A Novel Neuroprotective Strategy for Ischemic Stroke: Transient Mild Acidosis Treatment by CO₂ Inhalation at Reperfusion

Abstract

Acidosis is one of the key components in cerebral ischemic postconditioning that has emerged recently as an endogenous strategy for neuroprotection. We set out to test whether acidosis treatment at reperfusion can protect against cerebral ischemia/reperfusion injury. Adult male C57BL/6 J mice were subjected to 60-minute middle cerebral arterial occlusion followed by 24-hour reperfusion. Acidosis treatment by inhaling 10%, 20%, or 30% CO 2 for 5 or 10 minutes at 5, 50, or 100 minutes after reperfusion was applied. Our results showed that inhaling 20% CO 2 for 5 minutes at 5 minutes after reperfusion-induced optimal neuroprotection, as revealed by reduced infarct volume. Attenuating brain acidosis with NaHCO 3 significantly compromised the acidosis or ischemic postconditioning-induced neuroprotection. Consistently, both acidosis-treated primary cultured cortical neurons and acute corticostriatal slices were more resistant to oxygen-glucose deprivation/reperfusion insult. In addition, acidosis inhibited ischemia/reperfusion-induced apoptosis, caspase-3 expression, cytochrome c release to cytoplasm, and mitochondrial permeability transition pore (mPTP) opening. The neuroprotection of acidosis was inhibited by the mPTP opener atractyloside both in vivo and in vitro. Taken together, these findings indicate that transient mild acidosis treatment at reperfusion protects against cerebral ischemia/reperfusion injury. This neuroprotection is likely achieved, at least partly, by inhibiting mPTP opening and mitochondria-dependent apoptosis. Keywords: acidosis treatment; cerebral ischemia; mPTP; neuroprotection INTRODUCTION Ischemic postconditioning, a neuroprotective strategy for ischemic stroke, has attracted much attention over the past 7 years. It is defined as a series of cycles of brief reperfusion and ischemia applied at the onset of reperfusion, which can mobilize the brain's own endogenous adaptive mechanisms and limit the extent of reperfusion injury. In 2006, Zhao et al 1 first reported that ischemic postconditioning reduces infarct volume in experimental cerebral ischemia and reperfusion (I/R) injury. Since then, its cerebral protective effects have been confirmed in several distinct ischemia models.2-5 However, the clinical application of ischemic postconditioning may be limited because of the difficulties in achieving well-controlled interruptions of reperfusion and the risks associated with these manipulations.6 Hypoxia, as a key component of ischemia, is a feasible treatment for ischemic stroke in theory. Surprisingly, we have recently found that an early application of hypoxic postconditioning after reperfusion does not induce neuroprotection against cerebral I/R injury.7 Thus, attempts should be undertaken to identify the other key factors, which are responsible for the protection afforded by ischemic postconditioning, and to develop a new therapeutic strategy, which is relatively safe and practical.Acidosis is another key component of ischemia besides hypoxia and glucose depletion. Extracellular pH can fall...

PARK2-dependent mitophagy induced by acidic postconditioning protects against focal cerebral ischemia and extends the reperfusion window

“…1D). Consistent with our results, NaHCO 3 also abolishes the neuroprotective effects of APC, 8 suggesting that acidosis is critical for both APC-mediated activation of mitophagy and neuroprotection.

Figure 1.Acidic postconditioning activates mitophagy after cerebral ischemia in vivo and in vitro.

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“…A 3-min APC treatment of corticostriatal slices applied after 5 min of reperfusion (5/3 min), which is one of the most prominent neuroprotection strategies, 8 reduced TOMM20 and COX4I1 protein levels to a greater extent compared with other APC treatments (Fig. 1E and F).…”

“…To overcome these disadvantages, we have developed acidic postconditioning (APC) as a therapy for cerebral ischemia. Induced simply by inhaling 20% CO 2 , APC significantly reduces focal cerebral ischemia and allows a wider time window for reperfusion than ischemic postconditioning (IPC) 8 . APC can preserve mitochondrial membrane potential, and, similarly, acidic pre-conditioning, where CO 2 is inhaled before ischemia, also alleviates mitochondrial dysfunction 9 .…”

Protease Omi cleaving Hax-1 protein contributes to OGD/R-induced mitochondrial damage in neuroblastoma N2a cells and cerebral injury in MCAO mice

“…In addition, increased Ca 2+ stimulates calpain to cleave Bid to form tBid that binds to pro-apoptotic proteins such as Bax, Bad, and Bak after I/R [31] . The formation of tBid heterodimers with other pro-apoptotic protein opens the mitochondrial permeability transition pore (mPTP) and releases cytochrome c, thereby activating caspases, leading to apoptosis in the ischemic brain [32,33] . The Bcl-2 protein family includes both anti-apoptotic (such as Bcl-2 and Bcl-xL) and pro-apoptotic proteins (such as Bax, Bad and Bak).…”

Neuroprotection and acidosis induced by cortical spreading depression

“…In the literature, brief acidosis in ischemic conditions when oxygen supply is low has been shown to be cytoprotective and neuroprotective,6–10 a result related sometimes to the concept of the “pH paradox.”9,11,12 Acidosis had been reported to reduce stroke infarct size with CO 2 applied for a short time after reperfusion6 as well as during ischemic stroke 10. Brief acidosis applied after birth asphyxia7 was reported to successfully suppress brain alkalosis, which led to seizures.…”