Anosmin-1 modulates fibroblast growth factor receptor 1 signaling in human gonadotropin-releasing hormone olfactory neuroblasts through a heparan sulfate-dependent mechanism

Mutations in fibroblast growth factor receptor 1 cause Kallmann syndrome with a wide spectrum of reproductive phenotypes

BACKGROUND

Kallmann's syndrome (KS) is a clinically and genetically heterogeneous disorder consisting of idiopathic hypogonadotropic hypogonadism (IHH) and anosmia. Mutations in KAL1 causing the X-linked form of KS have been identified in 10% of all KS patients and consistently result in a severe reproductive phenotype. KAL1 gene encodes for anosmin-1, a key protein involved in olfactory and GnRH neuronal migration through a putative interaction with FGFR1. Heterozygous mutations in the FGFR1 gene accompanied by a high frequency of cleft palate and other facial dysmorphisms were recently identified in 8% of a large KS cohort, yet the reproductive phenotype of KS patients harboring FGFR1 mutations has not been described.

RESULTS

One hundred and fifty probands with KS (130 males and 20 females) were studied to determine the frequency and distribution of FGFR1 mutations and their detailed reproductive phenotypes. Fifteen heterozygous mutations in unrelated probands were identified. Twelve missense mutations (p.R78C, p.V102I, p.D224H, p.G237D, p.R254Q, p.V273M, p.E274G, p.Y339C, p.S346C, p.I538V, p.G703S and p.G703R) were distributed among the first, second and third immunoglobulin-like domains (D1-D3), as well as the tyrosine kinase domain (TKD). The mutations Y339C and S346C are located in exon 8B and code for the isoform FGFR1c. Additionally, two nonsense mutations (p.T585X and p.R622X) were documented in the TKD of the protein. A wide spectrum of reproductive function was observed among KS probands including: (1) a severe phenotype demonstrated by microphallus, cryptorchidism, no pubertal development, undetectable serum gonadotropins and low serum testosterone (T) and inhibin B; (2) partial pubertal development; (3) the fertile eunuch variant of IHH with normal testicular size and active spermatogenesis with a reversal of HH after T therapy. In addition, we found an even wider spectrum of reproductive function within pedigrees carrying an FGFR1 mutation ranging from IHH to delayed puberty to normal reproductive function (anosmia only or asymptomatic carriers). These observations strongly suggest a role for other genes that modify the phenotype of FGFR1 mutations.

CONCLUSION

KS patients and family members carrying an FGFR1 mutation present a broad spectrum of pubertal development in contrast to the almost uniform severe clinical phenotype described in KS subjects with a KAL1 mutation. Additionally, this report implicates the isoform FGFR1c in the pathogenesis of KS.

N-glycosylation of fibroblast growth factor receptor 1 regulates ligand and heparan sulfate co-receptor binding

The regulation of cell function by fibroblast growth factors (FGF) occurs through a dual receptor system consisting of a receptor-tyrosine kinase, FGFR and the glycosaminoglycan heparan sulfate (HS). Mutations of some potential N-glycosylation sites in human fgfr lead to phenotypes characteristic of receptor overactivation. To establish how N-glycosylation may affect FGFR function, soluble- and membrane-bound recombinant receptors corresponding to the extracellular ligand binding domain of FGFR1-IIIc were produced in Chinese Hamster Ovary cells. Both forms of FGFR1-IIIc were observed to be heavily N-glycosylated and migrated on SDS-PAGE as a series of multiple bands between 50 and 75 kDa, whereas the deglycosylated receptors migrated at 32 kDa, corresponding to the expected molecular weight of the polypeptides. Optical biosensor and quartz crystal microbalance-dissipation binding assays show that the removal of the N-glycans from FGFR1-IIIc caused an increase in the binding of the receptor to FGF-2 and to heparin-derived oligosaccharides, a proxy for cellular HS. This effect is mediated by N-glycosylation reducing the association rate constant of the receptor for FGF-2 and heparin oligosaccharides. N-Glycans were analyzed by mass spectrometry, which demonstrates a predominance of bi- and tri-antennary core-fucosylated complex type structures carrying one, two, and/or three sialic acids. Modeling of such glycan structures on the receptor protein suggests that at least some may be strategically positioned to interfere with interactions of the receptor with FGF ligand and/or the HS co-receptor. Thus, the N-glycans of the receptor represent an additional pathway for the regulation of the activity of FGFs.

C. elegans Kallmann syndrome protein KAL-1 interacts with syndecan and glypican to regulate neuronal cell migrations

The anosmin-1 protein family regulates cell migration, axon guidance, and branching, by mechanisms that are not well understood. We show that the C. elegans anosmin-1 ortholog KAL-1 promotes migrations of ventral neuroblasts prior to epidermal enclosure. KAL-1 does not modulate FGF signaling in neuroblast migration and acts in parallel to other neuroblast migration pathways. Defects in heparan sulfate (HS) synthesis or in specific HS modifications disrupt neuroblast migrations and affect the KAL-1 pathway. KAL-1 binds the cell surface HS proteoglycans syndecan/SDN-1 and glypican/GPN-1. This interaction is mediated via HS side chains and requires specific HS modifications. SDN-1 and GPN-1 are expressed in ventral neuroblasts and have redundant roles in KAL-1-dependent neuroblast migrations. Our findings suggest that KAL-1 interacts with multiple HSPGs to promote cell migration.

Mechanisms of Disease: insights into X-linked and autosomal-dominant Kallmann syndrome

Kallmann syndrome (KS) is a disorder characterized by hypogonadotropic hypogonadism and anosmia. Although KS is genetically heterogeneous, only two causal genes have been identified to date. These include an X-linked gene that encodes anosmin 1 and an autosomal gene that encodes fibroblast growth factor receptor 1. Mutations in these two genes result in disorders that often include, but are not limited to, severe defects in olfactory and reproductive functions. In this respect, KS can be regarded as a 'human model' for understanding critical factors that regulate olfactory and reproductive development. Here we give an overview of the disorders that stem from mutations in these two genes, with special emphasis on the cellular mechanisms underlying olfactory and reproductive anomalies. Other, less well-known aspects of KS, such as the convergence of symptoms in patients with different genetic forms of KS and the unpredictable manifestation of KS symptoms, are also discussed.

Minireview: recent progress in gonadotropin-releasing hormone neuronal migration

Neurons that synthesize GnRH are critical brain regulators of the reproductive axis, yet they originate outside the brain and must migrate over long distances and varied environments to get to their appropriate positions during development. Many studies, past and present, are providing clues for the types of molecules encountered and movements expected along the migratory route. Recent studies provide real-time views of the behavior of GnRH neurons in the context of in vitro preparations that model those in vivo. Live images provide direct evidence of the changing behavior of GnRH neurons in their different environments, showing that GnRH neurons move with greater frequency and with more alterations in direction after they enter the brain. The heterogeneity of molecular phenotypes for GnRH neurons likely ensures that multiple external factors will be found that regulate the migration of different portions of the GnRH neuronal population at different steps along the route. Molecules distributed in gradients both in the peripheral olfactory system and basal forebrain may be particularly influential in directing the appropriate movement of GnRH neurons along their arduous migration. Molecules that mediate the adhesion of GnRH neurons to changing surfaces may also play critical roles. It is likely that the multiple external factors converge on selective signal transduction pathways to engage the mechanical mechanisms needed to modulate GnRH neuronal movement and ultimately migration.

Role of endothelin-1 in the migration of human olfactory gonadotropin-releasing hormone-secreting neuroblasts

FNC-B4 neuroblasts that express both neuronal and olfactory markers have been established and cloned. These cells express GnRH and both the endothelin-1 (ET-1) gene and protein and respond in a migratory manner to GnRH in a dose-dependent manner. Previous research has shown that FNC-B4 cells produce and respond to ET-1 by regulating the secretion of GnRH through endothelin type A receptors and by stimulating their proliferation through endothelin type B (ETB) receptors. In this study, we found that FNC-B4 cells are able to migrate in response to ET-1 through the involvement of ETB receptors. Combined immunohistochemical and biochemical analyses showed that ET-1 triggered actin cytoskeletal remodeling and a dose-dependent increase in migration (up to 6-fold). Whereas the ETB receptor antagonist (B-BQ788) blunted the ET-1-induced effects, the ETA receptor antagonist (A-BQ123) did not. Moreover, we observed that FNC-B4 cells were independently and selectively stimulated by ET-1 and GnRH. We suggest that ET-1, through ETB receptor activation, may be required to maintain an adequate proliferative stem cell pool in the developing olfactory epithelium and the subsequent commitment to GnRH neuronal migratory pattern. The coordinate interaction between ET receptors and GnRH receptor participates in the fully expressed GnRH-secreting neuron phenotype.

MRC-Wellcome Trust Human Developmental Biology Resource: enabling studies of human developmental gene expression

A striking finding of the human and mouse genome sequencing projects is that, although there are many differences between the two species, they have similar numbers of genes. The differences arise during development and are driven, in part, by changes in gene expression. The MRC-Wellcome Trust Human Developmental Biology Resource (HDBR) is a unique resource that provides human embryonic and foetal tissues to the scientific community, enabling gene-expression studies at these crucial periods of development.

Extended and Flexible Domain Solution Structure of the Extracellular Matrix Protein Anosmin-1 by X-ray Scattering, Analytical Ultracentrifugation and Constrained Modelling

Kallmann's syndrome corresponds to a loss of sense of smell and hypogonadotrophic hypogonadism. Defects in anosmin-1 result in the X-linked inherited form of Kallmann's syndrome. Anosmin-1 is an extracellular matrix protein comprised of an N-terminal, cysteine-rich (Cys-box) domain and a whey acidic protein-like (WAP) domain, followed by four fibronectin type III (FnIII) domains. The solution structures of recombinant proteins containing the first three domains (PIWF1) and all six domains (PIWF4) were determined by X-ray scattering and analytical ultracentrifugation. Guinier analyses showed that PIWF1 and PIWF4 have different radii of gyration (R(G)) values of 3.1 nm and 6.7 nm, respectively, but similar cross-sectional radii of gyration (R(XS)) values of 1.5 nm and 1.9 nm, respectively. Distance distribution functions showed that the maximum lengths of PIWF1 and PIWF4 were 11 nm and 23 nm, respectively. Analytical ultracentrifugation gave sedimentation coefficients of 2.52 S and 3.55 S for PIWF1 and PIWF4, respectively. The interpretation of the scattering data by constrained modelling requires homology models for all six domains in anosmin-1. While models were already available for the WAP and FnIII domains, searches suggested the Cys-box domain may resemble the cysteine-rich region of the insulin-like growth factor receptor. Automated constrained molecular modelling based on joining the anosmin-1 domains with structurally randomised linkers resulted in 10,000 models for anosmin-1. A trial-and-error search showed that about 0.1-1.4% of these models fitted the X-ray data. The best models showed that the three domains and six domains in PIWF1 and PIWF4, respectively, were extended. The inter-domain linkers in anosmin-1 could not all be extended at the same time, and there was evidence for inter-domain flexibility. Models with folded-back domain arrangements do not fit the data. These solution structures account for the known biological function of anosmin-1, in particular its ability to interact with its three macromolecular ligands.

Reversible kallmann syndrome, delayed puberty, and isolated anosmia occurring in a single family with a mutation in the fibroblast growth factor receptor 1 gene

Kallmann syndrome (KS) is a clinically and genetically heterogeneous disorder. Recently, loss-of-function mutations in the fibroblast growth factor receptor 1 (FGFR1) gene have been shown to cause autosomal dominant KS. To date, the detailed reproductive phenotype of KS associated with mutations in the FGFR1 has yet to be described. We report a kindred comprising a male proband with KS and spontaneous reversibility, whose mother had delayed puberty and whose maternal grandfather isolated anosmia. The proband presented at age 18 yr with KS and was subsequently treated with testosterone (T) therapy. Upon discontinuation of T therapy, he recovered from his hypogonadotropic hypogonadism, as evidenced by a normal LH secretion pattern, sustained normal serum T levels, and active spermatogenesis. The three members of this single family harbor the same FGFR1 mutation (Arg(622)X) in the tyrosine kinase domain. This report demonstrates 1) the first genetic cause of the rare variant of reversible KS, 2) the reversal of hypogonadotropic hypogonadism in a proband carrying an FGFR1 mutation suggests a role of FGFR1 beyond embryonic GnRH neuron migration, and 3) a loss of function mutation in the FGFR1 gene causing delayed puberty.