Conditional Deletion of Activating Protein 2α (AP-2α) in the Developing Retina Demonstrates Non-Cell-Autonomous Roles for AP-2α in Optic Cup Development

Abstract

Activating protein 2␣ (AP-2␣) is known to be expressed in the retina, and AP-2␣-null mice exhibit defects in the developing optic cup, including patterning of the neural retina (NR) and a replacement of the dorsal retinal pigmented epithelium (RPE) with NR. In this study, we analyzed the temporal and spatial retinal expression patterns of AP-2␣ and created a conditional deletion of AP-2␣ in the developing retina. AP-2␣ exhibited a distinct expression pattern in the developing inner nuclear layer of the retina, and colocalization studies indicated that AP-2␣ was exclusively expressed in postmitotic amacrine cell populations. Targeted deletion of AP-2␣ in the developing retina did not result in observable retinal defects. Further examination of AP-2␣-null mutants revealed that the severity of the RPE defect was variable and, although defects in retinal lamination occur at later embryonic stages, earlier stages showed normal lamination and expression of markers for amacrine and ganglion cells. Together, these data demonstrate that, whereas AP-2␣ alone does not play an intrinsic role in retinogenesis, it has non-cell-autonomous effects on optic cup development. Additional expression analyses showed that multiple AP-2 proteins are present in the developing retina, which will be important to future studies.The retina is an extension of the central nervous system derived from the forebrain neural ectoderm. During vertebrate eye development, the diencephalon evaginates to form optic vesicles, which subsequently invaginate to form a bilayered optic cup. The inner layer of the optic cup will give rise to the neural retina (NR), and the outer layer becomes the retinal pigmented epithelium (RPE) (13). Six principal types of neurons and the Müller glia cells that comprise the NR are generated in a fixed, overlapping chronological order (69). Ganglion cells are "born" (i.e., become postmitotic) first, followed by amacrine, horizontal, and cone photoreceptor cells, and ending with bipolar and Müller glia cells. The birth of rod photoreceptors spans nearly the entire period of retinal histogenesis, which begins at embryonic day 10.5 (E10.5) in mice and continues for approximately 3 weeks, ending at postnatal day 11 (P11) (69). A "central-to-peripheral" gradient of differentiation has been described in the NR, where the genesis of a particular cell type begins in the central retina (near the optic nerve head) and spreads toward the peripheral retina (next to the ciliary body) (26,37,47).A range of extrinsic and intrinsic factors control the many steps that retinal progenitor cells (RPCs) progress through during development, including cell cycle exit, cell fate bias or commitment, and differentiation into a functional neuron or glial cell. The prevailing model to explain how different retinal cell fates are determined from multipotent progenitors suggests that RPCs progress through states of competence, in which their continually changing intrinsic properties determine how they will respond to external signals at given times duri...

Expression of AP‐2δ in the developing chick retina

“…The ocular defects in these mice are so severe and appear so early, that it has not been possible to evaluate the function of AP-2␣ in retinal development. Recently, conditional knockout mice with AP-2␣ inactivation specific to the retina have been generated (Bassett et al, 2007). Surprisingly, these mice have no obvious retinal defects.…”

“…The absence of a phenotype is likely due to compensatory effects by other members of the AP-2 family. In particular, AP-2␣ and AP-2␤ have very similar expression profiles in the developing retina (Bisgrove and Godbout, 1999;Bassett et al, 2007). In contrast to AP-2␣ knockout mice, AP-2␤Ϫ/Ϫ mice do not exhibit defects in craniofacial and ocular structures, but die perinatally due to massive apoptosis of renal tubular epithelia (Moser et al, 1997).…”

“…In mouse, AP-2␦ RNA is detected later than the other AP-2 genes and is mainly found in the CNS as well as heart (Zhao et al, 2003). In the mouse retina, AP-2␦ RNA is primarily detected in ganglion cells (Bassett et al, 2007). Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis has revealed AP-2␦ transcripts in a wide variety of human tissues (Cheng et al, 2002).…”

AP2 transcription factor induces apoptosis in retinoblastoma cells

“…Three of these genes, TFAP2A, TFAP2B, and TFAP2D, have been mapped to 6p24, 6p12, and 6p12.1, respectively (Gaynor et al, 1991;Williamson et al, 1996;Cheng et al, 2002). TFAP2A and TFAP2B are expressed in the amacrine cells of the developing murine and chick retina, with TFAP2B also found in the horizontal cells of chick retina (Bisgrove and Godbout, 1999;Bassett et al, 2007). TFAP2D is expressed in a subset of ganglion cells, whereas TFAP2C and TFAP2E have a poorly defined expression pattern in the retina (Bassett et al, 2007;Li et al, 2008).…”

“…The severity of the ocular defects, often resulting in the absence of eyes, precludes analysis of the role of TFAP2A during retinal development. Conditional Tcfap2a knock-out mice specifically targeting the retina have no obvious retinal defects, leading to the hypothesis that TFAP2B and possibly other TFAP2s may compensate for the loss of TFAP2A in mouse retina (Bassett et al, 2007). In contrast to Tcfap2a, Tcfap2b knock-out mice have no defects in craniofacial and ocular structures, but die perinatally due to massive apoptosis of renal tubular epithelia (Moser et al, 1997).…”