Urinary collagen peptides: Source of markers for bone metabolic processes in kidney transplant recipients

Abstract

Introduction: Kidney transplant recipients (KTRs) are at an increased risk of fractures.Total urinary hydroxyproline excretion served as marker for bone resorption (BR) but was replaced by β-CrossLaps (CTX), a C-terminal collagen α-1(I) chain (COL1A1) telopeptide. We investigated the low-molecular-weight urinary proteome for peptides associated with changes in bone metabolism after kidney transplantation. Methods: Clinical and laboratory data including serum levels of CTX in 96 KTR from two nephrology centers were correlated with signal intensities of urinary peptides identified by capillary electrophoresis mass spectrometry. Results: Eighty-two urinary peptides were significantly correlated with serum CTX levels. COL1A1 was the predominant peptide source. Oral bisphosphonates were administered for decreased bone density in an independent group of 11 KTR and their effect was evaluated on the aforementioned peptides. Study of the peptides cleavage sites revealed a signature of Cathepsin K and MMP9. Seventeen of these peptides were significantly associated with bisphosphonate treatment, all showing a marked reduction in their excretion levels compared to baseline. Discussion: This study provides strong evidence for the presence of collagen peptides in the urine of KTR that are associated with BR and that are sensitive to bisphosphonate treatment. Their assessment might become a valuable tool to monitor bone status in KTR.

Identification of novel bone-resorption markers by osteoclastsin vitroandin vivo

“…Consistent with previous studies [3][4][5][6][7][8] , our observed bone peptide fragment products were rich in hydroxyproline residues and more frequently observed and at higher abundance in urine versus serum. The products spanned a wide range in molecular weight (174-1540 Da), from di-peptides to peptide fragments comprising 17 amino acid residues (Table 1).…”

“…Amongst the COL1A1 specific products, two overlapped with the sequences of previously reported urine COL1A1 fragments and one reproduced exactly another previously reported fragment. 5 Three of the COL1A1-specific resorption products are not previously described. Six of the seven highest molecular weight products did not map to known COL1A1 protease cleavage sites, potentially indicating new COL1A1 proteolytic degradation pathways.…”

“…The LC/MS technique used here is different to platforms used in other studies reporting collagen peptide fragments, which have frequently used capillary electrophoresis coupled to mass spectrometry (CE/MS). [4][5][6][7][8]23 Our LC/MS approach has been employed extensively in small molecule analyses including metabolomic applications and is possibly more optimal for detection of low, versus high, molecular-weight peptides. 12 This contrasts the proteomic approaches used in previous studies and shows the potential to detect low-medium molecular weight bone collagen fragments in serum and urine metabolomic datasets.…”

“…Multiple studies have shown that even in healthy subjects, human serum and urine contain a range of peptide products derived from collagens, particularly collagen I alpha-1 chain (COL1A1). [3][4][5][6][7][8] The variety of urine collagen fragments indicates that the protease cleavage sites in type I collagen I are extensive and not limited to the wellcharacterised Cathepsin K cleavage sites. 5 It is likely that some type I collagen fragments are products of other proteolytic enzymes.…”

“…[3][4][5][6][7][8] The variety of urine collagen fragments indicates that the protease cleavage sites in type I collagen I are extensive and not limited to the wellcharacterised Cathepsin K cleavage sites. 5 It is likely that some type I collagen fragments are products of other proteolytic enzymes. For example, a group of noncysteine protease enzymes expressed in bone and known to act on collagen is the matrix metalloproteinases (MMPs).…”