Development. Longing for ligand: hedgehog, patched, and cell death

Molecular pathogenesis and treatment of cavernous nerve injury-induced erectile dysfunction: A narrative review

Introduction: Erectile dysfunction (ED) is a common complication after radical prostatectomy (RP), and it seriously affects the quality of life in patients and their partners. The primary trigger of postoperative ED is surgical injury to the cavernous nerves that control penile erection and run along the anterolateral aspect of the prostate. Despite the introduction and ongoing innovation of nerve-sparing techniques, a significant number of patients still suffer from moderate cavernous nerve injury (CNI), which is thought to be transient and reversible. Therefore, early postoperative penile rehabilitation therapy may salvage patients’ erectile function by promoting cavernous nerve regeneration and preventing penile structural alterations.

Aims: To present a comprehensive overview of the current molecular pathogenesis of CNI-induced ED, as well as novel therapeutic strategies and their potential mechanisms.

Methods: A literature search was performed using PubMed. Search terms included erectile dysfunction, cavernous nerve injury, pathogenesis, pathway, and treatment.

Results: The NOS/NO pathway, oxidative stress-related pathway, RhoA/ROCK pathway, transforming growth factor-β (TGF-β), sonic hedgehog (Shh), and hydrogen sulfide (H2S) are involved in the molecular pathogenesis of CNI-induced ED. Multiple neurotrophins, including brain-derived nerve growth factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), and neurturin (NTN), were found to promote cavernous nerve regeneration. Emerging therapeutic approaches can be roughly summarized into four categories, namely small molecule and drug, stem cell-based therapy (SCT), micro-energy therapy and platelet-rich plasma (PRP) therapy.

Conclusion: These pathways collectively lead to the irreversible damage to the penile structure after CNI. The combined early rehabilitation strategies of promoting upstream nerve regeneration and recovering abnormal molecular signals of downstream penis are presumed to save patients’ erectile function after RP. In future studies, the cross-talk between these molecular pathways needs to be further clarified, and the questions of how denervation injury induces the molecular alterations in the penis also need to be addressed.

The Hedgehog receptor Patched1 regulates proliferation, neurogenesis, and axon guidance in the embryonic spinal cord

The formation of the vertebrate nervous system depends on the complex interplay of morphogen signaling pathways and cell cycle progression to establish distinct cell fates. The Sonic hedgehog (Shh) signaling pathway is well understood to promote ventral cell fates in the developing spinal cord. A key regulator of Shh signaling is its receptor Patched1 (Ptch1). However, because the Ptch1 null mutation is lethal early in mouse embryogenesis, its role in controlling cell cycle progression, neurogenesis, and axon guidance in the developing spinal cord is not fully understood. An allele of Ptch1 called Wiggable (Ptch1^Wig), which was previously shown to enhance Shh signaling, was used to test its ability to regulate neurogenesis and proliferation in the developing spinal cord. Ptch1^Wig/Wig mutants displayed enhanced ventral proneural gene activation, and aberrant proliferation of the neural tube and floor plate cells, the latter normally being a quiescent population. The expression of the cell cycle regulators p27^Kip1 and p57^Kip2 were expanded in Ptch1^Wig/Wig mutant spinal cords, as was the number of mitotic and S-phase nuclei, suggesting enhanced cell cycle progression. However, Ptch1^Wig/Wig mutants also showed enhanced apoptosis in the ventral embryonic spinal cord, which resulted in thinner spinal cords at later embryonic stages. Commissural axons largely failed to cross the floor plate of Ptch1^Wig/Wig mutant embryos, suggesting enhanced Shh signaling in these mutants led to a dorsal expansion of the chemoattraction front. These findings are consistent with a role of Ptch1 in regulating neurogenesis and proliferation of neural progenitors, and in restricting the influence of Shh signaling in commissural axon guidance to the floor plate.

Apontic directly activates hedgehog and cyclin E for proper organ growth and patterning

Hedgehog (Hh) signaling pathway and Cyclin E are key players in cell proliferation and organ development. Hyperactivation of hh and cyclin E has been linked to several types of cancer. However, coordination of the expression of hh and cyclin E was not well understood. Here we show that an evolutionarily conserved transcription factor Apontic (Apt) directly activates hh and cyclin E through its binding site in the promoter regions of hh and cyclin E. This Apt-dependent proper expression of hh and cyclin E is required for cell proliferation and development of the Drosophila wing. Furthermore, Fibrinogen silencer-binding protein (FSBP), a mammalian homolog of Apt, also positively regulates Sonic hh (Shh), Desert hh (Dhh), Cyclin E1 (CCNE1) and Cyclin E2 (CCNE2) in cultured human cells, suggesting evolutionary conservation of the mechanism. Apt-mediated expression of hh and cyclin E can direct proliferation of Hh-expressing cells and simultaneous growth, patterning and differentiation of Hh-recipient cells. The discovery of the simultaneous expression of Hh and principal cell-cycle regulator Cyclin E by Apt implicates insight into the mechanism by which deregulated hh and cyclin E promotes tumor formation.

Evaluation of the 'Hedgehog' signaling pathways in squamous and basal cell carcinomas of the eyelids and conjunctiva

The purpose of the present study was to assess the role of hedgehog signaling pathway in the carcinogenesis of eyelid skin and conjunctival epithelial malignant tumors. The study was conducted on specimens from 41 patients with cutaneous eyelid basal cell carcinoma, 22 with bulbar conjunctival squamous cell carcinoma, 12 with bulbar conjunctival intraepithelial neoplasia. Major molecules of Hedgehog signaling pathway (Sonic Hedgehog [Shh] and Patched-1 [Ptch-1] and Glioma-associated oncogene [Gli-1]) were evaluated in paraffin-embedded tissue specimens using immunohistochemical staining. For each specimen, the percentage (<10%, 10–50%, >50%) and the intensity of the immunohistochemical staining (graded from 0 to 3) were calculated and the scores obtained by multiplication of two values were analyzed using the Kruskall-Wallis test. Shh and Ptch-1 expression levels were statistically significantly lower in the basal cell carcinoma group compared with the squamous cell carcinoma group (P=0.043 for Shh; P=0.030 for Ptch-1). In the conjunctival squamous cell carcinoma group, the Ptch-1 score was 0 in ~25% of specimens and the Gli-1 score was ≤2 in ~45% of cases. In the conjunctival intraepithelial neoplasia group, the Ptch-1 score was ≥2 in 66% of specimens, the Gli-1 score was ≤2 in ~92% of cases. Ptch-1 mutations contribute to the development of cutaneous eyelid basal cell carcinoma. The present study provides evidence that alterations in hedgehog signaling pathways may lead to transformation of the conjunctival intraepithelial neoplasia into invasive squamous cell carcinoma.

Hedgehog regulates cerebellar progenitor cell and medulloblastoma apoptosis

The external granule layer (EGL) is a proliferative region that produces over 90% of the neurons in the cerebellum but can also malignantly transform into a cerebellar tumor called the medulloblastoma (the most common malignant brain tumor in children). Current dogma considers Hedgehog stimulation a potent proliferative signal for EGL neural progenitor cells (NPCs) and medulloblastomas. However, the Hedgehog pathway also acts as a survival signal in the neural tube where it regulates dorsoventral patterning by controlling NPC apoptosis. Here we show that Hedgehog stimulation is also a potent survival signal in the EGL and medulloblastomas that produces a massive apoptotic response within hours of signal loss in mice. This toxicity can be produced by numerous Hedgehog antagonists (vismodegib, cyclopamine, and jervine) and is Bax/Bak dependent but p53 independent. Finally, since glucocorticoids can also induce EGL and medulloblastoma apoptosis, we show that Hedgehog's effects on apoptosis can occur independent of glucocorticoid stimulation. This effect may play a major role in cerebellar development by directing where EGL proliferation occurs thereby morphologically sculpting growth. It may also be a previously unknown major therapeutic effect of Hedgehog antagonists during medulloblastoma therapy. Results are discussed in terms of their implications for both cerebellar development and medulloblastoma treatment.

Hedgehog signaling in the normal and neoplastic mammary gland

The hedgehog signal transduction network is a critical regulator of metazoan development. Inappropriate activation of this network is implicated in several different cancers, including breast. Genetic evidence in mice as well as molecular biological studies in human cells clearly indicate that activated signaling can lead to mammary hyperplasia and, in some cases, tumor formation. However, the exact role(s) activated hedgehog signaling plays in the development or progression of breast cancer also remain unclear. In this review, we have discussed recent data regarding the mechanism(s) by which the hedgehog network may signal in the mammary gland, as well as the data implicating activated signaling as a contributing factor to breast cancer development. Finally, we provide a brief update on the available hedgehog signaling inhibitors with respect to ongoing clinical trials, some of which will include locally advanced or metastatic breast cancers. Given the growing intensity with which the hedgehog signaling network is being studied in the normal and neoplastic mammary gland, a more complete understanding of this network should allow more effective targeting of its activities in breast cancer treatment or prevention.

Protection from lipopolysaccharide-induced pulmonary microvascular endothelial cell injury by activation of hedgehog signaling pathway

Pulmonary microvascular endothelial cells (PMVECs) are critically involved in the pathogenesis of acute lung injury. Hedgehog signaling pathway plays a fundamental role in embryonic development as well as adult morphogenesis and carcinogenesis. As the priming protein of hedgehog signaling pathway, sonic hedgehog (Shh) may recently be advantage for decreasing endothelial injury and promoting the repair of endothelial barrier function. To investigate the expression and role of hedgehog signal pathway in PMVECs injured by lipopolysaccharide (LPS), cells were divided into six groups: control group, LPS group, rhShh group, LPS+rhShh group, rhShh+cyclopamine group, and LPS+rhShh+cyclopamine group. Real time RT-PCR and Western blotting were used to detect the mRNA and protein expression of hedgehog signal molecules including Shh, Patched-1 (Ptc-1) and Gli1 in nucleus. The activity of PMVECs was examined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. In this study, we found that Shh, Ptch1, and Gli1 were expressed in rat PMVECs and their expression decreased when cells were treated by LPS. In the other hand, LPS inhibited the activity of rat PMVECs and caused the cells injury. Activation of Hedgehog signaling pathway by Shh could elevate the activity of PMVECs with pretreatment by LPS. Therefore, hedgehog signaling pathway should play a protective role on injury PMVECs by LPS.

In the absence of Sonic hedgehog, p53 induces apoptosis and inhibits retinal cell proliferation, cell-cycle exit and differentiation in zebrafish

Background

Sonic hedgehog (Shh) signaling regulates cell proliferation during vertebrate development via induction of cell-cycle regulator gene expression or activation of other signalling pathways, prevents cell death by an as yet unclear mechanism and is required for differentiation of retinal cell types. Thus, an unsolved question is how the same signalling molecule can regulate such distinct cell processes as proliferation, cell survival and differentiation.

Methodology/Principal Findings

Analysis of the zebrafish shh^−/− mutant revealed that in this context p53 mediates elevated apoptosis during nervous system and retina development and interferes with retinal proliferation and differentiation. While in shh^−/− mutants there is activation of p53 target genes and p53-mediated apoptosis, an increase in Hedgehog (Hh) signalling by over-expression of dominant-negative Protein Kinase A strongly decreased p53 target gene expression and apoptosis levels in shh^−/− mutants. Using a novel p53 reporter transgene, I confirm that p53 is active in tissues that require Shh for cell survival. Proliferation assays revealed that loss of p53 can rescue normal cell-cycle exit and the mitotic indices in the shh^−/− mutant retina at 24, 36 and 48 hpf. Moreover, generation of amacrine cells and photoreceptors was strongly enhanced in the double p53^−/−shh^−/− mutant retina suggesting the effect of p53 on retinal differentiation.

Conclusions

Loss of Shh signalling leads to the p53-dependent apoptosis in the developing nervous system and retina. Moreover, Shh-mediated control of p53 activity is required for proliferation and cell cycle exit of retinal cells as well as differentiation of amacrine cells and photoreceptors.

Peptide amphiphile nanofiber delivery of sonic hedgehog protein to reduce smooth muscle apoptosis in the penis after cavernous nerve resection

INTRODUCTION

Erectile dysfunction (ED) is a serious medical condition that affects 16-82% of prostate cancer patients treated by radical prostatectomy and current treatments are ineffective in 50-60% of prostatectomy patients. The reduced efficacy of treatments makes novel therapeutic approaches to treat ED essential. The secreted protein Sonic hedgehog (SHH) is a critical regulator of penile smooth muscle and apoptosis that is decreased in cavernous nerve (CN) injury and diabetic ED models. Past studies using Affi-Gel beads have shown SHH protein to be effective in suppressing apoptosis caused by CN injury.

AIM

We hypothesize that SHH protein delivered via novel peptide amphiphile (PA) nanofibers will be effective in suppressing CN injury-induced apoptosis.

METHODS

Adult Sprague Dawley rats (n=50) were used to optimize PA injection in vivo. PA with SHH protein (n=16) or bovine serum albumin (BSA) (control, n=14) was injected into adult rats that underwent bilateral CN cut. Rats were sacrificed at 2, 4, and 7 days. Alexa Fluor-labeled SHH protein was used to determine the target of SHH signaling (n=3).

MAIN OUTCOME MEASURES

Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and semiquantitative immunohistochemical analysis for SHH protein and cluster differentiation protein three (CD3) were performed.

RESULTS

SHH-PA caused a 25% and 16% reduction in apoptosis at 4 and 7 days after CN injury and a 9.3% and 19% increase in SHH protein at 4 and 7 days after CN injury. CD3 protein was not observed in SHH-PA-treated penis. In vitro, 73% of SHH protein diffused from PA within 6 days. Labeled SHH was observed in smooth muscle.

CONCLUSIONS

PA technology is effective in delivering SHH protein to the penis and SHH is effective in suppressing CN injury-induced apoptosis. These results suggest substantial translational potential of this methodology and show that only a short duration of SHH treatment is required to impact the apoptotic index.

Ptch1 is required locally for mammary gland morphogenesis and systemically for ductal elongation

Systemic hormones and local growth factor-mediated tissue interactions are essential for mammary gland development. Using phenotypic and transplantation analyses of mice carrying the mesenchymal dysplasia (mes) allele of patched 1 (Ptch1(mes)), we found that Ptch1(mes) homozygosity led to either complete failure of gland development, failure of post-pubertal ductal elongation, or delayed growth with ductal dysplasia. All ductal phenotypes could be present in the same animal. Whole gland and epithelial fragment transplantation each yielded unique morphological defects indicating both epithelial and stromal functions for Ptch1. However, ductal elongation was rescued in all cases, suggesting an additional systemic function. Epithelial function was confirmed using a conditional null Ptch1 allele via MMTV-Cre-mediated disruption. In Ptch1(mes) homozygotes, failure of ductal elongation correlated with diminished estrogen and progesterone receptor expression, but could not be rescued by exogenous ovarian hormone treatment. By contrast, pituitary isografts were able to rescue the ductal elongation phenotype. Thus, Ptch1 functions in the mammary epithelium and stroma to regulate ductal morphogenesis, and in the pituitary to regulate ductal elongation and ovarian hormone responsiveness.

Toward understanding the development of scleral ossicles in the chicken, Gallus gallus

Scleral ossicles are dermal bones that are present in the eye of many vertebrates. Despite this, little is understood about their development. This study investigates the cellular dynamics during and after induction, and attempts to identify inducing factors. Both cell death and proliferation were found to play limited roles in mesenchymal condensation formation, but are involved in development of the inducing epithelium overlying the presumptive ossicle. Real-time reverse transcriptase polymerase chain reaction of candidate genes identified significant increases in sonic hedgehog (SHH) expression. In situ hybridization confirmed that SHH is exclusively expressed in the conjunctival (scleral) papillae and not in the mesenchyme. Direct localized inhibition of Hedgehog signaling, by means of cyclopamine, supports the finding that SHH may play a role in scleral ossicle induction. In addition, a nonfluctuating asymmetry with respect to the number of ossicles per eye was found. This study provides significant insight into understanding the development of the neural crest derived dermal bones.

Neural influences on sonic hedgehog and apoptosis in the rat penis

The role of sonic hedgehog (SHH) in maintaining corpora cavernosal morphology in the adult penis has been established; however, the mechanism of how SHH itself is regulated remains unclear. Since decreased SHH protein is a cause of smooth muscle apoptosis and erectile dysfunction (ED) in the penis, and SHH treatment can suppress cavernous nerve (CN) injury-induced apoptosis, the question of how SHH signaling is regulated is significant. It is likely that neural input is involved in this process since two models of neuropathy-induced ED exhibit decreased SHH protein and increased apoptosis in the penis. We propose the hypothesis that SHH abundance in the corpora cavernosa is regulated by SHH signaling in the pelvic ganglia, neural activity, or neural transport of a trophic factor from the pelvic ganglia to the corpora. We have examined each of these potential mechanisms. SHH inhibition in the penis shows a 12-fold increase in smooth muscle apoptosis. SHH inhibition in the pelvic ganglia causes significantly increased apoptosis (1.3-fold) and decreased SHH protein (1.1-fold) in the corpora cavernosa. SHH protein is not transported by the CN. Colchicine treatment of the CN resulted in significantly increased smooth muscle apoptosis (1.2-fold) and decreased SHH protein (1.3-fold) in the penis. Lidocaine treatment of the CN caused a similar increase in apoptosis (1.6-fold) and decrease in SHH protein (1.3-fold) in the penis. These results show that neural activity and a trophic factor from the pelvic ganglia/CN are necessary to regulate SHH protein and smooth muscle abundance in the penis.

Purification and bioassay of hedgehog ligands for the study of cell death and survival

The Hedgehog (Hh) family of secreted ligands-composed of Sonic Hedgehog (Shh), Indian Hedgehog (Ihh), and Desert Hedgehog (Dhh)-possesses many roles during embryonic development, adult homeostasis, and cancer. The specific functions of the Hh proteins are intertwined with their requirement as survival factors in Hh-responsive cells. However, studies designed to dissect the anti-apoptotic role of Hhs have been hindered by the lack of simple approaches to purify large quantities of recombinant ligands in the average laboratory setting because of the natural modifications of these proteins with palmitic acid and cholesterol. In this chapter, we provide a comprehensive protocol for the expression of Shh, Ihh, and Dhh in Escherichia coli as fusion proteins with calmodulin-binding peptide to allow easy and rapid purification. The ligands are engineered with a new N-terminus containing two isoleucine residues to provide an essential hydrophobic interphase for achieving high biologic activity. The protocol includes a detailed description of a method for determination of the specific activity of the generated proteins by use of a cell culture-based luciferase approach.

Expression of EIF3-p48/INT6, TID1 and Patched in cancer, a profiling of multiple tumor types and correlation of expression

Alterations in eIF3-p48/INT6 gene expression have been implicated in murine and human mammary carcinogenesis. We examined levels of INT6 protein in human tumors and determined that breast and colon tumors clustered into distinct groups based on levels of INT6 expression and clinicopathological variables. We performed multiplex tissue immunoblotting of breast, colon, lung, and ovarian tumor tissues and found that INT6 protein levels positively correlated with those of TID1, Patched, p53, c-Jun, and phosphorylated-c-Jun proteins in a tissue-specific manner. INT6 and TID1 showed significant positive correlation in all tissue types tested. These findings were confirmed by immunohistochemical staining of INT6 and TID1. Further evidence supporting a cooperative role for INT6 and TID1 is the presence of endogenous INT6 and TID1 proteins as complexes. We detected co-immunoprecipitation between INT6 and TID1, as well as between INT6 and Patched. These findings suggest potential integrated roles for INT6, TID1, and Patched proteins in cell growth, development, and tumorigenesis. Additionally, these data suggest that the combination of INT6, TID1, and Patched protein levels may be useful biomarkers for the development of diagnostic assays.

Promotion of Hras-induced squamous carcinomas by a polymorphic variant of the Patched gene in FVB mice

Mice of the C57BL/6 strain are resistant to the development of skin squamous carcinomas (SCCs) induced by an activated Ras oncogene, whereas FVB/N mice are highly susceptible. The genetic basis of this difference in phenotype is unknown. Here we show that susceptibility to SCC is under the control of a carboxy-terminal polymorphism in the mouse Ptch gene. F1 hybrids between C57BL/6 and FVB/N strains ((B6FVB)F1) are resistant to Ras-induced SCCs, but resistance can be overcome either by elimination of the C57BL/6 Ptch allele (Ptch(B6)) or by overexpression of the FVB/N Ptch allele (Ptch(FVB)) in the epidermis of K5Hras-transgenic (B6FVB)F1 hybrid mice. The human Patched (PTCH) gene is a classical tumour suppressor gene for basal cell carcinomas and medulloblastomas, the loss of which causes increased signalling through the Sonic Hedgehog (SHH) pathway. SCCs that develop in PtchB6+/- mice do not lose the wild-type Ptch gene or show evidence of increased SHH signalling. Although Ptch(FVB) overexpression can promote SCC formation, continued expression is not required for tumour maintenance, suggesting a role at an early stage of tumour cell lineage commitment. The Ptch polymorphism affects Hras-induced apoptosis, and binding to Tid1, the mouse homologue of the Drosophila l(2)tid tumour suppressor gene. We propose that Ptch occupies a critical niche in determining basal or squamous cell lineage, and that both tumour types can arise from the same target cell depending on carcinogen exposure and host genetic background.

The hedgehog signaling network, mammary stem cells, and breast cancer: connections and controversies

Several signal transduction networks have been implicated in the regulation of mammary epithelial stem cell self-renewal and maintenance (Kalirai and Clarke 2006; Liu et al. 2005). These signaling networks include those of the Wnt, Notch, TGFO, EGF, FGF, IGF, and most recently, the Hedgehog (Hh) families of secreted ligands. However, we currently know very little about the cellular and molecular mechanisms by which these signaling pathways function to regulate normal epithelial stem/progenitor cells. What is clear is that the regulatory signaling networks thought to control normal stem/progenitor cell self-renewal and maintenance are, with the current sole exception of the hedgehog network, well-documented to have contributory roles in mammary cancer development and disease progression when misregulated. In this review, genetic regulation of mammary gland development by hedgehog network genes is outlined, highlighting a developing controversy as to whether activated hedgehog signaling regulates normal regenerative mammary epithelial stem cells or, indeed, whether activated hedgehog signaling functions at all in ductal development. In addition, the question of whether inappropriate hedgehog network activation influences breast cancer development is addressed, with emphasis on the prospects for using hedgehog signaling antagonists clinically for breast cancer treatment or prevention.

Regulation of cavernous nerve injury-induced apoptosis by sonic hedgehog

Thirty to eighty-seven percent of patients treated by radical prostatectomy experience erectile dysfunction (ED). The reduced efficacy of treatments in this population makes novel therapeutic approaches to treat ED essential. We propose that abundant apoptosis observed in penile smooth muscle when the cavernous nerve (CN) is cut (mimicking the neural injury which can result from prostatectomy) is a major contributing factor to ED development. We hypothesize that decreased Sonic hedgehog (SHH) signaling is a cause of ED in neurological models of impotence by increasing apoptosis in penile smooth muscle. We examined this hypothesis in a bilateral CN injury model of ED. We found that the active form of SHH protein was significantly decreased 1.2-fold following CN injury, that SHH inhibition causes a 12-fold increase in smooth muscle apoptosis in the penis, and that SHH treatment at the time of CN injury was able to decrease CN injury-induced apoptosis (1-3-fold) in a dose-dependent manner. These results show that SHH stabilizes the alterations of the corpora cavernosal smooth muscle following nerve injury.

Contributions of PTCH gene variants to isolated cleft lip and palate

OBJECTIVE

Mutations in patched (PTCH) cause the nevoid basal cell carcinoma syndrome (NBCCS), or Gorlin syndrome. Nevoid basal cell carcinoma syndrome may present with developmental anomalies, including rib and craniofacial abnormalities, and predisposes to several tumor types, including basal cell carcinoma and medulloblastoma. Cleft palate is found in 4% of individuals with nevoid basal cell carcinoma syndrome. Because there might be specific sequence alterations in PTCH that limit expression to orofacial clefting, a genetic study of PTCH was undertaken in cases with cleft lip and/or palate (CL/P) known not to have nevoid basal cell carcinoma syndrome.

RESULTS

Seven new normal variants spread along the entire gene and three missense mutations were found among cases with cleft lip and/or palate. One of these variants (P295S) was not found in any of 1188 control samples. A second variant was found in a case and also in 1 of 1119 controls. The third missense (S827G) was found in 5 of 1369 cases and in 5 of 1104 controls and is likely a rare normal variant. Linkage and linkage desequilibrium also was assessed using normal variants in and adjacent to the PTCH gene in 220 families (1776 individuals), each with two or more individuals with isolated clefting. Although no statistically significant evidence of linkage (multipoint HLOD peak = 2.36) was uncovered, there was borderline evidence of significant transmission distortion for one haplotype of two single nucleotide polymorphisms located within the PTCH gene (p = .08).

CONCLUSION

Missense mutations in PTCH may be rare causes of isolated cleft lip and/or palate. An as yet unidentified variant near PTCH may act as a modifier of cleft lip and/or palate.

Morphogens and cell survival during development

The notion of "morphogens" is an important one in developmental biology. By definition, a morphogen is a molecule that emanates from a specific set of cells that is present in a concentration gradient and that specifies the fate of each cell along this gradient. The strongest candidate morphogens are members of the transforming growth factor-beta (TGF-beta), Hedgehog (Hh), and Wnt families. While these morphogens have been extensively described as differentiation inducers, some reports also suggest their possible involvement in cell death and cell survival. It is frequently speculated that the cell death induction that is found associated with experimental removal of morphogens is the manifestation of abnormal differentiation signals. However, several recent reports have raised controversy about this death by default, suggesting that cell death regulation is an active process for shaping tissues and organs. In this review, we will present morphogens, with a specific emphasis on Sonic Hedgehog, a mammalian member of the Hh family, not as a positive regulators of cell differentiation but as key regulators of cell survival.

Early neural cell death: dying to become neurons

The importance of programmed cell death (PCD) during vertebrate development has been well established. During the development of the nervous system in particular, neurotrophic cell death in innervating neurons matches the number of neurons to the size of their target field. However, PCD also occurs during earlier stages of neural development, within populations of proliferating neural precursors and newly postmitotic neuroblasts, all of which are not yet fully differentiated. This review addresses early neural PCD, which is distinct from neurotrophic death in differentiated neurons. Although early neural PCD is observed in a range of organisms, from Caenorhabditis elegans to mouse, the role and the regulation of early neural PCD are not well understood. The regulation of early neural PCD can be inferred from the function of factors such as bone morphogenetic proteins (BMPs), Wnts, fibroblast growth factors (FGFs), and Sonic Hedgehog (Shh), which regulate both early neural development and PCD occurring in other developmental processes. Cell number control, removal of damaged or misspecified cells (spatially or temporally), and selection are the proposed roles early neural PCDs play during neural development. Data from developmental PCD in C. elegans and Drosophila provide insights into the possible signaling pathways integrating PCD with other processes during early neural development and the roles they might play.

Next stop, the twilight zone: hedgehog network regulation of mammary gland development

The hedgehog signal transduction network is a critical mediator of cell-cell communication during embryonic development. Evidence also suggests that properly regulated hedgehog network function is required in some adult organs for stem cell maintenance or renewal. Mutation, or misexpression, of network genes is implicated in the development of several different types of cancer, particularly that of skin, brain, lung, and pancreas. Recent studies in the mouse mammary gland have demonstrated roles for hedgehog network genes at virtually every phase of mammary gland development where it regulates such diverse processes as embryonic mammary gland induction, establishment of ductal histoarchitecture, and functional differentiation in lactation. Further, studies suggest a role for misregulated network function in the progression of breast cancer.

Sonic hedgehog signaling plays an essential role during embryonic salivary gland epithelial branching morphogenesis

Gene targeting studies indicate that sonic hedgehog (Shh) signaling plays an essential role during craniofacial development. Because numerous mandibular derivatives (e.g., teeth, tongue, Meckel's cartilage) are absent in Shh null mice and the embryonic submandibular salivary gland (SMG) develops from the mandibular arch, we postulated that Shh signaling is important for embryonic SMG development. To address this question, we first determined the spatiotemporal distribution of Shh; two transmembrane proteins, patched 1 (Ptc) and Smoothened (Smo), which act as a negative or a positive regulator of the Shh signal, respectively; and the Gli 3 transcription factor, which is downstream of the Shh signal. The epithelial localization of Shh, Ptc, Smo, and Gli 3 suggests that Shh signaling may act within the epithelium in a juxtacrine manner. The SMG phenotype in our embryonic day (E) 18.5 Shh null mice can be characterized as "paedomorphic," that is, it fails to progress to ontogenic stages beyond the Early Pseudoglandular ( approximately E14). In a complementary set of experiments, we used organ culture to evaluate the effect of enhanced or abrogated Shh signaling on embryonic SMG development in vitro. Paired E13 (Late Initial Bud stage) or E14 (Pseudoglandular stage) SMGs were cultured in the presence or absence of exogenous Shh peptide supplementation; Shh-supplemented explants exhibit a significant stage-dependent increase in branching morphogenesis compared with control explants. Furthermore, by using cyclopamine, a steroidal alkaloid that specifically disrupts the Shh pathway, to abrogate endogenous Shh signaling in vitro, we found a significant decrease in branching in cyclopamine-treated explants compared with controls, as well as a significant decrease in epithelial cell proliferation. Our results indicate that Shh signaling plays an essential role during embryonic SMG branching morphogenesis. Exogenous FGF8 peptide supplementation in vitro rescues the abnormal SMG phenotype seen in cyclopamine-treated explants, demonstrating that overexpression of a parallel, but related, downstream signaling pathway can compensate for diminished Shh signaling and restore embryonic SMG branching morphogenesis.