Lesion‐Induced Changes in the Production of Newly Synthesized and Secreted Apo‐E and Other Molecules Are Independent of the Concomitant Recruitment of Blood‐Borne Macrophages into Injured Peripheral Nerves

Abstract: Peripheral nerve injury produces Wallerian degeneration characterized by a change in the composition of resident nonneuronal cells: macrophages are recruited from the circulation to join Schwann, fibroblast, and endothelial cells. At the same time, the nonneuronal cell population exhibits, as a whole, alterations in synthesis and secretion of diffusible molecules, some of which are instrumental in nerve repair mechanisms. In this study, we determined whether changes in the production of secreted molecules depend on the concomitant modification in cell composition. Therefore, we studied the secretion of newly synthesized molecules by defined cell populations of intact nerves, intact nerve explants undergoing in vitro axonal degeneration, in vivo degenerating nerves, and recruited cells. Nerves were incubated in serum‐free, [³⁵S]methionine‐containing media. Secreted, radioactively labeled proteins were precipitated from the medium and analyzed by gel electrophoresis. Reduced production of 43‐, 46‐, and 48‐kDa proteins and increased production of 33–34‐, 37‐, 49‐, 59–, and 67‐kDa proteins were detected in in situ degenerating nerves. High‐density ultracentrifugation and immunoblot analysis revealed that the 33–34‐kDa protein is apolipoprotein‐E (apo‐E). Similar alterations in the production of these molecules were detected in intact nerve explants from which blood‐borne cells were excluded. Apo‐E, 37‐, 49‐, 59‐, and 67‐kDa proteins were also produced in frozen nerves that lacked the intact nerve nonneuronal cell population. Instead, these preparations contained blood‐borne cells, primarily macrophages. Thus, change in the production of a substantial number of secreted molecules, apo‐E included, is a characteristic response to axonal disintegration of the nonneuronal cells resident in intact nerves. Recruited macrophages, although not required, contribute to the production of apo‐E and other secreted molecules. The production of apo‐E and 45‐kDa proteins was inhibited, and that of 37‐kDa proteins increased in the presence of NH₄Cl, further suggesting that lysosomal activity plays a role in the regulation of the production of these molecules.