Posttranslational protein modification by amino acid addition in regenerating optic nerves of goldfish

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.

Effects of different vitamin D supplementation strategies in reversing metabolic syndrome and its component risk factors in adolescents

There is little evidence on the efficacy of various vitamin D supplementation strategies in reversing metabolic syndrome (MetS) in adolescents. The present study aims to fill this gap. A total of 535 (243/292) out of 650 apparently healthy Saudi adolescents were randomly selected from the Vitamin D School Project database which has baseline and post-intervention information of more than 1000 Saudi adolescents 12-18 years old attending 34 schools in Riyadh, Saudi Arabia from Nov 2014-May 2015. Allocation of intervention was done in 3 groups using cluster randomization: vitamin D tablet, 1000IU/day (N = 180; 69 boys, 111 girls); vitamin D fortified milk consumption, 200 ml/day, 40IU/100 ml (N = 189; 93 boys, 96 girls) and control (educational awareness) (N = 166; 81 boys, 85 girls). All groups were given educational awareness on how to increase vitamin D levels. All groups were matched for BMI and analysis adjusted for age. Post-intervention and using intent-to-treat approach, within-group analysis revealed a statistically significant increase in 25(OH)D levels in all groups, and a clinically significant increase in favor of the tablet group (between-group) [10.7 nmol/l (34.7%) versus 6.3 nmol/l (19.8%) in milk and 2.1 nmol/l (7.0%) in control; p < 0.001], adjusted for age and BMI-matched. Between group analysis also revealed a clinically significant decrease in triglycerides (p = 0.05), glucose (p < 0.001) and systolic blood pressure (p = 0.005) as well as a clinically significant increase in HDL-cholesterol (p = 0.004) over time, all in favor of the tablet group. Within-group comparison showed a significant decrease in the incidence of MetS in the tablet group (9.4% versus 4.4%; p < 0.05) only. In conclusion, oral vitamin D supplementation is superior to vitamin D fortified milk in improving vitamin D status. Reduction in the incidence of MetS in the Arab adolescent population secondary to vitamin D correction may be dose-dependent.

Prevalence of Vitamin D Deficiency and Calcium Homeostasis in Saudi Children

OBJECTIVE

Vitamin D deficiency (VDD) and vitamin D insufficiency (VDI) are significant health problems all over the world. The aim of this study was to determine the prevalence of VDD and VDI in children and adolescents residing in 8 provinces in the Kingdom of Saudi Arabia and to also investigate calcium homeostasis in these subjects.

METHODS

A cross-sectional study was conducted in 2110 participants aged between 6 and 15 years. Information on socio-demographic status, anthropometric measurements, knowledge about vitamin D, color of the skin, dietary intake, sun exposure experience, smoking, and physical activity were collected through a questionnaire given to the parents of all subjects. The subjects were divided into three groups as vitamin D deficient, vitamin D insufficient, and vitamin sufficient according to their blood level of vitamin D [VDD ≤25 nmol/L (25 hydroxy vitamin D), VDI >25-50 nmol/L, and VDS >50 nmol/L].

RESULTS

VDD was highly prevalent in this group of children. 95.3 of the subjects had either VDD (45.5%) or VDI (49.9%). The prevalence rate of VDD combined with VDI was higher in females (97.8%) compared to males (92.8%) (p<0.001). Only 1.6% had significant hypocalcaemia. Children with dark skin had lower concentrations of vitamin D and higher concentrations of parathormone. A positive correlation was observed between 25 hydroxy vitamin D level and serum calcium, inorganic phosphate, and alkaline phosphatase concentrations. onclusion: The results showed a high prevalence of VDD and VDI in Saudi children with significantly higher prevalence in girls. These findings necessitate the set-up of a national program for vitamin D supplementation and health education for this vulnerable group.

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.

Protein synthesis in axons and terminals: significance for maintenance, plasticity and regulation of phenotype. With a critique of slow transport theory

This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and expression of plasticity in axons and terminals. Recent evidence of discrete ribosomal domains, subjacent to the axolemma, which are distributed at intermittent intervals along axons, are described. Studies of locally synthesized proteins, and proteins encoded by RNA transcripts in axons indicate that the latter comprise constituents of the so-called slow transport rate groups. A comprehensive review and analysis of published data on synaptosomes and identified presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modifiable synapses. The concept that all axonal proteins are supplied by slow transport after synthesis in the perikaryon is challenged because the underlying assumptions of the model are discordant with known metabolic principles. The flawed slow transport model is supplanted by a metabolic model that is supported by evidence of local synthesis and turnover of proteins in axons. A comparison of the relative strengths of the two models shows that, unlike the local synthesis model, the slow transport model fails as a credible theoretical construct to account for axons and terminals as we know them. Evidence for a dynamic anatomy of axons is presented. It is proposed that a distributed "sprouting program," which governs local plasticity of axons, is regulated by environmental cues, and ultimately depends on local synthesis. In this respect, nerve regeneration is treated as a special case of the sprouting program. The term merotrophism is proposed to denote a class of phenomena, in which regional phenotype changes are regulated locally without specific involvement of the neuronal nucleus.

Incorporation of amino acids into the axoplasm is enhanced by electrical stimulation of the fiber

The effect of sustained electrical stimulation upon the incorporation of amino acids into the axoplasm was studied in the goldfish Mauthner (M) axon with light autoradiography. An extracellular pulse of tracers applied between M-axons in the medulla resulted in a local and substantial labeling of the M-axoplasm and a faint labeling of the M-perikaryon 4-5 mm away from the site of injection. After 18 h of direct electrical stimulation of the M-axon at 0.3-0.8 Hz, the local incorporation of amino acids into the M-axoplasm doubled. This enhancement declined to reach the baseline within 24 h. A 4 h electrical stimulation did not enhance the incorporation. Transynaptic activation of the M-neuron through the auditory input at 0.1-0.2 Hz for 18 h did not raise the amino acid incorporation in the M-axoplasm. We conclude that electrical discharge of the axon modulates the local incorporation of amino acids into the axoplasm.

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.

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.

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.

Changes in alpha-tubulin and actin gene expression during optic nerve regeneration in frog retina

The optic nerve of the bullfrog was transected and the regeneration process was investigated. We previously reported that alpha-tubulin mRNA in the retina increased to a maximum 1-2 h after optic nerve transection with no specific change in actin mRNA. In the present investigation, we examined the long-term effect of optic nerve transection. Northern blot analysis revealed that alpha-tubulin mRNA increased again gradually after the rapid and transient increase and actin mRNA increased to a maximum at 7 days (more than twofold compared to the control retinas). The period during which actin mRNA reaches a maximal increase almost corresponds to the time lag between the axotomy and the initiation of axonal outgrowth. The main cytoskeletons of neuronal growth cones have been shown to consist of actin-containing microfilaments. Therefore, the transient increase of actin mRNA may have a relationship to the initial outgrowth of axons. On the other hand, the rapid and transient increase of alpha-tubulin mRNA observed in our previous studies is probably one of the initial responses of retinal ganglion cells to the axotomy, and the gradual increase in alpha-tubulin mRNA observed in this study can probably be interpreted as provision of the structural materials necessary for axonal elongation.

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.

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.

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

Posttranslational protein modifications by the addition of amino acids are reactions which occur in intact sciatic and optic nerves of rats. The nerves differ, however, in that 2 h after crush injury these reactions are activated in sciatic but not in optic nerves. As sciatic nerves will eventually regenerate, whereas optic nerves will not, we have proposed that the activation of these reactions is correlated with the ability of a nerve to regenerate. The current experiments examined the posttranslational addition of amino acids to proteins at times greater than 2 h after nerve crush, during sciatic nerve regeneration and optic nerve degeneration. We also examined the optic nerve for morphologic correlates to changes in protein modification and partially characterized the proteins modified by [3H]Lys in the regenerating sciatic nerve using two-dimensional sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE). In a segment of sciatic nerve taken from a region just proximal to the site of crush, protein modification by covalent addition of [3H]Arg, [3H]Lys and [3H]Leu increased during both posttraumatic (2 h postcrush) and regenerative (6 days and 14 days postcrush) stages. Two-dimensional PAGE of [3H]Lys modified sciatic nerve proteins 6 days after crush injury showed labeling of proteins having molecular masses in the 18,000- to 20,000-, 30,000- to 40,000-, and 80,000- to 100,000-Da ranges, with neutral or basic isoelectric points (pI 7.1 to 8.0). In the retinal portion of the crushed optic nerve, incorporation of the same amino acids was unchanged or depressed to 21 days postcrush, except at 6 days postcrush when the incorporation of all three amino acids into proteins was increased threefold. These increases correlated with the appearance of terminal end bulbs in the portion of nerve analyzed. Histological examination of each nerve 2 h postcrush showed marked edema in the optic but not the sciatic nerve, a condition which may be related to the ability of sciatic and inability of optic nerves to activate protein modification reactions.

Phosphorylation of proteins in normal and regenerating goldfish optic nerve

Within 6 h after radiolabeled phosphate was injected into the eye of goldfish, labeled acid-soluble and acid-precipitable material began to appear in the optic nerve and subsequently also in the lobe of the optic tectum, to which the optic axons project. From the rate of appearance of the acid-precipitable material, a maximal velocity of axonal transport of 13-21 mm/day could be calculated, consistent with fast axonal transport group II. Examination of individual proteins by two-dimensional gel electrophoresis revealed that approximately 20 proteins were phosphorylated in normal and regenerating nerves. These ranged in molecular weight from approximately 18,000 to 180,000 and in pI from 4.4 to 6.9. Among them were several fast transported proteins, including protein 4, which is the equivalent of the growth-associated protein GAP-43. In addition, there was phosphorylation of some recognizable constituents of slow axonal transport, including alpha-tubulin, a neurofilament constituent (NF), and another intermediate filament protein characteristic of goldfish optic axons (ON2). At least some axonal proteins, therefore, may become phosphorylated as a result of the axonal transport of a phosphate carrier. Some of the proteins labeled by intraocular injection of 32P showed changes in phosphorylation during regeneration of the optic axons. By 3-4 weeks after an optic tract lesion, five proteins, including protein 4, showed a significant increase in labeling in the intact segment of nerve between the eye and the lesion, whereas at least four others (including ON2) showed a significant decrease. When local incorporation of radiolabeled phosphate into the nerve was examined by incubating nerve segments in 32P-containing medium, there was little or no labeling of the proteins that showed changes in phosphorylation during regeneration. Segments of either normal or regenerating nerves showed strong labeling of several other proteins, particularly a group ranging in molecular weight from 46,000 to 58,000 and in pI from 4.9 to 6.4. These proteins were presumably primarily of nonneuronal origin. Nevertheless, if degeneration of the axons had been caused by removal of the eye 1 week earlier, most of the labeling of these proteins was abolished. This suggests that phosphorylation of these proteins depends on the integrity of the optic axons.

Posttranslational protein modification by polyamines in intact and regenerating nerves

A 150,000-g supernatant from axoplasm of the giant axon of the stellate nerve of the squid and from rat sciatic and goldfish optic nerves was found to be able to incorporate covalently [3H]putrescine and [3H]spermidine into an exogenous protein (N,N'-dimethylcasein). Incorporation of radioactivity was inhibited by CuSO4, a specific inhibitor of transglutaminases, the enzymes mediating these reactions in other tissues. Analysis of pH and temperature range and enzyme kinetics displayed characteristics predicted for transglutaminase-mediated reactions. Transglutaminase activity increased during regeneration of both vertebrate nerves, but greater activity was found in segments of nerve containing no intact axons than in either intact segments or in segments containing regenerating axons. Polyacrylamide gel electrophoresis of endogenous modified proteins (in the absence of N,N'-dimethylcasein) showed labeling of 18-, 46- and 200-kilodalton proteins by both [3H]putrescine and [3H]spermidine. Analysis of the protein-bound radioactivity from intact and regenerating rat sciatic nerves demonstrated it to be predominantly in the form of the parent radioactive polyamine. These experiments demonstrate the covalent modification of proteins by polyamines at low levels in squid axoplasm and at relatively higher levels in rat sciatic and goldfish optic nerves. In the latter two cases, the activity of these modification reactions may be due in part to the modification of axonal proteins, but the majority of the activity occurs in nonneuronal cells of the nerve.

Protein modification by amino acid addition is increased in crushed sciatic but not optic nerves

Rat optic and sciatic nerves were crushed, and 10 minutes to 3 days later nerve segments between the crushed site and the cell body were removed and assayed for posttranslational protein modification by amino acid addition. Protein modification was comparable in intact optic and sciatic nerves, but in sciatic nerves increased to 1.6 times control levels 10 minutes after crushing and reached a maximum of ten times control levels by 2 hours. In optic nerves activity was decreased throughout the time course studied. The results indicate that, in a nerve which is capable of regeneration (sciatic), protein modification by the addition of amino acids increases immediately after injury, but a nerve incapable of regeneration (optic) is incapable of activating the modification reaction. These findings may be important in understanding the reasons for the lack of a regenerative response after injury to central mammalian nerves.