There is currently a great interest in identifying laminin isoforms expressed in developing and regenerating skeletal muscle. Laminin ␣1 has been reported to localize to human fetal muscle and to be induced in muscular dystrophies based on immunohistochemistry with the monoclonal antibody 4C7, suggested to recognize the human laminin ␣1 chain. Nevertheless, there seems to be no expression of laminin ␣1 protein or mRNA in developing or dystrophic mouse skeletal muscle fibers. To address the discrepancy between the results obtained in developing and dystrophic human and mouse muscle we expressed the E3 domain of human laminin ␣1 chain as a recombinant protein and made antibodies specific for human laminin ␣1 chain (anti-hLN-␣1G4/G5). We also made antibodies to the human laminin ␣5 chain purified from placenta. In the present report we show that hLN-␣1G4/G5 antibodies react with a 400-kDa laminin ␣1 chain and that 4C7 reacts with a 380-kDa laminin ␣5 chain. Immunohistochemistry with the hLN-␣1G4/G5 antibody and 4C7 revealed that the two antibodies stained human kidney, developing and dystrophic muscle in distinct patterns. Our data indicate that the previously reported expression patterns in developing, adult, and dystrophic human muscle tissues with 4C7 should be re-interpreted as an expression of laminin ␣5 chain. Our data are also consistent with earlier work in mouse, indicating that laminin ␣1 is largely an epithelial laminin chain not present in developing or dystrophic muscle fibers.
S U M M A R YMuscle spindle density is extremely high in the deep muscles of the human neck. However, there is a paucity of information regarding the morphology and immunoreactivity of these muscle spindles. The objective of this study was to investigate the intrafusal fiber content and to assess the myosin heavy chain (MyHC) composition of muscle spindles from human deep neck muscles. In addition to the conventional spindles containing bag 1 , bag 2 , and chain fibers (b 1 b 2 c spindle), we observed a number of spindles lacking bag 1 (b 2 c spindle) or bag 2 (b 1 c spindle) fibers. Both bag 1 and bag 2 fibers contained slow tonic MyHCs along their entire fiber length and MyHCI, MyHCIIa, embryonic, and ␣ -cardiac MyHC isoforms along a variable length of the fibers. Fetal MyHC was present in bag 2 fibers but not in bag 1 fibers. Nuclear chain fibers contained MyHCIIa, embryonic, and fetal isoforms with regional variations. We also compared the present data with our previous results obtained from muscle spindles in human biceps brachii and the first lumbrical muscles. The allotment of numbers of intrafusal fibers and the MyHC composition showed some muscle-related differences, suggesting functional specialization in the control of movement among different human muscles.
Data on the myosin heavy chain (MyHC) composition of human muscle spindles are scarce in spite of the well-known correlation between MyHC composition and functional properties of skeletal muscle fibers. The MyHC composition of intrafusal fibers from 36 spindles of human biceps brachii muscle was studied in detail by immunocytochemistry with a large battery of antibodies. The MyHC content of isolated muscle spindles was assessed with SDS-PAGE and immunoblots. Four major MyHC isoforms (MyHCI, IIa, embryonic, and intrafusal) were detected with SDS-PAGE. Immunocytochemistry revealed very complex staining patterns for each intrafusal fiber type. The bag(1) fibers contained slow tonic MyHC along their entire fiber length and MyHCI, alpha-cardiac, embryonic, and fetal isoforms along a variable part of their length. The bag(2) fibers contained MyHC slow tonic, I, alpha-cardiac, embryonic, and fetal isoforms with regional variations. Chain fibers contained MyHCIIa, embryonic, and fetal isoforms throughout the fiber, and MyHCIIx at least in the juxtaequatorial region. Virtually each muscle spindle had a different allotment of numbers of bag(1), bag(2) and chain fibers. Taken together, the complexity in intrafusal fiber content and MyHC composition observed indicate that each muscle spindle in the human biceps has a unique identity.
The abundant and synaptic-specific binding of anti-GQ1b, -GT1a, and -GD1b ganglioside antibodies and the rich capillary supply in the human EOMs may partly explain the selective paralysis of these muscles in Miller Fisher syndrome.
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