Genetic rescue of cell number in a mouse model of microphthalmia:interactions between Chx10 and G1-phase cell cycle regulators

Abstract

Insufficient cell number is a primary cause of failed retinal development in the Chx10 mutant mouse. To determine if Chx10 regulates cell number by antagonizing p27 Kip1 activity, we generated Chx10, p27 Kip1 double null mice. The severe hypocellular defect in Chx10 single null mice is alleviated in the double null, and while Chx10-null retinas lack lamination, double null retinas have near normal lamination. Bipolar cells are absent in the double null retina, a defect that is attributable to a requirement for Chx10 that is independent of p27 Kip1 . We find that p27 Kip1 is abnormally present in progenitors of Chx10-null retinas, and that its ectopic localization is responsible for a significant amount of the proliferation defect in this microphthalmia model system. mRNA and protein expression patterns in these mice and in cyclin D1-null mice suggest that Chx10 influences p27 Kip1 at a posttranscriptional level, through a mechanism that is largely dependent on cyclin D1. This is the first report of rescue of retinal proliferation in a microphthalmia model by deletion of a cell cycle regulatory gene.

Chx10 is required to block photoreceptor differentiation but is dispensable for progenitor proliferation in the postnatal retina

“…The central retina is less affected, but data here show that, by P3, division is severely reduced. The early defect in RPC expansion probably ref lects a requirement for Chx10 to regulate one or more genes that directly or indirectly affect proliferation (9,13,15,17). Deregulation of these genes early in retinogenesis probably has enormous indirect consequences on late-stage development; thus, the or J model cannot reveal whether Chx10 plays a role either in proliferation of late-stage RPCs or in driving bipolar cell differentiation.…”

“…The or J proliferation defect is partially rescued by inactivating the cycle-dependent kinase inhibitor p27 Kip1 (9). We asked whether this rescue also increases the number of Chx10 mRNA ϩ rods in the or J retina.…”

“…There are 19-fold fewer cells in the or J vs. WT postnatal retina (9). Chx10 is expressed in all RPCs (10), but it is unclear whether Chx10 regulates RPC division throughout retinogenesis or whether its loss specifically perturbs early RPC division, which indirectly affects late-stage RPC production.…”

“…Nevertheless, Müller glia are closely related to RPCs, both in terms of markers and their potential to divide and generate multiple cell types (18,19). In view of the failure of most of the or J retina to differentiate at all (4,9,20), some RPCs in the central retina may default toward the overlapping glial fate. The idea that Chx10 may directly regulate bipolar cell differentiation is supported by the observation that inactivating the cyclin-dependent kinase inhibitor p27 Kip1 partially rescues division in the or J retina but does not rescue bipolar cell genesis (9).…”

“…In view of the failure of most of the or J retina to differentiate at all (4,9,20), some RPCs in the central retina may default toward the overlapping glial fate. The idea that Chx10 may directly regulate bipolar cell differentiation is supported by the observation that inactivating the cyclin-dependent kinase inhibitor p27 Kip1 partially rescues division in the or J retina but does not rescue bipolar cell genesis (9). However, cell numbers in the Chx10 Ϫ/Ϫ ;p27 Kip1Ϫ/Ϫ retina are still 4-fold lower than WT (9); therefore, this large cell-cycle defect, which likely perturbs many pathways, may indirectly impair late-stage neuronal differentiation.…”