Increased IL-1β in cortex of aged rats is accompanied by downregulation of ERK and PI-3 kinase

Abstract

Ageing is accompanied by a myriad of changes, which lead to deficits in synaptic function and recent studies have identified an increase in concentration of the proinflammatory cytokine, interleukin-1β (IL-1β), as a factor which significantly contributes to deterioration of cell function. Here, we consider that increased IL-1β concentration and upregulation of IL-1β-induced cell signalling cascades may be accompanied by downregulation of survival signals, perhaps as a consequence of decreased neurotrophins-associated signalling. The data indicate that increased IL-1β concentration was coupled with downregulation of ERK and phosphoinositide-3 kinase (PI-3 kinase) in cortical tissue prepared from aged rats. These changes could not be attributed to decreased concentration of NGF or BDNF but the evidence suggested that they may be a consequence of an age-related change in the anti-inflammatory cytokine, IL-4. Significantly, treatment of aged rats with eicosapentaenoic acid reversed the age-related increases in IL-1β and IL-1β-induced signalling and also the age-related changes in IL-4, ERK and PI-3 kinase.

Introduction

Several changes have been reported in the aged brain that may contribute to a deterioration in synaptic functions, for example changes in membrane fluidity, calcium homeostasis and neuronal signalling have been reported [4], [13], [23], [27], [42], [70], [72]. In addition, it has recently been shown that there is an increase in hippocampal concentration of the proinflammatory cytokine, interleukin-1β (IL-1β) that is accompanied by an increase in reactive oxygen species production [53]. These changes are considered to be indicators of inflammation and oxidative stress [51], and predictably have been coupled with increased activity of the stress-activated kinases, c-jun N-terminal kinase (JNK) and p38. Upregulation of these kinases is associated with cell death in several cells and tissues [50], [69] and also in hippocampus [40], [66].

Recent findings have indicated that treatment of aged rats with eicosapentaneoic acid (EPA) reversed the age-related changes in cell viability and the age-related enhancement in p38 activation, providing support for the proposal that these changes are functionally coupled [46]. Enhancement of JNK activity in γ-irradiated rats was also reversed following EPA treatment [39]. These findings are consistent with the idea that EPA has anti-inflammatory properties [1], [18] which may be associated with its ability to reduce production of proinflammatory cytokines [3], [10], [11], [67], like IL-1β [46].

An increase in stress-induced cell signalling in hippocampus may lead to cell death in the aged brain but it seems reasonable to propose that this may be coupled with a decrease in cell survival signalling pathways. Two neurotrophins, NGF and BDNF, which bind preferentially to TrkA and TrkB [7], [33] are known to play a major role in development and growth of neurons [22] and also in modulation of synaptic transmission. For example, BDNF increases the frequency of mEPSCs and enhances paired-pulse facilitation [16] and it increases synaptic transmission in Xenopus nerve-muscle preparations [38], [63] and cultured hippocampal neurons [37]. Trk-induced activation of ERK has been reported in hippocampus and this, in turn, enhances neurotransmitter release [6], [14], [44]. Consistently, incubation of synaptosomes in the presence of BDNF, which enhances phosphorylation of TrkB, increases glutamate release in hippocampus and this is inhibited by K252a, suggesting that the effect is dependent on Trk activation [14], [15]. NGF, by acting through TrkA, has been shown to enhance glutamate release in the rat hippocampus [30], however, NGF also interacts with the p75 neurotrophin receptor. Recent evidence suggests that p75 receptor activation impacts on neuronal survival [26]; indeed ligand-dependent receptor activation has been shown to cause apoptosis in various cells in culture [68].

Among the substrates which are activated following stimulation of TrkA and TrkB is ERK [15], [20], [31], [45], [47] and the evidence suggests that ERK activation modulates glutamate release in hippocampus [6], [15] where it is necessary for expression of LTP [12], [48]. Significantly ERK activation has been shown to play a role in neuronal survival during development [32] and also in the adult brain [26]. Phosphoinositide-3 kinase (PI-3 kinase), which is activated by growth factors [17], is an intracellular signalling enzyme that has been implicated in proliferation, differentiation and cell survival [17], [25]. It has been shown that NGF activates PI-3 kinase in PC12 cells [5], [58], [62] and also in hippocampus [29]. PI-3 kinase can be activated in a number of ways; one possibility is that neurotrophin-induced activation of Trk results in translocation and activation of the p110 catalytic subunit [36], [58] but G-protein activation, either through a ras-activated pathway [59] or by the βγ-subunit [17], [36], [64], [65] has been reported.

There is no consensus on changes in neurotrophin expression with age. An age-related decrease in BDNF mRNA has been described by one group [8], but not others [34], [49], while BDNF protein concentration has been reported to increase with age [28]. Similarly, hippocampal NGF concentration has been reported to increase by one group [9] and to decrease by another [35]. This lack of consensus is paralleled by similar equivocal findings relating to neurotrophin-induced signalling cascades in the aged brain. Therefore, we set out to assess parallel age-related changes in IL-1β and IL-1β-induced signalling, and neurotrophins and neurotrophin-induced signalling, with the objective of establishing (a) whether the observed increase in IL-1β-induced signalling was coupled with a decrease in cell survival signalling pathways, and (b) whether EPA treatment was effective in reversing these age-related changes.