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Sonic hedgehog signaling is decoded by calcium spike activity in the developing spinal cord

Department of Physiology and Membrane Biology and Institute for Pediatric Regenerative Medicine, Shriners Hospital for Children and University of California Davis School of Medicine, Sacramento, CA 95817, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 02/2011; 108(11):4482-7. DOI: 10.1073/pnas.1018217108
Source: PubMed

ABSTRACT

Evolutionarily conserved hedgehog proteins orchestrate the patterning of embryonic tissues, and dysfunctions in their signaling can lead to tumorigenesis. In vertebrates, Sonic hedgehog (Shh) is essential for nervous system development, but the mechanisms underlying its action remain unclear. Early electrical activity is another developmental cue important for proliferation, migration, and differentiation of neurons. Here we demonstrate the interplay between Shh signaling and Ca(2+) dynamics in the developing spinal cord. Ca(2+) imaging of embryonic spinal cells shows that Shh acutely increases Ca(2+) spike activity through activation of the Shh coreceptor Smoothened (Smo) in neurons. Smo recruits a heterotrimeric GTP-binding protein-dependent pathway and engages both intracellular Ca(2+) stores and Ca(2+) influx. The dynamics of this signaling are manifested in synchronous Ca(2+) spikes and inositol triphosphate transients apparent at the neuronal primary cilium. Interaction of Shh and electrical activity modulates neurotransmitter phenotype expression in spinal neurons. These results indicate that electrical activity and second-messenger signaling mediate Shh action in embryonic spinal neurons.

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Available from: Yesser Belgacem
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    • "Also, BMP4 induces the dopaminergic phenotype in cultured GABAergic neurons derived from the mouse cortical striatum during a sensitive period in vitro (Stull et al., 2001). Considering that BMPs (Swapna and Borodinsky, 2012), Shh (Belgacem and Borodinsky, 2011), Wnts (Varela-Nallar et al., 2010) and FGF modulate Ca 2+ dynamics and kinase activity in developing neurons together with the electrical activitydependent plasticity of neurotransmitter phenotype, the potential role of morphogenetic proteins in postmitotic neurons participating in neurotransmitters respecification through Ca 2+ -mediated signaling becomes apparent. "
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    ABSTRACT: Morphogenetic proteins are responsible for patterning the embryonic nervous system by enabling cell proliferation that will populate all the neural structures and by specifying neural progenitors that imprint different identities in differentiating neurons. The adoption of specific neurotransmitter phenotypes is crucial for the progression of neuronal differentiation, enabling neurons to connect with each other and with target tissues. Preliminary neurotransmitter specification originates from morphogen-driven neural progenitor specification through the combinatorial expression of transcription factors according to morphogen concentration gradients, which progressively restrict the identity that born neurons adopt. However, neurotransmitter phenotype is not immutable, instead trophic factors released from target tissues and environmental stimuli change expression of neurotransmitter-synthesizing enzymes and specific vesicular transporters modifying neuronal neurotransmitter identity. Here we review studies identifying the mechanisms of catecholaminergic, GABAergic, glutamatergic, cholinergic and serotonergic early specification and of the plasticity of these neurotransmitter phenotypes during development and in the adult nervous system. The emergence of spontaneous electrical activity in developing neurons recruits morphogenetic proteins in the process of neurotransmitter phenotype plasticity, which ultimately equips the nervous system and the whole organism with adaptability for optimal performance in a changing environment.
    Preview · Article · Dec 2015 · Journal of chemical neuroanatomy
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    • "Several non-canonical pathways have been described for Hedgehog signaling in various experimental models. SMOdependent , but GLI1-independent signaling downstream of Shh triggers calcium transients in spinal cord neurons (Belgacem and Borodinsky, 2011), or activates small GTPases in endothelial cells (Chinchilla et al., 2010; Pathi et al., 2001; Polizio et al., 2011). Interestingly, cytoskeletal rearrangements necessary for lamellipodia formation and migration in mesenchymal fibroblasts required omega-6 polyunsaturated fatty acid metabolites, but not GLI1 activation (Bijlsma et al., 2007). "
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    ABSTRACT: Axonogenesis, a process for the establishment of neuron connectivity, is central to brain function. The role of metabolites derived from docosahexaenoic acid (DHA, 22:6n-3) that is specifically enriched in the brain, has not been addressed in axon development. In this study, we tested if synaptamide (N-docosahexaenoylethanolamine), an endogenous metabolite of DHA, affects axon growth in cultured cortical neurons. We found that synaptamide increased the average axon length, inhibited GLI family zinc finger 1 (GLI1) transcription and sonic hedgehog (Shh) target gene expression while inducing cAMP elevation. Similar effects were produced by cyclopamine, a regulator of the Shh pathway. Conversely, Shh antagonized elevation of cAMP and blocked synaptamide-mediated increase in axon length. Activation of Shh pathway by a smoothened (SMO) agonist (SAG) or overexpression of SMO did not inhibit axon growth mediated by synaptamide or cyclopamine. Instead, adenylate cyclase inhibitor SQ22536 abolished synaptamide-mediated axon growth indicating requirement of cAMP elevation for this process. Our findings establish that synaptamide promotes axon growth while Shh antagonizes synaptamide-mediated cAMP elevation and axon growth by a SMO-independent, non-canonical pathway.
    Preview · Article · Nov 2015 · Biology Open
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    • "In contrast, Gli3 is supposed to act more likely as an inhibitor of transcription [15]. While the hedgehog protein in Drosophila is a key player in the formation of segment polarity, its best characterized mammalian homologue Sonic hedgehog is involved in the development of the CNS, limbs, skeleton, and internal organs [16] [17] [18] [19] [20]. In the mammalian central nervous system (CNS) SHH is responsible for the patterning of the spinal cord and brain as well as for the proliferation of the cerebellar granule precursor cells [21]. "
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    ABSTRACT: The Sonic Hedgehog (SHH) pathway plays a central role in the developing mammalian CNS. In our study, we aimed to investigate the spatiotemporal SHH pathway expression pattern in human fetal brains. We analyzed 22 normal fetal brains for Shh, Patched, Smoothened, and Gli1-3 expression by immunohistochemistry. In the telencephalon, strongest expression of Shh, Smoothened, and Gli2 was found in the cortical plate (CP) and ventricular zone. Patched was strongly upregulated in the ventricular zone and Gli1 in the CP. In the cerebellum, SHH pathway members were strongly expressed in the external granular layer (EGL). SHH pathway members significantly decreased over time in the ventricular and subventricular zone and in the cerebellar EGL, while increasing levels were found in more superficial telencephalic layers. Our findings show that SHH pathway members are strongly expressed in areas important for proliferation and differentiation and indicate a temporal expression gradient in telencephalic and cerebellar layers probably due to decreased proliferation of progenitor cells and increased differentiation. Our data about the spatiotemporal expression of SHH pathway members in the developing human brain serves as a base for the understanding of both normal and pathological CNS development.
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