Quantifying extracellular solute clearance from the murine hippocampus

The clearance of solutes from the brain plays a crucial role in maintaining its homeostasis. An imbalance between the production and elimination of toxic protein aggregates has been linked to several neurodegenerative pathologies, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS), among others. Numerous studies have sought to characterize the mechanisms underlying the efflux of solutes and waste products from the brain to the cerebrospinal fluid (CSF), yet these efforts have yielded contrasting results. This has generated criticisms that undermine the validity of proposed models such as the glymphatic, intramural periarterial drainage (IPAD), and mixing models. In this thesis, I have applied a combination of advanced microscopy techniques, various fluorescent probes and in vivo surgical procedures on mice to create a platform aimed at clarifying these controversies. I used this platform to quantitatively analyse how factors—including solute size and structure, animal age, and extracellular space (ECS) complexity—influence the efficiency of solutes elimination from the brain to the CSF and dura mater. My results revealed strong size- and structure-dependent effects on the drainage of both fluorescent nanoparticles and fluorescent Aβ1-42 aggregates. Additionally, older animals showed a drastic decrease in their ability to eliminate solutes from the brain into the CSF compared to younger mice. I used fluorescent probes to create a detailed map of solute movement within the brain, which allowed me to clarify and confirm the differential roles of the periarterial spaces as entry routes and the perivenous spaces as exit routes for solutes in the glymphatic system. Finally, I have refined my study of solute clearance by characterizing the elimination of chronically produced toxic proteins in the 5xFAD animal model for AD, thereby establishing this model as a valuable platform for solute clearance studies. This work expands knowledge of the mechanisms behind solute clearance enabling for the first time a quantitative approach.