beta-Amyloid precursor protein-deficient mice show reactive gliosis and decreased locomotor activity

All in the Family: How the APPs Regulate Neurogenesis

Recent intriguing evidence suggests that metabolites of amyloid precursor protein (APP), mutated in familial forms of Alzheimer’s disease (AD), play critical roles in developmental and postnatal neurogenesis. Of note is soluble APPα (sAPPα) that regulates neural progenitor cell proliferation. The APP family encompasses a group of ubiquitously expressed and evolutionarily conserved, type I transmembrane glycoproteins, whose functions have yet to be fully elucidated. APP can undergo proteolytic cleavage by mutually exclusive pathways. The subtle structural differences between metabolites generated in the different pathways, as well as their equilibrium, may be crucial for neuronal function. The implications of this new body of evidence are significant. Miscleavage of APP would readily impact developmental and postnatal neurogenesis, which might contribute to cognitive deficits characterizing Alzheimer’s disease. This review will discuss the implications of the role of the APP family in neurogenesis for neuronal development, cognitive function, and brain disorders that compromise learning and memory, such as AD.

Generation of Calhm1 knockout mouse and characterization of calhm1 gene expression

Alzheimer's disease (AD) is the most common neurodegenerative disease among elderly people worldwide. Several genes have been validated to be associated with AD, and calcium homeostasis modulator 1 (Calhm1) is the latest suspected one. To investigate the biological and pathological function of Calhm1 systematically, we generated a Calhm1 conventional knockout mouse. However, both the male and female of elderly Calhm1 knockout (KO) mice showed similar ability to their wild type littermates in spatial learning and memory retrieving. Surprisingly, we found that Calhm1 mRNA could not be detected in mouse brains at different ages, although it is expressed in the human brain tissues. We further found that CpG islands (CGIs) of both mouse and human Calhm1 were hypermethylated, whereas CGI of mouse Calhm2 was hypomethylated. In addition, transcriptional active marker H3K4Di occupied on promoters of human Calhm1 and mouse Calhm2 at a considerable level in brain tissues, while the occupancy of H3K4Di on promoter of mouse Calhm1 was rare. In sum, we found that mouse Calhm1 was of rare abundance in brain tissues. So it might not be suitable to utilize the knockout murine model to explore biological function of Calhm1 in the pathogenesis of AD.

The APP intracellular domain (AICD) inhibits Wnt signalling and promotes neurite outgrowth

β- and γ-secretase cleave the amyloid precursor protein (APP) to release the amyloidogenic β-amyloid peptides (Aβ) and the APP intracellular domain (AICD). Aβ has been widely believed to initiate pathogenic cascades culminating in Alzheimer's disease (AD). However, the physiological functions of the AICD remain elusive. In this study, we found the AICD to strongly inhibit Wnt-induced transcriptional reporter activity, and to counteract Wnt-induced c-Myc expression. Loss of the AICD resulted in an increased responsiveness to Wnt/β-catenin-mediated transcription. Mechanically, the AICD was found to interact with glycogen synthase kinase 3 beta (GSK3β) and promote its kinase activity. The subsequent AICD-strengthened Axin-GSK3β complex potentiates β-catenin poly-ubiquitination. Functional studies in N(2)a mouse neuroblastoma cells, rat pheochromocytoma PC12 cells and primary neurons showed that the AICD facilitated neurite outgrowth. And AICD antagonised Wnt3a-suppressed growth arrest and neurite outgrowth in N2a and PC12 cells. Taken together, our results identify the AICD as a novel inhibitory factor of the canonical Wnt signalling pathway and suggest its regulatory role in neuronal cell proliferation and differentiation.

Immunization targeting a minor plaque constituent clears β-amyloid and rescues behavioral deficits in an Alzheimer's disease mouse model

Although anti-human β-amyloid (Aβ) immunotherapy clears brain β-amyloid plaques in Alzheimer's disease (AD), targeting additional brain plaque constituents to promote clearance has not been attempted. Endogenous murine Aβ is a minor Aβ plaque component in amyloid precursor protein (APP) transgenic AD models, which we show is ∼3%-8% of the total accumulated Aβ in various human APP transgenic mice. Murine Aβ codeposits and colocalizes with human Aβ in amyloid plaques, and the two Aβ species coimmunoprecipitate together from brain extracts. In the human APP transgenic mouse model Tg2576, passive immunization for 8 weeks with a murine-Aβ-specific antibody reduced β-amyloid plaque pathology, robustly decreasing both murine and human Aβ levels. The immunized mice additionally showed improvements in two behavioral assays, odor habituation and nesting behavior. We conclude that passive anti-murine Aβ immunization clears Aβ plaque pathology--including the major human Aβ component--and decreases behavioral deficits, arguing that targeting minor endogenous brain plaque constituents can be beneficial, broadening the range of plaque-associated targets for AD therapeutics.

sAPPα rescues deficits in amyloid precursor protein knockout mice following focal traumatic brain injury

The amyloid precursor protein (APP) is thought to be neuroprotective following traumatic brain injury (TBI), although definitive evidence at moderate to severe levels of injury is lacking. In the current study, we investigated histological and functional outcomes in APP-/- mice compared with APP+/+ mice following a moderate focal injury, and whether administration of sAPPα restored the outcomes in knockout animals back to the wildtype state. Following moderate controlled cortical impact injury, APP-/- mice demonstrated greater impairment in motor and cognitive outcome as determined by the ledged beam and Barnes Maze tests respectively (p < 0.05). This corresponded with the degree of neuronal damage, with APP-/- mice having significantly greater lesion volume (25.0 ± 1.6 vs. 20.3 ± 1.6%, p < 0.01) and hippocampal damage, with less remaining CA neurons (839 ± 245 vs. 1353 ± 142 and 1401 ± 263). This was also associated with an impaired neuroreparative response, with decreased GAP-43 immunoreactivity within the cortex around the lesion edge compared with APP+/+ mice. The deficits observed in the APP-/- mice related to a lack of sAPPα, as treatment with exogenously added sAPPα post-injury improved APP-/- mice histological and functional outcome to the point that they were no longer significantly different to APP+/+ mice (p < 0.05). This study shows that endogenous APP is potentially protective at moderate levels of TBI, and that this neuroprotective activity is related to the presence of sAPPα. Importantly, it indicates that the mechanism of action of exogenously added sAPPα is independent of the presence of endogenous APP.

Knockdown of amyloid precursor protein in zebrafish causes defects in motor axon outgrowth

Amyloid precursor protein (APP) plays a pivotal role in Alzheimer's disease (AD) pathogenesis, but its normal physiological functions are less clear. Combined deletion of the APP and APP-like protein 2 (APLP2) genes in mice results in post-natal lethality, suggesting that APP performs an essential, if redundant, function during embryogenesis. We previously showed that injection of antisense morpholino to reduce APP levels in zebrafish embryos caused convergent-extension defects. Here we report that a reduction in APP levels causes defective axonal outgrowth of facial branchiomotor and spinal motor neurons, which involves disorganized axonal cytoskeletal elements. The defective outgrowth is caused in a cell-autonomous manner and both extracellular and intracellular domains of human APP are required to rescue the defective phenotype. Interestingly, wild-type human APP rescues the defective phenotype but APPswe mutation, which causes familial AD, does not. Our results show that the zebrafish model provides a powerful system to delineate APP functions in vivo and to study the biological effects of APP mutations.

APL-1, the Alzheimer's Amyloid precursor protein in Caenorhabditis elegans, modulates multiple metabolic pathways throughout development

Mutations in the amyloid precursor protein (APP) gene or in genes that process APP are correlated with familial Alzheimer's disease (AD). The biological function of APP remains unclear. APP is a transmembrane protein that can be sequentially cleaved by different secretases to yield multiple fragments, which can potentially act as signaling molecules. Caenorhabditis elegans encodes one APP-related protein, APL-1, which is essential for viability. Here, we show that APL-1 signaling is dependent on the activity of the FOXO transcription factor DAF-16 and the nuclear hormone receptor DAF-12 and influences metabolic pathways such as developmental progression, body size, and egg-laying rate. Furthermore, apl-1(yn5) mutants, which produce high levels of the extracellular APL-1 fragment, show an incompletely penetrant temperature-sensitive embryonic lethality. In a genetic screen to isolate mutants in which the apl-1(yn5) lethality rate is modified, we identified a suppressor mutation in MOA-1/R155.2, a receptor-protein tyrosine phosphatase, and an enhancer mutation in MOA-2/B0495.6, a protein involved in receptor-mediated endocytosis. Knockdown of apl-1 in an apl-1(yn5) background caused lethality and molting defects at all larval stages, suggesting that apl-1 is required for each transitional molt. We suggest that signaling of the released APL-1 fragment modulates multiple metabolic states and that APL-1 is required throughout development.

Characterisation of the effect of knockout of the amyloid precursor protein on outcome following mild traumatic brain injury

The amyloid precursor protein (APP) increases following traumatic brain injury (TBI), although the functional significance of this remains unclear largely because the functions of the subsequent APP metabolites are so different: Aβ is neurotoxic whilst sAPPα is neuroprotective. To investigate this further, APP wildtype and knockout mice were subjected to mild diffuse TBI and their outcomes compared. APP knockout mice displayed significantly worse cognitive and motor deficits, as demonstrated by the Barnes Maze and rotarod respectively, than APP wildtype mice. This was associated with a significant increase in hippocampal and cortical cell loss, as well as axonal injury, in APP knockout mice and an impaired neuroreparative response as indicated by diminished GAP-43 immunoreactivity when compared to APP wildtype mice. This study is the first to demonstrate that endogenous APP is beneficial following mild TBI, suggesting that the upregulation of APP observed following injury is an acute protective response.

Central CRF system perturbation in an Alzheimer's disease knockin mouse model

Alzheimer's disease (AD) is often accompanied by changes in mood as well as increases in circulating cortisol levels, suggesting that regulation of the stress responsive hypothalamic-pituitary-adrenal (HPA) axis is disturbed. Here, we show that amyloid precursor protein (APP) is endogenously expressed in important limbic, hypothalamic, and midbrain nuclei that regulate hypothalamic-pituitary-adrenal axis activity. Furthermore, in a knockin mouse model of AD that expresses familial AD (FAD) mutations of both APP with humanized amyloid beta (hAβ), and presenilin 1 (PS1), in their endogenous patterns (APP/hAβ/PS1 animals), corticotropin releasing factor (CRF) levels are increased in key stress-related nuclei, resting corticosteroid levels are elevated, and animals display increased anxiety-related behavior. Endocrine and behavioral phenotypes can be normalized by loss of 1 copy of CRF receptor type-1 (Crfr1), consistent with a perturbation of central CRF signaling in APP/hAβ/PS1 animals. However, reductions in anxiety and corticosteroid levels conferred by heterozygosity of CRF receptor type-1 do not improve a deficit in working memory observed in APP/hAβ/PS1 mice, suggesting that perturbations of the CRF system are not the primary cause of decreased cognitive performance.

Deletion of the amyloid precursor-like protein 2 (APLP2) does not affect hippocampal neuron morphology or function

Amyloid precursor protein (APP), the parent molecule to amyloid β peptide, is part of a larger gene family with two mammalian homologues, amyloid precursor-like protein 1 (APLP1) and amyloid precursor-like protein 2 (APLP2). Initial knock-out studies demonstrated that while single APP family gene deletions produced relatively mild phenotypes, deficiency of APLP2 and one other member of the gene family resulted in perinatal lethality, suggesting vital roles masked by functional redundancy of the other homologues. Because of the importance of APP in Alzheimer's disease, the vast majority of studies to date have concentrated on the neuronal functions of APP, leaving limited data on its homologues. APLP2 is of particular interest as it contains high sequence homology with APP, is processed similarly, is expressed in overlapping spatial and temporal patterns, and is obligatory for lethality when combined with deficiency of either APLP1 or APP but does not contain the toxic amyloid β sequence. Here we sought to test the role of APLP2 on neuronal structure and function using a combined approach involving in vitro and in vivo techniques in young and aged animals. Surprisingly, we found that unlike APP, APLP2 appears not to be essential for maintenance of dendritic structure, spine density, or synaptic function. Thus, there is clear divergence in the functional redundancy between APP and APLP2.

Deficits in axonal transport in hippocampal-based circuitry and the visual pathway in APP knock-out animals witnessed by manganese enhanced MRI

Mounting evidence implicates axonal transport defects, typified by the presence of axonal varicosities with aberrant accumulations of cargo, as an early event in Alzheimer's disease (AD) pathogenesis. Work identifying amyloid precursor protein (APP) as a vesicular motor receptor for anterograde axonal transport further implicates axonal transport in AD. Manganese-enhanced MRI (MEMRI) detects axonal transport dynamics in preclinical studies. Here we pursue an understanding of the role of APP in axonal transport in the central nervous system by applying MEMRI to hippocampal circuitry and to the visual pathway in living mice homozygous for either wild type or a deletion in the APP gene (n=12 for each genotype). Following intra-ocular or stereotaxic hippocampal injection, we performed time-lapse MRI to detect Mn(2+) transport. Three dimensional whole brain datasets were compared on a voxel-wise basis using within-group pair-wise analysis. Quantification of transport to structures connected to injection sites via axonal fiber tracts was also performed. Histology confirmed consistent placement of hippocampal injections and no observable difference in glial-response to the injections. APP-/- mice had significantly reduced transport from the hippocampus to the septal nuclei and amygdala after 7h and reduced transport to the contralateral hippocampus after 25 h; axonal transport deficits in the APP-/- animals were also identified in the visual pathway. These data support a system-wide role for APP in axonal transport within the central nervous system and demonstrate the power of MEMRI for assessing neuronal circuitry involved in memory and learning.

Amyloid precursor protein is required for normal function of the rod and cone pathways in the mouse retina

Amyloid precursor protein (APP) is a transmembrane glycoprotein frequently studied for its role in Alzheimer's disease. Our recent study in APP knockout (KO) mice identified an important role for APP in modulating normal neuronal development in the retina. However the role APP plays in the adult retina and whether it is required for vision is unknown. In this study we evaluated the role of APP in retinal function and morphology comparing adult wildtype (WT) and APP-KO mice. APP was expressed on neuronal cells of the inner retina, including horizontal, cone bipolar, amacrine and ganglion cells in WT mice. The function of the retina was assessed using the electroretinogram and although the rod photoreceptor responses were similar in APP-KO and WT mice, the post-photoreceptor, inner retinal responses of both the rod and cone pathways were reduced in APP-KO mice. These changes in inner retinal function did not translate to a substantial change in visual acuity as assessed using the optokinetic response or to changes in the gross cellular structure of the retina. These findings indicate that APP is not required for basic visual function, but that it is involved in modulating inner retinal circuitry.

Amyloid precursor protein revisited: neuron-specific expression and highly stable nature of soluble derivatives

APP processing and amyloid-β production play a central role in Alzheimer disease pathogenesis. APP has been considered a ubiquitously expressed protein. In addition to amyloid-β, α- or β-secretase-dependent cleavage of APP also generates soluble secreted APP (APPsα or APPsβ, respectively). Interestingly, APPsβ has been shown to be subject to further cleavage to create an N-APP fragment that binds to the DR6 death receptor and mediates axon pruning and degeneration under trophic factor withdrawal conditions. By performing APP immunocytochemical staining, we found that, unexpectedly, many antibodies yielded nonspecific staining in APP-null samples. Screening of a series of antibodies allowed us to identify a rabbit monoclonal antibody Y188 that is highly specific for APP and prompted us to re-examine the expression, localization, and stability of endogenous APP and APPsβ in wild-type and in APPsβ knock-in mice, respectively. In contrast to earlier studies, we found that APP is specifically expressed in neurons and that its expression cannot be detected in major types of glial cells under basal or neuroinflammatory conditions. Both APPsα and APPsβ are highly stable in the central nervous system (CNS) and do not undergo further cleavage with or without trophic factor support. Our results clarify several key questions with regard to the fundamental properties of APP and offer critical cellular insights into the pathophysiology of APP.

Proteolytic processing of Alzheimer's β-amyloid precursor protein

β-Amyloid precursor protein (APP) is a critical factor in the pathogenesis of Alzheimer's disease (AD). APP undergoes post-translational proteolysis/processing to generate the hydrophobic β-amyloid (Aβ) peptides. Deposition of Aβ in the brain, forming oligomeric Aβ and plaques, is identified as one of the key pathological hallmarks of AD. The processing of APP to generate Aβ is executed by β- and γ-secretase and is highly regulated. Aβ toxicity can lead to synaptic dysfunction, neuronal cell death, impaired learning/memory and abnormal behaviors in AD models in vitro and in vivo. Aside from Aβ, proteolytic cleavages of APP can also give rise to the APP intracellular domain, reportedly involved in multiple types of cellular events such as gene transcription and apoptotic cell death. In addition to amyloidogenic processing, APP can also be cleaved by α-secretase to form a soluble or secreted APP ectodomain (sAPP-α) that has been shown to be mostly neuro-protective. In this review, we describe the mechanisms involved in APP metabolism and the likely functions of its various proteolytic products to give a better understanding of the patho/physiological functions of APP.

Roles of amyloid precursor protein family members in neuroprotection, stress signaling and aging

The roles of amyloid precursor protein (APP) family members in normal brain function are poorly understood. Under physiological conditions the majority of APP appears to be processed along the non-amyloidogenic pathway leading to the formation of the secreted N-terminal APP fragment sAPPα. This cleavage product of APP has been implicated in several physiological processes such as neuroprotection, synaptic plasticity, neurite outgrowth and synaptogenesis. In this review we focus on the role of APP family members in neuroprotection and summarize the cellular and molecular mechanisms which are believed to mediate this effect. We propose that a reduction of APP processing along the non-amyloidogenic pathway during brain aging could result in an enhanced susceptibility of neurons to cellular stress and could contribute to neurodegeneration in Alzheimer's disease.

Functional consequences of the lack of amyloid precursor protein in the mouse dentate gyrus in vivo

The amyloid precursor protein (APP) plays a crucial role in the pathogenesis of Alzheimer's disease. Here, we studied whether the lack of APP affects the synaptic properties in the dentate gyrus by measuring granule cell field potentials evoked by perforant path stimulation in anesthetized 9-11-month-old APP-deficient mice in vivo. We found decreased paired-pulse facilitation, indicating altered presynaptic short-term plasticity in the APP-deficient dentate gyrus. In contrast, excitatory synaptic strength and granule cell firing were unchanged in APP knockout mice. Likewise, long-term potentiation (LTP) induced by a theta-burst stimulation protocol was not impaired in the absence of APP. These findings suggest that the deletion of APP may affect presynaptic plasticity of synaptic transmission at the perforant path-granule cell synapse but leaves synaptic efficacy intact and LTP preserved, possibly due to functional redundancy within the APP gene family.

Caenorhabditis elegans as a model organism to study APP function

The brains of Alzheimer's disease patients show an increased number of senile plaques compared with normal patients. The major component of the plaques is the β-amyloid peptide, a cleavage product of the amyloid precursor protein (APP). Although the processing of APP has been well-described, the physiological functions of APP and its cleavage products remain unclear. This article reviews the multifunctional roles of an APP orthologue, the C. elegans APL-1. Understanding the function of APL-1 may provide insights into the functions and signaling pathways of human APP. In addition, the physiological effects of introducing human β-amyloid peptide into C. elegans are also reviewed. The C. elegans system provides a powerful genetic model to identify genes regulating the molecular mechanisms underlying intracellular β-amyloid peptide accumulation.

Reversal of fragile X phenotypes by manipulation of AβPP/Aβ levels in Fmr1KO mice

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and the leading known genetic cause of autism. Fragile X mental retardation protein (FMRP), which is absent or expressed at substantially reduced levels in FXS, binds to and controls the postsynaptic translation of amyloid β-protein precursor (AβPP) mRNA. Cleavage of AβPP can produce β-amyloid (Aβ), a 39-43 amino acid peptide mis-expressed in Alzheimer's disease (AD) and Down syndrome (DS). Aβ is over-expressed in the brain of Fmr1(KO) mice, suggesting a pathogenic role in FXS. To determine if genetic reduction of AβPP/Aβ rescues characteristic FXS phenotypes, we assessed audiogenic seizures (AGS), anxiety, the ratio of mature versus immature dendritic spines and metabotropic glutamate receptor (mGluR)-mediated long-term depression (LTD) in Fmr1(KO) mice after removal of one App allele. All of these phenotypes were partially or completely reverted to normal. Plasma Aβ(1-42) was significantly reduced in full-mutation FXS males compared to age-matched controls while cortical and hippocampal levels were somewhat increased, suggesting that Aβ is sequestered in the brain. Evolving therapies directed at reducing Aβ in AD may be applicable to FXS and Aβ may serve as a plasma-based biomarker to facilitate disease diagnosis or assess therapeutic efficacy.

Amyloid precursor protein: more than just neurodegeneration

Amyloid precursor protein (APP) fascinates cell biologists because it is proteolytically processed to generate multiple peptides, including amyloid-β, which is implicated in Alzheimer's disease. However, a large body of data also shows that the extracellular soluble fragment of APP produced by α-secretase (sAPPα) is neuroprotective and promotes neuronal outgrowth. A study by Demars and colleagues appearing in the previous issue provides data showing that sAPPα is a general growth factor for stem cells of multiple lineages. Thus, APP seems to play complex and disparate roles in neurodegeneration and neuroprotection.

The gene or not the gene--that is the question: understanding the genetically engineered mouse phenotype

Embryonic stem cells have had a significant impact on understanding gene function and gene interactions through the use of genetically engineered mice. However, the genetic context (ie, mouse strain) in which these modifications in alleles are made may have a considerable effect on the phenotypic changes identified in these mice. In addition, tissue- and time-specific gene expression systems may generate unanticipated outcomes. This article discusses the history of embryonic stem cells, reviews how mouse strain can affect phenotype (using specific examples), and examines some of the caveats of conditional gene expression systems.

FE65 proteins regulate NMDA receptor activation-induced amyloid precursor protein processing

Amyloid precursor protein (APP) family members and their proteolytic products are implicated in normal nervous system function and Alzheimer's disease pathogenesis. APP processing and Aβ secretion are regulated by neuronal activity. Various data suggest that NMDA receptor (NMDAR) activity plays a role in both non-amyloidogenic and amyloidogenic APP processing depending on whether synaptic or extrasynaptic NMDARs are activated, respectively. The APP-interacting FE65 proteins modulate APP trafficking and processing in cell lines, but little is known about their contribution to APP trafficking and processing in neurons, either in vivo or in vitro. In this study, we examined the contribution of the FE65 protein family to APP trafficking and processing in WT and FE65/FE65L1 double knockout neurons under basal conditions and following NMDAR activation. We report that FE65 proteins facilitate neuronal Aβ secretion without affecting APP fast axonal transport to pre-synaptic terminals. In addition, FE65 proteins facilitate an NMDAR-dependent non-amyloidogenic APP processing pathway. Generation of high-molecular weight (HMW) species bearing an APP C-terminal epitope was also observed following NMDAR activation. These HMW species require proteasomal and calpain activities for their accumulation. Recovery of APP polypeptide fragments from electroeluted HMW species having molecular weights consistent with calpain I cleavage of APP suggests that HMW species are complexes formed from APP metabolic products. Our results indicate that the FE65 proteins contribute to physiological APP processing and accumulation of APP metabolic products resulting from NMDAR activation.

Functions of the APP gene family in the nervous system: insights from mouse models

The amyloid precursor protein (APP) plays a key role in the pathogenesis of Alzheimer's disease (AD), as proteolytical cleavage of APP gives rise to the β-amyloid peptide which is deposited in the brains of Alzheimer patients. During the past years, intense research efforts have been directed at elucidating the physiological function(s) of APP and the question of whether a perturbation of these functions contributes to AD pathogenesis. Indeed, a growing body of evidence has accumulated supporting a role of APP and the two closely related homologues APLP1 and APLP2 in various aspects of nervous system development and function, in particular, for synapse formation and function. This review summarizes recent insights into the in vivo role of the APP gene family from mice lacking individual or combinations of APP family members, with particular emphasis on recently generated knockin mice to examine the in vivo relevance of distinct functional domains.

Analysis of amyloid precursor protein function in Drosophila melanogaster

Amyloid precursor proteins (APPs) are evolutionary conserved from nematodes to man (Jacobsen and Iverfeldt in Cell Mol Life Sci 66:2299-2318, 2009) suggesting an important physiological function of these proteins. Human APP is a key factor in the pathogenesis of Alzheimer's Disease because its proteolytic processing results in the production of the neurotoxic Aβ-peptide, which accumulates in the amyloid plaques characteristic for this disease (Selkoe in Physiol Rev 81(2):741-766, 2001). However, the processing also leads to the production of several other fragments and the role of these products, as well as the function of the full-length protein is so far not well understood. The functional analysis of APP in vertebrates has been hampered by the fact that two close relatives, APLP1 and APLP2, exist and that knock-out mice for APP only show subtle defects. In contrast, invertebrates like Caenorhabditis elegans and Drosophila express only one APP-like protein but whereas a null mutation in the C. elegans APL-1 protein is lethal, flies lacking APPL (Amyloid Precursor Protein-like) are viable but show synaptic defects and behavioral abnormalities. Together with the analyses of flies that express APP proteins ectoptically or xenotopically, these studies show that APP proteins are involved in neuronal differentiation, neuritic outgrowth, and synapse formation. In addition, they play a role in long-term memory formation and maintaining brain integrity in adult flies.

Soluble amyloid precursor protein: a novel proliferation factor of adult progenitor cells of ectodermal and mesodermal origin

INTRODUCTION

Soluble amyloid precursor protein α (sAPPα) is a proteolyte of APP cleavage by α-secretase. The significance of the cleavage and the physiological role of sAPPα are unknown. A crystal structure of a region of the amino terminal of sAPPα reveals a domain that is similar to cysteine-rich growth factors. While a previous study implicates sAPPα in the regulation of neural progenitor cell proliferation in the subventricular zone of adult mice, the ubiquitous expression of APP suggests that its role as a growth factor might be broader.

METHODS

sAPPα and α-secretase activities were determined in neural progenitor cells (NPCs), mesenchymal stem cells (MSC) and human decidua parietalis placenta stem cells (hdPSC). Inhibition of α-secretase was achieved by treatment with the matrixmetalloproteinase inhibitor GM6001, and proliferation was determined using clonogenic and immunocytochemical analysis of cell-lineage markers. Recovery of proliferation was achieved by supplementing GM6001-treated cells with recombinant soluble APPα. Expression of APP and its cellular localization in the subventricular zone was determined by Western blot and immunohistochemical analyses of APP wild type and knockout tissue. Alterations in pERK and pAKT expression as a function of soluble APPα production and activity in NPCs were determined by Western blot analysis.

RESULTS

Here we show that sAPPα is a proliferation factor of adult NPCs, MSCs and hdpPSC. Inhibition of α-secretase activity reduces proliferation of these stem cell populations in a dose-dependent manner. Stem cell proliferation can be recovered by the addition of sAPPα in a dose-dependent manner, but not of media depleted of sAPPα. Importantly, sAPPα operates independently of the prominent proliferation factors epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF), but in association with ERK signaling and MAP-kinase signaling pathways. Levels of sAPPα and putative α-secretase, ADAM10, are particularly high in the subventricular zone of adult mice, suggesting a role for sAPPα in regulation of NPCs in this microenvironment.

CONCLUSIONS

These results determine a physiological function for sAPPα and identify a new proliferation factor of progenitor cells of ectodermal and mesodermal origin. Further, our studies elucidate a potential pathway for sAPPα signaling through MAP kinase activation.

APP physiological and pathophysiological functions: insights from animal models

The amyloid precursor protein (APP) has been under intensive study in recent years, mainly due to its critical role in the pathogenesis of Alzheimer's disease (AD). β-Amyloid (Aβ) peptides generated from APP proteolytic cleavage can aggregate, leading to plaque formation in human AD brains. Point mutations of APP affecting Aβ production are found to be causal for hereditary early onset familial AD. It is very likely that elucidating the physiological properties of APP will greatly facilitate the understanding of its role in AD pathogenesis. A number of APP loss- and gain-of-function models have been established in model organisms including Caenorhabditis elegans, Drosophila, zebrafish and mouse. These in vivo models provide us valuable insights into APP physiological functions. In addition, several knock-in mouse models expressing mutant APP at a physiological level are available to allow us to study AD pathogenesis without APP overexpression. This article will review the current physiological and pathophysiological animal models of APP.

The miRNA pathway in neurological and skeletal muscle disease: implications for pathogenesis and therapy

RNA interference (RNAi) represents a powerful post-transcriptional gene silencing network which fine-tunes gene expression in all eukaryotic cells. The endogenous triggers of RNAi, microRNAs (miRNAs), are proposed to regulate expression of up to a third of all protein-coding genes, and have been shown to have critical roles in developmental processes including in the central nervous system and skeletal muscle. Further, many have been reported to display differential expression in various disease states. Here we describe present understanding of the biogenesis and function of miRNAs, review current knowledge of miRNA abnormalities in both human neurological and skeletal muscle disease and discuss their potential as novel disease biomarkers. Finally, we highlight the many ways in which the miRNA pathway may be targeted for therapeutic benefit.

The roles of amyloid precursor protein (APP) in neurogenesis: Implications to pathogenesis and therapy of Alzheimer disease

The amyloid-beta (Aβ) peptide is the derivative of amyloid precursor protein (APP) generated through sequential proteolytic processing by β- and γ-secretases. Excessive accumulation of Aβ, the main constituent of amyloid plaques, has been implicated in the etiology of Alzheimer's disease (AD). It was found recently that the impairments of neurogenesis in brain were associated with the pathogenesis of AD. Furthermore recent findings implicated that APP could function to influence proliferation of neural progenitor cells (NPC) and might regulate transcriptional activity of various genes. Studies demonstrated that influence of neurogenesis by APP is conferred differently via its two separate domains, soluble secreted APPs (sAPPs, mainly sAPPα) and APP intracellular domain (AICD). The sAPPα was shown to be neuroprotective and important to neurogenesis, whereas AICD was found to negatively modulate neurogenesis. Furthermore, it was demonstrated recently that microRNA could function to regulate APP expression, APP processing, Aβ accumulation and subsequently influence neurotoxicity and neurogenesis related to APP, which was implicated to AD pathogenesis, especially for sporadic AD. Based on data accumulated, secretase balances were proposed. These secretase balances could influence the downstream balance related to regulation of neurogenesis by AICD and sAPPα as well as balance related to influence of neuron viability by Aβ and sAPPα. Disruption of these secretase balances could be culprits to AD onset.

Alzheimer's disease

Over the last three decades, advances in biochemical pathology and human genetics have illuminated one of the most enigmatic subjects in biomedicine--neurodegeneration. Eponymic diseases of the nervous system such as Alzheimer's, Parkinson's, and Huntington's diseases that were long characterized by mechanistic ignorance have yielded striking progress in our understanding of their molecular underpinnings. A central theme in these and related disorders is the concept that certain normally soluble neuronal proteins can misfold and aggregate into oligomers and amyloid fibrils which can confer profound cytotoxicity. Perhaps the foremost example, both in terms of its societal impact and how far knowledge has moved toward the clinic, is that of Alzheimer's disease (AD). Here, we will review the classical protein lesions of the disorder that have provided a road map to etiology and pathogenesis. We will discuss how elucidating the genotype-to-phenotype relationships of familial forms of Alzheimer's disease has highlighted the importance of the misfolding and altered proteostasis of two otherwise soluble proteins, amyloid β-protein and tau, suggesting mechanism-based therapeutic targets that have led to clinical trials.

Crystal structure of the E2 domain of amyloid precursor protein-like protein 1 in complex with sucrose octasulfate

Missense mutations in the amyloid precursor protein (APP) gene can cause familial Alzheimer disease. It is thought that APP and APP-like proteins (APLPs) may play a role in adhesion and signal transduction because their ectodomains interact with components of the extracellular matrix. Heparin binding induces dimerization of APP and APLPs. To help explain how these proteins interact with heparin, we have determined the crystal structure of the E2 domain of APLP1 in complex with sucrose octasulfate (SOS). A total of three SOS molecules are bound to the E2 dimer. Two SOSs are bound inside a narrow intersubdomain groove, and the third SOS is bound near the two-fold axis of the protein. Mutational analyses show that most residues interacting with SOS also contribute to heparin binding, although in varying degrees; a deep pocket, defined by His-376, Lys-422, and Arg-429, and an interfacial site between Lys-314 and its symmetry mate are most important in the binding of the negatively charged polysaccharide. Comparison with a lower resolution APP structure shows that all key heparin binding residues are conserved and identically positioned, suggesting that APLP1 and APP may bind heparin similarly. In transfected HEK-293 cells, mutating residues responsible for heparin binding causes little change in the proteolysis of APP by the secretases. However, mutating a pair of conserved basic residues (equivalent to Arg-414 and Arg-415 of APLP1) immediately adjacent to the heparin binding site affects both the maturation and the processing of APP.

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.

Biology and pathophysiology of the amyloid precursor protein

The amyloid precursor protein (APP) plays a central role in the pathophysiology of Alzheimer's disease in large part due to the sequential proteolytic cleavages that result in the generation of β-amyloid peptides (Aβ). Not surprisingly, the biological properties of APP have also been the subject of great interest and intense investigations. Since our 2006 review, the body of literature on APP continues to expand, thereby offering further insights into the biochemical, cellular and functional properties of this interesting molecule. Sophisticated mouse models have been created to allow in vivo examination of cell type-specific functions of APP together with the many functional domains. This review provides an overview and update on our current understanding of the pathobiology of APP.

Endogenous amyloid-β is necessary for hippocampal synaptic plasticity and memory

OBJECTIVE

The goal of this study was to investigate the role of endogenous amyloid-β peptide (Aβ) in healthy brain.

METHODS

Long-term potentiation (LTP), a type of synaptic plasticity that is thought to be associated with learning and memory, was examined through extracellular field recordings from the CA1 region of hippocampal slices, whereas behavioral techniques were used to assess contextual fear memory and reference memory. Amyloid precursor protein (APP) expression was reduced through small interfering RNA (siRNA) technique.

RESULTS

We found that both antirodent Aβ antibody and siRNA against murine APP reduced LTP as well as contextual fear memory and reference memory. These effects were rescued by the addition of human Aβ₄₂, suggesting that endogenously produced Aβ is needed for normal LTP and memory. Furthermore, the effect of endogenous Aβ on plasticity and memory was likely due to regulation of transmitter release, activation of α7-containing nicotinic acetylcholine receptors, and Aβ₄₂ production.

INTERPRETATION

Endogenous Aβ₄₂ is a critical player in synaptic plasticity and memory within the normal central nervous system. This needs to be taken into consideration when designing therapies aiming at reducing Aβ levels to treat Alzheimer disease.

The amyloid precursor protein and its homologues: structural and functional aspects of native and pathogenic oligomerization

Over the last 25 years, remarkable progress has been made not only in identifying key molecules of Alzheimer's disease but also in understanding their meaning in the pathogenic state. One hallmark of Alzheimer pathology is the amyloid plaque. A major component of the extracellular deposit is the amyloid-β (Aβ) peptide which is generated from its larger precursor molecule, i.e., the amyloid precursor protein (APP) by consecutive cleavages. Processing is exerted by two enzymes, i.e., the β-secretase and the γ-secretase. We and others have found that the self-association of the amyloid peptide and the dimerization and oligomerization of these proteins is a key factor under native and pathogenic conditions. In particular, the Aβ homodimer represents a nidus for plaque formation and a well defined therapeutic target. Further, dimerization of the APP was reported to increase generation of toxic Aβ whereas heterodimerization with its homologues amyloid precursor like proteins (APLP1 and APLP2) decreased Aβ formation. This review mainly focuses on structural features of the homophilic and heterophilic interactions among APP family proteins. The proposed contact sites are described and the consequences of protein dimerization on their functions and in the pathogenesis of Alzheimer's disease are discussed.

The intracellular threonine of amyloid precursor protein that is essential for docking of Pin1 is dispensable for developmental function

Background

Processing of Aβ-precursor protein (APP) plays an important role in Alzheimer's Disease (AD) pathogenesis. Thr residue at amino acid 668 of the APP intracellular domain (AID) is highly conserved. When phosphorylated, this residue generates a binding site for Pin1. The interaction of APP with Pin1 has been involved in AD pathogenesis.

Methodology/Principal Findings

To dissect the functions of this sequence in vivo, we created an APP knock-in allele, in which Thr⁶⁶⁸ is replaced by an Ala (T⁶⁶⁸A). Doubly deficient APP/APP-like protein 2 (APLP2) mice present postnatal lethality and neuromuscular synapse defects. Previous work has shown that the APP intracellular domain is necessary for preventing early lethality and neuromuscular junctions (NMJ) defects. Crossing the T⁶⁶⁸A allele into the APLP2 knockout background showed that mutation of Thr⁶⁶⁸ does not cause a defective phenotype. Notably, the T⁶⁶⁸A mutant APP is able to bind Mint1.

Conclusions/Significance

Our results argue against an important role of the Thr⁶⁶⁸ residue in the essential function of APP in developmental regulation. Furthermore, they indicate that phosphorylation at this residue is not functionally involved in those APP-mediated functions that prevent (NMJ) defects and early lethality in APLP2 null mice.

Dysregulation of Ca2+ signaling in astrocytes from mice lacking amyloid precursor protein

The relationship between altered metabolism of the amyloid-β precursor protein (APP) and Alzheimer's disease is well established but the physiological roles of APP still remain unclear. Here, we studied Ca(2+) signaling in primary cultured and freshly dissociated cortical astrocytes from APP knockout (KO) mice and from Tg5469 mice overproducing by five- to sixfold wild-type APP. Resting cytosolic Ca(2+) (measured with fura-2) was not altered in cultured astrocytes from APP KO mice. The stored Ca(2+) evaluated by measuring peak amplitude of cyclopiazonic acid [CPA, endoplasmic reticulum (ER) Ca(2+) ATPase inhibitor]-induced Ca(2+) transients in Ca(2+)-free medium was significantly smaller in APP KO astrocytes than in wild-type cells. Store-operated Ca(2+) entry (SOCE) activated by ER Ca(2+) store depletion with CPA was also greatly reduced in APP KO astrocytes. This reflected a downregulated expression in APP KO astrocytes of TRPC1 (C-type transient receptor potential) and Orai1 proteins, essential components of store-operated channels (SOCs). Indeed, silencer RNA (siRNA) knockdown of Orai1 protein expression in wild-type astrocytes significantly attenuated SOCE. SOCE was also essentially reduced in freshly dissociated APP KO astrocytes. Importantly, knockdown of APP with siRNA in cultured wild-type astrocytes markedly attenuated ATP- and CPA-induced ER Ca(2+) release and extracellular Ca(2+) influx. The latter correlated with downregulation of TRPC1. Overproduction of APP in Tg5469 mice did not alter, however, the stored Ca(2+) level, SOCE, and expression of TRPC1/4/5 in cultured astrocytes from these mice. The data demonstrate that the functional role of APP in astrocytes involves the regulation of TRPC1/Orai1-encoded SOCs critical for Ca(2+) signaling.

APP involvement in retinogenesis of mice

Very few studies have examined expression and function of amyloid precursor protein (APP) in the retina. We showed that APP mRNA and protein are expressed according to the different waves of retinal differentiation. Depletion of App led to an absence of amacrine cells, a 50% increase in the number of horizontal cells and alteration of the synapses. The retinas of adult APP(-/-) mice showed only half as many glycinergic amacrine cells as wild-type retinas. We identified Ptf1a, which plays a role in controlling both amacrine and horizontal cell fates, as a downstream effector of APP. The observation of a similar phenotype in sorLA knockout mice, a major regulator of APP processing, suggests that regulation of APP functions via sorLA controls the determination of amacrine and horizontal cell fate. These findings provide novel insights that indicate that APP plays an important role in retinal differentiation.

Drosophila amyloid precursor protein-like is required for long-term memory

The amyloid precursor protein (APP) plays an important role in Alzheimer's disease (AD), a progressive neurodegenerative pathology that first manifests as a decline of memory. While the main hypothesis for AD pathology centers on the proteolytic processing of APP, very little is known about the physiological function of the APP protein in the adult brain. Likewise, whether APP loss of function contributes to AD remains unclear. Drosophila has been used extensively as a model organism to study neuronal function and pathology. In addition, many of the molecular mechanisms underlying memory are thought to be conserved from flies to mammals, prompting us to study the function of APPL, the fly APP ortholog, during associative memory. It was previously shown that APPL expression is highly enriched in the mushroom bodies (MBs), a specialized brain structure involved in olfactory memory. We analyzed memory in flies in which APPL expression has been silenced specifically and transiently in the adult MBs. Our results show that in adult flies, APPL is not required for learning but is specifically involved in long-term memory, a long lasting memory whose formation requires de novo protein synthesis and is thought to require synaptic structural plasticity. These data support the hypothesis that disruption of normal APP function may contribute to early AD cognitive impairment.

Loss of amyloid precursor protein in a mouse model of Niemann-Pick type C disease exacerbates its phenotype and disrupts tau homeostasis

Niemann-Pick type C disease (NPC) is a lysosomal storage disorder which, at the cellular level, shows amyloid Aβ and tau pathologies comparable to those seen in the AD brain. Here, we have investigated, in a mouse model of NPC, the impact of removing the source of Aβ, namely APP, on the disease phenotype and on the expression levels and phosphorylation patterns of tau. We reasoned that removing APP from the NPC brain might help to unveil its impact on the disease phenotype and shed light on the mechanisms governing the interaction, both physiological and pathological, between APP function and tau homeostasis, at least in NPC. We show that, unexpectedly, loss of APP in NPC mice leads to poorer neuromuscular coordination and cumulative survival rates; exacerbation of their cholesterol abnormalities; higher levels of astrocytosis and dysregulation of tau homeostasis. Our results are consistent with a mechanism of neurodegeneration in the NPC and AD brains in which cholesterol dysregulation is a key early pathogenic event affecting tau homeostasis in parallel with, and independently of, amyloid accumulation.

A single tyrosine residue in the amyloid precursor protein intracellular domain is essential for developmental function

The Aβ-precursor protein (APP) intracellular domain is highly conserved and contains many potentially important residues, in particular the (682)YENPTY(687) motif. To dissect the functions of this sequence in vivo, we created an APP knock-in allele mutating Tyr(682) to Gly (Y682G). Crossing this allele to APP-like protein 2 (APLP2) knock-out background showed that mutation of Tyr(682) results in postnatal lethality and neuromuscular synapse defects similar to doubly deficient APP/APLP2 mice. Our results demonstrate that a single residue in the APP intracellular region, Tyr(682), is indispensable for the essential function of APP in developmental regulation.

APP processing in Alzheimer's disease

An important pathological feature of Alzheimer's disease (AD) is the presence of extracellular senile plaques in the brain. Senile plaques are composed of aggregations of small peptides called β-amyloid (Aβ). Multiple lines of evidence demonstrate that overproduction/aggregation of Aβ in the brain is a primary cause of AD and inhibition of Aβ generation has become a hot topic in AD research. Aβ is generated from β-amyloid precursor protein (APP) through sequential cleavages first by β-secretase and then by γ-secretase complex. Alternatively, APP can be cleaved by α-secretase within the Aβ domain to release soluble APPα and preclude Aβ generation. Cleavage of APP by caspases may also contribute to AD pathologies. Therefore, understanding the metabolism/processing of APP is crucial for AD therapeutics. Here we review current knowledge of APP processing regulation as well as the patho/physiological functions of APP and its metabolites.

Tyr(682) in the intracellular domain of APP regulates amyloidogenic APP processing in vivo

BACKGROUND

The pathogenesis of Alzheimer's disease is attributed to misfolding of Amyloid-β (Aβ) peptides. Aβ is generated during amyloidogenic processing of Aβ-precursor protein (APP). Another characteristic of the AD brain is increased phosphorylation of APP amino acid Tyr(682). Tyr(682) is part of the Y(682)ENPTY(687) motif, a docking site for interaction with cytosolic proteins that regulate APP metabolism and signaling. For example, normal Aβ generation and secretion are dependent upon Tyr(682) in vitro. However, physiological functions of Tyr(682) are unknown.

METHODOLOGY/PRINCIPAL FINDINGS

To this end, we have generated an APP Y682G knock-in (KI) mouse to help dissect the role of APP Tyr(682) in vivo. We have analyzed proteolytic products from both the amyloidogenic and non-amyloidogenic processing of APP and measure a profound shift towards non-amyloidogenic processing in APP KI mice. In addition, we demonstrate the essential nature of amino acid Tyr(682) for the APP/Fe65 interaction in vivo.

CONCLUSIONS/SIGNIFICANCE

Together, these observations point to an essential role of APP intracellular domain for normal APP processing and function in vivo, and provide rationale for further studies into physiological functions associated with this important phosphorylation site.

Iron-export ferroxidase activity of β-amyloid precursor protein is inhibited by zinc in Alzheimer's disease

Alzheimer's Disease (AD) is complicated by pro-oxidant intraneuronal Fe(2+) elevation as well as extracellular Zn(2+) accumulation within amyloid plaque. We found that the AD β-amyloid protein precursor (APP) possesses ferroxidase activity mediated by a conserved H-ferritin-like active site, which is inhibited specifically by Zn(2+). Like ceruloplasmin, APP catalytically oxidizes Fe(2+), loads Fe(3+) into transferrin, and has a major interaction with ferroportin in HEK293T cells (that lack ceruloplasmin) and in human cortical tissue. Ablation of APP in HEK293T cells and primary neurons induces marked iron retention, whereas increasing APP695 promotes iron export. Unlike normal mice, APP(-/-) mice are vulnerable to dietary iron exposure, which causes Fe(2+) accumulation and oxidative stress in cortical neurons. Paralleling iron accumulation, APP ferroxidase activity in AD postmortem neocortex is inhibited by endogenous Zn(2+), which we demonstrate can originate from Zn(2+)-laden amyloid aggregates and correlates with Aβ burden. Abnormal exchange of cortical zinc may link amyloid pathology with neuronal iron accumulation in AD.

Dysregulation of the mTOR pathway mediates impairment of synaptic plasticity in a mouse model of Alzheimer's disease

Background

The mammalian target of rapamycin (mTOR) is an evolutionarily conserved Ser/Thr protein kinase that plays a pivotal role in multiple fundamental biological processes, including synaptic plasticity. We explored the relationship between the mTOR pathway and β-amyloid (Aβ)-induced synaptic dysfunction, which is considered to be critical in the pathogenesis of Alzheimer's disease (AD).

Methodology/Principal Findings

We provide evidence that inhibition of mTOR signaling correlates with impairment in synaptic plasticity in hippocampal slices from an AD mouse model and in wild-type slices exposed to exogenous Aβ1-42. Importantly, by up-regulating mTOR signaling, glycogen synthase kinase 3 (GSK3) inhibitors rescued LTP in the AD mouse model, and genetic deletion of FK506-binding protein 12 (FKBP12) prevented Aβ-induced impairment in long-term potentiation (LTP). In addition, confocal microscopy demonstrated co-localization of intraneuronal Aβ42 with mTOR.

Conclusions/Significance

These data support the notion that the mTOR pathway modulates Aβ-related synaptic dysfunction in AD.

Intracellular trafficking and synaptic function of APL-1 in Caenorhabditis elegans

Background

Alzheimer's disease (AD) is a neurodegenerative disorder primarily characterized by the deposition of β-amyloid plaques in the brain. Plaques are composed of the amyloid-β peptide derived from cleavage of the amyloid precursor protein (APP). Mutations in APP lead to the development of Familial Alzheimer's Disease (FAD), however, the normal function of this protein has proven elusive. The organism Caenorhabditis elegans is an attractive model as the amyloid precursor-like protein (APL-1) is the single ortholog of APP, and loss of apl-1 leads to a severe molting defect and early larval lethality.

Methodology/Principal Findings

We report here that lethality and molting can be rescued by full length APL-1, C-terminal mutations as well as a C-terminal truncation, suggesting that the extracellular region of the protein is essential for viability. RNAi knock-down of apl-1 followed by drug testing on the acetylcholinesterase inhibitor aldicarb showed that loss of apl-1 leads to aldicarb hypersensitivity, indicating a defect in synaptic function. The aldicarb hypersensitivity can be rescued by full length APL-1 in a dose dependent fashion. At the cellular level, kinesins UNC-104/KIF-1A and UNC-116/kinesin-1 are positive regulators of APL-1 expression in the neurons. Knock-down of the small GTPase rab-5 also leads to a dramatic decrease in the amount of apl-1 expression in neurons, suggesting that trafficking from the plasma membrane to the early endosome is important for apl-1 function. Loss of function of a different small GTPase, UNC-108, on the contrary, leads to the retention of APL-1 in the cell body.

Conclusions/Significance

Our results reveal novel insights into the intracellular trafficking of APL-1 and we report a functional role for APL-1 in synaptic transmission.

Soluble amyloid precursor protein (APP) regulates transthyretin and Klotho gene expression without rescuing the essential function of APP

Amyloidogenic processing of the amyloid precursor protein (APP) generates a large secreted ectodomain fragment (APPsβ), β-amyloid (Aβ) peptides, and an APP intracellular domain (AICD). Whereas Aβ is viewed as critical for Alzheimer's disease pathogenesis, the role of other APP processing products remains enigmatic. Of interest, the AICD has been implicated in transcriptional regulation, and N-terminal cleavage of APPsβ has been suggested to produce an active fragment that may mediate axonal pruning and neuronal cell death. We previously reported that mice deficient in APP and APP-like protein 2 (APLP2) exhibit early postnatal lethality and neuromuscular synapse defects, whereas mice with neuronal conditional deletion of APP and APLP2 are viable. Using transcriptional profiling, we now identify transthyretin (TTR) and Klotho as APP/APLP2-dependent genes whose expression is decreased in loss-of-function states but increased in gain-of-function states. Significantly, by creating an APP knockin allele that expresses only APPsβ protein, we demonstrate that APPsβ is not normally cleaved in vivo and is fully capable of mediating the APP-dependent regulation of TTR and Klotho gene expression. Despite being an active regulator of gene expression, APPsβ did not rescue the lethality and neuromuscular synapse defects of APP and APLP2 double-KO animals. Our studies identify TTR and Klotho as physiological targets of APP that are regulated by soluble APPsβ independent of developmental APP functions. This unexpected APP-mediated signaling pathway may play an important role in maintaining TTR and Klotho levels and their respective functions in Aβ sequestration and aging.

Developmental changes of gene expression after spinal cord injury in neonatal opossums

Changes in gene expression have been measured 24h after injury to mammalian spinal cords that can and cannot regenerate. In opossums there is a critical period of development when regeneration stops being possible: at 9 days postnatal cervical spinal cords regenerate, at 12 days they do not. By the use of marsupial cDNA microarrays, we detected 158 genes that respond differentially to injury at the two ages critical for regeneration. For selected candidates additional measurements were made by real-time PCR and sites of their expression were shown by immunostaining. Candidate genes have been classified so as to select those that promote or prevent regeneration. Up-regulated by injury at 8 days and/or down-regulated by injury at 13 days were genes known to promote growth, such as Mitogen-activated protein kinase kinase 1 or transcription factor TCF7L2. By contrast, at 13 days, up-regulation occurred of inhibitory molecules, including annexins, ephrins, and genes related to apoptosis and neurodegenerative diseases. Certain genes such as calmodulin 1 and NOGO, changed expression similarly in animals that could and could not regenerate without any additional changes in response to injury. These findings confirmed and extended changes of gene expression found in earlier screens on 9 and 12 ay preparations without lesions and provide a comprehensive list of genes that serve as a basis for testing how identified molecules, singly or in combination, promote and prevent central nervous system regeneration.

Regulation of cholesterol efflux by amyloid beta secretion

Amyloid beta (Abeta) is a key molecule in the pathogenesis of Alzheimer's disease, but its physiological function remains unclear. Abeta is produced from amyloid precursor protein (APP) by beta- and gamma-secretases, which is enhanced by high levels of cellular cholesterol, so cholesterol is a risk factor for Alzheimer's disease. This linkage led us to hypothesize that Abeta is produced to regulate cellular cholesterol levels in response to high-cholesterol stimulation. Here we show that Abeta production caused a reduction of cellular cholesterol levels in transfected HEK293 cells and neuronal IMR-32 and Neuro2a cells, which was accompanied by an increase in efflux of cholesterol from cells. Fractionation of the culture media by ultracentrifugation and subsequent immunoelectron microscopic observation revealed that Abeta assembled high-density lipoprotein-like particles with cellular cholesterol during its secretion. This assembly was mediated by the ATP-binding cassette transporter A1. APP transgenic and knockout mice exhibited lower and higher levels of cellular cholesterol in their brains, suggesting that Abeta-mediated regulation of cellular cholesterol is physiological. Furthermore, we found that, when injected into mouse cerebral ventricle, reconstituted lipoproteins with Abeta were excreted into the peripheral tissues more efficiently than those without Abeta. This result suggests that Abeta mediates cholesterol transport from the brain to the circulation. We propose, based on these findings, a novel, apolipoprotein-like function for Abeta that is involved in maintenance of cellular and cerebral cholesterol homeostasis.