Functional Analysis Using Interdisciplinary Methods Centered on Organic Synthetic Chemistry: Immunomodulation via Complex Lipids

Takanori Matsumaru (Assistant Professor, Department of Chemistry)

Complex lipids, which are widely found in nature and also present in our bodies, have long been thought to function as part of the cell membrane. However, recent research has revealed that complex lipids play an important role not only as membrane components but also as "signaling molecules." Specifically, they are involved in important bodily functions, such as regulating the immune system. However, complex lipids from natural sources often exist as diverse mixtures of molecules with similar structures, making it difficult to understand the detailed functions of a single molecular species of complex lipid.

Meanwhile, innate immunity is driven by "receptor" proteins on immune cells that function as sensors to detect specific molecules. Furthermore, it is believed that innate immunity precisely controls the immune response through the cooperative and complex action of multiple receptors. Among these, Toll-like receptors (TLRs) and C-type lectin receptors (CLRs) are known as receptors that recognize complex lipids and regulate the innate immune response. However, many aspects of the control mechanisms involving complex lipids for these innate immune receptors remain to be elucidated.

Against this backdrop, our research aims to elucidate the mechanisms of immunomodulation by complex lipids. Our approach first involves establishing an efficient synthetic method to obtain complex lipids as single molecular species through chemical synthesis. Using the established method, we synthesize various derivatives and then evaluate their immune-activating potential to clarify the correlation between lipid structure and immune activation. We also elucidate the functions of complex lipids or innate immune receptors using various immunological analyses that leverage computational chemistry, physicochemical analysis, or the creation of molecular probes.

As an example, we will introduce our research related to Mincle, a type of CLR. Mincle has been reported to recognize naturally derived complex lipids, such as trehalose dimycolate (TDM) from *Mycobacterium tuberculosis*, and self-glycolipids, thereby activating innate immunity. However, the details of the interaction between the fatty acid portion of the ligand and Mincle were not clear.

Based on a binding model of Mincle and its ligand, we chemically synthesized lipid-modified derivatives of trehalose diester with a polar group introduced into the fatty acid portion. When we proceeded to evaluate their activity, we found that a derivative with a hydroxy group (OH group) introduced into the fatty acid portion retained its activity. Furthermore, binding simulation analysis suggested that the OH group on the ligand's fatty acid portion could form a hydrogen bond with an aspartic acid (Asp) residue near the Mincle surface, and we believe this interaction contributes to the retention of the derivative's activity. Thus, this research has established a new and important guiding principle for the molecular design of Mincle ligands. This achievement also provides a foundation for understanding the interaction between Mincle and its ligands at the molecular level and for further advancing research on immune activation.

The immune system is an extremely complex mechanism, and many parts of it have yet to be elucidated. However, by analyzing it through an interdisciplinary approach that uses various knowledge and techniques, including organic synthetic chemistry as in this study, it is possible to obtain new insights that were not visible with conventional methods. This kind of interdisciplinary approach is what makes research interesting, and I feel that the true pleasure of scientific research lies in challenging unknown territories and enjoying new discoveries. We intend to continue developing our research in pursuit of further discoveries.