Free heme induces neuroinflammation and cognitive impairment by microglial activation via the TLR4/MyD88/NF-κB signaling pathway

BACKGROUND

Red blood cells (RBCs) transfusion is related to perioperative neurocognitive disorders. The toxic effect of free heme has been identified in many pathologies. However, the underlying mechanisms of RBCs transfusion or free heme in cognitive impairment have not been clearly explored. Therefore, this research was conducted to determine the mechanism of free heme-induced neuroinflammation and cognitive impairment.

METHODS

Rats were received intraperitoneal injection of hemin alone or combined with intracerebroventricular injection of Hemopexin (HPX), and MWM test was conducted to measure cognitive function. The amount of heme-HPX complexes was evaluated by flow cytometry for CD91 + cells. The microglial inflammatory response in rat brain was observed by immunofluorescence staining of Iba-1, and the inflammatory factors of TNF-α, IL-1β and IL-6 in rat brain and BV2 cells were detected by ELISA analysis. Furthermore, neuronal apoptosis in HT22 cells alone and in HT22 + BV2 coculture system was detected by flow cytometry and immunofluorescence staining. Finally, western blot was conducted to detect TLR4/MyD88/NF-κB proteins in rat brain and BV2 cells treated with hemin or combined with pathway inhibitors. Additionally, the M1 surface marker CD86 was observed in BV2 cells to further confirm neuroinflammation.

RESULTS

Intraperitoneal injection of hemin induced cognitive impairment, increase of CD91 + cells, up-regulation of TNF-α and IL-1β, down-regulation of IL-6, activation of microglia, and activation of the TLR4/MyD88/NF-κB signaling pathway in rat brain. Significantly, intracerebroventricular injection of HPX reduced the above effects. Hemin induced boost of TNF-α, IL-1β and IL-6 in BV2 cells, as well as apoptosis in HT22 cells. Notably, when HT22 cells were cocultured with BV2 cells, apoptosis was significantly increased. Hemin also induced activation of the TLR4/MyD88/NF-κB signaling pathway and increased the M1 surface marker CD86 in BV2 cells, and inhibiting this pathway reduced the inflammatory responses.

CONCLUSIONS

Free heme induces cognitive impairment, and the underlying mechanism may involve neuronal apoptosis and microglial inflammation via the TLR4/MyD88/NF-κB signaling pathway. HPX may have potential therapeutic effects. Video Abstract.

Severe homocysteinemia in two givosiran-treated porphyria patients: is free heme deficiency the culprit?

Givosiran is a novel approach to treat patients with acute intermittent porphyrias (AIP) by silencing of ∂-ALA-synthase 1, the first enzyme of heme biosynthesis in the liver. We included two patients in the Envision study who responded clinically well to this treatment. However, in both patients, therapy had to be discontinued because of severe adverse effects: One patient (A) developed local injection reactions which continued to spread all over her body with increasing number of injections and eventually caused a severe systemic allergic reaction. Patient B was hospitalized because of a fulminant pancreatitis. Searching for possible causes, we also measured the patients plasma homocysteine (Hcy) levels in fluoride-containing collection tubes: by LC-MS/MS unexpectedly, plasma Hcy levels were 100 and 200 in patient A and between 100 and 400 μmol/l in patient B. Searching for germline mutations in 10 genes that are relevant for homocysteine metabolism only revealed hetero- and homozygous polymorphisms in the MTHFR gene. Alternatively, an acquired inhibition of cystathionine-beta-synthase which is important for homocysteine metabolism could explain the plasma homocysteine increase. This enzyme is heme-dependent: when we gave heme arginate to our patients, Hcy levels rapidly dropped. Hence, we conclude that inhibition of ∂-ALA-synthase 1 by givosiran causes a drop of free heme in the hepatocyte and therefore the excessive increase of plasma homocysteine. Hyperhomocysteinemia may contribute to the adverse effects seen in givosiran-treated patients which may be due to protein-N-homocysteinylation.

Heterozygous disruption of ALAS1 in mice causes an accelerated age-dependent reduction in free heme, but not total heme, in skeletal muscle and liver

5-Aminolevulinic acid (ALA) is the rate-limiting intermediate in heme biosynthesis in vertebrate species; a reaction catalyzed by the mitochondrial ALA synthase 1 (ALAS1) enzyme. Previously we reported that knockdown of the ubiquitously expressed ALAS1 gene in mice disrupts normal glucose metabolism, attenuates mitochondrial function and results in a prediabetic like phenotype when animals pass 20-weeks of age (Saitoh et al., 2018). Contrary to our expectations, the cytosolic and mitochondrial heme content of ALAS1 heterozygous (A1+/-) mice were similar to WT animals. Therefore, we speculated that regulatory "free heme" may be reduced in an age dependent manner in A1+/- mice, but not total heme. Here, we examine free and total heme from the skeletal muscle and liver of WT and A1+/- mice using a modified acetone extraction method and examine the effects of aging on free heme by comparing the amounts at 8-12 weeks and 30-36 weeks of age, in addition to the mRNA abundance of ALAS1. We found an age-dependent reduction in free heme in the skeletal muscle and liver of A1+/- mice, while WT mice showed only a slight decrease in the liver. Total heme levels showed no significant difference between young and aged WT and A1+/- mice. ALAS1 mRNA levels showed an age-dependent reduction similar to that of free heme levels, indicating that ALAS1 mRNA expression levels are a major determinant for free heme levels. The free heme pools in skeletal muscle tissue were almost 2-fold larger than that of liver tissue, suggesting that the heme pool varies across different tissue types. The expression of heme oxygenase 1 (HO-1) mRNA, which is expressed proportionally to the amount of free heme, were similar to those of free heme levels. Taken together, this study demonstrates that the free heme pool differs across tissues, and that an age-dependent reduction in free heme levels is accelerated in mice heterozygous for ALAS1, which could account for the prediabetic phenotype and mitochondrial abnormality observed in these animals.

Phosgene inhalation causes hemolysis and acute lung injury

Phosgene (Carbonyl Chloride, COCl₂) remains an important chemical intermediate in many industrial processes such as combustion of chlorinated hydrocarbons and synthesis of solvents (degreasers, cleaners). It is a sweet smelling gas, and therefore does not prompt escape by the victim upon exposure. Supplemental oxygen and ventilation are the only available management strategies. This study was aimed to delineate the pathogenesis and identify novel biomarkers of acute lung injury post exposure to COCl₂ gas. Adult male and female C57BL/6 mice (20-25 g), exposed to COCl₂ gas (10 or 20 ppm) for 10 min in environmental chambers, had a dose dependent reduction in P_aO₂ and an increase in P_aCO₂, 1 day post exposure. However, mortality increased only in mice exposed to 20 ppm of COCl₂ for 10 min. Correspondingly, these mice (20 ppm) also had severe acute lung injury as indicated by an increase in lung wet to dry weight ratio, extravasation of plasma proteins and neutrophils into the bronchoalveolar lavage fluid, and an increase in total lung resistance. The increase in acute lung injury parameters in COCl₂ (20 ppm, 10 min) exposed mice correlated with simultaneous increase in oxidation of red blood cells (RBC) membrane, RBC fragility, and plasma levels of cell-free heme. In addition, these mice had decreased plasmalogen levels (plasmenylethanolamine) and elevated levels of their breakdown product, polyunsaturated lysophosphatidylethanolamine, in the circulation suggesting damage to cellular plasma membranes. This study highlights the importance of free heme in the pathogenesis of COCl₂ lung injury and identifies plasma membrane breakdown product as potential biomarkers of COCl₂ toxicity.

Heme Oxygenase Activity and Heme Binding in a Neonatal Mouse Model

BACKGROUND

Severe hemolytic disease of the newborn leads to the release of pro-oxidative free heme (FH). Heme oxygenase (HO) is primarily responsible for detoxifying FH.

OBJECTIVE

To investigate the protective effects of HO in a model of heme overload.

METHODS

For in vitro studies, NIH3T3 HO-1-luc cells were incubated with 10, 30, or 60 µM FH or methemalbumin (MHA). HO-1 promoter activity was assessed 3, 6, and 24 h after treatment. Cell survival was indexed by viability assays. For in vivo studies, 1- and 5-week-old wild-type (Wt) or HO-1-heterozygous (Het, HO-1+/-) mice were given 60 µmol FH or MHA/kg intraperitoneally. After 24 h, plasma aspartate aminotransferease (AST)/alanine transaminase (ALT) and hemopexin, liver HO activity, and lipid peroxidation (LP) were determined.

RESULTS

In HO-1-luc cells, HO-1 promoter activity peaked 6 h after incubation with 30 µM FH (1.6-fold) or 60 µM MHA (2.1-fold) over baseline. Twenty-four hours after exposure to 60 µM FH, a decrease in viability of 80% was found, compared with no decrease after exposure to 60 µM MHA. In 1-week-old Wt and HO-1 Het pups given 60 µmol FH/kg, HO activity significantly increased 3.5- and 3.1-fold, respectively. No changes in LP or AST/ALT levels were observed. In adult Wt and HO-1 Het mice, HO activity increased (3.0- and 2.6-fold, respectively). LP and AST levels significantly increased 28.4- and 2.7-fold, respectively, in adult HO-1 Het mice. Hemopexin levels at baseline were higher in adults compared with newborns for both Wt and Het mice. In addition, FH induced hemopexin levels in both adults and newborns, but to a lesser degree in newborns.

CONCLUSIONS

FH is highly toxic in vitro, but its toxicity is abolished when bound to albumin. Newborns appear to be protected from the pro-oxidative effects of FH, which may be mediated by heme binding and a higher absolute HO activity at baseline and after FH-mediated induction.