Genetic and pharmacological intervention of the p75NTR pathway alters morphological and behavioural recovery following traumatic brain injury in mice

Temporal expression of brainstem neurotrophic proteins following mild traumatic brain injury

BDNF, a neurotrophic factor, and its receptors have been implicated in the pathophysiology of mild traumatic brain injury (mTBI). The brainstem houses many vital functions, that are also associated with signs and symptoms of mTBI, but has been understudied in mTBI animal models. We determined the extent to which neurotrophic protein and associated receptor expression is affected within the brainstem of adult rats following mTBI. Their behavioral function was assessed and temporal expression of the 'negative' regulators of neuronal function (p75, t-TrkB, and pro-BDNF) and 'positive' neuroprotective (FL-TrkB and m-BDNF) protein isoforms were determined via western blot and immunohistochemistry at 1, 3, 7, and 14 post-injury days (PID) following mTBI or sham (control) procedure. Within the brainstem, p75 expression increased at PID 1 vs. sham animals. t-TrkB and pro-BDNF expression increased at PID 7 and 14. The 'positive' protein isoforms of FL-TrkB and m-BDNF expression were increased only at PID 7. The ratio of t-TrkB:FL-TrkB (negative:positive) was substantial across groups and time points, suggesting a negative impact of neurotrophic signaling on neuronal function. Additional NeuN experiments revealed cell death occurring within a subset of neurons within the medulla. While behavioral measures improved by PID 7-14, negative neurotrophic biochemical responses persisted. Despite the assertion that mTBI produces "mild" injury, evidence of cell death was observed in the medulla. Ratios of TrkB and BDNF isoforms with conflicting functions suggest that future work should specifically measure each subtype since they induce opposing downstream effects on neuronal function.

Microglia and Astrocytes Responses Contribute to Alleviating Inflammatory Damage by Repetitive Transcranial Magnetic Stimulation in Rats with Traumatic Brain Injury

Repetitive transcranial magnetic stimulation (rTMS) is a therapeutic strategy that shows promise in ameliorating the clinical sequelae following traumatic brain injury (TBI). These improvements are associated with neuroplastic changes in neurons and their synaptic connections. However, it has been hypothesized that rTMS may also modulate microglia and astrocytes, potentially potentiating their neuroprotective capabilities. This study aims to investigate the effects of high-frequency rTMS on microglia and astrocytes that may contribute to its neuroprotective effects. Feeney's weight-dropping method was used to establish rat models of moderate TBI. To evaluate the neuroprotective effect of high frequency rTMS on rats by observing the synaptic ultrastructure and the level of neuron apoptosis. The levels of several important inflammation-related proteins within microglia and astrocytes were assessed through immunofluorescence staining and western blot. Our findings demonstrate that injured neurons can be rescued through the modulation of microglia and astrocytes by rTMS. This modulation plays a key role in preserving the synaptic ultrastructure and inhibiting neuronal apoptosis. Among microglia, we observed that rTMS inhibited the levels of proinflammatory factors (CD16, IL-6 and TNF-α) and promoted the levels of anti-inflammatory factors (CD206, IL-10 and TNF-β). rTMS also reduced the levels of pyroptosis within microglia and pyroptosis-related proteins (NLRP3, Caspase-1, GSDMD, IL-1β and IL-18). Moreover, rTMS downregulated P75NTR expression and up-regulated IL33 expression in astrocytes. These findings suggest that regulation of microglia and astrocytes is the mechanism through which rTMS attenuates neuronal inflammatory damage after moderate TBI.

Single Nucleotide Polymorphism in Cell Adhesion Molecule L1 Affects Learning and Memory in a Mouse Model of Traumatic Brain Injury

The L1 cell adhesion molecule (L1) has demonstrated a range of beneficial effects in animal models of spinal cord injury, neurodegenerative disease, and ischemia; however, the role of L1 in TBI has not been fully examined. Mutations in the L1 gene affecting the extracellular domain of this type 1 transmembrane glycoprotein have been identified in patients with L1 syndrome. These patients suffer from hydrocephalus, MASA (mental retardation, adducted thumbs, shuffling gait, aphasia) symptoms, and corpus callosum agenesis. Clinicians have observed that recovery post-traumatic brain injury (TBI) varies among the population. This variability may be explained by the genetic differences present in the general population. In this study, we utilized a novel mouse model of L1 syndrome with a mutation at aspartic acid position 201 in the extracellular domain of L1 (L1-201). We assessed the impact of this specific single nucleotide polymorphism (SNP) localized to the X-chromosome L1 gene on recovery outcomes following TBI by comparing the L1-201 mouse mutants with their wild-type littermates. We demonstrate that male L1-201 mice exhibit significantly worse learning and memory outcomes in the Morris water maze after lateral fluid percussion (LFP) injury compared to male wild-type mice and a trend to worse motor function on the rotarod. However, no significant changes were observed in markers for inflammatory responses or apoptosis after TBI.

Axonal injury mediated by neuronal p75NTR/TRAF6/JNK pathway contributes to cognitive impairment after repetitive mTBI

Repetitive mild traumatic brain injury (rmTBI) is one of the leading causes of cognitive disorders. The impairment of axonal integrity induced by rmTBI is speculated to underlie the progression of cognitive dysfunction. However, few studies have uncovered the cellular mechanism regulating axonal impairment. In this study, we showed that after rmTBI, the activation of neuronal p75NTR signaling contributes to abnormal axonal morphology and impaired axonal transport, which further leads to cognitive dysfunction in mice. By neuron-specific knockdown of p75NTR or treatment with p75NTR inhibitor LM11A-31, we observed better recovery of axonal integrity and cognitive function after brain trauma. Further analysis revealed that p75NTR relies on its adaptor protein TRAF6 to activate downstream signaling via TAK1 and JNK. Overall, our results provide novel insight into the role of neuronal p75NTR in axonal injury and suggest that p75NTR may be a promising target for cognitive function recovery after rmTBI.

Lost in Translation: Neurotrophins Biology and Function in the Neurovascular Unit

The neurovascular unit (NVU) refers to the functional building unit of the brain and the retina, where neurons, glia, and microvasculature orchestrate to meet the demand of the retina's and brain's function. Neurotrophins (NTs) are structural families of secreted proteins and are known for exerting neurotrophic effects on neuronal differentiation, survival, neurite outgrowth, synaptic formation, and plasticity. NTs include several molecules, such as nerve growth factor, brain-derived neurotrophic factor, NT-3, NT-4, and their precursors. Furthermore, NTs are involved in signaling pathways such as inflammation, apoptosis, and angiogenesis in a nonneuronal cell type. Interestingly, NTs and the precursors can bind and activate the p75 neurotrophin receptor (p75NTR) at low and high affinity. Mature NTs bind their cognate tropomyosin/tyrosine-regulated kinase receptors, crucial for maintenance and neuronal development in the brain and retina axis. Activation of p75NTR results in neuronal apoptosis and cell death, while tropomysin receptor kinase upregulation contributes to differentiation and cell growth. Recent findings indicate that modulation of NTs and their receptors contribute to neurovascular dysfunction in the NVU. Several chronic metabolic and acute ischemic diseases affect the NVU, including diabetic and ischemic retinopathy for the retina, as well as stroke, acute encephalitis, and traumatic brain injury for the brain. This work aims to review the current evidence through published literature studying the impact of NTs and their receptors, including the p75NTR receptor, on the injured and healthy brain-retina axis.

Cortical Brain Injury Causes Retrograde Degeneration of Afferent Basal Forebrain Cholinergic Neurons via the p75NTR

Traumatic brain injury (TBI) elicits neuronal loss at the site of injury and progressive neuronal loss in the penumbra. However, the consequences of TBI on afferent neurons projecting to the injured tissue from distal locations is unknown. Basal forebrain cholinergic neurons (BFCNs) extend long projections to multiple brain regions including the cortex, regulate many cognitive functions, and are compromised in numerous neurodegenerative disorders. To determine the consequence of cortical injury on these afferent neurons, we used the fluid percussion injury model of traumatic brain injury and assessed the effects on BFCN survival and axon integrity in male and female mice. Survival or death of BF neurons can be regulated by neurotrophins or proneurotrophins, respectively. The injury elicited an induction of proNGF and proBDNF in the cortex and a loss of BFCNs ipsilateral to the injury compared with sham uninjured mice. The p75NTR knock-out mice did not show loss of BFCN neurons, indicating a retrograde degenerative effect of the cortical injury on the afferent BFCNs mediated through p75NTR. In contrast, locus ceruleus neurons, which also project throughout the cortex, were unaffected by the injury, suggesting specificity in retrograde degeneration after cortical TBI. Proneurotrophins (proNTs) provided directly to basal forebrain axons in microfluidic cultures triggered retrograde axonal degeneration and cell death, which did not occur in the absence of p75NTR. This study shows that after traumatic brain injury, proNTs induced in the injured cortex promote BFCN axonal degeneration and retrograde neuron loss through p75NTR.

Electroacupuncture promotes apoptosis and inhibits axonogenesis by activating p75 neurotrophin receptor for triple-negative breast xenograft in mice

PURPOSE

The aim of this study was to investigate the anti-tumor effect of electroacupuncture (EA) on mice bearing breast tumors by regulating p75 neurotrophin receptor (p75NTR) and remodelling intratumoral innervation.

METHODS

Female BALB/c mice were implanted with 4T1 breast tumor cells to establish a murine mammary cancer model. Tumor volume and weight were measured to evaluate tumor growth. Cell apoptosis was assessed by TUNEL assay. The relative expression of p75NTR, TrkA, TrkB, NGF and proNGF were detected by immunohistochemistry. Neurotransmitter and neurotrophin were detected by enzyme-linked immunosorbent assay. Intratumoral innervation was confirmed by β3-tubulin and TH labeling immunohistochemistry. The antagonist TAT-Pep5 was employed to determine if the effects of EA on tumor growth and cell apoptosis were mediated by p75NTR.

RESULTS

Peritumoral EA alleviated tumor growth especially after 14 days of intervention. Apoptosis index in the tumor tissue was obviously decreased after EA. Meanwhile, EA intervention significantly upregulated the expression of p75NTR and proNGF, along with a decline in the tumor growth and an increase in the cell apoptosis. Besides, EA reduced local sympathetic innervation and downregulated sympathetic neurotransmitter NE level in the local tumor. Furthermore, p75NTR antagonist alleviated EA-mediated cell apoptosis and intratumoral innervation.

CONCLUSIONS

One mechanism of EA intervention for alleviating tumor progression is mediated by p75NTR to promote apoptosis and decrease intratumoral axonogenesis in the tumor microenvironment.

Apelin alleviated neuroinflammation and promoted endogenous neural stem cell proliferation and differentiation after spinal cord injury in rats

Background

Spinal cord injury (SCI) causes devastating neurological damage, including secondary injuries dominated by neuroinflammation. The role of Apelin, an endogenous ligand that binds the G protein-coupled receptor angiotensin-like receptor 1, in SCI remains unclear. Thus, our aim was to investigate the effects of Apelin in inflammatory responses and activation of endogenous neural stem cells (NSCs) after SCI.

Methods

Apelin expression was detected in normal and injured rats, and roles of Apelin in primary NSCs were examined. In addition, we used induced pluripotent stem cells (iPSCs) as a carrier to prolong the effective duration of Apelin and evaluate its effects in a rat model of SCI.

Results

Co-immunofluorescence staining suggested that Apelin was expressed in both astrocytes, neurons and microglia. Following SCI, Apelin expression decreased from 1 to 14 d and re-upregulated at 28 d. In vitro, Apelin promoted NSCs proliferation and differentiation into neurons. In vivo, lentiviral-transfected iPSCs were used as a carrier to prolong the effective duration of Apelin. Transplantation of transfected iPSCs in situ immediately after SCI reduced polarization of M1 microglia and A1 astrocytes, facilitated recovery of motor function, and promoted the proliferation and differentiation of endogenous NSCs in rats.

Conclusion

Apelin alleviated neuroinflammation and promoted the proliferation and differentiation of endogenous NSCs after SCI, suggesting that it might be a promising target for treatment of SCI.

Status of precision medicine approaches to traumatic brain injury

Traumatic brain injury (TBI) is a serious condition in which trauma to the head causes damage to the brain, leading to a disruption in brain function. This is a significant health issue worldwide, with around 69 million people suffering from TBI each year. Immediately following the trauma, damage occurs in the acute phase of injury that leads to the primary outcomes of the TBI. In the hours-to-days that follow, secondary damage can also occur, leading to chronic outcomes. TBIs can range in severity from mild to severe, and can be complicated by the fact that some individuals sustain multiple TBIs, a risk factor for worse long-term outcomes. Although our knowledge about the pathophysiology of TBI has increased in recent years, unfortunately this has not been translated into effective clinical therapies. The U.S. Food and Drug Administration has yet to approve any drugs for the treatment of TBI; current clinical treatment guidelines merely offer supportive care. Outcomes between individuals greatly vary, which makes the treatment for TBI so challenging. A blow of similar force can have only mild, primary outcomes in one individual and yet cause severe, chronic outcomes in another. One of the reasons that have been proposed for this differential response to TBI is the underlying genetic differences across the population. Due to this, many researchers have begun to investigate the possibility of using precision medicine techniques to address TBI treatment. In this review, we will discuss the research detailing the identification of genetic risk factors for worse outcomes after TBI, and the work investigating personalized treatments for these higher-risk individuals. We highlight the need for further research into the identification of higher-risk individuals and the development of personalized therapies for TBI.

Potential Therapeutic Strategies and Substances for Facial Nerve Regeneration Based on Preclinical Studies

Despite advances in microsurgical technology and an improved understanding of nerve regeneration, obtaining satisfactory results after facial nerve injury remains a difficult clinical problem. Among existing peripheral nerve regeneration studies, relatively few have focused on the facial nerve, particularly how experimental studies of the facial nerve using animal models play an essential role in understanding functional outcomes and how such studies can lead to improved axon regeneration after nerve injury. The purpose of this article is to review current perspectives on strategies for applying potential therapeutic methods for facial nerve regeneration. To this end, we searched Embase, PubMed, and the Cochrane library using keywords, and after applying exclusion criteria, obtained a total of 31 qualifying experimental studies. We then summarize the fundamental experimental studies on facial nerve regeneration, highlighting recent bioengineering studies employing various strategies for supporting facial nerve regeneration, including nerve conduits with stem cells, neurotrophic factors, and/or other therapeutics. Our summary of the methods and results of these previous reports reveal a common feature among studies, showing that various neurotrophic factors arising from injured nerves contribute to a microenvironment that plays an important role in functional recovery. In most cases, histological examinations showed that this microenvironmental influence increased axonal diameter as well as myelination thickness. Such an analysis of available research on facial nerve injury and regeneration represents the first step toward future therapeutic strategies.

The Role of BDNF in Experimental and Clinical Traumatic Brain Injury

Traumatic brain injury is one of the leading causes of mortality and morbidity in the world with no current pharmacological treatment. The role of BDNF in neural repair and regeneration is well established and has also been the focus of TBI research. Here, we review experimental animal models assessing BDNF expression following injury as well as clinical studies in humans including the role of BDNF polymorphism in TBI. There is a large heterogeneity in experimental setups and hence the results with different regional and temporal changes in BDNF expression. Several studies have also assessed different interventions to affect the BDNF expression following injury. Clinical studies highlight the importance of BDNF polymorphism in the outcome and indicate a protective role of BDNF polymorphism following injury. Considering the possibility of affecting the BDNF pathway with available substances, we discuss future studies using transgenic mice as well as iPSC in order to understand the underlying mechanism of BDNF polymorphism in TBI and develop a possible pharmacological treatment.

Effect of Bumetanide on Facial Nerve Regeneration in Rat Model

OBJECTIVE

We investigated the effects of bumetanide alone and in combination with dexamethasone on facial nerve regeneration in rats with facial paralysis.

STUDY DESIGN

A prospective controlled animal study.

SETTING

An animal laboratory.

SUBJECTS AND METHODS

Facial paralysis was induced in 32 Wistar rats that we then divided into 4 groups: group 1, control; group 2, bumetanide; group 3, dexamethasone; group 4, bumetanide and dexamethasone. Electroneurography was performed 1, 2, and 4 weeks later, and nerve regeneration was evaluated by electron and light microscopy and Western blotting in week 4.

RESULTS

Regarding the comparison between preoperative values and week 4, the latency difference in group 1 (1.25 milliseconds) was significantly higher than those of groups 2 to 4 (0.56, 0.34, and 0.10 milliseconds, respectively; P = .001). The latency increment in groups 2 and 3 was higher than that of group 4 (P = .002 and P = .046) in week 4, whereas groups 2 and 3 did not differ significantly (P = .291). Amplitude difference was not statistically significant from week 4 among all groups (all P > .05). The number of myelinated axons was significantly higher in all treatment groups than in the control group (P = .001). Axon number and intensity were significantly higher in group 4 as compared with groups 2 and 3 (P = .009, P = .005).

CONCLUSION

After primary neurorrhaphy, dexamethasone and bumetanide alone promoted nerve recovery based on electrophysiologic and histologic measures. Combination therapy was, however, superior.

Edaravone attenuates neuronal apoptosis in hippocampus of rat traumatic brain injury model via activation of BDNF/TrkB signaling pathway

INTRODUCTION

The purpose of our study was to explore the effects of edaravone on rats with traumatic brain injury (TBI) and investigate the underlying mechanism.

MATERIAL AND METHODS

All rats were separated randomly into 3 groups as follows: sham group (n = 25), TBI group (n = 25), TBI + edaravone group (n = 25). Edaravone was administered intraperitoneally (i.p.) at a dose of 3 mg/kg at 30 min, 12 h, and 24 h after TBI. The neurological impairment and spatial cognitive function were assessed by the neurologic severity score (NSS) and Morris water maze (MWM), respectively. Western blot and reverse transcription polymerase chain reaction (RT-PCR) were used to determine the expression levels of caspase-3, B-cell lymphoma-2 (Bcl-2), Bcl-2 associated X protein (Bax), brain-derived neurotrophic factor (BDNF) and tyrosine kinase receptor B (TrkB). Transferase-mediated dUTP-biotin nick end labeling (TUNEL) assay as well as flow cytometry assay was used to determine the apoptosis rate of cells.

RESULTS

Edaravone administration significantly attenuated neurological impairment induced by TBI and promoted cognitive function outcome. The expression of BDNF and TrkB was elevated with treatment of edaravone, which was increased after TBI. The expression of apoptosis related proteins such as caspase-3 and Bax-2 was decreased while that of Bcl-2 was enhanced with edaravone administration following TBI. In addition, edaravone treatment reduced TBI-induced cell apoptosis in the hippocampus.

CONCLUSIONS

Our study showed that administration with edaravone was able to inhibit neuronal apoptosis in the hippocampus in a rat TBI model. The neuroprotective function of edaravone may relate to modulation of the BDNF/TrkB signaling pathway.

APOE4 genetic polymorphism results in impaired recovery in a repeated mild traumatic brain injury model and treatment with Bryostatin-1 improves outcomes

After traumatic brain injury (TBI), some people have worse recovery than others. Single nucleotide polymorphisms (SNPs) in Apolipoprotein E (APOE) are known to increase risk for developing Alzheimer’s disease, however there is controversy from human and rodent studies as to whether ApoE4 is a risk factor for worse outcomes after brain trauma. To resolve these conflicting studies we have explored the effect of the human APOE4 gene in a reproducible mouse model that mimics common human injuries. We have investigated cellular and behavioral outcomes in genetically engineered human APOE targeted replacement (TR) mice following repeated mild TBI (rmTBI) using a lateral fluid percussion injury model. Relative to injured APOE3 TR mice, injured APOE4 TR mice had more inflammation, neurodegeneration, apoptosis, p-tau, and activated microglia and less total brain-derived neurotrophic factor (BDNF) in the cortex and/or hippocampus at 1 and/or 21 days post-injury. We utilized a novel personalized approach to treating APOE4 susceptible mice by administering Bryostatin-1, which improved cellular as well as motor and cognitive behavior outcomes at 1 DPI in the APOE4 injured mice. This study demonstrates that APOE4 is a risk factor for poor outcomes after rmTBI and highlights how personalized therapeutics can be a powerful treatment option.

Bumetanide: A review of its neuroplasticity and behavioral effects after stroke

Stroke often leads to neuronal injury and neurological functional deficits. Whilst spontaneous neurogenesis and axon regeneration are induced by ischemic stroke, effective pharmacological treatments are also essential for the improvement of neuroplasticity and functional recovery after stroke. However, no pharmacological therapy has been demonstrated to be able to effectively improve the functional recovery after stroke. Bumetanide is a specific Na+-K+-Cl- co-transporter inhibitor which can maintain chloride homeostasis in neurons. Therefore, many studies have focused on this drug's effect in stroke recovery in recent years. Here, we first review the function of Na+-K+-Cl- co-transporter in neurons, then how bumetanide's role in reducing brain damage, promoting neuroplasticity, leading to functional recovery after stroke, is elucidated. Finally, we discuss current limitations of bumetanide's efficiency and their potential solutions. These results may provide new avenues for further exploring mechanisms of post-stroke functional recovery as well as promising therapeutic targets for functional disability rehabilitation after ischemic stroke.

Proneurotrophins Induce Apoptotic Neuronal Death After Controlled Cortical Impact Injury in Adult Mice

The p75 neurotrophin receptor (p75NTR) can regulate multiple cellular functions including proliferation, survival, and apoptotic cell death. The p75NTR is widely expressed in the developing brain and is downregulated as the nervous system matures, with only a few neuronal subpopulations retaining expression into adulthood. However, p75NTR expression is induced following damage to the adult brain, including after traumatic brain injury, which is a leading cause of mortality and disability worldwide. A major consequence of traumatic brain injury is the progressive neuronal loss that continues secondary to the initial trauma, which ultimately contributes to cognitive decline. Understanding mechanisms governing this progressive neuronal death is key to developing targeted therapeutic strategies to provide neuroprotection and salvage cognitive function. In this study, we demonstrate that a cortical impact injury to the sensorimotor cortex elicits p75NTR expression in apoptotic neurons in the injury penumbra, confirming previous studies. To establish whether preventing p75NTR induction or blocking the ligands would reduce the extent of secondary neuronal cell death, we used a noninvasive intranasal strategy to deliver either siRNA to block the induction of p75NTR, or function-blocking antibodies to the ligands pro-nerve growth factor and pro-brain-derived neurotrophic factor. We demonstrate that either preventing the induction of p75NTR or blocking the proneurotrophin ligands provides neuroprotection and preserves sensorimotor function.

BDNF Val66Met Genetic Polymorphism Results in Poor Recovery Following Repeated Mild Traumatic Brain Injury in a Mouse Model and Treatment With AAV-BDNF Improves Outcomes

Clinicians have long noticed that some Traumatic Brain Injury (TBI) patients have worse symptoms and take a longer time to recover than others, for reasons unexplained by known factors. Identifying what makes some individuals more susceptible is critical to understanding the underlying mechanisms through which TBI causes deleterious effects. We have sought to determine the effect of a single nucleotide polymorphism (SNP) in Brain-derived neurotrophic factor (BDNF) at amino acid 66 (rs6265) on recovery after TBI. There is controversy from human studies as to whether the BDNF Val66Val or Val66Met allele is the risk factor for worse outcomes after brain trauma. We therefore investigated cellular and behavioral outcomes in genetically engineered mice following repeated mild TBI (rmTBI) using a lateral fluid percussion (LFP) injury model. We found that relative to injured Val66Val carriers, injured Val66Met carriers had a larger inflammation volume and increased levels of neurodegeneration, apoptosis, p-tau, activated microglia, and gliosis in the cortex and/or hippocampus at 1 and/or 21 days post-injury (DPI). We therefore concluded that the Val66Met genetic polymorphism is a risk factor for poor outcomes after rmTBI. In order to determine the mechanism for these differences, we investigated levels of the apoptotic-inducing pro BDNF and survival-inducing mature BDNF isoforms and found that Met carriers had less total BDNF in the cortex and a higher pro/mature ratio of BDNF in the hippocampus. We then developed a personalized approach to treating genetically susceptible individuals by overexpressing wildtype BDNF in injured Val66Met mice using an AAV-BDNF virus. This intervention improved cellular, motor, and cognitive behavior outcomes at 21 DPI and increased levels of mature BDNF and phosphorylation of mature BDNF's receptor trkB. This study lays the groundwork for further investigation into the genetics that play a role in the extent of injury after rmTBI and highlights how personalized therapeutics may be targeted for recovery in susceptible individuals.

Regulation of BACE1 expression after injury is linked to the p75 neurotrophin receptor

BACE1 is a transmembrane aspartic protease that cleaves various substrates and it is required for normal brain function. BACE1 expression is high during early development, but it is reduced in adulthood. Under conditions of stress and injury, BACE1 levels are increased; however, the underlying mechanisms that drive BACE1 elevation are not well understood. One mechanism associated with brain injury is the activation of injurious p75 neurotrophin receptor (p75), which can trigger pathological signals. Here we report that within 72 h after controlled cortical impact (CCI) or laser injury, BACE1 and p75 are increased and tightly co-expressed in cortical neurons of mouse brain. Additionally, BACE1 is not up-regulated in p75 null mice in response to focal cortical injury, while p75 over-expression results in BACE1 augmentation in HEK-293 and SY5Y cell lines. A luciferase assay conducted in SY5Y cell line revealed that BACE1 expression is regulated at the transcriptional level in response to p75 transfection. Interestingly, this effect does not appear to be dependent upon p75 ligands including mature and pro-neurotrophins. In addition, BACE1 activity on amyloid precursor protein (APP) is enhanced in SY5Y-APP cells transfected with a p75 construct. Lastly, we found that the activation of c-jun n-terminal kinase (JNK) by p75 contributes to BACE1 up-regulation. This study explores how two injury-induced molecules are intimately connected and suggests a potential link between p75 signaling and the expression of BACE1 after brain injury.

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

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

Resident Microglia Activate before Peripheral Monocyte Infiltration and p75NTR Blockade Reduces Microglial Activation and Early Brain Injury after Subarachnoid Hemorrhage

Early brain injury (EBI) after aneurysmal subarachnoid hemorrhage (SAH) contributes to high morbidity and mortality. Although it is well recognized that acute neuroinflammation reaction is one of the most important triggers of EBI, pharmacotherapy proved to be clinically effective against the initiating of neuroinflammation after SAH is lacking. The resident microglia and infiltrated peripheral monocyte are two main types of immune cells in central nervous system (CNS) and control the inflammation process in brain after SAH. But the time course and relative contributions of these two immune cell activations after SAH are unknown. The p75 neurotrophin receptor (p75NTR), member of TNF receptor superfamily, expresses on infiltrated peripheral monocytes and suppresses their proinflammatory action after brain insults. But the p75NTR expression on resident microglia in vivo is rarely explored and their function keeps elusive. Therefore, we designed this study to investigate the time course of resident microglia activation and peripheral monocyte infiltration, as well as the microglial expression of p75NTR by using CX3C-chemokine receptor 1 (Cx3cr1) and chemokine receptor 2 (Ccr2) double transgenic mice (Cx3cr1GFP/+Ccr2RFP/+) after SAH. The results showed activated microglia was observed in cortex as early as 24 h and further increased at 48 and 72 h post SAH, while the infiltrated monocyte was not found until 72h. In addition, activated microglia expressed p75NTR acutely and p75NTR specific antagonist TAT-Pep5 significantly reduced microglia activation, neuroinflammation and EBI from 24 to 72 h. Together, these data suggest that the early neuroinflammation reaction might be initiated and intensified mainly by resident microglia rather than infiltrated monocyte at least in the first 48 h after SAH and p75NTR blockading by TAT-Pep5P might alleviate EBI through mediating microglial activation.

BDNF, Brain, and Regeneration: Insights from Zebrafish

Zebrafish (Danio rerio) is a teleost fish widely accepted as a model organism for neuroscientific studies. The adults show common basic vertebrate brain structures, together with similar key neuroanatomical and neurochemical pathways of relevance to human diseases. However, the brain of adult zebrafish possesses, differently from mammals, intense neurogenic activity, which can be correlated with high regenerative properties. Brain derived neurotrophic factor (BDNF), a member of the neurotrophin family, has multiple roles in the brain, due also to the existence of several biologically active isoforms, that interact with different types of receptors. BDNF is well conserved in the vertebrate evolution, with the primary amino acid sequences of zebrafish and human BDNF being 91% identical. Here, we review the available literature regarding BDNF in the vertebrate brain and the potential involvement of BDNF in telencephalic regeneration after injury, with particular emphasis to the zebrafish. Finally, we highlight the potential of the zebrafish brain as a valuable model to add new insights on future BDNF studies.

Role of brain-derived neurotrophic factor during the regenerative response after traumatic brain injury in adult zebrafish

Several mammalian animal models of traumatic brain injury have been used, mostly rodents. However, reparative mechanisms in mammalian brain are very limited, and newly formed neurons do not survive for long time. The brain of adult zebrafish, a teleost fish widely used as vertebrate model, possesses high regenerative properties after injury due to the presence of numerous stem cells niches. The ventricular lining of the zebrafish dorsal telencephalon is the most studied neuronal stem cell niche because its dorso-lateral zone is considered the equivalent to the hippocampus of mammals which contains one of the two constitutive neurogenic niches of mammals. To mimic TBI, stab wound in the dorso-lateral telencephalon of zebrafish was used in studies devoted to fish regenerative properties. Brain-derived neurotrophic factor, which is known to play key roles in the repair process after traumatic brain lesions, persists around the lesioned area of injured telencephalon of adult zebrafish. These results are extensively compared to reparative processes in rodent brain. Considering the complete repair of the damaged area in fish, it could be tempting to consider brain-derived neurotrophic factor as a factor contributing to create a permissive environment that enables the establishment of new neuronal population in damaged brain.

Neuronal expression of brain derived neurotrophic factor in the injured telencephalon of adult zebrafish

The reparative ability of the central nervous system varies widely in the animal kingdom. In the mammalian brain, the regenerative mechanisms are very limited and newly formed neurons do not survive longer, probably due to a non-suitable local environment. On the opposite, fish can repair the brain after injury, with fast and complete recovery of damaged area. The brain of zebrafish, a teleost fish widely used as vertebrate model, also possesses high regenerative properties after injury. Taking advantage of this relevant model, the aim of the present study was to investigate the role of brain-derived neurotrophic factor (BDNF) in the regenerative ability of adult brain, after stab wound telencephalic injury. BDNF is involved in many brain functions and plays key roles in the repair process after traumatic brain lesions. It has been reported that BDNF strengthens the proliferative activity of neuronal precursor cells, facilitates the neuronal migration toward injured areas, and shows survival properties due to its anti-apoptotic effects. BDNF mRNA levels, assessed by quantitative PCR and in situ hybridization at 1, 4, 7, and 15 days after the lesion, were increased in the damaged telencephalon, mostly suddenly after the lesion. Double staining using in situ hybridization and immunocytochemistry revealed that BDNF mRNA was restricted to cells identified as mature neurons. BDNF mRNA expressing neurons mostly increased in the area around the lesion, showing a peak 1 day after the lesion. Taken together, these results highlight the role of BDNF in brain repair processes and reinforce the value of zebrafish for the study of regenerative neurogenesis.

Neurotrauma: The Crosstalk between Neurotrophins and Inflammation in the Acutely Injured Brain

Traumatic brain injury (TBI) is a major cause of morbidity and mortality among young individuals worldwide. Understanding the pathophysiology of neurotrauma is crucial for the development of more effective therapeutic strategies. After the trauma occurs, immediate neurologic damage is produced by the traumatic forces; this primary injury triggers a secondary wave of biochemical cascades together with metabolic and cellular changes, called secondary neural injury. In the scenario of the acutely injured brain, the ongoing secondary injury results in ischemia and edema culminating in an uncontrollable increase in intracranial pressure. These areas of secondary injury progression, or areas of “traumatic penumbra”, represent crucial targets for therapeutic interventions. Neurotrophins are a class of signaling molecules that promote survival and/or maintenance of neurons. They also stimulate axonal growth, synaptic plasticity, and neurotransmitter synthesis and release. Therefore, this review focuses on the role of neurotrophins in the acute post-injury response. Here, we discuss possible endogenous neuroprotective mechanisms of neurotrophins in the prevailing environment surrounding the injured areas, and highlight the crosstalk between neurotrophins and inflammation with focus on neurovascular unit cells, particularly pericytes. The perspective is that neurotrophins may represent promising targets for research on neuroprotective and neurorestorative processes in the short-term following TBI.

Natural Compounds as a Therapeutic Intervention following Traumatic Brain Injury: The Role of Phytochemicals

There has been a tremendous focus on the discovery and development of neuroprotective agents that might have clinical relevance following traumatic brain injury (TBI). This type of brain injury is very complex and is divided into two major components. The first component, a primary injury, occurs at the time of impact and is the result of the mechanical insult itself. This primary injury is thought to be irreversible and resistant to most treatments. A second component or secondary brain injury, is defined as cellular damage that is not immediately obvious after trauma, but that develops after a delay of minutes, hours, or even days. This injury appears to be amenable to treatment. Because of the complexity of the secondary injury, any type of therapeutic intervention needs to be multi-faceted and have the ability to simultaneously modulate different cellular changes. Because of diverse pharmaceutical interactions, combinations of different drugs do not work well in concert and result in adverse physiological conditions. Research has begun to investigate the possibility of using natural compounds as a therapeutic intervention following TBI. These compounds normally have very low toxicity and have reduced interactions with other pharmaceuticals. In addition, many natural compounds have the potential to target numerous different components of the secondary injury. Here, we review 33 different plant-derived natural compounds, phytochemicals, which have been investigated in experimental animal models of TBI. Some of these phytochemicals appear to have potential as possible therapeutic interventions to offset key components of the secondary injury cascade. However, not all studies have used the same scientific rigor, and one should be cautious in the interpretation of studies using naturally occurring phytochemical in TBI research.

Oxidation of KCNB1 Potassium Channels Causes Neurotoxicity and Cognitive Impairment in a Mouse Model of Traumatic Brain Injury

The delayed rectifier potassium (K+) channel KCNB1 (Kv2.1), which conducts a major somatodendritic current in cortex and hippocampus, is known to undergo oxidation in the brain, but whether this can cause neurodegeneration and cognitive impairment is not known. Here, we used transgenic mice harboring human KCNB1 wild-type (Tg-WT) or a nonoxidable C73A mutant (Tg-C73A) in cortex and hippocampus to determine whether oxidized KCNB1 channels affect brain function. Animals were subjected to moderate traumatic brain injury (TBI), a condition characterized by extensive oxidative stress. Dasatinib, a Food and Drug Administration-approved inhibitor of Src tyrosine kinases, was used to impinge on the proapoptotic signaling pathway activated by oxidized KCNB1 channels. Thus, typical lesions of brain injury, namely, inflammation (astrocytosis), neurodegeneration, and cell death, were markedly reduced in Tg-C73A and dasatinib-treated non-Tg animals. Accordingly, Tg-C73A mice and non-Tg mice treated with dasatinib exhibited improved behavioral outcomes in motor (rotarod) and cognitive (Morris water maze) assays compared to controls. Moreover, the activity of Src kinases, along with oxidative stress, were significantly diminished in Tg-C73A brains. Together, these data demonstrate that oxidation of KCNB1 channels is a contributing mechanism to cellular and behavioral deficits in vertebrates and suggest a new therapeutic approach to TBI.

SIGNIFICANCE STATEMENT

This study provides the first experimental evidence that oxidation of a K+ channel constitutes a mechanism of neuronal and cognitive impairment in vertebrates. Specifically, the interaction of KCNB1 channels with reactive oxygen species plays a major role in the etiology of mouse model of traumatic brain injury (TBI), a condition associated with extensive oxidative stress. In addition, a Food and Drug Administration-approved drug ameliorates the outcome of TBI in mouse, by directly impinging on the toxic pathway activated in response to oxidation of the KCNB1 channel. These findings elucidate a basic mechanism of neurotoxicity in vertebrates and might lead to a new therapeutic approach to TBI in humans, which, despite significant efforts, is a condition that remains without effective pharmacological treatments.

Cognitive assessment of pycnogenol therapy following traumatic brain injury

We have previously shown that pycnogenol (PYC) increases antioxidants, decreases oxidative stress, suppresses neuroinflammation and enhances synaptic plasticity following traumatic brain injury (TBI). Here, we investigate the effects of PYC on cognitive function following a controlled cortical impact (CCI). Adult Sprague-Dawley rats received a CCI injury followed by an intraperitoneal injection of PYC (50 or 100mg/kg). Seven days post trauma, subjects were evaluated in a Morris water maze (MWM) and evaluated for changes in lesion volume. Some animals were evaluated at 48h for hippocampal Fluoro-jade B (FJB) staining. The highest dose of PYC therapy significantly reduced lesion volume, with no improvement in MWM compared to vehicle controls. PYC failed to reduce the total number of FJB positive neurons in the hippocampus. These results suggest that the reduction of oxidative stress and neuroinflammation are not the key components of the secondary injury that contribute to cognitive deficits following TBI.