Oestrogen is an excellent partner of the brain, protecting this organ against injury and degeneration. The end of this wonderful relationship is marked by the menopause that provokes a dramatic decline of oestrogen levels. One of the consequences of this decline is Alzheimer’s disease (AD), a degenerative pathology of the nervous system characterized by a progressive loss of memory and cognitive functions. A prominent feature of AD is the presence of extracellular neuritic plaques mainly formed by amyloid-β peptide (Aβ) that contribute to dysfunction in septal cholinergic circuits. In an attempt to maintain these friends together, oestrogen replacement therapy administrated to menopausal women has been shown to partially palliate this damage (Brinton, 2001). However, the beneficial effects of this therapy in long-term treatments are still controversial as they need to be quantified after a prolonged follow-up. Oestradiol-mediated prevention of cell degeneration has also been described in cellular paradigms of AD neurotoxicity. These neuroprotective effects have been ascribed to oxidative stress in the injured brain, suppression of intracellular Ca²⁺ elevation induced by Aβ, inhibition of toxicity related to acetylcholinesterase-Aβ complexes, modulation of Aβ-induced apoptosis and increase of Aβ microglial uptake to decrease Aβ load in the brain (Garcia-Segura et al. 2001). However, the importance of oestrogen receptors (ERs) and the potential mechanisms of action of these proteins in prevention of brain injury are largely unknown.

Here we present recent evidences demonstrating that oestrogen may exert neuroprotection against Aβ-induced toxicity by modulation of classical ER-mediated mechanisms and alternative membrane-related pathways. These studies have been performed in an SN56 neuronal line. These murine septal cells are considered a good model to study the mechanisms of oestrogenic prevention of neurotoxicity as they show cholinergic, peptidergic and nitrergic properties (Martinez-Morales et al. 2001), respond to Aβ-related toxicity and constitutively express ERs which show transcriptional activity (Marin et al. 2001). Using the trypan blue exclusion method, we found that cell death provoked by the 1-40 residue of Aβ (Aβ1-40) was significantly reduced in a dose-dependent manner by long exposure (24 h) to physiological concentrations of oestradiol. Palliation of cell death was blocked in the presence of the specific ER antagonist ICI182,780, suggesting the participation of classical ERs in the modulation of Aβ-induced toxicity. ER levels of expression were up-regulated by the hormone during injury, as observed by RT-PCR, Western blot and confocal microscopy. Interestingly, part of oestradiol neuroprotection was obtained within 15 min after hormone application and was reproduced with oestradiol coupled to HRP. We propose that, in this model, cell protection by oestradiol also requires the involvement of a putative membrane form of ER (mER) that may be structurally related to classical ER. This affirmation is based first on the fact that, using specific anti-ERα antibodies, we have detected by plasma membrane fractionation and immunocytochemistry what appears to be an ERα at the surface of SN56 neurons. Second, oestrogen protection triggered at the plasma membrane was palliated by both ICI182,780 and a specific antiserum directed to canonical ERα. In addition, affinity cytochemistry employing two membrane-impermeant oestrogen conjugates (E-HRP and E-BSA-FITC) revealed binding sites on the surface of SN56 cells that were competitively inhibited by oestrogen, anti-oestrogen and ERα antibodies. These results let us to hypothesize that oestrogenic actions in the prevention of Aβ-induced injury may be initiated via an ER residing at the membrane and co-ordinated to intracellular ERs that may ultimately modulate some of the pathophysiological processes related to AD. Further understanding of the discrete actions by which steroids act in both classical and alternative mechanisms to induce protection of brain integrity may provide alternative targets to maintain this love story in the best terms.

This work was supported by grant SAF200-3614-C03-01. Partial financial support was from Lilly S.A. and Astra-Zeneca.