A CD14 monocyte receptor polymorphism and genetic susceptibility to Parkinson's disease for females

Disease mechanisms as subtypes: Immune dysfunction in Parkinson's disease

In recent years, the contraposition between inflammatory and neurodegenerative processes has been increasingly challenged. Inflammation has been emphasized as a key player in the onset and progression of Parkinson disease (PD) and other neurodegenerative disorders. The strongest indicators of the involvement of the immune system derived from evidence of microglial activation, profound imbalance in phenotype and composition of peripheral immune cells, and impaired humoral immune responses. Moreover, peripheral inflammatory mechanisms (e.g., involving the gut-brain axis) and immunogenetic factors are likely to be implicated. Even though several lines of preclinical and clinical studies are supporting and defining the complex relationship between the immune system and PD, the exact mechanisms are currently unknown. Similarly, the temporal and causal connections between innate and adaptive immune responses and neurodegeneration are unsettled, challenging our ambition to define an integrated and holistic model of the disease. Despite these difficulties, current evidence is providing the unique opportunity to develop immune-targeted approaches for PD, thus enriching our therapeutic armamentarium. This chapter aims to provide an extensive overview of past and present studies that explored the implication of the immune system in neurodegeneration, thus paving the road for the concept of disease modification in PD.

Supramolecular organizing centers at the interface of inflammation and neurodegeneration

The pathogenesis of neurodegenerative diseases involves the accumulation of misfolded protein aggregates. These deposits are both directly toxic to neurons, invoking loss of cell connectivity and cell death, and recognized by innate sensors that upon activation release neurotoxic cytokines, chemokines, and various reactive species. This neuroinflammation is propagated through signaling cascades where activated sensors/receptors, adaptors, and effectors associate into multiprotein complexes known as supramolecular organizing centers (SMOCs). This review provides a comprehensive overview of the SMOCs, involved in neuroinflammation and neurotoxicity, such as myddosomes, inflammasomes, and necrosomes, their assembly, and evidence for their involvement in common neurodegenerative diseases. We discuss the multifaceted role of neuroinflammation in the progression of neurodegeneration. Recent progress in the understanding of particular SMOC participation in common neurodegenerative diseases such as Alzheimer's disease offers novel therapeutic strategies for currently absent disease-modifying treatments.

Sexual Dimorphic Role of CD14 (Cluster of Differentiation 14) in Salt-Sensitive Hypertension and Renal Injury

Genomic sequence and gene expression association studies in animals and humans have identified genes that may be integral in the pathogenesis of various diseases. CD14 (cluster of differentiation 14)-a cell surface protein involved in innate immune system activation-is one such gene associated with cardiovascular and hypertensive disease. We previously showed that this gene is upregulated in renal macrophages of Dahl salt-sensitive animals fed a high-salt diet; here we test the hypothesis that CD14 contributes to the elevated pressure and renal injury observed in salt-sensitive hypertension. Using CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeat-associated 9), we created a targeted mutation in the CD14 gene on the Dahl SS (SS/JrHSDMcwi) background and validated the absence of CD14 peptides via mass spectrometry. Radiotelemetry was used to monitor blood pressure in wild-type and CD14^-/- animals challenged with high salt and identified infiltrating renal immune cells via flow cytometry. Germline knockout of CD14 exacerbated salt-sensitive hypertension and renal injury in female animals but not males. CD14^-/- females demonstrated increased infiltrating macrophages but no difference in infiltrating lymphocytes. Transplant of CD14^+/+ or CD14^-/- bone marrow was used to isolate the effects of CD14 knockout to hematopoietic cells and confirmed that the differential phenotype observed was due to knockout of CD14 in hematopoietic cells. Ovariectomy was used to remove the influence of female sex hormones, which completely abrogated the effect of CD14 knockout. These studies provide a novel treatment target and evidence of a new dichotomy in immune activation between sexes within the context of hypertensive disease where CD14 regulates immune cell activation and renal injury.

Association of CD14-260 (-159) C/T and Alzheimer's disease: systematic review and trial sequential analyses

Current studies have evaluated the association between CD14-260 (also known as -159) C/T polymorphism and Alzheimer's disease (AD) susceptibility. However, the association remains inconclusive. The aim of this study was to draw an accurate conclusion of the association. The literature search was conducted using PubMed, Embase, Chinese National Knowledge Infrastructure, China Biological Medicine Database, and Wanfang Databases for related articles. Four case-control studies with a total of 868 cases and 766 controls were eligible to be included in this meta-analysis. The association was evaluated by calculating the odds ratios (ORs) with the corresponding 95% confidence intervals (CIs). Overall, there was no significant association between CD14-260C/T polymorphism and AD risk in all genetic models (the allele model T vs. C: OR = 1.06, 95% CI 0.92-1.21, p = 0.44; the homozygous model TT vs. CC: OR = 1.09, 95% CI 0.83-1.44, p = 0.53; the heterozygote model CT vs. CC: OR = 0.95, 95% CI 0.75-1.22, p = 0.71; the dominant model TT + CT vs. CC: OR = 1.05, 95% CI 0.84-1.32, p = 0.66; the recessive model TT vs. CT + CC: OR = 1.14, 95% CI 0.92-1.43, p = 0.24). The sample size of 5064 was calculated by applying trial sequential analysis. Cumulative z curve does not cross trial sequential monitoring boundary. In conclusion, the present meta-analysis suggests that the CD14-260C/T polymorphism may not be associated with genetic susceptibility of AD, but the association remains indeterminate due to the insufficient evidence.

Common gene-network signature of different neurological disorders and their potential implications to neuroAIDS

The neurological complications of AIDS (neuroAIDS) during the infection of human immunodeficiency virus (HIV) are symptomized by non-specific, multifaceted neurological conditions and therefore, defining a specific diagnosis/treatment mechanism(s) for this neuro-complexity at the molecular level remains elusive. Using an in silico based integrated gene network analysis we discovered that HIV infection shares convergent gene networks with each of twelve neurological disorders selected in this study. Importantly, a common gene network was identified among HIV infection, Alzheimer's disease, Parkinson's disease, multiple sclerosis, and age macular degeneration. An mRNA microarray analysis in HIV-infected monocytes showed significant changes in the expression of several genes of this in silico derived common pathway which suggests the possible physiological relevance of this gene-circuit in driving neuroAIDS condition. Further, this unique gene network was compared with another in silico derived novel, convergent gene network which is shared by seven major neurological disorders (Alzheimer's disease, Parkinson's disease, Multiple Sclerosis, Age Macular Degeneration, Amyotrophic Lateral Sclerosis, Vascular Dementia, and Restless Leg Syndrome). These networks differed in their gene circuits; however, in large, they involved innate immunity signaling pathways, which suggests commonalities in the immunological basis of different neuropathogenesis. The common gene circuits reported here can provide a prospective platform to understand how gene-circuits belonging to other neuro-disorders may be convoluted during real-time neuroAIDS condition and it may elucidate the underlying-and so far unknown-genetic overlap between HIV infection and neuroAIDS risk. Also, it may lead to a new paradigm in understanding disease progression, identifying biomarkers, and developing therapies.

Polymorphism in MIR4697 but not VPS13C, GCH1, or SIPA1L2 is associated with risk of Parkinson's disease in a Han Chinese population

A large meta-analysis recently identified six new loci associated with risk of PD, but subsequent studies have given discrepant results. Here we conducted a case-control study in a Han Chinese population in an attempt to clarify risk associations in Chinese. Among the four single-nucleotide polymorphisms (SNPs) that we examined - VPS13C-rs2414739, MIR4697-rs329648, GCH1-rs11158026, and SIPA1L2- rs10797576 we detected a significant association between rs329648 and risk of developing PD in a recessive model. This association remained significant after adjusting for gender and age (OR 1.87, 95%CI 1.295-2.694, p=8.21×10^-4) or Bonferroni correction. The T allele of rs329648 occurred significantly more frequently among patients with PD than among healthy controls (OR 1.22, 95%CI 1.033-1.443, p=0.02), while there was no statistic significant after Bonferroni correction. Subgroup analysis showed a significant association specifically among males in a recessive model (OR 1.943, 95%CI 1.200-3.147, p=0.007). In contrast, genotye and allele frequencies at rs329648 did not differ significantly between female patients with PD and healthy female controls, or between patients with early-onset or late-onset PD. Our results suggest that rs329648 is associated with risk of developing PD in the Han Chinese population. Our findings should be verified in further studies, and they highlight the need for functional studies of MIR4697.

The role of inflammation in sporadic and familial Parkinson's disease

The etiology of Parkinson's disease (PD) is complex and most likely involves numerous environmental and heritable risk factors. Interestingly, many genetic variants, which have been linked to familial forms of PD or identified as strong risk factors, also play a critical role in modulating inflammatory responses. There has been considerable debate in the field as to whether inflammation is a driving force in neurodegeneration or simply represents a response to neuronal death. One emerging hypothesis is that inflammation plays a critical role in the early phases of neurodegeneration. In this review, we will discuss emerging aspects of both innate and adaptive immunity in the context of the pathogenesis of PD. We will highlight recent data from genetic and functional studies that strongly support the theory that genetic susceptibility plays an important role in modulating immune pathways and inflammatory reactions, which may precede and initiate neuronal dysfunction and subsequent neurodegeneration. A detailed understanding of such cellular and molecular inflammatory pathways is crucial to uncover pathogenic mechanisms linking sporadic and hereditary PD and devise tailored neuroprotective interventions.

Absence of CD14 delays progression of prion diseases accompanied by increased microglial activation

Prion diseases are fatal neurodegenerative disorders characterized by accumulation of PrP(Sc), vacuolation of neurons and neuropil, astrocytosis, and microglial activation. Upregulation of gene expressions of innate immunity-related factors, including complement factors and CD14, is observed in the brains of mice infected with prions even in the early stage of infections. When CD14 knockout (CD14(-/-)) mice were infected intracerebrally with the Chandler and Obihiro prion strains, the mice survived longer than wild-type (WT) mice, suggesting that CD14 influences the progression of the prion disease. Immunofluorescence staining that can distinguish normal prion protein from the disease-specific form of prion protein (PrP(Sc)) revealed that deposition of PrP(Sc) was delayed in CD14(-/-) mice compared with WT mice by the middle stage of the infection. Immunohistochemical staining with Iba1, a marker for activated microglia, showed an increased microglial activation in prion-infected CD14(-/-) mice compared to WT mice. Interestingly, accompanied by the increased microglial activation, anti-inflammatory cytokines interleukin-10 (IL-10) and transforming growth factor β (TGF-β) appeared to be expressed earlier in prion-infected CD14(-/-) mice. In contrast, IL-1β expression appeared to be reduced in the CD14(-/-) mice in the early stage of infection. Double immunofluorescence staining demonstrated that CD11b- and Iba1-positive microglia mainly produced the anti-inflammatory cytokines, suggesting anti-inflammatory status of microglia in the CD14(-/-) mice in the early stage of infection. These results imply that CD14 plays a role in the disease progression by suppressing anti-inflammatory responses in the brain in the early stage of infection.

Toll like receptor 4 mediates cell death in a mouse MPTP model of Parkinson disease

In mammalians, toll-like receptors (TLR) signal-transduction pathways induce the expression of a variety of immune-response genes, including inflammatory cytokines. It is therefore plausible to assume that TLRs are mediators in glial cells triggering the release of cytokines that ultimately kill DA neurons in the substantia nigra in Parkinson disease (PD). Accordingly, recent data indicate that TLR4 is up-regulated by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment in a mouse model of PD. Here, we wished to evaluate the role of TLR4 in the acute mouse MPTP model of PD: TLR4-deficient mice and wild-type littermates control mice were used for the acute administration way of MPTP or a corresponding volume of saline. We demonstrate that TLR4-deficient mice are less vulnerable to MPTP intoxication than wild-type mice and display a decreased number of Iba1+ and MHC II+ activated microglial cells after MPTP application, suggesting that the TLR4 pathway is involved in experimental PD.

Inflammation and neurodegeneration: the story 'retolled'

Toll-like receptors (TLRs) play a crucial role in innate immunity by recognizing conserved motifs predominantly found in microorganisms. Increasing evidence supports a role for TLRs in sterile inflammation as observed in neurodegenerative disorders. This includes work suggesting a contribution for these receptors to the pathophysiology of Alzheimer's disease (AD), Parkinson's disease (PD), and related disorders. In this review, the potential role of TLRs in the context of protein aggregation, neuronal degeneration, and genetic risk factors is addressed. In particular, we discuss the evidence derived from experimental models of both AD and PD which suggests that activation of TLRs can have beneficial and detrimental effects on pathological features such as protein aggregation and neuronal death. A deeper understanding of these dichotomous observations could be used for therapeutic benefit.

Inflammation in Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disease that is characterized by the degeneration of dopaminergic neurons in the substantia nigra pars compacta. Inflammatory responses manifested by glial reactions, T cell infiltration, and increased expression of inflammatory cytokines, as well as other toxic mediators derived from activated glial cells, are currently recognized as prominent features of PD. The consistent findings obtained by various animal models of PD suggest that neuroinflammation is an important contributor to the pathogenesis of the disease and may further propel the progressive loss of nigral dopaminergic neurons. Furthermore, although it may not be the primary cause of PD, additional epidemiological, genetic, pharmacological, and imaging evidence support the proposal that inflammatory processes in this specific brain region are crucial for disease progression. Recent in vitro studies, however, have suggested that activation of microglia and subsequently astrocytes via mediators released by injured dopaminergic neurons is involved. However, additional in vivo experiments are needed for a deeper understanding of the mechanisms involved in PD pathogenesis. Further insight on the mechanisms of inflammation in PD will help to further develop alternative therapeutic strategies that will specifically and temporally target inflammatory processes without abrogating the potential benefits derived by neuroinflammation, such as tissue restoration.

The endotoxin-induced neuroinflammation model of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by the progressive loss of dopaminergic (DA) neurons in the substantia nigra. Although the exact cause of the dopaminergic neurodegeneration remains elusive, recent postmortem and experimental studies have revealed an essential role for neuroinflammation that is initiated and driven by activated microglial and infiltrated peripheral immune cells and their neurotoxic products (such as proinflammatory cytokines, reactive oxygen species, and nitric oxide) in the pathogenesis of PD. A bacterial endotoxin-based experimental model of PD has been established, representing a purely inflammation-driven animal model for the induction of nigrostriatal dopaminergic neurodegeneration. This model, by itself or together with genetic and toxin-based animal models, provides an important tool to delineate the precise mechanisms of neuroinflammation-mediated dopaminergic neuron loss. Here, we review the characteristics of this model and the contribution of neuroinflammatory processes, induced by the in vivo administration of bacterial endotoxin, to neurodegeneration. Furthermore, we summarize the recent experimental therapeutic strategies targeting endotoxin-induced neuroinflammation to elicit neuroprotection in the nigrostriatal dopaminergic system. The potential of the endotoxin-based PD model in the development of an early-stage specific diagnostic biomarker is also emphasized.

Pharmacological therapy in Parkinson's disease: focus on neuroprotection

Although the number of available therapeutic approaches in Parkinson's disease (PD) is steadily increasing the search for effective neuroprotective agent is continuing. Such research is directed at influencing the key steps in the pathomechanism: the mitochondrial dysfunction, the oxidative stress, the neuroinflammatory processes and the final common apoptotic pathway. Earlier-developed symptomatic therapies were implicated to be neuroprotective, and promising novel disease modifying approaches were brought into the focus of interest. The current review presents a survey of our current knowledge relating to the pathomechanism of PD and discusses the putative neuroprotective therapy.

Expression of TLR4 and CD14 in the central nervous system (CNS) in a MPTP mouse model of Parkinson's-like disease

Systemic infections are often associated with neurodegenerative processes in many diseases of the central nervous system (CNS), including Parkinson's disease. Toll-like receptor (TLR)4 and CD14 act as receptors for lipopolysaccharide (LPS) released by gram-negative bacteria. In this contest, CD14 functions as the main LPS ligand and TLR4 transmits the LPS signal into the cell. In this paper, we investigated the expression of TLR4 and CD14, in different anatomical areas of the CNS, in an experimental model of Parkinson's-like disease, represented by MPTP-treated mouse. In particular, we analyzed the gene transcripts and proteins expression of CD14 and TLR4, in the substantia nigra and caudate-putamen nuclei of these animals. Results demonstrated an augmented expression of both CD14 and TLR4 in the substantia nigra of mice treated with MPTP in comparison to untreated animals, suggesting that the endotoxin receptors are over expressed in different manner in specific areas of the CNS during Parkinson's-like disease.

Neuroinflammation in Parkinson's disease: a target for neuroprotection?

Parkinson's disease is characterised by a slow and progressive degeneration of dopaminergic neurons in the substantia nigra. Despite intensive research, the cause of the neuronal loss in Parkinson's disease is poorly understood. Neuroinflammatory mechanisms might contribute to the cascade of events leading to neuronal degeneration. In this Review, we describe the evidence for neuroinflammatory processes from post-mortem and in vivo studies in Parkinson's disease. We further identify the cellular and molecular events associated with neuroinflammation that are involved in the degeneration of dopaminergic neurons in animal models of the disease. Overall, available data support the importance of non-cell-autonomous pathological mechanisms in Parkinson's disease, which are mostly mediated by activated glial and peripheral immune cells. This cellular response to neurodegeneration triggers deleterious events (eg, oxidative stress and cytokine-receptor-mediated apoptosis), which might eventually lead to dopaminergic cell death and hence disease progression. Finally, we highlight possible therapeutic strategies (including immunomodulatory drugs and therapeutic immunisation) aimed at downregulating these inflammatory processes that might be important to slow the progression of Parkinson's disease.

Sex, genes, hormones and nigral neurodegeneration: two different Parkinson’s diseases in males and in females

Evaluation of: Cantuti-Castelvetri I, Keller-McGandy C, Bouzou B et al.: Effects of gender on nigral gene expression and Parkinson disease. Neurobiol. Dis. 26(3), 606–614 (2007). Evidence from clinical, anatomical, neurophysiological, endocrinological and genetic studies demonstrates that Parkinson’s disease (PD) is a gender-specific pathology. The study by Cantuti-Castelvetri et al. points out the central role of sexual dimorphisms in gene expression for understanding the pathophysiology of gender differences in PD. They identified several genes differentially expressed in female and male PD patients. Their results support the hypothesis that PD in females and in males are two different pathologies possibly associated with specific therapies. Gender differences in PD could be seen in the wider perspective of gender medicine for neurological disorders and their results suggest that basic and clinical research should examine sex differences for both understanding the disease and optimizing treatments.

Therapeutic approaches to inflammation in neurodegenerative disease

PURPOSE OF REVIEW

According to the neuroinflammatory hypothesis of neurodegenerative diseases, drugs with an anti-inflammatory mode of action should slow the disease progression. Here we review recent advances in our understanding of one such disorder, Parkinson's disease, in which anti-inflammatory drugs are now becoming a new therapeutic focus.

RECENT FINDINGS

The involvement of inflammatory mechanisms in Parkinson's disease has been revealed through in-vitro and in-vivo experimental studies supported by pathological and epidemiological findings. Several of the demonstrated inflammatory mechanisms are shared by other neurodegenerative disorders but some Parkinson's disease-specific mechanisms have also emerged. These include inflammatory stimulation by interaction of alpha-synuclein with microglia and astrocytes and a suppressive action by nonsteroidal anti-inflammatory drugs on dopamine quinone formation.

SUMMARY

It can be anticipated that a more detailed understanding of neuroinflammatory mechanisms in Parkinson's disease will lead to new cellular and molecular targets, which may, in turn, permit design of Parkinson's disease modifying drugs. Future treatment may involve combination therapies with drugs directed at both inflammatory and non-inflammatory mechanisms.