MAGI-2 orchestrates the localization of backbone proteins in the slit diaphragm of podocytes

Podocytes are highly specialized cells within the glomerulus that are essential for ultrafiltration. The slit diaphragm between the foot processes of podocytes functions as a final filtration barrier to prevent serum protein leakage into urine. The slit-diaphragm consists mainly of Nephrin and Neph1, and localization of these backbone proteins is essential to maintaining the integrity of the glomerular filtration barrier. However, the mechanisms that regulate the localization of these backbone proteins have remained elusive. Here, we focused on the role of membrane-associated guanylate kinase inverted 2 (MAGI-2) in order to investigate mechanisms that orchestrate localization of slit-diaphragm backbone proteins. MAGI-2 downregulation coincided with a reduced expression of slit-diaphragm backbone proteins in human kidneys glomerular disease such as focal segmental glomerulosclerosis or IgA nephropathy. Podocyte-specific deficiency of MAGI-2 in mice abrogated localization of Nephrin and Neph1 independently of other scaffold proteins. Although a deficiency of zonula occuldens-1 downregulated the endogenous Neph1 expression, MAGI-2 recovered Neph1 expression at the cellular edge in cultured podocytes. Additionally, overexpression of MAGI-2 preserved Nephrin localization to intercellular junctions. Co-immunoprecipitation and pull-down assays also revealed the importance of the PDZ domains of MAGI-2 for the interaction between MAGI-2 and slit diaphragm backbone proteins in podocytes. Thus, localization and stabilization of Nephrin and Neph1 in intercellular junctions is regulated mainly via the PDZ domains of MAGI-2 together with other slit-diaphragm scaffold proteins. Hence, these findings may elucidate a mechanism by which the backbone proteins are maintained.

MAGI-2 downregulation: a potential predictor of tumor progression and early recurrence in Han Chinese patients with prostate cancer

Membrane-associated guanylate kinase (MAGUK) family protein MAGUK invert 2 (MAGI-2) has been demonstrated to be involved in the tumorigenic mechanism of prostate cancer. The objective of this study was to investigate the expression of MAGI-2 at mRNA and protein levels. The prognostic value of MAGI-2 in Han Chinese patients with prostate cancer was also investigated. The expression data of MAGI-2 were assessed through database retrieval, analysis of sequencing data from our group, and tissue immunohistochemistry using digital scoring system (H-score). The clinical, pathological, and follow-up data were collected. The expression of MAGI-2 in prostate tumor tissues and prostate normal tissues was evaluated and compared. MAGI-2 expression was associated with clinical parameters including tumor stage, lymph node status, Gleason score, PSA level, and biochemical recurrence of prostate cancer. The relative expression of MAGI-2 mRNA was lower in the tumor tissue in The Cancer Genome Atlas (TCGA) database and sequencing data (P < 0.001). There was no difference in MAGI-2 protein expression between tumor and normal tissues in tissue microarray (TMA) results. MAGI-2 expression was associated with pathological tumor stage (P = 0.02), Gleason score (P = 0.05), and preoperation prostate-specific antigen (PSA; P = 0.04). A positive correlation was identified between MAGI-2 and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) expressions through the analysis of TCGA and TMA data (P < 0.0001). Patients with higher MAGI-2 expression had longer biochemical recurrence-free survival in the univariate analysis (P = 0.005), which indicates an optimal prognostic value of MAGI-2 in Han Chinese patients with prostate cancer. In conclusion, MAGI-2 expression gradually decreases with tumor progression, and can be used as a predictor of tumor recurrence in Chinese patients.

Neuroligin 3 Regulates Dendritic Outgrowth by Modulating Akt/mTOR Signaling

Neuroligins (NLs) are a group of postsynaptic cell adhesion molecules that function in synaptogenesis and synaptic transmission. Genetic defects in neuroligin 3 (NL3), a member of the NL protein family, are associated with autism. Studies in rodents have revealed that mutations of NL3 gene lead to increased growth and complexity in dendrites in the central nervous system. However, the detailed mechanism is still unclear. In our study, we found that deficiency of NL3 led to morphological changes of the pyramidal neurons in layer II/III somatosensory cortex in mice, including enlarged somata, elongated dendritic length, and increased dendritic complexity. Knockdown of NL3 in cultured rat neurons upregulated Akt/mTOR signaling, resulting in both increased protein synthesis and dendritic growth. Treating neurons with either rapamycin to inhibit the mTOR or LY294002 to inhibit the PI3K/Akt activity rescued the morphological abnormalities resulting from either NL3 knockdown or knockout (KO). In addition, we found that the hyperactivated Akt/mTOR signaling associated with NL3 defects was mediated by a reduction in phosphatase and tensin (PTEN) expression, and that MAGI-2, a scaffold protein, interacted with both NL3 and PTEN and could be a linker between NL3 and Akt/mTOR signaling pathway. In conclusion, our results suggest that NL3 regulates neuronal morphology, especially dendritic outgrowth, by modulating the PTEN/Akt/mTOR signaling pathway, probably via MAGI-2. Thereby, this study provides a new link between NL3 and neuronal morphology.

MAGI-2 and scaffold proteins in glomerulopathy

In many cells and tissues, including the glomerular filtration barrier, scaffold proteins are critical in optimizing signal transduction by enhancing structural stability and functionality of their ligands. Recently, mutations in scaffold protein membrane-associated guanylate kinase inverted 2 (MAGI-2) encoding gene were identified among the etiology of steroid-resistant nephrotic syndrome. MAGI-2 interacts with core proteins of multiple pathways, such as transforming growth factor-β signaling, planar cell polarity pathway, and Wnt/β-catenin signaling in podocyte and slit diaphragm. Through the interaction with its ligand, MAGI-2 modulates the regulation of apoptosis, cytoskeletal reorganization, and glomerular development. This review aims to summarize recent findings on the role of MAGI-2 and some other scaffold proteins, such as nephrin and synaptopodin, in the underlying mechanisms of glomerulopathy.

Glomerulosclerosis Induced by Deficiency of Membrane-Associated Guanylate Kinase Inverted 2 in Kidney Podocytes

Membrane-associated guanylate kinase inverted 2 (MAGI-2) is a component of the slit diaphragm (SD) of glomerular podocytes. Here, we investigated the podocyte-specific function of MAGI-2 using newly generated podocyte-specific MAGI-2-knockout (MAGI-2-KO) mice. Compared with podocytes from wild-type mice, podocytes from MAGI-2-KO mice exhibited SD disruption, morphologic abnormalities of foot processes, and podocyte apoptosis leading to podocyte loss. These pathologic changes manifested as massive albuminuria by 8 weeks of age and glomerulosclerosis and significantly higher plasma creatinine levels at 12 weeks of age; all MAGI-2-KO mice died by 20 weeks of age. Loss of MAGI-2 in podocytes associated with decreased expression and nuclear translocation of dendrin, which is also a component of the SD complex. Dendrin translocates from the SD to the nucleus of injured podocytes, promoting apoptosis. Our coimmunoprecipitation and in vitro reconstitution studies showed that dendrin is phosphorylated by Fyn and dephosphorylated by PTP1B, and that Fyn-induced phosphorylation prevents Nedd4-2-mediated ubiquitination of dendrin. Under physiologic conditions in vivo, phosphorylated dendrin localized at the SDs; in the absence of MAGI-2, dephosphorylated dendrin accumulated in the nucleus. Furthermore, induction of experimental GN in rats led to the downregulation of MAGI-2 expression and the nuclear accumulation of dendrin in podocytes. In summary, MAGI-2 and Fyn protect dendrin from Nedd4-2-mediated ubiquitination and from nuclear translocation, thereby maintaining the physiologic homeostasis of podocytes, and the lack of MAGI-2 in podocytes results in FSGS.

MAGI-2 Is a Sensitive and Specific Marker of Prostatic Adenocarcinoma: A Comparison With AMACR

OBJECTIVES

We compared the utility of membrane-associated guanylate kinase, WW and PDZ domain-containing protein 2 (MAGI-2) and α-methylacyl CoA (AMACR) by immunohistochemistry in diagnosing prostatic adenocarcinoma.

METHODS

Seventy-eight radical prostatectomies were used to construct three tissue microarrays with 512 cores, including benign prostatic tissue, benign prostatic hyperplasia, high-grade prostatic intraepithelial neoplasia (HGPIN), and adenocarcinoma. AMACR and MAGI-2 immunohistochemistry were evaluated by visual and image analysis.

RESULTS

MAGI-2 and AMACR were significantly higher in adenocarcinoma and HGPIN compared with benign tissue. At H-score cutoffs of 300 and 200, MAGI-2 was more accurate in distinguishing benign from malignant glands than AMACR. Areas under the curve by image and visual analysis were 0.846 and 0.818 for MAGI-2 and 0.937 and 0.924 for AMACR, respectively. The accuracy of MAGI-2 in distinguishing benign from malignant glands on the same core was higher (95% vs 88%).

CONCLUSIONS

MAGI-2 could represent a useful adjunct for diagnosis of prostatic adenocarcinoma, especially when AMACR is not discriminatory.