B-50/GAP-43 phosphorylation in hippocampal slices from aged rats: effects of phosphatidylserine administration

Protein posttranslational modifications in health and diseases: Functions, regulatory mechanisms, and therapeutic implications

Protein posttranslational modifications (PTMs) refer to the breaking or generation of covalent bonds on the backbones or amino acid side chains of proteins and expand the diversity of proteins, which provides the basis for the emergence of organismal complexity. To date, more than 650 types of protein modifications, such as the most well-known phosphorylation, ubiquitination, glycosylation, methylation, SUMOylation, short-chain and long-chain acylation modifications, redox modifications, and irreversible modifications, have been described, and the inventory is still increasing. By changing the protein conformation, localization, activity, stability, charges, and interactions with other biomolecules, PTMs ultimately alter the phenotypes and biological processes of cells. The homeostasis of protein modifications is important to human health. Abnormal PTMs may cause changes in protein properties and loss of protein functions, which are closely related to the occurrence and development of various diseases. In this review, we systematically introduce the characteristics, regulatory mechanisms, and functions of various PTMs in health and diseases. In addition, the therapeutic prospects in various diseases by targeting PTMs and associated regulatory enzymes are also summarized. This work will deepen the understanding of protein modifications in health and diseases and promote the discovery of diagnostic and prognostic markers and drug targets for diseases.

Research progress on the age-related changes in proteins of the synaptic active zone

Neurotransmitter release during synaptic transmission is mediated by the presynaptic active zone. Multiple protein components at the active zone region interact to regulate docking, priming and fusion of the synaptic vesicles with the presynaptic membrane to maintain normal neurotransmitter release. This review discusses research progress in questions of protein transcript and expression pattern changes at the synaptic active zone related to aging and whether these changes have the effects on learning and memory. We will specifically address normal synaptic structure and proteins; active zone structure and components; active zone functional regulation and age-related changes in active zone proteins.

Protein kinase C activity is associated with prefrontal cortical decline in aging

Aging is associated with deficiencies in the prefrontal cortex, including working memory impairment and compromised integrity of neuronal dendrites. Although protein kinase C (PKC) is implicated in structural plasticity, and overactivation of PKC results in working memory impairments in young animals, the role of PKC in prefrontal cortical impairments in the aged has not been examined. This study provides the first evidence that PKC activity is associated with prefrontal cortical dysfunction in aging. Pharmacological inhibition of PKC with chelerythrine rescued working memory impairments in aged rats and enhanced working memory in aged rhesus monkeys. Improvement correlated with age, with older monkeys demonstrating a greater degree of improvement following PKC inhibition. Furthermore, PKC activity within the prefrontal cortex was inversely correlated with the length of basal dendrites of prefrontal cortical neurons, as well as with working memory performance in aged rats. Together these findings indicate that PKC is dysregulated in aged animals and that PKC inhibitors may be useful in the treatment of cognitive deficits in the elderly.

Catalpol increases hippocampal neuroplasticity and up-regulates PKC and BDNF in the aged rats

Rehmannia, a traditional Chinese medical herb, has a long history in age-related disease therapy. Previous work has indicated that catalpol is a main active ingredient performing neuroprotective effect in rehmannia, while the mechanism underlying the effect remains poorly understood. In this study, we attempt to investigate the effect of catalpol on presynaptic proteins and explore a potential mechanism. The hippocampal levels of GAP-43 and synaptophysin in 3 groups of 4 months (young group), 22-24 months (aged group) and catalpol-treated 22-24 months (catalpol-treated group) rats were evaluated by western blotting. Results clearly showed a significant decrease in synaptophysin (46.6%) and GAP-43 (61.4%) levels in the aged group against the young animals and an increase (45.0% and 31.8% respectively) in the catalpol-treated aged rats in comparison with the untreated aged group. In particular, synaptophysin immunoreactivity (OD) in the dentate granule layer of the hippocampus was increased 0.0251 in the catalpol-treated group as compared with the aged group. The study also revealed a catalpol-associated increase of PKC and BDNF in the hippocampus of the catalpol-treated group in comparison with the aged rats and highly correlated with synaptophysin and GAP-43. Such positive correlations between presynaptic proteins and signaling molecules also existed in the young group. These results suggested that catalpol could increase presynaptic proteins and up-regulate relative signaling molecules in the hippocampus of the aged rats. Consequently, it seemed to indicate that catalpol might ameliorate age-related neuroplasticity loss by "normalizing" presynaptic proteins and their relative signaling pathways in the aged rats.

Enhanced learning of normal adult rodents by repeated oral administration of soybean transphosphatidylated phosphatidylserine

Soybean lecithin transphosphatidylated phosphatidylserine (SB-tPS) is already known to improve the learning ability of aged or drug-induced amnesic rodents. In this study, its effect on normal adult rodents was evaluated using several learning tasks. Firstly, three behavioral tests (open-field, Y-maze, and active avoidance test) were consecutively carried out after the daily oral administration of SB-tPS (50 mg/kg per day, for 34 days). Repeated oral administration of SB-tPS did not affect either exploratory behavior in the open-field test or spontaneous alternation behavior in the Y-maze test, while mice pretreated with SB-tPS showed significant enhancement of conditioned avoidance response. Secondly, the brightness discrimination test was used to evaluate the effect of SB-tPS on learning ability. The daily oral administration of SB-tPS (50 mg/kg per day, for 27 days) to normal rats significantly increased the correct response ratio in the brightness discrimination test. Finally, to elucidate the necessity of SB-tPS pretreatment, another active avoidance test was carried out, and no enhancement of conditioned avoidance response was observed in non-pretreated mice. These results suggest that repeated administration of SB-tPS could enhance the learning ability of normal adult rodents as those of aged ones.

Selective age-related changes in the PKC-sensitive, calmodulin-binding protein, neurogranin, in the mouse brain

Brain ageing is associated with a dysregulation of intracellular calcium (Ca(2+)) homeostasis which leads to deficits in Ca(2+)-dependent signalling pathways and altered neuronal functions. Given the crucial role of neurogranin/RC3 (Ng) in the post-synaptic regulation of Ca(2+) and calmodulin levels, age-dependent changes in the levels of Ng mRNA and protein expression were analysed in 3, 12, 24 and 31-month-old mouse brains. Ageing produced significant decreases in Ng mRNA expression in the dorsal hippocampal subfields, retrosplenial and primary motor cortices, whereas no reliable changes were seen in any other cortical regions examined. Western blot indicated that Ng protein expression was also down-regulated in the ageing mouse brain. Analysis of Ng immunoreactivity in both hippocampal CA1 and retrosplenial areas indicated that Ng protein in aged mice decreased predominantly in the dendritic segments of pyramidal neurones. These data suggest that age-related changes of post-synaptic Ng in selected brain areas, and particularly in hippocampus, may contribute to altered Ca(2+)/calmodulin-signalling pathways and to region-specific impairments of synaptic plasticity and cognitive decline.

Cognition-enhancing properties of subchronic phosphatidylserine (PS) treatment in middle-aged rats: comparison of bovine cortex PS with egg PS and soybean PS

There are various clinical and non-clinical studies that have indicated that phosphatidylserine (PS) treatment can improve cognitive functions in humans and other animals. However, treatment with PS derived from bovine cortex is not desirable because of possible transfer of infectious diseases. The present study investigated the cognition-enhancing properties of different types of PS in rats. Seventeen-month-old male Fischer 344 rats were treated daily with a dose of 15 mg/kg of PS derived from bovine cortex (BC-PS), soybean (S-PS), egg (E-PS), or vehicle (n = 9 for each group). The effects of treatment were evaluated in three different behavioral tests. An open field test was conducted to examine the effects of treatment on psychomotor behavior. Two other tests (Morris water escape task and two-way active avoidance) assessed treatment effects on the cognitive performance of rats. Treatment with the different forms of PS did not affect the psychomotor or spatial discrimination performance of the rats. In accordance with previous studies, the cognition-enhancing effects of BC-PS were observed in the two-way active avoidance task. It appeared that the cognition-enhancing effects of S-PS were not different from those of BC-PS. The performance of rats treated with E-PS did not deviate from that of vehicle-treated rats. On the basis of the present study, it was concluded that S-PS, but not E-PS, may have comparable effects on cognition when compared with BC-PS.

Spatial memory is related to hippocampal subcellular concentrations of calcium-dependent protein kinase C isoforms in young and aged rats

Relationships were examined between spatial learning and hippocampal concentrations of the alpha, beta2, and gamma isoforms of protein kinase C (PKC), an enzyme implicated in neuronal plasticity and memory formation. Concentrations of PKC were determined for individual 6-month-old (n = 13) and 24-month-old (n = 27) male Long-Evans rats trained in the water maze on a standard place-learning task and a transfer task designed for rapid acquisition. The results showed significant relationships between spatial learning and the amount of PKC among individual subjects, and those relationships differed according to age, isoform, and subcellular fraction. Among 6-month-old rats, those with the best spatial memory were those with the highest concentrations of PKCgamma in the particulate fraction and of PKCbeta2 in the soluble fraction. Aged rats had increased hippocampal PKCgamma concentrations in both subcellular fractions in comparison with young rats, and memory impairment was correlated with higher PKCgamma concentrations in the soluble fraction. No age difference or correlations with behavior were found for concentrations of PKCgamma in a comparison structure, the neostriatum, or for PKCalpha in the hippocampus. Relationships between spatial learning and hippocampal concentrations of calcium-dependent PKC are isoform-specific. Moreover, age-related spatial memory impairment is associated with altered subcellular concentrations of PKCgamma and may be indicative of deficient signal transduction and neuronal plasticity in the hippocampal formation.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

The role of anchoring protein RACK1 in PKC activation in the ageing rat brain

High levels of expression of Ca2+/phospholipid-dependent protein kinase C (PKC) occur in neuronal tissues and play a strategic role in the modulation of short- and long-term functions (ion channels, receptor desensitization, neurotransmitter release and synaptic efficiency) that become modified during the brain ageing process. Recent studies have clarified the key role played by the anchoring proteins in mediating subcellular PKC location, that is, in driving the enzyme to specific sites of action. The protein, receptor for activated C-kinase 1 (RACK1) is involved in PKC-mediated signal transduction. A postnatal developmental increase in RACK1 levels indicates their significance in the outgrowth of neuronal processes. In a physiological model of impairment in PKC translocation-the aged rat brain cortex-RACK1 levels are reduced and the PKC isoenzymes known to interact with it do not translocate to membrane compartments upon stimulation. Anchoring proteins might represent new targets for compounds that modulate PKC signal transduction processes.

Amelioration of age-related deficits in the stimulation of synapsin phosphorylation

In a previous report we demonstrated that aged (24-26 month) rats have deficits in long-term potentiation, a form of synaptic enhancement that is dependent on protein phosphorylation (Moore et al., Hippocampus, 3:57-66; 1993). In the present study we demonstrate that aged rats have a deficit in the phosphorylation of the synaptic vesicle associated protein synapsin I. Specifically, aged animals exhibit defective phorbol ester-induced stimulation of synapsin phosphorylation at its calcium/calmodulin dependent protein kinase II sites. We also examined the effects of caloric restriction and antioxidant therapy on this age-related deficit. We found that either life-long caloric restriction or treatment with 16 mg/kg N-tert-butyl-alpha-phenylnitrone (PBN) for 2 weeks can at least partially ameliorate the age-related deficit in the phorbol ester stimulation of synapsin phosphorylation.

Stimulation of calcium uptake in cultured rat hippocampal neurons by 2,3,7,8-tetrachlorodibenzo-p-dioxin

This study examined the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and related compounds on the uptake of intracellular calcium ([Ca2+]i) in primary cultures of rat hippocampal neuronal cells. [Ca2+]i levels were detected and quantified by interactive laser cytometry with microscopic image analysis. Cells were noninvasively labeled with fluo-3/AM and all experiments were conducted on cultured rat hippocampal neurons 14 days in culture. Treatment of cell cultures with 2,3,7,8-TCDD (10-100 nM) resulted in a rapid concentration-dependent increase in [Ca2+]i associated with a decrease in mitochondrial membrane potential and activation of alpha-protein kinase C (alpha-PKC). In contrast, 1,2,3,4-TCDD, a weak Ah receptor agonist, had no effect on [Ca2+]i at concentrations as high as 10 microM and similar results were also observed for 2,2',5,5'-tetrachlorobiphenyl. Maximal [Ca2+]i was observed within 30 s after addition of 2,3,7,8-TCDD and remained elevated (at higher concentrations) above resting levels for the duration of the experiment. This rapid increase in [Ca2+]i was blocked by addition of EDTA (2 mM) to the external medium or by pretreatment of the cells with the calcium channel antagonist nifedipine (10 microM). However, pretreatment of the cells with 100 microM cycloheximide failed to block calcium uptake in neuronal cells. These data indicate that rat hippocampal neuronal cells are responsive to 2,3,7,8-TCDD; however, the mechanism is not associated with altered gene transcription and may involve cellular targets.

The need for cellular, biochemical, and mechanistic studies

The results of diverse in vitro neurotoxicity studies demonstrate that there are variations in cellular responsiveness between different types of neural cells. In contrast to experimental systems that have reported cellular responses to relatively high concentrations of various PCB congeners, our studies with rat hippocampal neural cells indicate that neurons and astroglia are responsive to relatively low levels of TCDD. However, these responses are probably not mediated through the classical Ah receptor pathway, which involves nuclear Ah receptor-mediated modulation of gene expression. It has recently been reported that TCDD-induced phosphorylation and other responses can be observed in some cell lines within minutes after treatment (20), and that cell membrane or cytosolic receptors may also play a role in mediating these effects. Future studies are required to determine both Ah receptor-dependent and -independent pathways associated with the neurotoxicity of PCBs, TCDD, and related compounds. The report that low-level dietary or background exposure to HAHs (32) results in neurobehavioral deficits is still a perplexing problem also requiring additional research and consideration of other dietary factors that may contribute to these effects. For example, we have recently been comparing the toxic effects and relative potencies of TCDD (exodioxins) and other "natural occurring" compounds (endodioxins) such as indole-3-carbinol (vegetables) and polynuclear aromatic hydrocarbons (PAHs, cooked foods), which also bind to the Ah receptor. Dr. Clynn Wilker has shown that in utero exposure of rats to indole-3-carbinol and chrysene (a PAH) cause demasculinization of the adult offspring as previously reported for TCDD (19). Thus, the neurotoxicity of low-level dietary exposure to HAHs should at least consider other possible confounding factors, including dietary endodioxins.

Protein kinase C activity, translocation, and conventional isoforms in aging rat brain

Protein kinase C was studied in various brain areas in aging Wistar rats. Histone-directed kinase activity from the cortex, hippocampus and cerebellum did not change with aging. Using purified protein B-50 as a substrate, between 3 and 8 months a decrease in in vitro phosphorylation was detected in the membrane fraction of the cortex but after this age values remained stable. In hippocampal membranes, B-50 phosphorylation was increased in aged rats. PKC translocation was impaired in aged rats in both the cortex and the hippocampus. PKC alpha and beta mRNA decreased in the cortex between 3 and 8 months with no further decline in aged animals. Hippocampal mRNA for calcium-dependent PKC isoforms was not modified during aging, as assessed by Northern and in situ hybridization. Western blot analysis revealed a change in PKC gamma protein only, which was increased in hippocampal membranes from aged rats. The data indicate that the key PKC function that is impaired in aged rats is enzyme translocation irrespective of the brain area investigated.