Toll-like receptor 4 deficiency facilitates α-synuclein propagation and neurodegeneration in a mouse model of prodromal Parkinson's disease

Alpha-synuclein pathology and Parkinson's disease-related olfactory dysfunctions: an update on preclinical models and therapeutic approaches

Olfactory dysfunction (OD) is considered one of the early signs of Parkinson's disease (PD), affecting over 90% of PD patients. OD often appears several years before the onset of motor symptoms and is therefore considered an early biomarker of PD. Recent studies have shown that COVID-19 infection might lead to worsening of symptoms and acceleration of disease progression in neurodegenerative disorders, where OD is a common symptom to both. Hence, it is essential to accurately monitor olfactory fitness in clinical settings using any of the currently available olfactory function tests. Even after a quarter of a century of the discovery of α-synuclein (α-syn) pathogenesis in PD, many aspects related to the α-syn pathogenesis in OD remain unknown. Currently, there is no definitive cure for PD; the disease management options include dopaminergic medications, deep brain stimulations, stem cells, and immunotherapy. Generating reliable PD animal models is critical for understanding the molecular pathways and neural circuits affected by disease conditions. This might contribute to the development and validation of new therapeutic approaches. This review discusses the known mechanisms of α-syn aggregated forms causing neuronal death, the recent developments in the PD preclinical models with ODs, and the treatment strategies employed.

Emerging targets of α-synuclein spreading in α-synucleinopathies: a review of mechanistic pathways and interventions

α-Synucleinopathies constitute a spectrum of neurodegenerative disorders, including Parkinson's disease (PD), Lewy body dementia (LBD), Multiple System Atrophy (MSA), and Alzheimer's disease concurrent with LBD (AD-LBD). These disorders are unified by a pathological hallmark: aberrant misfolding and accumulation of α-synuclein (α-syn). This review delves into the pivotal role of α-syn, the key agent in α-synucleinopathy pathophysiology, and provides a survey of potential therapeutics that target cell-to-cell spread of pathologic α-syn. Recognizing the intricate complexity and multifactorial etiology of α-synucleinopathy, the review illuminates the potential of various membrane receptors, proteins, intercellular spreading pathways, and pathological agents for therapeutic interventions. While significant progress has been made in understanding α-synucleinopathy, the pursuit of efficacious treatments remains challenging. Several strategies involving decreasing α-syn production and aggregation, increasing α-syn degradation, lowering extracellular α-syn, and inhibiting cellular uptake of α-syn are presented. The paper underscores the necessity of meticulous and comprehensive investigations to advance our knowledge of α-synucleinopathy pathology and ultimately develop innovative therapeutic strategies for α-synucleinopathies.

Uptake of alpha-synuclein preformed fibrils is suppressed by inflammation and induces an aberrant phenotype in human microglia

Microglia are brain resident immune cells that maintain proteostasis and cellular homeostasis. Recent findings suggest that microglia dysfunction could contribute to the pathogenesis of Parkinson's disease (PD). One of the hallmarks of PD is the aggregation and accumulation of alpha-synuclein (αSyn) into Lewy bodies inside nerve cells. Microglia may worsen the neuronal microenvironment by persistent inflammation, resulting in deficient clearing of aggregated αSyn. To model microglial behavior in PD, we utilized human induced pluripotent stem cells to generate functionally active microglia. We studied the microglial uptake of alpha-synuclein preformed fibrils (PFFs) and the effect of pro-inflammatory stimulation by interferon gamma. We demonstrate that combined exposure disrupts the phagosome maturation pathway while inflammatory stimuli suppress chaperone mediated autophagy and mitochondrial function. Furthermore, inflammatory stimulation impairs PFF uptake in microglia and increases cytokine production. Moreover, excessive PFF uptake by microglia results in induction of inducible nitric oxide synthase. Taken together, we demonstrate that this model is valuable for investigating the behavior of microglia in PD and provide new insights on how human microglia process aggregated αSyn.

Emerging role of microglia in inter-cellular transmission of α-synuclein in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease worldwide, significantly prejudicing the health and quality of life of elderly patients. The main pathological characteristics of PD are the loss of dopaminergic neurons in the substantia nigra (SN) as well as abnormal aggregation of α-synuclein (α-syn) monomers and oligomers, which results in formation of Lewy bodies (LBs). Intercellular transmission of α-syn is crucial for PD progression. Microglia play diverse roles in physiological and pathological conditions, exhibiting neuroprotective or neurotoxic effects; moreover, they may directly facilitate α-syn propagation. Various forms of extracellular α-syn can be taken up by microglia through multiple mechanisms, degraded or processed into more pathogenic forms, and eventually released into extracellular fluid or adjacent cells. This review discusses current literature regarding the molecular mechanisms underlying the uptake, degradation, and release of α-syn by microglia.

The immune system in Parkinson's disease: what we know so far

Parkinson's disease is characterized neuropathologically by the degeneration of dopaminergic neurons in the ventral midbrain, the accumulation of α-synuclein (α-syn) aggregates in neurons and chronic neuroinflammation. In the past two decades, in vitro, ex vivo and in vivo studies have consistently shown the involvement of inflammatory responses mediated by microglia and astrocytes, which may be elicited by pathological α-syn or signals from affected neurons and other cell types, and are directly linked to neurodegeneration and disease development. Apart from the prominent immune alterations seen in the CNS, including the infiltration of T cells into the brain, more recent studies have demonstrated important changes in the peripheral immune profile within both the innate and adaptive compartments, particularly involving monocytes, CD4+ and CD8+ T cells. This review aims to integrate the consolidated understanding of immune-related processes underlying the pathogenesis of Parkinson's disease, focusing on both central and peripheral immune cells, neuron-glia crosstalk as well as the central-peripheral immune interaction during the development of Parkinson's disease. Our analysis seeks to provide a comprehensive view of the emerging knowledge of the mechanisms of immunity in Parkinson's disease and the implications of this for better understanding the overall pathogenesis of this disease.

Microglia signaling in health and disease - Implications in sex-specific brain development and plasticity

Microglia, the intrinsic neuroimmune cells residing in the central nervous system (CNS), exert a pivotal influence on brain development, homeostasis, and functionality, encompassing critical roles during both aging and pathological states. Recent advancements in comprehending brain plasticity and functions have spotlighted conspicuous variances between male and female brains, notably in neurogenesis, neuronal myelination, axon fasciculation, and synaptogenesis. Nevertheless, the precise impact of microglia on sex-specific brain cell plasticity, sculpting diverse neural network architectures and circuits, remains largely unexplored. This article seeks to unravel the present understanding of microglial involvement in brain development, plasticity, and function, with a specific emphasis on microglial signaling in brain sex polymorphism. Commencing with an overview of microglia in the CNS and their associated signaling cascades, we subsequently probe recent revelations regarding molecular signaling by microglia in sex-dependent brain developmental plasticity, functions, and diseases. Notably, C-X3-C motif chemokine receptor 1 (CX3CR1), triggering receptors expressed on myeloid cells 2 (TREM2), calcium (Ca2+), and apolipoprotein E (APOE) emerge as molecular candidates significantly contributing to sex-dependent brain development and plasticity. In conclusion, we address burgeoning inquiries surrounding microglia's pivotal role in the functional diversity of developing and aging brains, contemplating their potential implications for gender-tailored therapeutic strategies in neurodegenerative diseases.

Proximity Labeling Proteomics Reveals Kv1.3 Potassium Channel Immune Interactors in Microglia

Microglia are resident immune cells of the brain and regulate its inflammatory state. In neurodegenerative diseases, microglia transition from a homeostatic state to a state referred to as disease-associated microglia (DAM). DAM express higher levels of proinflammatory signaling molecules, like STAT1 and TLR2, and show transitions in mitochondrial activity toward a more glycolytic response. Inhibition of Kv1.3 decreases the proinflammatory signature of DAM, though how Kv1.3 influences the response is unknown. Our goal was to identify the potential proteins interacting with Kv1.3 during transition to DAM. We utilized TurboID, a biotin ligase, fused to Kv1.3 to evaluate potential interacting proteins with Kv1.3 via mass spectrometry in BV-2 microglia following TLR4-mediated activation. Electrophysiology, Western blotting, and flow cytometry were used to evaluate Kv1.3 channel presence and TurboID biotinylation activity. We hypothesized that Kv1.3 contains domain-specific interactors that vary during a TLR4-induced inflammatory response, some of which are dependent on the PDZ-binding domain on the C terminus. We determined that the N terminus of Kv1.3 is responsible for trafficking Kv1.3 to the cell surface and mitochondria (e.g., NUDC, TIMM50). Whereas, the C terminus interacts with immune signaling proteins in a lipopolysaccharide-induced inflammatory response (e.g., STAT1, TLR2, and C3). There are 70 proteins that rely on the C-terminal PDZ-binding domain to interact with Kv1.3 (e.g., ND3, Snx3, and Sun1). Furthermore, we used Kv1.3 blockade to verify functional coupling between Kv1.3 and interferon-mediated STAT1 activation. Overall, we highlight that the Kv1.3 potassium channel functions beyond conducting the outward flux of potassium ions in an inflammatory context and that Kv1.3 modulates the activity of key immune signaling proteins, such as STAT1 and C3.

Alpha-synuclein shapes monocyte and macrophage cell biology and functions by bridging alterations of autophagy and inflammatory pathways

Introduction: Abnormal spreading of alpha-synuclein (αS), a hallmark of Parkinson's disease, is known to promote peripheral inflammation, which occurs in part via functional alterations in monocytes/macrophages. However, underlying intracellular mechanisms remain unclear. Methods: Herein we investigate the subcellular, molecular, and functional effects of excess αS in human THP-1 monocytic cell line, THP-1-derived macrophages, and at least preliminarily, in primary monocyte-derived macrophages (MDMs). In cells cultured w/wo recombinant αS (1 μM) for 4 h and 24 h, by Confocal microscopy, Western Blot, RT-qPCR, Elisa, and Flow Cytometry we assessed: i) αS internalization; ii) cytokine/chemokine expression/secretion, and C-C motif chemokine receptor 2 (CCR2) levels; iii) autophagy (LC3II/I, LAMP1/LysoTracker, p62, pS6/total S6); and iv) lipid droplets (LDs) accumulation, and cholesterol pathway gene expression. Transwell migration assay was employed to measure THP-1 cell migration/chemotaxis, while FITC-IgG-bead assay was used to analyze phagocytic capacity, and the fate of phagocytosed cargo in THP-1-derived macrophages. Results: Extracellular αS was internalized by THP-1 cells, THP-1-derived macrophages, and MDMs. In THP1 cells, αS induced a general pro-inflammatory profile and conditioned media from αS-exposed THP-1 cells potently attracted unstimulated cells. However, CCL2 secretion peaked at 4 h αS, consistent with early internalization of its receptor CCR2, while this was blunted at 24 h αS exposure, when CCR2 recycled back to the plasma membrane. Again, 4 h αS-exposed THP-1 cells showed increased spontaneous migration, while 24 h αS-exposed cells showed reduced chemotaxis. This occurred in the absence of cell toxicity and was associated with upregulation of autophagy/lysosomal markers, suggesting a pro-survival/tolerance mechanism against stress-related inflammation. Instead, in THP-1-derived macrophages, αS time-dependently potentiated the intracellular accumulation, and release of pro-inflammatory mediators. This was accompanied by mild toxicity, reduced autophagy-lysosomal markers, defective LDs formation, as well as impaired phagocytosis, and the appearance of stagnant lysosomes engulfed with phagocytosed cargo, suggesting a status of macrophage exhaustion reminiscent of hypophagia. Discussion: In summary, despite an apparently similar pro-inflammatory phenotype, monocytes and macrophages respond differently to intracellular αS accumulation in terms of cell survival, metabolism, and functions. Our results suggest that in periphery, αS exerts cell- and context-specific biological effects bridging alterations of autophagy, lipid dynamics, and inflammatory pathways.

From Immunity to Neurogenesis: Toll-like Receptors as Versatile Regulators in the Nervous System

Toll-like receptors (TLRs) are among the main components of the innate immune system. They can detect conserved structures in microorganisms and molecules associated with stress and cellular damage. TLRs are expressed in resident immune cells and both neurons and glial cells of the nervous system. Increasing evidence is emerging on the participation of TLRs not only in the immune response but also in processes of the nervous system, such as neurogenesis and cognition. Below, we present a review of the literature that evaluates the expression and role of TLRs in processes such as neurodevelopment, behavior, cognition, infection, neuroinflammation, and neurodegeneration.

Immunological dimensions of neuroinflammation and microglial activation: exploring innovative immunomodulatory approaches to mitigate neuroinflammatory progression

The increasing life expectancy has led to a higher incidence of age-related neurodegenerative conditions. Within this framework, neuroinflammation emerges as a significant contributing factor. It involves the activation of microglia and astrocytes, leading to the release of pro-inflammatory cytokines and chemokines and the infiltration of peripheral leukocytes into the central nervous system (CNS). These instances result in neuronal damage and neurodegeneration through activated nucleotide-binding domain and leucine-rich repeat containing (NLR) family pyrin domain containing protein 3 (NLRP3) and nuclear factor kappa B (NF-kB) pathways and decreased nuclear factor erythroid 2-related factor 2 (Nrf2) activity. Due to limited effectiveness regarding the inhibition of neuroinflammatory targets using conventional drugs, there is challenging growth in the search for innovative therapies for alleviating neuroinflammation in CNS diseases or even before their onset. Our results indicate that interventions focusing on Interleukin-Driven Immunomodulation, Chemokine (CXC) Receptor Signaling and Expression, Cold Exposure, and Fibrin-Targeted strategies significantly promise to mitigate neuroinflammatory processes. These approaches demonstrate potential anti-neuroinflammatory effects, addressing conditions such as Multiple Sclerosis, Experimental autoimmune encephalomyelitis, Parkinson's Disease, and Alzheimer's Disease. While the findings are promising, immunomodulatory therapies often face limitations due to Immune-Related Adverse Events. Therefore, the conduction of randomized clinical trials in this matter is mandatory, and will pave the way for a promising future in the development of new medicines with specific therapeutic targets.

Single-cell somatic copy number variants in brain using different amplification methods and reference genomes

The presence of somatic mutations, including copy number variants (CNVs), in the brain is well recognized. Comprehensive study requires single-cell whole genome amplification, with several methods available, prior to sequencing. We compared PicoPLEX with two recent adaptations of multiple displacement amplification (MDA): primary template-directed amplification (PTA) and droplet MDA, across 93 human brain cortical nuclei. We demonstrated different properties for each, with PTA providing the broadest amplification, PicoPLEX the most even, and distinct chimeric profiles. Furthermore, we performed CNV calling on two brains with multiple system atrophy and one control brain using different reference genomes. We found that 38% of brain cells have at least one Mb-scale CNV, with some supported by bulk sequencing or single-cells from other brain regions. Our study highlights the importance of selecting whole genome amplification method and reference genome for CNV calling, while supporting the existence of somatic CNVs in healthy and diseased human brain.

Toll-like Receptor 4 Is Upregulated in Parkinson's Disease Patients and Co-Localizes with pSer129αSyn: A Possible Link with the Pathology

Growing evidence suggests a crucial role of neuroinflammation in the pathophysiology of Parkinson's disease (PD). Neuroinflammation is linked to the accumulation and aggregation of a-synuclein (αSyn), the primary pathological hallmark of PD. Toll-like receptors 4 (TLR4) can have implications in the development and progression of the pathology. In this study, we analyzed the expression of TLR4 in the substantia nigra (SN) and medial temporal gyrus (GTM) of well-characterized PD patients and age-matched controls. We also assessed the co-localization of TLR4 with pSer129 αSyn. Using qPCR, we observed an upregulation of TLR4 expression in the SN and GTM in PD patients compared to controls, which was accompanied by a reduction in αSyn expression likely due to the depletion of dopaminergic (DA) cells. Additionally, using immunofluorescence and confocal microscopy, we observed TLR4-positive staining and co-localization with pSer129-αSyn in Lewy bodies of DA neurons in the SN, as well as in pyramidal neurons in the GTM of PD donors. Furthermore, we observed a co-localization of TLR4 and Iba-1 in glial cells of both SN and GTM. Our findings provide evidence for the increased expression of TLR4 in the PD brain and suggest that the interaction between TLR4 and pSer129-αSyn could play a role in mediating the neuroinflammatory response in PD.

A functional role for alpha-synuclein in neuroimmune responses

Alpha-synuclein is a neuronal protein with unclear function but is associated with the pathogenesis of Parkinson's disease and other synucleinopathies. In this review, we discuss the emerging functional role of alpha-synuclein in support of the unique immune responses in the nervous system. Recent data now show that alpha-synuclein functions to support interferon signaling within neurons and is released from neurons to support chemoattraction and activation of local glial cells and infiltrating immune cells. Inflammatory activation and interferon signaling also induce post-translational modifications of alpha-synuclein that are commonly associated with Parkinson's disease pathogenesis. Taken together, emerging data implicate complex interactions between alpha-synuclein and host immune responses that may contribute to the pathogenesis of Parkinson's disease. Additional study of the function of alpha-synuclein in the brain's immune response may provide disease-modifying therapeutic targets for Parkinson's disease in the future.

Aggregated alpha-synuclein transcriptionally activates pro-inflammatory canonical and non-canonical NF-κB signaling pathways in peripheral monocytic cells

Parkinson's disease (PD) is a neurodegenerative disorder characterized by chronic neuroinflammation, loss of dopaminergic neurons in the substantia nigra, and in several cases accumulation of alpha-synuclein fibril (α-syn) containing Lewy-bodies (LBs). Peripheral inflammation may play a causal role in inducing and perpetuating neuroinflammation in PD and accumulation of fibrillar α-syn has been reported at several peripheral sites including the gut and liver. Peripheral fibrillar α-syn may induce activation of monocytes via recognition by toll-like receptors (TLRs) and stimulation of downstream NF-κB signaling; however, the specific mechanism by which this occurs is not defined. In this study we utilized the THP-1 monocytic cell line to model the peripheral transcriptional response to preformed fibrillar (PFF) α-syn. Compared to monomeric α-syn, PFF α-syn displays overt inflammatory gene upregulation and pathway activation including broad pan-TLR signaling pathway activation and increases in TNF and IL1B gene expression. Notably, the non-canonical NF-κB signaling pathway gene and PD genome wide association study (GWAS) candidate NFKB2 was upregulated. Additionally, non-canonical NF-κB activation-associated RANK and CD40 pathways were also upregulated. Transcriptional-phenotype analysis suggests PFFs induce transcriptional programs associated with differentiation of monocytes towards macrophages and osteoclasts via non-canonical NF-κB signaling as a potential mechanism in which myeloid/monocyte cells may contribute to peripheral inflammation and pathogenesis in PD.

Microglial autophagy in Alzheimer's disease and Parkinson's disease

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common neurodegenerative diseases, characterized by gradual and selective loss of neurons in the central nervous system. They affect more than 50 million people worldwide, and their incidence increases with age. Although most cases of AD and PD are sporadic, some are caused by genetic mutations that are inherited. Both sporadic and familial cases display complex neuropathology and represent the most perplexing neurological disorders. Because of the undefined pathogenesis and complex clinical manifestations, there is still no effective treatment for both AD and PD. Understanding the pathogenesis of these important neurodegenerative diseases is important for developing successful therapies. Increasing evidence suggests that microglial autophagy is associated with the pathogenesis of AD and PD, and its dysfunction has been implicated in disease progression. In this review, we focus on the autophagy function in microglia and its dysfunction in AD and PD disease models in an attempt to help our understanding of the pathogenesis and identifying new therapeutic targets of AD and PD.

Microglia in Parkinson’s Disease

Microglia are the primary resident immune cells of the central nervous system. Neuropathological reports have identified augmented microglial activation in brains of patients with neurodegenerative disorders including Parkinson’s disease (PD). Extensive research over the years has strengthened the current view on microglia as a player in the pathogenesis of PD and other α-synucleinopathies. In this review, we summarize key findings of the recent three years on microglia in PD with specific relevance to understanding its heterogeneity, dual nature, and specific interactions with pathological α-synuclein strains to mediate its clearance and spreading. This review provides evidence on the relevance of microglia as a putative biomarker and therapeutic target in PD and related disorders.

Toll-like receptor-mediated neuroinflammation: relevance for cognitive dysfunctions

Toll-like receptors (TLRs) are pattern recognition receptors (PRRs) with a well-documented role in the innate and adaptive immune responses. Interestingly, TLR activation has also been linked to several brain functions including neurogenesis and synaptogenesis. Increasing evidence supports TLR involvement in peripheral and central inflammation underlying normal aging and the pathogenesis of clinical conditions characterized by cognitive decline. These include not only major neurodegenerative diseases but also traumatic brain injuries, surgeries, and alcohol consumption- and chemotherapy-induced cognitive impairment. We first summarize the physiological roles of TLRs in the nervous system, and then illustrate the emerging involvement of TLRs in cognitive functions, pointing to these receptors as novel enticing pharmacological targets to develop more efficient drugs for the treatment of cognitive impairment.

Serum NfL and CHI3L1 for ALS and parkinsonian disorders in the process of diagnosis

Serum neurofilament light chain (NfL) and chitinase 3-like 1 (CHI3L1, also called YKL-40) concentrations are attractive candidate biomarkers for neurodegenerative disorders, which include amyotrophic lateral sclerosis (ALS) and parkinsonian disorders. We aimed to assess the diagnostic power of serum NfL and CHI3L1 concentrations with regard to the early diagnosis of ALS and Parkinson's disease (PD). We studied 157 individuals, which included 41 healthy controls, 8 patients with ALS mimics, 18 patients initially diagnosed with ALS (ID-ALS), 32 patients late-diagnosed with ALS (LD-ALS), 29 patients with PD, 12 patients with PD mimics, and 17 patients initially diagnosed with atypical parkinsonian disorders (ID-APDs) at the initial stage of diagnosis. Electrochemiluminescence was used to measure the concentrations of serum NfL and CHI3L1, the diagnostic performance of which was assessed using the area under the receiver operating curves (AUCs). The AUCs of serum NfL were 0.90 for discriminating ALS mimics from LD-ALS at the initial stage of diagnosis and 0.89 for discriminating ALS mimics from ALS (LD/ID-ALS). The AUCs of serum NfL were 0.76 for discriminating PD from PD mimics at the initial stage of diagnosis, and 0.80 for discriminating PD from APD. No significant difference existed in serum CHI3L1 concentrations between individuals with suspected ALS or parkinsonism (p = 0.14, and p = 0.44, respectively). Serum NfL had excellent and almost good diagnostic performances for patients with ALS and PD, respectively, at the initial stage of diagnosis, whereas no significant difference existed in serum CHI3L1 between any groups.