Protein Uptake at Glomerular Level: Possible Involvement of an Endocytic Machinery in Cell Culture and in Patients with Lupus Nephritis

ClC-5 with megalin (LRP2), cubilin, Disabled 2 (Dab2), and Amnionless (AMN), is part of the molecular complex involved at proximal tubular level in the endocytic re-uptake of low-molecular-weight proteins and albumin. ClC-5, megalin and cubilin expression in podocytes of human renal biopsies was already reported. Moreover, it was demonstrated that podocytes are able to internalize albumin through an endocytic process. It is reasonable to assume that there should be an involvement of this system in protein uptake mediated by podocytes. A disruption of this system can lead to proteinuria which is one of the first manifestation of kidney disease in Systemic Lupus Erythematous (SLE). Aims of this study were to explore the presence of the tubular endocytic machinery components also in human podocytes in vitro and to evaluate whether albumin modulates this system. Moreover, we wanted to verify and quantify the expression of ClC-5, megalin and cubilin in both glomerular and tubular compartments in renal biopsies of patients presenting Lupus nephritis and observing the presence of a relationship with clinical data. To verify the presence of an uptake mechanism in human podocytes in vitro, we performed time lapse experiments with a low dose of FITC albumin (10 µg/ml). We observed albumin internalization starting from 2 to 15 hours. To evaluate the uptake kinetic, we stimulated podocytes at different time (30 min and 2 hours) and doses (10 µg/ml, 100 µg/ml and 1 mg/ml) at 37°C and 4°C. We observed a significant dose dependent increase in fluorescence vs controls after 2 hours’ stimulation with a typical receptor-mediated kinetic since it is inhibited at 4 °C. Moreover, we disclosed the presence of ClC-5, Dab2 and AMN beyond megalin and cubilin, in this in vitro system using immunohistochemistry (IHC) and immunofluorescence (IF) techniques, highlighting a co-localization between albumin and both receptors. To analyze whether the proteinuric environment modulates CLCN5, LRP2, CUBN, DAB2 and AMN expression, we stimulated human podocytes with increasing concentrations of BSA (range 10 µg/ml - 30 mg/ml) and we evaluated the mRNA expression at different time points (2, 4, 8, 24, 48 and 72 hours). Using Real Time PCR we observed a significant time and dose dependent increase in CLCN5, CUBN and AMN expression and an up-regulation of DAB2 only at 24 hours. We collected 23 SLE renal biopsies, 6 control biopsies and 1 case of Minimal Change Disease. As clinical parameters we considered proteinuria and pharmacological therapy. IHC and IF were used to analyze ClC-5, megalin and cubilin protein expression in serial sections. Morphometric quantification revealed a direct correlation between tubular and glomerular expression of all molecules in SLE patients, highlighting a relationship between glomerular and tubular compartments independently from proteinuria levels. Furthermore, preliminary data on patients without ACEi/ARB and immunosuppressive drugs disclosed a positive trend among these molecules at glomerular level. Interestingly, we revealed megalin and cubilin expression in hypertrophic PECs of some SLE patients and characterization experiments identified a subpopulation with an intermediate phenotype between mature and progenitor cells. In conclusion, for the first time we demonstrated that human podocytes are naturally committed to perform albumin endocytosis via a receptor-mediated mechanism. Moreover, protein overload upregulates CUBN, AMN and CLCN5 in these cells. Functional studies regarding the role of cubilin in albumin uptake underlined its participation in this mechanism even if this is not the only pathway involved. Further studies will be necessary to analyze which may be the partner(s) of the CUBAM complex in this mechanism. For the first time we demonstrated the presence of ClC-5, megalin and cubilin in glomeruli of patients with SLE and MCD in addition to controls confirming in vitro data. Furthermore, in SLE biopsies we highlighted a strong correlation between the two renal compartments in the expression of the protein uptake system, supporting the idea of a partnership between tubular and glomerular cells in albumin uptake via the same mechanism of internalization. In addition, preliminary data on patients without ACEi/ARB and immunosuppressive drugs lead us to suppose that the pharmacological therapy could affect the expression of this system, in particular at glomerular level. The differences observed from in vivo and in vitro data, especially on megalin expression, underline the involvement of other glomerular cell types in addition to podocytes in protein uptake. Finally, megalin and cubilin expression in PECs of SLE patients is a real interesting but complex data, since characterization experiments identified a subpopulation with an intermediate phenotype between mature and progenitor cells. Further studies will be performed to better characterize the role of these double-positive cells and their correlation with clinical data and/or disease progression.