A fluorescent protein-readout for transcriptional activity reveals regulation of APP nuclear signaling by phosphorylation sites.

Signaling pathways that originate at the plasma membrane, including regulated intramembrane proteolysis (RIP), enable extracellular cues to control transcription. We modified the yeast Gal4 transcription system to study the nuclear translocation of transcriptionally active complexes using the fluorescent protein citrine (Cit) as a reporter. This enabled highly sensitive quantitative analysis of transcription in situ at the single cell level. The Gal4/UAS-Cit transcription assay displayed a sigmoidal response limited by the number of integrated reporter cassettes. We validated the assay by analyzing nuclear translocation of the amyloid precursor protein (APP) intracellular domain (AICD) and confirmed the requirement of Fe65 for nuclear translocation of AICD. In addition to the strong on-off effects on transcriptional activity, the results of this assay establish that phosphorylation modifies nuclear signaling. The Y682F mutation in APP showed the strongest increase in Cit expression, underscoring its role in regulating Fe65 binding. Together, we established a highly sensitive fluorescent protein-based assay that can monitor transcriptional activity at the single cell level and demonstrate that AICD phosphorylation affects Fe65 nuclear activity. This assay also introduces a platform for future single cell-based drug screening methods for nuclear translocation.

Visualization and quantification of APP intracellular domain-mediated nuclear signaling by bimolecular fluorescence complementation.

BACKGROUND: The amyloid precursor protein (APP) intracellular domain (AICD) is released from full-length APP upon sequential cleavage by either α- or ß-secretase followed by γ-secretase. Together with the adaptor protein Fe65 and the histone acetyltransferase Tip60, AICD forms nuclear multiprotein complexes (AFT complexes) that function in transcriptional regulation. OBJECTIVE: To develop a medium-throughput machine-based assay for visualization and quantification of AFT complex formation in cultured cells. METHODS: We used cotransfection of bimolecular fluorescence complementation (BiFC) fusion constructs of APP and Tip60 for analysis of subcellular localization by confocal microscopy and quantification by flow cytometry (FC). RESULTS: Our novel BiFC-constructs show a nuclear localization of AFT complexes that is identical to conventional fluorescence-tagged constructs. Production of the BiFC signal is dependent on the adaptor protein Fe65 resulting in fluorescence complementation only after Fe65-mediated nuclear translocation of AICD and interaction with Tip60. We applied the AFT-BiFC system to show that the Swedish APP familial Alzheimer's disease mutation increases AFT complex formation, consistent with the notion that AICD mediated nuclear signaling mainly occurs following APP processing through the amyloidogenic ß-secretase pathway. Next, we studied the impact of posttranslational modifications of AICD on AFT complex formation. Mutation of tyrosine 682 in the YENPTY motif of AICD to phenylalanine prevents phosphorylation resulting in increased nuclear AFT-BiFC signals. This is consistent with the negative impact of tyrosine phosphorylation on Fe65 binding to AICD. Finally, we studied the effect of oxidative stress. Our data shows that oxidative stress, at a level that also causes cell death, leads to a reduction in AFT-BiFC signals. CONCLUSION: We established a new method for visualization and FC quantification of the interaction between AICD, Fe65 and Tip60 in the nucleus based on BiFC. It enables flow cytometric analysis of AICD nuclear signaling and is characterized by scalability and low background fluorescence.

Turnover of amyloid precursor protein family members determines their nuclear signaling capability.

The amyloid precursor protein (APP) as well as its homologues, APP-like protein 1 and 2 (APLP1 and APLP2), are cleaved by α-, ß-, and γ-secretases, resulting in the release of their intracellular domains (ICDs). We have shown that the APP intracellular domain (AICD) is transported to the nucleus by Fe65 where they jointly bind the histone acetyltransferase Tip60 and localize to spherical nuclear complexes (AFT complexes), which are thought to be sites of transcription. We have now analyzed the subcellular localization and turnover of the APP family members. Similarly to AICD, the ICD of APLP2 localizes to spherical nuclear complexes together with Fe65 and Tip60. In contrast, the ICD of APLP1, despite binding to Fe65, does not translocate to the nucleus. In addition, APLP1 predominantly localizes to the plasma membrane, whereas APP and APLP2 are detected in vesicular structures. APLP1 also demonstrates a much slower turnover of the full-length protein compared to APP and APLP2. We further show that the ICDs of all APP family members are degraded by the proteasome and that the N-terminal amino acids of ICDs determine ICD degradation rate. Together, our results suggest that different nuclear signaling capabilities of APP family members are due to different rates of full-length protein processing and ICD proteasomal degradation. Our results provide evidence in support of a common nuclear signaling function for APP and APLP2 that is absent in APLP1, but suggest that APLP1 has a regulatory role in the nuclear translocation of APP family ICDs due to the sequestration of Fe65.

Reduced sodium channel Na(v)1.1 levels in BACE1-null mice.

The Alzheimer BACE1 enzyme cleaves numerous substrates, with largely unknown physiological consequences. We have previously identified the contribution of elevated BACE1 activity to voltage-gated sodium channel Na(v)1.1 density and neuronal function. Here, we analyzed physiological changes in sodium channel metabolism in BACE1-null mice. Mechanistically, we first confirmed that endogenous BACE1 requires its substrate, the ß-subunit Na(v)ß(2), to regulate levels of the pore-forming α-subunit Na(v)1.1 in cultured primary neurons. Next, we analyzed sodium channel α-subunit levels in brains of BACE1-null mice at 1 and 3 months of age. At both ages, we found that Na(v)1.1 protein levels were significantly decreased in BACE1-null versus wild-type mouse brains, remaining unchanged in BACE1-heterozygous mouse brains. Interestingly, levels of Na(v)1.2 and Na(v)1.6 α-subunits also decreased in 1-month-old BACE1-null mice. In the hippocampus of BACE1-null mice, we found a robust 57% decrease of Na(v)1.1 levels. Next, we performed surface biotinylation studies in acutely dissociated hippocampal slices from BACE1-null mice. Hippocampal surface Na(v)1.1 levels were significantly decreased, but Na(v)1.2 surface levels were increased in BACE1-null mice perhaps as a compensatory mechanism for reduced surface Na(v)1.1. We also found that Na(v)ß(2) processing and Na(v)1.1 mRNA levels were significantly decreased in brains of BACE1-null mice. This suggests a mechanism consistent with BACE1 activity regulating mRNA levels of the α-subunit Na(v)1.1 via cleavage of cell-surface Na(v)ß(2). Together, our data show that endogenous BACE1 activity regulates total and surface levels of voltage-gated sodium channels in mouse brains. Both decreased Na(v)1.1 and elevated surface Na(v)1.2 may result in a seizure phenotype. Our data caution that therapeutic BACE1 activity inhibition in Alzheimer disease patients may affect Na(v)1 metabolism and alter neuronal membrane excitability in Alzheimer disease patients.

Identification of BACE1 cleavage sites in human voltage-gated sodium channel beta 2 subunit.

BACKGROUND: The voltage-gated sodium channel ß2 subunit (Navß2) is a physiological substrate of BACE1 (ß-site APP cleaving enzyme) and γ-secretase, two proteolytic enzymes central to Alzheimer's disease pathogenesis. Previously, we have found that the processing of Navß2 by BACE1 and γ-secretase regulates sodium channel metabolism in neuronal cells. In the current study we identified the BACE1 cleavage sites in human Navß2. RESULTS: We found a major (147-148 L↓M, where ↓ indicates the cleavage site) and a minor (144145 L↓Q) BACE1 cleavage site in the extracellular domain of human Navß2 using a cell-free BACE1 cleavage assay followed by mass spectrometry. Next, we introduced two different double mutations into the identified major BACE1 cleavage site in human Navß2: 147LM/VI and 147LM/AA. Both mutations dramatically decreased the cleavage of human Navß2 by endogenous BACE1 in cell-free BACE1 cleavage assays. Neither of the two mutations affected subcellular localization of Navß2 as confirmed by confocal fluorescence microscopy and subcellular fractionation of cholesterol-rich domains. Finally, wildtype and mutated Navß2 were expressed along BACE1 in B104 rat neuroblastoma cells. In spite of α-secretase still actively cleaving the mutant proteins, Navß2 cleavage products decreased by ~50% in cells expressing Navß2 (147LM/VI) and ~75% in cells expressing Navß2 (147LM/AA) as compared to cells expressing wildtype Navß2. CONCLUSION: We identified a major (147-148 L↓M) and a minor (144-145 L↓Q) BACE1 cleavage site in human Navß2. Our in vitro and cell-based results clearly show that the 147-148 L↓M is the major BACE1 cleavage site in human Navß2. These findings expand our understanding of the role of BACE1 in voltage-gated sodium channel metabolism.

Alzheimer's secretases regulate voltage-gated sodium channels.

BACE1 and presenilin (PS)/γ-secretase are primary proteolytic enzymes responsible for the generation of pathogenic amyloid ß-peptides (Aß) in Alzheimer's disease. We and others have found that ß-subunits of the voltage-gated sodium channel (Na(v)ßs) also undergo sequential proteolytic cleavages mediated by BACE1 and PS/γ-secretase. In a follow-up study, we reported that elevated BACE1 activity regulates total and surface expression of voltage-gated sodium channels (Na(v)1 channels) and thereby modulates sodium currents in neuronal cells and mouse brains. In this review, we focus on the molecular mechanism of how BACE1 and PS/γ-secretase regulate Na(v)1 channels in neuronal cells. We will also discuss potential physiological and pathological roles of BACE1- and PS/γ-secretase-mediated processing of Na(v)ßs in relation to Na(v)1 channel function.