A glutamine metabolic switch in red blood cell development and disease
Blood stem cells develop through different stages to become fully mature red blood cells. This fundamental biologic process is defined by a series of complex metabolic processes, which are often dysregulated in blood disorders such as sickle cell disease and β-thalassemia.
Scientists identified a previously unrecognized role for the amino acid glutamine in this process. In particular, the work reveals that modulating glutamine metabolism is a potential therapeutic route for common red blood cell disorders. Additionally, glutamine abundance may serve as a tool for evaluating therapeutic efficacy. The findings were published today in Science.
These findings result from fundamental biologic research to understand red blood cell metabolism. Despite accounting for 84% of the total cells of a mature adult, red blood cells are unique in their metabolic function.
“Mature red blood cells don’t have organelles, such as mitochondria,” said a co-corresponding author. “This raises an important question about how mature red blood cells deal with metabolic needs in order to build such massive biomass.”
The authors set out to understand the metabolic processes that regulate normal red blood cell maturation and how this might be altered in various disorders. The collaborative research team systematically profiled metabolic changes through each red blood cell maturation stage.
“We noted a very surprising finding regarding glutamine, which is usually broken down by stem cells for various metabolic needs,” the author explained. “We found that this process is completely reversed during later differentiation. The cells stop breaking down glutamine and begin to synthesize it by completely reversing the reaction.”
Glutamine is broken down as an energy source in early blood cell maturation. However, the researchers found that the enzyme glutamine synthetase flips the script at later stages of development. Heme is the main component of hemoglobin, the protein in red blood cells that carries oxygen. Red blood cell maturation depends on heme production, but ammonium (a byproduct of heme production) can accumulate if not removed, causing oxidative stress. The researchers found that red blood cells begin producing glutamine synthetase to facilitate the removal of ammonium by combining glutamate with the ammonium to produce glutamine.
“We have genetic and biochemical molecular studies to show that this metabolic process is completely reversed in order to detoxify ammonium that is generated to support heme biosynthesis,” the author said.
This work impacts the treatment of red blood cell disorders, such as β-thalassemia, because drugs used to treat these disorders are associated with improved red blood cell maturation. However, because glutamine synthetase is found throughout the body, its removal is lethal.
“In our genetic studies, we rarely see patients with glutamine synthetase gene mutations,” explained the author. “This gene is also critical for embryonic development, so the entire body is affected.”
Through conditional inactivation of the enzyme, the researchers identified a direct link between disruption of glutamine metabolism and red blood cell disorders. “We show this process is impaired in various red blood cell disorders, such as β-thalassemia,” the author said. “This causes a metabolic phenotype that resembles a glutamine synthetase deficiency, which can be characterized by increased glutamate and ammonia levels and decreased glutamine levels.”
The researchers identified glutamine synthetase oxidation as the source of this deficiency in β-thalassemia. Further, by increasing the protein’s expression, the researchers regained enzyme activity to treat the condition. Currently, anemia in β-thalassemia can be treated with the drug luspatercept; however, how this drug works is not fully understood. The authors identified increased glutamine–to–glutamate ratios in studies involving luspatercept, indicating that the drug likely works by recovering glutamine levels.
“This will be the first report to make a connection that the therapeutic benefit of this drug is associated with improved glutamine metabolism,” the author said.
L-glutamine is also used to alleviate symptoms of sickle cell disease, but how this drug works has similarly been contentious. The findings indicate that the efficacy of L-glutamine supplements likely occurs by fixing glutamine synthetase disruption.
“By supplementing glutamine, you’re providing the red blood cells with additional glutamine for various needs,” the author explained. “This may explain why L-glutamine is beneficial for sickle cell patients; this is something we are actively studying.”
Beyond its direct impact on treating red blood cell disorders, the findings suggest that metabolic features such as glutamine–to–glutamate ratios can be used as biomarkers for many other diseases.
