Tomohiro Kurisaki

A metalloprotease-disintegrin, MDC9/meltrin-gamma/ADAM9 and PKCdelta are involved in TPA-induced ectodomain shedding of membrane-anchored heparin-binding EGF-like growth factor.

The ectodomains of many proteins located at the cell surface are shed upon cell stimulation. One such protein is the heparin‐binding EGF‐like growth factor (HB‐EGF) that exists in a membrane‐anchored form which is converted to a soluble form upon cell stimulation with TPA, an activator of protein kinase C (PKC). We show that PKCδ binds in vivo and in vitro to the cytoplasmic domain of MDC9/meltrin‐γ/ADAM9, a member of the metalloprotease–disintegrin family. Furthermore, the presence of constitutively active PKCδ or MDC9 results in the shedding of the ectodomain of proHB‐EGF, whereas MDC9 mutants lacking the metalloprotease domain, as well as kinase‐negative PKCδ, suppress the TPA‐induced shedding of the ectodomain. These results suggest that MDC9 and PKCδ are involved in the stimulus‐coupled shedding of the proHB‐EGF ectodomain.

Roles of meltrin beta/ADAM19 in the processing of neuregulin

Meltrin β/ADAM19 is a member of ADAMs (a d isintegrin andmetalloproteases), which are a family of membrane-anchored glycoproteins that play important roles in fertilization, myoblast fusion, neurogenesis, and proteolytic processing of several membrane-anchored proteins. The expression pattern ofmeltrin β during mouse development coincided well with that of neuregulin-1 (NRG), a member of the epidermal growth factor family. Then we examined whether meltrin β participates in the proteolytic processing of membrane-anchored NRGs. When NRG-β1 was expressed in mouse L929 cells, its extracellular domain was constitutively processed and released into the culture medium. This basal processing activity was remarkably potentiated by overexpression of wild-type meltrin β, which lead to the significant decrease in the cell surface exposure of extracellular domains of NRG-β1. Furthermore, expression of protease-deficient mutants of meltrin β exerted dominant negative effects on the basal processing of NRG-β1. These results indicate that meltrin β participates in the processing of NRG-β1. Since meltrin β affected the processing of NRG-β4 but not that of NRG-α2, meltrin β was considered to have a preference for β-type NRGs as substrate. Furthermore, the effects of the secretory pathway inhibitors suggested that meltrin β participates in the intracellular processing of NRGs rather than the cleavage on the cell surface.

Phenotypic Analysis of Meltrin α (ADAM12)-Deficient Mice: Involvement of Meltrin α in Adipogenesis and Myogenesis

ABSTRACT Meltrin α (ADAM12) is a metalloprotease-disintegrin whose specific expression patterns during development suggest that it is involved in myogenesis and the development of other organs. To determine the roles Meltrin α plays in vivo, we generated Meltrin α-deficient mice by gene targeting. Although the number of homozygous embryos are close to the expected Mendelian ratio at embryonic days 17 to 18, ca. 30% of the null pups born die before weaning, mostly within 1 week of birth. The viable homozygous mutants appear normal and are fertile. Most of the muscles in the homozygous mutants appear normal, and regeneration in experimentally damaged skeletal muscle is unimpeded. In some Meltrin α-deficient pups, the interscapular brown adipose tissue is reduced, although the penetrance of this phenotype is low. Impaired formation of the neck and interscapular muscles is also seen in some homozygotes. These observations suggest Meltrin α may be involved in regulating adipogenesis and myogenesis through a linked developmental pathway. Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is a candidate substrate of Meltrin α, and we found that TPA (12-O-tetradecanoylphorbol-13-acetate)-induced ectodomain shedding of HB-EGF is markedly reduced in embryonic fibroblasts prepared from Meltrin α-deficient mice. We also report here the chromosomal locations of Meltrin α in the mouse and rat.

Spatially- and temporally-restricted expression of meltrin α (ADAM12) and β (ADAM19) in mouse embryo

Abstract The cloning of the full-length cDNA encoding meltrin β (ADAM19), one of the metalloprotease-disintegrins expressed in mouse myogenic cells, revealed that the meltrin β gene encodes a membrane protein closely related to meltrin α (ADAM12) which participates in myotube formation in vitro. To delineate the functions of meltrin α and β, we examined the expression patterns of their transcripts during embryogenesis. The meltrin α gene is activated in condensed mesenchymal cells that give rise to skeletal muscle, bones and visceral organs. Meltrin β mRNA, in contrast, is markedly expressed in craniofacial and dorsal root ganglia and ventral horns of the spinal cord, where peripheral neuronal cell lineages differentiate. Heart, skeletal muscle, intestine and lung also express meltrin β mRNA transiently. Although the meltrin α and β transcripts exhibit distinct expression patterns during embryogenesis, both genes are mainly activated in mesenchymal cells that are derived from both mesoderm and ectoderm.

Lipid rafts identified as locations of ectodomain shedding mediated by meltrin β/ADAM19

Meltrin β (Mel β, also called ADAM19) is a member of the ADAM (adisintegrin and metalloprotease) family, which are membrane‐anchored glycoproteins that play crucial roles in various biological processes. Many intercellular signaling molecules are membrane‐anchored proteins, which are proteolytically processed after becoming membrane‐bound, to liberate their extracellular domains (ectodomain shedding). Genetic and biochemical studies have shown that some ADAMs participate in these events. We found previously that Mel β can cleave the extracellular region of the membrane‐anchored β‐exon‐containing Neuregulin‐1 (NRG β1) protein, which is one of the main ligands for the neural ErbB receptor. Mel β‐deficient mice showed developmental defects in the nervous system. These observations raise the possibility that the NRG ectodomain shedding mediated by Mel β is closely related to the neural development. Here we show that Mel β‐mediated ectodomain shedding of NRG β1 takes place in the lipid rafts of neurons. The lipid rafts localization of Mel β requires its membrane‐anchoring region, and NRG β1 ectodomain shedding is not enhanced if Mel β cannot reach the lipid rafts. These results indicate that localization of Mel β in lipid rafts is critical for its ectodomain shedding.

Dynamic clustering and dispersion of lipid rafts contribute to fusion competence of myogenic cells

Recent research indicates that the leading edge of lamellipodia of myogenic cells (myoblasts and myotubes) contains presumptive fusion sites, yet the mechanisms that render the plasma membrane fusion-competent remain largely unknown. Here we show that dynamic clustering and dispersion of lipid rafts contribute to both cell adhesion and plasma membrane union during myogenic cell fusion. Adhesion-complex proteins including M-cadherin, beta-catenin, and p120-catenin accumulated at the leading edge of lamellipodia, which contains the presumptive fusion sites of the plasma membrane, in a lipid raft-dependent fashion prior to cell contact. In addition, disruption of lipid rafts by cholesterol depletion directly prevented the membrane union of myogenic cell fusion. Time-lapse recording showed that lipid rafts were laterally dispersed from the center of the lamellipodia prior to membrane fusion. Adhesion proteins that had accumulated at lipid rafts were also removed from the presumptive fusion sites when lipid rafts were laterally dispersed. The resultant lipid raft- and adhesion complex-free area at the leading edge fused with the opposing plasma membrane. These results demonstrate a key role for dynamic clustering/dispersion of lipid rafts in establishing fusion-competent sites of the myogenic cell membrane, providing a novel mechanistic insight into the regulation of myogenic cell fusion.

Meltrin β/ADAM19 Interacting with EphA4 in Developing Neural Cells Participates in Formation of the Neuromuscular Junction

Background Development of the neuromuscular junction (NMJ) is initiated by the formation of postsynaptic specializations in the central zones of muscles, followed by the arrival of motor nerve terminals opposite the postsynaptic regions. The post- and presynaptic components are then stabilized and modified to form mature synapses. Roles of ADAM (A Disintegrin And Metalloprotease) family proteins in the formation of the NMJ have not been reported previously. Principal Findings We report here that Meltrin β, ADAM19, participates in the formation of the NMJ. The zone of acetylcholine receptor α mRNA distribution was broader and excess sprouting of motor nerve terminals was more prominent in meltrin β–deficient than in wild-type embryonic diaphragms. A microarray analysis revealed that the preferential distribution of ephrin-A5 mRNA in the synaptic region of muscles was aberrant in the meltrin β–deficient muscles. Excess sprouting of motor nerve terminals was also found in ephrin-A5 knockout mice, which lead us to investigate a possible link between Meltrin β and ephrin-A5-Eph signaling in the development of the NMJ. Meltrin β and EphA4 interacted with each other in developing motor neurons, and both of these proteins localized in the NMJ. Coexpression of Meltrin β and EphA4 strongly blocked vesicular internalization of ephrin-A5–EphA4 complexes without requiring the protease activity of Meltrin β, suggesting a regulatory role of Meltrin β in ephrin-A5-Eph signaling. Conclusion Meltrin β plays a regulatory role in formation of the NMJ. The endocytosis of ephrin-Eph complexes is required for efficient contact-dependent repulsion between ephrin and Eph. We propose that Meltrin β stabilizes the interaction between ephrin-A5 and EphA4 by regulating endocytosis of the ephrinA5-EphA complex negatively, which would contribute to the fine-tuning of the NMJ during development.

Meltrin β mini, a new ADAM19 isoform lacking metalloprotease and disintegrin domains, induces morphological changes in neuronal cells1

Meltrin β (ADAM19) is a metalloprotease‐disintegrin expressed in the peripheral nervous system and other organs during embryogenesis. We report here an alternatively spliced isoform, meltrin β mini, that lacks the prodomain, metalloprotease and disintegrin domains. A comparison of the cDNA and genomic sequences suggested the existence of a new exon. This isoform was detected in murine dorsal root ganglion and neuronal cell lines by RT‐PCR. Overexpression of meltrin β mini but not meltrin β induced neurite outgrowth in neuronal cells. These studies suggest that the novel meltrin β isoform has a distinct function related to neurogenesis.

Generation of a monoclonal antibody reactive to prefusion myocytes

We established a novel monoclonal antibody, Yaksa that is specific to a subpopulation of myogenic cells. The Yaksa antigen is not expressed on the surface of growing myoblasts but only on a subpopulation of myogenin-positive myocytes. When Yaksa antigen-positive mononucleated cells were freshly prepared from a murine myogenic cell by a cell sorter, they fused with each other and formed multinucleated myotubes shortly after replating while Yaksa antigen-negative cells scarcely generated myotubes. These results suggest that Yaksa could segregate fusion-competent, mononucleated cells from fusion–incompetent cells during muscle differentiation. The Yaksa antigen was also expressed in developing muscle and regenerating muscle in vivo and it was localized at sites of cell–cell contact between mono-nucleated muscle cells and between mono-nucleated muscle cells and myotubes. Thus, Yaksa that marks prefusion myocytes before myotube formation can be a useful tool to elucidate the cellular and molecular mechanisms of myogenic cell fusion.