Neurotrophic factor structures reveal clues to evolution, binding, specificity, and receptor activation

The neutrophin family, the glial-derived neurotrophic factor family, and the ciliary neurotrophic factor are the best described growth factors specific for developing neurons and neutral crest cells. As might be expected for regulatory molecules of the complex central and peripheral nervous system, these factors show considerable receptor specificity and cross-talk. Thanks to a decade of intense research by numerous laboratories, the structures of many of these factors are now available. This review discusses the structural bases of receptor binding, specificity, and activation in each of these systems. Using structure-based sequence alignments, the evolutionary implications of these molecules and their receptors are discussed, followed by suggestions for further directions for research on the structure and function of these neurotrophic factors.     

The neurotrophic factors in non-neuronal tissues

Although neurotrophic factors are defined as molecules that maintain neuronal cells, they possess a range of functions outside the nervous system. For example, glial cell line-derived neurotrophic factor is essential for ureteric branching in kidney morphogenesis and for regulating the fate of stem cells during spermatogenesis. Leukemia inhibitory factor, a member of the interleukin-6 (IL-6) ciliary neurotrophic factor family, inhibits differentiation of embryonic stem cells, induces tubulogenesis in the embryonic kidney, and regulates sperm differentiation. Other IL-6 family members are important in cardiac differentiation and they have pleiotropic functions in the hematopoietic and immune systems. Although neurotrophin receptors have been found on a number of non-neuronal tissues, they represent mostly truncated receptor isoforms that are incapable of signal transduction and may have scavenger or dominant negative functions. However, several examples can be presented of essential non-neuronal functions played by neurotrophins in e.g., cardiac, hair follicle, and vascular differentiation, and the maintenance of immune cells.     

Expression of nerve growth factor-like polypeptides and immunoreactivity related to the two types of neurotrophin receptors in earthworm tissues

Nerve growth factor (NGF) belongs by sequence homology to the neurotrophins, a family of proteins binding the same p75 receptor and closely related members of the Trk family of receptor tyrosine kinases. Fundamental in the vertebrate nervous system, neurotrophin signals have also been suggested as essential for relatively complex nervous systems occurring in invertebrate species that live longer than Caenorhabditis elegans and Drosophila melanogaster. Mammalian neurotrophins have been found to influence invertebrate neuronal growth. However, there are only a few data on the presence of molecules related to neurotrophin signalling components in invertebrates. Our studies provide evidence that analogues of neurotrophins and neurotrophin receptors are expressed in Eisenia foetida earthworms. In particular, NGF-like and Trk-like immunoreactive proteins are both expressed in the nervous system, whereas p75-like positivity identifies tubular structures associated with dorsal pores that are involved in the earthworm response to mechanical irritation or stress. Received 12 November 2001; received after revision 8 January 2002; accepted 8 January 2002     

Nerve growth factor: structure and function

Neurotrophins are critical for the development and maintenance of the peripheral and central nervous system. These highly homologous, homodimeric growth factors control cell survival, differentiation, growth cessation, and apoptosis of sensory neurons. The biological functions of the neurotrophins are mediated through two classes of cell surface receptors, the Trk receptors and the p75 neurotrophin receptor (p75NTR). Nerve growth factor (NGF), the best characterized member of the neurotrophin family, sends its survival signals through activation of TrkA and can induce cell death by binding to p75NTR. Recent domain deletion and mutagenesis studies have identified the membrane-proximal domain of the Trks as necessary and sufficient for ligand binding. Crystal structures of this domain of TrkA, TrkB, and TrkC, and an alanine scanning analysis of this domain of TrkA and TrkC have allowed identification of the ligand-binding site. The recent crystal structure of the complex between NGF and the ligand-binding domain of TrkA defines the orientation of NGF in the signaling complex, and eludicates the structural basis for binding and specificity in the family. Further structural work on NGF-TrkA-p7SNTR complexes will be necessary to address the many remaining questions in this complex signaling system.     

Sensing life: regulation of sensory neuron survival by neurotrophins

Neurotrophins are a family of structurally and functionally related neurotrophic factors which, in mammals, include: nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 (NT-3), and NT-4/5. In addition to their canonical role in promoting neuronal survival, these molecules appear to regulate multiple aspects of the development of the nervous system in vertebrates, including neuronal differentiation, axon elongation and target innervation, among others. Actions of neurotrophins and of their receptors in vivo are being analyzed by loss-of-function or gain-of-function experiments in mice. Here, we review the phenotypes of the primary sensory system in these mutant mouse strains and the different strategies specifically involved in the regulation of neuronal survival by neurotrophins in this portion of the nervous system. Received 10 December 2001; received after revision 11 May 2002; accepted 13 May 2002 RID="*" ID="*"Corresponding author.     

Local and target-derived actions of neurotrophins during peripheral nervous system development

Neurotrophic factors are present in limiting quantities, and neurons that obtain an adequate supply of the required neurotrophic factor survive whereas those that compete unsuccessfully die. Analysis of null mutant mice for neurotrophins and Trk receptors as well as in vivo experiments in ovo in the chick applying exogenous neurotrophins or neutralising antisera have significantly increased knowledge of the roles they play during development. This review focuses on recent advances in understanding the various roles of neurotrophins in dorsal root ganglion sensory neuron development at different times in embryonic development - an early local role for differentiation of the sensory precursor cells and a later survival-promoting target-derived role for the mature neurons. Neurotrophic factors are present in limiting quantities, and neurons that obtain an adequate supply of the required neurotrophic factor survive whereas those that compete unsuccessfully die. Analysis of null mutant mice for neurotrophins and Trk receptors as well as in vivo experiments in ovo in the chick applying exogenous neurotrophins or neutralising antisera have significantly increased knowledge of the roles they play during development. This review focuses on recent advances in understanding the various roles of neurotrophins in dorsal root ganglion sensory neuron development at different times in embryonic development - an early local role for differentiation of the sensory precursor cells and a later survival-promoting target-derived role for the mature neurons.     

The role of VEGF receptors in angiogenesis; complex partnerships

Vascular endothelial growth factors (VEGFs) regulate blood and lymphatic vessel development and homeostasis but also have profound effects on neural cells. VEGFs are predominantly produced by endothelial, hematopoietic and stromal cells in response to hypoxia and upon stimulation with growth factors such as transforming growth factors, interleukins or platelet-derived growth factor. VEGFs bind to three variants of type III receptor tyrosine kinases, VEGF receptor 1, 2 and 3. Each VEGF isoform binds to a particular subset of these receptors giving rise to the formation of receptor homo- and heterodimers that activate discrete signaling pathways. Signal specificity of VEGF receptors is further modulated upon recruitment of coreceptors, such as neuropilins, heparan sulfate, integrins or cadherins. Here we summarize the knowledge accumulated since the discovery of these proteins more than 20 years ago with the emphasis on the signaling pathways activated by VEGF receptors in endothelial cells during cell migration, growth and differentiation. Received 15 September 2005; received after revision 11 November; accepted 24 November 2005     

The Vps10p-domain receptor family

The family of mammalian type-I transmembrane receptors containing a Vps10p domain contains five members, Sortilin, SorCS1, SorCS2, SorCS3, and SorLA. The common characteristic of these receptors is an N-terminal Vps10p domain, which either represents the only module of the luminal/extracellular moiety or is combined with additional domains. Family members play roles in protein transport and signal transduction. The individual receptors bind and internalize a variety of ligands, such as neuropeptides and trophic factors, and Sortilin and SorLA mediate trans-Golgi network-to-endosome sorting. Their prominent neuronal expression, several of the identified ligands, and recent results support the notion that members of this receptor family have important functions in neurogenesis, plasticity-related processes, and functional maintenance of the nervous system. For instance, it has been demonstrated that Sortilin partakes in the transduction of proapoptotic effects, and there is converging biochemical and genetic evidence that implies that SorLA is an Alzheimer’s disease risk factor.     

Neurotrophin signalling pathways regulating neuronal apoptosis

Recent evidence indicates that naturally occurring neuronal death in mammals is regulated by the interplay between receptor-mediated prosurvival and proapoptotic signals. The neurotrophins, a family of growth factors best known for their positive effects on neuronal biology, have now been shown to mediate both positive and negative survival signals, by signalling through the Trk and p75 neurotrophin receptors, respectively. The mechanisms whereby these two neurotrophin receptors interact to determine neuronal survival have been difficult to decipher, largely because both can signal independently or coincidentally, depending upon the cell or developmental context. Nonetheless, the past several years have seen significant advances in our understanding of this receptor signalling system. In this review, we focus on the proapoptotic actions of the p75 neurotrophin receptor (p75NTR), and on the interplay between Trk and p75NTR that determines neuronal survival.     

Cytosolic factors in mitochondrial protein import

In vitro import studies have confirmed the participation of cytosolic protein factors in the import of various precursor proteins into mitochondria. The requirement for extramitochondrial adenosine triphosphate for the import of a group of precursor proteins seems to be correlated with the chaperone activity of the cytosolic protein factors. One of the cytosolic protein factors is hsp70, which generally recognizes and binds unfolded proteins in the cytoplasm. Hsp70 keeps the newly synthesized mitochondrial precursor proteins in import-competent unfolded conformations. Another cytosolic protein factor that has been characterized is mitochondrial import stimulation factor (MSF), which seems to be specific to mitochondrial precursor proteins. MSF recognizes the mitochondrial precursor proteins, forms a complex with them and targets them to the receptors on the outer surface of mitochondria.     

CD20-related signaling pathway is differently activated in normal and dystrophic circulating CD133+ stem cells

Among the heterogeneous population of circulating hematopoietic and endothelial progenitors, we identified a subpopulation of CD133⁺ cells displaying myogenic properties. Unexpectedly, we observed the expression of the B-cell marker CD20 in blood-derived CD133⁺ stem cells. The CD20 antigen plays a role in the modulation of intracellular calcium homeostasis through signaling pathways activation. Several observations suggest that an increase in intracellular calcium concentration ([Ca²⁺]_i) could be involved in the etiology of the Duchenne muscular dystrophy (DMD). Here, we show that a CD20-related signaling pathway able to induce an increase in [Ca²⁺]_i is differently activated after brain derived neurotrophic factor (BDNF) stimulation of normal and dystrophic blood-derived CD133⁺ stem cells, supporting the assumption of a “CD20-related calcium impairment-affecting dystrophic cells. Presented findings represent the starting point toward the expansion of knowledge on pathways involved in the pathology of DMD and in the behavior of dystrophic blood-derived CD133⁺ stem cells. Received 15 October 2008; received after revision 27 November 2008; accepted 05 December 2008     

New concepts in regulation and function of the insulin-like growth factors: implications for understanding normal growth and neoplasia

The insulin-like growth factors (IGFs) are a ubiquitous family of growth factors, binding proteins and receptors that are involved in normal growth and development. They are also implicated in numerous pathological states, including malignancy. IGF-II is a commonly expressed growth factor in many tumors and may enhance tumor growth, acting via the overexpressed IGF-I receptor, a cell-surface tyrosine kinase receptor. The IGF-I receptor may be overexpressed due to mutations in tumor suppression gene products such as p53 and WT-1 or growth factors such as bFGF and PDGF. Thus, this family of growth factors, especially the IGF-I receptor, may present an excellent target for new therapeutic agents in the treatment of cancer and other disorders of excessive cellular proliferation.     

RANKL,RANK, osteoprotegerin: key partners of osteoimmunology and vascular diseases

1997 saw the identification of a novel set of proteins within the tumor necrosis factor (TNF)/TNF receptor families that are required for the control of bone remodeling. Therefore, these receptors, receptor activator of nuclear factor kappa B (RANK), osteoprotegerin (OPG) and their ligand RANK ligand (RANKL) became the critical molecular triad controlling osteoclastogenesis and pathophysiologic bone remodeling. However, the establishment of the corresponding knock-out and transgenic mice revealed unexpected results, most particularly, the involvement of these factors in the vascular system and immunity. Thus, the OPG/RANK/RANKL molecular triad appears to be associated with vascular calcifications and plays a pivotal function in the development of the immune system through dendritic cells. OPG/RANK/RANKL thus constitute a molecular bridge spanning bone metabolism, vascular biology and immunity. This review summarizes recent knowledge of OPG/RANK/RANKL interactions and activities as well as the current evidence for their participation in osteoimmunology and vascular diseases. In fine, the targeting of the OPG/RANK/RANKL axis as novel therapeutic approaches will be discussed. Received 27 February 2007; accepted 4 April 2007     

The epidermal growth factor receptor family: Biology driving targeted therapeutics

The epidermal growth factor family of receptor tyrosine kinases (ErbBs) plays essential roles in regulating cell proliferation, survival, differentiation and migration. The ErbB receptors carry out both redundant and restricted functions in mammalian development and in the maintenance of tissues in the adult mammal. Loss of regulation of the ErbB receptors underlies many human diseases, most notably cancer. Our understanding of the function and complex regulation of these receptors has fueled the development of targeted therapeutic agents for human malignancies in the last 15 years. Here we review the biology of ErbB receptors, including their structure, signaling, regulation, and roles in development and disease, then briefly touch on their increasing roles as targets for cancer therapy.     

TRAF4 functions as an intermediate of GITR-induced NF-κB activation

Members of the tumor necrosis factor receptor (TNFR) family regulate the activation, differentiation, and function of many cell types, including cells of the immune system. TNFR-associated factors (TRAFs) function as adapter molecules controlling signaling pathways triggered by TNFR family members, such as activation of nuclear factor B (NF-B). Despite intensive research, the function of TRAF4 in signaling pathways triggered by TNFR-related proteins remains enigmatic. Intriguingly, our functional studies indicated that TRAF4 augments NF-B activation triggered by glucocorticoid-induced TNFR (GITR), a receptor expressed on T cells, B cells, and macrophages. Further analyses revealed that TRAF4-mediated NF-B activation downstream of GITR depends on a previously mapped TRAF-binding site in the cytoplasmic domain of the receptor and is inhibited by the cytoplasmic protein A20. GITR is thought to inhibit the suppressive function of regulatory T cells (T_reg cells) and to promote activation of T cells. Taken together, our studies provide the first indications that TRAF4 elaborates GITR signaling and suggest that TRAF4 can modulate the suppressive functions of T_reg cells.Received 20 September 2004; received after revision 8 October 2004; accepted 18 October 2004     

From MDR to MXR: new understanding of multidrug resistance systems, their properties and clinical significance

The ATP binding cassette (ABC) superfamily of membrane transporters is one of the largest protein classes known, and counts numerous proteins involved in the trafficking of biological molecules across cell membranes. The first known human ABC transporter was P-glycoprotein (P-gp), which confers multidrug resistance (MDR) to anticancer drugs. In recent years, we have obtained an increased understanding of the mechanism of action of P-gp as its ATPase activity, substrate specificity and pharmacokinetic interactions have been investigated. This review focuses on the functional characterization of P-gp, as well as other ABC transporters involved in MDR: the family of multidrug-resistance-associated proteins (MRP1-7), and the recently discovered ABC half-transporter MXR (also known as BCRP, ABCP and ABCG2). We describe recent progress in the analysis of protein structure-function relationships, and consider the conceptual problem of defining and identifying substrates and inhibitors of MDR. An in-depth discussion follows of how coupling of nucleotide hydrolysis to substrate transport takes place, and we propose a scheme for the mechanism of P-gp function. Finally, the clinical correlations, both for reversal of MDR in cancer and for drug delivery, are discussed.     

Developmental actions of natriuretic peptides in the brain and skeleton

The concept that atrial natriuretic peptide (ANP) and the closely related peptides BNP and CNP might be involved in the ontogeny of several organ systems emerged in the late 1980s. While many of the reported in vitro actions have not been examined in the context of organ development in vivo, recent studies demonstrate that mice which lack or overexpress natriuretic peptides or receptors exhibit pronounced skeletal growth defects. This article discusses how natriuretic peptides and other factors appear to regulate bone growth as an example of how natriuretic peptides might participate in the ontogeny of other organ systems. Evidence indicating that natriuretic peptides regulate neural development is then reviewed. Natriuretic peptides and receptors exhibit complex expression patterns in the developing nervous system, where they have been shown to act on neural cells as early as at the embryonic neural tube stage. Interestingly, both bone and brain growth appear to utilize primarily CNP and the CNP-specific type B receptor, and perhaps the type C receptor. In vitro data indicate that CNP may act on developing neurons, astrocytes and Schwann cells like a classical growth factor, regulating proliferation, patterning, phenotypic specification, survival and axonal pathfinding. Natriuretic peptides might also have roles in the vascularization of the embryonic brain, establishment of the blood-brain and blood-nerve barriers, and perhaps in nerve regeneration.Received 13 April 2004; received after revision 20 May 2004; accepted 27 May 2004