title: Expression of insulin-like growth factor 1 isoforms in the rabbit oculomotor system.
abstract: OBJECTIVE
  The insulin-like growth factor-1 (IGF-1) gene encodes two isoforms, IGF-1Ea and IGF-1Eb. Both isoforms can regulate skeletal muscle growth and strength. It has been suggested that IGF-Eb may be more potent in promoting skeletal muscle hypertrophy. Precise contractile force regulation is particularly important in the oculomotor system. However, expression of these isoforms in mammalian extraocular muscles (EOMs) is unknown. Here, we examined their expression in rabbit EOMs and the innervating nerve, two potential sources for myogenic growth factors, and compared isoform expression between EOMs and limb skeletal muscles.
  
  
  METHODS
  Expression of IGF-1 isoforms was quantified by real-time RT-PCR in adult rabbit EOMs, trochlear and ophthalmic nerves, and compared with expression in rabbit limb skeletal muscles. The presence of mature IGF-1 peptide in the muscles was further examined by Western blot.
  
  
  RESULTS
  Both IGF-1Ea and IGF-1Eb were expressed in the EOM and the trochlear nerve. Both isoforms were expressed at significantly higher levels (9-fold) in EOM than in limb skeletal muscle. Transcripts of IGF-1 isoforms, of IGF-1 receptor and of IGF binding proteins showed a gradient distribution along the EOM from proximal to distal. The mature IGF-1 protein showed the same gradient distribution in the EOM.
  
  
  CONCLUSIONS
  Expression of relatively abundant amounts of both IGF-1 splicing isoforms in EOMs, and at a significantly higher level than in limb skeletal muscle, underscores the potential relevance of these myogenic growth factors in EOM plasticity and force regulation.
authors.affiliation: Department of Physiology and Cell Biology, University of Nevada School of Medicine, Reno, NV 89557, USA.
authors.forename: Cheng-Yuan
authors.initials: CY
authors.lastname: Feng
authors.forename: Christopher S
authors.initials: CS
authors.lastname: von Bartheld
mesh_terms: D000818:Animals; D015153:Blotting, Western; D005786:Gene Expression Regulation; D007334:Insulin-Like Growth Factor I / Q000235:genetics* / Q000378:metabolism*; D008297:Male; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism*; D009801:Oculomotor Muscles / Q000166:cytology / Q000378:metabolism*; D020033:Protein Isoforms; D012326:RNA Splicing; D012333:RNA, Messenger / Q000235:genetics; D011817:Rabbits; D017526:Receptor, IGF Type 1 / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D020033:Protein Isoforms; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1