Faulty bony repair causes dysrepair of injured growth plate cartilage and bone growth defects in children; however, the underlying mechanisms are unclear. Recently, we observed the prominent induction of neurotrophin‑3 (NT-3) and its important roles as an osteogenic and angiogenic factor promoting the bony repair. The current study investigated its roles in regulating injury site remodelling. In a rat tibial growth plate drill-hole injury repair model, NT-3 was expressed prominently in osteoblasts at the injury site. Recombinant NT-3 (rhNT-3) systemic treatment enhanced, but NT-3 immunoneutralization attenuated, expression of cartilage-removal proteases (MMP-9 and MMP-13), presence of bone-resorbing osteoclasts and expression of osteoclast protease cathepsin K, and remodelling at the injury site. NT-3 was also highly induced in cultured mineralizing rat bone marrow stromal cells, and the conditioned medium augmented osteoclast formation and resorptive activity, an ability that was blocked by presence of anti-NT-3 antibody. Moreover, NT-3 and receptor TrkC were induced during osteoclastogenesis, and rhNT-3 treatment activated TrkC downstream kinase Erk1/2 in differentiating osteoclasts although rhNT-3 alone did not affect activation of osteoclastogenic transcription factors NF-κB or NFAT in RAW264.7 osteoclast precursor cells. Furthermore, rhNT-3 treatment increased, but NT-3 neutralization reduced, expression of osteoclastogenic cytokines (RANKL, TNF-α, and IL-1) in mineralizing osteoblasts and in growth plate injury site, and rhNT-3 augmented the induction of these cytokines caused by RANKL treatment in RAW264.7 cells. Thus, injury site osteoblast-derived NT-3 is important in promoting growth plate injury site remodelling, as it induces cartilage proteases for cartilage removal and augments osteoclastogenesis and resorption both directly (involving activing Erk1/2 and substantiating RANKL-induced increased expression of osteoclastogenic signals in differentiating osteoclasts) and indirectly (inducing osteoclastogenic signals in osteoblasts).

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive deposition of amyloid beta (Aβ) and dysregulation of neurotrophic signaling, causing synaptic dysfunction, loss of memory, and cell death. The expression of p75 neurotrophin receptor is elevated in the brain of AD patients, suggesting its involvement in this disease. However, the exact mechanism of its action is not yet clear. Here, we show that p75 interacts with beta-site amyloid precursor protein cleaving enzyme-1 (BACE1), and this interaction is enhanced in the presence of Aβ. Our results suggest that the colocalization of BACE1 and amyloid precursor protein (APP) is increased in the presence of both Aβ and p75 in cortical neurons. In addition, the localization of APP and BACE1 in early endosomes is increased in the presence of Aβ and p75. An increased phosphorylation of APP-Thr668 and BACE1-Ser498 by c-Jun N-terminal kinase (JNK) in the presence of Aβ and p75 could be responsible for this localization. In conclusion, our study proposes a potential involvement in amyloidogenesis for p75, which may represent a future therapeutic target for AD. Cover Image for this Issue: doi. 10.1111/jnc.14163.

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.

p75 neurotrophin receptor (p75NTR) has been implicated in Alzheimer's disease (AD). However, whether p75NTR is involved in Tau hyperphosphorylation, one of the pathologies observed in AD, remains unclear. In our previous study, the extracellular domain of p75NTR blocked amyloid beta (Aβ) toxicity and attenuated Aβ-induced Tau hyperphosphorylation. Here we show that, in the absence of Aβ, p75NTR regulates Tau phosphorylation in the transgenic mice with the P301L human Tau mutation (pR5). The knockout of p75NTR in pR5 mice attenuated the phosphorylation of human Tau. In addition, the elevated activity of kinases responsible for Tau phosphorylation including glycogen synthase kinase 3 beta; cyclin-dependent-kinase 5; and Rho-associated protein kinase was also inhibited when p75NTR is knocked out in pR5 mice at 9 months of age. The increased caspase-3 activity observed in pR5 mice was also abolished in the absence of p75NTR. Our study also showed that p75NTR is required for Aβ- and pro-brain derived neurotrophin factor (proBDNF)-induced Tau phosphorylation, in vitro. Overall, our data indicate that p75NTR is required for Tau phosphorylation, a key event in the formation of neurofibrillary tangles, another hallmark of AD. Thus, targeting p75NTR could reduce or prevent the pathologic hyperphosphorylation of Tau.

Schwann cell migration, including collective migration and chemotaxis, is essential for the formation of coordinate interactions between Schwann cells and axons during peripheral nerve development and regeneration. Moreover, limited migration of Schwann cells imposed a serious obstacle on Schwann cell-astrocytes intermingling and spinal cord repair after Schwann cell transplantation into injured spinal cords. Recent studies have shown that mature brain-derived neurotrophic factor, a member of the neurotrophin family, inhibits Schwann cell migration. The precursor form of brain-derived neurotrophic factor, proBDNF, was expressed in the developing or degenerating peripheral nerves and the injured spinal cords. Since "the yin and yang of neurotrophin action" has been established as a common sense, proBDNF would be expected to promote Schwann cell migration. However, we found, in the present study, that exogenous proBDNF also inhibited in vitro collective migration and chemotaxis of RSC 96 cells, a spontaneously immortalized rat Schwann cell line. Moreover, proBDNF suppressed adhesion and spreading of those cells. At molecular level, proBDNF inhibits F-actin polymerization and focal adhesion dynamics in cultured RSC 96 cells. Therefore, our results suggested a special case against the classical opinion of "the yin and yang of neurotrophin action" and implied that proBDNF might modulate peripheral nerve development or regeneration and spinal cord repair through perturbing native or transplanted Schwann cell migration.

Inappropriate expansion of antibody-secreting cells (ASCs) is typical of systemic lupus erythematosus (SLE), but the regulatory signaling of pathogenic ASCs is unclear. The present study shows that brain-derived neurotrophic factor precursor (proBDNF) and its high-affinity pan-75 neurotrophin receptor (p75NTR) are highly expressed in CD19+CD27hiCD38hi ASCs in patients with SLE and in CD19+CD44hiCD138+ ASCs in lupus-like mice. The increased proBDNF+ ASCs were positively correlated with clinical symptoms and higher titers of autoantibodies in SLE. Administration of monoclonal antibodies against proBDNF or specific knockout of p75NTR in CD19+ B cells exerted a therapeutic effect on lupus mice by limiting the proportion of ASCs, reducing the production of autoantibodies and attenuating kidney injury. Blocking the biological function of proBDNF or p75NTR also inhibits ASC differentiation and antibody production in vitro. Together, these findings suggest that proBDNF-p75NTR signaling plays a critical pathogenic role in SLE through promoting ASC dysfunction.

ProBDNF, a precursor of brain-derived neurotrophic factor (BDNF), is an important regulator of neurodegeneration, hippocampal long-term depression, and synaptic plasticity. ProBDNF and its receptors pan-neurotrophin receptor p75 (p75NTR), vps10p domain-containing receptor Sortilin and tropomyosin receptor kinase B (TrkB) are expressed in neuronal and glial cells. The role of proBDNF in regulation of neurogenesis is not fully defined. This study aims to uncover the function of proBDNF in regulating the differentiation, migration and proliferation of mouse neural stem cells (NSCs) in vitro. We have found that proBDNF and its receptors are constitutively expressed in NSCs when assessed by immunocytochemistry and western blotting. MTT (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide) assay showed that exogenous proBDNF treatment reduced mouse NSCs viability by 38% at 10ng/mL. The migration of NSCs was also reduced by exogenous proBDNF treatment in a concentration-dependent manner (by 90% at 10ng/mL) but increased by anti-proBDNF antibody treatment (by 50%). BrdU (5-Bromo-2'-Deoxyuridine) incorporation was performed for detection of newborn cells. We have found that proBDNF significantly inhibited proliferation of NSCs and reduced the number of differentiated neurons, oligodendrocytes and astrocytes, while anti-proBDNF antibody treatment promoted proliferation and differentiation of NSCs. In conclusion, proBDNF may oppose the functions of mature BDNF by inhibiting the proliferation, differentiation and migration of NSCs during development. Conversely, anti-proBDNF antibody treatment promoted proliferation and differentiation of NSCs.

The precursor of brain derived neurotrophic factor (proBDNF), the unprocessed BDNF gene product, binds to its receptors and exerts the opposing biologic functions of mature BDNF. proBDNF is expressed in the peripheral tissues but the functions of peripheral proBDNF remain elusive. Here we showed that proBDNF and its predominant receptor, p75 pan-neurotrophin receptor were upregulated in the nerve fibers and inflammatory cells in the local tissue in inflammatory pain. Neutralization of proBDNF by polyclonal antibody attenuated pain in different models of inflammatory pain. Unilateral intra-plantar supplementation of proBDNF by injecting exogenous proBDNF or ectopic overexpression resulted in pain hypersensitivity and induced spinal phosphorylated extracellular signal-regulated kinase activation. Exogenous proBDNF injection induced the infiltration of inflammatory cells and the activation of proinflammatory cytokines, suggesting that inflammatory reaction contributed to the pro-algesic effect of proBDNF. Finally, we generated monoclonal anti-proBDNF antibody that could biologically block proBDNF. Administration of monoclonal Ab-proBDNF attenuated various types of inflammatory pain and surgical pain. Thus, peripheral proBDNF is a potential pain mediator and anti-proBDNF pretreatment may alleviate the development of inflammatory pain.