Palatal fusion - where do the midline cells go? A review on cleft palate, a major human birth defect

Apoptotic force: active mechanical function of cell death during morphogenesis

Apoptosis, or programmed cell death, is an essential process for the elimination of unnecessary cells during embryonic development, tissue homeostasis, and certain pathological conditions. Recently, an active mechanical function of apoptosis called apoptotic force has been demonstrated during a tissue fusion process of Drosophila embryogenesis. The mechanical force produced during apoptosis is used not only to force dying cells out from tissues in order to keep tissue integrity, but also to change the morphology of neighboring cells to fill the space originally occupied by the dying cell. Furthermore, the occurrence of apoptosis correlates with tissue movement and tension of the tissue. This finding suggests that apoptotic forces might be harnessed throughout cell death-related morphogenesis; however, this concept remains to be fully investigated. While the investigation of this active mechanical function of apoptosis has just begun, here we summarize the current understandings of this novel function of apoptosis, and discuss some possible developmental processes in which apoptosis may play a mechanical role. The concept of apoptotic force prompts a necessity to rethink the role of programmed cell death during morphogenesis.

Tgf-beta-mediated FasL-Fas-Caspase pathway is crucial during palatogenesis

Programmed cell death, or apoptosis, is one of the fates of the medial edge epithelium (MEE) during palatal fusion. Transforming growth factor β (Tgf-β) signaling (such as Tgf-β3) is required for the disappearance of the MEE, but the relationship between Tgf-β3 and apoptosis remains unclear. Here we show that the Fas ligand (FasL)-Fas-Caspase extrinsic apoptosis pathway functions during palatal fusion in wild-type mice, but is not detectable in mice lacking Tgf-β3 (Tgf-β3 (-/-) ) or Tgfβr2 in the MEE (K14-Cre;Tgfbr2 (fl/fl)). Inhibition of the FasL-Fas system results in persistence of the midline epithelial seam (MES) and inhibition of caspase activity during palatal organ culture. Moreover, ectopic FasL protein induces apoptosis in MES of K14-Cre;Tgfbr2 (fl/fl) mice. Thus, we conclude that the FasL-Fas-caspase extrinsic apoptosis pathway is regulated by the Tgf-β3 signaling cascade and is essential for palatal fusion during craniofacial development.

Effects of phenytoin and Echinacea purpurea extract on proliferation and apoptosis of mouse embryonic palatal mesenchymal cells

Cleft palate is one of the most common birth defects. Several environment factors are involved in the disorder, such as smoking, vitamin deficiency and teratogens. We investigated the teratogenic agent phenytoin and extract of the immunostimulant Echinacea purpurea in the etiology of cleft palate associated with the proliferation and apoptosis of mouse embryonic palatal mesenchymal (MEPM) cells. We measured the effects of phenytoin, E. purpurea extract, and the mixture of phenytoin and E. purpurea extract on the cell viability of MEPM cells by CCK-8 assay and on the proliferation and apoptosis of MEPM cells by BrdU labeling assay, flow cytometry, and TUNEL assay. Exposure to phenytoin for 24 h inhibited cell proliferation and increased cell apoptosis of MEPM cells, and E. purpurea extract had the reverse effect. Importantly, treatment with the mixture of phenytoin and E. purpurea extract increased the proliferation and decreased the apoptosis of MEPM cells as compared with treatment with phenytoin alone. The teratogenic effect of phenytoin on cleft palate is associated with the proliferation and apoptosis of MEPM cells, and E. purpurea extract may have a protective effect.

Cooperation of two ADAMTS metalloproteases in closure of the mouse palate identifies a requirement for versican proteolysis in regulating palatal mesenchyme proliferation

We have identified a role for two evolutionarily related, secreted metalloproteases of the ADAMTS family, ADAMTS20 and ADAMTS9, in palatogenesis. Adamts20 mutations cause the mouse white-spotting mutant belted (bt), whereas Adamts9 is essential for survival beyond 7.5 days gestation (E7.5). Functional overlap of Adamts9 with Adamts20 was identified using Adamts9(+/-);bt/bt mice, which have a fully penetrant cleft palate. Palate closure was delayed, although eventually completed, in both Adamts9(+/-);bt/+ and bt/bt mice, demonstrating cooperation of these genes. Adamts20 is expressed in palatal mesenchyme, whereas Adamts9 is expressed exclusively in palate microvascular endothelium. Palatal shelves isolated from Adamts9(+/-);bt/bt mice fused in culture, suggesting an intact epithelial TGFβ3 signaling pathway. Cleft palate resulted from a temporally specific delay in palatal shelf elevation and growth towards the midline. Mesenchyme of Adamts9(+/-);bt/bt palatal shelves had reduced cell proliferation, a lower cell density and decreased processing of versican (VCAN), an extracellular matrix (ECM) proteoglycan and ADAMTS9/20 substrate, from E13.5 to E14.5. Vcan haploinsufficiency led to greater penetrance of cleft palate in bt mice, with a similar defect in palatal shelf extension as Adamts9(+/-);bt/bt mice. Cell density was normal in bt/bt;Vcan(hdf)(/+) mice, consistent with reduced total intact versican in ECM, but impaired proliferation persisted in palate mesenchyme, suggesting that ADAMTS-cleaved versican is required for cell proliferation. These findings support a model in which cooperative versican proteolysis by ADAMTS9 in vascular endothelium and by ADAMTS20 in palate mesenchyme drives palatal shelf sculpting and extension.

Developmental changes in cellular and extracellular structural macromolecules in the secondary palate and in the nasal cavity of the mouse

The aim of this study was to analyse the hitherto largely unknown expression patterns of some specific cellular and extracellular molecules during palate and nasal cavity development. We showed that epithelia of the developing palate and the vomerine epithelium express similar sets of structural proteins. With the exception of keratin 15, which becomes barely detectable in the elevated palatal shelves, nearly all of these proteins become upregulated at the presumptive areas of fusion and in the adhering epithelia of the palate and nasal septum. In vivo and in vitro analyses indicated that reduction in the amount of keratin 15 protein is independent of Tgfbeta-Alk5 signalling. Foxa1 expression also highlighted the regionalization of the palatal and nasal epithelia. Owing to the lack of reliable markers of the palatal periderm, the fate of peridermal cells has been controversial. We identified LewisX/stage-specific embryonic antigen-1 as a specific peridermal marker, and showed that numerous peridermal cells remain trapped in the medial epithelial seam (MES). The fate of these cells is probably apoptosis together with the rest of the MES cells, as we provided strong evidence for this event. Heparan sulphate, chondroitin-6-sulphate, and versican displayed dynamically changing distribution patterns. The hitherto-unknown innervation pattern of the developing palate was revealed. These findings may be of value for unravelling the pathogenesis of palatal clefting.

Palate morphogenesis: current understanding and future directions

In the past, most scientists conducted their inquiries of nature via inductivism, the patient accumulation of "pieces of information" in the pious hope that the sum of the parts would clarify the whole. Increasingly, modern biology employs the tools of bioinformatics and systems biology in attempts to reveal the "big picture." Most successful laboratories engaged in the pursuit of the secrets of embryonic development, particularly those whose research focus is craniofacial development, pursue a middle road where research efforts embrace, rather than abandon, what some have called the "pedestrian" qualities of inductivism, while increasingly employing modern data mining technologies. The secondary palate has provided an excellent paradigm that has enabled examination of a wide variety of developmental processes. Examination of cellular signal transduction, as it directs embryogenesis, has proven exceptionally revealing with regard to clarification of the "facts" of palatal ontogeny-at least the facts as we currently understand them. Herein, we review the most basic fundamentals of orofacial embryology and discuss how functioning of TGFbeta, BMP, Shh, and Wnt signal transduction pathways contributes to palatal morphogenesis. Our current understanding of palate medial edge epithelial differentiation is also examined. We conclude with a discussion of how the rapidly expanding field of epigenetics, particularly regulation of gene expression by miRNAs and DNA methylation, is critical to control of cell and tissue differentiation, and how examination of these epigenetic processes has already begun to provide a better understanding of, and greater appreciation for, the complexities of palatal morphogenesis.

CRISPLD2 variants including a C471T silent mutation may contribute to nonsyndromic cleft lip with or without cleft palate

OBJECTIVE

To assess the association between nonsyndromic (NS) cleft lip with or without cleft palate (CL(P)) and single-nucleotide polymorphisms (SNPs) within the CRISPLD2 gene (cysteine-rich secretory protein LCCL domain containing 2).

DESIGN

Four SNPs within the CRISPLD2 gene domain (rs1546124, rs8061351, rs2326398, rs4783099) were genotyped to test for association via family-based association methods.

PARTICIPANTS

A total of 5826 individuals from 1331 families in which one or more family member is affected with CL(P).

RESULTS

Evidence of association was seen for SNP rs1546124 in U.S. (p  =  .02) and Brazilian (p  =  .04) Caucasian cohorts. We also found association of SNP rs1546124 with cleft palate alone (CP) in South Americans (Guatemala and ECLAMC) and combined Hispanics (Guatemala, ECLAMC, and Texas Hispanics; p  =  .03 for both comparisons) and with both cleft lip with cleft palate (CLP; p  =  .04) and CL(P) (p  =  .02) in North Americans. Strong evidence of association was found for SNP rs2326398 with CP in Asian populations (p  =  .003) and with CL(P) in Hispanics (p  =  .03) and also with bilateral CL(P) in Brazilians (p  =  .004). In Brazilians, SNP rs8061351 showed association with cleft subgroups incomplete CL(P) (p  =  .004) and unilateral incomplete CL(P) (p  =  .003). Prediction of SNP functionality revealed that the C allele in the C471T silent mutation (overrepresented in cases with CL(P) presents two putative exonic splicing enhancer motifs and creates a binding site AP-2 alpha, a transcription factor involved in craniofacial development.

CONCLUSIONS

Our results support the hypothesis that variants in the CRISPLD2 gene may be involved in the etiology of NS CL(P).

Tbx1 is necessary for palatal elongation and elevation

The transcription factor TBX1 is a key mediator of developmental abnormalities associated with DiGeorge/Velocardiofacial Syndrome. Studies in mice have demonstrated that decreased dosage of Tbx1 results in defects in pharyngeal arch, cardiovascular, and craniofacial development. The role of Tbx1 in cardiac development has been intensely studied; however, its role in palatal development is poorly understood. By studying the Tbx1-/- mice we found defects during the critical points of palate elongation and elevation. The intrinsic palate defects in the Tbx1-/- mice were determined by measuring changes in palate shelf length, proliferation, apoptosis, expression of relevant growth factors, and in palate fusion assays. Tbx1-/- embryos exhibit cleft palate with failed palate elevation in 100% and abnormal palatal-oral fusions in 50%. In the Tbx1-/- mice the palate shelf length was reduced and tongue height was greater, demonstrating a physical impediment to palate elevation and apposition. In vitro palate fusion assays demonstrate that Tbx1-/- palate shelves are capable of fusion but a roller culture assay showed that the null palatal shelves were unable to elongate. Diminished hyaluronic acid production in the Tbx1-/- palate shelves may explain failed palate shelf elevation. In addition, cell proliferation and apoptosis were perturbed in Tbx1-/- palates. A sharp decrease of Fgf8 expression was detected in the Tbx1-/- palate shelves, suggesting that Fgf8 is dependent on Tbx1 in the palate. Fgf10 is also up-regulated in the Tbx1-/- palate shelves and tongue. These data demonstrate that Tbx1 is a critical transcription factor that guides palatal elongation and elevation and that Fgf8 expression in the palate is Tbx1-dependent.

Epithelial-mesenchymal transitions in development and disease

The epithelial to mesenchymal transition (EMT) plays crucial roles in the formation of the body plan and in the differentiation of multiple tissues and organs. EMT also contributes to tissue repair, but it can adversely cause organ fibrosis and promote carcinoma progression through a variety of mechanisms. EMT endows cells with migratory and invasive properties, induces stem cell properties, prevents apoptosis and senescence, and contributes to immunosuppression. Thus, the mesenchymal state is associated with the capacity of cells to migrate to distant organs and maintain stemness, allowing their subsequent differentiation into multiple cell types during development and the initiation of metastasis.

Molecular pathology of the fibroblast growth factor family

The human fibroblast growth factor (FGF) family contains 22 proteins that regulate a plethora of physiological processes in both developing and adult organism. The mutations in the FGF genes were not known to play role in human disease until the year 2000, when mutations in FGF23 were found to cause hypophosphatemic rickets. Nine years later, seven FGFs have been associated with human disorders. These include FGF3 in Michel aplasia; FGF8 in cleft lip/palate and in hypogonadotropic hypogonadism; FGF9 in carcinoma; FGF10 in the lacrimal/salivary glands aplasia, and lacrimo-auriculo-dento-digital syndrome; FGF14 in spinocerebellar ataxia; FGF20 in Parkinson disease; and FGF23 in tumoral calcinosis and hypophosphatemic rickets. The heterogeneity in the functional consequences of FGF mutations, the modes of inheritance, pattern of involved tissues/organs, and effects in different developmental stages provide fascinating insights into the physiology of the FGF signaling system. We review the current knowledge about the molecular pathology of the FGF family.

Maturation of ureter-bladder connection in mice is controlled by LAR family receptor protein tyrosine phosphatases

Congenital anomalies affecting the ureter-bladder junction are frequent in newborns and are often associated with other developmental defects. However, the molecular and morphological processes underlying these malformations are still poorly defined. In this study, we identified the leukocyte antigen-related (LAR) family protein tyrosine phosphatase, receptor type, S and F (Ptprs and Ptprf [also known as Lar], respectively), as crucially important for distal ureter maturation and craniofacial morphogenesis in the mouse. Embryos lacking both Ptprs and Ptprf displayed severe urogenital malformations, characterized by hydroureter and ureterocele, and craniofacial defects such as cleft palate, micrognathia, and exencephaly. The detailed analysis of distal ureter maturation, the process by which the ureter is displaced toward its final position in the bladder wall, leads us to propose a revised model of ureter maturation in normal embryos. This process was deficient in embryos lacking Ptprs and Ptprf as a result of a marked reduction in intrinsic programmed cell death, thereby causing urogenital system malformations. In cell culture, Ptprs bound and negatively regulated the phosphorylation and signaling of the Ret receptor tyrosine kinase, whereas Ptprs-induced apoptosis was inhibited by Ret expression. Together, these results suggest that ureter positioning is controlled by the opposing actions of Ret and LAR family phosphatases regulating apoptosis-mediated tissue morphogenesis.

Palatal seam disintegration: to die or not to die? that is no longer the question

Formation of the medial epithelial seam (MES) by palatal shelf fusion is a crucial step of palate development. Complete disintegration of the MES is the final essential phase of palatal confluency with surrounding mesenchymal cells. In general, the mechanisms of palatal seam disintegration are not overwhelmingly complex, but given the large number of interacting constituents; their complicated circuitry involving feedforward, feedback, and crosstalk; and the fact that the kinetics of interaction matter, this otherwise simple mechanism can be quite difficult to interpret. As a result of this complexity, apparently simple but highly important questions remain unanswered. One such question pertains to the fate of the palatal seam. Such questions may be answered by detailed and extensive quantitative experimentation of basic biological studies (cellular, structural) and the newest molecular biological determinants (genetic/dye cell lineage, gene activity, kinase/enzyme activity), as well as animal model (knockouts, transgenic) approaches. System biology and cellular kinetics play a crucial role in cellular MES function; omissions of such critical contributors may lead to inaccurate understanding of the fate of MES. Excellent progress has been made relevant to elucidation of the mechanism(s) of palatal seam disintegration. Current understanding of palatal seam disintegration suggests epithelial-mesenchymal transition and/or programmed cell death as two most common mechanisms of MES disintegration. In this review, I discuss those two mechanisms and the differences between them.

Runx1 is involved in the fusion of the primary and the secondary palatal shelves

Runx1 is expressed in medial edge epithelial (MEE) cells of the palatal shelf. Conditionally rescued Runx1(-/-) mice showed limited clefting in the anterior junction between the primary and the secondary palatal shelves, but not in the junction between the secondary palates. In wild type mice, the fusing epithelial surface exhibited a rounded cobblestone-like appearance, while such cellular prominence was less evident in the Runx1 mutants. We also found that Fgf18 was expressed in the mesenchyme underlying the MEE and that locally applied FGF18 induced ectopic Runx1 expression in the epithelium of the palatal explants, indicating that Runx1 was induced by mesenchymal Fgf18 signaling. On the other hand, unpaired palatal explant cultures revealed the presence of anterior-posterior (A-P) differences in the MEE fates and fusion mechanism. Interestingly, the location of anterior clefting in Runx1 mutants corresponded to the region with different MEE behavior. These data showed a novel function of Runx1 in morphological changes in the MEE cells in palatal fusion, which is, at least in part, regulated by the mesenchymal Fgf signaling via an epithelial-mesenchymal interaction.

DNA Methylation Changes during Cleft Palate Formation Induced by Retinoic Acid in Mice

Objectives:

The aim of this study was to analyze epigenetic (specifically, DNA methylation) participation in the mechanisms of cleft palate only induced by maternal exposure to all-trans retinoic acid in mice.

Design:

Cleft palate only was induced in fetuses by maternal exposure to all-trans retinoic acid. Their secondary palates were excised for analysis. Cytosine extension assay and restriction landmark genomic scanning were performed to analyze DNA methylation status. The expression levels of the DNA methyltransferases were examined by real-time reverse transcriptase–polymerase chain reaction.

Results:

Using cytosine extension assay, on gestation day 14.5, the status of DNA methylation within CpG islands and in global DNA was decreased significantly in all-trans retinoic acid–treated groups compared with the controls (p < .01 and p < .05). In the controls, the status within CpG islands on gestation day 14.5 was significantly increased compared with gestation days 13.5 and 18.5 (p < .01). Using real-time reverse transcriptase–polymerase chain reaction, there was no significant change in the expression of DNA methyltransferases, except on gestation day 18.5. Using restriction landmark genomic scanning on gestation day 18.5, five spots (0.49%) in the controls and one spot (0.1%) in all-trans retinoic acid–treated groups were specifically detected.

Conclusions:

These results indicate that changes in DNA methylation may play an important role in the manifestation of cleft palate only caused by environmental factors such as maternal exposure to all-trans retinoic acid.

Memory encoded throughout our bodies: molecular and cellular basis of tissue regeneration

When a sheep loses its tail, it cannot regenerate it in the manner of lizards. On the other hand, it is possible to clone mammals from somatic cells, showing that a complete developmental program is intact in a wounded sheep's tail the same way it is in a lizard. Thus, there is a requirement for more than only the presence of the entire genetic code in somatic cells for regenerative abilities. Thoughts like this have motivated us to assemble more than just a factographic synopsis on tissue regeneration. As a model, we review skin wound healing in chronological order, and when possible, we use that overview as a framework to point out possible mechanisms of how damaged tissues can restore their original structure. This article postulates the existence of tissue structural memory as a complex distributed homeostatic mechanism. We support such an idea by referring to an extremely fragmented literature base, trying to synthesize a broad picture of important principles of how tissues and organs may store information about their own structure for the purposes of regeneration. Selected developmental, surgical, and tissue engineering aspects are presented and discussed in the light of recent findings in the field. When a sheep loses its tail, it cannot regenerate it in the manner of lizards. On the other hand, it is possible to clone mammals from somatic cells, showing that a complete developmental program is intact in a wounded sheep's tail the same way it is in a lizard. Thus, there is a requirement for more than only the presence of the entire genetic code in somatic cells for regenerative abilities. Thoughts like this have motivated us to assemble more than just a factographic synopsis on tissue regeneration. As a model, we review skin wound healing in chronological order, and when possible, we use that overview as a framework to point out possible mechanisms of how damaged tissues can restore their original structure. This article postulates the existence of tissue structural memory as a complex distributed homeostatic mechanism. We support such an idea by referring to an extremely fragmented literature base, trying to synthesize a broad picture of important principles of how tissues and organs may store information about their own structure for the purposes of regeneration. Selected developmental, surgical, and tissue engineering aspects are presented and discussed in the light of recent findings in the field.

Cleft lip and/or palate with monogenic autosomal recessive transmission in Pyrenees shepherd dogs

OBJECTIVE

To document the genetic background of Pyrenees shepherd dogs as it relates to the incidence of cleft lip and/or cleft palate, to describe the phenotype, and to determine possible candidate genes.

DESIGN

Pedigree analysis was performed and blood samples were taken from five affected pups, their siblings, and parents. Seven candidate genes were selected and linkage analysis was performed. Further methods used included sequencing and histology.

RESULTS

In 37 litters consisting of 163 pups, we found 47 affected pups in a total population of 2104. The male:female ratio was 1:0.96. Affected pups showed isolated cleft lip and/or cleft palate; no attendant disorders have been reported. Despite a high degree of relationship, two affected pups displayed a cleft palate (- H S H -) and a cleft lip with or without cleft palate (L A -) cleft formation. Histology of affected pups showed that the medial edge epithelium remained intact and did not undergo an epithelial-mesenchymal transformation. There was no evidence for linkage between the trait and TGFb3 or Msx1. Subsequent sequencing excluded the coding sequence of Fst as well.

CONCLUSION

Pedigree analysis showed that cleft palate is not genetically distinct from cleft lip with or without cleft palate but is inherited in this breed as a monogenic autosomal recessive trait. Linkage analysis and sequencing excluded TGFb3, Msx1, and Fst as candidate genes. Histology of affected pups showed that the medial edge epithelium is still intact.

Epithelial--mesenchymal and mesenchymal--epithelial transitions in carcinoma progression

Like a set of bookends, cellular, molecular, and genetic changes of the beginnings of life mirror those of one of the most common cause of death--metastatic cancer. Epithelial to mesenchymal transition (EMT) is an important change in cell phenotype which allows the escape of epithelial cells from the structural constraints imposed by tissue architecture, and was first recognized by Elizabeth Hay in the early to mid 1980's to be a central process in early embryonic morphogenesis. Reversals of these changes, termed mesenchymal to epithelial transitions (METs), also occur and are important in tissue construction in normal development. Over the last decade, evidence has mounted for EMT as the means through which solid tissue epithelial cancers invade and metastasize. However, demonstrating this potentially rapid and transient process in vivo has proven difficult and data connecting the relevance of this process to tumor progression is still somewhat limited and controversial. Evidence for an important role of MET in the development of clinically overt metastases is starting to accumulate, and model systems have been developed. This review details recent advances in the knowledge of EMT as it occurs in breast development and carcinoma and prostate cancer progression, and highlights the role that MET plays in cancer metastasis. Finally, perspectives from a clinical and translational viewpoint are discussed.

The etiopathogenesis of cleft lip and cleft palate: usefulness and caveats of mouse models

Cleft lip and cleft palate are frequent human congenital malformations with a complex multifactorial etiology. These orofacial clefts can occur as part of a syndrome involving multiple organs or as isolated clefts without other detectable defects. Both forms of clefting constitute a heavy burden to the affected individuals and their next of kin. Human and mouse facial traits are utterly dissimilar. However, embryonic development of the lip and palate are strikingly similar in both species, making the mouse a model of choice to study their normal and abnormal development. Human epidemiological and genetic studies are clearly important for understanding the etiology of lip and palate clefting. However, our current knowledge about the etiopathogenesis of these malformations has mainly been gathered throughout the years from mouse models, including those with mutagen-, teratogen- and targeted mutation-induced clefts as well as from mice with spontaneous clefts. This review provides a comprehensive description of the numerous mouse models for cleft lip and/or cleft palate. Despite a few weak points, these models have revealed a high order of molecular complexity as well as the stringent spatiotemporal regulations and interactions between key factors which govern the development of these orofacial structures.

Mechanisms of palatal epithelial seam disintegration by transforming growth factor (TGF) beta3

TGFbeta3 signaling initiates and completes sequential phases of cellular differentiation that is required for complete disintegration of the palatal medial edge seam, that progresses between 14 and 17 embryonic days in the murine system, which is necessary in establishing confluence of the palatal stroma. Understanding the cellular mechanism of palatal MES disintegration in response to TGFbeta3 signaling will result in new approaches to defining the causes of cleft palate and other facial clefts that may result from failure of seam disintegration. We have isolated MES primary cells to study the details of MES disintegration mechanism by TGFbeta3 during palate development using several biochemical and genetic approaches. Our results demonstrate a novel mechanism of MES disintegration where MES, independently yet sequentially, undergoes cell cycle arrest, cell migration and apoptosis to generate immaculate palatal confluency during palatogenesis in response to robust TGFbeta3 signaling. The results contribute to a missing fundamental element to our base knowledge of the diverse roles of TGFbeta3 in functional and morphological changes that MES undergo during palatal seam disintegration. We believe that our findings will lead to more effective treatment of facial clefting.