferase 1 (Ate1) mediates protein arginylation, a poorly understood protein posttranslational modification (PTM) in eukaryotic cells. Previous evidence suggest a potential involvement of arginylation in stress response and this PTM was traditionally considered anti-apoptotic based on the studies of individual substrates. However, here we found that arginylation promotes cell death and/or growth arrest, depending on the nature and intensity of the stressing factor. Specifically, in yeast, mouse and human cells, deletion or downregulation of the ATE1 gene disrupts typical stress responses by bypassing growth arrest and suppressing cell death events in the presence of disease-related stressing factors, including oxidative, heat, and osmotic stresses, as well as the exposure to heavy metals or radiation. Conversely, in wild-type cells responding to stress, there is an increase of cellular Ate1 protein level and arginylation activity. Furthermore, the increase of Ate1 protein directly promotes cell death in a manner dependent on its arginylation activity. Finally, we found Ate1 to be required to suppress mutation frequency in yeast and mammalian cells during DNA-damaging conditions such as ultraviolet irradiation. Our study clarifies the role of Ate1/arginylation in stress response and provides a new mechanism to explain the link between Ate1 and a variety of diseases including cancer. This is also the first example that the modulation of the global level of a PTM is capable of affecting DNA mutagenesis.

The role of local protein synthesis and degradation in axon regeneration

In axotomised regenerating axons, the first step toward successful regeneration is the formation of a growth cone. This requires a variety of dynamic morphological and biochemical changes in the axon, including the appearance of many new cytoskeletal, cell surface and signalling molecules. These changes suggest the activation of coordinated complex cellular processes. A recent development has been the demonstration that the regenerative ability of some axons depends on their capacity to locally synthesise new proteins and degrade others at the injury site autonomously from the cell body. There are also events involving the degradation of cytoskeletal and other molecules, and activation of signalling pathways, with axotomy-induced calcium changes probably being an initiating event. A future challenge will be to understand how this complex network of processes interacts in order to find therapeutic ways of promoting the regeneration of CNS axons.

Increased nerve growth factor and its receptors in atopic dermatitis: an immunohistochemical study

Evidence suggests that neurotrophins may regulate certain immune functions and inflammation. In the present study, the localization and distribution of nerve growth factor (NGF) and its receptors were explored using immunohistochemical methods, with the aim of detecting the cause of the neurohyperplasia in early lesions of atopic dermatitis (AD). In AD involved skin, strong NGF-immunoreactive (IR) cells were observed in the epidermis. In some cases, a huge number of infiltrating cells with stronger NGF immunoreactivity was seen mainly in the dermal papillae. Some trkA immunoreactivity was observed in the outer membrane of cells in the basal and spinal layers of the epidermis. In the papillary dermis, a larger number of cells demonstrated strong trkA immunoreactivity. The p75 NGFr-IR nerve fibre profiles were increased (900 per mm(2); p<0.001) compared to normal [the involved skin also differed from the uninvolved skin (p<0.05)] in the dermal papillae. These nerve fibres were larger, coarser and branched, some of them terminated at p75 NGFr-IR basal cells, and also revealed a stronger fluorescence staining than the controls or the uninvolved skin. In normal healthy volunteers and AD uninvolved skin, the NGF immunoreactivity was weak in the basal layer of epidermis. Only a few trkA positive cells were seen in the basal layer of the epidermis and upper dermis. The IR epidermal basal cells revealed a striking patchy arrangement with strong p75 NGFr immunostaining in the peripheral part of the cells, and short and thick NGFr-IR nerve fibre profiles appeared as smooth endings scattered in the dermis including the cutaneous accessory organs. Using NGF and p75 NGFr double staining, both immunoreactivities showed a weak staining in the epidermis and dermis in normal and uninvolved skin. In the involved dermis of AD, the intensity of p75 NGFr-IR nerves was stronger in areas where there were also increased numbers of NGF-IR cells. These findings indicate that NGF and its receptors may contribute to the neurohyperplasia of AD.

N-terminal arginylation of sciatic nerve and brain proteins following injury

N-terminal protein arginylation has been demonstrated in vitro and in situ and has been reported to increase following injury to sciatic nerves of rats. The present study attempts to demonstrate these reactions in vivo by applying [3H]Arg to the cut end of sciatic nerves in anesthetized rats and assaying for N-terminal arginylation using Edman chemistry and acid precipitation of labeled proteins in the proximal nerve segment. No evidence was found for arginylation in an aqueous soluble fraction. However, N-terminal arginylation was detected in a urea soluble fraction at 2 hours after nerve crush. The data show that arginylation of rat sciatic nerve proteins occurs in vivo and suggest that the arginylated proteins formed an aqueous insoluble/urea soluble aggregate after arginylation. In other experiments, rat brains were injured and assayed for arginylation in vitro to test the hypothesis that injury causes an up-regulation of these reactions. Results showed an activation of the reaction at 2 hours post crush and indicate that increases in N-terminal arginylation are likely to be a general response to injury in nervous tissue.

Posttranslational arginylation of brain proteins

The knowledge of brain protein metabolism is important in understanding nervous system brain function. Protein synthesis rates are high in young brain, decline rapidly at adult stages, and thereafter continue falling slowly with age. The breakdown of protein appears to follow a similar rate (1). Protein synthesis and degradation however, are only the two extremes of a complex phenomena which includes a variety of other protein modifications. Proteolytic cleavage is the most common covalent modification of proteins; probably all proteins that have been isolated were modified by proteolysis, since only few are found with the starting amino acid (methionine) attached. This suggests that most proteins were subject to one or more co- and/or posttranslational modifications (2). One of these posttranslational modifications is the arginylation of proteins, described 30 years ago, which now is being recognized as a widespread modification of proteins. In this review, the current status of posttranslational arginylation of brain proteins is discussed.

Differential increases in rat retinal ganglion cell size with various methods of optic nerve lesion

Optic nerve injury is a well established paradigm for studying a variety of neuronal responses, although the actual method of nerve severance is rarely taken into account. This study assessed changes to ganglion cell size in three different methods of optic nerve lesions. Adult rats underwent either one of two types of mechanical nerve crush, or an axotomy. Ganglion cells were visualised by retrograde labelling from the optic nerve with 4Di-10ASP, and soma size measured. Two weeks after lesion, mean soma size was increased in all groups. However, at 4 weeks, the crush groups continued to show an increase (60.5% larger than normal), while the mean cell size in the axotomy group was almost at normal levels (0.2% smaller than normal). This study supports the hypothesis that axotomy, and not simple crush, deprives ganglion cells of substances beneficial to cell survival.

Neuronal regeneration and estrous cycle restoration after locus coeruleus-periventricular gray substance section

The locus coeruleus (LC) was anatomically separated from the periventricular gray substance (PVG) by means of knife cuts in the adult female rat presenting regular estrous cycling. This resulted in a transient suppression of the estrous cycling that lasted 10-13 days after surgery. After this period, irregular or regular cycling activity was observed. The regular cycling was restored 30-45 days after the knife cuts. Golgi impregnation of some of the brains of these rats revealed regenerative elements in the knife-cut-insulted area. Thus, blood vessels, macrophagic-like elements, and glial-like elements were observed in close relation with the knife-cut pathway. Additionally, well-defined stained neurons typical of the LC and PVG were observed in close proximity to the knife-cut pathway. Dendritic and axon projections towards the insulted area were observed. Well defined axons were seen across the knife-cut pathway. These data support, first, that the LC-PVG communication is part of a circuitry for the modulation of gonadotropic activity, and second, that in the restoration of the estrous cyclicity after the knife cut, regenerative processes leading to a LC-PVG functional reconnection occurred after the knife cut.

Effects of arginine, S-adenosylmethionine and polyamines on nerve regeneration

INTRODUCTION

Axon growth and axon regeneration are complex processes requiring an adequate supply of certain metabolic precursors and nutrients.

MATERIAL AND METHODS

This article reviews the studies examining some of the processes of protein modification fundamental to both nerve regeneration and to the continuous and adequate supply of specific factors such as arginine, S-adenosylmethionine and polyamines.

RESULTS

The process of arginylation notably increases following nerve injury and during subsequent regeneration of the nerve, with the most likely function of arginine-modification of nerve proteins being the degradation of proteins damaged through injury. It appears that defective methyl group metabolism may be one of the leading causes of demyelination, as suggested by the observation of reduced cerebrospinal fluid concentrations of s-adenosylmethionine (SAMe) and 5-methyltetrahydrofolate, the key metabolites in methylation processes, in patients with a reduction in myelination of corticospinal tracts. Polyamine synthesis, which depends strongly on the availability of both SAMe and arginine, markedly increases in neurons soon after an injury. This "polyamine-response" has been found to be essential for the survival of the parent neurons after injury to their axons. Polyamines probably exert their effects through involvement in DNA, RNA and protein synthesis, or through post-translational modifications that are indicated as the most relevant events of the "axon reaction."

CONCLUSIONS

Nerve regeneration requires the presence of arginine, s-adenosylmethionine, and polyamines. Further studies are needed to explore the mechanisms involved in these processes.

N-terminal arginylation of proteins in explants of injured sciatic nerves and embryonic brains of rats

Posttranslational modification of proteins by arginine and lysine has been demonstrated in crude extracts of vertebrate nerves and brain but not in intact cells. In the present experiments we have exploited the fact that Arg is added posttranslationally only at the N-terminus of target proteins, to demonstrate these reactions in intact cells of sciatic nerves and embryonic brains of rats. Sciatic nerves were crushed in anaesthesized rats and 2 hrs later segments of nerve, including the site of the crush, were removed and incubated in media containing [3H]Arg. Incorporation of [3H]Arg into total proteins was analyzed by acid precipitation and the presence of label at the N-terminus was determined by a modification of the Edman degradation procedure. Approximately 25% of protein bound [3H]Arg was released from the N-terminus by the Edman reaction indicating that it was added posttranslationally rather than through protein synthesis. N-terminal labeling was not detectable in nerves not crushed prior to explant and incubation. Slices of embryonic day 20 visual cortex, when incubated under similar conditions as injured sciatic nerves, also showed approximately 25% of the protein incorporated [3H]Arg at the N-terminus, while arginylation was not detectable in adult rat brain slices. Since Lys is not added posttranslationally to the N-terminus, we have attempted to observe lysylation of proteins in intact cells by using cycloheximide (Cx) to block protein synthesis without interfering with protein modification. The posttranslational incorporation of Arg/Lys into proteins was found to be insensitive to up to 2.0 mM Cx in tissue extracts (in vitro).(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of a peptide that inhibits the posttranslational arginylation of proteins in rat brain

All eukaryotic cells contain enzymes that are able to catalyze the transfer of Arg from tRNA to the N-terminus of naturally short lived or damaged cytosolic proteins. For certain test proteins, it has been shown that the addition of Arg to the N-terminus leads to their degradation via the ubiquitin proteolytic pathway. The mechanisms used by cells for identifying proteins for arginylation and regulating arginylation are not known. The present study reports the isolation of a peptide from rat brain that is able to inhibit the arginylation of proteins in brain extracts. We suggest that this peptide is the physiological regulator of arginylation in rat brain.

N-terminal arginylation and ubiquitin-mediated proteolysis in nerve regeneration

Damaged sciatic nerves of rats respond to injury within minutes by activating reactions that result in the transfer RNA-mediated posttranslational addition of several amino acids to a variety of cytoplasmic proteins. For the most part, the site of addition of individual amino acids and the identity of the target proteins is not known. However, arginine, one of the amino acids added in greatest amounts, has been shown to be covalently linked to the N-terminus of acceptor proteins. In other simpler eukaryotic cells, N-terminal arginylation results in degradation of the arginylated proteins via the ubiquitin proteolytic pathway. Recent experiments have shown that when proteins, obtained from sciatic nerves 2 h after injury, are arginylated in vitro, they form high molecular weight aggregates. Other experiments have shown that these arginylated proteins are immunoreactive to a monoclonal antibody to ubiquitin. These findings suggest that following injury to the sciatic nerve, proteins which are arginylated are candidates for ubiquitin mediated proteolysis. Injury to a nerve incapable of regeneration without experimental intervention, the rat optic nerve, does not result in activation of the arginylation reactions until 6 days following injury. Based on the temporal differences in response to injury of sciatic and optic nerves (2 h vs. 6 days), we propose that the lack of arginylation following injury to the CNS is related to its inability to mount a regenerative response. The association of Arg modification of damaged proteins with the ubiquitin-mediated degradation of those proteins, suggests that regenerative failure in the CNS may be related, in part, to a failure to degrade intracellular proteins at the site of injury.

Local radiolabeling of the 68 kDa neurofilament protein in rat sciatic nerves

Rat sciatic nerve segments, 4.5 cm distal to the dorsal root ganglion (DRG), were incubated in vivo with [35S]methionine. Fluorography of 2-D polyacrylamide gels of the nerve proteins demonstrated the labeling of the 68-kDa neurofilament protein, which was identified by immunoblotting. This experimental design excludes the dorsal root ganglion as the source of the radiolabeled neurofilament protein and suggests that this neuron-specific protein may be synthesized in axons.

Evidence that axonal tRNAs are resistant to RNase and ATPase and can be aminoacylated in the absence of exogenous ATP

A high molecular weight (HMW) fraction of the 150,000 g supernatant of rat brain homogenates contains protein-tRNA complexes which are able to incorporate [3H]Arg and [3H]Lys into tRNA. The aminoacylation of tRNA(Arg) was found to be dependent on ATP and inhibited by RNase. Conversely, the aminoacylation of tRNA(Lys) did not require exogenous ATP and was resistant to RNase and ATPase. In HMW fractions of regenerating rat sciatic nerves, the charging of both tRNA(Arg) and tRNA(Lys) was resistant to RNase and ATPase and did not require exogenous ATP. Because sciatic nerves are rich in axoplasm and tRNAs are known to be present in axons, we tested the hypothesis that degradative enzyme-resistant, ATP-tRNA complexes were of axonal origin. In HMW fractions from rat liver (containing no axons), both tRNA(Arg) and tRNA(Lys) were sensitive to RNase and required exogenous ATP for charging. But, in similar fractions of axoplasm obtained from the giant axon of squid, both tRNAs were insensitive to RNase and ATPase and did not require exogenous ATP for charging. These results suggest that tRNAs in axons are present in protected HMW complexes and contain endogenous stores of ATP. The presence of ATP in the HMW complexes was demonstrated by the luciferase-luciferin assay for ATP. The nature of the protection of tRNAs from RNases was examined by dissociating proteins from HMW complexes by boiling, treating with proteinase K, or overhomogenizing the tissue. These procedures failed to render brain tRNA(Lys) susceptible to RNase. But phenol-extracted, ethanol-precipitated brain tRNA(Lys) was sensitive to RNase, suggesting that the protection of tRNA(Lys) may be by a protease- and heat-resistant polypeptide or by a nonproteinaceous mechanism.

Post-translational arginylation in the bovine lens

This study demonstrates post-translational arginylation of bovine serum albumin and endogenous lens proteins by bovine lens arginyl-tRNA:protein transferase. This reaction has been proposed to be the first step in marking specific proteins for degradation by the non-lysosomal, ATP-dependent, ubiquitin-mediated proteolytic pathway. The transferase was obtained by the method used for isolation of the same enzyme from reticulocytes (Ferber and Ciechanover, 1987, Nature 326, 808-11). Incorporation of [3H]Arg was linear for at least 2 hr at 37 degrees C. The amount of incorporation was directly proportional to the amount of lens enzyme or substrate added. Arginylation was ATP-dependent. A requirement for tRNA was demonstrated by inhibition upon pretreatment of the enzyme preparation with nuclease to hydrolyse endogenous tRNA, and restoration of activity upon replacement of tRNA. [3H]Leu, [3H]Lys and [3H]His were not incorporated, demonstrating specificity of the reaction for arginine. This is the first demonstration of post-translational modification of proteins by arginylation in the lens.

The posttranslational arginylation of proteins in different regions of the rat brain

The posttranslational incorporation of arginine into proteins catalyzed by arginyl-tRNA protein transferase was determined in vitro in different rat brain regions. The incorporation was found in all the regions studied, although with different specific activities (pmol [14C]arginine incorporated/mg protein). Of the regions studied, hippocampus had the highest specific activity followed by striatum, medulla oblongata, cerebellum, and cerebral cortex. Electrophoretic analysis of the [14C]arginyl proteins from the different regions followed by autoradiography and scanner densitometry showed at least 13 polypeptide bands that were labeled with [14C]arginine. The radioactive bands were qualitatively coincident with protein bands revealed by Coomassie Blue. There were peaks that showed different proportions of labeling in comparison with peaks of similar molecular mass from total brain. Most notable because of their high proportions were those of molecular mass 125 kDa in hippocampus, striatum, and cerebral cortex; 112 and 98 kDa in striatum and cerebellum; and 33 kDa in hippocampus and striatum. In lower proportions than in total brain were the peaks of 33 kDa in medulla oblongata and cerebral cortex and of 125 kDa in medulla oblongata.

Prostaglandin E2 changes in the retina and optic nerve of an eye with injured optic nerve

Changes in arachidonic acid metabolism were studied in the optic nerve, the chorioretina, and in the vitreous following crush injury to the optic nerve of rats. Crush injury led to: (i) a 3.9-fold increase in optic nerve prostaglandin type E2 in vitro production which peaked on day 5 and was followed by a gradual decline, but was still significantly higher than baseline levels by day 12; (ii) a two-fold increase in the chorioretina prostaglandin type E2 in vitro production which peaked on day 1, and resumed baseline levels by day 3; (iii) a 3.5-fold increase in vitreous prostaglandin type E2 levels on day 1 which remained at 1.5-2 times higher than baseline levels for the rest of the study period (12 days). The findings indicate that the pattern of changes in prostaglandin type E2 production by the optic nerve (consisting mostly of white matter) is different from that described for injured brain tissues. The prolonged accumulation of vitreal prostaglandin type E2 in eyes with damaged optic nerve may lead to undesirable effects on the retina beyond those directly manifested in the retina by altered axonal flow in the injured optic nerve.

Effects of gangliosides on the methionine uptake in crushed sciatic nerves of rats with alloxan diabetes

Effect of gangliosides on the transmembrane transport of methionine in crushed sciatic nerves was studied in alloxan diabetic and control rats until the 4th day after nerve crushing. Methionine uptake rate decreases while time passes after injury in both control and diabetic rats, but the decrement is greater in diabetic rats than in controls. Ganglioside treatment causes an enhancement on the transmembrane transport in both diabetic and control animals, but the effect in time was greater in diabetic rats than in controls.

The site of amino acid addition to posttranslationally modified proteins of regenerating rat sciatic nerves

The posttranslational modification of proteins by amino acids has been described in a variety of biological systems. These reactions occur at low levels in intact sciatic nerves of rats but are increased 10-fold following nerve injury and during subsequent regeneration of the nerve. While it has been shown in brain and liver that the site of addition of Arg is to the N-terminus, there is no information on the location at which the other amino acids add on to targeted proteins nor the site of addition of Arg in regenerating nerves. In the present study, we have used manual micro-Edman degradation combined with HPLC, and digestion with carboxypeptidase A and B to determine the site of addition of various amino acids to targeted proteins. Of the 3H-labelled amino acids incorporated posttranslationally into proteins of regenerating sciatic nerves (Arg, Lys, Leu, Phe, Val, Ala, Pro and Ser), only [3H]Arg was found to be present at the N-terminus. To determine whether amino acid additions were occurring at the C-terminus, proteins modified by two of the amino acids incorporated in greatest amounts (Lys and Leu) were incubated with specific carboxypeptidases. [3H]Leucine was not liberated following incubation with carboxypeptidase, suggesting that Leu is not added at the C-terminus of modified proteins. Under similar conditions, some [3H]Lys was liberated, but in amounts not significantly different from controls incubated without carboxypeptidase, indicating a non-specific degradation of Lys modified proteins rather than a specific release of Lys from the C-terminus. These experiments show that in regenerating sciatic nerves of rats, Arg is the only amino acid added posttranslationally to the amino terminus of target proteins, and that Leu, and probably Lys, are not conjugated to proteins at the C-terminus.

Amino acid modification of proteins in regenerating sciatic nerves of rats

Recent experiments have shown that Arg, Lys, and Leu can be incorporated posttranslationally into proteins of regenerating sciatic nerves of rats. The present experiments investigate a mixture of 15 radioactive amino acids to determine if additional amino acids can be conjugated posttranslationally to proteins of regenerating nerves. Proteins of regenerating sciatic nerves of rats were able to incorporate Arg, Lys, Leu, Pro, Val, Ala, Phe, and Ser in relatively large amounts and Asp, Glu, Thr, Gly, Ile, His, and Tyr in relatively low or undetectable amounts, in the most advanced portion of the regenerating nerves. Two-dimensional SDS PAGE showed incorporation of the amino acid mixture into distinct radioactive peaks with molecular weights in the 80-90 kD, 53-66 kD, 22-46 kD, and 17 kD ranges with isoelectric points between 5.0 and 7.9. Most of the amino acids were incorporated into proteins in all of the molecular weight ranges. But Ser was incorporated in highest amounts in the 17 kD range, and Val was most abundant in the 22-46 kD range. In some cases results indicated that single proteins were modified by several amino acids. While we do not yet know which amino acids modify specific nerve proteins or the function of the modifications in nerve regeneration, these studies demonstrate the participation of some but not all amino acids in posttranslational modification reactions and the selective modification of specific groups of nerve proteins by these amino acids.

Changes in contralateral protein metabolism following unilateral sciatic nerve section

Changes in nerve biochemistry, anatomy, and function following injuries to the contralateral nerve have been repeatedly reported, though their significance is unknown. The most likely mechanisms for their development are either substances carried by axoplasmic flow or electrically transmitted signals. This study analyzes which mechanism underlies the development of a contralateral change in protein metabolism. The incorporation of labelled amino acids (AA) into proteins of both sciatic nerves was assessed by liquid scintillation after an unilateral section. AA were offered locally for 30 min to the distal stump of the sectioned nerves and at homologous levels of the intact contralateral nerves. At various times, from 1 to 24 h, both sciatic nerves were removed and the proteins extracted with trichloroacetic acid (TCA). An increase in incorporation was found in both nerves 14-24 h after section. No difference existed between sectioned and intact nerves, which is consistent with the contralateral effect. Lidocaine, but not colchicine, when applied previously to the nerves midway between the sectioning site and the spinal cord, inhibited the contralateral increase in AA incorporation. It is concluded that electrical signals, crossing through the spinal cord, are responsible for the development of the contralateral effect. Both the nature of the proteins and the significance of the contralateral effect are matters for speculation.

Regulation of the post-translational conjugation of amino acids to rat brain proteins

Post-translational conjugation of arginine (but not other amino acids) to proteins has been reported to occur in a high speed supernatant fraction of rat brain homogenates from which molecules of less than 5000 mol. wt have been removed. In the present study we report that removal of molecules of less than 1000 mol. wt by dialysis, does not result in incorporation of arginine into protein in amounts significantly different than in the undialysed supernatant. The addition of molecules with molecular weights greater than 1000 and less than 5000 to the active fraction, inhibits the incorporation of arginine into proteins in a concentration dependent manner suggesting that the post-translational incorporation of arginine into brain is regulated by a molecule(s) of greater than 1000 and less than 5000 mol. wt. Incorporation of lysine into proteins did not occur following removal of molecules of less than 5000 mol. wt, but did occur in the void volume fraction of a Sephacryl S-200 column (molecular weight cut-off 125,000), suggesting that the incorporation of lysine into proteins is regulated by molecules retained by the S-200 column but greater than 5000 mol. wt. When experiments were repeated using the void volume of a Sephacryl S-300 column (molecular weight exclusion, approximately 200 k), leucine and proline were incorporated in amounts similar to arginine and lysine and serine, alanine, valine, phenylalanine and histidine were incorporated at lower but measurable levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative analysis of labelled inner retinal proteins in experimental optic neuritis

In order to determine if axonal transport changes in chronic experimental allergic encephalomyelitis (EAE) were due to blockade or increased discharge of fast transported proteins from the inner retina, we examined the presence of pulse labeled proteins in autoradiograms of the optic nerve head, retinal ganglion cell and nerve fiber layers of juvenile strain-13 guinea pigs with chronic EAE and normal controls. Quantitative analysis of silver grains, performed six and twenty-four hours following the intravitreal injection of tritiated leucine, showed a decrease in inner retinal radioactivity in those with EAE, whereas no difference was detected between the two groups after three days. Grain counts within the optic nerve heads of guinea pigs with EAE were reduced at all time intervals studied. These results are consistent with an increase in discharge of fast transported proteins from retinal ganglion cells into optic nerve axons and support our previous observations of increased radioactivity at the foci of optic nerve demyelination.

Survival and Axonal Elongation of Adult Rat Retinal Ganglion Cells

A peripheral nerve exudate, collected in situ from the proximal end of a severed rat sciatic nerve, can induce substantial axonal elongation from ganglion cells when tested on explanted adult rat retinae. The responsive cells are identified on the basis of their Thy 1.1 immunostaining properties. Similar outgrowth can be obtained from explants when the culture medium is supplemented with brain-derived neurotrophic factor (BDNF). In addition, both BDNF and the sciatic nerve exudate can prevent ganglion cell degeneration as shown by the retrograde transport of a fluorescent dye that had been applied to the superior colliculus prior to explantation. The results demonstrate that soluble components, released by lesioned peripheral nerves, can effect adult retinal ganglion cells in a way that is reminiscent of that which has been described in vivo using sciatic nerve grafts after sectioning of the optic nerve. The molecular nature of these components is discussed.