Proliferative and Synthetic Activity of Nerve Cells after Combined or Individual Exposure to Hypoxia and Hypercapnia

Molecular Mechanisms of Neuroprotection after the Intermittent Exposures of Hypercapnic Hypoxia

The review introduces the stages of formation and experimental confirmation of the hypothesis regarding the mutual potentiation of neuroprotective effects of hypoxia and hypercapnia during their combined influence (hypercapnic hypoxia). The main focus is on the mechanisms and signaling pathways involved in the formation of ischemic tolerance in the brain during intermittent hypercapnic hypoxia. Importantly, the combined effect of hypoxia and hypercapnia exerts a more pronounced neuroprotective effect compared to their separate application. Some signaling systems are associated with the predominance of the hypoxic stimulus (HIF-1α, A1 receptors), while others (NF-κB, antioxidant activity, inhibition of apoptosis, maintenance of selective blood-brain barrier permeability) are mainly modulated by hypercapnia. Most of the molecular and cellular mechanisms involved in the formation of brain tolerance to ischemia are due to the contribution of both excess carbon dioxide and oxygen deficiency (ATP-dependent potassium channels, chaperones, endoplasmic reticulum stress, mitochondrial metabolism reprogramming). Overall, experimental studies indicate the dominance of hypercapnia in the neuroprotective effect of its combined action with hypoxia. Recent clinical studies have demonstrated the effectiveness of hypercapnic-hypoxic training in the treatment of childhood cerebral palsy and diabetic polyneuropathy in children. Combining hypercapnic hypoxia with pharmacological modulators of neuro/cardio/cytoprotection signaling pathways is likely to be promising for translating experimental research into clinical medicine.

Hypercapnia and Hypoxia Stimulate Proliferation of Astrocytes and Neurons In Vitro

We compared proliferative activity and hypoxic tolerance in a co-culture of neurons and astrocytes in vitro after preliminary exposure to normobaric hypoxia and/or permissive hypercapnia in vivo. Preliminary hypoxic exposure increased the cell index throughout the 72-h period of observation, the effect of hypercapnia was observed on days 1 and 3 of the experiment, and the effect of hypercapnic hypoxia was noted only on day 1. Preliminary hypoxic exposure has a protective effect on nerve cells under conditions of chemical hypoxia. This suggests that hypercapnia and hypoxia activate proliferative activity of nerve cells, which can be viewed as a mechanism of their neuroprotective effectiveness.

Inhibition of Apoptosis is a Potential Way to Improving Ischemic Brain Tolerance in Combined Exposure to Hypercapnia and Hypoxia

We compared the intensity of apoptosis in the peri-infarction area of the brain after isolated and combined exposure to hypoxia and hypercapnia prior to focal ischemic stroke modeling. Hypoxia and hypercapnia reduced the number of TUNEL-positive cells in the peri-infarction area, and their combination was most effective in comparison with effects of isolated exposures. The maximum neuroprotective effect of combined exposure to hypoxia and hypercapnia in comparison with isolated exposures was determined by inhibition of apoptosis in the peri-infarction zone.

Stress of the Endoplasmic Reticulum of Neurons in Stroke Can Be Maximally Limited by Combined Exposure to Hypercapnia and Hypoxia

We studied the expression of chaperone GRP-78 and transcription factor NF-kB during the development of ischemic tolerance of the brain after combined and isolated exposure to hypoxia and hypercapnia. Combined exposure to hypoxia and hypercapnia maximally increased the expression of chaperone GRP-78 and transcription factor NF-kB, while the formation of ischemia-induced tolerance under conditions of hypercapnic hypoxia can be associated with activation of adaptive stress mechanisms in the endoplasmic reticulum. Under these conditions, hypercapnia in combination with hypoxia is a priority factor for activation of GRP-78 and transcription factor NF-kB.