Rescue of a dystrophin-like protein by exon skipping normalizes synaptic plasticity in the hippocampus of the mdx mouse.

Duchenne muscular dystrophy (DMD) is caused by the absence of dystrophin, a protein that fulfills important functions in both muscle and brain. The mdx mouse model of DMD, which also lacks dystrophin, shows a marked reduction in γ-aminobutyric acid type A (GABA(A))-receptor clustering in central inhibitory synapses and enhanced long-term potentiation (LTP) at CA3-CA1 synapses of the hippocampus. We have recently shown that U7 small nuclear RNAs modified to encode antisense sequences and expressed from recombinant adeno-associated viral (rAAV) vectors are able to induce skipping of the mutated exon 23 and to rescue expression of a functional dystrophin-like product both in the muscle and nervous tissue in vivo. In the brain, this rescue was accompanied by restoration of both the size and number of hippocampal GABA(A)-receptor clustering. Here, we report that 25.2±8% of re-expression two months after intrahippocampal injection of rAAV reverses the abnormally enhanced LTP phenotype at CA3-CA1 synapses of mdx mice. These results suggests that dystrophin expression indirectly influences synaptic plasticity through modulation of GABA(A)-receptor clustering and that re-expression of the otherwise deficient protein in the adult can significantly alleviate alteration of neural functions in DMD.

Rescue of a dystrophin-like protein by exon skipping in vivo restores GABAA-receptor clustering in the hippocampus of the mdx mouse.

Dystrophin, the cytoskeletal protein whose defect is responsible for Duchenne muscular dystrophy (DMD), is normally expressed in both muscles and brain. Genetic loss of brain dystrophin in the mdx mouse model of DMD reduces the capacity for type A gamma-aminobutyric acid (GABA(A))-receptor clustering in central inhibitory synapses, which is thought to be a main molecular defect leading to brain and cognitive alterations in this syndrome. U7 small nuclear RNAs modified to encode antisense sequences and expressed from recombinant adeno-associated viral (rAAV) vectors have proven efficient after intramuscular injection to induce skipping of the mutated exon 23 and rescue expression of a functional dystrophin-like product in muscle tissues of mdx mice in vivo. Here, we report that intrahippocampal injection of a single dose of rAAV2/1-U7 can rescue substantial levels of brain dystrophin expression (15-25%) in mdx mice for months. This is sufficient to completely restore GABA(A)-receptor clustering in pyramidal and dendritic layers of CA1 hippocampus, suggesting exon-skipping strategies offer the prospect to investigate and correct both brain and muscle alterations in DMD. This provides new evidence that in the adult brain dystrophin is critical for the control of GABA(A)-receptor clustering, which may have an important role in activity-dependent synaptic plasticity in hippocampal circuits.

FGF6 mediated expansion of a resident subset of cells with SP phenotype in the C2C12 myogenic line.

Fibroblast growth factor 6 (FGF6) is selectively expressed during muscle development and regeneration. We examined its effect on muscle precursor cells (mpc) by forcing stable FGF6 expression in C2C12 cells in vitro. FGF6 produced in genetically engineered mpc was active, inducing strong morphological changes, altering cell adhesion and compromising their ability to differentiate into myotubes. Expression of MyoD and myogenin, but not of Myf5, was abrogated in FGF6 engineered mpc. These effects were reversed by FGF inhibitors. Ectopic expression of MyoD also restored fiber formation indicating that FGF6 interferes with the myogenic differentiation pathway upstream of MyoD. We also report that in the presence of FGF6, the minor (0.5-2%) subpopulation of cells actively excluding Hoechst 33342 in a verapamil-dependent manner (SP phenotype) was increased to 15-20% and the expression of the mdr1a gene (but not mdr1b) was upregulated by 400-fold. Our data establish a previously undescribed link between FGF6--a muscle specific growth factor--and a multidrug resistance gene expressed in stem cells, and suggest a role for FGF6 in the maintenance of a reserve pool of progenitor cells in the skeletal muscle.

Neurotropism and retrograde axonal transport of a canine adenoviral vector: a tool for targeting key structures undergoing neurodegenerative processes.

Viral tropism refers to the ability of a virus to selectively infect a given subset of cells. It relies on a variety of viral and host determinants that entail virus binding and entry into target cells, in addition to the presence of genetic elements that allow or enhance viral gene expression in a specific manner. Here we report the results of neuroanatomical studies in rat brains injected in different cerebral structures with vectors derived from the canine adenovirus type 2 (CAV2), whose natural target is the respiratory epithelium. Control animals injected with vectors derived from the human adenovirus type 5 (Ad5) displayed the previously documented pattern of gene transfer into both neurons and glial cells. Injection of CAV2 vectors resulted in selective transduction of neuronal cells. Cy3-labeled CAV2 particles allowed us to establish the high affinity of this vector for neuronal processes in vitro and their rapid uptake and retrograde axonal transport in vivo. After intrahippocampal injections, labeled particles were found, within 1 hour, closely associated to the nuclei of the neurons in layer II of the entorhinal cortex. Injections into the striatum resulted in a massive transduction of dopaminergic neurons in the substantia nigra compacta. The high efficiency with which CAV2 vectors are retrogradely transported opens the possibility of targeting a transgene to neuron populations remote from the injection site and difficult to access. Our data support the possibility to target key structures undergoing a degenerative process: the enthorhinal cortex, which is affected first in Alzheimer's disease; and the substantia nigra compacta, which undergoes degeneration in Parkinson's disease.