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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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Developmental regulation of nidogens and laminin α chains in synaptic basal lamina (BL). Cross-sections of muscle from P0 (A, D, G, J), P14 (B, E, H, K) and P21 (C, F, I, L) mice, stained for α-bungarotoxin (BTX) and nidogens or laminins. (A-F) While nidogen-1 expression changes little during development (A-C), nidogen-2 is present in both synaptic and extrasynaptic BLs at birth (D) and becomes restricted to synaptic BL postnatally (E, F). (G-L) Laminins α4 (G-I) and α5 (J-L) become restricted to synaptic BL in parallel with nidogen-2. Scale bar is 15 μm for (A, D, G, J) and 10 μm for all other panels.
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Figure 4: Developmental regulation of nidogens and laminin α chains in synaptic basal lamina (BL). Cross-sections of muscle from P0 (A, D, G, J), P14 (B, E, H, K) and P21 (C, F, I, L) mice, stained for α-bungarotoxin (BTX) and nidogens or laminins. (A-F) While nidogen-1 expression changes little during development (A-C), nidogen-2 is present in both synaptic and extrasynaptic BLs at birth (D) and becomes restricted to synaptic BL postnatally (E, F). (G-L) Laminins α4 (G-I) and α5 (J-L) become restricted to synaptic BL in parallel with nidogen-2. Scale bar is 15 μm for (A, D, G, J) and 10 μm for all other panels.

Mentions: The distribution of some laminins, collagens IV, and HSPGs in muscle fiber BL changes as development proceeds [9,11,12,44]. We asked whether the same was true of nidogens. At birth, both nidogen-1 and nidogen-2 were present throughout both synaptic and extrasynaptic muscle fiber BL (Figure 4A, D). Similar patterns were present through the first postnatal week (data not shown). Moreover, levels of nidogen-1 in synaptic and extrasynaptic BL remained similar into adulthood (Figures 4B, C). In contrast, levels of nidogen-2 decreased in extrasynaptic BL during the second postnatal week, so synaptic and Schwann cell BL contained more nidogen-2 than extrasynaptic BL by postnatal day (P) 14 (Figure 4E). By P21, nidogen-2 was undetectable in extrasynaptic BL (Figure 4F). Thus, nidogen-2 becomes restricted to synaptic sites as the NMJ matures. Because our methods are not quantitative, we do not know whether the decreased abundance of nidogen-2 in extrasynaptic BL is accompanied by an increased abundance in synaptic and Schwann cell BLs.


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)

Developmental regulation of nidogens and laminin α chains in synaptic basal lamina (BL). Cross-sections of muscle from P0 (A, D, G, J), P14 (B, E, H, K) and P21 (C, F, I, L) mice, stained for α-bungarotoxin (BTX) and nidogens or laminins. (A-F) While nidogen-1 expression changes little during development (A-C), nidogen-2 is present in both synaptic and extrasynaptic BLs at birth (D) and becomes restricted to synaptic BL postnatally (E, F). (G-L) Laminins α4 (G-I) and α5 (J-L) become restricted to synaptic BL in parallel with nidogen-2. Scale bar is 15 μm for (A, D, G, J) and 10 μm for all other panels.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
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Figure 4: Developmental regulation of nidogens and laminin α chains in synaptic basal lamina (BL). Cross-sections of muscle from P0 (A, D, G, J), P14 (B, E, H, K) and P21 (C, F, I, L) mice, stained for α-bungarotoxin (BTX) and nidogens or laminins. (A-F) While nidogen-1 expression changes little during development (A-C), nidogen-2 is present in both synaptic and extrasynaptic BLs at birth (D) and becomes restricted to synaptic BL postnatally (E, F). (G-L) Laminins α4 (G-I) and α5 (J-L) become restricted to synaptic BL in parallel with nidogen-2. Scale bar is 15 μm for (A, D, G, J) and 10 μm for all other panels.
Mentions: The distribution of some laminins, collagens IV, and HSPGs in muscle fiber BL changes as development proceeds [9,11,12,44]. We asked whether the same was true of nidogens. At birth, both nidogen-1 and nidogen-2 were present throughout both synaptic and extrasynaptic muscle fiber BL (Figure 4A, D). Similar patterns were present through the first postnatal week (data not shown). Moreover, levels of nidogen-1 in synaptic and extrasynaptic BL remained similar into adulthood (Figures 4B, C). In contrast, levels of nidogen-2 decreased in extrasynaptic BL during the second postnatal week, so synaptic and Schwann cell BL contained more nidogen-2 than extrasynaptic BL by postnatal day (P) 14 (Figure 4E). By P21, nidogen-2 was undetectable in extrasynaptic BL (Figure 4F). Thus, nidogen-2 becomes restricted to synaptic sites as the NMJ matures. Because our methods are not quantitative, we do not know whether the decreased abundance of nidogen-2 in extrasynaptic BL is accompanied by an increased abundance in synaptic and Schwann cell BLs.

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