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

Deep Transcriptomics Reveals Cell-Specific Isoforms of Pan-Neuronal Genes

Profiling gene expression in single neurons using single-cell RNA-Seq is a powerful method for understanding the molecular diversity of the nervous system. Profiling alternative splicing in single neurons using these methods is more challenging, however, due to low capture efficiency and sensitivity. As a result, we know much less about splicing patterns and regulation across neurons than we do about gene expression. Here we leverage unique attributes of the C. elegans nervous system to investigate deep cell-specific transcriptomes complete with biological replicates generated by the CeNGEN consortium, enabling high-confidence assessment of splicing across neuron types even for lowly-expressed genes. Global splicing maps reveal several striking observations, including pan-neuronal genes that harbor cell-specific splice variants, abundant differential intron retention across neuron types, and a single neuron highly enriched for upstream alternative 3' splice sites. We develop an algorithm to identify unique cell-specific expression patterns and use it to discover both cell-specific isoforms and potential regulatory RNA binding proteins that establish these isoforms. Genetic interrogation of these RNA binding proteins in vivo identifies three distinct regulatory factors employed to establish unique splicing patterns in a single neuron. Finally, we develop a user-friendly platform for spatial transcriptomic visualization of these splicing patterns with single-neuron resolution.

Simple model systems reveal conserved mechanisms of Alzheimer's disease and related tauopathies

The lack of effective therapies that slow the progression of Alzheimer's disease (AD) and related tauopathies highlights the need for a more comprehensive understanding of the fundamental cellular mechanisms underlying these diseases. Model organisms, including yeast, worms, and flies, provide simple systems with which to investigate the mechanisms of disease. The evolutionary conservation of cellular pathways regulating proteostasis and stress response in these organisms facilitates the study of genetic factors that contribute to, or protect against, neurodegeneration. Here, we review genetic modifiers of neurodegeneration and related cellular pathways identified in the budding yeast Saccharomyces cerevisiae, the nematode Caenorhabditis elegans, and the fruit fly Drosophila melanogaster, focusing on models of AD and related tauopathies. We further address the potential of simple model systems to better understand the fundamental mechanisms that lead to AD and other neurodegenerative disorders.

Caenorhabditis elegans as a Model System to Study Human Neurodegenerative Disorders

In recent years, advances in science and technology have improved our quality of life, enabling us to tackle diseases and increase human life expectancy. However, longevity is accompanied by an accretion in the frequency of age-related neurodegenerative diseases, creating a growing burden, with pervasive social impact for human societies. The cost of managing such chronic disorders and the lack of effective treatments highlight the need to decipher their molecular and genetic underpinnings, in order to discover new therapeutic targets. In this effort, the nematode Caenorhabditis elegans serves as a powerful tool to recapitulate several disease-related phenotypes and provides a highly malleable genetic model that allows the implementation of multidisciplinary approaches, in addition to large-scale genetic and pharmacological screens. Its anatomical transparency allows the use of co-expressed fluorescent proteins to track the progress of neurodegeneration. Moreover, the functional conservation of neuronal processes, along with the high homology between nematode and human genomes, render C. elegans extremely suitable for the study of human neurodegenerative disorders. This review describes nematode models used to study neurodegeneration and underscores their contribution in the effort to dissect the molecular basis of human diseases and identify novel gene targets with therapeutic potential.

Physiology and pharmacology of amyloid precursor protein

Amyloid precursor protein (APP) is an evolutionarily conserved transmembrane protein and a well-characterized precursor protein of amyloid-beta (Aβ) peptides, which accumulate in the brains of individuals with Alzheimer's disease (AD)-related pathologies. Aβ has been extensively investigated since the amyloid hypothesis in AD was proposed. Besides Aβ, previous studies on APP and its proteolytic cleavage products have suggested their diverse pathological and physiological functions. However, their roles still have not been thoroughly understood. In this review, we extensively discuss the evolutionarily-conserved biology of APP, including its structure and processing pathway, as well as recent findings on the physiological roles of APP and its fragments in the central nervous system and peripheral nervous system. We have also elaborated upon the current status of APP-targeted therapeutic approaches for AD treatment by discussing inhibitors of several proteases participating in APP processing, including α-, β-, and γ-secretases. Finally, we have highlighted the future perspectives pertaining to further research and the potential clinical role of APP.

Invited review: Unearthing the mechanisms of age-related neurodegenerative disease using Caenorhabditis elegans

As human life expectancy increases, neurodegenerative diseases present a growing public health threat, for which there are currently few effective treatments. There is an urgent need to understand the molecular and genetic underpinnings of these disorders so new therapeutic targets can be identified. Here we present the argument that the simple nematode worm Caenorhabditis elegans is a powerful tool to rapidly study neurodegenerative disorders due to their short lifespan and vast array of genetic tools, which can be combined with characterization of conserved neuronal processes and behavior orthologous to those disrupted in human disease. We review how pre-existing C. elegans models provide insight into human neurological disease as well as an overview of current tools available to study neurodegenerative diseases in the worm, with an emphasis on genetics and behavior. We also discuss open questions that C. elegans may be particularly well suited for in future studies and how worms will be a valuable preclinical model to better understand these devastating neurological disorders.

Modeling Alzheimer's Disease in Caenorhabditis elegans

Alzheimer’s disease (AD) is the most frequent cause of dementia. After decades of research, we know the importance of the accumulation of protein aggregates such as β-amyloid peptide and phosphorylated tau. We also know that mutations in certain proteins generate early-onset Alzheimer’s disease (EOAD), and many other genes modulate the disease in its sporadic form. However, the precise molecular mechanisms underlying AD pathology are still unclear. Because of ethical limitations, we need to use animal models to investigate these processes. The nematode Caenorhabditis elegans has received considerable attention in the last 25 years, since the first AD models overexpressing Aβ peptide were described. We review here the main results obtained using this model to study AD. We include works studying the basic molecular mechanisms of the disease, as well as those searching for new therapeutic targets. Although this model also has important limitations, the ability of this nematode to generate knock-out or overexpression models of any gene, single or combined, and to carry out toxicity, recovery or survival studies in short timeframes with many individuals and at low cost is difficult to overcome. We can predict that its use as a model for various diseases will certainly continue to increase.

Structural Studies Providing Insights into Production and Conformational Behavior of Amyloid-β Peptide Associated with Alzheimer's Disease Development

Alzheimer's disease is the most common type of neurodegenerative disease in the world. Genetic evidence strongly suggests that aberrant generation, aggregation, and/or clearance of neurotoxic amyloid-β peptides (Aβ) triggers the disease. Aβ accumulates at the points of contact of neurons in ordered cords and fibrils, forming the so-called senile plaques. Aβ isoforms of different lengths are found in healthy human brains regardless of age and appear to play a role in signaling pathways in the brain and to have neuroprotective properties at low concentrations. In recent years, different substances have been developed targeting Aβ production, aggregation, interaction with other molecules, and clearance, including peptide-based drugs. Aβ is a product of sequential cleavage of the membrane glycoprotein APP (amyloid precursor protein) by β- and γ-secretases. A number of familial mutations causing an early onset of the disease have been identified in the APP, especially in its transmembrane domain. The mutations are reported to influence the production, oligomerization, and conformational behavior of Aβ peptides. This review highlights the results of structural studies of the main proteins involved in Alzheimer's disease pathogenesis and the molecular mechanisms by which perspective therapeutic substances can affect Aβ production and nucleation.

Effect of policosanol from insect wax on amyloid β-peptide-induced toxicity in a transgenic Caenorhabditis elegans model of Alzheimer's disease

Background

Alzheimer’s disease (AD), an age-related neurodegenerative disorder and a serious public health concern, is mainly caused by β-amyloid (Aβ)-induced toxicity. Currently, a limited number of drugs are effective against AD, and only a few are used for its treatment. According to traditional Chinese medicine, white wax is mainly composed of policosanol, hexacosanol, and octacosanol. Policosanol has been shown to reduce lipid levels in blood and alleviate the symptoms associated with diabetic complications and neurodegenerative disorders, such as Parkinson’s disease and AD. However, the efficacy of policosanol depends on the purity and composition of the preparation, and the therapeutic efficacy of policosanol derived from insect wax (PIW) in AD is unknown.

Methods

Here, we identified the main components of PIW and investigated the effects of PIW on Aβ-induced toxicity and life-span in a transgenic Caenorhabditis elegans model of AD, CL4176. Furthermore, we estimated the expression of amyloid precursor-like protein (apl-1) and the genes involved in various pathways associated with longevity and alleviation of AD-related symptoms in PIW-fed CL4176.

Results

PIW mainly consists of tetracosanol, hexacosanol, octacosanol, and triacontanol; it could decrease the Aβ-induced paralysis rate from 86.87 to 66.97% (P < 0.01) and extend the life-span from 6.2 d to 7.8 d (P < 0.001) in CL4176 worms. Furthermore, PIW downregulated apl-1, a gene known to be associated with the levels of Aβ deposits in C. elegans. Additionally, our results showed that PIW modulated the expression of genes associated with longevity-related pathways such as heat shock response, anti-oxidative stress, and glutamine cysteine synthetase.

Conclusion

Our findings suggest that PIW may be a potential therapeutic agent for the prevention and treatment of AD. However, its effects on murine models and patients with AD need to be explored further.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12906-021-03278-2.

Deciphering the neuroprotective and neurogenic potential of soluble amyloid precursor protein alpha (sAPPα)

Amyloid precursor protein (APP) is a transmembrane protein expressed largely within the central nervous system. Upon cleavage, it does not produce the toxic amyloid peptide (Aβ) only, which is involved in neurodegenerative progressions but via a non-amyloidogenic pathway it is metabolized to produce a soluble fragment (sAPPα) through α-secretase. While a lot of studies are focusing on the role played by APP in the pathogenesis of Alzheimer's disease, sAPPα is reported to have numerous neuroprotective effects and it is being suggested as a candidate with possible therapeutic potential against Alzheimer's disease. However, the mechanisms through which sAPPα precisely works remain elusive. We have presented a comprehensive review of how sAPPα is regulating the neuroprotective effects in different biological models. Moreover, we have focused on the role of sAPPα during different developmental stages of the brain, neurogenic microenvironment in the brain and how this metabolite of APP is regulating the neurogenesis which is regarded as a compelling approach to ameliorate the impaired learning and memory deficits in dementia and diseases like Alzheimer's disease. sAPPα exerts beneficial physiological, biochemical and behavioral effects mitigating the detrimental effects of neurotoxic compounds. It has shown to increase the proliferation rate of numerous cell types and promised the synaptogenesis, neurite outgrowth, cell survival and cell adhesion. Taken together, we believe that further studies are warranted to investigate the exact mechanism of action so that sAPPα could be developed as a novel therapeutic target against neuronal deficits.

Role of Rab GTPases in Alzheimer's Disease

Alzheimer's disease (AD) comprises two major pathological hallmarks: extraneuronal deposition of β-amyloid (Aβ) peptides ("senile plaques") and intraneuronal aggregation of the microtubule-associated protein tau ("neurofibrillary tangles"). Aβ is derived from sequential cleavage of the β-amyloid precursor protein by β- and γ-secretases, while aggregated tau is hyperphosphorylated in AD. Mounting evidence suggests that dysregulated trafficking of these AD-related proteins contributes to AD pathogenesis. Rab proteins are small GTPases that function as master regulators of vesicular transport and membrane trafficking. Multiple Rab GTPases have been implicated in AD-related protein trafficking, and their expression has been observed to be altered in postmortem AD brain. Here we review current implicated roles of Rab GTPase dysregulation in AD pathogenesis. Further elucidation of the pathophysiological role of Rab GTPases will likely reveal novel targets for AD therapeutics.

Effects of 5-Aminolevulinic Acid on Gene Expression, Immunity, and ATP Levels in Pacific White Shrimp, Litopenaeus vannamei

With the emergence of several infectious diseases in shrimp aquaculture, there is a growing interest in the use of feed additives to enhance shrimp immunity. Recently, the use of 5-aminolevulinic acid (5-ALA), a non-protein amino acid that plays a rate-limiting role in heme biosynthesis, has received attention for its positive effect on immunity in livestock animals. To evaluate the effect of 5-ALA in the Pacific white shrimp, Litopenaeus vannamei, we conducted microarray analysis, a Vibrio parahaemolyticus immersion challenge test, an ATP level assay, and gene expression analysis of some hemoproteins and genes associated with heme synthesis and degradation. Out of 15,745 L. vannamei putative genes on the microarray, 101 genes were differentially expressed by more than fourfold (p < 0.05) between 5-ALA-supplemented and control shrimp hepatopancreas. 5-ALA upregulated 99 of the 101 genes, 41 of which were immune- and defense-related genes based on sequence homology. Compared to the control, the 5-ALA-supplemented group had a higher survival rate in the challenge test, higher transcript levels of porphobilinogen synthase, ferrochelatase, catalase, nuclear receptor E75, and heme oxygenase-1 and higher levels of ATP. These findings suggest that dietary 5-ALA enhanced the immune response of L. vannamei to V. parahaemolyticus, upregulated immune- and defense-related genes, and enhanced aerobic energy metabolism, respectively. Further studies are needed to elucidate the extent of 5-ALA use in shrimp culture.

Lower vertebrate and invertebrate models of Alzheimer's disease - A review

Alzheimer's disease is a common neurodegenerative disorder which is characterized by the presence of beta- amyloid protein and neurofibrillary tangles (NFTs) in the brain. Till now, various higher vertebrate models have been in use to study the pathophysiology of this disease. But, these models possess some limitations like ethical restrictions, high cost, difficult maintenance of large quantity and lesser reproducibility. Besides, various lower chordate animals like Danio rerio, Drosophila melanogaster, Caenorhabditis elegans and Ciona intestinalis have been proved to be an important model for the in vivo determination of targets of drugs with least limitations. In this article, we reviewed different studies conducted on theses models for the better understanding of the pathophysiology of AD and their subsequent application as a potential tool in the preclinical evaluation of new drugs.

The Amyloid Precursor Protein Is a Conserved Receptor for Slit to Mediate Axon Guidance

The amyloid precursor protein (APP) is a receptor-like membrane protein. Although APP processing and β-amyloid production play a central role in Alzheimer's disease (AD) pathogenesis, the physiological function of APP remains elusive. Here, we identify APP as a novel receptor for Slit that mediates axon guidance and neural circuit formation. APP deficiency abolishes the Slit repulsive effect in a 3D olfactory explant culture, consistent with its callosal projection deficit in vivo and reminiscent of Slit loss. Inactivation of APP ortholog APL-1 in Caenorhabditis elegans results in pioneer axon mistargeting and genetic analysis places APL-1 in the SLT-1 (Slit)/SAX-3 (Robo) repulsive pathway. Slit binds to APP through the E1 domain, which triggers APP ectodomain shedding and recruitment of the intracellular FE65 and Pak1 complex and associated Rac1 GTPase activation. Our study establishes APP as a novel receptor for Slit ligand mediating axon guidance and neural circuit formation.

Alzheimer-related protein APL-1 modulates lifespan through heterochronic gene regulation in Caenorhabditis elegans

Alzheimer's disease (AD) is an age‐associated disease. Mutations in the amyloid precursor protein (APP) may be causative or protective of AD. The presence of two functionally redundant APP‐like genes (APLP1/2) has made it difficult to unravel the biological function of APP during aging. The nematode Caenorhabditis elegans contains a single APP family member, apl‐1. Here, we assessed the function of APL‐1 on C. elegans’ lifespan and found tissue‐specific effects on lifespan by overexpression of APL‐1. Overexpression of APL‐1 in neurons causes lifespan reduction, whereas overexpression of APL‐1 in the hypodermis causes lifespan extension by repressing the function of the heterochronic transcription factor LIN‐14 to preserve youthfulness. APL‐1 lifespan extension also requires signaling through the FOXO transcription factor DAF‐16, heat‐shock factor HSF‐1, and vitamin D‐like nuclear hormone receptor DAF‐12. We propose that reinforcing APL‐1 expression in the hypodermis preserves the regulation of heterochronic lin‐14 gene network to improve maintenance of somatic tissues via DAF‐16/FOXO and HSF‐1 to promote healthy aging. Our work reveals a mechanistic link of how a conserved APP‐related protein modulates aging.

Imaging organelle transport in primary hippocampal neurons treated with amyloid-β oligomers

We describe a strategy for fluorescent imaging of organelle transport in primary hippocampal neurons treated with amyloid-β (Aβ) peptides that cause Alzheimer's disease (AD). This method enables careful, rigorous analyses of axonal transport defects, which are implicated in AD and other neurodegenerative diseases. Moreover, we present and emphasize guidelines for investigating Aβ-induced mechanisms of axonal transport disruption in the absence of nonspecific, irreversible cellular toxicity. This approach should be accessible to most laboratories equipped with cell culture facilities and a standard fluorescent microscope and may be adapted to other cell types.

The Amyloid Precursor Protein Controls PIKfyve Function

While the Amyloid Precursor Protein (APP) plays a central role in Alzheimer's disease, its cellular function still remains largely unclear. It was our goal to establish APP function which will provide insights into APP's implication in Alzheimer's disease. Using our recently developed proteo-liposome assay we established the interactome of APP's intracellular domain (known as AICD), thereby identifying novel APP interactors that provide mechanistic insights into APP function. By combining biochemical, cell biological and genetic approaches we validated the functional significance of one of these novel interactors. Here we show that APP binds the PIKfyve complex, an essential kinase for the synthesis of the endosomal phosphoinositide phosphatidylinositol-3,5-bisphosphate. This signalling lipid plays a crucial role in endosomal homeostasis and receptor sorting. Loss of PIKfyve function by mutation causes profound neurodegeneration in mammals. Using C. elegans genetics we demonstrate that APP functionally cooperates with PIKfyve in vivo. This regulation is required for maintaining endosomal and neuronal function. Our findings establish an unexpected role for APP in the regulation of endosomal phosphoinositide metabolism with dramatic consequences for endosomal biology and important implications for our understanding of Alzheimer's disease.

Cellular functions of the amyloid precursor protein from development to dementia

Amyloid precursor protein (APP) is a key player in Alzheimer's disease (AD). The Aβ fragments of APP are the major constituent of AD-associated amyloid plaques, and mutations or duplications of the gene coding for APP can cause familial AD. Here we review the roles of APP in neuronal development, signaling, intracellular transport, and other aspects of neuronal homeostasis. We suggest that APP acts as a signaling nexus that transduces information about a range of extracellular conditions, including neuronal damage, to induction of intracellular signaling events. Subtle disruptions of APP signaling functions may be major contributors to AD-causing neuronal dysfunction.

Role of MicroRNA Let-7 in Modulating Multifactorial Aspect of Neurodegenerative Diseases: an Overview

The multifactorial aspect of neurodegenerative diseases has posed challenges in terms of understanding various mechanistic cues behind these ailments. The fact that single microRNA (miRNA) molecules can regulate multiple genes and associated pathways makes these molecules interesting for studies within the area of age-associated neurodegenerative diseases. miRNAs are endogenous, evolutionarily conserved, 20-23 nucleotide non-coding RNAs, which were first discovered in Caenorhabditis elegans. They play a key role in gene regulation and are known to be deregulated in many disease conditions. Steady regulations of miRNAs are required for normal biological processes. One of the crucial miRNA molecules let-7 is highly conserved and is known to be required for development and viability. It acts as a regulator for oncogenes and insulin-PI3K-mTOR pathway genes. Upregulation of let-7 impairs glucose homeostasis and results in degeneration of neurons, while its downregulation leads to cancer. Maturation of let-7 in cancer subjects is inhibited by lin-28, an RNA-binding protein inhibitor. This highlights the importance of let-7 miRNAs in various diseases and developmental processes. This article provides an overview on the functions of let-7 and its probable association with various neurodegenerative diseases.

Use of Caenorhabditis elegans as a model to study Alzheimer's disease and other neurodegenerative diseases

Advances in research and technology has increased our quality of life, allowed us to combat diseases, and achieve increased longevity. Unfortunately, increased longevity is accompanied by a rise in the incidences of age-related diseases such as Alzheimer’s disease (AD). AD is the sixth leading cause of death, and one of the leading causes of dementia amongst the aged population in the USA. It is a progressive neurodegenerative disorder, characterized by the prevalence of extracellular Aβ plaques and intracellular neurofibrillary tangles, derived from the proteolysis of the amyloid precursor protein (APP) and the hyperphosphorylation of microtubule-associated protein tau, respectively. Despite years of extensive research, the molecular mechanisms that underlie the pathology of AD remain unclear. Model organisms, such as the nematode, Caenorhabditis elegans, present a complementary approach to addressing these questions. C. elegans has many advantages as a model system to study AD and other neurodegenerative diseases. Like their mammalian counterparts, they have complex biochemical pathways, most of which are conserved. Genes in which mutations are correlated with AD have counterparts in C. elegans, including an APP-related gene, apl-1, a tau homolog, ptl-1, and presenilin homologs, such as sel-12 and hop-1. Since the neuronal connectivity in C. elegans has already been established, C. elegans is also advantageous in modeling learning and memory impairments seen during AD. This article addresses the insights C. elegans provide in studying AD and other neurodegenerative diseases. Additionally, we explore the advantages and drawbacks associated with using this model.

Toward a unified model of developmental timing: A "molting" approach

Animal development requires temporal coordination between recurrent processes and sequential events, but the underlying timing mechanisms are not yet understood. The molting cycle of C. elegans provides an ideal system to study this basic problem. We recently characterized LIN-42, which is related to the circadian clock protein PERIOD, as a key component of the developmental timer underlying rhythmic molting cycles. In this context, LIN-42 coordinates epithelial stem cell dynamics with progression of the molting cycle. Repeated actions of LIN-42 may enable the reprogramming of seam cell temporal fates, while stage-specific actions of LIN-42 and other heterochronic genes select fates appropriate for upcoming, rather than passing, life stages. Here, we discuss the possible configuration of the molting timer, which may include interconnected positive and negative regulatory loops among lin-42, conserved nuclear hormone receptors such as NHR-23 and -25, and the let-7 family of microRNAs. Physiological and environmental conditions may modulate the activities of particular components of this molting timer. Finding that LIN-42 regulates both a sleep-like behavioral state and epidermal stem cell dynamics further supports the model of functional conservation between LIN-42 and mammalian PERIOD proteins. The molting timer may therefore represent a primitive form of a central biological clock and provide a general paradigm for the integration of rhythmic and developmental processes.

Physiological functions of APP family proteins

Biochemical and genetic evidence establishes a central role of the amyloid precursor protein (APP) in Alzheimer disease (AD) pathogenesis. Biochemically, deposition of the β-amyloid (Aβ) peptides produced from proteolytic processing of APP forms the defining pathological hallmark of AD; genetically, both point mutations and duplications of wild-type APP are linked to a subset of early onset of familial AD (FAD) and cerebral amyloid angiopathy. As such, the biological functions of APP and its processing products have been the subject of intense investigation, and the past 20+ years of research have met with both excitement and challenges. This article will review the current understanding of the physiological functions of APP in the context of APP family members.

Redundancy and divergence in the amyloid precursor protein family

Gene duplication provides genetic material required for functional diversification. An interesting example is the amyloid precursor protein (APP) protein family. The APP gene family has experienced both expansion and contraction during evolution. The three mammalian members have been studied quite extensively in combined knock out models. The underlying assumption is that APP, amyloid precursor like protein 1 and 2 (APLP1, APLP2) are functionally redundant. This assumption is primarily supported by the similarities in biochemical processing of APP and APLPs and on the fact that the different APP genes appear to genetically interact at the level of the phenotype in combined knockout mice. However, unique features in each member of the APP family possibly contribute to specification of their function. In the current review, we discuss the evolution and the biology of the APP protein family with special attention to the distinct properties of each homologue. We propose that the functions of APP, APLP1 and APLP2 have diverged after duplication to contribute distinctly to different neuronal events. Our analysis reveals that APLP2 is significantly diverged from APP and APLP1.

Advances with RNA interference in Alzheimer's disease research

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characterized clinically by memory and cognitive dysfunction. Unfortunately, there is no effective therapeutic method for AD treatment or ways to halt disease progression. Many mechanisms are involved in the disease, including genes mutation and protein dysfunction. RNA interference (RNAi) technology may potentially be able to control AD. It can inhibit the protein expression of specific genes by activating a sequence-specific RNA degradation process. This is a powerful tool with which to study gene function, investigate the mechanism of the disease, and validate drug targets. In this review, we highlight the advances in RNAi technology in the investigation and treatment of AD.

Worms under stress: C. elegans stress response and its relevance to complex human disease and aging

Many organisms have stress response pathways, components of which share homology with players in complex human disease pathways. Research on stress response in the nematode worm Caenorhabditis elegans has provided detailed insights into the genetic and molecular mechanisms underlying complex human diseases. In this review we focus on four different types of environmental stress responses - heat shock, oxidative stress, hypoxia, and osmotic stress - and on how these can be used to study the genetics of complex human diseases. All four types of responses involve the genetic machineries that underlie a number of complex human diseases such as cancer and neurodegenerative diseases, including Alzheimer's and Parkinson's. We highlight the types of stress response experiments required to detect the genes and pathways underlying human disease and suggest that studying stress biology in worms can be translated to understanding human disease and provide potential targets for drug discovery.

Steroid regulation of C. elegans diapause, developmental timing, and longevity

Hormones play a critical role in driving major stage transitions and developmental timing events in many species. In the nematode C. elegans the steroid hormone receptor, DAF-12, works at the confluence of pathways regulating developmental timing, stage specification, and longevity. DAF-12 couples environmental and physiologic signals to life history regulation, and it is embedded in a rich architecture governing diverse processes. Here, we highlight the molecular insights, extraordinary circuitry, and signaling pathways governing life stage transitions in the worm and how they have yielded fundamental insights into steroid regulation of biological time.

Pan-neuronal expression of APL-1, an APP-related protein, disrupts olfactory, gustatory, and touch plasticity in Caenorhabditis elegans

Patients with Alzheimer's disease show age-related cognitive decline. Postmortem autopsy of their brains shows the presence of large numbers of senile plaques, whose major component is the β-amyloid peptide. The β-amyloid peptide is a cleavage product of the amyloid precursor protein (APP). In addition to the neurodegeneration associated with β-amyloid aggregation in Alzheimer's disease patients, mutations in APP in mammalian model organisms have also been shown to disrupt several behaviors independent of visible amyloid plaque formation. However, the pathways in which APP function are unknown and difficult to unravel in mammals. Here we show that pan-neuronal expression of APL-1, the Caenorhabditis elegans ortholog of APP, disrupts several behaviors, such as olfactory and gustatory learning behavior and touch habituation. These behaviors are mediated by distinct neural circuits, suggesting a broad impact of APL-1 on sensory plasticity in C. elegans. Furthermore, we found that disruption of these three behaviors requires activity of the TGFβ pathway and reduced activity of the insulin pathway. These results suggest pathways and molecular components that may underlie behavioral plasticity in mammals and in patients with Alzheimer's disease.

Alzheimer's disease models and functional genomics-How many needles are there in the haystack?

Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD) are complex human brain disorders that affect an increasing number of people worldwide. With the identification first of the proteins that aggregate in AD and FTLD brains and subsequently of pathogenic gene mutations that cause their formation in the familial cases, the foundation was laid for the generation of animal models. These recapitulate essential aspects of the human conditions; expression of mutant forms of the amyloid-β protein-encoding APP gene in mice reproduces amyloid-β (Aβ) plaque formation in AD, while that of mutant forms of the tau-encoding microtubule-associated protein tau (MAPT) gene reproduces tau-containing neurofibrillary tangle formation, a lesion that is also prevalent in FTLD-Tau. The mouse models have been complemented by those in lower species such as C. elegans or Drosophila, highlighting the crucial role for Aβ and tau in human neurodegenerative disease. In this review, we will introduce selected AD/FTLD models and discuss how they were instrumental, by identifying deregulated mRNAs, miRNAs and proteins, in dissecting pathogenic mechanisms in neurodegenerative disease. We will discuss some recent examples, which includes miRNA species that are specifically deregulated by Aβ, mitochondrial proteins that are targets of both Aβ and tau, and the nuclear splicing factor SFPQ that accumulates in the cytoplasm in a tau-dependent manner. These examples illustrate how a functional genomics approach followed by a careful validation in experimental models and human tissue leads to a deeper understanding of the pathogenesis of AD and FTLD and ultimately, may help in finding a cure.

The kinesin-3 family motor KLP-4 regulates anterograde trafficking of GLR-1 glutamate receptors in the ventral nerve cord of Caenorhabditis elegans

The transport of glutamate receptors from the cell body to synapses is essential during neuronal development and may contribute to the regulation of synaptic strength in the mature nervous system. We previously showed that cyclin-dependent kinase-5 (CDK-5) positively regulates the abundance of GLR-1 glutamate receptors at synapses in the ventral nerve cord (VNC) of Caenorhabditis elegans. Here we identify a kinesin-3 family motor klp-4/KIF13 in a cdk-5 suppressor screen for genes that regulate GLR-1 trafficking. klp-4 mutants have decreased abundance of GLR-1 in the VNC. Genetic analysis of klp-4 and the clathrin adaptin unc-11/AP180 suggests that klp-4 functions before endocytosis in the ventral cord. Time-lapse microscopy indicates that klp-4 mutants exhibit decreased anterograde flux of GLR-1. Genetic analysis of cdk-5 and klp-4 suggests that they function in the same pathway to regulate GLR-1 in the VNC. Interestingly, GLR-1 accumulates in cell bodies of cdk-5 but not klp-4 mutants. However, GLR-1 does accumulate in klp-4-mutant cell bodies if receptor degradation in the multivesicular body/lysosome pathway is blocked. This study identifies kinesin KLP-4 as a novel regulator of anterograde glutamate receptor trafficking and reveals a cellular control mechanism by which receptor cargo is targeted for degradation in the absence of its motor.

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.

Regulation of neuronal APL-1 expression by cholesterol starvation

Background

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the deposition of β-amyloid plaques composed primarily of the amyloid-β peptide, a cleavage product of amyloid precursor protein (APP). While mutations in APP lead to the development of Familial Alzheimer's Disease (FAD), sporadic AD has only one clear genetic modifier: the ε4 allele of the apolipoprotein E (ApoE) gene. Cholesterol starvation in Caenorhabditis elegans leads to molting and arrest phenotypes similar to loss-of-function mutants of the APP ortholog, apl-1 (amyloid precursor-like protein 1), and lrp-1 (lipoprotein receptor-related protein 1), suggesting a potential interaction between apl-1 and cholesterol metabolism.

Methodology/Principal Findings

Previously, we found that RNAi knock-down of apl-1 leads to aldicarb hypersensitivity, indicating a defect in synaptic function. Here we find the same defect is recapitulated during lrp-1 knock-down and by cholesterol starvation. A cholesterol-free diet or loss of lrp-1 directly affects APL-1 levels as both lead to loss of APL-1::GFP fluorescence in neurons. However, loss of cholesterol does not affect global transcription or protein levels as seen by qPCR and Western blot.

Conclusions

Our results show that cholesterol and lrp-1 are involved in the regulation of synaptic transmission, similar to apl-1. Both are able to modulate APL-1 protein levels in neurons, however cholesterol changes do not affect global apl-1 transcription or APL-1 protein indicating the changes are specific to neurons. Thus, regulation of synaptic transmission and molting by LRP-1 and cholesterol may be mediated by their ability to control APL-1 neuronal protein expression.

Amyloid precursor proteins, neural differentiation of pluripotent stem cells and its relevance to Alzheimer's disease

Alzheimer's disease (AD) is a leading cause of age-related dementia that is characterized by an extensive loss of neurons and synaptic transmission. The pathological hallmarks of AD are neurofibrillary tangles and deposition of β-amyloid (Aβ) plaques. Previous research has investigated how Aβ fragments disrupt synaptic mechanisms in the vulnerable regions of the brain. There is a tremendous potential for stem cell technology to extend upon this research, not only in terms of developing therapeutic applications, but also in modeling AD. Indeed, the advent of induced pluripotent stem cell technology has opened up exciting new avenues for generating patient and disease-specific cell lines from somatic cells that may be used to model AD. Amyloid precursor protein (APP) is a key protein in neuronal development and this article reviews the role of APP in AD. Stem cell technology offers the opportunity to make use of APP in the directed differentiation of induced pluripotent stem cells into functional neurons, a process that may help generate a model of AD and thereby facilitate an understanding of the mechanisms underlying this disease.

Caenorhabditis elegans as an experimental tool for the study of complex neurological diseases: Parkinson's disease, Alzheimer's disease and autism spectrum disorder

The nematode Caenorhabditis elegans has a very well-defined and genetically tractable nervous system which offers an effective model to explore basic mechanistic pathways that might be underpin complex human neurological diseases. Here, the role C. elegans is playing in understanding two neurodegenerative conditions, Parkinson's and Alzheimer's disease (AD), and a complex neurological condition, autism, is used as an exemplar of the utility of this model system. C. elegans is an imperfect model of Parkinson's disease because it lacks orthologues of the human disease-related genes PARK1 and LRRK2 which are linked to the autosomal dominant form of this disease. Despite this fact, the nematode is a good model because it allows transgenic expression of these human genes and the study of the impact on dopaminergic neurons in several genetic backgrounds and environmental conditions. For AD, C. elegans has orthologues of the amyloid precursor protein and both human presenilins, PS1 and PS2. In addition, many of the neurotoxic properties linked with Aβ amyloid and tau peptides can be studied in the nematode. Autism spectrum disorder is a complex neurodevelopmental disorder characterised by impairments in human social interaction, difficulties in communication, and restrictive and repetitive behaviours. Establishing C. elegans as a model for this complex behavioural disorder is difficult; however, abnormalities in neuronal synaptic communication are implicated in the aetiology of the disorder. Numerous studies have associated autism with mutations in several genes involved in excitatory and inhibitory synapses in the mammalian brain, including neuroligin, neurexin and shank, for which there are C. elegans orthologues. Thus, several molecular pathways and behavioural phenotypes in C. elegans have been related to autism. In general, the nematode offers a series of advantages that combined with knowledge from other animal models and human research, provides a powerful complementary experimental approach for understanding the molecular mechanisms and underlying aetiology of complex neurological diseases.

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.

Sterol regulation of metabolism, homeostasis, and development

Sterol metabolites are critical signaling molecules that regulate metabolism, development, and homeostasis. Oxysterols, bile acids (BAs), and steroids work primarily through cognate sterol-responsive nuclear hormone receptors to control these processes through feed-forward and feedback mechanisms. These signaling pathways are conserved from simple invertebrates to mammals. Indeed, results from various model organisms have yielded fundamental insights into cholesterol and BA homeostasis, lipid and glucose metabolism, protective mechanisms, tissue differentiation, development, reproduction, and even aging. Here, we review how sterols act through evolutionarily ancient mechanisms to control these processes.

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.

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.