Synemin expression in developing normal and pathological human retina and lens

Synemin (Syn) is an intermediate filament (IF) protein. To gain insight into a morphogenetic role of Syn, we have studied its expression patterns in the developing human retina and lens and compared it with those of other IF proteins. In addition, we have tested Syn expression in fetuses (23 and 28 weeks) affected by Walker-Warburg syndrome (WWS), Meckel syndrome, and trisomy 13. In the retina, Syn expression starts in the nerve fiber and ganglion cell layers (NFL and GCL) at 15 weeks, remains there in up to 20 weeks, and spreads to other layers and may be colocalized with vimentin, GFA, or neurofilaments in the subsequent 16 weeks. This expansion of Synemin expression from 20 to 28 weeks is not observed in WWS in which Syn immunoreactivity in NFL is reduced and Vim expression is increased. Changes are seen in Syn or vimentin expressions in the retinae of 23-week-old Meckel syndrome or 28-week-old trisomy 13 fetuses. Syn expression in the lens is, at first (16 weeks), uniformly distributed, becoming stronger in the epithelium of the anterior part at 25 weeks and later. As in the retina, Syn expression in lens is also selectively affected in WWS. The colocalization of Synemin with vimentin, GFA, or NF supports the idea that Syn is a key cross-linking protein that connects different cytoskeletal structures. Moreover, stagnant Syn expression in WWS retina and lens reinforces the notion of a significant role of this protein in morphogenesis.