The expression and putative role of brain-derived neurotrophic factor and its receptor in bovine sperm

The neurotrophin family of proteins promote the survival and differentiation of nerve cells and are thought to play an important role in development of reproductive tissues. The objective of the present study was to detect the presence of Brain-derived neurotrophic factor (BDNF) and its receptor TrkB in bovine sperm, and explore the potential role of BDNF in sperm function. We demonstrated that both the neorotrophin BDNF and the tyrosine kinase receptor protein TrkB were expressed in ejaculated bovine sperm. Furthermore, BDNF per se was secreted by sperm. Insulin and leptin secretion by bovine sperm were increased (P < 0.01) when cells were exposed to exogenous BDNF, whereas insulin was decreased by K252a. Therefore, we inferred that BDNF could be a regulator of sperm secretion of insulin and leptin through the TrkB receptor. Sperm viability and mitochondrial activity were both decreased (P < 0.05) when the BDNF/TrkB signaling pathway was blocked with K252a. Furthermore, BDNF promoted apoptosis of bovine sperm through TrkB binding (P < 0.05). In conclusion, these observations provided evidence that BDNF secreted by bovine sperm was important in regulation of insulin and leptin secretion in ejaculated bovine sperm. Furthermore, BDNF may affect sperm mitochondrial activity and apoptosis, as well as their viability.     

Paracrine and autocrine functions of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in brain-derived endothelial cells

Brain-derived neurotrophic factor (BDNF) is expressed by endothelial cells. We investigated the characteristics of BDNF expression by brain-derived endothelial cells and tested the hypothesis that BDNF serves paracrine and autocrine functions affecting the vasculature of the central nervous system. In addition to expressing TrkB and p75NTR and BDNF under normoxic conditions, these cells increased their expression of BDNF under hypoxia. While the expression of TrkB is unaffected by hypoxia, TrkB exhibits a base-line phosphorylation under normoxic conditions and an increased phosphorylation when BDNF is added. TrkB phosphorylation is decreased when endogenous BDNF is sequestered by soluble TrkB. Exogenous BDNF elicits robust angiogenesis and survival in three-dimensional cultures of these endothelial cells, while sequestration of endogenous BDNF caused significant apoptosis. The effects of BDNF engagement of TrkB appears to be mediated via the phosphatidylinositol (PI) 3-kinase-Akt pathway. Modulation of BDNF levels directly correlate with Akt phosphorylation and inhibitors of PI 3-kinase abrogate the BDNF responses. BDNF-mediated effects on endothelial cell survival/apoptosis correlated directly with activation of caspase 3. These endothelial cells also express p75NTR and respond to its preferred ligand, pro-nerve growth factor (pro-NGF), by undergoing apoptosis. These data support a role for neurotrophins signaling in the dynamic maintenance/differentiation of central nervous system endothelia.     

Yeast polypeptide fusion surface display levels predict thermal stability and soluble secretion efficiency.

Efficiency of yeast cell surface display can serve as a proxy screening variable for enhanced thermal stability and soluble secretion efficiency of mutant proteins. Several single-chain T cell receptor (scTCR) single-site mutants that enable yeast surface display, along with their double and triple mutant combinations, were analyzed for soluble secretion from the yeast Saccharomyces cerevisiae. While secretion of the wild-type scTCR was not detected, each of the single, double, and triple mutants were produced in yeast supernatants, with increased expression resulting from the double and triple mutants. Soluble secretion levels were strongly correlated with the quantity of active scTCR displayed as a fusion to Aga2p on the surface of yeast. Thermal stability of the scTCR mutants correlated directly with the secreted and surface levels of scTCR, with evidence suggesting that intracellular proteolysis by the endoplasmic reticulum quality control apparatus dictates display efficiency. Thus, yeast display is a directed evolution scaffold that can be used for the identification of mutant eucaryotic proteins with significantly enhanced stability and secretion properties.     

Polysialic acid limits choline acetyltransferase activity induced by brain-derived neurotrophic factor

Choline acetyltransferase (ChAT), the enzyme synthesizing acetylcholine, is known to be activated by brain derived neurotrophic factor (BDNF). We found that the specific removal of the carbohydrate polysialic acid (PSA) significantly increased BDNF-induced ChAT-activity in embryonic septal neurons. Using a p75 neurotrophin receptor (p75(NTR)) function-blocking antibody and K252a, a-pan tropomyosin related kinase (Trk) inhibitor, we demonstrate that BDNF-induced ChAT activity requires the stimulation of p75(NTR) and TrkB. PSA removal drastically increased radioactive iodinated ([(125)I])BDNF's maximal binding capacity (Bmax), derived from concentrations of [(125)I]BDNF ranging from 1 pM to 3.2 nM. In the presence of unlabeled nerve growth factor to prevent the binding of [(125)I]BDNF to p75(NTR) sites, the impact of PSA removal on the binding capacity of [(125)I]BDNF was greatly reduced. In conclusion, PSA limits BDNF-induced ChAT activity and BDNF-receptor interactions. BDNF-induced ChAT activity is TrkB and p75(NTR) dependent, and upon PSA removal the additional binding of BDNF to its receptors, especially p75(NTR), likely contributes to the maximal ChAT activity observed. In vivo, the ontogenetic loss of PSA in the postnatal period may allow more interactions between BDNF and its receptors to increase ChAT activity and assure the proper development of the cholinergic septal neurons.     

Overcoming the inhibitors of myelin with a novel neurotrophin strategy

Myelin inhibitors activate a p75(NTR)-dependent signaling cascade in neurons that not only inhibits axonal growth but also prevents neurotrophins (NT) from stimulating growth. Most intriguingly, in addition to Trk receptors, neurotrophins also bind to p75(NTR). We have designed a "mini-neurotrophin" called B(AG) to activate TrkB in the absence of p75(NTR) binding. We find that B(AG) is as effective as the natural TrkB ligands (brain-derived neurotrophic factor (BDNF) and NT-4) at promoting neurite outgrowth from cerebellar neurons. Furthermore, the neurite outgrowth responses stimulated by BDNF and B(AG) are inhibited by a common set of reagents, including the Trk receptor inhibitor K252a, as well as protein kinase A and phosphoinositide 3-kinase inhibitors. However, in contrast to BDNF, B(AG) promotes growth in the presence of a myelin inhibitor or when antibodies directly activate the p75(NTR) inhibitory pathway. On the basis of this observation, we postulated that the binding of BDNF to the p75(NTR) might compromise the ability of BDNF to stimulate neurite outgrowth in an inhibitory environment. To test this, we used NGF, and an NGF-derived peptide, to compete for the BDNF/p75(NTR) interaction; remarkably, in the presence of either agent, BDNF acquired the ability to promote neurite outgrowth in the presence of a myelin inhibitor. The data suggest that in an inhibitory environment, the BDNF/p75(NTR) interaction compromises regeneration. Agents that activate Trk receptors in the absence of p75(NTR) binding, or agents that inhibit neurotrophin/p75(NTR) binding, might therefore be better therapeutic candidates than neurotrophins.     

Functional analysis of mutant neurotrophins deficient in low-affinity binding reveals a role for p75LNGFR in NT-4 signalling.

The neurotrophins mediate their effects through binding to two classes of receptors, a tyrosine kinase receptor, member of the Trk family, and the low-affinity neurotrophin receptor, p75LNGFR, of as yet undefined signalling capacity. The need for a two-component receptor system in neurotrophin signalling is still not understood. Using site-directed mutagenesis, we have identified positively charged surfaces in BDNF, NT-3 and NT-4 that mediate binding to p75LNGFR. Arg31 and His33 in NT-3, and Arg34 and Arg36 in NT-4, located in an exposed hairpin loop, were found to be essential for binding to p75LNGFR. In BDNF, however, positively charged residues critical for p75LNGFR binding (Lys95, Lys96 and Arg97) were found in a spatially close but distinct loop region. Models of each neurotrophin were built using the coordinates of NGF. Analysis of their respective electrostatic surface potentials revealed similar clusters of positively charged residues in each neurotrophin but with differences in their precise spatial locations. Disruption of this positively charged interface abolished binding to p75LNGFR but not activation of cognate Trk receptors or biological activity in Trk-expressing fibroblasts. Unexpectedly, loss of low-affinity binding in NT-4, but not in BDNF or NT-3, affected receptor activation and biological activity in neuronal cells co-expressing p75LNGFR and TrkB, suggesting a role for p75LNGFR in regulating biological responsiveness to NT-4. These findings reveal a possible mechanism of ligand discrimination by p75LNGFR and suggest this receptor may selectively modulate the biological actions of specific neurotrophin family members.     

HAP1 Is Required for Endocytosis and Signalling of BDNF and Its Receptors in Neurons

When BDNF binds to its receptors, TrkB and p75^NTR, the BDNF-receptor complex is endocytosed and trafficked to the cell body for downstream signal transduction, which plays a critical role in neuronal functions. Huntingtin-associated protein 1 (HAP1) is involved in trafficking of vesicles intracellularly and also interacts with several membrane proteins including TrkB. Although it has been known that HAP1 has functions in vesicular trafficking and receptor stabilisation, it is not yet established whether HAP1 has a role in BDNF and its receptor endocytosis. In the present study, we found that HAP1 is in an interacting complex with p75^NTR, TrkB and BDNF, especially newly endocytosed BDNF. BDNF and TrkB internalisation is abolished in HAP1 knock-out (KO) cortical neurons. TrkB downstream signalling pathways such as ERK, Akt and PLCγ-1 are also impaired in HAP1 KO cortical neurons upon BDNF stimulation. Proliferation of cerebellar granule cells is also impaired in cell culture and cerebellum of HAP1 KO mice. Our findings suggest that HAP1 may play a key role in BDNF and its receptor endocytosis and may promote neuronal survival and proliferation.     

The effects of chronic alcoholization on the expression of brain-derived neurotrophic factor and its receptors in the brains of mice genetically predisposed to depressive-like behavior

Brain-derived neurotropic factor (BDNF) plays an important role in mechanisms of depression. Precursor protein of this factor (proBDNF) can initiate apoptosis in the brain, while the mature form of BDNF is involved in neurogenesis. It is known that chronic alcoholization leads to the activation of apoptotic processes, neurodegeneration, brain injury, and cognitive dysfunction. In this work, we have studied the influence of long-term ethanol exposure on the proBDNF and BDNF protein levels, as well as on the expression of genes that encode these proteins in the brain structures of ASC mice with genetic predisposition to depressive-like behavior and in mice from parental nondepressive CBA strain. It was shown that chronic alcoholization results in a reduction of the BDNF level in the hippocampus and an increase in the amount of TrkB and p75 receptors in the frontal cortex of nondepressive CBA mice. At the same time, the long-term alcoholization of depressive ASC mice results in an increase of the proBDNF level in the frontal cortex and a reduction in the p75 protein level in the hippocampus. It has also been shown that, in depressive ASC mice, proBDNF and BDNF levels are significantly lower in the hippocampus and the frontal cortex compared with nondepressive CBA strain. However, no significant differences in the expression of genes encoding the studied proteins were observed. Thus, changes in the expression patterns of proBDNF, BDNF, and their receptors under the influence of alcoholization in the depressive ASC strain and nondepressive CBA strain mice are different.     

Brain-derived neurotrophic factor promotes central nervous system myelination via a direct effect upon oligodendrocytes

The extracellular factors that are responsible for inducing myelination in the central nervous system (CNS) remain elusive. We investigated whether brain-derived neurotrophic factor (BDNF) is implicated, by first confirming that BDNF heterozygous mice exhibit delayed CNS myelination during early postnatal development. We next established that the influence of BDNF upon myelination was direct, by acting on oligodendrocytes, using co-cultures of dorsal root ganglia neurons and oligodendrocyte precursor cells. Importantly, we found that BDNF retains its capacity to enhance myelination of neurons or by oligodendrocytes derived from p75NTR knockout mice, indicating the expression of p75NTR is not necessary for BDNF-induced myelination. Conversely, we observed that phosphorylation of TrkB correlated with myelination, and that inhibiting TrkB signalling also inhibited the promyelinating effect of BDNF, suggesting that BDNF enhances CNS myelination via activating oligodendroglial TrkB-FL receptors. Together, our data reveal a previously unknown role for BDNF in potentiating the normal development of CNS myelination, via signalling within oligodendrocytes.     

Brain-derived neurotrophic factor promotes interaction of the Nck2 adaptor protein with the TrkB tyrosine kinase receptor

Brain-derived neurotrophic factor (BDNF) binds to and activates the TrkB tyrosine kinase receptor to regulate cell differentiation, survival, and neural plasticity in the nervous system. However, the identities of the downstream signaling proteins involved in this process remain unclear. Using a yeast two-hybrid screen with the intracellular domain (ICD-TrkB) of the TrkB BDNF receptor, we identified the Nck2 adaptor protein as a novel interaction partner of the active form of TrkB. Additionally, we identified three tyrosines in ICD-TrkB (Y694, Y695, and Y771) that are crucial for this interaction. Similar results were obtained for Nck1, an Nck2 homolog. We also found that TrkB could be co-precipitated with GST-Nck2 recombinant protein or anti-Nck antibody in BDNF-activated cortical neurons. These results suggest that BDNF stimulation promotes interaction of Ncks with TrkB in cortical neurons.     

Effects of castration on the immunoreactivity to NGF, BDNF and their receptors in the pelvic ganglia of the male rat

Nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) and are members of the neurotrophin family, a family of neurotrophic factors that also includes neurotrophin (NT) 3 and NT4/5. Neurotrophins have essential roles in the survival, development and differentiation of neurons in the central and peripheral nervous systems. Neurotrophins exert their effects by binding to corresponding receptors which are formed by the tyrosine protein kinases TrkA, TrkB and TrkC, and the low affinity neurotrophic receptor (p75NTR). In the present study, using immunohistochemistry and quantitative analysis, we have investigated immunoreactivity to BDNF, NGF, TrkB, p75NTR and TrkA in the pelvic ganglia of normal and castrated rats. Neurons of the pelvic ganglia expressed both these neurotrophins and their receptors. After castration the immunoreactivity persisted. However, the number of BDNF- and p75NTR-IR cells statistically significant decreased after castration. These results suggest that castration modulates the expression of neurotrophins and their receptors in pelvic autonomic neurons.     

TrkB mutant lacking the amino-terminal half of the extracellular portion acts as a functional brain-derived neurotrophic factor receptor.

A series of mutants with deletion in the extracellular portion of TrkB were expressed transiently and stably in mammalian cells to examine the brain-derived neurotrophic factor (BDNF)-binding properties of TrkB. We found that these binding activities were retained by the TrkB deletion mutant (TrkBDelta4) lacking most of the extracellular portion, cysteine-rich cluster 1 and 2, leucine-rich motif and most of the first immunoglobulin-like domain (Ig1). Furthermore, the results of the neurotrophin selectivity, the equilibrium binding constant, auto-phosphorylation and BDNF dependent cell survival indicate that TrkBDelta4 acts as a functional BDNF receptor comparable to wild-type TrkB. Thus, our findings showed that only the carboxyl-terminal half of the extracellular portion of TrkB, which includes the Ig2 domain, is essential for the functional BDNF receptor.     

Bicaudal‐D1 regulates the intracellular sorting and signalling of neurotrophin receptors

We have identified a new function for the dynein adaptor Bicaudal D homolog 1 (BICD1) by screening a siRNA library for genes affecting the dynamics of neurotrophin receptor‐containing endosomes in motor neurons (MNs). Depleting BICD1 increased the intracellular accumulation of brain‐derived neurotrophic factor (BDNF)‐activated TrkB and p75 neurotrophin receptor (p75^NTR) by disrupting the endosomal sorting, reducing lysosomal degradation and increasing the co‐localisation of these neurotrophin receptors with retromer‐associated sorting nexin 1. The resulting re‐routing of active receptors increased their recycling to the plasma membrane and altered the repertoire of signalling‐competent TrkB isoforms and p75^NTR available for ligand binding on the neuronal surface. This resulted in attenuated, but more sustained, AKT activation in response to BDNF stimulation. These data, together with our observation that Bicd1 expression is restricted to the developing nervous system when neurotrophin receptor expression peaks, indicate that BICD1 regulates neurotrophin signalling by modulating the endosomal sorting of internalised ligand‐activated receptors.     

Down-regulation of brain-derived neurotrophic factor and its signaling components in the brain tissues of scrapie experimental animals

Prion is a unique nucleic acid-free pathogen that causes human and animal fatal neurodegenerative diseases. Brain-derived neurotrophic factor (BDNF) is a prototypic neurotrophin that helps to support the survival of existing neurons, and encourage the growth and differentiation of new neurons and synapses through axonal and dendritic sprouting. There are two distinct classes of glycosylated receptors, neurotrophin receptor p75 (p75NTR) and tropomyosin-related kinase (Trk), that can bind to BDNF. To obtain insights into the possible alterations of brain BDNF and its signaling pathway in prion disease, the levels of BDNF and several molecules in the BDNF pathway in the brain tissues of scrapie agents 263K-infected hamsters were separately evaluated. Western blots and/or immunohistochemical (IHC) assays revealed that BDNF, TrkB, GRB2 and p75NTR, were significantly downregulated in the brain tissues of scrapie-infected rodents at terminal stage. Double-stained immunofluorescent assay (IFA) demonstrated that BDNF and phospho-TrkB predominately expressed in neurons. Dynamic analyses of the brain samples collected at the different time-points during the incubation period illustrated continuous decreases of BDNF, TrkB, phospho-TrkB, GRB2 and p75NTR, which correlated well with neuron loss. However, these proteins remained almost unchanged in the prion infected cell line SMB-S15 compared with those of its normal cell line SMB-PS. These data suggest that the BDNF signaling pathway is severely hindered in the brains of prion disease, which may contribute, at least partially, to the neuron death.     

Post-translational regulation of expression and conformation of an immunoglobulin domain in yeast surface display

Display of heterologous proteins on the surface of Saccharomyces cerevisiae is increasingly being exploited for directed evolution because of straightforward cell screens. However, yeast post-translationally modifies proteins in ways that must be factored into library engineering and refinement. Here, we express the extracellular immunoglobulin domain of an ubiquitous mammalian membrane protein, CD47, which is implicated in cancer, immunocompatibility, and motility. CD47 has multiple sites of glycosylation and a core disulfide bond. We assess the effects of both of these post-translational modifications on expression and antibody binding. CD47's extracellular domain is fused to the yeast mating protein Aga2p on the cell wall, and the resulting fusion protein binds several key antibodies, including a conformation-sensitive antibody. Site-by-site mutagenesis of CD47's five N-linked glycosylation sites progressively decreases expression levels on yeast, but folding appears stable. Cysteine mutations disrupt the expected core disulfide, and also decrease protein expression levels, though not to the extent seen with complete deglycosylation. However, with the core disulfide mutants, antibody binding proves to be lower than expected from expression levels and glycosylation is clearly reduced compared to wild-type. The results indicate that glycosylation regulates heterologous display on yeast more than core disulfides do and thus suggest bounds on directed evolution by post-translational processing.     

The p75 receptor is required for BDNF-induced differentiation of neural precursor cells

Epidermal growth factor (EGF)-treated neurospheres from fetal forebrain contain multipotential cells capable of neuronal, astrocytic, and oligodendroglial differentiation. These neural precursor cells express the TrkB as well as the neurotrophin receptor p75 (p75NTR), suggesting that they are BDNF responsive. In this study, we test whether the p75NTR plays a role in the differentiation of these neural precursor cells in vitro. Activation of the TrkB and the p75NTR by the addition of BDNF facilitates neuronal commitment and marked neurite genesis. However, no promotion of neuronal commitment by BDNF was observed in the neural precursor cells from mice carrying a mutation in the p75NTR gene. In addition, we observed a significant increase in the number of nestin-positive cells and the proliferation of the cells lacking functional p75NTR. These findings suggest that the p75NTR is required for proper neuronal fate decision as well as the differentiation of the neural precursor cells.     

Analysis of tyrosine phosphorylation-dependent protein-protein interactions in TrkB-mediated intracellular signaling using modified yeast two-hybrid system.

Activated receptor tyrosine kinases induce a large number of tyrosine phosphorylation-dependent protein-protein interactions through which they mediate their various ligand-exerted functions including regulation of proliferation, differentiation and survival. TrkB receptor tyrosine kinase activated by binding of brain-derived neurotrophic factor (BDNF) also stimulates various protein interactions in a tyrosine phosphorylation-dependent manner in neuronal cells. To examine tyrosine phosphorylation-dependent interactions stimulated by active TrkB, we developed a modified yeast two-hybrid system, which we call the yeast two-and-a-half-hybrid system. In this system, yeast was engineered to express a tyrosine kinase domain of TrkB as an effector, in addition to two fusion proteins with GAL4 DNA-binding and GAL4 activation domains as bait and prey proteins, respectively. Using this system with Shp2 as the bait, we demonstrated that Shp2 interacts directly with BIT/SHPS-1 (also called SIRP) and Grb2 depending on tyrosine phosphorylation mediated by TrkB. Furthermore, we screened an adult human brain cDNA library with the yeast two-and-a-half-hybrid system in order to identify other Shp2-binding proteins in TrkB-stimulated tyrosine phosphorylation signaling. We found that fibroblast growth factor receptor substrate 2beta (FRS2beta), also called SNT2, interacts with Shp2 dependently on TrkB-mediated tyrosine phosphorylation of FRS2beta/SNT2. Therefore, we show that the two-and-a-half-hybrid system is a powerful tool for studying tyrosine phosphorylation-dependent protein-protein interactions in intracellular signaling pathways stimulated by TrkB receptor tyrosine kinase.     

A novel trimeric peptide, Neuropep-1-stimulating brain-derived neurotrophic factor expression in rat brain improves spatial learning and memory as measured by the Y-maze and Morris water maze

Abundant studies have shown possible links between low levels of brain-derived neurotrophic factor (BDNF) and neurological diseases such as Alzheimer's disease, Parkinson's disease, and depression, as well as stress and anxiety; therefore, BDNF could be a therapeutic target for neurological disorders. In the present study, a positional scanning-synthetic peptide combinatorial library was utilized to identify a peptide modulator of BDNF expression in the hippocampal neuronal cell line, H19-7. A novel tripeptide (Neuropep-1) induced a significant increase of BDNF mRNA and protein levels in H19-7 cells. Pre-treatment of TrkB inhibitor (K252a) did not block Neuropep-1-induced BDNF up-regulation. These results indicate that Neuropep-1 may up-regulate BDNF expression that might be independent of the TrkB receptor pathway. Tail vein injection of Neuropep-1 significantly up-regulated BDNF expression, TrkB phosphorylation, and its downstream signals including activation of Akt, ERK, and cAMP response element binding in the rat hippocampus. To evaluate improvement of spatial learning and memory (SLM) by Neuropep-1-induced BDNF up-regulation, the Y-maze and Morris water maze tests were performed. These results showed Neuropep-1 injection improved SLM performance with increase of BDNF and TrkB expression, activation of TrkB downstream signals in rat hippocampus compared with the control group. However, phosphorylation levels of TrkB were not changed when it was normalized to the level of TrkB expression. The difference on TrkB phosphorylation in Neuropep-1-injected rats may be affected by behavioral tests. These results suggest that Neuropep-1 may improve SLM via activation of the BDNF/TrkB signaling pathway in the rat hippocampus. Therefore, our findings represent that Neuropep-1 might be a potential candidate for treatment of learning and memory disorders as well as neurological diseases involving the abnormal expression of BDNF.