Developmental regulation of muscleblind-like (MBNL) gene expression in the chicken embryo retina

Muscleblind-like (MBNL) is a CCCH zinc finger-containing RNA-binding protein required for the development of both muscle and photoreceptors in Drosophila; it is conserved evolutionarily, and it is associated in humans with the muscular disease myotonic dystrophy. Its role in the development of vertebrate retinal cells, however, remains unknown. As an initial approach to its investigation, we have cloned three chick muscleblind genes, characterized their isoforms, and examined their expression patterns in the chick embryo retina. The relative levels of expression of the MBNL genes increased during embryonic development. In situ hybridization (ISH) showed that the three MBNL mRNAs had widespread patterns of expression at all the developmental stages examined. Of interest, the temporal and spatial patterns of protein expression, detected by immunocytochemistry with antibodies against MBNL1 and MBNL2, were much more restricted than those seen by ISH. At early stages (ED5-7), for example, MBNL1 and MBNL2 mRNAs were present throughout the retina, but immunoreactivity for the corresponding proteins was largely restricted to the periphery of the optic cup (presumptive iris/ciliary epithelium/ciliary margin zone). MBNL1 and MBNL2 immunoreactivity became detectable at the fundus at later stages, but was limited to a very small subset of the cells that had ISH signals for the cognate mRNAs (particularly ganglion cells and photoreceptors). Within photoreceptors, MBNL1 and MBNL2 immunoreactivity first appeared in their inner segments; MBNL2 remained there, but MBNL1 became subsequently localized to their synaptic terminals. These expression patterns are consistent with the possibility that MBNLs may regulate photoreceptor development in the chick retina, much as MBL does in Drosophila, and suggest that the expression of MBNL1 and MBNL2 may be regulated posttranscriptionally.