The glycocalyx plays a crucial role in vascular pathology. Damage to the glycocalyx is related to a variety of conditions, such as sepsis (29–31), renal failure (10, 32), and diabetes (13, 17, 31, 33, 34). Digestion of hyaluronan on glomerular endothelial cells by administration of hyaluronidase was recently shown to induce proteinuria ex vivo, which can exclude the influence of circulating mediators, and this has also been shown in vivo systems (15, 16). In vascular endothelial growth factor (VEGF) signaling inhibition–related glomerular diseases, such as thrombotic microangiopathy associated with anti-VEGF therapy or pre-eclampsia (35, 36), glomerular hyaluronan is diminished in association with proteinuria (15, 37, 38). Loss of hyaluronan in the glycocalyx also plays a role in the development of diabetic nephropathy (17, 33). These findings indicate that degradation of hyaluronan in the glomerulus leads to proteinuria and structural damage (15–17). These phenomena led us to hypothesize that loss of hyaluronan induces leakage of large molecules and protein into the dialysate, leading to a number of related complications in PD patients.
Only a few reports have been published to date concerning the expression of glycocalyx in the peritoneal membrane in PD (39). To the best of our knowledge, there are no reports describing the roles and expression of hyaluronan in the endothelial glycocalyx on the peritoneal membrane. The present study showed that hyaluronan in the glycocalyx on peritoneal vascular endothelial cells plays a crucial role in preventing leakage of protein into the dialysate (Figs. 1–3). Previously, it was difficult to identify the loss of hyaluronan and glycocalyx, which was judged by measuring thickness using sophisticated techniques (12, 40–42). The present study successfully demonstrated the presence of hyaluronan in the vascular endothelial surface layer using biotin-labeled HABP on paraffin-embedded sections. The present study also demonstrated that peritoneal permeability for large molecules, such as proteins, albumin, and β2MG, increases after degradation of hyaluronan but improves after recovery of hyaluronan (Figs. 1 and 3). Our data showed that hyaluronan itself plays an important role in preventing the leakage of macromolecules, including albumin and β2MG (Fig. 6). These phenomena are similar to proteinuria induced by degradation of hyaluronan in glomerular endothelial cell injury.
It is widely known that there are three sizes of pores between peritoneal vascular endothelial cells. Ultra-small pores (2.5 Å in diameter) are formed by aquaporin-1, which is involved in water passage. Small pores (diameter 45 Å) and large pores (diameter 250 Å) are involved in the passage of small solutes or large molecules (43) (Fig. 6). The number of large pores is very low, and the ratios of the pore areas of ultra-small, small, and large pores are reportedly 87.8%, 12.2%, and 0.03%, respectively (43). One function of peritoneal glycocalyx is thought to be regulation of the transport of macromolecules across the endothelial layer rather than small solutes (44). Our data indicate that hyaluronan regulates macromolecule transport across endothelial cells, as shown in Fig. 6. The glycocalyx does not appear to be a barrier to small molecules.
Previous research in PD primarily focused on mesothelial cell–derived hyaluronan. Hyaluronan is known to function as a lubricant, a component of the mesothelial glycocalyx, which protects the mesothelium from abrasion and adhesion (45, 46). Intraperitoneal administration of high-molecular-weight hyaluronan or PD solution containing hyaluronic acid has been attempted, but the results have been controversial, and there was no interpretation or discussion in reports of those studies regarding the role of hyaluronan in the glycocalyx on vascular endothelial cells (47, 48).
In this study, we measured the expression of hyaluronan in human peritoneal biopsy samples to investigate the possible role of hyaluronan in disease conditions involving protein leakage into PD fluid (Figs. 4 and 5). Low-GDP, pH-neutral PD solutions reportedly ameliorate structural and functional injuries to a greater degree than conventional solutions (18, 19, 49). We previously reported that severe vasculopathy involving damage to vascular endothelial cells can lead to leakage of plasma and fibrin, resulting in intestinal adhesions (18, 26). These effects are more pronounced with the use of conventional solutions, resulting in a high incidence of EPS (18, 26). We also recently reported that peritoneal vascular endothelial HS assessed using three antibodies that identify different domains of HS and fucose-containing sugar chains is better preserved during exposure to low-GDP, pH-neutral solutions than conventional solutions (39). The present human data indicate that loss of hyaluronan is more often observed in patients receiving conventional solutions. Our data support the hypothesis that degradation of the glycocalyx, which is more pronounced with conventional solutions, results in greater protein leakage, leading to the development of EPS (Supplementary Fig. S6).
In contrast, in peritonitis, hyaluronan is preserved in the vessels in human and animal CG models of peritonitis-related fibrosis (28) (Supplementary Figs. S4 and S5). A large amount of protein can leak into the dialysate during peritonitis. The altered permeability is reportedly due to endotoxin and complement activation, prostaglandins, IL-6, TNFα, and nitric oxide (50). Changes in hyaluronan were not observed, suggesting that chemical mediators are the main cause of protein loss in peritonitis.
The present study has some limitations. The number of patient samples involving EPS-neutral solutions was small, because the occurrence rate of EPS is very low in treatment with pH-neutral solutions in Japan (51). Previous studies have suggested that peritoneal membrane damage decreases after changing to low-GDP, pH-neutral solutions (18, 19, 49). All patients in this study were Japanese, so it may be difficult to generalize these data to other populations.
In summary, our data suggest that hyaluronan, a component of the glycocalyx, plays an important role in peritoneal membrane transport of macromolecules. Loss of hyaluronan in the glycocalyx may induce leakage of proteins and macromolecules, ultimately leading to the development of EPS (Supplementary Fig. S6).