Brian R. Roberts

Loss of the alpha-isoform of calcineurin is sufficient to induce nephrotoxicity and altered expression of transforming growth factor-beta.

Background. Use of calcineurin inhibitors is frequently limited by fibrosis, closely linked with increased transforming growth factor (TGF)-β. However, mechanisms of extracellular matrix expansion and TGFβ regulation following calcineurin inhibition are unknown. Mice lacking specific calcineurin catalytic subunit isoforms may offer important insight into this pathway. Methods. We compared mice lacking the α or β isoform to a model of cyclosporin nephrotoxicity. Histological features common with cyclosporin nephrotoxicity including matrix expansion, arteriole hyalinization, and inflammation were assessed. Next, regulation specifically of fibronectin and TGFβ was examined in vivo and in vitro. Finally, the role of TGFβ in upregulation of fibronectin with loss of calcineurin activity was examined. Results. Loss of the α isoform results in histologic features and matrix expansion similar to cyclosporin, whereas loss of the β does not. Fibronectin and TGFβ are increased and renal function is impaired in α-null and aged α+/−. In primary α−/− renal fibroblasts, nuclear translocation of the calcineurin substrate NFATc is normal but regulation is lost in β-null fibroblasts, confirming that the isoforms have distinct functions. Consistent with in vivo findings, α-null cells have increased fibronectin and TGFβ. However, neutralizing TGFβ antibody did not reduce fibronectin accumulation. Conclusions. Our data show that calcineurin-α is key to regulation of fibrosis and TGFβ and loss of this isoform reproduces features of cyclosporine nephrotoxicity in vivo and in vitro. In addition, we show that upregulation of TGFβ and fibronectin likely result from a shared mechanism, but changes in fibronectin expression are independent of TGFβ in renal fibroblasts.

Angiotensin II Stimulates Calcineurin Activity in Proximal Tubule Epithelia through AT-1 Receptor-Mediated Tyrosine Phosphorylation of the PLC-γ1 Isoform

Angiotensin II (AngII) contributes to the maintenance of extracellular fluid volume by regulating sodium transport in the nephron. In nonepithelial cells, activation of phospholipase C (PLC) by AT-1 receptors stimulates the generation of 1,4,5-trisphosphate (IP(3)) and the release of intracellular calcium. Calcineurin, a serine-threonine phosphatase, is activated by calcium and calmodulin, and both PLC and calcineurin have been linked to sodium transport in the proximal tubule. An examination of whether AngII activates calcineurin in a model of proximal tubule epithelia (LLC-PK1 cells) was performed; AngII increased calcineurin activity within 30 s. An examination of whether AngII activates PLC in proximal tubule epithelia was also performed after first showing that all three families of PLC isoforms are present in LLC-PK1 cells. Application of AngII increased IP(3) generation by 60% within 15 s, which coincided with AngII-induced tyrosine phosphorylation of the PLC-gamma1 isoform also observed at 15 s. AngII-induced tyrosine phosphorylation was blocked by the AT-1 receptor antagonist, Losartan. Subsequently, an inhibitor of tyrosine phosphorylation blocked the AngII-induced activation of calcineurin, as did coincubation with an inhibitor of PLC activity and with an antagonist of the AT-1 receptor. It is therefore concluded that AngII stimulates calcineurin phosphatase activity in proximal tubule epithelial cells through a mechanism involving AT-1 receptor-mediated tyrosine phosphorylation of the PLC isoform.

Calcineurin A‐β is required for hypertrophy but not matrix expansion in the diabetic kidney

Calcineurin is an important signalling protein that regulates a number of molecular and cellular processes. Previously, we found that inhibition of calcineurin with cyclosporine reduced renal hypertrophy and blocked glomerular matrix expansion in the diabetic kidney. Isoforms of the catalytic subunit of calcineurin are reported to have tissue specific expression and functions. In particular, the β isoform has been implicated in cardiac and skeletal muscle hypertrophy. Therefore, we examined the role of calcineurin β in diabetic renal hypertrophy and glomerular matrix expansion. Type I diabetes was induced in wild‐type and β−/− mice and then renal function, extracellular matrix expansion and hypertrophy were evaluated. The absence of β produced a significant decrease in total calcineurin activity in the inner medulla (IM) and reduced nuclear factor of activated T‐cells (NFATc) activity. Loss of β did not alter diabetic renal dysfunction assessed by glomerular filtration rate, urine albumin excretion and blood urea nitrogen. Similarly, matrix expansion in the whole kidney and glomerulus was not different between diabetic wild‐type and β−/− mice. In contrast, whole kidney and glomerular hypertrophy were significantly reduced in diabetic β−/− mice. Moreover, β−/− renal fibroblasts demonstrated impaired phosphorylation of Erk1/Erk2, c‐Jun N‐terminal kinases (JNK) and mammalian target of rapamycin (mTOR) following stimulation with transforming growth factor‐β and did not undergo hypertrophy with 48 hrs culture in high glucose. In conclusion, loss of the β isoform of calcineurin is sufficient to reproduce beneficial aspects of cyclosporine on diabetic renal hypertrophy but not matrix expansion. Therefore, while multiple signals appear to regulate matrix, calcineurin β appears to be a central mechanism involved in organ hypertrophy.

Calcineurin phosphatase activity: activation by glucocorticoids and role of intracellular calcium.

Background. Glucocorticoids stimulate release of intracellular calcium in peripheral lymphocytes, but their effects on calcineurin phosphatase activity are unknown. Methods. Calcineurin phosphatase activity was measured in permeabilized Jurkat T cells using a specific orthophosphate substrate. Changes in intracellular calcium were measured by FURA-2 fluorescence. Inositol triphosphate levels were measured by radioimmunoassay. Transfection with luciferase reporter plasmids linked to glucocorticoid response elements were used to evaluate glucocorticoid receptor function in Jurkat T cells. Results. Dexamethasone significantly (P <0.004) increased calcineurin activity within 15 sec, peaking at 10 min (P <0.001) and returning to basal levels by 180 min. Inhibition of DNA transcription with actinomycin D failed to block calcineurin activation, but co-incubation with RU-486 completely blocked enzyme stimulation. To determine whether Jurkat T cells express active glucocorticoid receptors, cells were transfected with a luciferase reporter plasmid linked to a glucocorticoid response element (GRE). Jurkat T cells incubated with dexamethasone (10 &mgr;M) for 24 hr failed to stimulate luciferase activity, whereas cells co-transfected with a transcriptionally active glucocorticoid receptor resulted in a doubling of luciferase activity. Dexamethasone rapidly increases intracellular inositol triphosphate (IP3) and intracellular calcium within 15 sec. Cells incubated with U-73122 (a nonspecific phospholipase C [PLC] antagonist) completely blocked dexamethasone-induced activation of calcineurin, whereas U-73343 failed to block enzyme activation. Dexamethasone-induced activation of calcineurin activity stimulates dephosphorylation of the proapoptotic protein BAD and augments apoptosis through a calcineurin-dependent pathway. Conclusion. Dexamethasone rapidly increases calcineurin activity through a transcription-independent mechanism involving activation of phospholipase C and the release of IP3-dependent calcium stores.

TGF-β upregulation drives tertiary lymphoid organ formation and kidney dysfunction in calcineurin A-α heterozygous mice

Calcineurin is an important intracellular signaling molecule which can be inhibited by cyclosporin resulting in immune suppression and nephrotoxicity. Previously, we reported that homozygous loss of the alpha isoform of calcineurin impairs kidney development and function and mimics many features of cyclosporin nephrotoxicity. However, early lethality of null mice prevented further study of renal changes. Alternatively, we examined aged heterozygous (CnAalpha(+/-)) mice. In addition to renal dysfunction and inflammation, we find that CnAalpha(+/-) mice spontaneously develop tertiary lymphoid aggregates in the kidney, small intestine, liver, and lung. Lymphoid aggregates contain both T cells and B cells and exhibited organization suggestive of tertiary lymphoid organs (TLOs). Kidney function and TLO formation were highly correlated suggesting that this process may contribute to nephrotoxicity. Consistent with previous findings, transforming growth factor (TGF)-beta is significantly increased in CnAalpha(+/-) mice. Neutralization of TGF-beta attenuated TLO formation and improved kidney function. In conclusion, we report that haploinsufficiency of CnAalpha causes uregulation of TGF-beta which contributes to chronic inflammation and formation of TLOs. While the process that leads to TLOs formation in transplant allografts is unknown, TLOs are associated with poor clinical prognosis. This study suggests that calcineurin inhibition itself may lead to TLO formation and that TGF-beta may be a novel therapeutic target.

T-Cell Receptor-Stimulated Calcineurin Activity Is Inhibited in Isolated T Cells from Transplant Patients

The addition of calcineurin inhibitors, including cyclosporine A (CsA) and FK-506 (tacrolimus), to transplant protocols has markedly reduced acute allograft rejection and prolonged patient survival. Although monitoring of serum drug levels has been shown to be a poor indicator of efficacy, there is little data on calcineurin enzymatic activity in humans. Therefore, we measured calcineurin in isolated CD3+/4+ T cells from 81 non-transplant controls and 39 renal allograft patients by using a 32PO4-labeled calcineurin-specific substrate. A gender difference was observed in the control cohort, with activity in males significantly higher than that in females (1073 ± 134 versus 758 ± 75 fmol/μg/min, respectively). Activity of both groups was comparably inhibited by 5 ng/ml tacrolimus (27 ± 4 versus 30 ± 4%). Calcineurin is a downstream target of the T-cell receptor (TCR). Therefore, activity was measured in isolated T cells after incubation with anti-CD3/CD28 antibodies to stimulate the TCR. Calcineurin activity increased significantly from 1214 ± 111 to 1652 ± 138 fmol/μg/min; addition of either tacrolimus or CsA (500 ng/ml) blocked CD3/CD28 stimulation. Despite therapeutic levels of tacrolimus and CsA (mean 11.4 and 172 ng/ml), basal calcineurin activity was significantly higher among renal transplant recipients than controls (1776 ± 175 versus 914 ± 78 fmol/μg/min). In contrast, anti-CD3/CD28 antibodies failed to stimulate calcineurin activity in transplant subjects. Finally, we found that basal and stimulated calcineurin activities are inversely related. Consistent with this finding, basal activity in resting T cells rose over time after transplant but stimulation fell (r2 = 0.785, p < 0.05). These data suggest that examination of TCR-stimulated calcineurin activity after renal transplantation may be useful for monitoring immunosuppression of individual patients.