Effects of TNFα-Converting Enzyme Inhibition on Amyloid β Production and APP ProcessingIn VitroandIn Vivo

Abstract

Tumor necrosis factor-␣ (TNF␣) is a proinflammatory cytokine that is elevated in Alzheimer's disease (AD) brains. Because TNF␣ is released from cell membranes by the TNF␣-converting enzyme (TACE), inhibition of TACE has the potential to mitigate TNF␣ effects in AD brain. TACE also cleaves amyloid precursor protein (APP) and generates sAPP␣, precluding the formation of potentially harmful amyloid ␤ (A␤) peptides by ␤-site APP cleaving enzymes (BACE). Hence, the anti-inflammatory benefits of TACE inhibition might be offset by an increase in A␤. We have examined the effects of the highly selective TACE inhibitor, BMS-561392, on APP processing in vitro and in vivo. In Chinese hamster ovary cells expressing APP, BMS-561392 significantly reduced secretion of sAPP␣ without a corresponding increase in A␤ production. Conversely, a BACE inhibitor decreased sAPP␤ and A␤ peptides with no change in the secretion of sAPP␣. These data indicate an absence of TACE and BACE competition for the APP substrate. Despite this, we observed competition for APP when TACE activity was enhanced via phorbol ester treatment or if APP was modified such that it was retained within the trans-Golgi network (TGN). These results suggest that BACE and TACE share a common TGN localization, but under normal conditions do not compete for APP. To confirm this finding in vivo, BMS-561392 was infused into the brains of Tg2576 and wild-type mice. Although decreased brain sAPP␣ levels were observed, steady-state A␤ levels were not significantly changed. Accordingly, it is possible that TACE inhibitors could reduce TNF␣ levels without increasing A␤ levels within the AD brain.

Soluble Amyloid Precursor Protein 770 Is Released from Inflamed Endothelial Cells and Activated Platelets

“…Since a selective TACE inhibitor, TAPI-0, partially blocked the secretion of sAPP770 from BMECs (Fig. 2B), TACE is at least partially involved in the production of sAPP770␣ in endothelial cells, similar to the case for sAPP695 production (20). Next, to clarify the extent to which sAPP770 accounts for the total sAPP in in vivo samples, we measured the sAPPtotal and sAPP770 levels in human serum and CSF samples.…”

“…We anticipate that measurement of sAPP770␣/␤ in in vivo samples will enable us to judge whether the increased sAPP secretion is caused by increased processing of endothelial APP770 or neuronal APP695. In fact, both sAPP␣ and sAPP␤ have been extensively analyzed for their ␣-and ␤-cleavage activities, respectively (20,21). However, the currently available APP ELISA systems detect APP695, APP751, and APP770 mixed together.…”

Reduction of Synaptojanin 1 Accelerates Aβ Clearance and Attenuates Cognitive Deterioration in an Alzheimer Mouse Model

“…4 Promoting APP processing through the nonamyloidogenic pathway by synj1 reduction can further strengthen rescue of AD related changes (43,44). For example, the ␣-secretase-derived soluble APP N-terminal fragment, sAPP␣ has been suggested to have neurotrophic and neuroprotective functions, further supporting the therapeutic value of reducing synj1 levels in the brain (45).…”

The Mitochondrial Secret(ase) of Alzheimer's Disease

“…Whether this reduction in α-secretase cleavage product is a response to increased β-secretase cleavage and reduced availability of the α-secretase substrate [55] or an otherwise unknown cause of reduced α-secretase activity with a subsequent response in increased β-secretase activity still remains to be understood, but the identification of a direct relationship in the activities of α-and β-secretase proteins would assist in determining which enzyme's activity is affected first by the inciting event for AD pathogenesis. Results from Kim and colleagues [72] suggest AβPP cleavage activity by α-secretase and β-secretase were not directly linked when inhibitors against TACE or BACE were used and the β-and α-secretase cleavage products were measured. Though brain β-secretase levels have been reported as increased, to our knowledge, no reports of alterations in brain α-secretase levels in AD individuals have been made.…”