The role of TREM2 in Alzheimer's disease and other neurodegenerative disorders

The Immune System's Role in the Consequences of Mild Traumatic Brain Injury (Concussion)

Mild traumatic brain injury (mild TBI), often referred to as concussion, is the most common form of TBI and affects millions of people each year. A history of mild TBI increases the risk of developing emotional and neurocognitive disorders later in life that can impact on day to day living. These include anxiety and depression, as well as neurodegenerative conditions such as chronic traumatic encephalopathy (CTE) and Alzheimer's disease (AD). Actions of brain resident or peripherally recruited immune cells are proposed to be key regulators across these diseases and mood disorders. Here, we will assess the impact of mild TBI on brain and patient health, and evaluate the recent evidence for immune cell involvement in its pathogenesis.

Microvascular Alterations in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder associated with continual decline in cognition and ability to perform routine functions such as remembering familiar places or understanding speech. For decades, amyloid beta (Aβ) was viewed as the driver of AD, triggering neurodegenerative processes such as inflammation and formation of neurofibrillary tangles (NFTs). This approach has not yielded therapeutics that cure the disease or significant improvements in long-term cognition through removal of plaques and Aβ oligomers. Some researchers propose alternate mechanisms that drive AD or act in conjunction with amyloid to promote neurodegeneration. This review summarizes the status of AD research and examines research directions including and beyond Aβ, such as tau, inflammation, and protein clearance mechanisms. The effect of aging on microvasculature is highlighted, including its contribution to reduced blood flow that impairs cognition. Microvascular alterations observed in AD are outlined, emphasizing imaging studies of capillary malfunction. The review concludes with a discussion of two therapies to protect tissue without directly targeting Aβ for removal: (1) administration of growth factors to promote vascular recovery in AD; (2) inhibiting activity of a calcium-permeable ion channels to reduce microglial activation and restore cerebral vascular function.

Apoptotic neurons and amyloid-beta clearance by phagocytosis in Alzheimer's disease: Pathological mechanisms and therapeutic outlooks

Neuronal survival and axonal renewal following central nervous system damage and in neurodegenerative illnesses, such as Alzheimer's disease (AD), can be enhanced by fast clearance of neuronal apoptotic debris, as well as the removal of amyloid beta (Aβ) by phagocytic cells through the process of efferocytosis. This process quickly inhibits the release of proinflammatory and antigenic autoimmune constituents, enhancing the formation of a microenvironment vital for neuronal survival and axonal regeneration. Therefore, the detrimental features associated with microglial phagocytosis uncoupling, such as the accumulation of apoptotic cells, inflammation and phagoptosis, could exacerbate the pathology in brain disease. Some mechanisms of efferocytosis could be targeted by several promising agents, such as curcumin, URMC-099 and Y-P30, which have emerged as potential treatments for AD. This review aims to investigate and update the current research regarding the signaling molecules and pathways involved in efferocytosis and how these could be targeted as a potential therapy in AD.

Gene therapy-mediated enhancement of protective protein expression for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is the leading form of dementia but lacks curative treatments. Current understanding of AD aetiology attributes the development of the disease to the misfolding of two proteins; amyloid-β (Aβ) and hyperphosphorylated tau, with their pathological accumulation leading to concomitant oxidative stress, neuroinflammation, and neuronal death. These processes are regulated at multiple levels to maintain homeostasis and avert disease. However, many of the relevant regulatory proteins appear to be downregulated in the AD-afflicted brain. Enhancement/restoration of these 'protective' proteins, therefore, represents an attractive therapeutic avenue. Gene therapy is a desirable means of achieving this because it is not associated with the side-effects linked to systemic protein administration, and sustained protein expression virtually eliminates compliance issues. The current article represents a focused and succinct review of the better established 'protective' protein targets for gene therapy enhancement/restoration rather than being designed as an exhaustive review incorporating less validated protein subjects. In addition, we will discuss how the risks associated with uncontrolled or irreversible gene expression might be mitigated through combining neuronal-specific promoters, inducible expression systems and localised injections. Whilst many of the gene therapy targets reviewed herein are yet to enter clinical trials, preclinical testing has thus far demonstrated encouraging potential for the gene therapy-based treatment of AD.

Neuropathological Alzheimer's Disease Lesions in Nasu-Hakola Disease with TREM2 Mutation: Atypical Distribution of Neurofibrillary Changes

Nasu-Hakola disease is a rare autosomal recessive disorder associated to mutations in TREM2 and DAP12 genes, neuropathologically characterized by leukoencephalopathy with axonal spheroids. We report the neuropathologic findings of a 51-year-old female with a homozygous mutation (Q33X) of TREM2 gene. Beside severe cerebral atrophy and hallmarks of Nasu-Hakola disease, significant Alzheimer's disease lesions were present. Neurofibrillary changes showed an atypical topographic distribution being severe at spots in the neocortex while sparing the mesial temporal structures. Our finding suggests that TREM2 genetic defects may favor Alzheimer's disease pathology with neurofibrillary changes not following the hierarchical staging of cortical involvement identified by Braak.

The Potential Roles of Redox Enzymes in Alzheimer's Disease: Focus on Thioredoxin

Alzheimer's disease (AD) is the most common neurodegenerative diseases. Increasing studies have demonstrated the critical importance for redox proteins mediating neuronal protection in models of AD. This review briefly describes some of the risk factors contributing to AD, specifically highlighting the important roles of oxidative stress in the pathology of AD. Then this article concisely introduces the dysregulation and functions of two main redox enzymes, peroxiredoxins and glutaredoxins, in AD models. This review emphasizes the neuroprotective role of the third redox enzyme thioredoxin (Trx), an important multifunctional protein regulating cellular redox status. This commentary not only summarizes the alterations of Trx expression in AD patients and models, but also reviews the potential effects and mechanisms of Trx, Trx-related molecules and Trx-inducing compounds against AD. In conclusion, Trx has a potential neuroprotection in AD and may be very promising for clinical therapy of AD in the future.

Amyloid-beta uptake by blood monocytes is reduced with ageing and Alzheimer's disease

Deficits in the clearance of amyloid β-protein (Aβ) play a pivotal role in the pathogenesis of sporadic Alzheimer's disease (AD). The roles of blood monocytes in the development of AD remain unclear. In this study, we sought to investigate the alterations in the Aβ phagocytosis function of peripheral monocytes during ageing and in AD patients. A total of 104 cognitively normal participants aged 22-89 years, 24 AD patients, 25 age- and sex-matched cognitively normal (CN) subjects, 15 Parkinson's disease patients (PD), and 15 age- and sex-matched CN subjects were recruited. The Aβ uptake by blood monocytes was measured and its alteration during ageing and in AD patients were investigated. Aβ_1-42 uptake by monocytes decreased during ageing and further decreased in AD but not in PD patients. Aβ_1-42 uptake by monocytes was associated with Aβ_1-42 levels in the blood. Among the Aβ uptake-related receptors and enzymes, the expression of Toll-like receptor 2 (TLR2) was reduced in monocytes from AD patients. Our findings suggest that monocytes regulate the blood levels of Aβ and might be involved in the development of AD. The recovery of the Aβ uptake function by blood monocytes represents a potential therapeutic strategy for AD.

A high cerebrospinal fluid soluble TREM2 level is associated with slow clinical progression of Alzheimer's disease

INTRODUCTION

The progression rate of Alzheimer's disease (AD) varies and might be affected by the triggering receptor expressed on myeloid cells (TREM2) activity. We explored if cerebrospinal fluid (CSF) soluble TREM2 (sTREM2), a proxy of microglial activity, is associated with clinical progression rate.

METHODS

Patients with clinical AD (N = 231) were followed for up to 3 years after diagnosis. Cognitively healthy controls (N = 42) were followed for 5 years. CSF sTREM2 was analyzed by enzyme-linked immunosorbent assay. Group-based trajectory modeling revealed distinct clinical progression groups.

RESULTS

Higher CSF sTREM2 was associated with slow clinical progression. The slow- and medium-progressing groups had higher CSF sTREM2 than the cognitively healthy, who had a similar level to patients with rapid clinical progression.

DISCUSSION

CSF sTREM2 levels were associated with clinical progression in AD, regardless of core biomarkers. This could be useful in assessing disease development in relation to patient care and clinical trial recruitment.

Neuroinflammation in neurodegenerative disorders: the roles of microglia and astrocytes

Neuroinflammation is associated with neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Microglia and astrocytes are key regulators of inflammatory responses in the central nervous system. The activation of microglia and astrocytes is heterogeneous and traditionally categorized as neurotoxic (M1-phenotype microglia and A1-phenotype astrocytes) or neuroprotective (M2-phenotype microglia and A2-phenotype astrocytes). However, this dichotomized classification may not reflect the various phenotypes of microglia and astrocytes. The relationship between these activated glial cells is also very complicated, and the phenotypic distribution can change, based on the progression of neurodegenerative diseases. A better understanding of the roles of microglia and astrocytes in neurodegenerative diseases is essential for developing effective therapies. In this review, we discuss the roles of inflammatory response in neurodegenerative diseases, focusing on the contributions of microglia and astrocytes and their relationship. In addition, we discuss biomarkers to measure neuroinflammation and studies on therapeutic drugs that can modulate neuroinflammation.

Cerebrospinal fluid sTREM2 in Alzheimer's disease: comparisons between clinical presentation and AT classification

Triggering receptor expressed on myeloid cells 2 (TREM2) is an innate immune receptor expressed by microglia. Its cleaved fragments, soluble TREM2 (sTREM2), can be measured in the cerebrospinal fluid (CSF). Previous studies indicate higher CSF sTREM2 in symptomatic AD; however most of these studies have included biomarker positive AD cases and biomarker negative controls. The aim of the study was to explore potential differences in the CSF level of sTREM2 and factors associated with an increased sTREM2 level in patients diagnosed with mild cognitive impairment (MCI) or dementia due to AD compared with cognitively unimpaired controls as judged by clinical symptoms and biomarker category (AT). We included 299 memory clinic patients, 62 (20.7%) with AD-MCI and 237 (79.3%) with AD dementia, and 113 cognitively unimpaired controls. CSF measures of the core biomarkers were applied to determine AT status. CSF sTREM2 was analyzed by ELISA. Patients presented with comparable CSF sTREM2 levels as the cognitively unimpaired (9.6 ng/ml [SD 4.7] versus 8.8 ng/ml [SD 3.6], p = 0.27). We found that CSF sTREM2 associated with age-related neuroinflammation and tauopathy irrespectively of amyloid β, APOE ε4 status or gender. The findings were similar in both symptomatic and non-symptomatic individuals.

A Unifying Hypothesis for Alzheimer's Disease: From Plaques to Neurodegeneration

Evidence suggests that amyloid β is highly toxic to synapses in a phospho-Tau-dependent manner. Here, I present a hypothesis that links previous evidence from the first rise of amyloid β through to Tau tangles and neurodegeneration. In the immediate vicinity of plaques, concentrated soluble amyloid β occurs in equilibrium with deposited forms. Initially, plaques cover only a small percentage of brain volume. Microglia, by efficiently removing damaged synapses, may prevent spread of damage along the axon, restricting damage to the immediate vicinity of plaques. However, as plaque load increases, as seen in Alzheimer's disease, an individual axon may suffer multiple points of damage, leading to dissociation of Tau, formation of a tangle, and loss of the axon. As more axons suffer this fate, the network eventually degenerates. According to this hypothesis, the degree of plaque load that an individual can tolerate would depend on the efficiency of their microglia in removing amyloid-β-damaged synapses and the distribution of plaques, relative to axon trajectories, would determine the eventual cognitive symptoms.

Quinoline-Pyrazole Scaffold as a Novel Ligand of Galectin-3 and Suppressor of TREM2 Signaling

Galectin-3 has been identified as a critical player in driving the neuroinflammatory responses in Alzheimer's disease (AD). A key feature of this function of galectin-3 is associated with its interaction with the triggering receptor expressed on myeloid cells-2 (TREM2). Herein, we report a high-throughput screening (HTS) platform that can be used for the identification of inhibitors of TREM2 and galectin-3 interaction. We have utilized this HTS assay to screen a focused library of compounds optimized for the central nervous system (CNS)-related diseases. MG-257 was identified from this screen as the first example of a small molecule that can attenuate TREM2 signaling based on its high affinity to galectin-3 (endogenous ligand of TREM2). Remarkably, MG-257 reduced the levels of proinflammatory cytokines in activated microglial cells, which highlights its ability to inhibit the neuroinflammatory response associated with AD.

The Other Side of Alzheimer's Disease: Influence of Metabolic Disorder Features for Novel Diagnostic Biomarkers

Nowadays, the amyloid cascade hypothesis is the dominant model to explain Alzheimer's disease (AD) pathogenesis. By this hypothesis, the inherited genetic form of AD is discriminated from the sporadic form of AD (SAD) that accounts for 85-90% of total patients. The cause of SAD is still unclear, but several studies have shed light on the involvement of environmental factors and multiple susceptibility genes, such as Apolipoprotein E and other genetic risk factors, which are key mediators in different metabolic pathways (e.g., glucose metabolism, lipid metabolism, energetic metabolism, and inflammation). Furthermore, growing clinical evidence in AD patients highlighted the presence of affected systemic organs and blood similarly to the brain. Collectively, these findings revise the canonical understating of AD pathogenesis and suggest that AD has metabolic disorder features. This review will focus on AD as a metabolic disorder and highlight the contribution of this novel understanding on the identification of new biomarkers for improving an early AD diagnosis.

Tau-Mediated Dysregulation of Neuroplasticity and Glial Plasticity

The inability of individual neurons to compensate for aging-related damage leads to a gradual loss of functional plasticity in the brain accompanied by progressive impairment in learning and memory. Whereas this loss in neuroplasticity is gradual during normal aging, in neurodegenerative diseases such as Alzheimer’s disease (AD), this loss is accelerated dramatically, leading to the incapacitation of patients within a decade of onset of cognitive symptoms. The mechanisms that underlie this accelerated loss of neuroplasticity in AD are still not completely understood. While the progressively increasing proteinopathy burden, such as amyloid β (Aβ) plaques and tau tangles, definitely contribute directly to a neuron’s functional demise, the role of non-neuronal cells in controlling neuroplasticity is slowly being recognized as another major factor. These non-neuronal cells include astrocytes, microglia, and oligodendrocytes, which through regulating brain homeostasis, structural stability, and trophic support, play a key role in maintaining normal functioning and resilience of the neuronal network. It is believed that chronic signaling from these cells affects the homeostatic network of neuronal and non-neuronal cells to an extent to destabilize this harmonious milieu in neurodegenerative diseases like AD. Here, we will examine the experimental evidence regarding the direct and indirect pathways through which astrocytes and microglia can alter brain plasticity in AD, specifically as they relate to the development and progression of tauopathy. In this review article, we describe the concepts of neuroplasticity and glial plasticity in healthy aging, delineate possible mechanisms underlying tau-induced plasticity dysfunction, and discuss current clinical trials as well as future disease-modifying approaches.

Mitochondrial Dysfunction and Alzheimer's Disease: Role of Microglia

In 1907, Alois Alzheimer observed, as he quoted, development of "numerous fibers" and "adipose saccules" in the brain of his diseased patient Auguste Deter. The neurodegenerative disease became known as Alzheimer's disease (AD) and is the most common cause of dementia worldwide. AD normally develops with aging and is mostly initiated because of the imbalance between the formation and clearance of amyloid-β (Aβ). Formation of neurofibrillary tangles (NFTs) of hyperphosphorylated tau is another hallmark of AD. Neuroinflammation plays a significant role in the development and pathology of AD. This chapter explores the role of mitochondrial dysfunction in microglia in case of AD. Mitochondrial oxidative stress in microglia has been linked to the development of AD. Elevated generation of reactive oxygen species (ROS) and loss of mitochondrial membrane potential through various mechanisms have been observed in AD. Aβ interacts with microglial receptors, such as triggering receptor expressed in myeloid cells 2 (TREM2), activating downstream pathways causing mitochondrial damage and aggravating inflammation and cytotoxicity. Fibrillar Aβ activates nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in microglia leading to elevated induction of mitochondrial ROS which further causes neurotoxicity. Elevated ROS in microglia causes activation of inflammatory and cell death pathways. Production of ATP, regulation of mitochondrial health, autophagy, and mitophagy in microglia play significant roles in the AD pathology. Understanding microglial physiology and mitochondrial dysfunction will enable better therapeutic interventions.

A 20-Year Journey from Axonal Injury to Neurodegenerative Diseases and the Prospect of Immunotherapy for Combating Alzheimer's Disease

The understanding of the dialogue between the brain and the immune system has undergone dramatic changes over the last two decades, with immense impact on the perception of neurodegenerative diseases, mental dysfunction, and many other brain pathologic conditions. Accumulated results have suggested that optimal function of the brain is dependent on support from the immune system, provided that this immune response is tightly controlled. Moreover, in contrast to the previous prevailing dogma, it is now widely accepted that circulating immune cells are needed for coping with brain pathologies and that their optimal effect is dependent on their type, location, and activity. In this perspective, we describe our own scientific journey, reviewing the milestones in attaining this understanding of the brain-immune axis integrated with numerous related studies by others. We then explain their significance in demonstrating the possibility of harnessing the immune system in a well-controlled manner for the treatment of neurodegenerative diseases.

Repetitive Head Trauma Induces Chronic Traumatic Encephalopathy by Multiple Mechanisms

Exposure to repetitive neurotrauma increases lifetime risk for developing progressive cognitive deficits, neurobehavioral abnormalities, and chronic traumatic encephalopathy (CTE). CTE is a tau protein neurodegenerative disease first identified in boxers and recently described in athletes participating in other contact sports (notably American football, ice hockey, rugby, and wrestling) and in military veterans with blast exposure. Currently, CTE can only be diagnosed by neuropathological examination of the brain after death. The defining diagnostic lesion of CTE consists of patchy perivascular accumulations of hyperphosphorylated tau protein that localize in the sulcal depths of the cerebral cortex. Neuronal abnormalities, axonopathy, neurovascular dysfunction, and neuroinflammation are triggered by repetitive head impacts (RHIs) and likely act as catalysts for CTE pathogenesis and progression. However, the specific mechanisms that link RHI to CTE are unknown. This review will explore two important areas of CTE pathobiology. First, we will review what is known about the biomechanical properties of RHI that initiate CTE-related pathologies. Second, we will provide an overview of key features of CTE neuropathology and how these contribute to abnormal tau hyperphosphorylation, accumulation, and spread.

Surgery-induced cognitive dysfunction is alleviated through triggering receptor expressed on myeloid cells 2

Neuroinflammation plays a key role in perioperative neurocognitive disorders (PND). Increased evidences indicate that triggering receptor expressed on myeloid cells 2 (TREM2) can mitigate inflammatory response in the brain, and the aim of this study is to investigate whether TREM2 is involved in surgery-induced cognitive dysfunction in adult mice. We used adult C57BL/6 mice subjected to intramedullary fixation surgery, and found that surgery did not impair the motor ability of mice, but worsened the learning and memory function, and reduced the expression of TREM2. Meanwhile, up-regulated TREM2 expression in the brain of mice, induced by selective TREM2 agonist HSP60, significantly improved the learning and memory, alleviated the neuroinflammation, and decreased the neuronal cell apoptosis in mice. Meanwhile, TREM2-siRNA abolished the increased expression of TREM2 induced by HSP60, and reversed all the HSP60-induced beneficial effects. Therefore, our study indicated that up-regulation of TREM2 alleviated neuroinflammation and improved learning and memory function after surgery in mice.

Cellular senescence and Alzheimer disease: the egg and the chicken scenario

Globally, 50 million people live with dementia, with Alzheimer disease (AD) being responsible for two-thirds of the total cases. As ageing is the main risk factor for dementia-related neurodegeneration, changes in the timing or nature of the cellular hallmarks of normal ageing might be key to understanding the events that convert normal ageing into neurodegeneration. Cellular senescence is a candidate mechanism that might be important for this conversion. Under persistent stress, as occurs in ageing, both postmitotic cells - including neurons - and proliferative cells - such as astrocytes and microglia, among others - can engender a state of chronic cellular senescence that is characterized by the secretion of pro-inflammatory molecules that promote the functional decline of tissues and organs. Ablation of senescent cells has been postulated as a promising therapeutic venue to target the ageing phenotype and, thus, prevent or mitigate ageing-related diseases. However, owing to a lack of evidence, it is not possible to label cellular senescence as a cause or a consequence of neurodegeneration. This Review examines cellular senescence in the context of ageing and AD, and discusses which of the processes - cellular senescence or AD - might come first.

Low-Dose Ionizing Radiation Modulates Microglia Phenotypes in the Models of Alzheimer's Disease

Alzheimer’s disease (AD) is the most common type of dementia. AD involves major pathologies such as amyloid-β (Aβ) plaques and neurofibrillary tangles in the brain. During the progression of AD, microglia can be polarized from anti-inflammatory M2 to pro-inflammatory M1 phenotype. The activation of triggering receptor expressed on myeloid cells 2 (TREM2) may result in microglia phenotype switching from M1 to M2, which finally attenuated Aβ deposition and memory loss in AD. Low-dose ionizing radiation (LDIR) is known to ameliorate Aβ pathology and cognitive deficits in AD; however, the therapeutic mechanisms of LDIR against AD-related pathology have been little studied. First, we reconfirm that LDIR (two Gy per fraction for five times)-treated six-month 5XFAD mice exhibited (1) the reduction of Aβ deposition, as reflected by thioflavins S staining, and (2) the improvement of cognitive deficits, as revealed by Morris water maze test, compared to sham-exposed 5XFAD mice. To elucidate the mechanisms of LDIR-induced inhibition of Aβ accumulation and memory loss in AD, we examined whether LDIR regulates the microglial phenotype through the examination of levels of M1 and M2 cytokines in 5XFAD mice. In addition, we investigated the direct effects of LDIR on lipopolysaccharide (LPS)-induced production and secretion of M1/M2 cytokines in the BV-2 microglial cells. In the LPS- and LDIR-treated BV-2 cells, the M2 phenotypic marker CD206 was significantly increased, compared with LPS- and sham-treated BV-2 cells. Finally, the effect of LDIR on M2 polarization was confirmed by detection of increased expression of TREM2 in LPS-induced BV2 cells. These results suggest that LDIR directly induced phenotype switching from M1 to M2 in the brain with AD. Taken together, our results indicated that LDIR modulates LPS- and Aβ-induced neuroinflammation by promoting M2 polarization via TREM2 expression, and has beneficial effects in the AD-related pathology such as Aβ deposition and memory loss.

Time to test antibacterial therapy in Alzheimer's disease

Alzheimer's disease is associated with cerebral accumulation of amyloid-β peptide and hyperphosphorylated tau. In the past 28 years, huge efforts have been made in attempting to treat the disease by reducing brain accumulation of amyloid-β in patients with Alzheimer's disease, with no success. While anti-amyloid-β therapies continue to be tested in prodromal patients with Alzheimer's disease and in subjects at risk of developing Alzheimer's disease, there is an urgent need to provide therapeutic support to patients with established Alzheimer's disease for whom current symptomatic treatment (acetylcholinesterase inhibitors and N-methyl d-aspartate antagonist) provide limited help. The possibility of an infectious aetiology for Alzheimer's disease has been repeatedly postulated over the past three decades. Infiltration of the brain by pathogens may act as a trigger or co-factor for Alzheimer's disease, with Herpes simplex virus type 1, Chlamydia pneumoniae, and Porphyromonas gingivalis being most frequently implicated. These pathogens may directly cross a weakened blood-brain barrier, reach the CNS and cause neurological damage by eliciting neuroinflammation. Alternatively, pathogens may cross a weakened intestinal barrier, reach vascular circulation and then cross blood-brain barrier or cause low grade chronic inflammation and subsequent neuroinflammation from the periphery. The gut microbiota comprises a complex community of microorganisms. Increased permeability of the gut and blood-brain barrier induced by microbiota dysbiosis may impact Alzheimer's disease pathogenesis. Inflammatory microorganisms in gut microbiota are associated with peripheral inflammation and brain amyloid-β deposition in subjects with cognitive impairment. Oral microbiota may also influence Alzheimer's disease risk through circulatory or neural access to the brain. At least two possibilities can be envisaged to explain the association of suspected pathogens and Alzheimer's disease. One is that patients with Alzheimer's disease are particularly prone to microbial infections. The other is that microbial infection is a contributing cause of Alzheimer's disease. Therapeutic trials with antivirals and/or antibacterials could resolve this dilemma. Indeed, antiviral agents are being tested in patients with Alzheimer's disease in double-blind placebo-controlled studies. Although combined antibiotic therapy was found to be effective in animal models of Alzheimer's disease, antibacterial drugs are not being widely investigated in patients with Alzheimer's disease. This is because it is not clear which bacterial populations in the gut of patients with Alzheimer's disease are overexpressed and if safe, selective antibacterials are available for them. On the other hand, a bacterial protease inhibitor targeting P. gingivalis toxins is now being tested in patients with Alzheimer's disease. Clinical studies are needed to test if countering bacterial infection may be beneficial in patients with established Alzheimer's disease.

Genetic architecture of common non-Alzheimer's disease dementias

Frontotemporal dementia (FTD), dementia with Lewy bodies (DLB) and vascular dementia (VaD) are the most common forms of dementia after Alzheimer’s disease (AD). The heterogeneity of these disorders and/or the clinical overlap with other diseases hinder the study of their genetic components. Even though Mendelian dementias are rare, the study of these forms of disease can have a significant impact in the lives of patients and families and have successfully brought to the fore many of the genes currently known to be involved in FTD and VaD, starting to give us a glimpse of the molecular mechanisms underlying these phenotypes. More recently, genome-wide association studies have also pointed to disease risk-associated loci. This has been particularly important for DLB where familial forms of disease are very rarely described. In this review we systematically describe the Mendelian and risk genes involved in these non-AD dementias in an effort to contribute to a better understanding of their genetic architecture, find differences and commonalities between different dementia phenotypes, and uncover areas that would benefit from more intense research endeavors.

The therapeutic potential of galectin-1 and galectin-3 in the treatment of neurodegenerative diseases

Introduction: Neuroinflammation has been proposed as a common factor and one of the main inducers of neuronal degeneration. Galectins are a group of β-galactoside-binding lectins, that play an important role in the immune response, adhesion, proliferation, differentiation, migration and cell growth. Up to 15 members of the galectin's family have been identified; however, the expression of galectin-1 and galectin-3 has been considered a key factor in neuronal regeneration and modulation of the inflammatory response. Galectin-1 is necessary to stimulate the secretion of neurotrophic factors in astrocytes and promoting neuronal regeneration. In contrast, galectin-3 fosters the proliferation of microglial cells and modulates cellular apoptosis, therefore these proteins are considered a useful alternative for the treatment of degenerative diseases.Areas covered: This review describes the roles of galectin-1 and galectin-3 in the modulation of neuroinflammation and their potential as therapeutic targets in the treatment for neurodegenerative diseases.Expert opinion: Although data in the literature vary, the effects of galectin-1 and galectin-3 on the activation and modulation of astrocytes and microglia has been described. Due to its anti-inflammatory effects, galectin-1 is proposed as a molecule with therapeutic potential, whereas the inhibition of galectin-3 could contribute to reduce the neuroinflammatory response in neurodegenerative diseases.

The Genetics of Alzheimer's Disease in the Chinese Population

Alzheimer’s disease (AD) is a neurodegenerative disease characterized by progressive cognitive dysfunction and behavioral impairment. In China, the number of AD patients is growing rapidly, which poses a considerable burden on society and families. In recent years, through the advancement of genome-wide association studies, second-generation gene sequencing technology, and their application in AD genetic research, more genetic loci associated with the risk for AD have been discovered, including KCNJ15, TREM2, and GCH1, which provides new ideas for the etiology and treatment of AD. This review summarizes three early-onset AD causative genes (APP, PSEN1, and PSEN2) and some late-onset AD susceptibility genes and their mutation sites newly discovered in China, and briefly introduces the potential mechanisms of these genetic susceptibilities in the pathogenesis of AD, which would help in understanding the genetic mechanisms underlying this devastating disease.

Can mouse models mimic sporadic Alzheimer's disease?

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common form of dementia worldwide. As age is the main risk factor, > 97% of all AD cases are of sporadic origin, potentiated by various risk factors associated with life style and starting at an age > 60 years. Only < 3% of AD cases are of genetic origin caused by mutations in the amyloid precursor protein or Presenilins 1 or 2, and symptoms already start at an age < 30 years. In order to study progression of AD, as well as therapeutic strategies, mouse models are state-of-the-art. So far many transgenic mouse models have been developed and used, with mutations in the APP or presenilin or combinations (3×Tg, 5×Tg). However, such transgenic mouse models more likely mimic the genetic form of AD and no information can be given how sporadic forms develop. Several risk genes, such as Apolipoprotein E4 and TREM-2 enhance the risk of sporadic AD, but also many risk factors associated with life style (e.g., diabetes, hypercholesterolemia, stress) may play a role. In this review we discuss the current situation regarding AD mouse models, and the problems to develop a sporadic mouse model of AD.

Abrogation of type-I interferon signalling alters the microglial response to Aβ1-42

Neuroinflammation and accompanying microglial dysfunction are now appreciated to be involved in Alzheimer’s disease (AD) pathogenesis. Critical to the process of neuroinflammation are the type-I interferon (IFN) family of cytokines. Efforts to phenotypically characterize microglia within AD identify distinct populations associated with type-I IFN signalling, yet how this affects underlying microglial function is yet to be fully elucidated. Here we demonstrate that Aβ_1–42 exposure increases bioactive levels of type-I IFN produced by primary microglia alongside increased expression of type-I IFN related genes. Primary microglia isolated from brains of APP_swePS1_ΔE9 mice with ablated type-I IFN signalling show an increased phagocytic ability to uptake FITC-Aβ_1–42. Correlative assessment of plaque sizes in aged APP_swePS1_ΔE9 mice with abrogated type-I IFN signalling show unchanged deposition levels. Microglia from these mice did however show alterations in morphology. This data further highlights the role of type-I IFN signalling within microglia and identifies a role in phagocytosis. As such, targeting both microglial and global type-I IFN signalling presents as a novel therapeutic strategy for AD management.

Therapeutic approaches targeting Apolipoprotein E function in Alzheimer's disease

One of the primary genetic risk factors for Alzheimer’s disease (AD) is the presence of the Ɛ4 allele of apolipoprotein E (APOE). APOE is a polymorphic lipoprotein that is a major cholesterol carrier in the brain. It is also involved in various cellular functions such as neuronal signaling, neuroinflammation and glucose metabolism. Humans predominantly possess three different allelic variants of APOE, termed E2, E3, and E4, with the E3 allele being the most common. The presence of the E4 allele is associated with increased risk of AD whereas E2 reduces the risk. To understand the molecular mechanisms that underlie APOE-related genetic risk, considerable effort has been devoted towards developing cellular and animal models. Data from these models indicate that APOE4 exacerbates amyloid β plaque burden in a dose-dependent manner. and may also enhance tau pathogenesis in an isoform-dependent manner. Other studies have suggested APOE4 increases the risk of AD by mechanisms that are distinct from modulation of Aβ or tau pathology. Further, whether plasma APOE, by influencing systemic metabolic pathways, can also possibly alter CNS function indirectly is not complete;y understood. Collectively, the available studies suggest that APOE may impact multiple signaling pathways and thus investigators have sought therapeutics that would disrupt pathological functions of APOE while preserving or enhancing beneficial functions. This review will highlight some of the therapeutic strategies that are currently being pursued to target APOE4 towards preventing or treating AD and we will discuss additional strategies that holds promise for the future.

Late-onset Alzheimer Disease

PURPOSE OF REVIEW

Alzheimer disease (AD) is the most common cause of late-onset dementia. This article describes the epidemiology, genetic and environmental risk factors, clinical diagnosis, biomarkers, and treatment of late-onset AD, defined by age of onset of 65 years or older.

RECENT FINDINGS

An estimated 5.7 million Americans are living with AD dementia, with the number of affected individuals growing rapidly because of an aging population. Vascular risk factors, sleep disorders, and traumatic brain injury are associated with an increased risk of AD, while increased cognitive and physical activity throughout the lifespan reduce the risk of disease. The primary genetic risk factor for late-onset AD is the apolipoprotein E (APOE) ε4 allele. AD typically presents with early and prominent episodic memory loss, although this clinical syndrome is neither sensitive nor specific for underlying AD neuropathology. Emerging CSF and imaging biomarkers can now detect the key neuropathologic features of the disease (amyloid plaques, neurofibrillary tangles, and neurodegeneration) in living people, allowing for characterization of patients based on biological measures. A comprehensive treatment plan for AD includes use of symptomatic medications, optimal treatment of comorbid conditions and neuropsychiatric symptoms, counseling about safety and future planning, and referrals to community resources.

SUMMARY

AD is very common in older neurologic patients. Neurologists should set the standard for the diagnosis and care of patients with AD and should be familiar with emerging biomarkers that have transformed AD research and are primed to enter the clinical arena.

The Ubiquitin System in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, most prevalent in the elderly population and has a significant impact on individuals and their family as well as the health care system and the economy. While the number of patients affected by various forms of dementia including AD is on the increase, there is currently no cure. Although genome-wide association studies have identified genetic markers for familial AD, the molecular mechanisms underlying the initiation and development of both familial and sporadic AD remain poorly understood. Most neurodegenerative diseases and in particular those associated with dementia have been defined as proteinopathies due to the presence of intra- and/or extracellular protein aggregates in the brain of affected individuals. Although loss of proteostasis in AD has been known for decades, it is only in recent years that we have come to appreciate the role of ubiquitin-dependent mechanisms in brain homeostasis and in brain diseases. Ubiquitin is a highly versatile post-translational modification which regulates many aspects of protein fate and function, including protein degradation by the Ubiquitin-Proteasome System (UPS), autophagy-mediated removal of damaged organelles and proteins, lysosomal turnover of membrane proteins and of extracellular molecules brought inside the cell through endocytosis. Amyloid-β (Aβ) fragments as well as hyperphosphorylation of Tau are hallmarks of AD, and these are found in extracellular plaques and intracellular fibrils in the brain of individuals with AD, respectively. Yet, whether it is the oligomeric or the soluble species of Aβ and Tau that mediate toxicity is still unclear. These proteins impact on mitochondrial energy metabolism, inflammation, as well as a number of housekeeping processes including protein degradation through the UPS and autophagy. In this chapter, we will discuss the role of ubiquitin in neuronal homeostasis as well as in AD; summarise crosstalks between the enzymes that regulate protein ubiquitination and the toxic proteins Tau and Aβ; highlight emerging molecular mechanisms in AD as well as future strategies which aim to exploit the ubiquitin system as a source for next-generation therapeutics.

Increased levels of Aβ42 decrease the lifespan of ob/ob mice with dysregulation of microglia and astrocytes

Clinical studies have indicated that obesity and diabetes are associated with Alzheimer's disease (AD) and neurodegeneration. Although the mechanisms underlying these associations remain elusive, the bidirectional interactions between obesity/diabetes and Alzheimer's disease (AD) may be involved in them. Both obesity/diabetes and AD significantly reduce life expectancy. We generated App^NL-F/wt knock-in; ob/ob mice by crossing App^NL-F/wt knock-in mice and ob/ob mice to investigate whether amyloid-β (Aβ) affects the lifespan of ob/ob mice. App^NL-F/wt knock-in; ob/ob mice displayed the shortest lifespan compared to wild-type mice, App^NL-F/wt knock-in mice, and ob/ob mice. Notably, the Aβ42 levels were increased at minimum levels before deposition in App^NL-F/wt knock-in mice and App^NL-F/wt knock-in; ob/ob mice at 18 months of age. No differences in the levels of several neuronal markers were observed between mice at this age. However, we observed increased levels of glial fibrillary acidic protein (GFAP), an astrocyte marker, in App^NL-F/wt knock-in; ob/ob mice, while the levels of several microglial markers, including CD11b, TREM2, and DAP12, were decreased in both ob/ob mice and App^NL-F/wt knock-in; ob/ob mice. The increase in GFAP levels was not observed in young App^NL-F/wt knock-in; ob/ob mice. Thus, the increased Aβ42 levels may decrease the lifespan of ob/ob mice, which is associated with the dysregulation of microglia and astrocytes in an age-dependent manner. Based on these findings, the imbalance in these neuroinflammatory cells may provide a clue to the mechanisms by which the interaction between obesity/diabetes and early AD reduces life expectancy.

Advances and considerations in AD tau-targeted immunotherapy

The multifactorial and complex nature of Alzheimer’s disease (AD) has made it difficult to identify therapeutic targets that are causally involved in the disease process. However, accumulating evidence from experimental and clinical studies that investigate the early disease process point towards the required role of tau in AD etiology. Importantly, a large number of studies investigate and characterize the plethora of pathological forms of tau protein involved in disease onset and propagation. Immunotherapy is one of the most clinical approaches anticipated to make a difference in the field of AD therapeutics. Tau –targeted immunotherapy is the new direction after the failure of amyloid beta (Aß)-targeted immunotherapy and the growing number of studies that highlight the Aß-independent disease process. It is now well established that immunotherapy alone will most likely be insufficient as a monotherapy. Therefore, this review discusses updates on tau-targeted immunotherapy studies, AD-relevant tau species, updates on promising biomarkers and a prospect on combination therapies to surround the disease propagation in an efficient and timely manner.

The Radiogenomics of Late-onset Alzheimer Disease

Radiogenomics, defined as the integrated analysis of radiologic imaging and genetic data, is a well-established tool shown to augment neuroimaging in the clinical diagnosis, prognostication, and scientific study of late-onset Alzheimer disease (LOAD). Early work using candidate single nucleotide polymorphisms (SNPs) identified genetic variation in APOE, BIN1, CLU, and CR1 as key modifiers of brain structure and function using magnetic resonance imaging (MRI). More recently, polygenic risk scores used in conjunction with MRI and positron emission tomography have shown great promise as a risk-stratification tool for clinical trials and care-management decisions. In addition, recent work using multimodal MRI and positron emission tomography as proxies of LOAD progression has identified novel risk variants that are enhancing our understanding of LOAD pathophysiology and progression. Herein, we highlight key studies and trends in the radiogenomics of LOAD over the past two decades and their implications for clinical practice and scientific research.

Recent advances from metabolomics and lipidomics application in alzheimer's disease inspiring drug discovery

Introduction: Although age is a major risk factor for Alzheimer's disease (AD), it is not an inevitable consequence of aging nor is it exclusively an old-age disease. Several other major risk factors for AD are strongly associated with metabolism and include lack of exercise, obesity, diabetes, high blood pressure and cholesterol, over-consumption of alcohol and depression in addition to low educational level, social isolation, and cognitive inactivity. Approaches for Alzheimer prevention and treatment through manipulation of metabolism and utilization of active metabolites have great potential either as a primary or secondary treatment avenue or as a preventative strategy in high-risk individuals.Areas covered: This review outlines the current knowledge concerning the relationship between AD and metabolism and the novel treatments attempting to correct changes in AD patients determined through metabolomics or lipidomic analyses.Expert opinion: Metabolites are one of the main driving factors and indicators of AD and can offer many possible avenues for prevention and treatment. However, with the highly interconnected effects of metabolites and metabolism, as well as the many different routes for metabolism dysfunction, successful treatment would have to include the correction of metabolic errors as well as errors in transport and metabolite processing in order to affect and revert AD progression.

Transcriptional Networks of Microglia in Alzheimer's Disease and Insights into Pathogenesis

Microglia, the main immune cells of the central nervous system, are increasingly implicated in Alzheimer's disease (AD). Manifold transcriptomic studies in the brain have not only highlighted microglia's role in AD pathogenesis, but also mapped crucial pathological processes and identified new therapeutic targets. An important component of many of these transcriptomic studies is the investigation of gene expression networks in AD brain, which has provided important new insights into how coordinated gene regulatory programs in microglia (and other cell types) underlie AD pathogenesis. Given the rapid technological advancements in transcriptional profiling, spanning from microarrays to single-cell RNA sequencing (scRNA-seq), tools used for mapping gene expression networks have evolved to keep pace with the unique features of each transcriptomic platform. In this article, we review the trajectory of transcriptomic network analyses in AD from brain to microglia, highlighting the corresponding methodological developments. Lastly, we discuss examples of how transcriptional network analysis provides new insights into AD mechanisms and pathogenesis.

Distinct Signaling Pathways Regulate TREM2 Phagocytic and NFκB Antagonistic Activities

Several genetic variants of the Triggering Receptor Expressed on Myeloid Cells-2 (TREM2) have been shown to increase the risk of developing Alzheimer’s disease (AD) supporting a role of microglia and immune cells in the pathobiology of AD. We have employed an ectopic model of TREM2 and DAP12 expression in HEK293 cells to study selectively TREM2 dependent signaling and phagocytic functions and evaluated the effects of some of the TREM2 mutations associated with AD. We show that shedding of the TREM2 N-terminal domain does not affect the inhibition of NFκB activation induced by TREM2 while it completely blocks phagocytosis suggesting that TREM2 anti-inflammatory properties can be mediated by the TREM2 C-terminal fragment while the phagocytic activity requires the full-length receptor. In addition, we confirm in that model that apolipoprotein E (APOE) is a ligand for TREM2 and triggers TREM2 signaling. In particular, we show that APOE4 stimulates spleen tyrosine kinase (SYK) activation more potently than APOE2 in a TREM2 dependent manner. Interestingly, TREM2 appears to antagonize NFκB activation induced by phorbol ester but is unable to prevent TNFα induction of NFκB activation suggesting that TREM2 antagonizes inflammatory events triggered downstream of PKC. TREM2 mutations drastically impact TREM2 phagocytosis as well as its ability to antagonize NFκB activation and notably prevent the activation of the PI3K/AKT pathway observed with wild-type TREM2. Overall our data suggest that TREM2 dependent phagocytosis requires an activation of the SYK/PI3K/AKT/PLCγ pathways while the suppression of NFκB activation by TREM2 is independent of SYK, PI3K, and PLCγ activities. This model of ectopic TREM2-DAP12 co-expression appears suitable to study TREM2 signaling as several biological functions of TREM2 and TREM2 mutations that have been previously described in myeloid and microglial cells were also replicated in this model.

Exosomal miRNAs in central nervous system diseases: biomarkers, pathological mediators, protective factors and therapeutic agents

Exosomes are small bilipid layer-enclosed extracellular vesicles that can be found in tissues and biological fluids. As a key cell-to-cell and distant communication mediator, exosomes are involved in various central nervous system (CNS) diseases, potentially through transferring their contents such as proteins, lipids and nucleic acids to the target cells. Exosomal miRNAs, which are small non-coding RNAs in the exosomes, are known to be more stable than free miRNAs and therefore have lasting effects on disease-related gene expressions. There are distinct profiles of exosomal miRNAs in different types of CNS diseases even before the onset of irreversible neurological damages, indicating that exosomal miRNAs within tissues and biological fluids could serve as promising biomarkers. Emerging evidence has also demonstrated the pathological effects of several exosomal miRNAs in CNS diseases via specific modulation of disease-related factors. Moreover, exosomes carry therapeutically beneficial miRNAs across the blood-brain-barrier, which can be exploited as a powerful drug delivery tool to help alleviating multiple CNS diseases. In this review, we summarize the recent progress made in understanding the biological roles of exosomal miRNAs as potential diagnostic biomarkers, pathological regulators, and therapeutic targets/drugs for CNS diseases. A comprehensive discussion of the main concerns and challenges for the applications of exosomal miRNAs in the clinical setting is also provided.

Stroke and Amyloid-β Downregulate TREM-2 and Uch-L1 Expression that Synergistically Promote the Inflammatory Response

Neuroinflammation is involved in the pathogenesis of Alzheimer's disease, and the transcription factor NF-κB is a player in this event. We found here that the ischemic damage alone or in association with Aβ1-42 activates the NF-κB pathway, induces an increase of BACE1 and a parallel inhibition of Uch-L1 and TREM2, both in vitro and in vivo, in Tg 5XFAD and in human brains of sporadic AD. This mechanism creates a synergistic loop that fosters inflammation. We also demonstrated a significant protection exerted by the restoration of Uch-L1 activity. The rescue of the enzyme is able to abolish the decrease of TREM2 and the parameters of neuroinflammation.

Single-Cell Quantification of mRNA Expression in The Human Brain

RNA analysis at the cellular resolution in the human brain is challenging. Here, we describe an optimised approach for detecting single RNA transcripts in a cell-type specific manner in frozen human brain tissue using multiplexed fluorescent RNAscope probes. We developed a new robust analytical approach for RNAscope quantification. Our method shows that low RNA integrity does not significantly affect RNAscope signal, recapitulates bulk RNA analysis and provides spatial context to transcriptomic analysis of human post-mortem brain at single-cell resolution. In summary, our optimised method allows the usage of frozen human samples from brain banks to perform quantitative RNAscope analysis.

The MS4A gene cluster is a key modulator of soluble TREM2 and Alzheimer's disease risk

Soluble triggering receptor expressed on myeloid cells 2 (sTREM2) in cerebrospinal fluid (CSF) has been associated with Alzheimer's disease (AD). TREM2 plays a critical role in microglial activation, survival, and phagocytosis; however, the pathophysiological role of sTREM2 in AD is not well understood. Understanding the role of sTREM2 in AD may reveal new pathological mechanisms and lead to the identification of therapeutic targets. We performed a genome-wide association study (GWAS) to identify genetic modifiers of CSF sTREM2 obtained from the Alzheimer's Disease Neuroimaging Initiative. Common variants in the membrane-spanning 4-domains subfamily A (MS4A) gene region were associated with CSF sTREM2 concentrations (rs1582763; P = 1.15 × 10-15); this was replicated in independent datasets. The variants associated with increased CSF sTREM2 concentrations were associated with reduced AD risk and delayed age at onset of disease. The single-nucleotide polymorphism rs1582763 modified expression of the MS4A4A and MS4A6A genes in multiple tissues, suggesting that one or both of these genes are important for modulating sTREM2 production. Using human macrophages as a proxy for microglia, we found that MS4A4A and TREM2 colocalized on lipid rafts at the plasma membrane, that sTREM2 increased with MS4A4A overexpression, and that silencing of MS4A4A reduced sTREM2 production. These genetic, molecular, and cellular findings suggest that MS4A4A modulates sTREM2. These findings also provide a mechanistic explanation for the original GWAS signal in the MS4A locus for AD risk and indicate that TREM2 may be involved in AD pathogenesis not only in TREM2 risk-variant carriers but also in those with sporadic disease.

Hypothesis: cerebrospinal fluid protein markers suggest a pathway toward symptomatic resilience to AD pathology

INTRODUCTION

We sought biological pathways that explained discordance between Alzheimer's disease (AD) pathology and symptoms.

METHODS

In 306 Alzheimer's Disease Neuroimaging Initiative (ADNI)-1 participants across the AD clinical spectrum, we investigated association between cognitive outcomes and 23 cerebrospinal fluid (CSF) analytes associated with abnormalities in the AD biomarkers amyloid β_1-42 and total-tau. In a 200-person "training" set, Least Absolute Shrinkage and Selection Operator regression estimated model weights for the 23 proteins, and for the AD biomarkers themselves, as predictors of ADAS-Cog₁₁ scores. In the remaining 106 participants ("validation" set), fully adjusted regression models then tested the Least Absolute Shrinkage and Selection Operator-derived models and a related protein marker summary score as predictors of ADAS-Cog₁₁, ADNI diagnostic category, and longitudinal cognitive trajectory.

RESULTS

AD biomarkers alone explained 26% of the variance in validation set cognitive scores. Surprisingly, the 23 AD-related proteins explained 31% of this variance. The biomarkers and protein markers appeared independent in this respect, jointly explaining 42% of test score variance. The composite protein marker score also predicted ADNI diagnosis and subsequent cognitive trajectory. Cognitive outcome prediction redounded principally to ten markers related to lipid or vascular functions or to microglial activation or chemotaxis. In each analysis, apoE protein and four markers in the latter immune-activation group portended better outcomes.

DISCUSSION

CSF markers of vascular, lipid-metabolic and immune-related functions may explain much of the disjunction between AD biomarker abnormality and symptom severity. In particular, our results suggest the hypothesis that innate immune activation improves cognitive outcomes in persons with AD pathology. This hypothesis should be tested by further study of cognitive outcomes related to CSF markers of innate immune activation.

Metalloproteinases and their tissue inhibitors in Alzheimer's disease and other neurodegenerative disorders

As life expectancy increases worldwide, age-related neurodegenerative diseases will increase in parallel. The lack of effective treatment strategies may soon lead to an unprecedented health, social and economic crisis. Any attempt to halt the progression of these diseases requires a thorough knowledge of the pathophysiological mechanisms involved to facilitate the identification of new targets and the application of innovative therapeutic strategies. The metzincin superfamily of metalloproteinases includes matrix metalloproteinases (MMP), a disintegrin and metalloproteinase (ADAM) and ADAM with thrombospondin motifs (ADAMTS). These multigenic and multifunctional proteinase families regulate the functions of an increasing number of signalling and scaffolding molecules involved in neuroinflammation, blood-brain barrier disruption, protein misfolding, synaptic dysfunction or neuronal death. Metalloproteinases and their physiological inhibitors, the tissue inhibitors of metalloproteinases (TIMPs), are therefore, at the crossroads of molecular and cellular mechanisms that support neurodegenerative processes, and emerge as potential new therapeutic targets. We provide an overview of current knowledge on the role and regulation of metalloproteinases and TIMPs in four major neurodegenerative diseases: Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and Huntington's disease.

TREM2 R47H (rs75932628) variant is unlikely to contribute to Multiple Sclerosis susceptibility and severity in a large Greek MS cohort

BACKGROUND

Multiple Sclerosis is a multifactorial autoimmune disease of the central nervous system, characterized by focal inflammation, demyelination and secondary axonal injury. TREM2 is a signaling protein which participates in the innate immune system by implication to inflammation, proliferation and phagocytosis. The R47H (rs75392628) rare variant of the TREM2 gene has been related to various neurological diseases and leads to impaired signaling, lipoprotein binding, lipoprotein uptake and surface uptake.

AIM

To assess the role of TREM2 rs75932628 on MS risk through a genetic candidate gene association case-control study in a Greek population.

METHODS

1246 MS cases and 398 controls were genotyped for this variant.

RESULTS

No MS or healthy subjects carried the variant.

CONCLUSION

This variant does not seem to play a determining role in the pathogenesis of MS, although further studies examining the presence of TREM2 mutations in other, phylogenetically different populations and the epigenetic regulation of this gene are needed in order to thoroughly investigate its role in MS.

Proximity extension assay testing reveals novel diagnostic biomarkers of atypical parkinsonian syndromes

OBJECTIVE

The high degree of clinical overlap between atypical parkinsonian syndromes (APS) and Parkinson's disease (PD) makes diagnosis challenging. We aimed to identify novel diagnostic protein biomarkers of APS using multiplex proximity extension assay (PEA) testing.

METHODS

Cerebrospinal fluid (CSF) samples from two independent cohorts, each consisting of APS and PD cases, and controls, were analysed for neurofilament light chain (NF-L) and Olink Neurology and Inflammation PEA biomarker panels. Whole-cohort comparisons of biomarker concentrations were made between APS (n=114), PD (n=37) and control (n=34) groups using logistic regression analyses that included gender, age and disease duration as covariates.

RESULTS

APS versus controls analyses revealed 11 CSF markers with significantly different levels in cases and controls (p<0.002). Four of these markers also reached significance (p<0.05) in APS versus PD analyses. Disease-specific analyses revealed lower group levels of FGF-5, FGF-19 and SPOCK1 in multiple system atrophy compared with progressive supranuclear palsy and corticobasal syndrome. Receiver operating characteristic curve analyses suggested that the diagnostic accuracy of NF-L was superior to the significant PEA biomarkers in distinguishing APS, PD and controls. The biological processes regulated by the significant proteins include cell differentiation and immune cell migration. Delta and notch-like epidermal growth factor-related receptor (DNER) had the strongest effect size in APS versus controls and APS versus PD analyses. DNER is highly expressed in substantia nigra and is an activator of the NOTCH1 pathway which has been implicated in the aetiology of other neurodegenerative disorders including Alzheimer's disease.

CONCLUSIONS

PEA testing has identified potential novel diagnostic biomarkers of APS.

Different pattern of CSF glial markers between dementia with Lewy bodies and Alzheimer's disease

The role of innate immunity in dementia with Lewy bodies (DLB) has been little studied. We investigated the levels in cerebrospinal fluid (CSF) of glial proteins YKL-40, soluble TREM2 (sTREM2) and progranulin in DLB and their relationship with Alzheimer’s disease (AD) biomarkers. We included patients with DLB (n = 37), prodromal DLB (prodDLB, n = 23), AD dementia (n = 50), prodromal AD (prodAD, n = 53), and cognitively normal subjects (CN, n = 44). We measured levels of YKL-40, sTREM2, progranulin, Aβ_1–42, total tau (t-tau) and phosphorylated tau (p-tau) in CSF. We stratified the group DLB according to the ratio t-tau/Aβ_1–42 (≥0.52, indicative of AD pathology) and the A/T classification. YKL-40, sTREM2 and progranulin levels did not differ between DLB groups and CN. YKL-40 levels were higher in AD and prodAD compared to CN and to DLB and prodDLB. Patients with DLB with a CSF profile suggestive of AD copathology had higher levels of YKL-40, but not sTREM2 or PGRN, than those without. T+ DLB patients had also higher YKL-40 levels than T−. Of these glial markers, only YKL-40 correlated with t-tau and p-tau in DLB and in prodDLB. In contrast, in prodAD, sTREM2 and PGRN also correlated with t-tau and p-tau. In conclusion, sTREM2 and PGRN are not increased in the CSF of DLB patients. YKL-40 is only increased in DLB patients with an AD biomarker profile, suggesting that the increase is driven by AD-related neurodegeneration. These data suggest a differential glial activation between DLB and AD.

GBA, Gaucher Disease, and Parkinson's Disease: From Genetic to Clinic to New Therapeutic Approaches

Parkinson’s disease (PD) is the second most common degenerative disorder. Although the disease was described more than 200 years ago, its pathogenetic mechanisms have not yet been fully described. In recent years, the discovery of the association between mutations of the GBA gene (encoding for the lysosomal enzyme glucocerebrosidase) and PD facilitated a better understating of this disorder. GBA mutations are the most common genetic risk factor of the disease. However, mutations of this gene can be found in different phenotypes, such as Gaucher’s disease (GD), PD, dementia with Lewy bodies (DLB) and rapid eye movements (REM) sleep behavior disorders (RBDs). Understanding the pathogenic role of this mutation and its different manifestations is crucial for geneticists and scientists to guide their research and to select proper cohorts of patients. Moreover, knowing the implications of the GBA mutation in the context of PD and the other associated phenotypes is also important for clinicians to properly counsel their patients and to implement their care. With the present review we aim to describe the genetic, clinical, and therapeutic features related to the mutation of the GBA gene.

Immune Signaling in Neurodegeneration

Neurodegenerative diseases of the central nervous system progressively rob patients of their memory, motor function, and ability to perform daily tasks. Advances in genetics and animal models are beginning to unearth an unexpected role of the immune system in disease onset and pathogenesis; however, the role of cytokines, growth factors, and other immune signaling pathways in disease pathogenesis is still being examined. Here we review recent genetic risk and genome-wide association studies and emerging mechanisms for three key immune pathways implicated in disease, the growth factor TGF-β, the complement cascade, and the extracellular receptor TREM2. These immune signaling pathways are important under both healthy and neurodegenerative conditions, and recent work has highlighted new functional aspects of their signaling. Finally, we assess future directions for immune-related research in neurodegeneration and potential avenues for immune-related therapies.

The vexing complexity of the amyloidogenic pathway

The role of the amyloidogenic pathway in the etiology of Alzheimer's disease (AD), particularly the common sporadic late onset forms of the disease, is controversial. To some degree, this is a consequence of the failure of drug and therapeutic antibody trials based either on targeting the proteases in this pathway or its amyloid end products. Here, we explore the formidable complexity of the biochemistry and cell biology associated with this pathway. For example, we review evidence that the immediate precursor of amyloid-β, the C99 domain of the amyloid precursor protein (APP), may itself be toxic. We also review important new results that appear to finally establish a direct genetic link between mutations in APP and the sporadic forms of AD. Based on the complexity of amyloidogenesis, it seems possible that a major contributor to the failure of related drug trials is that we have an incomplete understanding of this pathway and how it is linked to Alzheimer's pathogenesis. If so, this highlights a need for further characterization of this pathway, not its abandonment.

Recent advances in the genetics of frontotemporal dementia

Purpose of review

In this review we highlight recent advances in the human genetics of frontotemporal dementia (FTD). In addition to providing a broad survey of genes implicated in FTD in the last several years, we also discuss variation in genes implicated in both hereditary leukodystrophies and risk for FTD (e.g., TREM2, TMEM106B, CSF1R, AARS2, NOTCH3).

Recent findings

Over the past five years, genetic variation in approximately 50 genes has been confirmed or suggested to cause or influence risk for FTD and FTD-spectrum disorders. We first give background and discuss recent findings related to C9ORF72, GRN and MAPT, the genes most commonly implicated in FTD. We then provide a broad overview of other FTD-associated genes and go on to discuss new findings in FTD genetics in East Asian populations, including pathogenic variation in CHCHD10, which may represent a frequent cause of disease in Chinese populations. Finally, we consider recent insights gleaned from genome-wide association and genetic pleiotropy studies.

Summary

Recent genetic discoveries highlight cellular pathways involving autophagy, the endolysosomal system and neuroinflammation, and reveal an intriguing overlap between genes that confer risk for leukodystrophy and FTD.