Upregulation of c-mip is closely related to podocyte dysfunction in membranous nephropathy.

Membranous nephropathy is a glomerular disease typified by a nephrotic syndrome without infiltration of inflammatory cells or proliferation of resident cells. Although the cause of the disease is unknown, the primary pathology involves the generation of autoantibodies against antigen targets on the surface of podocytes. The mechanisms of nephrotic proteinuria, which reflect a profound podocyte dysfunction, remain unclear. We previously found a new gene, c-mip (c-maf-inducing protein), that was associated with the pathophysiology of idiopathic nephrotic syndrome. Here we found that c-mip was not detected in the glomeruli of rats with passive-type Heymann nephritis given a single dose of anti-megalin polyclonal antibody, yet immune complexes were readily present, but without triggering of proteinuria. Rats reinjected with anti-megalin develop heavy proteinuria a few days later, concomitant with c-mip overproduction in podocytes. This overexpression was associated with the downregulation of synaptopodin in patients with membranous nephropathy, rats with passive Heymann nephritis, and c-mip transgenic mice, while the abundance of death-associated protein kinase and integrin-linked kinase was increased. Cyclosporine treatment significantly reduced proteinuria in rats with passive Heymann nephritis, concomitant with downregulation of c-mip in podocytes. Thus, c-mip has an active role in the podocyte disorders of membranous nephropathy.

c-mip down-regulates NF-κB activity and promotes apoptosis in podocytes.

The mechanisms of podocyte disorders in cases of idiopathic nephrotic syndrome (INS) are complex and remain incompletely elucidated. The abnormal regulation of NF-κB may play a key role in the pathophysiology of these podocyte diseases, but at present, NF-κB has not been thoroughly investigated. In this study, we report that induction of c-mip in podocytes of patients with INS is associated with a down-regulation of RelA, a potent antiapoptotic factor that belongs to the NF-κB family. Overexpression of c-mip in differentiated podocytes promotes apoptosis by inducing caspase-3 activity and up-regulating the proapoptotic protein Bax, whereas the overall levels of the antiapoptotic protein Bcl-2 was concomitantly decreased. The associated overexpression of RelA prevented the proapoptotic effects of c-mip. In addition, the targeted induction of c-mip in podocytes in vivo inhibited the expression of the RelA protein and increased the Bax/Bcl-2 ratio. The expression of both c-mip and active caspase-3 increased in focal and segmental glomerulosclerosis biopsies, and both proteins displayed a close spatial relationship. These results suggest that alterations in NF-κB activity might result from the up-regulation of c-mip and are likely to contribute to podocyte disorders in cases of INS.

Truncation of C-mip (Tc-mip), a new proximal signaling protein, induces c-maf Th2 transcription factor and cytoskeleton reorganization.

Several arguments suggest that minimal change nephrotic syndrome (MCNS) results from yet unknown systemic disorder of T cell function. By screening a cDNA library from T cell relapse, we identified a new pleckstrin homology (PH) domain-containing protein encoded by a gene located on chromosome 16q24. Two alternative transcripts were identified. The first species (c-mip) was expressed in fetal liver, kidney, and peripheral blood mononuclear cells (PBMCs), but weakly detected in PBMCs from MCNS patients. The second form (Tc-mip, standing for truncated c-maf inducing protein), corresponds to subtracted transcript and lacks the NH2-terminal PH domain. The expression of Tc-mip was restricted to fetal liver, thymus, and MCNS PBMCs where it was specifically recruited in CD4+ T cells subset. Overexpression of Tc-mip in T cell Jurkat induced c-maf, transactivated the interleukin 4 gene and down-regulated the interferon gamma expression, characteristic of a Th2 commitment. Moreover, the overexpression of Tc-mip induced Src phosphorylation, T cell clustering, and a cellular redistribution of the cytoskeleton-associated L-plastin, by a PI3 kinase independent pathway. Tc-mip represents therefore the first identified protein, which links proximal signaling to c-maf induction.

C-mip interacts physically with RelA and inhibits nuclear factor kappa B activity.

The fine regulation of NF-kappaB activity is crucial for both resting and stimulated cells and relies on complex balance between multiple activators and inhibitors. We report here that c-mip, a recently identified pleckstrin homology (PH) and leucine-rich repeat (LRR)-domain-containing protein, inactivates GSKbeta and interacts with RelA, a key member of the NF-kappaB family. We show that c-mip inhibits the degradation of I-kappaBalpha and impedes the dissociation of the NF-kappaB/I-kappaBalpha complexes. C-mip acts downstream signaling of classical NF-kappaB pathway and may represent one of the missing links in the control of NF-kappaB activity.

h-Goliath, paralog of GRAIL, is a new E3 ligase protein, expressed in human leukocytes.

In Drosophila, the RING finger protein d-Goliath was originally identified as a transcription factor involved in the embryo mesoderm formation [Bouchard, M.L., Cote, S., 1993. The Drosophila melanogaster developmental gene g1 encodes a variant zinc-finger-motif protein. Gene 125, 205-209]. In mouse, the m-Goliath mRNA level was shown to be increased in growth factor withdrawal-induced apoptosis of myeloid cells [Baker, S.J., Reddy, E.P., 2000. Cloning of murine G1RP, a novel gene related to Drosophila melanogaster g1. Gene 248, 33-40]. Due to its putative function of transcription factor in apoptosis, we cloned the human cDNA for h-Goliath and characterized the expression of the protein in blood and bone marrow cells. The human protein of 419 aa (44 kDa) contains a protease-associated domain, a transmembrane domain and a RING-H2 motif. This structure classifies h-Goliath as a new member of a human family of ubiquitin ligases with GRAIL (gene related to anergy in lymphocytes) as founder. This E3 ligase controls the development of T cell clonal anergy by ubiquitination [Anandasabapathy, N., Ford, G.S., Bloom, D., Holness, C., Paragas, V., Seroogy, C., Skrenta, H., Hollenhorst, M., Fathman, C.G., Soares, L., 2003. GRAIL: an E3 ubiquitin ligase that inhibits cytokine gene transcription is expressed in anergic CD4+ T cells. Immunity 18, 535-547]. In vitro ubiquitination studies support the E3 ubiquitin ligase activity of h-Goliath. In human, the protein is expressed under 3 isoforms, a major one at 28 kDa and two others at 46 and 55 kDa. These proteins come from a common precursor (44 kDa) as we observed using in vitro transcription-translation. Using immunohistochemistry on blood or bone marrow smears, of healthy or leukemia samples, we found that the protein expression was restricted to the cytoplasm of progenitors and fully differentiated leukocyte populations. We did not observe any modification of h-Goliath expression or localization in leukemia. In these cells, this new E3 ubiquitin ligase protein does not seem associated with a differentiation state of the cell or with apoptosis.

The Filamin-A is a partner of Tc-mip, a new adapter protein involved in c-maf-dependent Th2 signaling pathway.

Using a yeast two-hybrid screen, we identified Filamin-A as a binding partner of the new adapter protein c-mip (c-maf inducing protein) and it's splice variant Tc-mip (truncated c-maf inducing protein). We have previously shown that Tc-mip is involved in Th2 signaling pathway and cytoskeletal reorganization in patients with minimal change nephrotic syndrome (MCNS), the most frequent glomerular disease in children. We showed that Filamin-A and c-mip or Tc-mip co-immunoprecipitate from c-mip or Tc-mip Jurkat transfected cells using antibodies directed against both types of proteins. In co-immunoprecipitate Jurkat cells, Filamin-A and c-mip were distributed evenly in the cytoplasm, whereas in Tc-mip-transfected Jurkat cells, Filamin-A was expressed in zones facing the cell contact. Moreover, we found that Filamin-A was upregulated in T lymphocytes of MCNS patients, as compared to normal subjects. These findings suggest that Filamin-A interacts with c-mip/Tc-mip in this new T-cell signaling pathway.

Absence of the Adaptor Protein PEA-15 Is Associated with Altered Pattern of Th Cytokines Production by Activated CD4+ T Lymphocytes In Vitro, and Defective Red Blood Cell Alloimmune Response In Vivo.

TCR-dependent and costimulation signaling, cell division, and cytokine environment are major factors driving cytokines expression induced by CD4(+) T cell activation. PEA-15 15 (Protein Enriched in Astrocyte / 15 kDa) is an adaptor protein that regulates death receptor-induced apoptosis and proliferation signaling by binding to FADD and relocating ERK1/2 to the cytosol, respectively. By using PEA-15-deficient mice, we examined the role of PEA-15 in TCR-dependent cytokine production in CD4(+) T cells. TCR-stimulated PEA-15-deficient CD4(+) T cells exhibited defective progression through the cell cycle associated with impaired expression of cyclin E and phosphoRb, two ERK1/2-dependent proteins of the cell cycle. Accordingly, expression of the division cycle-dependent cytokines IL-2 and IFNγ, a Th1 cytokine, was reduced in stimulated PEA-15-deficient CD4(+) T cells. This was associated with abnormal subcellular compartmentalization of activated ERK1/2 in PEA-15-deficient T cells. Furthermore, in vitro TCR-dependent differentiation of naive CD4(+) CD62L(+) PEA-15-deficient T cells was associated with a lower production of the Th2 cytokine, IL-4, whereas expression of the Th17-associated molecule IL4I1 was enhanced. Finally, a defective humoral response was shown in PEA-15-deficient mice in a model of red blood cell alloimmunization performed with Poly IC, a classical adjuvant of Th1 response in vivo. Collectively, our data suggest that PEA-15 contributes to the specification of the cytokine pattern of activated Th cells, thus highlighting a potential new target to interfere with T cell functional polarization and subsequent immune response.

[New insights about immunopathology of lipoid nephrosis]. / Immunopathologie de la néphrose lipoïdique. Données récentes.

Clinical and experimental observations suggest that Lipoid Nephrosis (Minimal change nephrotic syndrome) results from T cell dysfunction due to still unknown mechanisms. By subtractive screening library, we identified 84 transcripts, of which twelve match with proteins of yet unknown function and thirty are unknown clones. Among the 42 known transcripts, at least 18 are closely involved in the TCR-mediated complex signaling cascade. This includes genes encoding components of the T cell receptor and proteins associated with the cytoskeleton scaffold, as well as transcription factors such as NF-kappa B and c-maf. During the relapse phase, we have found significant alterations of the NF-kappa B/I kappa Ba regulatory pathway, whereas very low levels of IL12R beta 2 mRNA were detected suggesting that T cell activation evolves toward a Th2 phenotype. We have shown that c-maf is highly induced, shuttling between nuclear and cytoplasmic compartment during the relapse and the remission phases, respectively. Contrasting with the nuclear expression of c-maf, low IL4 levels were detected in relapse. This suggests that the downstream target gene of c-maf in Lipoid Nephrosis, is not IL4 and provides new directions in research leading to identify the target gene, possibly an unknown Th2 cytokine, which might play a critical role in the pathophysiology of this disease. Thus, the combination of subtractive cloning and differential screening constitutes an efficient approach to identify genes likely to be involved in the pathophysiology of MCNS.

C-mip interacts with the p85 subunit of PI3 kinase and exerts a dual effect on ERK signaling via the recruitment of Dip1 and DAP kinase.

In naive T cells, Lck exerts a negative control on the ERK/MAPK pathway. We show that c-mip (c-maf inducing protein) interacts with the p85 subunit of PI3 kinase and inactivates Lck, which results in Erk1/2 and p38 MAPK activation. This effect is not enough to activate AP1 given the inability of ERK to migrate into the nucleus and to transactivate its target genes. We demonstrate that c-mip interacts with Dip1 and upregulates DAPK, which blocks the nuclear translocation of ERK1/2. This dual effect of c-mip is unique and might represent a potential mechanism to prevent the development of an immune response.

c-mip impairs podocyte proximal signaling and induces heavy proteinuria.

Idiopathic nephrotic syndrome comprises several podocyte diseases of unknown origin that affect the glomerular podocyte, which controls the permeability of the filtration barrier in the kidney to proteins. It is characterized by the daily loss of more than 3 g of protein in urine and the lack of inflammatory lesions or cell infiltration. We found that the abundance of c-mip (c-maf inducing protein) was increased in the podocytes of patients with various acquired idiopathic nephrotic syndromes in which the podocyte is the main target of injury. Mice engineered to have excessive c-mip in podocytes developed proteinuria without morphological alterations, inflammatory lesions, or cell infiltration. Excessive c-mip blocked podocyte signaling by preventing the interaction of the slit diaphragm transmembrane protein nephrin with the tyrosine kinase Fyn, thereby decreasing phosphorylation of nephrin in vitro and in vivo. Moreover, c-mip inhibited interactions between Fyn and the cytoskeletal regulator N-WASP (neural Wiskott-Aldrich syndrome protein) and between the adaptor protein Nck and nephrin, potentially accounting for cytoskeletal disorganization and the effacement of foot processes seen in idiopathic nephrotic syndromes. The intravenous injection of small interfering RNA targeting c-mip prevented lipopolysaccharide-induced proteinuria in mice. Together, these results identify c-mip as a key component in the molecular pathogenesis of acquired podocyte diseases.

Abnormal RNA processing and altered expression of serin-rich proteins in minimal-change nephrotic syndrome.

Mechanisms underlying the pathophysiology of minimal-change nephrotic syndrome (MCNS), the most frequent glomerular disease in children, remain elusive, but recent findings argue for a T cell dysfunction. Starting from a differential cDNA library from T cells of a patient under relapse and remission, we identified 16 transcripts specific for MCNS. All of these transcripts that were selectively up-regulated during the relapse phase of the disease were generated by alternative splicing of known genes. This abnormal RNA expression was associated with a down-regulation of serin-rich protein 75 and serin-rich protein 40, two proteins involved in mRNA splicing. Taken together, these data suggest that T cell dysfunction in MCNS is associated with abnormal mRNA splicing.

Transcriptional and post-transcriptional alterations of IkappaBalpha in active minimal-change nephrotic syndrome.

Minimal-change nephrotic syndrome (MCNS) is a renal disease characterized by heavy glomerular proteinuria and increased production of cytokines by immune cells. Because of the central role of nuclear factor-kappaB (NF-kappaB) in the regulation of cytokine expression, its activity during the relapse and remission phases of steroid-sensitive MCNS was analyzed. During relapse, nuclear extracts from peripheral blood mononuclear cells displayed high levels of NF-kappaB DNA-binding activity, consisting primarily of p50/RelA (p65) complexes. NF-kappaB p65 and IkappaBalpha proteins were barely detected or not detected in cytosolic fractions during relapse, in contrast to remission. The lack of expression of IkappaBalpha protein was associated with downregulation of IkappaBalpha mRNA and increases in the levels of the mRNA encoding the proteasome alpha2 subunit proteolytic pathway. In addition, inhibition of proteasome activity induced cytosolic accumulation of phosphorylated IkappaBalpha and significant reductions in the NF-kappaB binding activity in nuclear extracts from peripheral blood mononuclear cells from patients experiencing relapses. These results suggest that alterations in the NF-kappaB/IkappaBalpha regulatory feedback loop may contribute to the immunologic abnormalities that occur in steroidsensitive MCNS.

NF-kappa B p65 antagonizes IL-4 induction by c-maf in minimal change nephrotic syndrome.

Mechanisms underlying the pathophysiology of minimal change nephrotic syndrome (MCNS), the most frequent of glomerular diseases in children, remain elusive, although recent arguments suggest that T cell dysfunction may be involved in the pathogenesis of this disease. Recently, we reported that activated T cells of these patients display a down-regulation of IL-12R beta2 chain, suggesting an early commitment toward Th2 phenotype. In this study, we show that the short form of the proto-oncogene c-maf, a known activator of the IL-4 gene, is highly induced in MCNS T cells during relapse, where it translocates to the nuclear compartment and binds to the DNA responsive element. Unexpectedly, the nuclear localization of c-maf did not promote the IL-4 gene transcription in relapse. Using several approaches, we show in this study that RelA blunts IL-4 induction in T cells during the relapse in these patients. We demonstrate that the ex vivo inhibition of proteasome activity in T cells from relapse, which blocks NF-kappaB activity, strongly increases the IL-4 mRNA levels. Overexpression of c-maf in T cells induces a high level of IL-4 promoter-driven luciferase activity. In contrast, coexpression of c-maf with NF-kappaB RelA/p50, or RelA, but not p50, inhibits the c-maf-dependent IL-4 promoter activity. Finally, we demonstrated that, in T cell overexpressing RelA and c-maf, RelA expelled c-maf from its DNA binding site on IL-4 gene promoter, which results in active inhibition of IL-4 gene transcription. Altogether, these results suggest that the involvement of c-maf in Th2 commitment in MCNS operates through IL-4-independent mechanisms.