Intermingled cAMP, cGMP and calcium spatiotemporal dynamics in developing neuronal circuits

Abstract

cAMP critically modulates the development of neuronal connectivity. It is involved in a wide range of cellular processes that require independent regulation. However, our understanding of how this single second messenger achieves specific modulation of the signaling pathways involved remains incomplete. The subcellular compartmentalization and temporal regulation of cAMP signals have recently been identified as important coding strategies leading to specificity. Dynamic interactions of this cyclic nucleotide with other second messenger including calcium and cGMP are critically involved in the regulation of spatiotemporal control of cAMP. Recent technical improvements of fluorescent sensors facilitate cAMP monitoring, whereas optogenetic tools permit spatial and temporal control of cAMP manipulations, all of which enabled the direct investigation of spatiotemporal characteristics of cAMP modulation in developing neurons. Focusing on neuronal polarization, neurotransmitter specification, axon guidance, and refinement of neuronal connectivity, we summarize herein the recent advances in understanding the features of cAMP signals and their dynamic interactions with calcium and cGMP involved in shaping the nervous system.

Keywords: axon guidance; axon outgrowth; cAMP; cGMP; calcium; kinetics; subcellular compartmentalization; topographic maps.

Figure 1

Mechanisms underlying the generation of subcellular domains of cAMP. Distinct subcellular localizations of adenylyl cyclases and phosphodiesterases enable the generation of subcellular domains of cAMP. Ca²⁺-sensitive transmembrane ACs and their regulating Ca²⁺ channels are restricted to lipid rafts whereas the Ca²⁺-independent ACs are excluded from this plasma membrane territory. Phosphodiesterases degrade cAMP close to its site of synthesis and limit the diffusion of the signal. The soluble AC is responsible of cAMP synthesis in discrete locations of the cell including the mitochondria and the nucleus.

Figure 2

Spatiotemporal dynamics of cAMP during neuronal polarization and neurotransmitter specification. (A) Local interactions between cAMP and cGMP determine neurite maturation into axon or dendrite. High cAMP and low cGMP concentrations promote axonogenesis, whereas low cAMP and high cGMP lead to dentritogenesis. (B) In developing neurons, cAMP transients and calcium spikes are interdependent. The choice of expression of excitatory or an inhibitory neurotransmitter is modulated by the frequency of calcium spikes, which is regulated by the activity of a set of kinases including the cAMP effector PKA.

Figure 3

Spatiotemporal dynamics of cAMP during axon outgrowth, axon guidance and topographic maps development. (A₁) Axon elongation is enhanced in cultured retinal ganglion cell with an increased cAMP concentration. (A₂) Electrical activity and calcium transients modulates cAMP synthesis by the soluble adenylyl cyclase (sAC/AC10). Both signals cooperate to regulate axon outgrowth. It is still unclear whether or not this signal transduction pathway is spatiotemporally restricted. (B₁) Transient and local cAMP synthesis by light-mediated activation of an optogenetic AC (within the blue region) is sufficient to induce axon turning. (B₂) Brief cAMP signal in filopodia induces an increase in the frequency of filopodia-restricted Ca²⁺ transients and a change of direction in axon outgrowth. cAMP/cGMP ratio sets the polarity of netrin-1-induced axon guidance. A high cAMP/cGMP ratio leads to attraction whereas a low ratio converts attraction into repulsion. (C₁) cAMP signaling perturbation (lack of AC1, pharmacological blockade of ACs, or PKA blockade) prevents the axon repellents ephrin-As to induce axon retraction. (C₂) Transient electrical activity- and calcium-dependent modulation of cAMP is required in growth cones for the backward movement of axons when exposed to ephrin-As.

Figure 4

Spatiotemporal dynamics of cAMP during developmental pruning of ectopic axonal branches, and synaptic competition. cAMP is required in two subcellular compartments during the synaptic refinement of neuronal projections. In the soma of retinal ganglion cells, cAMP oscillations and spontaneous calcium waves are interdependent. Spontaneous calcium waves are required for competition between axons in their targets. In the synapses, cAMP signaling mediated by adenylyl cyclases 1 and 8 is involved in synaptic potentiation and hebbian mechanisms, which are crucial to eliminate misplaced synapses.