Human CRB2 inhibits gamma-secretase cleavage of amyloid precursor protein by binding to the presenilin complex

Human CRB1 and CRB2 form homo- and heteromeric protein complexes in the retina

Crumbs homolog 1 (CRB1) is one of the key genes linked to retinitis pigmentosa and Leber congenital amaurosis, which are characterized by a high clinical heterogeneity. The Crumbs family member CRB2 has a similar protein structure to CRB1, and in zebrafish, Crb2 has been shown to interact through the extracellular domain. Here, we show that CRB1 and CRB2 co-localize in the human retina and human iPSC-derived retinal organoids. In retina-specific pull-downs, CRB1 was enriched in CRB2 samples, supporting a CRB1-CRB2 interaction. Furthermore, novel interactors of the crumbs complex were identified, representing a retina-derived protein interaction network. Using co-immunoprecipitation, we further demonstrate that human canonical CRB1 interacts with CRB1 and CRB2, but not with CRB3, which lacks an extracellular domain. Next, we explored how missense mutations in the extracellular domain affect CRB1-CRB2 interactions. We observed no or a mild loss of CRB1-CRB2 interaction, when interrogating various CRB1 or CRB2 missense mutants in vitro. Taken together, our results show a stable interaction of human canonical CRB2 and CRB1 in the retina.

Crumbs2 Is an Essential Slit Diaphragm Protein of the Renal Filtration Barrier

BACKGROUND

Crumbs2 is expressed at embryonic stages as well as in the retina, brain, and glomerular podocytes. Recent studies identified CRB2 mutations as a novel cause of steroid-resistant nephrotic syndrome (SRNS).

METHODS

To study the function of Crb2 at the renal filtration barrier, mice lacking Crb2 exclusively in podocytes were generated. Gene expression and histologic studies as well as transmission and scanning electron microscopy were used to analyze these Crb2^podKO knockout mice and their littermate controls. Furthermore, high-resolution expansion microscopy was used to investigate Crb2 distribution in murine glomeruli. For pull-down experiments, live cell imaging, and transcriptome analyses, cell lines were applied that inducibly express fluorescent protein-tagged CRB2 wild type and mutants.

RESULTS

Crb2^podKO mice developed proteinuria directly after birth that preceded a prominent development of disordered and effaced foot processes, upregulation of renal injury and inflammatory markers, and glomerulosclerosis. Pull-down assays revealed an interaction of CRB2 with Nephrin, mediated by their extracellular domains. Expansion microscopy showed that in mice glomeruli, Crb2 and Nephrin are organized in adjacent clusters. SRNS-associated CRB2 protein variants and a mutant that lacks a putative conserved O-glycosylation site were not transported to the cell surface. Instead, mutants accumulated in the ER, showed altered glycosylation pattern, and triggered an ER stress response.

CONCLUSIONS

Crb2 is an essential component of the podocyte's slit diaphragm, interacting with Nephrin. Loss of slit diaphragm targeting and increasing ER stress are pivotal factors for onset and progression of CRB2-related SRNS.

SERP1 is an assembly regulator of γ-secretase in metabolic stress conditions

The enzyme γ-secretase generates β-amyloid (Aβ) peptides by cleaving amyloid protein precursor (APP); the aggregation of these peptides is associated with Alzheimer's disease (AD). Despite the development of various γ-secretase regulators, their clinical use is limited by coincident disruption of other γ-secretase-regulated substrates, such as Notch. Using a genome-wide functional screen of γ-secretase activity in cells and a complementary DNA expression library, we found that SERP1 is a previously unknown γ-secretase activator that stimulates Aβ generation in cells experiencing endoplasmic reticulum (ER) stress, such as is seen with diabetes. SERP1 interacted with a subcomplex of γ-secretase (APH1A/NCT) through its carboxyl terminus to enhance the assembly and, consequently, the activity of the γ-secretase holoenzyme complex. In response to ER stress, SERP1 preferentially recruited APP rather than Notch into the γ-secretase complex and enhanced the subcellular localization of the complex into lipid rafts, increasing Aβ production. Moreover, SERP1 abundance, γ-secretase assembly, and Aβ production were increased both in cells exposed to high amounts of glucose and in diabetic AD model mice. Conversely, Aβ production was decreased by knocking down SERP1 in cells or in the hippocampi of mice. Compared to postmortem samples from control individuals, those from patients with AD showed increased SERP1 expression in the hippocampus and parietal lobe. Together, our findings suggest that SERP1 is an APP-biased regulator of γ-secretase function in the context of cell stress, providing a possible molecular explanation for the link between diabetes and sporadic AD.

Distinct functions of Crumbs regulating slit diaphragms and endocytosis in Drosophila nephrocytes

Mammalian podocytes, the key determinants of the kidney's filtration barrier, differentiate from columnar epithelial cells and several key determinants of apical-basal polarity in the conventional epithelia have been shown to regulate podocyte morphogenesis and function. However, little is known about the role of Crumbs, a conserved polarity regulator in many epithelia, for slit-diaphragm formation and podocyte function. In this study, we used Drosophila nephrocytes as model system for mammalian podocytes and identified a conserved function of Crumbs proteins for cellular morphogenesis, nephrocyte diaphragm assembly/maintenance, and endocytosis. Nephrocyte-specific knock-down of Crumbs results in disturbed nephrocyte diaphragm assembly/maintenance and decreased endocytosis, which can be rescued by Drosophila Crumbs as well as human Crumbs2 and Crumbs3, which were both expressed in human podocytes. In contrast to the extracellular domain, which facilitates nephrocyte diaphragm assembly/maintenance, the intracellular FERM-interaction motif of Crumbs is essential for regulating endocytosis. Moreover, Moesin, which binds to the FERM-binding domain of Crumbs, is essential for efficient endocytosis. Thus, we describe here a new mechanism of nephrocyte development and function, which is likely to be conserved in mammalian podocytes.

Whole Exome Sequencing in Psoriasis Patients Contributes to Studies of Acitretin Treatment Difference

Psoriasis vulgaris is an immune-mediated inflammatory skin disease. Although acitretin is a widely used synthetic retinoid for moderate to severe psoriasis, little is known about patients’ genetics in response to this drug. In this study, 179 patients were enrolled in either the discovery set (13 patients) or replication set (166 patients). The discovery set was sequenced by whole exome sequencing and sequential validation was conducted in the replication set by MassArray assays. Four SNPs (single nucleotide polymorphisms) (rs1105223T>C in CRB2, rs11086065A>G in ANKLE1, rs3821414T>C in ARHGEF3, rs1802073 T>G in SFRP4) were found to be significantly associated with acitretin response in either co-dominant or dominant models via multivariable logistic regression analysis, while CRB2 rs1105223CC (OR = 4.10, 95% CI = 1.46–11.5, p = 0.007) and ANKLE1 rs11086065AG/GG (OR = 2.76, 95% CI = 1.42–5.37, p = 0.003) were associated with no response to acitretin after 8-week treatment. Meanwhile, ARHGEF3 rs3821414CT/CC (OR = 0.25, 95% CI = 0.10–0.68, p = 0.006) and SFRP4 rs1802073GG/GT (OR = 2.40, 95% CI, 1.23–4.70, p = 0.011) were associated with a higher response rate. Four new genetic variations with potential influences on the response to acitretin were found in this study which may serve as genetic markers for acitretin in psoriasis patients.

Alzheimer disease: modeling an Aβ-centered biological network

In genetically complex diseases, the search for missing heritability is focusing on rare variants with large effect. Thanks to next generation sequencing technologies, genome-wide characterization of these variants is now feasible in every individual. However, a lesson from current studies is that collapsing rare variants at the gene level is often insufficient to obtain a statistically significant signal in case-control studies, and that network-based analyses are an attractive complement to classical approaches. In Alzheimer disease (AD), according to the prevalent amyloid cascade hypothesis, the pathology is driven by the amyloid beta (Aβ) peptide. In past years, based on experimental studies, several hundreds of proteins have been shown to interfere with Aβ production, clearance, aggregation or toxicity. Thanks to a manual curation of the literature, we identified 335 genes/proteins involved in this biological network and classified them according to their cellular function. The complete list of genes, or its subcomponents, will be of interest in ongoing AD genetic studies.

CRB2 completes a fully expressed Crumbs complex in the Retinal Pigment Epithelium

The CRB proteins CRB1, CRB2 and CRB3 are members of the cell polarity complex Crumbs in mammals that together with Scribble and Par complexes stablish the polarity of a variety of cell types. Although many members of the Crumbs complex proteins are expressed in the retinal pigment epithelium (RPE), and even though the mRNA of CRB2 has been detected in ARPE-19 cells and in the RPE/Choroid, to date no CRB protein has yet been found in this tissue. To investigate this possibility, we generated an antibody that specifically recognize the mouse CRB2 protein, and we demonstrate the expression of CRB2 in mouse RPE. Confocal analysis shows that CRB2 is restricted to the apicolateral membrane of RPE cells, and more precisely, in the tight junctions. Our study identified CRB2 as the member of the CRB protein family that is present together with the rest of the components of the Crumbs complex in the RPE apico-lateral cell membrane. Considering that the functions of CRB proteins are decisive in the establishment and maintenance of cell-cell junctions in several epithelial-derived cell types, we believe that these findings are a relevant starting point for unraveling the functions that CRB2 might perform in the RPE.

CRB3A Controls the Morphology and Cohesion of Cancer Cells through Ehm2/p114RhoGEF-Dependent Signaling

The transmembrane protein CRB3A controls epithelial cell polarization. Elucidating the molecular mechanisms of CRB3A function is essential as this protein prevents the epithelial-to-mesenchymal transition (EMT), which contributes to tumor progression. To investigate the functional impact of altered CRB3A expression in cancer cells, we expressed CRB3A in HeLa cells, which are devoid of endogenous CRB3A. While control HeLa cells display a patchy F-actin distribution, CRB3A-expressing cells form a circumferential actomyosin belt. This reorganization of the cytoskeleton is accompanied by a transition from an ameboid cell shape to an epithelial-cell-like morphology. In addition, CRB3A increases the cohesion of HeLa cells. To perform these functions, CRB3A recruits p114RhoGEF and its activator Ehm2 to the cell periphery using both functional motifs of its cytoplasmic tail and increases RhoA activation levels. ROCK1 and ROCK2 (ROCK1/2), which are critical effectors of RhoA, are also essential to modulate the cytoskeleton and cell shape downstream of CRB3A. Overall, our study highlights novel roles for CRB3A and deciphers the signaling pathway conferring to CRB3A the ability to fulfill these functions. Thereby, our data will facilitate further investigation of CRB3A functions and increase our understanding of the cellular defects associated with the loss of CRB3A expression in cancer cells.

OCIAD2 activates γ-secretase to enhance amyloid β production by interacting with nicastrin

The gamma (γ)-secretase holoenzyme is composed of four core proteins and cleaves APP to generate amyloid beta (Aβ), a key molecule that causes major neurotoxicity during the early stage of Alzheimer's disease (AD). However, despite its important role in Aβ production, little is known about the regulation of γ-secretase. OCIAD2, a novel modulator of γ-secretase that stimulates Aβ production, and which was isolated from a genome-wide functional screen using cell-based assays and a cDNA library comprising 6,178 genes. Ectopic expression of OCIAD2 enhanced Aβ production, while reduction of OCIAD2 expression suppressed it. OCIAD2 expression facilitated the formation of an active γ-secretase complex and enhanced subcellular localization of the enzyme components to lipid rafts. OCIAD2 interacted with nicastrin to stimulate γ-secretase activity. OCIAD2 also increased the interaction of nicastrin with C99 and stimulated APP processing via γ-secretase activation, but did not affect Notch processing. In addition, a cell-permeable Tat-OCIAD2 peptide that interfered with the interaction of OCIAD2 with nicastrin interrupted the γ-secretase-mediated AICD production. Finally, OCIAD2 expression was significantly elevated in the brain of AD patients and PDAPP mice. This study identifies OCIAD2 as a selective activator of γ-secretase to increase Aβ generation.

The FAM3 superfamily member ILEI ameliorates Alzheimer's disease-like pathology by destabilizing the penultimate amyloid-β precursor

Accumulation of amyloid-β peptide (Aβ) in the brain underlies the pathogenesis of Alzheimer's disease (AD). Aβ is produced by β- and γ-secretase-mediated sequential proteolysis of amyloid-β precursor protein (APP). Here we identify a secretory protein named interleukin-like epithelial-mesenchymal transition inducer (ILEI, also known as FAM3 superfamily member C) as a negative regulator of Aβ production. ILEI destabilizes the β-secretase-cleaved APP carboxy-terminal fragment, the penultimate precursor of Aβ, by binding to the γ-secretase complex and interfering with its chaperone properties. Notch signalling and γ-secretase activity are not affected by ILEI. We also show neuronal expression of ILEI and its induction by transforming growth factor-β signalling. The level of secreted ILEI is markedly decreased in the brains of AD patients. Transgenic (Tg) overexpression of ILEI significantly reduces the brain Aβ burden and ameliorates the memory deficit in AD model mice. ILEI may be a plausible target for the development of disease-modifying therapies.

The CRB1 and adherens junction complex proteins in retinal development and maintenance

The early developing retinal neuroepithelium is composed of multipotent retinal progenitor cells that differentiate in a time specific manner, giving rise to six major types of neuronal and one type of glial cells. These cells migrate and organize in three distinct nuclear layers divided by two plexiform layers. Apical and adherens junction complexes have a crucial role in this process by the establishment of polarity and adhesion. Changes in these complexes disturb the spatiotemporal aspects of retinogenesis, leading to retinal degeneration resulting in mild or severe impairment of retinal function and vision. In this review, we summarize the mouse models for the different members of the apical and adherens junction protein complexes and describe the main features of their retinal phenotypes. The knowledge acquired from the different mutant animals for these proteins corroborate their importance in retina development and maintenance of normal retinal structure and function. More recently, several studies have tried to unravel the connection between the apical proteins, important cellular signaling pathways and their relation in retina development. Still, the mechanisms by which these proteins function remain largely unknown. Here, we hypothesize how the mammalian apical CRB1 complex might control retinogenesis and prevents onset of Leber congenital amaurosis or retinitis pigmentosa.

Microarray and morphological analysis of early postnatal CRB2 mutant retinas on a pure C57BL/6J genetic background

In humans, the Crumbs homologue-1 (CRB1) gene is mutated in progressive types of autosomal recessive retinitis pigmentosa and Leber congenital amaurosis. The severity of the phenotype due to human CRB1 or mouse Crb1 mutations is dependent on the genetic background. Mice on C57BL/6J background with Crb1 mutations show late onset of retinal spotting phenotype or no phenotype. Recently, we showed that conditional deletion of mouse Crb2 in the retina results in early retinal disorganization leading to severe and progressive retinal degeneration with concomitant visual loss that mimics retinitis pigmentosa due to mutations in the CRB1 gene. Recent studies in the fruit fly and zebrafish suggest roles of the Crumbs (CRB) complex members in the regulation of cellular signalling pathways including the Notch1, mechanistic target of rapamycin complex 1 (mTORC1) and the Hippo pathway. Here, we demonstrate that mice backcrossed to C57BL/6J background with loss of CRB2 in the retina show a progressive disorganization and degeneration phenotype during late retinal development. We used microarray gene profiling to study the transcriptome of retinas lacking CRB2 during late retinal development. Unexpectedly, the retinas of newborn mice lacking CRB2 showed no changes in the transcriptome during retinal development. These findings suggest that loss of CRB2 in the developing retina results in retinal disorganization and subsequent degeneration without major changes in the transcriptome of the retina. These mice might be an interesting model to study the onset of retinal degeneration upon loss of CRB proteins.

Targeted ablation of CRB1 and CRB2 in retinal progenitor cells mimics Leber congenital amaurosis

Development in the central nervous system is highly dependent on the regulation of the switch from progenitor cell proliferation to differentiation, but the molecular and cellular events controlling this process remain poorly understood. Here, we report that ablation of Crb1 and Crb2 genes results in severe impairment of retinal function, abnormal lamination and thickening of the retina mimicking human Leber congenital amaurosis due to loss of CRB1 function. We show that the levels of CRB1 and CRB2 proteins are crucial for mouse retinal development, as they restrain the proliferation of retinal progenitor cells. The lack of these apical proteins results in altered cell cycle progression and increased number of mitotic cells leading to an increased number of late-born cell types such as rod photoreceptors, bipolar and Müller glia cells in postmitotic retinas. Loss of CRB1 and CRB2 in the retina results in dysregulation of target genes for the Notch1 and YAP/Hippo signaling pathways and increased levels of P120-catenin. Loss of CRB1 and CRB2 result in altered progenitor cell cycle distribution with a decrease in number of late progenitors in G1 and an increase in S and G2/M phase. These findings suggest that CRB1 and CRB2 suppress late progenitor pool expansion by regulating multiple proliferative signaling pathways.

Crumbs3 is essential for proper epithelial development and viability

First identified in Drosophila, the Crumbs (Crb) proteins are important in epithelial polarity, apical membrane formation, and tight junction (TJ) assembly. The conserved Crb intracellular region includes a FERM (band 4.1/ezrin/radixin/moesin) binding domain (FBD) whose mammalian binding partners are not well understood and a PDZ binding motif that interacts with mammalian Pals1 (protein associated with lin seven) (also known as MPP5). Pals1 binds Patj (Pals1-associated tight-junction protein), a multi-PDZ-domain protein that associates with many tight junction proteins. The Crb complex also binds the conserved Par3/Par6/atypical protein kinase C (aPKC) polarity cassette that restricts migration of basolateral proteins through phosphorylation. Here, we describe a Crb3 knockout mouse that demonstrates extensive defects in epithelial morphogenesis. The mice die shortly after birth, with cystic kidneys and proteinaceous debris throughout the lungs. The intestines display villus fusion, apical membrane blebs, and disrupted microvilli. These intestinal defects phenocopy those of Ezrin knockout mice, and we demonstrate an interaction between Crumbs3 and ezrin. Taken together, our data indicate that Crumbs3 is crucial for epithelial morphogenesis and plays a role in linking the apical membrane to the underlying ezrin-containing cytoskeleton.

Sticking together the Crumbs - an unexpected function for an old friend

Cell polarity and cell-cell junctions have pivotal roles in organizing cells into tissues and in mediating cell-cell communication. The transmembrane protein Crumbs has a well-established role in the maintenance of epithelial polarity, and it can also regulate signalling via the Notch and Hippo pathways to influence tissue growth. The functions of Crumbs in epithelial polarity and Hippo-mediated growth depend on its short intracellular domain. Recent evidence now points to a conserved and fundamental role for the extracellular domain of Crumbs in mediating homophilic Crumbs-Crumbs interactions at cell-cell junctions.

The apical polarity protein network in Drosophila epithelial cells: regulation of polarity, junctions, morphogenesis, cell growth, and survival

Epithelial tissue formation and function requires the apical-basal polarization of individual epithelial cells. Apical polarity regulators (APRs) are an evolutionarily conserved group of key factors that govern polarity and several other aspects of epithelial differentiation. APRs compose a diverse set of molecules including a transmembrane protein (Crumbs), a serine/threonine kinase (aPKC), a lipid phosphatase (PTEN), a small GTPase (Cdc42), FERM domain proteins (Moesin, Yurt), and several adaptor or scaffolding proteins (Bazooka/Par3, Par6, Stardust, Patj). These proteins form a dynamic cooperative network that is engaged in negative-feedback regulation with basolateral polarity factors to set up the epithelial apical-basal axis. APRs support the formation of the apical junctional complex and the segregation of the junctional domain from the apical membrane. It is becoming increasingly clear that APRs interact with the cytoskeleton and vesicle trafficking machinery, regulate morphogenesis, and modulate epithelial cell growth and survival. Not surprisingly, APRs have multiple fundamental links to human diseases such as cancer and blindness.

The apical polarity determinant Crumbs 2 is a novel regulator of ESC-derived neural progenitors

ESCs undergoing neural differentiation in vitro display an intrinsic heterogeneity with a large subset of the cells forming polarized neural rosettes that maintain the neural progenitor microenvironment. This heterogeneity is not only necessary for normal development but also causes substantial technical challenges for practical applications. Here, we report a novel regulator of early neural progenitors, the apical polarity protein Crb2 (Crumbs homologue 2). Employing monolayer differentiation of mouse ESCs to model neurogenesis in vitro, we find that Crb2 is upregulated with Sox1 and Musashi at the onset of neuroepithelial specification and localizes to the apical side of neural rosettes. Stable Crb2-knockdown (KD) lines die at the onset of neural specification and fail to stabilize several apical polarity proteins. However, these cells are able to proliferate under self-renewing conditions and can be differentiated into mesodermal and endodermal lineages. Conversely, Crb2 overexpression during neural differentiation results in elevated levels of other apical polarity proteins and increases proliferation. Additionally, sustained overexpression of Crb2 reduces terminal differentiation into TuJ1-positive neurons. Furthermore, we demonstrate that Crb2 overexpression under self-renewing conditions increases glycogen synthase kinase (GSK)-3β inhibition, correlating with an increase in clonogenicity. To confirm the importance of GSK-3β inhibition downstream of Crb2, we show that Crb2-KD cells can be forced into neural lineages by blocking GSK-3β function and supplementing Epidermal Growth Factor (EGF) and basic Fibroblast Growth Factor (bFGF). Thus, this is the first demonstration that a member of the Crumbs family is essential for survival and differentiation of ESC-derived neural progenitors.

[Advances on Drosophila presenilin gene]

In Drosophila, presenilin is an aspartyl protease that plays important roles in the development and calcium homeostasis. It has been expressed all through the fly developmental process. The loss of Drosophila presenilin (DPS) function causes significantly decreased Notch signaling and neuron apoptosis and increased cytoplasm calcium. This subsequently led to impaired long term memory and cognitive deficits. Therefore, study of DPS is one of the most popular models for Alzheimer's disease research, and has provided important insights into the pathological mechanisms of AD. This review is to summarize the AD related function of DPS gene.

Emerging role for epithelial polarity proteins of the Crumbs family as potential tumor suppressors

Defects in apical-basal polarity regulation are associated with tissue overgrowth and tumorogenesis, yet the molecular mechanisms linking epithelial polarity regulators to hyperplasia or neoplasia remain elusive. In addition, exploration of the expression and function of the full complement of proteins required for the polarized architecture of epithelial cells in the context of cancer is awaited. This paper provides an overview of recent studies performed on Drosophila and vertebrates showing that apical polarity proteins of the Crumbs family act to repress tissue growth and epithelial to mesenchymal transition. Thus, these proteins emerge as potential tumor suppressors. Interestingly, analysis of the molecular function of Crumbs proteins reveals a function for these polarity regulators in junctional complexes stability and control of signaling pathways regulating proliferation and apoptosis. Thereby, these studies provide a molecular basis explaining how regulation of epithelial polarity is coupled to tumorogenesis.

Retromer controls epithelial cell polarity by trafficking the apical determinant Crumbs

The evolutionarily conserved apical determinant Crumbs (Crb) is essential for maintaining apicobasal polarity and integrity of many epithelial tissues [1]. Crb levels are crucial for cell polarity and homeostasis, yet strikingly little is known about its trafficking or the mechanism of its apical localization. Using a newly established, liposome-based system described here, we determined Crb to be an interaction partner and cargo of the retromer complex. Retromer is essential for the retrograde transport of numerous transmembrane proteins from endosomes to the trans-Golgi network (TGN) and is conserved between plants, fungi, and animals [2]. We show that loss of retromer function results in a substantial reduction of Crb in Drosophila larvae, wing discs, and the follicle epithelium. Moreover, loss of retromer phenocopies loss of crb by preventing apical localization of key polarity molecules, such as atypical protein kinase C (aPKC) and Par6 in the follicular epithelium, an effect that can be rescued by overexpression of Crb. Additionally, loss of retromer results in multilayering of the follicular epithelium, indicating that epithelial integrity is severely compromised. Our data reveal a mechanism for Crb trafficking by retromer that is vital for maintaining Crb levels and localization. We also show a novel function for retromer in maintaining epithelial cell polarity.

Dilysine retrieval signal-containing p24 proteins collaborate in inhibiting γ-cleavage of amyloid precursor protein

γ-Secretase mediates intramembranous γ-cleavage and ε-cleavage of β-amyloid precursor protein (APP) to liberate β-amyloid peptide (Aβ) and APP intracellular domain respectively from the membrane. Although the regulatory mechanism of γ-secretase cleavage remains unresolved, a member of the p24 cargo protein family, named p24δ(1) or TMP21, has been identified as an activity-modulating component. The p24 family proteins are divided into four subfamilies (p24α, β, δ and γ). In contrast to p24δ(1), p24β(1) has reportedly no effect on γ-cleavage. In this study, we determined whether p24α(2), p24γ(3) or p24γ(4) modulates APP processing. Knockdown of cellular p24α(2) induced a significant increase in Aβ generation but not in APP intracellular domain production in cell-based and cell-free assays, whereas p24α(2) over-expression suppressed Aβ secretion. By contrast, Aβ secretion was not altered by p24γ(3) or p24γ(4) knockdown. Endogenous p24α(2) co-immunoprecipitated with core components of the γ-secretase complex, and the anti-p24α(2) immunoprecipitate exhibited γ-secretase activity. Mutational disruption of the conserved dilysine ER-retrieval motifs of p24α(2) and p24δ(1) perturbed inhibition of γ-cleavage. Simultaneous knockdown, or co-over-expression, of these proteins had no additive or synergistic effect on Aβ generation. Our findings suggest that dilysine ER-retrieval signal-containing p24 proteins, p24α(2) and p24δ(1), bind with γ-secretase complexes and collaborate in attenuating γ-cleavage of APP.