Effects of peroxidation and aging on rat neocortical ACh-release and protein kinase C

Dehydroepiandrosterone (DHEA) and the aging brain: flipping a coin in the "fountain of youth"

The physiological role of dehydroepiandrosterone (DHEA) and its sulphated ester DHEA(S) has been studied for nearly 2 decades and still eludes final clarification. The major interest in DHEA derives from its unique pattern of activity. Its levels exhibit a dramatic age-related decline that supports significant involvement of DHEA(S) in the aging process. Particularly relevant to the aging process is the functional decline that involves memory and cognitive abilities. DHEA is derived mainly from synthesis in the adrenal glands and gonads. It can also be detected in the brain where it is derived from a synthesis that is independent from peripheral steroid sources. For this reason DHEA and other steroid molecules have been named "neurosteroids." Pharmacological studies on animals provided evidence that neurosteroids could be involved in learning and memory processes because they can display memory-enhancing properties in aged rodents. However, human studies have reported contradictory results that so far do not directly support the use of DHEA in aging-related conditions. As such, it is important to remember that plasma levels of DHEA(S) may not reflect levels in the central nervous system (CNS), due to intrinsic ability of the brain to produce neurosteroids. Thus, the importance of neurosteroids in the memory process and in age-related cognitive impairment should not be dismissed. Furthermore, the fact that the compound is sold in most countries as a health food supplement is hampering the rigorous scientific evaluation of its potential. We will describe the effect of neurosteroids, in particular DHEA, on neurochemical mechanism involved in memory and learning. We will focus on a novel effect on a signal transduction mechanism involving a classical "cognitive kinase" such as protein kinase C. The final objective is to provide additional tools to understand the physiological role and therapeutic potentials of neurosteroids in normal and/or pathological aging, such as Alzheimer's disease.

Dehydroepiandrosterone and the relationship with aging and memory: a possible link with protein kinase C functional machinery

A progressive decline of cognitive and memory functions, compared to the average young-life performance, characterizes brain aging. The changes in performance may depend upon altered activity of neurotransmitters acting on attention and memory trace formation (acetylcholine, catecholamines, glutamate, for example) or the failure of the transduction mechanisms linked to receptor activation. One of the fundamental cellular changes associated with brain aging is the alteration of mechanisms involving the activity of the calcium-phospholipid-dependent protein kinase C (PKC). A crucial event for the activation of protein kinase C is its translocation from the cytosol to different intracellular sites and recent studies have demonstrated the key role played by several anchoring proteins in this mechanism. The defective activation of PKC-dependent pathways during aging is due to a defective mechanism of translocation of the kinase because of reduced levels of the major anchoring protein RACK-1 (receptor for activated C kinase). Pharmacological strategies aimed at the correction of age-associated memory deficits have been mostly focused on neurotransmitters using direct or indirect agonists. More recently, attention has been paid to the memory enhancing properties of some steroid hormones, namely 'neurosteroids'. Among these the activities of dehydroepiandrosterone (DHEA), pregnenolone (PREG) and their sulfates, have been extensively studied. These neuroactive steroids, can regulate neuronal function through their concurrent influence on transmitter-gated ion channels and gene expression. We addressed the possibility that DHEA, among other neurosteroids, could modulate directly the age-associated impairment of PKC signal transduction and provide experimental evidence that DHEA can revert the alteration of RACK-1 anchoring protein expression.

Protein kinase C anchoring deficit in postmortem brains of Alzheimer's disease patients

Protein kinase C (PKC) has been implicated in the pathophysiology of Alzheimer's disease (AD). The levels of particular isoforms and the activation of PKC are reduced in postmortem brain cortex of AD subjects. Receptors for activated C kinase (RACK) are a family of proteins involved in anchoring activated PKCs to relevant subcellular compartments. Recent evidence has indicated that the impaired activation (translocation) of PKC in the aging brain is associated with a deficit in RACK1, the most well-characterized member of this family. The present study was conducted to determine whether alterations in RACK1 occurred in cortical areas where an impaired translocation of PKC has been demonstrated in AD. Here we report the presence of RACK1 immunoreactivity in human brain frontal cortex for the first time and demonstrate a decrease in RACK1 content in cytosol and membrane extracts in AD when compared with non-AD controls. By comparison, the levels of the RACK1-related PKCbetaII were not modified in the same membrane extracts. These observations add a new perspective in understanding the disease-associated defective PKC signal transduction and indicate that a decrease in an anchoring protein for PKC is an additional determinant of this deficit.

Characterization of the neuroprotective and toxic effects of alpha7 nicotinic receptor activation in PC12 cells

The alpha7 nicotinic receptor partial agonist DMXB protected differentiated PC12 cells from NGF+ serum deprivation over a concentration range (1-10 microM) that correlated with activation of protein kinase C. Increased toxicity was observed at a higher concentration of DMXB (30 microM) that did not elevate protein kinase C activity, but did increase tyrosine protein kinase activity. Neuroprotection was blocked with the protein kinase C-inhibitor bis-indolemaleimide, while toxicity was attenuated with the tyrosine protein kinase-antagonists herbimycin and genistein. The alpha7-selective antagonist methyllyconitine attenuated both the protective and toxic actions of DMXB, but in temporally distinct manners. Methyllyconitine (1 microM) attenuated toxicity when added 10 s before, but not 10 s after, 30 microM DMXB. In contrast, it blocked neuroprotection when added 10 min post-agonist addition. This temporal difference in receptor-activation that was necessary for protection vs. toxicity reflected the time courses for agonist-induced desensitization of the receptor expressed in Xenopus oocytes. These results indicate that alpha7 nicotinic receptors act through different intracellular transduction processes to protect or kill cells. Further, they suggest that the transduction processes may be differentially activated depending on the amplitude and duration of calcium signals.

Antioxidant protection of cerebellar beta-adrenergic receptor function in aged F344 rats

We examined whether a 2-week treatment with the spin-trapping agent MDL 101.002 (MDL) or a diet supplemented with vitamin E (Vit E) would alleviate age-related deficits in cerebellar noradrenergic function of male 18-20 month old F344 rats compared to age-matched controls. Cerebellar beta-adrenergic receptor function was assessed using extracellular recordings of Purkinje cells during iontophoresis of GABA and isoproterenol (ISO). Noradrenergic receptor function of MDL and Vit E-treated rats was similar to young rats whereas for non-treated rats it was typical of that previously recorded in aged rats. Thus, treatment with MDL or Vit E reverses age-related deficits in cerebellar noradrenergic receptor function.

Age-related alteration of PKC, a key enzyme in memory processes: physiological and pathological examples

Brain aging is characterized by a progressive decline of the cognitive and memory functions. It is becoming increasingly clear that protein phosphorylation and, in particular, the activity of the calcium-phospholipid-dependent protein kinase C (PKC) may be one of the fundamental cellular changes associated with memory function. PKC is a multigene family of enzymes highly expressed in brain tissues. The activation of kinase C is coupled with its translocation from the cytosol to different intracellular sites and recent studies have demonstrated the key role played by several anchoring proteins in this mechanism. PKC-phosphorylating activity appears to be impaired during senescence at brain level in a strain-dependent fashion in rodents. Whereas the levels of the various isoforms do not show age-related alterations, the enzyme translocation upon phorbol-ester treatment is deficitary among all strains investigated. Anchoring proteins may contribute to this activation deficit. We discuss also modifications of the PKC system in Alzheimer's disease that may be related to pathological alterations in neurotransmission. A better insight of the different factors controlling brain-PKC activation may be important not only for elucidating the molecular basis of neuronal transmission, but also for identifying new approaches for correcting or even preventing age-dependent changes in brain function.

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.

Genes for Prader Willi syndrome/Angelman syndrome and fragile X syndrome are homologous, with genetic imprinting and unstable trinucleotide repeats causing mental retardation, autism and aggression

Genes for Prader Willi syndrome/Angelman syndrome are homologous to genes for fragile X syndrome. Genetic imprinting and expanded trinucleotide repeats cause mental retardation, autism and aggression.

Regulation of protein kinase C activity in cerebral microvessels

Regulation of protein kinase C (PKC)-mediated responses may occur by inhibition of PKC-dependent phosphorylation or by dephosphorylation of targets by specific phosphatases. Mechanisms for the regulation of PKC were examined in isolated cerebral microvessels and compared to those in brain. The data demonstrated that inhibitors of phosphorylation are responsible for the regulation in brain microvessels while dephosphorylation by protein phosphatases accounts for a substantial portion of the regulation of the PKC response in brain. In addition, the inhibitory activity apparently increases with age. These results suggest that the control of PKC may be cell-type specific and developmentally regulated.

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.

The effects of chronic treatment with N-tert-butyl-alpha-phenylnitrone on cerebellar noradrenergic receptor function in aged F344 rats

We examined if a 2 week treatment with the spin trapping agent N-tert-butyl-alpha-phenylnitrone (PBN) would alleviate age-related deficits in cerebellar noradrenergic function of male 21-22 month old F344 rats compared to age-matched controls. Noradrenergic receptor function of PBN-treated rats was similar to young rats whereas for non-treated rats it was typical of that previously recorded in aged rats. Thus, treatment with PBN may reverse age-related deficits in cerebellar noradrenergic receptor function.