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Organization of Synaptic Myonuclei by Syne Proteins and Their Role During the Formation of the Nerve–Muscle Synapse

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Organization of Synaptic Myonuclei by Syne Proteins and Their Role During the Formation of the Nerve–Muscle Synapse COMMENTARY Organization of synaptic myonuclei by Syne proteins and their role during the formation of the nerve–muscle synapse Markus A. Ruegg* Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland n most textbooks, the nucleus is drawn as a circle in the center of a cell. However, this is far from the truth, as nuclei can occupy many Idifferent locations in a given cell depend- ing on its stage during cell division, devel- opment, and differentiation. In polarized cells, nuclei are often positioned at the cell boundary (reviewed in ref. 1). Posi- tioning of nuclei and their differential gene expression profile is also a source of cellular differentiation. One of the promi- nent examples is the Drosophila embryo, where differential gene expression in the nuclei of the syncytial blastoderm is the basis for the segmentation of the develop- ing embryo (2). In mammalian muscle, myonuclei are localized at the periphery of individual muscle fibers. At the neuro- muscular junction (NMJ), myonuclei are aggregated underneath the site of contact Fig. 1. Development of the NMJ and the role of Syne-1. (A Left) Motor axons approach muscle fibers with between the presynaptic motor nerve ter- myonuclei (gray) expressing high levels of clustered AChRs (green). (A Right) Mature NMJ with the minal and the postsynaptic muscle fiber presynaptic terminal containing aggregates of synaptic vesicles (white) and many mitochondria (orange). (Fig. 1A). These subsynaptic myonuclei Nerve terminals are wrapped by Schwann cells. At the postsynaptic apparatus, AChRs (green) are are functionally specialized as they synthe- concentrated at the crest of postsynaptic folds and myonuclei (black) and mitochondria accumulate. size proteins localized to the postsynaptic Extrasynaptic myonuclei (white) are transcriptionally distinct form subsynaptic ones. (B Upper) Structure apparatus (black nuclei in Fig. 1A), of mouse Syne-1, the brain-specific alternative splice variant CPG2, and the dominant-negative form encoding the KASH domain used by Grady et al. (3). Red, calponin-type domains; blue, spectrin-like whereas gene expression for these pro- repeats; yellow, KASH domain. (B Lower) At the NMJ, agrin binds to the dystrophin-glycoprotein complex teins is silenced in the myonuclei outside (DGC) via dystroglycan (red), activates MuSK, and aggregates AChRs. Utrophin binds to the DGC and the of the NMJ (white nuclei in Fig. 1A). f-actin cytoskeleton. Syne-1 binds to f-actin and links it with the nuclear envelope (NE) via its KASH domain. Thus, it is hypothesized that the aggrega- Scheme is modified from ref. 16. (C) Mice overexpressing the KASH domain of Syne-1 lack subsynaptic tion of myonuclei is essential for the myonuclei, and mitochondria are moving to the vicinity of the postsynaptic membrane. Gene expression proper development of the NMJ. In a in the perisynaptic (black) and extrasynaptic (white) myonuclei is as in wild-type mice. recent issue of PNAS, Grady et al. (3) provide strong evidence that the myonu- clei-associated protein Syne-1 (for synaptic that contains all components responsible AChR clusters requires the muscle- specific receptor tyrosine kinase MuSK nuclear envelope-1) is a structural compo- to alter the membrane potential upon re- and the AChR-scaffolding molecule nent that is important for the tethering of lease of acetylcholine from the presynaptic rapsyn (9). Upon innervation, the nerve- myonuclei to the NMJ. However, the nerve terminal. Improper function of the released splice version of the large blocking of this aggregation does not im- NMJ is the cause of many diseases that heparansulfate proteoglycan agrin aggre- pair the formation and the function of the are often severe and eventually lead to premature death. They affect the pre- or gates and maintains postsynaptic AChR NMJ. clusters underneath the nerve terminal The NMJ is the best studied synapse in postsynaptic site and can have a genetic basis or might occur sporadically (6). (10). If agrin is not expressed in motor the vertebrate nervous system because of neurons, innervation (i.e., electrical activ- its good accessibility. Both the basic prin- During embryonic development, muscle fibers are formed by the fusion of precur- ity) causes AChR aggregates to disassem- ciple of chemical synaptic transmission ble, which results in perinatal death due sor myoblasts. Basic helix–loop–helix myo- (e.g., quantal release of neurotransmitter; to respiratory failure (reviewed in ref. 11). genic factors expressed in early myotubes ref. 4) and the molecular basis of synapse The AChR-aggregating function of agrin formation (reviewed in ref. 5) have been induce the expression of acetylcholine is mediated by MuSK, although the de- discovered by using the NMJ. The NMJ is receptor (AChR) subunits that compose tailed mechanisms involved in agrin- a highly organized structure destined for the heteropentameric ligand-gated ion MuSK signaling are not known (11). chemical neurotransmission. The presyn- channel. It was observed more than two aptic nerve terminal contains a molecular decades ago (7), and has more recently apparatus for the evoked release of neu- been studied in greater detail (8, 9), that See companion article on page 4359 in issue 12 of volume rotransmitter (acetylcholine at the verte- AChRs form clusters before innervation 102. brate NMJ). The postsynaptic muscle or when innervation is prevented (Fig. *E-mail: [email protected]. fiber assembles a postsynaptic apparatus 1A). The formation of these prepatterned © 2005 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0501516102 PNAS ͉ April 19, 2005 ͉ vol. 102 ͉ no. 16 ͉ 5643–5644 Downloaded by guest on September 30, 2021 Electrical activity also affects the Grady et al. (3) made use of this domi- there are many questions that await fur- many myonuclei outside of the NMJ and nant-negative effect of the KASH domain ther experiments. For example, it has re- causes the repression of gene expression by generating mice overexpressing this cently been shown that variants of Syne-1 for postsynaptic proteins (white nuclei in fragment in skeletal muscle (Fig. 1B). generated by alternative mRNA splicing Fig. 1A). In contrast, gene transcription Myonuclei were not clustered anymore, are localized to intracellular organelles for these proteins—AChR subunits be- but rather remained in a perisynaptic po- such as the Golgi complex in kidney epi- ing the best studied example—is main- sition near the site of innervation (black thelial cells (19). Most interestingly, CPG2 tained in subsynaptic myonuclei (black nuclei in Fig. 1C). Although Syne-1 binds (for candidate plasticity gene 2; see also nuclei in Fig. 1B). This transcriptional to the nuclear envelope protein lamin Fig. 1B) is a brain-specific splice variant of specialization requires agrin–MuSK sig- A͞C (18), staining for lamin A was not Syne-1 (20). This transcript was originally naling, is thought to also involve the altered in the transgenic mice. These re- discovered by a screen for genes that are neuregulin–ErbB pathway (11, 12), and sults clearly show that the KASH domains up-regulated by kainic acid-induced sei- is mediated by the ETS-related tran- of Syne-1 and Syne-2 are involved in the zures in the rat dentate gyrus (21). CPG2 scription factor GA-binding protein tethering of myonuclei to the NMJ but localizes to endocytic zones of excitatory (GABP; ref. 13). not in the organization of the nuclear en- postsynaptic spines in cultured hippocam- Although the evidence is strong that velope per se. It is also interesting that the pal neurons. Importantly, knockdown of agrin–MuSK signaling, in conjunction with extrasynaptic myonuclei were still targeted CPG2 by RNA interference inhibits both the scaffolding molecule rapsyn and the to the periphery of the muscle fibers. This the constitutive and the activity-dependent AChR subunits, constitutes the core com- finding is in contrast to C. elegans, where internalization of AMPA receptors. These ponents in the formation of the postsyn- overexpression of the KASH domain of results therefore expand the potential aptic apparatus at the NMJ (5), the ANC-1 results in ‘‘floating nuclei’’ in the function of Syne proteins and their mechanisms involved in the late steps of cytoplasm of the syncytial cells (17). Simi- alternatively spliced forms and suggest NMJ formation are less well defined. larly, and again in contrast to results in that vertebrates may have adapted this Therefore, it is interesting that Syne-1 was C. elegans, mitochondria were still concen- gene family for tasks involving protein discovered in a yeast two-hybrid screen trated at the NMJ of the transgenic mice. trafficking. using the cytoplasmic domain of MuSK Because myonuclei do not occupy the The paper by Cottrell et al. (20), to- (14), providing a potential link between space just underneath the postsynaptic gether with the work of Grady et al. (3), is agrin–MuSK signaling and the aggregation apparatus, the mitochondria move closer also an example that the NMJ is a good of subsynaptic myonuclei. Syne-1 (also to postsynaptic membrane (Fig. 1C). The model for discovering molecular mecha- called nesprin-1, Myne-1, enaptin, fact that mice show a less severe pheno- nisms that may also act at interneuronal NUANCE, ANC-1, and MSP-300; re- type than C. elegans indicates that higher synapses. Because the mice described by viewed in ref. 15) is a large nuclear enve- vertebrates may use additional compensa- Grady et al. (3) still synthesize alternative lope protein that is expressed as several tory pathways to localize nuclei to a par- forms derived from the syne-1 gene, it will alternatively spliced forms in different ticular position in the cell. be necessary to examine mice in which the tissues (Fig. 1B). Among the tissues with To me, the most interesting findings of entire gene was deleted. Last but not highest expression of Syne-1 are the heart Grady et al.
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