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

The RET Receptor Is Linked to Stress Response Pathways

RET is a transmembrane receptor required for the development of neuroendocrine and urogenital cell types. Activation of RET has roles in cell growth, migration, or differentiation, yet little is known about the gene expression patterns through which these processes are mediated. We have generated cell lines stably expressing either the RET9 or RET51 protein isoforms and have used these to investigate RET-mediated gene expression patterns by cDNA microarray analyses. As seen for many oncogenes, we identified altered expression of genes associated generally with cell-cell or cell-substrate interactions and up-regulation of tumor-specific transcripts. We also saw increased expression of transcripts normally associated with neural crest or other RET-expressing cell types, suggesting these genes may lie downstream of RET activation in development. The most striking pattern of expression was up-regulation of stress response genes. We showed that RET expression significantly up-regulated the genes for heat shock protein (HSP) 70 family members, HSPA1A, HSPA1B, and HSPA1L. Other members of several HSP families and HSP70-interacting molecules that were associated with stress response protein complexes involved in protein maturation were also specifically up-regulated by RET, whereas those associated with the roles of HSP70 in protein degradation were down-regulated or unaffected. The major mechanism of stress response induction is activation of the heat shock transcription factor HSF1. We showed that RET expression leads to increased HSF1 activation, which correlates with increased expression of stress response genes. Together, our data suggest that RET may be directly responsible for expression of stress response proteins and the initiation of stress response.

Spatiotemporal regulation of endothelin receptor-B by SOX10 in neural crest-derived enteric neuron precursors

Hirschsprung disease (HSCR) is a multigenic, congenital disorder that affects 1 in 5,000 newborns and is characterized by the absence of neural crest-derived enteric ganglia in the colon. One of the primary genes affected in HSCR encodes the G protein-coupled endothelin receptor-B (EDNRB). The expression of Ednrb is required at a defined time period during the migration of the precursors of the enteric nervous system (ENS) into the colon. In this study, we describe a conserved spatiotemporal ENS enhancer of Ednrb. This 1-kb enhancer is activated as the ENS precursors approach the colon, and partial deletion of this enhancer at the endogenous Ednrb locus results in pigmented mice that die postnatally from megacolon. We identified binding sites for SOX10, an SRY-related transcription factor associated with HSCR, in the Ednrb ENS enhancer, and mutational analyses of these sites suggested that SOX10 may have multiple roles in regulating Ednrb in the ENS.

The influence of Hox genes and three intercellular signalling pathways on enteric neuromuscular development

Normal intestinal motility requires orderly development of the complex nerve plexuses and smooth muscular layers in the gut wall. Organization of these structures results, in part, from cell autonomous programmes directed by transcription factors, which orchestrate appropriate temporal and spatial expression of specific target genes. Hox proteins appear to function in combination to dictate regional codes that establish major structural landmarks in the gut such as sphincters and muscle layers. These codes are translated in part by intercellular signals, which allow populations of cells in the embryonic gut wall to alter the developmental fate of their neighbours. Some of the best characterized intercellular signalling pathways involved in enteric neurodevelopment are mediated by GDNF/GFRa1/RET, EDN3/ENDRB, and NETRINS/DCC. These signals affect enteric neural precursors as they colonize the gut, and perturbations of these molecules are associated with various types of intestinal neuropathology.

Neurturin signalling via GFRα2 is essential for innervation of glandular but not muscle targets of sacral parasympathetic ganglion neurons

Neurturin, a member of the glial cell-derived neurotrophic factor familys of ligands, is important for development of many cranial parasympathetic ganglion neurons. We have investigated the sacral component of the parasympathetic nervous system in mice with gene deletions for neurturin or its preferred receptor, GFRalpha2. Disruption of neurturin signalling decreased cholinergic VIP innervation to the mucosa of the reproductive organs, but not to the smooth muscle layers of these organs or to the urinary bladder. Thus, neurturin and its receptor are involved in parasympathetic innervation of a select group of pelvic visceral tissues. In contrast, noradrenergic innervation was not affected by the gene ablations. The epithelium of reproductive organs from knockout animals was atrophied, indicating that cholinergic innervation may be important for the maintenance of normal structure. Cholinergic neurons express GFRalpha2 on their terminals and somata, indicating they can respond to neurotrophic support, and their somata are smaller when neurturin signalling is disrupted. Colocalisation studies showed that many peripheral glia express GFRalpha2 although its role in these cells is yet to be determined. Our results indicate that neurturin, acting through GFRalpha2, is essential for parasympathetic innervation of the mucosae of reproductive organs, as well as for maintenance of a broader group of sacral parasympathetic neurons.

Neuron and glia generating progenitors of the mammalian enteric nervous system isolated from foetal and postnatal gut cultures

Cultures of dissociated foetal and postnatal mouse gut gave rise to neurosphere-like bodies, which contained large numbers of mature neurons and glial cells. In addition to differentiated cells, neurosphere-like bodies included proliferating progenitors which, when cultured at clonal densities, gave rise to colonies containing many of the neuronal subtypes and glial cells present in the mammalian enteric nervous system. These progenitors were also capable of colonising wild-type and aganglionic gut in organ culture and had the potential to generate differentiated progeny that localised within the intrinsic ganglionic plexus. Similar progenitors were also derived from the normoganglionic small intestine of mice with colonic aganglionosis. Our findings establish the feasibility of expanding and isolating early progenitors of the enteric nervous system based on their ability to form distinct neurogenic and gliogenic structures in culture. Furthermore, these experiments provide the rationale for the development of novel approaches to the treatment of congenital megacolon (Hirschsprung's disease) based on the colonisation of the aganglionic gut with progenitors derived from normoganglionic bowel segments.

Enteric nervous system progenitors are coordinately controlled by the G protein-coupled receptor EDNRB and the receptor tyrosine kinase RET

The enteric nervous system (ENS) in vertebrates is derived mainly from vagal neural crest cells that enter the foregut and colonize the entire wall of the gastrointestinal tract. Failure to completely colonize the gut results in the absence of enteric ganglia (Hirschsprung's disease). Two signaling systems mediated by RET and EDNRB have been identified as critical players in enteric neurogenesis. We demonstrate that interaction between these signaling pathways controls ENS development throughout the intestine. Activation of EDNRB specifically enhances the effect of RET signaling on the proliferation of uncommitted ENS progenitors. In addition, we reveal novel antagonistic roles of these pathways on the migration of ENS progenitors. Protein kinase A is a key component of the molecular mechanisms that integrate signaling by the two receptors. Our data provide strong evidence that the coordinate and balanced interaction between receptor tyrosine kinases and G protein-coupled receptors controls the development of the nervous system in mammals.

Highly recurrent RET mutations and novel mutations in genes of the receptor tyrosine kinase and endothelin receptor B pathways in Chinese patients with sporadic Hirschsprung disease

BACKGROUND

Hirschsprung disease (HSCR) is a congenital disorder characterized by an absence of ganglion cells in the nerve plexuses of the lower digestive tract. HSCR has a complex pattern of inheritance and is sometimes associated with mutations in genes of the receptor tyrosine kinase (RET) and endothelin receptor B (EDNRB) signaling pathways, which are crucial for development of the enteric nervous system.

METHODS

Using PCR amplification and direct sequencing, we screened for mutations and polymorphisms in the coding regions and intron/exon boundaries of the RET, GDNF, EDNRB, and EDN3 genes of 84 HSCR patients and 96 ethnically matched controls.

RESULTS

We identified 10 novel and 2 previously described mutations in RET, and 4 and 2 novel mutations in EDNRB and in EDN3, respectively. Potential disease-causing mutations were detected in 24% of the patients. The overall mutation rate was 41% in females and 19% in males (P = 0.06). RET mutations occurred in 19% of the patients. R114H in RET was the most prevalent mutation, representing 7% of the patients or 37% of the patients with RET mutations. To date, such a high frequency of a single mutation has never been reported in unrelated HSCR patients. Mutations in EDNRB, EDN3, and GDNF were found in four, two, and none of the patients, respectively. Two patients with mutations in genes of the EDNRB pathway also harbored a mutation in RET. Three novel and three reported polymorphisms were found in EDNRB, EDN3, and GDNF.

CONCLUSION

This study identifies additional HSCR disease-causing mutations, some peculiar to the Chinese population, and represents the first comprehensive genetic analysis of sporadic HSCR disease in Chinese.

Novel functions and signalling pathways for GDNF

Glial-cell-line-derived neurotrophic factor (GDNF) was originally identified as a survival factor for midbrain dopaminergic neurons. GDNF and related ligands, neurturin (NRTN), artemin (ARTN) and persephin (PSPN), maintain several neuronal populations in the central nervous systems, including midbrain dopamine neurons and motoneurons. In addition, GDNF, NRTN and ARTN support the survival and regulate the differentiation of many peripheral neurons, including sympathetic, parasympathetic, sensory and enteric neurons. GDNF has further critical roles outside the nervous system in the regulation of kidney morphogenesis and spermatogenesis. GDNF family ligands bind to specific GDNF family receptor alpha (GFRalpha) proteins, all of which form receptor complexes and signal through the RET receptor tyrosine kinase. The biology of GDNF signalling is much more complex than originally assumed. The neurotrophic effect of GDNF, except in motoneurons, requires the presence of transforming growth factor beta, which activates the transport of GFRalpha1 to the cell membrane. GDNF can also signal RET independently through GFR1alpha. Upon ligand binding, GDNF in complex with GFRalpha1 may interact with heparan sulphate glycosaminoglycans to activate the Met receptor tyrosine kinase through cytoplasmic Src-family kinases. GDNF family ligands also signal through the neural cell adhesion molecule NCAM. In cells lacking RET, GDNF binds with high affinity to the NCAM and GFRalpha1 complex, which activates Fyn and FAK.

HOXB5 expression is spatially and temporarily regulated in human embryonic gut during neural crest cell colonization and differentiation of enteric neuroblasts

HOX genes from paralogous groups 4 and 5 are particularly relevant to the gut neuromusculature development because these genes are expressed at the splanchnic mesoderm surrounding the gut diverticulum, and at the level of the neural tube from where the vagal neural crest cells (NCCs) originate. In this study, we examined the migration and differentiation of NCCs, and investigated the expression patterns of HOXB5 in human embryonic guts. Human embryos of gestational week-4 to -8.5 were studied. Vagal NCCs enter the esophagus, migrate, and colonize the entire gut in a rostrocaudal manner between week-4 and week-7. The migrating NCCs in gut express HOXB5. Two separate and discontinuous mesenchymal expression domains of HOXB5 were detected in the gut: the distal domain preceding the migratory NCCs; and the proximal domain overlapping with the NCCs. The two expression domains shift caudally in parallel with the rostrocaudal migration of NCCs between week-4 and week-5. Neuron and glia differentiation of NCCs are concomitant with HOXB5 down-regulation in NCCs and the mesenchyme. By week-7, myenteric plexuses have formed; HOXB5 expression is switched on in the plexuses. We found that (1) the migratory route of NCCs in human embryonic gut was similar to that in mice and chicks; and (2) the expression pattern of HOXB5 correlated with the migration and differentiation of NCCs, suggesting a regulatory role of HOXB5 in the development of NCCs.

Collecting duct-specific gene inactivation of alphaENaC in the mouse kidney does not impair sodium and potassium balance

Aldosterone controls the final sodium reabsorption and potassium secretion in the kidney by regulating the activity of the epithelial sodium channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN). ASDN consists of the last portion of the distal convoluted tubule (late DCT), the connecting tubule (CNT), and the collecting duct (CD) (i.e., the cortical CD [CCD] and the medullary CD [MCD]). It has been proposed that the control of sodium transport in the CCD is essential for achieving sodium and potassium balance. We have tested this hypothesis by inactivating the alpha subunit of ENaC in the CD but leaving ENaC expression in the late DCT and CNT intact. Under salt restriction or under aldosterone infusion, whole-cell voltage clamp of principal cells of CCD showed no detectable ENaC activity, whereas large amiloride-sensitive currents were observed in control littermates. The animals survive well and are able to maintain sodium and potassium balance, even when challenged by salt restriction, water deprivation, or potassium loading. We conclude that the expression of ENaC in the CD is not a prerequisite for achieving sodium and potassium balance in mice. This stresses the importance of more proximal nephron segments (late DCT/CNT) to achieve sodium and potassium balance.

Rescue of human RET gene expression by sodium butyrate: a novel powerful tool for molecular studies in Hirschsprung disease

BACKGROUND

The RET gene encodes a tyrosine kinase receptor involved in different human neurocristopathies, such as specific neuroendocrine tumours and Hirschsprung disease (HSCR). Gene expression is developmentally regulated and the RET transcript is undetectable in most adult cells, including lymphocytes. The impossibility of performing functional studies on RET mRNA has to date limited the detection and characterisation of an indefinite proportion of gene anomalies that cannot be identified by conventional DNA genomic screening in HSCR cases.

AIMS

Development of a protocol suitable to activate RET expression in RET negative cell lines and therefore to investigate directly RET mRNA, extending the conventional gene mutation analysis to detection of splicing anomalies and impaired expression of the RET gene.

METHODS

The effect of sodium butyrate (NaB), a histone deacetylase inhibitor, on rescuing RET expression was tested by one round of reverse transcription- polymerase chain reaction from total RNA of treated lymphoblasts from both HSCR patients and control individuals.

RESULTS

Analysis of RET expression was possible by NaB treatment of RET negative cells, such as lymphoblasts. This treatment allowed us to detect impaired RET expression as well as a splicing defect in two HSCR patients previously believed to be devoid of any gene abnormality.

CONCLUSIONS

The full application of the proposed protocol in most of the unexplained HSCR cases will allow us to establish the precise role of RET not only in causing but also in predisposing to HSCR pathogenesis.

Stromal progenitors are important for patterning epithelial and mesenchymal cell types in the embryonic kidney

Growth and expansion of the embryonic kidney is driven in large part by continuous branching morphogenesis and nephron induction that occurs in a restricted domain beneath the renal capsule called the nephrogenic zone. Here, new ureteric bud branches and nephron aggregates form surrounded by a layer of cortical stromal cell progenitors. The boundaries and inductive activities of the nephrogenic zone are maintained as the kidney grows. As new ureteric bud branches and nephrogenic aggregates form, older generations of ureteric bud branches, renal vesicles and stromal progenitors are displaced from the nephrogenic zone and undergo further differentiation surrounded by medullary stroma, a different population of stromal cells. Recent studies suggest that cortical and medullary stromal progenitors may be an important source of signals that maintain outer and inner zones of differentiation in the embryonic kidney, and regulate distinct events important for differentiation of nephrons and the collecting duct system.

Real time PCR quantification of GFRalpha-2 alternatively spliced isoforms in murine brain and peripheral tissues

The neurotrophic factor neurturin (NTN) is structurally related to the glial-derived neurotrophic factor (GDNF) and has been shown to prevent the degeneration of dopaminergic neurons both in vitro and in vivo. The preferred receptor for NTN is the GDNF family receptor alpha 2 (GFRalpha-2). To date, three protein-coding alternatively spliced GFRalpha-2 isoforms (GFRalpha-2a, GFRalpha-2b, GFRalpha-2c) have been identified in mammalian tissues. An accurate quantification of the expression levels is necessary when determining the contributions of these isoforms to NTN signaling in tissues. In this report, sequence independent real time RT-PCR is used to determine the expression levels of GFRalpha-2 isoforms at different developmental stages of the murine embryos, and in various adult tissues. In the adult murine brain, GFRalpha-2a was found to be the most abundant, GFRalpha-2c was slightly less and GFRalpha-2b was 10-fold lower. The testis did not appear to express significant levels of GFRalpha-2a, 2b or 2c, compared to the brain. A novel finding in this study is that in some tissues, including the adult brain, the expression levels of GFRalpha-2, as quantified by the amplification of the 3' sequences encoding the putative glycosyl-phosphatidylinositol anchor signal sequence, were significantly higher than the combined levels of GFRalpha-2a, GFRalpha-2b and GFRalpha-2c. This indicates the existence of yet to be identified forms of GFRalpha-2 in some tissues that may be of physiological significance.

Cell signalling and gene expression mediated by RET tyrosine kinase

Germline mutations of the RET proto-oncogene cause multiple endocrine neoplasia (MEN) 2A or 2B by different mechanisms. As is the case for other receptor tyrosine kinases, mutant RET recruits a variety of signalling molecules via phosphorylated tyrosine residues present in the kinase domain and carboxy-terminal tail. As we previously reported, the signaling via phosphorylated tyrosine 1062 plays a crucial role in the transforming activities of both RET-MEN2A and RET-MEN2B mutant protein. Interestingly, this single tyrosine residue represents a binding site for several signalling molecules including SHC, Enigma, SNT/FRS2, DOK and IRS1 and is responsible for activation of the RAS/ERK, PI3-K/AKT, JNK, p38MAPK and ERK5 signalling pathways. Amongst these, the PI3-K/AKT and JNK pathways appeared to be more strongly activated in the cells expressing RET-MEN2B than in the cells expressing RET-MEN2A, suggesting the possibility that these pathways may be involved in the disease phenotype. In addition, RET is alternatively spliced to produce three isoforms and the splicing site is present just downstream of tyrosine 1062. These isoforms play different roles for the tumour development associated with MEN 2 or the development of the kidney and the enteric nervous system. Moreover, using differential display analysis, we identified several genes whose expression is highly induced by RET-MEN2B mutant proteins. The differential gene expression by RET-MEN2A and RET-MEN2B may also be important for the development of their phenotypes.

3' Splicing variants of ret receptor tyrosine kinase are differentially expressed in mouse embryos and in adult mice

The RET protooncogene encodes for a transmembrane receptor tyrosine kinase and plays a crucial role in nephrogenesis and the enteric nervous system (ENS) development. Alternative splicing at the 3' end of the RET gene generates 3' splicing variants that encode RET 9, RET 51 and RET 43 isoforms. It has been hypothesized that these isoforms perform distinct functions and that their expressions are differentially regulated during mammalian development. To gain an insight into the expression patterns of various ret isoforms during embryogenesis, we investigate the temporal and spatial expressions of ret gene in mouse embryos and in adult mice. We characterized the 3' end of the mouse ret gene and localized the alternatively spliced exons. Using 3' rapid amplification of cDNA ends (3' RACE) and reverse transcription-polymerase chain reaction (RT-PCR), ret 9 and ret 51 transcripts were identified in both mouse embryos and adult mouse tissues. However, the ret 43 transcript was not. Using in situ hybridization, we showed that ret 9 was the dominant ret encoding transcript in mouse embryos. Transcripts of ret 9 were detected in all cranial ganglia; in the sensory and autonomic ganglia of the trunk; in a subset of neurons of the dorsal root ganglion (DRG); in the motor neurons of the spinal cord; in the developing lung and excretory systems; in the enteric neuroblasts of the ENS; and in the thyroid lobes. In contrast, ret 51 expression was weak and restricted to the motor column of the spinal cord, the DRG, the enteric neuroblasts, the lung bud and the kidney. In adult mice, ret 9 expression was relatively widespread in many organs while that of ret 51 was rather restricted. Our data indicated that ret isoforms are temporally and spatially regulated in mouse embryos and adult mouse.

RET and NTRK1 proto-oncogenes in human diseases

RET and NTRK1 are receptor tyrosine kinase (RTK) proteins which play a role in the development and maturation of specific component of the nervous system. Their alterations have been associated to several human diseases, including some forms of cancer and developmental abnormalities. These features have contributed to the concept that one gene can be responsible for more than one disease. Moreover, both genes encoding for the two RTKs show genetic alterations that belong to either "gain of function" or "loss of function" class of mutations. In fact, receptor rearrangements or point mutations convert RET and NTRK1 in dominantly acting transforming genes leading to thyroid tumors, whereas inactivating mutations, associated with Hirschsprung's disease (HSCR) and congenital insensitivity to pain with anhidrosis (CIPA), impair RET and NTRK1 functions, respectively. In this review we have summarized the main features of the two receptors, their physiological and pathological roles. In addition, we attempted to identify the correlations between the different genetic alterations and the related pathogenetic mechanisms.

Requirement of signalling by receptor tyrosine kinase RET for the directed migration of enteric nervous system progenitor cells during mammalian embryogenesis

The majority of neurones and glia of the enteric nervous system (ENS) are derived from the vagal neural crest. Shortly after emigration from the neural tube, ENS progenitors invade the anterior foregut and, migrating in a rostrocaudal direction, colonise in an orderly fashion the rest of the foregut, the midgut and the hindgut. We provide evidence that activation of the receptor tyrosine kinase RET by glial cell line-derived neurotrophic factor (GDNF) is required for the directional migration of ENS progenitors towards and within the gut wall. We find that neural crest-derived cells present within foetal small intestine explants migrate towards an exogenous source of GDNF in a RET-dependent fashion. Consistent with an in vivo role of GDNF in the migration of ENS progenitors, we demonstrate that Gdnf is expressed at high levels in the gut of mouse embryos in a spatially and temporally regulated manner. Thus, during invasion of the foregut by vagal-derived neural crest cells, expression of Gdnf was restricted to the mesenchyme of the stomach, ahead of the invading NC cells. Twenty-four hours later and as the ENS progenitors were colonising the midgut,Gdnf expression was upregulated in a more posterior region —the caecum anlage. In further support of a role of endogenous GDNF in enteric neural crest cell migration, we find that in explant cultures GDNF produced by caecum is sufficient to attract NC cells residing in more anterior gut segments. In addition, two independently generated loss-of-function alleles of murine Ret, Ret.k— and miRet51, result in characteristic defects of neural crest cell migration within the developing gut. Finally, we identify phosphatidylinositol-3 kinase and the mitogen-activated protein kinase signalling pathways as playing crucial roles in the migratory response of enteric neural crest cells to GDNF.

A rare haplotype of the RET proto-oncogene is a risk-modifying allele in hirschsprung disease

Hirschsprung disease (HSCR) is a common genetic disorder characterized by intestinal obstruction secondary to enteric aganglionosis. HSCR demonstrates a complex pattern of inheritance, with the RET proto-oncogene acting as a major gene and with several additional susceptibility loci related to the Ret-signaling pathway or to other developmental programs of neural crest cells. To test how the HSCR phenotype may be affected by the presence of genetic variants, we investigated the role of a single-nucleotide polymorphism (SNP), 2508C-->T (S836S), in exon 14 of the RET gene, characterized by low frequency among patients with HSCR and overrepresentation in individuals affected by sporadic medullary thyroid carcinoma. Typing of several different markers across the RET gene demonstrated that a whole conserved haplotype displayed anomalous distribution and nonrandom segregation in families with HSCR. We provide genetic evidence about a protective role of this low-penetrant haplotype in the pathogenesis of HSCR and demonstrate a possible functional effect linked to RET messenger RNA expression.

The long and short isoforms of Ret function as independent signaling complexes

Ret, the receptor tyrosine kinase for the glial cell line-derived neurotrophic factor family ligands (GFLs), is alternatively spliced to yield at least two isoforms, Ret9 and Ret51, which differ only in their C termini. To identify tyrosines in Ret that are autophosphorylation sites in neurons, we generated antibodies specific to phosphorylated Y905Ret, Y1015Ret, Y1062Ret, and Y1096Ret, all of which are autophosphorylated in cell lines. All four of these tyrosines in Ret became phosphorylated rapidly upon activation by GFLs in sympathetic neurons. These tyrosines remained phosphorylated in sympathetic neurons in the continued presence of GFLs, albeit at a lower level than immediately after GFL treatment. Comparison of GFL activation of Ret9 and Ret51 revealed that phosphorylation of Tyr(905) and Tyr(1062) was greater and more sustained in Ret9 as compared with Ret51. In contrast, Tyr(1015) was more highly phosphorylated over time in Ret51 than in Ret9. Surprisingly, Ret9 and Ret51 did not associate with each other in sympathetic neurons after glial cell line-derived neurotrophic factor stimulation, even though they share identical extracellular domains. Furthermore, the signaling complex associated with Ret9 was markedly different from the Ret51-associated signaling complex. Taken together, these data provide a biochemical basis for the dramatic functional differences between Ret9 and Ret 51 in vivo.

Differential interaction of Enigma protein with the two RET isoforms

The receptor tyrosine kinase RET, with a known role in embryonic development and in human pathologies, is alternatively spliced to yield at least two functional isoforms, which differ only in their carboxyl terminal. Enigma protein is a member of the PDZ-LIM family and is known to interact with the short isoform of RET/PTC2, a chimeric oncoprotein isolated from papillary thyroid carcinoma. Here, we show that Enigma also interacts in intact cells with the short isoform of RET-wt and of its pathologic mutants associated to MEN2 syndromes, RET-C634R and RET-M918T. In contrast, Enigma binds all the corresponding RET long isoforms very poorly and colocalizes with short but not long RET/PTC2 isoforms. The RET docking tyrosine for Enigma is the last but one before the divergence between the two isoforms and we demonstrated that short-isoform-specific amino acid residues +2 to +4 to this tyrosine are required for the interaction of RET/PTC2 with Enigma.

RET receptor tyrosine kinase isoforms in kidney function and disease

The RET proto-oncogene encodes two major isoforms, RET9 and RET51, which differ at the carboxyl-terminal. Loss-of-function mutations in RET result in gut aganglionosis while gain of function mutations result in cancer syndromes. From studies on transgenic mice, RET9 is important for early development of the kidney and the enteric nervous system. Little is known about the function of RET isoforms in later life. Here we report the expression of RET isoforms and its signalling complex, GDNF and GFRalpha1, in foetal and adult human kidneys. We found their expression in both the developing and the adult renal collecting system. We further show that only RET51 but not RET9 could promote the survival and tubulogenesis of mIMCD3 (mouse inner medullary collecting duct) cells in collagen gel. Our results agree with the hypothesis that RET51 signalling is related to differentiation events in later kidney organogenesis. In addition, it may also have a function in the adult kidney. We further extend our study by showing increased RET and GDNF expression in collecting duct cysts of polycystic kidney patients. This suggests that GDNF/RET signalling may contribute to proliferation of the collecting duct epithelium in an autocrine/paracrine manner.

Genetic insights into familial cancers-- update and recent discoveries

While the vast majority of cancers are believed to occur sporadically, most forms of cancer, both adult and paediatric, have a hereditary equivalent. In the case of adult malignancies, these include hereditary breast and ovarian cancer and syndromes such as the multiple endocrine neoplasias types 1 and 2 characterised by specific tumours of the endocrine gland system. In the case of paediatric malignancies, these include syndromes such as retinoblastoma and Wilms tumour. In a little over a single decade, we have seen a tremendous increase in the knowledge of the primary genetic basis of many of the familial cancer syndromes. The majority of familial syndromes are inherited as autosomal dominant traits including hereditary colon cancer and familial malignant melanoma, however, the genetics behind autosomal recessive disorders such as Bloom syndrome and Fanconi anaemia are also being elucidated. A third mode of inheritance less well understood in the setting of familial cancer is that of imprinting recently observed in a subset of families with inherited paraganglioma. In this review, we discuss 31 genes inherited in an autosomal dominant manner associated with 20 familial cancer syndromes. Genes inherited in an autosomal recessive manner linked to familial cancer syndromes are also discussed. The identification of genes associated with familial cancer syndromes has in some families enabled a 'molecular diagnosis' that complements clinical assessment and allows directed cancer surveillance for those individuals determined to be at-risk of disease.

Conservation of RET proto-oncogene splicing variants and implications for RET isoform function

The RET proto-oncogene encodes a receptor tyrosine kinase required for development of the kidney and neural crest-derived cell types. Alternative splicing of the 3' exons of human RET results in three protein isoforms with distinct C-termini: RET9, RET51, and RET43. These RET isoforms show differential binding to downstream adapter molecules, suggesting they may have distinct signaling functions. We have characterized Ret 3' sequences in mouse and investigated alternative splicing of this region. We found that the organization of Ret 3' sequences is very similar to human RET. The mouse locus also has alternatively spliced C-terminal coding regions, and the sequences corresponding to RET9 and RET51 are highly conserved in both position and sequence with the human locus. Further, we compared the predicted C-terminal amino acids of RET9 and RET51 in seven vertebrate species, and found that they are well conserved. We have identified sequence encoding a putative ret43 isoform in mouse, however the predicted amino acid sequence showed low homology to human RET43. Our data suggest that RET isoforms are evolutionarily highly conserved over a broad range of species, which may indicate that each isoform has a distinct role in normal RET function.

Coordinating early kidney development: lessons from gene targeting

The kidney is widely used to study the mechanisms of organogenesis. Its development involves fundamental processes, such as epithelial branching, induced morphogenesis and cytodifferentiation, which are common to the development of many other organs. Gene-targeting experiments have greatly improved our understanding of kidney development, and have revealed many important genes that regulate early kidney organogenesis, some of which have a role in inherited human kidney disorders. Although our understanding of how the kidney is assembled is still limited, these studies are beginning to provide insights into the genetic and cellular interactions that regulate early organogenesis.

The GDNF family: signalling, biological functions and therapeutic value

Members of the nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF) families comprising neurotrophins and GDNF-family ligands (GFLs), respectively are crucial for the development and maintenance of distinct sets of central and peripheral neurons. Knockout studies in the mouse have revealed that members of these two families might collaborate or act sequentially in a given neuron. Although neurotrophins and GFLs activate common intracellular signalling pathways through their receptor tyrosine kinases, several clear differences exist between these families of trophic factors.

NGF utilizes c-Ret via a novel GFL-independent, inter-RTK signaling mechanism to maintain the trophic status of mature sympathetic neurons

During postnatal development, sympathetic neurons lose their dependence upon NGF for survival but continue to require NGF for soma and process growth and for development of a mature neurotransmitter phenotype. Although c-Ret is expressed in sympathetic neurons during this period, its function in these transitional processes is unclear. The level of Ret phosphorylation markedly increased with postnatal age in SCG neurons in vitro and in vivo. Postnatal Ret phosphorylation did not require either GFLs or GFR(alpha) coreceptors. Instead, NGF promoted age-dependent Ret phosphorylation with delayed kinetics both in vitro and in vivo. Functionally, maximal NGF-dependent trophism of mature sympathetic neurons required Ret, but not GFR(alpha) coreceptors. Therefore, NGF promotes phosphorylation of the heterologous RTK Ret resulting in augmented growth, metabolism, and gene expression.