Trends in Neurosciences
Volume 37, Issue 8, August 2014, Pages 424-432
Journal home page for Trends in Neurosciences

Review
Calcium signaling in axon guidance

https://doi.org/10.1016/j.tins.2014.05.008Get rights and content

Highlights

  • ā€¢

    Calcium signaling is crucial for guiding axons to their targets.

  • ā€¢

    Calcium enters the growth cone through several different routes.

  • ā€¢

    The calcium concentration helps to determine turning responses to graded guidance cues.

  • ā€¢

    Guidance by Wnt5a illustrates many concepts of calcium-mediated guidance.

Guidance of axons to their targets in the developing nervous system requires a myriad of downstream signaling molecules to coordinate growth cone movement. One of the most important of these is calcium, and over the past few years many new insights have been gained into the role of calcium in axon guidance. In this review we focus on mechanisms of calcium entry into the growth cone and its downstream effects on both growth cone motility and turning. We particularly highlight the role of calcium concentrations in determining attractive versus repulsive responses to graded guidance cues, and their role in guidance by the morphogen Wnt5a.

Introduction

For the brain to be wired correctly during development axons must navigate appropriately to their targets, often over long distances. Axons achieve this feat by detecting and responding appropriately to numerous molecular and physical guidance cues in their local environment, usually via the growth cone [1]. This includes guidance by growth through permissive conduits, by substrate-bound guidance cues, or by gradients of diffusible cues. Several molecular guidance cue families involved in axon guidance have been identified including the netrins, semaphorins, ephrins, slits, neurotrophins, Wnts, and some morphogens 2, 3. These cues are detected by receptors on the axon surface, which then activate a variety of downstream signaling pathways to cause axon turning and/or changes in growth rate [4].

One of the most crucial components of these pathways is calcium 5, 6, 7, 8. Calcium concentrations in growth cones are regulated both by calcium influx through the plasma membrane and by release from intracellular calcium stores 9, 10 (see Glossary). A calmodulin-dependent protein kinase II (CaMKII)ā€“calcineurin (CaN) switch often initiates either an attractive (turning towards the highest guidance cue concentration) or repulsive (turning away from the highest guidance cue concentration) response to these intracellular calcium elevations [8]. These responses are effected via the regulation of cytoskeletal components, such as microtubules and actin filaments 11, 12, 13, 14, 15, and by membrane dynamics, including vesicle trafficking 16, 17, 18. In the past few years many new insights have emerged regarding the central and multifaceted roles of calcium in mediating axon growth and guidance. We review here some of these developments, focusing particularly on new mechanisms for producing and mediating calcium elevations, the involvement of calcium in the attraction/repulsion switch, how calcium modulates growth cone motility, and the role of calcium in guidance by Wnt5a.

Section snippets

Mechanisms of calcium entry

The spatial distribution and level of intracellular calcium is crucial for axon guidance [19], and regulation of this distribution begins with calcium entry into the growth cone. Methods of entry particularly important for axon guidance include voltage-dependent calcium channels (VDCCs), release of intracellular calcium stores, and store-operated calcium entry (SOCE) from extracellular sources (Figure 1). The most important VDCCs for growth cone turning are the L-type Ca2+ channels 20, 21.

Role of calcium in turning

A turning response requires asymmetric deployment of signaling molecules between the two sides of the growth cone. Zheng [44] used focal laser-induced photolysis (FLIP) of caged calcium to generate a spatially restricted elevation of intracellular calcium concentration on one side of the growth cone. This asymmetric calcium elevation is sufficient to cause growth cone turning; however, the direction is dependent on the steepness of the calcium gradient and background calcium concentrations [44]

Downstream effects of calcium

Although a molecular switch mediated by CaMKII and CaN likely determines the growth cone response to intracellular calcium distributions, it is downstream effectors which induce membrane trafficking and cytoskeletal changes that create this response 16, 17, 48. Repulsive guidance cues usually induce asymmetric endocytosis [17], whereas attractive cues induce asymmetric exocytosis [50]. Notably, these responses are not merely the same response with the sign reversed; attraction to the left of a

Calcium, motility, and growth rate

The role of calcium signaling in overall growth cone motility is less well understood than its role in turning. Calcium signaling has an effect on the growth rate of growth cones, first characterized as a narrow extracellular calcium concentration range favorable for growth 43, 60, 61, 62. Surprisingly, calcium transients 63, 64 and calcium entry through mechanosensitive channels slow axon growth, whereas calcium release from ER stores enhances growth [41]. The relation between calcium

Concluding remarks

Although Ca2+ has long been implicated as a second messenger in growth cone turning, it is only over the past few years that the precise distributions and mechanisms for attraction and repulsion have been elucidated. Many different mechanisms for intracellular Ca2+ elevation have been discovered, including release from ER stores (CICR and IICR) and entry from extracellular sources (CRAC, SOCE, and voltage-sensitive Ca2+ channels) 22, 33, 34, 38. A complex pathway which interprets these

Acknowledgments

We gratefully acknowledge support from National Health and Medical Research Council (NHMRC) Project Grant 1043044, and thank Beth Kita and Rowan Tweedale for providing helpful feedback on earlier versions.

Glossary

Axon guidance by growth rate modulation
a type of chemotactic movement whereby axon guidance up the gradient is mediated not by turning, but instead by the axon taking larger steps when it is pointed up the gradient than when it is pointed down the gradient; [67] for more details.
Calcium-induced calcium release (CICR)
the release of calcium from intracellular stores in response to rising calcium concentration (i.e., a positive feedback process).
Calcium release-activated channel (CRAC)
calcium

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