Sprouting and abnormal contacts of nonmedullated axons, and deposition of extracellular material induced by the amyloid precursor protein (APP) and other protease inhibitors

We have reported that the local administration of serine protease inhibitors (amyloid precursor protein with the Kunitz insert (APP K +), aprotinin, and leupeptin) to the rat sciatic nerve determines a sprouting response of myelinated axons, proliferation of Schwann cells, and demyelination, 5 to 7 days later. Further study of these nerves with the electron microscope revealed (i) a sprouting response of nonmedullated axons, (ii) the appearance of fine axons with a few turns of compact myelin, (iii) abnormal contacts of axons with basal laminae, with fibroblast-like cells, and between them, (iv) the occurrence of hemidesmosome- and desmosome-like junctions between Schwann cell processes, and between Schwann cells and axons, and (v) the appearance of amorphous and fibrillary extracellular deposits alongside the axolemma. The adjacent proximal and distal segments were normal, i.e., axons remained continuous, and the alterations were confined to the segment exposed to the protease inhibitors, Heated APP K+, APP without the Kunitz insert (APP K-), bovine serum albumin, and saline, did not elicit cytological alterations. Our results suggest that these inhibitors of serine proteases (i) set free a sprouting drive of axons by disrupting an ongoing repressive mechanism; (ii) modify the adhesive properties of axons and Schwann cells, and (iii) alter the natural history of an extracellular material. The imbalance of an extracellular protease system may participate in the pathogenesis of Alzheimer's disease.