Pleiotropic functions of neurotrophins in development

Immunocytochemical evidence for the presence and location of the neurotrophin-Trk receptor family in adult human preovulatory ovarian follicles

OBJECTIVE

This study was undertaken to evaluate the presence or absence of neurotrophins and their respective receptors within adult human preovulatory follicles.

STUDY DESIGN

Prospective study of neurotrophins and their receptors in follicular cells and unfertilized oocytes from women undergoing aspiration for in vitro fertilization/intracytoplasmic sperm injection. Cells (mural and cumulus granulosa cells, unfertilized oocytes) were examined for immunocytochemical staining of neurotrophin and receptor proteins.

RESULTS

Mural and cumulus granulosa cells were positive for BDNF, NT-4/5, NT-3, and NGF, as well as for Trk B, Trk C, and Trk A receptors. Unfertilized oocytes were positive for Trk B, Trk C, and Trk A receptors.

CONCLUSION

Neurotrophins and their respective receptor proteins are present within the mural and cumulus granulosa cells of adult human preovulatory follicles. Neurotrophin receptors are present in human unfertilized oocytes. The location of the neurotrophins and their receptors suggest both an autocrine and paracrine function within the adult human ovarian follicle.

Sortilin controls intracellular sorting of brain-derived neurotrophic factor to the regulated secretory pathway

Brain-derived neurotrophic factor (BDNF), after activity-dependent secretion from neurons, modulates critical nervous system functions. Recently, a variant in the human bdnf gene, resulting in a valine to methionine substitution in the prodomain, has been shown to lead to defective regulated secretion from neurons and memory impairment. Here, we report a novel function for a Vps10p domain protein, sortilin, in controlling BDNF sorting to the regulated secretory pathway. Sortilin interacts specifically with BDNF in a region encompassing the methionine substitution and colocalizes with BDNF in secretory granules in neurons. A truncated form of sortilin causes BDNF missorting to the constitutive secretory pathway without affecting neurotrophin-4 (NT-4) secretion. In addition, sortilin small interfering RNA introduced into primary neurons also led to BDNF missorting from the regulated to the constitutive secretory pathway. Together, these data suggest a mechanism to understand the defect associated with variant BDNF and provide a framework, based on divergent presynaptic regulation of sorting to secretory pathways, to explain how two ligands for tropomyosin-related kinase B, BDNF and NT-4, can mediate diverse biological responses.

Expression of nerve growth factor (NGF), and its receptors TrkA and p75 in the reproductive organs of the adult male rats

Immunolocalization of nerve growth factor (NGF) and its receptors, TrkA and p75 in the reproductive organs of adult male rats was investigated. Sections of the testis, efferent duct, epididymis, deferent duct, seminal vesicle, coagulating gland and prostate of adult male rats were immunostained by the avidin-biotin-peroxidase complex methods (ABC). NGF was expressed in Leydig cells, primary spermatocytes and pachytene spermatocytes in the testis. TrkA only immunoreacted to elongate spermatids and p75 showed positive immunostaining in the Sertoli cells, Leydig cells, the pachytene spermatocytes and elongate spermatids. Immunoreactions for NGF and its two receptors were detected in epithelial cells of efferent duct, deferent duct and epididymis. In addition, immunoreactions for NGF and its two receptors were also observed in columnar secretory epithelium lines of the seminal vesicles, prostate and coagulating gland. These results suggest that NGF is an important growth factor in gonadal function of adult male rats.

Identification and characterization of compounds that potentiate NT-3-mediated Trk receptor activity

Neurotrophins are a family of secreted proteins that play an important role in the development, differentiation, and survival of neurons. Studies also suggest that aberrant neurotrophin signaling may play a role in processes underlying disease states such as schizophrenia, Alzheimer's disease, and depression. Whereas the development of agents that selectively stimulate neurotrophin signaling has proven to be difficult, compounds have been identified that potentiate neurotrophin 3 (NT-3)-mediated activation of trk A. In the present studies, we extend those initial observations to identify compounds that also potentiate NT-3-mediated activation of trk B. Compound potentiation of NT-3 was observed using several readouts of transfected and endogenous trk receptor activity, including trk receptor phosphorylation, mitogen-activated protein kinase phosphorylation, reporter assay activity (beta-lactamase and luciferase), cell survival and neurite extension assays. Studies using chimeric trk receptors demonstrated that the extracellular domain is essential for compound potentiation and rule out interaction with intracellular signaling molecules as a mechanism of compound activity. Thus, the present studies demonstrate that trk B receptor activity can be potentiated by small-molecule compounds via the extracellular domain of the receptor and provide reagents for further evaluating the role of NT-3-mediated trk A and trk B activity in vivo.

Expression of TrkB in the murine kidney

Neurotrophins acting through Trk signal-transducing receptors play essential roles in the nervous system, and probably in some nonneuronal tissues. In the present study we used Western-blot and immunohistochemistry to investigate the occurrence and cellular localization of TrkB in the mouse kidney. Furthermore, the structure and ultrastructure of the kidney in mice carrying a mutation in the trkB gene were analyzed. TrkB in the kidney was identical to the cerebral one (145 kDa). Since the antibody used recognize a sequence within the tyrosine-kinase domain of TrkB, the renal TrkB receptor identified here must be regarded as able to mediate biological effects of their ligands. TrkB immunoreactivity was restricted to the juxtaglomerular apparatus, including differentiated vascular cells and extaglomerular mesangial cells. In these cells, TrkB colocalized with renin. The structural analysis revealed no major changes in the kidney structure of TrkB-deficient mice, with the exception of a significant reduction of the glomerular area. Nevertheless, in these animals there was an apparent increase in the number of extraglomerular mesangial cells (which retain the ability to synthesize renin) and absence of the macula densa. Taken together, these results strongly suggest a role of TrkB and their ligands in the control of the normal development and maintenance of the juxtaglomerular apparatus.

TrkA and p75NTR in the ovary of adult cow and pig

Neurotrophins play a critical role in the development of the mammalian ovary, oogenesis and folliculogenesis. In this study we investigated the cell localization of the two main receptors for nerve growth factor (NGF), TrkA and p75NTR, in the cow and pig ovary, using immunohistochemistry. Specific immunoreactivity for TrkA and p75NTR was detected in the ovary of both species, but the pattern and intensity of immunostaining were marginally different between them. The follicular cells regularly expressed immunoreactivity for both receptors. Immunoreactivity was also detected in the oocytes, independently of the maturational stage of the follicles, with the exception of primordial and primary follicles of the pig which did not display p75NTR. Taken together, these results suggest a possible direct role of NGF on oocytes expressing TrkA and p75NTR, in addition to the well-known roles in other ovary functions. The practical relevance of these data remains to be clarified.

A novel endocytic recycling signal distinguishes biological responses of Trk neurotrophin receptors

Endocytic trafficking of signaling receptors to alternate intracellular pathways has been shown to lead to diverse biological consequences. In this study, we report that two neurotrophin receptors (tropomyosin-related kinase TrkA and TrkB) traverse divergent endocytic pathways after binding to their respective ligands (nerve growth factor and brain-derived neurotrophic factor). We provide evidence that TrkA receptors in neurosecretory cells and neurons predominantly recycle back to the cell surface in a ligand-dependent manner. We have identified a specific sequence in the TrkA juxtamembrane region, which is distinct from that in TrkB receptors, and is both necessary and sufficient for rapid recycling of internalized receptors. Conversely, TrkB receptors are predominantly sorted to the degradative pathway. Transplantation of the TrkA recycling sequence into TrkB receptors reroutes the TrkB receptor to the recycling pathway. Finally, we link these divergent trafficking pathways to alternate biological responses. On prolonged neurotrophin treatment, TrkA receptors produce prolonged activation of phosphatidylinositol 3-kinase/Akt signaling as well as survival responses, compared with TrkB receptors. These results indicate that TrkA receptors, which predominantly recycle in signal-dependent manner, have unique biological properties dictated by its specific endocytic trafficking itinerary.

Distribution of NGF and NT-3-like protein immunoreactivity in the teleost kidney

By means of immunochemistry and immunohistochemistry, we investigated in the kidney of freshwater and marine teleostean species for the presence and localization of three neurotrophins: nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin (NT)-3. In both species studied, NGF-like and NT-3-like immunoreactivity were present in the kidney with different distribution patterns, while BDNF-like immunoreactivity was never detected. In goldfish, NGF-like and NT-3-like immunoreactivity were identified extensively in cells along part of the arterial branches adjacent to the afferent arterioles. In scorpion fish, NGF-like and NT-3-like immunoreactive cells were observed both on afferent arterioles and on adjacent secondary branches derived from renal arteries. No immunoreactivity was detected in other renal structures. A staining pattern of immunoreactivity similar to that obtained for NGF and NT-3 was detected utilizing S100 antibody as a juxtaglomerular (JG) cell marker. Double immunolabellings NGF/S100 and NT-3/S100 evidenced the coexistence of neurotrophin-like proteins and S100-like protein in the same immunoreactive cells, thus identifying them as juxtaglomerular cells. Western blot analysis revealed the presence of molecules immunoreactive to NGF and NT-3, whose molecular weights were very similar to those of the corresponding mammalian neurotrophins. These findings extend the presence and distribution of NGF-like and NT-3-like IR in the kidney to teleost species, suggesting a probable participation of these proteins in the renal functions of freshwater and marine teleosts.

Involvement of neurotrophins and their receptors in spondyloarthritis synovitis: relation to inflammation and response to treatment

OBJECTIVE

To investigate whether expression of the four members of the neurotrophin (NT) family and their four corresponding receptors is related to synovial inflammation in patients with spondyloarthritis (SpA).

MATERIAL AND METHODS

Synovial fluid (SF) and serum NTs and their receptors were measured by ELISA. Immunohistochemistry was used for synovial tissue biopsy specimens from patients with SpA, rheumatoid arthritis, and osteoarthritis (OA). In SpA synovium, immunoreactivity of the receptors trkA and NGFRp75 was also assessed before and after 12 weeks of treatment with the monoclonal anti-tumour necrosis factor alpha antibody, infliximab.

RESULTS

mRNA transcripts of all NTs and receptors were expressed in the inflamed synovium. At the protein level, brain derived neurotrophic factor and NT-3 were significantly higher in the SF of patients with SpA than in those with OA. In contrast, ELISA of serum samples showed that the highest member in SpA was NT-4. Immunohistochemistry demonstrated that the NT receptors trkA and NGFRp75 were highly expressed in the inflamed synovium of patients with SpA, correlating with vascularity and lymphoid aggregates, respectively. Additionally, immunoreactivity of both receptors was significantly decreased after infliximab treatment.

CONCLUSIONS

NTs and their receptors are expressed in inflamed peripheral joints of patients with SpA. Their expression is not constitutive but related to inflammation and they may be involved in the local disease processes.

Expression of nerve growth factor (NGF) isoforms in the rat uterus during pregnancy: accumulation of precursor proNGF

The mechanisms that promote the transient degenerative changes in the uterus innervation during pregnancy remain incompletely understood. Signaling by the nerve growth factor (NGF)-beta is important for maintaining the density of peripheral sympathetic innervation. Here, we analyzed the spatial and temporal expression of NGF isoforms in the rat uterus using RT-PCR, immunoblot analysis, and immunohistochemistry during pregnancy (d 7, 14, and 21), and postpartum (d 1, 8, and 22). Western blot analysis using antibodies to mature NGF-beta and to proNGF domain demonstrated a significant decrease in mature NGF-beta at gestational d 14 and 21 (term pregnancy) and 1 d postpartum, which paralleled a remarkable accumulation of the 26-28-, 32-, and 60-kDa proNGF forms. There were diminished ratios of mature NGF-beta to proNGF independent of uterus growth on the same gestational days. Immunohistochemistry revealed a progressive NGF-beta decline throughout pregnancy in the myometrium and a near absence at term pregnancy, which contrasted with increased NGF immunostaining in the intermyometrial connective tissue layers. More importantly, proNGF-specific antibodies identified the increased NGF immunoreactivity in the intermyometrial layers at term pregnancy as proNGF and not mature NGF-beta. Alterations in the processing of NGF and accumulation of proNGF in the intermyometrial layers, where axonal degeneration occurs, may contribute significantly to the pregnancy-related uterine denervation and to the control of myometrial activity.

A neurotrophin axis in myeloma: TrkB and BDNF promote tumor-cell survival

Multiple myeloma (MM) is a B-cell neoplasm that is characterized by the clonal expansion of malignant plasma cells and is frequently associated with chromosomal translocations placing an oncogene under the control of the immunoglobulin heavy chain enhancer. Despite these pathogenic translocations, MM cells remain dependent on external cues for survival. We present evidence that brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family of growth factors, and its high-affinity receptor, tropomyosin receptor kinase B (TrkB), contribute to these survival cues. MM cells express TrkB, and respond to BDNF by activating mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase-a PI3K target (PI3K/Akt) signaling cascades. Addition of BDNF protects human MM cell lines (HMCLs) from apoptosis induced by dexamethasone or bortezomib and prolongs the survival of primary MM cells cultured alone or with human bone marrow (BM) stroma. As BDNF and TrkB are expressed by osteoblasts, stromal cells, and endothelial cells within the BM microenvironment, a BDNF-TrkB axis may be critical to the interactions of MM with bone and stroma that contribute to MM tumor progression. Finally, BDNF is expressed by malignant plasma cells isolated from a subset of patients with MM, as well as by most HMCLs, suggesting a potential role for this neurotrophin axis in autocrine as well as paracrine support of MM.

Role of nerve growth factor and FSH receptor in epithelial ovarian cancer

The neurotrophins (NT) including nerve growth factor (NGF) are a family of related growth factors that are of major importance in the regulation of neuronal survival and differentiation. In the ovary, they can help in follicular maturation and ovulation by inducing the FSH receptor (FSHR). Current literature shows that perimenopausal ovarian surface epithelium (OSE) can also express FSHR. By G protein link, this FSHR is capable of precipitating neoplasia of OSE, which is the commonest in the ovary. NT are implicated as the cause of this aberrant expression of FSHR in OSE. By central action NT can lower serum FSH, as is found in ovarian cancer. Thus, NGF deregulates expression of FSHR in OSE and secretion of FSH from the pituitary. This phenomenon may hold the key to the hitherto unexplained carcinogenic process of sporadic epithelial ovarian cancer.

Ablation of TrkA function in the immune system causes B cell abnormalities

The nerve growth factor (NGF) receptor TrkA is widely expressed in non-neural tissues suggesting pleiotropic functions outside the nervous system. Based on pharmacological and immuno-depletion experiments, it has been hypothesized that NGF plays an important role in the normal development and function of the immune system. However, attempts to unravel these functions by conventional gene targeting in mice have been hampered by the early postnatal lethality caused by null mutations. We have developed a novel 'reverse conditional' gene targeting strategy by which TrkA function is restored specifically in the nervous system. Mice lacking TrkA in non-neuronal tissues are viable and appear grossly normal. All major immune system cell populations are present in normal numbers and distributions. However, mutant mice have elevated serum levels of certain immunoglobulin classes and accumulate B1 cells with aging. These data, confirmed in a classical reconstitution model using embryonic fetal liver from TrkA-null mice, demonstrate that endogenous NGF modulates B cell development through TrkA in vivo. Furthermore, they demonstrate that many of the dramatic effects previously reported by pharmacological or immuno-depletion approaches do not reflect physiological developmental roles of TrkA in the immune system.

Variant brain-derived neurotrophic factor (BDNF) (Met66) alters the intracellular trafficking and activity-dependent secretion of wild-type BDNF in neurosecretory cells and cortical neurons

Brain-derived neurotrophic factor (BDNF) plays a critical role in nervous system and cardiovascular development and function. Recently, a common single nucleotide polymorphism in the bdnf gene, resulting in a valine to methionine substitution in the prodomain (BDNF(Met)), has been shown to lead to memory impairment and susceptibility to neuropsychiatric disorders in humans heterozygous for the variant BDNF. When expressed by itself in hippocampal neurons, less BDNF(Met) is secreted in an activity-dependent manner. The nature of the cellular defect when both BDNF(Met) and wild-type BDNF (BDNF(Val)) are present in the same cell is not known. Given that this is the predominant expression profile in humans, we examined the effect of coexpressed BDNF(Met) on BDNF(Val) intracellular trafficking and processing. Our data indicate that abnormal trafficking of BDNF(Met) occurred only in neuronal and neurosecretory cells and that BDNF(Met) could alter the intracellular distribution and activity-dependent secretion of BDNF(Val). We determined that, when coexpressed in the same cell, approximately 70% of the variant BDNF forms BDNF(Val).BDNF(Met) heterodimers, which are inefficiently sorted into secretory granules resulting in a quantitative decreased secretion. Finally, we determined the form of BDNF secreted in an activity-dependent manner and observed no differences in the forms of BDNF(Met) or the BDNF(Val).BDNF(Met) heterodimer compared with BDNF(Val). Together, these findings indicate that components of the regulated secretory machinery interacts specifically with a signal in the BDNF prodomain and that perturbations in BDNF trafficking may lead to selective impairment in CNS function.

Support of trigeminal sensory neurons by nonneuronal p75 neurotrophin receptors

The p75 neurotrophin receptor (p75NTR) binds all four mammalian neurotrophins, including neurotrophin-3 (NT-3) required for the development of select sensory neurons. This study demonstrated that many gustatory and somatosensory neurons of the tongue depend upon p75NTR. Each of thousands of filiform papillae at the front of the tongue as well as each somatosensory prominence at the back of the tongue has a small cluster of p75NTR-positive epithelial cells that is targeted by somatosensory innervation. This expression of p75NTR by epithelial target cells required NT-3 but not adult innervation. NT-3-secreting cells were adjacent to the p75NTR-positive target cells of each somatosensory organ, as demonstrated in NT-3(lacZneo) transgenic mice. In NT-3 null mutant mice, there were few lingual somatosensory neurons. In p75NTR null mutant mice, the lingual somatosensory axons were likewise absent or had deficient terminal arborizations. Cell culture indicated that substrate p75NTR can influence neuronal outgrowth. Specifically, dissociated trigeminal sensory neurons more than doubled their neurite lengths when grown on a lawn of p75NTR-overexpressing fibroblasts. This enhancement of neurite outgrowth by fibroblast p75NTR raises the possibility that epithelial target cell p75NTR may help to promote axonal arborization in vivo. The co-occurrence in p75NTR null mice of a 35% reduction in geniculate ganglion taste neurons and a shortfall of taste buds is consistent with the established role of gustatory innervation in prompting mammalian taste receptor cell differentiation.

TrkB receptors are required for follicular growth and oocyte survival in the mammalian ovary

Although it is well established that both follicular assembly and the initiation of follicle growth in the mammalian ovary occur independently of pituitary hormone support, the factors controlling these processes remain poorly understood. We now report that neurotrophins (NTs) signaling via TrkB receptors are required for the growth of newly formed follicles. Both neurotrophin-4/5 (NT-4) and brain-derived neurotrophic factor (BDNF), the preferred TrkB ligands, are expressed in the infantile mouse ovary. Initially, they are present in oocytes, but this site of expression switches to granulosa cells after the newly assembled primordial follicles develop into growing primary follicles. Full-length kinase domain-containing TrkB receptors are expressed at low and seemingly unchanging levels in the oocytes and granulosa cells of both primordial and growing follicles. In contrast, a truncated TrkB isoform lacking the intracellular domain of the receptor is selectively expressed in oocytes, where it is targeted to the cell membrane as primary follicles initiate growth. Using gene-targeted mice lacking all TrkB isoforms, we show that the ovaries of these mice or those lacking both NT-4 and BDNF suffer a stage-selective deficiency in early follicular development that compromises the ability of follicles to grow beyond the primary stage. Proliferation of granulosa cells-required for this transition-and expression of FSH receptors (FSHR), which reflects the degree of biochemical differentiation of growing follicles, are reduced in trkB-null mice. Ovaries from these animals grafted under the kidney capsule of wild-type mice fail to sustain follicular growth and show a striking loss of follicular organization, preceded by massive oocyte death. These results indicate that TrkB receptors are required for the early growth of ovarian follicles and that they exert this function by primarily supporting oocyte development as well as providing granulosa cells with a proliferative signal that requires oocyte-somatic cell bidirectional communication. The predominance of truncated TrkB receptors in oocytes and their developmental pattern of subcellular expression suggest that a significant number of NT-4/BDNF actions in the developing mammalian ovary are mediated by these receptors.

Expression of Nerve Growth Factor (NGF), and Its Receptors trkA and p75 in Ovaries of the Cyclic Golden Hamster (Mesocricetus auratus) and the Regulation of Their Production by Luteinizing Hormone

In the present study, changes in localization of nerve growth factor (NGF) and its receptors, trkA and p75 in the ovary were investigated during the estrous cycle of the golden hamster. The effect of LH surge on changes in localization of NGF, trkA and p75 in the ovary was also investigated. NGF and its receptors trkA and p75 were localized in oocytes, granulosa cells and theca cells of various stages of follicles throughout the estrous cycle. NGF and its two receptors were also present in numerous interstitial cells and luteal cells. The number of interstitial cells staining positively for NGF and its two receptors was greater in ovaries of day 1 (day 1=day of ovulation) than the other days during the estrous cycle. Treatment with the antiserum against luteinizing hormone releasing hormone (LHRH-AS) at 1100 h on day 4 completely blocked ovulation. There were few positive reactions for NGF and its two receptors in interstitial cells 24 hr after LHRH-AS injection. The effect of LHRH-AS treatment was blocked by a single injection of 10 IU human chorionic gonadotropin. The distinct widespread distribution of NGF and its two receptors in the ovary of golden hamsters suggest that NGF may be an important growth factor for regulation of ovarian function. Furthermore, the LH surge may be an important factor for inducing production of NGF and its two receptors in interstitial cells of the cyclic golden hamster.

Vertebrate thymus and the neurotrophin system

An immunomodulary role has been proposed for growth factors included in the family of neurotrophins. This is supported by the presence of both neurotrophins and neurotrophin receptors in the immune organs and some immunocompetent cells, the in vitro and in vivo effects of the neurotrophins on the immune cells, and the structural changes of lymphoid organs in mice deficient in neurotrophins and their receptors. The current data strongly indicate that neurotrophins regulate the biology of thymic stromal cells and T cells, including survival, and are involved in the thymic organogenesis. This review compiles the available data about the occurrence and distribution of neurotrophins and their signaling receptors (Trk proteins and p75(NTR)) in the vertebrate thymus and the possible contribution of these molecules to the thymic microenvironment and, therefore, to the T cells differentiation.

Involvement of growth factors in thymic involution

The thymus undergoes an age-dependent degenerative process which is mainly characterized by a progressive loss of lymphoid tissue. Thymic involution is particularly important in relation to immunosenescence and its various associated diseases; this fact has prompted many studies aimed at understanding the causes and mechanisms of thymic degeneration which may, ultimately, lead to the possibility of manipulating it. In this sense, one of the aspects which has deserved most attention is the thymic microenvironment, and more precisely, the many growth factors to which the cells present in the organ are exposed. Thus, the levels of several of such factors have been reported to undergo age-dependent changes in the thymus, which may point at an influence on the regression of the organ. In this article we consider which growth factors and growth factor receptors occur in the vertebrate thymus. Then, focusing on those whose influences are better documented, i.e., neurotrophins, cytokines and IGFs, we discuss their potential role in the organ and the possibility of their being involved in thymic involution.

Factors influencing the differentiation of dopaminergic traits in transplanted neural stem cells

1. Our previous studies demonstrated that when neural stem cells (NSCs) of the C17.2 clonal line are transplanted into the intact or 6-hydroxydopamine (6-OHDA) lesioned rat striatum, in most, but not all grafts, cells spontaneously express the dopamine (DA) biosynthetic enzymes, tyrosine hydroxylase (TH), and aromatic L-amino acid decarboxylase (Yang, M., Stull, N. D., Snyder. E. Y., Berk, M. A., and Iacovitti, L. (2002). Exp. Neurol.). 2. These results suggested that there were certain conditions which were more conducive to the development of DA traits in NSCs and possibly other neurotransmitter phenotypes. 3. In the present study, we modified a number of variables in vitro (i.e. passage number, confluence) and/or in vivo (degree, type, and site of injury) before assessing the survival, migration. and differentiation of engrafted NSCs. 4. We found that low confluence cultures were comprised exclusively of flattened polygonal cells, which when transplanted, migrated widely in the brain but did not express TH. 5. In contrast, high confluence cultures contained both polygonal cells and an overlying bed of fusiform cells. 6. When these NSCs were maintained for 12-20 passages and then transplanted, virtually all engrafted cells in 65% of the grafts expressed TH but not markers of other neurotransmitter systems. 7. Importantly, all TH+ grafts were accompanied by significant physical damage to the brain while TH- grafts were not, suggesting that local injury-related factors were also important. 8. Of no apparent influence on TH expression, regardless of how cells were grown prior to implantation, was the site of transplantation (cortex or striatum) or the degree of chemical lesion (intact, partial or full). 9. We conclude that transplanted NSCs can express traits specifically associated with DA neurons but only when cells are grown under certain conditions in vitro and then transplanted in proximity to injury-induced factors present in vivo.

Neurotrophin 3 and its receptor TrkC immunoreactivity in glucagon cells of buffalo pancreas

Neurotrophin 3 (NT3), a member of the neurotrophin family, and its specific receptor tyrosine kinase C (TrkC) are involved in the differentiation, survival, and maintenance of many neuronal populations. Recently, NT3 and TrkC were also retained involved in the biology of non-neuronal tissues. In this study, we report the presence of NT3- and TrkC-immunoreactive cells in the endocrine pancreas of adult buffalos. They were usually distributed at the periphery of islets and showed intense immunoreactivity. By double immunohistochemical staining, NT3- and TrkC-IR resulted to be colocalized in glucagon immunoreactive cells. These findings suggest endocrine and/or autocrine roles of NT3 in pancreatic A cells.

Neurotrophins and the immune system

The neurotrophins are a family of polypeptide growth factors that are essential for the development and maintenance of the vertebrate nervous system. In recent years, data have emerged indicating that neurotrophins could have a broader role than their name might suggest. In particular, the putative role of NGF and its receptor TrkA in immune system homeostasis has become a much studied topic, whereas information on the other neurotrophins is scarce in this regard. This paper reviews what is known about the expression and possible functions of neurotrophins and their receptors in different immune tissues and cells, as well as recent data obtained from studies of transgenic mice in our laboratory. Results from studies to date support the idea that neurotrophins may regulate some immune functions. They also play an important role in the development of the thymus and in the survival of thymocytes.

p75NTR in the spleen: age-dependent changes, effect of NGF and 4-methylcatechol treatment, and structural changes in p75NTR-deficient mice

In addition to their well-known actions within the nervous system, neurotrophins and their receptors are involved in immune system functioning, as demonstrated by their wide distribution in lymphoid tissues and their in vitro actions on immunocompetent cells. Nevertheless, the in vivo roles of neurotrophin-receptor systems in lymphoid tissues, as well as the scope of their influence throughout development and adulthood, are yet to be clarified. In the present study, we used combined morphological and immunohistochemical techniques to investigate the presence and cellular localization of p75NTR, the pan-neurotrophin receptor protein, in rat spleen from newborns to aging individuals, and the structural and innervation changes in the spleens of p75NTR-deficient mice. In rats, p75NTR was expressed by splenic nerve fibers and dendritic cells in an age-regulated fashion, with maximal expression detected at 2 weeks. Consistently, the spleens of newborn mice lacking this receptor protein showed no signs of ingrowing sympathetic fibers, along with an absence of defined white pulp areas. The present findings suggest a prolonged role of p75NTR in the physiology of the spleen; at least during the embryonic development period, the receptor may be critical for correct innervation and compartmentalization processes to occur.

Neurotrophic control of ovarian development

Substantial evidence now exists indicating that the neurotrophins, a family of growth factors required for the survival, development, and differentiation of various neuronal populations of the nervous system, are also important for the development of nonneuronal tissues. Such a function was first suggested by studies showing the presence of high-affinity neurotrophin receptors in a variety of nonneuronal tissues including those of the cardiovascular, endocrine, immune, and reproductive systems. Within the latter, the gonads appear to be a preferential site of neurotrophin action as suggested by the presence in the mammalian ovary of at least four of the five known neurotrophins and all of the neurotrophin receptors thus far identified. While the various functions that the neurotrophins may have in the ovary are still being elucidated, it is now clear that in addition to recruiting the ovarian innervation, they play a direct role in the regulation of two different maturational periods that are critical for the acquisition of female reproductive function: early follicular development and ovulation. Neurotrophins facilitate the development of newly formed follicles by promoting the initial differentiation and the subsequent growth of primordial follicles. These actions appear to be related to the ability of neurotrophins to sustain the proliferation of both mesenchymal and granulosa cells, and to induce the synthesis of follicle stimulating hormone (FSH) receptors. At the time of the first ovulation, neurotrophins contribute to the ovulatory cascade by increasing prostaglandin E(2) release, reducing gap junction communication, and inducing cell proliferation within the thecal compartment of preovulatory follicles.

Tyrosine kinase activity of nerve growth factor and estrogen in embryonic septal neurons cultured from the rat

Alzheimer's disease (AD) is a neurodegenerative disease characterized by dementia, senile plaques, fibrillary tangles, and a reduction of cholinergic neurons in the septal nucleus of the brain. Nerve growth factor (NGF) and estrogen were studied to observe effects on tyrosine kinase activity in septal neurons. The time course of tyrosine kinase activation and number of cells in which tyrosine kinase was activated were measured. Tissue from embryonic day 16 rats was microdissected and the septal neurons obtained were treated with estrogen (10 microM) or NGF (100 ng/mL) at intervals of 1, 2, 3, 4, 5, or 10 min. Immunostaining for phosphotyrosine revealed that cells treated with NGF showed an increase in phosphotyrosine activity within 2-4 min followed by a decline to control levels of enzyme activity. Treatment with estrogen led to an increase in phosphotyrosine immunostaining within 2-3 min followed by a decline to control levels. This time course suggests a mechanism for estrogen activity other than the traditional method involving binding to nuclear receptors followed by protein synthesis.

Massive lymphocyte apoptosis in the thymus of functionally deficient TrkB mice

The occurrence of TrkB in the murine thymus (15-day and 3-month old) was investigated by Northern blot, Western blot and immunohistochemistry. Furthermore, the thymus of 15-day-old mice carrying a non-functional mutation on trkB was analyzed. Both trkB mRNA and 145 kDa TrkB protein were detected. In addition, isolated lymphocytes and stromal cells also expressed this protein. The thymus of homozygous functionally TrkB-deficient animals showed structural and ultrastructural changes consistent with massive death of cortical lymphocytes, confirmed with TUNEL. Present results suggest a role for TrkB in maintaining the survival or preventing massive death of lymphocytes in the mammalian thymus.

Testis developmental phenotypes in neurotropin receptor trkA and trkC null mutations: role in formation of seminiferous cords and germ cell survival

The objective of the present study was to determine if the neurotropin receptors trkC and trkA are involved in embryonic testis development. These receptors bind neurotropin 3 and nerve growth factor, respectively. The hypothesis tested was that the absence of trkC or trkA receptors will have detrimental effects on testis development and morphology. The trkA and trkC homozygote knockout (KO) mice generally die either at or shortly after birth. Therefore, heterozygote mice were mated to obtain homozygote gene KO mice at Embryonic Day (E) 13, E14, E17, and E19 of gestation, with E0 being the plug date. Gonads from approximately 80 embryos were collected and fixed, and each embryo was genotyped. To determine gonadal characteristics for each genotype, the number of germ cells, number of seminiferous cords, seminiferous cord area, and interstitial area were calculated at each developmental age. Germ cell numbers varied in trkA gene KO mice from those of wild-type mice at each age evaluated. In trkC gene KO mice, differences were detected in germ cell numbers when compared to wild-type mice at E17 and E19. At E19, germ cell numbers were reduced in both trkA and trkC gene KO mice when compared to wild-type animals. Apoptosis was evaluated in testes of wild-type, trkC gene KO, and trkA gene KO mice to determine if the alteration in germ cell numbers at each developmental age was influenced by different patterns of germ cell survival or apoptosis. No differences were found in germ cell apoptosis during embryonic testis development. Interestingly, trkA gene KO mice that survived to Postnatal Day 19 had a 10-fold increase in germ cell apoptosis when compared to germ cells in wild-type mice. Evaluation of other morphological testis parameters demonstrated that trkC KO testes had reduced interstitial area at E13, reduced number of seminiferous cords at E14, and reduced seminiferous cord area at E19. The trkA gene KO testes had a reduction in the number of seminiferous cords at E14. Histology of both trkA and trkC gene KO testes demonstrated that these gonads appear to be developmentally delayed when compared to their wild-type testis counterparts at E13 during testis development. The current study demonstrates that both trkA and trkC neurotropin receptors influence germ cell numbers during testis development and events such as seminiferous cord formation.

Nerve growth factor induces the expression of functional FSH receptors in newly formed follicles of the rat ovary

The neurotrophin nerve growth factor (NGF) and its two membrane-anchored receptors are expressed in the developing ovary before the organization of the first primordial follicles. In the absence of NGF, the growth of primordial follicles is retarded, indicating that NGF contributes to facilitating early follicular development. The present experiments were undertaken to determine whether NGF can also be involved in the differentiation process by which ovarian follicles become responsive to gonadotropins. Treatment of 2-d-old rat ovaries in organ culture with NGF increased FSH receptor (FSHR) mRNA within 8 h of exposure. This effect was cAMP-independent but additive to the cAMP-mediated increase in FSHR gene expression induced by either forskolin or vasoactive intestinal peptide, a neurotransmitter previously shown to induce FSHR formation in neonatal rat ovaries. After NGF treatment, the ovary acquired the capacity of responding to FSH with cAMP formation and preantral follicular growth, indicating that exposure to the neurotrophin resulted in the formation of biologically active FSHRs. Quantitative measurement of FSHR mRNA demonstrated that the content of FSHR mRNA is reduced in the ovaries of mice carrying a null mutation of the NGF gene. These results indicate that one of the functions of NGF in the developing ovary is to facilitate the differentiation process by which early growing follicles become gonadotropin-dependent during postnatal life, and that it does so by increasing the synthesis of FSHRs.

Nerve growth factor selectively regulates expression of transcripts encoding ribosomal proteins

BACKGROUND

NGF exerts a variety of actions including promotion of neuronal differentiation and survival. The PC12 rat pheochromocytoma cell line has proved valuable for studying how NGF works and has revealed that the NGF mechanism includes regulation of gene expression. Accordingly, we used SAGE (Serial Analysis of Gene Expression) to compare levels of specific transcripts in PC12 cells before and after long-term NGF exposure. Of the approximately 22,000 transcripts detected and quantified, 4% are NGF-regulated by 6-fold or more. Here, we used database information to identify transcripts in our SAGE libraries that encode ribosomal proteins and have compared the effect of NGF on their relative levels of expression.

RESULTS

Among the transcripts detected in our SAGE analysis, 74 were identified as encoding ribosomal proteins. Ribosomal protein transcripts were among the most abundantly expressed and, for naive and NGF-treated PC12 cells, represented 5.2% and 3.5%, respectively, of total transcripts analyzed. Surprisingly, nearly half of ribosomal protein transcripts underwent statistically significant NGF-promoted alterations in relative abundance, with changes of up to 5-fold. Of the changes, approximately 2/3 represented decreases. A time course revealed that the relative abundance of transcripts encoding RPL9 increases within 1 hr of NGF treatment and is maximally elevated by 8 hr.

CONCLUSIONS

These data establish that NGF selectively changes expression of ribosomal protein transcripts. These findings raise potential roles for regulation of ribosomal protein transcripts in NGF-promoted withdrawal from the cell cycle and neuronal differentiation and indicate that regulation of individual ribosomal protein transcripts is cell- and stimulus-specific.

Neurotrophins are key mediators of the myelination program in the peripheral nervous system

Although knowledge of the functions of neurotrophins has advanced rapidly in recent years, studies concerning the involvement of neurotrophins in glial-neuronal interactions rarely extend further than their roles in supporting the survival and differentiation of neuronal cells. In this study endogenous brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) were identified in Schwann cell/dorsal root ganglia neuronal cocultures and shown to modulate the myelination program of the peripheral nervous system. The differential expression of BDNF and NT3 were examined and compared with the expression profiles of myelin proteins in the cocultures throughout the myelination process. BDNF levels correlated with active myelin formation, whereas NT3 expression was initially high and then down regulated throughout the proliferation and premyelination periods. Addition of exogenous BDNF enhanced myelination, whereas the removal of the endogenous BDNF by using the BDNF receptor TrkB-Fc fusion protein inhibited the formation of mature myelin internodes. Interestingly, exogenous NT3 significantly inhibited myelination, whereas the removal of the endogenous NT3 by using the NT3 receptor TrkC-Fc fusion protein resulted in an enhancement similar to that obtained with the addition of BDNF. In addition, in vivo studies were performed during the development of the mouse sciatic nerve. Subcutaneous injections of BDNF resulted in an enhancement of myelin formation in the sciatic nerve, whereas the removal of the endogenous BDNF dramatically inhibited myelination. Injections of NT3 inhibited myelin formation, and the removal of the endogenous NT3 enhanced myelination. These results demonstrate that BDNF and NT3 possess different modulatory roles in the myelination program of the peripheral nervous system and that their mechanisms of action are specific and highly regulated.

Dissection of NT3 functions in vivo by gene replacement strategy

The development of the peripheral nervous system is governed in part by a family of neurotrophic factors that signal through Trk tyrosine kinase receptors. Neurotrophin 3 (NT3) ablation in mice causes a more severe neuronal phenotype than deletion of its receptor TrkC, suggesting that NT3 acts also through other non-preferred Trk receptors. To study the role of low-affinity ligand receptor interactions in vivo, we have replaced the Nt3 gene with the gene for brain-derived neurotrophic factor (BDNF), a TrkB ligand. As in NT3 and TrkC null mice, the proprioception system of these mutants failed to assemble. However, sensory fiber projections in the embryonic spinal cord suggest chemotropic effects of BDNF in vivo. In the dorsal root ganglia, the developmental dynamic of neuron numbers demonstrates that NT3 is required for activation of TrkB during neurogenesis and that TrkA is required during target tissue innervation. In the inner ear, the ectopic BDNF rescued the severe neuronal deficits caused by NT3 absence, indicating that TrkB and TrkC activate equivalent pathways to promote survival of cochlear neurons. However, specific increased innervation densities suggest unique functions for BDNF and NT3 beyond promoting neuronal survival. This mouse model has allowed the dissection of specific spatiotemporal Trk receptor activation by NT3. Our analysis provides examples of how development can be orchestrated by complex high- and low-affinity interactions between ligand and receptor families.

TrkA and TrkB neurotrophin receptor immunoreactivity in the teleost (Scorpaena porcus) endocrine pancreas

Mammalian-like and specific neurotrophins, as well as their cognate tyrosine kinase Trk-like receptors have been identified in teleosts. They are mainly distributed in neuronal tissues, but evidence suggests that some non-neuronal tissues also express Trks. In this study we used immunohistochemistry to investigate the occurrence and cell distribution of Trks in the pancreas of teleosts (Scorpaena porcus). Immunoreactivity for TrkA and TrkB, but not for TrkC receptors, was found in the scorpionfish pancreas. TrkA-like positive cells were exclusively observed within pancreatic islets, among insulin- and glucagon-containing cells, but apparently did not co-localize with these hormones. TrkB immunostaining was found in islet cells, presumably colocalized with glucagon, as well as in some cells of the exocrine portion. These data provide a morphological basis for a role of the TrkA and TrkB ligands in the endocrine pancreas of teleosts.

Neurotrophin receptor-like proteins in the bovine (Bos taurus) lymphoid organs, with special reference to thymus and spleen

Increasing evidence suggests that neurotrophins could regulate immune functions acting directly or indirectly on immunocompetent cells. The indirect pathway involves stromal cells of the primary and secondary lymphoid organs. In the present study the occurrence of Trk proteins (TrkA, TrkB and TrkC), regarded as the high-affinity signal-transducing receptors for neurotrophins, was investigated in cow lymphoid organs using immunohistochemistry. The thymus and spleen of both fetal and adult animals, and the palatine tonsils, lymph nodes and Peyer's patches of adult animals, were analysed. Unidentified cells displaying TrkA-like immunoreactivity were found in the fetal thymus, whereas those expressing this protein in the adult gland were identified as epithelial cells. In the spleen, immunoreactive TrkA was observed in cells of the white pulp. TrkB immunoreactivity in both fetal and adult thymus and spleen was localized in monocyte/macrophage cells. As a rule, TrkC was absent from the thymus and the spleen independent of the animal's age. Different types of stromal cells, but never the lymphocytes themselves, displayed TrkA, TrkB, or TrkC immunoreactivity in the other lymphoid organs analysed. As in other vertebrate species, Trk proteins in the lymphoid organs of the cow were localized in the stromal, non-lymphoid cells, thus suggesting that neurotrophins might regulate the immune function acting indirectly on lymphocytes.

Nerve growth factor is required for early follicular development in the mammalian ovary

Nerve growth factor (NGF) epitomizes a family of proteins known as the neurotrophins (NTs), which are required for the survival and differentiation of neurons within both the central and peripheral nervous system. Synthesis of NGF in tissues innervated by the peripheral nervous system is consistent with its function as a target-derived trophic factor. However, the presence of low- and high-affinity NGF receptors in the gonads suggests another function for the NTs within the reproductive endocrine system. We now report that NGF is required for the growth of primordial ovarian follicles, a process known to occur independently of pituitary gonadotropins. Both the NT receptor p75(NTR) and the NGF tyrosine kinase receptor trkA were found to be expressed in the ovaries of infantile normal mice and mice carrying a null mutation of the NGF gene. The ovaries from homozygote NGF-null (-/-) mutant animals, analyzed after completion of ovarian histogenesis, exhibited a markedly reduced population of primary and secondary follicles in the presence of normal serum gonadotropin levels, and an increased number of oocytes that failed to be incorporated into a follicular structure. Assessment of mitogenic activity using two complementary proliferation markers revealed a conspicuous reduction in somatic cell proliferation in the ovaries of NGF-deficient mice. These results suggest that the delay in follicular growth observed in NGF(-/-) mice may be related to the loss of a proliferative signal provided by NGF to the nonneural endocrine component of the ovary.

Direct effects of nerve growth factor on thecal cells from antral ovarian follicles

TrkA, the nerve growth factor (NGF) tyrosine kinase receptor, is expressed not only in the nervous system, but also in nonneural cells, including discrete cellular subsets of the endocrine and immune system. In the rat ovary, trkA receptor abundance increases strikingly in thecal-interstitial cells during the hours preceding the first ovulation. Blockade of either trkA transducing capacity or NGF biological activity inhibited ovulation, suggesting a role for NGF in the ovulatory process of this species. To identify some of the processes that may be affected by trkA activation in the thecal compartment, we used purified thecal cells/thecal fibroblasts from bovine ovaries (heretofore referred to as thecal cells). Ribonuclease protection assays employing bovine-specific cRNA probes demonstrated the presence of the messenger RNAs (mRNAs) encoding NGF and its receptors, p75 NTR and trkA, in the thecal compartment of small, medium, and large antral follicles and showed that trkA mRNA is also expressed in granulosa cells. In situ hybridization and immunohistochemical examination of intact ovaries confirmed these cellular sites of NGF and trkA synthesis. TrkA mRNA, but not NGF mRNA, was lost within 48 h of placing thecal cells in culture. Thus, to study trkA-mediated actions of NGF on these cells we transiently expressed the receptor by transfection with a vector containing a full-length rat trkA complementary DNA under transcriptional control of the cytomegalovirus promoter. Because ovulation is preceded by an LH-dependent increase in androgen and progesterone production, the ability of NGF to modify the release of these steroids was determined in freshly plated cells still containing endogenous trkA receptors and in cells undergoing luteinization in culture that were transiently transfected with the trkA-encoding plasmid. NGF stimulated both androgen and progesterone release in freshly plated thecal cells, but not in luteinizing cells provided with trkA receptors. As ovulation in rodents requires an increased formation of PGE2 and has been shown to be antedated by proliferation of thecal fibroblasts, we determined the ability of NGF to affect these parameters in trkA-transfected thecal cells. The neurotrophin rapidly stimulated PGE2 release and amplified the early steroidal response to hCG in trkA-expressing cells, but not in cells lacking the receptor. Likewise, NGF stimulated [3H]thymidine incorporation into trkA-containing cells, but not into cells that had lost the receptor in culture. Induction of ovulation in immature rats by gonadotropin treatment verified that an increased cell proliferation in the thecal compartment, determined by the incorporation of bromodeoxyuridine into cell nuclei, occurs 4-5 h before ovulation in this species. These results suggest that the contribution of NGF to the ovulatory process includes a stimulatory effect of the neurotrophin on steroidogenesis, PGE2 formation, and proliferative activity of thecal compartment cells.

Nerve growth factor-mediated expression of galectin-3 in mouse dorsal root ganglion neurons

Galectin-3, a member of the galectin family of beta-galactoside-specific lectins has been found to be expressed by subsets of dorsal root ganglion (DRG) neurons during development and in adulthood. Here we show that (i) after 3-7 days in vitro, DRG neurons derived from neonatal mice express galectin-3 intra- and extracellularly and (ii) lectin expression requires the presence of nerve growth factor (NGF). After 3 days in vitro, a higher number of DRG neurons expressed galectin-3 in the presence of NGF (65 +/- 7%) than in the presence of brain-derived neurotrophic factor (BDNF, 30 +/- 3%) or neurotrophin-3 (NT-3, 34 +/- 3%). After 7 days in vitro, these numbers dropped to 51 +/- 3% (for NGF), 0% (for BDNF) and 8 +/- 4% (for NT-3), respectively. Our findings provide first evidence for the contribution of a neurotrophin to the neuronal expression of galectins and suggest an NGF/TrkA-mediated expression of galectin-3 by early postnatal DRG neurons.

Identification of diverse nerve growth factor-regulated genes by serial analysis of gene expression (SAGE) profiling

Neurotrophic factors such as nerve growth factor (NGF) promote a wide variety of responses in neurons, including differentiation, survival, plasticity, and repair. Such actions often require changes in gene expression. To identify the regulated genes and thereby to more fully understand the NGF mechanism, we carried out serial analysis of gene expression (SAGE) profiling of transcripts derived from rat PC12 cells before and after NGF-promoted neuronal differentiation. Multiple criteria supported the reliability of the profile. Approximately 157,000 SAGE tags were analyzed, representing at least 21,000 unique transcripts. Of these, nearly 800 were regulated by 6-fold or more in response to NGF. Approximately 150 of the regulated transcripts have been matched to named genes, the majority of which were not previously known to be NGF-responsive. Functional categorization of the regulated genes provides insight into the complex, integrated mechanism by which NGF promotes its multiple actions. It is anticipated that as genomic sequence information accrues the data derived here will continue to provide information about neurotrophic factor mechanisms.

Regulation of retinoic acid signaling in the embryonic nervous system: a master differentiation factor

This review describes some of the properties of retinoic acid (RA) in its functions as a locally synthesized differentiation factor for the developing nervous system. The emphasis is on the characterization of the metabolic enzymes that synthesize and inactivate RA, and which determine local RA concentrations. These enzymes create regions of autocrine and paracrine RA signaling in the embryo. One mechanism by which RA can act as a differentiation agent is through the induction of growth factors and their receptors. Induction of growth factor receptors in neural progenitor cells can lead to growth factor dependency, and the consequent developmental fate of the cell will depend on the local availability of growth factors. Because RA activates the early events of cell differentiation, which then induce context-specific differentiation programs, RA may be called a master differentiation factor.

TrkA is necessary for the normal development of the murine thymus

Nerve growth factor (NGF) and its signal-transducing receptor TrkA are expressed in the thymus. However, their possible role during thymic organogenesis is unknown. Here we analyze the thymus of trkA-kinase deficient 2-week-old mice. trkA-kinase +/+ and +/- mice had a normal thymus, whereas the thymus of trkA-kinase -/- mice showed lack of delimitation between the cortex and medulla, lower thymocyte density, and the presence of epithelial cell islands and numerous cysts lined with endodermal epithelium. The present results indicate that TrkA is necessary for the normal development of the thymus, and that its absence causes an arrest in the differentiation of endodermal epithelial cells. Whether this lack of differentiation has functional implication has yet to be determined.

Identification of novel trkA variants with deletions in leucine-rich motifs of the extracellular domain

The peripheral expression of trkA encoding for NGF receptor was investigated by RNase protection assay. A thymus-specific protected fragment was identified. Using 5' rapid amplification of cDNA ends, three different trkA fragments were characterized. The longer fragment corresponded to the classical trkA L3 transcripts while the two shorter fragments lacked sequences encoding for leucine-rich motifs of the extracellular domain of TrkA, similarly to the trkB L1 and L0 variants. RT-PCR analysis of adult rat tissues showed the expression of trkA L1 transcripts in the thymus, testis, lung and kidney but not in the central nervous system. Their combined expression with trkA L3 transcripts suggests that specific peripheral TrkA oligomers may modulate NGF binding and function in non-neuronal cells.

Co-activation of TGF-ss and cytokine signaling pathways are required for neurotrophic functions

This article summarizes and interprets recent data from our laboratories suggesting that transforming growth factor-ss (TGF-ss1, -ss2, -ss3) is essentially required, in vitro and in vivo, for the neurotrophic signaling of glial cell line-derived neurotrophic factor (GDNF). TGF-ss, which is synthesized by and released from neurons, also synergizes with neurotrophins and members of the neurokine and fibroblast growth factor families by increasing their efficacies. However, when applied to purified neuron populations without other factors being added, TGF-ss does not promote survival or differentiation. Together, these data suggest that neither TGF-ss nor GDNF fulfil essential criteria of a typical neurotrophic factor, as e.g. nerve growth factor (NGF). Moreover, the neurotrophic activity of NGF and other classic neurotrophic factors is apparently based, to a significant extent, on their co-operativity with TGF-ss. Mechanisms, by which TGF-ss generates neurotrophic effects and synergizes with other cytokines are beginning to emerge. Recruitment and/or stabilization of receptors and cross-talks at different levels of signal transduction are likely to be implied in generating the neurotrophic potential of the TGF-ss/cytokine synergisms. Together, these data outline a novel role of TGF-ss in a key event of nervous system development, ontogenetic neuron death. Conceptually more important, however, may be the broadening of the neurotrophic factor concept, which now has to imply the possibility that two cytokines, each being ineffective by itself, become neurotrophically active when acting in concert.

Intraovarian excess of nerve growth factor increases androgen secretion and disrupts estrous cyclicity in the rat

A single injection of estradiol valerate induces a form of cystic ovary resembling some aspects of the human polycystic ovarian syndrome. Preceding the development of follicular cysts, there is an increase in intraovarian synthesis of nerve growth factor (NGF) and the low affinity NGF receptor (p75 NGFR). Selective blockade of NGF actions and p75 NGFR synthesis in the ovary restored estrous cyclicity and ovulatory capacity in estradiol valerate-treated rats, suggesting that an increase in NGF-dependent, p75 NGFR-mediated actions within the ovary contributes to the development of cystic ovarian disease. We have tested this hypothesis by grafting NGF-producing neural progenitor cells into the ovary of juvenile rats that have been induced to ovulate precociously by a single injection of PMSG. The NGF-producing cells, detected by their content of immunoreactive p75 NGFR material, were found scattered throughout the ovary with some of them infiltrating the granulosa cell compartment of large, precystic follicles. Ovarian NGF content was 2-fold higher than in the ovary of rats receiving control cells. Estrous cyclicity was disrupted, with the animals showing prolonged periods of persistent estrus, and an almost continuous background of vaginal cornified cells at other phases of the estrous cycle. Morphometric analysis revealed that the presence of NGF-producing cells neither reduced the total number of corpora lutea per ovary nor significantly increased the formation of follicular cysts. However, the ovaries receiving these cells showed an increased incidence of precystic, type III follicles, accompanied by a reduced number of healthy antral follicles, and an increased size of both healthy and atretic follicles. These changes in follicular dynamics were accompanied by a selective increase in serum androstenedione levels. The results show that an abnormally elevated production of NGF within the ovary suffices to initiate several of the structural and functional alterations associated with the development of follicular cysts in the rat ovary.

Neurotrophic factors and neuro-muscular disease: II. GDNF, other neurotrophic factors, and future directions

This is the second of two reviews in which we discuss the essential aspects of neurotrophic factor neurobiology, the characteristics of each neurotrophic factor, and their clinical relevance to neuromuscular diseases. The previous paper reviewed the neurotrophin family and neuropoietic cytokines. In the present article, we focus on the GDNF family and other neurotrophic factors and then consider future approaches that may be utilized in neurotrophic factor treatment.

Evidence for a role of truncated trkC receptor isoforms in mouse development

The trkC locus encodes several receptors for neurotrophin-3, including the well studied full-length tyrosine kinase isoform, in addition to receptor isoforms lacking the kinase active domain. TrkC receptors are widely expressed throughout mouse development in many different organs. To investigate the function of truncated receptors in vivo and to identify cell types that are biologically responsive to this gene product, we have overexpressed a physiological truncated trkC isoform in the mouse. Mice overexpressing this receptor develop to term but die in the first postnatal days. High levels of transgene expression result in severe developmental defects in the peripheral nervous system and in the heart. The severity of neuronal losses observed in these animals suggests that truncated receptors may act by sequestering neurotrophin, thus, closely relating this mouse model to the neurotrophin-3-deficient one. Lower levels of exogenous truncated receptor in transgenic mice result in a more modest phenotype and, in some neuronal populations, do not cause neural deficits. Taken together, these data suggest that truncated trkC receptor isoforms may have modulatory functions in development.