Autocrine regulation of nerve growth factor expression by Trk receptors

EZH2 can be used as a therapeutic agent for inhibiting endothelial dysfunction

Enhancer of zeste homolog 2 (EZH2) is a catalytic subunit of polycomb repressor complex 2 and plays important roles in endothelial cell homeostasis. EZH2 functionally methylates lysine 27 of histone H3 and represses gene expression through chromatin compaction. EZH2 mediates the effects of environmental stimuli by regulating endothelial functions, such as angiogenesis, endothelial barrier integrity, inflammatory signaling, and endothelial mesenchymal transition. Numerous studies have been conducted to determine the significance of EZH2 in endothelial function. The aim of this review is to provide a concise summary of the roles EZH2 plays in endothelial function and elucidate its therapeutic potential in cardiovascular diseases.

Expression and Dendritic Trafficking of BDNF-6 Splice Variant are Impaired in Knock-In Mice Carrying Human BDNF Val66Met Polymorphism

BACKGROUND

The human Val66Met polymorphism in brain-derived neurotrophic factor (BDNF), a key factor in neuroplasticity, synaptic function, and cognition, has been implicated in the pathophysiology of neuropsychiatric and neurodegenerative disorders. BDNF is encoded by multiple transcripts with distinct regulation and localization, but the impact of the Val66Met polymorphism on BDNF regulation remains unclear.

METHODS

In BDNF Val66Met knock-in mice, which recapitulate the phenotypic hallmarks of individuals carrying the BDNF(Met) allele, we measured expression levels, epigenetic changes at promoters, and dendritic trafficking of distinct BDNF transcripts using quantitative PCR, chromatin immunoprecipitation (ChIP), and in situ hybridization.

RESULTS

BDNF-4 and BDNF-6 transcripts were reduced in BDNF(Met/Met) mice, compared with BDNF(Val/Val) mice. ChIP for acetyl-histone H3, a marker of active gene transcription, and trimethyl-histone-H3-Lys27 (H3K27me3), a marker of gene repression, showed higher H3K27me3 binding to exon 5, 6, and 8 promoters in BDNF(Met/Met). The H3K27 methyltransferase enhancer of zeste homolog 2 (EZH2) is involved in epigenetic regulation of BDNF expression, because in neuroblastoma cells BDNF expression was increased both by short interference RNA for EZH2 and incubation with 3-deazaneplanocin A, an inhibitor of EZH2. In situ hybridization for BDNF-2, BDNF-4, and BDNF-6 after pilocarpine treatment showed that BDNF-6 transcript was virtually absent from distal dendrites of the CA1 and CA3 regions in BDNF(Met/Met) mice, while no changes were found for BDNF-2 and BDNF-4.

CONCLUSIONS

Impaired BDNF expression and dendritic targeting in BDNF(Met/Met) mice may contribute to reduced regulated secretion of BDNF at synapses, and may be a specific correlate of pathology in individuals carrying the Met allele.

The Transcriptional Response of Neurotrophins and Their Tyrosine Kinase Receptors in Lumbar Sensorimotor Circuits to Spinal Cord Contusion is Affected by Injury Severity and Survival Time

Traumatic spinal cord injury (SCI) results in changes to the anatomical, neurochemical, and physiological properties of cells in the central and peripheral nervous system. Neurotrophins, acting by binding to their cognate Trk receptors on target cell membranes, contribute to modulation of anatomical, neurochemical, and physiological properties of neurons in sensorimotor circuits in both the intact and injured spinal cord. Neurotrophin signaling is associated with many post-SCI changes including maladaptive plasticity leading to pain and autonomic dysreflexia, but also therapeutic approaches such as training-induced locomotor improvement. Here we characterize expression of mRNA for neurotrophins and Trk receptors in lumbar dorsal root ganglia (DRG) and spinal cord after two different severities of mid-thoracic injury and at 6 and 12 weeks post-SCI. There was complex regulation that differed with tissue, injury severity, and survival time, including reversals of regulation between 6 and 12 weeks, and the data suggest that natural regulation of neurotrophins in the spinal cord may continue for months after birth. Our assessments determined that a coordination of gene expression emerged at the 12-week post-SCI time point and bioinformatic analyses address possible mechanisms. These data can inform studies meant to determine the role of the neurotrophin signaling system in post-SCI function and plasticity, and studies using this signaling system as a therapeutic approach.

Expression of tyrosine kinase receptors in cultured dorsal root ganglion neurons in the presence of monosialoganglioside and skeletal muscle cells

The neurotrophic factor-like activity of monosialoganglioside (GM1) has been shown to activate tyrosine kinase receptors (Trk). Targets of neuronal innervation play a vital role in regulating the survival and differentiation of innervating neurotrophin-responsive neurons. Both GM1 and target skeletal muscle (SKM) cells are essential for the maintenance of the function of neurons. However, much less is known about the effects of GM1 or/and target SKM cells on the expression of Trk receptors in dorsal root ganglion (DRG) neurons. Here we have tested what extent to the expression of TrkA, TrkB, and TrkC receptors in primary cultured of DRG neurons in absence or presence of GM1 or/and SKM cells. In this experiment, we found that: (1) GM1 promoted expression of TrkA and TrkB but not TrkC in primary cultured DRG neurons; (2) target SKM cells promoted expression of TrkC but not TrkA and TrkB in neuromuscular cocultures without GM1 treatment; and (3) GM1 and target SKM cells had additional effects on expression of these three Trk receptors. The results of the present study offered new clues for a better understanding of the association of GM1 and target SKM on the expression of Trk receptors.

Effect of low-intensity pulsed ultrasound on the expression of neurotrophin-3 and brain-derived neurotrophic factor in cultured Schwann cells

It is generally known that low-intensity pulsed ultrasound (LIPUS) accelerates peripheral nerve tissue regeneration. However, the precise cellular mechanism involved is still unclear. The purpose of this study was to determine how the Schwann cells respond directly to LIPUS stimuli. Thus, we investigated the effect of LIPUS on cell proliferation, neurotrophin-3 (NT-3), and brain-derived neurotrophic factor (BDNF) mRNA expression in rat Schwann cells. Schwann cells were enzymatically isolated from postnatal 1-3 day rat sciatic nerve tissue and cultured in the six-well plate. The ultrasound was applied at a frequency of 1 MHz and an intensity of 100 mW/cm(2) spatial average temporal average for 5 minutes/day. The control group was cultured in the same way but without the administration of ultrasound. Immunohistochemistry demonstrated that more than 98% of the experimental and control cells were positive for S-100, NT-3, and BDNF. With 5-bromo-2'-deoxyuridine (BrdU) assay, the stimulated cells also exhibited an increase in the rate of cell proliferation on days 4, 7, 10, and 14. Further investigation found that mRNA expression of NT-3 was significantly upregulated in experimental groups compared with the control 14 days after the LIPUS stimulation (the ratio of NT-3/beta-actin was 0.56 +/- 0.13 vs. 0.41 +/- 0.09, P < 0.01), whereas the mRNA expression of BDNF was significantly downregulated in experimental groups compared with the control (the ratio of BDNF/beta-actin was 0.51 +/- 0.05 vs. 0.60 +/- 0.08, P < 0.05). These results demonstrated that the application of LIPUS promotes cell proliferation and NT-3 gene expression in Schwann cells, and involved in the alteration of BDNF gene expression.

Nerve growth factor enhances voltage-gated Na+ channel activity and Transwell migration in Mat-LyLu rat prostate cancer cell line

The highly dynamic nature of voltage-gated Na+ channel (VGSC) expression and its controlling mechanism(s) are not well understood. In this study, we investigated the possible involvement of nerve growth factor (NGF) in regulating VGSC activity in the strongly metastatic Mat-LyLu cell model of rat prostate cancer (PCa). NGF increased peak VGSC current density in a time- and dose-dependent manner. NGF also shifted voltage to peak and the half-activation voltage to more positive potentials, and produced currents with faster kinetics of activation; sensitivity to the VGSC blocker tetrodotoxin (TTX) was not affected. The NGF-induced increase in peak VGSC current density was suppressed by both the pan-trk antagonist K252a, and the protein kinase A (PKA) inhibitor KT5720. NGF did not affect the Nav1.7 mRNA level, but the total VGSC alpha-subunit protein level was upregulated. NGF potentiated the cells' migration in Transwell assays, and this was not affected by TTX. We concluded that NGF upregulated functional VGSC expression in Mat-LyLu cells, with PKA as a signaling intermediate, but enhancement of migration by NGF was independent of VGSC activity.

THIS ARTICLE HAS BEEN RETRACTED: Functional nerve growth factor and trkA autocrine/paracrine circuits in adult rat cortex are revealed by episodic ethanol exposure and withdrawal

The hypothesis tested is that cortical neurotrophins communicate through an inducible autocrine/paracrine mechanism. As ethanol (Et) can induce cortical nerve growth factor (NGF) expression, adult rats were challenged with Et on three consecutive days per week for 6 weeks. The focus of the study was layer V, the chief repository of receptor-expressing neuronal cell bodies. Brains were collected immediately after the sixth Et exposure or 72 h later [i.e., following withdrawal (WD)]. Double-label in situ hybridization-immunohistochemistry studies showed that many neuronal somata co-expressed NGF mRNA with NGF, trkA, or phosphorylated trk (p-trk), essential components of an inducible autocrine system. The frequencies of co-labeling were affected by neither Et nor WD. On the contrary, Et increased the number of NGF mRNA-expressing neurons and the amount of NGF mRNA expressed per cell. Et also increased total cortical concentration of NGF protein, the number of layer V neurons expressing trkA transcript, the amount of trkA mRNA expressed per neuron, and trkA phosphorylation. Following WD, the frequency of NGF-mRNA-expressing cells increased, although transcript and protein content fell. WD induced an increase in trkA mRNA and protein expression, however, p-trk expression was unaffected. Thus, Et treatment reveals that layer V has inducible autocrine/paracrine and anterograde neurotrophin systems. WD unveils the dynamism and recruitability of these systems.

Semisynthetic sphingoglycolipid LIGA20 is neuroprotective against human immunodeficiency virus-gp120-mediated apoptosis

Apoptosis and neuronal atrophy are commonly seen in patients infected with the human immunodeficiency virus type 1 (HIV-1) in the late phase of infection. The HIV-1 envelope glycoprotein gp120 has been suggested to be a causal agent of neuronal loss. Therefore, blocking gp120 neurotoxicity may be an effective way to reduce the neuronal degeneration seen in HIV patients. Brain-derived neurotrophic factor (BDNF) prevents gp120-mediated apoptosis in cerebellar granule cells. However, BDNF poorly crosses the blood-brain barrier and therefore may not be a suitable therapy for HIV patients. LIGA20 is a semisynthetic sphingoglycolipid that may be a valid alternative to BDNF. In fact, it has been shown that LIGA20 mimics the neuroprotective properties of BDNF. The present study was undertaken to characterize the relative potency of LIGA20 to antagonize gp120-mediated apoptosis. Cerebellar granule cells were exposed to gp120IIIB (5 nM) or stromal-cell derived factor-1 (SDF), the natural ligand for the CXCR4 receptor to which gp120 binds, alone or in combination with LIGA20 (5 microM), and cell death/survival was determined 12 and 24 hr later by various markers of apoptosis. LIGA20 blocked the neurotoxic effect of gp120 and SDF. The neurotrophic effect of LIGA20 was reversed by K252a, a tyrosine kinase inhibitor used to block TrkB signaling, suggesting the involvement of TrkB activation. These findings provide the rationale for exploring the ability of compounds that mimic BDNF activity to reduce neuronal cell death in HIV-1-positive patients.

The role of tropomyosin-related kinase receptors in neurotrophin-induced rapid eye movement sleep in the cat

The microinjection of nerve growth factor and neurotrophin-3 into the rostro-dorsal pontine tegmentum of the cat evokes a state that is comparable to naturally-occurring rapid eye movement sleep. Using two experimental paradigms, we tested the hypothesis that neurotrophin high-affinity receptors (trkA and trkC, tropomyosin-related kinase A and C, respectively) mediate this effect. First, trk and fos immunohistochemistry were combined to determine whether tyrosine kinase receptor-containing neurons in the dorsal pontine tegmentum are active in cats that exhibit long-lasting periods of rapid eye movement sleep following the local microinjection of nerve growth factor. During approximately two hours of recording, nerve growth factor-treated cats spent 59.8% of the time in a rapid eye movement sleep-like state; vehicle-injected (control) animals remained in quiet wakefulness and non-rapid eye movement sleep. Whereas control and nerve growth factor-treated cats exhibited a similar mean number of trkA- and trkC-immunoreactive neurons in the dorsal pontine tegmentum, the number of trkA- and trkC-immunoreactive neurons that expressed Fos, i.e. double-labeled cells that are presumably activated, was significantly larger in cats that were injected with nerve growth factor. Axon terminals contained tyrosine kinase receptor immunoreactivity in this region; many were apposed to Fos-immunoreactive neurons. In addition, patterns of tyrosine kinase receptor and Fos immunoreactivity similar to those observed in nerve growth factor-injected cats were present, in conjunction with long-lasting rapid eye movement sleep, following the microinjection of carbachol into the dorsal pons. In a second series of studies, nerve growth factor or neurotrophin-3 was injected alone or after K-252a, a blocker of tyrosine kinase receptors, into the rostro-dorsal pontine tegmentum. Nerve growth factor or neurotrophin-3 alone produced, with a mean latency of 4 min, a rapid eye movement sleep-like state. However, neurotrophin injections preceded by K-252a were not effective in inducing rapid eye movement sleep. These results indicate that the activation of trkA and trkC receptors in neurons in the pontine tegmentum is responsible, at least in part, for the rapid eye movement sleep-inducing effect of nerve growth factor and neurotrophin-3. Furthermore, the data suggest that these neurotrophins are capable of acting both pre- and postsynaptically to activate pontine neurons that are involved in the generation of rapid eye movement sleep.