Targeted inactivation of dystrophin gene product Dp71: phenotypic impact in mouse retina

Abstract

The abnormal retinal neurotransmission observed in Duchenne muscular dystrophy (DMD) patients and in some genotypes of mice lacking dystrophin has been attributed to altered expression of short products of the dystrophin gene. We have investigated the potential role of Dp71, the most abundant C-terminal dystrophin gene product, in retinal electrophysiology. Comparison of the scotopic electroretinograms (ERG) between Dp71-null mice and wild-type (wt) littermates revealed a normal ERG in Dp71-null mice with no significant changes of the b-wave amplitude and kinetics. Analysis of DMD gene products, utrophin and dystrophin-associated proteins (DAPs), showed that Dp71 and utrophin were localized around the blood vessels, in the ganglion cell layer (GCL), and the inner limiting membrane (ILM). Dp71 deficiency was accompanied by an increased level of utrophin and decreased level of beta-dystroglycan localized in the ILM, without any apparent effect on the other DAPs. Dp71 deficiency was also associated with an impaired clustering of two Müller glial cell proteins-the inwardly rectifying potassium channel Kir4.1 and the water pore aquaporin 4 (AQP4). Immunostaining of both proteins decreased around blood vessels and in the ILM of Dp71-null mice, suggesting that Dp71 plays a role in the clustering and/or stabilization of the two proteins. AQP4 and Kir4.1 may also be involved in the regulation of the ischemic process. We found that a transient ischemia resulted in a greater damage in the GCL of mice lacking Dp71 than in wt mice. This finding points at a crucial role played by Dp71 in retinal function.

Potassium channel Kir4.1 macromolecular complex in retinal glial cells

“…The staining for AQP4 also shows this highly polarized distribution; however, notable staining can also be seen in the outer plexiform layer (OPL) and throughout the inner plexiform layer (IPL). These results are in agreement with previous findings (Dalloz et al, 2003;Nagelhus et al, 1999), confirming that Kir4.1 and AQP4 are specifically localized to particular subcellular regions within M€ uller cells. Similar results were also observed in rat retinas (results not shown).…”

“…Direct interactions between AQP4, a-syntrophin, Dp71 and b-dystroglycan have been shown by IP assays (Neely et al, 2001;Amiry-Moghaddam et al, 2003a,b). Finally, the specific loss of Dp71 eliminates the polarized distribution of both Kir4.1 and AQP4 in retina (Dalloz et al, 2003). Overall, these reports describe a convincing body of evidence that Kir4.1, AQP4 and DGC proteins are closely interrelated in some way.…”

“…Furthermore, AQP4 was shown to interact with the DGC in M€ uller cells (Dalloz et al, 2003) and brain astrocytes (Amiry-Moghaddam et al, 2003aNeely et al, 2001). Based on this evidence, experiments were conducted to examine whether AQP4 can interact with DGC proteins and also with Kir4.1 in retina.…”

“…For example, a mutant form of a-dystroglycan results in structural anomalies within the brain and prevents the onset of hippocampal LTP (Moore et al, 2002). Dystro-phin mutants also show abnormal electroretinogram (ERG) profiles (Pillers et al, 1999a,b;Dalloz et al, 2003), and the elimination of Dp71 results in the mislocalization of both Kir4.1 and AQP4 in mouse retina (Connors and Kofuji, 2002;Dalloz et al, 2003).…”

Dystrophin and utrophin isoforms are expressed in glia, but not neurons, of the avian parasympathetic ciliary ganglion

“…Specifically, the short dystrophin isoform, Dp71, is expressed at glial endfeet that surround blood vessels in the CNS and is essential for maintenance of blood-brain barrier integrity Connors and Kofuji, 2002;Dalloz et al, 2003;AmiryMoghaddam et al, 2004;Connors et al, 2004). Similarly, full-length utrophin is expressed in glia, blood vessels, choroid plexus and pia mater Kamakura et al, 1994;Imamura and Ozawa, 1998;Claudepierre et al, 2000;Knuesel et al, 2000).…”