Intracellular uptake of macromolecules by brain lymphatic endothelial cells during zebrafish embryonic development

Abstract

The lymphatic system controls fluid homeostasis and the clearance of macromolecules from interstitial compartments. In mammals brain lymphatics were only recently discovered, with significant implications for physiology and disease. We examined zebrafish for the presence of brain lymphatics and found loosely connected endothelial cells with lymphatic molecular signature covering parts of the brain without forming endothelial tubular structures. These brain lymphatic endothelial cells (BLECs) derive from venous endothelium, are distinct from macrophages, and are sensitive to loss of Vegfc. BLECs endocytose macromolecules in a selective manner, which can be blocked by injection of mannose receptor ligands. This first report on brain lymphatic endothelial cells in a vertebrate embryo identifies cells with unique features, including the uptake of macromolecules at a single cell level. Future studies will address whether this represents an uptake mechanism that is conserved in mammals and how these cells affect functions of the embryonic and adult brain.DOI: http://dx.doi.org/10.7554/eLife.25932.001

Energy-dependent transport at dural lymphatic vessels is necessary for Aβ brain clearance in Alzheimer’s disease

“…This is even more so with lymphatic endothelia; it is currently accepted dogma in textbooks that lymphatic paracellular transport through discontinuous junctions is the primary means of lymphatic transport of protein and fluid. However, some recent studies have shown that the transcellular route is a major mechanism of solute transport by lymphatic endothelial cells (70,71). In support of this mechanism, we also noted in our transmission electron microscopy (TEM) studies (see below) the presence of numerous caveolae and occasional larger vesicles that may be clathrin coated pits.…”

Cerebrovascular endothelial cells form transient Notch‐dependent cystic structures in zebrafish

“…Although very different in size and behaviour to other vesicular structures, inhibition of the cytoskeletal components filamentous actin (F‐actin) formation or Myosin II reduced kugeln formation, as previously shown for cellular blebs . Kugeln do not interact with brain lymphatic endothelial cells (BLECs) or macrophages, nor do they serve as scavengers. Kugeln formation is not influenced by either increased membrane rigidity, nor osmotic pressure, which are known to impact platelet ballooning and cellular blebbing .…”

“…Next, we examined whether kugeln serve an uptake function, similar to BLECs . We studied whether IgG‐conjugated Alexa Fluor 674, injected into the tectum, would be taken up by kugeln .…”

“…These findings were confirmed by imaging the double-transgenic Tg(kdrl:HRAS-mCherry) s916, Tg(flt4 BAC :mCitrine) hu7135 , which more specifically labels BLECs ( Fig EV3A). Again, ccbe1 MO knockdown had no effect on kugel number ( Next, we examined whether kugeln serve an uptake function, similar to BLECs [14]. We studied whether IgG-conjugated Alexa Fluor 674, injected into the tectum, would be taken up by kugeln.…”

Live Imaging of Intracranial Lymphatics in the Zebrafish

“…The importance of lymphatic vessels during heart regeneration in zebrafish has also been described 16,17 . Close examination of the developing zebrafish brain has led to a thorough description of unusual perivascular lymphatic-related cells called "Fluorescent Granular Perithelial cells" (FGPs) aka "Mato cells" 18 , "meningeal mural Lymphatic Endothelial Cells" (muLECs) 19 , or "Brain Lymphatic Endothelial cells" (BLECs) 20 with a macrophage-like morphology and a high capacity to take up macromolecules. However, while these cells have a gene expression profile very similar to that of lymphatic endothelial cells they do not form tubes under normal physiological conditions, and bona fide intracranial lymphatics have not been described in the zebrafish.…”