The C-Terminal Fragment of Agrin (CAF), a Novel Marker of Renal Function, Is Filtered by the Kidney and Reabsorbed by the Proximal Tubule

Abstract

Agrin, a multidomain proteoglycan and neurotrypsin, a neuronal serine protease, are important for forming (neuromuscular) synapses. Proteolytical activity of neurotrypsin produces a C-terminal fragment of agrin, termed CAF, of approximately 22 kDA molecular size which also circulates in blood. The presence of CAF in urine suggests either glomerular filtration or secretion into urine. Blood levels of CAF have been identified as a potential novel marker of kidney function. Here we describe that several nephron segments in the mouse kidney express agrin and neutrotrypsin in addition to the localization of both protein in the glomerulum. Agrin mRNA and protein was detected in almost all nephron segments and mRNA abundance was highest in the inner medullary collecting duct. Neurotrypsin mRNA was mostly detected in the thick ascending limb of the loop of Henle, the distal convoluted tubule, and the inner medullary collecting duct. Moreover, we show that the proximal tubule absorbs injected recombinant CAF by a process shared with receptor-mediated and fluid phase endocytosis. Co-injection of CAF with recombinant human transferrin, a substrate of the receptor-mediated endocytic pathway as well as with FITC-labelled dextran (10 kDa), a marker of fluid phase endocytosis, showed partial colocalization of CAF with both markers. Further colocalization of CAF with the lysosomal marker cathepsin B suggested degradation of CAF by the lysosome in the proximal tubule. Thus, the murine kidney expresses agrin and neurotrypsin in nephron segments beyond the glomerulum. CAF is filtered by the glomerulum and is reabsorbed by endocytosis by the proximal tubule. Thus, impaired kidney function could impair glomerular clearance of CAF and thereby increase circulating CAF levels.

The Reference Intervals for Serum C‐Terminal Agrin Fragment in Healthy Individuals and as a Biomarker for Renal Function in Kidney Transplant Recipients

“…Meanwhile, Goldberg et al reported that mice lacking agrin exhibited normal glomerular structure and normal renal function including glomerular function. Moreover, recent studies have demonstrated that CAF was likely cleared from circulation by glomerular filtration and subsequent endocytosis in the proximal tubule in their animal experiments. Thus, we could preliminarily infer that changes in serum CAF concentrations could most likely to be explained by extra‐renal source and decreased renal clearance.…”

Plasma CAF22 Levels as a Useful Predictor of Muscle Health in Patients with Chronic Obstructive Pulmonary Disease

“…Subjects with stable COPD were included while those with unstable COPD (infection, exacerbation and/or hospitalization in the past month), arthritis, myopathies and neurological diseases were excluded [ 19 ]. Further, impaired renal function can reduce glomerular filtration and increase circulating levels of CAF [ 20 ]. However, our study cohort had no clinical or laboratory signs of renal failure.…”

Proteomics and Metabolomics for AKI Diagnosis

“…214 In human beings, CAF was detected both in urine and blood, suggesting its role as a marker for renal clearance with increased serum levels indicating impaired kidney function. 215 In a study of 61 patients with severe sepsis and septic shock, serum levels of CAF were associated with AKI (AUC, 0.721) independent of sepsis. 216 Hepcidin is a 25-amino acid, cystein…”