Two-photon microscopy in vivo reveals brain vessel type-specific loss of glycocalyx caused by apoM/S1P signaling impairment

Abstract

Increases in adsorptive mediated transcytosis (AMT) at the blood-brain barrier (BBB) are linked to many brain disorders. In a healthy brain, AMT is suppressed by sphingosine-1-phosphate (S1P) receptor 1 (S1PR1) signaling. Low levels of S1P lead to a rise in AMT, but the mechanisms are incompletely understood. Here, we explored whether the rises in AMT are caused by the loss of the endothelial glycocalyx (gcx). We used two-photon microscopy in mice with low S1P plasma levels (Apom-/-) and developed a novel photobleaching approach to measure gcx in vivo at distinct classes of cerebral microvessels, i.e., arterioles, capillaries, and venules. We show that S1P signaling impairment reduced gcx in arterioles but not in other vessel segments. The location of gcx loss corresponded to the vascular topology of AMT increases. The S1PR1 agonist SEW2871 restores low levels of AMT in Apom-/- mice but did not restore the gcx within the same time window. We propose that while the gcx loss may contribute to AMT increase, restoring gcx is not necessary for AMT to return to normal. These data establish a new imaging method to study gcx in the living mouse brain, demonstrate zonation of gcx in cerebral microvessels, and suggest differences in vascular susceptibility to gcx loss in disease states.

Ultrastructure of precapillary sphincters and the neurovascular unit

“…The luminal side of endothelial cells is coated by a layer of glycoproteins (glycocalyx) a component of the tripartite blood-brain barrier ( 14 ). The glycocalyx has previously been shown to be nonuniform along the vascular tree, with the highest presence at arterioles and ‘hot spots’ at arteriole branchpoints ( 15 ), possibly reflecting the tissue’s shear stress. In the MICrONS dataset, the glycocalyx can be seen as an electron-dense hairy structure, also lining the precapillary sphincter ( Fig.…”

Ultrastructure of Precapillary Sphincters and the Neurovascular Unit

“…The luminal side of endothelial cells are coated by a layer of glycoproteins (glycocalyx) which is a part of the tripartite blood-brain barrier (13). The glycocalyx is non-uniform along the vascular tree, with the highest presence at arterioles and with "hot-spots" at arteriole branchpoints (14), perhaps reflecting the amount of shear stress in the tissue. In the MICrONS dataset, the glycocalyx appears darker and/or thicker at arterioles compared with nearby capillaries and venules, with an abrupt transition at the precapillary sphincter (Figure 1c), consistent with a higher density of glycocalyx in arterioles.…”