Oligodendrocyte apoptosis mediated by caspase activation

Cellular inhibitor of apoptosis 2 (cIAP2) restricts neuroinflammation during experimental autoimmune encephalomyelitis

Background

Immune activation, neuroinflammation, and cell death are the hallmarks of multiple sclerosis (MS), which is an autoimmune demyelinating disease of the central nervous system (CNS). It is well-documented that the cellular inhibitor of apoptosis 2 (cIAP2) is induced by inflammatory stimuli and regulates adaptive and innate immune responses, cell death, and the production of inflammatory mediators. However, the impact of cIAP2 on neuroinflammation associated with MS and disease severity remains unknown.

Methods

We used experimental autoimmune encephalomyelitis (EAE), a widely used mouse model of MS, to assess the effect of cIAP2 deletion on disease outcomes. We performed a detailed analysis on the histological, cellular, and molecular levels. We generated and examined bone-marrow chimeras to identify the cIAP2-deficient cells that are critical to the disease outcomes.

Results

cIAP2^−/− mice exhibited increased EAE severity, increased CD4⁺ T cell infiltration, enhanced proinflammatory cytokine/chemokine expression, and augmented demyelination. This phenotype was driven by cIAP2-deficient non-hematopoietic cells. cIAP2 protected oligodendrocytes from cell death during EAE by limiting proliferation and activation of brain microglia. This protective role was likely exerted by cIAP2-mediated inhibition of the non-canonical NLRP3/caspase-8-dependent myeloid cell activation during EAE.

Conclusions

Our findings suggest that cIAP2 is needed to modulate neuroinflammation, cell death, and survival during EAE. Significantly, our data demonstrate the critical role of cIAP2 in limiting the activation of microglia during EAE, which could be explored for developing MS therapeutics in the future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02527-6.

Chlorogenic Acid Prevents AMPA-Mediated Excitotoxicity in Optic Nerve Oligodendrocytes Through a PKC and Caspase-Dependent Pathways

In the CNS, including the optic nerve, oligodendrocytes play a critical role in the myelination of axons. Oligodendrocytes are exceptionally sensitive to insults to the CNS, such as injury, ischemia, or inflammation, which result in the loss of oligodendrocytes and myelin and eventually secondary axon degeneration. Oligodendrocytes are sensitive to excitotoxic insults mediated by overactivation of their AMPA ionotropic glutamate receptors. Phenolic compounds, which are widely distributed in fruits and vegetables, received the great attention of scientists due to their antioxidant activities and free radical scavenging abilities. Chlorogenic acid (CGA) has been demonstrated to possess potent neuroprotective activities against oxidative stress in various cellular models and pathological conditions. Hence, CGA protect against oxidative stress and excitotoxic insults mediated by AMPA receptors and that the protective mechanisms involve free radical scavenging, Ca²⁺ handling in the cytosol, and modulating antioxidant enzyme system. CGA was associated with the protein kinase A (PKC) signaling pathways transduction. Caspases and calpains have been studied as apoptotic mediators and cell death in this model of AMPA toxicity. Inhibitors of caspases initiators, caspases 1, 8, and 9, the upstream of caspase 3 effectors, have totally abrogated the protective activity of CGA. Inhibitors of calpains also totally abrogated the protective activity of CGA. In addition, a potential role for the CGA in inhibiting Bax in oligodendrocyte cell model undergoing AMPA is inducing excitotoxic death. Our results indicate that CGA exhibits a protective potential via antioxidant and apoptosis caspases and calpains dependent against AMPA-mediated excitotoxicity, and these finding indicate that CGA is able to be a good candidate for preventive approach for neurodegenerative disorders associated with loss and damage in oligodendrocytes and AMPA-mediated excitotoxicity.

BNN27, a 17-Spiroepoxy Steroid Derivative, Interacts With and Activates p75 Neurotrophin Receptor, Rescuing Cerebellar Granule Neurons from Apoptosis

Neurotrophin receptors mediate a plethora of signals affecting neuronal survival. The p75 pan-neurotrophin receptor controls neuronal cell fate after its selective activation by immature and mature isoforms of all neurotrophins. It also exerts pleiotropic effects interacting with a variety of ligands in different neuronal or non-neuronal cells. In the present study, we explored the biophysical and functional interactions of a blood-brain-barrier (BBB) permeable, C17-spiroepoxy steroid derivative, BNN27, with p75^NTR receptor. BNN27 was recently shown to bind to NGF high-affinity receptor, TrkA. We now tested the p75^NTR-mediated effects of BNN27 in mouse Cerebellar Granule Neurons (CGNs), expressing p75^NTR, but not TrkA receptors. Our findings show that BNN27 physically interacts with p75^NTR receptors in specific amino-residues of its extracellular domain, inducing the recruitment of p75^NTR receptor to its effector protein RIP2 and the simultaneous release of RhoGDI in primary neuronal cells. Activation of the p75^NTR receptor by BNN27 reverses serum deprivation-induced apoptosis of CGNs resulting in the decrease of the phosphorylation of pro-apoptotic JNK kinase and of the cleavage of Caspase-3, effects completely abolished in CGNs, isolated from p75^NTR null mice. In conclusion, BNN27 represents a lead molecule for the development of novel p75^NTR ligands, controlling specific p75^NTR-mediated signaling of neuronal cell fate, with potential applications in therapeutics of neurodegenerative diseases and brain trauma.

Review ■ : Insulin-like Growth Factor-I and Apoptosis in Glial Cell Biology

Insulin-like growth factor-I (IGF-I) is a potent trophic factor capable of promoting both survival and differentiation of neurons and glia. This review examines the role of IGF-I and apoptosis in oligodendrocyte and Schwann cell biology in vitro and in vivo. Apoptosis is an essential element of development, homeostasis, and disease. IGF-I protects oligodendrocytes and Schwann cells from apoptosis during development and after apoptotic stimuli. Transgenic mouse models, which ablate or increase expression of IGF-I, have abnormal oligodendrocytes and myelin formation. A more thorough understanding of the protective mechanism of IGF-I in oligodendrocytes and Schwann cells will aid in its precise application in treating a variety of neurologic disorders. NEUROSCIENTIST 6:39-47, 2000

Protective effects of matrine on experimental autoimmune encephalomyelitis via regulation of ProNGF and NGF signaling

Inflammation, demyelination, oligodendrocyte (OLG) death, and axonal degeneration are primary characteristics of multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). OLGs generate myelin sheaths that surround axons, while damage to OLGs leads to demyelination and neurological functional deficit. Matrine (MAT), a quinolizidine alkaloid derived from the herb Radix Sophorae Flave, has been recently found to effectively ameliorate clinical signs in EAE. Its therapeutic mechanism has, however, not been completely elucidated. In the present study, we found that MAT retarded the disease process, attenuated the clinical severity of EAE rats, ameliorated inflammation and demyelination, and suppressed the apoptosis of OLGs in the central nervous system (CNS) of EAE rats. In addition, MAT markedly blocked increased expression of the proNGF-p75(NTR) death signaling complex, which is known to mediate OLG death in EAE animals. At the same time, MAT also prevented a decrease in the levels of NGF and its receptor TrkA, which together mediate the cell survival pathway and differentiation of OLGs. ProNGF, NGF, and the downstream effector proteins play an important role in the growth, differentiation, and apoptosis of OLGs as well as the reparative response to neuronal damage. These findings thus indicate that MAT improves clinical severity of EAE in part by reducing OLG apoptosis via restoring the ratios of proNGF:NGF and the respective receptors p75(NTR):TrkA in vivo. Taken together, these results suggest that MAT may be a promising agent for MS treatment based on its protective effect on OLGs.

Brain-Expressed X-linked (BEX) proteins in human cancers

The Brain-Expressed X-linked (BEX) family proteins are comprised of five human proteins including BEX1, BEX2, BEX3, BEX4 and BEX5. BEX family proteins are expressed in a wide range of tissues and are known to play a role in neuronal development. Recent studies suggest a role of BEX family proteins in cancers. BEX1 expression is lost in a subgroup of patients with acute myeloid leukemia (AML) and chronic myeloid leukemia (CML). Expression of BEX1 controls cell surface receptor signaling and restores imatinib response in resistant cells. BEX2 is overexpressed in a group of breast cancer patients and also in gliomas. Increased BEX2 expression led to enhanced NF-κB signaling as well as cell proliferation. Although BEX2 acts as tumor promoter in a subset of breast cancer, BEX3 expression displayed an opposite role. Overexpression of BEX3 resulted in inhibition of tumor formation in breast cancer mouse xenograft models. The role of BEX4 and BEX5 in cancer has not yet been defined. Collectively this suggests that BEX family members have distinct roles in cancers. While BEX1 and BEX3 act as tumor suppressors, BEX2 seems to act as an oncogene.

Radioprotective effects of valproic acid, a histone deacetylase inhibitor, in the rat brain

Radiotherapy is commonly used in the treatment of brain tumors but can cause significant damage to surrounding normal brain. The radioprotective effects of valproic acid (VPA) on normal tissue in the rat brain were evaluated following irradiation. Male Wistar rats were used in the present study and 48 rats were randomly divided into four groups consisting of 12 rats each. The whole-brain irradiation (WBI) was delivered by X-ray and the rats received the following treatment once a day for 5 days. The control group (sham-exposed group) received sham irradiation plus physiological saline. The VPA group received sham irradiation plus 150 mg VPA/kg. The X-ray group received WBI plus physiological saline. The combined group received WBI plus 150 mg/kg intraperitoneally VPA. A total of 6 months post-irradiation, the rats were sacrificed and the brains were harvested. Cell apoptosis in the cortex was determined by immunohistochemistry 24 h post-irradiation using an antibody for protein caspase-3. Transmission electron microscope (TEM) analyses were used to assess the effects of VPA on the radioprotection of rat normal brain cells 6 months post-irradiation. The weights of the animals in the TEM group measured over the two weeks after the first injection of VPA were also observed. Histological findings demonstrated that apoptosis occurred on the cortex 1 day after treatment, peaking in the X-ray group. The cells of the combined group showed a moderate caspase-3 staining compared to the X-ray group. There was a trend towards a lower body weight of the X-ray group following irradiation compared to either no-irradiation or rats of the combined group, although there was no significant difference in the average weight between the combined group and irradiated rats. Mild swelling of the capillary endothelial cells in the irregular lumen was observed in the combined group, whereas the X-ray group showed a severe structural disorder. In conclusion, VPA supplementation during radiotherapy may be beneficial for radioprotection following WBI by reducing normal brain cell injury.

The biological functions and signaling mechanisms of the p75 neurotrophin receptor

The p75 neurotrophin receptor (p75(NTR)) regulates a wide range of cellular functions, including programmed cell death, axonal growth and degeneration, cell proliferation, myelination, and synaptic plasticity. The multiplicity of cellular functions governed by the receptor arises from the variety of ligands and co-receptors which associate with p75(NTR) and regulate its signaling. P75(NTR) promotes survival through interactions with Trk receptors, inhibits axonal regeneration via partnerships with Nogo receptor (Nogo-R) and Lingo-1, and promotes apoptosis through association with Sortilin. Signals downstream of these interactions are further modulated through regulated intramembrane proteolysis (RIP) of p75(NTR) and by interactions with numerous cytosolic partners. In this chapter, we discuss the intricate signaling mechanisms of p75(NTR), emphasizing how these signals are differentially regulated to mediate these diverse cellular functions.

Human immunodeficiency virus-associated dementia: a link between accumulation of viral proteins and neuronal degeneration

In the late stage of human immunodeficiency virus-1 (HIV) infection, a subset of individuals develops HIV associated neurocognitive disorders (HAND), which in its severe form, is characterized by motor and cognitive dysfunction. Dendritic pruning, synaptic abnormalities and neuronal apoptosis are observed in these patients. There are numerous advances in our understanding of HIV interactions with cells of the central nervous system. However, the underlying causes of neurological symptoms and pathological alterations observed in HIV positive subjects are poorly understood. Moreover, little is still known about the molecular mechanisms by which HIV induces synaptic dysfunction and degeneration. HAND resembles other common neurological diseases such as Alzheimer's and Huntington's diseases. These neurodegenerative disorders are characterized by accumulation of toxic proteins such as tau and huntingtin, respectively, which promote axonal degeneration by impairing axonal transport. Axonal degeneration precedes neuronal death. Therefore, a better understanding of the mechanisms whereby HIV triggers axonal degeneration has potential implications for developing therapeutic compounds to prevent synaptic failure in HAND. This article highlights and reviews evidence showing that neuronal accumulation of viral proteins promotes axonal damage.

Caffeic acid phenethyl ester synergistically enhances docetaxel and paclitaxel cytotoxicity in prostate cancer cells

Evidence is growing for the beneficial role of selective estrogen receptor modulators (SERM) in prostate diseases. Caffeic acid phenethyl ester (CAPE) is a promising component of propolis that possesses SERM activity. This study aimed at investigating the modulatory impact of CAPE on docetaxel (DOC) and paclitaxel (PTX) cytotoxicity in prostate cancer cells and exploring the possible underlying mechanisms for this chemomodulation. CAPE significantly increased DOC and PTX potency in PC-3, DU-145 and LNCaP prostate cancer cells. Combination index calculations showed synergistic interaction of CAPE/DOC and CAPE/PTX cotreatments in all the tested cell lines. Subsequent mechanistic studies in PC-3 cells indicated that cyclin D1 and c-myc were significantly reduced in the combined treatment groups with concurrent increase in p27kip. DNA-ploidy analysis indicated a significant increase in the percentage of cells in pre-G1 in CAPE/DOC and CAPE/PTX cotreatments. Decreased Bcl-2/Bax ratio together with increased caspase-3 activity and protein abundance were observed in the same groups. Estrogen receptor-β (ER-β) and its downstream tumor suppressor forkhead box O1 levels were significantly elevated in CAPE and combination groups compared to DOC or PTX-alone. ER-α and insulin-like growth factor-1 receptor protein abundance were reduced in the same groups. CAPE significantly reduced AKT, ERK and ER-α (Ser-167) phosphorylation in PC-3 cells. CAPE-induced inhibition of AKT phosphorylation was more prominent (1.7-folds higher) in cells expressing ER-α such as PC-3 compared to LNCaP. In conclusion, CAPE enhances the antiproliferative and cytotoxic effects of DOC and PTX in prostate cancer cells. This can be, at least partly, attributed to CAPE augmentation of DOC and PTX proapoptotic effects in addition to CAPE-induced alterations in ER-α and ER-β abundance.

Implementing neuronal plasticity in NeuroAIDS: the experience of brain-derived neurotrophic factor and other neurotrophic factors

Human immunodeficiency virus type-1 (HIV) causes mild or severe neurological problems, termed HIV-associated neurocognitive disorder (HAND), even when HIV patients receive antiretroviral therapy. Thus, novel adjunctive therapies are necessary to reduce or abolish the neurotoxic effect of HIV. However, new therapies require a better understanding of the molecular and cellular mechanisms of HIV-induced neurotoxicity. HAND subjects are characterized by being profoundly depressed, and they experience deficits in memory, learning and movements. Experimental evidence has also shown that HIV reduces neurogenesis. These deficits resemble those occurring in premature brain aging or in a brain with impaired neural repair properties. Thus, it appears that HIV diminishes neuronal survival, along with reduced neuronal connections. These two phenomena should not occur in the adult and developing brain when synaptic plasticity is promoted by neurotrophic factors, polypeptides that are present in adult synapses. This review will outline experimental evidence as well as present emerging concepts for the use of neurotrophic factors and in particular brain-derived neurotrophic factor as an adjunct therapy to prevent HIV-mediated neuronal degeneration and restore the loss of synaptic connections.

Synaptic dysfunction in human immunodeficiency virus type-1-positive subjects: inflammation or impaired neuronal plasticity?

Many people infected with the human immunodeficiency virus type-1 (HIV) exhibit mild or severe neurological problems, termed HIV-associated neurocognitive disorder (HAND), even when receiving antiretroviral therapy. Thus, novel adjunctive therapies must be developed to overcome the neurotoxic effect of HIV. New therapies require a better understanding of the molecular and cellular mechanisms of HIV-induced neurotoxicity and the risk factors that, besides inflammation and T-cell depletion and drugs of abuse, render the central nervous system (CNS) a target of HIV-induced neurotoxicity. HIV appears to impair neuronal plasticity, which refers to the innate ability of the CNS respond to injury and promote recovery of function. The availability of brain-derived neurotrophic factor (BDNF), a potent neurotrophic factor that is present in abundance in the adult brain, is essential for neuronal plasticity. BDNF acts through a receptor system composed of Trk and p75NTR. Here, we present experimental evidence that some of the clinical features of HIV-mediated neurological impairment could result from altered BDNF/TrkB/p75NTR regulation and function.

Administration of dexamethasone to neonatal rats induces hypomyelination and changes in the morphology of oligodendrocyte precursors

To examine whether hypomyelination in neonatal rats might be related to apoptosis of oligodendrocyte progenitors, we administered dexamethasone (0.5 mg/kg SC) to neonatal rats on postnatal (P) days 1 through 5. Immunofluorescent staining and Western blotting for myelin basic protein (MBP) were performed on P14. Morphologic changes associated with apoptotic death of oligodendrocyte progenitors were assessed by using immunofluorescent staining on P5 of surface markers present at different developmental stages of oligodendrocyte progenitors (O4 and O1) and by double-staining with terminal deoxynucleotidyl transferase-mediated digoxigenin-dUTP nick end-labeling (TUNEL) and O4 or O1. Administration of dexamethasone to neonatal rats reduced the expression of MBP in the white matter by P14. In addition, dexamethasone reduced the expression of O4-positive cells, presumably preoligodendrocytes, in the corpus callosum and induced degenerative changes, such as cytoplasmic condensation and fragmented, tortuous processes, in oligodendrocyte progenitors, and increased the number of TUNEL-positive pyknotic nuclei of oligodendrocyte progenitors. These findings suggest that the dexamethasone-induced decreased expression of MBP in the cerebral hemispheres of the neonatal rats is due to apoptotic degeneration of oligodendrocyte progenitors. Administration of dexamethasone during the critical period of brain development may increase the risk of apoptosis in oligodendrocyte progenitors, subsequently resulting in hypomyelination.

17β-Estradiol inhibits apoptotic cell death of oligodendrocytes by inhibiting RhoA-JNK3 activation after spinal cord injury

A delayed oligodendrocyte cell death after spinal cord injury (SCI) contributes to chronic demyelination of spared axons, leading to a permanent neurological deficit. Therefore, therapeutic approaches to prevent oligodendrocyte cell death after SCI should be considered. Estrogens are well known to have a broad neuroprotective effect, but the protective effect of estrogens on oligodendrocytes after injury is largely unknown. Here, we demonstrated that 17β-estradiol attenuates apoptosis of oligodendrocytes by inhibiting RhoA and c-Jun-N-terminal kinase activation after SCI. Estrogen receptor (ER)-α and -β were expressed in oligodendrocytes of the spinal cord, and 17β-estradiol treatment significantly inhibited oligodendrocyte cell death at 7 d after injury as compared with vehicle (cyclodextrin) control. 17β-Estradiol also attenuated caspase-3 and -9 activation at 7 d and reduced the loss of axons from progressive degeneration. In addition, 17β-estradiol inhibited RhoA and JNK3 activation, which were activated and peaked at 3 and/or 5 d after injury. Furthermore, administration of Rho inhibitor, PEP-1-C3 exoenzyme, inhibited RhoA and JNK3 activation, and decreased phosphorylated c-Jun level at 5 d after injury. Additionally, the attenuation of RhoA and JNK3 activation as well as oligodendrocyte cell death by 17β-estradiol was reversed by ER antagonist, ICI182780. Our results thus indicate that 17β-estradiol treatment improves functional recovery after SCI in part by reducing oligodendrocyte cell death via inhibition of RhoA and JNK3 activation, which were ER dependent. Furthermore, improvement of hindlimb motor function by posttreatment of 17β-estradiol suggests its potential as a therapeutic agent for SCI patients.

Death receptor signalling in central nervous system inflammation and demyelination

Death receptors (DRs) are members of the tumor necrosis factor receptor (TNF-R) superfamily that are characterised by the presence of a conserved intracellular death domain and are able to trigger a signalling pathway leading to apoptosis. Strong evidence suggests that DRs contribute to the pathology of tissue destructive diseases, including multiple sclerosis (MS), the most common inflammatory demyelinating disease of the central nervous system (CNS). Here, we review the evidence supporting a role for DRs in MS pathology and its implications for the development of therapeutic strategies for MS and other demyelinating pathologies of the CNS.

4-Methlycatechol prevents NGF/p75(NTR)-mediated apoptosis via NGF/TrkA system in pancreatic β cells

In this study, it was aimed to investigate whether 4-methylcatechol (4-MC) could serve as an autocrine antiapoptotic agent by increasing nerve growth factor (NGF) in β cells of hyperglycemic rats. Rats were divided into four groups: the first group was given citrate buffer and saline, the second group was administered 4-MC, the third group received streptozotocin (STZ), and the fourth group was given both 4-MC and STZ. 4-MC (10 μg/kg) was administered by daily intraperitoneal injection for 10 days before the animals were rendered hyperglycemic by administration of STZ (75 mg/kg). With 4-MC pretreatment on hyperglycemic rats the following results were noted: (i) Increase in plasma glucose, β cell apoptosis and caspase-8 activation was prevented. (ii) Reduction of NGF+ and tyrosine receptor kinase A (TrkA)+ β cell number was blocked. (iii) p75 neurotrophin receptor (p75(NTR))+ β cell number was increased. These data suggest that 4-MC might exert its antiapoptotic actions through NGF/TrkA system which may block NGF/p75(NTR) activation in pancreatic β cells of hyperglycemic rats.

Developmentally regulated ceramide synthase 6 increases mitochondrial Ca2+ loading capacity and promotes apoptosis

Ceramides, which are membrane sphingolipids and key mediators of cell-stress responses, are generated by a family of (dihydro) ceramide synthases (Lass1-6/CerS1-6). Here, we report that brain development features significant increases in sphingomyelin, sphingosine, and most ceramide species. In contrast, C(16:0)-ceramide was gradually reduced and CerS6 was down-regulated in mitochondria, thereby implicating CerS6 as a primary ceramide synthase generating C(16:0)-ceramide. Investigations into the role of CerS6 in mitochondria revealed that ceramide synthase down-regulation is associated with dramatically decreased mitochondrial Ca(2+)-loading capacity, which could be rescued by addition of ceramide. Selective CerS6 complexing with the inner membrane component of the mitochondrial permeability transition pore was detected by immunoprecipitation. This suggests that CerS6-generated ceramide could prevent mitochondrial permeability transition pore opening, leading to increased Ca(2+) accumulation in the mitochondrial matrix. We examined the effect of high CerS6 expression on cell survival in primary oligodendrocyte (OL) precursor cells, which undergo apoptotic cell death during early postnatal brain development. Exposure of OLs to glutamate resulted in apoptosis that was prevented by inhibitors of de novo ceramide biosynthesis, myriocin and fumonisin B1. Knockdown of CerS6 with siRNA reduced glutamate-triggered OL apoptosis, whereas knockdown of CerS5 had no effect: the pro-apoptotic role of CerS6 was not stimulus-specific. Knockdown of CerS6 with siRNA improved cell survival in response to nerve growth factor-induced OL apoptosis. Also, blocking mitochondrial Ca(2+) uptake or decreasing Ca(2+)-dependent protease calpain activity with specific inhibitors prevented OL apoptosis. Finally, knocking down CerS6 decreased calpain activation. Thus, our data suggest a novel role for CerS6 in the regulation of both mitochondrial Ca(2+) homeostasis and calpain, which appears to be important in OL apoptosis during brain development.

Oligodendrocyte-specific FADD deletion protects mice from autoimmune-mediated demyelination

Apoptosis of oligodendrocytes (ODCs), the myelin-producing glial cells in the CNS, plays a central role in demyelinating diseases such as multiple sclerosis and experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. To investigate the mechanism behind ODC apoptosis in EAE, we made use of conditional knockout mice lacking the adaptor protein FADD specifically in ODCs (FADD(ODC-KO)). FADD mediates apoptosis by coupling death receptors with downstream caspase activation. In line with this, ODCs from FADD(ODC-KO) mice were completely resistant to death receptor-induced apoptosis in vitro. In the EAE model, FADD(ODC-KO) mice followed an ameliorated clinical disease course in comparison with control littermates. Lymphocyte and macrophage infiltration into the spinal cord parenchyma was significantly reduced, as was the extent of demyelination and proinflammatory gene expression. Collectively, our data show that FADD is critical for ODC apoptosis and the development of autoimmune demyelinating disease.

Distinct signaling pathways of precursor BDNF and mature BDNF in cultured cerebellar granule neurons

Recent studies have focused on a distinctive contrast between bioactivities of precursor brain-derived neurotrophic factor (proBDNF) and mature BDNF (matBDNF). In this study, using a proteolytic cleavage-resistant proBDNF mutant (CR-proBDNF), signaling mechanisms underlying the proapoptotic effect of proBDNF and antiapoptotic effect of matBDNF on the low potassium (LK)-inducing cell death of cultured cerebellar granule neurons (CGNs) were analyzed. A time course study demonstrated that unlike matBDNF, CR-proBDNF failed to induce TrkB phosphorylation for up to 360 min. CR-proBDNF did not activate ERK-1, ERK-2 and Akt, which are involved in TrkB-induced cell survival signaling, while matBDNF activated these kinases. On the other hand treatment of CGNs with CR-proBDNF led to a rapid activation of Rac-GTPase and phosphorylation of JNK which are involved in p75(NTR)-induced apoptosis. In addition, a JNK-specific inhibitor, SP600125, inhibited the CR-proBDNF-induced apoptosis but did not affect the antiapoptotic effect of matBDNF. CR-proBDNF treatment led to an earlier appearance of active caspase-3. In contrast, matBDNF dramatically postponed the appearance of active caspase-3. Not like other signaling molecules, activation of caspase-3 was conversely regulated by both CR-proBDNF and matBDNF. These results thus suggest that in CGNs proBDNF elicits apoptosis via activation of p75(NTR), Rac-GTPase, JNK, and caspase-3, while matBDNF signals cell survival via activation of TrkB, ERKs and Akt, and deactivation of caspase-3.

The role of caspase-2 in stress-induced apoptosis

Caspase-2 is the most evolutionarily conserved of all the caspases, yet it has a poorly defined role in apoptotic pathways. This is mainly due to a dearth of techniques to determine the activation status of caspase-2 and the lack of an abnormal phenotype in caspase-2 deficient mice. Nevertheless, emerging evidence suggests that caspase-2 may have important functions in a number of stress-induced cell death pathways, in cell cycle maintenance and regulation of tumour progression. This review discusses recent advances that have been made to help elucidate the true role of this elusive caspase and the potential contribution of caspase-2 to the pathology of human diseases including cancer.

Signaling of the neurotrophin receptor p75 in relation to Alzheimer's disease

The cellular mechanism of neuronal apoptosis in Alzheimer's disease (AD) is poorly understood. Many hypotheses have been put fourth to explain the underlying reason for neuro-degeneration in AD. Here, it is demonstrated that all neurotrophins that activated p75, without co-activation of the relevant Trk co-receptor, mediated apoptosis in hippocampal neurons. Thus, proneurotrophins and amyloid beta peptides (Abeta) can induce p75-mediated apoptosis in hippocampal neurons since they do not bind or activate Trk receptors. Based on the combined effects of aging, proneurotrophins, neurotrophins, and Abeta, a novel model of pathogenesis in AD is proposed. This mini-review explores the ligand and cell type based signaling pathways of the neurotrophin receptor p75 relating to Alzheimer's disease.

Triple-transgenic Alzheimer's disease mice exhibit region-specific abnormalities in brain myelination patterns prior to appearance of amyloid and tau pathology

Alzheimer's disease (AD) is a progressively debilitating brain disorder pathologically defined by extracellular amyloid plaques, intraneuronal neurofibrillary tangles, and synaptic disintegrity. AD has not been widely considered a disease of white matter, but more recent evidence suggests the existence of abnormalities in myelination patterns and myelin attrition in AD-afflicted human brains. Herein, we demonstrate that triple-transgenic AD (3xTg-AD) mice, which harbor the human amyloid precursor protein Swedish mutant transgene, presenilin knock-in mutation, and tau P301L mutant transgene, exhibit significant region-specific alterations in myelination patterns and in oligodendrocyte marker expression profiles at time points preceding the appearance of amyloid and tau pathology. These immunohistochemical signatures are coincident with age-related alterations in axonal and myelin sheath ultrastructure as visualized by comparative electron microscopic examination of 3xTg-AD and nontransgenic mouse brain tissue. Overall, these findings indicate that 3xTg-AD mice represent a viable model in which to examine mechanisms underlying AD-related myelination and neural transmission defects that occur early during presymptomatic stages of the disease process.

Multiple roles of the p75 neurotrophin receptor in the nervous system

The p75 neurotrophin receptor (p75NTR) is a transmembrane protein that binds nerve growth factor (NGF) and has multiple functions in the nervous system where it is expressed widely during the developmental stages of life, although expression decreases dramatically by adulthood. Expression of p75NTR can increase in pathological states related to neural cell death. p75NTR is a member of the tumour necrosis factor (TNF) receptor family and it consists of intracellular, transmembrane and extracellular domains which are different from other TNF receptors. Either by interacting with tropomyosin receptor kinase (Trk) receptors or via the independent binding of neurotrophin, p75NTR can induce neurite outgrowth and cellular survival or cell apoptosis through several complicated signal transduction pathways. Most of these signalling pathways remain to be elucidated. By interacting with different cellular factors, p75NTR can induce neuron growth cone collapse or regrowth. p75NTR is also expressed in a variety of glial populations. The many functions of p75NTR require further study.

Differential effects of Th1, monocyte/macrophage and Th2 cytokine mixtures on early gene expression for molecules associated with metabolism, signaling and regulation in central nervous system mixed glial cell cultures

BACKGROUND

Cytokines secreted by immune cells and activated glia play central roles in both the pathogenesis of and protection from damage to the central nervous system (CNS) in multiple sclerosis (MS).

METHODS

We have used gene array analysis to identify the initial direct effects of cytokines on CNS glia by comparing changes in early gene expression in CNS glial cultures treated for 6 hours with cytokines typical of those secreted by Th1 and Th2 lymphocytes and monocyte/macrophages (M/M).

RESULTS

In two previous papers, we summarized effects of these cytokines on immune-related molecules, and on neural and glial related proteins, including neurotrophins, growth factors and structural proteins. In this paper, we present the effects of the cytokines on molecules involved in metabolism, signaling and regulatory mechanisms in CNS glia. Many of the changes in gene expression were similar to those seen in ischemic preconditioning and in early inflammatory lesions in experimental autoimmune encephalomyelitis (EAE), related to ion homeostasis, mitochondrial function, neurotransmission, vitamin D metabolism and a variety of transcription factors and signaling pathways. Among the most prominent changes, all three cytokine mixtures markedly downregulated the dopamine D3 receptor, while Th1 and Th2 cytokines downregulated neuropeptide Y receptor 5. An unexpected finding was the large number of changes related to lipid metabolism, including several suggesting a switch from diacylglycerol to phosphatidyl inositol mediated signaling pathways. Using QRT-PCR we validated the results for regulation of genes for iNOS, arginase and P glycoprotein/multi-drug resistance protein 1 (MDR1) seen at 6 hours with microarray.

CONCLUSION

Each of the three cytokine mixtures differentially regulated gene expression related to metabolism and signaling that may play roles in the pathogenesis of MS, most notably with regard to mitochondrial function and neurotransmitter signaling in glia.

The life, death, and replacement of oligodendrocytes in the adult CNS

Oligodendrocytes (OLs) are mature glial cells that myelinate axons in the brain and spinal cord. As such, they are integral to functional and efficient neuronal signaling. The embryonic lineage and postnatal development of OLs have been well-studied and many features of the process have been described, including the origin, migration, proliferation, and differentiation of precursor cells. Less clear is the extent to which OLs and damaged/dysfunctional myelin are replaced following injury to the adult CNS. OLs and their precursors are very vulnerable to conditions common to CNS injury and disease sites, such as inflammation, oxidative stress, and elevated glutamate levels leading to excitotoxicity. Thus, these cells become dysfunctional or die in multiple pathologies, including Alzheimer's disease, spinal cord injury, Parkinson's disease, ischemia, and hypoxia. However, studies of certain conditions to date have detected spontaneous OL replacement. This review will summarize current information on adult OL progenitors, mechanisms that contribute to OL death, the consequences of their loss and the pathological conditions in which spontaneous oligodendrogenesis from endogenous precursors has been observed in the adult CNS.

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.

The multiple sclerosis degradome: enzymatic cascades in development and progression of central nervous system inflammatory disease

An array of studies implicate different classes of protease and their endogenous inhibitors in multiple sclerosis (MS) pathogenesis based on expression patterns in MS lesions, sera, and/or cerebrospinal fluid (CSF). Growing evidence exists regarding their mechanistic roles in inflammatory and neurodegenerative aspects of this disease. Proteolytic events participate in demyelination, axon injury, apoptosis, and development of the inflammatory response including immune cell activation and extravasation, cytokine and chemokine activation/inactivation, complement activation, and epitope spreading. The potential significance of proteolytic activity to MS therefore relates not only to their potential use as important biomarkers of disease activity, but additionally as prospective therapeutic targets. Experimental data indicate that understanding the net physiological consequence of altered protease levels in MS development and progression necessitates understanding protease activity in the context of substrates, endogenous inhibitors, and proteolytic cascade interactions, which together make up the MS degradome. This review will focus on evidence regarding the potential physiologic role of those protease families already identified as markers of disease activity in MS; that is, the metallo-, serine, and cysteine proteases.

Sphingosine 1-phosphate in neural signalling and function

Complex sphingolipids are particularly enriched in the central nervous system. Although they were long considered to be structural components of membranes, in the last decades it has become apparent that they have other important functions. More recently, attention has been given to the sphingolipid metabolites ceramide, sphingosine and sphingosine 1-phosphate (S1P), which have been implicated in the regulation of many aspects of neuronal proliferation, differentiation, survival and apoptosis. Dysregulation of the relative levels of these bioactive sphingolipid metabolites may have implications for a wide array of neurodegenerative disorders and neural malformations. In this paper, we will focus on studies from our laboratory over the past few years using cultured neurons to examine the roles of the sphingolipid metabolite S1P in neuronal survival and differentiation.

CONCLUSION

Identification of potential intracellular targets of S1P remains a crucial objective for attaining a better understanding of the potent role this molecule plays in regulating cell fate.

Loss of p75 neurotrophin receptor expression accompanies malignant progression to human and murine retinoblastoma

We studied the expression of pro-apoptotic neurotrophin receptor p75 (p75(NTR)) in human and murine retinoblastoma, compared to normal retina, and examined changes in p75(NTR) expression with the onset of apoptosis in the course of murine retinoblastoma progression, using immunohistochemistry and quantitative real-time RT-PCR. The murine retinoblastoma is induced by retinal specific expression of SV40 T-antigen (TAg), which blocks the function of the retinoblastoma protein (pRB) and related proteins, and is a well-studied model that closely simulates human retinoblastoma. The majority of human retinoblastoma either lacked or expressed decreased levels of p75(NTR) mRNA, compared to human retina. Moreover, p75(NTR) protein was not detected in any tumor studied, unlike normal retina. Like human retinoblastoma, advanced murine retinoblastoma did not express p75(NTR). However, before tumors emerged, small clusters of TAg-positive cells coexpressed p75(NTR) and activated caspase-3, a marker of apoptosis. Furthermore, in three rare human eyes containing retinoblastoma adjacent to regions resembling the benign retinal tumor retinoma, both normal retina and retinoma-like tissue expressed p75(NTR) protein, while the retinoblastoma did not. We suggest that p75(NTR) loss accompanies progression from retinoma to retinoblastoma.

Tumour necrosis factor-induced death of adult human oligodendrocytes is mediated by apoptosis inducing factor

Tumour necrosis factor (TNF)-induced death of oligodendrocytes, the cell type targeted in multiple sclerosis, is mediated by TNF receptor p55 (TNFR-p55). The ligation of TNFR-p55 induces several signal transduction pathways; however, the precise mechanism involved in human oligodendrocyte (hOL) death is unknown. We defined that TNF-induced death of hOLs is non-caspase dependent, as evidenced by lack of generation of caspases 8, 1 and 3 active subunits; lack of cleavage of caspases 1 and 3 fluorogenic substrates; and lack of hOL death inhibition by the general caspase inhibitor, ZVAD.FMK. Electrophoresis of TNF-exposed hOL DNA revealed large-scale DNA fragmentation characteristic of apoptosis-inducing factor (AIF)-mediated cell death, and co-localization experiments showed that AIF translocation to the nucleus occurred upon exposure to TNF. AIF depletion by an antisense strategy prevented TNF-induced hOL death. These results indicate that TNF-induced death of hOLs is dependent on AIF, information of significance for the design strategies to protect hOLs during immune-mediated demyelination.

Fetal origin of adverse pregnancy outcome: the water disinfectant by-product chloroacetonitrile induces oxidative stress and apoptosis in mouse fetal brain

Epidemiological studies indicate a relationship between water disinfectant by-products (DBP) and adverse pregnancy outcomes (APO) including neural tube defects. These studies suggest that fetal brain may be vulnerable to DBP during early stages of development. Therefore, we examined several molecular markers commonly known to indicate chemical-induced neurotoxicity during fetal brain development following prenatal exposure to the DBP; chloroacetonitrile (CAN). Pregnant mice, at gestation day 6 (GD6), were treated with a daily oral dose of CAN (25 mg/kg). At GD12, two groups of animals were treated with an i.v. tracer dose of [2-14C]-CAN. These animals were sacrificed at 1 and 24 h after treatment and processed for quantitative in situ micro-whole-body autoradiography. The remaining groups of animals continued to receive CAN. At GD18, control and treated animals were weighed, anesthetized, and fetuses were obtained and their brains were removed for biochemical and immunohistochemical analyses. Whole-body autoradiography studies indicate a significant uptake and retention of [2-14C]-CAN/metabolites (M) in fetal brain (cerebral cortex, hippocampus, cerebellum) at 1 and 24 h. There was a 20% reduction in body weight and a 22% reduction in brain weight of fetuses exposed to CAN compared to controls. A significant increase in oxidative stress markers was observed in various fetal brain regions in animals exposed to CAN compared to controls. This was indicated by a 3- to 4-fold decrease in the ratio of the reduced to oxidized form of glutathione (GSH/GSSG), increased lipid peroxidation (1.3-fold), and increased 8-hydroxy-2-deoxyguanosine levels (1.4-fold). Cupric silver staining indicated a significant increase in the number of degenerating neurons in cortical regions in exposed animals. In animals exposed to CAN there was increase in nuclear DNA fragmentation (TUNEL staining) detected in the cerebral cortex and cerebellum (2-fold increase in apoptotic indices). Caspase-3 activity in cerebral cortex and cerebellum of treated animals were also increased (1.7- and 1.5-fold, respectively). In conclusion, this study indicates that CAN/M crossed the placenta and accumulated in fetal brain tissues where it caused oxidative stress and neuronal apoptosis. These events could predispose the fetus to altered brain development leading to APO as well as behavioral and learning and memory deficits.

The role of nerve growth factor receptors in cholinergic basal forebrain degeneration in prodromal Alzheimer disease

Dysfunction of nerve growth factor (NGF) and its high (TrkA) and low (p75NTR) affinity receptors has been suggested to underlie the selective degeneration of the nucleus basalis (NB) cholinergic cortical projection neurons in end stage Alzheimer disease (AD). Whether the NGF system is dysfunctional during the prodromal stages of AD has only recently been evaluated. Surprisingly, the number of choline acetyltransferase-containing neurons remains stable despite a significant reduction in NGF receptor-positive cells in people with mild cognitive impairment (MCI), suggesting a phenotypic NGF receptor downregulation but not a frank loss of NB neurons during prodromal AD. Moreover, there is a loss of cortical TrkA in the face of stable p75NTR and increased proNGF levels, the precursor molecule of mature NGF, in early AD. Depending upon the cellular context these changes may result in increased pro-apoptotic signaling, cell survival, or a defect in retrograde transport mechanisms. Alterations in NGF and its receptors within the cholinotrophic NB system in early AD suggest that NGF-mediated cell signaling is required for the longterm survival of these neurons. Therapeutic neurotrophic intervention might delay or prevent NB neuron degeneration and preserve cholinergic cortical function during prodromal AD.

Identification of oligodendrocytes in experimental disease models

The ability to identify oligodendrocytes in culture, in fixed tissue, and in vivo using unique markers is a requisite step to understanding their responses in any damage, recovery, or developmental process. Their nuclei are readily seen in histological preparations of healthy white and gray matter, and their cell bodies can be reliably identified with a variety of immunocytochemical markers. However, there is little consensus regarding optimal methods to assess oligodendrocyte survival or morphology under experimental injury conditions. We review common approaches for histological and immunocytochemical identification of these cells. Transgenic and viral methods for cell type-selective transfer of genes encoding fluorescent proteins offer promising new approaches for manipulating and visualizing oligodendrocytes in models of health and disease.

c-Cbl binds to tyrosine-phosphorylated neurotrophin receptor p75 and induces its ubiquitination

The p75 neurotrophin receptor (p75NTR) has dual functions in cell survival and cell death but its intracellular signalling pathways are not understood. Here we describe that in rat brain and in pervanadate-stimulated PCNA and HEK293 cells p75NTR is phosphorylated at a single tyrosine residue within the cytosolic C-terminus. Phosphorylated tyrosine 308 constitutes a binding site for the ubiquitin ligase c-Cbl. This interaction is a prerequisite for ubiquitination of p75NTR. Our data suggest a c-Cbl-dependent ubiquitination of p75NTR involved in the regulation of p75NTR signalling.

Expression of the low-affinity neurotrophin receptor, p75(NTR), is upregulated by oligodendroglial progenitors adjacent to the subventricular zone in response to demyelination

Precursor cells have the capacity to repopulate the demyelinated brain, but the molecular mechanisms that facilitate their recruitment are largely unknown. The low-affinity neurotrophin receptor, p75(NTR), may be one of these regulators; however, its expression profile by oligodendroglia within the multiple sclerosis (MS) brain remains uncertain. We therefore assessed the expression profile of this receptor within 8 MS and 4 control brains. We found no evidence of expression of p75(NTR) by mature oligodendrocytes. Instead, we demonstrated the presence of p75(NTR) on a subgroup of NG2-positive oligodendroglial progenitors in a periventricular plaque in one MS sample. Notably, p75(NTR)-expressing cells were also detected within the subventricular zone (SVZ) of this brain, adjacent to the periventricular plaque. In animals with experimental demyelination we observed similar patterns of p75(NTR) expression, initially confined to precursor cells within the SVZ, followed at later stages in the disease course by its expression amongst a subset of oligodendroglial progenitors within the corpus callosum. These data suggest that a population of precursor cells within the SVZ can be induced to express p75(NTR) and to subsequently assume an oligodendroglial progenitor phenotype in response to demyelination in the adjacent white matter.

Cell death mechanisms following traumatic brain injury

Neuronal and glial cell death and traumatic axonal injury contribute to the overall pathology of traumatic brain injury (TBI) in both humans and animals. In both head-injured humans and following experimental brain injury, dying neural cells exhibit either an apoptotic or a necrotic morphology. Apoptotic and necrotic neurons have been identified within contusions in the acute post-traumatic period, and in regions remote from the site of impact in the days and weeks after trauma, while degenerating oligodendrocytes and astrocytes have been observed within injured white matter tracts. We review and compare the regional and temporal patterns of apoptotic and necrotic cell death following TBI and the possible mechanisms underlying trauma-induced cell death. While excitatory amino acids, increases in intracellular calcium and free radicals can all cause cells to undergo apoptosis, in vitro studies have determined that neural cells can undergo apoptosis via many other pathways. It is generally accepted that a shift in the balance between pro- and anti-apoptotic protein factors towards the expression of proteins that promote death may be one mechanism underlying apoptotic cell death. The effect of TBI on cellular expression of survival promoting-proteins such as Bcl-2, Bcl-xL, and extracellular signal-regulated kinases, and death-inducing proteins such as Bax, c-Jun N-terminal kinase, tumor-suppressor gene, p53, and the calpain and caspase families of proteases are reviewed. In light of pharmacologic strategies that have been devised to reduce the extent of apoptotic cell death in animal models of TBI, our review also considers whether apoptosis may serve a protective role in the injured brain. Together, these observations suggest that cell death mechanisms may be representative of a continuum between apoptotic and necrotic pathways.

Mitochondrial pathway is involved in hydrogen-peroxide-induced apoptotic cell death of oligodendrocytes

Oligodendrocytes, the myelin-forming cells of the CNS, are specifically sensitive to oxidative stress and respond by the onset of programmed cell death (PCD). To further unravel the molecular events underlying their enhanced susceptibility, we have investigated whether mitochondrial damage occurs during oxidative stress-induced PCD in cultured rat brain oligodendrocytes. Mitochondria are considered as a central control point of apoptosis, and mitochondrial dysfunction has been linked to neurodegenerative disease. Upon a number of stimuli through the release of cytochrome c, they coordinate caspase activation, causing morphological and biochemical changes associated with PCD. Oxidative stress was exerted by the application of hydrogen peroxide. The data show that hydrogen peroxide-induced apoptosis in oligodendrocytes involves mitochondrial damage and cytochrome c release and is accompanied by the activation of the death-related caspases 3 and 9. Concomitantly, the activation and nuclear translocation of extracellular signal regulated kinases ERK1,2 are observed, which have been implicated to participate in the regulation of cell death and survival. DNA fragmentation could not be attenuated by the ERK1,2 inhibitor PD 98059, indicating that the ERK1,2- pathway in oligodendrocytes may be involved in the initial survival response after exposure to stressful stimuli.

Degenerative and regenerative mechanisms governing spinal cord injury

Spinal cord injury (SCI) is a major cause of disability, and at present, there is no universally accepted treatment. The functional decline following SCI is contributed to both direct mechanical injury and secondary pathophysiological mechanisms that are induced by the initial trauma. These mechanisms initially involve widespread haemorrhage at the site of injury and necrosis of central nervous system (CNS) cellular components. At later stages of injury, the cord is observed to display reactive gliosis. The actions of astrocytes as well as numerous other cells in this response create an environment that is highly nonpermissive to axonal regrowth. Also manifesting important effects is the immune system. The early recruitment of neutrophils and at later stages, macrophages to the site of insult cause exacerbation of injury. However, at more chronic stages, macrophages and recruited T helper cells may potentially be helpful by providing trophic support for neuronal and non-neuronal components of the injured CNS. Within this sea of injurious mechanisms, the oligodendrocytes appear to be highly vulnerable. At chronic stages of SCI, a large number of oligodendrocytes undergo apoptosis at sites that are distant to the vicinity of primary injury. This leads to denudement of axons and deterioration of their conductive abilities, which adds significantly to functional decline. By indulging into the molecular mechanisms that cause oligodendrocyte apoptosis and identifying potential targets for therapeutic intervention, the prevention of this apoptotic wave will be of tremendous value to individuals living with SCI.

AMPA receptors are the major mediators of excitotoxic death in mature oligodendrocytes

Myelination of axons is important for central nervous system function, but oligodendrocytes, which constitute CNS myelin, are vulnerable to excitotoxic injury and death. Although mature oligodendrocytes express functional alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) and kainate-type glutamate receptors, the relative roles of these subtypes in excitotoxicity are not well understood. Using recently developed selective antagonists for subtypes of ionotropic non-NMDA receptors, we addressed this issue. By examining the pharmacological, biochemical, and morphologic features of kainite-induced excitotoxic death, we also determined whether it occurs by apoptosis, necrosis, or both. We conclude that when mature oligodendrocytes die after exposure to kainate: (1) AMPA receptors are the most important mediators, (2) kainate receptors play a smaller role, and (3) death occurs predominantly by necrosis, not apoptosis.

The role of neurotransmission and the Chopper domain in p75 neurotrophin receptor death signaling

The role of p75 neurotrophin receptor (p75NTR) in mediating cell death is now well characterized, however, it is only recently that details of the death signaling pathway have become clearer. This review focuses on the importance of the juxtamembrane Chopper domain region of p75NTR in this process. Evidence supporting the involvement of K+ efflux, the apoptosome (caspase-9, apoptosis activating factor-1, APAF-1, and Bcl-xL), caspase-3, c-jun kinase, and p53 in the p75NTR cell death pathway is discussed and regulatory roles for the p75NTR ectodomain and death domain are proposed. The role of synaptic activity is also discussed, in particular the importance of neutrotransmitter-activated K+ channels acting as the gatekeepers of cell survival decisions during development and in neurodegenerative conditions.

The p75 neurotrophin receptor: multiple interactors and numerous functions

The neurotrophin receptor p75 (p75NTR), is involved in a diverse array of cellular responses, including apoptosis, neurite outgrowth and myelination. Stimulation of p75NTR with neurotrophin can activate multiple downstream signals, including the small GTP binding protein Rac, the transcription factor NF-kappa B and the stress activated kinase, JNK. How these signals are generated and regulated to produce a specific cellular effect has yet to be fully elucidated. A number of proteins have recently been shown to interact with the intracellular domain of p75NTR. Here, we review these p75NTR interacting factors and the current evidence as to how they contribute to the functional effects of p75NTR activation.

p75 neurotrophin receptor signaling in the nervous system

The neurotrophin receptor p75(NTR) has long been known as a receptor for neurotrophins that promote survival and differentiation. Consistent with the role of neurotrophins, p75(NTR) is expressed during the developmental stages of the nervous system. However, p75(NTR) is re-expressed in various pathological conditions in the adult. We now know that p75(NTR) has the ability to elicit bi-directional signals, that result in the inhibition as well as the promotion of the neurite outgrowth. p75(NTR) is a key receptor for myelin-derived inhibitory cues that contribute to the lack of regeneration of the central nervous system.

Apoptosis induced by p75NTR overexpression requires Jun kinase-dependent phosphorylation of Bad

The p75 neurotrophin receptor (p75NTR), a member of the tumor necrosis factor receptor superfamily, facilitates apoptosis during development and after injury to the CNS. The signaling cascades activated by p75NTR that result in apoptosis remain poorly understood. In this study, we show that overexpression of p75NTR in primary cortical neurons, in pheochromocytoma cell line (PC12) cells, and in glioma cells results in activation of Jun kinase (JNK), accumulation of cytochrome c within the cytosol, and activation of caspases 9, 6, and 3. To link p75NTR-dependent JNK activation to mitochondrial cytochrome c release, regulation of BH3-domain-only family members was examined. Transcription of BH3-domain-only family members was not induced by p75NTR, but p75NTR-dependent JNK activation resulted in phosphorylation and oligomerization of the BH3-domain-only family member Bad. Loss of function experiments using Bad dominant negatives or RNA interference demonstrated a requirement for Bad in p75NTR-induced apoptosis. Together, these studies provide the first data linking apoptosis induced by p75NTR to the phosphorylation of BH3-domain-only family members.

Neurotrophins regulate proliferation and survival of two microglial cell lines in vitro

Microglia are thought to play a key role in the development and regeneration of the central nervous system although the mechanisms regulating their presence and activity are not fully understood. Substantial evidence suggests that members of the neurotrophin family such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 and -4 (NT-3/4) have a dramatic effect on both neurons and perineuronal cells. This study employed two murine microglial lines, BV-2 and N9, to examine the action of these neurotrophins on the mitotic activity and survival of microglia in vitro. Neurotrophins were incorporated into the media at the time of plating and cell number and levels of mitochondrial dehydrogenase activity (MTT) were determined at various time points in vitro. NGF increased cell number and MTT levels of both cell lines in a dose-dependent manner. BV-2 was more sensitive to NGF than N9. Similar responses were elicited by BDNF, although the sensitivity of each cell line was different than that found for NGF. NT-3 and NT-4 had no effect on cell proliferation. However, NT-4 had an effect on the survival of BV-2 and N9 cells. The response of these cells to neurotrophins was blocked by K252a, a tyrosine kinase inhibitor, suggesting that actions of neurotrophins were mediated by high-affinity tyrosine kinase receptors (Trk). Immunolocalization studies revealed positive Trk (pan) reactivity in the above cell lines and in primary microglia, but an absence of the low-affinity p75 neurotrophin receptor. Western blot analysis supported the above observations. These studies suggest that in addition to their neurotrophic actions, NGF and BDNF may also regulate microglial dynamics, thereby influencing the surrounding milieu during neuronal regeneration.