Cholesterol-24-hydroxylase (CYP46) in the old brain: Analysis of positive populations and factors triggering its expression in astrocytes.

Cholesterol-24-hydroxylase (CYP46), a member of the cytochrome P450 superfamily of enzymes, is selectively expressed in the brain and is mainly responsible for cholesterol turnover in the central nervous system. Although increased cyp46A1 gene expression has been linked to cognitive alterations in aging and observed in neurodegenerative diseases and after traumatic brain injury, a detailed characterization of the brain regions and cell types in which CYP46 is expressed in old individuals has not been performed. Using immunohistochemistry and immunofluorescence, we investigated the specific regions and cell populations in the brain, in which cyp46A1 is expressed in 24-month-old mice. We found that CYP46 is localized in the same neuronal populations in young and old brains, mainly in the hippocampus, in cortical layers, and in Purkinje neurons of the cerebellum. No increase in CYP46 levels was found in astrocytes in old mice brains, in primary astrocyte-neuron cocultures aged in vitro, or in primary cultures of senescent astrocytes. However, interleukin-6 treatment strongly induced cyp46A1 expression in reactive astrocytes characterized by high GFAP levels but had no effect in nonactivated astrocytes. Our data suggest that cholesterol-24-hydroxylase expression is triggered in reactive astrocytes in response to proinflammatory signals, probably as part of a response mechanism to injury.

Understanding new molecular and cell biology findings based on progressive scientific practices and interconnected activities in undergraduate students.

Nowadays Molecular Cell Biology (MCB) must be taught as science is practiced. Even though there are several approaches based on scientific practices, a key aspect is to define the purpose of each of these teaching strategies and, most importantly, their implementation. Our goal was to train students to acquire, understand, and communicate new scientific knowledge in the field. The main feature of our new teaching methodology was progressive training in scientific practices associated with a back-and-forward interplay between activities and assessments. The methodology was implemented over 4 years, in students attending the MCB course of the undergraduate degree in Biological Sciences. In the first two modules, the students were prepared to comprehend MCB concepts and techniques and to experience activities based on scientific practices. In the third module, the students analyzed a primary paper in-depth. They were assessed by midterm exams based on a primary paper, written laboratory reports, and the oral presentation of a scientific paper. Our teaching proposal was evaluated through the students' academic performance and by their opinion on the teaching methodology. Most students were satisfied since they improved their acquisition of concepts, their interpretation and integration of scientific knowledge, and developed skills to communicate scientific knowledge in writing and orally. The novelty of transversal interconnections and progressive training in scientific practices provides students with skills in acquiring and understanding new scientific information, even beyond the MCB course.

Nerve growth factor induces cell cycle arrest of astrocytes.

Neurotrophins can influence multiple cellular functions depending on the cellular context and the specific receptors they interact with. These neurotrophic factors have been extensively studied for their ability to support neuronal survival via Trk receptors and to induce apoptosis via the p75(NTR). However, the p75(NTR) is also detected on cell populations that do not undergo apoptosis in response to neurotrophins. In particular, the authors have detected p75(NTR) expression on astrocytes during development and after seizure-induced injury. In this study, the authors investigated the role of Nerve growth factor (NGF) in regulating astrocyte proliferation and in influencing specific aspects of the cell cycle. The authors have demonstrated that NGF prevents the induction of cyclins and their association with specific cyclin-dependent kinases, and thereby prevents progression through the G1 phase of the cell cycle. Since the authors have previously shown that p75(NTR) but not TrkA, is expressed in astrocytes, these data suggest that activation of p75(NTR) promotes withdrawal of astrocytes from the cell cycle, which may have important consequences during development and after injury.

ProNGF induces PTEN via p75NTR to suppress Trk-mediated survival signaling in brain neurons.

Proneurotrophins and mature neurotrophins activate different signaling pathways with distinct effects on their target cells: proneurotrophins can induce apoptotic signaling via p75(NTR), whereas mature neurotrophins activate Trk receptors to influence survival and differentiation. Here, we demonstrate that the PTEN (phosphatase and tensin homolog deleted on chromosome 10) phosphatase represents a novel switch between the survival and apoptotic signaling pathways in rat CNS neurons. Simultaneous activation of p75(NTR) by proNGF and TrkB signaling by BDNF elicited apoptosis despite TrkB phosphorylation. Apoptosis induced by p75(NTR) required suppression of TrkB-induced phosphoinositide-3 kinase signaling, mediated by induction of PTEN, for apoptosis to proceed. Inhibition of PTEN restored the ability of BDNF to phosphorylate Akt and protect cultured basal forebrain neurons from proNGF-induced death. In vivo, inhibition or knockdown of PTEN after pilocarpine-induced seizures protected CNS neurons from p75(NTR)-mediated death, demonstrating that PTEN is a crucial factor mediating the balance between p75(NTR)-induced apoptotic signaling and Trk-mediated survival signaling in brain neurons.

Nerve growth factor attenuates proliferation of astrocytes via the p75 neurotrophin receptor.

The p75 neurotrophin receptor has been implicated in the regulation of multiple cellular functions that differ depending on the cell context. We have observed that p75(NTR) is strongly induced on astrocytes as well as neurons in the hippocampal CA3 region after seizures; however, the function of this receptor on these glial cells has not been defined. We have employed a primary culture system to investigate the effects of neurotrophins on astrocytes. Treatment of hippocampal astrocytes with nerve growth factor (NGF) caused a reduction in cell number, but did not elicit an apoptotic response, in contrast to hippocampal neurons. Instead, activation of p75(NTR) by NGF attenuated proliferation induced by mitogens such as EGF or serum. These studies demonstrate the cell type specificity of neurotrophin functions in the brain.

The function of p75NTR in glia.

The p75 neurotrophin receptor (p75(NTR)) is expressed on many cell types and can influence a variety of cellular functions. This receptor can mediate cell survival or cell death, can promote or inhibit axonal growth and can facilitate or attenuate proliferation, depending on the cell context. The emerging picture regarding p75(NTR) indicates that it can partner with different coreceptors to dictate specific responses. It then signals by recruiting intracellular binding proteins to activate different signaling pathways. The function of p75(NTR) has mainly been studied in neurons; however, it is also expressed in a variety of glial populations, especially during development and after injury, where its roles have been poorly defined. In this review, we will examine the potential roles for p75(NTR) in glial function.

Evidence that the effect of melanocortins on female sexual behavior in preoptic area is mediated by the MC3 receptor; Participation of nitric oxide.

alpha-MSH is involved in reproductive processes and can regulate the expression of lordosis, an important component of female reproductive behavior in rats and many other species. In this study, we investigated the effects of MSH peptides on lordosis behavior when injected in medial preoptic area (POA) of ovariectomised rats primed with estradiol. The results show an increase in lordotic activity after bilateral administration of alpha-MSH and gamma-MSH. Interestingly, the treatment with the MC4 receptor antagonist HS014 did not block the stimulatory effect of alpha-MSH. Moreover, the injection of HS014 did not itself modify the lordosis quotient. Nitric oxide has been suggested to play a crucial role in the regulation of lordosis behavior via stimulation of guanylyl cyclase to synthesize cGMP. In order to determine the participation of NO in the effect of the melanocortins, another group of rats were treated with L-NAME, an inhibitor of NOS, alone or 15 min before the injection of alpha-MSH or gamma-MSH. The injection of L-NAME into the POA of E-primed rats 15 min before the test for sexual receptivity did not modify significantly the lordosis quotient at the two doses examined. The treatment with L-NAME at the lowest dose completely abolished the stimulatory effect of alpha-MSH and gamma-MSH on sexual behavior. The results indicate that the effects of MSH peptides on female sexual behavior in this area are mediated through specific MC receptor, that could be the MC3 receptor and that NO mediates the melanocortins effects.

Differential role of the hippocampus, amygdala, and dorsal raphe nucleus in regulating feeding, memory, and anxiety-like behavioral responses to ghrelin.

Ghrelin is a peptide hormone produced and secreted from the stomach. Hypothalamic injection of the peptide increases food intake but it is not known if the peptide affects other brain regions. We measured several behavioral parameters such as anxiety (elevated plus maze), memory retention (step down test), and food intake after injections of different doses of the peptide in the hippocampus, amygdala, and dorsal raphe nucleus (DRN). The injection of ghrelin in the hippocampus and DRN significantly and dose dependently increased food intake in relation to controls rats, while injections into the amygdala did not affect the food intake. We also show for the first time that ghrelin clearly and dose dependently increases memory retention in the hippocampus, amygdala, and DRN. Moreover, ghrelin at different potencies induced anxiogenesis in these brain structures while the highest dose of 3 nmol/microl was effective in all of them. The comparison of sensitivity of each brain structure indicates a specific role of them for each of the behaviors studied. The results provide new insight in to the anatomical substrate and the functional role of extrahypothalamic ghrelin targets in the CNS.

Ghrelin increases anxiety-like behavior and memory retention in rats.

Ghrelin is a peptide found in the hypothalamus and stomach that stimulates food intake and whose circulating concentrations are affected by nutritional state. Very little is known about other central behavioral effects of ghrelin, and thus, we investigated the effects of ghrelin on anxiety and memory retention. The peptide was injected intracerebroventricularly in rats and we performed open-field, plus-maze, and step-down tests (inhibitory avoidance). The administration of ghrelin increased freezing in the open field and decreased the number of entries into the open spaces and the time spent on the open arms in the plus-maze, indicating an anxiogenic effect. Moreover, the peptide increased in a dose-dependent manner the latency time in the step-down test. A rapid and prolonged increase in food intake was also observed. Our results indicate that ghrelin induces anxiogenesis in rats. Moreover, we show for the first time that ghrelin increases memory retention, suggesting that the peptide may influence processes in the hippocampus.