Expression and alternative splicing of mouse Gfra4 suggest roles in endocrine cell development

Downregulation of miR-144 blocked the proliferation and invasion of nerve cells in Hirschsprung disease by regulating Transcription Factor AP 4 (TFAP4)

BACKGROUND

Hirschsprung's disease (HSCR) is characterized by a dysfunction of enteric neural crest cells (ENCCs) proliferation, migration and premature apoptosis during embryonic development, resulting in aganglionic colon. Our aim is to explore the role of miR-144 with its target gene Transcription Factor AP 4 (TFAP4) in nerve cells in HSCR.

METHODS

The relative expression levels of miR-144 in HSCR colon samples were detected by quantitative real-time PCR (RT-qPCR). Western blot assays were conducted to investigate the TFAP4 protein expressing level. The interaction of miR-144 and TFAP4 was predicted with bioinformatics analysis and examined with luciferase reporter assays. Overexpression or knockdown of miR-144 and TFAP4 in 293T and SH-SY5Y cell lines was applied. Cell proliferation, migration and invasion were detected by CCK-8 assays, Transwell migration and invasion assays. Cell cycle and apoptosis was examined by flow cytometric analysis.

RESULTS

Downregulation of miR-144 and upregulation of TFAP4 were shown in HSCR. Luciferase reporter assay indicated that miR-144 reduced luciferase activity in 293T and SH-SY5Y transfected with TFAP4-WT-3UTR luciferase reporter and confirmed TFAP4 was the downstream target gene of miR-144. Data showed that miR-144 promoted the cell proliferation, migration and invasion of 293T and SH-SY5Y, while TFAP4 blocked the cell proliferation, migration and invasion. TFAP4 overexpression reversed the miR-144-mediated cell proliferation, migration and invasion of 293T and SH-SY5Y.

CONCLUSIONS

Downregulation of miR-144 blocked the cell proliferation and migration of nerve cells via targeting TFAP4 and contributed to the pathogenesis of HSCR. This provides an innovative and candidate target for treatment of HSCR.

MiR-195-5p inhibits proliferation and invasion of nerve cells in Hirschsprung disease by targeting GFRA4

Studies have reported that miR-195-5p plays a role in the Hirschsprung disease (HSCR). Our previous work found GDNF family receptor alpha 4 (GFRA4) is also associated with HSCR. In this study, we focused on whether miR-195-5p induces the absence of enteric neurons and enteric neural crest in HSCR by regulating GFRA4. The expression levels of GFRA4 and miR-195-5p in colon tissues were evaluated by real-time PCR (RT-PCR) assay. We overexpressed GFRA4 or miR-195-5p in SH-SY5Y cells, the cell proliferation, cell cycle, apoptosis and invasion were subsequently investigated by CCK-8 assay, EdU staining, Flow cytometry analysis and Transwell assay, respectively. We also established the xenograft model to detect the effect of miR-195-5p on tumor growth and GFRA4 and p-RET expressions. GFRA4 expression was significantly downregulated in the HSCR colon tissues when compared with that in the control tissues. Overexpression of GFRA4 significantly promoted proliferation, invasion and cell cycle arrest, and inhibited apoptosis of SH-SY5Y cells. We also proved that GFRA4 is a direct target of miR-195-5p, and miR-195-5p inhibited proliferation, invasion, cell cycle arrest and differentiation, and accelerated apoptosis in SH-SY5Y cells which can be reversed by GFRA4 overexpression. Furthermore, we demonstrated that miR-195-5p suppressed tumor growth, and observably decreased GFRA4 and p-RET expressions. Our findings suggest that miR-195-5p plays an important role in the pathogenesis of HSCR. MiR-195-5p inhibited proliferation, invasion and cell cycle arrest, and accelerated apoptosis of nerve cells by targeting GFRA4.

Regulation of GDNF expression in Sertoli cells

Sertoli cells regulate male germ cell proliferation and differentiation and are a critical component of the spermatogonial stem cell (SSC) niche, where homeostasis is maintained by the interplay of several signaling pathways and growth factors. These factors are secreted by Sertoli cells located within the seminiferous epithelium, and by interstitial cells residing between the seminiferous tubules. Sertoli cells and peritubular myoid cells produce glial cell line-derived neurotrophic factor (GDNF), which binds to the RET/GFRA1 receptor complex at the surface of undifferentiated spermatogonia. GDNF is known for its ability to drive SSC self-renewal and proliferation of their direct cell progeny. Even though the effects of GDNF are well studied, our understanding of the regulation its expression is still limited. The purpose of this review is to discuss how GDNF expression in Sertoli cells is modulated within the niche, and how these mechanisms impact germ cell homeostasis.

First High-Density Linkage Map and QTL Fine Mapping for Growth-Related Traits of Spotted Sea bass (Lateolabrax maculatus)

Possessing powerful adaptive capacity and a pleasant taste, spotted sea bass (Lateolabrax maculatus) has a broad natural distribution and is one of the most popular mariculture fish in China. However, the genetic improvement program for this fish is still in its infancy. Growth is the most economically important trait and is controlled by quantitative trait loci (QTL); thus, the identification of QTLs and genetic markers for growth-related traits is an essential step for the establishment of marker-assisted selection (MAS) breeding programs. In this study, we report the first high-density linkage map of spotted sea bass constructed by sequencing 333 F1 generation individuals in a full-sib family using 2b-RAD technology. A total of 6883 SNP markers were anchored onto 24 linkage groups, spanning 2189.96 cM with an average marker interval of 0.33 cM. Twenty-four growth-related QTLs, including 13 QTLs for body weight and 11 QTLs for body length, were successfully detected, with phenotypic variance explained (PVE) ranging from 5.1 to 8.6%. Thirty potential candidate growth-related genes surrounding the associated SNPs were involved in cell adhesion, cell proliferation, cytoskeleton reorganization, calcium channels, and neuromodulation. Notably, the fgfr4 gene was detected in the most significant QTL; this gene plays a pivotal role in myogenesis and bone growth. The results of this study may facilitate marker-assisted selection for breeding populations and establish the foundation for further genomic and genetic studies investigating spotted sea bass.

Genomic signatures of domestication in Old World camels

Domestication begins with the selection of animals showing less fear of humans. In most domesticates, selection signals for tameness have been superimposed by intensive breeding for economical or other desirable traits. Old World camels, conversely, have maintained high genetic variation and lack secondary bottlenecks associated with breed development. By re-sequencing multiple genomes from dromedaries, Bactrian camels, and their endangered wild relatives, here we show that positive selection for candidate genes underlying traits collectively referred to as 'domestication syndrome' is consistent with neural crest deficiencies and altered thyroid hormone-based signaling. Comparing our results with other domestic species, we postulate that the core set of domestication genes is considerably smaller than the pan-domestication set - and overlapping genes are likely a result of chance and redundancy. These results, along with the extensive genomic resources provided, are an important contribution to understanding the evolutionary history of camels and the genomic features of their domestication.

Correlation of Artemin and GFRα3 With Osteoarthritis Pain: Early Evidence From Naturally Occurring Osteoarthritis-Associated Chronic Pain in Dogs

Arthritis, including osteoarthritis (OA) and other musculoskeletal-associated pain, is a worldwide problem, however, effective drug options are limited. Several receptors, neurotransmitters, and endogenous mediators have been identified in rodent models, but the relevance of these molecules in disease-associated pain is not always clear. Artemin, a neurotrophic factor, and its receptor, glial-derived neurotrophic factor (GDNF) family receptor alpha-3 (GFRα3), have been identified as involved in pain in rodents. Their role in OA-associated pain is unknown. To explore a possible association, we analyzed tissue from naturally occurring OA in dogs to characterize the correlation with chronic pain. We used behavioral assessment, objective measures of limb use, and molecular tools to identify whether artemin and GFRα3 might be associated with OA pain. Our results using banked tissue from well-phenotyped dogs indicates that artemin/GFRα3 may play an important, and hitherto unrecognized, role in chronic OA-associated pain. Elevated serum levels of artemin from osteoarthritic humans compared to healthy individuals suggest translational relevance. Our data provide compelling evidence that the artemin/GFRα3 signaling pathway may be important in OA pain in both non-humans and humans and may ultimately lead to novel therapeutics.

Unmasking a new prognostic marker and therapeutic target from the GDNF-RET/PIT1/p14ARF/p53 pathway in acromegaly

Background

Acromegaly is produced by excess growth hormone secreted by a pituitary adenoma of somatotroph cells (ACRO). First-line therapy, surgery and adjuvant therapy with somatostatin analogs, fails in 25% of patients. There is no predictive factor of resistance to therapy. New therapies are investigated using few dispersed tumor cells in acute primary cultures in standard conditions where the cells do not grow, or using rat pituitary cell lines that do not maintain the full somatotroph phenotype. The RET/PIT1/p14ARF/p53 pathway regulates apoptosis in normal pituitary somatotrophs whereas the RET/GDNF pathway regulates survival, controlling PIT1 levels and blocking p14ARF (ARF) and p53 expression.

Methods

We investigated these two RET pathways in a prospective series of 32 ACRO and 63 non-functioning pituitary adenomas (NFPA), studying quantitative RNA and protein gene expression for molecular-clinical correlations and how the RET pathway might be implicated in therapeutic success. Clinical data was collected during post-surgical follow-up. We also established new'humanized’ pituitary cultures, allowing 20 repeated passages and maintaining the pituitary secretory phenotype, and tested five multikinase inhibitors (TKI: Vandetanib, Lenvatinib, Sunitinib, Cabozantinib and Sorafenib) potentially able to act on the GDNF-induced RET dimerization/survival pathway. Antibody arrays investigated intracellular molecular pathways.

Findings

In ACRO, there was specific enrichment of all genes in both RET pathways, especially GDNF. ARF and GFRA4 gene expression were found to be opposing predictors of response to first-line therapy. ARF cut-off levels, calculated categorizing by GNAS mutation, were predictive of good response (above) or resistance (below) to therapy months later. Sorafenib, through AMPK, blocked the GDNF/AKT survival action without altering the RET apoptotic pathway.

Interpretation

Tumor ARF mRNA expression measured at the time of the surgery is a prognosis factor in acromegaly. The RET inhibitor, Sorafenib, is proposed as a potential treatment for resistant ACRO.

Fund

This project was supported by national grants from Agencia Estatal de Investigación (AEI) and Instituto Investigación Carlos III, with participation of European FEDER funds, to IB (PI150056) and CVA (BFU2016-76973-R). It was also supported initially by a grant from the Investigator Initiated Research (IIR) Program (WI177773) and by a non-restricted Research Grant from Pfizer Foundation to IB. Some of the pituitary acromegaly samples were collected in the framework of the Spanish National Registry of Acromegaly (REMAH), partially supported by an unrestricted grant from Novartis to the Spanish Endocrine Association (SEEN).

CVA is also supported from a grant of Medical Research Council UK MR/M018539/1.

The GDNF Family: A Role in Cancer?

The glial cell line-derived neurotrophic factor (GDNF) family of ligands (GFLs) comprising of GDNF, neurturin, artemin, and persephin plays an important role in the development and maintenance of the central and peripheral nervous system, renal morphogenesis, and spermatogenesis. Here we review our current understanding of GFL biology, and supported by recent progress in the area, we examine their emerging role in endocrine-related and other non-hormone-dependent solid neoplasms. The ability of GFLs to elicit actions that resemble those perturbed in an oncogenic phenotype, alongside mounting evidence of GFL involvement in tumor progression, presents novel opportunities for therapeutic intervention.

Downregulation of microRNA-483-5p Promotes Cell Proliferation and Invasion by Targeting GFRA4 in Hirschsprung's Disease

Recent studies have suggested the critical roles of miRNAs for disease progression. miRNA-483-5p (miR-483-5p) was previously found to have a relationship with tumor cell behavior, but its biological function in Hirschsprung's disease (HSCR) remains undefined. Thus, we explored the role of miR-483-5p in the pathogenesis of HSCR. Histological changes of colonic tissues were evaluated by hematoxylin and eosin (HE) staining. Quantitative real-time PCR and western blotting were used to determine relative expression levels of miRNA, mRNA, and proteins in 20 HSCR patients and 20 normal colon tissues. In this study, we found that miR-483-5p expression in HSCR tissues was significantly increased and their downregulation promoted cell proliferation, cell cycle progression and invasion and inhibited cell apoptosis in human 293T and SH-SY5Y cell lines by the CCK-8, flow cytometry, and Transwell assay. GNDF family receptor alpha 4 (GFRA4) was confirmed as a downstream target of miR-483-5p by dual-luciferase reporter gene assay and inversely correlated with miR-483-5p expression in cell lines. Taken together, miR-483-5p may play a crucial role in the pathogenesis of HSCR by targeting GFRA4.

Ret function in muscle stem cells points to tyrosine kinase inhibitor therapy for facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) involves sporadic expression of DUX4, which inhibits myogenesis and is pro-apoptotic. To identify target genes, we over-expressed DUX4 in myoblasts and found that the receptor tyrosine kinase Ret was significantly up-regulated, suggesting a role in FSHD. RET is dynamically expressed during myogenic progression in mouse and human myoblasts. Constitutive expression of either RET9 or RET51 increased myoblast proliferation, whereas siRNA-mediated knockdown of Ret induced myogenic differentiation. Suppressing RET activity using Sunitinib, a clinically-approved tyrosine kinase inhibitor, rescued differentiation in both DUX4-expressing murine myoblasts and in FSHD patient-derived myoblasts. Importantly, Sunitinib also increased engraftment and differentiation of FSHD myoblasts in regenerating mouse muscle. Thus, DUX4-mediated activation of Ret prevents myogenic differentiation and could contribute to FSHD pathology by preventing satellite cell-mediated repair. Rescue of DUX4-induced pathology by Sunitinib highlights the therapeutic potential of tyrosine kinase inhibitors for treatment of FSHD.

DOI:http://dx.doi.org/10.7554/eLife.11405.001

The Candidate Neuroprotective Agent Artemin Induces Autonomic Neural Dysplasia without Preventing Peripheral Nerve Dysfunction

Artemin (ART) signals through the GFR α—3/RET receptor complex to support sympathetic neuron development. Here we show that ART also influences autonomic elements in adrenal medulla and enteric and pelvic ganglia. Transgenic mice over-expressing Art throughout development exhibited systemic autonomic neural lesions including fusion of adrenal medullae with adjacent paraganglia, adrenal medullary dysplasia, and marked enlargement of sympathetic (superior cervical and sympathetic chain ganglia) and parasympathetic (enteric, pelvic) ganglia. Changes began by gestational day 12.5 and formed progressively larger masses during adulthood. Art supplementation in wild type adult mice by administering recombinant protein or an Art-bearing retroviral vector resulted in hyperplasia or neuronal metaplasia at the adrenal corticomedullary junction. Expression data revealed that Gfr α—3 is expressed during development in the adrenal medulla, sensory and autonomic ganglia and their projections, while Art is found in contiguous mesenchymal domains (especially skeleton) and in certain nerves. Intrathecal Art therapy did not reduce hypalgesia in rats following nerve ligation. These data (1) confirm that ART acts as a differentiation factor for autonomic (chiefly sympathoadrenal but also parasympathetic) neurons, (2) suggest a role for ART overexpression in the genesis of pheochromocytomas and paragangliomas, and (3) indicate that ART is not a suitable therapy for peripheral neuropathy.

Neurotrophic factors and their effects in the treatment of multiple sclerosis

Neurotrophins are small molecules of polypeptides, which include nerve growth factor (NGF) family, glial cell line–derived neurotrophic factor (GDNF) family ligands, and neuropoietic cytokines. These factors have an important role in neural regeneration, remyelination, and regulating the development of the peripheral and central nervous systems (PNS and CNS, respectively) by intracellular signaling through specific receptors. It has been suggested that the pathogenesis of human neurodegenerative disorders may be due to an alteration in the neurotrophic factors and their receptors. The use of neurotrophic factors as therapeutic agents is a novel strategy for restoring and maintaining neuronal function during neurodegenerative disorders such as multiple sclerosis. Innate and adaptive immune responses contribute to pathology of neurodegenerative disorders. Furthermore, autoimmune and mesenchymal stem cells, by the release of neurotrophic factors, have the ability to protect neuronal population and can efficiently suppress the formation of new lesions. So, these cells may be an alternative source for delivering neurotrophic factors into the CNS.

Thyroid C-Cell Biology and Oncogenic Transformation

The thyroid parafollicular cell, or commonly named "C-cell," functions in serum calcium homeostasis. Elevations in serum calcium trigger release of calcitonin from the C-cell, which in turn functions to inhibit absorption of calcium by the intestine, resorption of bone by the osteoclast, and reabsorption of calcium by renal tubular cells. Oncogenic transformation of the thyroid C-cell is thought to progress through a hyperplastic process prior to malignancy with increasing levels of serum calcitonin serving as a biomarker for tumor burden. The discovery that multiple endocrine neoplasia type 2 is caused by activating mutations of the RET gene serves to highlight the RET-RAS-MAPK signaling pathway in both initiation and progression of medullary thyroid carcinoma (MTC). Thyroid C-cells are known to express RET at high levels relative to most cell types; therefore, aberrant activation of this receptor is targeted primarily to the C-cell, providing one possible cause of tissue-specific oncogenesis. The role of RET signaling in normal C-cell function is unknown though calcitonin gene transcription appears to be sensitive to RET activation. Beyond RET, the modeling of oncogenesis in animals and screening of human tumors for candidate gene mutations have uncovered mutation of RAS family members and inactivation of Rb1 regulatory pathway as potential mediators of C-cell transformation. A growing understanding of how RET interacts with these pathways, both in normal C-cell function and during oncogenic transformation, will help in the development of novel molecular-targeted therapies.

Artemin augments survival and axon regeneration in axotomized retinal ganglion cells

Artemin, a recently discovered member of the glial cell line-derived neurotrophic factor (GDNF) family, has neurotrophic effects on damaged neurons, including sympathetic neurons, dopamine neurons, and spiral ganglion neurons both in vivo and in vitro. However, its effects on retinal cells and its intracellular signaling remain relatively unexplored. During development, expression of GFRα3, a specific receptor for artemin, is strong in the immature retina and gradually decreases during maturation, suggesting a possible role in the formation of retinal connections. Optic nerve damage in mature rats causes levels of GFRα3 mRNA to increase tenfold in the retina within 3 days. GFRα3 mRNA levels continue to rise within the first week and then decline. Artemin, a specific ligand for GFRα3, has a neuroprotective effect on axotomized retinal ganglion cells (RGCs) in vivo and in vitro via activation of the extracellular signal-related kinase- and phosphoinositide 3-kinase-Akt signaling pathways. Artemin also has a substantial effect on axon regeneration in RGCs both in vivo and in vitro, whereas other GDNF family members do not. Therefore, artemin/GFRα3, but not other GDNF family members, may be of value for optic nerve regeneration in mature mammals.

Differential RET signaling pathways drive development of the enteric lymphoid and nervous systems

During the early development of the gastrointestinal tract, signaling through the receptor tyrosine kinase RET is required for initiation of lymphoid organ (Peyer's patch) formation and for intestinal innervation by enteric neurons. RET signaling occurs through glial cell line-derived neurotrophic factor (GDNF) family receptor α co-receptors present in the same cell (signaling in cis). It is unclear whether RET signaling in trans, which occurs in vitro through co-receptors from other cells, has a biological role. We showed that the initial aggregation of hematopoietic cells to form lymphoid clusters occurred in a RET-dependent, chemokine-independent manner through adhesion-mediated arrest of lymphoid tissue initiator (LTin) cells. Lymphoid tissue inducer cells were not necessary for this initiation phase. LTin cells responded to all RET ligands in trans, requiring factors from other cells, whereas RET was activated in enteric neurons exclusively by GDNF in cis. Furthermore, genetic and molecular approaches revealed that the versatile RET responses in LTin cells were determined by distinct patterns of expression of the genes encoding RET and its co-receptors. Our study shows that a trans RET response in LTin cells determines the initial phase of enteric lymphoid organ morphogenesis, and suggests that differential co-expression of Ret and Gfra can control the specificity of RET signaling.

Functional characterization of the MTC-associated germline RET-K666E mutation: evidence of oncogenic potential enhanced by the G691S polymorphism

Activating mutations of RET, a gene encoding two isoforms of a tyrosine kinase receptor physiologically expressed in several neural crest-derived cell lineages, are associated with the inherited forms of medullary thyroid carcinoma (MTC). The identification and characterization of novel RET mutations involved in MTC is valuable, as RET gene testing plays a crucial role in the management of these patients. In an MTC patient, we have identified a germline c.1996A>G transition in heterozygosis leading to K666E substitution. In addition, the conservative S904S (c.2712C>G) and the non-conservative functional G691S (c.2071G>A) polymorphisms have been identified. Through functional studies, we demonstrate for the first time that K666E is a gain-of-function mutation with oncogenic potential, based on its ability to transform NIH3T3 cells. It was not possible to define whether K666E is a de novo or inherited RET variant in the patient, as the family history was negative for MTC, and the carrier status of family members could not be tested. Our results, together with a recent report of co-segregation of the mutation in three MTC families, suggest that K666E is a causative MTC mutation. As we have shown that the same patient allele carries both K666E and G691S variants, the latter known to increase downstream RET signaling, a possible role for the G691S polymorphism has also been investigated. We have demonstrated that, although RET-G691S is not oncogenic per se, it enhances the transforming activity of the RET-K666E mutant, thus suggesting a modifier role for this functional polymorphism.

Role of EphA4 receptor signaling in thyroid development: regulation of folliculogenesis and propagation of the C-cell lineage

Transcriptome analysis revealed that the tyrosine kinase receptor EphA4 is enriched in the thyroid bud in mouse embryos. We used heterozygous EphA4-EGFP knock-in mice in which enhanced green fluorescent protein (EGFP) replaced the intracellular receptor domain (EphA4(+/EGFP)) to localize EphA4 protein in thyroid primordial tissues. This showed that thyroid progenitors originating in the pharyngeal floor express EphA4 at all embryonic stages and when follicles are formed in late development. Also, the ultimobranchial bodies developed from the pharyngeal pouch endoderm express EphA4, but the ultimobranchial epithelium loses the EGFP signal before it merges with the median thyroid primordium. Embryonic C cells invading the thyroid are exclusively EphA4-negative. EphA4 expression continues in the adult thyroid. EphA4 knock-out mice and EphA4-EGFP homozygous mutants are euthyroid and have a normal thyroid anatomy but display subtle histological alterations regarding number, size, and shape of follicles. Of particular interest, the pattern of follicular abnormality differs between EphA4(-/-) and EphA4(EGFP/EGFP) thyroids. In addition, the number of C cells is reduced by >50% exclusively in animals lacking EphA4 forward signaling (EphA4(EGFP/EGFP)). Heterozygous EphA4 mutants have no apparent thyroid phenotype. We conclude that EphA4 is a novel regulator of thyroid morphogenesis that impacts on postnatal development of the two endocrine cell lineages of the differentiating gland. In this process both EphA4 forward signaling (in the follicular epithelium) and reverse signaling mediated by its cognate ligand(s) (A- and/or B-ephrins expressed in follicular cells and C cells, respectively) are probably functionally important.

Heparan sulfate proteoglycan syndecan-3 is a novel receptor for GDNF, neurturin, and artemin

Syndecan-3 may act alone or as a coreceptor with RET to promote cell spreading, neurite outgrowth, and migration of cortical neurons by GNDF, NRTN, and ARTN.

Glial cell line–derived neurotrophic factor (GDNF) family ligands (GFLs) are potent survival factors for dopaminergic neurons and motoneurons with therapeutic potential for Parkinson’s disease. Soluble GFLs bind to a ligand-specific glycosylphosphatidylinositol-anchored coreceptor (GDNF family receptor α) and signal through the receptor tyrosine kinase RET. In this paper, we show that all immobilized matrix-bound GFLs, except persephin, use a fundamentally different receptor. They interact with syndecan-3, a transmembrane heparan sulfate (HS) proteoglycan, by binding to its HS chains with high affinity. GFL–syndecan-3 interaction mediates both cell spreading and neurite outgrowth with the involvement of Src kinase activation. GDNF promotes migration of cortical neurons in a syndecan-3–dependent manner, and in agreement, mice lacking syndecan-3 or GDNF have a reduced number of cortical γ-aminobutyric acid–releasing neurons, suggesting a central role for the two molecules in cortical development. Collectively, syndecan-3 may directly transduce GFL signals or serve as a coreceptor, presenting GFLs to the signaling receptor RET.

The evolving field of tyrosine kinase inhibitors in the treatment of endocrine tumors

Activation of tyrosine kinase receptors (TKRs) and their related pathways has been associated with development of endocrine tumors. Compounds that target and inactivate the kinase function of these receptors, tyrosine kinase inhibitors (TKIs), are now being applied to the treatment of endocrine tumors. Recent clinical trials of TKIs in patients with advanced thyroid cancer, islet cell carcinoma, and carcinoid have shown promising preliminary results. Significant reductions in tumor size have been described in medullary and papillary thyroid carcinoma, although no complete responses have been reported. Case reports have described significant tumor volume reductions of malignant pheochromocytomas and paragangliomas. In addition, these compounds showed an initial tumoricidal or apoptotic response followed by long-term static effects on tumor growth. Despite the promising preliminary results, this class of therapeutic agents has a broad spectrum of adverse effects, mediated by inhibition of kinase activities in normal tissues. These adverse effects will have to be balanced with their benefit in clinical use. New strategies will have to be applied in clinical research to achieve optimal benefits. In this review, we will address the genetic alterations of TKRs, the rationale for utilizing TKIs for endocrine tumors, and current information on tumor and patient responses to specific TKIs. We will also discuss the adverse effects related to TKI treatment and the mechanisms involved. Finally, we will summarize the challenges associated with use of this class of compounds and potential solutions.

Medullary thyroid carcinoma cell lines contain a self-renewing CD133+ population that is dependent on ret proto-oncogene activity

CONTEXT

Medullary thyroid carcinoma (MTC) is a cancer of the parafollicular C cells commonly caused by an inherited or acquired RET proto-oncogene mutation. Therapeutic resistance and recurrence of the disease imply the presence of cancer stem cells in MTC.

OBJECTIVE

In this study, we sought to identify and characterize cancer stem cell-like cells in MTC.

MAIN OUTCOME MEASURES

The characterization of stem cell properties was performed using immunostaining, flow cytometry, sphere formation assay, rederivation assay, Western blotting, and quantitative RT-PCR of defined markers of neural stem and progenitor cells. The role of ret proto-oncogene activation was assessed through RNA interference knockdown.

RESULTS

CD133 positivity was identified by immunostaining patient MTC. Flow cytometry confirmed a subpopulation of CD133(+) cells in two MTC cell lines. The CD133(+) cells could be expanded by sphere formation assay, passaged multiple times, and expressed neural progenitor markers beta-tubulin 3 and glial fibrillary acidic protein. The MZ-CRC-1 cell line, which harbors a M918T RET mutation, had greater CD133(+) cell numbers and sphere-forming ability than the TT cell line, which harbors the less active C634W mutation. Sphere formation was more dependent on ret proto-oncogene activity than epidermal growth factor or fibroblast growth factor.

CONCLUSION

Our data support the existence of cancer stem-like cells in MTC, which exhibit the features of self-renewal and of multiple lineage differentiation that is dependent on ret proto-oncogene receptor activity. These findings may provide new insights to develop more promising therapy for MTC.

Molecular identification of rapidly adapting mechanoreceptors and their developmental dependence on ret signaling

In mammals, the first step in the perception of form and texture is the activation of trigeminal or dorsal root ganglion (DRG) mechanosensory neurons, which are classified as either rapidly (RA) or slowly adapting (SA) according to their rates of adaptation to sustained stimuli. The molecular identities and mechanisms of development of RA and SA mechanoreceptors are largely unknown. We found that the "early Ret(+)" DRG neurons are RA mechanoreceptors, which form Meissner corpuscles, Pacinian corpuscles, and longitudinal lanceolate endings. The central projections of these RA mechanoreceptors innervate layers III through V of the spinal cord and terminate within discrete subdomains of the dorsal column nuclei. Moreover, mice lacking Ret signaling components are devoid of Pacinian corpuscles and exhibit a dramatic disruption of RA mechanoreceptor projections to both the spinal cord and medulla. Thus, the early Ret(+) neurons are RA mechanoreceptors and Ret signaling is required for the assembly of neural circuits underlying touch perception.

Glial cell line-derived neurotrophic factor family ligands enhance capsaicin-stimulated release of calcitonin gene-related peptide from sensory neurons

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are a group of peptides that have been implicated as important factors in inflammation, since they are released in increased amounts during inflammation and induce thermal hyperalgesia upon injection. Mouse isolated sensory neurons in culture and freshly dissociated spinal cord slices were used to examine the enhancement in stimulated-release of the neuropeptide, calcitonin gene-related peptide (CGRP), as a measure of sensitization. Exposure of isolated sensory neurons in culture to GDNF, neurturin, and artemin enhanced the capsaicin-stimulated release of immunoreactive calcitonin gene-related peptide (iCGRP) two- to threefold, but did not increase potassium-stimulated release of iCGRP. A similar profile of sensitization was observed in freshly dissociated spinal cord slices. Persephin, another member of the GFL family thought to be important in development, was unable to induce an enhancement in the release of iCGRP. These results demonstrate that specific GFLs are important mediators affecting sensory neuronal sensitivity, likely through modulation of the capsaicin receptor. The sensitization of sensory neurons during inflammation, and the pain and neurogenic inflammation resulting from this sensitization, may be due in part to the effects of these selected GFLs.

Ape1/Ref-1 induces glial cell-derived neurotropic factor (GDNF) responsiveness by upregulating GDNF receptor alpha1 expression

Apurinic/apyrimidinic endonuclease 1 (Ape1/Ref-1) dysregulation has been identified in several human tumors and in patients with a variety of neurodegenerative diseases. However, the function of Ape1/Ref-1 is unclear. We show here that Ape1/Ref-1 increases the expression of glial cell-derived neurotropic factor (GDNF) receptor alpha1 (GFRalpha1), a key receptor for GDNF. Expression of Ape1/Ref-1 led to an increase in the GDNF responsiveness in human fibroblast. Ape1/Ref-1 induced GFRalpha1 transcription through enhanced binding of NF-kappaB complexes to the GFRalpha1 promoter. GFRalpha1 levels correlate proportionally with Ape1/Ref-1 in cancer cells. The knockdown of endogenous Ape1/Ref-1 in pancreatic cancer cells markedly suppressed GFRalpha1 expression and invasion in response to GNDF, while overexpression of GFRalpha1 restored invasion. In neuronal cells, the Ape1/Ref-1-mediated increase in GDNF responsiveness not only stimulated neurite outgrowth but also protected the cells from beta-amyloid peptide and oxidative stress. Our results show that Ape1/Ref-1 is a novel physiological regulator of GDNF responsiveness, and they also suggest that Ape1/Ref-1-induced GFRalpha1 expression may play important roles in pancreatic cancer progression and neuronal cell survival.

Glial cell-line derived neurotrophic factor (GDNF) family of ligands confer chemoresistance in a ligand-specific fashion in malignant gliomas

Glial cell-line derived neurotrophic factor (GDNF) is a neurotrophic factor known to promote neuronal survival of dopaminergic neurons in the embryonic midbrain as well as contribute to carcinogenesis in many cancers. Its ubiquitous presence in the central nervous system suggests a role in the mitogenesis of high-grade astrocytoma. GDNF is overexpressed in glioblastoma cell lines and human gliomas. GFRalpha1b is the predominant spliced receptor isoform in human gliomas and RET9 is the predominant co-receptor. Significantly there is differential overexpression of the GFRalpha1b spliced isoform compared to the GFRalpha1a spliced variant. Pre-treatment of glioblastoma cell lines with GDNF but not the alternative ligand neurturin, promoted mitogenic behaviour and conferred chemoresistance to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). Signaling mapping of BCNU and GDNF suggest that the ability of GDNF to promote Akt activity and inhibit JNK activity may contribute to the increased cellular survival after BCNU chemotherapy.

CD2AP and Cbl-3/Cbl-c constitute a critical checkpoint in the regulation of ret signal transduction

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are critical for nervous system development and maintenance. GFLs promote survival and growth via activation of the receptor tyrosine kinase (RTK) Ret. In sympathetic neurons, the duration of Ret signaling is governed by how rapidly Ret is degraded after its activation. In an effort to elucidate mechanisms that control the half-life of Ret, we have identified two novel Ret interactors, CD2-associated protein (CD2AP) and Cbl-3. CD2AP, an adaptor molecule involved in the internalization of ubiquitinated RTKs, is associated with Ret under basal, unstimulated conditions in neurons. After Ret activation by GDNF, CD2AP dissociates. Similarly, the E3-ligase Cbl-3 interacts with unphosphorylated Ret and dissociates from Ret after Ret activation. In contrast to their dissociation from autophosphorylated Ret, an interaction between CD2AP and Cbl-3 is induced by GDNF stimulation of sympathetic neurons, suggesting that CD2AP and Cbl-3 dissociate from Ret as a complex. In neurons, the overexpression of CD2AP enhances the degradation of Ret and inhibits GDNF-dependent survival, and gene silencing of CD2AP blocks Ret degradation and promotes GDNF-mediated survival. Surprisingly, Cbl-3 overexpression dramatically stabilizes activated Ret and enhances neuronal survival, even though Cbl-family E3 ligases normally function to trigger RTK downregulation. In combination with CD2AP, however, Cbl-3 promotes Ret degradation rapidly and almost completely blocks survival promotion by GDNF, suggesting that Cbl-3 acts as a switch that is triggered by CD2AP and oscillates between inhibition and promotion of Ret degradation. Consistent with the hypothesis, Cbl-3 silencing in neurons only inhibited Ret degradation and enhanced neuronal survival in combination with CD2AP silencing. CD2AP and Cbl-3, therefore, constitute a checkpoint that controls the extent of Ret downregulation and, thereby, the sensitivity of neurons to GFLs.

Does the RET variant G691S influence the features of sporadic medullary thyroid carcinoma?

OBJECTIVE

The RET (rearranged during transfection) proto-oncogene G691S variant is over-represented in the germline of patients with sporadic medullary thyroid carcinoma (sMTC) vs. normal controls but so far is not associated with any medical or pathological features of the tumour. The aim of our study was to assess the influence of this variant on the age of onset, clinical, biological and pathological features of sMTC.

DESIGN AND PATIENTS

One hundred patients with histologically proven MTC, for whom the germline genetic analysis of RET was negative and medical records were available, were included in the study.

RESULTS

Patients with the heterozygous GS variant or the homozygous SS variant (n = 36) were on average 8.0 years younger than patients with the wild-type GG variant (n = 64, mean age 43.9 vs. 51.9 years, P < 0.01). The former group did not differ from the wild-type group in terms of MTC size, prevalence of C-cell hyperplasia (CCH) or papillary thyroid carcinoma (PTC). However, the prevalence of an increased preoperative basal calcitonin (bCT) level (> 1000 pg/ml) was 2.75-fold higher in the patients with the GS or SS variant than in those with the wild-type variant (P < 0.001). The proportion of patients with lymph node metastases was also higher in the former group (P < 0.05). Multivariate analysis confirmed that the presence of the RET variant is independently associated with higher preoperative bCT values (P = 0.011).

CONCLUSIONS

Our data demonstrate that the RET G691S variant could modulate the age of onset of sMTC as demonstrated previously for familial tumours. Moreover, this variant is an independent predictor of a higher basal calcitonin synthesis rate in patients with sMTC.

Transforming growth factor beta cooperates with persephin for dopaminergic phenotype induction

The aim of the present study was to investigate the putative cooperative effects of transforming growth factor beta (TGF-beta) and glial cell line-derived neurotrophic factor (GDNF) family ligands in the differentiation of midbrain progenitors toward a dopaminergic phenotype. Therefore, a mouse midbrain embryonic day (E) 12 neurospheres culture was used as an experimental model. We show that neurturin and persephin (PSPN), but not GDNF, are capable of transient induction of dopaminergic neurons in vitro. This process, however, requires the presence of endogenous TGF-beta. In contrast, after 8 days in vitro GDNF rescued the TGF-beta neutralization-dependent loss of the TH-positive cells. In vivo, at E14.5, no apparent phenotype concerning dopaminergic neurons was observed in Tgf-beta2(-/-)/gdnf(-/-) double mutant mice. In vitro, combined TGF-beta/PSPN treatment achieved a yield of approximately 20% TH-positive cells that were less vulnerable against 1-methyl-4-phenyl pyridinium ion toxicity. The underlying TGF-beta/PSPN differentiation signaling is receptor-mediated, involving p38 mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways. These results indicate that phenotype induction and survival of fully differentiated neurons are accomplished through distinct pathways and individual factor requirement. TGF-beta is required for the induction of dopaminergic neurons, whereas GDNF is required for regulating and/or maintaining a differentiated neuronal phenotype. Moreover, this study suggests that the combination of TGF-beta with PSPN is a potent inductive cocktail for the generation of dopaminergic neurons that should be considered in tissue engineering and cell replacement therapies for Parkinson's disease.

Specificity in the crosstalk of TGFbeta/GDNF family members is determined by distinct GFR alpha receptors

Glial cell line-derived neurotrophic factor (GDNF) and neurturin (NRTN) are neurotrophic factors for parasympathetic neurons including ciliary ganglion (CG) neurons. Recently, we have shown that survival and signaling mediated by GDNF in CG neurons essentially requires transforming growth factor beta (TGFbeta). We have provided evidence that TGFbeta regulates the availability of the glycosyl phosphatidylinositol (GPI)-anchored GDNF receptor alpha 1 (GFRalpha1) by promoting the recruitment of the receptor to the plasma membrane. We report now that in addition to GDNF, NRTN, but not persephin (PSPN) or artemin (ARTN), is able to promote survival of CG neurons. Interestingly, in contrast to GDNF, NRTN is not dependent on cooperation with TGFbeta, but efficiently promotes neuronal survival and intracellular signaling in the absence of TGFbeta. Additional treatment with TGFbeta does not further increase the NRTN response. Both NRTN and GDNF exclusively bind to and activate their cognate receptors, GFRalpha2 and GFRalpha1, respectively, as shown by the use of receptor-specific neutralizing antibodies. Immunocytochemical staining for the two receptors on the surface of CG neurons reveals that, in contrast to the effect on GFRalpha1, TGFbeta is not required for recruitment of GFRalpha2 to the plasma membrane. Moreover, binding of radioactively labeled GDNF but not NRTN is increased upon treatment of CG neurons with TGFbeta. Disruption of TGFbeta signaling does interfere with GDNF-, but not NRTN-mediated signaling and survival. We propose a model taking into account data from GFRalpha1 crystallization and ontogenetic development of the CG that may explain the differences in TGFbeta-dependence of GDNF and NRTN.

Glial cell line-derived neurotrophic factor and neurturin inhibit neurite outgrowth and activate RhoA through GFR alpha 2b, an alternatively spliced isoform of GFR alpha 2

The glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) belong to a structurally related family of neurotrophic factors. NTN exerts its effect through a multicomponent receptor system consisting of the GDNF family receptor alpha2 (GFR alpha2), RET, and/or NCAM (neural cell adhesion molecule). GFR alpha2 is alternatively spliced into at least three isoforms (GFR alpha2a, GFR alpha2b, and GFR alpha2c). It is currently unknown whether these isoforms share similar functional and biochemical properties. Using highly specific and sensitive quantitative real-time PCR, these isoforms were found to be expressed at comparable levels in various regions of the human brain. When stimulated with GDNF and NTN, both GFR alpha2a and GFR alpha2c, but not GFR alpha2b, promoted neurite outgrowth in transfected Neuro2A cells. These isoforms showed ligand selectivity in MAPK (mitogen-activated protein kinase) [ERK1/2 (extracellular signal-regulated kinase 1/2)] and Akt signaling. In addition, the GFR alpha2 isoforms regulated different early-response genes when stimulated with GDNF or NTN. In coexpression studies, GFR alpha2b was found to inhibit ligand-induced neurite outgrowth by GFR alpha2a and GFR alpha2c. Stimulation of GFR alpha2b also inhibited the neurite outgrowth induced by GFR alpha1a, another member of the GFR alpha. Furthermore, activation of GFR alpha2b inhibited neurite outgrowth induced by retinoic acid and activated RhoA. Together, these data suggest a novel paradigm for the regulation of growth factor signaling and neurite outgrowth via an inhibitory splice variant of the receptor. Thus, depending on the expressions of specific GFR alpha2 receptor spliced isoforms, GDNF and NTN may promote or inhibit neurite outgrowth through the multicomponent receptor complex.

Computational and molecular characterization of multiple isoforms of lfe-2 gene in nematode C. elegans

C. elegans C46H11.4 gene encodes a Let-23 fertility effector/regulator protein of the EGF-receptor class of the tyrosine kinase family. Alternative splicing is a major mechanism of generating protein diversity in higher eukaryotes. C. elegans genome sequencing consortium has reported three alternatively spliced transcripts of C46H11.4 gene which encodes for three hypothetical proteins namely, C46H11.4a, C46H11.4b and C46H11.4c. Using a combination of various bioinformatics tools like gene or exon finding programmes, blast searches, alignment tools etc followed by experimental validation, we report the presence of three more alternatively spliced transcripts which encode for novel hypothetical proteins C46H11.4d, C46H11.4e and C46H11.4f. These isoforms arise as a result of alternative splicing in the pre-mRNA encoded by gene C46H11.4. These novel un-reported spliced variants not only point towards the extent of alternative splicing in C. elegans genes but also hint towards the complex nature of alternative splicing.

The mouse soluble GFRalpha4 receptor activates RET independently of its ligand persephin

Glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) all signal through the transmembrane receptor tyrosine kinase RET. The signalling complex consists of GFLs, GPI-anchored ligand binding GDNF family receptor alphas (GFRalphas) and RET. Signalling via RET is required for the development of the nervous system and the kidney, as well as for spermatogenesis. However, constitutive activation of RET is implicated as a cause in several diseases. Mutations of the RET proto-oncogene cause the inherited cancer syndrome multiple endocrine neoplasia type 2 (MEN 2). Recently, it has been suggested that mutations in the persephin binding GFRalpha4 receptor may have a potentially modifying role in MEN 2. Several naturally occurring, different splice variants of the mammalian GFRalpha4 have been reported. A 7 bp insertion-mutation in the human GFRalpha4 gene causes a shift of reading frame and thereby changes the balance between the transcripts encoding GPI-anchored and soluble GFRalpha4 receptors. We report here that the mammalian soluble GFRalpha4 can activate RET independently of its preferential ligand, persephin. Our data show that soluble GFRalpha4 can associate with, and induce, phosphorylation of RET. In addition, our data show that this isoform of GFRalpha4 can induce downstream signalling, as well as neuronal survival and differentiation, in the absence of persephin. These results suggest that, in line with the previous report, GFRalpha4 may be a candidate gene for, or modifier of, the MEN 2 diseases.

RET signaling in endocrine tumors: delving deeper into molecular mechanisms

The rearranged during transfection (RET) proto-oncogene encodes a receptor tyrosine kinase that is implicated in the development of endocrine tumors of the thyroid and adrenal glands. In humans, activating RET mutations are found in the inherited cancer syndrome multiple endocrine neoplasia 2 and in sporadic medullary and papillary thyroid carcinomas. The specific type and location of RET mutations are strongly correlated with the disease phenotype and have both diagnostic and prognostic value. Recent advances in the molecular characterization of the RET receptor and its mutants have begun to define the mechanisms underlying the transforming ability of the different RET mutant forms. This information has revealed key functional features of these mutant proteins that distinguish the different clinically recognized mutations and provide clues as to the functional origins of the phenotypes associated with specific RET mutations. The elucidation of molecular mechanisms involved in RET-mediated transformation is a key step in the development of much needed therapeutics that target RET's oncogenic properties. Recent advances have begun to provide a deeper understanding of the receptor's function, and dysfunction, in human tumors that may guide this process.

Loss of neurturin in frog--comparative genomics study of GDNF family ligand-receptor pairs

Four different GDNF family ligand (GFL)-receptor (GFRalpha) binding pairs exist in mammals, and they all signal via the RET receptor tyrosine kinase. However, the evolution of these molecules is poorly understood. We identified orthologs of all four GFRalpha receptors and GRAL (GDNF Receptor Alpha-Like) in all vertebrate classes, and a predicted GFR-like protein in several invertebrates. In addition, Gas1 (growth arrest-specific 1), a distant member of the GFR-superfamily, is present in both vertebrates and invertebrates. Analysis of exon structures suggests a common origin of GFR-superfamily proteins and early divergence of Gas1 from the common ancestor. Bony fishes have orthologs of all four mammalian GFLs, consistent with genome duplications in early vertebrates. Surprisingly, the clawed frog and chicken have only three GFLs: synteny analysis indicates loss of neurturin in frog and of persephin in chicken. Evolutionary trace analysis and protein structure homology modeling points at GDNF as the endogenous ligand of frog GFRalpha2.

Evolution of the GDNF family ligands and receptors

Four different ligand-receptor binding pairs of the GDNF (glial cell line-derived neurotrophic factor) family exist in mammals, and they all signal via the transmembrane RET receptor tyrosine kinase. In addition, GRAL (GDNF Receptor Alpha-Like) protein of unknown function and Gas1 (growth arrest specific 1) have GDNF family receptor (GFR)-like domains. Orthologs of the four GFRalpha receptors, GRAL and Gas1 are present in all vertebrate classes. In contrast, although bony fishes have orthologs of all four GDNF family ligands (GFLs), one of the ligands, neurturin, is absent in clawed frog and another, persephin, is absent in the chicken genome. Frog GFRalpha2 has selectively evolved possibly to accommodate GDNF as a ligand. The key role of GDNF and its receptor GFRalpha1 in enteric nervous system development is conserved from zebrafish to humans. The role of neurturin, signaling via GFRalpha2, for parasympathetic neuron development is conserved between chicken and mice. The role of artemin and persephin that signal via GFRalpha3 and GFRalpha4, respectively, is unknown in non-mammals. The presence of RET- and GFR-like genes in insects suggests that a ProtoGFR and a ProtoRET arose early in the evolution of bilaterian animals, but when the ProtoGFL diverged from existing transforming growth factor (TGFbeta)-like proteins remains unclear. The four GFLs and GFRalphas were presumably generated by genome duplications at the origin of vertebrates. Loss of neurturin in frog and persephin in chicken suggests functional redundancy in early tetrapods. Functions of non-mammalian GFLs and prechordate RET and GFR-like proteins remain to be explored.

Artemin overexpression in skin enhances expression of TRPV1 and TRPA1 in cutaneous sensory neurons and leads to behavioral sensitivity to heat and cold

Artemin, a neuronal survival factor in the glial cell line-derived neurotrophic factor family, binds the glycosylphosphatidylinositol-anchored protein GFRalpha3 and the receptor tyrosine kinase Ret. Expression of the GFRalpha3 receptor is primarily restricted to the peripheral nervous system and is found in a subpopulation of nociceptive sensory neurons of the dorsal root ganglia (DRGs) that coexpress the Ret and TrkA receptor tyrosine kinases and the thermosensitive channel TRPV1. To determine how artemin affects sensory neuron properties, transgenic mice that overexpress artemin in skin keratinocytes (ART-OE mice) were analyzed. Expression of artemin caused a 20.5% increase in DRG neuron number and increased the level of mRNA encoding GFRalpha3, TrkA, TRPV1, and the putative noxious cold-detecting channel TRPA1. Nearly all GFRalpha3-positive neurons expressed TRPV1 immunoreactivity, and most of these neurons were also positive for TRPA1. Interestingly, acid-sensing ion channel (ASIC) 1, 2a, 2b, and 3 mRNAs were decreased in the DRG, and this reduction was strongest in females. Analysis of sensory neuron physiological properties using an ex vivo preparation showed that cutaneous C-fiber nociceptors of ART-OE mice had reduced heat thresholds and increased firing rates in response to a heat ramp. No change in mechanical threshold was detected. Behavioral testing of ART-OE mice showed that they had increased sensitivity to both heat and noxious cold. These results indicate that the level of artemin in the skin modulates gene expression and response properties of afferents that project to the skin and that these changes lead to behavioral sensitivity to both hot and cold stimuli.

Ablation of persephin receptor glial cell line-derived neurotrophic factor family receptor alpha4 impairs thyroid calcitonin production in young mice

Glial cell line-derived neurotrophic factor family receptor (GFRalpha) 4, the binding receptor for persephin, is coexpressed with the signaling Ret receptor tyrosine kinase predominantly in thyroid calcitonin-producing C cells. We show by in situ hybridization and immunohistochemistry that the functional, glycolipid-anchored form of GFRalpha4 is produced in mouse only in the C cells but not in parathyroid gland or in the brain. C cells expressed functional GFRalpha4 throughout postnatal development, whereas Ret expression in these cells decreased postnatally and was undetectable in adults. To understand the physiological role of GFRalpha4, we produced GFRalpha4-deficient [knockout (KO)] mice. No differences were observed between wild-type and GFRalpha4-KO littermate animals in growth, gross behavior, or viability. The number and morphology of the thyroid C cells were indistinguishable between the genotypes in both newborn and adult age. However, thyroid tissue calcitonin content was reduced by 60% in newborn and by 45% in 3-wk-old GFRalpha4-KO mice compared with wild-type controls. In contrast, thyroid calcitonin levels were similar in adult animals. Consistent with the reduced calcitonin levels, bone formation rate in juvenile GFRalpha4-KO mice was increased. In conclusion, this study indicates a novel role for endogenous GFRalpha4 signaling in regulating calcitonin production in thyroid C cells of young mice.

The first cysteine-rich domain of the receptor GFRalpha1 stabilizes the binding of GDNF

The GDNF (glial cell line-derived neurotrophic factor)-binding receptor GFRalpha1 (GDNF family receptor alpha1) is attached to the membrane by a GPI (glycosylphosphatidylinositol) anchor and consists of three cysteine-rich domains. The region corresponding to the second and third domains has been shown previously to participate in ligand binding, and to interact with the transmembrane tyrosine kinase receptor RET. No function has so far been found for the N-terminal, first domain (D1). Here we show that the GPI-anchored full-length receptor binds 125I-GDNF two times more tightly than does a GPI-anchored truncated receptor lacking D1. Scintillation proximity assays with purified receptor proteins also show that the GDNF-binding capacity of the soluble full-length GFRalpha1 is two times higher than the GDNF-binding capacity of the soluble D1-truncated GFRalpha1. As RET stabilizes the binding of GDNF equally well to the full-length and truncated receptors, D1 seems not to be involved in the interaction between GFRalpha1 and RET. Moreover, soluble full-length GFRalpha1 mediates GDNF-promoted neurite outgrowth in PC6-3 cells more efficiently than the soluble truncated GFRalpha1 protein. At low concentrations, the soluble fulllength receptor mediates the phosphorylation of RET more efficiently than the soluble truncated receptor. However, when the receptors are overexpressed on the cell surface as GPI-anchored proteins, or added to the growth medium at high concentrations as soluble proteins, full-length and truncated GFRalpha1 are indistinguishable in GDNF-dependent RET-phosphorylation assays. High levels of the receptors can thus mask a slightly impaired function in the phosphorylation assay. Based on assays with both GPI-anchored and soluble receptors, we therefore conclude that D1 contributes to the optimal function of GFRalpha1 by stabilizing the interaction between GFRalpha1 and GDNF.

Differential effects of glial cell line-derived neurotrophic factor and neurturin in RET/GFRalpha1-expressing cells

The c-ret protooncogene, RET, encodes a receptor tyrosine kinase. RET is activated by members of the glial cell line-derived neurotrophic factor (GDNF) family of ligands, which include GDNF, neurturin, artemin, and persephin. The ligands bind RET through GDNF family receptor alpha, termed GFRalpha1-4. Despite the importance of RET signaling in the development of the enteric nervous system and the kidney, the differential signaling mechanisms between RET ligands are poorly established. It has been suggested that signal specificity is achieved through binding of the ligand to its preferred GFRalpha. To compare the signaling profiles of GDNF and neurturin, we have identified a cell line, NG108-15, which endogenously expresses RET and GFRalpha1 but not GFRalpha2-4. Immunoblot data showed that GDNF caused a transient activation, whereas neurturin caused a sustained activation, of both p44/p42 MAP kinases and PLCgamma. Under serum starvation, NG108-15 cells differentiate and form neurites. Neurturin but not GDNF stimulated neurite outgrowth, which could be blocked by the selective PLC inhibitor U73122. On the other hand, GDNF but not neurturin promoted cell survival, and this could be blocked by the p44/p42 MAP kinase inhibitor PD98059. Our findings not only show the differential signaling of GDNF and neurturin but also suggest that this can be achieved through binding to the same GFRalpha subtype, leading to distinct biological responses.

Tissue expression of alternatively spliced GFRα1, NCAM and RET isoforms and the distinct functional consequence of ligand-induced activation of GFRα1 isoforms

Glial-cell-line-derived neurotrophic factor (GDNF) exerts its effect through a multi-component receptor system consisting of GFRalpha1, RET and NCAM. Two highly homologous alternatively spliced GFRalpha1 isoforms (GFRalpha1a and GFRalpha1b) have previously been identified. In this study, isoform specific real-time PCR assays were used to quantify the expression levels of GFRalpha1, RET and NCAM isoforms in murine embryonic and adult tissues. The expression levels of GFRalpha1b were found to be comparable to that of GFRalpha1a in peripheral tissues. However, GFRalpha1a was the predominant isoform expressed in the whole brain. The co-expressions of GFRalpha1 and the co-receptors were developmentally regulated and differentially expressed in some tissues. Microarray analyses of GFRalpha1 isoforms transfected cells stimulated with NTN showed distinct and non-overlapping gene profiles. These observations are consistent with the emerging view that the combinatorial interactions of the spliced isoforms of GFRalpha, RET and NCAM may contribute to the pleiotropic biological responses.

Up-regulation of ret by reserpine in the adult rat adrenal medulla

The receptor tyrosine kinase, ret, is activated by glial cell line-derived neurotrophic factor, neurturin and related ligands that bind to glycosylphosphatidylinositol-tailed receptors GFRalpha1-4. Ret expression is developmentally regulated and detectable only at very low levels in adult adrenal medulla. However, mutations of ret that cause constitutive activation or alter signal transduction give rise to adrenal medullary hyperplasia and pheochromocytomas in humans with hereditary multiple endocrine neoplasia (MEN) syndromes 2A and 2B and in animal models. These discordant observations pose the conundrum of how a molecule barely detectable in the adult adrenal can contribute to development of adrenal medullary pathology that typically occurs in adults. We recently reported that depolarization and phorbol esters that activate protein kinase C act synergistically with neurturin to up-regulate ret protein and mRNA expression in adult rat chromaffin cell cultures. Those findings suggested that ret expression in vivo is not static and might be regulated in part by neurally derived signals. We show here that the anti-hypertensive agent reserpine, which is known to cause a reflex increase in trans-synaptic stimulation of chromaffin cells, increases expression of ret mRNA and protein in adult rat adrenal medullary tissue in vivo. Elevated ret protein levels are detectable both by immunoblots and immunohistochemistry, which shows immunoreactive ret in chromaffin cells and neurons after reserpine administration. The finding that ret expression is subject to up-regulation by environmental signals in vivo suggests that epigenetic factors might influence the development of adrenal medullary disease by affecting the expression of ret. It is known that long-term administration of reserpine leads to the development of adrenal medullary hyperplasia and pheochromocytomas in rats. Our findings suggest potential utility of the rat model for studying the roles of ret in the adrenal medulla and the mechanisms of its involvement in MEN 2 and other pheochromocytoma syndromes.

Regulation of neural development by glial cell line-derived neurotrophic factor family ligands

Glial cell line-derived neurotrophic factor (GDNF) and its three relatives constitute a novel family of neurotrophic factors, the GDNF family ligands. These factors signal through a multicomponent receptor complex comprising a glycosylphosphatidylinositol-anchored cell surface molecule (GDNF family receptor (GFR) alpha) and RET tyrosine kinase, triggering the activation of multiple signaling pathways in responsive cells. Recent gene-targeting studies have demonstrated that GDNF family ligands are essential for the development of a diverse set of neuronal populations and we have now started to understand how these ligands uniquely regulate the formation and sculpting of the nervous system. Recent studies have also revealed interactions by multiple extracellular signals during neural development. The deciphering of GDNF family ligand signaling in neural cells promises to provide vital new insights into the development and pathology of the nervous system.

Expression of GFRalpha1 receptor splicing variants with different biochemical properties is modulated during kidney development

The glial cell line-derived neurotrophic factor (GDNF) family coreceptor alpha1 (GFRalpha1) is a critical component of the RET receptor kinase signal-transducing complex. The activity of this multicomponent receptor is stimulated by the glial cell line-derived neurotrophic factor (GDNF) and is involved in neuronal cells survival and kidney development. GFRalpha1 pre-mRNA is alternatively spliced and produces two isoforms: GFRalpha1a, which includes the exon 5; and GFRalpha1b, which excludes it. Here we show that the Gfralpha1a isoform is predominantly expressed in neuronal tissues and in PC12 cells differentiated toward a neuronal phenotype. GFRalpha1 splicing is also regulated during kidney development, GFRalpha1a is the minor isoform before birth and then rapidly becomes the major form after birth. We established cell lines expressing either GFRalpha1 isoforms and demonstrated that the GFRalpha1b isoform binds GDNF more efficiently than GFRalpha1a. Consistently, GFRalpha1b promotes a stronger RET phosphorylation than GFRalpha1a. These results indicate that specific inclusion of the GFRalpha1 exon 5 in neuronal tissues or during kidney development may alter the binding properties of GDNF to GFRalpha1, and thus could constitute an additional regulatory mechanism of the RET signaling pathway.

Neurotrophic factor receptor expression and in vitro nerve growth of geniculate ganglion neurons that supply divergent nerves

We investigated which neurotrophic factors may contribute to the divergence of two peripheral nerves emanating from the geniculate ganglion. We compared receptor mRNA profiles of the neurons that supply the nerves, and also the growth of their neurites in response to neurotrophic factors in culture. Three mRNAs, Gfra2, TrkA, and TrkC, were differentially expressed. Only one ligand, Neurturin, promoted substantially different nerve regrowth from the nerves, and therefore may contribute to nerve divergence. Three receptor mRNAs were expressed in 100% of the neurons: TrkB, TrkB.T2 (kinase-lacking isoform), and NCAM-140. Ligands for these Trks and FRalpha-1 promoted more outgrowth than ligands for the other receptors. NT-3 and BDNF synergistically promoted outgrowth. Finally, receptors are coexpressed at random rates, arguing against the existence of neuronal subtypes defined by a combinatorial code of these receptors.

Neurotrophin and GDNF family ligands promote survival and alter excitotoxic vulnerability of neurons derived from murine embryonic stem cells

Embryonic stem (ES) cells are genetically manipulable pluripotential cells that can be differentiated in vitro into neurons, oligodendrocytes, and astrocytes. Given their potential utility as a source of replacement cells for the injured nervous system and the likelihood that transplantation interventions might include co-application of growth factors, we examined the effects of neurotrophin and GDNF family ligands on the survival and excitotoxic vulnerability of ES cell-derived neurons (ES neurons) grown in vitro. ES cells were differentiated down a neural lineage in vitro using the 4-/4+ protocol (Bain et al., Dev Biol 168:342-57, 1995). RT-PCR demonstrated expression of receptors for neurotrophins and GDNF family ligands in ES neural lineage cells. Neuronal expression of GFRalpha1, GFRalpha2, and ret was confirmed by immunocytochemistry. Exposure to 30-100 ng/ml GDNF or neurturin (NRTN) resulted in activation of ret. Addition of NT-3 and GDNF did not increase cell division but did increase the number of neurons in the cultures 7 days after plating. Pretreatment with NT-3 enhanced the vulnerability of ES neurons to NMDA-induced death (100 microM NMDA for 10 min) and enhanced the NMDA-induced increase in neuronal [Ca2+]i, but did not alter expression of NMDA receptor subunits NR2A or NR2B. In contrast, pretreatment with GDNF reduced the vulnerability of ES neurons to NMDA-induced death while modestly enhancing the NMDA-induced increase in neuronal [Ca2+]i. These findings demonstrate that the response of ES-derived neurons to neurotrophins and GDNF family ligands is largely similar to that of other cultured central neurons.

A model for GFRα4 function and a potential modifying role in multiple endocrine neoplasia 2

Mutations of the RET proto-oncogene are found in the majority of patients with the inherited cancer syndrome multiple endocrine neoplasia type 2 (MEN 2). A minority of cases, however, have no detectable RET mutation and there is considerable phenotypic variation within and among MEN 2 families with the same RET mutation, suggesting a role for other loci in this disease. A candidate for such a gene is glial cell line-derived neurotrophic factor receptor alpha 4 (GFRA4), which encodes a cell surface-bound co-receptor (GFR alpha 4) required for interaction of RET with its ligand persephin. The GFRA4 gene has multiple alternative splices leading to three distinct protein isoforms that are prominently expressed in thyroid. We postulated that mutations of GFRA4 contribute to MEN 2 in the absence of RET mutations or modify the RET mutation phenotype. We screened patients with MEN 2 or MEN 2-like phenotypes, with and without RET mutations, for variants of GFRA4. We identified 10 variants, one of which was over represented in, and two of which were found exclusively in, our patient populations. One of these was a single-base substitution upstream of the GFR alpha 4 coding region, where it may alter gene expression. The second was a 7 bp insertion, which results in a change in reading frame for all three GFR alpha 4 isoforms. This would cause a relative shift in membrane bound and soluble forms of GFR alpha 4, which would significantly alter the formation of RET signalling complexes. Our data suggest a model of wild-type GFR alpha 4 isoform expression that includes both activating and inhibiting co-receptors for RET.

Identification of BRCA1-IRIS, a BRCA1 locus product

Breast cancer is the most common malignancy among women, and mutations in the BRCA genes produce increased susceptibility to these malignancies in certain families. Here we identify BRCA1-IRIS as a 1,399-amino-acid BRCA1 gene product encoded by an uninterrupted open reading frame that extends from codon 1 of the known BRCA1 open reading frame to a termination point 34 triplets into intron 11. Unlike full-length BRCA1 (p220), BRCA1-IRIS is exclusively chromatin-associated, fails to interact with BARD1 in vivo or in vitro and exhibits unique nuclear immunostaining. Unlike BRCA1FL (or p220), BRCA1-IRIS also co-immunoprecipitated with DNA-replication-licensing proteins and with known replication initiation sites. Suppression of BRCA1-IRIS expression hindered the normal departure of geminin from pre-replication complexes, and depressed the rate of cellular DNA replication and possibly initiation-related synthesis. In contrast, BRCA1-IRIS overexpression stimulated DNA replication. These data imply that endogenous BRCA1-IRIS positively influences the DNA replication initiation machinery.

PSPN/GFRalpha4 has a significantly weaker capacity than GDNF/GFRalpha1 to recruit RET to rafts, but promotes neuronal survival and neurite outgrowth

Previously, it was shown that the recruitment of RET into lipid rafts by glial cell line-derived neurotrophic factor (GDNF)/GFRalpha1 is crucial for efficient signal transduction. Here, we show that the mouse GFRalpha4 is a functional, N-glycosylated, glycosylphosphatidylinositol (GPI)-anchored protein, which mediates persephin (PSPN)-induced phosphorylation of RET, but has an almost undetectable capacity to recruit RET into the 0.1% Triton X-100 insoluble membrane fraction. In spite of this, PSPN/mGFRalpha4 promotes neurite outgrowth in PC6-3 cells and survival of cerebellar granule neurons. As we show that also human PSPN/GFRalpha4 is unable to recruit RET into lipid rafts, we propose that the mammalian GFRalpha4 in this respect differs from GFRalpha1.

Temporally induced Nurr1 can induce a non-neuronal dopaminergic cell type in embryonic stem cell differentiation

The nuclear transcription factor Nurr1 is involved in the development and maintenance of the midbrain dopaminergic (DA) neuronal phenotype. We analysed the cellular and biological effects of Nurr1 during embryonic stem (ES) cell differentiation using the ROSA26-engineered Tet-inducible ES cell line J1-rtTA that does not express transgenes in mature neurons. Induction of Nurr1 at nestin-positive precursor and later stages of ES cell differentiation produced a non-neuronal DA cell type including functional DA transporters. In these cells, we found a clear correlation between Nurr1 and TH gene expression and specific midbrain DA cellular markers such as AADC, AHD2 and calbindin. Nurr1 did not alter gene expression of non-DA neuronal phenotypes and did not influence other midbrain developmental transcription factors, such as Otx1, Otx2, En-1, GBX2, Pitx3 and lmx1b. In addition, Nurr1 expression was required for maintenance of the DA phenotype and mediated up-regulation of the tyrosine kinase Ret and associated trophic factor GDNF-family receptors alpha 1, 2, and 4. This demonstrates that Nurr1 is sufficient to induce and maintain a midbrain-like DA biochemical and functional cellular phenotype independent of neurogenesis.

The structure of GFRalpha1 domain 3 reveals new insights into GDNF binding and RET activation

Glial cell line-derived neurotrophic factor (GDNF) binds to the GDNF family co-receptor alpha1 (GFRalpha1) and activates RET receptor tyrosine kinase. GFRalpha1 has a putative domain structure of three homologous cysteine-rich domains, where domains 2 and 3 make up a central domain responsible for GDNF binding. We report here the 1.8 A crystal structure of GFRalpha1 domain 3 showing a new protein fold. It is an all-alpha five-helix bundle with five disulfide bridges. The structure was used to model the homologous domain 2, the other half of the GDNF-binding fragment, and to construct the first structural model of the GDNF-GFRalpha1 interaction. Using site-directed mutagenesis, we identified closely spaced residues, Phe213, Arg224, Arg225 and Ile229, comprising a putative GDNF-binding surface. Mutating each one of them had slightly different effects on GDNF binding and RET phosphorylation. In addition, the R217E mutant bound GDNF equally well in the presence and absence of RET. Arg217 may thus be involved in the allosteric properties of GFRalpha1 or in binding RET.