Differential activities of the RET tyrosine kinase receptor isoforms during mammalian embryogenesis

Future horizons in the treatment of enteric neuropathies

Enteric neuropathies comprise a vast and disparate array of congenital and acquired disorders of the enteric nervous system (ENS), reflecting both the complexity of its neuronal composition and the many interactions that modulate its function. Although present therapeutic strategies, largely limited to surgery and the provision of artificial nutrition, have transformed the early survival and life of sufferers, levels of morbidity and mortality remain unacceptably high. This highlights the need to develop new treatments for enteric neuropathies. In the last decade, the tremendous advances in molecular biology and genetics have significantly enhanced our understanding of ENS development and function. Coupled with equivalent progress in the fields of pharmacology and stem-cell biology, this has led to the identification of novel tools and targets for therapy, which either aim to optimise the function of the intrinsic ENS or replace/replenish components of an inadequate or dysfunctional ENS. This article reviews current work on a number of these interventions with a particular focus on the use of ENS stem cells as potential therapeutic tools for enteric neuropathies.

New horizons in the pathogenesis of gastrointestinal neuromuscular disease

The term "gastrointestinal neuromuscular disease" can be interpreted variably and encompasses a spectrum of paediatric and adult conditions including achalasia, pseudoobstruction, idiopathic constipation, irritable bowel syndrome, megacolon, and Hirschsprung disease. Although progress has been made in the understanding of the pathophysiology of some conditions, the aetiopathogenesis has been elucidated only in the rare minority. This review critically considers the available evidence for possible pathogenic mechanisms in these disorders.

The receptor tyrosine kinase RET regulates hindgut colonization by sacral neural crest cells

The enteric nervous system (ENS) is formed from vagal and sacral neural crest cells (NCC). Vagal NCC give rise to most of the ENS along the entire gut, whereas the contribution of sacral NCC is mainly limited to the hindgut. This, and data from heterotopic quail-chick grafting studies, suggests that vagal and sacral NCC have intrinsic differences in their ability to colonize the gut, and/or to respond to signalling cues within the gut environment. To better understand the molecular basis of these differences, we studied the expression of genes known to be essential for ENS formation, in sacral NCC within the chick hindgut. Our results demonstrate that, as in vagal NCC, Sox10, EdnrB, and Ret are expressed in sacral NCC within the gut. Since we did not detect a qualitative difference in expression of these ENS genes we performed DNA microarray analysis of vagal and sacral NCC. Of 11 key ENS genes examined from the total data set, Ret was the only gene identified as being highly differentially expressed, with a fourfold increase in expression in vagal versus sacral NCC. We also found that over-expression of RET in sacral NCC increased their ENS developmental potential such that larger numbers of cells entered the gut earlier in development, thus promoting the fate of sacral NCC towards that of vagal NCC.

Neural crest and the development of the enteric nervous system

The formation of the enteric nervous system (ENS) is a particularly interesting example of the migratory ability of the neural crest and of the complexity of structures to which neural crest cells contribute. The distance that neural crest cells migrate to colonize the entire length of the gastrointestinal tract exceeds that of any other neural crest cell population. Furthermore, this migration takes a long time--over 25% of the gestation period for mice and around 3 weeks in humans. After colonizing the gut, neural crest-derived cells within the gut wall then differentiate into glial cells plus many different types of neurons, and generate the most complex part of the peripheral nervous system.

PTEN modulates GDNF/RET mediated chemotaxis and branching morphogenesis in the developing kidney

The RET receptor tyrosine kinase is activated by GDNF and controls outgrowth and invasion of the ureteric bud epithelia in the developing kidney. In renal epithelial cells and in enteric neuronal precursor cells, activation of RET results in chemotaxis as Ret expressing cells invade the surrounding GDNF expressing tissue. One potential downstream signaling pathway governing RET mediated chemotaxis may require phosphatidylinositol 3-kinase (PI3K), which generates PI(3,4,5) triphosphate. The PTEN tumor suppressor gene encodes a protein and lipid phosphatase that regulates cell growth, apoptosis and many other cellular processes. PTEN helps regulate cellular chemotaxis by antagonizing the PI3K signaling pathway through dephosphorylation of phosphotidylinositol triphosphates. In this report, we show that PTEN suppresses RET mediated cell migration and chemotaxis in cell culture assays, that RET activation results in asymmetric localization of inositol triphosphates and that loss of PTEN affects the pattern of branching morphogenesis in developing mouse kidneys. These data suggest a critical role for the PI3K/PTEN axis in shaping the pattern of epithelial branches in response to RET activation.

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.

Enteric nervous system development and Hirschsprung's disease: advances in genetic and stem cell studies

The enteric nervous system (ENS) has been explored by developmental neurobiologists and medical researchers for decades. Whereas developmental biologists have been unravelling the molecular mechanisms underlying the migration, proliferation and differentiation of the neural crest derivatives that give rise to the ENS, human geneticists have been uncovering the genetic basis for diseases of the ENS, notably Hirschsprung's disease. Here we discuss the exciting recent advances, including novel transgenic and genetic tools, a broadening range of model organisms, and the pursuit of ENS stem cells as a therapeutic tool, that are bringing these fields closer together.

Effect of Gdnf haploinsufficiency on rate of migration and number of enteric neural crest-derived cells

The enteric nervous system arises predominantly from vagal level neural crest cells that migrate into the foregut and then colonize the entire length of the gastrointestinal tract. Previous studies have demonstrated that glial cell line-derived neurotrophic factor (GDNF) promotes the migration of enteric neural crest-derived cells (ENCs) in vitro, but a role for GDNF in the migration of ENCs in vivo has yet to be demonstrated. In this study, the effects of Gdnf haploinsufficiency on ENC rate of migration and number during mid embryonic development were examined. Although the entire gut of embryonic Gdnf(+/-) mice was colonized, a significant delay in the migration of ENCs along the embryonic hindgut was found. However, significant effects of Gdnf haploinsufficiency on ENC number were detected before the stage at which migration defects were first evident. As previous studies have shown a relationship between ENC number and migration, the effects of Gdnf haploinsufficiency on migration may be due to an indirect effect on cell number and/or a direct effect of GDNF on ENC migration. Gdnf haploinsufficiency did not cause any detectable change in the rate of neuronal differentiation of ENCs.

SH2B1beta adaptor is a key enhancer of RET tyrosine kinase signaling

The RET gene encodes two main isoforms of a receptor tyrosine kinase (RTK) implicated in various human diseases. Activating germ-line point mutations are responsible for multiple endocrine neoplasia type 2-associated medullary thyroid carcinomas, inactivating germ-line mutations for Hirschsprung's disease, while somatic rearrangements (RET/PTCs) are specific to papillary thyroid carcinomas. SH2B1beta, a member of the SH2B adaptors family, and binding partner for several RTKs, has been recently described to interact with proto-RET. Here, we show that both RET isoforms and its oncogenic derivatives bind to SH2B1beta through the SRC homology 2 (SH2) domain and a kinase activity-dependent mechanism. As a result, RET phosphorylates SH2B1beta, which in turn enhances its autophosphorylation, kinase activity, and downstream signaling. RET tyrosine residues 905 and 981 are important determinants for functional binding of the adaptor, as removal of both autophosphorylation sites displaces its recruitment. Binding of SH2B1beta appears to protect RET from dephosphorylation by protein tyrosine phosphatases, and might represent a likely mechanism contributing to its upregulation. Thus, overexpression of SH2B1beta, by enhancing phosphorylation/activation of RET transducers, potentiates the cellular differentiation and the neoplastic transformation thereby induced, and counteracts the action of RET inhibitors. Overall, our results identify SH2B1beta as a key enhancer of RET physiologic and pathologic activities.

Tyrosine kinase receptor RET is a key regulator of Peyer's patch organogenesis

Normal organogenesis requires co-ordinate development and interaction of multiple cell types, and is seemingly governed by tissue specific factors. Lymphoid organogenesis during embryonic life is dependent on molecules the temporal expression of which is tightly regulated. During this process, haematopoietic 'inducer' cells interact with stromal 'organizer' cells, giving rise to the lymphoid organ primordia. Here we show that the haematopoietic cells in the gut exhibit a random pattern of motility before aggregation into the primordia of Peyer's patches, a major component of the gut-associated lymphoid tissue. We further show that a CD45+CD4-CD3-Il7Ralpha-c-Kit+CD11c+ haematopoietic population expressing lymphotoxin has an important role in the formation of Peyer's patches. A subset of these cells expresses the receptor tyrosine kinase RET, which is essential for mammalian enteric nervous system formation. We demonstrate that RET signalling is also crucial for Peyer's patch formation. Functional genetic analysis revealed that Gfra3-deficiency results in impairment of Peyer's patch development, suggesting that the signalling axis RET/GFRalpha3/ARTN is involved in this process. To support this hypothesis, we show that the RET ligand ARTN is a strong attractant of gut haematopoietic cells, inducing the formation of ectopic Peyer's patch-like structures. Our work strongly suggests that the RET signalling pathway, by regulating the development of both the nervous and lymphoid system in the gut, has a key role in the molecular mechanisms that orchestrate intestine organogenesis.

Role of fibroblast growth factor receptor signaling in kidney development

Fibroblast growth factor receptors (Fgfrs) are expressed in the ureteric bud and metanephric mesenchyme of the developing kidney. Furthermore, in vitro and in vivo studies have shown that exogenous fibroblast growth factors (Fgfs) increase growth and maturation of the metanephric mesenchyme and ureteric bud. Deletion of fgf7, fgf10, and fgfr2IIIb (the receptor isoform that binds Fgf7 and Fgf10) in mice lead to smaller kidneys with fewer collecting ducts and nephrons. Overexpression of a dominant negative receptor isoform in transgenic mice has revealed more striking defects including renal aplasia or severe dysplasia. Moreover, deletion of many fgf ligands and receptors in mice results in early embryonic lethality, making it difficult to determine their roles in kidney development. Recently, conditional targeting approaches revealed that deletion of fgf8 from the metanephric mesenchyme interrupts nephron formation. Furthermore, deletion of fgfr2 from the ureteric bud resulted in both ureteric bud branching and stromal mesenchymal patterning defects. Deletion of both fgfr1 and fgfr2 in the metanephric mesenchyme resulted in renal aplasia, characterized by defects in metanephric mesenchyme formation and initial ureteric bud elongation and branching. Thus, Fgfr signaling is critical for growth and patterning of all renal lineages at early and later stages of kidney development.

Disruption of the GDNF binding site in NCAM dissociates ligand binding and homophilic cell adhesion

Most plasma membrane proteins are capable of sensing multiple cell-cell and cell-ligand interactions, but the extent to which this functional versatility is founded on their modular design is less clear. We have identified the third immunoglobulin domain of the Neural Cell Adhesion Molecule (NCAM) as the necessary and sufficient determinant for its interaction with Glial Cell Line-derived Neurotrophic Factor (GDNF). Four charged contacts were identified by molecular modeling as the main contributors to binding energy. Their mutation abolished GDNF binding to NCAM but left intact the ability of NCAM to mediate cell adhesion, indicating that the two functions are genetically separable. The GDNF-NCAM interface allows complex formation with the GDNF family receptor alpha1, shedding light on the molecular architecture of a multicomponent GDNF receptor.

Distribution of RET immunoreactivity in the rodent spinal cord and changes after nerve injury

RET (for "rearranged during transfection") is a transmembrane tyrosine kinase signaling receptor for members of the glial cell line-derived neurotrophic factor (GDNF) family of ligands. We used RET immunohistochemistry (IHC), double-labeling immunofluorescence (IF), and in situ hybridization (ISH) in adult naïve and nerve-injured rats to study the distribution of RET in the spinal cord. In the dorsal horn, strong RET-immunoreactive (-ir) fibers were abundant in lamina II-inner (II(i)), although this labeling was preferentially observed after an antigen-unmasking procedure. After dorsal rhizotomy, RET-ir fibers in lamina II(i) completely disappeared from the dorsal horn, indicating that they were all primary afferents. After peripheral axotomy, RET-ir in primary afferents decreased in lamina II(i) and appeared to increase slightly in laminae III and IV. RET-ir was also observed in neurons and dendrites throughout the dorsal horn. Some RET-ir neurons in lamina I had the morphological appearance of nociceptive projection neurons, which was confirmed by the finding that 53% of RET-ir neurons in lamina I colocalized with neurokinin-1. GDNF-ir terminals were in close proximity to RET-ir neurons in the superficial dorsal horn. In the ventral horn, RET-ir was strongly expressed by motoneurons, with the strongest staining in small, presumably gamma-motoneurons. Increased RET expression following peripheral axotomy was most pronounced in alpha-motoneurons. The expression and regulation pattern of RET in the spinal cord are in line with its involvement in regenerative processes following nerve injury. The presence of RET in dorsal horn neurons, including nociceptive projection neurons, suggests that RET also has a role in signal transduction at the spinal level. This role may include mediating the effects of GDNF released from nociceptive afferent fibers.

A common variant located in the 3'UTR of the RET gene is associated with protection from Hirschsprung disease

Complex diseases are common genetic disorders showing familial aggregation but no typical Mendelian inheritance. Hirschsprung disease (HSCR), a developmental disorder characterized by the absence of enteric neurons in distal segments of the gut, shows a complex pattern of inheritance, with the RET protooncogene acting as a major gene and additional susceptibility loci playing minor roles. In the last years, we have identified a "protective" RET haplotype, which is underrepresented in HSCR patients with respect to controls. Here, we demonstrate that the protective effect of this haplotype is due to a variant located in the 3' untranslated region (UTR) of the RET gene, which slows down the physiological mRNA decay of the gene transcripts. Such a functional effect of this common RET variant explains the under-representation of the whole haplotype and its role as a modifying factor in HSCR pathogenesis.

NGF augments the autophosphorylation of Ret via inhibition of ubiquitin-dependent degradation

Nerve growth factor (NGF) is required for the trophic maintenance of postnatal sympathetic neurons. A significant portion of the growth-promoting activity of NGF is from NGF-dependent phosphorylation of the heterologous receptor tyrosine kinase, Ret. We found that NGF applied selectively to distal axons of sympathetic neurons maintained in compartmentalized cultures activated Ret located in these distal axons. Inhibition of either proteasomal or lysosomal degradation pathways mimicked the effect of NGF on Ret activation. Likewise, NGF inhibited the degradation of Ret induced by glial cell line-derived neurotrophic factor-dependent activation, a process that requires ubiquitination and proteasomal degradation. NGF induced the accumulation of autophosphorylated Ret predominantly in the plasma membrane, in contrast to GDNF, which promoted the internalization of activated Ret. An accretion of monoubiquitinated, but not polyubiquitinated, Ret occurred in NGF-treated neurons, in contrast to glial cell line-derived neurotrophic factor that promoted the robust polyubiquitination of Ret. Thus, NGF stimulates Ret activity in mature sympathetic neurons by inhibiting the ongoing ubiquitin-mediated degradation of Ret before its internalization and polyubiquitination.

Neurotrophic factor receptor RET: structure, cell biology, and inherited diseases

RET (REarranged during Transfection) is a transmembrane receptor tyrosine kinase that is activated by a complex consisting of a soluble glial cell line-derived neurotrophic factor (GDNF) family ligand (GFL) and a glycosyl phosphatidylinositol-anchored co-receptor, GDNF family receptors alpha (GFRalpha). RET signalling is crucial for the development of the enteric nervous system. RET also regulates the development of sympathetic, parasympathetic, motor, and sensory neurons, and is necessary for the postnatal maintenance of dopaminergic neurons. The effect of GFLs on sensory, motor, and dopaminergic neurons has raised clinical interest towards these ligands. Outside the nervous system, RET is crucial for development of the kidney and plays a key role in spermatogenesis. Inactivating mutations in RET cause the Hirschsprung's disease characterized by megacolon aganglionosis. In contrast, activating mutations give rise to different types of cancer, multiple endocrine neoplasia type 2A and type 2B, familial medullary thyroid carcinoma, and papillary thyroid carcinoma. The multiple disease phenotypes correlate with differences in the molecular and cell biological functions of different oncogenic RET proteins. In this review we summarize how the different domains of the RET protein contribute to its normal function and how mutations in these domains affect the function of the receptor.

The cellular basis of kidney development

Mammalian kidney development has helped elucidate the general concepts of mesenchymal-epithelial interactions, inductive signaling, epithelial cell polarization, and branching morphogenesis. Through the use of genetically engineered mouse models, the manipulation of Xenopus and chick embryos, and the identification of human renal disease genes, the molecular bases for many of the early events in the developing kidney are becoming increasingly clear. Early patterning of the kidney region depends on interactions between Pax/Eya/Six genes, with essential roles for lim1 and Odd1. Ureteric bud outgrowth and branching morphogenesis are controlled by the Ret/Gdnf pathway, which is subject to positive and negative regulation by a variety of factors. A clear role for Wnt proteins in induction of the kidney mesenchyme is now well established and complements the classic literature nicely. Patterning along the proximal distal axis as the nephron develops is now being investigated and must involve aspects of Notch signaling. The development of a glomerulus requires interactions between epithelial cells and infiltrating endothelial cells to generate a unique basement membrane. The integrity of the glomerular filter depends in large part on the proteins of the nephrin complex, localized to the slit diaphragm. Despite the kidney's architectural complexity, with the advent of genomics and expression arrays, it is becoming one of the best-characterized organ systems in developmental biology.

Branching morphogenesis of the ureteric epithelium during kidney development is coordinated by the opposing functions of GDNF and Sprouty1

Branching of ureteric bud-derived epithelial tubes is a key morphogenetic process that shapes development of the kidney. Glial cell line-derived neurotrophic factor (GDNF) initiates ureteric bud formation and promotes subsequent branching morphogenesis. Exactly how GDNF coordinates branching morphogenesis is unclear. Here we show that the absence of the receptor tyrosine kinase antagonist Sprouty1 (Spry1) results in irregular branching morphogenesis characterized by both increased number and size of ureteric bud tips. Deletion of Spry1 specifically in the epithelium is associated with increased epithelial Wnt11 expression as well as increased mesenchymal Gdnf expression. We propose that Spry1 regulates a Gdnf/Ret/Wnt11-positive feedback loop that coordinates mesenchymal-epithelial dialogue during branching morphogenesis. Genetic experiments indicate that the positive (GDNF) and inhibitory (Sprouty1) signals have to be finely balanced throughout renal development to prevent hypoplasia or cystic hyperplasia. Epithelial cysts develop in Spry1-deficient kidneys that share several molecular characteristics with those observed in human disease, suggesting that Spry1 null mice may be useful animal models for cystic hyperplasia.

Targeted mutation of serine 697 in the Ret tyrosine kinase causes migration defect of enteric neural crest cells

The RET receptor tyrosine kinase plays a critical role in the development of the enteric nervous system (ENS) and the kidney. Upon glial-cell-line-derived neurotrophic factor (GDNF) stimulation, RET can activate a variety of intracellular signals, including the Ras/mitogen-activated protein kinase, phosphatidylinositol 3-kinase (PI3K)/AKT, and RAC1/JUN NH(2)-terminal kinase (JNK) pathways. We recently demonstrated that the RAC1/JNK pathway is regulated by serine phosphorylation at the juxtamembrane region of RET in a cAMP-dependent manner. To determine the importance of cAMP-dependent modification of the RET signal in vivo, we generated mutant mice in which serine residue 697, a putative protein kinase A (PKA) phosphorylation site, was replaced with alanine (designated S697A mice). Homozygous S697A mutant mice lacked the ENS in the distal colon, resulting from a migration defect of enteric neural crest cells (ENCCs). In vitro organ culture showed an impaired chemoattractant response of the mutant ENCCs to GDNF. JNK activation by GDNF but not ERK, AKT and SRC activation was markedly reduced in neurons derived from the mutant mice. The JNK inhibitor SP600125 and the PKA inhibitor KT5720 suppressed migration of the ENCCs in cultured guts from wild-type mice to comparable degrees. Thus, these findings indicated that cAMP-dependent modification of RET function regulates the JNK signaling responsible for proper migration of the ENCCs in the developing gut.

Renal branching morphogenesis: concepts, questions, and recent advances

The ureteric bud (UB) is an outgrowth of the Wolffian duct, which undergoes a complex process of growth, branching, and remodeling, to eventually give rise to the entire urinary collecting system during kidney development. Understanding the mechanisms that control this process is a fascinating problem in basic developmental biology, and also has considerable medical significance. Over the past decade, there has been significant progress in our understanding of renal branching morphogenesis and its regulation, and this review focuses on several areas in which there have been recent advances. The first section deals with the normal process of UB branching morphogenesis, and methods that have been developed to better observe and describe it. The next section discusses a number of experimental methodologies, both established and novel, that make kidney development in the mouse a powerful and attractive experimental system. The third section discusses some of the cellular processes that are likely to underlie UB branching morphogenesis, as well as recent data on cell lineages within the growing UB. The fourth section summarizes our understanding of the roles of two groups of growth factors that appear to be particularly important for the regulation of UB outgrowth and branching: GDNF and FGFs, which stimulate this process via tyrosine kinase receptors, and members of the TGFbeta family, including BMP4 and Activin A, which generally inhibit UB formation and branching.

Engineering the Recruitment of Phosphotyrosine Binding Domain-containing Adaptor Proteins Reveals Distinct Roles for RET Receptor-mediated Cell Survival

The RET receptor tyrosine kinase is important for several different biological functions during development. The recruitment at the phosphorylated Tyr(1062) site in RET of a number of different phosphotyrosine binding (PTB) domain-containing adaptor proteins, including Shc and Frs2, plays a dominant role for the multiple different biological functions of the RET receptor during development, including stimulation of cell survival. Here, we demonstrate that a competitive recruitment of Shc as opposed to Frs2 mediates the survival signaling arising from RET activation. Based on results from a peptide array, we have genetically engineered the PTB domain binding site of RET to rewire its recruitment of the PTB proteins Shc and Frs2. An engineered RET that has a competitive interaction with Shc at the expense of Frs2, but not a RET receptor that only recruits Frs2, activates cell survival signaling pathways and is protective from cell death in neuronal SK-N-MC cells. Thus, cell type-specific functions involve a competitive recruitment of different PTB adaptor molecules by RET that activate selective signaling pathways.

Advances in ontogeny of the enteric nervous system

The neurons and glia that comprise the enteric nervous system (ENS), the intrinsic innervation of the gastrointestinal tract, are derived from vagal and sacral regions of the neural crest. In order to form the ENS, neural crest-derived precursors undergo a number of processes including survival, migration and proliferation, prior to differentiation into neuronal subtypes, some of which form functional connections with the gut smooth muscle. Investigation of the developmental processes that underlie ENS formation has progressed dramatically in recent years, in no small part due to the attention of scientists from a range of disciplines on the genesis of Hirschsprung's disease (aganglionic megacolon), the major congenital abnormality of the ENS. This review summarizes recent advances in the field of early ENS ontogeny and focuses on: (i) the spatiotemporal migratory pathways followed by vagal and sacral neural crest-derived ENS precursors, including recent in vivo imaging of migrating crest cells within the gut, (ii) the roles of the RET and EDNRB signalling pathways and how these pathways interact to control ENS development, and (iii) how perpendicular migrations of neural crest cells within the gut lead to the formation of the myenteric and submucosal plexi located between the smooth muscle layers of the gut wall.

RET as a diagnostic and therapeutic target in sporadic and hereditary endocrine tumors

The RET gene encodes a receptor tyrosine kinase that is expressed in neural crest-derived cell lineages. The RET receptor plays a crucial role in regulating cell proliferation, migration, differentiation, and survival through embryogenesis. Activating mutations in RET lead to the development of several inherited and noninherited diseases. Germline point mutations are found in the cancer syndromes multiple endocrine neoplasia (MEN) type 2, including MEN 2A and 2B, and familial medullary thyroid carcinoma. These syndromes are autosomal dominantly inherited. The identification of mutations associated with these syndromes has led to genetic testing to identify patients at risk for MEN 2 and familial medullary thyroid carcinoma and subsequent implementation of prophylactic thyroidectomy in mutation carriers. In addition, more than 10 somatic rearrangements of RET have been identified from papillary thyroid carcinomas. These mutations, as those found in MEN 2, induce oncogenic activation of the RET tyrosine kinase domain via different mechanisms, making RET an excellent candidate for the design of molecular targeted therapy. Recently, various kinds of therapeutic approaches, such as tyrosine kinase inhibition, gene therapy with dominant negative RET mutants, monoclonal antibodies against oncogene products, and nuclease-resistant aptamers that recognize and inhibit RET have been developed. The use of these strategies in preclinical models has provided evidence that RET is indeed a potential target for selective cancer therapy. However, a clinically useful therapeutic option for treating patients with RET-associated cancer is still not available.

Molecular advances in medullary thyroid cancer diagnostics

Germline activating mutations in the RET proto-oncogene cause inherited medullary thyroid cancer (MTC) and the multiple endocrine neoplasia type 2 (MEN2) syndrome. Identification of a RET mutation in an individual with MEN2 allows pre-symptomatic genetic testing of other at-risk family members, and guides early intervention to prevent death and serious morbidity from MTC. Developments in the understanding of downstream RET receptor signalling pathways and how activating mutations disturb receptor function has led to insights into the possible molecular mechanisms underlying the different MEN2 phenotypes. Mutation analysis of RET in individuals with MEN2 has identified a number of different mutations, and correlation with cancer biology and clinical outcome has led to tailoring of management according to the mutation detected.

Maintenance of mammalian enteric nervous system progenitors by SOX10 and endothelin 3 signalling

The transcriptional regulator SOX10 and the signalling molecule endothelin 3 have important roles in the development of the mammalian enteric nervous system (ENS). Using a clonal cell culture system, we show that SOX10 inhibits overt neuronal and glial differentiation of multilineage ENS progenitor cells (EPCs), without interfering with their neurogenic commitment. We also demonstrate that endothelin 3 inhibits reversibly the commitment and differentiation of EPCs along the neurogenic and gliogenic lineages, suggesting a role for this factor in the maintenance of multilineage ENS progenitors. Consistent with such a role, the proportion of Sox10-expressing progenitors in the total population of enteric neural crest cells is reduced in the gut of endothelin 3-deficient embryos. This reduction may be related to the requirement of endothelin signalling for the proliferation of ENS progenitors. The dependence of ENS progenitors on endothelin 3 is more pronounced at the migratory front of enteric neural crest cells, which is associated with relatively high levels of endothelin 3 mRNA. Our findings indicate that SOX10 and endothelin 3 have a crucial role in the maintenance of multilineage enteric nervous system progenitors.

GDNF/Ret signaling and the development of the kidney

Signaling by GDNF through the Ret receptor is required for normal growth of the ureteric bud during kidney development. However, the precise role of GDNF/Ret signaling in renal branching morphogenesis and the specific responses of ureteric bud cells to GDNF remain unclear. Recent studies have provided new insight into these issues. The localized expression of GDNF by the metanephric mesenchyme, together with several types of negative regulation, is important to elicit and correctly position the initial budding event from the Wolffian duct. GDNF also promotes the continued branching of the ureteric bud. However, it does not provide the positional information required to specify the pattern of ureteric bud growth and branching, as its site of synthesis can be drastically altered with minimal effects on kidney development. Cells that lack Ret are unable to contribute to the tip of the ureteric bud, apparently because GDNF-driven proliferation is required for the formation and growth of this specialized epithelial domain.

Critical and distinct roles for key RET tyrosine docking sites in renal development

Molecular mechanisms that lead to congenital anomalies of kidneys and the lower urinary tract (CAKUT) are poorly understood. To elucidate the molecular basis for signaling specificity of GDNF-mediated RET signaling in kidney development, we characterized mice that exclusively express either the human RET9 or RET51 isoform, or express these isoforms with individual mutations in docking tyrosines for PTB and SH2-domain-containing adaptors Src (Y981), PLCgamma (Y1015), and Shc (Y1062). Our results provide evidence for differential and isoform-specific roles of these docking sites in murine kidney development. Homozygous Ret(RET9) and Ret(RET51) mice were viable and show normally developed kidneys, indicating redundant roles of human RET isoforms in murine kidney development. In the context of the RET51 isoform, only mutation of the docking Tyr 1015 (Y1015F) resulted in severe renal anomalies. These included bilateral megaureters and multicystic kidneys that were caused by supernumerary ureteric buds that fail to separate from the wolffian duct as well as decreased branching morphogenesis. Similar kidney and ureter defects were observed in RET9(Y1015F) mice that contain the Y1015F mutation in the RET9 isoform. Interestingly, loss of RET9(Y1062)-mediated AKT/MAPK activation resulted in renal agenesis or kidney rudiments, whereas mutation of this residue in RET51 had no obvious effect on AKT/MAPK activity and renal development. These results reveal novel roles of key RET-dependent signaling pathways in embryonic kidney development and provide murine models and new insights into the molecular basis for CAKUT.

Molecular mechanisms of RET-induced Hirschsprung pathogenesis

The RET proto-oncogene is the major gene involved in the pathogenesis of Hirschsprung (HSCR), a complex genetic disease characterized by lack of ganglia along variable lengths of the gut. Here we present a survey of the different molecular mechanisms through which RET mutations lead to the disease development. Among these, loss of function, gain of function, apoptosis, aberrant splicing and decreased gene expression are exemplified and considered with respect to their pathogenetic impact. In particular, RET transcription regulation represents a new insight into the outline of HSCR susceptibility, and having reached important progress in the last few years, deserves to be reviewed. Notably, gene expression impairment seems to be at the basis of the association of HSCR disease with several RET polymorphisms, allowing us to define a predisposing haplotype spanning from the promoter to exon 2. Putative functional variants, in the promoter and in intron 1, and proposed as low penetrant predisposing alleles, are presented and discussed. Finally, based on the RET mutation effects thus summarized, we attempt to derive conclusions which may be useful for HSCR risk prediction and genetic counselling.

RET and neuroendocrine tumors

The RET proto-oncogene encodes a receptor tyrosine kinase that is a main component of the signaling pathway activated by the glial cell line-derived neurotrophic factor family ligands. Gene targeting studies revealed that signaling through RET plays a crucial role in neuronal and renal organogenesis. It is well-known that germline mutations in RET lead to the human inherited diseases, multiple endocrine neoplasia type 2 (MEN 2) and Hirschsprung's disease, and that somatic rearrangements of RET cause papillary thyroid carcinoma. Due to marked advances in understanding of the molecular mechanisms of the development of MEN 2, a consensus on MEN 2 management associated with RET status is being reached and currently put into general use as a guideline. In this review, we summarize progress in the study of RET from bench to bedside, focusing on pathophysiology of neuroendocrine tumors.

BMP signaling is necessary for neural crest cell migration and ganglion formation in the enteric nervous system

The enteric nervous system (ENS) is derived from neural crest cells that migrate along the gastrointestinal tract to form a network of neurons and glia that are essential for regulating intestinal motility. Despite the number of genes known to play essential roles in ENS development, the molecular etiology of congenital disorders affecting this process remains largely unknown. To determine the role of bone morphogenetic protein (BMP) signaling in ENS development, we first examined the expression of bmp2, bmp4, and bmprII during hindgut development and find these strongly expressed in the ENS. Moreover, functional BMP signaling, demonstrated by the expression of phosphorylated Smad1/5/8, is present in the enteric ganglia. Inhibition of BMP activity by noggin misexpression within the developing gut, both in ovo and in vitro, inhibits normal migration of enteric neural crest cells. BMP inhibition also leads to hypoganglionosis and failure of enteric ganglion formation, with crest cells unable to cluster into aggregates. Abnormalities of migration and ganglion formation are the hallmarks of two human intestinal disorders, Hirschsprung's disease and intestinal neuronal dysplasia. Our results support an essential role for BMP signaling in these aspects of ENS development and provide a basis for further investigation of these proteins in the etiology of neuro-intestinal disorders.

Phosphotyrosine 1062 is critical for the in vivo activity of the Ret9 receptor tyrosine kinase isoform

The receptor tyrosine kinase Ret plays a critical role in the development of the mammalian excretory and enteric nervous systems. Differential splicing of the primary Ret transcript results in the generation of two main isoforms, Ret9 and Ret51, whose C-terminal amino acid tails diverge after tyrosine (Y) 1062. Monoisoformic mice expressing only Ret9 develop normally and are healthy and fertile. In contrast, animals expressing only Ret51 have aganglionosis of the distal gut and hypoplastic kidneys. By generating monoisoformic mice in which Y1062 of Ret9 has been mutated to phenylalanine, we demonstrate that this amino acid has a critical role in Ret9 signaling that is necessary for the development of the kidneys and the enteric nervous system. These findings argue that the distinct activities of Ret9 and Ret51 result from the differential regulation of Y1062 by C-terminal flanking sequences. However, a mutation which places Y1062 of Ret51 in a Ret9 context improves only marginally the ability of Ret51 to support renal and enteric nervous system development. Finally, monoisoformic mice expressing a variant of Ret9 in which a C-terminal PDZ-binding motif was mutated develop normally and are healthy. Our studies identify Y1062 as a critical regulator of Ret9 signaling and suggest that Ret51-specific motifs are likely to inhibit the activity of this isoform.

RET tyrosine kinase signaling in development and cancer

The variety of diseases caused by mutations in RET receptor tyrosine kinase provides a classic example of phenotypic heterogeneity. Gain-of-function mutations of RET are associated with human cancer. Gene rearrangements juxtaposing the tyrosine kinase domain to heterologous gene partners have been found in sporadic papillary carcinomas of the thyroid (PTC). These rearrangements generate chimeric RET/PTC oncogenes. In the germline, point mutations of RET are responsible for multiple endocrine neoplasia type 2 (MEN 2A and 2B) and familial medullary thyroid carcinoma (FMTC). Both MEN 2 mutations and PTC gene rearrangements potentiate the intrinsic tyrosine kinase activity of RET and, ultimately, activate the RET downstream targets. Loss-of-function mutations of RET cause Hirschsprung's disease (HSCR) or colonic aganglionosis. A deeper understanding of the molecular signaling of normal versus abnormal RET activity in cancer will enable the development of potential new treatments for patients with sporadic and inherited thyroid cancer or MEN 2 syndrome. We now review the role and mechanisms of RET signaling in development and carcinogenesis.

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.

Intrathecal injection of GDNF and BDNF induces immediate early gene expression in rat spinal dorsal horn

Glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) are potent trophic factors for dorsal root ganglion cells. In addition, these factors are produced in subsets of dorsal root ganglion cells and transported anterogradely to their terminals in the superficial dorsal horn of the spinal cord, where they constitute the only source of GDNF and BDNF. We investigated the effect of 10 mug GDNF and BDNF injected by lumbar puncture on the expression of the immediate early gene (IEG) products c-Fos, c-Jun, and Krox-24 in the adult rat dorsal horn. In the dorsal horn of S1 spinal segments, GDNF and BDNF induced a strong increase in IEG expression, which was most pronounced in laminae I and II (2.9- to 4.5-fold). More distal from the injection site, in the dorsal horn of L1/L2 spinal segments, the increase in IEG expression was less pronounced, suggesting a concentration-dependent effect. In order to explain the effects of intrathecally injected GDNF, we investigated whether lumbo-sacral dorsal horn neurons expressed RET protein, the signal-transducing element of the receptor complex for GDNF. It was found that several of these neurons contained RET immunoreactivity and that some of the RET-labeled neurons had the appearance of nociceptive-specific cells, confirming their presumed role in pain transmission. Additionally, using double-labeling immunofluorescence combined with confocal microscopy, it was found that after intrathecal GDNF injection 35% of c-Fos-labeled cells were also labeled for RET. These results demonstrate that intrathecally administered GDNF and BDNF induce IEG expression in dorsal horn neurons in the adult rat, supposedly by way of their cognate receptors, which are present on these neurons. We further suggest that the endogenous release of GDNF and BDNF, triggered by nociceptive stimuli, is involved in the induction of changes in spinal nociceptive transmission as in various pain states.

The role of GDNF in patterning the excretory system

Mesenchymal-epithelial interactions are an important source of information for pattern formation during organogenesis. In the developing excretory system, one of the secreted mesenchymal factors thought to play a critical role in patterning the growth and branching of the epithelial ureteric bud is GDNF. We have tested the requirement for GDNF as a paracrine chemoattractive factor by altering its site of expression during excretory system development. Normally, GDNF is secreted by the metanephric mesenchyme and acts via receptors on the Wolffian duct and ureteric bud epithelium. Misexpression of GDNF in the Wolffian duct and ureteric buds resulted in formation of multiple, ectopic buds, which branched independently of the metanephric mesenchyme. This confirmed the ability of GDNF to induce ureter outgrowth and epithelial branching in vivo. However, in mutant mice lacking endogenous GDNF, kidney development was rescued to a substantial degree by GDNF supplied only by the Wolffian duct and ureteric bud. These results indicate that mesenchymal GDNF is not required as a chemoattractive factor to pattern the growth of the ureteric bud within the developing kidney, and that any positional information provided by the mesenchymal expression of GDNF may provide for renal branching morphogenesis is redundant with other signals.

RET proto-oncogene: a review and update of genotype-phenotype correlations in hereditary medullary thyroid cancer and associated endocrine tumors

Hereditary medullary thyroid carcinoma (MTC) is caused by autosomal dominant gain-of-function mutations in the RET proto-oncogene. Associations between specific RET mutations (genotype) and the aggressiveness of MTC and presence or absence of other endocrine neoplasms (phenotype) are well documented. Mutations in six exons (10, 11, 13, 14, 15, and 16) located in either cysteine-rich or tyrosine kinase domains cause one of three distinctive clinical subtypes: familial MTC, multiple endocrine neoplasia (MEN) type 2A (including variants with Hirschsprung's disease and cutaneous lichen amyloidosis), and MEN 2B. Hallmarks of MEN 2A include MTC, pheochromocytoma, and hyperparathyroidism. MEN 2B is associated with an earlier onset of MTC and pheochromocytoma, the absence of hyperparathyroidism, and the presence of striking physical stigmata (e.g., coarse facies, ganglioneuromatosis, and marfanoid habitus). Familial MTC is not associated with other endocrine neoplasms; however, the accurate distinction between familial MTC and MEN 2A may be difficult in kindreds with small size, incomplete histories, or a predominance of young individuals who may not have yet fully manifested the syndrome. Genetic testing detects greater than 95% of mutation carriers and is considered the standard of care for all first-degree relatives of patients with newly diagnosed MTC. Recommendations on the timing of prophylactic thyroidectomy and the extent of surgery are based upon a model that utilizes genotype- phenotype correlations to stratify mutations into three risk levels.

Distinct Turnover of Alternatively Spliced Isoforms of the RET Kinase Receptor Mediated by Differential Recruitment of the Cbl Ubiquitin Ligase

Alternative splicing of transcripts encoding the RET kinase receptor leads to isoforms differing in their cytoplasmic tail. Although in vitro studies have demonstrated a higher transforming activity of the long RET isoform (RET51), only the short isoform (RET9) can rescue the effects of a RET null mutation in the enteric nervous system and kidney development. The molecular basis underlying the distinct functions of the two RET isoforms is not understood. Here we demonstrated that activated RET51 associated more strongly with the ubiquitin ligase Cbl than did RET9, leading to increased ubiquitylation and faster turnover of RET51. The association of Cbl with RET was indirect and was mediated through Grb2. A constitutive complex of Grb2 and Cbl could be recruited to both receptor isoforms via docking of Shc to phosphorylated Tyr-1062 in RET. A mutant Shc protein unable to recruit the Grb2.Cbl complex decreased the turnover and prolonged the half-life of RET9, thus ascribing a previously unknown negative role to the Shc adaptor molecule. In addition, phosphorylation of Tyr-1096, which is present in RET51 but absent in RET9, endowed the longer isoform with a second route to recruit the Grb2.Cbl complex. These findings establish a mechanism for the differential down-regulation of RET9 and RET51 signaling that could explain the apparently paradoxical activities of these two RET isoforms. More generally, these results illustrate how alternative splicing can regulate the half-life and function of a growth factor receptor.

Role of fibroblast growth factor receptors 1 and 2 in the ureteric bud

Fibroblast growth receptors (FGFRs) consist of four signaling family members. Mice with deletions of fgfr1 or fgfr2 are embryonic lethal prior to the onset of kidney development. To determine roles of FGFR1 and FGFR2 in the ureteric bud, we used a conditional targeting approach. First, we generated transgenic mice using the Hoxb7 promoter to drive cre recombinase and green fluorescent protein expression throughout ureteric bud tissue. We crossed Hoxb7creEGFP mice with mice carrying lox-p sites flanking critical regions of fgfr1 and/or fgfr2. Absence of fgfr1 from the ureteric bud (fgfr1(UB-/-)) results in no apparent renal abnormalities. In contrast, fgfr2(UB-/-) mice have very aberrant ureteric bud branching, thin ureteric bud stalks, and fewer ureteric bud tips. Fgfr2(UB-/-) ureteric bud tips also demonstrate inappropriate regions of apoptosis and reduced proliferation. The nephrogenic mesenchymal lineage in fgfr2(UB-/-) mice develops normal-appearing glomeruli and tubules, and only slightly fewer nephrons than controls. In contrast, fgfr2(UB-/-) kidneys have abnormally thickened subcapsular cortical stromal mesenchyme. Ultimately, fgfr2(UB-/-) adult kidneys are small and abnormally shaped or are hydronephrotic. Finally, there are no additional abnormalities in the fgfr1/2(UB-/-) kidneys versus the fgfr2(UB-/-) kidneys. In conclusion, FGFR2, but not FGFR1, appears crucial for ureteric bud branching morphogenesis and stromal mesenchyme patterning.

Sprouty1 is a critical regulator of GDNF/RET-mediated kidney induction

Intercellular signaling molecules and their receptors, whose expression must be tightly regulated in time and space, coordinate organogenesis. Regulators of intracellular signaling pathways provide an additional level of control. Here we report that loss of the receptor tyrosine kinase (RTK) antagonist, Sprouty1 (Spry1), causes defects in kidney development in mice. Spry1(-/-) embryos have supernumerary ureteric buds, resulting in the development of multiple ureters and multiplex kidneys. These defects are due to increased sensitivity of the Wolffian duct to GDNF/RET signaling, and reducing Gdnf gene dosage correspondingly rescues the Spry1 null phenotype. We conclude that the function of Spry1 is to modulate GDNF/RET signaling in the Wolffian duct, ensuring that kidney induction is restricted to a single site. These results demonstrate the importance of negative feedback regulation of RTK signaling during kidney induction and suggest that failures in feedback control may underlie some human congenital kidney malformations.

A human yeast artificial chromosome containing the multiple endocrine neoplasia type 2B Ret mutation does not induce medullary thyroid carcinoma but does support the growth of kidneys and partially rescues enteric nervous system development in Ret-deficient mice

We generated a line of transgenic mice using a yeast artificial chromosome containing the Ret mutation responsible for the multiple endocrine neoplasia type 2B syndrome (MEN 2B). The resulting animals did not develop any of the expected neoplasms associated with MEN 2B. Transgenic animals were then bred with animals lacking murine Ret (Ret(M)) to further evaluate the function of human mutated Ret (Ret(H)(2B)) in the murine context. Whereas mice lacking Ret(M) exhibit intestinal aganglionosis and the absence of kidneys with other genitourinary anomalies, expression of the Ret(H)(2B) transgene in Ret(M)-deficient mice allowed significant renal development with a partial rescue of the enteric nervous system. These Ret(H)(2B)-positive/Ret(M)-deficient mice exhibit normal Ret expression and survive longer than Ret(M)-deficient mice, but still die at 3 to 5 days of age with evidence of enterocolitis. We conclude that the normal expression of a human Ret proto-oncogene with the MEN 2B mutation does not cause any features of MEN 2B in mice. Although the gene is normally expressed in the appropriate target tissues, there is incomplete phenotypic rescue in mice lacking murine Ret. These results suggest important interspecies differences between humans and mice in the function of the Ret oncogene.

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.

All-trans retinoic acid-responsive genes identified in the human SH-SY5Y neuroblastoma cell line and their regulated expression in the nervous system of early embryos

The vitamin A metabolite, all-trans retinoic acid (atRA), is required for embryonic development. atRA binds to the nuclear retinoic acid receptors and regulates the transcription of specific target genes. In order to identify atRA-induced genes that play a role in neural development, a subtractive library was created from SH-SY5Y neuroblastoma cells, a human cell line that exhibits changes in cell adhesion and neurite outgrowth after exposure to the vitamin A acid. We report here the identification of 14 genes that are rapidly induced by atRA (retinoic acid induced in neuroblastoma or RAINB), eight of which were previously not known to be atRA responsive (BTBD11, calmin, cyclin M2, ephrin B2, HOXD10, NEDD9, RAINB6 and tenascin R). mRNA regulation by atRA was confirmed in SH-SY5Y cells by Northern blotting, and gene regulation was studied in additional human cell lines using the quantitative polymerase chain reaction. The majority of the atRA-responsive clones revealed in this screen are highly expressed in the nervous system of developing rat embryos. Further, the expression of several of these genes is perturbed in developing rat embryos exposed to excess atRA or conversely, deprived of sufficient retinoid during early development. We propose that a subset of these genes lie downstream of atRA and its receptors in the regulation of neurite outgrowth and cell adhesion in both neural and non-neural tissues within the developing embryo.

Function of alternative splicing

Alternative splicing is one of the most important mechanisms to generate a large number of mRNA and protein isoforms from the surprisingly low number of human genes. Unlike promoter activity, which primarily regulates the amount of transcripts, alternative splicing changes the structure of transcripts and their encoded proteins. Together with nonsense-mediated decay (NMD), at least 25% of all alternative exons are predicted to regulate transcript abundance. Molecular analyses during the last decade demonstrate that alternative splicing determines the binding properties, intracellular localization, enzymatic activity, protein stability and posttranslational modifications of a large number of proteins. The magnitude of the effects range from a complete loss of function or acquisition of a new function to very subtle modulations, which are observed in the majority of cases reported. Alternative splicing factors regulate multiple pre-mRNAs and recent identification of physiological targets shows that a specific splicing factor regulates pre-mRNAs with coherent biological functions. Therefore, evidence is now accumulating that alternative splicing coordinates physiologically meaningful changes in protein isoform expression and is a key mechanism to generate the complex proteome of multicellular organisms.

Minireview: RET: normal and abnormal functions

The RET gene encodes a single-pass transmembrane receptor tyrosine kinase. RET is the oncogene that causes papillary thyroid carcinoma and medullary thyroid carcinoma. The latter may arise as a component of multiple endocrine neoplasia type 2 syndromes; germline mutations in RET are responsible for multiple endocrine neoplasia type 2 inheritance. In this report we review data on the mechanisms leading to RET oncogenic conversion and on RET targeting as a strategy in thyroid cancer treatment.

The neuronal scaffold protein Shank3 mediates signaling and biological function of the receptor tyrosine kinase Ret in epithelial cells

Shank proteins, initially also described as ProSAP proteins, are scaffolding adaptors that have been previously shown to integrate neurotransmitter receptors into the cortical cytoskeleton at postsynaptic densities. We show here that Shank proteins are also crucial in receptor tyrosine kinase signaling. The PDZ domain–containing Shank3 protein was found to represent a novel interaction partner of the receptor tyrosine kinase Ret, which binds specifically to a PDZ-binding motif present in the Ret9 but not in the Ret51 isoform. Furthermore, we show that Ret9 but not Ret51 induces epithelial cells to form branched tubular structures in three-dimensional cultures in a Shank3-dependent manner. Ret9 but not Ret51 has been previously shown to be required for kidney development. Shank3 protein mediates sustained Erk–MAPK and PI3K signaling, which is crucial for tubule formation, through recruitment of the adaptor protein Grb2. These results demonstrate that the Shank3 adaptor protein can mediate cellular signaling, and provide a molecular mechanism for the biological divergence between the Ret9 and Ret51 isoform.

Differential requirement of Tyr1062 multidocking site by RET isoforms to promote neural cell scattering and epithelial cell branching

The receptor tyrosine kinase RET is alternatively spliced to yield two main isoforms, RET9 and RET51, which differ in their carboxyl terminal. Activated RET induces different biological responses such as morphological transformation, neurite outgrowth, proliferation, cell migration and branching. The two isoforms have been suggested to have separate intracellular signaling pathways and different roles in mouse development. Here we show that both isoforms are able to induce cell scattering of SK-N-MC neuroepithelioma cell line and branching tubule formation in MDCK cell line. However, the Y1062F mutation, which abrogates the transforming activity of both activated RET isoforms in NIH3T3 cells, does not abolish scattering and branching morphogenesis of RET51, whereas impairs these biological effects of RET9. The GDNF-induced biological effects of RET51 are inhibited by the simultaneous abrogation of both Tyr1062 and Tyr1096 docking sites. Thus, Tyr1096 may substitute the functions of Tyr1062. GRB2 is the only known adaptor protein binding to Tyr1096. Dominant-negative GRB2 expressed in MDCK cells together with RET9 or RET51 significantly reduces branching. Therefore, GRB2 is necessary for RET-mediated branching of MDCK cells.

Mice expressing a dominant-negative Ret mutation phenocopy human Hirschsprung disease and delineate a direct role of Ret in spermatogenesis

The Ret receptor tyrosine kinase mediates physiological signals of glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) and is essential for postnatal survival in mice. It is implicated in a number of human diseases and developmental abnormalities. Here, we describe our analyses of mice expressing a Ret mutant (RetDN) with diminished kinase activity that inhibits wild-type Ret activity, including its activation of AKT. All RetDN/+ mice died by 1 month of age and had distal intestinal aganglionosis reminiscent of Hirschsprung disease (HSCR) in humans. The RetDN/+ proximal small intestine also had severe hypoganglionosis and reduction in nerve fiber density, suggesting a potential mechanism for the continued gastric dysmotility in postsurgical HSCR patients. Unlike Ret-null mice, which have abnormalities in the parasympathetic and sympathetic nervous systems, the RetDN/+ mice only had defects in the parasympathetic nervous system. A small proportion of RetDN/+ mice had renal agenesis, and the remainder had hypoplastic kidneys and developed tubulocystic abnormalities postnatally. Postnatal analyses of the testes revealed a decreased number of germ cells, degenerating seminiferous tubules,maturation arrest and apoptosis, indicating a crucial role for Ret in early spermatogenesis.

A targeting mutation of tyrosine 1062 in Ret causes a marked decrease of enteric neurons and renal hypoplasia

The Ret receptor tyrosine kinase plays a crucial role in the development of the enteric nervous system and the kidney. Tyrosine 1062 in Ret represents a binding site for the phosphotyrosine-binding domains of several adaptor and effector proteins that are important for the activation of intracellular signaling pathways, such as the RAS/ERK, phosphatidylinositol 3-kinase/AKT, and Jun-associated N-terminal kinase pathways. To investigate the importance of tyrosine 1062 for organogenesis in vivo, knock-in mice in which tyrosine 1062 in Ret was replaced with phenylalanine were generated. Although homozygous knock-in mice were born normally, they died by day 27 after birth and showed growth retardation. The development of the enteric nervous system was severely impaired in homozygous mutant mice, about 40% of which lacked enteric neurons in the whole intestinal tract, as observed in Ret-deficient mice. The rest of the mutant mice developed enteric neurons in the intestine to various extents, although the size and number of ganglion cells were significantly reduced. Unlike Ret-deficient mice, a small kidney developed in all knock-in mice, accompanying a slight histological change. The reduction of kidney size was due to a decrease of ureteric bud branching during embryogenesis. Thus, these findings demonstrated that the signal via tyrosine 1062 plays an important role in histogenesis of the enteric nervous system and nephrogenesis.

Dok-6, a Novel p62 Dok family member, promotes Ret-mediated neurite outgrowth

Activation of Ret, the receptor-tyrosine kinase for the glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs), results in the recruitment and assembly of adaptor protein complexes that function to transduce signals downstream of the receptor. Here we identify Dok-6, a novel member of the Dok-4/5 subclass of the p62 Dok family of intracellular adaptor molecules, and characterize its interaction with Ret. Expression analysis reveals that Dok-6 is highly expressed in the developing central nervous system and is co-expressed with Ret in several locations, including sympathetic, sensory, and parasympathetic ganglia, as well as in the ureteric buds of the developing kidneys. Pull-down assays using the Dok-6 phosphotyrosine binding (PTB) domain and GDNF-activated Ret indicate that Dok-6 binds to the phosphorylated Ret Tyr(1062) residue. Moreover, ligand activation of Ret resulted in phosphorylation of tyrosine residue(s) located within the unique C terminus of Dok-6 predominantly through a Src-dependent mechanism, indicating that Dok-6 is a substrate of the Ret-Src signaling pathway. Interestingly, expression of Dok-6 potentiated GDNF-induced neurite outgrowth in GDNF family receptor alpha1 (GFRalpha1)-expressing Neuro2A cells that was dependent upon the C-terminal residues of Dok-6. Taken together, these data identify Dok-6 as a novel Dok-4/5-related adaptor molecule that may function in vivo to transduce signals that regulate Ret-mediated processes such as axonal projection.