SAT-123 THE FRACTIONAL EXCRETION OF HYALURONAN AND SYNDECAN-1 IN DETECTING ENDOTHELIAL GLYCOCALYX DAMAGE IN KIDNEY DISEASE

Abstract

The endothelial glycocalyx is a glycoprotein layer in the luminal surface on blood vessels with protective properties. Damage to this layer can be detected as an increase in the levels of its shed components such as hyaluronan and syndecan-1. Previous studies from our group have demonstrated elevated hyaluronan and syndecan-1 in patients with kidney disease. However, it is not clear whether an increase in these constituents in this population reflects damage to the glycocalyx or reduced renal clearance. Hyaluronan has a molecular weight of 4000kDa and is predominantly hepatically metabolised but is degraded into smaller fragments which can be detected in urine. The metabolism of syndecan-1 is unknown and has a variable molecular weight (32-200kDa) due to its attachment to variable heparan sulphate chain lengths. We aimed to determine the fractional excretion of hyaluronan and syndecan-1 in people with different levels of kidney function. We recruited healthy controls, CKD patients (eGFR 15-60), and kidney transplant recipients. Blood and spot urine samples were collected for enzyme-linked immunosorbent assays for hyaluronan (R&D Systems, Minneapolis) and syndecan-1 (Diaclone, France). Blood samples were centrifuged at 1000g for 15 minutes. Urine samples were centrifuged at 3000g for 10 minutes as well as 1000g for 15 minutes for comparison. Serum used for hyaluronan and syndecan-1 assays were diluted at 1:4 and 1:1, respectively. The fractional excretion of hyaluronan and syndecan were calculated from serum and urine levels using the formula: 100 × (urine level × serum creatinine)/(serum level × urine creatinine). Healthy controls (n=32), CKD patients (n=32) and transplants (n=36) had median eGFRs of 98 (70-127), 31 (10-59) and 59 (32-90) mL/min/1.73m2, p<0.0001, respectively. Liver function as measured by alanine transferase was normal in all groups (see table). No difference was detected in urinary hyaluronan and syndecan-1 values between urine centrifuged at 3000g or 1000g. Serum hyaluronan levels were highest in the CKD group, followed by transplant and healthy controls, whereas urine hyaluronan levels were unchanged between groups. The fractional excretion of hyaluronan was higher in healthy controls compared to CKD patients, and eGFR correlated negatively with serum hyaluronan (r=-0.47, p<0.0001); both indicating a higher clearance of hyaluronan with a higher GFR. Serum and urine syndecan-1 levels as well as the fractional excretion of syndecan were not different between groups. Furthermore, there was no correlation between serum syndecan-1 and eGFR. Both hyaluronan and syndecan-1 are detectable in urine which may represent the detection of smaller fragments of these molecules. The higher fractional excretion of hyaluronan in people with better kidney function suggests an element of renal excretion whereas this was not evident with syndecan-1. Given the normal liver function of this cohort and the known relatively small contribution of renal clearance of hyaluronan, the high serum levels in this study likely represent increased release from the endothelial glycocalyx. Syndecan-1 is present in renal tubular cells which may have affected the calculation of its fractional excretion. However, the absence of association between serum syndecan-1 and eGFR indicates little contribution of renal function to serum concentrations.