Aging does not affect steady-state expression of the Na+/Ca2+ exchanger in rat brain

Age Influences the Expression of GAP-43 in the Rat Hippocampus following Seizure

BACKGROUND

Normal aging is associated with impairments in learning and memory and motor function. One viable hypothesis is that these changes reflect an age-related decrease in brain plasticity.

OBJECTIVE

The aim of the present study was to identify age-related changes in the time course of expression of the axonal growth associated protein 43 (GAP-43) in a rat model of brain plasticity.

METHODS

We examined by Northern blotting, in situ hybridization, and immunohistochemistry the effects of age on the time course of the expression GAP-43 following pentylenetetrazole-induced seizure in the hippocampus of 3-, 18-, and 28-month-old rats.

RESULTS

In this model of brain plasticity, young rats displayed a decrease in GAP-43 mRNA levels in CA1, CA3, and polymorphic regions, lasting from 10 h to 3 days after seizure. This was followed by recovery, with peak expression between days 10 and 20. The baseline levels of GAP-43 mRNA decreased with age, especially in the CA3 region. Despite lower baseline levels, middle-aged rats showed the same pattern of upregulation of GAP-43 mRNA expression as the young animals. Old rats showed only minimal upregulation, however, and this occurred only in the polymorphic layer. The level GAP-43 protein itself was higher in old control rats than in the other two control groups, a condition that was transiently reversed by seizure activity.

CONCLUSIONS

Middle-aged rats are still capable of a sustained, though diminished, response to seizure activity, while old rats lose this ability. Disruption of the temporal and anatomical coordination of expression of GAP-43 may contribute to the general decline in brain plasticity with age.

Dynamics of gene expression for immediate early- and late genes after seizure activity in aged rats

The ability of the rodent brain to support plasticity-related phenomena declines with increasing age. A decreased coordination of genes implicated in brain plasticity may be one factor contributing to this decline. Synaptic rearrangement that occurs after seizure activity is regarded as a model of brain plasticity. In a rat model of seizure-related brain plasticity, we found that the induction of immediate-early genes, as exemplified by c-fos and tissue plasminogen activator ( tPA), is not impaired in the aged rat brain. However, the aged rat brain responded more slowly to chemically induced seizure, and the levels of c-fos and tPA mRNAs induction are decreased in the cortex and in the hippocampus of 30 month old rats, as compared to the levels expressed by 3 month old rats. In addition, at the peak induction, the TPA transcripts were restricted to certain cortical layers of the older rats. Surprisingly, in applying the same experimental paradigm to late genes, we found that there was a shift toward earlier times in the maximum expression of growth-related molecules, the microtubule-associated protein 1B (MAP1B) mRNA, which was very evident in 18 month old rats. Aberrant immunolabeling of MAP1B occurred in cortical layer VI of the aged rats where, unlike in young rats, there was heavy staining of neuronal somata. These results suggest that (1) one consequence of aging, besides decreases in the levels of mRNA, is a progressive loss of coordination in gene activity following the administration of a stimulus; (2) since c-fos, TPA and MAP1B have been implicated in neuronal plasticity, these findings could explain, in part, the limited plasticity of the aging brain.

Brain plasticity: to what extent do aged animals retain the capacity to coordinate gene activity in response to acute challenges

The ability of the rodent brain to support plasticity-related phenomena declines with increasing age. A decreased coordination of genes implicated in brain plasticity may be one factor contributing to this decline. Synaptic rearrangement that occurs after seizure activity is regarded as a model of brain plasticity. In a rat model of seizure-related brain plasticity, we found that the induction of immediate-early genes, as exemplified by c-fos and tissue plasminogen activator (TPA) is not impaired in the aged rat brain. However, the aged rat brain responded more slowly to chemically induced seizure and the levels of c-fos and TPA mRNAs induction are decreased in the cortex and in the hippocampus of 30-month-old rats, as compared to the levels expressed by 3-month-old rats. In addition, at the peak induction the TPA transcripts were restricted to certain cortical layers of the older rats. Surprisingly, in applying the same experimental paradigm to late genes we found that there was a shift toward earlier times in the maximum expression of growth-related molecule, the microtubule-associated protein 1B (MAP1B) mRNA, which was very evident in 18-month-old rats. Aberrant immunolabeling of MAP1B occurred in cortical layer VI of the aged rats where, unlike in young rats, there was heavy staining of neuronal somata. These results suggest that (i) one consequence of aging, besides decreases in the levels of mRNA, is a progressive loss of coordination in gene activity following the administration of a stimulus; (ii) since c-fos, TPA and MAP1B have been implicated in neuronal plasticity, these findings could explain, in part, the limited plasticity of the aging brain.

Anomalous expression of microtubule-associated protein 1B in the hippocampus and cortex of aged rats treated with pentylenetetrazole

The aim of the present study was to assess the age-dependent response of microtubule-associated protein 1B, a plasticity-associated protein deriving from a late gene, following administration of an epileptogenic stimulus. The effect of a single administration of the convulsant pentylenetetrazole on microtubule-associated protein 1B expression in the hippocampal formation and cortex of three-, 18- and 28-month-old rats was assessed using northern blot analysis, in situ hybridization and immunohistochemistry. In three-month-old rats, we detected initial increases in microtubule-associated protein 1B messenger RNA at 15 h following pentylenetetrazole administration in the granule cells of the dentate gyrus, in the CA3 region of the hippocampus and in layers II/III of the entorhinal cortex, and these reached a maximum at 44 h. However, in the hippocampus and cortex of 18-month-old rats, the peak occurred at 15 h, and in the brains of 28-month-old rats a blunted peak was reached at 3 h. Pentylenetetrazole treatment in young rats resulted in a robust induction of microtubule-associated protein 1B immunoreactivity in the granule cells of the dentate gyrus and in layers II/III of the entorhinal cortex, but also produced a large decrease in the retrosplenial cortex. However, following pentylenetetrazole treatment in older rats, the granule cells of the dentate gyrus were nearly devoid of microtubule-associated protein 1B immunoreactivity, whereas the retrosplenial cortex showed no changes at all, and the entorhinal cortex had an expression pattern similar to that of young rats. Aberrant immunolabeling of microtubule-associated protein 1B occurred in cortical layer VI of the aged rats where, unlike in young rats, there was heavy staining of neuronal somata. These results suggest that the regulation of the plasticity-associated protein microtubule-associated protein 1B is altered in the ageing rat brain, with the peak of expression shifted to earlier times in 18-month-old rats and blunted, variable increases at even earlier times in 28-month-old rats.

Regional differences in expression of transcripts for Na+/Ca2+ exchanger isoforms in rat brain

The Na+/Ca2+ exchanger has a primary role in maintaining intraneuronal Ca2+ homeostasis. There are three distinct Na+/Ca2+ exchanger isoforms cloned from rat brain, NCX1, NCX2 and NCX3, which are the products of three different genes. In the present study, isoform expression in different regions of rat brain was determined by using reverse transcription PCR (RT-PCR) and Northern analysis. RT-PCR detected all three Na+/Ca2+ exchanger isoforms in each region studied (brainstem/spinal cord, cerebellum, cerebral cortex, striatum/septum and hippocampus). Northern analysis was performed to determine the steady-state mRNA levels of each isoform. NCX1 had two transcripts, 14 and 7 kb, and the 7-kb transcript was predominant in brainstem/spinal cord, cerebellum and hippocampus. NCX2 expression (4.8-kb transcript) was an order of magnitude higher than NCX1 or NCX3 expression in all the five areas except brainstem/spinal cord where the 4.8-kb transcript was nearly absent. The third isoform (NCX3) had two transcripts, one was 6 kb and the other was 4 kb. The 6-kb transcript was predominant in brainstem/spinal cord and cerebellum. The results suggest that Na+/Ca2+ exchanger isoforms are expressed ubiquitously in rat brain but that each isoform shows a unique distribution within the brain. The exchanger probably participates in the regulation of intracellular calcium homeostasis in a wide range of cell types within the brain. Furthermore, individual cells may contain more than one type of exchanger isoform with distinct subcellular distributions.

Alzheimer's amyloid-beta peptide inhibits sodium/calcium exchange measured in rat and human brain plasma membrane vesicles

Na+/Ca2+ exchange activity was measured by monitoring vesicular Ca2+ content after incubation in buffers containing 45Ca2+. When Na+-loaded vesicles were placed into Na+-free buffer, vesicular Ca2+ content increased rapidly and reached a plateau after two to three minutes. Only preaggregated amyloid-beta1-40 (Abeta1-40) and Abeta25-35 reduced vesicular Ca2+ content. Both peptides produced a maximal reduction in Ca2+ content of approximately 50%. The peptides reduced Ca2+ content with similar potency and half maximal effects were seen at less than 10 microM for Abeta25-35. Calcium-loaded vesicles mediate a rapid Ca2+/Ca2+ exchange, which also was inhibited by aggregated Abeta25-35. Aggregated Abeta25-35 did not affect the passive Ca2+ permeability of the vesicles. Aggregated Abeta25-35 reduced Ca2+ content in plasma membrane vesicles isolated from normal and Alzheimer's disease frontal cortex with less potency but the same efficacy as seen in rat brain. Aggregated Abeta25-35 did not produce nonspecific effects on vesicle morphology such as clumping or loss of intact vesicles. When placed in the buffer used to measure Ca2+ content, Congo Red at molar ratios of less than one blocked the inhibitory effect of preaggregated Abeta25-35. When added in equimolar concentrations to freshly dissolved and unaggregated Abeta25-35, Congo Red also was effective at blocking the inhibitory effect on Ca2+ content. In contrast, vitamin E (antioxidant) and N-tert-butyl-alpha-phenylnitrone (spin trapping agent) failed to block the inhibitory action of aggregated Abeta25-35. The exact mechanisms of Abeta-induced neurotoxicity in cell culture has yet to be solved. Accumulation of free radicals play a necessary role, but disruptions of Ca2+ homeostasis are also important. The data presented here are consistent with a proposed mechanism where aggregated Abeta peptides directly interact with hydrophobic surfaces of the exchanger protein and/or lipid bilayer and interfere with plasma membrane Ca2+ transport.

Genistein inhibits Na+/Ca2+ exchange activity in primary rat cortical neuron culture

We have examined the possible regulatory effect of tyrosine kinase activity on Ca2+ transport observed in the cultured rat cortical neurons. Na+/Ca2+ exchange was studied using cells cultured for various time periods. A nearly two fold increase in Ca2+ uptake was seen when comparing 3 day and 9 day cultures. Western blot analysis also showed a two fold increase in Na+/Ca2+ exchanger (NCX1) protein levels as cells matured in culture. To study the effect of genistein (a specific tyrosine kinase inhibitor) cells were incubated with 100 microM genistein (in 1% DMSO) for 1 hour before the assay of Na+/Ca2+ exchange activity. There was a significant decrease of Ca2+ uptake in genistein treated neurons (control: 4.596+/-0.205 nmol/mg protein/15 min, n=12; genistein: 1.420+/-0.131 nmol/mg protein/15 min, n=12, mean+/-S.E. P<0.001). Daidzein, an inactive analog of genistein and phorbol myristate acetate (PMA), a PKC activator were without effect. The results suggest that as cells mature in culture, Na+/Ca2+ exchange capacity increases, as a result of greater protein expression. Exposure to genistein inhibited Ca2+ uptake suggesting that the exchanger may be modulated by tyrosine phosphorylation.