Protein citrullination marks myelin protein aggregation and disease progression in mouse ALS models

Molecular Mechanisms of Neurofilament Alterations and its Application in Assessing Neurodegenerative Disorders

Neurofilaments are intermediate filaments present in neurons. These provide structural support and maintain the size and shape of the neurons. Dysregulation, mutation, and aggregation of neurofilaments raise the levels of these proteins in the blood and cerebrospinal fluid, which are characteristic features of axonal damage and certain rare neurological diseases, such as Giant axonal neuropathy and Charcot-Mare-Tooth disease. Understanding the structure, dynamics, and function of neurofilaments has been greatly enhanced by a diverse range of biochemical and preclinical investigations conducted over more than four decades. Recently, there has been a resurgence of interest in post-translational modifications of neurofilaments, such as phosphorylation, aggregation, mutation, oxidation, etc. Over the past twenty years, several rare disorders have been studied from structural alterations of neurofilaments. These disorders are monitored by fluid biomarkers such as neurofilament light chains. Currently, there are many tools, such as Enzyme-Linked Immunosorbent Assay, Electrochemiluminescence Assay, Single-Molecule Array, Western/immunoblotting, etc., in use to assess the neurofilament proteins in Blood and CSF. However, all these techniques utilize expensive, non-specific, or antibody-based methods, which make them unsuitable for routine screening of neurodegenerative disorders. This provides room to search for newer sensitive, cost-effective, point-of-care tools for rapid screening of the disease. For a long time, the molecular mechanisms of neurofilaments have been poorly understood due to insufficient research attempts, and a deeper understanding of them remains elusive. Therefore, this review aims to highlight the available literature on molecular mechanisms of neurofilaments and the function of neurofilaments in axonal transport, axonal conduction, axonal growth, and neurofilament aggregation, respectively. Further, this review discusses the role of neurofilaments as potential biomarkers for the identification of several neurodegenerative diseases in clinical laboratory practice.

Brain-Region-Specific Differences in Protein Citrullination/Deimination in a Pre-Motor Parkinson's Disease Rat Model

The detection of early molecular mechanisms and potential biomarkers in Parkinson's disease (PD) remains a challenge. Recent research has pointed to novel roles for post-translational citrullination/deimination caused by peptidylarginine deiminases (PADs), a family of calcium-activated enzymes, in the early stages of the disease. The current study assessed brain-region-specific citrullinated protein targets and their associated protein-protein interaction networks alongside PAD isozymes in the 6-hydroxydopamine (6-OHDA) induced rat model of pre-motor PD. Six brain regions (cortex, hippocampus, striatum, midbrain, cerebellum and olfactory bulb) were compared between controls/shams and the pre-motor PD model. For all brain regions, there was a significant difference in citrullinated protein IDs between the PD model and the controls. Citrullinated protein hits were most abundant in cortex and hippocampus, followed by cerebellum, midbrain, olfactory bulb and striatum. Citrullinome-associated pathway enrichment analysis showed correspondingly considerable differences between the six brain regions; some were overlapping for controls and PD, some were identified for the PD model only, and some were identified in control brains only. The KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways identified in PD brains only were associated with neurological, metabolic, immune and hormonal functions and included the following: "Axon guidance"; "Spinocerebellar ataxia"; "Hippo signalling pathway"; "NOD-like receptor signalling pathway"; "Phosphatidylinositol signalling system"; "Rap1 signalling pathway"; "Platelet activation"; "Yersinia infection"; "Fc gamma R-mediated phagocytosis"; "Human cytomegalovirus infection"; "Inositol phosphate metabolism"; "Thyroid hormone signalling pathway"; "Progesterone-mediated oocyte maturation"; "Oocyte meiosis"; and "Choline metabolism in cancer". Some brain-region-specific differences were furthermore observed for the five PAD isozymes (PADs 1, 2, 3, 4 and 6), with most changes in PAD 2, 3 and 4 when comparing control and PD brain regions. Our findings indicate that PAD-mediated protein citrullination plays roles in metabolic, immune, cell signalling and neurodegenerative disease-related pathways across brain regions in early pre-motor stages of PD, highlighting PADs as targets for future therapeutic avenues.

Genome-wide association analysis reveals potential genetic correlation and causality between circulating inflammatory proteins and amyotrophic lateral sclerosis

BACKGROUND

Amyotrophic Lateral Sclerosis (ALS), a fatal neurodegenerative disease, continues to elude complete comprehension of its pathological underpinnings. Recent focus on inflammation in ALS pathogenesis prompts this investigation into the genetic correlation and potential causal relationships between circulating inflammatory proteins and ALS.

METHODS

Genome-wide association study (GWAS) data encompassing 91 circulating inflammatory protein measures from 14,824 individuals of European ancestry, alongside records from 27,205 ALS cases and 110,881 controls, were employed. Assessment of genetic correlation and overlap utilized LD score regression (LDSC), high-definition likelihood (HDL), and genetic analysis integrating pleiotropy and annotation (GPA) methodologies. Identification of shared genetic loci involved pleiotropy analysis, functional mapping and annotation (FUMA), and co-localization analysis. Finally, Mendelian randomization was applied to probe causal relationships between inflammatory proteins and ALS.

RESULTS

Our investigation revealed significant genetic correlation and overlap between ALS and various inflammatory proteins, including C-C motif chemokine 28, Interleukin-18, C-X-C motif chemokine 1, and Leukemia inhibitory factor receptor (LIFR). Pleiotropy analysis uncovered shared variations at specific genetic loci, some of which bore potential harm. Mendelian randomization analysis suggested that alterations in specific inflammatory protein levels, notably LIFR, could impact ALS risk.

CONCLUSIONS

Our findings uncover a genetic correlation between certain circulating inflammatory proteins and ALS, suggesting their possible causal involvement in ALS pathogenesis. Moreover, the identification of LIFR as a crucial protein may yield new insights into ALS pathomechanisms and offer a promising avenue for therapeutic interventions. These discoveries provide novel perspectives for advancing the comprehension of ALS pathophysiology and exploring potential therapeutic avenues.

Oligodendrocytes in amyotrophic lateral sclerosis and frontotemporal dementia: the new players on stage

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are fatal adult-onset neurodegenerative disorders that share clinical, neuropathological and genetic features, which forms part of a multi-system disease spectrum. The pathological process leading to ALS and FTD is the result of the combination of multiple mechanisms that operate within specific populations of neurons and glial cells. The implication of oligodendrocytes has been the subject of a number of studies conducted on patients and related animal models. In this review we summarize our current knowledge on the alterations specific to myelin and the oligodendrocyte lineage occurring in ALS and FTD. We also consider different ways by which specific oligodendroglial alterations influence neurodegeneration and highlight the important role of oligodendrocytes in these two intrinsically associated neurodegenerative diseases.

PAD2 dysregulation and aberrant protein citrullination feature prominently in reactive astrogliosis and myelin protein aggregation in sporadic ALS

Alteration in protein citrullination (PC), a common posttranslational modification (PTM), contributes to pathogenesis in various inflammatory disorders. We previously reported that PC and protein arginine deiminase 2 (PAD2), the predominant enzyme isoform that catalyzes this PTM in the central nervous system (CNS), are altered in mouse models of amyotrophic lateral sclerosis (ALS). We now demonstrate that PAD2 expression and PC are altered in human postmortem ALS spinal cord and motor cortex compared to controls, increasing in astrocytes while trending lower in neurons. Furthermore, PC is enriched in protein aggregates that contain the myelin proteins PLP and MBP in ALS. These results confirm our findings in ALS mouse models and suggest that altered PAD2 and PC contribute to neurodegeneration in ALS.

White matter injury across neurodegenerative disease

Oligodendrocytes (OLs), the myelin-generating cells of the central nervous system (CNS), are active players in shaping neuronal circuitry and function. It has become increasingly apparent that injury to cells within the OL lineage plays a central role in neurodegeneration. In this review, we focus primarily on three degenerative disorders in which white matter loss is well documented: Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). We discuss clinical data implicating white matter injury as a key feature of these disorders, as well as shared and divergent phenotypes between them. We examine the cellular and molecular mechanisms underlying the alterations to OLs, including chronic neuroinflammation, aggregation of proteins, lipid dysregulation, and organellar stress. Last, we highlight prospects for therapeutic intervention targeting the OL lineage to restore function.

Impact of GSK199 and GSK106 binding on protein arginine deiminase IV stability and flexibility: A computational approach

Protein arginine deiminase IV (PAD4) is a potential target for diseases including rheumatoid arthritis and cancers. Currently, GSK199 is a potent, selective yet reversible PAD4 inhibitor. Its derivative, GSK106, on the other hand, was reported as an inactive compound when tested against PAD4 assay. Although they had similar skeleton, their impact towards PAD4 structural and flexibility is unknown. In order to fill the research gap, the impact of GSK199 and GSK106 binding towards PAD4 stability and flexibility is investigated via a combination of computational methods. Molecular docking indicates that GSK199 and GSK106 are capable to bind at PAD4 pocket by using its back door with -10.6 kcal/mol and -9.6 kcal/mol, respectively. The simulations of both complexes were stable throughout 100 ns. The structure of PAD4 exhibited a tighter packing in the presence of GSK106 compared to GSK199. The RMSF analysis demonstrates significant changes between the PAD4-GSK199 and PAD4-GSK106 simulations in the regions containing residues 136, 160, 220, 438, and 606. The Molecular Mechanics Poisson-Boltzmann Surface Area (MMPBSA) analysis shows a marked difference in binding free energies, with -11.339 kcal/mol for the PAD4-GSK199 complex and 1.063 kcal/mol for the PAD4-GSK106 complex. The hydrogen bond analysis revealed that the GSK199 and GSK106 binding to PAD4 are assisted by six hydrogen bonds and three hydrogen bonds, respectively. The visualisation of the MD simulations revealed that GSK199 remained in the PAD4 pocket, whereas GSK106 shifted away from the catalytic site. Meanwhile, molecular dockings of benzoyl arginine amide (BAEE) substrate have shown that BAEE is able to bind to PAD4 catalytic site when GSK106 was present but not when GSK199 occupied the site. Overall, combination of computational approaches successfully described the behaviour of binding pocket of PAD4 structure in the presence of the active and inactive compounds.

Stability dynamics of neurofilament and GFAP networks and protein fragments

Neurofilaments (NFs) and GFAP are cytoskeletal intermediate filaments (IFs) that support cellular processes unfolding within the uniquely complex environments of neurons and astrocytes, respectively. This review highlights emerging concepts on the transitions between stable and destabilized IF networks in the nervous system. While self-association between transiently structured low-complexity IF domains promotes filament assembly, the opposing destabilizing actions of phosphorylation-mediated filament severing facilitate faster intracellular transport. Cellular proteases, including caspases and calpains, produce a variety of IF fragments, which may interact with N-degron and C-degron pathways of the protein degradation machinery. The rapid adoption of NF and GFAP-based clinical biomarker tests is contrasted with the lagging understanding of the dynamics between the native IF proteins and their fragments.

Increase in hnRNPA1 Expression Suffices to Kill Motor Neurons in Transgenic Rats

A dominant mutation in hnRNPA1 causes amyotrophic lateral sclerosis (ALS), but it is not known whether this mutation leads to motor neuron death through increased or decreased function. To elucidate the relationship between pathogenic hnRNPA1 mutation and its native function, we created novel transgenic rats that overexpressed wildtype rat hnRNPA1 exclusively in motor neurons. This targeted expression of wildtype hnRNPA1 caused severe motor neuron loss and subsequent denervation muscle atrophy in transgenic rats that recapitulated the characteristics of ALS. These findings demonstrate that the augmentation of hnRNPA1 expression suffices to trigger motor neuron degeneration and the manifestation of ALS-like phenotypes. It is reasonable to infer that an amplification of an as-yet undetermined hnRNPA1 function plays a pivotal role in the pathogenesis of familial ALS caused by pathogenic hnRNPA1 mutation.

Detergent-insoluble PFN1 inoculation expedites disease onset and progression in PFN1 transgenic rats

Accumulating evidence suggests a gain of elusive toxicity in pathogenically mutated PFN1. The prominence of PFN1 aggregates as a pivotal pathological hallmark in PFN1 transgenic rats underscores the crucial involvement of protein aggregation in the initiation and progression of neurodegeneration. Detergent-insoluble materials were extracted from the spinal cords of paralyzed rats afflicted with ALS and were intramuscularly administered to asymptomatic recipient rats expressing mutant PFN1, resulting in an accelerated development of PFN1 inclusions and ALS-like phenotypes. This effect diminished when the extracts derived from wildtype PFN1 transgenic rats were employed, as detergent-insoluble PFN1 was detected exclusively in mutant PFN1 transgenic rats. Consequently, the factor influencing the progression of ALS pathology in recipient rats is likely associated with the presence of detergent-insoluble PFN1 within the extracted materials. Noteworthy is the absence of disease course modification upon administering detergent-insoluble extracts to rats that already displayed PFN1 inclusions, suggesting a seeding rather than augmenting role of such extracts in initiating neuropathological changes. Remarkably, pathogenic PFN1 exhibited an enhanced affinity for the molecular chaperone DNAJB6, leading to the sequestration of DNAJB6 within protein inclusions, thereby depleting its availability for cellular functions. These findings shed light on a novel mechanism that underscores the prion-like characteristics of pathogenic PFN1 in driving neurodegeneration in the context of PFN1-related ALS.

Citrullination Post-Translational Modification: State of the Art of Brain Tumor Investigations and Future Perspectives

The present review aims to describe the state of the art of research studies investigating the citrullination post-translational modification in adult and pediatric brain tumors. After an introduction to the deimination reaction and its occurrence in proteins and polypeptide chains, the role of the citrullination post-translational modification in physiological as well as pathological states, including cancer, is summarized, and the recent literature and review papers on the topic are examined. A separate section deals with the specific focus of investigation of the citrullination post-translational modification in relation to brain tumors, examining the state of the art of the literature that mainly concerns adult and pediatric glioblastoma and posterior fossa pediatric tumors. We examined the literature on this emerging field of research, and we apologize in advance for any possible omission. Although only a few studies inspecting citrullination in brain tumors are currently available, the results interestingly highlighted different profiles of the citrullinome associated with different histotypes. The data outlined the importance of this post-translational modification in modulating cancer invasion and chemoresistance, influencing key factors involved in apoptosis, cancer cell communication through extracellular vesicle release, autophagy, and gene expression processes, which suggests the prospect of taking citrullination as a target of cancer treatment or as a source of potential diagnostic and prognostic biomarkers for potential clinical applications in the future.

The Extracellular Vesicle Citrullinome and Signature in a Piglet Model of Neonatal Seizures

Neonatal seizures are commonly associated with acute perinatal brain injury, while understanding regarding the downstream molecular pathways related to seizures remains unclear. Furthermore, effective treatment and reliable biomarkers are still lacking. Post-translational modifications can contribute to changes in protein function, and post-translational citrullination, which is caused by modification of arginine to citrulline via the calcium-mediated activation of the peptidylarginine deiminase (PAD) enzyme family, is being increasingly linked to neurological injury. Extracellular vesicles (EVs) are lipid-bilayer structures released from cells; they can be isolated from most body fluids and act as potential liquid biomarkers for disease conditions and response to treatment. As EVs carry a range of genetic and protein cargo that can be characteristic of pathological processes, the current study assessed modified citrullinated protein cargo in EVs isolated from plasma and CSF in a piglet neonatal seizure model, also following phenobarbitone treatment. Our findings provide novel insights into roles for PAD-mediated changes on EV signatures in neonatal seizures and highlight the potential of plasma- and CSF-EVs to monitor responses to treatment.

Differential, Stage Dependent Detection of Peptidylarginine Deiminases and Protein Deimination in Lewy Body Diseases-Findings from a Pilot Study

Over 10 million people worldwide live with Parkinson's disease (PD) and 4% of affected people are diagnosed before the age of 50. Research on early PD-related pathways is therefore of considerable importance. Peptidylarginine deiminases (PADs) are a family of calcium-activated enzymes that, through post-translational deimination of arginine to citrulline, contribute to changes in protein function, including in pathological processes. Recent studies have highlighted roles for PADs in a range of neurological disorders including PD, but overall, investigations on PADs in Lewy body disease (LBD), including PD, are still scarce. Hence, the current pilot study aimed at performing an immunohistochemistry screen of post-mortem human brain sections from Braak stages 4-6 from PD patients, as well as patients with incidental LBD (ILBD). We assessed differences in PAD isozyme detection (assessing all five PADs), in total protein deimination/citrullination and histone H3 deimination-which is an indicator of epigenetic changes and extracellular trap formation (ETosis), which can elicit immune responses and has involvement in pathogenic conditions. The findings of our pilot study indicate that PADs and deimination are increased in cingulate cortex and hippocampus, particularly in earlier stages of the disease. PAD2 and PAD3 were the most strongly upregulated PAD isozymes, with some elevation also observed for PAD1, while PAD4 and PAD6 increase was less marked in PD brains. Total protein deimination and histone H3 deimination were furthermore increased in PD brains, with a considerable increase at earlier Braak stages, compared with controls. Our findings point to a significant contribution of PADs, which may further aid early disease biomarker discovery, in PD and other LBDs.