ABSTRACT
Ischemic stroke is a leading cause of death and disability worldwide with limited therapeutic options. The newly identified proton-activated chloride channel (PAC) has received increasing attentions in recent years and has been reported to provide partial neuroprotection in tissue acidosis resulting from ischemic stroke. This review discusses two therapeutic strategies with high potential based on PAC properties, in order to indicate a new direction for the treatment of ischemic stroke.
Keywords: Ischemic Stroke; PAC; Hypothermia; Nano-Systems; Acidosis
Introduction
Ischemic stroke is a multifactorial disease with high rate of morbidity, mortality and disability rate worldwide [1]. When acute ischemia occurs, it leads to excessive oxidative stress, causing irreversible damage to nervous tissue [2,3]. However, current treatment strategies of ischemic stroke are relatively limited, actually, a large number of neuroprotective agents that have proved ineffective in clinical trials [4]. PAC, also known as ASOR (acid-sensitive outwardly rectifying anion channel) [5] or PAORAC (proton-activated outwardly rectifying anion channel) [6], which was first observed in rat Sertoli cells in 2003[7], has shown encouraging results in providing partial neuroprotection against tissue acidosis caused by ischemic stroke.
Pac Channel and its Therapeutic Potential
Under normal physiological condition, the pH of extracellular
fluid, including the blood plasma, is normally tightly regulated
between 7.32 and 7.42 by the chemical buffers, the respiratory
system, and the renal system [8]. However, in cerebral acidosis
caused by ischemic stroke, the pH of the ischemic core may be as low
as 6.0 [9]. Ischemic tissue acidosis is a sensitive metabolic indicator
for the progression of cerebral ischemic injury. PAC was reported
to be activated by extracellular acidity as well as its sensitivity to
temperature and pH [5]. The threshold pH for PAC activation is
relatively low, around pH 5.5 at room temperature and around
pH 6.0 at 37℃ [10]. As a highly conserved channel ubiquitously
expressed in mammals, the core protein of PAC (TMEM206) has
the highest molecular expression in the cerebral cortex of human
[11]. By cooperating with other ion transporters and channels
such as V-ATPase, acid-sensing ion channel 1a (ASIC1a), and Na+/
H+ exchanger (NHE), PAC is activated by extracellular acidity and
mediates Cl- influx into the cell, cytotoxic edema and cell necrosis
ensue [12-14]. The simultaneous increase in intracellular Ca2+ promotes cell death cascade associated with intracellular Ca2+
accumulation [15].
Several studies have reported that inhibition of PAC provides
partial neuroprotection against tissue acidosis after ischemic
stroke:
1. Neurons undergoing massive necrosis 1h after exposure
to acidic solution were not only protected by PAC channel
blockers but also by cooling down to 25 ℃ [10],
2. Knocking down the core component of PAC (TMEM206)
almost abolished proton activated Cl- current in rat neurons
and reduced neuronal cell death caused by acid treatment [16],
3. Application of the chloride channel blocker 4,4′-Diisothiocyano-
2,2′-stilbenedisulfonic acid (DIDS) in vivo attenuated cell
injury induced by ischemia-reperfusion in hippocampal CA1
neurons [17],
4. Knockout of mouse PAC abolished proton-activated Cl- current
in neurons and attenuated brain damage after ischemic stroke
[11]. Based on above lines of evidence, PAC is a promising
therapeutic target to protect neurons in cerebral ischemia.
Therapeutic Strategies of Ischemic Stroke Targeting PAC Channels
Metabolic acidosis occurring in ischemic stroke is a sensitive
metabolic indicator capable of targeting drugs to the salvageable
ischemic penumbra with high specificity. Targeting ischemic brain
tissue using pH sensitive marker polymers or nanoparticles has
enabled medical imaging to accurately distinguish ischemic tissues
from normal ones [18,19]. The development of bio-responsive
materials that undergo conformational or solubility changes under
acidic environments offers great promise for the development
of smart targeted drug delivery nano-systems [20], such as the
hydrophobically modified chitosan nanoparticles (Chit NPs) with
C6-side chains were released the Ca2+ channel blocker nimodipine
(NIMO) drug at the pH of ischemic tissue (=6.0), while at normal
pH (=7.4) the drug molecules was remained closed in polymer
shell [21]. pH directing effect and bio-responsive nanomaterials
have been applied in solid tumor therapy and inspired the
application of nanotechnology in the treatment of ischemic stroke
[22,23]. Selective delivery of PAC specific blockers to the ischemic
penumbra via a pH responsive smart nanosystem holds promise
for the treatment of injury sites without adverse nontargeted side
effects. However, currently there are only several universal drugs
available to target chloride channels, such as DIDS, niflumic acid
(NFA), and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB),
specific PAC blocker has not yet been reported [11,12].
Therapeutic hypothermia has been regarded as one of the
most effective neuroprotective strategies since 1987 [24,25]. Over
the past few decades, the neuroprotective effects of hypothermia
in cardiac arrest [26] and resuscitation from neonatal hypoxicischemic
encephalopathy [27] have been confirmed by multiple
clinical trials. Numerous preclinical studies based on vascular
recanalization models have shown that hypothermia exerts
neuroprotective effects mainly by reducing the cerebral metabolic
rate [28], but also involves cell death, inflammation and white
matter integrity [29], and the temperature sensitivity of PAC also
provides potential mechanisms [16]. Intra-arterial selective cooling
infusion (IA-SCI), as a novel therapeutic method of hypothermia,
utilizes the characteristics of high blood flow to the brain to directly
perfuse hypothermic fluid into the cerebral arteries for rapid and
selective cooling [30]. IA-SCI can precisely achieve regional cooling
of brain tissue with a much faster cooling compared with traditional
superficial cooling and systemic transvenous cooling and can be
combined with mechanical thrombectomy to avoid the side effects
brought by systemic cooling [31,32], which has been clinically
validated in rodent models [33], large animal models [34] and
ischemic stroke patients [35]. Precise cooling of the acidotic brain
tissue in ischemic stroke by IA-SCI can elevate opening threshold
of PAC, thereby shutting the channel, thereby reducing the influx
of Cl-, improving cytotoxic edema, and achieving neuroprotection
[10,12].
Conclusion
Targeted delivery of PAC blockers by smart nano-systems is highly specific and efficient and enables the gradual release of drugs to extend the time of drug exposure, reducing the risk of adverse side effects or off target toxicity. Compared with the simultaneous closure / blockade of multiple cationic channels associated with cellular edema (such as ASICs and NHEs, etc.), the blockade of a single and unique anion channel (PAC) by hypothermia has higher applicability and feasibility experimentally or clinically. Although there is currently a lack of sufficient experiments to validate the above two therapeutic strategies, smart drug targeting nanosystems and hypothermia therapy that inhibit PAC also pose an promising opportunity for translational trials to protect brain tissue.
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