Project Summary/Abstract The blood-brain barrier (BBB) maintains brain health by protecting the brain from the bloodstream. These barrier properties frustrate the treatment of nearly all brain disorders, representing one of the largest challenges in neuroscience and drug delivery. Yet, intriguingly, recent studies have discovered a variety of surprising peripheral influences on brain function, hinting at the existence of underappreciated modes of communication across the BBB. Indeed, while canonical BBB properties, such as paracellular tight junctions and minimal caveolin-mediated transcytosis, have been established via a handful of standard tracers, it remains unclear whether these tracers fully represent the BBB’s physiological interactions with and permeability to the thousands of circulating proteins and cells it is constantly exposed to. By developing methods to tag and track the blood plasma proteome, I recently observed an unexpected degree and diversity of protein transport into the healthy adult brain. Thus, I hypothesize that brain health is maintained not just by BBB impermeability—but by specific routes of blood-to-brain communication actively facilitated by the BBB. Specifically, I propose that there are three logical routes for how peripheral information is communicated across the healthy BBB: the direct transport of proteins into the brain; the responsive relay of proteins made by the BBB into the brain; and the BBB-licensed migration of peripheral immune cells into the brain. By combining proteome tagging techniques with bioorthogonal chemistries, each proposed aim explores one independent route to systematically reveal the identities and mechanisms of the signals transmitted via the healthy BBB. I will begin by creating a first catalog of plasma proteins that directly cross the BBB and quantifying their permeabilities (Aim 1). I will subsequently characterize a new BBB relay function by deducing the signals the BBB secretes into the brain in response to peripheral cues (Aim 2). Lastly, I will elucidate neuroimmune surveillance by determining the BBB sites and molecules enabling healthy immune cell migration into the brain (Aim 3). Together, these studies will expand our understanding of how the BBB maintains brain health and enable new studies exploring the neurological functions of BBB-permeable proteins and cells in health, aging, and disease. Our results will also provide a comprehensive set of functional targets to enhance drug delivery to the brain, reveal new mechanisms to understand and blunt neuroinflammation, and generate innovative tools to decipher intercellular communication for broad use across disciplines.

Project Narrative The blood-brain barrier impedes the effective treatment of nearly all brain disorders, and its dysfunction is a leading cause of death. I propose developing and applying new tools to tag, track, and identify which native proteins and cells in the blood can traverse the blood-brain barrier and learn the rules by which they do so. This will expand many-fold the set of therapeutic pathways into the brain, reveal new targets to block disease- provoking neuroinflammation, and provide a new understanding of how communication across the blood-brain barrier supports brain health.