Lens fibre cell differentiation and organelle loss: many paths lead to clarity

Abstract

The programmed removal of organelles from differentiating lens fibre cells contributes towards lens transparency through formation of an organelle-free zone (OFZ). Disruptions in OFZ formation are accompanied by the persistence of organelles in lens fibre cells and can contribute towards cataract. A great deal of work has gone into elucidating the nature of the mechanisms and signalling pathways involved. It is apparent that multiple, parallel and redundant pathways are involved in this process and that these pathways form interacting networks. Furthermore, it is possible that the pathways can functionally compensate for each other, for example in mouse knockout studies. This makes sense given the importance of lens clarity in an evolutionary context. Apoptosis signalling and proteolytic pathways have been implicated in both lens fibre cell differentiation and organelle loss, including the Bcl-2 and inhibitor of apoptosis families, tumour necrosis factors, p53 and its regulators (such as Mdm2) and proteolytic enzymes, including caspases, cathepsins, calpains and the ubiquitin-proteasome pathway. Ongoing approaches being used to dissect the molecular pathways involved, such as transgenics, lens-specific gene deletion and zebrafish mutants, are discussed here. Finally, some of the remaining unresolved issues and potential areas for future studies are highlighted.

Figure 1.

Carl Rabl's 1898 drawing (with additional labelling) of the developing sand lizard eye. The developing lens at different stages is illustrated in succession from left to right on both the top and bottom rows. (a–e) The lens invaginates from the ectoderm, then pinches off, thereby forming a hollow lens vesicle (LV). (f) Primary lens fibre cells (LFCs) then elongate to fill the lens vesicle. Subsequently, secondary LFCs form from lens epithelial cells (LECs) differentiating at the lens equator (Eq) or bow region (h). Both the primary and secondary LFC nuclei elongate and then become increasingly pyknotic and disappear, forming an organelle-free zone (OFZ), which progresses from the centre of the lens outwards as development proceeds. Image provided by Dr Ralf Dahm and used Courtesy of Zeitschrift für wissenschaftliche Zoologie.

Figure 2.

Semi-quantitative RT–PCRs with no RT controls for TNF-related genes expressed during chick embryo lens development. (a) PCRs presented are representative of three replicates. (b) Graphical representation of relative mean expression levels normalized with respect to Gapdh in each case, using densitometry. PCR reactions carried out in triplicate. Error bars represent standard error of the mean. See table 1 for details of primers, annealing temperatures and number of cycles used for each primer. RNA was isolated from pooled lenses using TRIzol (Invitrogen, Paisley, UK); reverse transcription (RT) reactions were carried out using the Superscript II Reverse Transcriptase kit (Invitrogen); PCR was carried out using GoTaq Flexi DNA Polymerase (Promega, UK). PCRs were visualized using 2% agarose gel electrophoresis; densitometry was carried out using Scion Image software [61]. (i) Trail; (ii) Traf1; (iii) Traf2; (iv) Traf3; (v) Traf4; (vi) Traf7.