The glomerular filtration barrier: a structural target for novel kidney therapies

Abstract

Loss of normal kidney function affects more than 10% of the population and contributes to morbidity and mortality. Kidney diseases are currently treated with immunosuppressive agents, antihypertensives and diuretics with partial but limited success. Most kidney disease is characterized by breakdown of the glomerular filtration barrier (GFB). Specialized podocyte cells maintain the GFB, and structure-function experiments and studies of intercellular communication between the podocytes and other GFB cells, combined with advances from genetics and genomics, have laid the groundwork for a new generation of therapies that directly intervene at the GFB. These include inhibitors of apolipoprotein L1 (APOL1), short transient receptor potential channels (TRPCs), soluble fms-like tyrosine kinase 1 (sFLT1; also known as soluble vascular endothelial growth factor receptor 1), roundabout homologue 2 (ROBO2), endothelin receptor A, soluble urokinase plasminogen activator surface receptor (suPAR) and substrate intermediates for coenzyme Q10 (CoQ₁₀). These molecular targets converge on two key components of GFB biology: mitochondrial function and the actin-myosin contractile machinery. This Review discusses therapies and developments focused on maintaining GFB integrity, and the emerging questions in this evolving field.

Fig. 1. Introduction to the GFB.

Sequentially magnified schematic showing the nephron, the functional unit of the kidney, followed by the glomerulus, the primary interface between the microvascular circulation and the nephron. Each glomerular capillary loop forms a glomerular filtration barrier (GFB) comprising the glycocalyx and fenestrated glomerular endothelial cells (GECs), which block negatively charged molecules and plasma proteins; the glomerular basement membrane (GBM), which provides a physical scaffold that supports the GECs and podocytes, but contributes little directly to GFB function; and podocytes, which form primary, secondary and tertiary (foot) processes. Foot processes attach to the GBM by matrix tethering receptors including dystroglycans and integrins that, in turn, regulate the actin–myosin contractile apparatus to maintain foot process projections. Primary and secondary processes have an elaborate actin–myosin contractile apparatus and cross-link proteins running inside foot processes. The slit diaphragm, an ultrastructural molecular barrier, connects interdigitating foot processes, and contains NEPH1/2, P-cadherin, protocadherin FAT1, nephrin and roundabout homologue 2 (ROBO2). This molecular bridge forms part of the GFB and connects with the actin–myosin contractile apparatus via CD2-associated protein (CD2AP) and podocin. External short transient receptor potential channels (TRPCs) open in response to diverse stimuli, including mechanical forces, to release Ca²⁺ and regulate cellular responses. The function and localization of mitochondria in both podocytes and GECs, particularly in tertiary processes, is important in the maintenance of the GFB. Adapted from ref..

Fig. 2. Therapeutic approaches targeting the GFB in clinical or late preclinical development.

Glomerular filtration barrier (GFB), toxic mechanisms that affect podocytes and strategies to address podocyte dysfunction currently in development are shown. a | Apolipoprotein L1 (APOL1) inhibition. APOL1 genetic variants G1 and G2 accumulate in intracellular membranes and structures, particularly mitochondria (inset), where they form oligomers that can function as cation pores and leak cations, such as K⁺, across membranes. Oligomers also bind to mitochondrial inner membrane proteins, activating the mitochondrial permeability transition pore (MPTP), depolarizing mitochondria and causing mitochondrial dysfunction. VX-147 inhibits these toxic functions of APOL1. b | Coenzyme Q10 (CoQ₁₀) restoration. Mutations in genes encoding components of the CoQ₁₀ biosynthetic pathway reduce levels of CoQ₁₀, an electron acceptor of the inner membrane electron transport chain (inset), leading to mitochondrial dysfunction. Pharmacological delivery of CoQ₁₀ or 2,4-dihydroxybenzoic acid (2,4-diHB) bypasses the defects and restores CoQ₁₀ levels in the mitochondrial inner membrane. c | Short transient receptor potential channel (TRPC) inhibition. Mutant forms of TRPC6, overactivated TRPC6 or overactivated TRPC5 increase levels of intracellular Ca²⁺, which acts via calmodulin and calcineurin to activate the transcription factor nuclear factor of activated T cells (NFAT). Intracellular calcium activates RhoA (mainly via TRPC6) and RAC1 (mainly via TRPC5 and calcineurin). These signalling events remodel actin to form stress fibres and disrupt the normal actin–myosin contractile apparatus, resulting in foot process effacement, proteinuria and cell dysfunction. GFB-887 and AM-1473 inhibit TRPC5 and TRPC6, respectively. d | Soluble urokinase plasminogen activator surface receptor (suPAR) inhibition. Elevated levels of circulating suPAR, particularly a fragment containing the D2 and D3 domains, can bind to and activate αvβ3 integrin at focal adhesions on podocytes. This interaction activates focal adhesion kinase (FAK), which remodels actin to form stress fibres and disrupts the normal actin–myosin contractile apparatus, resulting in foot process effacement, proteinuria and cell dysfunction. Antibodies blocking suPAR from binding to αvβ3 integrin are one potential therapeutic approach when suPAR is the disease driver. CytC, cytochrome c; Dyn, dynamin; GBM, glomerular basement membrane; GEC, glomerular endothelial cell.

Fig. 3. Therapeutic approaches targeting podocyte–endothelial cell crosstalk in clinical or late preclinical development.

a | Glomerular capillary loops showing injury to glomerular endothelial cells (GECs) from toxins, cellular stress or depletion of podocyte-derived trophic factors. Such injury can result in endotheliosis (left), as observed in pre-eclampsia. Endothelial injury may also result in the release of GEC-derived pro-apoptotic factors (right) causing focal segmental glomerulosclerosis (FSGS). In both settings there is breakdown of the glomerular filtration barrier (GFB). Foot process (FP) fusion reduces the number of slit diaphragms, causing protein leakage and, ultimately, podocyte detachment. Strategies to rebalance podocyte trophic factors or inhibit endothelial cell-derived apoptotic factors are under evaluation. Adapted from ref.. b | Roundabout homologue 2 (ROBO2) inhibition. Endothelial cell-derived SLIT2 activates ROBO2 in podocytes to negatively regulate actin polymerization and non-muscle myosin activity, acting as a counterbalance to nephrin signalling. ROBO2 signals via the adaptor protein slit-robo rho GTPase activating protein 1 (SRGAP1) and protein intermediates NCK and CDC42. A ROBO2–crystallizable fragment (Fc) fusion protein (PF-06730512) blocks ROBO2 activation, enhancing nephrin function in disease states. c | Endothelin type A (ET_A) receptor inhibition. In response to podocyte stress or injury, ET_A receptor is upregulated on GECs and the ligand endothelin (ET1) is released from podocytes. ET_A receptor activation in adjacent GECs causes mitochondrial depolarization, resulting in damage from reactive oxygen species (ROS) and a reduction in nitric oxide synthase (NOS). These changes cause loss of GEC fenestrae and degradation of the glycocalyx. Such GECs release pro-apoptotic factors, which in turn cause podocyte stress fibre formation and, ultimately, cell depletion. d | Soluble fms-like tyrosine kinase 1 (sFLT1) inhibition. In pre-eclampsia, high levels of sFLT1, an endogenous inhibitor of vascular endothelial growth factor (VEGF) signalling, are released from the placenta into the circulation. Differentiation and maintenance of specialized functions of GECs relies on high levels of VEGF, released from podocytes, engaging VEGF receptor 2 (VEGFR2) on GECs. High concentrations of sFLT1 ‘mop up’ podocyte VEGF and block this tonic signal, causing GEC swelling and dysfunction. Although podocyte foot processes remain intact, the glycocalyx and fenestrae of GECs are disrupted, which is sufficient to compromise the GFB. A silencing small interfering RNA (siRNA) directed at the placenta reduces circulating levels of sFLT1 in the setting of pre-eclampsia. GBM, glomerular basement membrane; MRLC-NM myosin, non-muscle myosin regulatory light chain.