Proteinuria: an enzymatic disease of the podocyte?

Abstract

Proteinuria is a major health-care problem that affects several hundred million people worldwide. Proteinuria is a cardinal sign and a prognostic marker of kidney disease, and also an independent risk factor for cardiovascular morbidity and mortality. Microalbuminuria is the earliest cue of renal complications of diabetes, obesity, and the metabolic syndrome. It can often progress to overt proteinuria that in 10-50% of patients is associated with the development of chronic kidney disease, ultimately requiring dialysis or transplantation. Therefore, reduction or prevention of proteinuria is highly desirable. Here we review recent novel insights into the pathogenesis and treatment of proteinuria, with a special emphasis on the emerging concept that proteinuria can result from enzymatic cleavage of essential regulators of podocyte actin dynamics by cytosolic cathepsin L (CatL), resulting in a motile podocyte phenotype. Finally, we describe signaling pathways controlling the podocyte actin cytoskeleton and motility and how these pathways can be manipulated for therapeutic benefit.

Figure 1. Podocyte structure in health and disease

(a) Left: the glomerulus contains a capillary tuft that receives primary structural support from the glomerular basement membrane (GBM). Glomerular endothelial cells (E) embracing the capillary lumen (CL), and mesangial cells (M) are located on the blood side of the GBM, whereas podocyte foot processes (FP) cover the outer aspect of the GBM. Podocyte cell bodies (CB) and major processes (MT) are floating in primary urine in Bowman’s space (BS). Along its route from the blood space to the urine space (red arrow), the plasma ultrafiltrate passes sequentially through the fenestrated glomerular capillary endothelium, the GBM, and the filtration slits between neighboring podocyte foot processes with the interposed slit diaphragms. AA, afferent arteriole; DT, distal tubule; EE, efferent arteriole; PT, proximal tubule. Middle: scanning electron microscopy (SEM) illustrates the complexity of podocyte morphology. Looking from Bowman’s space, a cell body (CB) is seen that is linked to the capillaries by major processes (MP). Podocyte foot processes (FP) arise from MP and form the signature interdigitating pattern with FPs of neighboring podocytes, leaving in between the filtration slits. Only FPs are in direct contact with the GBM. Right: transmission electron microscopy (TEM) view (top) of the glomerular filtration barrier consisting of fenestrated endothelium (E), GBM, and podocyte FP with the interposed slit diaphragm (SD) covering the filtration slits. Top view (bottom) of normal FPs. In healthy podocytes, FPs regularly interdigitate. The highly organized parallel contractile actin bundles of interdigitating FPs from two adjacent podocytes are shown in red and yellow. Microtubules of MPs are shown in blue. (b) Left: in nephrotic syndrome with proteinuria, FPs lose their normal interdigitating pattern and show effacement instead. Middle: the loss of the normal cytoarchitecture and the development of meandering cell borders can be best viewed by SEM. Right: effaced FPs develop a continuous band of cytoplasm containing a dense band of short branched actin filaments (*). Top view (bottom) of effaced podocyte FPs showing the loss of the replacement of the regular interdigitating pattern by a simplified meandering cell border between effaced FPs. A continuous sheet of cytoplasm develops that is filled with reorganized, short, branched actin filaments (green).

Figure 2. Consequences of podocyte injury

Podocytes can be injured in many human and experimental glomerular diseases, leading to structural changes, such as foot processes (FP) effacement and slit diaphragm (SD) disruption that are reversible. Persistence of podocyte injury can cause cell death or detachment of podocytes from the glomerular basement membrane (GBM). The resulting loss of podocyte will ultimately lead to irreversible glomerulosclerosis and end-stage renal failure (ESRD). The role of proteinuria in the progression of ESRD is a matter of debate. In some patients, nephrotic-range proteinuria can persist over years without progression to ESRD.

Figure 3. Induction of cathepsin L in podocytes precedes FP effacement and proteinuria

Upon an insult, stationary podocytes upregulate cytoplasm cytosolic cathepsin L expression and activity and develop motile podocyte foot processes (FPs). This migratory response leads to FP effacement, slit diaphragm remodeling, and proteinuria.

Figure 4. Integrative model for the regulation of podocyte actin dynamics in health and disease

Induction of cytoplasmic cytosolic cathepsin L (CatL) enzyme is a common downstream effector in many glomerular diseases. Lipopolysaccharide (LPS) or various other proximal signals induce the expression of B7-1 and CatL^, in podocytes, which cause proteinuria through the increased degradation of synaptopodin and dynamin. Phosphorylation of synaptopodin by PKA or CaMKII promotes 14-3-3 binding, which protects synaptopodin against CatL-mediated cleavage, thereby stabilizing synaptopodin steady-state levels. Synaptopodin suppresses IRSp53–Mena-mediated filopodia by blocking the binding of Cdc42 and Mena to IRSp53 and induces stress fibers by competitive blocking the Smurf-1-mediated ubiquitination of RhoA. Synaptopodin also prevents the CatL-mediated degradation of dynamin. Dephosphorylation of synaptopodin by calcineurin abrogates the interaction with 14-3-3. This renders the CatL cleavage sites of synaptopodin accessible and promotes the degradation of synaptopodin. LPS or other proximal signals can also activate Cdc42 and Rac1 through uPAR–β3-integrin signaling, through the loss of synaptopodin-mediated inhibition of Cdc42 signaling, or through Nef–Src-mediated activation of Rac1. As a consequence, the podocyte actin cytoskeleton shifts from a stationary to a motile phenotype, thereby causing foot process effacement and proteinuria. CsA and the CatL inhibitor E64 safeguard against proteinuria by stabilizing synaptopodin and dynamin steady-state protein levels in podocytes, FP(4)-Mito by blocking Cdcd42–IRSp53–Mena signaling, cycloRGDfV by blocking uPAR–β3-intergrin signaling, NSC23766 by blocking Rac1, and epleronone by blocking aldosterone signaling.