The Iroquois homeobox gene, Irx5, is required for retinal cone bipolar cell development

Abstract

In the mouse retina, at least ten distinct types of bipolar interneurons are involved in the transmission of visual signals from photoreceptors to ganglion cells. How bipolar interneuron diversity is generated during retinal development is poorly understood. Here, we show that Irx5, a member of the Iroquois homeobox gene family, is expressed in developing bipolar cells starting at postnatal day 5 and is localized to a subset of cone bipolar cells in the mature mouse retina. In Irx5-deficient mice, defects were observed in the expression of some, but not all, immunohistological markers that define mature Type 2 and Type 3 OFF cone bipolar cells, indicating a role for Irx5 in bipolar cell differentiation. The differentiation of these two bipolar cell types has previously been shown to require the homeodomain-CVC transcription factor, Vsx1. However, the defects observed in Irx5-deficient retinas do not coincide with a reduction of Vsx1 expression, and conversely, the expression of Irx5 in cone bipolar cells does not require the presence of a functional Vsx1 allele. These results indicate that there are at least two distinct genetic pathways (Irx5-dependent and Vsx1-dependent) regulating the development of Type 2 and Type 3 cone bipolar cells.

Genetic Control of Circuit Function:Vsx1andIrx5Transcription Factors Regulate Contrast Adaptation in the Mouse Retina

“…In addition, ERGs of Vsx1-deficient mice showed a reduced b-wave, a measure of bipolar cell activity, and OFF but not ON RGCs fired on average fewer spikes in response to steps in light intensity compared with their wild-type counterparts (Chow et al, 2004;Ohtoshi et al, 2004). In contrast, preliminary studies revealed no specific b-wave defects in ERGs from Irx5-deficient mice (Cheng et al, 2005). These findings argued that Vsx1 and Irx5 regulate the differentiation of OFF cone bipolar cells and suggested their possible importance for retinal function.…”

“…Mice that were heterozygous or null for Vsx1 and Irx5 were bred from single gene knock-out mice of Vsx1 (Chow et al, 2004) and Irx5 (Cheng et al, 2005) and genotyped using the following primer sets: Vsx1 genotyping: common 5Ј primer, TTC TAG GCT GTC TAG GTC TC; wild-type 3Ј primer, TGA TGG CAA AGC TTC GAA GG; mutant 3Ј primer, TTG CCT TTA CTG ACC ATG CG; Irx5 genotyping: wild-type 5Ј primer, ACC GCT TCC TCG TGC TTT AC; mutant 5Ј primer, GCC ACC CAA AAG ACT GAA ACC; common 3Ј primer, TAA ACC TAT CTT CGC AAT CC. These mice were in a mixed 129/Sv and CD1 background.…”

“…For example, Bhlhb4 is required for the survival of rod but not cone bipolar cells (Bramblett et al, 2004), whereas Bhlhb5 is necessary for the generation of type 2 OFF cone bipolar cells (Feng et al, 2006). Recently, two homeobox transcription fac-tors, Vsx1 and the Iroquois gene Irx5, were found to be expressed in overlapping sets of mouse OFF cone bipolar cells (Chow et al, 2001;Cheng et al, 2005). Deletion of either gene did not affect the number or gross morphology of bipolar cells but led to distinct changes in the expression profile of the several OFF cone bipolar cell types (Chow et al, 2004;Cheng et al, 2005).…”

“…Recently, two homeobox transcription fac-tors, Vsx1 and the Iroquois gene Irx5, were found to be expressed in overlapping sets of mouse OFF cone bipolar cells (Chow et al, 2001;Cheng et al, 2005). Deletion of either gene did not affect the number or gross morphology of bipolar cells but led to distinct changes in the expression profile of the several OFF cone bipolar cell types (Chow et al, 2004;Cheng et al, 2005). In addition, ERGs of Vsx1-deficient mice showed a reduced b-wave, a measure of bipolar cell activity, and OFF but not ON RGCs fired on average fewer spikes in response to steps in light intensity compared with their wild-type counterparts (Chow et al, 2004;Ohtoshi et al, 2004).…”

A cascade of irx1a and irx2a controls shh expression during retinogenesis

“…It has been suggested that the architecture and major class of neurons as well as the neurogenesis events in the fish eye are reminiscent of the progression of the morphogenetic furrow in the Drosophila eye. We previously demonstrated that mouse Irx5 plays a unique role in the differentiation of specific bipolar neurons (Cheng et al, 2005). Furthermore, we have showed that, in the zebrafish, irx1a (Cheng et al, 2001;Wang et al, 2001;Itoh et al, 2002;Dildrop and Rü ther, 2004;Feijoo et al, 2004) plays an early function in retinogenesis (Cheng et al, 2006) and serotonergic neuron differentiation (Cheng et al, 2007).…”