Defining the mechanisms that drive novel cyclic nucleotide signaling in immune defense

Project Summary- R35 Application PI: Benjamin R. Morehouse Ph.D. The innate immune system is the critical first line of defense against microbial pathogens and is also a key player in the response to cellular stress. A thorough understanding of innate immune function and disfunction is necessary if we are to meet the healthcare challenges that face modern human society. Innate immunity is common to all life on this planet, however how exactly diverse organisms, especially those considered to be ‘non-model’, defend themselves from infection is an underdeveloped field of research that may yield clues to understanding of our evolutionary history and provide new insights into the organization and functioning of human immunity. Surprisingly, we find that parts of the human innate immune system have ancient origins that can be traced as far back as the antiviral defense pathways of bacteria and archaea. Among these evolutionary connections we find many mechanistic similarities but also significant differences in form and function. Our particular interest lies in the shared strategies of cyclic nucleotide second messenger signaling pathways of immunity, the use of specialized chemical compounds that mediate antiviral immune defense. The antiviral and antitumoral cGAS-STING (cyclic GMP-AMP synthase-stimulator of interferon genes) cyclic dinucleotide signaling pathway in humans is analogous to cyclic oligonucleotide-based antiphage signaling systems (CBASS) in prokaryotes and we have uncovered many striking elements of conservation between these defensive mechanisms. Pycsar (pyrimidine cyclase system for antiphage resistance), is a similar prokaryotic immune defense pathway that operates through production of cyclic pyrimidine mononucleotides. Through detailed exploration and characterization of CBASS and Pycsar immunity that generate cyclic nucleotide signals, we will delineate the modes of viral activation, probe the mechanisms of nucleotide selectivity for both cyclase enzymes and cyclic nucleotide receptors, and define the functional consequences of effector activity on cell growth and viral infection outcomes. Using a combination of bioinformatic analysis, biochemical testing, and structural biology approaches, we will confirm activities and phylogenetic links between diverse bacterial, fungal, and mammalian homologs to establish the relationships shared between cyclic nucleotide signaling pathways. We will examine the connections between evolutionarily distant immune systems, potentially leading to identification of new strategies for pharmacological targeting and destruction of antimicrobial resistant pathogens, and we stand to uncover more of the mechanisms that shape our own immune system which will benefit humankind in the fight against infectious agents, cancer, and autoimmune disease.