Intercellular factors regulating hematopoiesis and maintaining neuromuscular junction in Drosophila

Abstract

As a genetic model organism, the fruit fly, Drosophila melanogaster, is utilized in a wide range of developmental and pathological studies. The process of hematopoiesis in Drosophila shares many similarities with primitive immune cells in humans, making it a suitable model for investigating the mechanisms of immune cell differentiation, particularly within hemocytes. However, many aspects of hematopoietic cell differentiation, especially the mechanisms by which external factors promote or inhibit differentiation, remain largely unresolved. Furthermore, fruit flies have been instrumental in studies of neurogenesis, and extensive research on the pathogenic mechanisms of neurodegenerative genes has been conducted. Particularly, in the role of a model organism for one of the most challenging neurodegenerative diseases, Alzheimer's disease, research on the Drosophila’s nervous system is actively ongoing. Nevertheless, the exact causes of Alzheimer's disease have not been fully elucidated, and many factors contributing to its pathological processes remain largely unknown. In this study, I investigated the role of iron in Drosophila hematopoiesis to validate whether iron is an extrinsic cue on hemocyte differentiation. Moreover, I studied the function of proteins involved in mitochondrial morphological changes to understand their participation in known neurodegenerative mechanisms in fruit flies. First, I confirmed that inhibiting the absorption of iron ions in Drosophila larvae's hematopoietic organ, the lymph gland, promotes the differentiation of immune cells. Then, I conducted a screening to inhibit proteins related to the absorption, storage, and distribution of iron ions in the intestine cell. As a result, I uncovered that the Fer1HCH protein, a component of ferritin involved in intracellular storage and transport of iron ions, affects the differentiation of hemocytes. Furthermore, I observed Fer1HCH expression in various types of Drosophila hematopoietic cells, and it was discovered that inhibiting Fer1HCH within differentiated hemocytes promotes their differentiation. Subsequently, when I overexpressed the mitochondrial fusion protein Marf in Drosophila, it exhibited neurodegenerative traits by reducing the number of boutons at the neuromuscular junction in muscle cells. Interestingly, the Marf protein was found to genetically interact with the PAR-1 kinase, known for inducing hyperphosphorylation of the tau protein, which is associated with Alzheimer's disease. Additionally, these two proteins were discovered to have a physical interaction within the mitochondria. Finally, I confirmed that the interaction between Marf and PAR-1 regulates mitochondrial morphology and cellular respiration within muscle cells. Moreover, I found that these two proteins also interact in the mammalian orthologs within mouse cells. Through these studies, I have identified novel factors contributing to the developmental or pathological regulation of hemocytes or neurons. I demonstrated how iron ion levels outside of the hematopoietic organ can impact differentiation, and I also found that mitochondrial proteins in muscle cells regulate neurodegenerative traits at neuromuscular junctions. By utilizing the Drosophila genetic model, I was able to conduct research on how external factors influence cells, highlighting the importance of intercellular interactions.