Identification of a beta-dystroglycan immunoreactive subcompartment in photoreceptor terminals

Mutations in the dystrophin-associated glycoprotein complex (DGC) cause various forms of muscular dystrophy. These diseases are characterized by progressive loss of skeletal muscle tissue and by dysfunctions in the central nervous system (CNS). The CNS deficits include an altered electroretinogram, caused by an impaired synaptic transmission between photoreceptors and their postsynaptic target cells in the outer plexiform layer (OPL). The DGC is concentrated in the OPL but its exact distribution is controversial. Therefore, the precise distribution of beta-dystroglycan, the central component of the DGC, within the OPL of the mature chick retina, was determined.

Double immunolabeling with antibodies against beta-dystroglycan and against Bassoon, a component of the presynaptic cytomatrix, concentrated at the insertion point of the synaptic ribbon into the active zone of the photoreceptor synapses, showed a nonoverlapping distribution of both proteins within individual rod and cone photoreceptor terminals. The three-dimensional distribution of the DGC within the photoreceptor terminals was determined by reconstruction of the beta-dystroglycan immunoreactivity from serial electron microscopic sections.

We found that beta-dystroglycan was not directly associated with the ribbon synapse but instead concentrated perisynaptically in processes extending from the photoreceptors into the OPL. The processes displayed dystroglycan immunoreactivity primarily along their lateral sides and at their tips. Processes from bipolar or horizontal cells were not labeled.

The perisynaptic concentration of beta-dystroglycan in photoreceptor terminals suggests a novel domain within photoreceptor terminals with functions in synaptic transmission.