Glomerular Function and Structural Integrity Depend on Hyaluronan Synthesis by Glomerular Endothelium
Authors: van den Berg Bernard M. , Wang Gangqi , Boels Margien G. S. , Avramut M. Cristina , Jansen Erik , Sol Wendy M. P. J. , Lebrin Franck , Jan van Zonneveld Anton , de Koning Eelco J. P. , Vink Hans , Gröne Hermann-Josef , Carmeliet Peter , van der Vlag Johan , Rabelink Ton J. 
Published: July 2019

Significance Statement
In previous work, the authors demonstrated that short-term loss of integrity of the luminal part of the glycocalyx envelope that covers endothelial cells resulted in impaired glomerular filtration barrier function. In this study, using knockout mice lacking endothelial hyaluronan synthase 2 (the enzyme that produces hyaluronan, the main structural component of the glycocalyx layer), they found that loss of glomerular endothelial hyaluronan leads to mesangiolysis and glomerulosclerosis. Tissue from patients with diabetic nephropathy showed loss of glomerular endothelial hyaluronan in association with lesion formation. The authors also demonstrated that hyaluronan loss results in disturbed signaling of the extracellular matrix stabilizer angiopoietin 1. These findings suggest that the loss of glomerular endothelial hyaluronan in diabetic nephropathy may be a new therapeutic target to restabilize glomerular lesions.

Background
A glycocalyx envelope consisting of proteoglycans and adhering proteins covers endothelial cells, both the luminal and abluminal surface. We previously demonstrated that short-term loss of integrity of the luminal glycocalyx layer resulted in perturbed glomerular filtration barrier function.

Methods
To explore the role of the glycocalyx layer of the endothelial extracellular matrix in renal function, we generated mice with an endothelium-specific and inducible deletion of hyaluronan synthase 2 (Has2), the enzyme that produces hyaluronan, the main structural component of the endothelial glycocalyx layer. We also investigated the presence of endothelial hyaluronan in human kidney tissue from patients with varying degrees of diabetic nephropathy.

Results
Endothelial deletion of Has2 in adult mice led to substantial loss of the glycocalyx structure, and analysis of their kidneys and kidney function showed vascular destabilization, characterized by mesangiolysis, capillary ballooning, and albuminuria. This process develops over time into glomerular capillary rarefaction and glomerulosclerosis, recapitulating the phenotype of progressive human diabetic nephropathy. Using a hyaluronan-specific probe, we found loss of glomerular endothelial hyaluronan in association with lesion formation in tissue from patients with diabetic nephropathy. We also demonstrated that loss of hyaluronan, which harbors a specific binding site for angiopoietin and a key regulator of endothelial quiescence and maintenance of EC barrier function results in disturbed angiopoietin 1 Tie2.

Conclusions
Endothelial loss of hyaluronan results in disturbed glomerular endothelial stabilization. Glomerular endothelial hyaluronan is a previously unrecognized key component of the extracelluar matrix that is required for glomerular structure and function and lost in diabetic nephropathy.