The role of neurotrophins during successive stages of sensory neuron development

Neurotrophins comprise a family of basic homodimeric proteins. The isolation of the first two neurotrophins, nerve growth factor and brain-derived neurotrophic factor, was based on the ability of these proteins to promote the survival of embryonic neurons. However, the identification of additional neurotrophins by homology screening together with recent work on these proteins has shown that neurotrophins do more than just regulate neuronal survival. Neurotrophins influence the proliferation and differentiation of neuron progenitor cells and regulate the expression of several differentiated traits of neurons throughout life. Moreover, the influence of neurotrophins on survival is more complex than originally thought; some neurons switch their survival requirements from one set of neurotrophins to another during development and several neurotrophins may be involved in regulating the survival of a population of neurons at any one time. Most of what is known of the developmental physiology of neurotrophins has come from studying neurons of the peripheral nervous system. Quite apart from the accessibility of these neurons and their progenitor cell populations, investigation of the actions of neurotrophins on several well-characterised populations of sensory neurons has permitted the age-related changes in the effects of neurotrophins to be interpreted in the appropriate developmental context. In this review I provide a chronological account of the action of neurotrophins in neuronal development with special reference to sensory neurons.     

Activity-dependent and hormonal regulation of neurotrophin mRNA levels in brain–implications for neuronal plasticity

The neurotrophins exhibit neurotrophic effects on specific, partially overlapping populations of neurons both in the peripheral and the central nervous system (CNS). In the periphery, they are synthesized by a variety of nonneuronal cells, and their synthesis seems to be independent of the neuronal input. In contrast, in the CNS all neurotrophins are expressed under physiological conditions primarily by neurons. The production of NGF and BDNF is controlled by neuronal activity: up-regulation by glutamate and acetylcholine, down-regulation by gamma-aminobutyric acid. In contrast, NT-3 regulation is independent of neuronal activity, but it is up-regulated by thyroid hormones and BDNF. The latter observation suggests that NT-3 might be controlled indirectly by neuronal activity via BDNF. In peripheral nonneuronal tissues, glucocorticoid hormones down-regulate NGF mRNA levels both in vitro and in vivo. In contrast, in the CNS, neuronal production of NGF is enhanced by glucocorticoids. The rapid regulation of NGF and BDNF by subtle physiological stimuli together with the recent demonstration that the neurotrophin release neurotransmitters such as acetylcholine opens up interesting perspectives for the function of neurotrophins as mediators of neuronal plasticity. 1994 John Wiley & Sons, Inc.     

A New Model for Developmental Neuronal Death and Excitatory/Inhibitory Balance in Hippocampus

The nervous system develops through a program that produces neurons in excess and then eliminates approximately half during a period of naturally occurring death. Neuronal activity has been shown to promote the survival of neurons during this period by stimulating the production and release of neurotrophins. In the peripheral nervous system (PNS), neurons depends on neurotrophins that activate survival pathways, which explains how the size of target cells influences number of neurons that innervate them (neurotrophin hypothesis). However, in the central nervous system (CNS), the role of neurotrophins has not been clear. Contrary to the neurotrophin hypothesis, a recent study shows that, in neonatal hippocampus, neurotrophins cannot promote survival without spontaneous network activity: Neurotrophins recruit neurons into spontaneously active networks, and this activity determines which neurons survive. By placing neurotrophin upstream of activity in the survival signaling pathway, these new results change our understanding of how neurotrophins promote survival. Spontaneous, synchronized network activity begins to spread through both principle neurons and interneurons in the hippocampus as they enter the death period. At this stage, neurotransmission mediated by γ-aminobutyric acid (GABA) is excitatory and drives the spontaneous activity. An important recent observation is that neurotrophins preferentially recruit GABAergic neurons into spontaneously active networks; thus, neurotrophins select for survival only those neurons joined to active networks with strong GABAergic inputs, which would later become inhibitory. A proper excitatory/inhibitory (E/I) balance is critical for normal adult brain function. This balance may be especially important in the hippocampus where impairments in E/I balance are associated with pathologies including epilepsy. Here, I discuss the molecular mechanisms for survival in neonatal neurons, how these mechanisms change during development, and how they may be linked to degenerative diseases.     

Development of trophic interactions in the vertebrate peripheral nervous system

During embryogenesis, the neurons of vertebrate sympathetic and sensory ganglia become dependent on neurotrophic factors, derived from their targets, for survival and maintenance of differentiated functions. Many of these interactions are mediated by the neurotrophins NGF, BDNF, and NT3 and the receptor tyrosine kinases encoded by genes of thetrk family. Both sympathetic and sensory neurons undergo developmental changes in their responsiveness to NGF, the first neurotrophin to be identified and characterized. Subpopulations of sensory neurons do not require NGF for survival, but respond instead to BDNF or NT3 with enhanced survival. In addition to their classic effects on neuron survival, neurotrophins influence the differentiation and proliferation of neural crest-derived neuronal precursors. In both sympathetic and sensory systems, production of neurotrophins by target cells and expression of neurotrophin receptors by neurons are correlated temporally and spatially with innervation patterns. In vitro, embryonic sympathetic neurons require exposure to environmental cues, such as basic FGF and retinoic acid to acquire neurotrophin-responsiveness; in contrast, embryonic sensory neurons acquire neurotrophin-responsiveness on schedule in the absence of these molecules.     

Differential regulation of survival and growth in adult sympathetic neurons: An invitro study of neurotrophin responsiveness

The survival and growth of embryonic and postnatal sympathetic neurons is dependent on both NGF and NT3. While it has been established that adult sensory neurons survive independently of neurotrophins, the case is less clear for adult sympathetic neurons, where the studies of survival responses to neurotrophins have relied upon using long‐term cultures of embryonic neurons. We have previously established a method to culture purified young (7 day) and adult (12 week) sympathetic neurons isolated from adult rat superior cervical ganglia (SCG) in order to examine their survival and growth responses to neurotrophins. We now show that by 12 weeks after birth virtually all neurons (90%) survive for 24 h in the absence of neurotrophins. Neuron survival is unaffected by treatment with anti‐NGF antibodies (anti‐NGF) or with the tyrosine kinase inhibitor, K252a, confirming the lack of dependence on extrinsic neurotrophins. Duration of neuron survival in culture increases significantly between E19 and day 7 and week 12 posnatally, and is similarly unaffected by the presence of anti‐NGF or K252a. Saturating concentrations of NGF and NT3 are equipotent in promoting neurite extension and branching. However, we find that NGF is more potent than NT3 in promoting neurite growth, irrespective of postnatal age. The growth‐promoting effects of NGF and NT3 are almost entirely blocked by K252a, demonstrating that these effects are mediated via activation of Trk receptors, which therefore appear to remain crucial to plasticity of adult neurons. Our results indicate that maturing neurons acquire protection against cell death, induced in the absence of neurotrophin, while retaining their growth responsiveness to these factors. © 2001 John Wiley & Sons, Inc. J Neurobiol 47: 295–305, 2001     

Neurotrophin-regulated signalling pathways

Neurotrophins are a family of closely related proteins that were identified initially as survival factors for sensory and sympathetic neurons, and have since been shown to control many aspects of survival, development and function of neurons in both the peripheral and the central nervous systems. Each of the four mammalian neurotrophins has been shown to activate one or more of the three members of the tropomyosin-related kinase (Trk) family of receptor tyrosine kinases (TrkA, TrkB and TrkC). In addition, each neurotrophin activates p75 neurotrophin receptor (p75NTR), a member of the tumour necrosis factor receptor superfamily. Through Trk receptors, neurotrophins activate Ras, phosphatidyl inositol-3 (PI3)-kinase, phospholipase C-gamma1 and signalling pathways controlled through these proteins, such as the MAP kinases. Activation of p75NTR results in activation of the nuclear factor-kappaB (NF-kappaB) and Jun kinase as well as other signalling pathways. Limiting quantities of neurotrophins during development control the number of surviving neurons to ensure a match between neurons and the requirement for a suitable density of target innervation. The neurotrophins also regulate cell fate decisions, axon growth, dendrite growth and pruning and the expression of proteins, such as ion channels, transmitter biosynthetic enzymes and neuropeptide transmitters that are essential for normal neuronal function. Continued presence of the neurotrophins is required in the adult nervous system, where they control synaptic function and plasticity, and sustain neuronal survival, morphology and differentiation. They also have additional, subtler roles outside the nervous system. In recent years, three rare human genetic disorders, which result in deleterious effects on sensory perception, cognition and a variety of behaviours, have been shown to be attributable to mutations in brain-derived neurotrophic factor and two of the Trk receptors.     

Neurotrophin-4: The odd one out in the neurotrophin family

Neurotrophin-4 (NT-4) is a member of a family of neurotrophic factors, the neurotrophins, that control survival and differentiation of vertebrate neurons (2–4). Besides being the most recently discovered neurotrophin in mammals, and the least well understood, several aspects distinguish NT-4 from other members of the neurotrophin family. It is the most divergent member and, in contrast to the other neurotrophins, its expression is ubiquitous and appears to be less influenced by environmental signals. NT-4 seems to have the unique requirement of binding to the lowaffinity neurotrophin receptor (p75^LNGFR) for efficient signalling and retrograde transport in neurons. Moreover, while all other neurotrophin knock-outs have proven lethal during early postnatal development, mice deficient in NT-4 have so far only shown minor cellular deficits and develop normally to adulthood. Is NT-4 a recent addition to the neurotrophic factor repertoire in search of a crucial function, or is it an evolutionary relic, a kind of wisdom tooth of the neurotrophin family?     

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.     

High-affinity TrkA and p75 neurotrophin receptor complexes: A twisted affair

Neurotrophin signaling is essential for normal nervous system development and adult function. Neurotrophins are secreted proteins that signal via interacting with two neurotrophin receptor types: the multifaceted p75 neurotrophin receptor and the tropomyosin receptor kinase receptors. In vivo, neurons compete for the limited quantities of neurotrophins, a process that underpins neural plasticity, axonal targeting, and ultimately survival of the neuron. Thirty years ago, it was discovered that p75 neurotrophin receptor and tropomyosin receptor kinase A form a complex and mediate high-affinity ligand binding and survival signaling; however, despite decades of functional and structural research, the mechanism of modulation that yields this high-affinity complex remains unclear. Understanding the structure and mechanism of high-affinity receptor generation will allow development of pharmaceuticals to modulate this function for treatment of the many nervous system disorders in which altered neurotrophin expression or signaling plays a causative or contributory role. Here we re-examine the key older literature and integrate it with more recent studies on the topic of how these two receptors interact. We also identify key outstanding questions and propose a model of inside-out allosteric modulation to assist in resolving the elusive high-affinity mechanism and complex.     

The nerve growth factor receptor: a multicomponent system that mediates the actions of the neurotrophin family of proteins

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3) are members of a family of structurally related proteins termed neurotrophins that promote the growth and survival of neurons in the central and peripheral nervous systems. Each of these proteins bind to at least two membrane receptors. One is the low affinity nerve growth factor receptor (p75), which binds each member of the neurotrophin family. The other is one of a family of tyrosine kinase receptors —trkA binds only NGF, the relatedtrkB receptor binds BDNF and NT-3, andtrkC binds NT-3 alone. This article reviews kinetic and biochemical information on p75 and its relationship to thetrk gene products.     

Structure–function relationships in the neurotrophin family

The study of structure–function relationships in the neurotrophin family has in recent years increased our understanding of several important aspects of neurotrophin function. Site-directed mutagenesis studies have localized amino acid residues important for binding to the low-affinity (p75^LNGFR), as well as to the members of the Trk family of tyrosine kinase receptors. A cluster of positively charged residues has been shown to form a surface for binding to p75^LNGFR in all four neurotrophins. Differences in the spatial distribution of these charges among the different neurotrophins may explain some of their distinct binding properties. Elimination of these positive charges drastically reduces binding to P75^LNGFR but not to the Trk family members, and it does not impair the biological properties of the neurotrophins in vitro, arguing that binding to and activation of Trk receptors is sufficient to mediate the biological responses of neurotrophins. In contrast. the binding sites to Trk receptors appear to be formed by discontinuous stretches of amino acid residues distributed throughout the primary sequence of the molecule. These include the N-terminus, some of the variable loop regions and a β-strand. Despite their apparent distribution, when viewed in the three-dimensional structure of NGF, these residues appear grouped on one side of the neurotrophin dimer, delineating a continuous surface extending approximately parallel to the twofold symmetry axis of the molecule. Two symmetrical surfaces are formed along the axis of the neurotrophin dimer providing a model for ligand-mediated receptor dimerization. In the neurotrophin family, co-evolution of cognate ligands and Trk receptors has developed specific contacts through different residues in the same variable regions of the neurotrophins. Thus, binding specificity is determined by the cooperation of distinct active and inhibitory binding determinants that restrict ligand-receptors interactions. Binding determinants to the Trk receptors can be manipulated independently in a rational fashion to create neurotrophin analogues with novel ligand-binding properties. In this way, second-generation chimeric neurotrophins with multiple specificities (pan-neurotrophins) have been engineered which may have valuable applications in the treatment of neurodegeneration and nerve damage. 1994 John Wiley & Sons, Inc.     

Brain neurotrophic supply in ontogenesis and during development of neurodegenerative diseases

Neurotrophic factors play a key role in ontogenetic changes of the nervous system’s functioning. The nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) were most completely characterized over six decades of active studies of neurotrophin family protein structure and functions. A complex coordination of synthesis, transport, secretion, and interaction of proneurotrophins and mature neurotrophins, as well as their receptors (Trk tyrosine kinase and p75^NTR receptor family proteins), cause a wide spectrum of their biological activity. In embryogenesis, neurotrophic factors are involved in the nervous system formation regulating both division, differentiation, survival, migration, and growth of neurons and their neurites and apoptosis activation. In the mature brain, neurotrophins are involved in the maintenance of the functional state of neurons and glial cells and synaptic plasticity regulation. It is natural that the development of processes typical for aging and neurodegenerative diseases is closely associated with a change in the brain neurotrophic supply caused both by a damage in neurotrophin metabolism and modification of their availability due to a change in the neuron microenvironment. The restoration of neurotrophic factor balance in the brain is considered as a promising approach to the therapy of neurodegenerative disorders.     

CNS neurotrophins are biologically active and expressed by multiple cell types

Neutrotrophins are increasingly appreciated as potential modulators of neuronal function in the adult central nervous system (CNS). To describe the neurotrophin environment within the adult CNS, mRNA and protein expression patterns of neurotrophins-3 and –4 and of brain-derived neurotrophin were investigated in adult rat spinal cord and brain. Co-localization studies with CNS cell type-specific markers demonstrates that multiple cell types, including both neurons and glia, express these neurotrophins in the normal adult CNS. Although widely implicated in important CNS functions such as synaptic plasticity, biological activity of endogenous CNS neurotrophins has not been directly demonstrated. With a sensitive neurite outgrowth bioassay we demonstrate that CNS neurotrophins elicit neurite outgrowth and are biologically active. Moreover, antibody-blocking studies suggest that these three neurotrophins may comprise the bulk of adult CNS neurotrophic activity.     

p75NTR--live or let die

During neuronal development, neurotrophins are essential factors that promote survival, differentiation and myelination of neurons. The trophic signals are relayed to the cells via binding to Trk receptor tyrosine kinases and the p75 neurotrophin receptor. Paradoxically, the p75 neurotrophin receptor also ensures rapid and appropriate apoptosis of neonatal neurons not reaching their proper targets and transmits death signals to injured neurons. Until recently, the mechanisms by which the p75 neurotrophin receptor governs these opposing functions have remained elusive. By the identification of new ligands and cytosolic interacting partners, receptor cleavage products and coreceptors, some of these mechanisms are now being unraveled. Here, we review recent progress in delineating the molecular networks that enable p75(NTR) to dictate life and death.     

Neurotrophins improve synaptic transmission in the adult rodent diaphragm

Neurotrophins are usually viewed as secreted proteins that control long-term survival and differentiation of neurons. However, recent studies have established that among the most important functions of neurotrophins is their capacity to regulate synaptic functions and plasticity. When altering synaptic function, neurotrophins are able to produce two types of outcomes, an immediate effect on synaptic transmission and long-term control of synaptic structure and function. The first effect occurs within seconds or minutes after the neurotrophic factor has been applied and usually involves acute modification of synaptic transmission. The second effect takes hours and days, as protein synthesis is required to complete the structural changes. Neurotrophins and their receptors are expressed within the neuromuscular system, making these agents ideal candidates for the short-and long-term regulation of skeletal muscle function. For instance, neurotrophins can alter neuromuscular function acutely, by modulating the amount of neurotransmitter released with each nerve impulse, or chronically, by changing postsynaptic properties or the content and size of synaptic vesicles. It is obvious that the effects of neurotrophins depend on the specific neurotrophin involved (four neurotrophins have been found in mammals; these are nerve growth factor, brain-derived neurotrophic factor, and neurotrophins-3 and-4) and on the specific synapse being studied. Growing evidence highlights the role of neurotrophins in the development and function of neuromuscular synapses. This review will examine the role of neurotrophins in the regulation of neuromuscular transmission. Neirofiziologiya/Neurophysiology, Vol. 39, Nos. 4/5, pp. 327–337, July–October, 2007.     

Defining Responsiveness of Avian Cochlear Neurons to Brain-Derived Neurotrophic Factor and Nerve Growth Factor by HSV-1-Mediated Gene Transfer

Abstract: The importance of individual members of the neurotrophin gene family for avian inner ear development is not clearly defined. Here we address the role of two neurotrophins, brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), for innervation of the chicken cochlea. We have used defective herpes simplex virus type 1 (HSV-1) vectors, or amplicons, to express these neurotrophins in dissociated cultures of cochlear neurons. HSV-1-mediated expression of BDNF promotes neuronal survival similar to the maximal level seen by exogenously added BDNF and exceeds its potency to produce neurite outgrowth. In contrast, cochlear neurons transduced with an amplicon producing bioactive NGF show no response. These results confirm BDNF as an important mediator of neurotrophin signaling inside avian cochlear neurons. However, these neurons can be rendered NGF-responsive by transducing them with the high-affinity receptor for NGF, TrkA. This study underlines the usefulness of amplicons to study and modify neurotrophin signaling inside neurons.     

Neurotrophins and cell death

The neurotrophins - NGF, BDNF, NT-3 - are secreted proteins that play a major role in neuron survival, differentiation and axon wiring toward target territories. They do so by interacting with their main tyrosine kinase receptors TrkA, TrkB, TrkC and p75(NTR). Even though there is a general consensus on the view that neurotrophins are survival factors, there are two fundamentally different views on how they achieve this survival activity. One prevailing view is that all neurons and more generally all normal cells are naturally committed to die unless a survival factor blocks this death. This death results from the engagement of a "default" apoptotic cell program. The minority report supports, on the opposite, that neurotrophin withdrawal is associated with an active signal of cell death induced by unbound dependence receptors. We will discuss here how neurotrophins regulate cell death and survival and how this has implications not only during nervous system development but also during cancer progression.     

Neurotrophins in inflammatory lung diseases: modulators of cell differentiation and neuroimmune interactions

Chronic inflammatory lung diseases represent a group of severe diseases with increasing prevalence as well as epidemiological importance. Inflammatory lung diseases could result from allergic or infectious genesis. There is growing evidence that the immune and nervous system are closely related not only in physiological but also in pathological reactions in the lung. Extensive communications between neurons and immune cells are responsible for the magnitude of airway inflammation and the development of airway hyperreactivity, a consequence of neuronal dysregulation. Neurotrophins are molecules regulating and controlling this crosstalk between the immune and peripheral nervous system (PNS) during inflammatory lung diseases. They are constitutively expressed by resident lung cells and produced in increasing quantities by immune cells invading the airways under inflammatory conditions. They act as activation, differentiation and survival factors for cells of both the immune and nervous system. This article will review the most recent data of neurotrophin signaling in the normal and inflamed lung and as yet unexplored, roles of neurotrophins in the complex communication within the neuroimmune network.