Emergence of a Chimeric Globin Pseudogene and Increased Hemoglobin Oxygen Affinity Underlie the Evolution of Aquatic Specializations in Sirenia

Glutathione Non-Covalent Binding Sites on Hemoglobin and Major Glutathionylation Target betaCys93 Are Conservative among Both Hypoxia-Sensitive and Hypoxia-Tolerant Mammal Species

Intracellular tripeptide glutathione is an important agent of cell survival under hypoxia. Glutathione covalently binds to SH groups of hemoglobin cysteine residues, protecting them from irreversible oxidation, and changes its affinity to oxygen. Reduced glutathione (GSH) can also form a noncovalent complex with hemoglobin. Previously, we showed that hemoglobin tetramer has four noncovalent binding sites of glutathione GSH molecules inside, two of which are released during hemoglobin transition to deoxy form. In this study, we characterized the conserved cysteine residues and residues of noncovalent glutathione binding sites in the sequences of a number of hypoxia-tolerant and hypoxia-sensitive mammals. The solvent accessibility of all HbA and HbB residues in oxy and deoxy forms was analyzed. The alpha subunit of all species considered was shown to have no conserved cysteines, whereas the beta subunit contains Cys93 residue, which is conserved across species and whose glutathionylation changes the affinity of hemoglobin for oxygen 5-6-fold. It was found that the key residues of noncovalent glutathione binding sites in both alpha and beta subunits are absolutely conserved in all species considered, suggesting a common mechanism of hemoglobin redox regulation for both hypoxia-sensitive and hypoxia-tolerant mammals.

dN/dS-H, a New Test to Distinguish Different Selection Modes in Protein Evolution and Cancer Evolution

One of the most popular measures in the analysis of protein sequence evolution is the ratio of nonsynonymous distance (d_N) to synonymous distance (d_S). Under the assumption that synonymous substitutions in the coding region are selectively neutral, the d_N/d_S ratio can be used to statistically detect the adaptive evolution (or purifying selection) if d_N/d_S > 1 (or d_N/d_S < 1) significantly. However, due to strong structural constraints and/or variable functional constraints imposed on amino acid sites, most encoding genes in most species have demonstrated d_N/d_S < 1. Consequently, the statistical power for testing d_N/d_S = 1 may be insufficient to distinguish between different selection modes. In this paper, we propose a more powerful test, called d_N/d_S-H, in which a new parameter H, a relative measure of rate variation among sites, was introduced. Given the condition of strong purifying selections at some sites, the d_N/d_S-H model predicts d_N/d_S = 1-H for neutral evolution, d_N/d_S < 1-H for nearly neutral selection, and d_N/d_S > 1-H for adaptive evolution. The potential of this new method for resolving the neutral-adaptive debates is illustrated by the protein sequence evolution in vertebrates, Drosophila and yeasts, as well as somatic cancer evolution (specialized as the C_N/C_S-H test).

Evolved increases in hemoglobin-oxygen affinity and the Bohr effect coincided with the aquatic specialization of penguins

In diving birds like penguins, physiologic considerations suggest that increased hemoglobin (Hb)-O₂ affinity may improve pulmonary O₂ extraction and enhance dive capacity. We integrated experimental tests on whole-blood and native Hbs of penguins with protein engineering experiments on reconstructed ancestral Hbs. The experiments involving ancestral protein resurrection enabled us to test for evolved changes in Hb function in the stem lineage of penguins after divergence from their closest nondiving relatives. We demonstrate that penguins evolved an increased Hb-O₂ affinity in conjunction with a greatly augmented Bohr effect (i.e., reduction in Hb-O₂ affinity at low pH) that should maximize pulmonary O₂ extraction without compromising O₂ delivery at systemic capillaries.

Dive capacities of air-breathing vertebrates are dictated by onboard O₂ stores, suggesting that physiologic specialization of diving birds such as penguins may have involved adaptive changes in convective O₂ transport. It has been hypothesized that increased hemoglobin (Hb)-O₂ affinity improves pulmonary O₂ extraction and enhances the capacity for breath-hold diving. To investigate evolved changes in Hb function associated with the aquatic specialization of penguins, we integrated comparative measurements of whole-blood and purified native Hb with protein engineering experiments based on site-directed mutagenesis. We reconstructed and resurrected ancestral Hb representing the common ancestor of penguins and the more ancient ancestor shared by penguins and their closest nondiving relatives (order Procellariiformes, which includes albatrosses, shearwaters, petrels, and storm petrels). These two ancestors bracket the phylogenetic interval in which penguin-specific changes in Hb function would have evolved. The experiments revealed that penguins evolved a derived increase in Hb-O₂ affinity and a greatly augmented Bohr effect (i.e., reduced Hb-O₂ affinity at low pH). Although an increased Hb-O₂ affinity reduces the gradient for O₂ diffusion from systemic capillaries to metabolizing cells, this can be compensated by a concomitant enhancement of the Bohr effect, thereby promoting O₂ unloading in acidified tissues. We suggest that the evolved increase in Hb-O₂ affinity in combination with the augmented Bohr effect maximizes both O₂ extraction from the lungs and O₂ unloading from the blood, allowing penguins to fully utilize their onboard O₂ stores and maximize underwater foraging time.

Neuroprotective effect of intermittent hypobaric hypoxia preconditioning on cerebral ischemia/reperfusion in rats

BACKGROUND

Ischemic tolerance is an endogenous protective mechanism in organs or tissues undergoing one or more short-term sublethal ischemias. Intermittent hypobaric hypoxia preconditioning (IHHP) can induce tolerance and thus protect brain tissues from cerebral ischemic injury (CIR). The current study evaluated the neuroprotective effect of IHHP.

METHODS

The established xenograft model was divided into the ischemia/reperfusion (I/R), IHHP, IHHP+I/R, and sham groups. Transmission electron microscopy was used to observe alterations in neuron ultrastructure. Neuron damage was detected using Nissl staining. Western blot and qRT-PCR were used to evaluate the relative expression of genes and proteins related to apoptosis. Immunohistochemistry was used to determine the expression of proteins involved in the processes of neuroprotection and repair.

RESULTS

Our results indicated that the damage to the neurons, organelles, and axons was significantly less following ischemia/reperfusion and intermittent hypobaric hypoxia reconditioning treatment than that in the ischemia/reperfusion group. Compared to the ischemia/reperfusion group, significant downregulation of pro-apoptotic gene/protein expressions along with upregulation of anti-apoptotic and nerve regeneration gene/protein expressions in the IHHP+I/R group were observed.

CONCLUSION

IHHP can significantly reduce ischemia/reperfusion injury in rat brain nerves and promote nerve repair.