Hosts Going Viral: Building The Tools And Systems To Determine How Human-Relevant Biology Evolved Through The Capture Of Viral Genes In Our Distant Ancestors

SUMMARY: Acquiring extracellular genetic and functional novelty can forever change the evolutionary trajectory of organisms, from mitochondria to antibiotic resistance to placental formation. Retroviruses heritable through the germline, endogenous retroviruses (ERVs), comprise eight percent of the human genome (four times more than

protein coding sequence). The vast majority of ERVs lack all protein coding potential. A select few ERV genes have been preserved over evolutionary timescales, a strong indication that these specific genes are beneficial. env is the most frequently preserved ERV gene. In retroviruses, env encodes a glycoprotein that interacts with

specific cellular receptors and is the virus-cell membrane fusogen. These activities have been repurposed by a pair of ERV envs that form the syncytiotrophoblast, an essential placental structure of fused cells at the maternal- fetal interface. Divergent envs have been coopted by different mammalian lineages for the same function.

Knowledge of how ERV genes are acquired and repurposed is largely restricted to DNA and mRNA, while it is typically the proteins that carry out their functions. A dearth of quality reagents for ERV proteins has impeded our ability to define when, where, and why these proteins are expressed. We will leverage our expertise as

evolutionary biologists, protein biochemists, and B cell immunologists to move ERV research into the proteome by generating, quality controlling, and disseminating the reagents required for this transition. Our primary focus is EnvP(b)1, an env preserved for >40 million years with no known function. Evolution has maintained its

membrane fusion activity across human, ape and monkey orthologs. Expression in cultured cells results in fused monolayers of multinucleated cells and rapid cell death. Expression in vivo must therefore be tightly regulated, and its activity tailored to its function. We find genetic and functional evidence of evolutionary refinement of

EnvP(b)1 over the past >40 million years. We will, in vitro, define the tissue and species specificities of membrane fusion by human and non-human primate (NHP) EnvP(b)1 orthologs. Using ancestral reconstruction, we will determine how EnvP(b)1 function evolved to its present form across primate lineages. Given its cytotoxicity and

ability to produce cells with variable chromosomal content, the preservation of EnvP(b)1 indicates it must perform critical functions. We will define the anatomical sites and context of EnvP(b)1 protein expression in humans and NHPs, and if cell-cell fusion occurs at these sites. This work will ultimately establish NHP models required to

define the function and explain why EnvP(b)1 is maintained. Using a carefully controlled antibody reagent, we find EnvP(b)1 protein expression in healthy human and NHP tissues, including placenta. The placenta, a rapidly evolving structure, is a hotbed of functional ERV Env expression. We will develop the reagents needed to dissect

the roles and regulation of at least eight unique ERV Envs expressed in this tissue. This work will define new human biology and raise the questions of how do species without these elements do the same thing and do standard small animal models fail to capture the biology of these elements that are largely primate-specific?