Vertebrate Lens Development and Notch Signaling
The Notch signaling pathway regulates proliferation, cell shape changes, differentiation and stem cell maintenance in many developing tissues of the body. It is one signaling pathway that is highly conserved among metazoans. Figure 1 depicts our current view of how canonical Notch signaling regulates and maintains secondary fiber cell differentiation in the mouse lens. Upon ligand binding Notch receptors undergo cleavage to free an intracellular domain (ICD) that translocates to the nucleus and binds with a transcription factor complex. A key component of this complex is the Rbpj transcription factor that directly activates Hes gene family transcription. Hes genes encode bHLH transcriptional repressors. We examined the requirements for Hes1 during early eye development and found that it is required for lens cell proliferation. This provoked us to understand more fully the requirements for Notch signaling during lens formation. Interestingly, multiple Notch ligands and receptors are expressed, along with Rbpj and Hes1, during mouse embryonic lens induction, vesicle formation and at later stages of lens formation (Figure 1).
Germline deletion of Jagged1, Notch2 and Hes1 each cause prenatal heterozygous mouse eye phenotypes, which could not be analyzed further since fuly mutant animals die before birth, some just after gastrulation. Additionally, mutations in the human Jagged1 or Notch2 genes have been associated with Alagille syndrome, wherein some patients have abnormal vision. We have conditionally deleted the Jagged1, Notch1 and Rbpj genes (individually) during lens development, and observed postnatal aphakia and a loss of the anterior chamber. This phenotype initiates with molecular marker changes at E12.5, in the forming anterior epithelium. At birth mutant mice have a profound loss of the anterior epithelium, severe disruption of the equatorial transitional zone and arrested lens size. Mutant eyes also have no pupillary openings. Currently we are characterizing the mutant phenotypes of additional Notch pathway genes, and testing for Notch functions during lens induction. The left diagram in Figure 1 indicates two hypothesized Jagged1-Notch signals, one that orchestrates proliferation and differentiation at the transition zone, the other a feedback signal from nascent fiber cells to proliferative progenitors. We propose that these signals are segregated between distinct Notch receptors. In the future, it will be important to place the Notch pathway in the known genetic hierarchy of lens development, particularly relative to genes whose human mutations cause aphakia or lens dysgenesis.
