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

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.     

The role of aquaporin-4 in the blood-brain barrier development and integrity: studies in animal and cell culture models

Aquaporin-4 (AQP4) is the major water channel expressed in brain perivascular astrocyte processes. Although the role of AQP4 in brain edema has been extensively investigated, little information exists regarding its functional role at the blood-brain barrier (BBB). The purpose of this work is to integrate previous and recent data regarding AQP4 expression during BBB formation and depending on BBB integrity, using several experimental models. Results from studies on the chick optic tectum, a well-established model of BBB development, and the effect of lipopolysaccharide on the BBB integrity and on perivascular AQP4 expression have been analyzed and discussed. Moreover, data on the BBB structure and AQP4 expression in murine models of Duchenne muscular dystrophy are reviewed. In particular, published results obtained from mdx(3cv) mice have been analyzed together with new data obtained from mdx mice in which all the dystrophin isoforms including DP71 are strongly reduced. Finally, the role of the endothelial component on AQP4 cellular expression and distribution has been investigated using rat primary astrocytes and brain capillary endothelial cell co-cultures as an in vitro model of BBB.     

Reduced levels of dystrophin associated proteins in the brains of mice deficient for Dp71

Duchenne muscular dystrophy (DMD) is a progressive degenerative lethal muscle disease. A significant proportion of DMD affected children suffer also from mental retardation. The rod shaped protein, dystrophin, which is absent from or defective in the muscle of DMD patients, binds to a number of membrane associated proteins (known collectively as dystrophin associated proteins [DAPs]). The levels of DAPs is greatly reduced in the muscle of DMD patients and mdx mice, which lack dystrophin. In addition to dystrophin isoforms, the DMD gene codes also for several smaller proteins. One of the small proteins, Dp71, is expressed in most or all non-muscle tissues and is the major DMD gene product in the brain. The function of the small DMD gene products is unknown. Here we show that mutant mice which do not express the smaller non-muscle products of the DMD gene have a reduced level of DAPs in their brain. This suggests that Dp71 is important for the formation and/or stabilization of a DAPs complex in brain.      

The role of utrophin and Dp71 for assembly of different dystrophin-associated protein complexes (DPCs) in the choroid plexus and microvasculature of the brain

In the brain, utrophin is present in the choroid plexus epithelium and vascular endothelial cells, whereas the short C-terminal isoform of dystrophin (Dp71) is localized in the glial end-feet surrounding blood vessels. Both proteins serve as anchors for the so-called dystrophin-associated protein complex (DPC), composed of isoforms of syntrophin, dystroglycan and dystrobrevin. Numerous transporter proteins and channels have a polarized distribution in vascular endothelial cells and in glial end-feet, suggesting an association with the DPC. We investigated the composition and localization of the DPC in dependence on the anchoring proteins in mice lacking either utrophin (utrophin0/0) or dystrophin isoforms (mdx3Cv). Three distinct complexes were identified: (i) associated with utrophin in the basolateral membrane of the choroid plexus epithelium, (ii) associated with utrophin in vascular endothelial cells, and (iii) associated with Dp71 in the glial end-feet. Upon ablation of utrophin or Dp71, the corresponding DPCs were disrupted and no compensation of the missing protein by its homologue was observed. Association of the water channel aquaporin 4 with the glial DPC likewise was disrupted in mdx3Cv mice. These results demonstrate the essential role of utrophin and Dp71 for assembly of the DPC and suggest that these proteins contribute to the proper functioning of the cerebrospinal fluid and blood-brain barriers.     

Redefinition of Dystrophin Isoform Distribution in Mouse Tissue by RT-PCR Implies Role in Nonmuscle Manifestations of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by a defect in a 427-kDa membrane-associated protein: dystrophin. The DMD gene also encodes several shorter isoforms which are believed to participate in nonmuscle manifestations of DMD, including abnormal retinal electrophysiology, dilated cardiomyopathy, mental retardation, and hearing defects. The purpose of this work was to determine the normal tissue expression of full-length dystrophin (Dp427) and the dystrophin isoforms Dp260, Dp140, Dp116, and Dp71, to aid in understanding what roles these isoforms might play in DMD nonmuscle manifestations. RT-PCR was performed on mRNA isolated from wild-type C57BL/6J mouse tissues, including brain, cardiac muscle, eye, intestine, kidney, liver, lung, skeletal muscle, spleen, stomach, testis, thymus, and uterus. RT-PCR amplification demonstrated that the isoforms were in a number of tissues which had not been revealed by previous Western and Northern blot analyses. Dp427 was expressed at equal levels in all tissues. Dp260 and Dp140 were present in all tissues tested, but the levels of expression varied. Dp116 was expressed in a subset of tissues and levels of expression varied. Dp71 was constitutively expressed in all tissues, suggesting that this isoform plays a basic role in normal tissue function. The expanded tissue distribution supports the hypothesis that dystrophin isoforms serve essential and unique functions, necessitating further investigation into their potential roles in DMD nonmuscle manifestations.     

Localization of dystrophin isoform Dp71 to the inner limiting membrane of the retina suggests a unique functional contribution of Dp71 in the retina

The electroretinograms (ERGs) of patients with Duchenne muscular dystrophy and an allelic variant of the mdx mouse (mdxCv3) have been shown to be abnormal. Analysis of five allelic variants of the mdx mouse with mutations in the dystrophin gene has shown that there is a correlation between the position of the mutation and the severity of the ERG abnormality. Three isoforms are expressed in the retina: Dp427, Dp260 and Dp71. Using indirect immunofluorescence and isoform-specific antibodies on retinal sections from three allelic mdx mouse strains, we have examined the localization of each of the isoforms. We show that Dp71 expression does not overlap with Dp427 and Dp260 expression at the outer plexiform layer (OPL). Instead, Dp71 is localized to the inner limiting membrane (ILM) and to retinal blood vessels. Moreover, we show that Dp260 and Dp71 differ structurally at their respective C-termini. In addition, we find that the proper localization of the beta- dystroglycan is dependent upon both Dp260 at the OPL and Dp71 expression at the ILM. Thus, Dp260 and Dp71 are non-redundant isoforms that are located at different sites within the retina yet have a common interaction with beta-dystroglycan. Our data suggest that both Dp71 and Dp260 contribute distinct but essential roles to retinal electrophysiology.      

Effects of dystrophin isoforms on signal transduction through neural retina: genotype-phenotype analysis of duchenne muscular dystrophy mouse mutants

Duchenne and Becker muscular dystrophy patients have mutations in the dystrophin gene. Most show reduced b-wave amplitudes in the dark-adapted electroretinogram (ERG). We studied normal C57BL/6J mice and five X-linked muscular dystrophy strains with different dystrophin mutations to determine whether the location of the mutation within the gene affects the mouse ERG and to correlate such effects with dystrophin isoform expression. Amplitudes and implicit times were measured for a-waves, b-waves, and digitally filtered oscillatory potentials. mdx and mdxCv5 mice, with mutations near the amino terminus and lacking expression of Dp427, had ERGs similar to those of C57BL/6J mice. mdxCv2 and mdxCv4 mice, with mutations in the center of dystrophin and who do not express isoforms Dp427, Dp260, or Dp140 (mdxCv4), had increased b-wave and oscillatory potential implicit times. mdxCv3 mice, with a mutation near the carboxy terminus resulting in deficiency of all dystrophin isoforms, had increased b-wave and oscillatory potential implicit times and reduced scotopic b-wave amplitudes. Fitting the a-wave data to a transduction activation phase mathematical model showed normal responses for all phenotypes, suggesting that the b-wave delays are due to defects beyond the rod outer segment, most likely at the rod to on-bipolar cell synapse. The variation in the ERG phenotype with the position of the dystrophin gene mutation suggests that there are different contributions by each isoform to retinal electrophysiology. Although Dp427 and Dp140 isoforms do not appear to be important contributors to the ERG, lack of Dp260 and possibly Dp71 isoforms is associated with an abnormal ERG.     

Increased Gs signalling in platelets and impaired collagen activation, due to a defect in the dystrophin gene, result in increased blood loss during spinal surgery

Controversy exists regarding the cause of the significantly increased blood loss during spinal surgery in Duchenne muscular dystrophy (DMD) patients compared with similar surgery in other patients. DMD is caused by a mutation in the cytoskeletal dystrophin, which binds to extracellular matrix laminin and which has been described as a G-protein-coupled receptor. We hypothesized that disturbed cytoskeleton organization in DMD patients would alter Gs protein and collagen signalling in platelets, leading to dysfunctional platelets and a haemorrhagic tendency during surgery. In the present study, we found that platelets and skin fibroblasts, respectively, express the Dp71 and Dp116 dystrophin isoforms. Absent or decreased expression of these isoforms in DMD patients correlates with significant Gs alpha upregulation. Moreover, dysfunctional dystrophin in these cells is accompanied with increased Gs signalling and higher cAMP levels after Gs stimulation. Functional analysis showed that DMD platelets responded slower to collagen with an extensive shape change in the aggregometer and with a significantly reduced platelet adhesion to coated collagen under flow. The decreased collagen activation was shown to result from both Gs activation and cytoskeletal disruption and not from decreased expression of platelet membrane receptors or impaired von Willebrand factor (vWF) activity. In conclusion, DMD platelets have a disorganized cytoskeleton and manifest Gs hyperactivity and reduced platelet collagen reactivity. Their increased bleeding during surgery will, at least partly, result from the increased platelet Gs activity after the release of natural Gs agonists as prostacyclin during surgery and an ineffective reactivity to collagen.     

Dp260 disrupted mice revealed prolonged implicit time of the b-wave in ERG and loss of accumulation of beta-dystroglycan in the outer plexiform layer of the retina

Dp260 is a C-terminal isoform of dystrophin and is expressed specifically in the retina. Abnormal electroretinograms (ERG) in some Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) patients are likely linked to a disruption of Dp260. To clarify the importance of Dp260 in the retina, we examined dystrophin exon 52 knock- out mice, whose expression of Dp260 is impaired. We also confirmed the localization of Dp260 in the outer plexiform layer (OPL) of the retina. Disruption of Dp260 causes a change in the localization of beta- dystroglycan, which is normally found in the OPL of the retina. This suggests a requirement for Dp260 for normal formation of the dystrophin- dystroglycan complex in the retina. Dp71, also expressed in the retina, was, however, not detected in the OPL. The difference in localization of Dp260 and Dp71 implies that the two isoforms have different functions. The dystrophin exon 52 knock-out mice had a prolonged implicit time of the b-wave in ERG, although no significant change was observed in amplitude. These ERG findings differed from those of DMD and BMD patients, especially with regard to amplitude of the b-wave, but make it clear that Dp260 is required for normal electrophysiology.      

Targeted inactivation of Dp71, the major non-muscle product of the DMD gene: differential activity of the Dp71 promoter during development

The dystrophin gene, which is defective in Duchenne muscular dystrophy (DMD), also encodes a number of smaller products controlled by internal promoters. Dp71, which consists of the two C-terminal domains of dystrophin, is the most abundant product of the gene in non-muscle tissues and is the major product in adult brain. To study the possible function of Dp71 and its expression during development, we specifically inactivated the expression of Dp71 by replacing its first and unique exon and a part of the concomitant intron with a beta-galactosidase reporter gene. X-Gal staining of Dp71-null mouse embryos and tissues revealed a very stage- and cell type-specific activity of the Dp71 promoter during development and during differentiation of various tissues, including the nervous system, eyes, limb buds, lungs, blood vessels, vibrissae and hair follicles. High activity of the Dp71 promoter often seemed to be associated with morphogenic events and terminal differentiation. In some tissues the activity greatly increased towards birth.      

Suppression of revertant fibers in mdx mice by expression of a functional dystrophin.

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle degeneration that results from the absence of dystrophin. Despite null mutations in the dystrophin gene, many DMD patients display a low percentage of dystrophin-positive fibers. These "revertant fibers" are also present in the dystrophin-deficient mdx mouse and are believed to result from alternative splicing or second mutation events that bypass the mutation and restore an open reading frame. However, it is unclear what role dystrophin and the dystrophic pathology might play in revertant fiber formation and accumulation. We have analyzed the role of dystrophin expression and the dystrophic pathology in this process by monitoring revertant fibers in transgenic mdx mice that express truncated dystrophins. We found that newborn transgenic mice displayed approximately the same number of revertant fibers as newborn mdx mice, indicating that expression of a functional dystrophin does not suppress the initiation of revertant fiber formation. Surprisingly, when the transgene encoded a functional dystrophin, revertant fibers were not detected in adult or old mdx mice. In contrast, adult transgenic mice expressing a non-functional dystrophin accumulated increasing numbers of revertant fibers, similar to mdx mice, suggesting that positive selection is required for the persistence of revertant fibers. Finally, we provide evidence that the loss of revertant dystrophin in transgenic mdx muscle fibers overexpressing a functional dystrophin results from displacement of the revertant protein by the transgene-encoded dystrophin.     

Nitric oxide generated by muscle corrects defects in hippocampal neurogenesis and neural differentiation caused by muscular dystrophy

Duchenne muscular dystrophy (DMD) results from null mutation of dystrophin, a membrane-associated structural protein that is expressed in skeletal muscle. Dystrophin deficiency causes muscle membrane lesions, muscle degeneration and eventually death in afflicted individuals. However, dystrophin deficiency also causes cognitive defects that are difficult to relate to the loss of dystrophin. We assayed neurogenesis in the dentate gyrus (DG) in the mdx mouse model of DMD, using bromodeoxyuridine incorporation as a marker of proliferation and NeuN expression as a marker of differentiation. Our findings show that dystrophin mutation disrupts adult neurogenesis by promoting cell proliferation in the DG and suppressing neuronal differentiation. Because loss of dystrophin from muscle results in the secondary loss of neuronal nitric oxide synthase (nNOS), and NO is able to modulate neurogenesis, we assayed whether the genetic restoration of nNOS to mdx muscles corrected defects in adult, hippocampal neurogenesis. Assays of NO in the sera of active mice showed significant reductions in NO caused by the dystrophin mutation. However, over-expression of nNOS in the muscles of mdx mice increased serum NO and normalized cell proliferation and neuronal differentiation in the DG. These findings indicate that muscle-derived NO regulates adult neurogenesis in the brain and loss of muscle nNOS may underlie defects in the central nervous system in DMD.     

A housekeeping type promoter, located in the 3' region of the Duchenne muscular dystrophy gene, controls the expression of Dp71, a major product of the gene

The 70.8 kDa protein product of the distal part of the giantDuchenne muscular dystrophy (DMD) gene, Dp71, is expressed inmany cell types and tissues. Anchored PCR, primer extensionand functional analysis of transfected constructs were usedto determine the 5' end of the mRNA and characterize the promoterof this major DMD gene product. The 5' untranslated region (5'UTR)of Dp71 is transcribed from a single exon; the promoter doesnot contain a TATA box, and has a very high GC content and severalpotential Sp1 binding sites. It is located more than 2000 kb3' to the muscle and brain type dystrophin promoters and only150 kb from the 3' end of the gene, suggesting that in mostDMD patients the expression of Dp71 Is unaffected.     

Effect of beta-dystroglycan processing on utrophin/Dp116 anchorage in normal and mdx mouse Schwann cell membrane

In the peripheral nervous system, utrophin and the short dystrophin isoform (Dp116) are co-localized at the outermost layer of the myelin sheath of nerve fibers; together with the dystroglycan complex. Dp116 is associated with multiple glycoproteins, i.e. sarcoglycans, and alpha- and beta-dystroglycan, which anchor the cytoplasmic protein subcomplex to the extracellular basal lamina. In peripheral nerve, matrix metalloproteinase activity disrupts the dystroglycan complex by cleaving the extracellular domain of beta-dystroglycan. Metalloproteinase creates a 30 kDa fragment of beta-dystroglycan, leading to a disruption of the link between the extracellular matrix and the cell membrane. Here we asked if the processing of the beta-dystroglycan could influence the anchorage of Dp116 and/or utrophin in normal and mdx Schwann cell membrane. We showed that metalloproteinase-9 was more activated in mdx nerve than in wild-type ones. This activation leads to an accumulation of the 30 kDa beta-dystroglycan isoform and has an impact on the anchorage of Dp116 and utrophin isoforms in mdx Schwann cells membrane. Our results showed that Dp116 had greater affinity to the full length form of beta-dystroglycan than the 30 kDa form. Moreover, we showed for the first time that the short isoform of utrophin (Up71) was over-expressed in mdx Schwann cells compared with wild-type. In addition, this utrophin isoform (Up71) seems to have greater affinity to the 30 kDa beta-dystroglycan which could explain the increased stabilization of this 30 kDa form at the membrane compartment. Our results highlight the potential participation of the short utrophin isoform and the cleaved form of beta-dystroglycan in mdx Schwann cell membrane architecture. We proposed that these two proteins could be implicated in Schwann cell proliferation in response to a microenvironment stress such as mediated by accumulating macrophages in mdx mouse muscle inflammation sites.