Age-dependent changes in murine protein kinase and protease enzymes

Senescent-induced dysregulation of cAMP/CREB signaling and correlations with cognitive decline

It is well known that alongside senescence there is a gradual decline in cognitive ability, most noticeably certain kinds of memory such as working, episodic, spatial, and long term memory. However, until recently, not much has been known regarding the specific mechanisms responsible for the decline in cognitive ability with age. Over the past decades, researchers have become more interested in cAMP signaling, and its downstream transcription factor cAMP response element binding protein (CREB) in the context of senescence. However, there is still a lack of understanding on what ultimately causes the cognitive deficits observed with senescence. This review will focus on the changes in intracellular signaling in the brain, more specifically, alterations in cAMP/CREB signaling in aging. In addition, the downstream effects of altered cAMP signaling on cognitive ability with age will be further discussed. Overall, understanding the senescent-related changes that occur in cAMP/CREB signaling could be important for the development of novel drug targets for both healthy aging, and pathological aging such as Alzheimer's disease.

Autophagy suppresses age-dependent ischemia and reperfusion injury in livers of mice

BACKGROUND & AIMS

As life expectancy increases, there are greater numbers of patients with liver diseases who require surgery or transplantation. Livers of older patients have significantly less reparative capacity following ischemia and reperfusion (I/R) injury, which occurs during these operations. There are no strategies to reduce the age-dependent I/R injury. We investigated the role of autophagy in the age dependence of sensitivity to I/R injury.

METHODS

Hepatocytes and livers from 3- and 26-month-old mice were subjected to in vitro and in vivo I/R, respectively. We analyzed changes in autophagy-related proteins (Atg). Mitochondrial dysfunction was visualized using confocal and intravital multi-photon microscopy of isolated hepatocytes and livers from anesthetized mice, respectively.

RESULTS

Immunoblot, autophagic flux, genetic, and imaging analyses all associated the increase in sensitivity to I/R injury with age with decreased autophagy and subsequent mitochondrial dysfunction due to calpain-mediated loss of Atg4B. Overexpression of either Atg4B or Beclin-1 recovered Atg4B, increased autophagy, blocked the onset of the mitochondrial permeability transition, and suppressed cell death after I/R in old hepatocytes. Coimmunoprecipitation analysis of hepatocytes and Atg3-knockout cells showed an interaction between Beclin-1 and Atg3, a protein required for autophagosome formation. Intravital multi-photon imaging revealed that overexpression of Beclin-1 or Atg4B attenuated autophagic defects and mitochondrial dysfunction in livers of older mice after I/R.

CONCLUSIONS

Loss of Atg4B in livers of old mice increases their sensitivity to I/R injury. Increasing autophagy might ameliorate liver damage and restore mitochondrial function after I/R.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

Differential changes of cAMP-dependent protein kinase activity and 3H-cAMP binding sites in rat hippocampus during maturation and aging

The cyclic AMP-dependent protein kinase (PKA) has been involved in the brain aging process and recent papers have reported age-associated changes in enzyme activity in rat brain. The present study was undertaken to assess simultaneously PKA activity and regulatory (R) subunit levels during maturation and aging. Five cohorts of rats of different ages were used, namely pups of 1 week and 3 weeks old, mature rats (2 months), postmature rats (1 year) and old rats (2 years or more). PKA activity and 3H-cAMP binding sites were determined in cytosolic fractions of hippocampus. Results showed a low PKA activity in newborn rats which increased in mature and postmature rats and finally declined in old rats (ANOVA, P<0.001). The maximum binding sites (Bmax) of 3H-cAMP which measure the PKA R subunit levels were elevated in newborn rats and declined in mature and old rats (ANOVA; P<0.001). It is suggested the changes in PKA R subunit levels reflect an adaptative role in maturing process, a role which is lost in aging phase.

Changes in the expression of protein kinase C (PKC), phospholipases C (PLC) and D (PLD) isoforms in spleen, brain and kidney of the aged rat: RT-PCR and Western blot analysis

The age-dependent changes of expression of protein kinase C (PKC), phospholipase C (PLC) and phospholipase D (PLD) isozymes were analyzed in spleen, brain and kidney of young-adult (12-16 week-old) and aged (82-88 week-old) rats. The activities of spleen cPKC and nPKC were significantly decreased by nearly 35 and 30% in aged rats compared to those of young adults, respectively (P < 0.05). The level of PKC beta1 was significantly decreased in aged rats as assessed by RT-PCR and Western blot analyses. In aged rat brain where the activity of cPKC was significantly decreased by nearly 25% (P < 0.05), PKC alpha and beta1 isozymes were significantly down-regulated. In kidney, the level of PKC beta2 was decreased. In spleen the both mRNA and protein levels of PLC beta2 and gamma2 were significantly down-regulated in aged rat (P < 0.05). PLC beta1 was also significantly lower in aged rat brain (P < 0.05) as assessed by RT-PCR and Western blotting. Moreover, PLC beta1 was significantly down-regulated in both mRNA and protein levels in aged rat kidney (P < 0.05). In contrast, the tissues examined, the expressions of PLD isozymes (PLD1a, 1b and 2) were rather stable in the course of aging. These results indicate that mRNAs of PLD isozymes were rather stable but that particular PKC and PLC isozymes were down-regulated in different tissues during aging, suggesting age-dependent decline of specific PKC and PLC isozymes in organs which may, at least in part, be implicated in tissue dysfunction with aging.

Pituitary expression of protein kinase C isotypes during early development

Protein kinase C (PKC) is a critical regulator of signal transduction and cell function in many tissues, including pituitary. Although PKC influences pituitary hormone secretion in adults, its role in determining characteristic perinatal patterns of hormone secretion and synthesis is not known, and the expression of major PKC isotypes in perinatal pituitary is poorly defined. We therefore determined the developmental, cell-specific expression of the major PKC isotypes, using Western analysis and double label immunohistochemistry, in pituitaries of perinatal and mature rats. Expression of specific PKC isotypes was strikingly age-dependent. Pituitary expression of PKC alpha was particularly high in neonates and declined significantly with age, with levels in adult rats approximately half those of neonates as assessed by Western analysis. Similarly, immunohistochemistry indicated that PKC alpha was less abundant in adult than in neonatal pituitaries; the most intensely staining cells of both age groups were identified as somatotrophs and gonadotrophs. In contrast to PKC alpha, pituitary expression of PKC epsilon increased approximately two-fold with advancing age as assessed by Western analysis; this age-dependent pattern was confirmed by immunohistochemistry. Perinatal pituitaries expressed PKC epsilon in some somatotrophs and in all gonadotrophs, whereas PKC epsilon expression was limited to gonadotrophs in the mature pituitary. Pituitary expression of PKC betaII, delta, and zeta did not differ with age, and PKC gamma was not detected in pituitaries of any age group. These results indicate that expression of PKC isotypes within the pituitary is developmentally regulated in a cell-specific and isotype-specific manner, and are consistent with the concept that PKC contributes to the regulation of pituitary function during early development.

Melatonin: immune modulation of spleen cells in young, middle-aged and senescent mice

We examined the effects of melatonin on the immune response in aged mice in an in vitro model system. Spleen cells were removed from 2 month old, 11 month old, and 19 month old C57/BL6 male mice and incubated with melatonin concentrations ranging from 50 to 5000 ng melatonin/ml. All age groups demonstrated increased ability to proliferation in response to mitogenic stimulation when melatonin and mitogen were added at the same time, and we demonstrated that 11 month old and 19 month old mice exhibited a greater increase in proliferation response than the young 2 month old mice. We further examined whether this increased responsiveness was due to increased levels of IL-2 and found that while there was a significant age-dependent difference in the levels of IL-2 in the different age groups, melatonin had no effect on IL-2 levels in any age groups despite the fact that proliferation increased in all groups. The underlying mechanism of melatonin action is being investigated further.

cAMP signalling mechanisms with aging in the Ceratitis capitata brain

Aging has been associated with alterations in protein phosphorylation. This study was undertaken to examine eventual changes in cAMP-dependent protein kinase (PKA) activity and enzyme regulatory subunit levels from the dipterous Ceratitis capitata brain with postmaturational aging and senescence. PKA activity was determined in cytosolic and membrane fractions of the C. capitata brain during the adult stage of the insect lifespan. PKA activity markedly increased at the first stages of the life of the fly both in cytosol and in membranes. A lower peak of PKA activity was evident both in particulate and cytosolic fractions in the terminal phase of the life of the fly. Thus, PKA activity was significantly higher in the brain of mature flies when compared to the brain of aged flies. It is possible that increases in cAMP-dependent protein phosphorylation levels characterize the terminal aging process in the insect nervous tissue. On the other hand, levels of regulatory (R) subunit were also measured in membranes and cytosol by immunoblotting. Cytosolic regulatory subunit levels were more elevated near the terminal phase of life, whereas in membranes, regulatory subunit levels decrease in senescence in parallel with particulate PKA activity. The increased R subunit level in cytosol may reflect a cellular response mechanisms to down-regulate the kinase system in aged flies.

Differential expression of the alpha- and beta-isoforms of protein kinase C in peripheral blood T and B cells from young and elderly adults

The expression of alpha- and beta-isoforms of protein kinase C (PKC) was analyzed in the peripheral blood T and B cells from 11 elderly and young humans. Immunoblot analysis with isoenzyme specific antibodies showed that T cells from five of 11 elderly subjects exhibited selective reductions in PKC alpha which was < 60% of those in young subjects whereas the levels of PKC beta were comparable to T cells of young subjects. No age-related reductions of PKC alpha or beta were observed in B cells. Among individual elderly subjects, the reductions in T cell PKC alpha were not associated with lower levels of PKC beta thereby resulting in only approximately 60-70% reductions of combined PKC alpha plus PKC beta. In addition, the functional properties of PKC in stimulated T cells of elderly subjects with respect to activation/translocation were comparable to T cells of young subjects. These results suggest that selective alterations in PKC isoenzymes can occur in human T cells during aging which may not be readily apparent in standard enzymatic assays and may contribute to aberrancies in intracellular signal transduction.

Aged lymphocyte proliferation following incorporation and retention of dietary omega-3 fatty acids

T cell activation involves events at the plasma membrane; therefore, molecules such as long chain omega-3 fatty acids that alter the structure of the plasma membrane may affect the activation of aged T cells. In this project we investigated whether the incorporation of omega-3 fatty acids (from fish oil), in the presence of vitamin E, improves age-diminished T cell proliferation. Young and old mice were fed diets rich in either fish (menhaden) oil or saturated fat for various lengths of time. Splenocytes were harvested from these mice and stimulated in culture with either mitogen or the antigen keyhole limpet hemocyanin (for a secondary response); proliferation was estimated by [3H]thymidine incorporation. We found no discernible effect of dietary omega-3 fatty acids (with vitamin E supplementation) on lymphocyte proliferation stimulated by the mitogens concanavalin A or phytohemagglutinin. We did, however, find that the saturated fat diet and the menhaden oil diet in young mice lowered protein kinase C activities in the particulate fractions of spleen cells when compared to chow-fed mice. Middle-aged and old mice were less affected by the experimental diets than young mice, but they demonstrated decreased protein kinase C activity as well. These alterations did not affect the ability of splenocytes to respond to mitogenic stimulation. Fatty acid analysis revealed that lymphocytes from mice fed saturated fat for 8.5 months retained significant amounts of the omega-3 fatty acid docosahexaenoic acid, despite the lack of dietary omega-3 fatty acids. However, when aged (but not young) lymphocytes were clonally expanded by antigen in vivo in the presence of dietary omega-3 fatty acids, they produced a greater secondary proliferative response than old lymphocytes expanded during a saturated fat diet. Although our results suggest that omega-3 fatty acids may enhance aged lymphocyte proliferation, the tenacious retention of these fatty acids makes comparison with omega-3-depleted lymphocytes difficult.

Changes in nitric oxide precursor, L-arginine, and metabolites, nitrate and nitrite, with aging

The present study was performed to test the hypothesis that excretion of nitric oxide metabolites, nitrate and nitrite, are decreased with progressive aging in rats and that a decrease in nitric oxide precursor, L-arginine, also decreases with aging. Urinary nitrate/nitrite excretory rates and serum L-arginine levels were measured in male Sprague Dawley rats, ranging in ages from 3 to 25 months. Proteinuria increased dramatically with aging. Conversely, urinary nitrate/nitrite excretion decreased by 50% and 80% in rats, aged 12 months and 17 months, respectively. There was no further decrease in urinary nitrate/nitrite excretion in very old rats, aged 23-24 months. Glomerular filtration rate (GFR) was also measured in some of the rats, aged 3-5 mos and 17 mos. GFR was not different between old and young rats, suggesting that a decrease in GFR could not account for the decrease in urinary nitrate/nitrite excretion in the old rats. However, serum L-arginine levels were decreased with aging, by 30% and 50% in rats, aged 13-15 months and 24-25 months, respectively, when compared with young rats. These data confirm our hypothesis and suggest that nitric oxide (NO) production may decrease with aging and that one mechanism by which nitric oxide production could be decreased with age is a lack of the endogenous substrate, L-arginine. Because NO has been implicated to be involved in many physiological processes, age-related decreases in NO production could have far-reaching adverse effects in the aging individual.

Serine/threonine protein kinases and calcium-dependent protease in senescent IMR-90 fibroblasts

Three enzymes relevant to signal transduction were compared in replicating, quiescent and senescent human diploid fibroblasts (HDF). These were Ca(2+)-dependent thiol protease (calpain), cAMP-dependent protein kinase (Pk-A), and calcium/phospholipid-dependent protein kinase C (Pk-C). The amounts of these enzymes in quiescent HDF were slightly greater or the same as in replicating HDF. In contrast, senescent HDF exhibited higher Pk-C, Pk-A and proteolytic activities than did either replicating or quiescent cells. While the elevated protein kinase activities could be accounted for by the larger size of senescent cells relative to younger cells, the increased calpain activity exceeded this size differential. Immunoblotting studies with antisera to both Pk-C and calpain demonstrated increased enzyme concentrations in parallel with the increased activities. Photolabeling cell extracts with an analog of cAMP, 8-N3-[32P]cAMP, provides an estimate of Pk-A concentration. By this criterion, senescent HDF have more Pk-A molecules than do young cells that are either replicating or quiescent. Only the type I isozyme of Pk-A (Pk-A-I) was observed in any of these cells. Photolabeling with 8-N3-[32P]cAMP demonstrated more degradative fragments of the Pk-A regulatory subunit (RI) in senescent cells also. This is a logical consequence of the increased calpain activity in senescent cells, since RI is a substrate for calpain. These results imply that senescent cells do not fail to enter S phase owing to inadequate concentrations of Pk-A or Pk-C. Rather, the increased quantities of these enzymes in senescent cells may reflect aberrations elsewhere along signal transduction pathways that coordinate cell size with cell proliferation.

Pregnancy-associated, lymphocyte-derived suppressor factor inhibits protein kinase C activity

For successful allogenic pregnancy to occur, suppression of maternal defense responses toward the fetus are vital. Suppressor factors elaborated by decidual cells or immune cells may facilitate this suppression. In order for appropriate cellular responses to occur an intact signal transduction/second messenger system must be present. The calcium/phospholipid-dependent protein kinase, Pk-C, plays an important role in regulating immune responses, and may also be important in regulating uterine cell responses and implantation events. Pk-C activation is necessary for IL-2 synthesis and IL-2 receptor synthesis through activation of the proto-oncogenes c-jun and c-fos. These proto-oncogene gene products combine to form the heterodimer AP-1 which then activates IL-2 gene transcription for both peptide and receptor. If Pk-C activity becomes abrogated then appropriate cell responsiveness is diminished. We have shown that Pk-C activity is decreased in the particulate fraction of 4-7 day pregnant spleen, thymus and draining lymph node (DLN) cells. Spleen cells did not exhibit any change in cytosolic Pk-C activity, the thymus was found to have a decrease in both cytosol and particulate fractions, and the DLN cells exhibited a translocation effect whereby particulate Pk-C decreased and cytosolic Pk-C activity increased. Supernatant from 3-day cultures of DLN cells from pregnant animals was shown to inhibit proliferation of spleen cells. In addition, the supernatant was able to directly lower Pk-C activity. We hypothesize that DLN cells secrete a factor(s) that is able to suppress immune response through abrogation of Pk-C activity, thereby decreasing AP-1 formation resulting in decreased IL-2 synthesis and IL-2 receptor synthesis.

The effect of isobutylmethylxanthine, forskolin, and cholera toxin on growth hormone release from pituitary cell cultures of perinatal and mature rats

The factors that regulate growth hormone (GH) release during the perinatal period are not well understood. Circulating GH levels are markedly elevated in mammalian fetuses and newborns compared with mature animals, and the immature pituitary is highly responsive to the GH-stimulatory effect of GH-releasing factor (GHRF). The etiology of these developmental changes in GH secretion is not known. In order to investigate the mechanisms underlying GH release from immature pituitaries, we tested the effects of agents that increase intracellular cyclic adenosine 3',5' monophosphate (cAMP) production independent of the GHRF receptor on GH release from pituitaries of developing and mature rats. Pituitary cell cultures from fetal (day 20 of gestation), newborn (postnatal day 2), juvenile (postnatal day 12-15), adult male (3-4 months), and adult female (3-4 months) rats were tested with isobutylmethylxanthine (IBMX; 0.001-1.0 mM), forskolin (0.01-10 microM), and cholera toxin (0.025-25 ng/ml). IBMX, forskolin, and cholera toxin stimulated GH release in a dose-dependent manner from pituitary cultures of all age groups. However, the magnitude of the GH responses to these agents was highly age-dependent. Perinatal pituitaries exhibited markedly greater GH responses to IBMX, forskolin, and cholera toxin than did those of mature animals (P < 0.001 for age effect with each agent). GH release in response to the highest dose of IBMX (1 mM) was 301 +/- 8, 389 +/- 37, 296 +/- 33, 198 +/- 14, and 187 +/- 19% of control values from pituitary cell cultures of fetal, newborn, juvenile, adult male, and adult female rats, respectively (P < 0.001). In response to the highest dose of forskolin (10 microM) GH release was 537 +/- 46, 601 +/- 75, 274 +/- 22, 270 +/- 37, and 248 +/- 35% of control values in the same respective age groups (P < 0.001). Similarly, the highest dose of cholera toxin (25 ng/ml) stimulated GH release to 407 +/- 55, 365 +/- 43, 249 +/- 26, 186 +/- 11, and 186 +/- 1% of controls in these respective age groups (P < 0.003). The marked stimulation of GH release from perinatal pituitaries by IBMX, forskolin, and cholera toxin is consistent with the concept that cAMP is a potent mediator of GH release from immature as well as mature somatotrophs. The developmental changes in the GH secretory response to these agents further suggest that signal transduction pathways mediating GH release may undergo maturation, at least in part, at intrasomatotroph loci distal to the GHRF receptor.

Protein synthesis, posttranslational modifications, and aging

Posttranslational modifications of proteins are involved in determining their activities, stability, and specificity of interaction. More than 140 major and minor modifications of proteins have been reported. Of these, only a few have been studied in relation to the aging of cells, tissues, and organisms. These include phosphorylation, methylation, ADP-ribosylation, oxidation, glycation, and deamidation. Several of these modifications occur on proteins involved in crucial cellular processes, such as DNA synthesis, protein synthesis, protein degradation, signal transduction, cytoskeletal organization, and the components of extracellular matrix. Some of the modifications are the markers of abnormal and altered proteins for rapid degradation. Others make them less susceptible to degradation by normal proteolytic enzymes, and hence these accumulate during aging.

Intracellular free Ca2+ fluxes and responses to phorbol ester in T lymphocytes from healthy elderly subjects

A group of healthy elderly subjects (greater than or equal to 75 years) was selected by the strict criteria of the SENIEUR protocol, and compared with healthy young (less than or equal to 35 years) volunteers. Mitogenic responses of peripheral blood mononuclear cells to phytohaemagglutinin and anti-CD3 were significantly reduced in the elderly (P less than 0.0002), thereby confirming that even though in perfect health, elderly individuals show impaired cell-mediated immunity. However, no abnormality of intracellular free Ca2+ fluxes could be detected in purified T cells from the elderly subjects when stimulated with anti-CD3 antibody. Nevertheless, both the proliferative responses of purified T cells to phorbol ester and calcium ionophore (Ionomycin) and the phorbol ester-induced inhibition of the Ca2+ response were defective in the elderly subjects (P less than 0.003 and P less than 0.0002, respectively). These data suggest that signal transduction and the generation of second messengers proceed normally in T cells from the elderly, but downstream events mediated by activation of protein kinase C are dysfunctional.