The Phosphoinosotide 3-Kinase Catalytic Subunit p110α is Required for Normal Lens Growth

Abstract

PurposeSignal transduction pathways influence lens growth, but little is known about the role(s) of the class 1A phosphoinositide 3-kinases (PI3Ks). To further investigate how signaling regulates lens growth, we generated and characterized mice in which the p110α and p110β catalytic subunits of PI3K were conditionally deleted in the mouse lens.MethodsFloxed alleles of the catalytic subunits of PI3K were conditionally deleted in the lens by using MLR10-cre transgenic mice. Lenses of age-matched animals were dissected and photographed. Postnatal lenses were fixed, paraffin embedded, sectioned, and stained with hematoxylin-eosin. Cell proliferation was quantified by labeling S-phase cells in intact lenses with 5-ethynyl-2′-deoxyuridine. Protein kinase B (AKT) activation was examined by Western blotting.ResultsLens-specific deletion of p110α resulted in a significant reduction of eye and lens size, without compromising lens clarity. Conditional knockout of p110β had no effect on lens size or clarity, and deletion of both the p110α and p110β subunits resulted in a phenotype that resembled the p110α single-knockout phenotype. Levels of activated AKT were decreased more in p110α- than in p110β-deficient lenses. A significant reduction in proliferating cells in the germinative zone was observed on postnatal day 0 in p110α knockout mice, which was temporally correlated with decreased lens volume.ConclusionsThese data suggest that the class 1A PI3K signaling pathway plays an important role in the regulation of lens size by influencing the extent and spatial location of cell proliferation in the perinatal period.

The lens growth process

“…The PI3K pathway has been implicated in Cx50 regulation, with increased PI3K signaling leading to specific increases in Cx50-mediated coupling (Martinez et al, 2015). Similarly, lens specific knockout of p110α (the catalytic subunit of PI3K) leads to a reduction in cell-cell coupling, reduced proliferation and a microphakic phenotype similar to that seen in Gja8 -null lenses (Sellitto et al, 2016). …”

“…In many cells, activation of the PI3K/AKT pathway promotes cellular proliferation and suppresses differentiation. In the mouse lens, targeting of the PI3K pathway results in a permanent growth deficit, due to reduced epithelial proliferation, suggesting that the probability of a cell dividing is regulated through the PI3K/AKT pathway and possibly MTOR, its downstream effector (Sellitto et al, 2016). As more is learned about the pathways that control lens cell behavior, we will be able to more confidently assign molecular identities to what are currently purely mathematical concepts.…”

“…In mouse embryos, genetic ablation experiments (utilizing lens-expressed diphtheria toxin) demonstrate that lens growth defects generally result in microphthalmia (Breitman et al, 1987). Similarly, conditional deletion of the p110α catalytic subunit of phosphoinositide 3-kinase (PI3K) in the lens causes reduced lens growth and a proportionate decrease in the size of the globe (Sellitto et al, 2016). These results suggest that the lens exerts powerful qualitative and quantitative effects on tissues in the developing eye.…”

A full lifespan model of vertebrate lens growth

“…It is not yet clear whether the ‘pedals’ have biological identities, but they could be analogous to the Hippo and TOR pathways which, in other organ systems, regulate organ growth by controlling cell number and cell size, respectively [ 18 ]. In support of this notion, it was recently shown that inactivation of PI3 K, an upstream regulator of mTOR, results in a lens that is too small but otherwise histologically normal [ 19 ].…”

“…an overly large lens), implying that either growth rates are already maximal or that the lens actively compensates for cellular overproduction (for example, by adjusting the fibre cell compaction factor). In mice, lens growth defects often lead to microphthalmia [ 19 , 23 ] suggesting that at least with regard to growth, the eye takes its cue from the lens.…”