Post-translational Processing and Turnover Kinetics of Presynaptically Targeted Amyloid Precursor Superfamily Proteins in the Central Nervous System

Abstract

The amyloid precursor superfamily is composed of three highly conserved transmembrane glycoproteins, the amyloid precursor protein (APP) and amyloid precursor-like proteins 1 and 2 (APLP1, APLP2), whose functions are unknown. Proteolytic cleavage of APP yields the ␤A4 peptide, the major component of cerebral amyloid in Alzheimer's disease. Here we show that five post-translationally modified, full-length species of APP and APLP2 (but not APLP1) arrive at the mature presynaptic terminal in the fastest wave of axonal transport and are subsequently rapidly cleared (mean halflife of 3.5 h). Rapid turnover of presynaptic APP and APLP2 occurs independently of visual activity. Turnover of the most rapidly arriving APP species was accompanied by a delayed accumulation of a 120-kDa, APP fragment lacking the C terminus, consistent with presynaptic APP turnover via constitutive proteolysis. Turnover of APLP2 was not accompanied by detectable APLP2 fragment peptides, suggesting either that APLP2 either is more rapidly degraded than is APP or is retrogradely transported shortly after reaching the terminus. A single 150-kDa APLP2 species containing the Kunitz protease inhibitor domain is the major amyloid precursor superfamily protein transported to the presynapse. Presynaptic APP and APLP2 are sialylated and N-and O-glycosylated, and some also carry chondroitin sulfate glycosaminoglycan and/or dermatan sulfate glycosaminoglycan. The rapid kinetics for turnover of APP and APLP2 predict a sensitive balance of synthesis, transport, and elimination rates that may be critical to normal neuronal functions and metabolic fates of these proteins.Alzheimer's disease is an age-related human dementia whose principle neuropathological signs are forebrain cholinergic neuronal death, neurofibrilliary tangles, and amyloidosis of cerebral vessels and senile plaques (1). Interest in the regulation of APP 1 expression arose from it being recognized as the parent protein of ␤A4, the major peptide constituent of cerebrovascular and senile plaque amyloid (2). APP is one of the three proteins encoded by genes in the mammalian amyloid precursor superfamily (APSF; see Fig. 1A), which also has two genes encoding APLP1 and APLP2 (3). Although the APP gene is the only APSF gene that encodes ␤A4, the conservation of gene structure (4) and domain homology (5) among the APSF proteins suggests that they may share significant overlap in function and expression, and thus, that amyloidogenic processing of APP may be influenced in parallel with processing of APLP1 and/or APLP2. For instance, APSF proteins from various phyla share highly conserved extracellular zinc and heparin binding domains (3), single membrane-spanning domains (4, 6), and intracellular phosphorylation sites (7). Additionally, APP and APLP2 genes both contain N-linked glycosylation sites (8), splice-specific chondroitin sulfate glycosaminoglycan (CSGAG) attachment sites (9 -11), and, alternatively, spliced exons that encode the Kunitz protease inhibitor (KPI) domain (4, 12). Various combinat...

β‐secretase‐cleaved amyloid precursor protein in Alzheimer brain: a morphologic study

“…Hematoxyline counterstaining revealed the linearly distributed granules to be intraaxonal, indicating presence and transport of -sAPP in the axon. The normal presence of -sAPP in the axon is supported by several previous studies [22][23][24][25][26] and the axonally transported APP can be processed in the axolemma or the axon [28,29]. Newly synthesised APP is believed to be delivered to the synaptic terminal by fast anterograde axonal transport and then transported to the dendrites by a transcytotic mechanism [23].…”

The Proteolytic Processing of the Amyloid Precursor Protein Gene Family Members APLP-1 and APLP-2 Involves α-, β-, γ-, and ϵ-Like Cleavages

“…We have shown that the 2z tag does not alter the proteolytic processing of APP but stabilizes its C-terminal fragments and enhances their detection (23). This method has enabled the detection of a novel ⑀-cleavage site of APP at the C-terminal end of the transmembrane domain, between Leu 49 and Val 50 (23). Thus, the 2z tag was fused to the C termini of APLP-1 and APLP-2-763, and these constructs, which we termed APLP-1-2z and APLP-2-2z, respectively, were overexpressed in SH-SY5Y cells.…”

“…An additional cleavage within the APP ectodomain at position Ϫ12 (relative to A␤), which was termed ␦-cleavage, has only been observed in hippocampal neurons so far (22). Recently, we and others have identified a presenilindependent cleavage at the C-terminal end of the APP transmembrane domain between Leu 49 and Val 50 , which we termed ⑀-cleavage (23)(24)(25)(26), that is homologous to the S3 cleavage of Notch (27,28). The exact mechanism of the two presenilin-dependent ␥-and ⑀-cleavages within the transmembrane domain of APP remains unclear.…”

Physiological role for amyloid precursor protein in adult experience-dependent plasticity

“…APP operates in a cell autonomous fashion to mediate axonal pruning in the adult after changes in sensory input, similar to its cell autonomous role in sensory neuron and retinal axon pruning (8,29). Previous work demonstrates that APP is expressed both pre-and postsynaptically (13)(14)(15)(16)(17)(18) and is present in growth cones (30,31). Furthermore, the E1 portion of the APP protein plays a role in cell adhesion (27).…”