Abstract
The amyloid beta-protein (A beta) is a major component of extracellular deposits that are characteristic features of Alzheimer's disease. A beta is derived from the large transmembrane beta-amyloid precursor protein (beta APP). In the rabbit optic nerve/optic tract (ON), beta APP is synthesized in vivo in retinal ganglion cell perikarya, rapidly transported into the ON axons in small transport vesicles and is subsequently transferred to the axonal plasma membrane as well as to the presynaptic nerve terminals (Morin, P. J., Abraham, C. R., Amaratunga, A., Johnson, R.J., Huber, G., Sandell, J. H., and Fine, R. E. (1993) J. Neurochem. 61, 464-473). Present results indicate that there is rapid processing of beta APP in the ON to generate a 14-kDa C-terminal membrane-associated fragment that contains the A beta sequence. By using equilibrium sucrose density gradient fractionation, this fragment, as well as non-amyloidogenic C-terminal fragments and intact beta APP, are detected in at least two classes of transport vesicles destined for the plasma membrane and the presynaptic nerve terminal. The two classes of transported vesicles are distinguished by labeling kinetics as well as by density. In contrast to the ON, only nonamyloidogenic C-terminal fragments are generated in the retina, which contains the perikarya of retinal ganglion cells and glial (Muller) cells which also produce beta APP.
Highlights
Mature I3APP can undergo proteolytic cleavage within the AI3 domain and this results in the secretion of soluble I3APP and a 9-12-kDa non-amyloidogenic fragment which remains membrane-bound (Selkoe et al, 1988; Schubert et al, 1989; Weidemann et al, 1989; Esch et al, 1990; Sisodia et al, 1990)
Employing a central nervous system neuron of rabbit, we demonstrated that I3APP695 as well as I3APP751/770 are synthesized in the retina; I3APP695 is rapidly transported in vivo into the optic nerve/optic tract (ON) in small vesicles and is subsequently transferred to a membrane fraction containing glucose transporters and other axonal plasma membrane markers, as well as to the presynaptic nerve terminals in the lateral geniculate (Morin et al, 1993)
In order to examine the generation of potentially amyloidogenic fragments of I3APP in the ON in vivo, we carried out a series of studies employing gel electrophoresis conditions capable of detecting small molecular weight fragments
Summary
A{3, amyloid {3-protein; {3APP, {3-amyloid precursor protein; ON, optic nerve/optic tract; PBS, phosphate-buffered saline; Tricine, N-[2-hydroxy-l,l-bis(hydroxymethyl)ethyl]glycine. C-terminal fragments containing the complete AI3 sequence of I3APP (of ~ 11 kDa and larger) have been detected in endosomes/lysosomes in cultured cells and brain tissue (Nordstedt et al, 1991; Estus et al, 1992; Tamaoka et al, 1992) and could serve as potential intermediates for AI3 formation Both neurons and astrocytes in cell culture generate and secrete soluble AI3 (Haass et al, 1992b; Seubert et al, 1992; Shoji et al, 1992), which is found in the cerebral spinal fluid of normal individuals (Seubert et al, 1992). We describe results indicating that there is rapid processing of I3APP695 in the ON to generate a C-terminal membrane-associated fragment that contains the AI3 sequence This fragment, as well as non-amyloidogenic fragments and intact I3APP, are seen in regions of an equilibrium sucrose density. We report that in the retina, where I3APP751 and I3APP77o are present, I3APP is processed differently from that which occurs in the ON, generating only non-amyloidogenic C-terminal fragments
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