Occurrence of glutamate receptor subunit 1-containing aggresome-like structures during normal development of rat spinal cord interneurons.

During a developmental study of the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) -type glutamate receptor subunits in rat spinal cord, we observed the existence of cytoplasmic inclusion bodies with positive immunoreactivity to glutamate receptor subunit 1 (GluR1) but not to other glutamate receptor subunits. GluR1-positive bodies have a diameter of between 1 and 3 microm and can be seen widely distributed throughout spinal cord gray matter, with the exception of the ventral horn region. They transiently appear within a definite developmental time-period between embryonic day 19 and postnatal day 17 and are only associated with neuronal cells. Ultrastructural analysis revealed that these inclusions were located adjacent to the nucleus and consisted of amorphous material without any limiting membrane. Immunocytochemical analysis revealed that the inclusions displayed positive immunoreactivity to ubiquitin, HSP70, and 20S proteasome. All these data indicate that GluR1-containing inclusions display all the ultrastructural and immunocytochemical characteristics of the recently described structure, which have been given the name aggresomes. Further studies are needed to determine the biological significance of these normally occurring aggresome-like inclusions, because aggresomes are usually considered in a pathologic context.