Modelling neurodegenerative diseases with 3D brain organoids

Neurodegenerative diseases are incurable and debilitating conditions characterized by the deterioration of brain function. Most brain disease models rely on human post‐mortem brain tissue, non‐human primate tissue, or in vitro two‐dimensional (2D) experiments. Resource limitations and the complexity of the human brain are some of the reasons that make suitable human neurodegenerative disease models inaccessible. However, recently developed three‐dimensional (3D) brain organoids derived from pluripotent stem cells (PSCs), including embryonic stem cells and induced PSCs, may provide suitable models for the study of the pathological features of neurodegenerative diseases. In this review, we provide an overview of existing 3D brain organoid models and discuss recent advances in organoid technology that have increased our understanding of brain development. Moreover, we explain how 3D organoid models recapitulate aspects of specific neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease, and explore the utility of these models, for therapeutic applications.     

Manganese homeostasis in the nervous system

Manganese (Mn) is an essential heavy metal that is naturally found in the environment. Daily intake through dietary sources provides the necessary amount required for several key physiological processes, including antioxidant defense, energy metabolism, immune function and others. However, overexposure from environmental sources can result in a condition known as manganism that features symptomatology similar to Parkinson's disease (PD). This disorder presents with debilitating motor and cognitive deficits that arise from a neurodegenerative process. In order to maintain a balance between its essentiality and neurotoxicity, several mechanisms exist to properly buffer cellular Mn levels. These include transporters involved in Mn uptake, and newly discovered Mn efflux mechanisms. This review will focus on current studies related to mechanisms underlying Mn import and export, primarily the Mn transporters, and their function and roles in Mn‐induced neurotoxicity.

     

Drosophila as a screening tool to study human neurodegenerative diseases

In an aging society, research involving neurodegenerative disorders is of paramount importance. Over the past few years, research on Alzheimer's and Parkinson's diseases has made tremendous progress. Experimental studies, however, rely mostly on transgenic animal models, preferentially using mice. Although experiments on mice have enormous advantages, they also have some inherent limitations, some of which can be overcome by the use of Drosophila melanogaster as an experimental animal. Among the major advantages of using the fly is its small genome, which can also be modified very easily. The fact that its genome lends itself to diverse alterations (e. g. mutagenesis, transposons) has made the fly a useful organism to perform large‐scale and genome‐wide screening approaches. This has opened up an entirely new field of experimental research aiming to elucidate genetic interactions and screen for modifiers of disease processes in vivo. Here, we provide a brief overview of how flies can be used to analyze molecular mechanisms underlying human neurodegenerative diseases.     

Autophagy and human diseases

Autophagy is a major intracellular degradative process that delivers cytoplasmic materials to the lysosome for degradation. Since the discovery of autophagy-related (Atg) genes in the 1990s, there has been a proliferation of studies on the physiological and pathological roles of autophagy in a variety of autophagy knockout models. However, direct evidence of the connections between ATG gene dysfunction and human diseases has emerged only recently. There are an increasing number of reports showing that mutations in the ATG genes were identified in various human diseases such as neurodegenerative diseases, infectious diseases, and cancers. Here, we review the major advances in identification of mutations or polymorphisms of the ATG genes in human diseases. Current autophagy-modulating compounds in clinical trials are also summarized.     

Understanding the importance of autophagy in human diseases using Drosophila

Autophagy is a lysosome-dependent intracellular degradation pathway that has been implicated in the pathogenesis of various human diseases, either positively or negatively impacting disease outcomes depending on the specific context. The majority of medical conditions including cancer, neurodegenerative diseases, infections and immune system disorders and inflammatory bowel disease could probably benefit from therapeutic modulation of the autophagy machinery. Drosophila represents an excellent model animal to study disease mechanisms thanks to its sophisticated genetic toolkit, and the conservation of human disease genes and autophagic processes. Here, we provide an overview of the various autophagy pathways observed both in flies and human cells(macroautophagy, microautophagy and chaperone-mediated autophagy), and discuss Drosophila models of the above-mentioned diseases where fly research has already helped to understand how defects in autophagy genes and pathways contribute to the relevant pathomechanisms.     

Glycomics of pediatric and adulthood diseases of the central nervous system

Glycosylation consists in the covalent linkage of a carbohydrate structure to membrane bound and secreted glycoconjugates. It is a common post-translational modification that serves multiple functions in cell differentiation, signaling and intercellular communication. Unlike DNA/RNA/protein, the addition of complex carbohydrates is not-template driven and it is conceivable that both genetics and environmental factors might interact to influence glycosylation machinery in several pathological processes. Over the last few decades, the recognition of Congenital Disorders of Glycosylation (CDG) as an increasing number of genetic diseases of glycosylation with almost constant nervous system involvement, dramatically illustrated the consequences of abnormal glycosylation as improper CNS development and function. In addition, CDG recognition contributed to postulate that aberrant glycosylation processes might play a role in multifactorial, complex CNS diseases. On this context, CNS glycomics explores the effects of possible aberrant glycosylation to identify potential glyco-biomarkers useful for the diagnosis and ultimately for potential intervention strategies in neurological diseases. Up to date, CNS glycomics is an emerging, still uncharted area because of the specificity of CNS glycosylation, the complexity of the neurological disorders and for the inaccessibility and invasiveness of disease relevant samples. Here we review current knowledge on clinical glycomics of nervous system diseases, starting with CDG to include those pediatric and adulthood neuropsychiatric diseases where some evidences suggest that multifactor determinants converge to dysregulate glycosylation. Conventional and mass spectrometry-based high throughput technology for glyco-biomarker detection in CNS diseases is reported.     

Vitamin C is a source of oxoaldehyde and glycative stress in age‐related cataract and neurodegenerative diseases

Oxoaldehyde stress has recently emerged as a major source of tissue damage in aging and age‐related diseases. The prevailing mechanism involves methylglyoxal production during glycolysis and modification of arginine residues through the formation of methylglyoxal hydroimidazolones (MG‐H1). We now tested the hypothesis that oxidation of vitamin C (ascorbic acid or ASA) contributes to this damage when the homeostatic redox balance is disrupted especially in ASA‐rich tissues such as the eye lens and brain. MG‐H1 measured by liquid chromatography mass spectrometry is several fold increased in the lens and brain from transgenic mice expressing human vitamin C transporter 2 (hSVCT2). Similarly, MG‐H1 levels are increased two‐ to fourfold in hippocampus extracts from individuals with Alzheimer's disease (AD), and significantly higher levels are present in sarkosyl‐insoluble tissue fractions from AD brain proteins than in the soluble fractions. Moreover, immunostaining with antibodies against methylglyoxal hydroimidazolones reveals similar increase in substantia nigra neurons from individuals with Parkinson's disease. Results from an in vitro incubation experiment suggest that accumulated catalytic metal ions in the hippocampus during aging could readily accelerate ASA oxidation and such acceleration was significantly enhanced in AD. Modeling studies and intraventricular injection of ¹³C‐labeled ASA revealed that ASA backbone carbons 4–6 are incorporated into MG‐H1 both in vitro and in vivo, likely via a glyceraldehyde precursor. We propose that drugs that prevent oxoaldehyde stress or excessive ASA oxidation may protect against age‐related cataract and neurodegenerative diseases.     

Human tissue transglutaminase is inhibited by pharmacologic and chemical acetylation

Human tissue transglutaminase (TGM2) is implicated in the pathogenesis of several neurodegenerative disorders including Alzheimer's, Parkinson's and expanded polyglutamine (polyQ) diseases. TGM2 promotes formation of soluble and insoluble high molecular weight aggregates by catalyzing a covalent linkage between peptide‐bound Q residues in polyQ proteins and a peptide‐bound Lys residue. Therapeutic approaches to modulate the activity of TGM2 are needed to proceed with studies to test the efficacy of TGM2 inhibition in disease processes. We investigated whether acetylation of Lys‐residues by sulfosuccinimidyl acetate (SNA) or aspirin (ASA) would alter the crosslinking activity of TGM2. Acetylation by either SNA and/or ASA resulted in a loss of >90% of crosslinking activity. The Lys residues that were critical for inhibition were identified by mass spectrometry as Lys⁴⁴⁴, Lys⁴⁶⁸, and Lys⁶⁶³. Hence, acetylation of Lys‐residues may modulate the enzymatic function of TGM2 in vivo and offer a novel approach to treatment of TGM2 mediated disorders.     

Regulation of autophagy by polyphenols: Paving the road for treatment of neurodegeneration

In the present paper, we will discuss on the importance of autophagy in the central nervous system, and outline the relation between autophagic pathways and the pathogenesis of neurodegenerative disorders. The potential therapeutic benefits of naturally occurring phytochemicals as pharmacological modulators of autophagy will also be addressed. Our findings provide renewed insight on the molecular modes of protection by polyphenols, which is likely to be at least in part mediated not only by their potent antioxidant and anti-inflammatory effects, but also through modulation of autophagic processes to remove the aberrant protein aggregates.     

Glycation modulates alpha-synuclein fibrillization kinetics: A sweet spot for inhibition

Glycation is a nonenzymatic posttranslational modification (PTM) known to be increased in the brains of hyperglycemic patients. Alpha-synuclein (αSN), a central player in the etiology of Parkinson’s disease, can be glycated at lysine residues, thereby reducing αSN fibril formation in vitro and modulating αSN aggregation in cells. However, the molecular basis for these effects is unclear. To elucidate this, we investigated the aggregation of αSN modified by eight glycating agents, namely the dicarbonyl compound methylglyoxal (MGO) and the sugars ribose, fructose, mannose, glucose, galactose, sucrose, and lactose. We found that MGO and ribose modify αSN to the greatest extent, and these glycation products are the most efficient inhibitors of fibril formation. We show glycation primarily inhibits elongation rather than nucleation of αSN and has only a modest effect on the level of oligomerization. Furthermore, glycated αSN is not significantly incorporated into fibrils. For both MGO and ribose, we discovered that a level of ∼5 modifications per αSN is optimal for inhibition of elongation. The remaining sugars showed a weak but optimal inhibition at ∼2 modifications per αSN. We propose that this optimal level balances the affinity for the growing ends of the fibril (which decreases with the extent of modification) with the ability to block incorporation of subsequent αSN subunits (which increases with modification). Our results are not only relevant for other αSN PTMs but also for understanding PTMs affecting other fibrillogenic proteins and may thus open novel avenues for therapeutic intervention in protein aggregation disorders.     

Effect of natural exogenous antioxidants on aging and on neurodegenerative diseases

Abstract

Aging and neurodegenerative diseases share oxidative stress cell damage and depletion of endogenous antioxidants as mechanisms of injury, phenomena that are occurring at different rates in each process. Nevertheless, as the central nervous system (CNS) consists largely of lipids and has a poor catalase activity, a low amount of superoxide dismutase and is rich in iron, its cellular components are damaged easily by overproduction of free radicals in any of these physiological or pathological conditions. Thus, antioxidants are needed to prevent the formation and to oppose the free radicals damage to DNA, lipids, proteins, and other biomolecules. Due to endogenous antioxidant defenses are inadequate to prevent damage completely, different efforts have been undertaken in order to increase the use of natural antioxidants and to develop antioxidants that might ameliorate neural injury by oxidative stress. In this context, natural antioxidants like flavonoids (quercetin, curcumin, luteolin and catechins), magnolol and honokiol are showing to be the efficient inhibitors of the oxidative process and seem to be a better therapeutic option than the traditional ones (vitamins C and E, and β-carotene) in various models of aging and injury in vitro and in vivo conditions. Thus, the goal of the present review is to discuss the molecular basis, mechanisms of action, functions, and targets of flavonoids, magnolol, honokiol and traditional antioxidants with the aim of obtaining better results when they are prescribed on aging and neurodegenerative diseases.     

Involvement of aberrant regulation of epigenetic mechanisms in the pathogenesis of Parkinson's disease and epigenetic-based therapies

Parkinson's disease (PD) is known as a progressive neurodegenerative disorder associated with the reduction of dopamine-secreting neurons and the formation of Lewy bodies in the substantia nigra and basal ganglia routes. Aging, as well as environmental and genetic factors, are considered as disease risk factors that can make PD as a complex one. Epigenetics means studying heritable changes in gene expression or function, without altering the underlying DNA sequence. Multiple studies have shown the association of epigenetic variations with onset or progression of various types of diseases. DNA methylation, posttranslational modifications of histones and presence of microRNA (miRNA) are among epigenetic processes involved in regulating pathways related to the development of PD. Unlike genetic mutations, most epigenetic variations may be reversible or preventable. Therefore, the return of aberrant epigenetic events in different cells is a growing therapeutic approach to treatment or prevention. Currently, there are several methods for treating PD patients, the most important of which are drug therapies. However, detection of genes and epigenetic mechanisms involved in the disease can develop appropriate diagnosis and treatment of the disease before the onset of disabilities and resulting complications. The main purpose of this study was to review the most important epigenetic molecular mechanisms, epigenetic variations in PD, and epigenetic-based therapies.