The dual nature of neurotrophins

Neurotrophins: are they involved in immune tolerance in pregnancy?

In this review, an attempt was made to substantiate the possibility for neurotrophins to be involved in the development of immune tolerance based on data accumulated on neurotrophin content and receptor expression in the trophoblast and immune cells, in particular, in natural killer cells. Numerous research results are reviewed to show that the expression and localization of neurotrophins along with their high-affinity tyrosine kinase receptors and low-affinity p75NTR receptor in the mother-placenta-fetus system indicate the important role of neurotrophins as binding molecules in regulating the crosstalk between the nervous, endocrine, and immune systems in pregnancy. An imbalance between these systems can occur with tumor growth and pathological processes observed in pregnancy complications and fetal development anomalies.

The NGF R100W Mutation, Associated with Hereditary Sensory Autonomic Neuropathy Type V, Specifically Affects the Binding Energetic Landscapes of NGF and of Its Precursor proNGF and p75NTR

Nerve Growth Factor (NGF), the prototype of the neurotrophin family, stimulates morphological differentiation and regulates neuronal gene expression by binding to TrkA and p75NTR receptors. It plays a critical role in maintaining the function and phenotype of peripheral sensory and sympathetic neurons and in mediating pain transmission and perception during adulthood. A point mutation in the NGFB gene (leading to the amino acid substitution R100W) is responsible for Hereditary Sensory and Autonomic Neuropathy type V (HSAN V), leading to a congenital pain insensitivity with no clear cognitive impairments, but with alterations in the NGF/proNGF balance. The available crystal structures of the p75NTR/NGF and 2p75NTR/proNGF complexes offer a starting point for Molecular Dynamics (MD) simulations in order to capture the impact of the R100W mutation on their binding energetic landscapes and to unveil the molecular determinants that trigger their different physiological and pathological outcomes. The present in silico studies highlight that the stability and the binding energetic fingerprints in the 2p75NTR/proNGF complex is not affected by R100W mutation, which on the contrary, deeply affects the energetic landscape, and thus the stability in the p75NTR/NGF complex. Overall, these findings present insights into the structural basis of the molecular mechanisms beyond the clinical manifestations of HSAN V patients.

Perineural invasion-associated biomarkers for tumor development

Perineural invasion (PNI) is the process of neoplastic invasion of peripheral nerves and is considered to be the fifth mode of cancer metastasis. PNI has been detected in head and neck tumors and pancreatic, prostate, bile duct, gastric, and colorectal cancers. It leads to poor prognostic outcomes and high local recurrence rates. Despite the increasing number of studies on PNI, targeted therapeutic modalities have not been proposed. The identification of PNI-related biomarkers would facilitate the non-invasive and early diagnosis of cancers, the establishment of prognostic panels, and the development of targeted therapeutic approaches. In this review, we compile information on the molecular mediators involved in PNI-associated cancers. The expression and prognostic significance of molecular mediators and their receptors in PNI-associated cancers are analyzed, and the possible mechanisms of action of these mediators in PNI are explored, as well as the association of cells in the microenvironment where PNI occurs.

Targeting the Cation-Chloride Co-Transporter NKCC1 to Re-Establish GABAergic Inhibition and an Appropriate Excitatory/Inhibitory Balance in Selective Neuronal Circuits: A Novel Approach for the Treatment of Alzheimer's Disease

GABA, the main inhibitory neurotransmitter in the adult brain, depolarizes and excites immature neurons because of an initially higher intracellular chloride concentration [Cl^-]i due to the delayed expression of the chloride exporter KCC2 at birth. Depolarization-induced calcium rise via NMDA receptors and voltage-dependent calcium channels is instrumental in shaping neuronal circuits and in controlling the excitatory (E)/inhibitory (I) balance in selective brain areas. An E/I imbalance accounts for cognitive impairment observed in several neuropsychiatric disorders. The aim of this review is to summarize recent data on the mechanisms by which alterations of GABAergic signaling alter the E/I balance in cortical and hippocampal neurons in Alzheimer's disease (AD) and the role of cation-chloride co-transporters in this process. In particular, we discuss the NGF and AD relationship and how mice engineered to express recombinant neutralizing anti-NGF antibodies (AD11 mice), which develop a neurodegenerative pathology reminiscent of that observed in AD patients, exhibit a depolarizing action of GABA due to KCC2 impairment. Treating AD and other forms of dementia with bumetanide, a selective KCC2 antagonist, contributes to re-establishing a proper E/I balance in selective brain areas, leading to amelioration of AD symptoms and the slowing down of disease progression.

Neurotrophin-3 and neurotrophin-4: The unsung heroes that lies behind the meninges

Neurotrophin is a growth factor that regulates the development and repair of the nervous system. From all factors, two pioneer groups, the nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF), have been widely explored for their role in disease pathogenesis and potential use as therapeutic agents. Nonetheless, neurotrophin-3 (NT3) and neurotrophin-4 (NT4) also have promising potential, albeit less popular than their counterparts. This review focuses on the latter two factors and their roles in the pathogenesis of brain disorders and potential therapies. An extensive literature search of NT3 and NT4 with their receptors, the TrkB and TrkC on the nervous system were extracted and analyzed. We found that NT3 and NT4 are not only involved in the pathogenesis of some neurodegenerative diseases, but also have promising therapeutic potential on injury- and vascular-related nervous system disease, neuropsychiatry, neurodegeneration and peripheral nerve diseases. In conclusion, the role of NT3 and NT4 should be further emphasized, and more studies could be explored on the potential use of these neurotrophins in the human study.

A combined evolutionary and structural approach to disclose the primary structural determinants essential for proneurotrophins biological functions

The neurotrophins, i.e., Nerve Growth Factor (NGF), Brain-Derived Neurotrophic Factor (BDNF), Neurotrophin 3 (NT3) and Neurotrophin 4 (NT4), are known to play a range of crucial functions in the developing and adult peripheral and central nervous systems. Initially synthesized as precursors, i.e., proneurotrophins (proNTs), that are cleaved to release C-terminal mature forms, they act through two types of receptors, the specific Trk receptors (Tropomyosin-related kinases) and the pan-neurotrophin receptor p75NTR, to initiate survival and differentiative responses. Recently, all the proNTs but proNT4 have been demonstrated to be not just inactive precursors, but signaling ligands that mediate opposing actions in fundamental aspects of the nervous system with respect to the mature counterparts through dual-receptor complexes formation with a member of the VPS10 family and p75NTR. Despite the functional relevance, the molecular determinants underpinning the interactions between the pro-domains and their receptors are still elusive probably due to their intrinsically disordered nature. Here we present an evolutionary approach coupled to an experimental study aiming to uncover the structural and dynamical basis of the biological function displayed by proNGF, proBDNF and proNT3 but missing in proNT4. A bioinformatic analysis allowed to elucidate the functional adaptability of the proNTs family in vertebrates, identifying conserved key structural features. The combined biochemical and SAXS experiments shed lights on the structure and dynamic behavior of the human proNTs in solution, giving insights on the evolutionary conserved structural motifs, essential for the multifaceted roles of proNTs in physiological as well as in pathological contexts.

New insights into perineural invasion of pancreatic cancer: More than pain

Pancreatic cancer is one of the most malignant human tumors. Perineural invasion, whereby a cancer cell invades the perineural spaces surrounding nerves, is acknowledged as a gradual contributor to cancer aggressiveness. Furthermore, perineural invasion is considered one of the root causes of the recurrence and metastasis observed after pancreatic resection, and it is also an independent predictor of prognosis. Advanced research has demonstrated that the neural microenvironment is closely associated with perineural invasion in pancreatic cancer. Therapy targeting the molecular mechanism of perineural invasion may enable the durable clinical treatment of this formidable disease. This review provides an overview of the present status of perineural invasion, the relevant molecular mechanisms of perineural invasion, pain and hyperglycemia associated with perineural invasion in pancreatic cancer, and the targeted therapeutics based on these studies.

Pro-region engineering for improved yeast display and secretion of brain derived neurotrophic factor

Brain derived neurotrophic factor (BDNF) is a promising therapeutic candidate for a variety of neurological diseases. However, it is difficult to produce as a recombinant protein. In its native mammalian context, BDNF is first produced as a pro-protein with subsequent proteolytic removal of the pro-region to yield mature BDNF protein. Therefore, in an attempt to improve yeast as a host for heterologous BDNF production, the BDNF pro-region was first evaluated for its effects on BDNF surface display and secretion. Addition of the wild-type pro-region to yeast BDNF production constructs improved BDNF folding both as a surface-displayed and secreted protein in terms of binding its natural receptors TrkB and p75, but titers remained low. Looking to further enhance the chaperone-like functions provided by the pro-region, two rounds of directed evolution were performed, yielding mutated pro-regions that further improved the display and secretion properties of BDNF. Subsequent optimization of the protease recognition site was used to control whether the produced protein was in pro- or mature BDNF forms. Taken together, we have demonstrated an effective strategy for improving BDNF compatibility with yeast protein engineering and secretion platforms.

L-type Calcium Channel Cav1.2 Is Required for Maintenance of Auditory Brainstem Nuclei

Cav1.2 and Cav1.3 are the major L-type voltage-gated Ca(2+) channels in the CNS. Yet, their individual in vivo functions are largely unknown. Both channel subunits are expressed in the auditory brainstem, where Cav1.3 is essential for proper maturation. Here, we investigated the role of Cav1.2 by targeted deletion in the mouse embryonic auditory brainstem. Similar to Cav1.3, loss of Cav1.2 resulted in a significant decrease in the volume and cell number of auditory nuclei. Contrary to the deletion of Cav1.3, the action potentials of lateral superior olive (LSO) neurons were narrower compared with controls, whereas the firing behavior and neurotransmission appeared unchanged. Furthermore, auditory brainstem responses were nearly normal in mice lacking Cav1.2. Perineuronal nets were also unaffected. The medial nucleus of the trapezoid body underwent a rapid cell loss between postnatal days P0 and P4, shortly after circuit formation. Phosphorylated cAMP response element-binding protein (CREB), nuclear NFATc4, and the expression levels of p75NTR, Fas, and FasL did not correlate with cell death. These data demonstrate for the first time that both Cav1.2 and Cav1.3 are necessary for neuronal survival but are differentially required for the biophysical properties of neurons. Thus, they perform common as well as distinct functions in the same tissue.

Propeptides as modulators of functional activity of proteases

Most proteases are synthesized in the cell as precursor-containing propeptides. These structural elements can determine the folding of the cognate protein, function as an inhibitor/activator peptide, mediate enzyme sorting, and mediate the protease interaction with other molecules and supramolecular structures. The data presented in this review demonstrate modulatory activity of propeptides irrespective of the specific mechanism of action. Changes in propeptide structure, sometimes minor, can crucially alter protein function in the living organism. Modulatory activity coupled with high variation allows us to consider propeptides as specific evolutionary modules that can transform biological properties of proteases without significant changes in the highly conserved catalytic domains. As the considered properties of propeptides are not unique to proteases, propeptide-mediated evolution seems to be a universal biological mechanism.

Brain-derived neurotrophic factor in urinary continence and incontinence

Urinary incontinence adversely affects quality of life and results in an increased financial burden for the elderly. Accumulating evidence suggests a connection between neurotrophins, such as brain-derived neurotrophic factor (BDNF), and lower urinary tract function, particularly with regard to normal physiological function and the pathophysiological mechanisms of stress urinary incontinence (SUI) and bladder pain syndrome/interstitial cystitis (BPS/IC). The interaction between BDNF and glutamate receptors affects both bladder and external urethral sphincter function during micturition. Clinical findings indicate reduced BDNF levels in antepartum and postpartum women, potentially correlating with postpartum SUI. Experiments with animal models demonstrate that BDNF is decreased after simulated childbirth injury, thereby impeding the recovery of injured nerves and the restoration of continence. Treatment with exogenous BDNF facilitates neural recovery and the restoration of continence. Serotonin and noradrenaline reuptake inhibitors, used to treat both depression and SUI, result in enhanced BDNF levels. Understanding the neurophysiological roles of BDNF in maintaining normal urinary function and in the pathogenesis of SUI and BPS/IC could lead to future therapies based on these mechanisms.

The serotonin-BDNF duo: developmental implications for the vulnerability to psychopathology

Serotonin (5-HT) and brain-derived neurotrophin factor (BDNF) are known to modulate behavioral responses to stress and to mediate the therapeutic efficacy of antidepressant agents through neuroplastic and epigenetic mechanisms. While the two systems interact at several levels, this scenario is complicated by a number of variants including brain region specificity, 5-HT receptor selectivity and timing. Based on recent insights obtained using 5-HT transporter (5-HTT) knockout rats we here set-out and discuss the crucial role of neurodevelopmental mechanisms and the contribution of transcription factors and epigenetic modifications to this interaction and its variants. 5-HTT knockout in rats, as well as the low activity short allelic variant of the serotonin transporter human polymorphism, consistently show reduced BDNF mRNA and protein levels in the hippocampus and in the prefrontal cortex. This starts during the second postnatal week, is preceded by DNA hypermethylation during the first postnatal week, and it is developmentally paralleled by reduced expression of key transcription factors. The reduced BDNF levels, in turn, affect 5-HT1A receptor-mediated intracellular signaling and thereby the serotonergic phenotype of the neurons. We propose that such a negative spiral of modifications may affect brain development and reduce its resiliency to environmental challenges during critical time windows, which may lead to phenotypic alterations that persist for the entire life. The characterization of 5-HT-BDNF interactions will eventually increase the understanding of mental illness etiology and, possibly, lead to the identification of novel molecular targets for drug development.

Effect of glial cell line-derived neurotrophic factor on behavior and key members of the brain serotonin system in mouse strains genetically predisposed to behavioral disorders

The effect of glial cell line-derived neurotrophic factor (GDNF) on behavior and on the serotonin (5-HT) system of a mouse strain predisposed to depressive-like behavior, ASC/Icg (Antidepressant Sensitive Cataleptics), in comparison with the parental "nondepressive" CBA/Lac mice was studied. Within 7 days after acute administration, GDNF (800 ng, i.c.v.) decreased cataleptic immobility but increased depressive-like behavioral traits in both investigated mouse strains and produced anxiolytic effects in ASC mice. The expression of the gene encoding the key enzyme for 5-HT biosynthesis in the brain, tryptophan hydroxylase-2 (Tph-2), and 5-HT1A receptor gene in the midbrain as well as 5-HT2A receptor gene in the frontal cortex were increased in GDNF-treated ASC mice. At the same time, GDNF decreased 5-HT1A and 5-HT2A receptor gene expression in the hippocampus of ASC mice. GDNF failed to change Tph2, 5-HT1A , or 5-HT2A receptor mRNA levels in CBA mice as well as 5-HT transporter gene expression and 5-HT1A and 5-HT2A receptor functional activity in both investigated mouse strains. The results show 1) a GDNF-induced increase in the expression of key genes of the brain 5-HT system, Tph2, 5-HT1A , and 5-HT2A receptors, and 2) significant genotype-dependent differences in the 5-HT system response to GDNF treatment. The data suggest that genetically defined cross-talk between neurotrophic factors and the brain 5-HT system underlies the variability in behavioral response to GDNF.

Involvement of brain-derived neurotrophic factor in late-life depression

Brain-derived neurotrophic factor (BDNF), one of the major neurotrophic factors, plays an important role in the maintenance and survival of neurons, synaptic integrity, and synaptic plasticity. Evidence suggests that BDNF is involved in major depression, such that the level of BDNF is decreased in depressed patients and that antidepressants reverse this decrease. Stress, a major factor in depression, also modulates BDNF expression. These studies have led to the proposal of the neurotrophin hypothesis of depression. Late-life depression is associated with disturbances in structural and neural plasticity as well as impairments in cognitive behavior. Stress and aging also play a crucial role in late-life depression. Many recent studies have suggested that not only expression of BDNF is decreased in the serum/plasma of patients with late-life depression, but structural abnormalities in the brain of these patients may be associated with a polymorphism in the BDNF gene, and that there is a relationship between a BDNF polymorphism and antidepressant remission rates. This review provides a critical review of the involvement of BDNF in major depression, in general, and in late-life depression, in particular.

[Precursors and propeptides of neurotrophic factors as the modulators of biological activity of its mature forms]

Here, we review the problems of neurotrophic factors' folding, the role of its precursors (proneurotrophins) and the contribution of elements deleted during its maturation (propeptides) in biological functioning of these growth factors.

Perineural invasion and associated pain in pancreatic cancer

Perineural invasion (PNI) is a prominent characteristic of pancreatic cancer. PNI is a process whereby cancer cells invade the surrounding nerves, thus providing an alternative route for metastatic spread and pain generation. PNI is thought to be an indicator of aggressive tumour behaviour and has been shown to correlate with poor prognosis of patients with pancreatic cancer. Recent studies demonstrated that some signalling molecules and pathways that are involved in PNI are also involved in pain generation. Targeting these signalling pathways has shown some promise in alleviating pain and reducing PNI, which could potentially improve treatment outcomes for patients with pancreatic cancer.

Prenatal stress differentially alters brain-derived neurotrophic factor expression and signaling across rat strains

BACKGROUND

Psychiatric illness and anxiety disorders have strong neurodevelopmental components. Environmental insults such as prenatal exposure to stress and genetic differences in stress responses may affect brain development.

METHODS

A rat model of random variable prenatal stress was used to study the expression and processing of hippocampal brain-derived neurotrophic factor (BDNF) in the offspring of the stressed rat dams. To account for unknown genetic influences that may play a role in the outcome of this prenatal stress paradigm, three different rat strains with known differences in stress responsivity were studied: Fischer, Sprague-Dawley, and Lewis rats (n=132).

RESULTS

Multiple disparities in mRNA expression levels of BDNF, and transcripts related to its processing and signaling were found in the three strains. Of the numerous splice variants transcribed from the BDNF gene, the transcript containing BDNF exon VI was most aberrant in the prenatally stressed animals. Protein levels of both uncleaved proBDNF and mature BDNF were also altered, as was intra-cellular signaling by phosphorylation of the neurotrophic tyrosine kinase receptor type 2 (NTRK2, TrkB) and mitogen-activated protein kinase (Erk 1/2). Changes were not only dependent on prenatal stress, but were also strain dependent, demonstrating the importance of genetic background.

CONCLUSION

BDNF signaling provides both positive neurotrophic support for neurons and negative apoptotic effects, both of which may contribute to behavioral or neurochemical outcomes after prenatal exposure to stress. Differential processing of BDNF after prenatal stress in the three rat strains has implications for human subjects where genetic differences may protect or exacerbate the effects of an environmental stressor during fetal development.

Strain dependent effects of prenatal stress on gene expression in the rat hippocampus

Multiple animal models have been developed to recapitulate phenotypes of the human disease, schizophrenia. A model that simulates many of the cognitive and sensory deficits of the disorder is the use of random variable prenatal stress (PS) in the rat. These deficits suggest a molecular origin in the hippocampus, a brain region that plays a role in the regulation of stress. To study both hippocampal gene expression changes in offspring of prenatally stressed dams and to address genetic variability, we used a random array of prenatal stressors in three different rat strains with diverse responses to stress: Fischer, Sprague-Dawley, and Lewis rats. Candidate genes involved in stress, schizophrenia, cognition, neurotrophic effects, and immunity were selected for assessment by real-time quantitative PCR under resting conditions and following a brief exposure to restraint stress. PS resulted in significant differences in gene expression in the offspring that were strain dependent. mRNA expression for the N-methyl-D-aspartate receptor subtype 2B (Grin2b) was increased, and tumor necrosis factor-alpha (Tnfα) transcript was decreased in PS Sprague-Dawley and Lewis rats, but not in the Fischer rats. Expression of brain-derived neurotrophic factor (Bdnf) mRNA in the hippocampus was increased after an acute stress in all controls of each strain, yet a decrease was seen after acute stress in the PS Sprague-Dawley and Lewis rats. Expression of the glucocorticoid receptor (Nr3c1) was decreased in the Fischer strain when compared to Lewis or Sprague-Dawley rats, though the Fischer rats had markedly higher α7 nicotinic receptor (Chrna7) expression. The expression differences seen in these animals may be important elements of the phenotypic differences seen due to PS and genetic background.

Development and neuronal dependence of cutaneous sensory nerve formations: Lessons from neurotrophins

Null mutations of genes from the NGF family of NTs and their receptors (NTRs) lead to loss/reduction of specific neurons in sensory ganglia; conversely, cutaneous overexpression of NTs results in skin hyperinnervation and increase or no changes in the number of sensory neurons innervating the skin. These neuronal changes are paralleled with loss of specific types of sensory nerve formations in the skin. Therefore, mice carrying mutations in NT or NTR genes represent an ideal model to identify the neuronal dependence of each type of cutaneous sensory nerve ending from a concrete subtype of sensory neuron, since the development, maintenance, and structural integrity of sensory nerve formations depend upon sensory neurons. Results obtained from these mouse strains suggest that TrkA positive neurons are connected to intraepithelial nerve fibers and other sensory nerve formations depending from C and Adelta nerve fibers; the neurons expressing TrkB and responding to BDNF and NT-4 innervate Meissner corpuscles, a subpopulation of Merkell cells, some mechanoreceptors of the piloneural complex, and the Ruffini's corpuscles; finally, a subpopulation of neurons, which are responsive to NT-3, support postnatal survival of some intraepithelial nerve fibers and Merkel cells in addition to the muscle mechanoreceptors. On the other hand, changes in NTs and NTRs affect the structure of non-nervous structures of the skin and are at the basis of several cutaneous pathologies. This review is an update about the role of NTs and NTRs in the maintenance of normal cutaneous innervation and maintenance of skin integrity.

ProNGF mediates death of Natural Killer cells through activation of the p75NTR-sortilin complex

The common neurotrophin receptor P75NTR, its co-receptor sortilin and ligand proNGF, have not previously been investigated in Natural Killer (NK) cell function. We found freshly isolated NK cells express sortilin but not significant amounts of P75NTR unless exposed to interleukin-12 (IL-12), or cultured in serum free conditions, suggesting this receptor is sequestered. A second messenger associated with p75NTR, neurotrophin-receptor-interacting-MAGE-homologue (NRAGE) was identified in NK cells. Cleavage resistant proNGF123 killed NK cells in the presence of IL-12 after 20h and without IL-12 in serum free conditions at 48h. This was reduced by blocking sortilin with neurotensin. We conclude that proNGF induced apoptosis of NK cells may have important implications for limiting the innate immune response.

TrkB and TrkC agonist antibodies improve function, electrophysiologic and pathologic features in Trembler J mice

Neurotrophic factors have been considered as potential therapeutics for peripheral neuropathies. Previously, we showed that neurotrophin-3 (NT-3) promotes nerve regeneration in Trembler(J) (Tr(J)) mice and in sural nerves from patients with Charcot-Marie-Tooth 1A (CMT1A). The relatively short plasma half-life of NT-3 and other neurotrophins, however, pose a practical difficulty in their clinical application. Therapeutic agonist antibodies (AAb) targeting the neurotrophic receptors may circumvent this obstacle due to their high specificity and long half-life. Using morphological, electrophysiological studies and functional motor testing, we assessed the efficacy of monoclonal TrkC AAb and TrkB AAb in the Tr(J) mice. Treatments of these AAbs individually or in combination over 20 weeks increased compound muscle action potential (CMAP) amplitude, which correlated with improved grip strength, as compared to the PBS control group. Improvements in CMAP amplitude were most prominent with TrkC AAb treatment. In all treatment groups, distal to the crush site of the sciatic nerves exhibited a significantly greater number of myelinated fibers (MFs) indicating improved regenerative response to injury. In the contralateral intact sciatic nerves, the number of MFs as well as the myelin thickness was also increased significantly by the AAb treatments, suggesting that the hypomyelination/amyelination state of the peripheral nerves in Tr(J) improved. Therapeutic response to AAb combination was often, albeit not always, the most prominent, indicating a non-redundant effect of TrkB and TrkC AAbs. An early functional recovery and the correlative morphological changes of enhanced regeneration were seen with TrkC AAb treatment. These results provide evidence for potential therapeutic use of monoclonal agonist antibodies for neurotrophin receptors in CMT1A and other neuropathies.

Intervertebral disc, sensory nerves and neurotrophins: who is who in discogenic pain?

The normal intervertebral disc (IVD) is a poorly innervated organ supplied only by sensory (mainly nociceptive) and postganglionic sympathetic (vasomotor efferents) nerve fibers. Interestingly, upon degeneration, the IVD becomes densely innervated even in regions that in normal conditions lack innervation. This increased innervation has been associated with pain of IVD origin. The mechanisms responsible for nerve growth and hyperinnervation of pathological IVDs have not been fully elucidated. Among the molecules that are presumably involved in this process are some members of the family of neurotrophins (NTs), which are known to have both neurotrophic and neurotropic properties and regulate the density and distribution of nerve fibers in peripheral tissues. NTs and their receptors are expressed in healthy IVDs but much higher levels have been observed in pathological IVDs, thus suggesting a correlation between levels of expression of NTs and density of innervation in IVDs. In addition, NTs also play a role in inflammatory responses and pain transmission by increasing the expression of pain-related peptides and modulating synapses of nociceptive neurons at the spinal cord. This article reviews current knowledge about the innervation of IVDs, NTs and NT receptors, expression of NTs and their receptors in IVDs as well as in the sensory neurons innervating the IVDs, the proinflammatory role of NTs, NTs as nociception regulators, and the potential network of discogenic pain involving NTs.

Identification of a linear epitope in sortilin that partakes in pro-neurotrophin binding

Sortilin acts as a cell surface receptor for pro-neurotrophins (pro-NT) that upon complex formation with the p75 neurotrophin receptor (p75(NTR)) is able to signal neuronal cell death. Here we screened a sortilin peptide library comprising 16-mer overlapping sequences for binding of the pro-domains of nerve growth factor and brain-derived neurotrophic factor. We find that a linear surface-exposed sequence, (163)RIFRSSDFAKNF(174), constitutes an important pro-NT binding epitope in sortilin. Systematic mutational analysis revealed residues Arg(163), Phe(165), Arg(166), and Phe(170) to be critical for the interaction. Expression of a sortilin mutant in which these four amino acids were substituted by alanines disrupted pro-NT binding without affecting receptor heterodimerization with p75(NTR) or binding of ligands that selectively engages the centrally located tunnel in the beta-propeller of sortilin. We furthermore demonstrate that a peptide comprising the ligand-binding epitope can prevent pro-NT-induced apoptosis in RN22 schwannoma cells.

Neuronal plasticity: a link between stress and mood disorders

Although stress represents the major environmental element of susceptibility for mood disorders, the relationship between stress and disease remains to be fully established. In the present article we review the evidence in support for a role of neuronal plasticity, and in particular of neurotrophic factors. Even though decreased levels of norepinephrine and serotonin may underlie depressive symptoms, compelling evidence now suggests that mood disorders are characterized by reduced neuronal plasticity, which can be brought about by exposure to stress at different stages of life. Indeed the expression of neurotrophic molecules, such as the neurotrophin BDNF, is reduced in depressed subjects as well as in experimental animals exposed to adverse experience at early stages of life or at adulthood. These changes show an anatomical specificity and might be sustained by epigenetic mechanisms. Pharmacological intervention may normalize such defects and improve neuronal function through the modulation of the same factors that are defective in depression. Several studies have demonstrated that chronic, but not acute, antidepressant treatment increases the expression of BDNF and may enhance its localization at synaptic level. Antidepressant treatment can normalize deficits in neurotrophin expression produced by chronic stress paradigms, but may also alter the modulation of BDNF under acute stressful conditions. In summary, there is good agreement in considering neuronal plasticity, and the expression of key proteins such as the neurotrophin BDNF, as a central player for the effects of stress on brain function and its implication for psychopathology. Accordingly, effective treatments should not limit their effects to the control of neurotransmitter and hormonal dysfunctions, but should be able to normalize defective mechanisms that sustain the impairment of neuronal plasticity.

Brain-derived neurotrophic factor: role in depression and suicide

Depression and suicidal behavior have recently been shown to be associated with disturbances in structural and synaptic plasticity. Brain-derived neurotrophic factor (BDNF), one of the major neurotrophic factors, plays an important role in the maintenance and survival of neurons and in synaptic plasticity. Several lines of evidence suggest that BDNF is involved in depression, such that the expression of BDNF is decreased in depressed patients. In addition, antidepressants up-regulate the expression of BDNF. This has led to the proposal of the "neurotrophin hypothesis of depression". Increasing evidence demonstrates that suicidal behavior is also associated with lower expression of BDNF, which may be independent from depression. Recent genetic studies also support a link of BDNF to depression/suicidal behavior. Not only BDNF, but abnormalities in its cognate receptor tropomycin receptor kinase B (TrkB) and its splice variant (TrkB.T1) have also been reported in depressed/suicidal patients. It has been suggested that epigenetic modulation of the Bdnf and Trkb genes may contribute to their altered expression and functioning. More recently, impairment in the functioning of pan75 neurotrophin receptor has been reported in suicide brain specimens. pan75 neurotrophin receptor is a low-affinity neurotrophin receptor that, when expressed in conjunction with low availability of neurotropins/Trks, induces apoptosis. Overall, these studies suggest the possibility that BDNF and its mediated signaling may participate in the pathophysiology of depression and suicidal behavior. This review focuses on the critical evidence demonstrating the involvement of BDNF in depression and suicide.

Drosophila neurotrophins reveal a common mechanism for nervous system formation

Neurotrophic interactions occur in Drosophila, but to date, no neurotrophic factor had been found. Neurotrophins are the main vertebrate secreted signalling molecules that link nervous system structure and function: they regulate neuronal survival, targeting, synaptic plasticity, memory and cognition. We have identified a neurotrophic factor in flies, Drosophila Neurotrophin (DNT1), structurally related to all known neurotrophins and highly conserved in insects. By investigating with genetics the consequences of removing DNT1 or adding it in excess, we show that DNT1 maintains neuronal survival, as more neurons die in DNT1 mutants and expression of DNT1 rescues naturally occurring cell death, and it enables targeting by motor neurons. We show that Spätzle and a further fly neurotrophin superfamily member, DNT2, also have neurotrophic functions in flies. Our findings imply that most likely a neurotrophin was present in the common ancestor of all bilateral organisms, giving rise to invertebrate and vertebrate neurotrophins through gene or whole-genome duplications. This work provides a missing link between aspects of neuronal function in flies and vertebrates, and it opens the opportunity to use Drosophila to investigate further aspects of neurotrophin function and to model related diseases.

Neurotrophins are secreted proteins that link nervous system structure and function in vertebrates. They regulate neuronal survival, thus adjusting cell populations, and connectivity, enabling the formation of neuronal circuits. They also regulate patterns of dendrites and axons, synaptic function, memory, learning, and cognition; and abnormal neurotrophin function underlies psychiatric disorders. Despite such relevance for nervous system structure and function, neurotrophins have been missing from invertebrates. We show here the identification and functional demonstration of a neurotrophin family in the fruit fly, Drosophila. Our findings imply that the neurotrophins may be present in all animals with a centralised nervous system (motor and sensory systems) or brain, supporting the notion of a common origin for the brain in evolution. This work bridges a void in the understanding of the Drosophila and human nervous systems, and it opens the opportunity to use the powerful fruit fly for neurotrophin related studies.

Members of the neurotrophin superfamily mediate critical roles in neuronal survival and targeting in the fruit flyDrosophila. Although this is an accepted role for neurotrophins in vertebrates, scant previous evidence has been able to demonstrate such a conserved role in invertebrates.

Association between Presenilin-1 and TRAF6 modulates regulated intramembrane proteolysis of the p75NTR neurotrophin receptor

The p75 neurotrophin receptor (p75(NTR)) is a member of the tumour necrosis factor superfamily, which relies on the recruitment of cytosolic protein partners including the tumour necrosis factor receptor-associated factor 6 (TRAF6) E3 ubiquitin ligase to produce cellular responses. Recently, p75(NTR) was also shown to undergo presenilin-dependent, gamma-secretase-mediated regulated intramembrane proteolysis. In this study, we report the characterization of a highly conserved TRAF6-binding site (PxExxAr/Ac) in presenilin-1 (PS1) that mediates nerve growth factor (NGF)-induced association between PS1 and TRAF6. We demonstrate that disruption of this interaction between PS1 and TRAF6 inhibits TRAF6 autoubiquitination and gamma-secretase cleavage of p75(NTR). Additionally, we show that PS1-deficiency antagonizes NGF-induced I-kappaB degradation. Finally, we also show that p75(NTR) is a substrate for TRAF6-mediated ubiquitination and that TRAF6 E3 ligase activity is required for regulated intramembrane proteolysis of p75(NTR). In summary, our data suggest that an NGF-induced association between PS1 and TRAF6 influences regulated intramembrane proteolysis of p75(NTR).

p75NTR expression in rat urinary bladder sensory neurons and spinal cord with cyclophosphamide-induced cystitis

A role for nerve growth factor (NGF) in contributing to increased voiding frequency and altered sensation from the urinary bladder has been suggested. Previous studies have examined the expression and regulation of tyrosine kinase receptors (Trks) in micturition reflexes with urinary bladder inflammation. The present studies examine the expression and regulation of another receptor known to bind NGF, p75(NTR), after various durations of bladder inflammation induced by cyclophosphamide (CYP). CYP-induced cystitis increased (P < or = 0.001) p75(NTR) expression in the superficial lateral and medial dorsal horn in L1-L2 and L6-S1 spinal segments. The number of p75(NTR)-immunoreactive (-IR) cells in the lumbosacral dorsal root ganglia (DRG) also increased (P < or = 0.05) with CYP-induced cystitis (acute, intermediate, and chronic). Quantitative, real-time polymerase chain reaction also demonstrated significant increases (P < or = 0.01) in p75(NTR) mRNA in DRG with intermediate and chronic CYP-induced cystitis. Retrograde dye-tracing techniques with Fastblue were used to identify presumptive bladder afferent cells in the lumbosacral DRG. In bladder afferent cells in DRG, p75(NTR)-IR was also increased (P < or = 0.01) with cystitis. In addition to increases in p75(NTR)-IR in DRG cell bodies, increases (P < or = 0.001) in pericellular (encircling DRG cells) p75(NTR)-IR in DRG also increased. Confocal analyses demonstrated that pericellular p75(NTR)-IR was not colocalized with the glial marker, glial fibrillary acidic protein (GFAP). These studies demonstrate that p75(NTR) expression in micturition reflexes is present constitutively and modified by bladder inflammation. The functional significance of p75(NTR) expression in micturition reflexes remains to be determined.

Nerve growth factor potentiates p53 DNA binding but inhibits nitric oxide-induced apoptosis in neuronal PC12 cells

NGF is recognized for its role in neuronal differentiation and maintenance. Differentiation of PC12 cells by NGF involves p53, a transcription factor that controls growth arrest and apoptosis. We investigated NGF influence over p53 activity during NO-induced apoptosis by sodium nitroprusside in differentiated and mitotic PC12 cells. NGF-differentiation produced increased p53 levels, nuclear localization and sequence-specific DNA binding. Apoptosis in mitotic cells also produced these events but the accompanying activation of caspases 1-10 and mitochondrial depolarization were inhibited during NGF differentiation and could be reversed in p53-silenced cells. Transcriptional regulation of PUMA and survivin expression were not inhibited by NGF, although NO-induced mitochondrial depolarization was dependent upon de novo gene transcription and only occurred in mitotic cells. We conclude that NGF mediates prosurvival signaling by increasing factors such as Bcl-2 and p21(Waf1/Cip1) without altering p53 transcriptional activity to inhibit mitochondrial depolarization, caspase activation and apoptosis.