Lysosomal cholesterol accumulation in aged astrocytes impairs cholesterol delivery to neurons and can be rescued by cannabinoids.

Cholesterol is crucial for the proper functioning of eukaryotic cells, especially neurons, which rely on cholesterol to maintain their complex structure and facilitate synaptic transmission. However, brain cells are isolated from peripheral cholesterol by the blood-brain barrier and mature neurons primarily uptake the cholesterol synthesized by astrocytes for proper function. This study aimed to investigate the effect of aging on cholesterol trafficking in astrocytes and its delivery to neurons. We found that aged astrocytes accumulated high levels of cholesterol in the lysosomal compartment, and this cholesterol buildup can be attributed to the simultaneous occurrence of two events: decreased levels of the ABCA1 transporter, which impairs ApoE-cholesterol export from astrocytes, and reduced expression of NPC1, which hinders cholesterol release from lysosomes. We show that these two events are accompanied by increased microR-33 in aged astrocytes, which targets ABCA1 and NPC1. In addition, we demonstrate that the microR-33 increase is triggered by oxidative stress, one of the hallmarks of aging. By coculture experiments, we show that cholesterol accumulation in astrocytes impairs the cholesterol delivery from astrocytes to neurons. Remarkably, we found that this altered transport of cholesterol could be alleviated through treatment with endocannabinoids as well as cannabidiol or CBD. Finally, according to data demonstrating that aged astrocytes develop an A1 phenotype, we found that cholesterol buildup is also observed in reactive C3+ astrocytes. Given that reduced neuronal cholesterol affects synaptic plasticity, the ability of cannabinoids to restore cholesterol transport from aged astrocytes to neurons holds significant implications in aging and inflammation.

Brain-region specific responses of astrocytes to an in vitro injury and neurotrophins.

Astrocytes are a heterogeneous population of glial cells that react to brain insults through a process referred to as astrogliosis. Reactive astrocytes are characterized by an increase in proliferation, size, migration to the injured zone and release of a plethora of chemical mediators such as NGF and BDNF. The aim of this study was to determine whether there are brain region-associated responses of astrocytes to an injury and to the neurotrophins NGF and BDNF. We used the scratch injury model to study the closure of a wound inflicted on a monolayer of astrocytes obtained from cortex, hippocampus or striatum. Our results indicate that the response of astrocytes to a mechanical lesion differ according to brain regions. Astrocytes from the striatum proliferate and repopulate the injury site more rapidly than astrocytes from cortex or hippocampus. We found that the scratch injury induced the upregulation of neurotrophin receptor p75NTR and TrkB.t in astrocytes from all brain regions studied. When astrocytes from all regions were treated with NGF, the neurotrophin induced migration of the astrocytes (assessed in Boyden chambers) and induced wound closure but did not affect proliferation. In contrast, BDNF induced wound closure but only in astrocytes from striatum. Our overall findings show the heterogeneity in astrocyte functions based on their brain region of origin, and how this functional diversity may determine their responses to an injury and to neurotrophins.

Phenotypically aberrant astrocytes that promote motoneuron damage in a model of inherited amyotrophic lateral sclerosis.

Motoneuron loss and reactive astrocytosis are pathological hallmarks of amyotrophic lateral sclerosis (ALS), a paralytic neurodegenerative disease that can be triggered by mutations in Cu-Zn superoxide dismutase (SOD1). Dysfunctional astrocytes contribute to ALS pathogenesis, inducing motoneuron damage and accelerating disease progression. However, it is unknown whether ALS progression is associated with the appearance of a specific astrocytic phenotype with neurotoxic potential. Here, we report the isolation of astrocytes with aberrant phenotype (referred as "AbA cells") from primary spinal cord cultures of symptomatic rats expressing the SOD1(G93A) mutation. Isolation was based on AbA cells' marked proliferative capacity and lack of replicative senescence, which allowed oligoclonal cell expansion for 1 y. AbA cells displayed astrocytic markers including glial fibrillary acidic protein, S100ß protein, glutamine synthase, and connexin 43 but lacked glutamate transporter 1 and the glial progenitor marker NG2 glycoprotein. Notably, AbA cells secreted soluble factors that induced motoneuron death with a 10-fold higher potency than neonatal SOD1(G93A) astrocytes. AbA-like aberrant astrocytes expressing S100ß and connexin 43 but lacking NG2 were identified in nearby motoneurons, and their number increased sharply after disease onset. Thus, AbA cells appear to be an as-yet unknown astrocyte population arising during ALS progression with unprecedented proliferative and neurotoxic capacity and may be potential cellular targets for slowing ALS progression.

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.

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.

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.

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.

Regional brain susceptibility to neurodegeneration: what is the role of glial cells?

The main pathological feature of the neurodegenerative diseases is represented by neuronal death that represents the final step of a cascade of adverse/hostile events. Early in the neurodegenerative process, glial cells (including astrocytes, microglial cells, and oligodendrocytes) activate and trigger an insidious neuroinflammatory reaction, metabolic decay, blood brain barrier dysfunction and energy impairment, boosting neuronal death. How these mechanisms might induce selective neuronal death in specific brain areas are far from being elucidated. The last two decades of neurobiological studies have provided evidence of the main role of glial cells in most of the processes of the central nervous system, from development to synaptogenesis, neuronal homeostasis and integration into, highly specific neuro-glial networks. In this mini-review, we moved from in vitro and in vivo models of neurodegeneration to analyze the putative role of glial cells in the early mechanisms of neurodegeneration. We report changes of transcriptional, genetic, morphological, and metabolic activity in astrocytes and microglial cells in specific brain areas before neuronal degeneration, providing evidence in experimental models of neurodegenerative disorders, including Parkinson's and Alzheimer's diseases. Understanding these mechanisms might increase the insight of these processes and pave the way for new specific glia-targeted therapeutic strategies for neurodegenerative disorders.

Induction of proneurotrophins and activation of p75NTR-mediated apoptosis via neurotrophin receptor-interacting factor in hippocampal neurons after seizures.

Seizure-induced damage elicits a loss of hippocampal neurons mediated to a great extent by the p75 neurotrophin receptor (NTR). Proneurotrophins, which are potent apoptosis-inducing ligands for p75(NTR), were increased in the hippocampus, particularly in astrocytes, by pilocarpine-induced seizures; and infusion of anti-pro-NGF dramatically attenuated neuronal loss after seizures. The p75(NTR) is expressed in many different cell types in the nervous system, and can mediate a variety of different cellular functions by recruiting specific intracellular binding proteins to activate distinct signaling pathways. In this study, we demonstrate that neurotrophin receptor-interacting factor (NRIF) mediates apoptotic signaling via p75(NTR) in hippocampal neurons in vitro and in vivo. After seizure-induced injury, NRIF(-/-) mice showed an increase in p75(NTR) expression in the hippocampus; however, these neurons failed to undergo apoptosis in contrast to wild-type mice. Treatment of cultured hippocampal neurons with proneurotrophins induced association of NRIF with p75(NTR) and subsequent translocation of NRIF to the nucleus, which was dependent on cleavage of the receptor. Neurons lacking NRIF were resistant to p75(NTR)-mediated apoptosis in vitro and in vivo. In addition, we demonstrate some mechanistic differences in p75(NTR) signaling in hippocampal neurons compared with other cell types. Overall, these studies demonstrate the requirement for NRIF to signal p75(NTR)-mediated apoptosis of hippocampal neurons and that blocking pro-NGF can inhibit neuronal loss after seizures.

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.

Loss of TrkB Signaling Due to Status Epilepticus Induces a proBDNF-Dependent Cell Death.

Neurotrophins (NTs) are secretory proteins that bind to target receptors and influence many cellular functions, such as cell survival and cell death in neurons. The mammalian NT brain-derived neurotrophic factor (matBDNF) is the C-terminal mature form released by cleavage from the proBDNF precursor. The binding of matBDNF to the tyrosine kinase receptor B (TrkB) activates different signaling cascades and leads to neuron survival and plasticity, while the interaction of proBDNF with the p75 NT receptor (p75NTR)/sortilin receptor complex has been highly involved in apoptosis. Many studies have demonstrated that prolonged seizures such as status epilepticus (SE) induce changes in the expression of NT, pro-NT, and their receptors. We have previously described that the blockage of both matBDNF and proBDNF signaling reduces neuronal death after SE in vivo (Unsain et al., 2008). We used an in vitro model as well as an in vivo model of SE to determine the specific role of TrkB and proBDNF signaling during neuronal cell death. We found that the matBDNF sequestering molecule TrkB-Fc induced an increase in neuronal death in both models of SE, and it also prevented a decrease in TrkB levels. Moreover, SE triggered the interaction between proBDNF and p75NTR, which was not altered by sequestering matBDNF. The intra-hippocampal administration of TrkB-Fc, combined with an antibody against proBDNF, prevented neuronal degeneration. In addition, we demonstrated that proBDNF binding to p75NTR exacerbates neuronal death when matBDNF signaling is impaired through TrkB. Our results indicated that both the mature and the precursor forms of BDNF may have opposite effects depending on the scenario in which they function and the signaling pathways they activate.

Anxiety-like behavior induced by IL-1beta is modulated by alpha-MSH through central melanocortin-4 receptors.

The proinflammatory cytokine interleukin-1beta (IL-1beta) influences neuroendocrine activity and produces other effects, including fever and behavioral changes such as anxiety. The melanocortin neuropeptides, such as alpha-melanocyte-stimulating hormone (alpha-MSH), antagonize many actions of IL-1, including fever, anorexia and hypothalamic-pituitary-adrenal (HPA) axis activation through specific melanocortin receptors (MC-R) in the central nervous system. The objective of the present study was to establish the effect of MSH peptides on IL-1beta-induced anxiety-like behavior and the melanocortin receptors involved. We evaluated the effects of intracerebroventricular (i.c.v.) administration of IL-1beta (30 ng) and melanocortin receptor agonists: alpha-MSH, an MC3/MC4-R agonist (0.2 microg) or gamma-MSH, an MC3-R agonist (2 microg) or HS014, an MC4-R antagonist (2 microg), on an elevated plus-maze (EPM) test. Injection of IL-1beta induced an anxiogenic-like response, as indicated by reduced open arms entries and time spent on open arms. The administration of alpha-MSH reversed IL-1beta-induced anxiety with co-administration of HS014 inhibiting the effect of alpha-MSH. However, the associated treatment with gamma-MSH did not affect the anxiety response to IL-1beta. These data suggest that alpha-MSH, through central MC4-R can modulate the anxiety-like behavior induced by IL-1beta.

Interleukin-1 beta-induced anorexia is reversed by ghrelin.

Interleukins, in particular interleukin-1beta (IL-1beta), reduce food intake after peripheral and central administration, which suggests that they contribute to anorexia during various infectious, neoplastic, and autoimmune diseases. On the other hand, ghrelin stimulates food intake by acting on the central nervous system (CNS) and is considered an important regulator of food intake in both rodents and humans. In the present study, we investigated if ghrelin could reverse IL-1beta-induced anorexia. Intracerebroventricular (i.c.v.) injection of 15, 30 or 45 ng/microl of IL-1beta caused significant suppression of food intake in 20 h fasting animals. This effect lasted for a 24h period. Ghrelin (0.15 nmol or 1.5 nmol/microl) produced a significant increase in cumulative food intake in normally fed animals. However, it did not alter food intake in 20 h fasting animals. Central administration of ghrelin reduced the anorexic effect of IL-1beta (15 ng/microl). The effect was observed 30 min after injection and lasted for the next 24h. This study provides evidence that ghrelin is an orexigenic peptide capable of antagonizing IL-1beta-induced anorexia.

Alpha-MSH and gamma-MSH modulate early release of hypothalamic PGE2 and NO induced by IL-1beta differently.

Interleukin-1beta (IL-1beta) stimulates corticotropin-releasing hormone (CRH) secretion in hypothalamus, which involves the release of prostaglandins (PGE2) and nitric oxide (NO). We have demonstrated that melanocortins can inhibit the early effects of IL-1beta on the HPA axis by acting on the central nervous system (CNS). Our study investigated whether alpha-melanocyte stimulating hormone (alpha-MSH) and gamma-MSH could inhibit IL-1beta-induced PGE2 and NO release in hypothalamus in the rapid activation of the HPA axis. An i.c.v. injection of 12.5 ng/microl of IL-1beta significantly increased the release of PGE2 and NOS activity in the hypothalamus. Treatment with alpha-MSH (0.1 microg/microl) inhibited the effect of IL-1beta on PGE2 release. Also, gamma-MSH (1 microg/microl) eliminated the increase in NOS activity induced by IL-1beta. Our data indicate the modulatory role of melanocortins in the early hypothalamic response to IL-1beta, with different regulation of PGE2 and NO release.

alpha-MSH and gamma-MSH inhibit IL-1beta induced activation of the hypothalamic-pituitary-adrenal axis through central melanocortin receptors.

Alpha-melanocyte-stimulating hormone (alpha-MSH) is a neuroimmunomodulatory peptide that is involved in the control of host responses trough modulation of production and action of proinflammatory cytokines in inflammatory cells in the periphery and within the central nervous system (CNS). However, little is known about the receptors that mediate the modulatory effects of alpha-MSH in the CNS. The objective of the present study was to establish the specific melanocortin receptors involved in the inhibition by MSH peptides of IL-1beta-induced activation of the HPA. i.c.v. injection of 12.5 ng of IL-1beta caused significant changes in plasma corticosterone, as compared to basal levels. The treatment with gamma-MSH (1 microg), an MC3 receptor agonist, resulted in significant reduction of the IL-1beta-induced plasma corticosterone levels. Administration of the MC3/MC4 receptor antagonist SHU9119 blocked this effect. Besides, treatment with a high dose of alpha-MSH (1 microg) increased plasma corticosterone. When alpha-MSH was given at a lower dose (0.1 microg), it did not modify corticosterone levels but caused an inhibitory effect on the corticosterone release induced by IL-1beta. The administration of SHU9119 or a more selective MC4 receptor antagonist like HS014 blocked the effects of alpha-MSH. In conclusion, our results suggest that both alpha-MSH and gamma-MSH are capable of inhibiting the effect of the IL-1beta on the activation of HPA axis acting at the CNS, and that this effect is mediated by specific central melanocortin receptors.

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.

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.

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.

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.