Traumatic injuries to the central nervous system (brain and spinal cord) affect young and middle age people causing premature death and disability [1,2]. These injuries, as well as peripheral nerve injuries because of wounds or surgeries, involve axotomy (AT), i.e., transection or disconnection of axons. AT is followed by either degeneration and death of neurons, or regeneration of axons and their reconnection with their targets. To fight the consequences of neurotraumas, it is necessary to stop the processes, leading to neuron death, as soon as possible. Unfortunately, to date, no sufficiently effective neuroprotectors have been discovered. A more profound study of molecular processes, induced by axon lesion, is required.

The neurons’ vulnerability to AT depends on a number of factors, such as localization, age, and distance. Neurons in the peripheral nervous system (PNS) usually survive axotomy and regenerate, while many neurons in the central nervous system (CNS) undergo degeneration and death after axotomy. This is related to neuronal factors, such as differences in gene expression in cell response to axotomy, and non-neuronal factors, such as immune proteins, which inhibit the regeneration, and the interaction of both types of factors [3,4]. In very young animals, axon damage results in retrograde degeneration and death both in the PNS and CNS [5]. Generally, the more remote the lesion is from the soma, the more resistant to axotomy is the neuron [6,7]. There are two main suggestions for possible mechanisms of how the information about the trauma is transmitted to a cell body: (a) a mechanism of double signals, where the distance to the lesion site is estimated based on the time delay between lesion-induced early and rapid ion fluxes and the later arrival of motor-dependent signal complexes; and (b) a mechanism involving continuous scanning and regulation of axonal transport in two directions [5,8].

Two forms of axotomy are now considered. “Primary” axotomy occurs when axons are broken apart or transected directly in the mechanical impact on nervous tissue. The physical transection of axons massively involves its microenvironment and creates a dramatic, immediate disturbance of ionic regulation. “Secondary” axotomy occurs after relatively minor lesions to axons, such as stretch injury or diffuse axonal injury in brain trauma, which trigger a cascade of events, ultimately resulting in cytoskeleton degradation and axonal rupture [9]. In the CNS, primary axotomy is more typical for spinal cord injury, while secondary axotomy is more typical for traumatic brain injury. There are very little data on secondary axotomy in the PNS.

Ca²⁺ is critically involved in a number of signal pathways, controlling cell homeostasis. It plays an important role in neurodegeneration [10,11], particularly, in the response of neurons to AT [2,8,9]. The elevation of cytosolic calcium concentration to 10⁻⁴–10⁻³ M and more can trigger processes of cell death, necrosis or apoptosis [12].

The injury-induced focal permeability leads to local Ca²⁺ influx with activation of cysteine proteases, calpain and caspases, which play an important role in resulting pathogenesis of traumatic axon injury via the proteolytic cleavage of cerebral spectrin, one of the components of subaxolemmal cytoskeleton. During this pathological process, a local calcium overload, together with calpain activation, also causes mitochondrial damage, resulting in the release of cytochrome c and caspase activation. Then the activated calpain and caspases are involved in the degradation of the local axonal cytoskeleton [13].

Ca²⁺ signaling is very important in the glial environment of neurons, which plays a substantial role in the survival and regeneration of neurons after injury [14]. The injury-induced Ca²⁺-related transcription factors can serve as useful biomarkers of pathological processes in reactive glia [15]. It has been shown that damage to glia may suppress neuronal functions and induce neuron loss [16]. Moreover, damage to nerves induces the death of surrounding glial cells [17]. What is important, damage to nerves not only causes death of glial cells, immediately adjacent to the affected area (or damaged collaterally in mechanical way), but also to glial cells, remote from the damage site (remote glial cells, further RGC). Ca²⁺ is one of the intermediaries between cellular–molecular events in the damage site and in RGC. The exact role of Ca²⁺, including extracellular calcium, the different mechanisms of cytosolic Ca²⁺ regulation (Ca²⁺ channels and pumps), and Ca²⁺-activated proteins in the survival and death of RGC are still unclear.

2. Calcium Dynamics in Neurons and Glia after Axotomy

In the experiments of Ziv and Spira on isolated Aplysia neurons [18], the spatiotemporal dynamics of intra-axonal calcium levels from the transection site was studied. The dynamics of Ca²⁺ were similar in both cut ends and went through the following stages: axolemmal disruption and up to a 30-fold Ca²⁺ elevation along the whole axon; and the slower process propagation of Ca²⁺ elevation front propagation with 11–16 pm/sec speed from the transection site towards intact end. After the sealing of the axonal lumen, the Ca²⁺ level recovered to initial values for 7–10 min, going from intact ends to lesion sites. In the absence of Ca²⁺ in the medium, axon transection does not result in Ca²⁺ elevation and lumen resealing. After the returning of normal levels of Ca²⁺ in the medium, Ca²⁺ is increased near the transected ends and the lumen is sealed.

The AT-induced Ca²⁺ elevation is mainly provided by the influx of Ca²⁺ through voltage-gated Ca²⁺ channels, inversion of Na⁺–Ca²⁺ exchanger, and the lumen. However, the spatiotemporal dynamics of Ca²⁺ after axon transection does not correspond to just diffusion, suggesting that Ca²⁺ gradients are created and restricted through some other mechanisms, making it possible for the neuron to survive the injury and ultimately recover. Ca²⁺ ions mediate early events in axo-somal communication (retrograde signaling) after nerve damage. Rishal and Fainzilber [8] consider in detail the mechanism of the retrograde Ca²⁺ front that is formed after axon transection, involving the possible reinforcement via the release of Ca²⁺ from intracellular storages, such as endoplasmic reticulum. The resulting Ca²⁺ waves propagate along the axon and serve as the initial damage signal for soma. In our experiments on crayfish stretch receptor neurons, axotomy induced the elevation in cytosolic calcium levels in soma and near-soma axoplasm within minutes [19].

The increase in cytosolic calcium concentration also induces calcium-activated chlorine currents [20]. In crayfish motor neurons, a high increase in the expression of chlorine channels was observed [21].

As a result of focal brain trauma, extracellular calcium decreases to 0.1 mM [22]. [Ca²⁺]_o elevation increases pH_i and decreases [Na⁺]_i, and vice versa [23].

AT promotes the activity of the plasmatic membrane Ca²⁺ ATPase (PMCA), which regulates intracellular Ca²⁺ concentration by taking calcium out from the cell [24]. In addition, AT promotes the expression of PMCA in dorsal root ganglia [25].

In mammal glial cells, ER lumen is one of the main sources of signal transduction Ca²⁺. Upon depletion, the lumen is filled with Ca²⁺ ions from the extracellular space via the SOCE mechanism (store-operated calcium entrance) [26].

Astrocytes exchange signals via ATP. IP3 molecule messengers diffuse between astrocytes through gap junctions. IP3 activates Ca²⁺ channels in cell organelles, which results in the release of Ca²⁺ to cytosol. This Ca²⁺ can additionally promote IP3 production and cause ATP release through membrane channels, formed by pannexins and innexins [14]. This eventually results in a Ca²⁺ wave, propagating from cell to cell. In addition, the wave can be mediated by the release of ATP to the extracellular medium and following purinergic receptor activation. The NFAT transcription factor links Ca²⁺ signaling with reactive transcriptional changes. Blockade of astrocytic calcineurin/NFAT signaling helps to normalize hippocampal synaptic function and plasticity in a rat model of traumatic brain injury [15].

Satellite glial cells are small cells surrounding neurons in sensory, sympathetic, and parasympathetic ganglia. These cells are involved in the chemical environment regulation, in particular, buffering it with the help of K⁺ and Ca²⁺ channels [27]. Similar to astrocytes, they are connected with each other via gap junctions and respond to ATP signals, increasing intracellular Ca²⁺. They are highly inflammation-sensitive and contribute to pathological conditions, such as chronic pain.

Although neurons and satellite glia are not strongly connected or coupled, their close location provides favorable conditions for effective communication. Bidirectional Ca²⁺ signaling between satellite glial cells and neurons, involving both gap junctions and ATP release, is detected [28,29]. However, it is still unknown how the activation of satellite glia after axonal damage is associated with neuroglial communication.

Unlike glial cells, located in the lesion site or in immediate proximity, which are directly damaged in axotomy, the sensibly distant glial cells can be susceptible to the injury only indirectly. The question of what is happening to this glia, in particular, with glial envelope around soma and proximal axon area of the neuron, is little studied. Very little is particularly known about how Ca²⁺ and Ca²⁺-dependent signal pathways are involved.

In experiments on the crayfish stretch receptor model, it is observed a significant Ca²⁺ level increase in the glial envelope of the soma and proximal axon segment [30].

3. Future Prospects and Targets

As we said above, AT promotes the activity and expression of PMCA. At present, direct PMCA inhibitors are not easily accessible, so indirect methods of inhibiting this pump are used, for example, the increase in saline pH against SERCA inhibition [24].

IP3 receptors (IP3R) are, together with RyR, a way of Ca²⁺ release from ER. Calcium signaling in astroglia is based on combined work of IP3R store-operated Ca²⁺ channels (SOCC) in plasma membrane, belonging to the Orai family and acting together with STIM 1 and 2 molecules, which transmit to them the ER depletion signal [117]. As in the case of PMCA, these channels can contribute to axotomy-induced increase in the Ca²⁺ concentration in remote glia.

Mitochondrial Ca²⁺ uniporter (MCU), through which Ca²⁺ enters the mitochondria, and mitochondrial Na⁺–Ca²⁺ exchanger (NCX), releasing Ca²⁺ from mitochondria, regulate Ca²⁺ movement between mitochondria and cytosol, thus being involved in Ca²⁺ homeostasis and the survival of the cell. For MCU inhibition, ruthenium complexes Ru360 and Ru265 are used [118]. There are also selective inhibitors for mitochondrial NCX (CGP-37157) [119].

There is a question about the nature of cause-and-effect processes, linking axon damage and the elevation of Ca²⁺ concentration in RGC. To study a possible mechanism of AT-induced retrograde propagating of Ca²⁺ wave along glial envelope, it seems reasonable to apply gap junction inhibitors, such as arachidonic acid.

Thus, the analysis of the involvement of PMCA, mitochondrial Ca²⁺ uniporter and NCX, IP3R with SOCC, and differentiated Ca²⁺-dependent potassium channels are a prospective direction in the research of Ca²⁺ mechanisms, regulating AT-induced death and the survival of RGC together with neurons. The wide spectrum of Ca²⁺-activated proteins also needs to be comprehensively studied for potential therapeutic targets.

4. Conclusions

Traumatic brain injury is one of main causes of death and disability in young and middle age, and spinal brain injury is one main causes of disability, limiting mobility in people of all ages. Peripheral nerve injuries remain a growing social-economic burden, mainly affecting the young working population. The existing methods of clinical treatment, aimed to prevent the death and degeneration of nerve cells in the first hour after a trauma, are, in general, insufficient, leaving a significant part of motor or sensory functions lost.

One of the recovery conditions after such injures is the preservation of viability of damaged neurons and glia, which, in turn, depends on the number of factors, including Ca²⁺ homeostasis and neuroglial interaction. To date, biomedical science has not created sufficiently effective agents and methods of treatment, aimed at both of these factors and considering their connection.

The death and survival of glia have substantial significance in the recovery after neurotrauma, in which, on the one hand, glia plays protective role, and on the other hand, glia should not hinder the regeneration processes. There should be some type of balance between survival (proliferation) and manageable cell death (apoptosis, controlled necrosis, and autophagy). Apparently, some ways of altering Ca²⁺ concentrations increase or decrease both apoptosis and necrosis, and others, depending on circumstances, act more selectively, preventing or promoting a certain type of death. The important fact is that AT induces susceptibility to activation or inhibition of certain signal pathways.

The already obtained data about the involvement of Ca²⁺ regulation mechanisms in AT-induced death of RGC indicate some possible directions for the search of novel and the development of existing methods of pharmacological applications for protecting neurons and glia from the consequences of neurotrauma. The critical clinical importance, especially for clinics, of the interplay between neurons, satellite glial cells, and Ca²⁺-dependent mechanisms in response to AT demands its further research.