An Fgf8 mutant allelic series generated by Cre- and Flp-mediated recombination

Versatile roles of V-ATPases accessory subunit Ac45 in osteoclast formation and function

Vacuolar-type H(+)-ATPases (V-ATPases) are macromolecular proton pumps that acidify intracellular cargos and deliver protons across the plasma membrane of a variety of specialized cells, including bone-resorbing osteoclasts. Extracellular acidification is crucial for osteoclastic bone resorption, a process that initiates the dissolution of mineralized bone matrix. While the importance of V-ATPases in osteoclastic resorptive function is well-defined, whether V-ATPases facilitate additional aspects of osteoclast function and/or formation remains largely obscure. Here we report that the V-ATPase accessory subunit Ac45 participates in both osteoclast formation and function. Using a siRNA-based approach, we show that targeted suppression of Ac45 impairs intracellular acidification and endocytosis, both are prerequisite for osteoclastic bone resorptive function in vitro. Interestingly, we find that knockdown of Ac45 also attenuates osteoclastogenesis owing to a reduced fusion capacity of osteoclastic precursor cells. Finally, in an effort to gain more detailed insights into the functional role of Ac45 in osteoclasts, we attempted to generate osteoclast-specific Ac45 conditional knockout mice using a Cathepsin K-Cre-LoxP system. Surprisingly, however, insertion of the neomycin cassette in the Ac45-Flox(Neo) mice resulted in marked disturbances in CNS development and ensuing embryonic lethality thus precluding functional assessment of Ac45 in osteoclasts and peripheral bone tissues. Based on these unexpected findings we propose that, in addition to its canonical function in V-ATPase-mediated acidification, Ac45 plays versatile roles during osteoclast formation and function.

Aspectos clínicos e moleculares do hipogonadismo hipogonadotrófico isolado congênito

O hipogonadismo hipogonadotrófico isolado (HHI) congênito caracteriza-se pela falta completa ou parcial de desenvolvimento puberal em decorrência de defeitos na migração, síntese, secreção ou ação do hormônio liberador de gonadotrofinas (GnRH). Baixas concentrações de esteroides sexuais e valores reduzidos ou inapropriadamente normais de gonadotrofinas hipofisárias (LH e FSH) definem, do ponto de vista laboratorial, essa condição clínica. A secreção dos demais hormônios hipofisários encontra-se normal, bem como a ressonância magnética de região hipotalâmica-hipofisária, demonstrando a ausência de uma causa anatômica. Alterações olfatórias, como anosmia ou hiposmia, podem estar associadas ao HHI, caracterizando a síndrome de Kallmann. Uma lista crescente de genes está envolvida na etiologia do HHI, sugerindo a heterogeneidade e a complexidade da base genética dessa condição. Distúrbios na rota de migração dos neurônios secretores de GnRH e dos neurônios olfatórios formam a base clínico-patológica da síndrome de Kallmann. Mutações nos genes KAL1, FGFR1/FGF8, PROK2/PROKR2, NELF, CHD7, HS6ST1 e WDR11 foram associadas a defeitos de migração neuronal, causando a síndrome de Kallmann. É notável que defeitos nos genes FGFR1, FGF8, PROKR2, CHD7 e WDR11 foram também associados ao HHI sem alterações olfatórias (HHI normósmico), porém em menor frequência. Adicionalmente, defeitos nos KISS1R, TAC3/TACR3 e GNRH1/GNRHR foram descritos exclusivamente em pacientes com HHI normósmico. Neste trabalho, revisaremos as características clínicas, hormonais e genéticas do HHI.

Characterization of two mutations, M287L and Q266I, in the α1 glycine receptor subunit that modify sensitivity to alcohols

Glycine receptors (GlyRs) are inhibitory ligand-gated ion channels. Ethanol potentiates glycine activation of the GlyR, and putative binding sites for alcohol are located in the transmembrane (TM) domains between and within subunits. To alter alcohol sensitivity of GlyR, we introduced two mutations in the GlyR α1 subunit, M287L (TM3) and Q266I (TM2). After expression in Xenopus laevis oocytes, both mutants showed a reduction in glycine sensitivity and glycine-induced maximal currents. Activation by taurine, another endogenous agonist, was almost abolished in the M287L GlyR. The ethanol potentiation of glycine currents was reduced in the M287L GlyR and eliminated in Q266I. Physiological levels of zinc (100 nM) potentiate glycine responses in wild-type GlyR and also enhance the ethanol potentiation of glycine responses. Although zinc potentiation of glycine responses was unchanged in both mutants, zinc enhancement of ethanol potentiation of glycine responses was absent in M287L GlyRs. The Q266I mutation decreased conductance but increased mean open time (effects not seen in M287L). Two lines of knockin mice bearing these mutations were developed. Survival of homozygous knockin mice was impaired, probably as a consequence of impaired glycinergic transmission. Glycine showed a decreased capacity for displacing strychnine binding in heterozygous knockin mice. Electrophysiology in isolated neurons of brain stem showed decreased glycine-mediated currents and decreased ethanol potentiation in homozygous knockin mice. Molecular models of the wild-type and mutant GlyRs show a smaller water-filled cavity within the TM domains of the Q266I α1 subunit. The behavioral characterization of these knockin mice is presented in a companion article (J Pharmacol Exp Ther 340:317-329, 2012).

Role of fibroblast growth factor (FGF) signaling in the neuroendocrine control of human reproduction

Fibroblast growth factor (FGF) signaling is critical for a broad range of developmental processes. In 2003, Fibroblast growth factor receptor 1 (FGFR1) was discovered as a novel locus causing both forms of isolate GnRH Deficiency, Kallmann syndrome [KS with anosmia] and normosmic idiopathic hypogonadotropic hypogonadism [nIHH] eventually accounting for approximately 10% of gonadotropin-releasing hormone (GnRH) deficiency cases. Such cases are characterized by a broad spectrum of reproductive phenotypes from severe congenital forms of GnRH deficiency to reversal of HH. Additionally, the variable expressivity of both reproductive and non-reproductive phenotypes among patients and family members harboring the identical FGFR1 mutations has pointed to a more complex, oligogenic model for GnRH deficiency. Further, reversal of HH in patients carrying FGFR1 mutations suggests potential gene-environment interactions in human GnRH deficiency disorders.

Cell-based therapies for Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease worldwide, classically characterized by a triad of motor features: bradykinesia, rigidity and resting tremor. Neurodegeneration in PD critically involves the dopaminergic neurons of the substantia nigra pars compacta, which results in a severe reduction in dopamine levels in the dorsal striatum. However, the disease also exhibits extensive non-nigral pathology and as many non-motor as motor features. Nevertheless, owing to the relatively circumscribed nature of the nigrostriatal lesion in PD, dopaminergic cell transplantation has emerged as a potentially reparative therapy for the disease. Sources for such cells are varied and include the developing ventral mesencephalon, several autologous somatic cell types, embryonic stem cells and induced pluripotent stem cells. In this article, we review the origins of dopaminergic transplantation for PD and the emergent hunt for a suitable long-term source of transplantable dopaminergic neurons.

HESX1- and TCF3-mediated repression of Wnt/β-catenin targets is required for normal development of the anterior forebrain

The Wnt/β-catenin pathway plays an essential role during regionalisation of the vertebrate neural plate and its inhibition in the most anterior neural ectoderm is required for normal forebrain development. Hesx1 is a conserved vertebrate-specific transcription factor that is required for forebrain development in Xenopus, mice and humans. Mouse embryos deficient for Hesx1 exhibit a variable degree of forebrain defects, but the molecular mechanisms underlying these defects are not fully understood. Here, we show that injection of a hesx1 morpholino into a 'sensitised' zygotic headless (tcf3) mutant background leads to severe forebrain and eye defects, suggesting an interaction between Hesx1 and the Wnt pathway during zebrafish forebrain development. Consistent with a requirement for Wnt signalling repression, we highlight a synergistic gene dosage-dependent interaction between Hesx1 and Tcf3, a transcriptional repressor of Wnt target genes, to maintain anterior forebrain identity during mouse embryogenesis. In addition, we reveal that Tcf3 is essential within the neural ectoderm to maintain anterior character and that its interaction with Hesx1 ensures the repression of Wnt targets in the developing forebrain. By employing a conditional loss-of-function approach in mouse, we demonstrate that deletion of β-catenin, and concomitant reduction of Wnt signalling in the developing anterior forebrain of Hesx1-deficient embryos, leads to a significant rescue of the forebrain defects. Finally, transcriptional profiling of anterior forebrain precursors from mouse embryos expressing eGFP from the Hesx1 locus provides molecular evidence supporting a novel function of Hesx1 in mediating repression of Wnt/β-catenin target activation in the developing forebrain.

Cell surface heparan sulfate chains regulate local reception of FGF signaling in the mouse embryo

Heparan sulfate (HS) proteoglycans modulate the activity of multiple growth factors on the cell surface and extracellular matrix. However, it remains unclear how the HS chains control the movement and reception of growth factors into targeted receiving cells during mammalian morphogenetic processes. Here, we found that HS-deficient Ext2 null mutant mouse embryos fail to respond to fibroblast growth factor (FGF) signaling. Marker expression analyses revealed that cell surface-tethered HS chains are crucial for local retention of FGF4 and FGF8 ligands in the extraembryonic ectoderm. Fine chimeric studies with single-cell resolution and expression studies with specific inhibitors for HS movement demonstrated that proteolytic cleavage of HS chains can spread FGF signaling to adjacent cells within a short distance. Together, the results show that spatiotemporal expression of cell surface-tethered HS chains regulate the local reception of FGF-signaling activity during mammalian embryogenesis.

A conserved Pbx-Wnt-p63-Irf6 regulatory module controls face morphogenesis by promoting epithelial apoptosis

Morphogenesis of mammalian facial processes requires coordination of cellular proliferation, migration, and apoptosis to develop intricate features. Cleft lip and/or palate (CL/P), the most frequent human craniofacial birth defect, can be caused by perturbation of any of these programs. Mutations of WNT, P63, and IRF6 yield CL/P in humans and mice; however, how these genes are regulated remains elusive. We generated mouse lines lacking Pbx genes in cephalic ectoderm and demonstrated that they exhibit fully penetrant CL/P and perturbed Wnt signaling. We also characterized a midfacial regulatory element that Pbx proteins bind to control the expression of Wnt9b-Wnt3, which in turn regulates p63. Altogether, we establish a Pbx-dependent Wnt-p63-Irf6 regulatory module in midfacial ectoderm that is conserved within mammals. Dysregulation of this network leads to localized suppression of midfacial apoptosis and CL/P. Ectopic Wnt ectodermal expression in Pbx mutants rescues the clefting, opening avenues for tissue repair.

Novel FGF8 mutations associated with recessive holoprosencephaly, craniofacial defects, and hypothalamo-pituitary dysfunction

CONTEXT

Fibroblast growth factor (FGF) 8 is important for GnRH neuronal development with human mutations resulting in Kallmann syndrome. Murine data suggest a role for Fgf8 in hypothalamo-pituitary development; however, its role in the etiology of wider hypothalamo-pituitary dysfunction in humans is unknown.

OBJECTIVE

The objective of this study was to screen for FGF8 mutations in patients with septo-optic dysplasia (n = 374) or holoprosencephaly (HPE)/midline clefts (n = 47).

METHODS

FGF8 was analyzed by PCR and direct sequencing. Ethnically matched controls were then screened for mutated alleles (n = 480-686). Localization of Fgf8/FGF8 expression was analyzed by in situ hybridization in developing murine and human embryos. Finally, Fgf8 hypomorphic mice (Fgf8(loxPNeo/-)) were analyzed for the presence of forebrain and hypothalamo-pituitary defects.

RESULTS

A homozygous p.R189H mutation was identified in a female patient of consanguineous parentage with semilobar HPE, diabetes insipidus, and TSH and ACTH insufficiency. Second, a heterozygous p.Q216E mutation was identified in a female patient with an absent corpus callosum, hypoplastic optic nerves, and Moebius syndrome. FGF8 was expressed in the ventral diencephalon and anterior commissural plate but not in Rathke's pouch, strongly suggesting early onset hypothalamic and corpus callosal defects in these patients. This was consolidated by significantly reduced vasopressin and oxytocin staining neurons in the hypothalamus of Fgf8 hypomorphic mice compared with controls along with variable hypothalamo-pituitary defects and HPE.

CONCLUSION

We implicate FGF8 in the etiology of recessive HPE and potentially septo-optic dysplasia/Moebius syndrome for the first time to our knowledge. Furthermore, FGF8 is important for the development of the ventral diencephalon, hypothalamus, and pituitary.

Collecting duct-specific knockout of adenylyl cyclase type VI causes a urinary concentration defect in mice

Collecting duct (CD) adenylyl cyclase VI (AC6) has been implicated in arginine vasopressin (AVP)-stimulated renal water reabsorption. To evaluate the role of CD-derived AC6 in regulating water homeostasis, mice were generated with CD-specific knockout (KO) of AC6 using the Cre/loxP system. CD AC6 KO and controls were studied under normal water intake, chronically water loaded, or water deprived; all of these conditions were repeated in the presence of continuous administration of 1-desamino-8-d-arginine vasopressin (DDAVP). During normal water intake or after water deprivation, urine osmolality (U(osm)) was reduced in CD AC6 KO animals vs. controls. Similarly, U(osm) was decreased in CD AC6 KO mice vs. controls after water deprivation+DDAVP administration. Pair-fed (with controls) CD AC6 KO mice also had lower urine osmolality vs. controls. There were no detectable differences between KO and control animals in fluid intake or urine volume under any conditions. CD AC6 KO mice did not have altered plasma AVP levels vs. controls. AVP-stimulated cAMP accumulation was reduced in acutely isolated inner medullary CD (IMCD) from CD A6 KO vs. controls. Medullary aquaporin-2 (AQP2) protein expression was lower in CD AC6 KO mice vs. controls. There were no differences in urinary urea excretion or IMCD UT-A1 expression; however, IMCD UT-A3 expression was reduced in CD AC6 KO mice vs. controls. In summary, AC6 in the CD regulates renal water excretion, most likely through control of AVP-stimulated cAMP accumulation and AQP2.

Temporally controlled modulation of FGF/ERK signaling directs midbrain dopaminergic neural progenitor fate in mouse and human pluripotent stem cells

Effective induction of midbrain-specific dopamine (mDA) neurons from stem cells is fundamental for realizing their potential in biomedical applications relevant to Parkinson's disease. During early development, the Otx2-positive neural tissues are patterned anterior-posteriorly to form the forebrain and midbrain under the influence of extracellular signaling such as FGF and Wnt. In the mesencephalon, sonic hedgehog (Shh) specifies a ventral progenitor fate in the floor plate region that later gives rise to mDA neurons. In this study, we systematically investigated the temporal actions of FGF signaling in mDA neuron fate specification of mouse and human pluripotent stem cells and mouse induced pluripotent stem cells. We show that a brief blockade of FGF signaling on exit of the lineage-primed epiblast pluripotent state initiates an early induction of Lmx1a and Foxa2 in nascent neural progenitors. In addition to inducing ventral midbrain characteristics, the FGF signaling blockade during neural induction also directs a midbrain fate in the anterior-posterior axis by suppressing caudalization as well as forebrain induction, leading to the maintenance of midbrain Otx2. Following a period of endogenous FGF signaling, subsequent enhancement of FGF signaling by Fgf8, in combination with Shh, promotes mDA neurogenesis and restricts alternative fates. Thus, a stepwise control of FGF signaling during distinct stages of stem cell neural fate conversion is crucial for reliable and highly efficient production of functional, authentic midbrain-specific dopaminergic neurons. Importantly, we provide evidence that this novel, small-molecule-based strategy applies to both mouse and human pluripotent stem cells.

Pallial origin of basal forebrain cholinergic neurons in the nucleus basalis of Meynert and horizontal limb of the diagonal band nucleus

The majority of the cortical cholinergic innervation implicated in attention and memory originates in the nucleus basalis of Meynert and in the horizontal limb of the diagonal band nucleus of the basal prosencephalon. Functional alterations in this system give rise to neuropsychiatric disorders as well as to the cognitive alterations described in Parkinson and Alzheimer's diseases. Despite the functional importance of these basal forebrain cholinergic neurons very little is known about their origin and development. Previous studies suggest that they originate in the medial ganglionic eminence of the telencephalic subpallium; however, our results identified Tbr1-expressing, reelin-positive neurons migrating from the ventral pallium to the subpallium that differentiate into cholinergic neurons in the basal forebrain nuclei projecting to the cortex. Experiments with Tbr1 knockout mice, which lack ventropallial structures, confirmed the pallial origin of cholinergic neurons in Meynert and horizontal diagonal band nuclei. Also, we demonstrate that Fgf8 signaling in the telencephalic midline attracts these neurons from the pallium to follow a tangential migratory route towards the basal forebrain.

Functional interaction of fibroblast growth factor-8, bone morphogenetic protein and estrogen receptor in breast cancer cell proliferation

Estrogen is involved in the development and progression of breast cancer. Here we investigated the effect of fibroblast growth factor (FGF)-8 on breast cancer cell proliferation caused by estrogen using human breast cancer MCF-7 cells. MCF-7 cells express estrogen receptor (ER)α, ERβ, FGF receptors, and Smad signaling molecules. Estradiol stimulated MCF-7 cell proliferation in a concentration-responsive manner, whereas BSA-bound estradiol had a weak effect on MCF-7 cell mitosis compared with the effect of free estradiol. It is notable that estrogen-induced cell proliferation was enhanced in the presence of FGF-8 and that the combined effects were reversed in the presence of an FGF-receptor kinase inhibitor or an ER antagonist. It was also revealed that FGF-8 increased the expression levels of ERα, ERβ and aromatase mRNAs, while estradiol reduced the expression levels of ERs, aromatase and steroid sulfatase in MCF-7 cells. FGF-8-induced phosphorylation of FGF receptors was augmented by estradiol, which was reversed by an ER antagonist. FGF-8-induced activation of MAPKs and AKT signaling was also upregulated in the presence of estrogen. On the other hand, FGF-8 suppressed BMP-7 actions that are linked to mitotic inhibition by activating the cell cycle regulator cdc2. FGF-8 was revealed to inhibit BMP receptor actions including Id-1 promoter activity and Smad1/5/8 phosphorylation by suppressing expression of BMP type-II receptors and by increasing expression of inhibitory Smads. Collectively, the results indicate that FGF-8 acts to facilitate cell proliferation by upregulating endogenous estrogenic actions as well as by suppressing BMP receptor signaling in ER-expressing breast cancer cells.

An RNA interference phenotypic screen identifies a role for FGF signals in colon cancer progression

In tumor cells, stepwise oncogenic deregulation of signaling cascades induces alterations of cellular morphology and promotes the acquisition of malignant traits. Here, we identified a set of 21 genes, including FGF9, as determinants of tumor cell morphology by an RNA interference phenotypic screen in SW480 colon cancer cells. Using a panel of small molecular inhibitors, we subsequently established phenotypic effects, downstream signaling cascades, and associated gene expression signatures of FGF receptor signals. We found that inhibition of FGF signals induces epithelial cell adhesion and loss of motility in colon cancer cells. These effects are mediated via the mitogen-activated protein kinase (MAPK) and Rho GTPase cascades. In agreement with these findings, inhibition of the MEK1/2 or JNK cascades, but not of the PI3K-AKT signaling axis also induced epithelial cell morphology. Finally, we found that expression of FGF9 was strong in a subset of advanced colon cancers, and overexpression negatively correlated with patients' survival. Our functional and expression analyses suggest that FGF receptor signals can contribute to colon cancer progression.

Sprouty genes are essential for the normal development of epibranchial ganglia in the mouse embryo

Fibroblast growth factor (FGF) signalling has important roles in the development of the embryonic pharyngeal (branchial) arches, but its effects on innervation of the arches and associated structures have not been studied extensively. We investigated the consequences of deleting two receptor tyrosine kinase (RTK) antagonists of the Sprouty (Spry) gene family on the early development of the branchial nerves. The morphology of the facial, glossopharyngeal and vagus nerves are abnormal in Spry1−/−;Spry2−/− embryos. We identify specific defects in the epibranchial placodes and neural crest, which contribute sensory neurons and glia to these nerves. A dissection of the tissue-specific roles of these genes in branchial nerve development shows that Sprouty gene deletion in the pharyngeal epithelia can affect both placode formation and neural crest fate. However, epithelial-specific gene deletion only results in defects in the facial nerve and not the glossopharyngeal and vagus nerves, suggesting that the facial nerve is most sensitive to perturbations in RTK signalling. Reducing the Fgf8 gene dosage only partially rescued defects in the glossopharyngeal nerve and was not sufficient to rescue facial nerve defects, suggesting that FGF8 is functionally redundant with other RTK ligands during facial nerve development.

Dynamic lineage analysis of embryonic morphogenesis using transgenic quail and 4D multispectral imaging

We describe the development of transgenic quail that express various fluorescent proteins in targeted manners and their use as a model system that integrates advanced imaging approaches with conventional and emerging molecular genetics technologies. We also review the progression and complications of past fate mapping techniques that led us to generate transgenic quail, which permit dynamic imaging of amniote embryogenesis with unprecedented subcellular resolution.

Numerous isoforms of Fgf8 reflect its multiple roles in the developing brain

Soluble growth factors play an important role in the coordination and integration of cell proliferation, differentiation, fate determination, and morphogenesis during development of multicellular organisms. Fibroblast growth factors (FGFs) are a large family of polypeptide growth factors that are present in organisms ranging from nematodes to humans. RNA alternative splicing of FGFs and their receptors further enhances the complexity of this ligand-receptor system. The mouse Fgf8 gene produces eight splice variants, which encode isoform proteins with different N-termini and distinct receptor-binding affinity and biological activity. In this article, we review the roles of Fgf8 in vertebrate development and summarize the recent findings on the in vivo function of different Fgf8 splice variants. We propose that multiple Fgf8 isoform proteins act in concert to regulate the overall function of Fgf8 and account for the diverse and essential role of Fgf8 during vertebrate development.

The connexin 40 A96S mutation causes renin-dependent hypertension

Deletion of the gap-junction-forming protein connexin40 leads to renin-dependent hypertension in mice, but whether observed human variants in connexin40, such as A96S, promote hypertension is unknown. Here, we generated mice with the A96S variant in the mouse connexin40 gene. Although mice homozygous for the A96S mutations had normal expression patterns of connexin40 in the kidney, they were hypertensive, had sixfold higher plasma renin concentrations, and had 40% higher levels of renin mRNA than controls. Renin-expressing cells were aberrantly located outside the media layer of afferent arterioles, and increased renal perfusion pressure did not inhibit renin secretion from kidneys isolated from homozygous A96S mice. Treatment with a low-salt diet in combination with an ACE inhibitor increased renin mRNA levels, plasma renin concentrations, and the number of aberrantly localized renin-producing cells. Taken together, these findings suggest that the A96S mutation in connexin40 leads to renin-dependent hypertension in mice. Modulation of renin secretion by BP critically depends on functional connexin40; with the A96S mutation, the aberrant extravascular localization of renin-secreting cells in the kidney likely impairs the pressure-mediated inhibition of renin secretion.

RAS-converting enzyme 1-mediated endoproteolysis is required for trafficking of rod phosphodiesterase 6 to photoreceptor outer segments

Prenylation is the posttranslational modification of a carboxyl-terminal cysteine residue of proteins that terminate with a CAAX motif. Following prenylation, the last three amino acids are cleaved off by the endoprotease, RAS-converting enzyme 1 (RCE1), and the prenylcysteine residue is methylated. Although it is clear that prenylation increases membrane affinity of CAAX proteins, less is known about the importance of the postprenylation processing steps. RCE1 function has been studied in a variety of tissues but not in neuronal cells. To approach this issue, we generated mice lacking Rce1 in the retina. Retinal development proceeded normally in the absence of Rce1, but photoreceptor cells failed to respond to light and subsequently degenerated in a rapid fashion. In contrast, the inner nuclear and ganglion cell layers were unaffected. We found that the multimeric rod phosphodiesterase 6 (PDE6), a prenylated protein and RCE1 substrate, was unable to be transported to the outer segments in Rce1-deficient photoreceptor cells. PDE6 present in the inner segment of Rce1-deficient photoreceptor cells was assembled and functional. Synthesis and transport of transducin, and rhodopsin kinase 1 (GRK1), also prenylated substrates of RCE1, was unaffected by Rce1 deficiency. We conclude that RCE1 is essential for the intracellular trafficking of PDE6 and survival of photoreceptor cells.

Identification and characterization of an inhibitory fibroblast growth factor receptor 2 (FGFR2) molecule, up-regulated in an Apert Syndrome mouse model

AS (Apert syndrome) is a congenital disease composed of skeletal, visceral and neural abnormalities, caused by dominant-acting mutations in FGFR2 [FGF (fibroblast growth factor) receptor 2]. Multiple FGFR2 splice variants are generated through alternative splicing, including PTC (premature termination codon)-containing transcripts that are normally eliminated via the NMD (nonsense-mediated decay) pathway. We have discovered that a soluble truncated FGFR2 molecule encoded by a PTC-containing transcript is up-regulated and persists in tissues of an AS mouse model. We have termed this IIIa–TM as it arises from aberrant splicing of FGFR2 exon 7 (IIIa) into exon 10 [TM (transmembrane domain)]. IIIa–TM is glycosylated and can modulate the binding of FGF1 to FGFR2 molecules in BIAcore-binding assays. We also show that IIIa–TM can negatively regulate FGF signalling in vitro and in vivo. AS phenotypes are thought to result from gain-of-FGFR2 signalling, but our findings suggest that IIIa–TM can contribute to these through a loss-of-FGFR2 function mechanism. Moreover, our findings raise the interesting possibility that FGFR2 signalling may be a regulator of the NMD pathway.

Focusing forward genetics: a tripartite ENU screen for neurodevelopmental mutations in the mouse

The control of growth, patterning, and differentiation of the mammalian forebrain has a large genetic component, and many human disease loci associated with cortical malformations have been identified. To further understand the genes involved in controlling neural development, we have performed a forward genetic screen in the mouse (Mus musculus) using ENU mutagenesis. We report the results from our ENU screen in which we biased our ascertainment toward mutations affecting neurodevelopment. Our screen had three components: a careful morphological and histological examination of forebrain structure, the inclusion of a retinoic acid response element-lacZ reporter transgene to highlight patterning of the brain, and the use of a genetically sensitizing locus, Lis1/Pafah1b1, to predispose animals to neurodevelopmental defects. We recovered and mapped eight monogenic mutations, seven of which affect neurodevelopment. We have evidence for a causal gene in four of the eight mutations. We describe in detail two of these: a mutation in the planar cell polarity gene scribbled homolog (Drosophila) (Scrib) and a mutation in caspase-3 (Casp3). We find that refining ENU mutagenesis in these ways is an efficient experimental approach and that investigation of the developing mammalian nervous system using forward genetic experiments is highly productive.

Primary cilia control telencephalic patterning and morphogenesis via Gli3 proteolytic processing

Primary cilia have essential functions in vertebrate development and signaling. However, little is known about cilia function in brain morphogenesis, a process that is severely affected in human ciliopathies. Here, we study telencephalic morphogenesis in a mouse mutant for the ciliopathy gene Ftm (Rpgrip1l). We show that the olfactory bulbs are present in an ectopic location in the telencephalon of Ftm(-/-) fetuses and do not display morphological outgrowth at the end of gestation. Investigating the developmental origin of this defect, we have established that E12.5 Ftm(-/-) telencephalic neuroepithelial cells lack primary cilia. Moreover, in the anterior telencephalon, the subpallium is expanded at the expense of the pallium, a phenotype reminiscent of Gli3 mutants. This phenotype indeed correlates with a decreased production of the short form of the Gli3 protein. Introduction of a Gli3 mutant allele encoding the short form of Gli3 into Ftm mutants rescues both telencephalic patterning and olfactory bulb morphogenesis, despite the persistence of cilia defects. Together, our results show that olfactory bulb morphogenesis depends on primary cilia and that the essential role of cilia in this process is to produce processed Gli3R required for developmental patterning. Our analysis thus provides the first in vivo demonstration that primary cilia control a developmental process via production of the short, repressor form of Gli3. Moreover, our findings shed light on the developmental origin of olfactory bulb agenesis and of other brain morphogenetic defects found in human diseases affecting the primary cilium.

Pluripotent stem cell studies elucidate the underlying mechanisms of early embryonic development

Early embryonic development is a multi-step process that is intensively regulated by various signaling pathways. Because of the complexity of the embryo and the interactions between the germ layers, it is very difficult to fully understand how these signals regulate embryo patterning. Recently, pluripotent stem cell lines derived from different developmental stages have provided an in vitro system for investigating molecular mechanisms regulating cell fate decisions. In this review, we summarize the major functions of the BMP, FGF, Nodal and Wnt signaling pathways, which have well-established roles in vertebrate embryogenesis. Then, we highlight recent studies in pluripotent stem cells that have revealed the stage-specific roles of BMP，FGF and Nodal pathways during neural differentiation. These findings enhance our understanding of the stepwise regulation of embryo patterning by particular signaling pathways and provide new insight into the mechanisms underlying early embryonic development.

Targeting fibroblast-growth-factor-receptor-dependent signaling for cancer therapy

INTRODUCTION

Fibroblast growth factors (FGF) exert a combination of biological effects that contribute to four of the six essential hallmarks of cancer. It is no surprise that FGF-dependent signaling has increasingly moved to the center of cancer therapy research during the past decade. This is illustrated by the large number of publications focusing on various aspects of this theme that have been published in the past 5 years.

AREAS COVERED

Information from these sources as well as ongoing work from the authors' groups is used to outline the physiological functions of FGF signaling and to highlight how the high oncogenic effects of deregulated FGFs and FGFRs derive from their physiological functions. The biological effect of deregulated FGFR signaling in malignant diseases is described and the current state of therapeutic targeting of FGFR is summarized.

EXPERT OPINION

Strategies for targeting FGFR-signaling for cancer therapy are very promising, but need to be carefully developed based on the physiological roles of FGF signaling. Preventive measures may be necessary for protection from FGF-related side effects. Combined targeting of several receptor tyrosine kinases or combination with other therapies may be a useful way of avoiding or ameliorating side effects. FGF-related markers of prognosis and therapy response still need to be investigated.

Gbx2 and Fgf8 are sequentially required for formation of the midbrain-hindbrain compartment boundary

In vertebrates, the common expression border of two homeobox genes, Otx2 and Gbx2, demarcates the prospective midbrain-hindbrain border (MHB) in the neural plate at the end of gastrulation. The presence of a compartment boundary at the MHB has been demonstrated, but the mechanism and timing of its formation remain unclear. We show by genetic inducible fate mapping using a Gbx2(CreER) knock-in mouse line that descendants of Gbx2(+) cells as early as embryonic day (E) 7.5 do not cross the MHB. Without Gbx2, hindbrain-born cells abnormally populate the entire midbrain, demonstrating that Gbx2 is essential for specifying hindbrain fate. Gbx2(+) and Otx2(+) cells segregate from each other, suggesting that mutually exclusive expression of Otx2 and Gbx2 in midbrain and hindbrain progenitors is responsible for cell sorting in establishing the MHB. The MHB organizer gene Fgf8, which is expressed as a sharp transverse band immediately posterior to the lineage boundary at the MHB, is crucial in maintaining the lineage-restricted boundary after E7.5. Partial deletion of Fgf8 disrupts MHB lineage separation. Activation of FGF pathways has a cell-autonomous effect on cell sorting in midbrain progenitors. Therefore, Fgf8 from the MHB may signal the nearby mesencephalic cells to impart distinct cell surface characteristics or induce local cell-cell signaling, which consequently prevents cell movements across the MHB. Our findings reveal the distinct function of Gbx2 and Fgf8 in a stepwise process in the development of the compartment boundary at the MHB and that Fgf8, in addition to its organizer function, plays a crucial role in maintaining the lineage boundary at the MHB by restricting cell movement.

Morphological and functional midbrain phenotypes in Fibroblast Growth Factor 17 mutant mice detected by Mn-enhanced MRI

With increasing efforts to develop and utilize mouse models of a variety of neuro-developmental diseases, there is an urgent need for sensitive neuroimaging methods that enable in vivo analysis of subtle alterations in brain anatomy and function in mice. Previous studies have shown that the brains of Fibroblast Growth Factor 17 null mutants (Fgf17(-/-)) have anatomical abnormalities in the inferior colliculus (IC)-the auditory midbrain-and minor foliation defects in the cerebellum. In addition, changes in the expression domains of several cortical patterning genes were detected, without overt changes in forebrain morphology. Recently, it has also been reported that Fgf17(-/-) mutants have abnormal vocalization and social behaviors, phenotypes that could reflect molecular changes in the cortex and/or altered auditory processing / perception in these mice. We used manganese (Mn)-enhanced magnetic resonance imaging (MEMRI) to analyze the anatomical phenotype of Fgf17(-/-) mutants in more detail than achieved previously, detecting changes in IC, cerebellum, olfactory bulb, hypothalamus and frontal cortex. We also used MEMRI to characterize sound-evoked activity patterns, demonstrating a significant reduction of the active IC volume in Fgf17(-/-) mice. Furthermore, tone-specific (16- and 40-kHz) activity patterns in the IC of Fgf17(-/-) mice were observed to be largely overlapping, in contrast to the normal pattern, separated along the dorsal-ventral axis. These results demonstrate that Fgf17 plays important roles in both the anatomical and functional development of the auditory midbrain, and show the utility of MEMRI for in vivo analyses of mutant mice with subtle brain defects.

FGF4 and FGF8 comprise the wavefront activity that controls somitogenesis

Somites form along the embryonic axis by sequential segmentation from the presomitic mesoderm (PSM) and differentiate into the segmented vertebral column as well as other unsegmented tissues. Somites are thought to form via the intersection of two activities known as the clock and the wavefront. Previous work has suggested that fibroblast growth factor (FGF) activity may be the wavefront signal, which maintains the PSM in an undifferentiated state. However, it is unclear which (if any) of the FGFs expressed in the PSM comprise this activity, as removal of any one gene is insufficient to disrupt early somitogenesis. Here we show that when both Fgf4 and Fgf8 are deleted in the PSM, expression of most PSM genes is absent, including cycling genes, WNT pathway genes, and markers of undifferentiated PSM. Significantly, markers of nascent somite cell fate expand throughout the PSM, demonstrating the premature differentiation of this entire tissue, a highly unusual phenotype indicative of the loss of wavefront activity. When WNT signaling is restored in mutants, PSM progenitor markers are partially restored but premature differentiation of the PSM still occurs, demonstrating that FGF signaling operates independently of WNT signaling. This study provides genetic evidence that FGFs are the wavefront signal and identifies the specific FGF ligands that encode this activity. Furthermore, these data show that FGF action maintains WNT signaling, and that both signaling pathways are required in parallel to maintain PSM progenitor tissue.

CXCR4 and CXCR7 have distinct functions in regulating interneuron migration

CXCL12/CXCR4 signaling is critical for cortical interneuron migration and their final laminar distribution. No information is yet available on CXCR7, a newly defined CXCL12 receptor. Here we demonstrated that CXCR7 regulated interneuron migration autonomously, as well as nonautonomously through its expression in immature projection neurons. Migrating cortical interneurons coexpressed Cxcr4 and Cxcr7, and Cxcr7(-/-) and Cxcr4(-/-) mutants had similar defects in interneuron positioning. Ectopic CXCL12 expression and pharmacological blockade of CXCR4 in Cxcr7(-/-) mutants showed that both receptors were essential for responding to CXCL12 during interneuron migration. Furthermore, live imaging revealed that Cxcr4(-/-) and Cxcr7(-/-) mutants had opposite defects in interneuron motility and leading process morphology. In vivo inhibition of Gα(i/o) signaling in migrating interneurons phenocopied the interneuron lamination defects of Cxcr4(-/-) mutants. On the other hand, CXCL12 stimulation of CXCR7, but not CXCR4, promoted MAP kinase signaling. Thus, we suggest that CXCR4 and CXCR7 have distinct roles and signal transduction in regulating interneuron movement and laminar positioning.

A mouse M-opsin monochromat: retinal cone photoreceptors have increased M-opsin expression when S-opsin is knocked out

Mouse cone photoreceptors, like those of most mammals including humans, express cone opsins derived from two ancient families: S-opsin (gene Opn1sw) and M-opsin (gene Opn1mw). Most C57Bl/6 mouse cones co-express both opsins, but in dorso-ventral counter-gradients, with M-opsin dominant in the dorsal retina and S-opsin in the ventral retina, and S-opsin 4-fold greater overall. We created a mouse lacking S-opsin expression by the insertion of a Neomycin selection cassette between the third and fourth exons of the Opn1sw gene (Opn1sw(Neo/Neo)). In strong contrast to published results characterizing mice lacking rhodopsin (Rho⁻/⁻) in which retinal rods undergo cell death by 2.5 months, cones of the Opn1sw(Neo/Neo) mouse remain viable for at least 1.5 yrs, even though many ventral cones do not form outer segments, as revealed by high resolution immunohistochemistry and electron microscopy. Suction pipette recordings revealed that functional ventral cones of the Opn1sw(Neo/Neo) mouse not only phototransduce light with normal kinetics, but are more sensitive to mid-wavelength light than their WT counterparts. Quantitative Western blot analysis revealed the basis of the heightened sensitivity to be increased M-opsin expression. Because S- and M-opsin transcripts must compete for the same translational machinery in cones where they are co-expressed, elimination of S-opsin mRNA in ventral Opn1sw(Neo/Neo) cones likely increases M-opsin expression by relieving competition for translational machinery, revealing an important consequence of eliminating a dominant transcript. Overall, our results reveal a striking capacity for cone photoreceptors to function with much reduced opsin expression, and to remain viable in the absence of an outer segment.

Hypomorphic mice

The use of genetically engineered mice has become a standard approach in order to study the physiological contribution of genes in a variety of life-science disciplines. Classical and conditional gene-targeting methods aimed at generating knock-out mice that lack gene products have been useful, but may be limited in their scope. If the gene of interest is essential for cell viability, little insight can be gained into the in vivo function of these genes. A hypomorphic approach, utilizing many of the same methods employed for traditional gene targeting, allows one to disrupt the function of genes to a lesser degree and bypass the lethality caused by many gene mutations. The purpose of this chapter is to introduce the concepts behind how hypomorphic alleles impede normal genetic function and provide information necessary to construct a targeting vector successfully for use in ES cells to generate ultimately mice with lower than normal amounts of an endogenous protein of interest.

Fibroblast growth factor signaling in the developing neuroendocrine hypothalamus

Fibroblast growth factor (FGF) signaling is pivotal to the formation of numerous central regions. Increasing evidence suggests FGF signaling also directs the development of the neuroendocrine hypothalamus, a collection of neuroendocrine neurons originating primarily within the nose and the ventricular zone of the diencephalon. This review outlines evidence for a role of FGF signaling in the prenatal and postnatal development of several hypothalamic neuroendocrine systems. The emphasis is placed on the nasally derived gonadotropin-releasing hormone neurons, which depend on neurotrophic cues from FGF signaling throughout the neurons' lifetime. Although less is known about neuroendocrine neurons derived from the diencephalon, recent studies suggest they also exhibit variable levels of dependence on FGF signaling. Overall, FGF signaling provides a broad spectrum of cues that ranges from genesis, cell survival/death, migration, morphological changes, to hormone synthesis in the neuroendocrine hypothalamus. Abnormal FGF signaling will deleteriously impact multiple hypothalamic neuroendocrine systems, resulting in the disruption of diverse physiological functions.

Gonadotropin-releasing hormone (GnRH) neuron migration: initiation, maintenance and cessation as critical steps to ensure normal reproductive function

GnRH neurons follow a carefully orchestrated journey from their birth in the olfactory placode area. Initially, they migrate along with the vomeronasal nerve into the brain at the cribriform plate, then progress caudally to sites within the hypothalamus where they halt and send projections to the median eminence to activate pituitary gonadotropes. Many factors controlling this precise journey have been elucidated by the silencing or over-expression of candidate genes in mouse models. Importantly, a number of these factors may not only play a role in normal physiology of the hypothalamic-pituitary-gonadal axis but also be mis-expressed to cause human disorders of GnRH deficiency, presenting as a failure to undergo normal pubertal development. This review outlines the current cadre of candidates thought to modulate GnRH neuronal migration. The further elucidation and characterization of these factors that impact GnRH neuron development may shed new light on human reproductive disorders and provide potential targets to develop new pro-fertility or contraceptive agents.

Extending the family table: Insights from beyond vertebrates into the regulation of embryonic development by FGFs

Since the discovery of fibroblast growth factors (FGFs) much focus has been placed on elucidating the roles for each vertebrate FGF ligand, receptor, and regulating molecules in the context of vertebrate development, human disorders and cancer. Studies in human, mouse, frog, chick, and zebrafish have made great contributions to our understanding of the role of FGFs in specific processes. However, in recent years, as more genomes are sequenced, information is becoming available from many non-vertebrate models and a more complete picture of the FGF superfamily as a whole is emerging. In some cases, less redundancy in these FGF signaling systems may allow for more mechanistic insights. Studies in sea anemones have highlighted how ancient FGF signaling is and helped provide insight into the evolution of the FGF gene family. Work in nematodes has shown that different splice forms can be used for functional specificity in invertebrate FGF signaling. Comparing FGFs between urochordates and vertebrates as well as between different insect species reveals important clues into the process of gene loss, duplication and subfunctionalization of FGFs throughout evolution. Finally, comparing all members of the FGF ligand superfamily reveals variability in many properties, which may point to a feature of FGFs as being highly adaptable with regards to protein structure and signaling mechanism. Further studies on FGF signaling outside of vertebrates is likely to continue to complement work in vertebrates by contributing additional insights to the FGF field and providing unexpected information that could be used for medical applications.

Behavioral and cerebellar transmission deficits in mice lacking the autism-linked gene islet brain-2

Deletion of the human SHANK3 gene near the terminus of chromosome 22q is associated with Phelan-McDermid syndrome and autism spectrum disorders. Nearly all such deletions also span the tightly linked IB2 gene. We show here that IB2 protein is broadly expressed in the brain and is highly enriched within postsynaptic densities. Experimental disruption of the IB2 gene in mice reduces AMPA and enhances NMDA receptor-mediated glutamatergic transmission in cerebellum, changes the morphology of Purkinje cell dendritic arbors, and induces motor and cognitive deficits suggesting an autism phenotype. These findings support a role for human IB2 mutation as a contributing genetic factor in Chr22qter-associated cognitive disorders.

The mouse KRAB zinc-finger protein CHATO is required in embryonic-derived tissues to control yolk sac and placenta morphogenesis

Yolk sac and placenta are required to sustain embryonic development in mammals, yet our understanding of the genes and processes that control morphogenesis of these extraembryonic tissues is still limited. The chato mutation disrupts ZFP568, a Krüppel-Associated-Box (KRAB) domain Zinc finger protein, and causes a unique set of extraembryonic malformations, including ruffling of the yolk sac membrane, defective extraembryonic mesoderm morphogenesis and vasculogenesis, failure to close the ectoplacental cavity, and incomplete placental development. Phenotypic analysis of chato embryos indicated that ZFP568 does not control proliferation or differentiation of extraembryonic lineages but rather regulates the morphogenetic events that shape extraembryonic tissues. Analysis of chimeric embryos showed that Zfp568 function is required in embryonic-derived lineages, including the extraembryonic mesoderm. Depleting Zfp568 affects the ability of extraembryonic mesoderm cells to migrate. However, explanted Zfp568 mutant cells could migrate properly when plated on appropriate extracellular matrix conditions. We show that expression of Fibronectin and Indian Hedgehog are reduced in chato mutant yolk sacs. These data suggest that ZFP568 controls the production of secreted factors required to promote morphogenesis of extraembryonic tissues. Our results support previously undescribed roles of the extraembryonic mesoderm in yolk sac morphogenesis and in the closure of the ectoplacental cavity and identify a novel role of ZFP568 in the development of extraembryonic tissues.

FGF signalling: diverse roles during early vertebrate embryogenesis

Fibroblast growth factor (FGF) signalling has been implicated during several phases of early embryogenesis, including the patterning of the embryonic axes, the induction and/or maintenance of several cell lineages and the coordination of morphogenetic movements. Here, we summarise our current understanding of the regulation and roles of FGF signalling during early vertebrate development.

Molecular regulation of the developing commissural plate

Coordinated transfer of information between the brain hemispheres is essential for function and occurs via three axonal commissures in the telencephalon: the corpus callosum (CC), hippocampal commissure (HC), and anterior commissure (AC). Commissural malformations occur in over 50 human congenital syndromes causing mild to severe cognitive impairment. Disruption of multiple commissures in some syndromes suggests that common mechanisms may underpin their development. Diffusion tensor magnetic resonance imaging revealed that forebrain commissures crossed the midline in a highly specific manner within an oblique plane of tissue, referred to as the commissural plate. This specific anatomical positioning suggests that correct patterning of the commissural plate may influence forebrain commissure formation. No analysis of the molecular specification of the commissural plate has been performed in any species; therefore, we utilized specific transcription factor markers to delineate the commissural plate and identify its various subdomains. We found that the mouse commissural plate consists of four domains and tested the hypothesis that disruption of these domains might affect commissure formation. Disruption of the dorsal domains occurred in strains with commissural defects such as Emx2 and Nfia knockout mice but commissural plate patterning was normal in other acallosal strains such as Satb2(-/-). Finally, we demonstrate an essential role for the morphogen Fgf8 in establishing the commissural plate at later developmental stages. The results demonstrate that correct patterning of the commissural plate is an important mechanism in forebrain commissure formation.

The role of FGF/Erk signaling in pluripotent cells

Fibroblast growth factor (FGF) signaling controls fundamental processes such as proliferation, differentiation and migration throughout mammalian development. Here we discuss recent discoveries that implicate FGF/Erk signaling in the control of pluripotency and lineage specification in several different stem cell states, including the separation of pluripotent epiblast and primitive endoderm in the blastocyst, the lineage priming of embryonic stem (ES) cells, and in the stabilization of the metastable state of mouse epiblast and human ES cells. Understanding how extrinsic signals such as FGF regulate different stem cell states will be crucial to harvest the clinical promise of induced pluripotent and embryo-derived stem cells.

Neuronal connexin-36 can functionally replace connexin-45 in mouse retina but not in the developing heart

The gap junction protein connexin-45 (Cx45) is expressed in the conduction system of the heart and in certain neurons of the retina and brain. General and cardiomyocyte-directed deficiencies of Cx45 in mice lead to lethality on embryonic day 10.5 as a result of cardiovascular defects. Neuron-directed deletion of Cx45 leads to defects in transmission of visual signals. Connexin-36 (Cx36) is co-expressed with Cx45 in certain types of retinal interneurons. To determine whether these two connexins have similar functions and whether Cx36 can compensate for Cx45, we generated knock-in mice in which DNA encoding Cx45 was replaced with that encoding Cx36. Neuron-directed replacement of Cx45 with Cx36 resulted in viable animals. Electroretinographic and neurotransmitter coupling analyses demonstrated functional compensation in the retina. By contrast, general and cardiomyocyte-directed gene replacement led to lethality on embryonic day 11.5. Mutant embryos displayed defects in cardiac morphogenesis and conduction. Thus, functional compensation of Cx45 by Cx36 did not occur during embryonic heart development. These data suggest that Cx45 and Cx36 have similar functions in the retina, whereas Cx45 fulfills special functions in the developing heart that cannot be compensated by Cx36.

Unexpected requirement for ELMO1 in clearance of apoptotic germ cells in vivo

Apoptosis and the subsequent clearance of dying cells occurs throughout development and adult life in many tissues. Failure to promptly clear apoptotic cells has been linked to many diseases. ELMO1 is an evolutionarily conserved cytoplasmic engulfment protein that functions downstream of the phosphatidylserine receptor BAI1, and, along with DOCK1 and the GTPase RAC1, promotes internalization of the dying cells. Here we report the generation of ELMO1-deficient mice, which we found to be unexpectedly viable and grossly normal. However, they had a striking testicular pathology, with disrupted seminiferous epithelium, multinucleated giant cells, uncleared apoptotic germ cells and decreased sperm output. Subsequent in vitro and in vivo analyses revealed a crucial role for ELMO1 in the phagocytic clearance of apoptotic germ cells by Sertoli cells lining the seminiferous epithelium. The engulfment receptor BAI1 and RAC1 (upstream and downstream of ELMO1, respectively) were also important for Sertoli-cell-mediated engulfment. Collectively, these findings uncover a selective requirement for ELMO1 in Sertoli-cell-mediated removal of apoptotic germ cells and make a compelling case for a relationship between engulfment and tissue homeostasis in vivo.

Abnormal hypothalamic oxytocin system in fibroblast growth factor 8-deficient mice

Oxytocin (OT) is a nonapeptide essential for maternal care. The development of the OT neuroendocrine system is a multi-step process dependent on the action of many transcription factors, but upstream signaling molecules regulating this process are still poorly understood. In this study, we examined if fibroblast growth factor 8 (FGF8), a signaling molecule critical for forebrain development, is essential for the proper formation of the OT system. Using immunohistochemistry, we showed a significant reduction in the number of neurons immunoreactive for the mature OT peptide in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) in the hypothalamus of homozygous (HOMO) FGF8 hypomorphic mice compared to wild-type mice. The number of neurons positive for oxyphysin prohormone in the SON but not the PVN was also significantly reduced in FGF8 HOMO hypomorphs. However, steady-state mRNA levels of the oxyphysin prohormone were not significantly different between FGF8 hypomorphs and WT mice. These data suggest that a global reduction in FGF8 signaling leads to an overall reduction of mature OT and oxyphysin prohormone levels that may have resulted from defects in multiple stages of the hormone-synthesis pathway. Since proper hormone synthesis is a hallmark of mature OT neurons, this study suggests that FGF8 signaling may contribute to the phenotypic maturation of a neuroendocrine system that originates within the diencephalon.

Cdc20 is critical for meiosis I and fertility of female mice

Chromosome missegregation in germ cells is an important cause of unexplained infertility, miscarriages, and congenital birth defects in humans. However, the molecular defects that lead to production of aneuploid gametes are largely unknown. Cdc20, the activating subunit of the anaphase-promoting complex/cyclosome (APC/C), initiates sister-chromatid separation by ordering the destruction of two key anaphase inhibitors, cyclin B1 and securin, at the transition from metaphase to anaphase. The physiological significance and full repertoire of functions of mammalian Cdc20 are unclear at present, mainly because of the essential nature of this protein in cell cycle progression. To bypass this problem we generated hypomorphic mice that express low amounts of Cdc20. These mice are healthy and have a normal lifespan, but females produce either no or very few offspring, despite normal folliculogenesis and fertilization rates. When mated with wild-type males, hypomorphic females yield nearly normal numbers of fertilized eggs, but as these embryos develop, they become malformed and rarely reach the blastocyst stage. In exploring the underlying mechanism, we uncover that the vast majority of these embryos have abnormal chromosome numbers, primarily due to chromosome lagging and chromosome misalignment during meiosis I in the oocyte. Furthermore, cyclin B1, cyclin A2, and securin are inefficiently degraded in metaphase I; and anaphase I onset is markedly delayed. These results demonstrate that the physiologically effective threshold level of Cdc20 is high for female meiosis I and identify Cdc20 hypomorphism as a mechanism for chromosome missegregation and formation of aneuploid gametes.

Aneuploidy, an abnormal number of chromosomes, is a common defect in sperm and egg cells that is responsible for human infertility, miscarriage, and congenital birth defects. Although these developmental outcomes are prevalent in human reproduction, little is known about the molecular defects that may cause aneuploidy in germ cells. In this study, we identify Cdc20, a critical activator of the APC/C E3 ubiquitin ligase that initiates sister chromosome separation by ordering the destruction of cyclin B1 and securin, as a female infertility gene. We show that female mice with low amounts of Cdc20 have normal fitness but almost exclusively produce aneuploid embryos that fail to thrive and die early in development. The aneuploidy primarily results from chromosome segregation errors in primary oocytes that may be caused by inefficient APC/C-mediated destruction of mitotic cyclins and securin during metaphase I. Thus, our studies reveal that primary oocytes are highly dependent on Cdc20 for accurate chromosome segregation and raise the possibility that Cdc20 insufficiency may be a cause of infertility in otherwise healthy women.

Setdb1-mediated histone H3K9 hypermethylation in neurons worsens the neurological phenotype of Mecp2-deficient mice

Rett syndrome (RTT, OMIM # 312750), a neurodevelopmental disorder of early childhood, is primarily caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MECP2). Various molecular functions have been ascribed to MECP2, including the regulation of histone modifications associated with repressive chromatin remodeling, but the role of these mechanisms for the pathophysiology of RTT remains unclear. Here, we explore whether or not neuronal expression of the histone H3-lysine 9 specific methyl-transferase, Setdb1 (Set domain, bifurcated 1)/Eset/Kmt1e, which is normally present only at low levels in differentiated neurons, rescues the RTT-like phenotype of Mecp2-deficient mice. A myc-tagged Setdb1 cDNA was expressed through the tau locus for ubiquitous expression in CNS neurons, or under control of the calcium/calmodulin-dependent protein kinase II (CK) promoter to selectively target postmitotic neurons in forebrain. However, the CK-Setdb1 transgene lead to an enhanced neurological deficit, and the tauSetdb1 allele further shortened life span of mice with a brain-wide deletion of Mecp2 during prenatal development. In contrast, no neurological deficits or premature death was observed in CK-Setdb1 and tauSetdb1 mice expressing wildtype Mecp2. However, levels of trimethylated H3K9 at pericentromeric repeats were fully maintained in differentiated neurons from symptomatic Mecp2 null mutant mice. Based on these results, we draw two conclusions: First, neuronal chromatin in RTT brain is not affected by a generalized deficit in H3K9 trimethylation. Second, artificial up-regulation of this repressive chromatin mark via Setdb1 gene delivery specifically to neurons is harmful for the Mecp2-deficient brain. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

When Cre-mediated recombination in mice does not result in protein loss

Cre/loxP recombination enables cellular specificity and, in the case of inducible systems, temporal control of genomic deletions. Here we used a SM22α tamoxifen-inducible Cre line to inactivate β1 integrin in adult smooth muscle. Interestingly, analysis of two distinct β1 loxP transgenic mice revealed vastly different outcomes after β1 integrin deletion. Lethality occurred 4 weeks postinduction in one Cre/loxP line, while no apparent phenotype was seen in the other line. Genetic analysis revealed appropriate DNA excision in both cases; however, differences were found in the degree of protein loss with absolutely no change in protein levels in the model that lacked a phenotype. Seeking to understand protein persistence despite appropriate recombination, we first validated the flox allele using a constitutive Cre line and demonstrated its ability to mediate effective protein inactivation. We then examined the possibility of heterozygous cell selection, protein turnover, and deletion efficiency with no success for explaining the phenotype. Finally, we documented the presence of the Cre-recombination episomal product, which persisted in tissue samples with no protein loss. The product was only noted in cells with low proliferative capacity. These findings highlight the potential for protein expression from the products of Cre-recombinase excised genes, particularly when deletion occurs in low turnover populations.

Partial rescue of the Tbx1 mutant heart phenotype by Fgf8: genetic evidence of impaired tissue response to Fgf8

Tbx1 is the candidate gene of DiGeorge syndrome and is required in humans and mice for the development of the cardiac outflow tract (OFT) and aortic arch arteries. Loss of function mutants present with reduced cell proliferation and premature differentiation of cardiac progenitor cells of the second heart field (SHF). Tbx1 regulates Fgf8 expression hence the hypothesis that the proliferation impairment may contribute to the heart phenotype of mutants. Here we show that forced Fgf8 expression modifies and partially rescues the OFT septation defects of Tbx1 mutants but only if there is some residual expression of Tbx1. This genetic experiment suggests that Tbx1, directly or indirectly, affects tissue response to Fgf8. Indeed, Tbx1(-/-) mouse embryonic fibroblasts were unable to respond to Fgf8 added to the culture media and showed defective response of Erk1/2 and Rsk1. Our data suggest a coordinated pathway modulating Fgf8 ligand expression and tissue response to it in the SHF.