In vivo imaging of cerebrospinal fluid circulation and outflow
Cerebrospinal fluid is produced with the ventricles of the brain and flows to the subarachnoid space surrounding the brain and spinal cord. The anatomical pathways for the circulation and drainage of CSF have been debated for decades. We have demonstrated in mice that outflow of CSF-injected tracers occurs predominantly through lymphatic pathways instead of through routes directly to the bloodstream as commonly believed. The major pathways for outflow of CSF were found along cranial nerves, such as olfactory or optic nerves, to reach lymphatic vessels outside the skull. We have developed dynamic near-infrared fluorescence imaging techniques to measure CSF outflow and are using these quantitative methods to evaluate how CSF turnover is altered under different physiological and pathological conditions. This may lead to new strategies to improve the clearance of this fluid, which may be of benefit for neurological conditions such as hydrocephalus, multiple sclerosis or neurodegenerative diseases.
Exchange of CNS fluids
At the same time, we are focused on elucidating the relationships between CSF and the interstitial fluid (ISF) of the brain and spinal cord. The exchange of these fluids is dynamically regulated to ensure that the neurons and supporting cells of the brain and spinal cord are provided with nutrients and can clear the waste products of their metabolism. However, how this process occurs is still very much unknown. There is considered to be a net production of fluid at the blood-brain barrier under normal conditions. There is also evidence that fluid and small molecules from the CSF may enter the parenchyma of the CNS. Since the brain and spinal cord lack a lymphatic system, other pathways must be utilized for the clearance of interstitial fluid from these organs. Several mechanisms may be at play for the exchange of CSF and ISF, including diffusion through CSF-brain interfaces and the extracellular spaces surrounding neurons and glial cells and/or convective (bulk) flow along the paravascular spaces of arteries and veins and within white matter tracts of the parenchyma. A better understanding of these processes could lead to improved drug delivery strategies targeting the CNS and to new methods to improve the clearance of brain edema fluid and pathological protein and cell aggregates.
Quantitative imaging of lymphatic vessel function
Our group has a long-standing interest in the development of methods for the visualization and quantification of lymphatic vessel function. In collaboration with investigators at ETH Zürich, we have designed a new class of vascular tracer that can be used with near-infrared imaging techniques to visualize several different aspects of the lymphatic drainage process. Quantifications of lymphatic clearance from skin, collecting lymphatic vessel contractility, flow through draining lymph nodes and lymphatic transport to the systemic bloodstream have been developed using fluorescence imaging approaches and have been validated in several mouse models. We also employ the use of transgenic reporter strains, such as Prox1-GFP mice, and whole-mount techniques to visualize the lymphatic system at the anatomic level. This combination of morphological and functional techniques allows assessment of the phenotype of transgenic mouse models of lymphatic modulator genes, investigation of the efficacy of novel pharmaceutical formulations and further elucidation of the role of the lymphatic system in a wide range of diseases.