PROJECTS & AWARDS

Project: Vascular changes in acute kidney injury and sepsis using a novel technique for imaging red blood cells.

Background:

Sepsis is a life-threatening loss of organ function caused by a dysregulated host response to infection(1). Sepsis is the commonest cause of acute kidney injury (AKI) in critically unwell patients, and is independently associated with higher odds of death and longer hospitalization(2–9). Albuminuria observed in sepsis-AKI suggests glomerular filtration barrier (GFB) dysfunction and may contribute to disease progression through renal epithelial injury(7,10). If so, the mechanisms that lead to albuminuria may be considered as therapeutic targets.

There is evidence that one potential mechanism is through loss of the glycocalyx(7,11,12). The glycocalyx is a gel like layer covering the luminal surface of the endothelium, acting as a filtration barrier at the fenestrated endothelium in the glomeruli(10). It is a dynamic structure, rapidly shed and synthesized facilitating endothelial adaptation to changes in local environment(13). Alterations to glomerular glycocalyx occurs in sepsis models in mice including lipopolysaccharide and TNF-alpha administration(7). In a rat sepsis-AKI model, increased urinary albumin is associated with GFB structural changes and decreased expression of key glycocalyx components(11). In humans, the depth of the sublingual glycocalyx is reduced in volunteers receiving low dose endotoxin and increased plasma levels of the glycocalyx constituent hyaluronan suggest glycocalyx shedding(12). Decreased glycocalyx thickness correlates with kidney dysfunction and hospital mortality in sepsis(14,15). Glycocalyx shedding is therefore likely to be a crucial step in the pathogenesis of sepsis-AKI.

Demonstrating glycocalyx loss in humans has been technically challenging. It is possible to detect glycocalyx proteoglycans in urine and plasma, suggestive of increased shedding, but the clinical value of these techniques is still unclear. Altered glomerular filtration and saturation of the intracellular pathways responsible for synthesis of glycocalyx proteoglycans during periods of increased shedding are two obvious confounders. Non-invasive imaging techniques, such as Glycocheck TM, indirectly measure glycocalyx dysfunction in the mucosal microcirculation of the mouth. This technique assumes glycocalyx shedding is a systemic phenomenon, but questions remain about its accuracy and reproducibility(16,17).

A novel technique measuring the depth of human red blood cell (RBC) glycocalyx has been developed in Bristol by Dr Butler and has the potential to unlock understanding of glycocalyx dysfunction in disease states. Preliminary data shows that RBC glycocalyx correlates with glomerular glycocalyx thickness and predicts glomerular albumin permeability. In humans, Glycocheck TM measurements correlate significantly with the RBC glycocalyx depth in pregnant women. Importantly the RBC assay is much easier to conduct, requiring only a blood sample, and has an improved intraclass correlation (0.66 vs 0.33 for GlycocheckTM). Moreover, there are distinct RBC glycocalyx appearances between albuminuric and non-albuminuric women. These findings suggest that RBC glycocalyx depth is a useful marker of systemic glycocalyx injury. Dr Butler has just been awarded a prestigious MRC Clinician Scientist fellowship to use the RBC assay in diabetes, demonstrating that the potential of the assay has been recognised following detailed scrutiny.

To test the hypothesise that albuminuria in sepsis-AKI results from loss of glycocalyx, we aim to measure RBC glycocalyx depth using a novel RBC assay and compare results from patients with sepsis-AKI to healthy controls.