M. Cristina Avramut

Early microscopic-scale pericyte dysfunction contributes to the initial stages of many neurological diseases and represents a strong candidate target for therapeutic intervention. A non-invasive imaging modality able to image microvascular alterations induced by pericyte dysfunction is needed. In addition, the development of pericyte-focused therapies remains challenging due to the lack of early biomarkers of disease progression. Here we show that cerebral microvascular alterations induced by pericyte dysfunction can be characterized non-invasively in mice using functional ultrasound localization microscopy (fULM). Depletion of endothelial endoglin in adult mice as a model of hereditary haemorrhagic telangiectasia, leads to pericyte detachment in the arteriole–capillary transition (ACT) zone. Imaging reveals that arteriolar capillaries have irregular shapes, increased diameters, reduced blood speed and neurovascular uncoupling mainly localized in the ACT zone. Transforming growth factor-β signalling activator C381 restores pericyte coverage and neurovascular response. Our study underscores the potential of fULM in characterizing early microvascular alterations. As super-resolution ultrasound transitions to the clinic, our data support its future use in monitoring pericyte-focused therapies in humans.

Repeated fasting mimicking diet (FMD) partially prevents glomerular damage in a diabetic mouse model; however, although endothelial glycocalyx heparan sulfate contents were preserved, hyaluronan surface expression was reduced in the presence of UDP-glucuronic acid. The weight loss observed was of lean mass, not fat mass, and increased glomerular oxidative stress argue whether such a diet is safe in patients with diabetes.

Proteinuria develops when specific components in the glomerular filtration barrier have impaired function. Although the precise components involved in maintaining this barrier have not been fully identified, heparan sulfate proteoglycans are believed to play an essential role in maintaining glomerular filtration. Although in situ studies showed that a loss of heparan sulfate glycosaminoglycans increases the permeability of the glomerular filtration barrier, recent studies using experimental models showed that podocyte-specific deletion of heparan sulfate glycosaminoglycan assembly does not lead to proteinuria. However, tubular reabsorption of leaked proteins might have masked an increase in glomerular permeability in these models. Furthermore, not only podocytes, but also glomerular endothelial cells are involved in heparan sulfate synthesis in the glomerular filtration barrier. Therefore, we investigated the effect of a global heparan sulfate glycosaminoglycan deficiency on glomerular permeability. We used a zebrafish embryo model carrying a homozygous germline mutation in the ext2 gene. Glomerular permeability was assessed with a quantitative dextran tracer injection method. In this model, we accounted for tubular reabsorption. Loss of anionic sites in the glomerular basement membrane was measured using polyethyleneimine (PEI) staining. Although mutant animals had significantly fewer negatively charged areas in the glomerular basement membrane, glomerular permeability was unaffected. Moreover, heparan sulfate glycosaminoglycan‒deficient embryos had morphologically intact podocyte foot processes. Glomerular filtration remains fully functional despite a global reduction of heparan sulfate.

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.