Excessive collagen deposition plays a critical role in the development of fibrosis, and early or active fibrosis may be more susceptible to therapeutic intervention than later stages of scarring. However, at present there is no simple method for assessing the collagen-synthesizing and secreting activity of fibroblasts in human tissues. Type I procollagen carboxyterminal domains are proteolytically removed during collagen secretion. Thus, antibodies to these domains should stain fibroblasts synthesizing type I collagen but not extracellular collagen fibrils which could mask the signal from the cells. We developed and characterized a monoclonal antibody (Anti-pC) specific for the carboxyterminal propeptide of type I procollagen.

Monensin converts the Golgi apparatus of rod photoreceptors into distended vacuoles, similar to those seen in other monensin-treated cell types, and leads to the accumulation of [3H]leucine in the distended vacuoles. As evaluated by quantitative, electron microscopic autoradiography, transport of newly made proteins--both to the outer segments and to the presynaptic terminals--is inhibited. These effects suggest that the Golgi apparatus is involved in transport in both principal directions within the highly polarized photoreceptors, a matter of interest since there seems to be only a single, extensive, Golgi apparatus in the cell body. Seemingly there are two distinguishable "sorting" routes, for proteins, out of the Golgi apparatus and, for the terminals, an additional non-Golgi route. Accumulation of newly made glycerolipids in the outer segments and terminals is less affected by monensin than is accumulation of new proteins, and glycerolipid accumulation is little affected by puromycin, an inhibitor of protein synthesis. These latter findings suggest that the routes or mechanisms of assembly of newly made lipids into membranes in the photoreceptors are at least partially dissociable from those for newly made proteins.

We have explored the use of the Na+-H+ ionophore monensin as a potential tool for the investigation of membrane assembly and transport in retinal photoreceptors. Autoradiographic analysis of frog retinas incubated with [3H]leucine in the presence of monensin revealed a lack of concentrated silver grains ("bands") at the base of the rod outer segments, in contrast to controls. This is indicative of a pronounced monensin-induced decrease in disc membrane assembly. Biochemical analyses of whole retinas and isolated rod outer segment membranes showed that protein synthesis (including opsin synthesis) was not significantly inhibited under these conditions, whereas passage of membrane protein to the rod outer segment was blocked. Glycerolipid synthesis was not significantly affected by monensin. The results suggest that membrane proteins (e.g., opsin) destined for incorporation into the rod outer segment must pass through the Golgi apparatus and demonstrate the potential utility of monensin for inhibiting aspects of marcomolecule transport in photoreceptors.