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A synaptic nidogen: developmental regulation and role of nidogen-2 at the neuromuscular junction.

Fox MA, Ho MS, Smyth N, Sanes JR - Neural Dev (2008)

Bottom Line: All four core components of the basal lamina have synaptically enriched isoforms.Together, they form a highly specialized synaptic cleft material.Individually, they play distinct roles in the formation, maturation and maintenance of the neuromuscular junction.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Molecular and Cellular Biology and Center for Brain Science, Harvard University, Cambridge, MA 02138, USA. mafox@vcu.edu

ABSTRACT

Background: The skeletal neuromuscular junction is a useful model for elucidating mechanisms that regulate synaptogenesis. Developmentally important intercellular interactions at the neuromuscular junction are mediated by the synaptic portion of a basal lamina that completely ensheaths each muscle fiber. Basal laminas in general are composed of four main types of glycosylated proteins: laminins, collagens IV, heparan sulfate proteoglycans and nidogens (entactins). The portion of the muscle fiber basal lamina that passes between the motor nerve terminal and postsynaptic membrane has been shown to bear distinct isoforms of the first three of these. For laminins and collagens IV, the proteins are deposited by the muscle; a synaptic proteoglycan, z-agrin, is deposited by the nerve. In each case, the synaptic isoform plays key roles in organizing the neuromuscular junction. Here, we analyze the fourth family, composed of nidogen-1 and -2.

Results: In adult muscle, nidogen-1 is present throughout muscle fiber basal lamina, while nidogen-2 is concentrated at synapses. Nidogen-2 is initially present throughout muscle basal lamina, but is lost from extrasynaptic regions during the first three postnatal weeks. Neuromuscular junctions in mutant mice lacking nidogen-2 appear normal at birth, but become topologically abnormal as they mature. Synaptic laminins, collagens IV and heparan sulfate proteoglycans persist in the absence of nidogen-2, suggesting the phenotype is not secondary to a general defect in the integrity of synaptic basal lamina. Further genetic studies suggest that synaptic localization of each of the four families of synaptic basal lamina components is independent of the other three.

Conclusion: All four core components of the basal lamina have synaptically enriched isoforms. Together, they form a highly specialized synaptic cleft material. Individually, they play distinct roles in the formation, maturation and maintenance of the neuromuscular junction.

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Morphological defects in nid2-/- neuromuscular junctions (NMJs) are due to improper maturation and maintenance. (A, B) NMJs from diaphragms of P7 (A) and P21 (B) nid2-/- mutants and aged-matched controls. Pre- and postsynaptic elements are labeled with anti-synaptotagmin 2 antibodies (syt 2) and α-bungarotoxin (BTX), respectively. No obvious defects were present at either age. (C) NMJs from diaphragms of 1-year-old mutants and age-matched controls. Mutant NMJs were fragmented at 1 year of age but were not appreciably more severely affected than in 2-month-old mutants (Figure 5). Scale bars are 20 μm.
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Figure 6: Morphological defects in nid2-/- neuromuscular junctions (NMJs) are due to improper maturation and maintenance. (A, B) NMJs from diaphragms of P7 (A) and P21 (B) nid2-/- mutants and aged-matched controls. Pre- and postsynaptic elements are labeled with anti-synaptotagmin 2 antibodies (syt 2) and α-bungarotoxin (BTX), respectively. No obvious defects were present at either age. (C) NMJs from diaphragms of 1-year-old mutants and age-matched controls. Mutant NMJs were fragmented at 1 year of age but were not appreciably more severely affected than in 2-month-old mutants (Figure 5). Scale bars are 20 μm.

Mentions: Neuromuscular abnormalities observed in nidogen-2 mutants could reflect a role of nidogen-2 in the formation, maturation or maintenance of the NMJ. To distinguish these possibilities, we examined mutant muscles at P7, when NMJs are quite immature, and at P21, soon after the early postnatal period of synapse elimination is complete [39]. No obvious defects were observed in nidogen-2 mutants at either age (Figure 6A, B). Thus, nidogen-2 appears to be dispensable for formation and remodelling of the NMJ, but required for its full maturation or maintenance. To ask whether the defects are progressive, we examined diaphragms from 1-year-old nidogen-2 mutants. Defects were not appreciably more severe in 12-month-old mutants than in 2-month-old mutants. Thus, nidogen-2 is required for final stages in the maturation of the NMJ.


A synaptic nidogen: developmental regulation and role of nidogen-2 at the neuromuscular junction.

Fox MA, Ho MS, Smyth N, Sanes JR - Neural Dev (2008)

Morphological defects in nid2-/- neuromuscular junctions (NMJs) are due to improper maturation and maintenance. (A, B) NMJs from diaphragms of P7 (A) and P21 (B) nid2-/- mutants and aged-matched controls. Pre- and postsynaptic elements are labeled with anti-synaptotagmin 2 antibodies (syt 2) and α-bungarotoxin (BTX), respectively. No obvious defects were present at either age. (C) NMJs from diaphragms of 1-year-old mutants and age-matched controls. Mutant NMJs were fragmented at 1 year of age but were not appreciably more severely affected than in 2-month-old mutants (Figure 5). Scale bars are 20 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2567315&req=5

Figure 6: Morphological defects in nid2-/- neuromuscular junctions (NMJs) are due to improper maturation and maintenance. (A, B) NMJs from diaphragms of P7 (A) and P21 (B) nid2-/- mutants and aged-matched controls. Pre- and postsynaptic elements are labeled with anti-synaptotagmin 2 antibodies (syt 2) and α-bungarotoxin (BTX), respectively. No obvious defects were present at either age. (C) NMJs from diaphragms of 1-year-old mutants and age-matched controls. Mutant NMJs were fragmented at 1 year of age but were not appreciably more severely affected than in 2-month-old mutants (Figure 5). Scale bars are 20 μm.
Mentions: Neuromuscular abnormalities observed in nidogen-2 mutants could reflect a role of nidogen-2 in the formation, maturation or maintenance of the NMJ. To distinguish these possibilities, we examined mutant muscles at P7, when NMJs are quite immature, and at P21, soon after the early postnatal period of synapse elimination is complete [39]. No obvious defects were observed in nidogen-2 mutants at either age (Figure 6A, B). Thus, nidogen-2 appears to be dispensable for formation and remodelling of the NMJ, but required for its full maturation or maintenance. To ask whether the defects are progressive, we examined diaphragms from 1-year-old nidogen-2 mutants. Defects were not appreciably more severe in 12-month-old mutants than in 2-month-old mutants. Thus, nidogen-2 is required for final stages in the maturation of the NMJ.

Bottom Line: All four core components of the basal lamina have synaptically enriched isoforms.Together, they form a highly specialized synaptic cleft material.Individually, they play distinct roles in the formation, maturation and maintenance of the neuromuscular junction.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Molecular and Cellular Biology and Center for Brain Science, Harvard University, Cambridge, MA 02138, USA. mafox@vcu.edu

ABSTRACT

Background: The skeletal neuromuscular junction is a useful model for elucidating mechanisms that regulate synaptogenesis. Developmentally important intercellular interactions at the neuromuscular junction are mediated by the synaptic portion of a basal lamina that completely ensheaths each muscle fiber. Basal laminas in general are composed of four main types of glycosylated proteins: laminins, collagens IV, heparan sulfate proteoglycans and nidogens (entactins). The portion of the muscle fiber basal lamina that passes between the motor nerve terminal and postsynaptic membrane has been shown to bear distinct isoforms of the first three of these. For laminins and collagens IV, the proteins are deposited by the muscle; a synaptic proteoglycan, z-agrin, is deposited by the nerve. In each case, the synaptic isoform plays key roles in organizing the neuromuscular junction. Here, we analyze the fourth family, composed of nidogen-1 and -2.

Results: In adult muscle, nidogen-1 is present throughout muscle fiber basal lamina, while nidogen-2 is concentrated at synapses. Nidogen-2 is initially present throughout muscle basal lamina, but is lost from extrasynaptic regions during the first three postnatal weeks. Neuromuscular junctions in mutant mice lacking nidogen-2 appear normal at birth, but become topologically abnormal as they mature. Synaptic laminins, collagens IV and heparan sulfate proteoglycans persist in the absence of nidogen-2, suggesting the phenotype is not secondary to a general defect in the integrity of synaptic basal lamina. Further genetic studies suggest that synaptic localization of each of the four families of synaptic basal lamina components is independent of the other three.

Conclusion: All four core components of the basal lamina have synaptically enriched isoforms. Together, they form a highly specialized synaptic cleft material. Individually, they play distinct roles in the formation, maturation and maintenance of the neuromuscular junction.

Show MeSH
Related in: MedlinePlus