Podocyte-associated talin1 is critical for glomerular filtration barrier maintenance

Podocytes are specialized actin-rich epithelial cells that line the kidney glomerular filtration barrier. The interface between the podocyte and the glomerular basement membrane requires integrins, and defects in either α₃ or β₁ integrin, or the α₃β₁ ligand laminin result in nephrotic syndrome in murine models. The large cytoskeletal protein talin1 is not only pivotal for integrin activation, but also directly links integrins to the actin cytoskeleton. Here, we found that mice lacking talin1 specifically in podocytes display severe proteinuria, foot process effacement, and kidney failure. Loss of talin1 in podocytes caused only a modest reduction in β₁ integrin activation, podocyte cell adhesion, and cell spreading; however, the actin cytoskeleton of podocytes was profoundly altered by the loss of talin1. Evaluation of murine models of glomerular injury and patients with nephrotic syndrome revealed that calpain-induced talin1 cleavage in podocytes might promote pathogenesis of nephrotic syndrome. Furthermore, pharmacologic inhibition of calpain activity following glomerular injury substantially reduced talin1 cleavage, albuminuria, and foot process effacement. Collectively, these findings indicate that podocyte talin1 is critical for maintaining the integrity of the glomerular filtration barrier and provide insight into the pathogenesis of nephrotic syndrome.

The glomerular filtration barrier is composed of podocytes and fenestrated endothelial cells separated by a glomerular basement membrane (GBM), which together function to filter plasma to generate urine devoid of large molecular weight proteins. A breach of this barrier at any level can result in nephrotic syndrome, a severe kidney disease characterized by massive protein loss in the urine. Podocyte-GBM interactions are mediated by heterodimeric transmembrane integrin adhesion receptors that connect the extracellular matrix with the actin cytoskeleton. There are 24 distinct integrins, consisting of 1 of 18 α subunits and 1 of 8 β subunits. Integrins are classified depending on their ligand preferences and the principal integrin that mediates podocytes adhesion to the GBM is the laminin-binding integrin α₃β₁ (1). The importance of integrins in podocyte biology is underscored by the observation that in mice, podocyte-specific deletion of β₁ or α₃ integrin develops into fatal proteinuria (2–4). Furthermore, deletion of the tetraspanin CD151 that interacts with integrin α₃β₁ and the α₃β₁ ligand β₂ laminin results in nephrotic syndrome (4, 5). Moreover, activation of α_Vβ₃ integrin by podocytes or circulating ligands can cause specific glomerular diseases such as focal and segmental glomerulosclerosis (6). Integrin heterodimer clustering and the assembly of multiprotein cytoplasmic adhesion complexes termed focal adhesions are necessary for integrins to generate high-affinity adhesion or to relay intracellular signals (7). There are numerous proteins found within the focal adhesions, including proteins such as integrin-linked kinase (ILK), which has been demonstrated to play a critical role in podocyte development and function (8–10). Another critical protein required for integrin function found in focal adhesions is talin, a large 270 kDa (2541 aa) dimeric adaptor protein that has been demonstrated to be critical in linking integrins to the actin cytoskeleton (11). Talin is comprises an atypical N-terminal FERM domain (the talin head; THD) with 4 subdomains, F0, F1, F2, and F3, linked to a large C-terminal flexible rod (12, 13). The F3 subdomain adopts a phosphotyrosine-binding (PTB) domain fold that interacts with β integrin tails and is required to activate integrins (14). THD has also been demonstrated to interact with the type 1 isoform of phosphatidylinositol phosphate kinase γ (PIPKγ) (15–17) and phosphoinositides (18). Conversely, the talin rod comprises a series of amphipathic helical bundles, a number of which contain binding sites for integrins (19), actin (20), and cytoskeletal proteins such as vinculin (21). Cells lacking talin1 have reduced cell adhesion, spreading, and migration, indicating its fundamental importance in focal adhesion assembly (22). Moreover, talin1 can also undergo calpain-mediated proteolytic cleavage-liberating THD and rod-promoting focal adhesion turnover (23).

While previous work has established a role for both integrins and actin-regulating proteins in podocyte biology (24–30), the key link between these proteins remains undetermined. Consequently, the goal of this study was to address the role of talin1, a key integrin- and actin-binding protein, in the development and stability of the glomerular filtration barrier. To dissect its functional role, we generated a conditional podocyte-specific Tln1-KO mouse. We showed that loss of talin1 specifically in podocytes resulted in a dramatic disruption of the glomerular permeability barrier and depleting talin1 in podocytes resulted in a striking disorganization of the actin network. We further demonstrated that podocyte injury in mouse models of glomerular disease revealed an unexpected cleavage of talin1 in the glomerulus due to an increase in the activity of calpain, a calcium-activated cysteine protease. In addition, we determined that in human samples from patients with focal segmental glomerulosclerosis (FSGS) or minimal change disease (MCD), there was a marked increase in calpain activity in the urine, akin to what was observed in our mouse models of glomerular injury. Finally, using a rabbit-anti-mouse GBM (NTS) model of glomerular injury, we showed that treatment with calpain inhibitors resulted in a reduction in proteinuria and foot process effacement. Collectively, our results not only implicate talin1 as a critical component for the development and maintenance of podocytes by linking integrins to actin, but also suggest that calpain-mediated talin1 cleavage may play a vital role in the pathogenesis of nephrotic syndrome following insult to the glomerular filtration barrier.

Generation of podocyte-specific Tln1-KO mice. As a first step toward investigating the role of talin1 in the glomerulus, we used immunogold electron microscopy (EM) to determine its localization. Talin1 localized primarily at the basal aspect of the podocytes (Figure 1A). Furthermore, in a subset of podocytes, talin1 was also found clustered at the base of the podocytes adjacent to the slit diaphragm (Figure 1A). Global loss of the Tln1 gene is embryonic lethal (31), so to study the role of talin1 in podocyte biology, we generated a podocyte-specific Tln1-KO using the Cre-Lox system based on podocin-Cre, which drives Cre expression at E13–E14 (32). Compound Tln1 heterozygote mice were mated to generate podocyte-specific Tln1-KO mice (Pod-Tln1–KO) (Figure 1B). Pod-Tln1–KO, Pod-Tln1^fl/+, and Tln1^fl/fl mice (used as controls) were born according to the expected Mendelian frequency as identified by tail genotyping (Figure 1C). Western blot analysis and immunofluorescence revealed markedly diminished expression of talin1 in the Dynabeads-extracted podocyte-enriched cell fractions obtained from the Pod-Tln1–KO mice (Figure 1, D and E), but no difference was observed in expression of talin2 in the podocytes (Figure 1D and Supplemental Figure 1, A and B; supplemental material available online with this article; doi: 10.1172/JCI69778DS1). Moreover, loss of podocyte talin1 immunoreactivity was also demonstrated in the Pod-Tln1–KO kidney sections costained with the podocyte-specific marker nephrin (Figure 1F).

Figure 1

Generation of podocyte-specific Tln1-KO mice. (A) Immunogold transmission EM staining demonstrates talin1 (arrowheads) localizes to the base of the foot process (fp) and adjacent to the slit diaphragm (SD) (arrows) of podocytes from WT mouse kidneys. Scale bars: 100 nm. (B) Schematic demonstrating breeding of the Podocin-Cre mice with Tln1^fl/fl mice to generate podocyte-specific Tln1-KO mice (Pod-Tln1–KO). Forward and reverse primers are denoted as a and b, respectively, for Cre and Tln1. (C) Identification of Tln1 and Podocin-Cre by tail genotyping (age P7). (D and E) Talin1 expression in purified control (Ctrl) podocytes and lack of talin1 expression in podocytes harvested from Pod-Tln1–KO mice (age P7), as detected by Western blotting. (D) and immunofluorescence (E). Note that Ctrl podocytes express both talin1 and talin2. Podocytes were plated on collagen type I–coated glass coverslips and stained for WT1 (red) and for talin1 using an isoform-specific talin1 monoclonal Ab. Scale bars: 10 μm. (F) Double-immunofluorescence detection of nephrin (red) and talin1 (green) on kidney sections of the indicated genotypes (age P14). Scale bars: 10 μm (upper panels) 2 μm (lower panels).

Loss of podocyte talin1 results in proteinuria and kidney failure. Pod-Tln1–KO mice appeared normal at birth, but by 4 weeks of age, their weight gain fell below that of Tln1^fl/fl littermate controls. By 8 weeks of age, the animals were smaller, hunched, and severely lethargic (Figure 2, A and B), with death resulting in more than 70% of mice by 8 weeks of age (Figure 2C). As vertebrates contain 2 highly conserved talin isoforms (talin1 and talin2), we also examined Pod-Tln1–KO mice crossed with constitutive Tln2-KO mice (33), Intriguingly, ablation of podocyte talin1 in mice lacking talin2 (33), which is also expressed in podocytes (Figure 1D and Supplemental Figure 1, A and B), did not significantly change the Pod-Tln1–KO phenotype (data not shown), suggesting that talin1 function most likely predominates in normal podocyte physiology. Urine analysis on Pod-Tln1–KO mice using SDS-PAGE followed by Coomassie blue staining demonstrated robust albuminuria (Figure 2D) beginning as early as postnatal week 1 and continuing to progress, as validated by ELISA, with normalization to urine creatinine (Figure 2E). Furthermore, plasma creatinine measurements in Pod-Tln1–KO mice demonstrated severe kidney failure when compared with littermate controls (Figure 2G), while the harvested kidney took on a shrunken and corrugated appearance at 8 weeks of age (Figure 2F).

Figure 2

Loss of talin1 in podocytes results in severe proteinuria and kidney failure. (A) Representative images of control and Pod-Tln1–KO mice at 8 weeks of age. (B) Pod-Tln1–KO mice, relative to controls, fail to gain weight by 4 weeks of age. n = 10 mice. *P < 0.01; **P < 0.001. (C) Survival curve of Pod-Tln1–KO mice demonstrates greater than 70% death by 8 weeks of age. n = 10 mice. (D) SDS-PAGE (Coomassie blue staining) of standard BSA (numbers indicate μg) and of urine from Pod-Tln1–KO mice, demonstrating albuminuria at 4 and 8 weeks of age. Equal volumes of standard BSA and urine (2 μl) were loaded in each lane. (E) Quantification of urinary albumin normalized to creatinine at 1, 2, 4, 6, and 8 weeks of age. n = 8 mice. (F) Kidneys of Pod-Tln1–KO mice are paler and smaller than those of controls and have a corrugated appearance at 8 weeks of age. (G) Elevated plasma creatinine in Pod-Tln1–KO mice at 4, 6, and 8 weeks of age. n = 5 mice. **P < 0.001.

Loss of podocyte talin1 results in proteinuria and foot process effacement. Histological examination of Pod-Tln1–KO kidneys revealed normal features at birth. However, by 2 weeks of age, approximately 15% of glomeruli revealed severe glomerular capillary dilatation (Figure 3A; quantified in Figure 3B). By 8 weeks of age, the glomeruli had undergone a process of global sclerosis (Figure 3A; quantified in Figure 3C) and the kidney further demonstrated severe interstitial fibrosis, tubular dilatation, and proteinaceous casts (Figure 3D; quantified in Figure 3E). To further determine the ultrastructural characteristics of the glomerulus, both scanning and transmission EM were performed on Pod-Tln1–KO and control kidneys. In Pod-Tln1–KO mice 10 days following birth, scanning EM analysis demonstrated a dramatic loss of podocyte foot process interdigitations and destruction of the major processes (Figure 4A). Interestingly, the transmission EM of glomeruli appeared normal at birth (P1) (Figure 4B), but by postnatal week 2, there was significant foot process effacement (Figure 4B; quantified in Figure 4D), validating our findings observed from the scanning EM. By 4 weeks of age, the architecture of the podocyte-endothelial interface was strikingly different from the classical architecture observed in age-matched controls (Figure 4B). In the Pod-Tln1–KO mice, there was a profound basement membrane splitting (Figure 4C), and by 8 weeks of age, thickening of the basement membrane and mesangial expansion were observed (Figure 4B; quantified in Figure 4E).

Figure 3

Podocyte-specific loss of talin1 results in glomerular capillary loop dilation with progressive glomerulosclerosis (scarring and fibrosis of the glomerulus) and interstitial fibrosis. (A) Representative light microscopy images (H&E, PAS, and trichrome) of glomeruli from Pod-Tln1–KO mice revealing histological evidence of dilated glomerular capillary loops at 2 weeks of age (arrowheads), which progresses to diffuse glomerulosclerosis by 8 weeks. Scale bar: 25 μm. (B) Quantification as percentage of glomeruli, with capillary loop dilation at 2 weeks of age, in Pod-Tln1–KO mouse kidney sections when compared with control. n = 3 mice. (C) Quantification of glomerulosclerosis at 2, 4, and 8 weeks of age. n = 4 mice (D) Proteinaceous casts (arrowhead), dilated tubules, and severe interstitial fibrosis are observed in Pod-Tln1–KO mice at 8 weeks of age. Scale bar: 50 μm. (E) Quantification of interstitial fibrosis at 2, 4, and 8 weeks of age. n = 4 mice. *P < 0.001.

Figure 4

Podocyte-specific loss of talin1 results in foot process effacement. (A) Scanning electron micrographs illustrating the ultrastructure of glomerulus of control and Pod-Tln1–KO mice at P10 and at 6 weeks. Arrowheads demonstrate loss of foot process interdigitations. Scale bars: 5 μm (left panels); 1 μm (right panels). (B) Transmission electron micrographs illustrating foot process effacement by 2 weeks of age (P1, P14, 4 weeks, and 8 weeks). Scale bar: 1 μm. The red arrows depict foot process effacement while the black arrow depicts mesangial expansion. (C) High-magnification micrographs from control and mutant glomeruli showing thickening and splitting of the basement membrane (4 weeks after birth), as indicated by the arrowheads. Scale bar: 200 nm. Note the effacement (podocyte collapsing on its basement membrane in the Pod-Tln1–KO mice). (D) Quantification of the number of foot processes per micrometer of GBM in the control and Pod-Tln1–KO mice at various ages. n = 2 mice at each time point. (E) Quantification of GBM thickness in the control and Pod-Tln1–KO mice at various ages. n = 2 mice at each time point.

Loss of podocyte talin1 results in a modest reduction in β₁ integrin activation and cell spreading. Given that the Pod-Tln1–KO mice demonstrated severe proteinuria as early as 1 week of age, we decided to examine the possibility that podocyte loss might occur in the Pod-Tln1–KO mice as a result of cell detachment and therefore generate defects in the glomerular filtration barrier. Compared with those from littermate control (Tln1^fl/fl) mice, kidney sections from P14 Pod-Tln1–KO mice, stained with the podocyte-specific transcription factor WT1, revealed similar podocyte numbers (Figure 5A; quantified in Figure 5B). Further validating our results, TUNEL staining demonstrated no evidence of apoptosis occurring within the Pod-Tln1–KO mice glomeruli, although increased apoptosis within the tubular segments was observed (Figure 5C). Talin1 binding to the cytoplasmic tail of the β integrin subunit is important for integrin activation (34, 35). Furthermore, as the podocyte-specific loss of β₁ integrin results in proteinuria, next we explored β₁ integrin expression and activation. No significant differences were observed in the expression levels of either β₁ or α₃ integrin from enriched primary podocytes harvested from Tln1^fl/fl and Pod-Tln1–KO mice (Figure 5D). We also detected no differences in the β₁ or α₃ integrin localization within the glomerulus and podocytes following immunostaining (Supplemental Figure 2, A and B). Moreover, podocytes from Pod-Tln1–KO mice showed only a moderate, albeit significant decrease in β₁ integrin activation as measured by binding of the activation-sensitive 9EG7 Ab (Figure 5E; quantified in Figure 5F). To analyze the effects of Tln1-KO on podocyte cell adhesion and spreading, primary podocytes isolated from control and mutant mice were seeded on plates coated with either fibronectin, laminin, or collagen type I. There was again a modest impairment in adhesion to laminin, but not to fibronectin or collagen type I (Figure 5G). Moreover, a modest reduction in cell spreading was observed (Figure 5H; quantified in Figure 5I) in the isolated mutant podocytes plated on laminin when compared with control but not to fibronectin or collagen type I (Supplemental Figure 2, C and D). However, there was no difference in the time course of integrin-dependent activation of pERK, pAKT, and p38MAPK at 10, 30, and 60 minutes on laminin as determined by Western blotting of podocytes isolated from Pod-Tln1–KO and Tln1^fl/fl mice (data not shown). The above results were further confirmed using WT podocytes exposed to lentivirus containing shRNAmir-Tln1 (Supplemental Figure 3A); Talin1 knockdown caused only a modest though significant reduction in cell spreading, supporting our results observed in the Pod-Tln1–KO podocytes (Supplemental Figure 3B; quantified in Figure 3C).

Figure 5

Lack of talin1 in podocytes results in reduced integrin activation, adhesion, and cell spreading. (A) Representative images of kidney sections from control and Pod-Tln1–KO mouse kidneys stained with WT1, a podocyte-specific transcription factor, at 2 weeks of age. Scale bar: 10 μm. (B) Quantification of WT1 staining per glomerulus in A. n = 6 experiments. (C) TUNEL staining of kidney sections from control and Pod-Tln1–KO mice at 2 weeks of age. G, glomerulus. Scale bar: 20 μm. (D) Primary podocytes isolated from control and Pod-Tln1–KO kidneys immunoblotted for β₁ integrin, α₃ integrin, and GAPDH. (E) Cell-surface expression of integrins on freshly isolated primary podocytes from control and Pod-Tln1–KO mice analyzed by FACS by staining with Abs against total β₁ integrin and 9EG7 epitope of active β₁ integrin. Pod-Tln1–KO podocytes are denoted in green, and control podocytes are denoted in red histograms. (F) Quantification of E expressed as percentage of β₁ integrin activation per total β₁ integrin. n = 4 experiments. (G) Primary podocytes isolated from control and Pod-Tln1–KO mice demonstrate a small but significant decrease in adhesion to laminin (LN) and not to fibronectin (FN) or collagen type I (Col I). n = 4 experiments. (H) Representative image of control and Pod-Tln1–KO podocytes plated on laminin and monitored by live cell imaging for 120 minutes. (I) Quantification of cell area was performed on 50 random cells in each experiment. n = 3 experiments. Scare bar: 20 μm. *P < 0.01.

Loss of podocyte talin1 results in defects of the actin cytoskeleton. To gain further insight into the function of talin1 in podocytes, we initially evaluated the localization of focal adhesions as previously, the loss of talin1 in cells has been shown to result in a mislocalization and a reduction of expression of focal adhesions (36). Thus we stained mouse primary podocytes for FAK and paxillin, 2 proteins localized at focal adhesions. However, the localization and expression of FAK and paxillin in the Tln1^fl/fl and Pod-Tln1–KO were very similar (Figure 6A; quantified in Figure 6B). Talin1 has also been demonstrated to interact with PIPKγ, the major phosphatidylinositol-4,5-bisphosphate–synthesizing ([PI4,5P2]-synthesizing) enzyme (15). This interaction has been suggested to play an integral role in clathrin-mediated endocytosis (37), and we have shown through genetic mouse models of disease that perturbation of endocytic proteins results in severe proteinuria (38). To determine whether endocytic defects exist in the Pod-Tln1–KO podocytes, an internalization assay of nephrin was performed. Podocytes were transfected with a protein chimera comprising the transmembrane and cytoplasmic domain of nephrin and the extracellular domain of CD8. Using an anti-CD8 Ab–based internalization assay, we found that the internalization of this probe was not significantly delayed in the Pod-Tln1–KO podocytes when compared with control (Figure 6C; quantified in Figure 6D). Given that talin1 has also been demonstrated to interact with and regulate the actin cytoskeleton (7), we explored the actin architecture in primary podocytes isolated from Tln1^fl/fl and Pod-Tln1–KO mice by staining with conjugated phalloidin-Alexa Fluor 488. Podocytes isolated from the Pod-Tln1–KO mice revealed a striking reduction in actin stress fiber formation, with an accentuated cortical localization in both confluent and nonconfluent cells (Figure 6, E and G; quantified in Figure 6F). To investigate whether the talin1 N-terminal head or the C-terminal rod plays an important role in the actin cytoskeleton in podocytes, GFP-Tln1 full-length (residues 1–2541), GFP-Tln1 head (residues 1–435), or GFP-Tln1 rod (residues 435–2541) was reexpressed in Pod-Tln1–KO primary podocytes. As expected, the actin cytoskeleton defects in Pod-Tln1–KO primary podocytes were rescued by expressing GFP-Tln1 full-length talin1, although KO cells expressing either GFP-Tln1 head or GFP-Tln1 rod alone had partial effects, suggesting that both the N- and C-terminal regions of talin1 are required for actin stress fiber formation in podocytes (Figure 7, A–C; quantified in Figure 7D). We further utilized fluorescently labeled mCh-Utrophin, a marker for F-actin (39), to visualize in real time the actin dynamics of control and Tln1-KO primary podocytes. Results recapitulated the phalloidin-staining pattern with a severe reduction in stress fibers in Tln1-KO (Supplemental Videos 1 and 2).

Figure 6

Actin stress fibers are reduced in Pod-Tln1–KO podocytes. (A) Representative images of paxillin and FAK in focal adhesions in isolated podocytes from control and Pod-Tln1–KO mice. Scale bar: 10 μm. (B) Quantification of focal adhesions represented as focal adhesions/1000 μm² of podocyte cell area in control and Pod-Tln1–KO podocytes. 25 cells were analyzed in each experiment for control and Pod-Tln1–KO podocytes. n = 3 experiments. (C) Representative image of internalized (red) versus cell surface CD8-nephrin chimera in control and Tln1-KO podocytes at 60 minutes. Scale bar: 10 μm; 3 μm (inset). (D) Quantification of the data shown in C. (n = 3 experiments, 25 cells evaluated per experiment). (E) Representative images of different types of phalloidin staining patterns observed in isolated control podocytes. Scale bar: 10 μm. (F) Quantification of phalloidin staining in control and Pod-Tln1–KO podocytes scored by an observer blind to the genotype. The Pod-Tln1–KO podocytes had fewer cells with types A (>90% of cell area filled with thick cables) and B (at least 2 thick cables running under nucleus and rest of cell area filled with fine cables) staining patterns and more in the type C (no thick cables, but some cables present) and type D (no cables visible in the central area of the cell) pattern when compared with control podocytes. n = 3 experiments. *P < 0.001. (G) Representative image of confluent control and Pod-Tln1–KO podocytes stained with phalloidin and WT1. Scale bar: 30 μm.

Figure 7

Reexpression of full-length GFP-Tln1 in Pod-Tln1–KO podocytes results in stress fiber reconstitution. (A–C) Representative images of phalloidin staining (red) following reexpression of GFP-Tln1 full-length (residues 1–2541) (A), GFP-Tln1 head (residues 1–435) (B), and GFP-Tln1 rod (residues 435–2541) (C) in Pod-Tln1–KO podocytes. Note loss of stress fibers and prominent cortical actin staining in untransfected Pod-Tln1–KO podocytes (arrowhead). Scale bar: 10 μm. (D) Quantification of phalloidin staining in podocyte-specific Tln1–KO podocytes following reexpression of the different GFP-Tln1 constructs indicated in A–C. n = 4 experiments *P < 0.01, comparing GFP-Tln1 full-length vs. GFP-Tln1 head, GFP-Tln1 rod, or Pod-Tln1–KO podocytes (untransfected); **P < 0.01, GFP-Tln1 head or GFP-Tln1 rod vs. untransfected podocytes.

Podocyte injury results in calpain-induced talin1 proteolysis. We next sought to determine the role of talin1 following podocyte injury in an intact organism. Therefore, we perfused mice with protamine sulfate, an agent that has been shown to induce podocyte foot process effacement (40). Compared with Tln1^fl/fl mice glomeruli perfused with HBSS, glomeruli isolated from mice treated with protamine sulfate consistently demonstrated an unexpectedly striking talin1 cleavage product (Figure 8A; quantified in Supplemental Figure 4A). To further explore whether talin1 cleavage within the kidney occurred following glomerular injury, we used another model where NTS was injected in mice, resulting in GBM immunoglobulin deposition (Figure 8B), proteinuria, and foot process effacement (data not shown). Mice treated with NTS also demonstrated a robust talin1 proteolysis (Figure 8C; quantified in Supplemental Figure 4B). Recent evidence suggests that proteinuric diseases may result from enhanced activity of proteolytic enzymes such as cathespin in podocytes (41). Given that talin1 proteolysis occurs following glomerular injury, we questioned whether this cleavage was due to increased calpain activity (23, 42). Glomeruli isolated from mice treated with protamine sulfate or NTS revealed a significant increase in calpain activity when compared with control (Figure 8, D and E). Such pathological differences were present despite equal levels of total calpain as demonstrated by Western blot analysis of both experimental and control glomeruli (data not shown). Moreover, urine samples collected from the NTS-injected mice also revealed an increase in calpain activity (Figure 8F) along with elevated levels of albuminuria (data not shown). To determine whether podocytes were the source of the increased calpain activity, isolated primary podocytes were incubated with or without protamine sulfate. A dose-dependent activation of calpain and concomitant talin1 cleavage (Figure 8, G and H) was observed, which was abrogated by pretreatment with the calpain inhibitor calpeptin (Figure 8H). Previous reports have suggested that immortalized proximal tubular cells treated with albumin induce calpain activity (43). These results were confirmed by our in vitro cell culture experiments on isolated primary proximal tubular cells treated with BSA at various concentrations (Supplemental Figure 4C). However, to determine whether calpain activity was increased in vivo, primary proximal tubular cells in NTS-treated mice were isolated using a Percoll gradient (validated by γGT enzyme activity and by immunoblotting for proximal tubular marker, megalin; Supplemental Figure 4, D and E). However, the proximal tubules isolated following NTS injection in mice demonstrated neither talin1 cleavage (Figure 8I) nor increased calpain activity (Figure 8J). This suggests that podocytes may play an important role in calpain secretion and cleavage of talin1 following injury by NTS. Furthermore, calpain-induced cleavage of other potential substrates, nephrin, FAK, vinculin, dynamin1, paxillin, vimentin, and β-catenin (44–49) in glomeruli, were not observed following infusion of protamine sulfate or injection of NTS (Supplemental Figure 4, F–I). This suggests that calpain-induced cleavage of talin1 is likely critical in the pathogenesis of proteinuria, although other potential calpain substrates may also participate in this process. To determine the potential effect of calpain-induced talin1 cleavage on the podocyte actin architecture, phalloidin staining was performed on podocytes incubated with protamine sulfate. Actin rearrangement along with the loss of stress fiber formation was observed (Figure 9A; quantified in Figure 9B). However, expression of a calpain cleavage–resistant talin1 mutant (talin L432G) in primary podocytes resulted in the maintenance of stress fibers following treatment with protamine sulfate (Figure 9C; quantified in Figure 9D). This result highlights the importance of calpain-mediated talin1 cleavage as one of the key determinants of the actin cytoarchitecture in podocytes. To better understand whether alterations in calpain activity might play a role in the pathophysiology of human glomerular disease, we turned our attention to human subjects with biopsy-proven glomerular disease FSGS or MCD. Similar to what was observed in our mouse models of glomerular injury, a robust increase in calpain activity was noted in urine samples from patients with FSGS or MCD when compared with control (Figure 9E).

Figure 8

Calpain-induced talin1 cleavage in injured podocytes. (A) Glomerular lysate isolated following protamine sulfate (PS) infusion into mice demonstrates talin1 cleavage (mouse anti-talin C terminus, clone TD77; Millipore). (B) Injection of NTS in mice demonstrates rabbit IgG deposition in the GBM. Scale bar: 10 μm. Alexa Fluor goat anti rabbit denoted by gαr. Alexa Fluor goat anti-mouse, denoted by gαm, demonstrates no signal. (C) Talin1 cleavage is observed in glomeruli following injection of NTS in mice. (D) Glomerular calpain activity is increased following protamine sulfate infusion, n = 7 experiments. (E) Glomeruli isolated from mice injected with NTS exhibit increased calpain activity, n = 5 experiments. (F) Increased urinary calpain activity is observed following NTS stimulation. n = 10 experiments. (G) Supernatant obtained from primary podocytes treated with protamine sulfate exhibit increased calpain activity. n = 3 experiments. (H) Podocytes stimulated with protamine sulfate (200 μg/ml) demonstrate increased talin1 and spectrin cleavage. (I) Isolated primary proximal tubules show no increase in talin1 cleavage following NTS injection in mice. (J) Isolated primary proximal tubules show no increase in calpain activity following NTS injection in mice. n = 4. *P < 0.05; **P < 0.01.

Figure 9

Increased calpain activity results in actin-cytoskeletal disorganization. (A) Representative image of podocyte phalloidin staining following stimulation with protamine sulfate with or without calpeptin. Scale bar: 10 μm. (B) Quantification of the phalloidin staining pattern. n = 4 experiments. *P < 0.05, control or protamine sulfate plus calpeptin vs. protamine sulfate alone. (C) Representative image of primary podocytes expressing the calpain-resistant talin1 mutant (GFP-Talin L432G) stained with phalloidin following stimulation with protamine sulfate. Note loss of stress fiber in neighboring untransfected WT control (arrow). Scale bar: 10 μm.(D) Quantification of phalloidin staining pattern of C. n = 3 experiments. (E) Increased urinary calpain activity is observed in patients with biopsy-proven FSGS and MCD when compared with healthy controls. n = 17 patients with FSGS, 15 patients with MCD, and 6 healthy controls. *P < 0.05; **P < 0.01.

Inhibiting calpain activity reduces glomerular injury and talin1 cleavage. To examine the role of the NTS-induced enhancement of calpain activity in the glomerulus, we next determined the expression of the endogenous calpain inhibitor calpastatin. A significant reduction of calpastatin expression in the glomerulus was observed following NTS injection (Figure 10A), which was not seen in the proximal tubules (Supplemental Figure 4J). As the increase in calpain activity may partly be due to the decrease in the expression of the endogenous inhibitor, we next sought to determine the difference in severity of glomerular injury by inhibiting calpain activity exogenously. Mice treated with calpain inhibitor at the time of NTS injection showed a significant reduction in calpain activity (Figure 10B) and talin1 cleavage in the glomerulus (Figure 10C; quantified in Figure 10D). Furthermore, urine analysis also demonstrated a significant reduction in albuminuria in mice that were treated with calpain inhibitor (Figure 10E). In addition, the evaluation of the podocyte ultrastructure by EM revealed a reduction in podocyte foot process effacement (Figure 10F; quantified in Figure 10G). Lastly, we returned to our Pod-Tln1–KO mice to evaluate whether calpain activity was increased in these nephrotic mice. We also found a significant increase in calpain activity in the urine and glomerulus (Figure 10, H and I). However, inhibiting calpain activity by treatment of the Pod-Tln1–KO mice with calpain inhibitor for 3 weeks failed to rescue the albuminuria in the mutant mice (Figure 10J) These results are consistent with talin1 being a key substrate of calpain following glomerular injury and with talin1 cleavage in the glomerulus likely playing a pivotal role in disrupting the integrity of the glomerular filtration barrier.

Figure 10

Treatment with calpain inhibitor reduces talin1 cleavage and glomerular injury following NTS treatment. (A) NTS injection in mice reduces calpastatin expression in podocytes. (B) Glomerulus isolated at day 7 following NTS-injected mice treatment with calpain inhibitor (CI) reduces calpain activity. (C) Talin1 cleavage is prevented in NTS-injected mice treated with calpain inhibitor at day 7 after treatment. (D) Quantification of talin1 cleavage in C. (E) Albuminuria is reduced in mice treated with calpain inhibitor at day 7 following NTS injection. (F) Transmission electron micrographs illustrating reduced foot process effacement at 7 days in mice treated with calpain inhibitor following NTS injection. Scale bar: 1 μm. (G) Quantification of F. (H). Urine isolated from Pod-Tln1–KO mice exhibits increased calpain activity. n = 4 experiments. (I) Glomeruli isolated from Pod-Tln1–KO mice exhibit increased calpain activity. n = 4 experiments. (J) Albuminuria is not reduced in Pod-Tln1–KO mice following treatment with calpain inhibitor for 3 weeks. n = 4 mice. *P < 0.05.

In this study, we demonstrate the importance of talin1 in both the development and maintenance of the glomerular filtration barrier and report that talin1 is cleaved by calpain following podocyte injury resulting in ne phrotic syndrome. Precisely how talin1 contributes to the formation and maintenance of podocyte foot processes, a key element of the permeability barrier in kidney, remains to be established. Given that talin1 activates integrins (50) and is a key link between integrins and the actin cytoskeleton, one plausible mechanism is that talin1 is required for integrin-dependent tight adhesion of podocytes to the basement membrane, allowing them to withstand the glomerular transcapillary filtration pressure. Furthermore, the loss of tetraspanin CD151, which associates with α₃β₁ integrin, has been demonstrated to be involved in strengthening of adhesion to laminin by α₃β₁ integrin (51). Consistent with this idea, approximately 15% of the glomeruli in Pod-Tln1–KO mice have dilated capillary loops similar to those observed in podocyte-specific β₁ integrin KO mice (2). However, our results on podocytes lacking talin1 expression compared with littermate controls demonstrate only a mild decrease in adhesion and β₁ integrin activation when compared with littermate controls. Furthermore, there was no obvious change in the total podocyte number in kidneys from the Pod-Tln1–KO mice, which is not consistent with the inability of podocytes to adhere tightly to the GBM. Another conceivable explanation is that the loss of talin1 in podocytes results in a modest reduction in cell spreading, thus preventing the podocytes from tightly covering the glomerular capillaries. However, previous results in mice specifically lacking focal adhesion proteins, FAK, or Crk in podocytes revealed no evidence of proteinuria, even though cell spreading was compromised (40, 52).

Interestingly, the phenotype of the Pod-Tln1 Cre mice was significantly more severe than that of mice lacking podocyte β₁ integrin expression (2), with worse glomerulosclerosis, mesangiolysis, and biochemical and histological evidence of end-stage kidney disease. This suggests that much of the phenotype observed in the Pod-Tln1–KO mice may possibly be independent of that caused by its alterations of integrin-dependent functions. An alternative explanation for the differences in the phenotype is that it is due to the potential difference in efficiency of the podocin transgene for the excision of integrin β₁ and talin1. Furthermore, differences in the half-life of the 2 proteins following excision may not be identical, accounting for further differences. Loss of talin1 also has the potential to disrupt the activation and cytoskeletal linkage of other integrins such as α_Vβ₃ in addition to α₃β₁ (12). Indeed, there was a dramatic and defective organization of the F-actin cytoskeleton in Pod-Tln1–KO podocytes that could only be rescued by expressing full-length talin1, further demonstrating that the intact talin1 head and rod play a critical role in maintaining the integrity of the glomerular filtration barrier. Yet podocytes isolated in culture do have imperfect foot processes and slit diaphragms, resulting in a caveat that in vitro mechanistic results may not always mimic in vivo findings.

It has been demonstrated that mice expressing calpastatin, an endogenous inhibitor of calpain, exhibit reduced proteinuria following NTS injection (43), although the relevant proteolytic substrates remained elusive. Upon examination of talin1 following podocyte injury by either protamine sulfate or NTS, we noted a robust increase in talin1 cleavage between the head and rod regions, which was likely due to elevated calpain activity in podocytes, which was not observed in proximal tubules in vivo. Consistent with this, urine analysis in both mouse models of podocyte injury revealed elevated calpain activity, and these findings were further validated in urine samples from patients with nephrotic syndrome (FSGS and MCD). However, we do acknowledge that other cells within the kidney may also secrete calpain following glomerular injury. Calpain-mediated talin cleavage between the head and rod domains is important in focal adhesion turnover and cell migration (23), as is the cleavage of the C-terminal dimerization domain (42). Therefore, degradation of talin1 mediated by elevated calpain activity observed in podocyte injury and nephrotic syndromes may destabilize podocyte cell-matrix junctions by disrupting the connection between integrins and actin and so contribute to the pathobiology of disease. Indeed, podocytes expressing a calpain-resistant mutant of talin1 maintained numerous actin stress fibers after treatment with protamine sulfate, unlike the WT controls. Furthermore, mice treated with calpain inhibitor markedly reduced NTS-induced talin1 cleavage and subsequent proteinuria and foot process effacement.

In summary, we provide evidence that talin1 is critical for the integrity of the glomerular filtration barrier and demonstrate that calpain-mediated talin1 cleavage may play a critical role in the pathology resulting following podocyte injury. These data suggest that either deleting talin1 or inducing talin1 cleavage by calpain decouples the intimate link between integrins at the GBM and the actin cytoskeleton. Calpain-mediated cleavage of additional focal adhesion components such as FAK, vinculin, or paxillin was not observed following NTS or protamine sulfate infusion. The exact mechanism of how calpain activity is increased in podocytes following injury requires further investigation. We have shown that following injury, calpastatin, the endogenous inhibitor of calpain, is remarkably reduced in the glomerulus. It has also been demonstrated that calpain 2 can be increased following Erk phosphorylation of serine 50. Further corroborating this possibility, recent evidence suggests that Erk phosphorylation is increased following glomerular injury (53). These results further emphasize the critical role of podocyte cell-matrix interactions in glomerular function in mice (2, 4), and humans (24, 25) and how therapeutic targeting of these pathways may benefit patients (54). Furthermore, it highlights the importance of future studies on other integrin-associated proteins such as kindlin, which can interact with integrins, phosphoinositides, and ILK (55). Collectively, these findings highlight the importance of developing a greater understanding of cell-matrix interactions as it relates to nephrotic syndromes and shed light on what we believe are novel therapeutic strategies, such as the potential blockade of calpain activity following glomerular injury.

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We are grateful to Pietro De Camilli (Yale University) for access to spinning disc confocal microscopy, and Eduardo Garin at the University of Florida for the patient samples. This work was supported in part from the following sources: NIH DK083294, NIH DK093629 and a Norman Coplon Satellite Healthcare grant (to S. Ishibe); NIH grants and a grant from the George O’Brien Kidney Center at Yale, P30DK079310 (to S. Ishibe); and NIH DK073495, NIH DK089394, and NIH 1R01 DK101350-01 (to J. Reiser). Ramiro Nandez was supported by NIH grant DK082700 to Pietro De Camilli.

Address correspondence to: Shuta Ishibe, 300 Cedar Street, S369C, P.O. Box 208209, New Haven, Connecticut 06520-8029, USA. Phone: 203.737.4170; Fax: 203.785.4904; E-mail: [email protected].

Conflict of interest: Jochen Reiser is an inventor on pending and issued patents on technologies concerning proteinuric kidney diseases.

Reference information: J Clin Invest. 2014;124(3):1098–1113. doi:10.1172/JCI69778.

See the related articles at Podocyte-specific RAP1GAP expression contributes to focal segmental glomerulosclerosis–associated glomerular injury and Podocyte-associated talin1 is critical for glomerular filtration barrier maintenance.

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