Simultaneous dystrophin and dysferlin deficiencies associated with high-level expression of the coxsackie and adenovirus receptor in transgenic mice

Early administration of L-arginine in mdx neonatal mice delays the onset of muscular dystrophy in tibialis anterior (TA) muscle

Duchenne muscular dystrophy (DMD) is a genetic disorder that results in the absence of dystrophin, a cytoskeletal protein. Individuals with this disease experience progressive muscle destruction, which leads to muscle weakness. Studies have been conducted to find solutions for the relief of individuals with this disease, several of which have shown that utrophin, a protein closely related to dystrophin, when overexpressed in mdx neonatal mice (the murine model of DMD), is able to prevent the progressive muscle destruction observed in the absence of dystrophin. Furthermore, recent studies have shown that L‐arginine induces utrophin upregulation in adult mdx mice. We hypothesized that L‐arginine treatment also induces utrophin upregulation to prevent the development of muscle weakness in neonatal mdx mice. Hence, L‐arginine should also prevent progressive muscle destruction via utrophin upregulation in mdx neonatal mice. Mdx neonatal mice were injected intraperitoneally daily with 800 mg/kg of L‐arginine for 6 weeks, whereas control mice were injected with a physiological saline. The following experiments were performed on the tibialis anterior (TA) muscle: muscle contractility and resistance to mechanical stress; central nucleation and peripheral nucleation, utrophin, and creatine kinase quantification as well as a nitric oxide (NO) assay. Our findings show that early administration of L‐arginine in mdx neonatal mice prevents the destruction of the tibialis anterior (TA) muscle. However, this improvement was related to nitric oxide (NO) production rather than the expected utrophin upregulation.

Dystrophin deficiency exacerbates skeletal muscle pathology in dysferlin-null mice

Background

Mutations in the genes coding for either dystrophin or dysferlin cause distinct forms of muscular dystrophy. Dystrophin links the cytoskeleton to the sarcolemma through direct interaction with β-dystroglycan. This link extends to the extracellular matrix by β-dystroglycan's interaction with α-dystroglycan, which binds extracellular matrix proteins, including laminin α2, agrin and perlecan, that possess laminin globular domains. The absence of dystrophin disrupts this link, leading to compromised muscle sarcolemmal integrity. Dysferlin, on the other hand, plays an important role in the Ca²⁺-dependent membrane repair of damaged sarcolemma in skeletal muscle. Because dysferlin and dystrophin play different roles in maintaining muscle cell integrity, we hypothesized that disrupting sarcolemmal integrity with dystrophin deficiency would exacerbate the pathology in dysferlin-null mice and allow further characterization of the role of dysferlin in skeletal muscle.

Methods

To test our hypothesis, we generated dystrophin/dysferlin double-knockout (DKO) mice by breeding mdx mice with dysferlin-null mice and analyzed the effects of a combined deficiency of dysferlin and dystrophin on muscle pathology and sarcolemmal integrity.

Results

The DKO mice exhibited more severe muscle pathology than either mdx mice or dysferlin-null mice, and, importantly, the onset of the muscle pathology occurred much earlier than it did in dysferlin-deficient mice. The DKO mice showed muscle pathology of various skeletal muscles, including the mandible muscles, as well as a greater number of regenerating muscle fibers, higher serum creatine kinase levels and elevated Evans blue dye uptake into skeletal muscles. Lengthening contractions caused similar force deficits, regardless of dysferlin expression. However, the rate of force recovery within 45 minutes following lengthening contractions was hampered in DKO muscles compared to mdx muscles or dysferlin-null muscles, suggesting that dysferlin is required for the initial recovery from lengthening contraction-induced muscle injury of the dystrophin-glycoprotein complex-compromised muscles.

Conclusions

The results of our study suggest that dysferlin-mediated membrane repair helps to limit the dystrophic changes in dystrophin-deficient skeletal muscle. Dystrophin deficiency unmasks the function of dysferlin in membrane repair during lengthening contractions. Dystrophin/dysferlin-deficient mice provide a very useful model with which to evaluate the effectiveness of therapies designed to treat dysferlin deficiency.

Multiple phenotypes in adult mice following inactivation of the Coxsackievirus and Adenovirus Receptor (Car) gene

To determine the normal function of the Coxsackievirus and Adenovirus Receptor (CAR), a protein found in tight junctions and other intercellular complexes, we constructed a mouse line in which the CAR gene could be disrupted at any chosen time point in a broad spectrum of cell types and tissues. All knockouts examined displayed a dilated intestinal tract and atrophy of the exocrine pancreas with appearance of tubular complexes characteristic of acinar-to-ductal metaplasia. The mice also exhibited a complete atrio-ventricular block and abnormal thymopoiesis. These results demonstrate that CAR exerts important functions in the physiology of several organs in vivo.

A Method of Utrophin Up-Regulation through RNAi-Mediated Knockdown of the Transcription Factor EN1

The aim of this study was to induce up-regulation of the dystrophin-related gene UTRN that encodes the protein utrophin, to determine whether this could compensate for the lack of dystrophin function in Duchenne muscular dystrophy. The human UTRN promoter, which contains two putative binding sites for homeobox protein engrailed-1 (EN1), was analysed. It was found that EN1 binding site 2 in the UTRN gene promoter directly interacted with transcription factor EN1 in vitro. Chromatin immunoprecipitation assays of the EN1– UTRN promoter complex from rhabdomyosarcoma and HeLa cell lines confirmed that endogenous EN1 interacted with this region in vivo. The findings suggest that EN1 directly interacts with the UTRN promoter. Small interfering RNA was used to inhibit EN1 gene expression. Higher utrophin mRNA levels were observed in EN1-inhibited cells compared with controls. The increase in utrophin mRNA in rhabdomyosarcoma cells and HeLa cells may have resulted from inhibition of EN1 expression.

Modulation of coxsackie and adenovirus receptor expression for gene transfer to normal and dystrophic skeletal muscle

BACKGROUND

Efficient adenovirus (AdV)-mediated gene transfer is possible only in immature muscle or regenerating muscle, suggesting that a developmentally regulated event plays a major role in limiting AdV uptake in mature skeletal muscle. Previously, we showed that the expression of the primary coxsackie and adenovirus receptor (CAR) is severely down-regulated during muscle maturation and that, in muscle-specific CAR transgenic mice, there is significant enhancement of AdV-mediated gene transfer to mature skeletal muscle.

METHODS

To evaluate whether increasing CAR expression can also augment gene transfer to dystrophic muscle that has many regenerating fibers, we crossed CAR transgenics with dystrophin-deficient mice (mdx/CAR). We also tested a two-step protocol in which CAR levels were increased in the target muscle, prior to administration of AdV, through the use of recombinant adeno-associated virus (AAV2) expressing CAR. Lastly, we assessed the effect of histone deacetylase inhibitors on CAR and AdV transduction efficiency in myoblasts and mdx muscle.

RESULTS

Although somewhat higher rates of transduction can be achieved in adult mdx mice than in normal mice as a result of ongoing muscle regeneration in these animals, CAR expression in the mdx background (mdx/CAR transgenics) still markedly improved the susceptibility of mature muscle to AdV-mediated gene transfer of dystrophin. Prior administration of AAV2-CAR to normal muscle led to significantly increased transduction by subsequent injection of AdV. The histone deacetylase inhibitor valproate increased CAR transcript and protein levels in myoblasts and mdx muscle, and also increased AdV-mediated gene transfer.

CONCLUSIONS

We have developed a method of increasing CAR levels in both normal and regenerating muscle.

Coxsackievirus and adenovirus receptor is up-regulated in migratory germ cells during passage of the blood-testis barrier

The coxsackievirus and adenovirus receptor (CAR) is a cell adhesion molecule expressed in epithelial tight junctions and other cell-cell contacts. Using indirect immunofluorescence, quantitative RT-PCR, and Western blots, the expression and distribution of CAR in developing and adult testis are examined. CAR is highly expressed in both Sertoli and germ cells during perinatal and postnatal development, followed by a rapid down-regulation of both mRNA and protein levels. Interestingly, we find that CAR is a previously unknown downstream target for FSH because CAR mRNA levels were induced in primary cultures of FSH-stimulated Sertoli cells. In contrast to other epithelia, CAR is not a general component of tight junctions in the seminiferous epithelium, and Sertoli cells in the adult testis do not express CAR. Instead, CAR expression is stage dependent and specifically found in migratory germ cells. RT-PCR also demonstrated the presence of junctional adhesion molecule-like (JAML) in the testis. JAML was previously reported by others to form a functional complex with CAR regulating transepithelial migration of leukocytes. The expression of JAML in the testis suggests that a similar functional complex might be present during germ cell migration across the blood-testis barrier. Finally, an intermediate compartment occupied by CAR-positive, migrating germ cells and flanked by two occludin-containing junctions is identified. Together, these results implicate a function for CAR in testis morphogenesis and in migration of germ cells across the blood-testis barrier during spermatogenesis.