One-trial in vitro conditioning regulates a cytoskeletal-related protein (CSP24) in the conditioned stimulus pathway of Hermissenda.

Hermissenda CSP24 (cytoskeletal-related protein 24) is a 24 kDa beta-thymosin-like protein that is associated with intermediate memory. We showed previously that one-trial conditioning resulted in a significant increase in the phosphorylation of CSP24 detected in lysates of the pathway supporting the conditioned stimulus (CS). Here we report the association of the protein with the actin cytoskeleton and the distribution of CSP24-immunoreactive neurons in two sensory structures and the circumesophageal nervous system. Identified photoreceptors, hair cells, and neurons in the cerebropleural and pedal ganglia were immunoreactive for CSP24. Immunoprecipitation experiments with 32PO4-labeled lysates of the circumesophageal nervous system identified a 44 kDa protein band (consistent with actin) that coprecipitates with CSP24. An analysis of immunoprecipitates on Western blots probed with anti-actin antibody also showed that actin coprecipitates with CSP24. Laser confocal microscopy of photoreceptors costained with fluorescently labeled anti-actin antibody and anti-CSP24 antibody, or fluorescent phalloidin and anti-CSP24 antibody showed that CSP24 is localized with actin in the cytosol of photoreceptor cell bodies and colocalized with presumed G-actin, but not F-actin, in regions adjacent to the plasma membrane. Although CSP24 is widely distributed in the Hermissenda nervous system, its regulation by one-trial conditioning was observed only in the CS pathway. Our findings suggest that CSP24 may interact with components of the actin cytoskeleton that contribute to structural changes underlying the formation and maintenance of enduring forms of memory.