Endocytosis mediated by an atypical CUBAM complex modulates slit diaphragm dynamics in nephrocytes

Abstract

The vertebrate endocytic receptor CUBAM, consisting of three cubilin monomers complexed with a single amnionless molecule, plays a major role in protein reabsorption in the renal proximal tubule. Here, we show that Drosophila CUBAM is a tripartite complex composed of Amnionless and two cubilin paralogues, Cubilin and Cubilin2, and that it is required for nephrocyte slit diaphragm (SD) dynamics. Loss of CUBAM-mediated endocytosis induces dramatic morphological changes in nephrocytes and promotes enlarged ingressions of the external membrane and SD mislocalisation. These phenotypes result in part from an imbalance between endocytosis, which is strongly impaired in CUBAM mutants, and exocytosis in these highly active cells. Of note, rescuing receptor-mediated endocytosis by Megalin/LRP2 or Rab5 expression only partially restores SD positioning in CUBAM mutants, suggesting a specific requirement of CUBAM in SD degradation and/or recycling. This finding and the reported expression of CUBAM in podocytes suggest a possible unexpected conserved role for this endocytic receptor in vertebrate SD remodelling.

Interaction of the sorting nexin 25 homologue Snazarus with Rab11 balances endocytic and secretory transport and maintains the ultrafiltration diaphragm in nephrocytes

“…In this experimental setup, excessive uptake manifests in an increase in these parameters compared with control cells ( Figure 1, J and N and Supplemental Figure S1, A and P), as seen in the case of Rab5 overexpression, a known enhancer of endocytic activity ( Bucci et al. , 1992 ; Atienza-Manuel et al. , 2021 ) that we used as a positive control ( Figure 1, K and N and Supplemental Figure S1, B and P).…”

Oxidative Stress Contributes to Slit Diaphragm Defects Caused by Disruption of Endocytosis

“… 13 , 14 The Cubn-Amn complex mediates protein uptake in renal proximal tubule cells, and is also critical to nephrocyte function. 38 , 39 , 40 It has been noted that the labyrinthine channels of fly nephrocytes, with their high levels of endocytosis and proximity to the slit diaphragm filtration barrier, bear functional similarity to proximal tubule cells of the vertebrate nephron. 40 …”

A protective role forDrosophilaFilamin in nephrocytes via Yorkie mediated hypertrophy

“…This finding suggests nephrocytes are not only a model for podocyte biology, but also for proximal tubular cells. However, in recent studies cubilin and amnionless have been identified to be expressed in mammalian podocytes and to be important for nephrocyte diaphragm remodelling in nephrocytes ( Prabakaran et al, 2012 ; Gianesello et al, 2017 ; Atienza-Manuel et al, 2021 ). Moreover, according to single nucleus RNA-seq in adult fly kidneys, nephrocytes do not show an overlap with murine proximal tubular cells, but with podocytes and parietal epithelium ( Xu et al, 2021 Preprint ).…”