Iñigo Azcoitia

Role of astroglia in estrogen regulation of synaptic plasticity and brain repair

Astroglia are targets for estrogen and testosterone and are apparently involved in the action of sex steroids on the brain. Sex hormones induce changes in the expression of glial fibrillary acidic protein, the growth of astrocytic processes, and the degree of apposition of astroglial processes to neuronal membranes in the rat hypothalamus. These changes are linked to modifications in the number of synaptic inputs to hypothalamic neurons. These findings suggest that astrocytes may participate in the genesis of androgen-induced sex differences in synaptic connectivity and in estrogen-induced synaptic plasticity in the adult brain. Astrocytes and tanycytes may also participate in the cellular effects of sex steroids by releasing neuroactive substances and by regulating the local accumulation of specific growth factors, such as insulin-like growth factor-I, that are involved in estrogen-induced synaptic plasticity and estrogen-mediated neuroendocrine control. Astroglia may also be involved in regenerative and neuroprotective effects of sex steroids, since astroglia formation after brain injury or after peripheral nerve axotomy is regulated by sex hormones. Furthermore, the expression of aromatase, the enzyme that produces estrogen, is induced de novo in astrocytes in lesioned brain areas of adult male and female rodents. Since astroglia do not express aromatase under normal circumstances, the induction of this enzyme may be part of the program of glial activation to cope with the new conditions of the neural tissue after injury. Given the neuroprotective and growth-promoting effects of estrogen after injury, the local production of this steroid may be a relevant component of the reparative process.

Microglial CB2 cannabinoid receptors are neuroprotective in Huntington's disease excitotoxicity

Cannabinoid-derived drugs are promising agents for the development of novel neuroprotective strategies. Activation of neuronal CB(1) cannabinoid receptors attenuates excitotoxic glutamatergic neurotransmission, triggers prosurvival signalling pathways and palliates motor symptoms in animal models of neurodegenerative disorders. However, in Huntingtons disease there is a very early downregulation of CB(1) receptors in striatal neurons that, together with the undesirable psychoactive effects triggered by CB(1) receptor activation, foster the search for alternative pharmacological treatments. Here, we show that CB(2) cannabinoid receptor expression increases in striatal microglia of Huntingtons disease transgenic mouse models and patients. Genetic ablation of CB(2) receptors in R6/2 mice, that express human mutant huntingtin exon 1, enhanced microglial activation, aggravated disease symptomatology and reduced mice lifespan. Likewise, induction of striatal excitotoxicity in CB(2) receptor-deficient mice by quinolinic acid administration exacerbated brain oedema, microglial activation, proinflammatory-mediator state and medium-sized spiny neuron degeneration. Moreover, administration of CB(2) receptor-selective agonists to wild-type mice subjected to excitotoxicity reduced neuroinflammation, brain oedema, striatal neuronal loss and motor symptoms. Studies on ganciclovir-induced depletion of astroglial proliferation in transgenic mice expressing thymidine kinase under the control of the glial fibrillary acidic protein promoter excluded the participation of proliferating astroglia in CB(2) receptor-mediated actions. These findings support a pivotal role for CB(2) receptors in attenuating microglial activation and preventing neurodegeneration that may pave the way to new therapeutic strategies for neuroprotection in Huntingtons disease as well as in other neurodegenerative disorders with a significant excitotoxic component.

Localization of estrogen receptor β-immunoreactivity in astrocytes of the adult rat brain

Estrogen receptors are direct regulators of transcription that function by binding to specific DNA sequences in promoters of target genes. The two cloned forms of estrogen receptors, α and β, are expressed in the central nervous system by different neuronal populations. Astrocytes in vitro are also reported to express estrogen receptor α; however, this expression has not been confirmed in the rat brain in vivo. The apparent absence of estrogen receptors in glia in vivo contrasts with the well‐known effects of this hormone on astrocytes of different brain areas, including the hippocampal formation. In this study, the expression of estrogen receptors in the hippocampal formation of adult male rats has been assessed by confocal microscopy. Estrogen receptor α‐immunoreactivity was localized in neuronal nuclei in the pyramidal cell layer of CA1‐CA3 fields. Estrogen receptor β‐immunoreactivity was observed in the perikarya, apical dendrites, and cell nuclei of pyramidal neurons in CA1 and CA2. Furthermore, estrogen receptor β‐immunoreactive glia were observed in CA1, CA2, CA3, and in the hilus of the dentate gyrus of male and female rats. Estrogen receptor β‐immunoreactivity was localized in glial processes and perikarya and, in some cases, in glial cell nuclei. Double immunocytochemical labeling of estrogen receptor β and the specific astroglial marker glial fibrillary acidic protein revealed that estrogen receptor β‐immunoreactive glial cells were astrocytes. Estrogen receptor α was not co‐localized with glial fibrillary acidic protein. The presence of estrogen receptor β in astrocytes of adult male and female rats demonstrates a possible mechanism by which estrogen can directly modulate gene expression in these cells. GLIA 26:260–267, 1999.

Neuroprotective effects of estradiol in the adult rat hippocampus: interaction with insulin-like growth factor-I signalling.

We have previously shown that 17‐β‐estradiol protects neurons in the dentate gyrus from kainic acid‐induced death in vivo. To analyse whether this effect is mediated through estrogen receptors and through cross‐talk between steroid and insulin‐like growth factor (IGF) systems, we have concomitantly administered antagonists of estrogen receptor (ICI 182,780) or the IGF‐I receptor (JB1) with estradiol. In addition, we have also administered IGF‐I with or without the estrogen receptor antagonist. JB1 (20 μg/ml), ICI 182,780 (10‐7 M), and IGF‐I (100 μg/ml) were delivered into the left lateral ventricle of young ovariectomized rats via an Alzet osmotic minipump (0.5 μl/hr) for 2 weeks. All rats received kainic acid (7 mg/Kg b.w.) or vehicle i.p. injections at day 7 after minipump implant. Also on day 7, rats treated i.c. v.with only ICI 182,780 or JB1 received a single i.p. injection of 17‐β‐estradiol (150 μg/rat) or vehicle. On day 14 after minipump implant, the rats were killed, brains processed, and the number of surviving hilar neurons estimated by the optical disector technique. Both IGF‐I and estradiol treatments resulted in over 90% survival of hilar neurons. The neuroprotective action of estradiol was blocked by ICI 182,780 and by JB1. Furthermore, IGF‐I enhancement of neuronal survival was significantly reduced by ICI 182,780. These results indicate that in this model of hippocampal lesion, the neuroprotective effect of estradiol depends both on estrogen receptors and IGF‐I receptors, while the protection exerted by IGF‐I depends also on estrogen receptors. In conclusion, an interaction of estrogen receptor and IGF‐I receptor signalling may mediate neuroprotection in the adult rat hippocampus. J. Neurosci. Res. 58:815–822, 1999.

The neuroprotective actions of oestradiol and oestrogen receptors

Hormones regulate homeostasis by communicating through the bloodstream to the bodys organs, including the brain. As homeostatic regulators of brain function, some hormones exert neuroprotective actions. This is the case for the ovarian hormone 17β-oestradiol, which signals through oestrogen receptors (ERs) that are widely distributed in the male and female brain. Recent discoveries have shown that oestradiol is not only a reproductive hormone but also a brain-derived neuroprotective factor in males and females and that ERs coordinate multiple signalling mechanisms that protect the brain from neurodegenerative diseases, affective disorders and cognitive decline.

Interactions of estrogens and insulin-like growth factor-I in the brain: implications for neuroprotection

Data from epidemiological studies suggest that the decline in estrogen following menopause could increase the risk of neurodegenerative diseases. Furthermore, experimental studies on different animal models have shown that estrogen is neuroprotective. The mechanisms involved in the neuroprotective effects of estrogen are still unclear. Anti-oxidant effects, activation of different membrane-associated intracellular signaling pathways, and activation of classical nuclear estrogen receptors (ERs) could contribute to neuroprotection. Interactions with neurotrophins and other growth factors may also be important for the neuroprotective effects of estradiol. In this review we focus on the interaction between insulin-like growth factor-I (IGF-I) and estrogen signaling in the brain and on the implications of this interaction for neuroprotection. During the development of the nervous system, IGF-I promotes the differentiation and survival of specific neuronal populations. In the adult brain, IGF-I is a neuromodulator, regulates synaptic plasticity, is involved in the response of neural tissue to injury and protects neurons against different neurodegenerative stimuli. As an endocrine signal, IGF-I represents a link between the growth and reproductive axes and the interaction between estradiol and IGF-I is of particular physiological relevance for the regulation of growth, sexual maturation and adult neuroendocrine function. There are several potential points of convergence between estradiol and IGF-I receptor (IGF-IR) signaling in the brain. Estrogen activates the mitogen-activated protein kinase (MAPK) pathway and has a synergistic effect with IGF-I on the activation of Akt, a kinase downstream of phosphoinositol-3 kinase. In addition, IGF-IR is necessary for the estradiol induced expression of the anti-apoptotic molecule Bcl-2 in hypothalamic neurons. The interaction of ERs and IGF-IR in the brain may depend on interactions between neural cells expressing ERs with neural cells expressing IGF-IR, or on direct interactions of the signaling pathways of alpha and beta ERs and IGF-IR in the same cell, since most neurons expressing IGF-IR also express at least one of the ER subtypes. In addition, studies on adult ovariectomized rats given intracerebroventricular (i.c.v.) infusions with antagonists for ERs or IGF-IR or with IGF-I have shown that there is a cross-regulation of the expression of ERs and IGF-IR in the brain. The interaction of estradiol and IGF-I and their receptors may be involved in different neural events. In the developing brain, ERs and IGF-IR are interdependent in the promotion of neuronal differentiation. In the adult, ERs and IGF-IR interact in the induction of synaptic plasticity. Furthermore, both in vitro and in vivo studies have shown that there is an interaction between ERs and IGF-IR in the promotion of neuronal survival and in the response of neural tissue to injury, suggesting that a parallel activation or co-activation of ERs and IGF-IR mediates neuroprotection.

Progesterone and its derivatives dihydroprogesterone and tetrahydroprogesterone reduce myelin fiber morphological abnormalities and myelin fiber loss in the sciatic nerve of aged rats

Previous studies indicate that steroid hormones may be protective for Schwann cells and promote the expression of myelin proteins in the sciatic nerve of adult rats. In this study, we have evaluated the effect of progesterone (P), dihydroprogesterone (DHP), tetrahydroprogesterone (THP), testosterone (T), dihydrotestosterone (DHT) and 5alpha-androstan-3alpha, 17beta-diol (3alpha-diol) on the morphological alterations of myelinated fibers in the sciatic nerve of 22-24-month-old male rats. The sciatic nerves of untreated old male rats, showed a general disorganization and a significant reduction in the density of myelinated fibers, compared to nerves from 3-month-old male rats. The effect of aging was particularly evident in myelinated fibers of small caliber (<5 microm in diameter). In addition, the sciatic nerves of old rats showed a significant increase in the number of fibers with myelin infoldings in the axoplasm and in the number of fibers with irregular shapes. Treatments of old rats with P, DHP and THP resulted in a significant increase in the number of myelinated fibers of small caliber, a significant reduction in the frequency of myelin abnormalities and a significant increase in the g ratio of small myelinated fibers. Furthermore, P treatment significantly reduced the frequency of myelinated fibers with irregular shapes. In contrast, treatments with T, DHT or 3alpha-diol did not significantly affect any of the morphological parameters examined. In conclusion, our data indicate that P, and its derivatives DHP and THP, are able to reduce aging-associated morphological abnormalities of myelin and aging-associated myelin fiber loss in the sciatic nerve. These data suggest that P, DHP and THP may represent useful therapeutic alternatives to maintain peripheral nerve integrity in aged animals.

Estrogen receptor alpha forms estrogen-dependent multimolecular complexes with insulin-like growth factor receptor and phosphatidylinositol 3-kinase in the adult rat brain

Estradiol and insulin-like growth factor-I (IGF-I) have numerous functional interactions in the brain, including the regulation of neuroendocrine events, the control of reproductive behavior and the promotion of synaptic plasticity and neuronal survival. To explore the mechanisms involved in these interdependent actions of estradiol and IGF-I in the adult brain, the potential interactions of estrogen receptors with components of the IGF-I signaling system were assessed in this study. Systemic estradiol administration resulted in a transient immunocoprecipitation of the IGF-I receptor with the estrogen receptor alpha and in a transient increase in tyrosine phosphorylation of the IGF-I receptor in the hypothalamus of adult ovariectomized Wistar rats. Both effects were coincident in time, with a peak between 1 and 3 h after systemic estradiol administration. Three hours after estradiol treatment, there was an enhanced immunocoprecipitation of estrogen receptor alpha with p85 subunit of phosphatidylinositol 3-kinase, as well as an enhanced immunocoprecipitation of p85 with insulin receptor substrate-1. The interaction with the IGF-I receptor was specific for the alpha form of the estrogen receptor and was also induced by intracerebroventricular injection of IGF-I. These hormonal actions may be part of the mechanism by which estradiol activates IGF-I receptor signaling pathways in the brain and may explain the interdependence of estrogen receptors and the IGF-I receptor in synaptic plasticity, neuroprotection and other neural events.

Actions of estrogens on glial cells: Implications for neuroprotection.

Glial cells are directly or indirectly affected by estradiol and by different estrogenic compounds, such as selective estrogen receptor modulators. Acting on oligodendrocytes, astrocytes and microglia, estrogens regulate remyelination, edema formation, extracellular glutamate levels and the inflammatory response after brain injury. In addition, estradiol induces the expression and release of growth factors by glial cells that promote neuronal survival. Therefore, glial cells are important players in the neuroprotective and reparative mechanisms of estrogenic compounds.

An antagonist of estrogen receptors blocks the induction of adult neurogenesis by insulin-like growth factor-I in the dentate gyrus of adult female rat

Interdependence between estradiol and insulin‐like growth factor‐I has been documented for different neural events, including neuronal differentiation, synaptic plasticity, neuroendocrine regulation and neuroprotection. In the present study we have assessed whether both factors interact in the regulation of neurogenesis in the adult rat dentate gyrus. Wistar albino female rats were bilaterally ovariectomized and treated with estradiol, insulin‐like growth factor‐I and/or the estrogen receptor antagonist ICI 182,780. Estradiol was administered in a subcutaneous silastic capsule. Insulin‐like growth factor‐I and ICI 182,780 were delivered in the lateral cerebral ventricle. Animals received six daily injections of 5‐bromo‐2‐deoxyuridine and were killed 24 h after the last injection. The total number of 5‐bromo‐2‐deoxyuridine‐positive neurons was significantly increased in animals treated with insulin‐like growth factor‐I, compared with rats treated with vehicles, while rats treated with both insulin‐like growth factor‐I and estradiol showed a higher number of 5‐bromo‐2‐deoxyuridine‐positive neurons than rats treated with insulin‐like growth factor‐I or estradiol alone. The antiestrogen ICI 182,780 blocked the effect of insulin‐like growth factor‐I on the number of 5‐bromo‐2‐deoxyuridine neurons with independence of whether the animals were treated or not with estradiol. These findings suggest that estrogen receptors are involved in the induction of adult neurogenesis by insulin‐like growth factor‐I in the dentate gyrus, and that estradiol and insulin‐like growth factor‐I have a cooperative interaction to promote neurogenesis. The interaction between insulin‐like growth factor‐I and estradiol may participate in changes in the rate of neurogenesis during different endocrine and physiological conditions, and may be related to the decline in neurogenesis with ageing.