Therapeutic potential of a TrkB agonistic antibody for ischemic brain injury

Crosstalk between ferroptosis and necroptosis in cerebral ischemia/reperfusion injury and Naotaifang formula exerts neuroprotective effect via HSP90-GCN2-ATF4 pathway

BACKGROUND

Cerebral ischemia/reperfusion injury (CIRI) is a sequence of pathophysiological processes after blood recanalization in the patients with ischemic stroke, and has become the hinder for the rehabilitation. Naotaifang formula (NTF) has exhibited the clinical effectiveness for this disease. However, its action effects and molecular mechanisms against CIRI are not fully elucidated.

PURPOSE

The research was to clarify the crosstalk between ferroptosis and necroptosis in CIRI, and uncover the mechanism underlying the neuroprotection of NTF.

METHODS

This study established MCAO/R rat models with various reperfusion times. Western blot, transmission electron microscope, laser speckle imaging, immunofluorescence, immunohistochemistry and pathological staining were conducted to detect and analyze the obtained results. Subsequently, various NTF doses were used to intervene in MCAO/R rats, and biology experiments, such as western blot, Evans blue, immunofluorescence and immunohistochemistry, were used to analyze the efficacy of NTF doses. The effect of NTF was further clarified through in vitro experiments. Eventually, HT22 cells that suffered OGD/R were subjected to pre-treatment with plasmids overexpressing HSP90, MLKL, and GPX4 to indicate the interaction among ferroptosis and necroptosis.

RESULTS

There was a gradual increase in the Zea Longa score and cerebral infarction volume following CIRI with prolonged reperfusion. Furthermore, the expression of factors associated with pro-ferroptosis and pro-necroptosis was upregulated in the cortex and hippocampus. NTF alleviated ferroptosis and necroptosis in a dose-dependent manner, downregulated HSP90 levels, reduced blood-brain barrier permeability, and thus protected nerve cells from CIRI. The results in vitro research aligned with those of the in vivo research. HSP90 and MLKL overexpression promoted necroptosis and ferroptosis while activating the GCN2-ATF4 pathway. GPX4 overexpression had no effect on necroptosis or the associated signaling pathway. The administration of NTF alone, as well as its combination with the overexpression of HSP90, MLKL, or GPX4 plasmids, decreased the expression levels of factors associated with pro-ferroptosis and pro-necroptosis and reduced the protein levels of the HSP90-GCN2-ATF4 pathway. Moreover, the regulatory effects of the NTF alone group on GSH, ferrous iron, and GCN2 were more significant compared with those of the HSP90 overexpression combination group.

CONCLUSION

Ferroptosis and necroptosis were gradually aggravated following CIRI with prolonged reperfusion. MLKL overexpression may promote ferroptosis and necroptosis, while GPX4 overexpression may have little effect on necroptosis. HSP90 overexpression accelerated both forms of cell death via the HSP90-GCN2-ATF4 pathway. NTF alleviated ferroptosis and necroptosis to attenuate CIRI by regulating the HSP90-GCN2-ATF4 pathway. Our research provided evidence for the potential of drug development by targeting HSP90, MLKL, and GPX4 to protect against ischemic stroke.

Regulated cell death in hypoxic-ischaemic encephalopathy: recent development and mechanistic overview

Hypoxic-ischaemic encephalopathy (HIE) in termed infants remains a significant cause of morbidity and mortality worldwide despite the introduction of therapeutic hypothermia. Depending on the cell type, cellular context, metabolic predisposition and insult severity, cell death in the injured immature brain can be highly heterogenous. A continuum of cell death exists in the H/I-injured immature brain. Aside from apoptosis, emerging evidence supports the pathological activation of necroptosis, pyroptosis and ferroptosis as alternative regulated cell death (RCD) in HIE to trigger neuroinflammation and metabolic disturbances in addition to cell loss. Upregulation of autophagy and mitophagy in HIE represents an intrinsic neuroprotective strategy. Molecular crosstalk between RCD pathways implies one RCD mechanism may compensate for the loss of function of another. Moreover, mitochondrion was identified as the signalling "hub" where different RCD pathways converge. The highly-orchestrated nature of RCD makes them promising therapeutic targets. Better understanding of RCD mechanisms and crosstalk between RCD subtypes likely shed light on novel therapy development for HIE. The identification of a potential RCD converging node may open up the opportunity for simultaneous and synergistic inhibition of cell death in the immature brain.

Periodontal tissue regeneration with cementogenesis after application of brain-derived neurotrophic factor in 3-wall inflamed intra-bony defect

OBJECTIVE

The purpose of this study is to investigate regenerative process by immunohistochemical analysis and evaluate periodontal tissue regeneration following a topical application of BDNF to inflamed 3-wall intra-bony defects.

BACKGROUND

Brain-derived neurotrophic factor (BDNF) plays a role in the survival and differentiation of central and peripheral neurons. BDNF can regulate the functions of non-neural cells, osteoblasts, periodontal ligament cells, endothelial cells, as well as neural cells. Our previous study showed that a topical application of BDNF enhances periodontal tissue regeneration in experimental periodontal defects of dog and that BDNF stimulates the expression of bone (cementum)-related proteins and proliferation of human periodontal ligament cells.

METHODS

Six weeks after extraction of mandibular first and third premolars, 3-wall intra-bony defects were created in mandibular second and fourth premolars of beagle dogs. Impression material was placed in all of the artificial defects to induce inflammation. Two weeks after the first operation, BDNF (25 and 50 μg/mL) immersed into atelocollagen sponge was applied to the defects. As a control, only atelocollagen sponge immersed in saline was applied. Two and four weeks after the BDNF application, morphometric analysis was performed. Localizations of osteopontin (OPN) and proliferating cell nuclear antigen (PCNA)-positive cells were evaluated by immunohistochemical analysis.

RESULTS

Two weeks after application of BDNF, periodontal tissue was partially regenerated. Immunohistochemical analyses revealed that cells on the denuded root surface were positive with OPN and PCNA. PCNA-positive cells were also detected in the soft connective tissue of regenerating periodontal tissue. Four weeks after application of BDNF, the periodontal defects were regenerated with cementum, periodontal ligament, and alveolar bone. Along the root surface, abundant OPN-positive cells were observed. Morphometric analyses revealed that percentage of new cementum length and percentage of new bone area of experimental groups were higher than control group and dose-dependently increased.

CONCLUSION

These findings suggest that BDNF could induce cementum regeneration in early regenerative phase by stimulating proliferation of periodontal ligament cells and differentiation into periodontal tissue cells, resulting in enhancement of periodontal tissue regeneration in inflamed 3-wall intra-bony defects.

Relationship between plasma brain-derived neurotrophic factor levels and neurological disorders: An investigation using Mendelian randomisation

Background

Altered brain-derived neurotrophic factor (BDNF) concentrations have been detected in the central nervous system tissues and peripheral blood. These alterations are associated with a series of neurological disorders.

Objective

To investigate the potential causal relationships between genetically determined plasma BDNF levels and various neurological diseases using a two-sample Mendelian randomisation study.

Methods

We selected single nucleotide polymorphisms strongly related to plasma BDNF levels as instrumental variables. Within the Mendelian randomisation framework, we used summary-level statistics for exposure (plasma BDNF levels) and outcomes (neurological disorders).

Results

We observed suggestive evidence of a relation between higher plasma BDNF levels and less risk of nontraumatic intracranial haemorrhage (nITH) (odds ratio [OR] = 0.861, 95 % confidence interval [CI]: 0.774-0.958, P = 0.006, PFDR = 0.078), epilepsy (OR = 0.927, 95 % CI: 0.880-0.976, P = 0.004, PFDR = 0.078), focal epilepsy (OR = 0.928, 95 % CI: 0.874-0.986, P = 0.016, PFDR = 0.139), and non-lesional focal epilepsy (OR = 0.981, 95 % CI: 0.964-0.999, P = 0.041, PFDR = 0.267). Combined with the UK Biobank dataset, the association of plasma BDNF levels with nITH remained significant (OR = 0.88, 95 % CI: 0.81-0.96, P < 0.01). The combined analysis of three consortium datasets demonstrated a considerable impact of plasma BDNF on epilepsy (OR = 0.94, 95 % CI: 0.90-0.98, P < 0.01) and a suggestive impact on focal epilepsy (OR = 0.94, 95 % CI: 0.89-0.99, P = 0.02). However, there was no apparent correlation between plasma BDNF levels and other neurological disorders or related subtypes.

Conclusions

Our study supports a possible causal relationship between elevated plasma BDNF levels and a reduced risk of nITH, epilepsy, and focal epilepsy.

Targeting synapse function and loss for treatment of neurodegenerative diseases

Synapse dysfunction and loss are hallmarks of neurodegenerative diseases that correlate with cognitive decline. However, the mechanisms and therapeutic strategies to prevent or reverse synaptic damage remain elusive. In this Review, we discuss recent advances in understanding the molecular and cellular pathways that impair synapses in neurodegenerative diseases, including the effects of protein aggregation and neuroinflammation. We also highlight emerging therapeutic approaches that aim to restore synaptic function and integrity, such as enhancing synaptic plasticity, preventing synaptotoxicity, modulating neuronal network activity and targeting immune signalling. We discuss the preclinical and clinical evidence for each strategy, as well as the challenges and opportunities for developing effective synapse-targeting therapeutics for neurodegenerative diseases.

Regulated necrosis pathways: a potential target for ischemic stroke

Globally, ischemic stroke causes millions of deaths per year. The outcomes of ischemic stroke are largely determined by the amount of ischemia-related and reperfusion-related neuronal death in the infarct region. In the infarct region, cell injuries follow either the regulated pathway involving precise signaling cascades, such as apoptosis and autophagy, or the nonregulated pathway, which is uncontrolled by any molecularly defined effector mechanisms such as necrosis. However, numerous studies have recently found that a certain type of necrosis can be regulated and potentially modified by drugs and is nonapoptotic; this type of necrosis is referred to as regulated necrosis. Depending on the signaling pathway, various elements of regulated necrosis contribute to the development of ischemic stroke, such as necroptosis, pyroptosis, ferroptosis, pathanatos, mitochondrial permeability transition pore-mediated necrosis and oncosis. In this review, we aim to summarize the underlying molecular mechanisms of regulated necrosis in ischemic stroke and explore the crosstalk and interplay among the diverse types of regulated necrosis. We believe that targeting these regulated necrosis pathways both pharmacologically and genetically in ischemia-induced neuronal death and protection could be an efficient strategy to increase neuronal survival and regeneration in ischemic stroke.

Necroptosis in CNS diseases: Focus on astrocytes

In the last few years, necroptosis, a recently described type of cell death, has been reported to play an important role in the development of various brain pathologies. Necroptosis is a cell death mechanism that has morphological characteristics similar to necrosis but is mediated by fundamentally different molecular pathways. Necroptosis is initiated by signaling through the interaction of RIP1/RIP3/MLKL proteins (receptor-interacting protein kinase 1/receptor-interacting protein kinase 3/mixed lineage kinase domain-like protein). RIPK1 kinase is usually inactive under physiological conditions. It is activated by stimulation of death receptors (TNFR1, TNFR2, TLR3, and 4, Fas-ligand) by external signals. Phosphorylation of RIPK1 results in the formation of its complex with death receptors. Further, complexes with the second member of the RIP3 and MLKL cascade appear, and the necroptosome is formed. There is enough evidence that necroptosis plays an important role in the pathogenesis of brain ischemia and neurodegenerative diseases. In recent years, a point of view that both neurons and glial cells can play a key role in the development of the central nervous system (CNS) pathologies finds more and more confirmation. Astrocytes play complex roles during neurodegeneration and ischemic brain damage initiating both impair and protective processes. However, the cellular and molecular mechanisms that induce pathogenic activity of astrocytes remain veiled. In this review, we consider these processes in terms of the initiation of necroptosis. On the other hand, it is important to remember that like other types of programmed cell death, necroptosis plays an important role for the organism, as it induces a strong immune response and is involved in the control of cancerogenesis. In this review, we provide an overview of the complex role of necroptosis as an important pathogenetic component of neuronal and astrocyte death in neurodegenerative diseases, epileptogenesis, and ischemic brain damage.

7,8-Dihydroxyflavone alleviates cardiac fibrosis by restoring circadian signals via downregulating Bmal1/Akt pathway

Brain-derived neurotrophic factor (BDNF)/tyrosine kinase receptor B (TrkB) pathway is a therapeutic target in cardiac diseases. A BDNF mimetic, 7,8-dihydroxyflavone (7,8-DHF), is emerging as a protective agent in cardiomyocytes; however, its potential role in cardiac fibroblasts (CFs) and fibrosis remains unknown. Thus, we aimed to explore the effects of 7,8-DHF on cardiac fibrosis and the possible mechanisms. Myocardial ischemia (MI) and transforming growth factor-β1 (TGF-β1) were used to establish models of cardiac fibrosis. Hematoxylin & eosin and Masson's trichrome stains were used for histological analysis and determination of collagen content in mouse myocardium. Cell viability kit, EdU (5-ethynyl-2'-deoxyuridine) assay and immunofluorescent stain were employed to examine the effects of 7,8-DHF on the proliferation and collagen production of CFs. The levels of collagen I, α-smooth muscle actin (α-SMA), TGF-β1, Smad2/3, and Akt as well as circadian rhythm-related signals including brain and muscle Arnt-like protein 1 (Bmal1), period 2 (Per2), and cryptochrome 2 (Cry2) were analyzed. Treatment with 7,8-DHF markedly alleviated cardiac fibrosis in MI mice. It inhibited the activity of CFs accompanied by decreasing number of EdU-positive cells and downregulation of collagen I, α-SMA, TGF-β1, and phosphorylation of Smad2/3. 7,8-DHF significantly restored the dysregulation of Bmal1, Per2, and Cry2, but inhibited the overactive Akt. Further, inhibition of Bmal1 by SR9009 effectively attenuated CFs proliferation and collagen production of CFs. In summary, these findings indicate that 7,8-DHF attenuates cardiac fibrosis and regulates circadian rhythmic signals, at least partly, by inhibiting Bmal1/Akt pathway, which may provide new insights into therapeutic cardiac remodeling.

Investigation of 91 proteins implicated in neurobiological processes identifies multiple candidate plasma biomarkers of stroke outcome

The inter-individual variation in stroke outcomes is large and protein studies could point to potential underlying biological mechanisms. We measured plasma levels of 91 neurobiological proteins in 209 cases included in the Sahlgrenska Academy Study on Ischemic Stroke using a Proximity Extension Assay, and blood was sampled in the acute phase and at 3-month and 7-year follow-ups. Levels were also determined once in 209 controls. Acute stroke severity and neurological outcome were evaluated by the National Institutes of Health Stroke Scale. In linear regression models corrected for age, sex, and sampling day, acute phase levels of 37 proteins were associated with acute stroke severity, and 47 with 3-month and/or 7-year outcome at false discovery rate < 0.05. Three-month levels of 8 proteins were associated with 7-year outcome, of which the associations for BCAN and Nr-CAM were independent also of acute stroke severity. Most proteins followed a trajectory with lower levels in the acute phase compared to the 3-month follow-up and the control sampling point. Conclusively, we identified multiple candidate plasma biomarkers of stroke severity and neurological outcome meriting further investigation. This study adds novel information, as most of the reported proteins have not been previously investigated in a stroke cohort.

Necroptosis Blockade Potentiates the Neuroprotective Effect of Hypothermia in Neonatal Hypoxic-Ischemic Encephalopathy

Neonatal encephalopathy (NE) caused by hypoxia-ischemia (HI) affects around 1 per 1000 term newborns and is the leading cause of acquired brain injury and neurodisability. Despite the use of hypothermia (HT) as a standard of care, the incidence of NE and its devastating outcomes remains a major issue. Ongoing research surrounding add-on neuroprotective strategies against NE is important as HT effects are limited, leaving 50% of treated patients with neurological sequelae. Little is known about the interaction between necroptotic blockade and HT in neonatal HI. Using a preclinical Lewis rat model of term human NE induced by HI, we showed a neuroprotective effect of Necrostatin-1 (Nec-1: a compound blocking necroptosis) in combination with HT. The beneficial effect of Nec-1 added to HT against NE injuries was observed at the mechanistic level on both pMLKL and TNF-α, and at the anatomical level on brain volume loss visualized by magnetic resonance imaging (MRI). HT alone showed no effect on activated necroptotic effectors and did not preserve the brain MRI volume. This study opens new avenues of research to understand better the specific cell death mechanisms of brain injuries as well as the potential use of new therapeutics targeting the necroptosis pathway.

Necroptosis: A Pathogenic Negotiator in Human Diseases

Over the past few decades, mechanisms of programmed cell death have attracted the scientific community because they are involved in diverse human diseases. Initially, apoptosis was considered as a crucial mechanistic pathway for programmed cell death; recently, an alternative regulated mode of cell death was identified, mimicking the features of both apoptosis and necrosis. Several lines of evidence have revealed that dysregulation of necroptosis leads to pathological diseases such as cancer, cardiovascular, lung, renal, hepatic, neurodegenerative, and inflammatory diseases. Regulated forms of necrosis are executed by death receptor ligands through the activation of receptor-interacting protein kinase (RIPK)-1/3 and mixed-lineage kinase domain-like (MLKL), resulting in the formation of a necrosome complex. Many papers based on genetic and pharmacological studies have shown that RIPKs and MLKL are the key regulatory effectors during the progression of multiple pathological diseases. This review focused on illuminating the mechanisms underlying necroptosis, the functions of necroptosis-associated proteins, and their influences on disease progression. We also discuss numerous natural and chemical compounds and novel targeted therapies that elicit beneficial roles of necroptotic cell death in malignant cells to bypass apoptosis and drug resistance and to provide suggestions for further research in this field.

A Single Domain Shark Antibody Targeting the Transferrin Receptor 1 Delivers a TrkB Agonist Antibody to the Brain and Provides Full Neuroprotection in a Mouse Model of Parkinson’s Disease

Single domain shark antibodies that bind to the transferrin receptor 1 (TfR1) on brain endothelial cells have been used to shuttle antibodies and other cargos across the blood brain barrier (BBB) to the brain. For these studies the TXB4 brain shuttle was fused to a TrkB neurotrophin receptor agonist antibody. The TXB4-TrkB fusion retained potent agonist activity at its cognate receptor and after systemic administration showed a 12-fold increase in brain levels over the unmodified antibody. Only the TXB4-TrkB antibody fusion was detected within the brain and localized to TrkB positive cells in the cortex and tyrosine hydroxylase (TH) positive dopaminergic neurons in the substantia nigra pars compacta (SNc), where it was associated with activated ERK1/2 signaling. When tested in the 6-hydroxydopamine (6-OHDA) mouse model of Parkinson’s disease (PD), TXB4-TrkB, but not the unmodified antibody, completely prevented the 6-OHDA induced death of TH positive neurons in the SNc. In conclusion, the fusion of the TXB4 brain shuttle allows a TrkB agonist antibody to reach neuroprotective concentrations in the brain parenchyma following systemic administration.

TrkB Truncated Isoform Receptors as Transducers and Determinants of BDNF Functions

Brain-derived neurotrophic factor (BDNF) belongs to the neurotrophin family of secreted growth factors and binds with high affinity to the TrkB tyrosine kinase receptors. BDNF is a critical player in the development of the central (CNS) and peripheral (PNS) nervous system of vertebrates and its strong pro-survival function on neurons has attracted great interest as a potential therapeutic target for the management of neurodegenerative disorders such as Amyotrophic Lateral Sclerosis (ALS), Huntington, Parkinson’s and Alzheimer’s disease. The TrkB gene, in addition to the full-length receptor, encodes a number of isoforms, including some lacking the catalytic tyrosine kinase domain. Importantly, one of these truncated isoforms, namely TrkB.T1, is the most widely expressed TrkB receptor in the adult suggesting an important role in the regulation of BDNF signaling. Although some progress has been made, the mechanism of TrkB.T1 function is still largely unknown. Here we critically review the current knowledge on TrkB.T1 distribution and functions that may be helpful to our understanding of how it regulates and participates in BDNF signaling in normal physiological and pathological conditions.

Necrostatin-1 Against Sevoflurane-Induced Cognitive Dysfunction Involves Activation of BDNF/TrkB Pathway and Inhibition of Necroptosis in Aged Rats

Postoperative cognitive dysfunction (POCD) induced by anesthesia or surgery has become a common complication in the aged population. Sevoflurane, a clinical inhalation anesthetic, could stimulate calcium overload and necroptosis to POCD. In addition, necroptosis inhibitor necrostatin-1 (Nec-1) alleviated cognitive impairment caused by multiple causes, including postoperative cognitive impairment. However, whether Nec-1 exerts a neuroprotective effect on POCD via calcium and necroptosis remains unclear. We anesthetized Sprague-Dawley rats with sevoflurane to construct the POCD model and to explore the mechanism underlying neuroprotective effects of Nec-1 in POCD. Rats were treated with Nec-1 (6.25 mg/kg) 1 h prior to anesthesia. Open field test and Morris water maze were employed to detect the cognitive function. In this study, rats exposed to sevoflurane displayed cognitive dysfunction without changes in spontaneous activity; however, the sevoflurane-induced POCD could be relieved by Nec-1 pretreatment. Nec-1 decreased sevoflurane-induced calcium overload and calpain activity in the hippocampus. In addition, Nec-1 alleviated the expression of p-RIPK1, RIPK1, p-RIPK3, RIPK3, p-MLKL and MLKL. Furthermore, Nec-1 remarkably increased BDNF and p-TrkB/TrkB expression in the hippocampus of aged rats. Ultimately, our research manifests evidence that Nec-1 may play a neuroprotective role against sevoflurane-induced cognitive impairment via the increase of BDNF/TrkB and suppression of necroptosis-related pathway.

Inhibition of P2X4R attenuates white matter injury in mice after intracerebral hemorrhage by regulating microglial phenotypes

Background

White matter injury (WMI) is a major neuropathological event associated with intracerebral hemorrhage (ICH). P2X purinoreceptor 4 (P2X4R) is a member of the P2X purine receptor family, which plays a crucial role in regulating WMI and neuroinflammation in central nervous system (CNS) diseases. Our study investigated the role of P2X4R in the WMI and the inflammatory response in mice, as well as the possible mechanism of action after ICH.

Methods

ICH was induced in mice via collagenase injection. Mice were treated with 5-BDBD and ANA-12 to inhibit P2X4R and tropomyosin-related kinase receptor B (TrkB), respectively. Immunostaining and quantitative polymerase chain reaction (qPCR) were performed to detect microglial phenotypes after the inhibition of P2X4R. Western blots (WB) and immunostaining were used to examine WMI and the underlying molecular mechanisms. Cylinder, corner turn, wire hanging, and forelimb placement tests were conducted to evaluate neurobehavioral function.

Results

After ICH, the protein levels of P2X4R were upregulated, especially on day 7 after ICH, and were mainly located in the microglia. The inhibition of P2X4R via 5-BDBD promoted neurofunctional recovery after ICH as well as the transformation of the pro-inflammatory microglia induced by ICH into an anti-inflammatory phenotype, and attenuated ICH-induced WMI. Furthermore, we found that TrkB blockage can reverse the protective effects of WMI as well as neuroprotection after 5-BDBD treatment. This result indicates that P2X4R plays a crucial role in regulating WMI and neuroinflammation and that P2X4R inhibition may benefit patients with ICH.

Conclusions

Our results demonstrated that P2X4R contributes to WMI by polarizing microglia into a pro-inflammatory phenotype after ICH. Furthermore, the inhibition of P2X4R promoted pro-inflammatory microglia polarization into an anti-inflammatory phenotype, enhanced brain-derived neurotrophic factor (BDNF) production, and through the BDNF/TrkB pathway, attenuated WMI and improved neurological function. Therefore, the regulation of P2X4R activation may be beneficial for the reducing of ICH-induced brain injury.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02239-3.

mPFC GABAergic transmission mediated the role of BDNF signaling in cognitive impairment but not anxiety induced by adolescent social stress

Depression with comorbid anxiety or cognitive symptoms can vary in terms of symptoms, pathophysiology and antidepressant efficacy, but the underlying neurobiological mechanisms remain to be elucidated. Previous studies from our group and others have shown that as a classic animal model of depression, adolescent social stress (ASS) could stably induce a variety of emotional and cognitive alterations in adult animals, and accompanied by transcriptional decrease in brain-derived neurotrophic factor (BDNF) total and promoter IV levels in the medial prefrontal cortex (mPFC). The present study further identified the GABAergic synaptic and molecular changes downstream of BDNF signaling impairment in the mPFC and roles in various behavioral phenotypes induced by ASS. We found that ASS induced a set of emotional and cognitive symptoms, including decreased social interest, impaired cognitive function, and increased anxiety-like behavior, as well as decreased GABAergic transmission in the mPFC. The specific deletion of BDNF promoter IV directly caused impairments in social interest, cognitive function, and inhibition of GABAergic transmission, but no changes in anxiety-like behavior. Acute microinjections of tropomyosin-related kinase B (TrkB) agonists into the mPFC and chronic antidepressant treatment ameliorated the changes in social behavior and cognition, as well as the reduction in GABAergic synaptic transmission in the mPFC, but not anxiety in previously stressed adult mice. These results suggest that the downstream GABAergic transmission of BDNF signaling in the mPFC involved in depression with comorbid cognitive dysfunction induced by ASS and can be used as a therapeutic target for the treatment of cognitive dysfunction in depression. This article is part of the special issue on Stress, Addiction and Plasticity.

The possible roles of necroptosis during cerebral ischemia and ischemia / reperfusion injury

Cell death is a process consequential to cerebral ischemia and cerebral ischemia/reperfusion (I/R) injury. Recent evidence suggest that necroptosis has been involved in the pathogenesis of ischemic brain injury. The mechanism of necroptosis is initiated by an activation of inflammatory receptors including tumor necrosis factor, toll like receptor, and fas ligands. The signals activate the receptor-interacting protein kinase (RIPK) 1, 3, and a mixed-lineage kinase domain-like pseudokinase (MLKL) to instigate necroptosis. RIPK1 inhibitor, necrostatin-1, was developed, and dramatically reduced brain injury following cerebral ischemia in mice. Consequently, necroptosis could be a novel therapeutic target for stroke, which aims to reduce long-term adverse outcomes after cerebral ischemia. Several studies have been conducted to test the roles of necroptosis on cerebral ischemia and cerebral I/R injury, and the efficacy of necrostatin-1 has been tested in those models. Evidence regarding the roles of necroptosis and the effects of necrostatin-1, from in vitro and in vivo studies, has been summarized and discussed. In addition, other therapeutic managements, involving in necroptosis, are also included in this review. We believe that the insights from this review might clarify the clinical perspective and challenges involved in future stroke treatment by targeting the necroptosis pathway.

Therapeutic potential of a TrkB agonistic antibody for Alzheimer's disease

Repeated failures of “Aβ-lowering” therapies call for new targets and therapeutic approaches for Alzheimer's disease (AD). We propose to treat AD by halting neuronal death and repairing synapses using a BDNF-based therapy. To overcome the poor druggability of BDNF, we have developed an agonistic antibody AS86 to mimic the function of BDNF, and evaluate its therapeutic potential for AD.

Method: Biochemical, electrophysiological and behavioral techniques were used to investigate the effects of AS86 in vitro and in vivo.

Results: AS86 specifically activated the BDNF receptor TrkB and its downstream signaling, without affecting its other receptor p75^NTR. It promoted neurite outgrowth, enhanced spine growth and prevented Aβ-induced cell death in cultured neurons, and facilitated Long-Term Potentiation (LTP) in hippocampal slices. A single-dose tail-vein injection of AS86 activated TrkB signaling in the brain, with a half-life of 6 days in the blood and brain. Bi-weekly peripheral administration of AS86 rescued the deficits in object-recognition memory in the APP/PS1 mouse model. AS86 also reversed spatial memory deficits in the 11-month, but not 14-month old AD mouse model.

Conclusion: These results demonstrate the potential of AS86 in AD therapy, suggesting that neuronal and/or synaptic repair as an alternative therapeutic strategy for AD.

Brain-Derived Neurotrophic Factor and Its Potential Therapeutic Role in Stroke Comorbidities

With the rise in the aging global population, stroke comorbidities have become a serious health threat and a tremendous economic burden on human society. Current therapeutic strategies mainly focus on protecting neurons from cytotoxic damage at the acute phase upon stroke onset, which not only is a difficult way to ameliorate stroke symptoms but also presents a challenge for the patients to receive effective treatment in time. The brain-derived neurotrophic factor (BDNF) is the most abundant neurotrophin in the adult brain, which possesses a remarkable capability to repair brain damage. Recent promising preclinical outcomes have made BDNF a popular late-stage target in the development of novel stroke treatments. In this review, we aim to summarize the latest progress in the understanding of the cellular/molecular mechanisms underlying stroke pathogenesis, current strategies and difficulties in drug development, the mechanism of BDNF action in poststroke neurorehabilitation and neuroplasticity, and recent updates in novel therapeutic methods.

A subpopulation of Bdnf-e1-expressing glutamatergic neurons in the lateral hypothalamus critical for thermogenesis control

Objective

Brown adipose tissue (BAT)–mediated thermogenesis plays a key role in energy homeostasis and the maintenance of body temperature. Previous work suggests that brain-derived neurotrophic factor (BDNF) is involved in BAT thermogenesis, but the underlying neural circuits and molecular mechanism remain largely unknown. This is in part due to the difficulties in manipulating BDNF expression in different brain regions through different promoters and the lack of tools to identify neurons in the brain specifically involved in BAT thermogenesis.

Methods

We have created several lines of mutant mice in which BDNF transcription from a specific promoter was selectively disrupted by replacing Bdnf with green fluorescent protein (GFP; Bdnf-e1, -e4, and -e6^−/− mice). As such, cells expressing Bdnf-e1, -e4, or -e6 were labeled with GFP. To identify BAT-connected thermogenesis neurons in brain, we applied the retrograde pseudorabies virus labeling method from BAT. We also used chemogenetic tools to manipulate specific neurons coupled with BAT temperature recording. Moreover, we developed a new TrkB agonist antibody to rescue the BAT thermogenesis deficits.

Results

We show that selective disruption of Bdnf expression from promoter 1 (Bdnf-e1) resulted in severe obesity and deficits of BAT-mediated thermogenesis. Body temperature response to cold was impaired in Bdnf-e1^−/− mice. BAT expression of Ucp1 and Pcg1a, genes known to regulate thermogenesis, was also reduced, accompanying a decrease in the sympathetic activity of BAT. Staining of cells expressing Bdnf-e1 transcript, combined with transsynaptic, retrograde-tracing labeling of BAT-connected neurons, identified a group of excitatory neurons in lateral hypothalamus (LH) critical for thermogenesis regulation. Moreover, an adaptive thermogenesis defect in Bdnf-e1^−/− mice was rescued by injecting an agonistic antibody for TrkB, the BDNF receptor, into LH. Remarkably, activation of the excitatory neurons (VGLUT2+) in LH through chemogenetic tools resulted in a rise of BAT temperature.

Conclusions

These results reveal a specific role of BDNF promoter I in thermogenesis regulation and define a small subset of neurons in LH that contribute to such regulation.

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Only Bdnf-e1^−/−, but not Bdnf-e4^−/− or Bdnf-e6^−/−, mutant mice exhibited deficiencies of BAT thermogenesis.

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Neurons that are both Bdnf-e1 expressing and BAT-connected were found only in LH.

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BAT-connected neurons in LH are glutamatergic.

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Activation of the LH glutamatergic neurons resulted in an increase in BAT temperature.

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Administration of TrkB agonist antibody in LH rescued thermogenesis deficits.

TrkB agonistic antibodies superior to BDNF: Utility in treating motoneuron degeneration

While Brain-derived Neurotrophic Factor (BDNF) has long been implicated in treating neurological diseases, recombinant BDNF protein has failed in multiple clinical trials. In addition to its unstable and adhesive nature, BDNF can activate p75^NTR, a receptor mediating cellular functions opposite to those of TrkB. We have now identified TrkB agonistic antibodies (TrkB-agoAbs) with several properties superior to BDNF: They exhibit blood half-life of days instead of hours, diffuse centimeters in neural tissues instead millimeters, and bind and activate TrkB, but not p75^NTR. In addition, TrkB-agoAbs elicit much longer TrkB activation, reduced TrkB internalization and less intracellular degradation, compared with BDNF. More importantly, some of these TrkB-agoAbs bind TrkB epitopes distinct from that by BDNF, and work cooperatively with endogenous BDNF. Unlike BDNF, the TrkB-agoAbs exhibit a half-life of days/weeks and diffused readily in nerve tissues. We tested one of TrkB-agoAbs further and showed that it enhanced motoneuron survival in the spinal-root avulsion model for motoneuron degeneration in vivo. Thus, TrkB-agoAbs are promising drug candidates for the treatment of neural injury.