Scientists have long been puzzled by how maturing red blood cells manage to produce all the hemoglobin they need to carry oxygen to tissues, even after shedding the vital structures they need to produce it. Now, new University of Maryland School of Medicine (UMSOM) research has found a missing piece to the puzzle: In the final stages of their maturation, red blood cells can import heme, a crucial, iron-containing component of hemoglobin, from other cells. This surprising finding not only overturns long-standing assumptions about heme and red blood cells but could eventually lead to new therapies for inherited genetic blood disorders such as β-thalassemia.
In the study published today in the journal Science, UMSOM researchers demonstrated for the first time that under certain conditions, immature red blood cells known as erythroblasts can import heme from surrounding cells even after discarding their own heme-producing mitochondria. They do this through a transporter protein known as Heme Responsive Gene 1, or HRG1, which Dr. Hamza's lab first discovered years earlier in a microscopic bloodless worm.
"We've shown that this transporter protein, HRG1, is essential for the production of healthy, mature red blood cells, particularly at a time when the body needs to produce red blood cells quickly due to certain stresses like being oxygen deprived at a high altitude or during blood loss," said study principal investigator Dr. Iqbal Hamza, PhD , a UMSOM Professor of Pediatrics in the Center for Blood Oxygen Transport and Hemostasis ( CBOTH ). "We found that in the absence of HRG1, the red blood cells that are produced are sub-optimal, which means that when the system is stressed and must make more red blood cells than usual, they may have a hard time doing so without HRG1."
The discovery is clinically relevant in the context of iron deficiency anemia, the world's most prevalent nutritional disorder. "A common hallmark of anemia is pale, hemoglobin-deficient red blood cells," said Dr. Hamza, who holds a joint appointment as a Professor of Animal and Avian Sciences at University of Maryland, College Park. "By identifying strategies to mitigate anemia, such as regulating heme delivery through HRG1 to enhance hemoglobin production, we could substantially reduce the morbidity and mortality associated with this condition."
Hemoglobin Enigma
The research attempted to address the question of how maturing red blood cells meet the extraordinary heme requirements needed to keep producing hemoglobin in the final stages of their maturation after losing their mitochondria. "We were attempting to identify the pathway involved in this process," Dr. Hamza said.
He and his colleagues hypothesized that these developing red blood cells import heme through HRG1 to provide a final boost to their hemoglobin levels.
To test their hypothesis, Dr. Hamza and his team first used single-cell RNA sequencing on immature red blood cells in mice to confirm that the HRG1 gene is expressed in developing red blood cells, and that HRG1 expression increased as the cells matured. They then used an animal model to study what occurred when the HRG1 gene was knocked out.
The researchers found that mice with the knock-out HRG1 gene were unable to adequately increase their red blood cell production and became anemic when they were put under stress and forced to create more red blood cells. Normal mice, on the other hand, were able to replenish their red blood cells.
The researchers then used flow cytometry to demonstrate that red blood cells made by the knockout mice failed to accumulate sufficient hemoglobin and died off before reaching full maturity, indicating that HRG1 was essential for the cells to accumulate enough heme to survive and function properly.
Implications for Treating β-thalassemia
Dr. Hamza and his team also examined a mouse model of β-thalassemia, an inherited genetic blood disorder that causes insufficient hemoglobin and red blood cell formation, leading to severe anemia. Since the blood disorder—estimated to affect 10 million people worldwide—is characterized by an accumulation of excess free heme that is not integrated into hemoglobin, the researchers theorized that reducing the activity of the HRG1 gene could limit the accumulation of toxic free heme and mitigate symptoms. They tested the idea by deleting one copy of the HRG1 gene in the β-thalassemia animal model, and found there were measurable improvements in red blood cell production and anemia.
"This work reveals a previously unrecognized intercellular heme-transfer pathway that helps sustain red blood cell production under stress," said Mark T. Gladwin, MD , Dean of the University of Maryland School of Medicine, Vice President for Medical Affairs, University of Maryland, Baltimore, and the John Z. and Akiko K. Bowers Distinguished Professor. "The implications extend to a wide spectrum of blood disorders—including sickle cell disease and β-thalassemia—where heme imbalance drives inflammation, oxidative stress, and organ damage. Identifying HRG1 as a regulator of heme availability opens exciting therapeutic possibilities for conditions in which the body struggles to maintain healthy red cell production."
Dr. Hamza's lab is now conducting new experiments to determine the relative importance of mitochondria-produced heme versus HRG1-imported heme in different diseases, including iron-deficiency anemia. His lab also recently received a prestigious grant from the Human Frontier Science Program to research how heme proliferates through cells after being produced in the mitochondria.
"We hope to elucidate how heme is transported, trafficked, and inserted into proteins," Dr. Hamza said. "This will require developing methods to finally 'see' heme inside individual cells."
Further down the road, he aims to search for new drug targets that could regulate and fine-tune how much HRG1 is expressed, potentially providing a new tool for treating β-thalassemia and other inherited genetic blood disorders, as well as anemia.
Dr. Hamza conducted the study with his UMSOM colleagues Xiaojing Yuan, PhD , Satoru Otsuru MD, PhD , Audrey Belot, PhD , and Amaury Maros, as well as graduate students Andrew Rock and Sohini Dutt, and Hamza lab Research Assistant Gia Haemmerle. David Bodine, PhD, Chief of the Genetics and Molecular Biology Branch and Head of the Hematopoiesis Section at the National Human Genome Research Institute, also co-authored the study.
