Axonal transport of antibodies to subcellular and protein fractions of rat brain

Experiments examined the feasibility of using the axonal transport of antibodies as a possible means to characterize nerve membrane composition and the fate of internalized macromolecules. Polyspecific antibodies were generated in rabbits against rat brain synaptosomal and microsomal subcellular fractions and against wheat germ agglutinin-binding proteins isolated by lectin affinity chromatography. Antisera were injected into the vitreal chamber of the eye and into the facial musculature of anesthetized rats to test, respectively, for anterograde transport in retinotectal neurons and for retrograde transport in facial motoneurons. Control injections of preimmune serum were made into the opposite side. After survival for 4–168 h, animals were perfused and the axonally transported rabbit immunoglobulins detected in frozen sections of the brainstem using a modified peroxidase-antiperoxidase immunocytochemical procedure. Antisera against all 3 classes of neuronal antigens contained antibodies that underwent retrograde axonal transport. No evidence of anterograde transport was seen. Neurons containing retrogradely transported immunoglobulins exhibited punctate as well as diffuse staining of the cytoplasm and proximal dendrites, exclusive of the nucleus. Following retrograde transport of antibodies to the synaptosomal fraction, staining of the neuropil around motoneurons was also observed, suggesting transcellular transport of these antibodies. Concentrations of injected antibodies as low as 1% of whole antiserum led to detectable retrograde transport. Increasing concentrations of antibodies above the amount in whole antiserum did not increase the intensity of staining in retrogradely labeled neurons, suggesting saturation. The findings support the view that antibodies to neural membranes are taken up and transported by binding to specific sites on nerve terminals. The development of monospecific antibodies for studies of axonal transport is thus likely to provide a feasible approach to characterize, in vivo, specific neuronal cell surface components and their internalization, transport and processing in neurons.