Jae-Kyun Ko

MG53 nucleates assembly of cell membrane repair machinery

Dynamic membrane repair and remodelling is an elemental process that maintains cell integrity and mediates efficient cellular function. Here we report that MG53, a muscle-specific tripartite motif family protein (TRIM72), is a component of the sarcolemmal membrane-repair machinery. MG53 interacts with phosphatidylserine to associate with intracellular vesicles that traffic to and fuse with sarcolemmal membranes. Mice null for MG53 show progressive myopathy and reduced exercise capability, associated with defective membrane-repair capacity. Injury of the sarcolemmal membrane leads to entry of the extracellular oxidative environment and MG53 oligomerization, resulting in recruitment of MG53-containing vesicles to the injury site. After vesicle translocation, entry of extracellular Ca2+ facilitates vesicle fusion to reseal the membrane. Our data indicate that intracellular vesicle translocation and Ca2+-dependent membrane fusion are distinct steps involved in the repair of membrane damage and that MG53 may initiate the assembly of the membrane repair machinery in an oxidation-dependent manner.

Membrane repair defects in muscular dystrophy are linked to altered interaction between MG53, caveolin-3, and dysferlin.

Defective membrane repair can contribute to the progression of muscular dystrophy. Although mutations in caveolin-3 (Cav3) and dysferlin are linked to muscular dystrophy in human patients, the molecular mechanism underlying the functional interplay between Cav3 and dysferlin in membrane repair of muscle physiology and disease has not been fully resolved. We recently discovered that mitsugumin 53 (MG53), a muscle-specific TRIM (Tri-partite motif) family protein (TRIM72), contributes to intracellular vesicle trafficking and is an essential component of the membrane repair machinery in striated muscle. Here we show that MG53 interacts with dysferlin and Cav3 to regulate membrane repair in skeletal muscle. MG53 mediates active trafficking of intracellular vesicles to the sarcolemma and is required for movement of dysferlin to sites of cell injury during repair patch formation. Mutations in Cav3 (P104L, R26Q) that cause retention of Cav3 in Golgi apparatus result in aberrant localization of MG53 and dysferlin in a dominant-negative fashion, leading to defective membrane repair. Our data reveal that a molecular complex formed by MG53, dysferlin, and Cav3 is essential for repair of muscle membrane damage and also provide a therapeutic target for treatment of muscular and cardiovascular diseases that are linked to compromised membrane repair.

Redox-dependent oligomerization through a leucine zipper motif is essential for MG53-mediated cell membrane repair

We recently discovered that MG53, a muscle-specific tripartite motif (TRIM) family protein, functions as a sensor of oxidation to nucleate the assembly of cell membrane repair machinery. Our data showed that disulfide bond formation mediated by Cys242 is critical for MG53-mediated translocation of intracellular vesicles toward the injury sites. Here we test the hypothesis that leucine zipper motifs in the coiled-coil domain of MG53 constitute an additional mechanism that facilitates oligomerization of MG53 during cell membrane repair. Two leucine zipper motifs in the coiled-coil domain of MG53 (LZ1 - L176/L183/L190/V197 and LZ2 - L205/L212/L219/L226) are highly conserved across the different animal species. Chemical cross-linking studies show that LZ1 is critical for MG53 homodimerization, whereas LZ2 is not. Mutations of the conserved leucines into alanines in LZ1, not in LZ2, diminish the redox-dependent oligomerization of MG53. Live cell imaging studies demonstrate that the movement of green fluorescent protein (GFP)-tagged MG53 mutants (GFP-LA1 and GFP-LA2) is partially compromised in response to mechanical damage of the cell membrane, and the GFP-LA1/2 double mutant is completely ineffective in translocation toward the injury sites. In addition to the leucine zipper-mediated intermolecular interaction, redox-dependent cross talk between MG53 appears to be an obligatory step for cell membrane repair, since in vivo modification of cysteine residues with alkylating reagents can prevent the movement of MG53 toward the injury sites. Our data show that oxidation of the thiol group of Cys242 and leucine zipper-mediated interaction among the MG53 molecules both contribute to the nucleation process for MG53-mediated cell membrane repair.

Increased Store-Operated Ca2+ Entry in Skeletal Muscle with Reduced Calsequestrin-1 Expression

Store-operated Ca(2+) entry (SOCE) contributes to Ca(2+) handling in normal skeletal muscle function, as well as the progression of muscular dystrophy and sarcopenia, yet the mechanisms underlying the change in SOCE in these states remain unclear. Previously we showed that calsequestrin-1 (CSQ1) participated in retrograde regulation of SOCE in cultured skeletal myotubes. In this study, we used small-hairpin RNA to determine whether knockdown of CSQ1 in adult mouse skeletal muscle can influence SOCE activity and muscle function. Small-hairpin RNA against CSQ1 was introduced into flexor digitorum brevis muscles using electroporation. Transfected fibers were isolated for SOCE measurements using the Mn(2+) fluorescence-quenching method. At room temperature, the SOCE induced by submaximal depletion of the SR Ca(2+) store was significantly enhanced in CSQ1-knockdown muscle fibers. When temperature of the bathing solution was increased to 39 degrees C, CSQ1-knockdown muscle fibers displayed a significant increase in Ca(2+) permeability across the surface membrane likely via the SOCE pathway, and a corresponding elevation in cytosolic Ca(2+) as compared to control fibers. Preincubation with azumolene, an analog of dantrolene used for the treatment of malignant hyperthermia (MH), suppressed the elevated SOCE in CSQ1-knockdown fibers. Because the CSQ1-knockout mice develop similar MH phenotypes, this inhibitory effect of azumolene on SOCE suggests that elevated extracellular Ca(2+) entry in skeletal muscle may be a key factor for the pathophysiological changes in intracellular Ca(2+) signaling in MH.

The tail-anchoring domain of Bfl1 and HCCS1 targets mitochondrial membrane permeability to induce apoptosis

Many Bcl2 family proteins target intracellular membranes by their C-terminal tail-anchor domain. Bfl1 is a bi-functional Bcl2 family protein with both anti- and pro-apoptotic activities and contains an amphipathic tail-anchoring peptide (ATAP; residues 147-175) with unique properties. Here we show that ATAP targets specifically to mitochondria, and induces caspase-dependent apoptosis that does not require Bax or Bak. Mutagenesis studies revealed that lysine residues flanking the ATAP sequence are involved in targeting of the peptide to the mitochondrial membrane, and charged residues that contribute to the amphipathic nature of ATAP are critical for its pro-apoptotic function. The ATAP sequence is present in another tumor suppressor gene, HCCS1, which contains an additional mitochondria-targeting signal (MTS) close to the ATAP. We propose that both ATAP and MTS could be used as therapeutic peptides to induce cell death in the treatment of cancer cells.

Glutamate at position 227 of junctophilin-2 is involved in binding to TRPC3

Canonical-type transient receptor potential cation channel type 3 (TRPC3) allows the entry of extracellular Ca2+ and Na+ into various cells. In mouse skeletal myotubes, functional interaction between TRPC3 and RyR1 (ryanodine receptor type 1/Ca2+-release channel on sarcoplasmic reticulum membrane) regulates the gain of excitation–contraction coupling. Junctophilin-2 (JP2) is a TRPC3-interacting protein in mouse skeletal myotubes. Based on these knowledge from bona-fide TRPC3-expressing cells, to identify critical binding region(s) of JP2 that participate in binding to TRPC3, various JP2 portions were subjected to co-immunoprecipitation assay with intact TRPC3 from rabbit skeletal muscle. A region covering 143 to 234 amino acids of JP2 (F1-2) was the most efficient portion binding to TRPC3. Through mutational studies, we found that the binding ability of JP2 to TRPC3 was mainly due to glutamate in the F1-2 region (E227). This substantial binding between JP2 and TRPC3 suggests that JP2 can be a regulatory protein of TRPC3 and/or TRPC3-mediated Ca2+ homeostasis in skeletal muscle.

C‐terminal region of Bfl‐1 induces cell death that accompanies caspase activation when fused with GFP

Previously, we reported that anti‐apoptotic Bfl‐1 is converted to a pro‐apoptotic protein following fusion at its N‐terminus with green fluorescent protein (GFP) (GFP‐Bfl‐1). In this study, we performed a Bfl‐1 deletion study in order to elucidate the underlying mechanism of GFP‐Bfl‐1‐induced cell death. We found that the Bcl‐2 homology (BH) domains in Bfl‐1 are dispensable with respect to cell death and that GFP fusion with the 29 amino acids of the C‐terminal region of Bfl‐1 (GFP‐BC) is sufficient to induce cell death. Moreover, when BC was fused with other tagging partners like GST or MBP, little cell death was observed, implying that the GFP region is as important as the BC region for GFP‐BC‐induced cell death. Further deletion analysis defined a region of GFP as a determinant of GFP‐BC‐induced cell death. Confocal microscopic analysis showed that GFP‐chimeras containing the BC region of Bfl‐1 are located mainly in mitochondria. The GFP‐BC‐induced cell death accompanied cellular caspase activation, and treatment with the pan‐caspase inhibitor, Boc‐D‐FMK, partially inhibited GFP‐BC‐induced cell death. However, the over‐expression of anti‐apoptotic molecules, such as Bcl‐xL and CrmA, did not block GFP‐BC‐induced cell death. In summary, GFP‐BC induces cell death with caspase activation through mitochondria dependent process.

A versatile single-plasmid system for tissue-specific and inducible control of gene expression in transgenic mice

We describe a novel transgenic system for tissue‐specific and inducible control of gene expression in mice. The system employs a tetracycline‐responsive CMV promoter that controls transcription of a short‐hairpin RNA (shRNA) that remains nonfunctional until an interrupting reporter cassette is excised by Cre recombinase. Insertion of Dicer and Drosha RNase processing sites within the shRNA allows generation of siRNA to knock down a target gene efficiently. Tissue‐specific shRNA expression is achieved through the use of appropriate inducer mice with tissue‐specific expression of Cre. We applied this system to regulate expression of junctophilins (JPs), genes essential for maintenance of membrane ultrastructure and Ca2+ signaling in muscle. Transgenic mice with skeletal muscle‐specific expression of shRNA against JP mRNAs displayed no basal change of JP expression before treatment with doxycycline (Dox), while inducible and reversible knockdown of JPs was achieved by feeding mice with Dox‐containing water. Dox‐induced knockdown of JPs led to abnormal junctional membrane structure and Ca2+ signaling in adult muscle fibers, consistent with essential roles of JPs in muscle development and function. This transgenic approach can be applied for inducible and reversible gene knockdown or gene overexpression in many different tissues, thus providing a versatile system for elucidating the physiological gene function in viable animal models.—Ko, J. ‐K., Choi, K. ‐H., Zhao, X., Komazaki, S., Pan, Z., Weisleder, N., Ma, J. A versatile single‐plasmid system for tissue‐specific and inducible control of gene expression in transgenic mice. FASEB J. 25, 2638–2649 (2011). www.fasebj.org

Zinc Binding to MG53 Protein Facilitates Repair of Injury to Cell Membranes

Background: MG53, a zinc finger protein, is essential to cell membrane repair. It is not known whether zinc contributes to MG53-mediated membrane repair. Results: Chelation of Zn2+ or mutation of Zn2+-binding motifs in MG53 affects membrane repair. Conclusion: Zn2+ binding to MG53 is required for membrane repair. Significance: This study establishes a base for Zn2+ interaction with MG53 in protection against injury to the cell membrane. Zinc is an essential trace element that participates in a wide range of biological functions, including wound healing. Although Zn2+ deficiency has been linked to compromised wound healing and tissue repair in human diseases, the molecular mechanisms underlying Zn2+-mediated tissue repair remain unknown. Our previous studies established that MG53, a TRIM (tripartite motif) family protein, is an essential component of the cell membrane repair machinery. Domain homology analysis revealed that MG53 contains two Zn2+-binding motifs. Here, we show that Zn2+ binding to MG53 is indispensable to assembly of the cell membrane repair machinery. Live cell imaging illustrated that Zn2+ entry from extracellular space is essential for translocation of MG53-containing vesicles to the acute membrane injury sites for formation of a repair patch. The effect of Zn2+ on membrane repair is abolished in mg53−/− muscle fibers, suggesting that MG53 functions as a potential target for Zn2+ during membrane repair. Mutagenesis studies suggested that both RING and B-box motifs of MG53 constitute Zn2+-binding domains that contribute to MG53-mediated membrane repair. Overall, this study establishes a base for Zn2+ interaction with MG53 in protection against injury to the cell membrane.

Abstract 2612: Zinc inhibits Orai1-mediated Ca2+ signals and proliferation in esophageal cancer cells

Intracellular Ca 2+ signals, including oscillations, regulate proliferation, migration, and other cellular events in cancer cells. Zinc (Zn), an essential micronutrient, has been studied for its chemopreventive effects in several cancers, including esophageal cancer. Although there is a growing research interest in the cross-talk between Ca 2+ and Zn signaling, it remains elusive as to how Zn prevents tumor growth and whether Ca 2+ signaling is involved. Our previous report demonstrated that Orai1, a store-operated Ca 2+ entry (SOCE) channel, is highly expressed in esophageal squamous cell carcinoma (ESCC) compared to normal tissues, and that the elevated expression of Orai1 is strongly associated with poor prognosis in patients. In the current study, we show that physiological levels of Zn can significantly suppress cell proliferation in KYSE-150, a human ESCC cell line. We also show that Zn is able to inhibit Orai1-mediated SOCE and intracellular Ca 2+ oscillations, both which are known as proliferation signals. Based on the topology information of Orai1, we hypothesized that the histidine residue in linker region between transmembrane 1 and 2 of Orai1 (H113) as well as three cysteine residues may play a critical role in Zn-inhibitory effects. Using a point mutation approach, we exhibit that the Zn-inhibitory effects on both SOCE and cell proliferation are vanished in KYSE-150 cells containing Orai1 H113A mutant. Furthermore, KYSE-150 cells expressing Orai1 with any mutation in cysteine residues (C126A, C143A and C195A) display significant loss of Zn-inhibitory functions. These results indicate that the four amino acid residues are likely to be involved in Zn-inhibitory effects on SOCE and cell proliferation. Taken together, our data suggest that dietary Zn may inhibit Orai1-mediated SOCE and intracellular Ca 2+ signaling, which in turn suppresses cell proliferation in ESCC. Further studies are required to search for novel and effective prevention strategies as well as therapeutic options targeting on Zn and Ca 2+ signaling in ESCC. Citation Format: Sangyong Choi, Chaochu Cui, Yanhong Luo, Jae-Kyun Ko, Jianjie Ma, Liwu Fu, Irina Korichneva, Zui Pan. Zinc inhibits Orai1-mediated Ca2+ signals and proliferation in esophageal cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2612.