# Activation of the alternative complement pathway and its relevance for sodium retention in experimental nephrotic syndrome

## TL;DR

This study investigates the role of the alternative complement pathway in sodium retention during nephrotic syndrome in mice.

## Contribution

The study demonstrates that the alternative complement pathway is activated in nephrotic syndrome but does not prevent sodium retention.

## Key findings

- Components of the alternative complement pathway (C3, FB, FD) are activated in the kidneys of nephrotic mice.
- Deletion of C3, FB, or FD does not prevent proteolytic activation of ENaC or sodium retention in nephrotic syndrome.
- C3 staining is observed in protein casts and proximal tubules in nephrotic mice.

## Abstract

The complement component C3, factor B (FB) and factor D (FD) belong to the alternative complement pathway and have been identified in urine samples from nephrotic mice. However, it is not yet known whether these factors are involved in mediating sodium retention in nephrotic syndrome (NS). Here we used a genetic mouse model of NS based on an inducible podocin deletion (Nphs2Δipod). These mice were intercrossed with mice deficient for FB, FD or C3, yielding Nphs2Δipod*Cfb−/−, Nphs2Δipod*Cfd−/−
or Nphs2Δipod*C3−/− mice, respectively. NS was induced after oral doxycycline treatment for 14 days. C3, FB and FD were detected in the nephrotic urine of Nphs2Δipod mice as well as fragments of C3 and FB, indicating intrarenal activation of the alternative complement pathway. Lack of FB and FD had no impact on the activation of C3. Immunohistochemistry demonstrated positive C3 staining in protein casts and within the proximal tubule. Nephrotic mice of all genotypes experienced similar proteolytic activation of the epithelial sodium channel ENaC, developed sodium retention (urinary sodium concentration < 20 mM) and body weight gain. This was associated with a stimulation of proteolytic processing of epithelial sodium channel ENaC in all genotypes. In conclusion, components of the alternative complement pathway are detectable and activated in nephrotic syndrome. Mice with deletion of C3, FB or FD are not protected from proteolytic ENaC activation and sodium retention in NS.

The online version contains supplementary material available at 10.1007/s00424-025-03136-x.