β-Sheet Structure within the Extracellular Domain of C99 Regulates Amyloidogenic Processing

Familial mutations in C99 can increase the total level of the soluble Aβ peptides produced by proteolysis, as well as the Aβ42/Aβ40 ratio, both of which are linked to the progression of Alzheimer’s disease. We show that the extracellular sequence of C99 forms β-sheet structure upon interaction with membrane bilayers. Mutations that disrupt this structure result in a significant increase in Aβ production and, in specific cases, result in an increase in the amount of Aβ42 relative to Aβ40. Fourier transform infrared and solid-state NMR spectroscopic studies reveal a central β-hairpin within the extracellular sequence comprising Y10-E11-V12 and L17-V18-F19 connected by a loop involving H13-H14-Q15. These results suggest how familial mutations in the extracellular sequence influence C99 processing and provide a structural basis for the development of small molecule modulators that would reduce Aβ production.

Glycines from the APP GXXXG/GXXXA Transmembrane Motifs Promote Formation of Pathogenic Aβ Oligomers in Cells

Alzheimer’s disease (AD) is the most common neurodegenerative disorder characterized by progressive cognitive decline leading to dementia. The amyloid precursor protein (APP) is a ubiquitous type I transmembrane (TM) protein sequentially processed to generate the β-amyloid peptide (Aβ), the major constituent of senile plaques that are typical AD lesions. There is a growing body of evidence that soluble Aβ oligomers correlate with clinical symptoms associated with the disease. The Aβ sequence begins in the extracellular juxtamembrane region of APP and includes roughly half of the TM domain. This region contains GXXXG and GXXXA motifs, which are critical for both TM protein interactions and fibrillogenic properties of peptides derived from TM α-helices. Glycine-to-leucine mutations of these motifs were previously shown to affect APP processing and Aβ production in cells. However, the detailed contribution of these motifs to APP dimerization, their relation to processing, and the conformational changes they can induce within Aβ species remains undefined. Here, we describe highly resistant Aβ42 oligomers that are produced in cellular membrane compartments. They are formed in cells by processing of the APP amyloidogenic C-terminal fragment (C99), or by direct expression of a peptide corresponding to Aβ42, but not to Aβ40. By a point-mutation approach, we demonstrate that glycine-to-leucine mutations in the G²⁹XXXG³³ and G³⁸XXXA⁴² motifs dramatically affect the Aβ oligomerization process. G33 and G38 in these motifs are specifically involved in Aβ oligomerization; the G33L mutation strongly promotes oligomerization, while G38L blocks it with a dominant effect on G33 residue modification. Finally, we report that the secreted Aβ42 oligomers display pathological properties consistent with their suggested role in AD, but do not induce toxicity in survival assays with neuronal cells. Exposure of neurons to these Aβ42 oligomers dramatically affects neuronal differentiation and, consequently, neuronal network maturation.

Analysis by a highly sensitive split luciferase assay of the regions involved in APP dimerization and its impact on processing

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Amyloid precursor protein (APP) dimerizes more than its C-terminal fragments in cells.

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Mutations of membrane GXXXG motifs affect Aβ production but not APP dimerization.

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Deletion of the APP intracellular domain increases APP dimerization.

Alzheimer’s disease (AD) is a neurodegenerative disease that causes progressive loss of cognitive functions, leading to dementia. Two types of lesions are found in AD brains: neurofibrillary tangles and senile plaques. The latter are composed mainly of the β-amyloid peptide (Aβ) generated by amyloidogenic processing of the amyloid precursor protein (APP). Several studies have suggested that dimerization of APP is closely linked to Aβ production. Nevertheless, the mechanisms controlling APP dimerization and their role in APP function are not known. Here we used a new luciferase complementation assay to analyze APP dimerization and unravel the involvement of its three major domains: the ectodomain, the transmembrane domain and the intracellular domain. Our results indicate that within cells full-length APP dimerizes more than its α and β C-terminal fragments, confirming the pivotal role of the ectodomain in this process. Dimerization of the APP transmembrane (TM) domain has been reported to regulate processing at the γ-cleavage site. We show that both non-familial and familial AD mutations in the TM GXXXG motifs strongly modulate Aβ production, but do not consistently change dimerization of the C-terminal fragments. Finally, we found for the first time that removal of intracellular domain strongly increases APP dimerization. Increased APP dimerization is linked to increased non-amyloidogenic processing.

Conformational changes induced by the A21G Flemish mutation in the amyloid precursor protein lead to increased Aβ production

Proteolysis of the β C-terminal fragment (β-CTF) of the amyloid precursor protein generates the Aβ peptides associated with Alzheimer's disease. Familial mutations in the β-CTF, such as the A21G Flemish mutation, can increase Aβ secretion. We establish how the Flemish mutation alters the structure of C55, the first 55 residues of the β-CTF, using FTIR and solid-state NMR spectroscopy. We show that the A21G mutation reduces β sheet structure of C55 from Leu17 to Ala21, an inhibitory region near the site of the mutation, and increases α-helical structure from Gly25 to Gly29, in a region near the membrane surface and thought to interact with cholesterol. Cholesterol also increases Aβ peptide secretion, and we show that the incorporation of cholesterol into model membranes enhances the structural changes induced by the Flemish mutant, suggesting a common link between familial mutations and the cellular environment.

What is the optimal regimen for BCG intravesical therapy? Are six weekly instillations necessary?

OBJECTIVE

For more than 20 years, BCG intravesical therapy schedule has included 6 weekly instillations. Very few studies have, however, analyzed the rationale of this regimen. We previously demonstrated that intravesical BCG induced an increased peripheral immune response against mycobacterial antigens as compared to pretreatment values. In the present work, we have studied the weekly evolution of this immune response induced by intravesical BCG instillations.

MATERIALS AND METHODS

The evolution of the lymphoproliferative response of peripheral blood mononuclear cells against BCG culture filtrate (CF), tuberculin (PPD) and BCG extract (EXT) was tested before, every week during the BCG instillations and at 3 and 6 months follow-up in 9 patients with superficial bladder cancer treated with 6 weekly BCG instillations. Lymphoproliferation was measured by means of a tritiated thymidine incorporation test.

RESULTS

A significant increase in the lymphoproliferative response against PPD, CF and EXT was observed in 9, 8 and 7 of the 9 patients, respectively, as compared to pre-BCG values. The maximal lymphoproliferation was achieved after 4 instillations in 4/5 patients initially reactive against mycobacterial antigens whereas 2 of 4 initially nonreactive patients required 6 instillations. At 6 months' follow-up, lymphoproliferation against BCG and the other mycobacterial antigens returned to pre-BCG values in all patients. In 3 patients who received additional instillations because of tumor recurrence within 1 year of follow-up, the maximum immune response was observed already after 2 instillations.

CONCLUSION

In most patients, the maximal peripheral immune response is already observed after 4 weekly instillations. However, patients not previously immunized against mycobacterial antigens may require 6 weekly instillations to achieve a maximum stimulation level. Our data support the need to further evaluate the role of this status before starting BCG instillations. It could be of interest to study whether 6 BCG instillations are really necessary in patients previously immune against mycobacterial antigens.

Humoral response against heat shock proteins and other mycobacterial antigens after intravesical treatment with bacille Calmette-Guérin (BCG) in patients with superficial bladder cancer

Few studies have analysed the antibody response during intravesical BCG immunotherapy for superficial bladder cancer. We have examined the evolution in serum antibody response against several heat shock proteins (hsp), including the recombinant mycobacterial hsp65 and the native protein P64 from BCG, GroEL from Escherichia coli (hsp60 family), recombinant mycobacterial hsp70 and the E. coli DnaK (hsp70 family), against purified protein derivative of tuberculin (PPD) and the AG85 complex of Mycobacterium bovis BCG, as well as against tetanus toxoid in 42 patients with a superficial bladder tumour, 28 treated with six intravesical BCG instillations and 14 patients used as controls. We also analysed the lymphoproliferative response of peripheral blood mononuclear cells against PPD in this population. Data of antibody responses at 6 weeks post BCG were available in all 28 patients, and at 4 month follow up in 17 patients. All patients who demonstrated a significant increase in IgG antibodies against PPD at 4 months follow up had a significant increase already at 6 weeks of follow up. In contrast, IgG antibodies against hsp increased significantly from 6 weeks to 4 months post-treatment. A significant increase in IgG antibodies against PPD, hsp65, P64, GroEL, and hsp70 at 4 months follow up was observed in 10/17, 8/17, 10/17, 4/17 and 8/17 patients. Native P64 protein elicited a higher antibody response than recombinant mycobacterial hsp65. No increase in antibody response was observed against Dnak from E. coli, against AG85 or tetanus toxoid after BCG therapy. An increase in IgG antibodies against P64 at 4 months follow up compared with pretreatment values was found to be a significant predictor of tumour recurrence (P<0.01). Further studies with a larger number of patients are needed to confirm the value of the antibody response against P64 as a clinical independent prognostic factor.