Pathological and physiological functions of presenilins

Mutations in PSEN1 and PSEN2 genes account for the majority of cases of early-onset familial Alzheimer disease. Since the first prediction of a genetic link between PSEN1 and PSEN2 with Alzheimer's disease, many research groups from both academia and pharmaceutical industry have sought to unravel how pathogenic mutations in PSEN cause presenile dementia. PSEN genes encode polytopic membrane proteins termed presenilins (PS1 and PS2), which function as the catalytic subunit of γ-secretase, an intramembrane protease that has a wide spectrum of type I membrane protein substrates. Sequential cleavage of amyloid precursor protein by BACE and γ-secretase releases highly fibrillogenic β-amyloid peptides, which accumulate in the brains of aged individuals and patients with Alzheimer's disease. Familial Alzheimer's disease-associated presenilin variants are thought to exert their pathogenic function by selectively elevating the levels of highly amyloidogenic Aβ42 peptides. In addition to Alzheimer's disease, several recent studies have linked PSEN1 to familiar frontotemporal dementia. Here, we review the biology of PS1, its role in γ-secretase activity, and discuss recent developments in the cell biology of PS1 with respect to Alzheimer's disease pathogenesis

Axonal BACE1 dynamics and targeting in hippocampal neurons: a role for Rab11 GTPase

BACKGROUND: BACE1 is one of the two enzymes that cleave amyloid precursor protein to generate Alzheimer's disease (AD) beta amyloid peptides. It is widely believed that BACE1 initiates APP processing in endosomes, and in the brain this cleavage is known to occur during axonal transport of APP. In addition, BACE1 accumulates in dystrophic neurites surrounding brain senile plaques in individuals with AD, suggesting that abnormal accumulation of BACE1 at presynaptic terminals contributes to pathogenesis in AD. However, only limited information is available on BACE1 axonal transport and targeting. RESULTS: By visualizing BACE1-YFP dynamics using live imaging, we demonstrate that BACE1 undergoes bi-directional transport in dynamic tubulo-vesicular carriers along axons in cultured hippocampal neurons and in acute hippocampal slices of transgenic mice. In addition, a subset of BACE1 is present in larger stationary structures, which are active presynaptic sites. In cultured neurons, BACE1-YFP is preferentially targeted to axons over time, consistent with predominant in vivo localization of BACE1 in presynaptic terminals. Confocal analysis and dual-color live imaging revealed a localization and dynamic transport of BACE1 along dendrites and axons in Rab11-positive recycling endosomes. Impairment of Rab11 function leads to a diminution of total and endocytosed BACE1 in axons, concomitant with an increase in the soma. Together, these results suggest that BACE1 is sorted to axons in endosomes in a Rab11-dependent manner. CONCLUSION: Our results reveal novel information on dynamic BACE1 transport in neurons, and demonstrate that Rab11-GTPase function is critical for axonal sorting of BACE1. Thus, we suggest that BACE1 transcytosis in endosomes contributes to presynaptic BACE1 localization

Transgenic neuronal overexpression reveals that stringently regulated p23 expression is critical for coordinated movement in mice

<p>Abstract</p> <p>Background</p> <p>p23 belongs to the highly conserved p24 family of type I transmembrane proteins, which participate in the bidirectional protein transport between the endoplasmic reticulum and Golgi apparatus. Mammalian p23 has been shown to interact with γ-secretase complex, and modulate secretory trafficking as well as intramembranous processing of amyloid precursor protein in cultured cells. Negative modulation of β-amyloid production by p23 in cultured cell lines suggested that elevation of p23 expression in neurons might mitigate cerebral amyloid burden.</p> <p>Results</p> <p>We generated several lines of transgenic mice expressing human p23 in neurons under the control of <it>Thy-1.2 </it>promoter. We found that even a 50% increase in p23 levels in the central nervous system of mice causes post-natal growth retardation, severe neurological problems characterized by tremors, seizure, ataxia, and uncoordinated movements, and premature death. The severity of the phenotype closely correlated with the level of p23 overexpression in multiple transgenic lines. While the number and general morphology of neurons in Hup23 mice appeared to be normal throughout the brain, abnormal non-Golgi p23 localization was observed in a subset of neurons with high transgene expression in brainstem. Moreover, detailed immunofluorescence analysis revealed marked proliferation of astrocytes, activation of microglia, and thinning of myelinated bundles in brainstem of Hup23 mice.</p> <p>Conclusions</p> <p>These results demonstrate that proper level of p23 expression is critical for neuronal function, and perturbing p23 function by overexpression initiates a cascade of cellular reactions in brainstem that leads to severe motor deficits and other neurological problems, which culminate in premature death. The neurological phenotype observed in Hup23 mice highlights significant adverse effects associated with manipulating neuronal expression of p23, a previously described negative modulator of γ-secretase activity and β-amyloid production. Moreover, our report has broader relevance to molecular mechanisms in several neurodegenerative diseases as it highlights the inherent vulnerability of the early secretory pathway mechanisms that ensure proteostasis in neurons.</p

Nicastrin is critical for stability and trafficking but not association of other presenilin/gamma-secretase components

gamma-Secretase, which is responsible for the intramembranous cleavage of Alzheimer beta-amyloid precursor protein and the signaling receptor Notch, is a multiprotein complex consisting of at least four components: presenilin ( PS); nicastrin (Nct); APH-1 ( anterior pharynx-defective-1); and presenilin enhancer-2 (PEN-2). Presenilin 1 (PS1) is known to be essential for the stability, interaction, and trafficking of the other PS1/gamma-secretase components. However, the precise functions of the other components remain elusive. Here, we investigated the functions of Nct within the PS1/gamma-secretase complex. We demonstrated that the loss of Nct expression in the embryonic fibroblast cells ( Nct KO cells) results in dramatically decreased levels of APH-1, PEN-2, and PS1 fragments accompanied by a significant accumulation of full-length PS1. In the absence of Nct, PEN-2 and full-length PS1 are subjected to proteasome-mediated degradation, whereas the degradation of APH-1 is mediated by both proteasomal and lysosomal pathways. Unlike the case of wild type cells in which the gamma-secretase complex mainly locates in the trans-Golgi network, the majority of residual PEN-2, APH-1, and the uncleaved full-length PS1 in Nct KO cells reside in the endoplasmic reticulum, which remain associated with each other in the absence of Nct. Interestingly, significant amounts of full-length PS1 and PEN-2, but not APH-1, are detected on the plasma membrane in Nct KO cells, suggesting the Nct-independent cell surface delivery of the PEN-2 center dot PS1. Finally, the diminished PEN-2 protein level in Nct-deficient cells can be partially restored by overexpression of exogenous PS1, APH-1, or PEN-2 individually or collectively, indicating a dispensable role for Nct in controlling PEN-2 level. Taken together, our study demonstrates a critical role of Nct in the stability and proper intracellular trafficking of other components of the PS1/ gamma-secretase complex but not in maintaining the association of PEN-2, APH-1, and full-length PS1

Lactic Acid Induces Aberrant Amyloid Precursor Protein Processing by Promoting Its Interaction with Endoplasmic Reticulum Chaperone Proteins

Lactic acid, a natural by-product of glycolysis, is produced at excess levels in response to impaired mitochondrial function, high-energy demand, and low oxygen availability. The enzyme involved in the production of β-amyloid peptide (Aβ) of Alzheimer's disease, BACE1, functions optimally at lower pH, which led us to investigate a potential role of lactic acid in the processing of amyloid precursor protein (APP).Lactic acid increased levels of Aβ40 and 42, as measured by ELISA, in culture medium of human neuroblastoma cells (SH-SY5Y), whereas it decreased APP metabolites, such as sAPPα. In cell lysates, APP levels were increased and APP was found to interact with ER-chaperones in a perinuclear region, as determined by co-immunoprecipitation and fluorescence microscopy studies. Lactic acid had only a very modest effect on cellular pH, did increase the levels of ER chaperones Grp78 and Grp94 and led to APP aggregate formation reminiscent of aggresomes.These findings suggest that sustained elevations in lactic acid levels could be a risk factor in amyloidogenesis related to Alzheimer's disease through enhanced APP interaction with ER chaperone proteins and aberrant APP processing leading to increased generation of amyloid peptides and APP aggregates

MicroRNA-Related Cofilin Abnormality in Alzheimer's Disease

Rod-like structures composed of actin and the actin-binding protein cofilin are found in Alzheimer's disease (AD) patients. However, the mechanisms underlying formation of these structures and their pathological consequences are still largely unknown. We found that microRNAs 103 and 107 repress translation of cofilin, and that reduced levels of miR-103 or miR-107 are associated with elevated cofilin protein levels and formation of rod-like structures in a transgenic mouse model of AD. These results suggest that microRNAs may play an important role in cytoskeletal pathology in AD

Determination of the Proteolytic Cleavage Sites of the Amyloid Precursor-Like Protein 2 by the Proteases ADAM10, BACE1 and γ-Secretase

Regulated intramembrane proteolysis of the amyloid precursor protein (APP) by the protease activities α-, β- and γ-secretase controls the generation of the neurotoxic amyloid β peptide. APLP2, the amyloid precursor-like protein 2, is a homolog of APP, which shows functional overlap with APP, but lacks an amyloid β domain. Compared to APP, less is known about the proteolytic processing of APLP2, in particular in neurons, and the cleavage sites have not yet been determined. APLP2 is cleaved by the β-secretase BACE1 and additionally by an α-secretase activity. The two metalloproteases ADAM10 and ADAM17 have been suggested as candidate APLP2 α-secretases in cell lines. Here, we used RNA interference and found that ADAM10, but not ADAM17, is required for the constitutive α-secretase cleavage of APLP2 in HEK293 and SH-SY5Y cells. Likewise, in primary murine neurons knock-down of ADAM10 suppressed APLP2 α-secretase cleavage. Using mass spectrometry we determined the proteolytic cleavage sites in the APLP2 sequence. ADAM10 was found to cleave APLP2 after arginine 670, whereas BACE1 cleaves after leucine 659. Both cleavage sites are located in close proximity to the membrane. γ-secretase cleavage was found to occur at different peptide bonds between alanine 694 and valine 700, which is close to the N-terminus of the predicted APLP2 transmembrane domain. Determination of the APLP2 cleavage sites enables functional studies of the different APLP2 ectodomain fragments and the production of cleavage-site specific antibodies for APLP2, which may be used for biomarker development

Ablation of prion protein in wild type human amyloid precursor protein (APP) transgenic mice does not alter the proteolysis of APP, levels of amyloid-β or pathologic phenotype

The cellular prion protein (PrPC) has been proposed to play an important role in the pathogenesis of Alzheimer's disease. In cellular models PrPC inhibited the action of the β-secretase BACE1 on wild type amyloid precursor protein resulting in a reduction in amyloid-β (Aβ) peptides. Here we have assessed the effect of genetic ablation of PrPC in transgenic mice expressing human wild type amyloid precursor protein (line I5). Deletion of PrPC had no effect on the α- and β-secretase proteolysis of the amyloid precursor protein (APP) nor on the amount of Aβ38, Aβ40 or Aβ42 in the brains of the mice. In addition, ablation of PrPC did not alter Aβ deposition or histopathology phenotype in this transgenic model. Thus using this transgenic model we could not provide evidence to support the hypothesis that PrPC regulates Aβ production

APLP2 Regulates Refractive Error and Myopia Development in Mice and Humans

Myopia is the most common vision disorder and the leading cause of visual impairment worldwide. However, gene variants identified to date explain less than 10% of the variance in refractive error, leaving the majority of heritability unexplained (“missing heritability”). Previously, we reported that expression of APLP2 was strongly associated with myopia in a primate model. Here, we found that low-frequency variants near the 5’-end of APLP2 were associated with refractive error in a prospective UK birth cohort (n = 3,819 children; top SNP rs188663068, p = 5.0 × 10−4) and a CREAM consortium panel (n = 45,756 adults; top SNP rs7127037, p = 6.6 × 10−3). These variants showed evidence of differential effect on childhood longitudinal refractive error trajectories depending on time spent reading (gene x time spent reading x age interaction, p = 4.0 × 10−3). Furthermore, Aplp2 knockout mice developed high degrees of hyperopia (+11.5 ± 2.2 D, p −4) compared to both heterozygous (-0.8 ± 2.0 D, p −4) and wild-type (+0.3 ± 2.2 D, p −4) littermates and exhibited a dose-dependent reduction in susceptibility to environmentally induced myopia (F(2, 33) = 191.0, p −4). This phenotype was associated with reduced contrast sensitivity (F(12, 120) = 3.6, p = 1.5 × 10−4) and changes in the electrophysiological properties of retinal amacrine cells, which expressed Aplp2. This work identifies APLP2 as one of the “missing” myopia genes, demonstrating the importance of a low-frequency gene variant in the development of human myopia. It also demonstrates an important role for APLP2 in refractive development in mice and humans, suggesting a high lev