In situ molecular characterization of endoneurial microvessels that form the blood‐nerve barrier in normal human adult peripheral nerves

Abstract

The blood‐nerve barrier (BNB) formed by tight junction‐forming endoneurial microvessels located in the innermost compartment of peripheral nerves, and the perineurium serve to maintain the internal microenvironment required for normal signal transduction. The specific molecular components that define the normal adult human BNB are not fully known. Guided by data derived from the adult human BNB transcriptome, we evaluated the in situ expression of 25 junctional complex, transporter, cell membrane, and cytoskeletal proteins in four histologically normal adult sural nerves by indirect fluorescent immunohistochemistry to determine proteins specifically expressed by restrictive endoneurial microvascular endothelium. Using Ulex Europaeus Agglutinin‐1 expression to detect endothelial cells, we ascertained that the selected proteins were uniformly expressed in ≥90% of endoneurial microvessels. P‐glycoprotein (also known as adenosine triphosphate‐binding cassette subfamily B member 1) and solute carrier family 1 member 1 demonstrated restricted expression by endoneurial endothelium only, with classic tight junction protein claudin‐5 also expressed on fenestrated epineurial macrovessels, and vascular‐specific adherens junction protein cadherin‐5 also expressed by the perineurium. The expression profiles of the selected proteins provide significant insight into the molecular composition of normal adult peripheral nerves. Further work is required to elucidate the human adult BNB molecular signature in order to better understand its development and devise strategies to restore function in peripheral neuropathies.

Treatable, motor‐sensory, axonal neuropathies with C5b‐9 complement on endoneurial microvessels

“…Endoneurial microvessels are the equivalent of capillaries in other tissues, but have distinct properties that include large size and endothelial cells linked by tight junctions (which form a blood‐nerve barrier) 21,22 . The nerve pathology feature that defined our clinically ascertained subpopulation of motor‐sensory axonal polyneuropathies was C5b‐9 staining of endoneurial microvessels.…”

Drug Permeability: From the Blood–Brain Barrier to the Peripheral Nerve Barriers

“…Consisting primarily of microvascular endothelial cells with intercellular tight junction complexes, the BNB expresses a variety of influx and efflux transporters, including solute, macromolecular, nutrient, and drug transporters, immunoglobulin transporters, and ionic channels, to mention a few, forming an effective diffusion barrier and active regulator of peripheral nerve homeostasis (Figure 2b). [39][40][41][42][43]…”

“…The BBB has been shown to claudin-1 and claudin-5, [19] and the human BNB has recently been shown to express claudin-4 at intercellular junctional contacts, consistent with tight junctions. [40][41][42]66] Claudin-5, which is shared by BNB-and the BBB-forming endothelial cells, [40] has been shown to be expressed on the membranes of both BNB-forming endothelial cells and epineurial macrovascular endothelial cells that do not form restrictive tight junction complexes in human sural nerve biopsies in situ. [41,42,66] This suggests that claudin-5 is not a component of the restrictive tight junctional complex in human peripheral nerves.…”

“…[40][41][42]66] Claudin-5, which is shared by BNB-and the BBB-forming endothelial cells, [40] has been shown to be expressed on the membranes of both BNB-forming endothelial cells and epineurial macrovascular endothelial cells that do not form restrictive tight junction complexes in human sural nerve biopsies in situ. [41,42,66] This suggests that claudin-5 is not a component of the restrictive tight junctional complex in human peripheral nerves. The relative permeabilities of the two barriers were investigated in vivo by Malmgren and Olsson in 1980.…”

Mechanisms Ensuring Endothelial Junction Integrity Beyond VE-Cadherin

“…VE-cadherin represents its major constituent, as antibodies against VE-cadherin or gene inactivation are sufficient to perturb endothelial monolayers in vitro and enhance vascular permeability for plasma proteins in vivo ( Gotsch et al, 1997 ; Matsuyoshi et al, 1997 ; Gulino et al, 1998 ; Corada et al, 1999 ; Frye et al, 2015 ; Duong et al, 2020 ). VE-cadherin is essential for the development of the vascular system ( Carmeliet et al, 1999 ; Gory-Faure et al, 1999 ) and is probably the adhesion molecule with highest endothelial specificity, with some expression on trophoblasts and fetal stem cells ( Aird, 2007a ), perineural cells ( Colom et al, 2012 ) and the perineurium of peripheral nerves ( Smith et al, 1998 ; Ouyang et al, 2019 ).…”