A new role for the neuronal ubiquitin C‐terminal hydrolase‐L1 (UCH‐L1) in podocyte process formation and podocyte injury in human glomerulopathies

Abstract

Glomerular epithelial cell (podocyte) injury is characterized by foot process retraction, slit diaphragm reorganization, and degradation of podocyte-specific proteins. However, the mechanisms underlying podocyte injury are largely unknown. The ubiquitin C-terminal hydrolase-L1 (UCH-L1) is a key modulator of ubiquitin modification in neurons. Like neurons, UCH-L1 expression was associated with an undifferentiated status in cultured human podocytes, whereas differentiation and arborization decreased UCH-L1 and monoUb expression. Inhibition of UCH-L1 induced time and concentration-dependent process formation with alpha-actinin-4 distribution to the cell membrane and processes. An immunohistochemical approach was used to evaluate whether UCH-L1 expression was associated with podocyte injury in 15 different human glomerular diseases. Whereas normal kidneys expressed no UCH-L1 and little ubiquitin, a subset of human glomerulopathies associated with podocyte foot process effacement (membranous nephropathy, SLE class V, FSGS) de novo expressed UCH-L1 in podocyte cell bodies, nuclei, and processes. Interestingly, UCH-L1 expression correlated with podocyte ubiquitin content and internalization of the podocyte-specific proteins nephrin and alpha-actinin-4. In contrast, minimal change glomerulonephritis, a reversible disease, demonstrated minimal UCH-L1 and ubiquitin expression with intact alpha-actinin-4 but internalized nephrin. Glomerular kidney diseases typically not associated with foot process effacement (SLE class IV, ANCA+ necrotizing GN, amyloidosis, IgA nephritis) expressed intermediate to no UCH-L1 and ubiquitin. These studies show a role for UCH-L1 and ubiquitin modification in podocyte differentiation and injury.

Ubiquitin C-Terminal Hydrolase-L1 Activity Induces Polyubiquitin Accumulation in Podocytes and Increases Proteinuria in Rat Membranous Nephropathy

“…22 Ubiquitin accumulation in podocytes correlated with increased UCH-L1 expression in podocytes in biopsy specimens from patients with MN. In addition, ubiquitin aggregates were observed in podocytes in three of eight MN cases ( Figure 1, B and C).…”

“…A strong correlation was described between ubiquitin accumulation and expression of UCH-L1 in podocytes. 22 Therefore, it was hypothesized that UCH-L1 is regulated in podocyte injury during MN and that regulation of ubiquitin through UCH-L1 may affect protein degradation and be involved in podocyte injury during MN. During early disease, podocytes reacted to antibody-mediated injury by induction of the UPS, and lower levels of monoubiquitin were observed.…”

“…We demonstrated that in vitro UCH-L1 expression was the feature of an undifferentiated podocyte and that podocyte differentiation into an arborized phenotype requires UCH-L1 down-regulation. 22 The present study investigated UCH-L1 expression and its functional role for ubiquitin homeostasis and proteasomal degradation in MN in the rat and in cultured podocytes.…”

MicroRNA-193a Regulates the Transdifferentiation of Human Parietal Epithelial Cells toward a Podocyte Phenotype

“…14 Others have shown that isolated glomerular cells, referred to as PECs or renal progenitor cells, have the potential to increase the expression of podocyte markers when cultured in a specific induction medium containing vitamin D 3 and retinoic acid (VRAD medium). By culturing sieved rat glomeruli and selecting for Claudin-1, Benigni et al 21 isolated rat PECs, which expressed Synaptopodin when cultured in VRAD medium.…”

“…We could, furthermore, show that expression of podocyte proteins was negatively regulated through UCH-L1/the ubiquitin proteasome and the autophagosomal/lysosomal degradation system in an immortalized rat PEC cell line 13 and cultured human podocytes. 14 However, inhibition of neither UCH-L1 nor the proteasome was sufficient to induce a full PEC-to-podocyte conversion 13 (C. MeyerSchwesinger, unpublished observation). This suggests the existence of an additional layer of regulation, which could be performed by a microRNA (miR).…”