Calcium signalling does not only occur in combination or cross talk with cyclic nucleotide signalling. Calcium is a major intracellular second messenger in downstream signalling cascades within many cell types (Chilton, 2006). In developing neurons, calcium is particularly important in the transduction of guidance cue signals (Hong et al., 2000, Zheng, 2000), and in the regulation of growth cone motility and turning (Kater and Mills, 1991, Kater et al., 1994, Gomez and Spitzer, 1999, Song and Poo, 1999). Calcium signalling can activate both attractive and repulsive guidance (Fig. 1.2). Higher concentrations of calcium are observed on the side of the growth cone exposed to a source of soluble guidance cue, regardless of whether it is an attractant or a repellent (Zheng, 2000, Tojima et al., 2011). There are two major sources of calcium that are utilised for growth cone function. The first is the extracellular environment, which requires calcium influx across the plasma membrane (Fig. 1.2). The second are internal stores, in particular the ER (Fig. 1.2). Both extracellular and intracellular calcium stores are necessary for the turning of growth cones in response to most guidance cues.
Cytosolic calcium levels can be altered by either source of calcium; by the influx of calcium through membrane channels, or by the release of calcium from internal stores (Berridge, 1998). Extracellular calcium is required to initiate and sustain rises in intracellular calcium. If extracellular calcium is removed, the turning responses of neurons to ACh, BDNF, MAG and netrin-1 are abolished (Zheng et al., 1994, Ming et al., 1997, Song et al., 1997, Song et al., 1998).
Changes in the concentration of calcium can change the growth cone response to a guidance cue from attraction to repulsion. Netrin-1, which is normally an attractive cue, can induce a repulsive response when calcium influx is blocked, resulting in a reduction in cytosolic calcium signals (Hong et al., 2000). Fluctuations in basal resting calcium concentration in the growth cone can direct the growth cone turning response. In a landmark studying in calcium signalling, focal laser-induced photolysis (FLIP) was utilised to uncage calcium from spatially restricted areas of Xenopus growth cones. Direct focal elevation of calcium induced growth cone attraction, when released in a locally defined area, even though no guidance cues were present. When the resting level of cytosolic calcium was decreased by removing calcium from the extracellular environment, local calcium release caused focal calcium elevations resulting in growth cone repulsion, instead
Figure 1.2 Intracellular second messenger signalling within growth cones
A simplified schematic of intracellular second messenger signalling within a growth cone. An example of a calcium-dependent guidance cue: BDNF binds to the TrKB receptor on the plasma membrane. This activates TRPC channels to open, allowing calcium (green circles) to influx. Calcium can activate the production of IP3. IP3 binds to the IP3R on the ER membrane, to release calcium from the ER (calcium-induced calcium release). Released calcium results in a relatively large change in cytoplasmic calcium concentration, activating CaMKII, resulting in growth cone attraction. The initial rise of calcium can be small, resulting in a small change in cytoplasmic calcium concentration, activating CaN, resulting in growth cone repulsion. An example of a calcium-independent guidance cue: Sema-3a binds to Npn1/PlxA1 to activate the downstream production of cGMP. cGMP activates cyclic nucleotide gated channels (CNGC) to allow a very small influx of calcium into the cytoplasm. Both this calcium influx, and cGMP production result in growth cone repulsion.
of attraction (Zheng, 2000). This work clearly demonstrates that the resting concentration of intracellular calcium, and not only the absolute change in the focal elevation of calcium, is a determining factor in the nature of the growth cone turning response. Although it has been known for a long time that calcium is important in growth cone motility and turning responses (Cohan et al., 1987, Kater and Mills, 1991, Gomez et al., 1995), this study revealed that spatial and temporal characteristics of calcium, in addition to the amount of calcium present, are critical for bidirectional growth cone turning responses.
Intracellular calcium fluctuations can occur in a temporal manner within growth cones. They can be spontaneous or agonist-induced (Gu et al., 1994, Gomez et al., 1995, Gu and Spitzer, 1995, Williams et al., 1995, Tang et al., 2003), and can be either global or highly localised signals within growth cone filopodia (Gomez et al., 2001, Lohmann et al., 2005). Global elevations of calcium concentration can regulate neurite growth by causing growth cones to slow their rate of growth, stop, or retract (Lankford and Letourneau, 1989, Bandtlow et al., 1993), while decreased levels of intracellular calcium resulted in increases in neurite extension (Mattson and Kater, 1987). The extracellular application of neurotransmitters causes an increase in intracellular calcium concentration in growth cones in a global manner, inhibiting neurite growth (Haydon et al., 1984, Mattson et al., 1988). However, if a gradient of neurotransmitter such as glutamate (Zheng et al., 1996) or Ach (Zheng et al., 1994) is applied locally to a growth cone, the subsequent localised rise in calcium specifically induces growth cone attraction. Local elevations of calcium on one side of the growth cone can promote extension of filopodia (Zheng et al., 1994, Zheng et
1988), demonstrating highly spatial, localised calcium elevations are important for growth cone navigation.
Distinct calcium transients and localised signals are activated by specific calcium channels, which are located in both the plasma membrane, and the ER membrane, to gate both major sources of calcium (Bandtlow et al., 1993, Gomez et al., 1995, Tang et al., 2003). Bidirectional growth cone turning most likely relies on the gating of different sets of calcium channels located on the plasma membrane, such as the Transient Receptor Potential Canonical (TRPC) channels, L-type voltage-gated calcium channels (VGCC), and the calcium release-activated calcium (CRAC) channels (Orai proteins). Each of these calcium channels has the ability to trigger select spatial and temporal patterns of calcium signals, and hence growth cone turning (Berridge et al., 2003, Nishiyama et al., 2003, Li et al., 2005, Shim et al., 2005, Wang and Poo, 2005, Tojima et al., 2011, Mitchell et al., 2012).
Calcium signals can cause a switch from attraction to repulsion, depending on which downstream effectors are activated. Calcium has a limited ability to diffuse throughout the cytoplasm (Gabso et al., 1997). Due to this, it is assumed that calcium elevations are confined to small spatial vicinities, in close proximity to the calcium channels that the calcium entered through. However, the influx of calcium can be both amplified and extended from its location of entry (Tojima et al., 2011). These amplifications and extensions of calcium signals occur via a secondary rise in cytoplasmic calcium levels, by
release of calcium from the ER through IP3Rs and RyRs (reviewed in Tojima et al., 2011). This process is termed calcium-induced calcium release (CICR), and it produces large elevations in intracellular calcium concentration (Hong et al., 2000, Jin et al., 2005, Akiyama et al., 2009). These increases in cytosolic calcium result in the activation of growth cone attraction. Conversely, when CICR is not activated, there is no secondary release of calcium, and relatively smaller elevations in calcium concentration occur (Henley and Poo, 2004, Ooashi et al., 2005). These smaller elevations in calcium concentration result in growth cone repulsion (Hong et al., 2000, McFarlane, 2000, Zheng, 2000). Higher or lower calcium concentrations result in differing growth cone behavioural responses, due to downstream calcium effectors that detect the different amplitudes of calcium (Fig. 1.2) (Wen et al., 2004, Gomez and Zheng, 2006). The two main calcium effectors are CaMKII and the calcium/calmodulin-dependent protein phosphatase, CaN (Song and Poo, 1999, Wen et al., 2004, Tojima et al., 2011). Calmodulin is a calcium binding protein that senses intracellular calcium (Clapham, 1995, Ghosh and Greenberg, 1995). When calcium is bound to calmodulin, the complex associates with kinases and phosphatases to act as downstream calcium effectors (Wen et al., 2004, Gomez and Zheng, 2006). CaMKII and CaN have differing affinities for calcium. CaMKII has a lower affinity for calcium, while CaN has a higher affinity for calcium (Rusnak and Mertz, 2000, Hudmon and Schulman, 2002, Wen et al., 2004). This means when there are relatively smaller elevations in calcium, CaN is preferentially bound by calcium, and when there are higher elevations in calcium, CaMKII is bound (Fig. 1.2). These two calcium effectors have very different growth cone motility responses. When CaMKII is bound and activated, it activates growth cone attraction (Wen et al., 2004). When CaN is bound and activated repulsion ensues (Fig.
1.2). The higher amplitude of calcium is sufficient to cause attractive turning on the side of the growth cone with these calcium signals, such as when calcium is released focally on one side of the growth cone (Zheng, 2000). It has also been hypothesised that the calcium effectors are localised to different compartments of the cell. CaMKII is thought to be localised within close proximity of the ER calcium channels, RyRs and IP3Rs (Tojima et al., 2011), which is consistent with the idea that calcium influx across the plasma membrane can be further amplified both spatially and temporally by a secondary release of calcium from the internal stores, resulting in attraction (Takei et al., 1998, Song and Poo, 1999, Tojima et al., 2011). Growth cone repulsion occurs when there are lower amplitude calcium signals, caused only by calcium influx through plasma membrane bound channels, with no additional calcium release from the ER (Henley and Poo, 2004, Ooashi et al., 2005, Tojima et al., 2011). CaN has been hypothesised to exist in a diffuse manner throughout the cytosol, in areas where it is more likely to bind to calcium as it has just entered the cytosol via membrane bound channels (Tojima et al., 2011). These initial and secondary increases in cytoplasmic calcium concentrations are crucial for bidirectional growth cone turning, in response to guidance cues.