SUPPRESSION OF CATHEPSIN-B AND CATHEPSIN-L CAUSES A PROLIFERATION OF LYSOSOMES AND THE FORMATION OF MEGANEURITES IN HIPPOCAMPUS

Cultured hippocampal slices exhibited prominent ultrastructural featur es of brain aging after exposure to an inhibitor of cathepsins B and L . Six days of treatment with N-CBZ-L-phenylalanyl-L-alanine-diazomethy lketone (ZPAD) resulted in a dramatic increase in the number of lysoso mes in the perikarya of neurons and glial cells throughout the slices. Furthermore, lysosomes in CA1 and CA3 pyramidal cells were not restri cted to the soma but instead were located throughout dendritic process es. Clusters of lysosomes were commonly found within bulging segments of proximal dendrites that were notable for an absence of microtubules and neurofilaments, Although pyknotic nuclei were sometimes encounter ed, most of the cells in slices exposed to ZPAD for 6 d appeared relat ively normal. Slices given 7 d of recovery contained several unique fe atures, compared with those processed immediately after incubation wit h the inhibitor. Cell bodies of CA1 neurons were largely cleared of th e excess lysosomes but had gained fusiform, somatic extensions that we re filled with fused lysosomes and related complex, dense bodies. Thes e appendages, similar in form and content to structures previously ref erred to as ''meganeurites,'' were not observed in CA3 neurons or gran ule cells. Because meganeurites were often interposed between cell bod y and axon, they have the potential to interfere with processes requir ing axonal transport. It is suggested that inactivation of cathepsins B and L results in a proliferation of lysosomes and that meganeurite g eneration provides a means of storing residual catabolic organelles. T he accumulated material could be eliminated by pinching off the megane urite but, at least in some cases, this action would result in axotomy . Reduced cathepsin L activity, increased numbers of lysosomes, and th e formation of meganeurites are all reported to occur during brain agi ng; thus, it is possible that the infusion of ZPAD into cultured slice s sets in motion a greatly accelerated gerontological sequence.