Ectopic expression of the sialic acid modifying enzyme CMAH in human THP1 macrophages

Abstract

Throughout evolutionary processes, organisms gain or lose functions because of emerging mutations. The frameshift mutation emerged in the human CMAH gene is one of these loss of function mutations and it caused the human glycocalyx to have distinct phenotype compared to other mammals. Owing to this loss of function mutation, humans lose the ability to process sialic acids as they are not capable of converting N-acetlyneuraminic Acid (Neu5Ac) to N-glycolylneuraminic Acid (Neu5Gc). <br /> The effects of Neu5Gc incorporation to immune cells have not been studied with their impacts on neurodegenerative diseases so far. Aim of this study is to figure out how Neu5Gc incorporation regulates and modifies innate immune cell responses and affects progress of neurodegeneration in in vitro culture system. With this purpose, murine CMAH gene has been expressed in human THP1 macrophages. Moreover, CMAH gene has been expressed in CD33KO macrophages since CD33 is one of the sialic acid binding protein and it has been found to have impacts in progression of Alzheimer's disease. <br /> There were two major outcomes in this study. Firstly, CMAH expression decreased Aβ, debris, and staphylococcus aureus bioparticle phagocytosis. The decrease in phagocytosis is not affected by the lack of CD33. This decrease could be explained by difficulty of clearance of Neu5Gc to Neu5Ac. Sialic acids must be cleaved prior to phagocytosis and Neu5Gc could not be cleaved as easy compared to Neu5Ac. Disrupted cleavage of Neu5Gc prior to phagocytosis could be the reason of accumulated Aβ plaques in Alzheimer's disease. Secondly, CMAH expressing macrophages exhibited increase in ROS production via CD33 dependent manner and stimulation with Aβ, debris, and staphylococcus aureus bioparticles did not cause increase in further ROS production. The increase in ROS production could be explained by disrupted ITIM signaling, increased lysosomal activation and increased metabolic turnover of Neu5Gc. Lysosomal activation was shown to be CD33 dependent and it could be one of the links to Alzheimer's disease in respect to ROS production. <br /> Another distinct phenotype of CMAH expressing macrophages is their inability to respond and perform immune functions. CMAH expression limited the responses of macrophages to certain level. CD33KO macrophages were known to have increased pro-inflammatory cytokine release, increased Aβ phagocytosis, and elevated response in ROS production after neural debris stimulation. However, when CMAH was expressed in CD33KO macrophages, all of these responses get deteriorated and changes related with CD33KO phenotype reverted. Combined with CMAH expression, CD33KO macrophages displayed decreased proinflammatory cytokine transcription, Aβ phagocytosis and ROS production to external stimuli such as neural debris.<br /> Results of this study might demonstrate the missing connection between Neu5Gc incorporation with macrophage functions in cellular level. Owing to close progeny, these results could shed light upon the connection between CD33 and microglia connection in Alzheimer's disease. Neu5Gc incorporation to macrophages in high levels demonstrated similar phenotype to microglia in Alzheimer's disease which have decreased phagocytic capability and increased oxidative stress in cellular level. However, these results are applicable to only in vitro level and more research in vivo is necessary to reach more conclusive findings. Afterwards, Neu5Gc replacement therapies could be considered as alternative therapy against Alzheimer's disease.