Postsynaptic Membranes at the Neuromuscular Junction: Molecular Organisation

Abstract

Accurate neurotransmission between motor nerve and muscle fibres at the vertebrate cholinergic neuromuscular junction (NMJ) depends on the differentiation of highly specialised structures both pre‐ and postsynaptically. The accumulation of nicotinic acetylcholine receptors (AChRs) and voltage‐gated sodium channels represents the hallmark of postsynaptic membrane differentiation. Several synaptic organising proteins are required for the aggregation of postsynaptic AChRs. The muscle‐specific receptor tyrosine kinase (MuSK) provides a structural scaffold necessary to initiate aggregates of postsynaptic molecules. Agrin – a nerve‐derived extracellular matrix glycoprotein – is required for AChR clustering probably by activating MuSK activity. Rapsyn – the AChR‐associated peripheral protein acting downstream of agrin–MuSK signalling – is essential for AChR clustering. These proteins, together with a wealth of other components of the synapse, cooperate in multiple ways to play both structural and signalling roles in synaptic differentiation. Synaptopathies of the NMJ result from mutations in several key players of synaptic differentiation.

Key Concepts:

  • Efficient communication between neurons or between neurons and their targets requires the enrichment of synaptic proteins, in particular, ligand‐gated ion channels at postsynaptic sites.

  • The NMJ represents a particularly striking example of accumulation of nicotinic acetylcholine receptors in the postsynaptic membrane and is an ideal model to study receptor clustering.

  • Formation and maintenance of the postsynaptic membrane is a complex mechanism involving synaptic organising molecules derived both from nerve and muscle.

  • The muscle tyrosine kinase, MuSK, is the master organiser of the NMJ capable to initiate clustering of postsynaptic components, even in the absence of neuronal cues.

  • Mutations in several components of the postsynaptic membrane (not only in AChR subunits genes but also in agrin, ColQ, DOK‐7, laminin, MuSK, rapsyn, etc.) result in severe dysfunction of the NMJ: the congenital myasthenic syndromes.

Keywords: acetylcholinesterase; agrin; congenital myasthenic syndromes; nicotinic acetylcholine receptor; voltage‐sensitive sodium channel; rapsyn; synapse

Figure 1.

Electron micrographs of the postsynaptic apparatus of the mouse NMJ. The postsynaptic membrane just beneath the nerve ending (NE) is folded up into numerous folds. Note the electron‐dense appearance of the membrane at the top of the folds where AChRs are concentrated (arrows in (b)). Magnifications: (a) ×20 000, (b) ×38 000. Inset: Double fluorescence image showing the localisation of AChRs (labelled with fluorescein‐conjugated α‐bungarotoxin, green fluorescence) and ankyrin‐G (red fluorescence), at the top and at the base of the folds, respectively. Magnification ×1600.

Figure 2.

(a) Electron micrograph showing the densely packed AChR rosettes in purified AChR‐rich membrane from Torpedo electric tissue (negative staining; magnification ×300 000). Inset: Detail of an AChR molecule exhibiting a pentameric structure; magnification ×600 000. (b) Electron micrograph of a freeze‐etched AChR‐rich postsynaptic membrane fragment showing the quasi‐geometrical arrangement of the AChRs in the plane of the membrane (magnification ×200 000).

Figure 3.

Electron micrographs of negatively stained AChE molecules. The asymmetric form A12 (inset) aggregates at low ionic strength and in the presence of polyanions into discrete assemblies containing up to six molecules organised head to tail at both ends of a bundle of collagenic subunits. Similar aggregates are likely to occur in situ in the basal lamina. Arrows point to the collagenic tail in the isolated A12 molecule in the inset and in one aggregate. Magnification ×300 000.

Figure 4.

Model of the molecular specialisation of the postsynaptic membrane and of the basal lamina at the vertebrate NMJ. Distinct sets of molecules are segregated in the crests (a) and the troughs (b) of the postjunctional folds. The representation of the molecular organisation of the membrane and extracellular matrix is oversimplified to highlight the interactions between the major structural and functional components in both domains. Molecules are not drawn at scale and the stoichiometry between the various elements is roughly indicated. ECM, extracellular matrix and PM, postsynaptic membrane.

Figure 5.

Schematic representation of the signalling pathways leading to AChR clustering and stabilisation. Agrin released from nerve terminal activates the Lrp4/MuSK complex that in turn activates multiple signalling pathways leading to AChR clustering through remodelling of the actin cytoskeleton. Tid1 operates downstream of MuSK by incorporating Dok‐7 in the complex in an agrin‐dependant manner. Tid1 may trigger reorganisation of the postsynaptic cytoskeleton by interacting with APC, and by activating small Rho GTPases through Dvl and PAK1 localised at synaptic sites via activation of MuSK. Rapsyn is also believed to be required for agrin‐induced AChR clustering and stabilisation via Src‐like kinase presentation to the AChR. Wnt signalling has recently been implicated in the formation of the NMJ. Acetylcholine may disperse AChR clusters possibly by a mechanism involving Cdk5. Stars represent tyrosine phosphorylation of MuSK and AChRs. Adapted from Song and Balice‐Gordon .

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References

Allen JA, Halverson‐Tamboli RA and Rasenick MM (2007) Lipid raft microdomains and neurotransmitter signalling. Nature Reviews. Neuroscience 8: 128–140.

Arikawa‐Hirasawa E, Rossi SG, Rotundo RL and Yamada Y (2002) Absence of acetylcholinesterase at the neuromuscular junctions of perlecan‐null mice. Nature Neuroscience 5: 119–123.

Ayalon G, Davis JQ, Scotland PB and Bennett V (2008) An ankyrin‐based mechanism for functional organization of dystrophin and dystroglycan. Cell 135: 1189–1200.

Bailey SJ, Stocksley MA, Buckel A, Young C and Slater CR (2003) Voltage‐gated sodium channels and ankyrinG occupy a different postsynaptic domain from acetylcholine receptors from an early stage of neuromuscular junction maturation in rats. Journal of Neuroscience 23: 2102–2111.

Banks GB, Chamberlain JS and Froehner SC (2009) Truncated dystrophins can influence neuromuscular synapse structure. Molecular and Cellular Neuroscience 40: 433–441.

Banks GB, Fuhrer C, Adams ME and Froehner SC (2003) The postsynaptic submembrane machinery at the neuromuscular junction: requirement for rapsyn and the utrophin/dystrophin‐associated complex. Journal of Neurocytology 32: 709–726.

Cartaud J, Benedetti EL, Sobel A and Changeux JP (1976) A morphological study of the cholinergic receptor protein from Torpedo marmorata in its membrane environment and its detergent‐extracted purified form. Journal of Cell Science 29: 313–337.

Cartaud A, Strochlic L, Guerra M et al. (2004) MuSK is required for anchoring acetylcholinesterase at the neuromuscular junction. Journal of Cell Biology 165: 505–515.

Colledge M and Froehner SC (1998) Signals mediating ion channel clustering at the neuromuscular junction. Current Opinion in Neurobiology 8: 357–363.

Côté PD, Moukhles H, Lindenbaum M and Carbonetto S (1999) Chimaeric mice deficient in dystroglycans develop muscular dystrophy and have disrupted myoneural synapses. Nature Genetics 23: 338–342.

Couteaux R (1981) Structure of the subsynaptic sarcoplasm in the interfolds of the frog neuromuscular junction. Journal of Neurocytology 10: 947–962.

DeChiara TM, Bowen DC, Valenzuela MV, Simmons MV and Poueymirou WT (1996) The receptor tyrosine kinase MuSK is required for neuromuscular junction in vivo. Cell 85: 501–512.

Engel AG, Ohno K and Sine SM (2003) Sleuthing molecular targets for neurological deseases at the neuromuscular junction. Nature Reviews. Neuroscience 4: 339–352.

Escher P, Lacazette E, Courtet M et al. (2005) Synapses form in skeletal muscles lacking neuregulin receptors. Science 308: 1920–1923.

Fertuck HC and Salpeter MM (1974) Localization of acetylcholine receptor by 125I‐labeled alpha‐bungarotoxin binding at mouse motor endplates. Proceedings of the National Academy of Sciences of the USA 71: 1376–1368.

Froehner SC (1993) Regulation of ion channel distribution at synapses. Annual Review of Neuroscience 16: 347–368.

Gautam M, Noakes PG, Moscoso L et al. (1996) Defective neuromuscular synaptogenesis in agrin‐deficient mutant mice. Cell 85: 525–535.

Gautam M, Noakes PG, Mudd J et al. (1995) Failure of postsynaptic specialization to develop at neuromuscular junctions of rapsyn‐deficient mice. Nature 377: 232–236.

Grady R, Zhou H, Cunningham J et al. (2000) Maturation and maintenance of the neuromuscular synapse: genetic evidence for roles of the dystrophin–glycoprotein complex. Neuron 25: 279–293.

Hantaï D, Richard P, Koenig J and Eymard B (2004) Congenital myasthenic syndromes. Current Opinion in Neurology 17: 539–551.

Heuser JE and Salpeter SR (1979) Organization of acetylcholine receptors in quick‐frozen, deep‐etched, and rotary‐replicated Torpedo postsynaptic membrane. Journal of Cell Biology 82: 150–173.

Herbst R and Burden SJ (2000) The juxtamembrane region of MuSK has a critical role in agrin‐mediated signaling. EMBO Journal 19: 67–77.

Kim N, Stiegler AL, Cameron TO et al. (2008) Lrp4 is a receptor for agrin and forms a complex with MuSK. Cell 135: 334–342.

Korkut C and Budnik V (2009) WNTs tune up the neuromuscular junction. Nature Reviews. Neuroscience 10: 627–634.

Lemaillet G, Walker B and Lambert S (2003) Identification of a conserved ankyrin‐binding motif in the family of sodium channel alpha subunits. Journal of Biological Chemistry 278: 27333–27339.

Lin W, Burgess RW, Dominguez B et al. (2001) Distinct roles of nerve and muscle in postsynaptic differentiation of the neuromuscular synapse. Nature 410: 1057–1064.

Linnoila J, Wang Y, Yao Y and Wang ZZ (2008) A mammalian homolog of Drosophila tumorous imaginal discs, Tid1, mediates agrin signaling at the neuromuscular junction. Neuron 60: 625–641.

Massoulié J and Millard CB (2009) Cholinesterases and the basal lamina at vertebrate neuromuscular junctions. Current Opinion in Pharmacology 9: 316–325.

McMahan UJ (1990) The agrin hypothesis. Cold Spring Harbor Symposia on Quantitative Biology 55: 407–418.

Misgeld T, Kummer TT, Lichtman JW and Sanes JR (2005) Agrin promotes synaptic differentiation by counteracting an inhibitory effect of neurotransmitter. Proceedings of the National Academy of Sciences of the USA 102: 11088–11093.

Okada K, Inoue A, Okada M et al. (2006) The muscle protein Dok‐7 is essential for neuromuscular synaptogenesis. Science 312: 1802–1805.

Patton BL (2000) Laminins of the neuromuscular system. Microscopy Research and Technique 5: 247–261.

Pilgram GS, Potikanond S, Baines RA, Fradkin LG and Noordermeer JN (2009) The roles of the dystrophin‐associated glycoprotein complex at the synapse. Molecular Neurobiology doi: 10.1007/s12035‐009‐8089‐5.

Ramarao MK and Cohen JB (1998) Mechanism of nicotinic acetylcholine receptor cluster formation by rapsyn. Proceedings of the National Academy of Sciences of the USA 95: 4007–4012.

Ruegg MA and Bixby JL (1998) Agrin orchestrates synaptic differentiation at the vertebrate neuromuscular junction. Trends in Neuroscience 21: 22–27.

Schaeffer L, de Kerchove d'Exaerde A and Changeux JP (2001) Targeting transcription to the neuromuscular synapse. Neuron 31: 15–22.

Simons K and Ikonen E (1997) Functional rafts in cell membranes. Nature 387: 569–572.

Song Y and Balice‐Gordon R (2008) New dogs in the dogma: Lrp4 and Tid1 in neuromuscular synapse formation. Neuron 60: 526–528.

Strochlic L, Cartaud A and Cartaud J (2005) The synaptic muscle‐specific kinase (MuSK) complex: new partners, new functions. BioEssays 27: 1129–1135.

Sunada Y, Campbell KP (1995) Dystrophin‐glycoprotein complex: molecular organization and critical roles in skeletal muscle. Current Opinion in Neurology 8: 379–384.

Wang Q, Zhang B, Xiong WC and Mei L (2006) MUSK signaling at the neuromuscular junction. Journal of Molecular Neuroscience 30: 223–226.

Yang X, Arber S, William C et al. (2001) Patterning of muscle acetylcholine receptor gene expression in the absence of motor innervation. Neuron 30: 399–410.

Zhang B, Luo S, Wang Q et al. (2008) LRP4 serves as a coreceptor of agrin. Neuron 60: 285–297.

Further Reading

Arber S, Burden SJ and Harris AJ (2002) Patterning of skeletal muscle. Current Opinion in Neurobiology 12: 100–103.

Bezakova G and Ruegg M (2003) New insights into the roles of agrin. Nature Reviews. Molecular Cell Biology 4: 295–308.

Burden SJ (2002) Building the vertebrate neuromuscular synapse. Journal of Neurobiology 53: 501–511.

Engel AG, Shen XM, Selcen D and Sine SM (2010) What have we learn on the congenital myasthenic syndromes. Journal of Molecular Neuroscience 40: 143–153.

Kummer TT, Misgeld T and Sanes JR (2006) Assembly of the postsynaptic membrane at the neuromuscular junction: paradigm lost. Current Opinion in Neurobiology 16: 74–82.

Shao D, Okuse K and Djamgoz MB (2009) Protein‐protein interactions involving voltage‐gated sodium channels: Post‐translational regulation, intracellular trafficking and functional expression. International Journal of Biochemistry & Cell Biology 41: 1471–1481.

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Cartaud, Jean, Kordeli, Ekaterini, and Cartaud, Annie(Apr 2010) Postsynaptic Membranes at the Neuromuscular Junction: Molecular Organisation. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000252.pub2]