The Sonic hedgehog pathway independently controls the patterning, proliferation and survival of neuroepithelial cells by regulating Gli activity

Cell cycle and cell fate in the developing nervous system: the role of CDC25B phosphatase

Deciphering the core machinery of the cell cycle and cell division has been primarily the focus of cell biologists, while developmental biologists have identified the signaling pathways and transcriptional programs controlling cell fate choices. As a result, until recently, the interplay between these two fundamental aspects of biology have remained largely unexplored. Increasing data show that the cell cycle and regulators of the core cell cycle machinery are important players in cell fate decisions during neurogenesis. Here, we summarize recent data describing how cell cycle dynamics affect the switch between proliferation and differentiation, with an emphasis on the roles played by the cell cycle regulators, the CDC25 phosphatases.

Sonic hedgehog signaling in the postnatal brain

Sonic hedgehog (Shh) is a pleiotropic factor in the developing central nervous system (CNS), driving proliferation, specification, and axonal targeting in multiple sites within the forebrain, hindbrain, and spinal cord. Studies in embryonic CNS have shown how gradients of this morphogen are translated by neuroepithelial precursors to determine the types of neurons and glial cells they produce [1,2]. Shh also has a well-characterized role as a mitogen for specific progenitor cell types in neural development [3,4]. As we begin to appreciate that Shh continues to act in the adult brain, a central question is what functional role this ligand plays when major morphogenetic and proliferative processes are no longer in operation. A second fundamental question is whether similar signaling mechanisms operate in embryonic and adult CNS. In the two major germinal zones of the adult brain, Shh signaling modulates the self-renewal and specification of astrocyte-like primary progenitors, frequently referred to as neural stem cells (NSCs). It also may regulate the response of the mature brain to injury, as Shh signaling has been variously proposed to enhance or inhibit the development of a reactive astrocyte phenotype. The identity of cells producing the Shh ligand, and the conditions that trigger its release, are also areas of growing interest; both germinal zones in the adult brain contain Shh-responsive cells but do not autonomously produce this ligand. Here, we review recent findings revealing the function of this fascinating pathway in the postnatal and adult brain, and highlight ongoing areas of investigation into its actions long past the time when it shapes the developing brain.

Glioblastoma stem cells: Molecular characteristics and therapeutic implications

Glioblastoma Multiforme (GBM) is a grade IV astrocytoma, with a median survival of 14.6 mo. Within GBM, stem-like cells, namely glioblastoma stem cells (GSCs), have the ability to self-renew, differentiate into distinct lineages within the tumor and initiate tumor xenografts in immunocompromised animal models. More importantly, GSCs utilize cell-autonomous and tumor microenvironment-mediated mechanisms to overcome current therapeutic approaches. They are, therefore, very important therapeutic targets. Although the functional criteria defining GSCs are well defined, their molecular characteristics, the mechanisms whereby they establish the cellular hierarchy within tumors, and their contribution to tumor heterogeneity are not well understood. This review is aimed at summarizing current findings about GSCs and their therapeutic importance from a molecular and cellular point of view. A better characterization of GSCs is crucial for designing effective GSC-targeted therapies.

ADAM10 negatively regulates neuronal differentiation during spinal cord development

Members of the ADAM (a disintegrin and metalloprotease) family are involved in embryogenesis and tissue formation via their proteolytic function, cell-cell and cell-matrix interactions. ADAM10 is expressed temporally and spatially in the developing chicken spinal cord, but its function remains elusive. In the present study, we address this question by electroporating ADAM10 specific morpholino antisense oligonucleotides (ADAM10-mo) or dominant-negative ADAM10 (dn-ADAM10) plasmid into the developing chicken spinal cord as well as by in vitro cell culture investigation. Our results show that downregulation of ADAM10 drives precocious differentiation of neural progenitor cells and radial glial cells, resulting in an increase of neurons in the developing spinal cord, even in the prospective ventricular zone. Remarkably, overexpression of the dn-ADAM10 plasmid mutated in the metalloprotease domain (dn-ADAM10-me) mimics the phenotype as found by the ADAM10-mo transfection. Furthermore, in vitro experiments on cultured cells demonstrate that downregulation of ADAM10 decreases the amount of the cleaved intracellular part of Notch1 receptor and its target, and increases the number of βIII-tubulin-positive cells during neural progenitor cell differentiation. Taken together, our data suggest that ADAM10 negatively regulates neuronal differentiation, possibly via its proteolytic effect on the Notch signaling during development of the spinal cord.

Targeting sonic hedgehog-associated medulloblastoma through inhibition of Aurora and Polo-like kinases

Medulloblastoma is the most common malignant brain tumor in children. Although aggressive surgery, radiation, and chemotherapy have improved outcomes, survivors suffer severe long-term side effects, and many patients still succumb to their disease. For patients whose tumors are driven by mutations in the sonic hedgehog (SHH) pathway, SHH antagonists offer some hope. However, many SHH-associated medulloblastomas do not respond to these drugs, and those that do may develop resistance. Therefore, more effective treatment strategies are needed for both SHH and non-SHH-associated medulloblastoma. One such strategy involves targeting the cells that are critical for maintaining tumor growth, known as tumor-propagating cells (TPC). We previously identified a population of TPCs in tumors from patched mutant mice, a model for SHH-dependent medulloblastoma. These cells express the surface antigen CD15/SSEA-1 and have elevated levels of genes associated with the G2-M phases of the cell cycle. Here, we show that CD15(+) cells progress more rapidly through the cell cycle than CD15(-) cells and contain an increased proportion of cells in G2-M, suggesting that they might be vulnerable to inhibitors of this phase. Indeed, exposure of tumor cells to inhibitors of Aurora kinase (Aurk) and Polo-like kinases (Plk), key regulators of G2-M, induces cell-cycle arrest, apoptosis, and enhanced sensitivity to conventional chemotherapy. Moreover, treatment of tumor-bearing mice with these agents significantly inhibits tumor progression. Importantly, cells from human patient-derived medulloblastoma xenografts are also sensitive to Aurk and Plk inhibitors. Our findings suggest that targeting G2-M regulators may represent a novel approach for treatment of human medulloblastoma.

Pax6 is required for normal cell-cycle exit and the differentiation kinetics of retinal progenitor cells

The coupling between cell-cycle exit and onset of differentiation is a common feature throughout the developing nervous system, but the mechanisms that link these processes are mostly unknown. Although the transcription factor Pax6 has been implicated in both proliferation and differentiation of multiple regions within the central nervous system (CNS), its contribution to the transition between these successive states remains elusive. To gain insight into the role of Pax6 during the transition from proliferating progenitors to differentiating precursors, we investigated cell-cycle and transcriptomic changes occurring in Pax6 (-) retinal progenitor cells (RPCs). Our analyses revealed a unique cell-cycle phenotype of the Pax6-deficient RPCs, which included a reduced number of cells in the S phase, an increased number of cells exiting the cell cycle, and delayed differentiation kinetics of Pax6 (-) precursors. These alterations were accompanied by coexpression of factors that promote (Ccnd1, Ccnd2, Ccnd3) and inhibit (P27 (kip1) and P27 (kip2) ) the cell cycle. Further characterization of the changes in transcription profile of the Pax6-deficient RPCs revealed abrogated expression of multiple factors which are known to be involved in regulating proliferation of RPCs, including the transcription factors Vsx2, Nr2e1, Plagl1 and Hedgehog signaling. These findings provide novel insight into the molecular mechanism mediating the pleiotropic activity of Pax6 in RPCs. The results further suggest that rather than conveying a linear effect on RPCs, such as promoting their proliferation and inhibiting their differentiation, Pax6 regulates multiple transcriptional networks that function simultaneously, thereby conferring the capacity to proliferate, assume multiple cell fates and execute the differentiation program into retinal lineages.

Sonic hedgehog signaling switches the mode of division in the developing nervous system

The different modes of stem cell division are tightly regulated to balance growth and differentiation during organ development and homeostasis, and these regulatory processes are subverted in tumor formation. Here, we developed markers that provided the single-cell resolution necessary to quantify the three modes of division taking place in the developing nervous system in vivo: self-expanding, PP; self-replacing, PN; and self-consuming, NN. Using these markers and a mathematical model that predicts the dynamics of motor neuron progenitor division, we identify a role for the morphogen Sonic hedgehog in the maintenance of stem cell identity in the developing spinal cord. Moreover, our study provides insight into the process linking lineage commitment to neurogenesis with changes in cell-cycle parameters. As a result, we propose a challenging model in which the external Sonic hedgehog signal dictates stem cell identity, reflected in the consequent readjustment of cell-cycle parameters.

Brain tumor stem cells: Molecular characteristics and their impact on therapy

Glioblastoma (GBM) is the most prevalent primary brain tumor and ranks among the most lethal of human cancers with conventional therapy offering only palliation. Great strides have been made in understanding brain cancer genetics and modeling these tumors with new targeted therapies being tested, but these advances have not translated into substantially improved patient outcomes. Multiple chemotherapeutic agents, including temozolomide, the first-line treatment for glioblastoma, have been developed to kill cancer cells. However, the response to temozolomide in GBM is modest. Radiation is also moderately effective but this approach is plagued by limitations due to collateral radiation damage to healthy brain tissue and development of radioresistance. Therapeutic resistance is attributed at least in part to a cell population within the tumor that possesses stem-like characteristics and tumor propagating capabilities, referred to as cancer stem cells. Within GBM, the intratumoral heterogeneity is derived from a combination of regional genetic variance and a cellular hierarchy often regulated by distinct cancer stem cell niches, most notably perivascular and hypoxic regions. With the recent emergence as a key player in tumor biology, cancer stem cells have symbiotic relationships with the tumor microenvironment, oncogenic signaling pathways, and epigenetic modifications. The origins of cancer stem cells and their contributions to brain tumor growth and therapeutic resistance are under active investigation with novel anti-cancer stem cell therapies offering potential new hope for this lethal disease.

Sox2 is required for embryonic development of the ventral telencephalon through the activation of the ventral determinants Nkx2.1 and Shh

The Sox2 transcription factor is active in stem/progenitor cells throughout the developing vertebrate central nervous system. However, its conditional deletion at E12.5 in mouse causes few brain developmental problems, with the exception of the postnatal loss of the hippocampal radial glia stem cells and the dentate gyrus. We deleted Sox2 at E9.5 in the telencephalon, using a Bf1-Cre transgene. We observed embryonic brain defects that were particularly severe in the ventral, as opposed to the dorsal, telencephalon. Important tissue loss, including the medial ganglionic eminence (MGE), was detected at E12.5, causing the subsequent impairment of MGE-derived neurons. The defect was preceded by loss of expression of the essential ventral determinants Nkx2.1 and Shh, and accompanied by ventral spread of dorsal markers. This phenotype is reminiscent of that of mice mutant for the transcription factor Nkx2.1 or for the Shh receptor Smo. Nkx2.1 is known to mediate the initial activation of ventral telencephalic Shh expression. A partial rescue of the normal phenotype at E14.5 was obtained by administration of a Shh agonist. Experiments in Medaka fish indicate that expression of Nkx2.1 is regulated by Sox2 in this species also. We propose that Sox2 contributes to Nkx2.1 expression in early mouse development, thus participating in the region-specific activation of Shh, thereby mediating ventral telencephalic patterning induction.

Slit/Robo1 signaling regulates neural tube development by balancing neuroepithelial cell proliferation and differentiation

Formation of the neural tube is the morphological hallmark for development of the embryonic central nervous system (CNS). Therefore, neural tube development is a crucial step in the neurulation process. Slit/Robo signaling was initially identified as a chemo-repellent that regulated axon growth cone elongation, but its role in controlling neural tube development is currently unknown. To address this issue, we investigated Slit/Robo1 signaling in the development of chick neCollege of Life Sciences Biocentre, University of Dundee, Dundee DD1 5EH, UKural tube and transgenic mice over-expressing Slit2. We disrupted Slit/Robo1 signaling by injecting R5 monoclonal antibodies into HH10 neural tubes to block the Robo1 receptor. This inhibited the normal development of the ventral body curvature and caused the spinal cord to curl up into a S-shape. Next, Slit/Robo1 signaling on one half-side of the chick embryo neural tube was disturbed by electroporation in ovo. We found that the morphology of the neural tube was dramatically abnormal after we interfered with Slit/Robo1 signaling. Furthermore, we established that silencing Robo1 inhibited cell proliferation while over-expressing Robo1 enhanced cell proliferation. We also investigated the effects of altering Slit/Robo1 expression on Sonic Hedgehog (Shh) and Pax7 expression in the developing neural tube. We demonstrated that over-expressing Robo1 down-regulated Shh expression in the ventral neural tube and resulted in the production of fewer HNK-1(+) migrating neural crest cells (NCCs). In addition, Robo1 over-expression enhanced Pax7 expression in the dorsal neural tube and increased the number of Slug(+) pre-migratory NCCs. Conversely, silencing Robo1 expression resulted in an enhanced Shh expression and more HNK-1(+) migrating NCCs but reduced Pax7 expression and fewer Slug(+) pre-migratory NCCs were observed. In conclusion, we propose that Slit/Robo1 signaling is involved in regulating neural tube development by tightly coordinating cell proliferation and differentiation during neurulation.

Smoothened antagonists: a promising new class of antitumor agents

BACKGROUND

Hedgehog signaling is essential for the development of most metazoans. In recent years, evidence has accumulated showing that many human tumors aberrantly re-activate this developmental signaling pathway and that interfering with it may provide a new strategy for the development of novel anti-cancer therapeutics. Smoothened is a G-protein coupled receptor-like protein that is essentially involved in hedgehog signal transduction and small molecule antagonists of Smoothened have started to show antitumor activity in preclinical models and in clinical trials.

OBJECTIVE

We critically review the role of hedgehog signaling in normal development and in human malignancies, the available drug discovery tools and the classes of small molecule inhibitors that are in development. We further aim to address the potential impact that pathway antagonists may have on the treatment options of cancer patients.

METHODS

Literature, patents and clinical trial results from the past 5 years were analyzed.

CONCLUSIONS

1) A large body of evidence suggests a frequent reactivation of hedgehog signaling in human cancer. 2) Smoothened is an attractive, highly druggable target with extensive preclinical and initial clinical validation in basal cell carcinoma. Several promising novel classes of Smoothened antagonists have been discovered and are being developed as anticancer agents. 3) Our knowledge of the biology of hedgehog signaling in cancer is still very incomplete and significant efforts will be required to understand how to use the emerging novel agents in the clinic.

Dorsal-ventral patterning of the neural tube: a tale of three signals

Development of the vertebrate nervous system begins with the acquisition of neural identity from the midline dorsal-ectodermal cells of the gastrulating embryos. The subsequent progressive specification of the neural plate along its anterior-posterior and dorsal-ventral (DV) axes allows the generation of the tremendous variety of neuronal and glial cells that compose the vertebrate central nervous system (CNS). Studies on the development of the spinal cord, the anatomically simplest part of the CNS, have generated most of our current knowledge on the signaling events and the genetic networks that orchestrate the DV patterning of the neural plate. In this review, we discuss the recent advances in our understanding of these events and highlight unresolved questions. We focused our attention on the activity and the integration of the three main instructive cues: Sonic hedgehog, the Wnts and the Bone Morphogenetic Proteins, giving particular attention to the less well understood dorsal signaling events.

Primary cilia and Gli3 activity regulate cerebral cortical size

During neural development patterning, neurogenesis, and overall growth are highly regulated and coordinated between different brain regions. Here, we show that primary cilia and the regulation of Gli activity are necessary for the normal expansion of the cerebral cortex. We show that loss of Kif3a, an important functional component of primary cilia, leads to the degeneration of primary cilia, marked overgrowth of the cortex, and altered cell cycle kinetics within cortical progenitors. The G1 phase of the cell cycle is shortened through a mechanism likely involving reduced Gli3 activity and a resulting increase in expression of cyclin D1 and Fgf15. The defects in Gli3 activity alone are sufficient to accelerate cell cycle kinetics and cause the molecular changes seen in brains that lack cilia. Finally, we show that levels of full-length and repressor Gli3 proteins are tightly regulated during normal development and correlate with changes in expression of two known Shh-target genes, CyclinD1 and Fgf15, and with the normal lengthening of the cell cycle during corticogenesis. These data suggest that Gli3 activity is regulated through the primary cilium to control cell cycle length in the cortex and thus determine cortical size.

Adaptor protein LNK is a negative regulator of brain neural stem cell proliferation after stroke

Ischemic stroke causes transient increase of neural stem and progenitor cell (NSPC) proliferation in the subventricular zone (SVZ), and migration of newly formed neuroblasts toward the damaged area where they mature to striatal neurons. The molecular mechanisms regulating this plastic response, probably involved in structural reorganization and functional recovery, are poorly understood. The adaptor protein LNK suppresses hematopoietic stem cell self-renewal, but its presence and role in the brain are poorly understood. Here we demonstrate that LNK is expressed in NSPCs in the adult mouse and human SVZ. Lnk(-/-) mice exhibited increased NSPC proliferation after stroke, but not in intact brain or following status epilepticus. Deletion of Lnk caused increased NSPC proliferation while overexpression decreased mitotic activity of these cells in vitro. We found that Lnk expression after stroke increased in SVZ through the transcription factors STAT1/3. LNK attenuated insulin-like growth factor 1 signaling by inhibition of AKT phosphorylation, resulting in reduced NSPC proliferation. Our findings identify LNK as a stroke-specific, endogenous negative regulator of NSPC proliferation, and suggest that LNK signaling is a novel mechanism influencing plastic responses in postischemic brain.

Mouse thalamic differentiation: gli-dependent pattern and gli-independent prepattern

Sonic hedgehog (Shh) signaling is essential for thalamic development. The Gli transcription factors act downstream of Shh – while Gli2 is the major activator (GliA), Gli3 acts primarily as a repressor (GliR). The thalamus is remarkable among dorsal structures because of its proximity to the mid-diencephalic organizer, a unique dorsal Shh source. This lends complexity to the interactions between Shh, Gli2, and Gli3, suggesting the presence of a dorsal Gli activator which elsewhere is found only ventrally, and making the dissection of thalamic Gli functions particularly interesting. A current model based on mutant phenotypes in telencephalon and midbrain postulates a degree of reciprocal antagonism of Shh and Gli3 in dorsal brain regions. To approach the role of Gli factors in thalamic specification we first analyzed mice deficient in Gli2 or Gli3. In Gli2 mutants, the thalamus is small and poorly differentiated with the exception of the medial and intralaminar nuclei which, in contrast, are specifically and severely affected by Gli3 inactivation. Gbx2 expression is very reduced in the Gli3 mutant. Most thalamic nuclei are present in both mutants, although incompletely differentiated, as reflected by the loss of specific markers. The ventral posterior group, revealed by novel specific marker Hes1, is present in both mutants and extends axons to the telencephalon. To test the Gli3/Shh interaction we generated a novel mutant deficient in Gli3 and neuroepithelial Shh. The thalamus of the n-Shh/Gli3 double mutants is very large and very poorly differentiated except for a broad domain of Gbx2, Lhx2, and Calb2 expression. In utero electroporation experiments on wild type embryos suggest that a stage-specific factor acting early is responsible for this prepattern. We show that, in the thalamus, GliA acts downstream of Shh to specify pattern and size of the thalamic nuclei to the exception of the medial and intralaminar groups. Gli3A can partially substitute for Gli2A in the Gli2 mutant. GliR is essential for specification and growth of the medial and intralaminar nuclei, contributes to the specification of other thalamic nuclei and reduces thalamic size. GliA (from neuroepithelial Shh signaling) and GliR do not show reciprocal antagonism in the thalamus, and their joint abolition does not rescue the wild type phenotype.

The CDC25B phosphatase shortens the G2 phase of neural progenitors and promotes efficient neuron production

During embryonic development, changes in cell cycle kinetics have been associated with neurogenesis. This observation suggests that specific cell cycle regulators may be recruited to modify cell cycle dynamics and influence the decision between proliferation and differentiation. In the present study, we investigate the role of core positive cell cycle regulators, the CDC25 phosphatases, in this process. We report that, in the developing chicken spinal cord, only CDC25A is expressed in domains where neural progenitors undergo proliferative self-renewing divisions, whereas the combinatorial expression of CDC25A and CDC25B correlates remarkably well with areas where neurogenesis occurs. We also establish that neural progenitors expressing both CDC25A and CDC25B have a shorter G2 phase than those expressing CDC25A alone. We examine the functional relevance of these correlations using an RNAi-based method that allows us to knock down CDC25B efficiently and specifically. Reducing CDC25B expression results in a specific lengthening of the G2 phase, whereas the S-phase length and the total cell cycle time are not significantly modified. This modification of cell cycle kinetics is associated with a reduction in neuron production that is due to the altered conversion of proliferating neural progenitor cells to post-mitotic neurons. Thus, expression of CDC25B in neural progenitors has two functions: to change cell cycle kinetics and in particular G2-phase length and also to promote neuron production, identifying new roles for this phosphatase during neurogenesis.

Jagged2 controls the generation of motor neuron and oligodendrocyte progenitors in the ventral spinal cord

In the developing spinal cord, motor neurons (MNs) and oligodendrocytes arise sequentially from a common pool of progenitors. However, the genetic network responsible for this neurogenesis to gliogenesis switch is largely unknown. A transcriptome analysis identified the Notch ligand Jagged2 (JAG2) as a Sonic hedgehog-regulated factor transiently expressed in MN progenitors (pMNs). In vivo loss- and gain-of-function experiments show that JAG2 schedules the differentiation of the pMN progenitors. At early developmental stages, Olig2 expressing pMN progenitors that enter the differentiation pathway exclusively generate MNs. At these times, the activation of the Notch pathway by JAG2 maintains selected pMN progenitors in an undifferentiated state by two mechanisms; first it inhibits MN generation by reducing Olig2 proteins levels, and second it directly inhibits the premature generation of oligodendrocyte progenitors (OLPs) by maintaining high levels of Hes5. Later, extinction of JAG2 from the pMN results in the loss of Hes5 expression, heralding the gliogenic phase of pMN progenitors. Strikingly, downregulation of JAG2 in pMN progenitors is sufficient to promote the precocious generation of OLPs. Together these data provide evidence that JAG2 is a key regulator of the timely and ordered generation of two of the defining cell types in the spinal cord, MNs and OLPs.

Sonic Hedgehog regulates Wnt activity during neural circuit formation

Gradients of secreted morphogens, such as Sonic hedgehog (Shh), Wnt, and TGFβ/Bmp, have classically been shown to control many aspects of early development by regulating cell proliferation and determining cell fate. However, recent studies demonstrate that these molecules also play important and evolutionarily conserved roles in later aspects of neural development. Depending on the context, these molecules can elicit gene transcription in the nucleus, or alternatively can provide instructive signals at the growth cone that induce local and rapid changes in cytoskeletal organization. Shh can activate different cellular transduction pathways via its binding to alternative coreceptor complexes or simply by adaptation of its "classical" signaling pathway. However, in most of its activities during neural development, Shh does not act alone but rather in concert with other morphogens, particularly the Wnts. This review provides an overview of the mechanisms by which Shh signaling acts in concert with Wnts to mediate a myriad of cellular processes that are required for neural circuit formation.

Noncanonical Hedgehog signaling

The notion of noncanonical hedgehog (Hh) signaling in mammals has started to receive support from numerous observations. By noncanonical, we refer to all those cellular and tissue responses to any of the Hh isoforms that are independent of transcriptional changes mediated by the Gli family of transcription factors. In this chapter, we discuss the most recent findings that suggest that Patched1 can regulate cell proliferation and apoptosis independently of Smoothened (Smo) and Gli and the reports that Smo modulates actin cytoskeleton-dependent processes such as fibroblast migration, endothelial cell tubulogenesis, axonal extension, and neurite formation by diverse mechanisms that exclude any involvement of Gli-dependent transcription. We also acknowledge the existence of less stronger evidence of noncanonical signaling in Drosophila.

Role of Sonic Hedgehog signaling during progression from inflammation to cancer in the stomach

Despite advances in treatment and the declining incidence, gastric cancer remains the second leading cause of cancer-related deaths in the world. Understanding the progression from inflammation to cancer in the stomach is crucial in the development of novel therapies and strategies for treating this disease. Chronic inflammation of the stomach is typically caused by Helicobacter pylori (H. pylori) and resulting lesions may lead to gastric cancer. During the progression from inflammation to cancer, the stomach epithelium changes with evidence of the disruption of normal epithelial cell differentiation and infiltrating inflammatory cells. Coincident with the development of atrophic gastritis and metaplasia, is the loss of the gastric morphogen Sonic Hedgehog (Shh). Given its critical role as a regulator of gastric tissue homeostasis, the disruption of Shh expression during inflammation correlates with the loss of normal epithelial cell differentiation, but this has only recently been rigorously tested in vivo using a unique mouse model of targeted gastric Shh deletion. While pre-neoplastic lesions such as atrophic gastritis and intestinal metaplasia are associated with the loss of Shh within the acid-secreting glands of the stomach, there is a clear link between elevated Shh and signaling to gastric cancers. The current review focuses on the effects of aberrant Shh expression and its role in the development of gastric cancer, specifically in response to H. pylori infection.

The hedgehog pathway conditions the bone microenvironment for osteolytic metastasis of breast cancer

The microenvironment at the site of tumor metastasis plays a key role in determining the fate of the metastasizing tumor cells. This ultimately has a direct impact on the progression of cancer. Bone is the preferred site of metastasis of breast cancer. Painful, debilitating osteolytic lesions are formed as a result of crosstalk between breast cancer cells and cells in the bone, predominantly the osteoblasts and osteoclasts. In this paper, we have discussed the temporal and spatial role of hedgehog (Hh) signaling in influencing the fate of metastatic breast cancer cells in bone. By virtue of its secreted ligands, the Hh pathway is capable of homotypic and heterotypic signaling and consequently altering the microenvironment in the bone. We also have put into perspective the therapeutic implications of using Hh inhibitors to prevent and/or treat bone metastases of breast cancer.

Otic ablation of smoothened reveals direct and indirect requirements for Hedgehog signaling in inner ear development

In mouse embryos lacking sonic hedgehog (Shh), dorsoventral polarity within the otic vesicle is disrupted. Consequently, ventral otic derivatives, including the cochlear duct and saccule, fail to form, and dorsal otic derivatives, including the semicircular canals, endolymphatic duct and utricle, are malformed or absent. Since inner ear patterning and morphogenesis are heavily dependent on extracellular signals derived from tissues that are also compromised by the loss of Shh, the extent to which Shh signaling acts directly on the inner ear for its development is unclear. To address this question, we generated embryos in which smoothened (Smo), an essential transducer of Hedgehog (Hh) signaling, was conditionally inactivated in the otic epithelium (Smo(ecko)). Ventral otic derivatives failed to form in Smo(ecko) embryos, whereas vestibular structures developed properly. Consistent with these findings, we demonstrate that ventral, but not dorsal, otic identity is directly dependent on Hh. The role of Hh in cochlear-vestibular ganglion (cvg) formation is more complex, as both direct and indirect signaling mechanisms are implicated. Our data suggest that the loss of cvg neurons in Shh(-/-) animals is due, in part, to an increase in Wnt responsiveness in the otic vesicle, resulting in the ectopic expression of Tbx1 in the neurogenic domain and subsequent repression of Ngn1 transcription. A mitogenic role for Shh in cvg progenitor proliferation was also revealed in our analysis of Smo(ecko) embryos. Taken together, these data contribute to a better understanding of the intrinsic and extrinsic signaling properties of Shh during inner ear development.

Tcf/Lef repressors differentially regulate Shh-Gli target gene activation thresholds to generate progenitor patterning in the developing CNS

During neural tube development, Shh signaling through Gli transcription factors is necessary to establish five distinct ventral progenitor domains that give rise to unique classes of neurons and glia that arise in specific positions along the dorsoventral axis. These cells are generated from progenitors that display distinct transcription factor gene expression profiles in specific domains in the ventricular zone. However, the molecular genetic mechanisms that control the differential spatiotemporal transcriptional responses of progenitor target genes to graded Shh-Gli signaling remain unclear. The current study demonstrates a role for Tcf/Lef repressor activity in this process. We show that Tcf3 and Tcf7L2 (Tcf4) are required for proper ventral patterning and function by independently regulating two Shh-Gli target genes, Nkx2.2 and Olig2, which are initially induced in a common pool of progenitors that ultimately segregate into unique territories giving rise to distinct progeny. Genetic and functional studies in vivo show that Tcf transcriptional repressors selectively elevate the strength and duration of Gli activity necessary to induce Nkx2.2, but have no effect on Olig2, and thereby contribute to the establishment of their distinct expression domains in cooperation with graded Shh signaling. Together, our data reveal a Shh-Gli-independent transcriptional input that is required to shape the precise spatial and temporal response to extracellular morphogen signaling information during lineage segregation in the CNS.

The alpha subunit of the G protein G13 regulates activity of one or more Gli transcription factors independently of smoothened

Smoothened (Smo) is a seven-transmembrane (7-TM) receptor that is essential to most actions of the Hedgehog family of morphogens. We found previously that Smo couples to members of the G(i) family of heterotrimeric G proteins, which in some cases are integral although alone insufficient in the activation of Gli transcription factors through Hedgehog signaling. In response to a report that the G(12/13) family is relevant to Hedgehog signaling as well, we re-evaluated the coupling of Smo to one member of this family, G(13), and investigated the capacity of this and other G proteins to activate one or more of forms of Gli. We found no evidence that Smo couples directly to G(13). We found nonetheless that Gα(13) and to some extent Gα(q) and Gα(12) are able to effect activation of Gli(s). This capacity is realized in some cells, e.g. C3H10T1/2, MC3T3, and pancreatic cancer cells, but not all cells. The mechanism employed is distinct from that achieved through canonical Hedgehog signaling, as the activation does not involve autocrine signaling or in any other way require active Smo and does not necessarily involve enhanced transcription of Gli1. The activation by Gα(13) can be replicated through a G(q)/G(12/13)-coupled receptor, CCK(A), and is attenuated by inhibitors of p38 mitogen-activated protein kinase and Tec tyrosine kinases. We posit that G proteins, and perhaps G(13) in particular, provide access to Gli that is independent of Smo and that they thus establish a basis for control of at least some forms of Gli-mediated transcription apart from Hedgehogs.

Breast cancer growth and metastasis: interplay between cancer stem cells, embryonic signaling pathways and epithelial-to-mesenchymal transition

Induction of epithelial-to-mesenchymal transition (EMT) in cancer stem cells (CSCs) can occur as the result of embryonic pathway signaling. Activation of Hedgehog (Hh), Wnt, Notch, or transforming growth factor-β leads to the upregulation of a group of transcriptional factors that drive EMT. This process leads to the transformation of adhesive, non-mobile, epithelial-like tumor cells into cells with a mobile, invasive phenotype. CSCs and the EMT process are currently being investigated for the role they play in driving metastatic tumor formation in breast cancer. Both are very closely associated with embryonic signaling pathways that stimulate self-renewal properties of CSCs and EMT-inducing transcription factors. Understanding these mechanisms and embryonic signaling pathways may lead to new opportunities for developing therapeutic agents to help prevent metastasis in breast cancer. In this review, we examine embryonic signaling pathways, CSCs, and factors affecting EMT.

From cell death to viral replication: the diverse functions of the membrane-associated FKBP38

FKBP38 is in many ways an exceptional member of the FK506-binding proteins. The calmodulin-regulated activity of FKBP38 for instance is unique within this protein family. The activated FKBP38 participates in apoptosis signaling by inhibiting the anti-apoptotic Bcl-2. Beyond this role in programmed cell death, FKBP38 seems to be involved in very different cellular processes that do not necessarily depend on the FKBP domain. These functions involve regulation of the kinase mTOR, regulation of neural tube formation, regulation of cellular hypoxia response, but also Hepatitis C virus replication. Pharmacological targeting of FKBP38 might therefore prove a successful strategy for intervention in different FKBP38-dependent processes, including programmed cell death in cancer or neurodegenerative diseases.

N-cadherin-mediated cell adhesion restricts cell proliferation in the dorsal neural tube

Neural progenitors are organized as a pseudostratified epithelium held together by adherens junctions (AJs), multiprotein complexes composed of cadherins and α- and β-catenin. Catenins are known to control neural progenitor division; however, it is not known whether they function in this capacity as cadherin binding partners, as there is little evidence that cadherins themselves regulate neural proliferation. We show here that zebrafish N-cadherin (N-cad) restricts cell proliferation in the dorsal region of the neural tube by regulating cell-cycle length. We further reveal that N-cad couples cell-cycle exit and differentiation, as a fraction of neurons are mitotic in N-cad mutants. Enhanced proliferation in N-cad mutants is mediated by ligand-independent activation of Hedgehog (Hh) signaling, possibly caused by defective ciliogenesis. Furthermore, depletion of Hh signaling results in the loss of junctional markers. We therefore propose that N-cad restricts the response of dorsal neural progenitors to Hh and that Hh signaling limits the range of its own activity by promoting AJ assembly. Taken together, these observations emphasize a key role for N-cad-mediated adhesion in controlling neural progenitor proliferation. In addition, these findings are the first to demonstrate a requirement for cadherins in synchronizing cell-cycle exit and differentiation and a reciprocal interaction between AJs and Hh signaling.

The role of cell cycle in retinal development: cyclin-dependent kinase inhibitors co-ordinate cell-cycle inhibition, cell-fate determination and differentiation in the developing retina

The mature retina is formed through multi-step developmental processes, including eye field specification, optic vesicle evagination, and cell-fate determination. Co-ordination of these developmental events with cell-proliferative activity is essential to achieve formation of proper retinal structure and function. In particular, the molecular and cellular dynamics of the final cell cycle significantly influence the identity that a cell acquires, since cell fate is largely determined at the final cell cycle for the production of postmitotic cells. This review summarizes our current understanding of the cellular mechanisms that underlie the co-ordination of cell-cycle and cell-fate determination, and also describes a molecular role of cyclin-dependent kinase inhibitors (CDKIs) as co-ordinators of cell-cycle arrest, cell-fate determination and differentiation.

The Gli3 hypomorphic mutation Pdn causes selective impairment in the growth, patterning, and axon guidance capability of the lateral ganglionic eminence

Previous studies have defined a requirement for Sonic hedgehog (Shh) signaling in patterning the ventral telencephalon, a major source of the neuronal diversity found in the mature telencephalon. The zinc finger transcription factor Gli3 is a critical component of the Shh signaling pathway and its loss causes major defects in telencephalic development. Gli3 is expressed in a graded manner along the dorsoventral axis of the telencephalon but it is unknown whether Gli3 expression levels are important for dorsoventral telencephalic patterning. To address this, we used the Gli3 hypomorphic mouse mutant Polydactyly Nagoya (Pdn). We show that in Pdn/Pdn embryos, the telencephalic expression of Gli3 remains graded, but Gli3 mRNA and protein levels are reduced, resulting in an upregulation of Shh expression and signaling. These changes mainly affect the development of the lateral ganglionic eminence (LGE), with some disorganization of the medial ganglionic eminence mantle zone. The pallial/subpallial boundary is shifted dorsally and the production of postmitotic neurons is reduced. Moreover, LGE pioneer neurons that guide corticofugal axons into the LGE do not form properly, delaying the entry of corticofugal axons into the ventral telencephalon. Pdn/Pdn mutants also show severe pathfinding defects of thalamocortical axons in the ventral telencephalon. Transplantation experiments demonstrate that the intrinsic ability of the Pdn ventral telencephalon to guide thalamocortical axons is compromised. We conclude that correct Gli3 levels are particularly important for the LGE's growth, patterning, and development of axon guidance capabilities.

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.

SUMOylation by Pias1 regulates the activity of the Hedgehog dependent Gli transcription factors

BACKGROUND

Hedgehog (Hh) signaling, a vital signaling pathway for the development and homeostasis of vertebrate tissues, is mediated by members of the Gli family of zinc finger transcription factors. Hh signaling increases the transcriptional activity of Gli proteins, at least in part, by inhibiting their proteolytic processing. Conversely, phosphorylation by cAMP-dependent protein kinase (PKA) inhibits Gli transcriptional activity by promoting their ubiquitination and proteolysis. Whether other post-translational modifications contribute to the regulation of Gli protein activity has been unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Here we provide evidence that all three Gli proteins are targets of small ubiquitin-related modifier (SUMO)-1 conjugation. Expression of SUMO-1 or the SUMO E3 ligase, Pias1, increased Gli transcriptional activity in cultured cells. Moreover, PKA activity reduced Gli protein SUMOylation. Strikingly, in the embryonic neural tube, the forced expression of Pias1 increased Gli activity and induced the ectopic expression of the Gli dependent gene Nkx2.2. Conversely, a point mutant of Pias1, that lacks ligase activity, blocked the endogenous expression of Nkx2.2.

CONCLUSIONS/SIGNIFICANCE

Together, these findings provide evidence that Pias1-dependent SUMOylation influences Gli protein activity and thereby identifies SUMOylation as a post-translational mechanism that regulates the hedgehog signaling pathway.

The zebrafish flotte lotte mutant reveals that the local retinal environment promotes the differentiation of proliferating precursors emerging from their stem cell niche

It is currently unclear how intrinsic and extrinsic mechanisms cooperate to control the progression from self-renewing to neurogenic divisions in retinal precursor cells. Here, we use the zebrafish flotte lotte (flo) mutant, which carries a mutation in the elys (ahctf1) gene, to study the relationship between cell cycle progression and neuronal differentiation by investigating how proliferating progenitor cells transition towards differentiation in a retinal stem cell niche termed the ciliary marginal zone (CMZ). In zebrafish embryos without Elys, CMZ cells retain the capacity to proliferate but lose the ability to enter their final neurogenic divisions to differentiate as neurons. However, mosaic retinae composed of wild-type and flo cells show that despite inherent cell cycle defects, flo mutant cells progress from proliferation to differentiation when in the vicinity of wild-type retinal neurons. We propose that the differentiated retinal environment limits the proliferation of precursors emerging from the CMZ in a manner that explains the spatial organisation of cells in the CMZ and ensures that proliferative retinal progenitors are driven towards differentiation.

SOX5 controls cell cycle progression in neural progenitors by interfering with the WNT-beta-catenin pathway

Genes of the SOX family of high-mobility group transcription factors are essential during nervous system development. In this study, we show that SOX5 is expressed by neural progenitors in the chick spinal cord and is turned off as differentiation proceeds. The overexpression of SOX5 in neural progenitors causes premature cell cycle exit and prevents terminal differentiation. Conversely, knocking down SOX5 protein extends the proliferative period of neural progenitors and causes marked cell death in a dorsal interneuron (dI3) population. Furthermore, SOX5 reduces WNT-beta-catenin signalling, thereby triggering the expression of the negative regulator of the pathway axin2. We propose that SOX5 regulates the timing of cell cycle exit by opposing WNT-beta-catenin activity on cell cycle progression.

CREBBP re-arrangements affect protein function and lead to aberrant neuronal differentiation

Biallelic inactivation of the CREB-binding protein (CREBBP) a transcriptional co-activator produces an embryonic lethal phenotype in mice. In humans, re-arrangements in CREBBP are associated with the Rubinstein-Taybi Syndrome (RSTS) that is characterised by craniofacial, skeletal and neuronal symptoms. Neuronal defects in RSTS can be attributed to genetic re-arrangements in CREBBP, which has been implicated in synaptic plasticity and long-term memory. The present study was designed to investigate the role of CREBBP re-arrangements during neuronal differentiation. Towards this, deletion constructs of pCREBBP, viz. pDeltaCB-HAT and pDeltaHAT-CT were generated and transfected into NT2 cells. Expression profiling of the components of Notch, Wnt, SHH and Retinoid signaling along with screening of the neuronal markers was carried out in the NT2 cells and their mutant derivatives. ChIP-PCRs along with co-immunoprecipitations were also performed in these cells to investigate defects due to inappropriate interaction of mutated CREEBP with the corresponding transcription factor and other transcription regulatory proteins both at steady state as well as during differentiation. Mutant NT2 cells lacking the CREB, BROMO and HAT domains (CB-HAT) were highly proliferative and showed limited differentiation; while mutant NT2 cells expressing CREBBP lacking the HAT and CTAD domains (HAT-CT) are proliferation deficient and differentiate rapidly albeit generating an insufficient number of neurons. Altered CREBBP structure resulted in changes in HAT activity, cell cycle profiles and expression of basal levels of components of Notch, SHH, Wnt and retinoid pathways known to be critical in the proliferation and differentiation of neuronal progenitors. At the chromatin level, aberrant signaling correlated with altered binding affinities of the (CREBBP-transcription factor) complexes to promoter regions of components of these pathways. Thus, differentiation defects are manifested early at the genomic level leading to aberrant transcription of the genes involved in differentiation along the neuronal lineage.

Hedgehog-dependent down-regulation of the tumor suppressor, vitamin D3 up-regulated protein 1 (VDUP1), precedes lamina development in Drosophila

The tumor suppressor vitamin D(3) up-regulated protein 1 (VDUP1) is expressed throughout the developing and mature Drosophila nervous system, but its regulatory pathways are not well understood. Within the developing Drosophila visual system, down-regulation of VDUP1 in lamina precursor cells (LPCs) coincided with the arrival of retinal axons into the lamina target field, suggesting VDUP1 regulation by an axonally transmitted signal. Hedgehog (Hh) is a signal well known to coordinate LPC proliferation and differentiation in response to retinal axon innervation, and analysis of orthologous dvdup1 promoters identified an evolutionarily conserved binding site for the Hh-dependent transcription factor cubitus interruptus (Ci). Hh-dependent regulation of VDUP1 in the developing lamina was confirmed in Hh loss-of-function backgrounds where VDUP1 expression was maintained in LPCs, inhibiting both cell proliferation and lamina neurogenesis. This putative coupling of VDUP1 to the Hh signaling pathway may provide novel insights into the mechanisms controlling brain growth and development.

The transcriptional repressor REST/NRSF modulates hedgehog signaling

The spatial and temporal control of gene expression is key to generation of specific cellular fates during development. Studies of the transcriptional repressor REST/NRSF (RE1 Silencing Transcription Factor or Neural Restrictive Silencing Factor) have provided important insight into the role that epigenetic modifications play in differential gene expression. However, the precise function of REST during embryonic development is not well understood. We have discovered a novel interaction between zebrafish Rest and the Hedgehog (Hh) signaling pathway. We observed that Rest knockdown enhances or represses Hh signaling in a context-dependant manner. In wild-type embryos and embryos with elevated Hh signaling, Rest knockdown augments transcription of Hh target genes. Conversely, in contexts where Hh signaling is diminished, Rest knockdown has the opposite effect and Hh target gene expression is further attenuated. Epistatic analysis revealed that Rest interacts with the Hh pathway at a step downstream of Smo. Furthermore, we present evidence implicating the bifunctional, Hh signaling component Gli2a as key to the Rest modulation of the Hh response. The role of Rest as a regulator of Hh signaling has broad implications for many developmental contexts where REST and Hh signaling act.

Sonic hedgehog regulates integrin activity, cadherin contacts, and cell polarity to orchestrate neural tube morphogenesis

In vertebrates, the embryonic nervous system is shaped and patterned by a series of temporally and spatially regulated cell divisions, cell specifications, and cell adhesions and movements. Morphogens of the Hedgehog, Wnt, and bone morphogenetic protein families have been shown to play a crucial role in the control of cell division and specification in the trunk neural tube, but their possible implication in the regulation of adhesive events has been poorly documented. In the present study, we demonstrate that Sonic hedgehog regulates neural epithelial cell adhesion and polarity through regulation of integrin activity, cadherin cell-cell contact, and cell polarity genes in immature neural epithelial cells before the specification of neuronal cells. We propose that Sonic hedgehog orchestrates neural tube morphogenesis by coordinating adhesive and motility events with cell proliferation and differentiation.

Hedgehog activation is required upstream of Wnt signalling to control neural progenitor proliferation

The canonical Wnt and sonic hedgehog (Shh) pathways have been independently linked to cell proliferation in a variety of tissues and systems. However, interaction of these signals in the control of cell cycle progression has not been studied. Here, we demonstrate that in the developing vertebrate nervous system these pathways genetically interact to control progression of the G1 phase of the cell cycle. By in vivo loss-of-function experiments, we demonstrate the absolute requirement of an upstream Shh activity for the regulation of Tcf3/4 expression. In the absence of Tcf3/4, the canonical Wnt pathway cannot activate target gene expression, including that of cyclin D1, and the cell cycle is necessarily arrested at G1. In addition to the control of G1 progression, Shh activity controls the G2 phase through the regulation of cyclin E, cyclin A and cyclin B expression, and this is achieved independently of Wnt. Thus, in neural progenitors, cell cycle progression is co-ordinately regulated by Wnt and Shh activities.

Neuronal differentiation in basal cell carcinoma: possible relationship to Hedgehog pathway activation?

Although deregulated Hedgehog signalling and elevated Gli transcription factor expression are known to promote the development of basal cell carcinoma (BCC), little is known about molecular mechanisms driving the development of specific growth pattern subtypes. Using gene array analysis, we have previously observed that over-expression of GLI1 in human keratinocytes promotes increased expression of the neuronal differentiation markers ARC and ULK1. We asked whether neuronal differentiation is a characteristic of BCC and whether there is any correlation with BCC subtype. Using RT-PCR and immunohistochemistry, we confirmed that the neuronal markers ARC, beta-tubulin III, GAP-43 and Neurofilament are expressed in human BCC but not in normal epidermis. Moreover, we found that expression of these neuronal differentiation markers showed strong correlation to BCC subtype, with more aggressive infiltrative and morphoeic BCC showing low levels or lack of expression compared to nodular, superficial and micronodular subtypes. Primary human keratinocytes retrovirally expressing GLI1(-) and GLI2(-) showed elevated levels of beta-tubulin III and ARC but not Neurofilament or GAP-43, suggesting that beta-tubulin III and Arc may be early targets of aberrant Gli expression in BCC, whereas expression of Neurofilament and GAP-43 are either later, downstream targets or under control of alternative pathways. We propose that neuronal differentiation is a feature of BCC and that expression of these markers is in part due to aberrant Hedgehog signalling. Moreover, we suggest that correlation between loss of expression of neuronal markers in infiltrative and morphoeic BCC subtypes reflects dedifferentiation of more aggressive BCC subtypes.

The insulin-like growth factor (IGF) receptor type 1 (IGF1R) as an essential component of the signalling network regulating neurogenesis

The insulin-like growth factor receptor type 1 (IGF1R) signalling pathway is activated in the mammalian nervous system from early developmental stages. Its major effect on developing neural cells is to promote their growth and survival. This pathway can integrate its action with signalling pathways of growth and morphogenetic factors that induce cell fate specification and selective expansion of specified neural cell subsets. This suggests that during developmental and adult neurogenesis cellular responses to many signalling factors, including ligands of Notch, sonic hedgehog, fibroblast growth factor family members, ligands of the epidermal growth factor receptor, bone morphogenetic proteins and Wingless and Int-1, may be modified by co-activation of the IGF1R. Modulation of cell migration is another possible role that IGF1R activation may play in neurogenesis. Here, I briefly overview neurogenesis and discuss a role for IGF1R-mediated signalling in the developing and mature nervous system with emphasis on crosstalk between the signalling pathways of the IGF1R and other factors regulating neural cell development and migration. Studies on neural as well as on non-neural cells are highlighted because it may be interesting to test in neurogenic paradigms some of the models based on the information obtained in studies on non-neural cell types.

Sonic hedgehog guides axons through a noncanonical, Src-family-kinase-dependent signaling pathway

Sonic hedgehog (Shh) plays essential roles in developmental events such as cell fate specification and axon guidance. Shh induces cell fate specification through canonical Shh signaling, mediated by transcription. However, the mechanism by which Shh guides axons is unknown. To study this, we developed an in vitro assay for axon guidance, in which neurons can be imaged while responding to a defined gradient of a chemical cue. Axons of dissociated commissural neurons placed in a Shh gradient turned rapidly toward increasing concentrations of Shh. Consistent with this rapid response, we showed that attraction by Shh does not require transcription. Instead, Shh stimulates the activity of Src family kinase (SFK) members in a Smoothened-dependent manner. Moreover, SFK activity is required for Shh-mediated guidance of commissural axons, but not for induction of Gli transcriptional reporter activity. Together, these results indicate that Shh acts via a rapidly acting, noncanonical signaling pathway to guide axons.

Distinct effects of Hedgehog signaling on neuronal fate specification and cell cycle progression in the embryonic mouse retina

Cell-extrinsic signals can profoundly influence the production of various neurons from common progenitors. Yet mechanisms by which extrinsic signals coordinate progenitor cell proliferation, cell cycle exit, and cell fate choices are not well understood. Here, we address whether Hedgehog (Hh) signals independently regulate progenitor proliferation and neuronal fate decisions in the embryonic mouse retina. Conditional ablation of the essential Hh signaling component Smoothened (Smo) in proliferating progenitors, rather than in nascent postmitotic neurons, leads to a dramatic increase of retinal ganglion cells (RGCs) and a mild increase of cone photoreceptor precursors without significantly affecting other early-born neuronal cell types. In addition, Smo-deficient progenitors exhibit aberrant expression of cell cycle regulators and delayed G(1)/S transition, especially during the late embryonic stages, resulting in a reduced progenitor pool by birth. Deficiency in Smo function also causes reduced expression of the basic helix-loop-helix transcription repressor Hes1 and preferential elevation of the proneural gene Math5. In Smo and Math5 double knock-out mutants, the enhanced RGC production observed in Smo-deficient retinas is abolished, whereas defects in the G(1)/S transition persist, suggesting that Math5 mediates the Hh effect on neuronal fate specification but not on cell proliferation. These findings demonstrate that Hh signals regulate progenitor pool expansion primarily by promoting cell cycle progression and influence cell cycle exit and neuronal fates by controlling specific proneural genes. Together, these distinct cellular effects of Hh signaling in neural progenitor cells coordinate a balanced production of diverse neuronal cell types.

Sonic hedgehog signaling is required for sympathetic nervous system development

Sonic hedgehog (Shh) plays critical roles during nervous system development, yet little is known about its function in the sympathetic nervous system. Using a mouse Shh null line, we examined the roles of Shh during SNS development. Loss of Shh did not prevent formation of the sympathetic trunk, but the ganglia are hypoplastic and misspatterned. Neuronal differentiation was delayed in Shh mutant embryos showing that Shh is required for correct developmental timing in addition to its role in sympathetic nervous system patterning. Immunohistochemical analyses of the ganglia for expression of the pan-neuronal marker beta3-tubulin, the noradrenergic biosynthetic enzyme tyrosine hydroxylase and the glial marker B-FABP showed that Shh is not required for differentiation of sympathetic neurons or glia.

Hedgehog signalling: emerging evidence for non-canonical pathways

Hedgehog (HH) signalling is involved in the development of numerous embryonic tissues. In humans,germline mutations in hedgehog pathway components cause congenital malformations and somatic mutations are associated with cancers. The basic framework of the HH pathway was elucidated in the fruitfly, Drosophila melanogaster, and this pathway is largely conserved in vertebrates, although some important differences have been noted. The current paradigm of the "canonical" pathway views HH signalling as a series of repressive interactions which culminates in GLI-mediated transcriptional regulation of a variety of cellular processes. Definitions of "non-canonical" signalling stem from examples where the response to HH morphogen deviates from this paradigm and, according to current reports, three general scenarios of noncanonical HH signalling can be defined: (1) Signalling that involves HH pathway components but which is independent of GLI-mediated transcription; (2) Direct interaction of HH signalling components with components of other molecular pathways; and (3) "Non-contiguous" or "atypical" interaction of core HH pathway components with one another. Currently, the evidence supporting non-canonical HH signalling is not conclusive. However, Sonic hedgehog (SHH) has been shown to regulate cell migration and axon guidance in several contexts, and some of these processes are independent of downstream components of the HH pathway, and presumably the transcriptional response to morphogen. Furthermore, biochemical studies have shown that the HH receptor, PTCH1, can directly interact both with Cyclin B1 and caspases, to inhibit cell proliferation and to promote apoptosis, respectively, and that these functions are inhibited in the presence of morphogen. Genetic analysis of orthologues of the HH pathway in nematode worms further supports the notion that PTCH1-related molecules can function independently of other components of the canonical HH pathway, and the phenotypes of mice with point mutations in the Ptch1 gene offer clues as to the processes that non-canonical HH signalling might regulate. While none of these evidences are conclusive,collectively they point to the existence of added complexity in the HH pathway in the form of non-canonical pathways. A major difficulty in studying this problem is that canonical and non-canonical pathways are likely to act in parallel, and so in many situations it will not be possible to implicate non-canonical responses in certain cellular processes simply by excluding a role for the canonical pathway-directed analyses of non-canonical HH signalling are therefore necessary. The aim of this review is to present the cumulative evidence supporting non-canonical HH signalling, with the hope of promoting further enquiry into this area.