Genome-wide transcriptional profiles are consistent with functional specialization of the extraocular muscle layers

Compartmentalization of the extraocular muscles into well-defined orbital and global layers is highly conserved. Recently, the active pulley hypothesis correlated the anatomic properties of orbital-global muscle layers with layer-specific division of labor. Microarray technology was used to identify muscle-layer-specific transcriptional profiles and, thereby, extend understanding of the structure-function characteristics of extraocular muscle layers.

Laser capture microdissection was used to obtain muscle layer samples from monkey medial rectus muscles. RNA was linearly amplified and hybridized to human U133 series microarrays (Affymetrix, Santa Clara, CA), which have sufficient sequence homology for use in subhuman primates. Data was analyzed using Affymetrix and Robust Multichip Average (RMA) algorithms. Select transcripts were verified by quantitative PCR and in situ hybridization.

A broad spectrum of transcriptional differences (> 181 transcripts) was identified between the two extraocular muscle layers. Patterned differences in the sarcomeric contractile machinery and cytoskeleton were suggestive of key layer differences in contraction speed. Differentially expressed transcript identities, however, extended well beyond those traditionally associated with muscle-fiber-group differences.

Muscle layer transcriptional profiles correlated with the different loads and usage patterns of extraocular muscle layers, as proposed in the active pulley hypothesis. The magnitude and breadth of orbital-global layer expression differences strongly suggests that oculomotor control systems may drive two distinct motor output pathways, each comprising separate motoneurons and muscle fibers, with one output path adapted to determining pulley position and the other to movement of the eye.