Oxidation of hemoproteins by
Streptococcus pneumoniae
collapses the cell cytoskeleton and disrupts mitochondrial respiration leading to the cytotoxicity of human lung cells
Authors: Scasny Anna , Alibayov Babek , Khan Faidad , Rao Shambavi J. , Murin Landon , Jop Vidal Ana G. , Smith Perriann , Li Wei , Edwards Kristin , Warncke Kurt , Vidal Jorge E. 
Published: December 2023

Streptococcus pneumoniae
(Spn) causes pneumonia that kills millions through acute toxicity and invasion of the lung parenchyma. During aerobic respiration, Spn releases hydrogen peroxide (Spn-H
2
O
2
) as a by-product of the enzymes pyruvate oxidase and lactate oxidase and causes cell death with signs of both apoptosis and pyroptosis by oxidizing unknown cell targets. Hemoproteins are molecules essential for life and prone to oxidation by H
2
O
2
. We recently demonstrated that during infection-mimicking conditions, Spn-H
2
O
2
oxidizes the hemoprotein hemoglobin (Hb), releasing toxic heme. In this study, we investigated details of the molecular mechanism(s) by which the oxidation of hemoproteins by Spn-H
2
O
2
causes human lung cell death. Spn strains, but not H
2
O
2
-deficient SpnΔ
spxB
Δ
lctO
strains, caused time-dependent cell cytotoxicity characterized by the rearrangement of the actin cytoskeleton, the loss of the microtubule cytoskeleton, and nuclear contraction. Disruption of the cell cytoskeleton is correlated with the presence of invasive pneumococci and an increase in intracellular reactive oxygen species. In cell culture, the oxidation of Hb or cytochrome
c
(Cyt
c
) caused DNA degradation and mitochondrial dysfunction from inhibition of complex I-driven respiration, which was cytotoxic to human alveolar cells. Oxidation of hemoproteins resulted in the creation of a radical, which was identified as a protein-derived side chain tyrosyl radical by using electron paramagnetic resonance. Thus, we demonstrate that Spn invades lung cells, releasing H
2
O
2
that oxidizes hemoproteins, including Cyt
c
, catalyzing the formation of a tyrosyl side chain radical on Hb and causing mitochondrial disruption that ultimately leads to the collapse of the cell cytoskeleton.

IMPORTANCE

Streptococcus pneumoniae
(Spn) colonizes the lungs, killing millions every year. During its metabolism, Spn produces abundant amounts of hydrogen peroxide. When produced in the lung parenchyma, Spn-hydrogen peroxide (H
2
O
2
) causes the death of lung cells, and details of the mechanism are studied here. We found that Spn-H
2
O
2
targets intracellular proteins, resulting in the contraction of the cell cytoskeleton and disruption of mitochondrial function, ultimately contributing to cell death. Intracellular proteins targeted by Spn-H
2
O
2
included cytochrome
c
and, surprisingly, a protein of the cell cytoskeleton, beta-tubulin. To study the details of oxidative reactions, we used, as a surrogate model, the oxidation of another hemoprotein, hemoglobin. Using the surrogate model, we specifically identified a highly reactive radical whose creation was catalyzed by Spn-H
2
O
2
. In sum, we demonstrated that the oxidation of intracellular targets by Spn-H
2
O
2
plays an important role in the cytotoxicity caused by Spn, thus providing new targets for interventions.