Loss of LMO4 in the Retina Leads to Reduction of GABAergic Amacrine Cells and Functional Deficits

Abstract

BackgroundLMO4 is a transcription cofactor expressed during retinal development and in amacrine neurons at birth. A previous study in zebrafish reported that morpholino RNA ablation of one of two related genes, LMO4b, increases the size of eyes in embryos. However, the significance of LMO4 in mammalian eye development and function remained unknown since LMO4 null mice die prior to birth.Methodology/Principal FindingsWe observed the presence of a smaller eye and/or coloboma in ∼40% LMO4 null mouse embryos. To investigate the postnatal role of LMO4 in retinal development and function, LMO4 was conditionally ablated in retinal progenitor cells using the Pax6 alpha-enhancer Cre/LMO4flox mice. We found that these mice have fewer Bhlhb5-positive GABAergic amacrine and OFF-cone bipolar cells. The deficit appears to affect the postnatal wave of Bhlhb5+ neurons, suggesting a temporal requirement for LMO4 in retinal neuron development. In contrast, cholinergic and dopaminergic amacrine, rod bipolar and photoreceptor cell numbers were not affected. The selective reduction in these interneurons was accompanied by a functional deficit revealed by electroretinography, with reduced amplitude of b-waves, indicating deficits in the inner nuclear layer of the retina.Conclusions/SignificanceInhibitory GABAergic interneurons play a critical function in controlling retinal image processing, and are important for neural networks in the central nervous system. Our finding of an essential postnatal function of LMO4 in the differentiation of Bhlhb5-expressing inhibitory interneurons in the retina may be a general mechanism whereby LMO4 controls the production of inhibitory interneurons in the nervous system.

The stage-dependent roles of Ldb1 and functional redundancy with Ldb2 in mammalian retinogenesis

“…Specifically, the phenotype of the Ldb1 loxP/loxP ;α-Cre retina mimicked the phenotype associated with the combined loss of Isl1 and Lmo4, which is characterized by loss of GCLs, and a reduction in the number of GABAergic amacrine cells and subsets of bipolar cells (Elshatory et al, 2007;Duquette et al, 2010).…”

“…The loss of Ldb2 in the retina has no apparent retinal phenotype, suggesting that Ldb1 may be sufficient to execute the functions of Ldb in the retina. However, the loss of both Ldb1 and Ldb2 resulted in early depletion of most RPCs, while the inactivation of Ldb1 (but not of Ldb2) resulted in a phenotype that resembled a combination of the phenotypes associated with mutation of Isl1 and Lmo4, including loss of GCLs, reduction in bipolar cell number and a marked reduction in the number of GABAergic amacrine cells (Mu et al, 2008;Pan et al, 2008;Duquette et al, 2010). By contrast, Lhx2-dependent functions seemed to be preserved following loss of function of Ldb1, RPCs were maintained throughout retinogenesis, and Müller glia differentiation occurred normally.…”

“…The LIM-class homeobox gene Lim1 (Lhx1) is required for the laminar positioning of horizontal cells (Poche et al, 2007). Finally, the LMO transcription factor Lmo4 has an important role in the generation of GABAergic amacrine and OFF-bipolar cells (Duquette et al, 2010).…”

LIM Domain Only 4 (LMO4) Regulates Calcium-Induced Calcium Release and Synaptic Plasticity in the Hippocampus

“…Tissues were processed for in situ hybridization with digoxigenin-labeled antisense or sense riboprobes, as previously described (Duquette et al, 2010). For more reliable comparisons of gene expression patterns, wild-type and mutant tissues were processed on the same slides.…”

Development of Retinal Amacrine Cells and Their Dendritic Stratification

“…The expression in the adult in cholinergic and glycinergic amacrine cells is downregulated but is retained in GABAergic amacrine cell subtypes [56]. Lmo4 is expressed in a subset of GABAergic amacrine cells, and a conditional deletion of Lmo4 leads to a reduction in the number of Bhlhb5 -expressing GABAergic amacrine cells [57]. Analysis of specific expression of individual members of the Neurod1 transcription factor family showed the expression of Neurod2 specifically in Dab1-expressing AII amacrine cells.…”