Dp71ab/DAPs complex composition changes during the differentiation process in PC12 cells

Expression of Dystrophin Dp71 Splice Variants Is Temporally Regulated During Rodent Brain Development

Dystrophin Dp71 is the major product of the Duchenne muscular dystrophy (DMD) gene in the brain, and its loss in DMD patients and mouse models leads to cognitive impairments. Dp71 is expressed as a range of proteins generated by alternative splicing of exons 71 to 74 and 78, classified in the main Dp71d and Dp71f groups that contain specific C-terminal ends. However, it is unknown whether each isoform has a specific role in distinct cell types, brain regions, and/or stages of brain development. In the present study, we characterized the expression of Dp71 isoforms during fetal (E10.5, E15.5) and postnatal (P1, P7, P14, P21 and P60) mouse and rat brain development. We finely quantified the expression of several Dp71 transcripts by RT-PCR and cloning assays in samples from whole-brain and distinct brain structures. The following Dp71 transcripts were detected: Dp71d, Dp71d∆71, Dp71d∆74, Dp71d∆71,74, Dp71d∆71-74, Dp71f, Dp71f∆71, Dp71f∆74, Dp71f∆71,74, and Dp71fΔ71-74. We found that the Dp71f isoform is the main transcript expressed at E10.5 (> 80%), while its expression is then progressively reduced and replaced by the expression of isoforms of the Dp71d group from E15.5 to postnatal and adult ages. This major finding was confirmed by third-generation nanopore sequencing. In addition, we found that the level of expression of specific Dp71 isoforms varies as a function of postnatal stages and brain structure. Our results suggest that Dp71 isoforms have different and complementary roles during embryonic and postnatal brain development, likely taking part in a variety of maturation processes in distinct cell types.

Dp71 Point Mutations Induce Protein Aggregation, Loss of Nuclear Lamina Integrity and Impaired Braf35 and Ibraf Function in Neuronal Cells

Dystrophin Dp71 is the most abundant product of the Duchenne muscular dystrophy gene in the nervous system, and mutations impairing its function have been associated with the neurodevelopmental symptoms present in a third of DMD patients. Dp71 is required for the clustering of neurotransmitter receptors and the neuronal differentiation of cultured cells; nonetheless, its precise role in neuronal cells remains to be poorly understood. In this study, we analyzed the effect of two pathogenic DMD gene point mutations on the Dp71 function in neurons. We engineered C272Y and E299del mutations to express GFP-tagged Dp71 protein variants in N1E-115 and SH-SY5Y neuronal cells. Unexpectedly, the ectopic expression of Dp71 mutants resulted in protein aggregation, which may be mechanistically caused by the effect of the mutations on Dp71 structure, as predicted by protein modeling and molecular dynamics simulations. Interestingly, Dp71 mutant variants acquired a dominant negative function that, in turn, dramatically impaired the distribution of different Dp71 protein partners, including β-dystroglycan, nuclear lamins A/C and B1, the high-mobility group (HMG)-containing protein (BRAF35) and the BRAF35-family-member inhibitor of BRAF35 (iBRAF). Further analysis of Dp71 mutants provided evidence showing a role for Dp71 in modulating both heterochromatin marker H3K9me2 organization and the neuronal genes’ expression, via its interaction with iBRAF and BRAF5.

Human Dystrophin Dp71ab Enhances the Proliferation of Myoblasts Across Species But Not Human Nonmyoblast Cells

Dystrophin Dp71 is an isoform produced from the Dp71 promoter in intron 62 of the DMD gene, mutations in which cause Duchenne muscular dystrophy. Dp71 is involved in various cellular processes and comprises more than 10 isoforms produced by alternative splicing. Dp71ab, in which both exons 71 and 78 are deleted, has a hydrophobic C-terminus that is hydrophilic in Dp71. Therefore, Dp71ab is believed to have different roles from Dp71. Previously, we reported that Dp71ab enhanced the proliferation of human myoblasts. Here, we further characterized Dp71ab, focusing on the activation of cell proliferation. Dp71ab increased the proliferation of immortalized human myoblasts in a dose-dependent manner. In contrast, Dp71 suppressed proliferation in a dose-dependent manner. Consistent with these opposite effects, eGFP-tagged Dp71ab and mCherry-tagged Dp71 showed different cellular distributions, with Dp71ab mostly in the nucleus. Notably, human Dp71ab enhanced the proliferation of rat and mouse myoblasts. Despite these findings, human Dp71ab did not enhance the proliferation of human nonmyoblast cells, including rhabdomyosarcoma cells. We concluded that Dp71ab is a myoblast-specific proliferation enhancer. In further studies, Dp71ab will be employed for the expansion of myoblasts in clinical settings.

Dystrophin Dp71ab is monoclonally expressed in human satellite cells and enhances proliferation of myoblast cells

Dystrophin Dp71 is the smallest isoform of the DMD gene, mutations in which cause Duchenne muscular dystrophy (DMD). Dp71 has also been shown to have roles in various cellular processes. Stem cell-based therapy may be effective in treating DMD, but the inability to generate a sufficient number of stem cells remains a significant obstacle. Although Dp71 is comprised of many variants, Dp71 in satellite cells has not yet been studied. Here, the full-length Dp71 consisting of 18 exons from exons G1 to 79 was amplified by reverse transcription-PCR from total RNA of human satellite cells. The amplified product showed deletion of both exons 71 and 78 in all sequenced clones, indicating monoclonal expression of Dp71ab. Western blotting of the satellite cell lysate showed a band corresponding to over-expressed Dp71ab. Transfection of a plasmid expressing Dp71ab into human myoblasts significantly enhanced cell proliferation when compared to the cells transfected with the mock plasmid. However, transfection of the Dp71 expression plasmid encoding all 18 exons did not enhance myoblast proliferation. These findings indicated that Dp71ab, but not Dp71, is a molecular enhancer of myoblast proliferation and that transfection with Dp71ab may generate a high yield of stem cells for DMD treatment.

Dystrophin Dp71 and the Neuropathophysiology of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by frameshift mutations in the DMD gene that prevent the body-wide translation of its protein product, dystrophin. Besides a severe muscle phenotype, cognitive impairment and neuropsychiatric symptoms are prevalent. Dystrophin protein 71 (Dp71) is the major DMD gene product expressed in the brain and mutations affecting its expression are associated with the DMD neuropsychiatric syndrome. As with dystrophin in muscle, Dp71 localises to dystrophin-associated protein complexes in the brain. However, unlike in skeletal muscle; in the brain, Dp71 is alternatively spliced to produce many isoforms with differential subcellular localisations and diverse cellular functions. These include neuronal differentiation, adhesion, cell division and excitatory synapse organisation as well as nuclear functions such as nuclear scaffolding and DNA repair. In this review, we first describe brain involvement in DMD and the abnormalities observed in the DMD brain. We then review the gene expression, RNA processing and functions of Dp71. We review genotype-phenotype correlations and discuss emerging cellular/tissue evidence for the involvement of Dp71 in the neuropathophysiology of DMD. The literature suggests changes observed in the DMD brain are neurodevelopmental in origin and that their risk and severity is associated with a cumulative loss of distal DMD gene products such as Dp71. The high risk of neuropsychiatric syndromes in Duchenne patients warrants early intervention to achieve the best possible quality of life. Unravelling the function and pathophysiological significance of dystrophin in the brain has become a high research priority to inform the development of brain-targeting treatments for Duchenne.

Overexpression of mutant dystrophin Dp71[INCREMENT]₇₈-₇₉ stimulates cell proliferation

Dp71 dystrophin is the main DMD gene product expressed in the central nervous system. Experiments using PC12 cells as a neuronal model have shown that Dp71 isoforms are involved in differentiation, adhesion, cell division, and nuclear architecture. To contribute to the knowledge of Dp71 domains function, we previously reported the isolation and partial characterization of the dystrophin Dp71[INCREMENT]78-79 (a mutant that lacks exons 71, 78, and 79), which stimulates the neuronal differentiation of PC12-C11 clone. In this article, we generated a doxycycline (Dox)-inducible expression system in PC12 Tet-On cells (B10 cells) to overexpress and control the transcription of Dp71[INCREMENT]78-79. Western blotting and confocal microscopy showed an increase in the amount of Dp71[INCREMENT]78-79 (217±75-fold) with the addition of Dox to growth medium. Cell proliferation assays and morphometric analyses demonstrated that Dp71[INCREMENT]78-79 increases the growth rate of B10 cells and reduces the nerve growth factor-neuronal differentiation. Western blotting analysis revealed an upregulation in the expression of proliferating cell nuclear antigen, focal adhesion kinase, and β-dystroglycan in B10 cells compared with control cells. Our results show that the inducible expression of Dp71[INCREMENT]78-79 increases the growth rate of PC12 Tet-On cells, suggesting a role of this protein in cell proliferation.

Nucleocytoplasmic shuttling of the Duchenne muscular dystrophy gene product dystrophin Dp71d is dependent on the importin α/β and CRM1 nuclear transporters and microtubule motor dynein

Even though the Duchenne muscular dystrophy (DMD) gene product Dystrophin Dp71d is involved in various key cellular processes through its role as a scaffold for structural and signalling proteins at the plasma membrane as well as the nuclear envelope, its subcellular trafficking is poorly understood. Here we map the nuclear import and export signals of Dp71d by truncation and point mutant analysis, showing for the first time that Dp71d shuttles between the nucleus and cytoplasm mediated by the conventional nuclear transporters, importin (IMP) α/β and the exportin CRM1. Binding was confirmed in cells using pull-downs, while in vitro binding assays showed direct, high affinity (apparent dissociation coefficient of c. 0.25nM) binding of Dp71d to IMPα/β. Interestingly, treatment of cells with the microtubule depolymerizing reagent nocodazole or the dynein inhibitor EHNA both decreased Dp71d nuclear localization, implying that Dp71d nuclear import may be facilitated by microtubules and the motor protein dynein. The role of Dp71d in the nucleus appears to relate in part to interaction with the nuclear envelope protein emerin, and maintenance of the integrity of the nuclear architecture. The clear implication is that Dp71d's previously unrecognised nuclear transport properties likely contribute to various, important physiological roles.

Expression of sarcoglycans in the human cerebral cortex: an immunohistochemical and molecular study

The sarcoglycan (SG) complex (SGC) is a subcomplex within the dystrophin-glycoprotein complex (DGC) and is composed of several transmembrane proteins (α, β, δ, γ, ε and ζ). The DGC supplies a transmembranous connection between the subsarcolemmal cytoskeleton networks and the basal lamina in order to protect the lipid bilayer and to provide a scaffold for signaling molecules in all muscle cells. In addition to its role in muscle tissue, dystrophin and some DGC components are expressed in neurons and glia. Very little is known about the SG subunits in the central nervous system (CNS) and some data suggested the presence of ε and ζ subunits only. In fact, mutations in the ε-SG gene cause myoclonus-dystonia, indicating its importance for brain function. To determine the presence and localization of SGC in the human cerebral cortex, we performed an investigation using immunofluorescence, immunoblotting and reverse transcriptase polymerase chain reaction. The results showed that all SG subunits are expressed in the human cerebral cortex, particularly in large neurons but also in astrocytes. These data suggest that the SG subcomplex may be involved in the organization of CNS synapses.

Characterization of Dp71Δ(78-79), a novel dystrophin mutant that stimulates PC12 cell differentiation

Dp71 has an important role in the central nervous system. To better understand the function of Dp71 domains in neuronal differentiation, PC12 cells were stably transfected with a dystrophin mutant, Dp71Δ(78-79) , which lacks exons 78 and 79. Based on the percentage of cells bearing neurites and neurite length analyses, we found that cells stably expressing Dp71Δ(78-79) (PC12-C11) differentiate more efficiently than non-transfected cells. While wild-type cells reach their maximum differentiation 9-12 days after initiating the differentiation process, the PC12-C11 cells reach differentiation in 4-6 days. Protein expression analysis showed a down-regulation of Dp71a and an up-regulation of Dp71ab and/or Up71, β-dystroglycan and neuron-specific enolase in undifferentiated and in neural growth factor differentiated PC12-C11 cells. No change was observed in the expression of Grb2 and Up400. The subcellular localization of Dp71Δ(78-79) was in the cell periphery, and there was no change in localization during the differentiation process, which was also observed throughout the neurite extensions.

Dystrophins and DAPs are expressed in adipose tissue and are regulated by adipogenesis and extracellular matrix

The dystrophin-associated protein complex (DAPC), consisting of dystrophin, dystroglycans, sarcoglycans, dystrobrevins and syntrophins, provides a linkage between the cytoskeleton and the extracellular matrix. The disruption of DAPC leads to Duchenne/Becker muscular dystrophy and other neuromuscular diseases. Although adipose-derived stem cells had been used for the experimental treatment of Duchenne/Becker disease with promising results, little is known on the expression and function of DAPC in adipose tissue. Here we show that visceral and subcutaneous rat adipose depots express mRNAs for all known dystrophin isoforms, utrophin, α- and β-dystrobrevins, and α-, βI-, βII-, and γII-syntrophins. Visceral and subcutaneous rat preadipocytes express Dp116 and Dp71 mRNAs and proteins, and this expression is differentially regulated during adipogenesis. Rat preadipocytes also express β-dystrobrevin, α-, βI-, βII- and γII-syntrophins, β-dystroglycan and β-, δ-, and ε-sarcoglycans with no changes during adipogenesis. We also show that α-dystrobrevin increases their expression during adipose differentiation and extracellular matrix differentially regulates the expression of dystrophin isoforms mRNAs during adipogenesis. Our results show that DAPC components are expressed in adipose tissues and suggest that this complex has a role on the adipose biology.

Dystrophin Dp71 is critical for stability of the DAPs in the nucleus of PC12 cells

We have adopted the PC12 cell line as in vitro cell model for studying Dp71 function in neuronal cells. These cells express a cytoplasmic (Dp71f) and a nuclear (Dp71d) isoform of Dp71 as well as various dystrophin-associated proteins (DAPs). In this study, we revealed by confocal microscopy analysis and Western blotting evaluation of cell fractions the presence of different DAPs (beta-dystroglycan, beta-dystrobrevin, epsilon-sarcoglycan and gamma1-syntrophin) in the nucleus of PC12 cells. Furthermore, we established by immunoprecipitation assays that Dp71d and the DAPs form a dystrophin-associated protein complex (DAPC) in the nucleus. Interestingly, depletion of Dp71 by antisense treatment (antisense-Dp71 cells) provoked a drastic reduction of nuclear DAPs, which indicates that Dp71d is critical for DAPs stability within the nucleus. Although Up71, the utrophin gene product homologous to Dp71, exhibited increased expression in the antisense-Dp71 cells, its scarce nuclear levels makes unlikely that could compensate for Dp71 nuclear deficiency.