As an integral part of the vascular system, the lymphatic vasculature is essential for tissue fluid homeostasis, nutritional lipid assimilation and immune regulation. The composition of the lymphatic vasculature includes fluid-absorbing initial lymphatic vessels (LVs), transporting collecting vessels and anti-regurgitation valves. Although, in recent decades, research has drastically enlightened our view of LVs, investigations of initial LVs, also known as lymphatic capillaries, have been stagnant due to technical limitations. In the kidney, the lymphatic vasculature mainly presents in the cortex, keeping the local balance of fluid, solutes and immune cells. The contribution of renal LVs to various forms of pathology, especially chronic kidney diseases, has been addressed in previous studies, however with diverging and inconclusive results. In this review, we discuss the most recent advances in the proliferation and permeability of lymphatic capillaries as well as their influencing factors. Novel technologies to visualize and measure LVs function are described. Then, we highlight the role of the lymphatic network in renal fibrosis and the crosstalk between kidney and other organs, such as gut and heart.

Lymphatic vessels are highly responsive to changes in the interstitial environment. Previously, we showed renal lymphatics express the Na-K-2Cl cotransporter. Since interstitial sodium retention is a hallmark of proteinuric injury, we examined whether renal sodium affects NKCC1 expression and the dynamic pumping function of renal lymphatic vessels. Puromycin aminonucleoside (PAN)-injected rats served as a model of proteinuric kidney injury. Sodium ²³Na/¹H-MRI was used to measure renal sodium and water content in live animals. Renal lymph, which reflects the interstitial composition, was collected, and the sodium analyzed. The contractile dynamics of isolated renal lymphatic vessels were studied in a perfusion chamber. Cultured lymphatic endothelial cells (LECs) were used to assess direct sodium effects on NKCC1. MRI showed elevation in renal sodium and water in PAN. In addition, renal lymph contained higher sodium, although the plasma sodium showed no difference between PAN and controls. High sodium decreased contractility of renal collecting lymphatic vessels. In LECs, high sodium reduced phosphorylated NKCC1 and SPAK, an upstream activating kinase of NKCC1, and eNOS, a downstream effector of lymphatic contractility. The NKCC1 inhibitor furosemide showed a weaker effect on ejection fraction in isolated renal lymphatics of PAN vs controls. High sodium within the renal interstitium following proteinuric injury is associated with impaired renal lymphatic pumping that may, in part, involve the SPAK-NKCC1-eNOS pathway, which may contribute to sodium retention and reduce lymphatic responsiveness to furosemide. We propose that this lymphatic vessel dysfunction is a novel mechanism of impaired interstitial clearance and edema in proteinuric kidney disease.

Lysyl oxidase (LOX) is a copper-dependent monoamine oxidase whose primary function is the covalent cross-linking of collagen and elastin in the extracellular matrix (ECM). However, the regulation of LOX activity in renal fibrosis is not well understood. Here, our study showed that (a) LOX expression and ECM cross-linking were markedly increased in fibrotic kidneys. Reduction of copper levels in the Golgi apparatus by treatment with the copper chelator tetrathiomolybdate or by specific knockdown of copper transporter 1 (CTR1) decreased LOX activity and ameliorated renal fibrosis. (b) Overexpression of ATP7A caused an elevation of copper ions within the Golgi apparatus, resulting in increased LOX activity and enhanced ECM crosslinking, thereby promoting the progression of renal fibrosis. Knockdown of ATP7A showed the opposite result. (c) FBLN4 was essential for the ATP7A-mediated transfer of copper to LOX and formed a ternary complex of ATP7A-FBLN4-LOX. Our research revealed that high ATP7A expression induced copper overload in the Golgi apparatuses. FBLN4 then assisted ATP7A in transporting this excess copper to LOX, resulting in LOX overactivation. This, in turn, catalyzed the cross-linking of ECM components, thereby accelerating renal fibrosis. ATP7A regulates LOX activity through the induction of intra-Golgi copper accumulation, and this process is involved in FBLN4-mediated the interaction of ATP7A with LOX

Although renal fibrosis is predominantly driven by the accumulated inflammatory cells that secrete proinflammatory factors within the kidney, the key mechanisms underlying macrophage clearance from the kidney are not well understood. The interaction of hyaluronan with lymphatic endothelial hyaluronan receptor 1 (LYVE1) constitutes a critical initial step in macrophage adhesion and removal by lymphatic vessels. This study investigates alterations in LYVE1 during kidney disease and elucidates its role in macrophage trafficking. Three renal fibrosis models demonstrated a reduction in full-length LYVE1 and an increase in the soluble LYVE1 fragment. Immunostaining of fibrotic kidneys showed significantly reduced expression of soluble LYVE1 compared with the intracellular fragment (Cyto-LYVE1), demonstrating ectodomain shedding of LYVE1 in vivo and in vitro. Functionally, human lymphatic endothelial cells exposed to TGF-β1 exhibited a significant decrease in macrophage adhesion and transendothelial migration compared with controls. Mechanistic analyses identified increased matrix metalloproteinase 9 (MMP9) in renal injury as a key upstream regulator of LYVE1 shedding. MMP9 inhibitors reduced LYVE1 shedding, enhanced macrophage adhesion and trafficking, and mitigated macrophage accumulation and disease progression. In conclusion, MMP9-induced LYVE1 shedding is linked to progressive kidney fibrosis and macrophage accumulation. LYVE1 shedding inhibitors offer potential as therapeutic agents for mitigating immune overload and kidney fibrosis.