Disruption of the PACAP gene promotes medulloblastoma in ptc1 mutant mice

Integrated analysis of the genomic and transcriptional profile of gliomas with isocitrate dehydrogenase-1 and tumor protein 53 mutations

Background: The gene mutation of isocitrate dehydrogenase-1 (IDH1) is commonly found in LGG and some GBM patients and usually carries tumor protein 53 (TP53) mutations. However, the underlying mechanisms on both mutations of glioma patients in IDH1 and TP53 are still unclear. Aim: To find the potential target markers in GBM and LGG patients with IDH1 and TP53 mutation.Method: A total of 1122 glioma patients from The Cancer Genome Atlas were enrolled and divided as wild-type (without IDH1 and TP53 mutations) or both mutant (both IDH1 and TP53 mutations). The data of clinicopathological characteristics, mRNA, mutations, and copy number alteration were analyzed. Results: IDH1 and TP53 mutations, not gene expression, affect the survival probability of GBM and LGG patients, which might be related to neuron function, immune function, tumor invasion, and metastasis. The effects of the selected gene (EMILIN3, SAA1, VSTM2A, HAMP, IFT80, and CHIC2) on glioma patients could be regulated by IDH1 and TP53 mutations and had a higher survival possibility in these patients. Conclusions: The selected genes in GBM and LGG patients with IDH1 and TP53 mutations could be a potential prognosis marker in the future.

Gene Expression Data Mining Reveals the Involvement of GPR55 and Its Endogenous Ligands in Immune Response, Cancer, and Differentiation

G protein-coupled receptor 55 (GPR55) is a recently deorphanized lipid- and peptide-sensing receptor. Its lipidic endogenous agonists belong to lysoglycerophospholipids, with lysophosphatidylinositol (LPI) being the most studied. Peptide agonists derive from fragmentation of pituitary adenylate cyclase-activating polypeptide (PACAP). Although GPR55 and its ligands were implicated in several physiological and pathological conditions, their biological function remains unclear. Thus, the aim of the study was to conduct a large-scale re-analysis of publicly available gene expression datasets to identify physiological and pathological conditions affecting the expression of GPR55 and the production of its ligands. The study revealed that regulation of GPR55 occurs predominantly in the context of immune activation pointing towards the role of the receptor in response to pathogens and in immune cell lineage determination. Additionally, it was revealed that there is almost no overlap between the experimental conditions affecting the expression of GPR55 and those modulating agonist production. The capacity to synthesize LPI was enhanced in various types of tumors, indicating that cancer cells can hijack the motility-related activity of GPR55 to increase aggressiveness. Conditions favoring accumulation of PACAP-derived peptides were different than those for LPI and were mainly related to differentiation. This indicates a different function of the two agonist classes and possibly the existence of a signaling bias.

Neuronal-driven glioma growth requires Gαi1 and Gαi3

Neuroligin-3 (NLGN3) is necessary and sufficient to promote glioma cell growth. The recruitment of Gαi1/3 to the ligand-activated receptor tyrosine kinases (RTKs) is essential for mediating oncogenic signaling.

Methods: Various genetic strategies were utilized to examine the requirement of Gαi1/3 in NLGN3-driven glioma cell growth.

Results: NLGN3-induced Akt-mTORC1 and Erk activation was inhibited by decreasing Gαi1/3 expression. In contrast ectopic Gαi1/3 overexpression enhanced NLGN3-induced signaling. In glioma cells, NLGN3-induced cell growth, proliferation and migration were attenuated by Gαi1/3 depletion with shRNA, but facilitated with Gαi1/3 overexpression. Significantly, Gαi1/3 silencing inhibited orthotopic growth of patient-derived glioma xenografts in mouse brain, whereas forced Gαi1/3-overexpression in primary glioma xenografts significantly enhanced growth. The growth of brain-metastatic human lung cancer cells in mouse brain was largely inhibited with Gαi1/3 silencing. It was however expedited with ectopic Gαi1/3 overexpression. In human glioma Gαi3 upregulation was detected, correlating with poor prognosis.

Conclusion: Gαi1/3 mediation of NLGN3-induced signaling is essential for neuronal-driven glioma growth.

Pituitary Adenylate Cyclase-Activating Polypeptide: 30 Years in Research Spotlight and 600 Million Years in Service

Emerging from the depths of evolution, pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptors (i.e., PAC1, VPAC1, VPAC2) are present in multicellular organisms from Tunicates to humans and govern a remarkable number of physiological processes. Consequently, the clinical relevance of PACAP systems spans a multifaceted palette that includes more than 40 disorders. We aimed to present the versatility of PACAP1-38 actions with a focus on three aspects: (1) when PACAP1-38 could be a cause of a malfunction, (2) when PACAP1-38 could be the cure for a malfunction, and (3) when PACAP1-38 could either improve or impair biology. PACAP1-38 is implicated in the pathophysiology of migraine and post-traumatic stress disorder whereas an outstanding protective potential has been established in ischemia and in Alzheimer’s disease. Lastly, PACAP receptors could mediate opposing effects both in cancers and in inflammation. In the light of the above, the duration and concentrations of PACAP agents must be carefully set at any application to avoid unwanted consequences. An enormous amount of data accumulated since its discovery (1989) and the first clinical trials are dated in 2017. Thus in the field of PACAP research: “this is not the end, not even the beginning of the end, but maybe the end of the beginning.”

Differential Expression of Mitochondrial Biogenesis Markers in Mouse and Human SHH-Subtype Medulloblastoma

Medulloblastoma is a brain tumor that arises predominantly in infants and children. It is the most common pediatric brain malignancy. Around 25% of medulloblastomas are driven by constitutive activation of the Hedgehog signaling pathway. Hedgehog-driven medulloblastoma is often studied in the laboratory using genetic mouse models with overactive Hedgehog signaling, which recapitulate many of the pathological features of human Hedgehog-dependent tumors. However, we show here that on a molecular level the human and mouse HH-dependent MB are quite distinct, with human, but not mouse, tumors characterized by the presence of markers of increased oxidative phosphorylation and mitochondrial biogenesis. The latter suggests that, unlike for many other types of tumors, a switch to glycolytic metabolism might not be co-opted by human SHH-MB to perpetuate their survival and growth. This needs to be taken into consideration and could potentially be exploited in the design of therapies.

PKA at a Cross-Road of Signaling Pathways Involved in the Regulation of Glioblastoma Migration and Invasion by the Neuropeptides VIP and PACAP

Glioblastoma (GBM) remains an incurable disease, mainly due to the high migration and invasion potency of GBM cells inside the brain. PI3K/Akt, Sonic Hedgehog (SHH), and PKA pathways play major regulatory roles in the progression of GBM. The vasoactive intestinal peptide (VIP) family of neuropeptides and their receptors, referred in this article as the “VIP-receptor system”, has been reported to regulate proliferation, differentiation, and migration in a number of tumor cell types and more particularly in GBM cells. These neuropeptides are potent activators of the cAMP/PKA pathway. The present study aimed to investigate the cross-talks between the above cited signaling cascades. Regulation by VIP-related neuropeptides of GBM migration and invasion was evaluated ex vivo in rat brain slices explanted in culture. Effects of different combinations of VIP-related neuropeptides and of pharmacological and siRNA inhibitors of PKA, Akt, and of the SHH/GLI1 pathways were tested on GBM migration rat C6 and human U87 GBM cell lines using the wound-healing technique. Quantification of nuclear GLI1, phospho-Akt, and phospho-PTEN was assessed by western-immunoblotting. The VIP-receptor system agonists VIP and PACAP-38 significantly reduced C6 cells invasion in the rat brain parenchyma ex vivo, and C6 and U87 migration in vitro. A VIP-receptor system antagonist, VIP_10-28 increased C6 cell invasion in the rat brain parenchyma ex vivo, and C6 and migration in vitro. These effects on cell migration were abolished by selective inhibitors of the PI3K/Akt and of the SHH pathways. Furthermore, VIP and PACAP-38 reduced the expression of nuclear GLI1 while VIP_10-28 increased this expression. Selective inhibitors of Akt and PKA abolished VIP, PACAP-38, and VIP_10-28 effects on nuclear GLI1 expression in C6 cells. PACAP-38 induced a time-dependent inhibition of phospho-Akt expression and an increased phosphorylation of PTEN in C6 cells. All together, these data indicate that triggering the VIP-receptor system reduces migration and invasion in GBM cells through a PKA-dependent blockade of the PI3K/Akt and of the SHH/GLI1 pathways. Therefore, the VIP-receptor system displays anti-oncogenic properties in GBM cells and PKA is a central core in this process.

The G protein Gαs acts as a tumor suppressor in sonic hedgehog signaling-driven tumorigenesis

G protein-coupled receptors (GPCRs) are critical players in tumor growth and progression. The redundant roles of GPCRs in tumor development confound effective treatment; therefore, targeting a single common signaling component downstream of these receptors may be efficacious. GPCRs transmit signals through heterotrimeric G proteins composed of Gα and Gβγ subunits. Hyperactive Gαs signaling can mediate tumor progression in some tissues; however, recent work in medulloblastoma and basal cell carcinoma revealed that Gαs can also function as a tumor suppressor in neoplasms derived from ectoderm cells including neural and epidermal stem/progenitor cells. In these stem-cell compartments, signaling through Gαs suppresses self-renewal by inhibiting the Sonic Hedgehog (SHH) and Hippo pathways. The loss of GNAS, which encodes Gαs, leads to activation of these pathways, over-proliferation of progenitor cells, and tumor formation. Gαs activates the cAMP-dependent protein kinase A (PKA) signaling pathway and inhibits activation of SHH effectors Smoothened-Gli. In addition, Gαs-cAMP-PKA activation negatively regulates the Hippo pathway by blocking the NF2-LATS1/2-Yap signaling. In this review, we will address the novel function of the signaling network regulated by Gαs in suppression of SHH-driven tumorigenesis and the therapeutic approaches that can be envisioned to harness this pathway to inhibit tumor growth and progression.

Neuropeptides shaping the central nervous system development: Spatiotemporal actions of VIP and PACAP through complementary signaling pathways

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are neuropeptides with wide, complementary, and overlapping distributions in the central and peripheral nervous systems, where they exert important regulatory roles in many physiological processes. VIP and PACAP display a large range of biological cellular targets and functions in the adult nervous system including regulation of neurotransmission and neuroendocrine secretion and neuroprotective and neuroimmune responses. As the main focus of the present review, VIP and PACAP also have been long implicated in nervous system development and maturation through their interaction with the seven transmembrane domain G protein-coupled receptors, PAC1, VPAC1, and VPAC2, initiating multiple signaling pathways. Compared with PAC1, which solely binds PACAP with very high affinity, VPACs exhibit high affinities for both VIP and PACAP but differ from each other because of their pharmacological profile for both natural accessory peptides and synthetic or chimeric molecules, with agonistic and antagonistic properties. Complementary to initial pharmacological studies, transgenic animals lacking these neuropeptides or their receptors have been used to further characterize the neuroanatomical, electrophysiological, and behavioral roles of PACAP and VIP in the developing central nervous system. In this review, we recapitulate the critical steps and processes guiding/driving neurodevelopment in vertebrates and superimposing the potential contribution of PACAP and VIP receptors on the given timeline. We also describe how alterations in VIP/PACAP signaling may contribute to both (neuro)developmental and adult pathologies and suggest that tuning of VIP/PACAP signaling in a spatiotemporal manner may represent a novel avenue for preventive therapies of neurological and psychiatric disorders. © 2016 Wiley Periodicals, Inc.

Vasoactive intestinal peptide/pituitary adenylate cyclase activating polypeptide, and their receptors and cancer

PURPOSE OF REVIEW

To summarize the roles of vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase activating polypeptide (PACAP) and their receptors (VPAC1, VPAC2, PAC1) in human tumors as well as their role in potential novel treatments.

RECENT FINDINGS

Considerable progress has been made in understanding of the effects of VIP/PACAP on growth of various tumors as well as in the signaling cascades involved, especially in the role of transactivation of the epidermal growth factor family. The overexpression of VPAC1/2 and PAC1 on a number of common neoplasms (breast, lung, prostate, central nervous system and neuroblastoma) is receiving increased attention both as a means of tumor imaging the location and extent of these tumors, as well as for targeted directed treatment, by coupling cytotoxic agents to VIP/PACAP analogues.

SUMMARY

VIP/PACAP has prominent growth effects on a number of common neoplasms, which frequently overexpressed the three subtypes of their receptors. The increased understanding of their signaling cascades, effect on tumor growth/differentiation and the use of the overexpression of these receptors for localization/targeted cytotoxic delivery are all suggesting possible novel tumor treatments.

Sonic hedgehog patterning during cerebellar development

The morphogenic factor sonic hedgehog (Shh) actively orchestrates many aspects of cerebellar development and maturation. During embryogenesis, Shh signaling is active in the ventricular germinal zone (VZ) and represents an essential signal for proliferation of VZ-derived progenitors. Later, Shh secreted by Purkinje cells sustains the amplification of postnatal neurogenic niches: the external granular layer and the prospective white matter, where excitatory granule cells and inhibitory interneurons are produced, respectively. Moreover, Shh signaling affects Bergmann glial differentiation and promotes cerebellar foliation during development. Here we review the most relevant functions of Shh during cerebellar ontogenesis, underlying its role in physiological and pathological conditions.

Targeting brain tumor cAMP: the case for sex-specific therapeutics

A relationship between cyclic adenosine 3′, 5′-monophosphate (cAMP) levels and brain tumor biology has been evident for nearly as long as cAMP and its synthetase, adenylate cyclase (ADCY) have been known. The importance of the pathway in brain tumorigenesis has been demonstrated in vitro and in multiple animal models. Recently, we provided human validation for a cooperating oncogenic role for cAMP in brain tumorigenesis when we found that SNPs in ADCY8 were correlated with glioma (brain tumor) risk in individuals with Neurofibromatosis type 1 (NF1). Together, these studies provide a strong rationale for targeting cAMP in brain tumor therapy. However, the cAMP pathway is well-known to be sexually dimorphic, and SNPs in ADCY8 affected glioma risk in a sex-specific fashion, elevating the risk for females while protecting males. The cAMP pathway can be targeted at multiple levels in the regulation of its synthesis and degradation. Sex differences in response to drugs that target cAMP regulators indicate that successful targeting of the cAMP pathway for brain tumor patients is likely to require matching specific mechanisms of drug action with patient sex.

Targeted treatment for sonic hedgehog-dependent medulloblastoma

Novel treatment options, including targeted therapies, are needed for patients with medulloblastoma (MB), especially for those with high-risk or recurrent/relapsed disease. Four major molecular subgroups of MB have been identified, one of which is characterized by activation of the sonic hedgehog (SHH) pathway. Preclinical data suggest that inhibitors of the hedgehog (Hh) pathway could become valuable treatment options for patients with this subgroup of MB. Indeed, agents targeting the positive regulator of the pathway, smoothened (SMO), have demonstrated efficacy in a subset of patients with SHH MB. However, because of resistance and the presence of mutations downstream of SMO, not all patients with SHH MB respond to SMO inhibitors. The development of agents that target these resistance mechanisms and the potential for their combination with traditional chemotherapy and SHH inhibitors will be discussed. Due to its extensive molecular heterogeneity, the future of MB treatment is in personalized therapy, which may lead to improved efficacy and reduced toxicity. This will include the development of clinically available tests that can efficiently discern the SHH subgroup. The preliminary use of these tests in clinical trials is also discussed herein.

G-protein-coupled receptors, Hedgehog signaling and primary cilia

The Hedgehog (Hh) pathway has become an important model to study the cell biology of primary cilia, and reciprocally, the study of ciliary processes provides an opportunity to solve longstanding mysteries in the mechanism of vertebrate Hh signal transduction. The cilium is emerging as an unique compartment for G-protein-coupled receptor (GPCR) signaling in many systems. Two members of the GPCR family, Smoothened and Gpr161, play important roles in the Hh pathway. We review the current understanding of how these proteins may function to regulate Hh signaling and also highlight some of the critical unanswered questions being tackled by the field. Uncovering GPCR-regulated mechanisms important in Hh signaling may provide therapeutic strategies against the Hh pathway that plays important roles in development, regeneration and cancer.

Pleiotropic functions of pituitary adenylyl cyclase-activating polypeptide on retinal ontogenesis: involvement of KLF4 in the control of progenitor cell proliferation

We showed previously that the neuropeptide pituitary adenylyl cyclase-activating polypeptide (PACAP) negatively regulates proliferation of postnatal rat retinal progenitor cells through the downregulation of cyclin D1 in a cAMP/protein kinase A dependent manner. In the present study, we describe by microarray analysis several putative PACAP targets regulated by different transcription factor families. One of these families is the Sp/Klf family of transcriptional factors capable of regulating cyclin D1, and among members, we demonstrate by immunocytochemistry that KLF4 is expressed throughout rat retinal development by retinal progenitor cells and in most differentiated cell types. Using retinal explants preparations, PACAP treatment can transiently increase Klf4 mRNA levels; from electrophoretic mobility shift assays, PACAP is also able to increase the nuclear KLF4 content. From these results, we suggest that KLF4 may be involved in the anti-proliferative effects of PACAP as one mechanism regulating progenitor cell transition from proliferation to differentiation throughout retinal development.

Perigestational dietary folic acid deficiency protects against medulloblastoma formation in a mouse model of nevoid basal cell carcinoma syndrome

Hereditary nevoid basal cell carcinoma syndrome (NBCCS) is caused by PTCH1 gene mutations that result in diverse neoplasms including medulloblastoma (MB). Epidemiological studies report reduced pediatric brain tumor risks associated with maternal intake of prenatal vitamins containing folic acid (FA) and FA supplements specifically. We hypothesized that low maternal FA intake during the perigestational period would increase MB incidence in a transgenic NBCCS mouse model, which carries an autosomal dominant mutation in the Ptch1 gene. Female wild-type C57BL/6 mice (n = 126) were randomized to 1 of 3 diets with differing FA amounts: 0.3 mg/kg (low), 2.0 mg/kg (control), and 8.0 mg/kg (high) 1 mo prior to mating with Ptch1 (+/-) C57BL/6 males. Females were maintained on the diet until pup weaning; the pups were then aged for tumor development. Compared to the control group, offspring MB incidence was significantly lower in the low FA group (Hazard Ratio = 0.47; 95% confidence interval 0.27-0.80) at 1 yr. No significant difference in incidence was observed between the control and high FA groups. Low maternal perigestational FA levels may decrease MB incidence in mice genetically predisposed to tumor development. Our results could have implications for prenatal FA intake recommendations in the presence of cancer syndromes.

Interaction of PACAP with Sonic hedgehog reveals complex regulation of the hedgehog pathway by PKA

Sonic hedgehog (Shh) signaling is essential for proliferation of cerebellar granule cell progenitors (cGCPs) and its aberrant activation causes a cerebellar cancer medulloblastoma. Pituitary adenylate cyclase activating polypeptide (PACAP) inhibits Shh-driven proliferation of cGCPs and acts as tumor suppressor in murine medulloblastoma. We show that PACAP blocks canonical Shh signaling by a mechanism that involves activation of protein kinase A (PKA) and inhibition of the translocation of the Shh-dependent transcription factor Gli2 into the primary cilium. PKA is shown to play an essential role in inhibiting gene transcription in the absence of Shh, but global PKA activity levels are found to be a poor predictor of the degree of Shh pathway activation. We propose that the core Shh pathway regulates a small compartmentalized pool of PKA in the vicinity of primary cilia. GPCRs that affect global PKA activity levels, such as the PACAP receptor, cooperate with the canonical Shh signal to regulate Gli protein phosphorylation by PKA. This interaction serves to fine-tune the transcriptional and physiological function of the Shh pathway.

The complexities of G-protein-coupled receptor kinase function in Hedgehog signaling

Hedgehog (Hh) signaling is essential for proper tissue patterning and maintenance and has a substantial impact on human disease. While many of the main components and mechanisms involved in transduction of the Hh signal have been identified, the details of how the pathway functions are continually being refined. One aspect that has attracted much attention recently is the involvement of G-protein-coupled receptor kinases (GRKs) in the pathway. These regulators of G-protein-coupled receptor (GPCR) signaling have an evolutionarily-conserved function in promoting high-threshold Hh target gene expression through regulation of Smoothened (Smo), a GPCR family member that activates intracellular Hh signaling. Several models of how GRKs impact on Smo to increase downstream signaling have been proposed. Recently, we demonstrated that these kinases have surprisingly complex and conflicting roles, acting to limit signaling through the pathway while also promoting Smo activity. In addition to the previously described direct effects of Gprk2 on Smo activation, Gprk2 also indirectly affects Hh signaling by controlling production of the second messenger cyclic AMP to influence Protein kinase A activity.

Signaling through the neuropeptide GPCR PAC₁ induces neuritogenesis via a single linear cAMP- and ERK-dependent pathway using a novel cAMP sensor

Both cAMP and ERK are necessary for neuroendocrine cell neuritogenesis, and pituitary adenylate cyclase-activating polypeptide (PACAP) activates each. It is important to know whether cAMP and ERK are arranged in a novel, linear pathway or in two parallel pathways using known signaling mechanisms. Native cellular responses [cAMP elevation, ERK phosphorylation, cAMP responsive element binding (CREB) phosphorylation, and neuritogenesis] and promoter-reporter gene activation after treatment with forskolin, cAMP analogs, and PACAP were measured in Neuroscreen-1 (NS-1) cells, a PC12 variant enabling simultaneous morphological, molecular biological, and biochemical analysis. Forskolin (25 μM) and cAMP analogs (8-bromo-cAMP, dibutyryl-cAMP, and 8-chlorophenylthio-cAMP) stimulated ERK phosphorylation and neuritogenesis in NS-1 cells. Both ERK phosphorylation and neuritogenesis were MEK dependent (blocked by 10 μM U0126) and PKA independent (insensitive to 30 μM H-89 or 100 nM myristoylated protein kinase A inhibitor). CREB phosphorylation induced by PACAP was blocked by H-89. The exchange protein activated by cAMP (Epac)-selective 8-(4-chlorophenylthio)-2'-O-Me-cAMP (100-500 μM) activated Rap1 without affecting the other cAMP-dependent processes. Thus, PACAP-38 potently stimulated two distinct and independent cAMP pathways leading to CREB or ERK activation in NS-1 cells. Drug concentrations for appropriate effect were derived from control data for all compounds. In summary, a novel PKA- and Epac-independent signaling pathway: PACAP → adenylate cyclase → cAMP → ERK → neuritogenesis has been identified.

Distinct patterns of human medulloblastoma dissemination in the developing chick embryo nervous system

Medulloblastoma (MB) is the most common malignant primary brain tumor in children. Aggressive tumors that disseminate along the leptomeninges carry extremely poor prognoses. Mechanisms that predict dissemination are poorly understood. Our objective was to develop a reliable and reproducible model to study MB dissemination. We have created a chicken-human xenograft to study features of MB with leptomeningeal dissemination. Human MB cell lines (D283, Daoy), primary human MB cells (SF8113), and primary genetic mouse model (Math1cre:SmoM2 flox/flox) MB cells were either transfected to express green fluorescent protein (GFP) or were labeled with a membrane permeable green fluorescent probe. Cells were then injected as aggregates or implanted as pellets into the developing chicken brain immediately after neural tube closure at embryonic day 2 (E2). Most embryos were harvested three days after implantation (E5) though some were harvested up to E15. The developing brain was analyzed via whole mount fluorescent imaging and tissue section immunohistochemistry. Human and mouse MBs survived in the developing chicken central nervous system (CNS). They exhibited distinct patterns of incorporation and dissemination into the CNS that were consistent with observed phenotypes of the corresponding human patient or mouse host. Specifically, metastatic D283 cells disseminated along the leptomeninges whereas Daoy, primary mouse MB, and primary human MB cells did not. This work supports an avian-human xenograft as a successful model to study patterns of MB dissemination. Our model provides a basis for manipulating cell signaling mechanisms to understand critical targets involved in MB dissemination.

Drosophila G-protein-coupled receptor kinase 2 regulates cAMP-dependent Hedgehog signaling

G-protein-coupled receptor kinases (GRKs) play a conserved role in Hedgehog (Hh) signaling. In several systems, GRKs are required for efficient Hh target gene expression. Their principal target appears to be Smoothened (Smo), the intracellular signal-generating component of the pathway and a member of the G-protein-coupled receptor (GPCR) protein family. In Drosophila, a GRK called Gprk2 is needed for internalization and downregulation of activated Smo, consistent with the typical role of these kinases in negatively regulating GPCRs. However, Hh target gene activation is strongly impaired in gprk2 mutant flies, indicating that Gprk2 must also positively regulate Hh signaling at some level. To investigate its function in signaling, we analyzed several different readouts of Hh pathway activity in animals or cells lacking Gprk2. Surprisingly, although target gene expression was impaired, Smo-dependent activation of downstream components of the signaling pathway was increased in the absence of Gprk2. This suggests that Gprk2 does indeed play a role in terminating Smo signaling. However, loss of Gprk2 resulted in a decrease in cellular cAMP concentrations to a level that was limiting for Hh target gene activation. Normal expression of target genes was restored in gprk2 mutants by stimulating cAMP production or activating the cAMP-dependent Protein kinase A (Pka). Our results suggest that direct regulation of Smo by Gprk2 is not absolutely required for Hh target gene expression. Gprk2 is important for normal cAMP regulation, and thus has an indirect effect on the activity of Pka-regulated components of the Hh pathway, including Smo itself.

Pituitary adenylyl cyclase-activating peptide counteracts hedgehog-dependent motor neuron production in mouse embryonic stem cell cultures

Pituitary adenylyl cyclase-activating peptide (PACAP; ADCYAP1) is a neuropeptide that regulates a wide array of functions within the brain and periphery. We and others have previously demonstrated that PACAP and its high-affinity receptor PAC1 are expressed in the embryonic mouse neural tube, suggesting that PACAP plays a role in early brain development. Moreover, we previously showed that PACAP antagonizes the mitotic action of Sonic hedgehog (Shh) in postnatal cerebellar granule precursors. In the present study, we demonstrate that PACAP completely blocked Shh-dependent motor neuron generation from embryonic stem cell cultures and reduced mRNA levels of the Shh target gene Gli-1 and several ventral spinal cord patterning genes. In vivo examination of motor neuron and other patterning markers in embryonic day 12.5 spinal cords of wild-type and PACAP-deficient mice by immunofluorescence, on the other hand, revealed no obvious alterations in expressions of Islet1/2, MNR2, Lim1/2, Nkx2.2, or Shh, although the Pax6-positive area was slightly expanded in PACAP-deficient spinal cord. Caspase-3 staining revealed low, and similar, numbers of cells undergoing apoptosis in embryonic wild-type vs. PACAP-deficient spinal cords, whereas a slight but significant increase in number of mitotic cells was observed in PACAP-deficient mice. Thus, although PACAP has a strong capacity to counteract Shh signaling and motor neuron production in vitro, corresponding patterning defects associated with PACAP loss may be obscured by compensatory mechanisms.

Treating brain tumors with PDE4 inhibitors

Speculation regarding dysregulation of cAMP metabolism in oncogenesis has existed since the discovery of cAMP more than 50 years ago. Recent data confirm the relevance of disordered cAMP metabolism to the genesis of multiple cancers and suggest that the mechanism might involve altered expression and activity of phosphodiesterases (PDEs). These discoveries coincide with the rapid development and clinical evaluation of PDE inhibitors for non-cancer indications. Thus, the time is ripe to evaluate PDE inhibitors as cancer chemotherapeutics. Here we highlight recent evidence that abnormal regulation of cAMP levels might be a determinant of brain tumorigenesis and that altered PDE expression is one the mechanisms of its dysregulation. Recent preclinical and clinical experience with inhibitors of PDE4 indicates that this might be a promising approach to brain tumor therapy.

Pituitary adenylyl cyclase activating polypeptide inhibits gli1 gene expression and proliferation in primary medulloblastoma derived tumorsphere cultures

BACKGROUND

Hedgehog (HH) signaling is critical for the expansion of granule neuron precursors (GNPs) within the external granular layer (EGL) during cerebellar development. Aberrant HH signaling within GNPs is thought to give rise to medulloblastoma (MB) - the most commonly-observed form of malignant pediatric brain tumor. Evidence in both invertebrates and vertebrates indicates that cyclic AMP-dependent protein kinase A (PKA) antagonizes HH signalling. Receptors specific for the neuropeptide pituitary adenylyl cyclase activating polypeptide (PACAP, gene name ADCYAP1) are expressed in GNPs. PACAP has been shown to protect GNPs from apoptosis in vitro, and to interact with HH signaling to regulate GNP proliferation. PACAP/ptch1 double mutant mice exhibit an increased incidence of MB compared to ptch1 mice, indicating that PACAP may regulate HH pathway-mediated MB pathogenesis.

METHODS

Primary MB tumorsphere cultures were prepared from thirteen ptch1+/-/p53+/- double mutant mice and treated with the smoothened (SMO) agonist purmorphamine, the SMO antagonist SANT-1, the neuropeptide PACAP, the PKA activator forskolin, and the PKA inhibitor H89. Gene expression of gli1 and [3H]-thymidine incorporation were assessed to determine drug effects on HH pathway activity and proliferation, respectively. PKA activity was determined in cell extracts by Western blotting using a phospho-PKA substrate antibody.

RESULTS

Primary tumor cells cultured for 1-week under serum-free conditions grew as tumorspheres and were found to express PAC1 receptor transcripts. Gli1 gene expression was significantly reduced by SANT-1, PACAP and forskolin, but was unaffected by purmorphamine. The attenuation of gli1 gene expression by PACAP was reversed by the PKA inhibitor H89, which also blocked PKA activation. Treatment of tumorsphere cultures with PACAP, forskolin, and SANT-1 for 24 or 48 hours reduced proliferation.

CONCLUSIONS

Primary tumorspheres derived from ptch1+/-/p53+/- mice exhibit constitutive HH pathway activity. PACAP antagonizes HH signalling in these cells in a manner blocked by the PKA antagonist H89. PACAP and pharmacological activation of PKA also inhibited proliferation. Our data suggests that regulation of HH signaling by PACAP/PKA signaling may provide an alternative to SMO inhibition for the treatment of MB.

Expression analysis of PAC1-R and PACAP genes in zebrafish embryos

This study describes the expression of the pituitary adenylate cyclase-activating polypeptide (PACAP1 and PACAP2) and PAC1 receptor genes (PAC1a-R and PAC1b-R) in the brain of zebrafish (Danio rerio) during development. In situ hybridization of the 24- and 48-hpf embryos revealed that PACAP genes were expressed in the telencephalon, the diencephalon, the rhombencephalon, and the neurons in the dorsal part of the spinal cord. PACAP2 mRNA appears to be the most abundant form during brain development. The two PAC1-R subtypes showed a similar expression pattern: mRNAs were detected in the forebrain, the thalamus, and the rhombencephalon. However, in the tectum, only PAC1b-R gene was detected. These results suggest that, in fish, PACAP may play a role in brain development.

Protein kinase A regulatory subunit distribution in medulloblastoma

Background

Previous studies showed a differential distribution of the four regulatory subunits of cAMP-dependent protein kinases inside the brain, that changed in rodent gliomas: therefore, the distribution of these proteins inside the brain can give information on the functional state of the cells. Our goal was to examine human brain tumors to provide evidence for a differential distribution of protein kinase A in different tumors.

Methods

The distribution of detergent insoluble regulatory (R1 and R2) and catalytic subunits of cAMP dependent kinases was examined in pediatric brain tumors by immunohistochemistry and fluorescent cAMP analogues binding.

Results

R2 is organized in large single dots in medulloblastomas, while it has a different appearance in other tumors. Fluorescent cAMP labelling was observed only in medulloblastoma.

Conclusions

A different distribution of cAMP dependent protein kinases has been observed in medulloblastoma.

Chemical modification of class II G protein-coupled receptor ligands: frontiers in the development of peptide analogs as neuroendocrine pharmacological therapies

Recent research and clinical data have begun to demonstrate the huge potential therapeutic importance of ligands that modulate the activity of the secretin-like, Class II, G protein-coupled receptors (GPCRs). Ligands that can modulate the activity of these Class II GPCRs may have important clinical roles in the treatment of a wide variety of conditions such as osteoporosis, diabetes, amyotrophic lateral sclerosis and autism spectrum disorders. While these receptors present important new therapeutic targets, the large glycoprotein nature of their cognate ligands poses many problems with respect to therapeutic peptidergic drug design. These native peptides often exhibit poor bioavailability, metabolic instability, poor receptor selectivity and resultant low potencies in vivo. Recently, increased attention has been paid to the structural modification of these peptides to enhance their therapeutic efficacy. Successful modification strategies have included d-amino acid substitutions, selective truncation, and fatty acid acylation of the peptide. Through these and other processes, these novel peptide ligand analogs can demonstrate enhanced receptor subtype selectivity, directed signal transduction pathway activation, resistance to proteolytic degradation, and improved systemic bioavailability. In the future, it is likely, through additional modification strategies such as addition of circulation-stabilizing transferrin moieties, that the therapeutic pharmacopeia of drugs targeted towards Class II secretin-like receptors may rival that of the Class I rhodopsin-like receptors that currently provide the majority of clinically used GPCR-based therapeutics. Currently, Class II-based drugs include synthesized analogs of vasoactive intestinal peptide for type 2 diabetes or parathyroid hormone for osteoporosis.

Negative regulation of Hedgehog signaling by liver X receptors

Hedgehog (Hh) signaling is indispensable in embryonic development, and its dysregulated activity results in severe developmental disorders as shown by genetic models of naturally occurring mutations in animal and human pathologies. Hh signaling also functions in postembryonic development and adult tissue homeostasis, and its aberrant activity causes various human cancers. Better understanding of molecular regulators of Hh signaling is of fundamental importance in finding new strategies for pathway modulation. Here, we identify liver X receptors (LXRs), members of the nuclear hormone receptor family, as previously unrecognized negative regulators of Hh signaling. Activation of LXR by specific pharmacological ligands, TO901317 and GW3965, inhibited the responses of pluripotent bone marrow stromal cells and calvaria organ cultures to sonic Hh, resulting in the inhibition of expression of Hh-target genes, Gli1 and Patched1, and Gli-dependent transcriptional activity. Moreover, LXR ligands inhibited sonic Hh-induced differentiation of bone marrow stromal cells into osteoblasts. Elimination of LXRs by small interfering RNA inhibited ligand-induced inhibition of Hh target gene expression. Furthermore, LXR ligand did not inhibit Hh responsiveness in mouse embryonic fibroblasts that do not express LXRs, whereas introduction of LXR into these cells reestablished the inhibitory effects. Daily oral administration of TO901317 to mice after 3 d significantly inhibited baseline Hh target-gene expression in liver, lung, and spleen. Given the importance of modulating Hh signaling in various physiological and pathological settings, our findings suggest that pharmacological targeting of LXRs may be a novel strategy for Hh pathway modulation.

Lot1 is a key element of the pituitary adenylate cyclase-activating polypeptide (PACAP)/cyclic AMP pathway that negatively regulates neuronal precursor proliferation

The tumor suppressor gene Lot1 is highly expressed during brain development. During cerebellar development, Lot1 is expressed by proliferating granule cells with a time course matching the expression of the pituitary adenylate cyclase-activating polypeptide (PACAP) receptor, a neuropeptide receptor that plays an important role in the regulation of granule cell proliferation/survival. Although it has become clear that Lot1 is a negative regulator of cell division in tumor cells, its role in neuronal proliferation is not understood. We previously demonstrated that in cerebellar granule cells Lot1 expression is regulated by the PACAP/cAMP system. The aim of this study was to investigate the role played by Lot1 in neuron proliferation/survival and to identify the molecular mechanisms underlying its actions. Using a Lot1-inducible expression system, we found that in PC12 cells Lot1 negatively regulates proliferation and favors differentiation by up-regulating the expression of the PACAP receptor. In cerebellar granule cells in culture, an increase in Lot1 expression was paralleled by inhibition of proliferation and up-regulation of the PACAP receptor, which in turn positively regulated Lot1 expression. Silencing of Lot1 leads to an increase in granule cell proliferation and a reduction in survival. Confirming the in vitro results, in vivo experiments showed that PACAP induced an increase in Lot1 expression that was paralleled by inhibition of cerebellar granule cell proliferation. These data show that Lot1 is a key element of the PACAP/cAMP pathway that negatively regulates neuronal precursor proliferation. The existence of a PACAP receptor/Lot1-positive feedback loop may powerfully regulate neural proliferation during critical phases of cerebellar development.

Patched1 deletion increases N-Myc protein stability as a mechanism of medulloblastoma initiation and progression

Medulloblastoma tumorigenesis caused by inactivating mutations in the PATCHED1 (PTCH1) gene is initiated by persistently activated Sonic Hedgehog (Shh) signaling in granule neuron precursors (GNPs) during the late stages of cerebellar development. Both normal cerebellar development and Shh-driven medulloblastoma tumorigenesis require N-Myc expression. However, the mechanisms by which N-Myc affects the stages of medulloblastoma initiation and progression are unknown. Here we used a mouse model of Ptch1 heterozygosity and medulloblastoma to show that increased N-Myc expression characterized the earliest selection of focal GNP hyperplasia destined for later tumor progression. Step-wise loss of Ptch1 expression, from tumor initiation to progression, led to incremental increases in N-Myc protein, rather than mRNA, expression. Increased N-Myc resulted in enhanced proliferation and death resistance of perinatal GNPs at tumor initiation. Sequential N-Myc protein phosphorylation at serine-62 and serine-62/threonine-58 characterized the early and late stages of medulloblastoma tumorigenesis, respectively. Shh pathway activation led to increased Myc protein stability and reduced expression of key regulatory factors. Taken together our data identify N-Myc protein stability as the result of loss of Ptch1, which distinguishes normal cerebellar development from medulloblastoma tumorigenesis.

Only in congenial soil: the microenvironment in brain tumorigenesis

Microenvironmental or stromal influences on tumor formation and growth have become an active area of research. The use of mouse models of human cancers to study the role of the microenvironment will yield unique insights into this aspect of tumor biology and should identify novel therapeutic targets for the treatment of human cancers. In the following, the author review the natural history of two pediatric brain tumors, optic pathway glioma in neurofibromatosis type 1 and medulloblastoma in Gorlin's Syndrome, whose patterns of growth suggest that microenvironmental factors are essential for tumor formation. Each of these brain tumors is faithfully modeled in genetically engineered mice and the use of these mouse models to investigate the role of the microenvironment should yield exciting new insights into this important field of study.

Discovery of pituitary adenylate cyclase-activating polypeptide-regulated genes through microarray analyses in cell culture and in vivo

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an evolutionarily well conserved neuropeptide with multiple functions in the nervous, endocrine, and immune systems. PACAP provides neuroprotection from ischemia and toxin exposure, is anti-inflammatory in gastric inflammatory disease and sepsis, controls proliferative signaling pathways involved in neural cell transformation, and modulates glucohomeostasis. PACAP-based, disease-targeted therapeutics might thus be both effective and benign, enhancing homeostatic responses to behavioral, metabolic, oncogenic, and inflammatory stressors. PACAP signal transduction employs synergistic regulation of calcium and cyclic adenosine monophosphate (cAMP), and noncanonical activation of both calcium- and cAMP-dependent processes. Pharmacological activation of PACAP signaling should consequently have highly specific effects even in vivo. Here, a combined cellular biochemical, pharmacologic, transcriptomic, and bioinformatic approach to understanding PACAP signal transduction by identifying PACAP target genes with oligonucleotide- and cDNA-based microarray is described. Calcium- and cAMP-dependent PACAP signaling pathways for regulation of genes encoding proteins required for neuritogenesis, changes in cell morphology, and cell survival have been traced in PC12 cells. Pharmacological experiments have linked gene expression to cell physiological responses in this system, in which gene silencing can also be employed to confirm the functional significance of induction of specific transcripts. Differential transcriptional responses to metabolic, ischemic, and other stressors in wild type compared to PACAP-deficient mice establish in principle which PACAP-responsive transcripts in culture are PACAP-dependent in vivo. Bioinformatic approaches aid in creating a pipeline for identifying neuropeptide-regulated genes, validating their cellular functions, and defining their expression in the context of neuropeptide signaling physiology, required for discovery of new targets for drug action.

Parathyroid hormone-related protein regulates glioma-associated oncogene transcriptional activation: lessons learned from bone development and cartilage neoplasia

Hedgehog and parathyroid hormone-related protein (PTHrP) signaling play important roles regulating the differentiation of chondrocytes, which form the template for growing bone. By studying the interaction of the pathways in normal and neoplastic growth-plate chondrocytes (from enchondromas, a benign cartilage tumor), an unexpected direct regulation of hedgehog-mediated transcriptional activation by parathyroid hormone-related protein was uncovered. This regulation acts through the processing of the hedgehog-activated transcription factor, glioma-associated oncogene-three (Gli3). When PTHrP activates its receptor, Gli3 is processed to its repressor form though a protein kinase A (PKA) -dependent mechanism. Thus, activation of a G protein-coupled receptor can negatively regulate hedgehog-mediated transcription independent of hedgehog ligand activity, raising intriguing therapeutic possibilities.

Pituitary adenylyl cyclase-activating polypeptide is an intrinsic regulator of Treg abundance and protects against experimental autoimmune encephalomyelitis

Pituitary adenylyl cyclase-activating polypeptide (PACAP) is a widely expressed neuropeptide originally discovered in the hypothalamus. It closely resembles vasoactive intestinal peptide (VIP), a neuropeptide well known to inhibit macrophage activity, promote Th2-type responses, and enhance regulatory T cell (Treg) production. Recent studies have shown that administration of PACAP, like VIP, can attenuate dramatically the clinical and pathological features of murine models of autoimmune diseases such as experimental autoimmune encephalomyelitis (EAE) and collagen-induced arthritis. However, specific roles (if any) of endogenous VIP and PACAP in the protection against autoimmune diseases have not been explored. Here, we subjected PACAP-deficient mice to myelin oligodendrocyte glycoprotein (MOG(35-55))-induced EAE. MOG immunization of PACAP-deficient mice triggered heightened clinical and pathological manifestations of EAE compared to wild-type mice. The increased sensitivity was accompanied by enhanced mRNA expression of proinflammatory cytokines (TNFalpha, IL-6, IFN-gamma, IL-12p35, IL-23p19, and IL-17), chemokines (MCP-1/CCL2, MIP-1alpha/CCL3, and RANTES/CCL5), and chemotactic factor receptors (CCR1, CCR2, and CCR5), but downregulation of the anti-inflammatory cytokines (IL-4, IL-10, and TGF-beta) in the spinal cord. Moreover, the abundance of CD4(+)CD25(+)FoxP3(+) Tregs in lymph nodes and levels of FoxP3 mRNA in the spinal cord were also diminished. The reduction in Tregs was associated with increased proliferation and decreased TGF-beta secretion in lymph node cultures stimulated with MOG. These results demonstrate that endogenous PACAP provides protection in EAE and identify PACAP as an intrinsic regulator of Treg abundance after inflammation.

VPAC and PAC receptors: From ligands to function

Vasoactive intestinal peptide (VIP) and the pituitary adenylate cyclase activating polypeptides (PACAPs) share 68% identity at the amino acid level and belong to the secretin peptide family. Following the initial discovery of VIP almost four decades ago a substantial amount of knowledge has been presented describing the mechanisms of action, distribution and pleiotropic functions of these related peptides. It is now known that the physiological actions of these widely distributed peptides are produced through activation of three common G-protein coupled receptors (VPAC(1), VPAC(2) and PAC(1)R) which preferentially stimulate adenylate cyclase and increase intracellular cAMP, although stimulation of other intracellular messengers, including calcium and phospholipase D, has been reported. Using a range of in vitro and in vivo approaches, including cell-based functional assays, transgenic animals and rodent models of disease, VPAC/PAC receptor activation has been associated with numerous physiological processes (e.g. control of circadian rhythms) and clinical conditions (e.g. pulmonary hypertension), which underlies on-going research efforts and makes these peptides and their cognate receptors attractive targets for the pharmaceutical industry. However, despite the considerable interest in VPAC/PAC receptors and the processes which they mediate, there is still a paucity of selective and available, non-peptide ligands, which has hindered further advances in this field both at the basic research and clinical level. This review summarises the current knowledge of VIP/PACAP and the VPAC/PAC receptors with regard to their distribution, pharmacology, signalling pathways, splice variants and finally, the utility of animal models in exploring their physiological roles.