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(American Journal of Pathology. 2000;156:1835-1840.)
© 2000 American Society for Investigative Pathology

Short Communications

Association of Active Extracellular Signal-Regulated Protein Kinase with Paired Helical Filaments of Inclusion-Body Myositis Muscle Suggests Its Role in Inclusion-Body Myositis Tau Phosphorylation

From the University of Southern California Neuromuscular Center, Department of Neurology, University of Southern California Keck School of Medicine, Good Samaritan Hospital, Los Angeles, California

The possible role of extracellular signal-regulated kinase (ERK) in the pathogenesis of inclusion-body myositis (IBM) was investigated by immunostaining the active phosphorylated form of ERK in muscle biopsies of six IBM and 14 control patients. Between 80% and 90% of IBM vacuolated muscle fibers contained well-defined ERK-immunoreactive inclusions, which were co-localized by light microscopy, with phosphorylated tau in 70 to 80% of those fibers. Immunoelectronmicroscopy colocalized ERK to small amorphous tufts adjacent to the muscle fiber paired-helical filaments. Strong ERK immunoreactivity was also present at the postsynaptic domain of all human neuromuscular junctions. Our study suggests 1) that ERK, a signal transducer, might play a role in IBM pathogenesis, including participation in the pathological phosphorylation of IBM tau; and 2) that signal transduction abnormalities may be a component of the IBM pathogenic cascade. Our novel immunolocalization of ERK at the postsynaptic domain of human neuromuscular junctions supports a role in transcription of junctional-protein genes. The ERK localized in nonjunctional regions of IBM fibers may underlie the known pathological up-regulation of junctional proteins there.

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