The cytosolic loop of the gamma-secretase component presenilin enhancer 2 protects zebrafish embryos from apoptosis

Notch pathway is required for protection against heat stress in spermatogonial stem cells in medaka

Gamete production is a fundamental process for reproduction; however, exposure to stress, such as increased environmental temperature, can decrease or even interrupt this process, affecting fertility. Thus, the survival of spermatogonial stem cells (SSCs) is crucial for the recovery of spermatogenesis upon stressful situations. Here, we show that the Notch pathway is implicated in such survival, by protecting the SSCs against thermal stress. First, we corroborated the impairment of spermatogenesis under heat stress in medaka, observing an arrest in metaphase I at 10 days of heat treatment, an increase in the number of spermatocytes, and downregulation of ndrg1b and sycp3. In addition, at 30 days of treatment, an interruption of spermatogenesis was observed with a strong loss of spermatocytes and spermatids. Then, the exposure of adult males to thermal stress condition induced apoptosis mainly in spermatogenic and supporting somatic cells, with the exception of the germinal region, where SSCs are located. Concomitantly, the Notch pathway-related genes were upregulated, including the ligands (dll4, jag1-2) and receptors (notch1a-3). Moreover, during thermal stress presenilin enhancer-2 (pen-2), the catalytic subunit of γ-secretase complex of the Notch pathway was restricted to the germinal region of the medaka testis, observed in somatic cells surrounding type A spermatogonia (SGa). The importance of Notch pathway was further supported by an ex vivo approach, in which the inhibition of this pathway activity induced a loss of SSCs. Overall, this study supports the importance of Notch pathways for the protection of SSCs under chronic thermal stress.

Diflubenzuron Induces Cardiotoxicity in Zebrafish Embryos

Diflubenzuron is an insecticide that serves as a chitin inhibitor to restrict the growth of many harmful larvae, including mosquito larvae, cotton bollworm and flies. The residue of diflubenzuron is often detected in aquaculture, but its potential toxicity to aquatic organisms is still obscure. In this study, zebrafish embryos (from 6 h to 96 h post-fertilization, hpf) were exposed to different concentrations of diflubenzuron (0, 0.5, 1.5, 2.5, 3.5 and 4.5 mg/L), and the morphologic changes, mortality rate, hatchability rate and average heart rate were calculated. Diflubenzuron exposure increased the distance between the venous sinus and bulbar artery (SV-BA), inhibited proliferation of myocardial cells and damaged vascular development. In addition, diflubenzuron exposure also induced contents of reactive oxygen species (ROS) and malondialdehyde (MDA) and inhibited the activity of antioxidants, including SOD (superoxide dismutase) and CAT (catalase). Moreover, acridine orange (AO) staining showed that diflubenzuron exposure increased the apoptotic cells in the heart. Q-PCR also indicated that diflubenzuron exposure promoted the expression of apoptosis-related genes (bax, bcl2, p53, caspase3 and caspase9). However, the expression of some heart-related genes were inhibited. The oxidative stress-induced apoptosis damaged the cardiac development of zebrafish embryos. Therefore, diflubenzuron exposure induced severe cardiotoxicity in zebrafish embryos. The results contribute to a more comprehensive understanding of the safety use of diflubenzuron.

Neuron-specific deletion of presenilin enhancer2 causes progressive astrogliosis and age-related neurodegeneration in the cortex independent of the Notch signaling

Introduction

Presenilin enhancer2 (Pen‐2) is an essential subunit of γ‐secretase, which is a key protease responsible for the cleavage of amyloid precursor protein (APP) and Notch. Mutations on Pen‐2 cause familial Alzheimer disease (AD). However, it remains unknown whether Pen‐2 regulates neuronal survival and neuroinflammation in the adult brain.

Methods

Forebrain neuron‐specific Pen‐2 conditional knockout (Pen‐2 cKO) mice were generated for this study. Pen‐2 cKO mice expressing Notch1 intracellular domain (NICD) conditionally in cortical neurons were also generated.

Results

Loss of Pen‐2 causes astrogliosis followed by age‐dependent cortical atrophy and neuronal loss. Loss of Pen‐2 results in microgliosis and enhanced inflammatory responses in the cortex. Expression of NICD in Pen‐2 cKO cortices ameliorates neither neurodegeneration nor neuroinflammation.

Conclusions

Pen‐2 is required for neuronal survival in the adult cerebral cortex. The Notch signaling may not be involved in neurodegeneration caused by loss of Pen‐2.

Effects of lincomycin hydrochloride on the neurotoxicity of zebrafish

Lincomycin hydrochloride is one of the commonly used drugs in clinic. However, it has many side effects on patients, and its mechanism is still poorly understood. In this study, 6 h post-fertilization (6 hpf) zebrafish embryos were exposed to several concentrations of lincomycin hydrochloride (15, 30, 60 μg/mL) for up to 24 or 96 hpf to detect their developmental toxicity and neurotoxicity, and to 6 days post-fertilization (6 dpf) to detect their behavioral toxicity. Our results showed that lincomycin hydrochloride could lead to embryonic head deformities (unclear ventricles, smaller ventricles, fewer new neurons). The studies showed that the frequency of spontaneous tail flick of zebrafish embryo increased at 24 hpf, and the lincomycin hydrochloride exposed zebrafish embryos showed increased heart rate, shorter body length, and yolk sac edema with severe pericardial edema at 96 hpf. The studies also showed that lincomycin hydrochloride increased oxidative stress level, Acetylcholinesterase (AChE) activity, ATPase activity and apoptosis in zebrafish larvae. In addition, the swimming behavior of zebrafish larvae decreased with the increase of lincomycin hydrochloride concentration, but the angular velocity and meandering degree increased, which might be due to the decreased activity of AChE and ATPase, as well as the decreased expression of genes related to neurodevelopment and neurotransmitter system, leading to the change of their motor behaviors. In summary, we found that lincomycin hydrochloride induced developmental toxicity and neurotoxicity in zebrafish larvae, contributing to a more comprehensive evaluation of the safety of the drug.

γ-Secretase and its modulators: Twenty years and beyond

Twenty years ago, Wolfe, Xia, and Selkoe identified two aspartate residues in Alzheimer's presenilin protein that constitute the active site of the γ-secretase complex. Mutations in the genes encoding amyloid precursor protein (APP) or presenilin (PS) cause early onset familial Alzheimer's disease (AD), and sequential cleavages of the APP by β-secretase and γ-secretase/presenilin generate amyloid β protein (Aβ), the major component of pathological hallmark, neuritic plaques, in brains of AD patients. Therapeutic strategies centered on targeting γ-secretase/presenilin to reduce amyloid were implemented and led to several high profile clinical trials. This review article focuses on the studies of γ-secretase and its inhibitors/modulators since the discovery of presenilin as the γ-secretase. While a lack of complete understanding of presenilin biology renders failure of clinical trials, the lessons learned from some γ-secretase modulators, while premature for human testing, provide new directions to develop potential therapeutics. Imbalanced Aβ homeostasis is an upstream event of neurodegenerative processes. Exploration of γ-secretase modulators for their roles in these processes is highly significant, e.g., decreasing neuroinflammation and levels of phosphorylated tau, the component of the other AD pathological hallmark, neurofibrillary tangles. Agents with excellent human pharmacology hold great promise in suppressing neurodegeneration in pre-symptomatic or early stage AD patients.

SiO2 nanoparticles change colour preference and cause Parkinson's-like behaviour in zebrafish

With advances in the development of various disciplines, there is a need to decipher bio-behavioural mechanisms via interdisciplinary means. Here, we present an interdisciplinary study of the role of silica nanoparticles (SiO2-NPs) in disturbing the neural behaviours of zebrafish and a possible physiological mechanism for this phenomenon. We used adult zebrafish as an animal model to evaluate the roles of size (15-nm and 50-nm) and concentration (300 μg/mL and 1000 μg/mL) in SiO2-NP neurotoxicity via behavioural and physiological analyses. With the aid of video tracking and data mining, we detected changes in behavioural phenotypes. We found that compared with 50-nm nanosilica, 15-nm SiO2-NPs produced greater significant changes in advanced cognitive neurobehavioural patterns (colour preference) and caused potentially Parkinson's disease-like behaviour. Analyses at the tissue, cell and molecular levels corroborated the behavioural results, demonstrating that nanosilica acted on the retina and dopaminergic (DA) neurons to change colour preference and to cause potentially Parkinson's disease-like behaviour.

Zebrafish as a model to understand autophagy and its role in neurological disease

In the past decade, the zebrafish (Danio rerio) has become a popular model system for the study of vertebrate development, since the embryos and larvae of this species are small, transparent and undergo rapid development ex utero, allowing in vivo analysis of embryogenesis and organogenesis. These characteristics can also be exploited by researchers interested in signaling pathways and disease processes and, accordingly, there is a growing literature on the use of zebrafish to model human disease. This model holds great potential for exploring how autophagy, an evolutionarily conserved mechanism for protein degradation, influences the pathogeneses of a range of different human diseases and for the evaluation of this pathway as a potential therapeutic strategy. Here we summarize what is known about the regulation of autophagy in eukaryotic cells and its role in neurodegenerative disease and highlight how research using zebrafish has helped further our understanding of these processes.

Presenilin mouse and zebrafish models for dementia: focus on neurogenesis

Autosomal dominant mutations in the presenilin gene PSEN cause familial Alzheimer's disease (AD), a neurological disorder pathologically characterized by intraneuronal accumulation and extracellular deposition of amyloid-β in plaques and intraneuronal, hyperphosphorylated tau aggregation in neurofibrillary tangles. Presenilins (PS/PSENs) are part of the proteolytic γ-secretase complex, which cleaves substrate proteins within the membrane. Cleavage of the amyloid precursor protein (APP) by γ-secretase releases amyloid-β peptides. Besides its role in the processing of APP and other transmembrane proteins, presenilin plays an important role in neural progenitor cell maintenance and neurogenesis. In this review, we discuss the role of presenilin in relation to neurogenesis and neurodegeneration and review the currently available presenilin animal models. In addition to established mouse models, zebrafish are emerging as an attractive vertebrate model organism to study the role of presenilin during the development of the nervous system and in neurodegenerative disorders involving presenilin. Zebrafish is a suitable model organism for large-scale drug screening, making this a valuable model to identify novel therapeutic targets for AD.

Proteomics profiling of single organs from individual adult zebrafish

The model organism zebrafish (Danio rerio) is extensively utilized in studies of developmental biology but is also being investigated in the context of a growing list of human age-related diseases. To facilitate such studies, we here present protein expression patterns of adult zebrafish organs, including blood, brain, fin, heart, intestine, liver, and skeletal muscle. Protein extracts were prepared from the different organs of two zebrafish and analyzed using liquid chromatography coupled to high-resolution tandem mass spectrometry. Zebrafish tissue was digested directly after minimal fractionation and cleaned up (the shotgun approach). Proteins were identified using Mascot software. In total, 1394 proteins were identified of which 644 were nonredundant. Of these, 373 demonstrated an organ-specific expression pattern and 57 had not been shown on protein level before. These data emphasize the need for increased research at the protein level to facilitate the selection of candidate proteins for targeted quantification and to refine systematic genetic network analysis in vertebrate development, biology, and disease.

Zebrafish as a tool in Alzheimer's disease research

Alzheimer's disease is the most prevalent form of neurodegenerative disease. Despite many years of intensive research our understanding of the molecular events leading to this pathology is far from complete. No effective treatments have been defined and questions surround the validity and utility of existing animal models. The zebrafish (and, in particular, its embryos) is a malleable and accessible model possessing a vertebrate neural structure and genome. Zebrafish genes orthologous to those mutated in human familial Alzheimer's disease have been defined. Work in zebrafish has permitted discovery of unique characteristics of these genes that would have been difficult to observe with other models. In this brief review we give an overview of Alzheimer's disease and transgenic animal models before examining the current contribution of zebrafish to this research area. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

Aspects of beta-amyloid as a biomarker for Alzheimer's disease

Alzheimer's disease is an age-related neurodegenerative disorder that results in progressive cognitive impairment and death. The accumulation of beta-amyloid (Abeta) in specific brain regions is believed by many to represent the earliest event in the pathogenesis of the disease. Here, we review the key aspects of Abeta as a biomarker for Alzheimer's disease, including the pathogenicity of Abeta, the possible biological functions of its precursor protein, the Abeta metabolism and homeostasis, the diagnostic performance of different Abeta assays in different settings and the potential usefulness of Abeta as a surrogate marker for treatment efficacy in clinical trials of novel Abeta-targeting drugs against Alzheimer's disease.

In vivo manifestation of Notch related phenotypes in zebrafish treated with Alzheimer's amyloid reducing gamma-secretase inhibitors

Gamma-secretase is responsible for the final cleavage of amyloid precursor protein to generate the amyloid-beta protein, the major component of plaques in the brains of Alzheimer's disease patients. gamma-Secretase inhibitors (GSI) have been explored for therapeutic inhibition of amyloid beta protein generation, but mechanistic toxicity has been documented because of its blockage of gamma-secretase cleavage of several dozens of substrates including Notch. This becomes the primary obstacle for most inhibitors during the pre-clinical development and the main concern for several compounds in the clinical trials. To predict potential side effects related to Notch signaling, we examined global effect of GSIs in vertebrate animal zebrafish. We have used two potent GSIs (GSI A and GSI 18) with a sub-microM effective concentration for 50% amyloid beta protein inhibition (EC(50)). Zebrafish embryos were treated with GSI A, 18 or a well characterized GSI N-[N-(3,5-difluorophenacetyl-L-alanyl)]-S-phenylglycine t-butyl ester (DAPT), and transparent animals were examined for up to 7 days. GSI A had less abnormal phenotype in zebrafish, compared to GSI 18-treated embryos that displayed curved tails, a loss of pigmentation, and reduced swim bladder and heart rate. To understand mechanistic effect at the molecular level, we examined Notch signaling in these GSI-treated zebrafish. Notch phenotypes were observed in embryos treated with 50 and 10 microM GSI 18, but not with 10 microM GSI A. In accordance, in situ hybridization with a probe against Notch target gene her6 showed a weaker staining in embryos treated with 10 microM GSI 18 than those treated with 10 microM GSI A. In conclusion, phenotypic profile in whole animals offers important information on Notch related pathways and provides prediction of safe compounds during early development stages of therapeutic GSIs.

A novel PSENEN mutation in a patient with complaints of memory loss and a family history of dementia

Presenilin enhancer-2 (PSENEN) is a fundamental component of the gamma-secretase protein complex involved in beta-amyloid precursor protein (beta APP) processing, a key event in Alzheimer's disease (AD) etiopathogenesis. In a mild cognitive impairment (MCI)-diagnosed woman, belonging to a family with a positive history for AD, we found that a novel PSENEN mutation (S73F) was the only genetic alteration of relevance. The mutation was absent in 253 age-matched controls. In an attempt to learn the biochemical effects of this mutation, we cultured skin primary fibroblasts from the patient and her daughter, and we assessed A beta(1-40) and Abeta(1-42) production. We did not find any relevant differences in comparison to age-matched, normal subjects. Although our data do not definitively support a pathogenetic role for this mutation, it does not appear to be a common polymorphism. Further follow-up is warranted in this family.

BACE and gamma-secretase characterization and their sorting as therapeutic targets to reduce amyloidogenesis

Secretases are named for enzymes processing amyloid precursor protein (APP), a prototypic type-1 membrane protein. This led directly to discovery of novel Aspartyl proteases (beta-secretases or BACE), a tetramer complex gamma-secretase (gamma-SC) containing presenilins, nicastrin, aph-1 and pen-2, and a new role for metalloprotease(s) of the ADAM family as a alpha-secretases. Recent advances in defining pathways that mediate endosomal-lysosomal-autophagic-exosomal trafficking now provide targets for new drugs to attenuate abnormal production of fibril forming products characteristic of AD. A key to success includes not only characterization of relevant secretases but mechanisms for sorting and transport of key metabolites to abnormal vesicles or sites for assembly of fibrils. New developments we highlight include an important role for an 'early recycling endosome' coated in retromer complex containing lipoprotein receptor LRP-II (SorLA) for switching APP to a non-amyloidogenic pathway for alpha-secretases processing, or to shuttle APP to a 'late endosome compartment' to form Abeta or AICD. LRP11 (SorLA) is of particular importance since it decreases in sporadic AD whose etiology otherwise is unknown.

An automated method for cell detection in zebrafish

Quantification of cells is a critical step towards the assessment of cell fate in neurological disease or developmental models. Here, we present a novel cell detection method for the automatic quantification of zebrafish neuronal cells, including primary motor neurons, Rohon-Beard neurons, and retinal cells. Our method consists of four steps. First, a diffused gradient vector field is produced. Subsequently, the orientations and magnitude information of diffused gradients are accumulated, and a response image is computed. In the third step, we perform non-maximum suppression on the response image and identify the detection candidates. In the fourth and final step the detected objects are grouped into clusters based on their color information. Using five different datasets depicting zebrafish cells, we show that our method consistently displays high sensitivity and specificity of over 95%. Our results demonstrate the general applicability of this method to different data samples, including nuclear staining, immunohistochemistry, and cell death detection.

Neurosecretases provide strategies to treat sporadic and familial Alzheimer disorders

Recent discoveries on neurosecretases and their trafficking to release fibril-forming neuropeptides or other products, are of interest to pathology, cell signaling and drug discovery. Nomenclature arose from the use of amyloid precursor protein (APP) as a prototypic type-1 substrate leading to the isolation of beta-secretase (BACE), multimeric complexes (gamma-secretase, gamma-SC) for intramembranal cleavage, and attributing a new function to well-characterized metalloproteases of the ADAM family (alpha-secretase) for normal APP turnover. While purified alpha/beta-secretases facilitate drug discovery, gamma-SC presents greater challenges for characterization and mechanisms of catalysis. The review comments on links between mutation or polymorphisms in relation to enzyme mechanisms and disease. The association between lipoprotein receptor LRP11 variants and sporadic Alzheimer's disease (SAD) offers scope to integrate components of pre- and post-Golgi membranes, or brain clathrin-coated vesicles within pathways for trafficking as targets for intervention. The presence of APP and metabolites in brain clathrin-coated vesicles as significant cargo with lipoproteins and adaptors focuses attention as targets for therapeutic intervention. This overview emphasizes the importance to develop new therapies targeting neurosecretases to treat a major neurological disorder that has vast economic and social implications.

Zebrafish as a powerful vertebrate model system for in vivo studies of cell death

Understanding and manipulating cell death pathways are critical to our ability to treat human degenerative diseases and cancer. The zebrafish Danio rerio, a common aquatic pet, has evolved as a powerful tool for the discovery of genes regulating cellular suicide both during normal vertebrate development and after genetic or environmental insult. In this review, we describe the techniques that can be applied to studying cell death in zebrafish as well as highlighting what has been discovered so far. Finally, we discuss future perspectives in the field and how they relate to human disease.

A Two Decade Contribution of Molecular Cell Biology to the Centennial of Alzheimer's Disease: Are We Progressing Toward Therapy?

Alzheimer's disease (AD), described for the first time 100 years ago, is a neurodegenerative disease characterized by two neuropathological hallmarks: neurofibrillary tangles containing hyperphosphorylated tau and senile plaques. These lesions are likely initiated by an imbalance between production and clearance of amyloid beta, leading to increased oligomerization of these peptides, formation of amyloid plaques in the brain of the patient, and final dementia. Amyloid beta is generated from amyloid precursor protein (APP) by subsequent beta- and gamma-secretase cleavage, the latter being a multiprotein complex consisting of presenilin-1 or -2, nicastrin, APH-1, and PEN-2. Alternatively, APP can be cleaved by alpha- and gamma-secretase, precluding the production of Abeta. In this review, we discuss the major breakthroughs during the past two decades of molecular cell biology and the current genetic and cell biological state of the art on APP proteolysis, including structure-function relationships and subcellular localization. Finally, potential directions for cell biological research toward the development of AD therapies are briefly discussed.