Tob1 and Tob2 mark distinct RNA processing granules in differentiating lens fibre cells

RC Perez1, M Familari2, G Martinez1, F Lovicu3, G Hime1 and RU de Iongh1

  1. Anatomy & Neuroscience, University of Melbourne
  2. School of Biosciences, University of Melbourne
  3. Anatomy & Histology, University of Sydney

Lens fibre cell differentiation involves a complex interplay of growth factor signals and tight control of gene expression via transcriptional and post-transcriptional regulators. Recent studies have demonstrated an important role for RNA-binding proteins, functioning in ribonucleoprotein granules, in regulating post-transcriptional expression during lens development. Here we have documented the expression of Tob1 and Tob2, members of the BTG/Tob family of RNA-binding proteins, in the developing lens. Both Tob1 and Tob2 mRNA were detected by RT-PCR in embryonic and postnatal lenses and were present in both epithelial and fibre cells in postnatal lenses. By in situ hybridisation, Tob1 and Tob2 mRNA were most intensely expressed in the early differentiating fibres, with weaker expression in the anterior epithelial cells and were downregulated in the germinative zone of E15.5 lenses. Tob1 protein was detected from E11.5 to E16.5 and was predominantly detected in large cytoplasmic puncta in early differentiating fibre cells, often colocalising with the P-granule marker, Dcp2. Occasional nuclear puncta were also observed. By contrast, Tob2 was detected in later differentiating fibre cells in the inner cortex and did not co-localise with Dcp2. The identity of these Tob2+ granules, which often appear as a series of interconnected peri-nuclear granules, is currently unknown. In vitro experiments using rat lens epithelial explants treated with or without a fibre differentiating dose of FGF2 showed that both Tob1 and Tob2 are up-regulated during FGF-induced differentiation. In differentiating explants, Tob1 also co-localised with Dcp2 in large cytoplasmic granules. These findings suggest that Tob proteins play important, but distinct, roles in RNA processing during lens fibre differentiation.