proBDNF and p75NTR Control Excitability and Persistent Firing of Cortical Pyramidal Neurons

Selenoprotein I is indispensable for ether lipid homeostasis and proper myelination

Selenoprotein I (SELENOI) catalyzes the final reaction of the CDP-ethanolamine branch of the Kennedy pathway, generating the phospholipids phosphatidylethanolamine (PE) and plasmenyl-PE. Plasmenyl-PE is a key component of myelin and is characterized by a vinyl ether bond that preferentially reacts with oxidants, thus serves as a sacrificial antioxidant. In humans, multiple loss-of-function mutations in genes affecting plasmenyl-PE metabolism have been implicated in hereditary spastic paraplegia, including SELENOI. Herein, we developed a mouse model of nervous system-restricted SELENOI deficiency that circumvents embryonic lethality caused by constitutive deletion and recapitulates phenotypic features of hereditary spastic paraplegia. Resulting mice exhibited pronounced alterations in brain lipid composition, which coincided with motor deficits and neuropathology including hypomyelination, elevated reactive gliosis, and microcephaly. Further studies revealed increased lipid peroxidation in oligodendrocyte lineage cells and disrupted oligodendrocyte maturation both in vivo and in vitro. Altogether, these findings detail a critical role for SELENOI-derived plasmenyl-PE in myelination that is of paramount importance for neurodevelopment.

The functional and molecular roles of p75 neurotrophin receptor (p75NTR) in epilepsy

Epilepsy is a chronic neurological disorder manifested by recurring unprovoked seizures resulting from an imbalance in the inhibitory and excitatory neurotransmitters in the brain. The process of epileptogenesis involves a complex interplay between the reduction of inhibitory gamma-aminobutyric acid (GABA) and the enhancement of excitatory glutamate. Pro-BDNF/p75NTR expression is augmented in both glial cells and neurons following epileptic seizures and status epileptics (SE). Over-expression of p75NTR is linked with the pathogenesis of epilepsy, and augmentation of pro-BDNF/p75NTR is implicated in the pathogenesis of epilepsy. However, the precise mechanistic function of p75NTR in epilepsy has not been completely elucidated. Therefore, this review aimed to revise the mechanistic pathway of p75NTR in epilepsy.

Extracellular mixed histones are neurotoxic and modulate select neuroimmune responses of glial cells

Although histone proteins are widely known for their intranuclear functions where they organize DNA, all five histone types can also be released into the extracellular space from damaged cells. Extracellular histones can interact with pattern recognition receptors of peripheral immune cells, including toll-like receptor 4 (TLR4), causing pro-inflammatory activation, which indicates they may act as damage-associated molecular patterns (DAMPs) in peripheral tissues. Very limited information is available about functions of extracellular histones in the central nervous system (CNS). To address this knowledge gap, we applied mixed histones (MH) to cultured cells modeling neurons, microglia, and astrocytes. Microglia are the professional CNS immunocytes, while astrocytes are the main support cells for neurons. Both these cell types are critical for neuroimmune responses and their dysregulated activity contributes to neurodegenerative diseases. We measured effects of extracellular MH on cell viability and select neuroimmune functions of microglia and astrocytes. MH were toxic to cultured primary murine neurons and also reduced viability of NSC-34 murine and SH-SY5Y human neuron-like cells in TLR4-dependent manner. MH did not affect the viability of resting or immune-stimulated BV-2 murine microglia or U118 MG human astrocytic cells. When applied to BV-2 cells, MH enhanced secretion of the potential neurotoxin glutamate, but did not modulate the release of nitric oxide (NO), tumor necrosis factor-α (TNF), C-X-C motif chemokine ligand 10 (CXCL10), or the overall cytotoxicity of lipopolysaccharide (LPS)- and/or interferon (IFN)-γ-stimulated BV-2 microglial cells towards NSC-34 neuron-like cells. We demonstrated, for the first time, that MH downregulated phagocytic activity of LPS-stimulated BV-2 microglia. However, MH also exhibited protective effect by ameliorating the cytotoxicity of LPS-stimulated U118 MG astrocytic cells towards SH-SY5Y neuron-like cells. Our data demonstrate extracellular MH could both damage neurons and alter neuroimmune functions of glial cells. These actions of MH could be targeted for treatment of neurodegenerative diseases.

The Possible Role of Brain-derived Neurotrophic Factor in Epilepsy

Epilepsy is a neurological disease characterized by repeated seizures. Despite of that the brain-derived neurotrophic factor (BDNF) is implicated in the pathogenesis of epileptogenesis and epilepsy, BDNF may have a neuroprotective effect against epilepsy. Thus, the goal of the present review was to highlight the protective and detrimental roles of BDNF in epilepsy. In this review, we also try to find the relation of BDNF with other signaling pathways and cellular processes including autophagy, mTOR pathway, progranulin (PGN), and α-Synuclein (α-Syn) which negatively and positively regulate BDNF/tyrosine kinase receptor B (TrkB) signaling pathway. Therefore, the assessment of BDNF levels in epilepsy should be related to other neuronal signaling pathways and types of epilepsy in both preclinical and clinical studies. In conclusion, there is a strong controversy concerning the potential role of BDNF in epilepsy. Therefore, preclinical, molecular, and clinical studies are warranted in this regard.

Acetylcholine synergizes with netrin-1 to drive persistent firing in the entorhinal cortex

The ability of the mammalian brain to maintain spatial representations of external or internal information for short periods of time has been associated with sustained neuronal spiking and reverberatory neural network activity in the medial entorhinal cortex. Here, we show that conditional genetic deletion of netrin-1 or the netrin receptor deleted-in-colorectal cancer (DCC) from forebrain excitatory neurons leads to deficits in short-term spatial memory. We then demonstrate that conditional deletion of either netrin-1 or DCC inhibits cholinergic persistent firing and show that cholinergic activation of muscarinic receptors expressed by entorhinal cortical neurons promotes persistent firing by recruiting DCC to the plasma membrane. Together, these findings indicate that normal short-term spatial memory function requires the synergistic actions of acetylcholine and netrin-1.

Pro-neuroinflammatory and neurotoxic potential of extracellular histones H1 and H3

Histones organize DNA within cellular nuclei, but they can be released from damaged cells. In peripheral tissues extracellular histones act as damage-associated molecular patterns (DAMPs) inducing pro-inflammatory activation of immune cells. Limited studies have considered DAMP-like activity of histones in the central nervous system (CNS); therefore, we studied the effects of extracellular histones on microglia, the CNS immunocytes, and on neuronal cells. Both the linker histone H1 and the core histone H3 induced pro-inflammatory activation of microglia-like cells by upregulating their secretion of NO and cytokines, including interferon-γ-inducible protein 10 (IP-10) and tumor necrosis factor-α (TNF). The selective inhibitors MMG-11 and TAK-242 were used to demonstrate involvement of toll-like receptors (TLR) 2 and 4, respectively, in H1-induced NO secretion by BV-2 microglia. H1, but not H3, downregulated the phagocytic activity of BV-2 microglia. H1 was also directly toxic to all neuronal cell types studied. We conclude that H1, and to a lesser extent H3, when released extracellularly, have the potential to act as a CNS DAMPs. Inhibition of the DAMP-like effects of extracellular histones on microglia and their neurotoxic activity represents a potential strategy for combating neurodegenerative diseases that are characterized by the adverse activation of microglia and neuronal death.

Neuroprotection of exercise: P2X4R and P2X7R regulate BDNF actions

The neurotrophin brain-derived neurotrophic factor (BDNF), which acts as a transducer, is responsible for improving cerebral stroke, neuropathic pain, and depression. Exercise can alter extracellular nucleotide levels and purinergic receptors in central nervous system (CNS) structures. This inevitably activates or inhibits the expression of BDNF via purinergic receptors, particularly the P2X receptor (P2XR), to alleviate pathological progression. In addition, the significant involvement of sensitive P2X4R in mediating increased BDNF and p38-MAPK for intracerebral hemorrhage and pain hypersensitivity has been reported. Moreover, archetypal P2X7R blockade induces mouse antidepressant-like behavior and analgesia by BDNF release. This review summarizes BDNF-mediated neural effects via purinergic receptors, speculates that P2X4R and P2X7R could be priming molecules in exercise-mediated changes in BDNF, and provides strategies for the protective mechanism of exercise in neurogenic disease.

The BDNF Val66Met Polymorphism is a Relevant, But not Determinant, Risk Factor in the Etiology of Neuropsychiatric Disorders - Current Advances in Human Studies: A Systematic Review

Brain-derived neurotrophic factor (BDNF) is the brain's most-produced neurotrophin during the lifespan, essentially involved in multiple mechanisms of nervous system development and function. The production/release of BDNF requires multi-stage processing that appears to be regulated at various stages in which the presence of a polymorphism "Val66Met" can exert a critical influence.

Aim

To synthesize the knowledge on the BDNF Val66Met polymorphism on intracellular processing and function of BDNF.

Methods

We performed a systematic review and collected all available studies on the post-translation processes of BDNF, regarding the Val66Met polymorphism. Searches were performed up to 21st March 2021.

Results

Out of 129 eligible papers, 18 studies addressed or had findings relating to BDNF post-translation processes and were included in this review.

Discussion

Compilation of experimental findings reveals that the Val66Met polymorphism affects BDNF function by slightly altering the processing, distribution, and regulated release of BDNF. Regarding the critical role of pro-BDNF as a pro-apoptotic factor, such alteration might represent a risk for the development of neuropsychiatric disorders.

P75 neurotrophin receptor as a therapeutic target for drug development to treat neurological diseases

The interaction of neurotrophins with their receptors is involved in the pathogenesis and progression of various neurological diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, spinal cord injury and acute and chronic cerebral damage. The p75 neurotrophin receptor (p75NTR) plays a pivotal role in the development of neurological dysfunctions as a result of its high expression, abnormal processing and signalling. Therefore, p75NTR represents as a vital therapeutic target for the treatment of neurodegeneration, neuropsychiatric disorders and cerebrovascular insufficiency. This review summarizes the current research progress on the p75NTR signalling in neurological deficits. We also summarize the present therapeutic approaches by genetically and pharmacologically targeting p75NTR for the attenuation of pathological changes. Based on the evolving knowledge, the role of p75NTR in the regulation of tau hyperphosphorylation, Aβ metabolism, the degeneration of motor neurons and dopaminergic neurons has been discussed. Its position as a biomarker to evaluate the severity of diseases and as a druggable target for drug development has also been elucidated. Several prototype small molecule compounds were introduced to be crucial in neuronal survival and functional recovery via targeting p75NTR. These small molecule compounds represent desirable agents in attenuating neurodegeneration and cell death as they abolish activation-induced neurotoxicity of neurotrophins via modulating p75NTR signalling. More comprehensive and in-depth investigations on p75NTR-based drug development are required to shed light on effective treatment of numerous neurological disorders.

Aluminium oxide nanoparticles compromise spatial memory performance and proBDNF-mediated neuronal function in the hippocampus of rats

Background

Alumina nanoparticles (aluminaNPs), which are widely used in a range of daily and medical fields, have been shown to penetrate blood-brain barrier, and distribute and accumulate in different brain areas. Although oral treatment of aluminaNPs induces hippocampus-dependent learning and memory impairments, characteristic effects and exact mechanisms have not been fully elucidated. Here, male adult rats received a single bilateral infusion of aluminaNPs (10 or 20 µg/kg of body weight) into the hippocampal region, and their behavioral performance and neural function were assessed.

Results

The results indicated that the intra-hippocampus infusions at both doses of aluminaNPs did not cause spatial learning inability but memory deficit in the water maze task. This impairment was attributed to the effects of aluminaNP on memory consolidation phase through activation of proBDNF/RhoA pathway. Inhibition of the increased proBDNF by hippocampal infusions of p75^NTR antagonist could effectively rescue the memory impairment. Incubation of aluminaNPs exaggerated GluN2B-dependent LTD induction with no effects on LTD expression in hippocampal slices. AluminaNP could also depress the amplitude of NMDA-GluN2B EPSCs. Meanwhile, increased reactive oxygen specie production was reduced by blocking proBDNF-p75^NTR pathway in the hippocampal homogenates. Furthermore, the neuronal correlate of memory behavior was drastically weakened in the aluminaNP-infused groups. The dysfunction of synaptic and neuronal could be obviously mitigated by blocking proBDNF receptor p75^NTR, implying the involvement of proBDNF signaling in aluminaNP-impaired memory process.

Conclusions

Taken together, our findings provide the first evidence that the accumulation of aluminaNPs in the hippocampus exaggeratedly activates proBDNF signaling, which leads to neural and memory impairments.

Prelimbic proBDNF Facilitates Retrieval-Dependent Fear Memory Destabilization by Regulation of Synaptic and Neural Functions in Juvenile Rats

Fear regulation changes as a function of the early life is a key developmental period for the continued maturation of fear neural circuitry. The mechanisms of fear retrieval-induced reconsolidation have been investigated but remain poorly understood. The involvement of prelimbic proBDNF in fear memory extinction and its mediated signaling have been reported previously. Specifically, blocking the proBDNF/p75^NTR pathway during the postnatal stage disrupts synaptic development and neuronal activity in adulthood. Given the inherent high expression of proBDNF during the juvenile period, we tested whether the prelimbic proBDNF regulated synaptic and neuronal functions allowing to influencing retrieval-dependent memory processing. By examining the freezing behavior of auditory fear-conditioned rats, we found the high level of the prelimbic proBDNF in juvenile rats enhanced the destabilization of the retrieval-dependent weak but not strong fear memory through activating p75^NTR-GluN2B signaling. This modification of fear memory traces was attributed to the increment in the proportion of thin-type spine and promotion in synaptic function, as evidenced by the facilitation of NMDA-mediated EPSCs and GluN2B-dependent synaptic depression at the prelimbic projection. Furthermore, the strong prelimbic theta- and gamma-oscillation coupling predicted the suppressive effect of juvenile proBDNF on the recall of postretrieval memory. Our results critically emphasize the importance of developmental proBDNF for modification of retrieval-dependent memory and provide a potential critical targeting to inhibit threaten memories associated with neurodevelopment disorders.

Fading memories in aging and neurodegeneration: Is p75 neurotrophin receptor a culprit?

Aging and age-related neurodegenerative diseases have become one of the major concerns in modern times as cognitive abilities tend to decline when we get older. It is well known that the main cause of this age-related cognitive deficit is due to aberrant changes in cellular, molecular circuitry and signaling pathways underlying synaptic plasticity and neuronal connections. The p75 neurotrophin receptor (p75^NTR) is one of the important mediators regulating the fate of the neurons in the nervous system. Its importance in neuronal apoptosis is well documented. However, the mechanisms involving the regulation of p75^NTR in synaptic plasticity and cognitive function remain obscure, although cognitive impairment has been associated with a higher expression of p75^NTR in neurons. In this review, we discuss the current understanding of how neurons are influenced by p75^NTR function to maintain normal neuronal synaptic strength and connectivity, particularly to support learning and memory in the hippocampus. We then discuss the age-associated alterations in neurophysiological mechanisms of synaptic plasticity and cognitive function. Furthermore, we also describe current evidence that has begun to elucidate how p75^NTR regulates synaptic changes in aging and age-related neurodegenerative diseases, focusing on the hippocampus. Elucidating the role that p75^NTR signaling plays in regulating synaptic plasticity will contribute to a better understanding of cognitive processes and pathological conditions. This will in turn provide novel approaches to improve therapies for the treatment of neurological diseases in which p75^NTR dysfunction has been demonstrated.

Sexual Dimorphism in Balance and Coordination in p75NTRexonIII Knock-Out Mice

The p75 neurotrophin receptor (p75NTR) is implicated in various biological functions during development and adulthood. Several animal models have been developed to identify the roles of p75NTR in vivo and in vitro. P75NTR^ExonIII knock-out mice are widely used to study the neurotrophin receptor and its signaling pathways. Similar to other models of p75NTR knock-out (p75NTR^{Exon IV} KO) or conditional knock-out (p75NTR^fl/fl) mice, p75NTR^ExonIII knock-out mice present severe abnormalities in walking, gait, balance and strength. The present study identifies a sexual dimorphism in the p75NTR^ExonIII knock-out strain regarding balance and coordination. Using Kondziela’s inverted grid test, we observed that p75NTR^ExonIII knock-out males performed poorly at the task, whereas p75NTR^ExonIII knock-out females did not exhibit any defects. We also observed that female p75NTR^ExonIII knock-out mice performed significantly better than male p75NTR^ExonIII knock-out mice at the beam balance test. There were no differences in strength, skin innervation, or the number of ulcers on the toes between p75NTR^ExonIII knock-out males and females. The literature regarding the role of p75NTR in behavior is controversial; our results suggest that studies investigating the role of p75NTR in vivo using p75NTR knock-out mice should systematically report data from males and females.

The Role of Brain-Derived Neurotrophic Factor in Epileptogenesis: an Update

Epilepsy, which is characterized by spontaneous recurrent seizures, is one of the most common and serious chronic neurological diseases in the world. 30% patients failed to control seizures with multiple anti-seizure epileptic drugs, leading to serious outcomes. The pathogenesis of epilepsy is very complex and remains unclear. Brain-derived neurotrophic factor (BDNF), as a member of the neurotrophic factor family, is considered to play an important role in the survival, growth and differentiation of neurons during the development of the central nervous system. Recent years, a series of studies have reported that BDNF can maintain the function of the nervous system and promotes the regeneration of neurons after injury, which is believed to be closely related to epileptogenesis. However, two controversial views (BDNF inhibits or promotes epileptogenesis) still exist. Thus, this mini-review focuses on updating the new evidence of the role of BDNF in epileptogenesis and discussing the possibility of BDNF as an underlying target for the treatment of epilepsy.

Negative regulation by proBDNF signaling of peripheral neurogenesis in the sensory ganglia of adult rats

Neurogenesis in the adult brain is well recognized and plays a critical role in the maintenance of brain function and homeostasis. However, whether neurogenesis also occurs in the adult peripheral nervous system remains unknown. Here, using sensory ganglia (dorsal root ganglia, DRGs) as a model, we show that neurogenesis also occurs in the peripheral nervous system, but in a manner different from that in the central nervous system. Satellite glial cells (SGCs) express the neuronal precursor markers Nestin, POU domain, class 4, transcription factor 1, and p75 pan-neurotrophin receptor. Following sciatic nerve injury, the suppression of endogenous proBDNF by proBDNF antibodies resulted in the transformation of proliferating SGCs into doublecortin-positive cells in the DRGs. Using purified SGCs migrating out from the DRGs, the inhibition of endogenous proBDNF promoted the conversion of SGCs into neuronal phenotypes in vitro. Our findings suggest that SGCs are neuronal precursors, and that proBDNF maintains the SGC phenotype. Furthermore, the suppression of proBDNF signaling is necessary for neuronal phenotype acquisition by SGCs. Thus, we propose that peripheral neurogenesis may occur via the direct conversion of SGCs into neurons, and that this process is negatively regulated by proBDNF.

Enhanced pro-BDNF-p75NTR pathway activity in denervated skeletal muscle

AIMS

Brain derived neurotrophic factor (BDNF) and the related receptors TrkB and p75NTR are expressed in skeletal muscle, yet their functions remain to be fully understood. Skeletal muscle denervation, which occurs in spinal injury, peripheral neuropathies, and aging, negatively affects muscle mass and function. In this study, we wanted to understand the role of BDNF, TrkB, and p75NTR in denervation-induced adverse effects on skeletal muscle.

MAIN METHODS

Mice with unilateral sciatic denervation were used. Protein levels of pro- and mature BDNF, TrkB, p75NTR, activations of their downstream signaling pathways, and inflammation in the control and denervated muscle were measured with Western blot and tissue staining. Treatment with a p75NTR inhibitor and BDNF skeletal muscle specific knockout in mice were used to examine the role of p75NTR and pro-BDNF.

KEY FINDINGS

In denervated muscle, pro-BDNF and p75NTR were significantly upregulated, and JNK and NF-kB, two major downstream signaling pathways of p75NTR, were activated, along with muscle atrophy and inflammation. Inhibition of p75NTR using LM11A-31 significantly reduced JNK activation and inflammatory cytokines in the denervated muscle. Moreover, skeletal muscle specific knockout of BDNF reduced pro-BDNF level, JNK activation and inflammation in the denervated muscle.

SIGNIFICANCE

These results reveal for the first time that the upregulation of pro-BDNF and activation of p75NTR pathway are involved in denervation-induced inflammation in skeletal muscle. The results suggest that inhibition of pro-BDNF-p75NTR pathway can be a new target to treat skeletal muscle inflammation.

In vivo functions of p75NTR: challenges and opportunities for an emerging therapeutic target

The p75 neurotrophin receptor (p75^NTR) functions at the molecular nexus of cell death, survival, and differentiation. In addition to its contribution to neurodegenerative diseases and nervous system injuries, recent studies have revealed unanticipated roles of p75^NTR in liver repair, fibrinolysis, lung fibrosis, muscle regeneration, and metabolism. Linking these various p75^NTR functions more precisely to specific mechanisms marks p75^NTR as an emerging candidate for therapeutic intervention in a wide range of disorders. Indeed, small molecule inhibitors of p75^NTR binding to neurotrophins have shown efficacy in models of Alzheimer's disease (AD) and neurodegeneration. Here, we outline recent advances in understanding p75^NTR pleiotropic functions in vivo, and propose an integrated view of p75^NTR and its challenges and opportunities as a pharmacological target.

Long-Term Social Isolation Reduces Expression of the BDNF Precursor and Prolyl Endopeptidase in the Rat Brain

Early-life stress is a risk factor for the development of behavioral and cognitive disorders in humans and animals. Such stressful situations include social isolation in early postnatal ontogenesis. Behavioral and cognitive impairments associated with neuroplastic changes in brain structures. We have found that after ten weeks of social isolation, male Wistar rats show behavioral abnormalities and cognitive deficit, accompanied by an increase in the relative expression of gene encoding serine protease prolyl endopeptidase (PREP, EC 3.4.21.26) in the brain frontal cortex. The present study aimed to assess synaptophysin (SYP), brain-derived neurotrophic factor precursor (proBDNF), and PREP expression using Western blot in the brain structures - the hippocampus, frontal cortex, and striatum of the rats subjected to prolonged social isolation compared with group-housed animals. Twenty Wistar rats were used for this study (10 males and 10 females). Experimental animals (5 males and 5 females) were kept one per cage for nine months, starting from the age of one month. Ten-month-old socially isolated rats showed memory deficit in passive avoidance paradigm and Morris Water Maze and reactivity to novelty reduction. We used monoclonal antibodies for the Western blot analysis of the expression of SYP, proBDNF, and PREP in the rat brain structures. Social isolation caused a proBDNF expression reduction in the frontal cortex in females and a reduction in PREP expression in the striatum in males. These data suppose that neurotrophic factors and PREP are involved in the mechanisms of behavioral and cognitive impairments observed in the rats subjected to prolonged social isolation with an early life onset.

ProBDNF induces sustained elevation of intracellular Ca2+ possibly mediated by TRPM7 channels in rodent microglial cells

Microglia are intrinsic immune cells that release factors including pro- and anti-inflammatory cytokines, nitric oxide (NO) and neurotrophins following activation in the brain. Elevation of intracellular Ca²⁺ concentration ([Ca²⁺ ]i) is important for microglial functions, such as the release of cytokines or NO from activated microglia. Brain-derived neurotrophic factor (BDNF) is a neurotrophin well known for its roles in the activation of microglia. Interestingly, proBDNF, the precursor form of mature BDNF, and mature BDNF elicit opposing neuronal responses in the brain. Mature BDNF induces sustained intracellular Ca²⁺ elevation through the upregulation of the surface expression of TRPC3 channels in rodent microglial cells. In addition, TRPC3 channels are important for the BDNF-induced suppression of NO production in activated microglia. In this study, we observed that proBDNF and mature BDNF have opposite effects on the relative expression of surface p75 neurotrophin receptor (p75^NTR ) in rodent microglial cells. ProBDNF induces a sustained elevation of [Ca²⁺ ]i through binding to the p75^NTR , which is possibly mediated by Rac 1 activation and TRPM7 channels in rodent microglial cells. Flow cytometry showed that proBDNF increased the relative surface expression of TRPM7. Although proBDNF did not affect either mRNA expression of pro- and anti-inflammatory cytokines or the phagocytic activity, proBDNF potentiates the generation of NO induced by IFN-γ and TRPM7 channels could be involved in the proBDNF-induced potentiation of IFN-γ-mediated production of NO. We show direct evidence that rodent microglial cells are able to respond to proBDNF, which might be important for the regulation of inflammatory responses in the brain.

Robustly High Hippocampal BDNF levels under Acute Stress in Mice Lacking the Full-length p75 Neurotrophin Receptor

BACKGROUND

Brain-derived neurotrophic factor (BDNF) exerts its effects on neural plasticity via 2 distinct receptor types, the tyrosine kinase TrkB and the p75 neurotrophin receptor (p75NTR). The latter can promote inflammation and cell death while TrkB is critically involved in plasticity and memory, particularly in the hippocampus. Acute and chronic stress have been associated with suppression of hippocampal BDNF expression and impaired hippocampal plasticity. We hypothesized that p75NTR might be involved in the hippocampal stress response, in particular in stress-induced BDNF suppression, which might be accompanied by increased neuroinflammation.

METHOD

We assessed hippocampal BDNF protein concentrations in wild-type mice compared that in mice lacking the long form of the p75NTR (p75NTR^ExIII-/-) with or without prior exposure to a 1-hour restraint stress challenge. Hippocampal BDNF concentrations were measured using an optimized ELISA. Furthermore, whole-brain mRNA expression of pro-inflammatory interleukin-6 (Il6) was assessed with RT-PCR.

RESULTS

Deletion of full-length p75NTR was associated with higher hippocampal BDNF protein concentration in the stress condition, suggesting persistently high hippocampal BDNF levels in p75NTR-deficient mice, even under stress. Stress elicited increased whole-brain Il6 mRNA expression irrespective of genotype; however, p75NTR^ExIII-/- mice showed elevated baseline Il6 expression and thus a lower relative increase.

CONCLUSIONS

Our results provide evidence for a role of p75NTR signaling in the regulation of hippocampal BDNF levels, particularly under stress. Furthermore, p75NTR signaling modulates baseline but not stress-related Il6 gene expression in mice. Our findings implicate p75NTR signaling as a potential pathomechanism in BDNF-dependent modulation of risk for neuropsychiatric disorders.

Involvement of proBDNF in Monocytes/Macrophages with Gastrointestinal Disorders in Depressive Mice

Major depressive disorders (MDD) are often comorbid with the gastrointestinal (GI) disorders. Brain-derived neurotrophic factor precursor (proBDNF) has been reported to contribute to the development of depression in mouse models. However, the role of proBDNF in depression-associated GI disorders is still unrevealed. Mice experienced unpredictable chronic mild stress (UCMS) procedure and were then intraperitoneally injected with fluoxetine (20 mg/kg). Open field test (OFT), forced swimming test (FST), and sucrose preference test (SPT) were performed to evaluate the severity of depression. Oral administration of food dye gel and histological staining were performed to assess GI transit and morphological alterations. QPCR was performed to assess the mRNA levels of inflammatory cytokines. Additionally, flow cytometry, immunohistochemistry, and immunofluorescence were performed to examine the expression and cellular localization of proBDNF. It was found that (a) in the peripheral blood, the expression of proBDNF and its receptor pan neurotrophin receptor 75 (p75^NTR) in CD11b⁺ cells in depressive mice was higher than in controls; (b) the GI motility was decreased after the UCMS procedure and partly reversed by fluoxetine treatment; (c) proBDNF/p75^NTR was highly expressed in macrophages in the intestinal lamina propria; (d) the upregulated proBDNF/p75^NTR and the activated cytokines, including IL (interleukin)-1β, IL-6, IL-10, and IFN (interferon)-γ, were positively correlated with the depression and GI disorders, and were inhibited by fluoxetine treatment. UCMS procedure upregulated the expression of proBDNF and p75^NTR in monocytes/macrophages of peripheral blood and intestinal lamina propria, which may be involved in the pathogenesis of depression-associated GI disorders. Fluoxetine reversed the GI dysfunction, infiltration of macrophages, and upregulation of proBDNF signaling in the depressive mice.

Astrocytic BDNF and TrkB regulate severity and neuronal activity in mouse models of temporal lobe epilepsy

Astrocytes have emerged as crucial regulators of neuronal network activity, synapse formation, and underlying behavioral and cognitive processes. Despite some pathways have been identified, the communication between astrocytes and neurons remains to be completely elucidated. Unraveling this communication is crucial to design potential treatments for neurological disorders like temporal lobe epilepsy (TLE). The BDNF and TrkB molecules have emerged as very promising therapeutic targets. However, their modulation can be accompanied by several off-target effects such as excitotoxicity in case of uncontrolled upregulation or dementia, amnesia, and other memory disorders in case of downregulation. Here, we show that BDNF and TrkB from astrocytes modulate neuronal dysfunction in TLE models. First, conditional overexpression of BDNF from astrocytes worsened the phenotype in the lithium-pilocarpine mouse model. Our evidences pointed out to the astrocytic pro-BDNF isoform as a major player of this altered phenotype. Conversely, specific genetic deletion of BDNF in astrocytes prevented the increase in the number of firing neurons and the global firing rate in an in vitro model of TLE. Regarding to the TrkB, we generated mice with a genetic deletion of TrkB specifically in hippocampal neurons or astrocytes. Interestingly, both lines displayed neuroprotection in the lithium-pilocarpine model but only the mice with genetic deletion of TrkB in astrocytes showed significantly preserved spatial learning skills. These data identify the astrocytic BDNF and TrkB molecules as promising therapeutic targets for the treatment of TLE.

BDNF Expression in Cortical GABAergic Interneurons

Brain-derived neurotrophic factor (BDNF) is a major neuronal growth factor that is widely expressed in the central nervous system. It is synthesized as a glycosylated precursor protein, (pro)BDNF and post-translationally converted to the mature form, (m)BDNF. BDNF is known to be produced and secreted by cortical glutamatergic principal cells (PCs); however, it remains a question whether it can also be synthesized by other neuron types, in particular, GABAergic interneurons (INs). Therefore, we utilized immunocytochemical labeling and reverse transcription quantitative PCR (RT-qPCR) to investigate the cellular distribution of proBDNF and its RNA in glutamatergic and GABAergic neurons of the mouse cortex. Immunofluorescence labeling revealed that mBDNF, as well as proBDNF, localized to both the neuronal populations in the hippocampus. The precursor proBDNF protein showed a perinuclear distribution pattern, overlapping with the rough endoplasmic reticulum (ER), the site of protein synthesis. RT-qPCR of samples obtained using laser capture microdissection (LCM) or fluorescence-activated cell sorting (FACS) of hippocampal and cortical neurons further demonstrated the abundance of BDNF transcripts in both glutamatergic and GABAergic cells. Thus, our data provide compelling evidence that BDNF can be synthesized by both principal cells and INs of the cortex.

Receptor dependence of BDNF actions in superficial dorsal horn: relation to central sensitization and actions of macrophage colony stimulating factor 1

Peripheral nerve injury elicits an enduring increase in the excitability of the spinal dorsal horn. This change, which contributes to the development of neuropathic pain, is a consequence of release and prolonged exposure of dorsal horn neurons to various neurotrophins and cytokines. We have shown in rats that nerve injury increases excitatory synaptic drive to excitatory neurons but decreases drive to inhibitory neurons. Both effects, which contribute to an increase in dorsal horn excitability, appear to be mediated by microglia-derived BDNF. We have used multiphoton Ca²⁺ imaging and whole cell recording of spontaneous excitatory postsynaptic currents in defined-medium organotypic cultures of GAD67-GFP⁺ mice spinal cord to determine the receptor dependence of these opposing actions of BDNF. In mice, as in rats, BDNF enhances excitatory transmission onto excitatory neurons. This is mediated via presynaptic TrkB and p75 neurotrophin receptors and exclusively by postsynaptic TrkB. By contrast with findings from rats, in mice BDNF does not decrease excitation of inhibitory neurons. The cytokine macrophage colony-stimulating factor 1 (CSF-1) has also been implicated in the onset of neuropathic pain. Nerve injury provokes its de novo synthesis in primary afferents, its release in spinal cord, and activation of microglia. We now show that CSF-1 increases excitatory drive to excitatory neurons via a BDNF-dependent mechanism and decreases excitatory drive to inhibitory neurons via BDNF-independent processes. Our findings complete missing steps in the cascade of events whereby peripheral nerve injury instigates increased dorsal horn excitability in the context of central sensitization and the onset of neuropathic pain. NEW & NOTEWORTHY Nerve injury provokes synthesis of macrophage colony-stimulating factor 1 (CSF-1) in primary afferents and its release in the dorsal horn. We show that CSF-1 increases excitatory drive to excitatory dorsal horn neurons via BDNF activation of postsynaptic TrkB and presynaptic TrkB and p75 neurotrophin receptors. CSF-1 decreases excitatory drive to inhibitory neurons via a BDNF-independent processes. This completes missing steps in understanding how peripheral injury instigates central sensitization and the onset of neuropathic pain.

Regulation of cholinergic basal forebrain development, connectivity, and function by neurotrophin receptors

Cholinergic basal forebrain (cBF) neurons are defined by their expression of the p75 neurotrophin receptor (p75^NTR) and tropomyosin-related kinase (Trk) neurotrophin receptors in addition to cholinergic markers. It is known that the neurotrophins, particularly nerve growth factor (NGF), mediate cholinergic neuronal development and maintenance. However, the role of neurotrophin signalling in regulating adult cBF function is less clear, although in dementia, trophic signalling is reduced and p75^NTR mediates neurodegeneration of cBF neurons. Here we review the current understanding of how cBF neurons are regulated by neurotrophins which activate p75^NTR and TrkA, B or C to influence the critical role that these neurons play in normal cortical function, particularly higher order cognition. Specifically, we describe the current evidence that neurotrophins regulate the development of basal forebrain neurons and their role in maintaining and modifying mature basal forebrain synaptic and cortical microcircuit connectivity. Understanding the role neurotrophin signalling plays in regulating the precision of cholinergic connectivity will contribute to the understanding of normal cognitive processes and will likely provide additional ideas for designing improved therapies for the treatment of neurological disease in which cholinergic dysfunction has been demonstrated.

Cholinergic basal forebrain neurons regulate fear extinction consolidation through p75 neurotrophin receptor signaling

Cholinergic basal forebrain (cBF)-derived neurotransmission plays a crucial role in regulating neuronal function throughout the cortex, yet the mechanisms controlling cholinergic innervation to downstream targets have not been elucidated. Here we report that removing the p75 neurotrophin receptor (p75NTR) from cBF neurons induces a significant impairment in fear extinction consolidation. We demonstrate that this is achieved through alterations in synaptic connectivity and functional activity within the medial prefrontal cortex. These deficits revert back to wild-type levels upon re-expression of the active domain of p75NTR in adult animals. These findings demonstrate a novel role for cholinergic neurons in fear extinction consolidation and suggest that neurotrophic signaling is a key regulator of cholinergic-cortical innervation and function.

Hippocampal proBDNF facilitates place learning strategy associated with neural activity in rats

Mature brain-derived neurotrophic factor has been shown to have a promotive effect on synaptic plasticity and spatial memory. The precursor of BDNF (proBDNF) has emerged as a protein against its mature form. However, it is unknown whether and how proBDNF regulates neural excitability and spatial behavior. Through infusion of cleavage-resistant proBDNF or its antibody into HPC, we sought evidence for the influences by employing multiple behavioral tests and recording hippocampal single-unit activity. Our behavioral findings showed that proBDNF induced beneficial effects on spatial learning by facilitating the use of the place strategy and inhibiting the response strategy, including (1) using more place search strategies but less response strategies, and (2) increasing the number of rats in choosing place strategies but not response strategies. Intriguingly, infusion of an anti-proBDNF antibody did not affect rats' training process but rendered the adaption to learning reversal training more difficult, indicating deficits in choosing the proper learning strategy. The training-induced increase in proBDNF promoted the firing rate of pyramidal neurons but not fast-spiking (FS) interneurons. Importantly, endogenous proBDNF facilitated the neural correlate of spatial, but not response, learning behavior. However, the anti-proBDNF antibody effectively reversed the strategy preference and inhibited neural activity. We herein propose that proBDNF exerts pivotal effects on neural excitability and the use of cognitive strategies to facilitate the spatial learning process.

Do TRPC channels support working memory? Comparing modulations of TRPC channels and working memory through G-protein coupled receptors and neuromodulators

Working memory is a crucial ability we use in daily life. However, the cellular mechanisms supporting working memory still remain largely unclear. A key component of working memory is persistent neural firing which is believed to serve short-term (hundreds of milliseconds up to tens of seconds) maintenance of necessary information. In this review, we will focus on the role of transient receptor potential canonical (TRPC) channels as a mechanism underlying persistent firing. Many years of in vitro work have been suggesting a crucial role of TRPC channels in working memory and temporal association tasks. If TRPC channels are indeed a central mechanism for working memory, manipulations which impair or facilitate working memory should have a similar effect on TRPC channel modulation. However, modulations of working memory and TRPC channels were never systematically compared, and it remains unanswered whether TRPC channels indeed contribute to working memory in vivo or not. In this article, we review the effects of G-protein coupled receptors (GPCR) and neuromodulators, including acetylcholine, noradrenalin, serotonin and dopamine, on working memory and TRPC channels. Based on comparisons, we argue that GPCR and downstream signaling pathways that activate TRPC, generally support working memory, while those that suppress TRPC channels impair it. However, depending on the channel types, areas, and systems tested, this is not the case in all studies. Further work to clarify involvement of specific TRPC channels in working memory tasks and how they are affected by neuromodulators is still necessary in the future.

G-Protein-Coupled Inwardly Rectifying Potassium (GIRK) Channel Activation by the p75 Neurotrophin Receptor Is Required for Amyloid β Toxicity

Alzheimer's disease is characterized by cognitive decline, neuronal degeneration, and the accumulation of amyloid-beta (Aβ). Although, the neurotoxic Aβ peptide is widely believed to trigger neuronal dysfunction and degeneration in Alzheimer's disease, the mechanism by which this occurs is poorly defined. Here we describe a novel, Aβ-triggered apoptotic pathway in which Aβ treatment leads to the upregulation of G-protein activated inwardly rectifying potassium (GIRK/Kir3) channels, causing potassium efflux from neurons and Aβ-mediated apoptosis. Although, GIRK channel activity is required for Aβ-induced neuronal degeneration, we show that it is not sufficient, with coincident signaling by the p75 neurotrophin receptor (p75^NTR) also required for potassium efflux and cell death. Our results identify a novel role for GIRK channels in mediating apoptosis, and provide a previously missing mechanistic link between the excitotoxicity of Aβ and its ability to trigger cell death pathways, such as that mediated by p75^NTR. We propose that this death-signaling pathway contributes to the dysfunction of neurons in Alzheimer's disease and is responsible for their eventual degeneration.

Neurotrophin receptors in the pathogenesis, diagnosis and therapy of neurodegenerative diseases

In the last few years, exciting properties have emerged regarding the activation, signaling, mechanisms of action, and therapeutic targeting of the two types of neurotrophin receptors: the p75^NTR with its intracellular and extracellular peptides, the Trks, their precursors and their complexes. This review summarizes these new developments, with particular focus on neurodegenerative diseases. Based on the evolving knowledge, innovative concepts have been formulated regarding the pathogenesis of these diseases, especially the Alzheimer's and two other, the Parkinson's and Huntington's diseases. The medical progresses include original procedures of diagnosis, started from studies in mice and now investigated for human application, based on innovative classes of receptor agonists and blockers. In parallel, comprehensive studies have been and are being carried out for the development of drugs. The relevance of these studies is based on the limitations of the therapies employed until recently, especially for the treatment of Alzheimer's patients. Starting from well known drugs, previously employed for non-neurodegenerative diseases, the ongoing progress has lead to the development of small molecules that cross rapidly the blood-brain barrier. Among these molecules the most promising are specific blockers of the p75^NTR receptor. Additional drugs, that activate Trk receptors, were shown effective against synaptic loss and memory deficits. In the near future such approaches, coordinated with treatments with monoclonal antibodies and with developments in the microRNA field, are expected to improve the therapy of neurodegenerative diseases, and may be relevant also for other human disease conditions.

Growth factors and hormones pro-peptides: the unexpected adventures of the BDNF prodomain

Most growth factors and hormones are synthesized as pre-pro-proteins which are processed to the biologically active mature protein. The pre- and prodomains are cleaved from the precursor protein in the secretory pathway or, in some cases, extracellularly. The canonical functions of these prodomains are to assist in folding and stabilization of the mature domain, to direct intra and extracellular localization, to facilitate storage, and to regulate bioavailability of their mature counterpart. Recently, exciting evidence has revealed that prodomains of certain growth factors, after cleaved from the precursor pro-protein, can act as independent active signaling molecules. In this review, we discuss the various classical functions of prodomains, and the biological consequences of these pro-peptides acting as ligands. We will focus our attention on the brain-derived neurotrophic factor prodomain (pBDNF), which has been recently described as a novel secreted ligand influencing neuronal morphology and physiology.

Neurotrophins and Proneurotrophins: Focus on Synaptic Activity and Plasticity in the Brain

Neurotrophins have been intensively studied and have multiple roles in the brain. Neurotrophins are first synthetized as proneurotrophins and then cleaved intracellularly and extracellularly. Increasing evidences demonstrate that proneurotrophins and mature neurotrophins exerts opposing role in the central nervous system. In the present review, we explore the role of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT3), and neurotrophin 4 (NT4) and their respective proform in cellular processes related to learning and memory. We focused on their roles in synaptic activity and plasticity in the brain with an emphasis on long-term potentiation, long-term depression, and basal synaptic transmission in the hippocampus and the temporal lobe area. We also discuss new findings on the role of the Val66Met polymorphism on the BDNF propeptide on synaptic activity.

Neurobiology of local and intercellular BDNF signaling

Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family of secreted proteins. Signaling cascades induced by BDNF and its receptor, the receptor tyrosine kinase TrkB, link neuronal growth and differentiation with synaptic plasticity. For this reason, interference with BDNF signaling has emerged as a promising strategy for potential treatments in psychiatric and neurological disorders. In many brain circuits, synaptically released BDNF is essential for structural and functional long-term potentiation, two prototypical cellular models of learning and memory formation. Recent studies have revealed an unexpected complexity in the synaptic communication of mature BDNF and its precursor proBDNF, not only between local pre- and postsynaptic neuronal targets but also with participation of glial cells. Here, we consider recent findings on local actions of the BDNF family of ligands at the synapse and discuss converging lines of evidence which emerge from per se conflicting results.

Towards the PET radiotracer for p75 neurotrophin receptor: [(11)C]LM11A-24 shows biological activity in vitro, but unfavorable ex vivo and in vivo profile

Mature neurotrophins as well as their pro forms are critically involved in the regulation of neuronal functions. They are signaling through three distinct types of receptors: tropomyosin receptor kinase family (TrkA/B/C), p75 neurotrophin receptor (p75(NTR)) and sortilin. Aberrant expression of p75(NTR) in the CNS is implicated in a variety of neurodegenerative diseases, including Alzheimer's disease. The goal of this work was to evaluate one of the very few reported p75(NTR) small molecule ligands as a lead compound for development of novel PET radiotracers for in vivo p75(NTR) imaging. Here we report that previously described ligand LM11A-24 shows significant inhibition of carbachol-induced persistent firing (PF) of entorhinal cortex (EC) pyramidal neurons in wild-type mice via selective interaction with p75(NTR). Based on this electrophysiological assay, the compound has very high potency with an EC50<10nM. We optimized the radiosynthesis of [(11)C]LM11A-24 as the first attempt to develop PET radioligand for in vivo imaging of p75(NTR). Despite some weak interaction with CNS tissues, the radiolabeled compound showed unfavorable in vivo profile presumably due to high hydrophilicity.

HBpF-proBDNF: A New Tool for the Analysis of Pro-Brain Derived Neurotrophic Factor Receptor Signaling and Cell Biology

Neurotrophins activate intracellular signaling pathways necessary for neuronal survival, growth and apoptosis. The most abundant neurotrophin in the adult brain, brain-derived neurotrophic factor (BDNF), is first synthesized as a proBDNF precursor and recent studies have demonstrated that proBDNF can be secreted and that it functions as a ligand for a receptor complex containing p75NTR and sortilin. Activation of proBDNF receptors mediates growth cone collapse, reduces synaptic activity, and facilitates developmental apoptosis of motoneurons but the precise signaling cascades have been difficult to discern. To address this, we have engineered, expressed and purified HBpF-proBDNF, an expression construct containing a 6X-HIS tag, a biotin acceptor peptide (BAP) sequence, a PreScission^™ Protease cleavage site and a FLAG-tag attached to the N-terminal part of murine proBDNF. Intact HBpF-proBDNF has activities indistinguishable from its wild-type counterpart and can be used to purify proBDNF signaling complexes or to monitor proBDNF endocytosis and retrograde transport. HBpF-proBDNF will be useful for characterizing proBDNF signaling complexes and for deciphering the role of proBDNF in neuronal development, synapse function and neurodegenerative disease.

Opposing presynaptic roles of BDNF and ProBDNF in the regulation of persistent activity in the entorhinal cortex

Background

Sustained, persistent firing (PF) of cortical pyramidal neurons following a short depolarization is a crucial cellular mechanism required for spatial and working memory. Pyramidal neurons in the superficial and deep layers of the medial and lateral entorhinal cortex (EC) display this property of prolonged firing activity. Here, we focused on the regulation of this activity in EC neurons by mature brain derived neurotrophic factor (BDNF) and its precursor proBDNF.

Results

Using patch clamp electrophysiology in acute mouse cortical slices, we observed that BDNF facilitates cholinergic PF in pyramidal neurons in layer V of the medial EC. Inhibition of TrkB with K252a blocks the potentiating effect of BDNF whereas inhibition of p75NTR with function-blocking antibodies does not. By recording spontaneous excitatory post-synaptic currents (sEPSC), we find that BDNF acts pre-synaptically via TrkB to increase glutamate release whereas proBDNF acting via p75NTR acts to reduce it. MPEP abolished the facilitating effect of BDNF on PF, demonstrating that the metabotropic glutamate receptor mGluR5 plays a critical role in the BDNF effect. In contrast, paired pulse ratio and EPSC measurements indicated that proBDNF, via presynaptic p75NTR, is a negative regulator of glutamate release in the EC.

Conclusions

Taken together, our findings demonstrate that the BDNF/TrkB pathway facilitates persistent activity whereas the proBDNF/p75NTR pathway inhibits this mnemonic property of entorhinal pyramidal neurons.