Role of Gli3 during intramembranous calvarial bone development

The flat bones of the skull, the calvarial bones, develop by intramembranous ossification during which mesenchymal cells first condense and subsequently differentiate into osteoblasts. Sutures separate the calvarial bones and facilitate the synchronized growth of the underlying brain and the calvaria. Hedgehog (Hh) signalling has an indisputable role in craniofacial development as well as during endochondral ossification. Yet, little is known about its function during intramembranous ossification of the calvarial bones. GLI-Kruppel family member 3 (Gli3) is a zinc-finger transcription factor that mediates Hh signalling. In the absence of Hh ligand Gli3 is proteolytically cleaved into a repressor that inhibits transcription of Hh target genes. Mutations in GLI3 cause Greig cephalopolysyndactyly syndrome in humans, in which an infrequent, but significant feature is premature fusion of the metopic suture (interfrontal suture in mice). We have used Gli3 loss-of- function mouse (Gli3Xt-J/Xt-J) as a model to investigate the effects of aberrant Hh signalling during calvarial development. In my thesis I describe how loss of Gli3 causes craniosynostosis of the lambdoid as well as interfrontal sutures in mice. Elevated proliferation and ectopic differentiation of osteoprogenitors underlies this phenomenon. We were able to rescue craniosynostosis in these mice by two mechanisms. Firstly, by elevating fibroblast growth factor (Fgf) signalling in the suture prior to its fusion by imbedding Fgf2 soaked beads in tissue culture. This induced Twist1 expression, which inhibits function of ectopically expressed Runx2. Secondly, craniosynostosis was prevented by genetically reducing Runx2 activity by generating Gli3Xt-J/Xt-J;Runx2+/- mice, which normalized elevated levels of Bmp signalling in the affected sutures. We also put forward a model of how Hh signalling helps to maintain the integrity of bone margins during calvarial development. The repressor isoform of Gli3 inhibits Runx2 activity in the early osteoprogenitor cells. Runx2, on the other hand, activates Ihh expression in the mature osteoblasts, which then induces osteogenesis by inhibiting the function of Gli3 repressor. Our findings indicate that Gli3 and Hh signalling have an important role in mediating the location of osteoblast differentiation and the speed of bone formation in the developing calvaria. Uncovering the cellular and molecular mechanisms that underlie normal calvarial development, as well as pathological processes, is a vital step in developing treatment strategies for patients with craniosynostosis.Kallon litteät peitinluut kehittyvät ns. suoran luutumisen mekanismilla eli intramembranoottisesti: mesenkymaaliset solut muodostavat ensin tiivistymän, jonka jälkeen ne erilaistuvat asteittain luusoluiksi eli osteoblasteiksi. Kasvavia kallon peitinluita erottavat kallon saumat, joissa luiden aktiivinen kasvu jatkuu mahdollistaen alla olevien aivojen samanaikaisen kasvun. Kraniosynostoosi on patologinen tila, jossa yksi tai useampi kallon sauma luutuu ennenaikaisesti. Tällöin kasvu kyseisessä kallon saumassa loppuu, mikä johtaa kallon, silmäkuoppien ja kasvojen epäsymmetriseen kasvuun, sekä neurologisiin oireisiin. Toistaiseksi ainoa hoitokeino on luutuneen sauman kirurginen avaus. Hedgehog (Hh) viestintäreitillä on merkittävä rooli sekä kraniofakiaalisen kehityksen että endokondraalisen luutumisen aikana. Kuitenkin, sen mahdollisesta tehtävästä intramembranoottisten kallon peitinluiden kehityksessä tiedetään varsin vähän. Gli3 on transkriptiotekijä, joka säätelee Hh signalointia. Hh-viestimolekyylin puuttuessa Gli3 pilkotaan proteolyyttisesti lyhyempään repressori-muotoon, mikä estää Hh viestintäreitin kohdegeenien ilmentymistä. Ihmisellä mutaatiot GLI3 geenissä aiheuttavat Greig kefalopolysyndaktylia-oireyhtymän, jonka harvinainen, mutta merkittävä piirre on metooppisen sauman (interfrontaalisauma hiirellä) kraniosynostoosi. Tässä väitöskirjatyössä käytimme mallina Gli3-poistogeenistä hiirtä (Gli3Xt-J/Xt-J) tutkiessamme epänormaalin Hh signaloinnin vaikutusta kallon peitinluiden kehitykseen. Hiirellä Gli3 geenin puutos johti sekä lambda- että interfrontaalisaumojen kraniosynostoosiin. Kraniosynostoosin taustalta paljastui kiihtynyt solujen jakautuminen sekä ektooppinen osteoblastien erilaistuminen. Pystyimme estämään saumojen ennenaikaisen luutumisen Gli3-poistogeenisessä hiiressä kahdella mekanismilla. 1) Lisäsimme fibroblastikasvutekijäperheeseen kuuluvaa Fgf2-proteiinia kudosviljelmässä paikallisesti lambdasaumaan ennen sen luutumista. Fgf2 indusoi saumassa Twist1-geenin ilmentymisen, mikä puolestaan esti ektooppisesti ilmentyvän Runx2:den toiminnan. Runx2 on välttämätön osteoblastien erilaistumiselle. 2) Vähensimme myös Runx2-geenin aktiivisuutta poistamalla Gli3-poistogeenisestä hiirestä yhden kopion Runx2-geeniä (Gli3Xt-J/Xt-J;Runx2+/-), mikä normalisoi lisääntyneen luun morfogeeninen kasvutekijä (Bmp)-viestintäreitin aktiivisuuden affektoituneissa saumoissa. Väitöskirjassani kuvaamme mallin, jossa Hh viestintäreitti säätelee kallon peitinluiden muodostusta kallon saumassa. Gli3:n repressori-muoto ilmentyy hyvin varhaisissa osteoblastien esiasteissa, luun sauman puoleisella reunalla, jossa se rajoittaa näiden solujen erilaistumista estämällä Runx2:den toimintaa. Toisaalta, myöhemmässä osteoblastien erilaistumisvaiheessa Runx2 aktivoi Hh-viestimolekyylin, Indian hedgehogin, ilmentymisen, mikä puolestaan edistää osteoblastien erilaistumista estämällä Gli3-repressorin muodostusta. Tämän väitöskirjan tulokset osoittavat, että Gli3:lla ja Hh viestintäreitillä on merkittävä rooli kallon kehityksen aikana, sekä osteoblastien erilaistumispaikan että erilaistumisnopeuden säätelyssä. Normaalin kallon kehityksen ja patologisten tilanteiden taustalla olevien solu- ja molekyylitason mekanismien selvittäminen on välttämätön askel uusien hoitomuotojen kehityksessä kraniosynostoosipotilaille

Regulation of Calvarial Osteogenesis by Concomitant De-repression of GLI3 and Activation of IHH Targets

Loss-of-function mutations in GLI3 and IHH cause craniosynostois and reduced osteogeneiss, respectively. In this study, we show that ihh ligand, the receptor Ptch1 and Gli transcription factors are differentialyy expressed in embryonic mouse calvaria osteogenic condenstions. We show that in both ihh(-/-) and Gli3(Xt-J/Xt-J) embroyonic mice, the normal gene expression architecture is lost and this results in disorganized calvarial bone developement. RUNX2 is a master regulatory transciption factor controlling osteogenesis. In the absence of Gli3, RUNX2 isoform II and IHH are upregulated, and RUNX2 isoform I downregulated. This is consistent with the expandeed and aberant osteogenesis observed in Gli3Xt-J/Xt-J mice, and consistent RunX2-t expression by relatively immature osteoprogenitors. ihh-/- mice exhibited small calvarial bones HH target genes, Ptch1 and Gli1, were absent. This indicates that IHH is the functional HH ligand, and that it is not compensated by another HH ligand. To decipher the roles and potential interaction of Gli3 and ihh. we generated ihh-/-; gli3Xt-J/Xt-J compound mutant mice. Even in the absence of ihh, Gli3 deletion was sufficient to induce aberrant precocious ossification across the developing suture, indicating that the carniosyostosis pehnotype of Gli3Xt-J/Xt-J mice is not dependent on IHH ligand. Also we found that ihh was not required for Runx2 expression as the expression of RUNX2 target genes was unaffected by deletion of Ihh. To test whether RUNX2 has a role upstream of IHH we performed RUNX2 siRNA knock down experiements in WT calvarial osteoblasts and explants and found that Ihh expression is suppressed. Our results show that IHH is the functional HH ligand in the embroynic mouse calvaria osteogenic condensations, where it regulates the progression of osteoblastic differentation. As GLI3 represses the expression of Runx2-II abd Ihh, and also elevats the Runx2-I expression, and as IHH may be regulated by RUNX2 these results raise the possibility of a regualtory feedback circuit to control calvarial osteogenesis and suture patency. Taken together RUNX2-controlled osteoblastic cell fate is regulated by IHH through concomitant inhibition of GLI3-repressor formation and activation of downstreams targets.Peer reviewe

RAB23 coordinates early osteogenesis by repressing FGF10-pERK1/2 and GLI1

Mutations in the gene encoding Ras-associated binding protein 23 (RAB23) cause Carpenter Syndrome, which is characterized by multiple developmental abnormalities including polysyndactyly and defects in skull morphogenesis. To understand how RAB23 regulates skull development, we generated Rab23-deficient mice that survive to an age where skeletal development can be studied. Along with polysyndactyly, these mice exhibit premature fusion of multiple sutures resultant from aberrant osteoprogenitor proliferation and elevated osteogenesis in the suture. FGF10-driven FGFR1 signaling is elevated in Rab23(-/-) sutures with a consequent imbalance in MAPK, Hedgehog signaling and RUNX2 expression. Inhibition of elevated pERK1/2 signaling results in the normalization of osteoprogenitor proliferation with a concomitant reduction of osteogenic gene expression, and prevention of craniosynostosis. Our results suggest a novel role for RAB23 as an upstream negative regulator of both FGFR and canonical Hh-GLI1 signaling, and additionally in the non-canonical regulation of GLI1 through pERK1/2.Peer reviewe

Noggin null allele mice exhibit a microform of holoprosencephaly

Holoprosencephaly (HPE) is a heterogeneous craniofacial and neural developmental anomaly characterized in its most severe form by the failure of the forebrain to divide. In humans, HPE is associated with disruption of Sonic hedgehog and Nodal signaling pathways, but the role of other signaling pathways has not yet been determined. In this study, we analyzed mice which, due to the lack of the Bmp antagonist Noggin, exhibit elevated Bmp signaling. Noggin−/− mice exhibited a solitary median maxillary incisor that developed from a single dental placode, early midfacial narrowing as well as abnormalities in the developing hyoid bone, pituitary gland and vomeronasal organ. In Noggin−/− mice, the expression domains of Shh, as well as the Shh target genes Ptch1 and Gli1, were reduced in the frontonasal region at key stages of early facial development. Using E10.5 facial cultures, we show that excessive BMP4 results in reduced Fgf8 and Ptch1 expression. These data suggest that increased Bmp signaling in Noggin−/− mice results in downregulation of the hedgehog pathway at a critical stage when the midline craniofacial structures are developing, which leads to a phenotype consistent with a microform of HP

Loss-of-function of Gli3 in mice causes abnormal frontal bone morphology and premature synostosis of the interfrontal suture

Peer reviewe

Mutations in multidomain protein MEGF8 identify a Carpenter syndrome subtype associated with defective lateralization

Carpenter syndrome is an autosomal-recessive multiple-congenital-malformation disorder characterized by multisuture craniosynostosis and polysyndactyly of the hands and feet; many other clinical features occur, and the most frequent include obesity, umbilical hernia, cryptorchidism, and congenital heart disease. Mutations of RAB23, encoding a small GTPase that regulates vesicular transport, are present in the majority of cases. Here, we describe a disorder caused by mutations in multiple epidermal-growth-factor-like-domains 8 (MEGF8), which exhibits substantial clinical overlap with Carpenter syndrome but is frequently associated with abnormal left-right patterning. We describe five affected individuals with similar dysmorphic facies, and three of them had either complete situs inversus, dextrocardia, or transposition of the great arteries; similar cardiac abnormalities were previously identified in a mouse mutant for the orthologous Megf8. The mutant alleles comprise one nonsense, three missense, and two splice-site mutations; we demonstrate in zebrafish that, in contrast to the wild-type protein, the proteins containing all three missense alterations provide only weak rescue of an early gastrulation phenotype induced by Megf8 knockdown. We conclude that mutations in MEGF8 cause a Carpenter syndrome subtype frequently associated with defective left-right patterning, probably through perturbation of signaling by hedgehog and nodal family members. We did not observe any subject with biallelic loss-of function mutations, suggesting that some residual MEGF8 function might be necessary for survival and might influence the phenotypes observed

Homolog-specific PCR primer design for profiling splice variants

To study functional diversity of proteins encoded from a single gene, it is important to distinguish the expression levels among the alternatively spliced variants. A variant-specific primer pair is required to amplify each alternatively spliced variant individually. For this purpose, we developed a new feature, homolog-specific primer design (HSPD), in our high-throughput primer and probe design software tool, PRIMEGENS-v2. The algorithm uses a de novo approach to design primers without any prior information of splice variants or close homologs for an input query sequence. It not only designs primer pairs but also finds potential isoforms and homologs of the input sequence. Efficiency of this algorithm was tested for several gene families in soybean. A total of 187 primer pairs were tested under five different abiotic stress conditions with three replications at three time points. Results indicate a high success rate of primer design. Some primer pairs designed were able to amplify all splice variants of a gene. Furthermore, by utilizing combinations within the same multiplex pool, we were able to uniquely amplify a specific variant or duplicate gene. Our method can also be used to design PCR primers to specifically amplify homologs in the same gene family. PRIMEGENS-v2 is available at: http://primegens.org

Mouth development

WIREs Developmental Biology published by Wiley Periodicals, Inc. A mouth is present in all animals, and comprises an opening from the outside into the oral cavity and the beginnings of the digestive tract to allow eating. This review focuses on the earliest steps in mouth formation. In the first half, we conclude that the mouth arose once during evolution. In all animals, the mouth forms from ectoderm and endoderm. A direct association of oral ectoderm and digestive endoderm is present even in triploblastic animals, and in chordates, this region is known as the extreme anterior domain (EAD). Further support for a single origin of the mouth is a conserved set of genes that form a ‘mouth gene program’ including foxA and otx2. In the second half of this review, we discuss steps involved in vertebrate mouth formation, using the frog Xenopus as a model. The vertebrate mouth derives from oral ectoderm from the anterior neural ridge, pharyngeal endoderm and cranial neural crest (NC). Vertebrates form a mouth by breaking through the body covering in a precise sequence including specification of EAD ectoderm and endoderm as well as NC, formation of a ‘pre-mouth array,’ basement membrane dissolution, stomodeum formation, and buccopharyngeal membrane perforation. In Xenopus, the EAD is also a craniofacial organizer that guides NC, while reciprocally, the NC signals to the EAD to elicit its morphogenesis into a pre-mouth array. Human mouth anomalies are prevalent and are affected by genetic and environmental factors, with understanding guided in part by use of animal models.National Institute of Dental and Craniofacial Research (U.S.) (Grant RO1 DE021109

Early epithelial signaling center governs tooth budding morphogenesis

During organogenesis, cell fate specification and patterning are regulated by signaling centers, specialized clusters of morphogen-expressing cells. In many organs, initiation of development is marked by bud formation, but the cellular mechanisms involved are ill defined. Here, we use the mouse incisor tooth as a model to study budding morphogenesis. We show that a group of nonproliferative epithelial cells emerges in the early tooth primordium and identify these cells as a signaling center. Confocal live imaging of tissue explants revealed that although these cells reorganize dynamically, they do not reenter the cell cycle or contribute to the growing tooth bud. Instead, budding is driven by proliferation of the neighboring cells. We demonstrate that the activity of the ectodysplasin/Edar/nuclear factor kappa B pathway is restricted to the signaling center, and its inactivation leads to fewer quiescent cells and a smaller bud. These data functionally link the signaling center size to organ size and imply that the early signaling center is a prerequisite for budding morphogenesis.Peer reviewe