Ion Channel Dysregulation Following Intracerebral Hemorrhage

Abstract

Injury to the brain after intracerebral hemorrhage (ICH) results from numerous complex cellular mechanisms. At present, effective therapy for ICH is limited and a better understanding of the mechanisms of brain injury is necessary to improve prognosis. There is increasing evidence that ion channel dysregulation occurs at multiple stages in primary and secondary brain injury following ICH. Ion channels such as TWIK-related K⁺ channel 1, sulfonylurea 1 transient receptor potential melastatin 4 and glutamate-gated channels affect ion homeostasis in ICH. They in turn participate in the formation of brain edema, disruption of the blood-brain barrier, and the generation of neurotoxicity. In this review, we summarize the interaction between ions and ion channels, the effects of ion channel dysregulation, and we discuss some therapeutics based on ion-channel modulation following ICH.

Introduction

Intracerebral hemorrhage (ICH) is a lethal subtype of stroke with high mortality and severe neurological deficits. ICH-related poor outcomes are caused by a cluster of pathological processes [1, 2]. Although there are numerous studies on ICH [3–8], effective therapy is still limited. Thus, there is a need to continue research on brain injury mechanisms following ICH to find more effective and efficient therapeutic strategies for the condition.

Primary and secondary injuries are involved in ICH. The primary injury is mainly caused by the hematoma, and the secondary injury comprises the reactions to clot degradation such as neuroinflammation, cell death, and blood-brain barrier (BBB) disruption [9]. The uneven distribution of ions is requisite and characteristic for the survival of cells and their proper functions. This includes the concentration gradients across cellular organellar membranes to gradients within the cytoplasm from one side to the other side of a cell. The localization and activation of various ion-specific channels, co-transporters, and pumps affect the distribution of ions. Typically, there is a gating mechanism to control whether ion channels are open or closed, and ions passively pass these channels down their concentration gradient [10]. The dysregulation of ion channels in the central nervous system (CNS) causes the dyshomeostasis of major ions, particularly sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and chloride (Cl⁻). These ions influence a series of cellular processes, many of which overlap with the features of ICH. Above all, the involvement of ions and ion channels in the pathology after ICH such as disruption of BBB, the generation of brain edema, and the death of neurons should attract our attention. This review summarizes the functions of some essential ions in ICH (Fig. 1), with an emphasis on the roles of their corresponding channels. In addition, this review focuses on the current understanding of ion-channel dysfunction in ICH and explores methods aiming to modulate ion channels as potential therapeutic strategies for ICH patients.

Fig. 1.

The roles of ions and some related ion channels following ICH. The disproportionate K⁺ influx and Na⁺ efflux through SUR1-TRPM4, and water entering the cell through aquaporin cause cytotoxic edema. The activation of TREK-1 may protect the blood-brain barrier. Voltage-gated sodium channels (VGSCs) may trigger seizures. Glutamate-gated channels contribute to neuronal death after ICH.

The Roles of Ion Channel Dysregulation in ICH

Ion Channel Dysregulation Participates in the Formation of Brain Edema

Brain edema is a common complication of ICH and is linked to neurological injury and unfavorable prognosis. It appears quickly after ICH as excess water accumulates in the brain parenchyma [11–13]. In the ICH rat model, edema peaks in 1-3 days and dissipates within 7 days [14–17]. There are two important types of brain edema, classified according to their underlying mechanism and time course: cytotoxic edema and vasogenic edema. Cytotoxic edema is cellular swelling that often appears at the early stage of perihematomal edema, within 24–48 h after brain ischemia or ICH onset. It is caused by the abnormal activation and dysregulation of ion channels (e.g., aquaporin-4 (AQP 4) and sulfonylurea 1 transient receptor potential melastatin 4 (SUR1-TRPM4)), which results in the imbalanced shifting of ions with the osmotic influx of water into cells [18–20]. Vasogenic edema manifests in the second phase of perihematomal edema, hours after the initial brain injury. It results from the breakdown of BBB and is formed by intravascular substances exuding into extracellular space [21]. After CNS injury, many cells in the vicinity of the damage will go through edema. It is especially notable in astrocytes, which appears to be a rapid response to CNS injury [22]. The accumulation of glutamate in the perihematomal region may also lead to cytotoxic edema [23]. Ionic channel dysregulation drives cytotoxic edema, which in turn further aggravates ionic imbalance, cell swelling, and BBB damage [18–20, 24], which then culminates in vasogenic edema [18, 19]. Swelling can be caused by vasogenic edema. The expanded mass caused by edema can increase intracranial pressure (ICP), resulting in brain herniation and further brain injury [20, 25].

SUR1-TRPM4 is a transient, non-selective monovalent cation channel, formed by the obligate binding of four SUR1 subunits to the pore-forming cation subunit, TRPM4 [26–29]. It is not expressed in physiological CNS tissue [27] but is upregulated in different cell types especially astrocytes after diverse forms of CNS injury including brain ischemia [27, 30, 31], TBI [32], and CNS metastases [33]. SUR1-TRPM4 is considered to cause cytotoxic edema, and inhibition of this channel can alleviate edema in the rat model of brain ischemia [30]. When ATP is depleted or the nanomolar concentration of Ca²⁺ is changed, the SUR1-TRPM4 channel will open, causing the influx of Na⁺ and efflux of K⁺ from cells [20, 34]. Many negatively charged macromolecules bind to K⁺ within the cell, however, for Na⁺, it is not the case. The influx of Na⁺ is out of proportion compared to the efflux of K⁺, thus generating a strong osmotic gradient for water influx [19, 20], leading to cell depolarization, intracellular edema, and cell death [30, 35, 36]. Disturbance of Na⁺, K⁺, and Cl⁻ occurs 24 h following ICH [37], and increased Cl⁻ and decreased K⁺ concentrations engaged in cellular edema can last for 14 days after ICH [38].

SUR1 possesses high-affinity connecting sites for sulfonylurea drugs and related compounds including glibenclamide and repaglinide, which can pharmacologically regulate the activity of the SUR1-regulated channel [27]. Previous studies have found that glibenclamide can improve outcomes and reduce mortality in severe brain ischemia models, while those studies that evaluate the pharmacological inhibition in ICH have shown mixed results [39–45]. The effects of glibenclamide in germinal matrix hemorrhage and hemorrhagic encephalopathy of prematurity are demonstrated in two early-stage research [40, 45]. The low-dose administration of glibenclamide acutely reduced the extent and severity of brain hemorrhage. However, the results of multiple studies which investigate whether glibenclamide can reduce brain edema and improve neurological function deficit after ICH turns out to be divergent. Notably, in an autologous blood-induced ICH rat model, SUR1 was upregulated and the treatment with low-dose glibenclamide resulted in marked benefits in both histopathological and functional outcomes. It improved neurological severity scores, protected the integrity of BBB, and decreased edema [42]. These benefits were independently consistent with two sets: in the same autologous blood-induced ICH model, it has been found that glibenclamide seemed to play a part via the NLRP3 inflammasome [39]. In a collagenase-induced ICH model, glibenclamide presented antioxidative and neuroprotective effects [41]. Conversely, another study reported that glibenclamide did not alleviate brain edema, protect BBB integrity, or improve the disturbance of ions in a rat ICH model [46]. Furthermore, some studies indicate that in moderate or severe ICH models caused by collagenase, glibenclamide did not exhibit obvious effects or improve the outcomes [43, 46]. The reason for the ambiguous results is probably owing to the differences in animal species or the scoring methods.

Ion Channel Dysregulation Contributes to Spreading Depolarization (SD)

ICH pathological processes may also arise from SD. SD happens in the cortex and is characterized by transmembrane ion gradient breakdown in cells, sustained depolarization in neurons, and swelling of neuronal and glia cells [47–49]. SD could influence microvascular perfusion, leading to local ischemia, increased ICP, and secondary brain injury [50, 51]. The main initiation factor for SD is the elevation of extracellular K⁺ because of its activity-dependent release via calcium-activated, ATP-sensitive potassium channels or two-pore domain potassium channels [52–54]. In this pathological environment, high extracellular K⁺ can result in spreading ischemia [55–58]. Possible mechanisms of extracellular accumulation of K⁺ in the brain include erythrocytosis, BBB breakdown [59], and membrane destruction due to parenchyma injury [60–62], which could occur after the increase in hematoma size post-ICH. These factors may be diminished but not eliminated by hematoma evacuation in an ICH trial [63].

SD aggravates the formation of neuronal swelling and recurrent SDs may result in dendritic beading in the ischemic penumbra, according to research outcomes [64–66]. Therefore, SD may trigger or facilitate the development of cytotoxic edema in the cortical gray matter and the brain tissue surrounding hematoma in ICH. In rare cases, SD could also produce BBB disruption and the ensuing vasogenic edema via the activation of matrix metalloproteinase-9 (MMP-9) [67]. Clinical trials have shown that SD is strongly correlated with perihematomal edema progression in ICH patients [63], which is related to a transient drop in cerebral blood flow [68].

Ion Channel Dysregulation Influences the Integrity of BBB

It has been established that BBB breakdown is one of the hallmarks of ICH brain injury [69]. The BBB regulates the interaction of ions or molecules between blood and the brain. As the uncontrolled flow of these substances into the parenchyma is potentially dangerous, sustained BBB breakdown deserves attention [70]. Ion dyshomeostasis, brain edema, and BBB permeability are inseparable as ICH progresses. Ion dyshomeostasis occurs in different ICH models [71]. It begins within 24 h of ictus in an autologous whole blood-induced ICH model [37] and within 1-3 days in the collagenase-induced model, with changes of Cl⁻, Fe²⁺, and K⁺ detected at day 14 [38, 72].

TREK-1 is a two-pore-domain potassium channel that is extensively expressed in neurons, astrocytes, and vascular endothelial cells [73, 74]. Numerous physiological or pathological promoters such as membrane stretch, heat, and intracellular acidosis can regulate the gating properties of TREK-1 [75, 76]. Increasing evidence has indicated that TREK-1 plays a strong part in the normal functioning of the nervous system by hyperpolarizing membrane potential [77]. TREK-1 is also crucial in the cellular mechanisms of neuroprotection in various CNS diseases [78–81]. For example, TREK-1 regulates inflammatory responses and BBB permeability in a model of multiple sclerosis, and experimental autoimmune encephalomyelitis (EAE) [82, 83]. Activated TREK-1 represses microglia activation in brain ischemia [80, 81, 84, 85]. In secondary brain injury of ICH mice, deficiency of TREK-1 increases the integrity of BBB and aggravates post-ICH brain edema, inflammatory cell infiltration, and neuronal apoptosis, thereby promoting neurodegeneration in ICH [77]. The increased BBB integrity also contributes to the infiltration of neutrophils and the formation of brain edema, which exacerbates neuronal death and neurological deterioration [15, 86, 87].

Ion Channel Dysregulation Participates in Neuronal Death

After ICH, numerous processes facilitate the occurrence of neuronal death, such as inflammatory responses [88], excitotoxicity, and dyshomeostasis of Ca²⁺ [89]. Apart from Na⁺, K⁺, Cl^-, and the action of water, Ca²⁺ is also involved in the formation of brain edema. More importantly, calcium overload is a catalyst for neuronal death in ICH. The dyshomeostasis of Ca²⁺, which leads to calpain activation, is characteristic of apoptosis and necrosis [90, 91]. Ca²⁺ and calmodulin regulate cell proliferation and influence the cell cycle, but when intracellular Ca²⁺ exceeds its normal concentration, as well as time and space limits, or irreversible damage occurs during cell progression, Ca²⁺ can promote cell death through necrosis and apoptosis [92–94] (Fig. 2).

Fig. 2.

Dysregulation of various calcium channels following ICH. α2δ-1 combines with N-methyl-D-aspartate receptor (NMDAR) under pathological conditions. Transient receptor potential melastatin 7, Acid-sensing ion channels, Purinergic 2X7 receptor, etc. also participate in neuronal death after ICH.

Whether it is brain ischemia or ICH, when the blood flow in the brain is interrupted, the depletion of cellular energy can cause ionic dyshomeostasis [95]. Inhibition of oxidative phosphorylation leads to the loss of ATP substrate for Na⁺- K⁺-ATPase, which results in the concentration changes of K⁺ and Na⁺, and membrane depolarization [96]. Continued depolarization leads to excessive Ca²⁺ entry through voltage-sensitive Ca²⁺ channels, which triggers an excessive release of the neurotransmitter glutamate [97–99]. Neuronal injury post-ICH is closely associated with neurotoxicity caused by the massive release of glutamate from injured neurons [100]. Glutamate is a common excitatory mediator in the CNS, which is involved in excitatory synaptic transmission and plays an essential role in maintaining the normal signal transmission of neurons [49, 100]. However, glutamate has toxic effects on neurons under pathological conditions. The role of glutamate is regulated by different types of channels (Fig. 2).

N-methyl-D-aspartate receptor (NMDAR) is the most prominent glutamate receptor that is involved in glutamate-mediated nervous system damage [101, 102]. It is a ligand-gated ion channel receptor consisting of two structural subunits GluN1 and two regulatory subunits GluN2, which is closely linked to synaptic plasticity and involved in various processes of CNS diseases. After ICH, blood clot degradation and other factors lead to the release of several endogenous substances such as glutamate, Ca²⁺, reactive oxygen species (ROS), thrombin, MMPs, heme, iron, tumor necrosis factor (TNF)-α, and others [102, 103]. Excess accumulation of glutamate in the injured brain tissues results in the over-activation of NMDAR, then mediates the influx of Ca²⁺ [104–106], triggering secondary signal cascades, activating proteases and phospholipases, and producing free radicals, which eventually cause delayed damage to neurons [107]. The expression of GluN1 and inflammatory factors are both significantly up-regulated compared with the control groups in pre-clinical experiments and clinical patients following ICH [108]. These molecules participate in excitatory and mitotic signal transduction, including mediating NMDAR. Mitotic signals trigger the normal cell division cycles in neural progenitor cells and microglia, while abnormal mitotic signals contribute to neuronal toxicity. Some clinical trials demonstrate neuroprotection in acute brain ischemia by blocking NMDA-mediated neurotoxicity; however, its clinical application is limited because it requires instant intervention after an endovascular repair procedure [109, 110].

Voltage-gated calcium channel subunit α2δ-1 is encoded by the Cacna2d1 gene and exerts a vital part in the development of muscle and the generation of synapses [111]. Down-regulation of α2δ-1 can cause migration, adhesion, and spread of myoblasts [112]. It has been found that most synaptic NMDARs are not connected with α2δ-1. However, following neuropathic pain, the expression of α2δ-1 is increased and forms a pathological interaction with NMDAR to increase NMDAR activity. In these circumstances, α2δ-1 can be regulated by gabapentin to treat neuropathic pain through the inhibition of synaptic trafficking of α2δ-1–NMDAR complexes [113, 114]. The expression level of α2δ-1 is increased after ICH [115], and the loss of α2δ-1 improves neuronal functions and reduces their apoptosis in the ICH mice model, while suppressing the activation of GluN1 [108]. According to a recent study, the serum concentration of α2δ-1 subunit is closely related to the severity of ICH. It may be used as a prognostic marker for ICH [116]. The detailed mechanisms of α2δ-1 action in ICH need further investigations.

Transient Receptor Potential Vanilloid 4 (TRPV4)

TRPV4 is one subtype of the six-transmembrane cation-permeable channels formed by transient receptor potential (TRP) channel proteins, mediating the transmembrane flow of Ca²⁺. TRPV4 can be activated by hypo-osmolarity and moderate change in temperature [117]. It is broadly expressed in neurons and glial cells [118]. TRPV4 channels induce the efflux of intracellular Ca²⁺ and play an important role in maintaining Ca²⁺ homeostasis under normal conditions [119–121]. They may participate in regulating neuronal excitability and behaviors [122], and the integrity of the blood-cerebrospinal fluid barrier [123]. TRPV4 channels are also involved in pathological processes such as neuronal apoptosis [124], inflammatory response [125], and brain edema [126]. Blocking TRPV4 can prevent NMDA receptor-mediated glutamate excitotoxicity [127]. Recently, numerous studies have reported the function of TRPV4 in ICH. For instance, SLC24A6 negatively mediated Ca²⁺ overload is closely related to experimental ICH [128]. After ICH, TRPV4 is upregulated in mouse neurons, which causes Ca²⁺ dyshomeostasis, and results in brain edema and neuronal apoptosis [129].

Blocking TRPV4 channels using TRPV4 antagonist HC-067047 and TRPV4 siRNA ameliorates brain edema, neuronal death, and BBB disruption in ICH rats [130]. Inhibiting the expression level of TRPV4 improves neuronal survival [129]. However, these results are opposite to another study which demonstrates that a selective TRPV4 agonist, GSK1016790A, ameliorates brain injury and improves neurological deficits in a collagenase-induced mouse model of ICH [131]. The contradictory outcomes may be due to the different severity of brain damage in particular types of ICH models because TRPV4 is sensitive to pressure changes in the brain [118, 132]. Overall, available results indicate that TRPV4 contributes to ICH brain injury and implicate TRPV4 as a novel therapeutic target for ICH treatment.

Transient Receptor Potential Melastatin 7 (TRPM7)

TRPM7 is an active divalent cation-selective channel that gives access to Ca²⁺, Mg^2+, and Zn²⁺ [133, 134]. TRPM7 can mediate the progression through the cell cycle and cell death, and a related study finds that Ca²⁺ in brain ischemia mice is regulated by TRPM7 [135]. TRPM7 may act as a potential non-glutamate target for hypoxic-ischemic neuronal injury since the inhibition of TRPM7 reduces anoxic neuronal cell death in vitro [136], and improves neuron survival and neurological functions in vivo after brain ischemia [137]. A previous study has also demonstrated the TRPM7 might participate in cell death via p38 and actin modulation through upstream calcium-dependent calcineurin [138]. It has been demonstrated that Ca²⁺-permeable TRPM7 channels participate in neuronal toxicity of Ca²⁺ overload after brain ischemia or ICH [139]. The signal transducer and activator of transcription 1 (Stat1) is a member of the Stats family and an important transcription factor for cells to respond to cytokines. Stat1 facilitates TRPM7 transcription by increasing H3K27ac in the TRPM7 promoter region, thus exacerbating Ca²⁺ overload in neurons after ICH [93]. Overexpression of TRPM7 can counteract the inhibition of Ca²⁺ overload in neurons after ICH by silencing Stat1 to some extent.

Acid-Sensing Ion Channels (ASICs)

In normal conditions, intracellular pH and extracellular pH are maintained stably through different H⁺-transporting mechanisms, which are crucial for normal cellular functions. In pathological conditions especially ischemia, the accumulation of lactic acid and release of H⁺ result in acidosis. Acidosis can activate ASIC1a, which is highly sensitive to protons and is permeable to Na⁺ and Ca²⁺, playing a crucial role in the development of acidosis-mediated ischemic damage [140, 141]. This channel causes a large influx of Ca²⁺ and Na⁺, leading to neuronal injury [142, 143]. Other studies have reported that the combination of ASIC1a and NMDA blockers results in additional neuroprotection compared to either alone, with the presence of the ASIC1a blocker extending the window of time in which the NMDA blocker is effective [142, 144]. Pharmacological and genetic interventions that limit ASIC1 activation can exert neuroprotective effects even in the presence of a glutamate receptor antagonist. Given the important role of ASICs in ischemic brain injury, and the novel therapeutic aspect of targeting ASICs [142, 144], we hope there will be more research concerning ASIC1a in ICH in the future.

Purinergic 2X7 Receptor (P2X7R)

P2X7R is a subtype of P2X superfamily of purinoreceptors. It is an ATP-gated, non-selective cation channel, known for its cytotoxic activity [145]. Functional P2X7 receptors are expressed on microglia, Schwann cells, and astrocytes within the CNS [146, 147]. ATP is the physiological agonist of P2X7R [148]. After brief exposure to ATP, P2X7R allows small cations (Na⁺, K⁺, and Ca²⁺) to pass through. However, prolonged stimulation of P2X7R leads to the formation of a non-selective pore that allows the entry of solutes up to 900 Da in size [149, 150]. P2X7 also plays a pivotal role in CNS pathological processes. Intracellular ion dyshomeostasis caused by P2X7R excites certain second messenger and enzyme cascades [145]. In microglia, P2X7R activation promptly excites c-Jun N-terminal kinases 1 and 2, extracellular signal-regulated kinases (ERK 1/2), and p38 mitogen-activated protein kinases (MAPK) [151–153].

Genetic deletion and pharmacological blockade of the P2X7R confer neuroprotection in multiple neurological diseases, including ICH. The P2X7R may regulate programmed cell death through various pathways [154]. It has been found that P2X7R can also activate downstream responses such as the NLRP3 inflammasome [155], and RhoA [156], as well as the activation of MAPK pathway [157] after ICH, further aggravating brain injury. The activation of MAPK may result from the influx of Ca²⁺ caused by P2X7R activation. The inhibition of P2X7R prevents NLRP3 inflammasome activation and brain injury [155]. Therefore, inhibiting the activation of P2X7R may be attractive to alleviate ICH injuries.

LRRC8A Channel

LRRC8A channel is an anion channel widely expressed in mammalian cells especially astrocytes [158]. This channel can be activated by cell swelling, and the pivotal role of this channel is to mediate the efflux of Cl⁻ for regulating cell volume [158]. It has been found that in brain ischemia, LRRC8A acts as a glutamate-releasing channel in astrocytes, and inhibition of the LRRC8A channel alleviates brain injury [159, 160]. But more than that, in ICH, by regulating microglia/macrophage phagocytosis and hematoma clearance, inhibiting the LRRC8A channel can decrease neuronal death and improve neurological functions [161].

These channels and/or exchangers are expressed in most cell types of the neurovascular unit. Continued activation of these proteins can lead to excessive inflow of cations, such as Ca²⁺, Na⁺, and Zn²⁺, resulting in ischemic-reperfusion brain injury. More than one-fifth of ICH patients have acute brain infarction [162]. Numerous studies have demonstrated that ICH results in ischemia in the surrounding tissue [163–165]. It is conceivable that a sharp drop in blood pressure during metabolic normalization may result in brain ischemia. Other factors related to hematoma volume may also be involved in ischemic injury in ICH patients [162]. In such a case, it is reasonable to speculate that the mechanism of Ca²⁺ dyshomeostasis in the ischemic microenvironment of ICH is similar to that in brain ischemia [135].

Ion Channel Dysregulation Affects White Matter Injury

The Transient Receptor Potential Ankyrin 1 (TRPA1) Channel

TRPA1 channel, composed of numerous N-terminal ankyrin repeats, is one kind of nonselective transmembrane cation channel and has a high permeability to calcium ions [166]. This channel is mainly expressed in sensory neurons, astrocytes, oligodendrocytes, ependymal cells located at ventricles and cardiomyocytes [167], as well as the endothelium of cerebral arteries [168]. It is sensitive to pain, cold, and environmental irritants [167, 169, 170]. Decreased pH and increased reactive oxygen species (ROS) can activate TRPA1, causing the entry of Ca²⁺ [171, 172]. Raised Ca²⁺ can eventually exacerbate oxidative stress [166, 173]. One study reported that endothelial cell TRPA1 channel activity increased cerebral blood flow thus causing the rise of extravasated blood during hypertension after ICH. However, blocking TRPA1 channels seems to be ineffective in treating hypertension-associated ICH in a clinical trial [174]. Low-frequency and low-intensity ultrasound can activate astrocytic TRPA1 channels to regulate neuronal activity [175]. Astrocytic TRPA1 can help astrocytes transform to the A2 phenotype and promote microglia activation after ICH. Knockout of astrocytic TRPA1 reduced neuroinflammation after ICH [176]. TRPA1 is also involved in myelin damage and oxidative stress injury in the ICH model, and blocking TRPA1 in oligodendrocytes can alleviate peri-hematoma white matter injury and motor dysfunction in the mice during acute ICH [177, 178].

Piezo 1

Piezo1 works as a cation-selective channel and is permeable to calcium; it is extensively expressed in the CNS on neurons, microglia, astrocytes, and endothelial cells [179–181]. Two main signaling pathways have been shown downstream of the activation of Piezo1: adenosine triphosphate (ATP) release [182] and calpain activation [183]. The activation of Calpain is related to several Ca²⁺-dependent processes, such as cell proliferation, apoptosis, and angiogenesis [184]. Calpain activity changes are related to several pathologies including stroke [185, 186].

Piezo1 is also expressed in oligodendrocyte precursor cells [187, 188]. By preventing overloaded Ca²⁺ from entering neuronal axons, suppressing Piezo1 can attenuate both psychosine- and lipopolysaccharide-induced demyelination, which may inhibit calpain-mediated destabilization of integrin attachments, resulting in enhanced myelin formation [189]. Piezo1 may be considered a potential target of ICH for the reason that Piezo1 inhibition can reduce intracellular endoplasmic reticulum stress and cell apoptosis, and protect myelin sheath, eventually improving neuronal function after ICH [190].

Ion Channel Dysregulation Affects Seizures and Epilepsy After ICH

Seizures are a common outcome of ICH and happen in 30% of ICH patients [191]. Acute seizures appear to reflect rapid and reversible enhancement of excitability involving changes in the level of electrolytes. Early seizures (≤ 7 days after ICH) [192] are associated with worse neurological consequences, including late seizures (> 7 days after ICH) and epilepsy. Epilepsy is considered to reflect a long-term change in the neuronal network, including modifications in intrinsic neuronal excitability such as the upregulation and downregulation of ion channels and/or synaptic connectivity [193–195].

Voltage-gated sodium channels (VGSCs) typically consist of a 9 pore-forming α-subunit and a 5 regulatory β-subunit. They are traditionally related to the initiation and conduction of action potentials in electrically excited cells such as neurons and muscle cells [196]. When the CNS is developing, VGSCs regulate the migration of neurite outgrowth. It has been indicated that the existence of thrombin contributes to seizures after ICH in vivo and in vitro [197, 198]. The properties of VGSCs can be altered by thrombin, thereby affecting seizures following ICH [199]. Furthermore, the change of BBB may result in the increase of extraneuronal K⁺ concentration and the decrease of Mg²⁺ level, both of which are conducive to the increase of excitability [200] and transforming growth factor (TGF)-β dependent activation of astrocytes. These may result in ion dyshomeostasis and glutamate disorder, thereby increasing the danger of seizures and transition to SDs [201–203].

Discussion and Future Perspectives

This review gives a summary of the consequences of ion dyshomeostasis and ion channel dysregulation in brain injuries including brain edema and neuronal death following ICH. We suggest some therapies that may be available to target the dysfunction of ions and their corresponding channels (Table 1). There are still many important issues to be discussed.

Table 1.

Experimental treatments after ICH targeting ion channels and the results.

Ion channel	Treatment	Effects	Reference
SUR1-TRPM4	Sulfonylurea drugs (Glibenclamide)	Improved histopathological and functional outcomes	Xu et al. [39], Zhou et al. [41], Jiang et al. [42]
		Did not show obvious benefits	Kung et al. [43], Wilkinson et al. [46]
TRPV4	Antagonist HC-067047/TRPV4 siRNA	Ameliorated brain edema, neuronal death, and BBB disruption	Shen et al. [129], Zhao et al. [130]
	Agonist GSK1016790A	Ameliorated brain injury and improved neurological deficits	Asao et al. [131]
TRPM7	Silencing Stat1	Inhibited Ca2+ overload in neurons	Li et al. [93]
ASICs	Pharmacological blocker/genetic interventions	Reduced neuronal injury	Xiong et al. [142], Pignataro et al. [144]
P2X7R	Genetic deletion and pharmacological blockade	Prevented inflammation and brain injury	Feng et al. [155]
LRRC8A	Conditional knockout	Decreased neuronal death and improved neurological functions	Liu et al. [161]
TRPA1	Knockout; Antagonists	Reduced neuroinflammation; alleviated white matter injury and motor dysfunction	Xia et al. [176], Xia et al. [178]
Piezo 1	Antagonist Dooku1	Reduced cell apoptosis and protected myelin sheath	Qu et al. [190]

Ion channel dysregulation influences various pathophysiological processes of ICH, and different treatments targeting these channels can benefit the outcome.

The disturbance in ions may persist after edema resolves, as occurs after other nervous system injuries [204]. Ion dyshomeostasis disrupts proper neuronal function [204]. Thus, ion disturbance in the peri-hematoma region may underlie neuronal damage and hinder the activity-dependent recovery processes. One of the participants in brain edema, AQP4, is the prominent water channel in the brain and is expressed in astrocytes, which also contribute to potassium spatial buffering [205, 206]. The study of aquaporin-mediated brain edema is basically the study of ion transport because aquaporins as passive channels are completely dependent upon the activity of ion transporters for water flux [22]. AQP4 deficiency increases apoptosis after ICH [207, 208]. The relationship between the mechanism of cell death and AQP4 functions awaits further research.

There are other points worth exploring in the future. The functions of astrocytes are indispensable for the ionic balance of Na⁺, K⁺, and Cl⁻, control of pH, and cell volume. These processes play crucial roles in the progression or recovery of ICH-induced brain damage. From the literature and consistent with K⁺ data, the activation of astrocytes decreases from the hematoma area into the peri-hematoma zone [209]. Astrocytes are activated in the perihematomal region within 72 h following ICH, and its reaction persists for 14 days [210, 211]. Astrocytes are involved in neurotransmitter metabolism and K⁺ buffering [212]. A better understanding of the various astrocytic ion channel functions holds large potential to improve the recovery of ICH patients.

Tissue compliance, or cell shrinkage owing to increased ICP may be regulated by ion channels including SUR1-TRPM4 [20, 24, 34]. The Monro-Kellie doctrine dictates that under physiological conditions, the blood, brain, and cerebrospinal fluid (CSF) have to be in balance for ICP regulation [213]. In ICH, to compensate for increased hematoma and minimize the increased ICP, the volume of cerebral venous blood and CSF volumes is redirected out of the skull. In addition, tissue compliance occurs wherein the density of neurons is increased and the size of them is decreased throughout the brain [25]. These cell volume changes are undoubtedly concerned with ions and regulated by multiple ion channels. Thus, existing treatments seem to affect not only the peri-hematoma edema but also the volume of the remaining brain [24, 214, 215], which remains to be further explored.

There is increasing interest in the roles of other ions such as Fe²⁺, Mg²⁺, and Zn²⁺ in ICH. Fe²⁺ participates in ferroptosis following ICH [7, 216]. MMP-9 functions are affected by the presence of zinc following ICH [217]. More details and mechanisms concerning these ions in ICH are warranted.

With regard to therapies, rehabilitation is worth consideration. A shift in the reversal potential of Cl⁻ leads to hyperexcitability. This excitatory switch also occurs after brain ischemia and ICH and may be the basis of seizures following ICH [218–220]. Rehabilitation can normalize Cl⁻ homeostasis and promote behavioral recovery by regulating the expression of Cl⁻ transporters [204]. Exercise after spinal cord injury normalizes spinal activity by moderating ionic imbalance [204]. Thus, ion concentrations or signaling pathways after ICH, including rehabilitation, can be advantageous [221].

Conclusion

In conclusion, ion channels regulate many processes after ICH and their activities may be promising prognostic biomarkers in ICH. More importantly, novel therapeutic interventions focusing on ion and ion channels post-ICH are becoming increasingly feasible and promising.

Conflict of interest

The authors declare no conflict of interest.