Nebulin isoforms of extraocular muscle

Abstract

The extraocular muscles (EOMs), which are responsible for reflexive and voluntary eye movements, have many unique biochemical, physiological, and ultrastructural features that set them apart from other skeletal muscles. For example, rodent EOMs lack M-lines and express EOM-specific myosin heavy chain (MYH13) and alpha-cardiac myosin heavy chain. Recent gene-expression profiling studies indicate the presence of other cardiac-specific proteins in adult EOMs. This interesting mixture of myofibrillar and cytoskeletal proteins poses the questions as to whether nebulette, as opposed to nebulin, might be expressed in EOM, and what isoforms of titin are expressed in the EOM. We have performed gel electrophoresis and immunological analyses to determine the titin and nebulin isoforms expressed in the EOM. We have found that the mass of the titin isoforms expressed in the EOM most closely resemble those found in the skeletal muscles tested, viz., the soleus and extensor digitorum longus (EDL). We also demonstrate that, although the EOM expresses cardiac isoforms of myosin, it does not express nebulette and contains a nebulin isoform with a mass consistent with that found in the prototypical fast hindlimb muscle EDL.

Development transitions of thin filament proteins in rat extraocular muscles

“…In our previous analysis of nebulin isoforms in the extraocular muscles, we failed to detect nebulette by western blot analysis [15]. In this study, we used a monoclonal antibody that recognizes four nebulin family members: nebulin, nebulette, LASP1 and (LASP2) and determined that LIM-nebulette, a splice variant of the nebulette locus, is indeed expressed in the extraocular muscles.…”

“…Immunolabeling was performed as described previously [15]. Briefly, cryostat sections were treated with 1% Triton X-100 in PBS for 15 min at 4°C, washed and then blocked in 1% normal goat serum in PBS for 1 hr at 4°C.…”

Nonmuscle myosin IIB, a sarcomeric component in the extraocular muscles

“…Recently, proteomic analysis of rodent EOMs confirmed the presence of α-cardiac actin [16]. We analyzed the nebulin isoforms present in EOMs and found that nebulin, not the cardiac-specific nebulette, is the major nebulin family member expressed [17]. The immunological detection of nebulin in EOMs does not, however, preclude the possibility that nebulette or its splice variant LIM-nebulette [18] is present in low abundance or within specialized domains in EOM fibers.…”