GRK2 kinases in the primary cilium initiate SMOOTHENED-PKA signaling in the Hedgehog cascade.

During Hedgehog (Hh) signal transduction in development and disease, the atypical G protein-coupled receptor (GPCR) SMOOTHENED (SMO) communicates with GLI transcription factors by binding the protein kinase A catalytic subunit (PKA-C) and physically blocking its enzymatic activity. Here, we show that GPCR kinase 2 (GRK2) orchestrates this process during endogenous mouse and zebrafish Hh pathway activation in the primary cilium. Upon SMO activation, GRK2 rapidly relocalizes from the ciliary base to the shaft, triggering SMO phosphorylation and PKA-C interaction. Reconstitution studies reveal that GRK2 phosphorylation enables active SMO to bind PKA-C directly. Lastly, the SMO-GRK2-PKA pathway underlies Hh signal transduction in a range of cellular and in vivo models. Thus, GRK2 phosphorylation of ciliary SMO and the ensuing PKA-C binding and inactivation are critical initiating events for the intracellular steps in Hh signaling. More broadly, our study suggests an expanded role for GRKs in enabling direct GPCR interactions with diverse intracellular effectors.

Numb positively regulates Hedgehog signaling at the ciliary pocket.

Hedgehog (Hh) signaling relies on the primary cilium, a cell surface organelle that serves as a signaling hub for the cell. Using proximity labeling and quantitative proteomics, we identify Numb as a ciliary protein that positively regulates Hh signaling. Numb localizes to the ciliary pocket and acts as an endocytic adaptor to incorporate Ptch1 into clathrin-coated vesicles, thereby promoting Ptch1 exit from the cilium, a key step in Hh signaling activation. Numb loss impedes Sonic hedgehog (Shh)-induced Ptch1 exit from the cilium, resulting in reduced Hh signaling. Numb loss in spinal neural progenitors reduces Shh-induced differentiation into cell fates reliant on high Hh activity. Genetic ablation of Numb in the developing cerebellum impairs the proliferation of granule cell precursors, a Hh-dependent process, resulting in reduced cerebellar size. This study highlights Numb as a regulator of ciliary Ptch1 levels during Hh signal activation and demonstrates the key role of ciliary pocket-mediated endocytosis in cell signaling.

GRK2 Kinases in the Primary Cilium Initiate SMOOTHENED-PKA Signaling in the Hedgehog Cascade.

During Hedgehog (Hh) signal transduction in development and disease, the atypical G protein-coupled receptor (GPCR) SMOOTHENED (SMO) communicates with GLI transcription factors by binding the protein kinase A catalytic subunit (PKA-C) and physically blocking its enzymatic activity. Here we show that GPCR kinase 2 (GRK2) orchestrates this process during endogenous Hh pathway activation in the primary cilium. Upon SMO activation, GRK2 rapidly relocalizes from the ciliary base to the shaft, triggering SMO phosphorylation and PKA-C interaction. Reconstitution studies reveal that GRK2 phosphorylation enables active SMO to bind PKA-C directly. Lastly, the SMO-GRK2-PKA pathway underlies Hh signal transduction in a range of cellular and in vivo models. Thus, GRK2 phosphorylation of ciliary SMO, and the ensuing PKA-C binding and inactivation, are critical initiating events for the intracellular steps in Hh signaling. More broadly, our study suggests an expanded role for GRKs in enabling direct GPCR interactions with diverse intracellular effectors.

Hedgehog Signaling in Cortical Development.

The Hedgehog (Hh) pathway plays a crucial role in embryonic development, acting both as a morphogenic signal that organizes tissue formation and a potent mitogenic signal driving cell proliferation. Dysregulated Hh signaling leads to various developmental defects in the brain. This article aims to review the roles of Hh signaling in the development of the neocortex in the mammalian brain, focusing on its regulation of neural progenitor proliferation and neuronal production. The review will summarize studies on genetic mouse models that have targeted different components of the Hh pathway, such as the ligand Shh, the receptor Ptch1, the GPCR-like transducer Smo, the intracellular transducer Sufu, and the three Gli transcription factors. As key insights into the Hh signaling transduction mechanism were obtained from mouse models displaying neural tube defects, this review will also cover some studies on Hh signaling in neural tube development. The results from these genetic mouse models suggest an intriguing hypothesis that elevated Hh signaling may play a role in the gyrification of the brain in certain species. Additionally, the distinctive production of GABAergic interneurons in the dorsal cortex in the human brain may also be linked to the extension of Hh signaling from the ventral to the dorsal brain region. Overall, these results suggest key roles of Hh signaling as both a morphogenic and mitogenic signal during the forebrain development and imply the potential involvement of Hh signaling in the evolutionary expansion of the neocortex.

A PKA inhibitor motif within SMOOTHENED controls Hedgehog signal transduction.

The Hedgehog (Hh) cascade is central to development, tissue homeostasis and cancer. A pivotal step in Hh signal transduction is the activation of glioma-associated (GLI) transcription factors by the atypical G protein-coupled receptor (GPCR) SMOOTHENED (SMO). How SMO activates GLI remains unclear. Here we show that SMO uses a decoy substrate sequence to physically block the active site of the cAMP-dependent protein kinase (PKA) catalytic subunit (PKA-C) and extinguish its enzymatic activity. As a result, GLI is released from phosphorylation-induced inhibition. Using a combination of in vitro, cellular and organismal models, we demonstrate that interfering with SMO-PKA pseudosubstrate interactions prevents Hh signal transduction. The mechanism uncovered echoes one used by the Wnt cascade, revealing an unexpected similarity in how these two essential developmental and cancer pathways signal intracellularly. More broadly, our findings define a mode of GPCR-PKA communication that may be harnessed by a range of membrane receptors and kinases.

Control of the Hedgehog pathway by compartmentalized PKA in the primary cilium.

The Hedgehog (Hh) signaling is one of the essential signaling pathways during embryogenesis and in adults. Hh signal transduction relies on primary cilium, a specialized cell surface organelle viewed as the hub of cell signaling. Protein kinase A (PKA) has been recognized as a potent negative regulator of the Hh pathway, raising the question of how such a ubiquitous kinase specifically regulates one signaling pathway. We reviewed recent genetic, molecular and biochemical studies that have advanced our mechanistic understanding of PKA's role in Hh signaling in vertebrates, focusing on the compartmentalized PKA at the centrosome and in the primary cilium. We outlined the recently developed genetic and optical tools that can be harvested to study PKA activities during the course of Hh signal transduction.

Myomegalin regulates Hedgehog pathway by controlling PDE4D at the centrosome.

Mutations in the hedgehog (Hh) signaling are implicated in birth defects and cancers, including medulloblastoma (MB), one of the most malignant pediatric brain tumors. Current Hh inhibitors face the challenge of drug resistance and tumor relapse, urging new insights in the Hh pathway regulation. Our previous study revealed how PDE4D controls global levels of cAMP in the cytoplasm to positively regulate Hh signaling; in the present study, we found that a specific isoform PDE4D3 is tethered to the centrosome by Myomegalin (Mmg), a centrosome/Golgi-associated protein. Mmg loss dislocates PDE4D3 from the centrosome, leading to local PKA overactivation and inhibition of the Hh signaling, leaving other PKA-related pathways unaffected. Mmg loss suppresses the proliferation of granule neuron precursors and blocks the growth of MB in mouse model. Our findings specify a new regulatory mechanism of the Hh pathway and highlight an exciting therapeutic avenue for Hh-related cancers with reduced side effects.

The lysosomal GPCR-like protein GPR137B regulates Rag and mTORC1 localization and activity.

Cell growth is controlled by a lysosomal signalling complex containing Rag small GTPases and mammalian target of rapamycin complex 1 (mTORC1) kinase. Here, we carried out a microscopy-based genome-wide human short interfering RNA screen and discovered a lysosome-localized G protein-coupled receptor (GPCR)-like protein, GPR137B, that interacts with Rag GTPases, increases Rag localization and activity, and thereby regulates mTORC1 translocation and activity. High GPR137B expression can recruit and activate mTORC1 in the absence of amino acids. Furthermore, GPR137B also regulates the dissociation of activated Rag from lysosomes, suggesting that GPR137B controls a cycle of Rag activation and dissociation from lysosomes. GPR137B-knockout cells exhibited defective autophagy and an expanded lysosome compartment, similar to Rag-knockout cells. Like zebrafish RagA mutants, GPR137B-mutant zebrafish had upregulated TFEB target gene expression and an expanded lysosome compartment in microglia. Thus, GPR137B is a GPCR-like lysosomal regulatory protein that controls dynamic Rag and mTORC1 localization and activity as well as lysosome morphology.

Developmental phosphoproteomics identifies the kinase CK2 as a driver of Hedgehog signaling and a therapeutic target in medulloblastoma.

A major limitation of targeted cancer therapy is the rapid emergence of drug resistance, which often arises through mutations at or downstream of the drug target or through intrinsic resistance of subpopulations of tumor cells. Medulloblastoma (MB), the most common pediatric brain tumor, is no exception, and MBs that are driven by sonic hedgehog (SHH) signaling are particularly aggressive and drug-resistant. To find new drug targets and therapeutics for MB that may be less susceptible to common resistance mechanisms, we used a developmental phosphoproteomics approach in murine granule neuron precursors (GNPs), the developmental cell of origin of MB. The protein kinase CK2 emerged as a driver of hundreds of phosphorylation events during the proliferative, MB-like stage of GNP growth, including the phosphorylation of three of the eight proteins commonly amplified in MB. CK2 was critical to the stabilization and activity of the transcription factor GLI2, a late downstream effector in SHH signaling. CK2 inhibitors decreased the viability of primary SHH-type MB patient cells in culture and blocked the growth of murine MB tumors that were resistant to currently available Hh inhibitors, thereby extending the survival of tumor-bearing mice. Because of structural interactions, one CK2 inhibitor (CX-4945) inhibited both wild-type and mutant CK2, indicating that this drug may avoid at least one common mode of acquired resistance. These findings suggest that CK2 inhibitors may be effective for treating patients with MB and show how phosphoproteomics may be used to gain insight into developmental biology and pathology.

LEAP2 Is an Endogenous Antagonist of the Ghrelin Receptor.

Ghrelin, an appetite-stimulatory hormone secreted by the stomach, was discovered as a ligand for the growth hormone secretagogue receptor (GHSR). Through GHSR, ghrelin stimulates growth hormone (GH) secretion, a function that evolved to protect against starvation-induced hypoglycemia. Though the biology mediated by ghrelin has been described in great detail, regulation of ghrelin action is poorly understood. Here, we report the discovery of liver-expressed antimicrobial peptide 2 (LEAP2) as an endogenous antagonist of GHSR. LEAP2 is produced in the liver and small intestine, and its secretion is suppressed by fasting. LEAP2 fully inhibits GHSR activation by ghrelin and blocks the major effects of ghrelin in vivo, including food intake, GH release, and maintenance of viable glucose levels during chronic caloric restriction. In contrast, neutralizing antibodies that block endogenous LEAP2 function enhance ghrelin action in vivo. Our findings reveal a mechanism for fine-tuning ghrelin action in response to changing environmental conditions.

Myt1l safeguards neuronal identity by actively repressing many non-neuronal fates.

Normal differentiation and induced reprogramming require the activation of target cell programs and silencing of donor cell programs. In reprogramming, the same factors are often used to reprogram many different donor cell types. As most developmental repressors, such as RE1-silencing transcription factor (REST) and Groucho (also known as TLE), are considered lineage-specific repressors, it remains unclear how identical combinations of transcription factors can silence so many different donor programs. Distinct lineage repressors would have to be induced in different donor cell types. Here, by studying the reprogramming of mouse fibroblasts to neurons, we found that the pan neuron-specific transcription factor Myt1-like (Myt1l) exerts its pro-neuronal function by direct repression of many different somatic lineage programs except the neuronal program. The repressive function of Myt1l is mediated via recruitment of a complex containing Sin3b by binding to a previously uncharacterized N-terminal domain. In agreement with its repressive function, the genomic binding sites of Myt1l are similar in neurons and fibroblasts and are preferentially in an open chromatin configuration. The Notch signalling pathway is repressed by Myt1l through silencing of several members, including Hes1. Acute knockdown of Myt1l in the developing mouse brain mimicked a Notch gain-of-function phenotype, suggesting that Myt1l allows newborn neurons to escape Notch activation during normal development. Depletion of Myt1l in primary postmitotic neurons de-repressed non-neuronal programs and impaired neuronal gene expression and function, indicating that many somatic lineage programs are actively and persistently repressed by Myt1l to maintain neuronal identity. It is now tempting to speculate that similar 'many-but-one' lineage repressors exist for other cell fates; such repressors, in combination with lineage-specific activators, would be prime candidates for use in reprogramming additional cell types.

Phosphodiesterase 4D acts downstream of Neuropilin to control Hedgehog signal transduction and the growth of medulloblastoma.

Alterations in Hedgehog (Hh) signaling lead to birth defects and cancers including medulloblastoma, the most common pediatric brain tumor. Although inhibitors targeting the membrane protein Smoothened suppress Hh signaling, acquired drug resistance and tumor relapse call for additional therapeutic targets. Here we show that phosphodiesterase 4D (PDE4D) acts downstream of Neuropilins to control Hh transduction and medulloblastoma growth. PDE4D interacts directly with Neuropilins, positive regulators of Hh pathway. The Neuropilin ligand Semaphorin3 enhances this interaction, promoting PDE4D translocation to the plasma membrane and cAMP degradation. The consequent inhibition of protein kinase A (PKA) enhances Hh transduction. In the developing cerebellum, genetic removal of Neuropilins reduces Hh signaling activity and suppresses proliferation of granule neuron precursors. In mouse medulloblastoma allografts, PDE4D inhibitors suppress Hh transduction and inhibit tumor growth. Our findings reveal a new regulatory mechanism of Hh transduction, and highlight PDE4D as a promising target to treat Hh-related tumors.

Control of activating transcription factor 4 (ATF4) persistence by multisite phosphorylation impacts cell cycle progression and neurogenesis.

Organogenesis is a highly integrated process with a fundamental requirement for precise cell cycle control. Mechanistically, the cell cycle is composed of transitions and thresholds that are controlled by coordinated post-translational modifications. In this study, we describe a novel mechanism controlling the persistence of the transcription factor ATF4 by multisite phosphorylation. Proline-directed phosphorylation acted additively to regulate multiple aspects of ATF4 degradation. Stabilized ATF4 mutants exhibit decreased ß-TrCP degron phosphorylation, ß-TrCP interaction, and ubiquitination, as well as elicit early G(1) arrest. Expression of stabilized ATF4 also had significant consequences in the developing neocortex. Mutant ATF4 expressing cells exhibited positioning and differentiation defects that were attributed to early G(1) arrest, suggesting that neurogenesis is sensitive to ATF4 dosage. We propose that precise regulation of the ATF4 dosage impacts cell cycle control and impinges on neurogenesis.

Centrosome motility is essential for initial axon formation in the neocortex.

The mechanisms underlying the normal development of neuronal morphology remain a fundamental question in neurobiology. Studies in cultured neurons have suggested that the position of the centrosome and the Golgi may predict the site of axon outgrowth. During neuronal migration in the developing cortex, however, the centrosome and Golgi are oriented toward the cortical plate at a time when axons grow toward the ventricular zone. In the current work, we use in situ live imaging to demonstrate that the centrosome and the accompanying polarized cytoplasm exhibit apical translocation in newborn cortical neurons preceding initial axon outgrowth. Disruption of centrosomal activity or downregulation of the centriolar satellite protein PCM-1 affects axon formation. We further show that downregulation of the centrosomal protein Cep120 impairs microtubule organization, resulting in increased centrosome motility. Decreased centrosome motility resulting from microtubule stabilization causes an aberrant centrosomal localization, leading to misplaced axonal outgrowth. Our results reveal the dynamic nature of the centrosome in developing cortical neurons, and implicate centrosome translocation and microtubule organization during the multipolar stage as important determinants of axon formation.

Dixdc1 is a critical regulator of DISC1 and embryonic cortical development.

The psychiatric illness risk gene Disrupted in Schizophrenia-1 (DISC1) plays an important role in brain development; however, it is unclear how DISC1 is regulated during cortical development. Here, we report that DISC1 is regulated during embryonic neural progenitor proliferation and neuronal migration through an interaction with DIX domain containing-1 (Dixdc1), the third mammalian gene discovered to contain a Disheveled-Axin (DIX) domain. We determined that Dixdc1 functionally interacts with DISC1 to regulate neural progenitor proliferation by co-modulating Wnt-GSK3beta/beta-catenin signaling. However, DISC1 and Dixdc1 do not regulate migration via this pathway. During neuronal migration, we discovered that phosphorylation of Dixdc1 by cyclin-dependent kinase 5 (Cdk5) facilitates its interaction with the DISC1-binding partner Ndel1. Furthermore, Dixdc1 phosphorylation and its interaction with DISC1/Ndel1 in vivo is required for neuronal migration. Together, these data reveal that Dixdc1 integrates DISC1 into Wnt-GSK3beta/beta-catenin-dependent and -independent signaling pathways during cortical development and further delineate how DISC1 contributes to neuropsychiatric disorders.

Hook3 interacts with PCM1 to regulate pericentriolar material assembly and the timing of neurogenesis.

Centrosome functions are important in multiple brain developmental processes. Proper functioning of the centrosome relies on assembly of protein components into the pericentriolar material. This dynamic assembly is mediated by the trafficking of pericentriolar satellites, which are comprised of centrosomal proteins. Here we demonstrate that trafficking of pericentriolar satellites requires the interaction between Hook3 and Pericentriolar Material 1 (PCM1). Hook3, previously shown to link the centrosome and the nucleus in C. elegans, is recruited to pericentriolar satellites through interaction with PCM1, a protein associated with schizophrenia. Disruption of the Hook3-PCM1 interaction in vivo impairs interkinetic nuclear migration, a featured behavior of embryonic neural progenitors. This in turn leads to overproduction of neurons and premature depletion of the neural progenitor pool in the developing neocortex. These results underscore the importance of centrosomal assembly in neurogenesis and provide potential insights into the etiology of brain developmental diseases related to the centrosome dysfunction.

Guiding neuronal cell migrations.

Neuronal migration is, along with axon guidance, one of the fundamental mechanisms underlying the wiring of the brain. As other organs, the nervous system has acquired the ability to grow both in size and complexity by using migration as a strategy to position cell types from different origins into specific coordinates, allowing for the generation of brain circuitries. Guidance of migrating neurons shares many features with axon guidance, from the use of substrates to the specific cues regulating chemotaxis. There are, however, important differences in the cell biology of these two processes. The most evident case is nucleokinesis, which is an essential component of migration that needs to be integrated within the guidance of the cell. Perhaps more surprisingly, the cellular mechanisms underlying the response of the leading process of migrating cells to guidance cues might be different to those involved in growth cone steering, at least for some neuronal populations.

Disrupted in schizophrenia 1 regulates neuronal progenitor proliferation via modulation of GSK3beta/beta-catenin signaling.

The Disrupted in Schizophrenia 1 (DISC1) gene is disrupted by a balanced chromosomal translocation (1; 11) (q42; q14.3) in a Scottish family with a high incidence of major depression, schizophrenia, and bipolar disorder. Subsequent studies provided indications that DISC1 plays a role in brain development. Here, we demonstrate that suppression of DISC1 expression reduces neural progenitor proliferation, leading to premature cell cycle exit and differentiation. Several lines of evidence suggest that DISC1 mediates this function by regulating GSK3beta. First, DISC1 inhibits GSK3beta activity through direct physical interaction, which reduces beta-catenin phosphorylation and stabilizes beta-catenin. Importantly, expression of stabilized beta-catenin overrides the impairment of progenitor proliferation caused by DISC1 loss of function. Furthermore, GSK3 inhibitors normalize progenitor proliferation and behavioral defects caused by DISC1 loss of function. Together, these results implicate DISC1 in GSK3beta/beta-catenin signaling pathways and provide a framework for understanding how alterations in this pathway may contribute to the etiology of psychiatric disorders.

The heparan sulfate proteoglycans Dally-like and Syndecan have distinct functions in axon guidance and visual-system assembly in Drosophila.

Heparan sulfate proteoglycans (HSPGs), a class of glycosaminoglycan-modified proteins, control diverse patterning events via their regulation of growth-factor signaling and morphogen distribution. In C. elegans, zebrafish, and the mouse, heparan sulfate (HS) biosynthesis is required for normal axon guidance, and mutations affecting Syndecan (Sdc), a transmembrane HSPG, disrupt axon guidance in Drosophila embryos. Glypicans, a family of glycosylphosphatidylinositol (GPI)-linked HSPGs, are expressed on axons and growth cones in vertebrates, but their role in axon guidance has not been determined. We demonstrate here that the Drosophila glypican Dally-like protein (Dlp) is required for proper axon guidance and visual-system function. Mosaic studies revealed that Dlp is necessary in both the retina and the brain for different aspects of visual-system assembly. Sdc mutants also showed axon guidance and visual-system defects, some that overlap with dlp and others that are unique. dlp+ transgenes were able to rescue some sdc visual-system phenotypes, but sdc+ transgenes were ineffective in rescuing dlp abnormalities. Together, these findings suggest that in some contexts HS chains provide the biologically critical component, whereas in others the structure of the protein core is also essential.