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

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.

Roles for Hedgehog signaling in adult organ homeostasis and repair

“…Smoothened (SMO), however, was recently found to function as a G-protein-coupled receptor (GPCR) (Riobo et al, 2006), which allows it to control axon guidance possibly through monomeric G proteins (Yam et al, 2009). In terms of the function of SMO as a GPCR, second messengers such as calcium (Ca 2+ ) have also been implicated (Belgacem and Borodinsky, 2011). Finally, although not commonly described as non-canonical HH signaling, a role for the GLI transcription factors independent of the traditional HH/SMO-signaling cascade has also been reported, particularly in cancer, where other signaling pathways appear to directly regulate the GLIs (Stecca and Ruiz, 2010).…”

Folate receptor 1 is necessary for neural plate cell apical constriction during Xenopus neural tube formation

“…mRNA was synthesized as previously described (Belgacem and Borodinsky, 2011;Borodinsky et al, 2004;Swapna and Borodinsky, 2012) using the Xenopus laevis folr1 template (XGC African clawed frog folr1 cDNA, Clone ID: 7012141, Open Biosystems, Thermo Fisher Scientific), subcloned into pCS2+ vector. Mutations were carried out using a site-directed mutagenesis kit (200515, Agilent Technologies;Swapna and Borodinsky, 2012) and PCR reaction to render Folr1-MO1-resistant mRNA.…”

Interplay between electrical activity and bone morphogenetic protein signaling regulates spinal neuron differentiation

“…This activity occurs prior and during synapse formation and modulates several aspects of nervous system development. It is important for the proliferation of mouse cortical progenitors (16), cell migration of cerebellar, hippocampal, and cortical cells in mice (17-19), differentiation of spinal and cerebral neurons in Xenopus and mouse (20)(21)(22)(23)(24)(25), and pathfinding of retinogeniculate projections in ferret and of motor neuron axons in chicken (26,27), demonstrating the universality of the relevance of early electrical activity for nervous system development. Although the role of electrical activity in neuronal development is now generally accepted, it is still considered to be important mostly in later stages of circuit formation; the early neuronal specialization is believed to occur solely based on morphogenetic protein signaling and independently of electrical activity.…”

“…It is important for the proliferation of mouse cortical progenitors (16), cell migration of cerebellar, hippocampal, and cortical cells in mice (17-19), differentiation of spinal and cerebral neurons in Xenopus and mouse (20)(21)(22)(23)(24)(25), and pathfinding of retinogeniculate projections in ferret and of motor neuron axons in chicken (26,27), demonstrating the universality of the relevance of early electrical activity for nervous system development. Although the role of electrical activity in neuronal development is now generally accepted, it is still considered to be important mostly in later stages of circuit formation; the early neuronal specialization is believed to occur solely based on morphogenetic protein signaling and independently of electrical activity.In the Xenopus embryonic spinal cord, Ca 2+ spike activity consisting of rapid transients in intracellular [Ca 2+ ] that are propagated throughout the whole neuronal cell body, lasting for 20 s, becomes apparent after neural tube closure (21) and is present in a ventrodorsal gradient (20,21) opposite to that of BMPs (4). In the present study, we show that there is an interplay between Ca 2+ -mediated electrical activity and BMP signaling that is important for the appropriate differentiation of spinal neurons.…”

“…In the Xenopus embryonic spinal cord, Ca 2+ spike activity consisting of rapid transients in intracellular [Ca 2+ ] that are propagated throughout the whole neuronal cell body, lasting for 20 s, becomes apparent after neural tube closure (21) and is present in a ventrodorsal gradient (20,21) opposite to that of BMPs (4). In the present study, we show that there is an interplay between Ca 2+ -mediated electrical activity and BMP signaling that is important for the appropriate differentiation of spinal neurons.…”