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MuSK induces in vivo acetylcholine receptor clusters in a ligand-independent manner.

Sander A, Hesser BA, Witzemann V - J. Cell Biol. (2001)

Bottom Line: Expression of kinase-inactive MuSK did not result in the formation of acetylcholine receptor (AChR) clusters, whereas a mutant MuSK lacking the ectodomain did induce AChR clusters.Thus, the kinase activity of MuSK initiates signals that are sufficient to induce the formation of AChR clusters.This process does not require additional determinants located in the ectodomain.

View Article: PubMed Central - PubMed

Affiliation: Abteilung Zellphysiologie, Max-Planck-Institut für Medizinische Forschung, D-69120 Heidelberg, Germany.

ABSTRACT
Muscle-specific receptor tyrosine kinase (MuSK) is required for the formation of the neuromuscular junction. Using direct gene transfer into single fibers, MuSK was expressed extrasynaptically in innervated rat muscle in vivo to identify its contribution to synapse formation. Spontaneous MuSK kinase activity leads, in the absence of its putative ligand neural agrin, to the appearance of epsilon-subunit-specific transcripts, the formation of acetylcholine receptor clusters, and acetylcholinesterase aggregates. Expression of kinase-inactive MuSK did not result in the formation of acetylcholine receptor (AChR) clusters, whereas a mutant MuSK lacking the ectodomain did induce AChR clusters. The contribution of endogenous MuSK was excluded by using genetically altered mice, where the kinase domain of the MuSK gene was flanked by loxP sequences and could be deleted upon expression of Cre recombinase. This allowed the conditional inactivation of endogenous MuSK in single muscle fibers and prevented the induction of ectopic AChR clusters. Thus, the kinase activity of MuSK initiates signals that are sufficient to induce the formation of AChR clusters. This process does not require additional determinants located in the ectodomain.

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Model of MuSK-initiated synapse formation. (1) MuSK accumulates to reach a critical threshold in central regions of developing muscle fibers. Spontaneous kinase activity induces small accumulations of AChR, similar to those we observe upon transgenic expression of MuSK. (2) Agrin released by incoming motor axons supports the formation of denser accumulations by locally activating MuSK, possibly by mechanisms suggested in previous models (Zhou et al., 1999). The local action of MuSK and the local secretion of agrin reinforce each other to generate the extreme density of postsynaptic proteins encountered at the neuromuscular junction. See Discussion for further information.
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fig9: Model of MuSK-initiated synapse formation. (1) MuSK accumulates to reach a critical threshold in central regions of developing muscle fibers. Spontaneous kinase activity induces small accumulations of AChR, similar to those we observe upon transgenic expression of MuSK. (2) Agrin released by incoming motor axons supports the formation of denser accumulations by locally activating MuSK, possibly by mechanisms suggested in previous models (Zhou et al., 1999). The local action of MuSK and the local secretion of agrin reinforce each other to generate the extreme density of postsynaptic proteins encountered at the neuromuscular junction. See Discussion for further information.

Mentions: These results lead to the hypothesis that MuSK signals mediate an increased expression of AChR subunits which are assembled to form complexes with endogenous rapsyn. Postsynaptic specializations develop only at sites where MuSK is concentrated, under natural conditions at contact sites of nerve and muscle. The clustering of AChR that we observe upon transgenic expression of MuSK may thus reflect a physiological process occurring before synapse formation. It was reported that in topoisomerase-deficient mice, motor axons are not developed. Yet, there are AChR concentrated in the central region of the diaphragm muscle (Yang et al., 2000). This suggests that the accumulation of AChR occurs in the absence of neuronal factors. However, AChR are not accumulated in muscle fibers lacking MuSK, although motor axons are present in these muscles (DeChiara et al., 1996), demonstrating that MuSK is required for the accumulation of AChR. While this paper was under review, two papers were published where AChR expression was investigated in embryonic mutant mice lacking agrin, MuSK, rapsyn, and/or motor nerves (Lin et al., 2001; Yang et al., 2001). The results show that MuSK and rapsyn are required for early postsynaptic differentiation which occurs in absence of agrin or motor axons. In this scenario (Fig. 9) MuSK accumulates in the central, developmentally oldest region of muscle fibers. Reaching a critical threshold, spontaneous kinase activity is strong enough to induce accumulations of AChR, similar to those we observe upon transgenic expression of MuSK. In a second step, agrin (and possibly other factors) released by incoming motor axons might favor the formation of denser accumulations by locally activating MuSK. The local action of MuSK and the local secretion of agrin might reinforce each other to generate the extreme density of postsynaptic proteins encountered at the neuromuscular junction.


MuSK induces in vivo acetylcholine receptor clusters in a ligand-independent manner.

Sander A, Hesser BA, Witzemann V - J. Cell Biol. (2001)

Model of MuSK-initiated synapse formation. (1) MuSK accumulates to reach a critical threshold in central regions of developing muscle fibers. Spontaneous kinase activity induces small accumulations of AChR, similar to those we observe upon transgenic expression of MuSK. (2) Agrin released by incoming motor axons supports the formation of denser accumulations by locally activating MuSK, possibly by mechanisms suggested in previous models (Zhou et al., 1999). The local action of MuSK and the local secretion of agrin reinforce each other to generate the extreme density of postsynaptic proteins encountered at the neuromuscular junction. See Discussion for further information.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2199313&req=5

fig9: Model of MuSK-initiated synapse formation. (1) MuSK accumulates to reach a critical threshold in central regions of developing muscle fibers. Spontaneous kinase activity induces small accumulations of AChR, similar to those we observe upon transgenic expression of MuSK. (2) Agrin released by incoming motor axons supports the formation of denser accumulations by locally activating MuSK, possibly by mechanisms suggested in previous models (Zhou et al., 1999). The local action of MuSK and the local secretion of agrin reinforce each other to generate the extreme density of postsynaptic proteins encountered at the neuromuscular junction. See Discussion for further information.
Mentions: These results lead to the hypothesis that MuSK signals mediate an increased expression of AChR subunits which are assembled to form complexes with endogenous rapsyn. Postsynaptic specializations develop only at sites where MuSK is concentrated, under natural conditions at contact sites of nerve and muscle. The clustering of AChR that we observe upon transgenic expression of MuSK may thus reflect a physiological process occurring before synapse formation. It was reported that in topoisomerase-deficient mice, motor axons are not developed. Yet, there are AChR concentrated in the central region of the diaphragm muscle (Yang et al., 2000). This suggests that the accumulation of AChR occurs in the absence of neuronal factors. However, AChR are not accumulated in muscle fibers lacking MuSK, although motor axons are present in these muscles (DeChiara et al., 1996), demonstrating that MuSK is required for the accumulation of AChR. While this paper was under review, two papers were published where AChR expression was investigated in embryonic mutant mice lacking agrin, MuSK, rapsyn, and/or motor nerves (Lin et al., 2001; Yang et al., 2001). The results show that MuSK and rapsyn are required for early postsynaptic differentiation which occurs in absence of agrin or motor axons. In this scenario (Fig. 9) MuSK accumulates in the central, developmentally oldest region of muscle fibers. Reaching a critical threshold, spontaneous kinase activity is strong enough to induce accumulations of AChR, similar to those we observe upon transgenic expression of MuSK. In a second step, agrin (and possibly other factors) released by incoming motor axons might favor the formation of denser accumulations by locally activating MuSK. The local action of MuSK and the local secretion of agrin might reinforce each other to generate the extreme density of postsynaptic proteins encountered at the neuromuscular junction.

Bottom Line: Expression of kinase-inactive MuSK did not result in the formation of acetylcholine receptor (AChR) clusters, whereas a mutant MuSK lacking the ectodomain did induce AChR clusters.Thus, the kinase activity of MuSK initiates signals that are sufficient to induce the formation of AChR clusters.This process does not require additional determinants located in the ectodomain.

View Article: PubMed Central - PubMed

Affiliation: Abteilung Zellphysiologie, Max-Planck-Institut für Medizinische Forschung, D-69120 Heidelberg, Germany.

ABSTRACT
Muscle-specific receptor tyrosine kinase (MuSK) is required for the formation of the neuromuscular junction. Using direct gene transfer into single fibers, MuSK was expressed extrasynaptically in innervated rat muscle in vivo to identify its contribution to synapse formation. Spontaneous MuSK kinase activity leads, in the absence of its putative ligand neural agrin, to the appearance of epsilon-subunit-specific transcripts, the formation of acetylcholine receptor clusters, and acetylcholinesterase aggregates. Expression of kinase-inactive MuSK did not result in the formation of acetylcholine receptor (AChR) clusters, whereas a mutant MuSK lacking the ectodomain did induce AChR clusters. The contribution of endogenous MuSK was excluded by using genetically altered mice, where the kinase domain of the MuSK gene was flanked by loxP sequences and could be deleted upon expression of Cre recombinase. This allowed the conditional inactivation of endogenous MuSK in single muscle fibers and prevented the induction of ectopic AChR clusters. Thus, the kinase activity of MuSK initiates signals that are sufficient to induce the formation of AChR clusters. This process does not require additional determinants located in the ectodomain.

Show MeSH
Related in: MedlinePlus