Consequences of axon guidance defects on the development of retinotopic receptive fields in the mouse colliculus.

Gradients of molecular factors pattern the developing retina and superior colliculus (SC) and guide retinal ganglion cell (RGC) axons to their appropriate central target perinatally. During and subsequent to this period, spontaneous waves of action potentials sweep across the retina, providing an instructive topographic signal based on the correlations of firing patterns of neighbouring RGCs. How these activity-independent and activity-dependent factors interact during retinotopic map formation remains unclear. A typical phenotype of mutant mice lacking genes for one or more RGC axon guidance molecules is the presence of topographically inappropriate projections or 'ectopic spots'. Here, we examine mice that lack functional bone morphogenetic protein receptors (BMPRs) in the retina. Retinal BMP controls the graded expression of RGC axon guidance molecules, resulting in some dorsal RGCs projecting ectopically to locations in the SC that normally receive input from ventral retina. We examine the consequences of this anatomical phenotype in vivo by studying the receptive field (RF) properties of neurons in the superficial SC. We observe a mixture of physiological phenotypes in BMPR mutant mice; notably we find some neurons with ectopic RFs displaced in elevation, corresponding to the observed anatomical defect. However, in a result not necessarily congruent with the presence of focal ectopic projections, some neurons have split, enlarged and patchy/distorted RFs. These results are consistent with the effects of spontaneous retinal waves acting upon a disrupted molecular template, and they place significant limits on the form of an activity-dependent learning rule for the development of retinocollicular projections.

Developmental potential of Gcn5(-/-) embryonic stem cells in vivo and in vitro.

Gcn5 is a prototypical histone acetyltransferase (HAT) that serves as a coactivator for multiple DNA-bound transcription factors. We previously determined that deletion of Gcn512 (hereafter referred to as Gcn5) causes embryonic lethality in mice. Gcn5 null embryos undergo gastrulation but exhibit high levels of apoptosis, leading to loss of mesodermal lineages. To further define the functions of Gcn5 during development, we created Gcn5(-/-) mouse embryonic stem (ES) cells. These cells survived in vitro and formed embryoid bodies (EBs) that expressed markers for ectodermal, mesodermal, and endodermal lineages. Gcn5(-/-) EBs were misshapen and smaller than wild-type EBs by day 6, with an increased proportion of cells in G2/M. Expression of Oct 4 and Nodal was prematurely curtailed in Gcn5(-/-) EBs, indicating early loss of pluripotent ES cells. Gcn5(-/-) EBs differentiated efficiently into skeletal and cardiac muscle, which derive from mesoderm. High percentage Gcn5(-/-) chimeric embryos created by injection of Gcn5(-/-) ES cells into wild-type blastocysts were delayed in development and died early. Interestingly, elevated levels of apoptosis were observed specifically in Gcn5 null cells within the chimeric embryos. Collectively, these data indicate that Gcn5 may be required to maintain pluripotent states and that loss of Gcn5 invokes a cell-autonomous pathway of cell death in vivo.