TGFβ1 regulates the inflammatory response during chronic neurodegeneration

Intranasally delivered mesenchymal stromal cells decrease glial inflammation early in prion disease

Mesenchymal stromal cells (MSCs) are an intriguing avenue for the treatment of neurological disorders due to their ability to migrate to sites of neuroinflammation and respond to paracrine signaling in those sites by secreting cytokines, growth factors, and other neuromodulators. We potentiated this ability by stimulating MSCs with inflammatory molecules, improving their migratory and secretory properties. We investigated the use of intranasally delivered adipose-derived MSCs (AdMSCs) in combating prion disease in a mouse model. Prion disease is a rare, lethal neurodegenerative disease that results from the misfolding and aggregation of the prion protein. Early signs of this disease include neuroinflammation, activation of microglia, and development of reactive astrocytes. Later stages of disease include development of vacuoles, neuronal loss, abundant aggregated prions, and astrogliosis. We demonstrate the ability of AdMSCs to upregulate anti-inflammatory genes and growth factors when stimulated with tumor necrosis factor alpha (TNFα) or prion-infected brain homogenates. We stimulated AdMSCs with TNFα and performed biweekly intranasal deliveries of AdMSCs on mice that had been intracranially inoculated with mouse-adapted prions. At early stages in disease, animals treated with AdMSCs showed decreased vacuolization throughout the brain. Expression of genes associated with Nuclear Factor-kappa B (NF-κB) and Nod-Like Receptor family pyrin domain containing 3 (NLRP3) inflammasome signaling were decreased in the hippocampus. AdMSC treatment promoted a quiescent state in hippocampal microglia by inducing changes in both number and morphology. Animals that received AdMSCs showed a decrease in both overall and reactive astrocyte number, and morphological changes indicative of homeostatic astrocytes. Although this treatment did not prolong survival or rescue neurons, it demonstrates the benefits of MSCs in combatting neuroinflammation and astrogliosis.

The Dialogue Between Neuroinflammation and Adult Neurogenesis: Mechanisms Involved and Alterations in Neurological Diseases

Adult neurogenesis occurs mainly in the subgranular zone of the hippocampal dentate gyrus and the subventricular zone of the lateral ventricles. Evidence supports the critical role of adult neurogenesis in various conditions, including cognitive dysfunction, Alzheimer's disease (AD), and Parkinson's disease (PD). Several factors can alter adult neurogenesis, including genetic, epigenetic, age, physical activity, diet, sleep status, sex hormones, and central nervous system (CNS) disorders, exerting either pro-neurogenic or anti-neurogenic effects. Compelling evidence suggests that any insult or injury to the CNS, such as traumatic brain injury (TBI), infectious diseases, or neurodegenerative disorders, can provoke an inflammatory response in the CNS. This inflammation could either promote or inhibit neurogenesis, depending on various factors, such as chronicity and severity of the inflammation and underlying neurological disorders. Notably, neuroinflammation, driven by different immune components such as activated glia, cytokines, chemokines, and reactive oxygen species, can regulate every step of adult neurogenesis, including cell proliferation, differentiation, migration, survival of newborn neurons, maturation, synaptogenesis, and neuritogenesis. Therefore, this review aims to present recent findings regarding the effects of various components of the immune system on adult neurogenesis and to provide a better understanding of the role of neuroinflammation and neurogenesis in the context of neurological disorders, including AD, PD, ischemic stroke (IS), seizure/epilepsy, TBI, sleep deprivation, cognitive impairment, and anxiety- and depressive-like behaviors. For each disorder, some of the most recent therapeutic candidates, such as curcumin, ginseng, astragaloside, boswellic acids, andrographolide, caffeine, royal jelly, estrogen, metformin, and minocycline, have been discussed based on the available preclinical and clinical evidence.

PAI-1 production by reactive astrocytes drives tissue dysfibrinolysis in multiple sclerosis models

BACKGROUND

In multiple sclerosis (MS), disturbance of the plasminogen activation system (PAS) and blood brain barrier (BBB) disruption are physiopathological processes that might lead to an abnormal fibrin(ogen) extravasation into the parenchyma. Fibrin(ogen) deposits, usually degraded by the PAS, promote an autoimmune response and subsequent demyelination. However, the PAS disruption is not well understood and not fully characterized in this disorder.

METHODS

Here, we characterized the expression of PAS actors during different stages of two mouse models of MS (experimental autoimmune encephalomyelitis-EAE), in the central nervous system (CNS) by quantitative RT-PCR, immunohistofluorescence and fluorescent in situ hybridization (FISH). Thanks to constitutive PAI-1 knockout mice (PAI-1 KO) and an immunotherapy using a blocking PAI-1 antibody, we evaluated the role of PAI-1 in EAE models and its impact on physiopathological processes such as fibrin(ogen) deposits, lymphocyte infiltration and demyelination.

RESULTS

We report a striking overexpression of PAI-1 in reactive astrocytes during symptomatic phases, in two EAE mouse models of MS. This increase is concomitant with lymphocyte infiltration and fibrin(ogen) deposits in CNS parenchyma. By genetic invalidation of PAI-1 in mice and immunotherapy using a blocking PAI-1 antibody, we demonstrate that abolition of PAI-1 reduces the severity of EAE and occurrence of relapses in two EAE models. These benefits are correlated with a decrease in fibrin(ogen) deposits, infiltration of T4 lymphocytes, reactive astrogliosis, demyelination and axonal damage.

CONCLUSION

These results demonstrate that a deleterious overexpression of PAI-1 by reactive astrocytes leads to intra-parenchymal dysfibrinolysis in MS models and anti-PAI-1 strategies could be a new therapeutic perspective for MS.

Human in vivo neuroimaging to detect reprogramming of the cerebral immune response following repeated systemic inflammation

Despite increasing evidence that immune training within the brain may affect the clinical course of neuropsychiatric diseases, data on cerebral immune tolerance are scarce. This study in healthy volunteers examined the trajectory of the immune response systemically and within the brain following repeated lipopolysaccharide (LPS) challenges. Five young males underwent experimental human endotoxemia (intravenous administration of 2 ng/kg LPS) twice with a 7-day interval. The systemic immune response was assessed by measuring plasma cytokine levels. Four positron emission tomography (PET) examinations, using the translocator protein (TSPO) ligand ¹⁸F-DPA-714, were performed in each participant, to assess brain immune cell activation prior to and 5 hours after both LPS challenges. The first LPS challenge caused a profound systemic inflammatory response and resulted in a 53% [95%CI 36-71%] increase in global cerebral ¹⁸F-DPA-714 binding (p < 0.0001). Six days after the first challenge, ¹⁸F-DPA-714 binding had returned to baseline levels (p = 0.399). While the second LPS challenge resulted in a less pronounced systemic inflammatory response (i.e. 77 ± 14% decrease in IL-6 compared to the first challenge), cerebral inflammation was not attenuated, but decreased below baseline, illustrated by a diffuse reduction of cerebral ¹⁸F-DPA-714 binding (-38% [95%CI -47 to -28%], p < 0.0001). Our findings constitute evidence for in vivo immunological reprogramming in the brain following a second inflammatory insult in healthy volunteers, which could represent a neuroprotective mechanism. These results pave the way for further studies on immunotolerance in the brain in patients with systemic inflammation-induced cerebral dysfunction.

Electrochemical Biosensors for Cytokine Profiling: Recent Advancements and Possibilities in the Near Future

Cytokines are soluble proteins secreted by immune cells that act as molecular messengers relaying instructions and mediating various functions performed by the cellular counterparts of the immune system, by means of a synchronized cascade of signaling pathways. Aberrant expression of cytokines can be indicative of anomalous behavior of the immunoregulatory system, as seen in various illnesses and conditions, such as cancer, autoimmunity, neurodegeneration and other physiological disorders. Cancer and autoimmune diseases are particularly adept at developing mechanisms to escape and modulate the immune system checkpoints, reflected by an altered cytokine profile. Cytokine profiling can provide valuable information for diagnosing such diseases and monitoring their progression, as well as assessing the efficacy of immunotherapeutic regiments. Toward this goal, there has been immense interest in the development of ultrasensitive quantitative detection techniques for cytokines, which involves technologies from various scientific disciplines, such as immunology, electrochemistry, photometry, nanotechnology and electronics. This review focusses on one aspect of this collective effort: electrochemical biosensors. Among the various types of biosensors available, electrochemical biosensors are one of the most reliable, user-friendly, easy to manufacture, cost-effective and versatile technologies that can yield results within a short period of time, making it extremely promising for routine clinical testing.

Is Decorin a Promising New Agent for Facial Nerve Regeneration? An Experimental Study

OBJECTIVE

The aim of this study was to investigate the effects of systemic administration of decorin (DC) on facial nerve (FN) regeneration.

METHODS

A total of 32 female albino Wistar rats were divided into 4 groups: control (C) group: no bilateral FN neurorrhaphy (B-FNN), no DC application, sham-operated group: B-FNN without DC application, DC group: DC application without B-FNN, and B-FNN + DC group: B-FNN and DC application. Nerve conduction studies were performed before and after skin incisions at 1st, 3rd, 5th, and 7th weeks in all groups. The amplitude and latency of compound muscle action potentials were recorded. FN samples were obtained and were investigated under light microscopy and immunohistochemical staining. The nerve and axon diameter, number of axons, H score, Schwann cell proliferation, and myelin and axonal degeneration were recorded quantitatively.

RESULTS

In the sham group, the 3rd and 5th postoperative week, amplitude values were significantly lower than those of the B-FNN + DC group (p < 0.05). Nerve diameters were found to be significantly larger in the sham, DC, and B-FNN + DC groups than in the C group (p < 0.05). The number of axons, the axon diameter, and the H scores were found to be significantly higher in the B-FNN + DC group than in the sham group (p < 0.05). The Schwann cell proliferation, myelin degeneration, and axonal degeneration scores were significantly lower in the B-FNN + DC group than in the sham group (p < 0.05).

CONCLUSION

Electrophysiological and histopathological evaluation revealed the potential benefits provided by DC. This agent may increase FN regeneration.

The effect of aged microglia on synaptic impairment and its relevance in neurodegenerative diseases

Microglia serve key functions in the central nervous system (CNS), participating in the establishment and regulation of synapses and the neuronal network, and regulating activity-dependent plastic changes. As the neuroimmune system, they respond to endogenous and exogenous signals to protect the CNS. In aging, one of the main changes is the establishment of inflamm-aging, a mild chronic inflammation that reduces microglial response to stressors. Neuroinflammation depends mainly on the increased activation of microglia. Microglia over-activation may result in a reduced capacity for performing normal functions related to migration, clearance, and the adoption of an anti-inflammatory state, contributing to an increased susceptibility for neurodegeneration. Oxidative stress contributes both to aging and to the progression of neurodegenerative diseases. Increased production of reactive oxygen species (ROS) and neuroinflammation associated with age- and disease-dependent mechanisms affect synaptic activity and neurotransmission, leading to cognitive dysfunction. Astrocytes prevent microglial cell cytotoxicity by mechanisms mediated by transforming growth factor β1 (TGFβ1). However, TGFβ1-Smad3 pathway is impaired in aging, and the age-related impairment of TGFβ signaling can reduce protective activation while facilitating cytotoxic activation of microglia. A critical analysis on the effect of aging microglia on neuronal function is relevant for the understanding of age-related changes on neuronal function. Here, we present evidence in the context of the "microglial dysregulation hypothesis", which leads to the reduction of the protective functions and increased cytotoxicity of microglia, to discuss the mechanisms involved in neurodegenerative changes and Alzheimer's disease.

Lentivirus-Mediated Expression of Human Secreted Amyloid Precursor Protein-Alpha Promotes Long-Term Induction of Neuroprotective Genes and Pathways in a Mouse Model of Alzheimer's Disease

BACKGROUND

Secreted amyloid precursor protein-alpha (sAPPα) can enhance memory and is neurotrophic and neuroprotective across a range of disease-associated insults, including amyloid-β toxicity. In a significant step toward validating sAPPα as a therapeutic for Alzheimer's disease (AD), we demonstrated that long-term overexpression of human sAPPα (for 8 months) in a mouse model of amyloidosis (APP/PS1) could prevent the behavioral and electrophysiological deficits that develop in these mice.

OBJECTIVE

To explore the underlying molecular mechanisms responsible for the significant physiological and behavioral improvements observed in sAPPα-treated APP/PS1 mice.

METHODS

We assessed the long-term effects on the hippocampal transcriptome following continuous lentiviral delivery of sAPPα or empty-vector to male APP/PS1 mice and wild-type controls using Affymetrix Mouse Transcriptome Assays. Data analysis was carried out within the Affymetrix Transcriptome Analysis Console and an integrated analysis of the resulting transcriptomic data was performed with Ingenuity Pathway analysis (IPA).

RESULTS

Mouse transcriptome assays revealed expected AD-associated gene expression changes in empty-vector APP/PS1 mice, providing validation of the assays used for the analysis. By contrast, there were specific sAPPα-associated gene expression profiles which included increases in key neuroprotective genes such as Decorin, betaine-GABA transporter and protocadherin beta-5, subsequently validated by qRT-PCR. An integrated biological pathways analysis highlighted regulation of GABA receptor signaling, cell survival and inflammatory responses. Furthermore, upstream gene regulatory analysis implicated sAPPα activation of Interleukin-4, which can counteract inflammatory changes in AD.

CONCLUSION

This study identified key molecular processes that likely underpin the long-term neuroprotective and therapeutic effects of increasing sAPPα levels in vivo.

CSF1R inhibitor JNJ-40346527 attenuates microglial proliferation and neurodegeneration in P301S mice

Neuroinflammation and microglial activation are significant processes in Alzheimer's disease pathology. Recent genome-wide association studies have highlighted multiple immune-related genes in association with Alzheimer's disease, and experimental data have demonstrated microglial proliferation as a significant component of the neuropathology. In this study, we tested the efficacy of the selective CSF1R inhibitor JNJ-40346527 (JNJ-527) in the P301S mouse tauopathy model. We first demonstrated the anti-proliferative effects of JNJ-527 on microglia in the ME7 prion model, and its impact on the inflammatory profile, and provided potential CNS biomarkers for clinical investigation with the compound, including pharmacokinetic/pharmacodynamics and efficacy assessment by TSPO autoradiography and CSF proteomics. Then, we showed for the first time that blockade of microglial proliferation and modification of microglial phenotype leads to an attenuation of tau-induced neurodegeneration and results in functional improvement in P301S mice. Overall, this work strongly supports the potential for inhibition of CSF1R as a target for the treatment of Alzheimer's disease and other tau-mediated neurodegenerative diseases.

In Situ Temporospatial Characterization of the Glial Response to Prion Infection

Mammalian transmissible spongiform encephalopathies (TSEs) display marked activation of astrocytes and microglia that precedes neuronal loss. Investigation of clinical parallels between TSEs and other neurodegenerative protein misfolding diseases, such as Alzheimer's disease, has revealed similar patterns of neuroinflammatory responses to the accumulation of self-propagating amyloids. The contribution of glial activation to the progression of protein misfolding diseases is incompletely understood, with evidence for mediation of both protective and deleterious effects. Glial populations are heterogeneously distributed throughout the brain and capable of dynamic transitions along a spectrum of functional activation states between pro- and antiinflammatory polarization extremes. Using a murine model of Rocky Mountain Laboratory scrapie, the neuroinflammatory response to prion infection was characterized by evaluating glial activation across 15 brain regions over time and correlating it to traditional markers of prion neuropathology, including vacuolation and PrP^Sc deposition. Quantitative immunohistochemistry was used to evaluate glial expression of iNOS and Arg1, markers of classical and alternative glial activation, respectively. The results indicate progressive upregulation of iNOS in microglia and a mixed astrocytic profile featuring iNOS expression in white matter tracts and detection of Arg1-positive populations throughout the brain. These data establish a temporospatial lesion profile for this prion infection model and demonstrate evidence of multiple glial activation states.

Microglial polarization: novel therapeutic mechanism against Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease that results in progressive dementia, and exhibits high disability and fatality rates. Recent evidence has demonstrated that neuroinflammation is critical in the pathophysiological processes of AD, which is characterized by the activation of microglia and astrocytes. Under different stimuli, microglia are usually activated into two polarized states, termed the classical 'M1' phenotype and the alternative 'M2' phenotype. M1 microglia are considered to promote inflammatory injury in AD; in contrast, M2 microglia exert neuroprotective effects. Imbalanced microglial polarization, in the form of excessive activation of M1 microglia and dysfunction of M2 microglia, markedly promotes the development of AD. Furthermore, an increasing number of studies have shown that the transition of microglia from the M1 to M2 phenotype could potently alleviate pathological damage in AD. Hence, this article reviews the current knowledge regarding the role of microglial M1/M2 polarization in the pathophysiology of AD. In addition, we summarize several approaches that protect against AD by altering the polarization states of microglia. This review aims to contribute to a better understanding of the pathogenesis of AD and, moreover, to explore the potential of novel drugs for the treatment of AD in the future.

PD-1 deficiency is not sufficient to induce myeloid mobilization to the brain or alter the inflammatory profile during chronic neurodegeneration

Innate immune activation is a major driver of neurodegenerative disease and immune regulatory pathways could be potential targets for therapeutic intervention. Recently, Programmed cell death-1 (PD-1) immune checkpoint inhibition has been proposed to mount an IFN-γ-dependent systemic immune response, leading to the recruitment of peripheral myeloid cells to the brain and neuropathological and functional improvements in mice with Alzheimer's disease-like β-amyloid pathology. Here we investigate the impact of PD-1 deficiency on murine prion disease (ME7 strain), a model of chronic neurodegeneration. Although PD-1 was found to be increased in the brain of prion mice, the absence of PD-1 did not cause myeloid cell infiltration into the brain or major changes in the inflammatory profile. However, we observed a slight exacerbation of the behavioural phenotype of ME7 mice upon PD-1 deficiency. These results do not support the possibility of using immune checkpoint blockade as a therapeutic strategy in neurodegenerative disease.

Microglia under psychosocial stressors along the aging trajectory: Consequences on neuronal circuits, behavior, and brain diseases

Mounting evidence indicates the importance of microglia for proper brain development and function, as well as in complex stress-related neuropsychiatric disorders and cognitive decline along the aging trajectory. Considering that microglia are resident immune cells of the brain, a homeostatic maintenance of their effector functions that impact neuronal circuitry, such as phagocytosis and secretion of inflammatory factors, is critical to prevent the onset and progression of these pathological conditions. However, the molecular mechanisms by which microglial functions can be properly regulated under healthy and pathological conditions are still largely unknown. We aim to summarize recent progress regarding the effects of psychosocial stress and oxidative stress on microglial phenotypes, leading to neuroinflammation and impaired microglia-synapse interactions, notably through our own studies of inbred mouse strains, and most importantly, to discuss about promising therapeutic strategies that take advantage of microglial functions to tackle such brain disorders in the context of adult psychosocial stress or aging-induced oxidative stress.

Lessons Learned about Neurodegeneration from Microglia and Monocyte Depletion Studies

While bone marrow-derived Ly6C^hi monocytes can infiltrate the central nervous system (CNS) they are developmentally and functionally distinct from resident microglia. Our understanding of the relative importance of these two populations in the distinct processes of pathogenesis and resolution of inflammation during neurodegenerative disorders was limited by a lack of tools to specifically manipulate each cell type. During recent years, the development of experimental cell-specific depletion models has enabled this issue to be addressed. Herein we compare and contrast the different depletion approaches that have been used, focusing on the respective functionalities of microglia and monocyte-derived macrophages in a range of neurodegenerative disease states, and discuss their prospects for immunotherapy.

The Role of Microglia in Prion Diseases: A Paradigm of Functional Diversity

Inflammation is a major component of neurodegenerative diseases. Microglia are the innate immune cells in the central nervous system (CNS). In the healthy brain, microglia contribute to tissue homeostasis and regulation of synaptic plasticity. Under disease conditions, they play a key role in the development and maintenance of the neuroinflammatory response, by showing enhanced proliferation and activation. Prion diseases are progressive chronic neurodegenerative disorders associated with the accumulation of the scrapie prion protein PrP^Sc, a misfolded conformer of the cellular prion protein PrP^C. This review article provides the current knowledge on the role of microglia in the pathogenesis of prion disease. A large body of evidence shows that microglia can trigger neurotoxic pathways contributing to progressive degeneration. Yet, microglia are also crucial for controlling inflammatory, repair and regenerative processes. This dual role of microglia is regulated by multiple pathways and evidences the ability of these cells to polarize into distinct phenotypes with characteristic functions. The awareness that the neuroinflammatory response is inextricably involved in producing tissue damage as well as repair in neurodegenerative disorders, opens new perspectives for the modulation of the immune system. A better understanding of this complex process will be essential for developing effective therapies for neurodegenerative diseases, in order to improve the quality of life of patients and mitigating the personal, economic and social consequences derived from these diseases.

Distribution of Misfolded Prion Protein Seeding Activity Alone Does Not Predict Regions of Neurodegeneration

Protein misfolding is common across many neurodegenerative diseases, with misfolded proteins acting as seeds for "prion-like" conversion of normally folded protein to abnormal conformations. A central hypothesis is that misfolded protein accumulation, spread, and distribution are restricted to specific neuronal populations of the central nervous system and thus predict regions of neurodegeneration. We examined this hypothesis using a highly sensitive assay system for detection of misfolded protein seeds in a murine model of prion disease. Misfolded prion protein (PrP) seeds were observed widespread throughout the brain, accumulating in all brain regions examined irrespective of neurodegeneration. Importantly, neither time of exposure nor amount of misfolded protein seeds present determined regions of neurodegeneration. We further demonstrate two distinct microglia responses in prion-infected brains: a novel homeostatic response in all regions and an innate immune response restricted to sites of neurodegeneration. Therefore, accumulation of misfolded prion protein alone does not define targeting of neurodegeneration, which instead results only when misfolded prion protein accompanies a specific innate immune response.

The distribution of misfolded prion protein seeding activity alone does not predict regions of neurodegeneration in prion disease; rather, a complex microglial response appears to determine selective vulnerability and provides new strategies for therapy.

Normal brain function requires tight regulation of protein folding; when this goes wrong, proteins can fold into abnormal conformations, which have severe impacts on brain performance, leading to conditions like dementia. Previous studies show that abnormally folded proteins are found in restricted parts of the brain, and neuronal cells in these specific brain regions have been shown to undergo degeneration. Recent technological advances have enhanced the detection of abnormally folded prion protein (PrP) during disease; we used these technologies to test whether distribution of abnormally folded proteins is indeed restricted to regions of the brain undergoing degeneration. Surprisingly, we observed abnormally folded proteins throughout the brain, demonstrating that these proteins can accumulate in parts of the brain that do not show degeneration. Thus, the distribution of abnormally folded protein, by itself, is not sufficient for neuronal degeneration. In addition, we found that microglia (one of the nonneuronal cell types in the brain) change their response during prion disease in two different ways; one response is associated with resilient brain regions, and the second, an inflammatory response is associated with regions susceptible to degeneration. Thus, the microglial response appears to be important in determining the outcome of disease.

Protective Microglia and Their Regulation in Parkinson's Disease

Microglia-mediated neuroinflammation is a hallmark of Parkinson’s disease (PD). In the brains of patients with PD, microglia have both neurotoxic and neuroprotective effects, depending on their activation state. In this review, we focus on recent research demonstrating the neuroprotective role of microglia in PD. Accumulating evidence indicates that the protective mechanisms of microglia may result from their regulation of transrepression pathways via nuclear receptors, anti-inflammatory responses, neuron–microglia crosstalk, histone modification, and microRNA regulation. All of these mechanisms work together to suppress the production of neurotoxic inflammatory components. However, during the progression of PD, the detrimental effects of inflammation overpower the protective actions of microglia. Therefore, an in-depth exploration of the mechanisms underlying microglial neuroprotection, and a means of promoting the transformation of microglia to the protective phenotype, are urgently needed for the treatment of PD.

Microglia regulate hippocampal neurogenesis during chronic neurodegeneration

Neurogenesis is altered in neurodegenerative disorders, partly regulated by inflammatory factors. We have investigated whether microglia, the innate immune brain cells, regulate hippocampal neurogenesis in neurodegeneration. Using the ME7 model of prion disease we applied gain- or loss-of CSF1R function, as means to stimulate or inhibit microglial proliferation, respectively, to dissect the contribution of these cells to neurogenesis. We found that increased hippocampal neurogenesis correlates with the expansion of the microglia population. The selective inhibition of microglial proliferation caused a reduction in neurogenesis and a restoration of normal neuronal differentiation, supporting a pro-neurogenic role for microglia. Using a gene screening strategy, we identified TGFβ as a molecule controlling the microglial pro-neurogenic response in chronic neurodegeneration, supported by loss-of-function mechanistic experiments. By the selective targeting of microglial proliferation we have been able to uncover a pro-neurogenic role for microglia in chronic neurodegeneration, suggesting promising therapeutic targets to normalise the neurogenic niche during neurodegeneration.

Differential Roles of M1 and M2 Microglia in Neurodegenerative Diseases

One of the most striking hallmarks shared by various neurodegenerative diseases, including Parkinson's disease, Alzheimer's disease (AD), and amyotrophic lateral sclerosis, is microglia-mediated neuroinflammation. Increasing evidence indicates that microglial activation in the central nervous system is heterogeneous, which can be categorized into two opposite types: M1 phenotype and M2 phenotype. Depending on the phenotypes activated, microglia can produce either cytotoxic or neuroprotective effects. In this review, we focus on the potential role of M1 and M2 microglia and the dynamic changes of M1/M2 phenotypes that are critically associated with the neurodegenerative diseases. Generally, M1 microglia predominate at the injury site at the end stage of disease, when the immunoresolution and repair process of M2 microglia are dampened. This phenotype transformation is very complicated in AD due to the phagocytosis of regionally distributed β-amyloid (Aβ) plaque and tangles that are released into the extracellular space. The endogenous stimuli including aggregated α-synuclein, mutated superoxide dismutase, Aβ, and tau oligomers exist in the milieu that may persistently activate M1 pro-inflammatory responses and finally lead to irreversible neuron loss. The changes of microglial phenotypes depend on the disease stages and severity; mastering the stage-specific switching of M1/M2 phenotypes within appropriate time windows may provide better therapeutic benefit.

Defining the Microglia Response during the Time Course of Chronic Neurodegeneration

Inflammation has been proposed as a major component of neurodegenerative diseases, although the precise role it plays has yet to be defined. We examined the role of key contributors to this inflammatory process, microglia, the major resident immune cell population of the brain, in a prion disease model of chronic neurodegeneration. Initially, we performed an extensive reanalysis of a large study of prion disease, where the transcriptome of mouse brains had been monitored throughout the time course of disease. Our analysis has provided a detailed classification of the disease-associated genes based on cell type of origin and gene function. This revealed that the genes upregulated during disease, regardless of the strain of mouse or prion protein, are expressed predominantly by activated microglia. In order to study the microglia contribution more specifically, we established a mouse model of prion disease in which the 79A murine prion strain was introduced by an intraperitoneal route into BALB/cJ(Fms-EGFP/-) mice, which express enhanced green fluorescent protein under the control of the c-fms operon. Samples were taken at time points during disease progression, and histological analysis of the brain and transcriptional analysis of isolated microglia was carried out. The analysis of isolated microglia revealed a disease-specific, highly proinflammatory signature in addition to an upregulation of genes associated with metabolism and respiratory stress. This study strongly supports the growing recognition of the importance of microglia within the prion disease process and identifies the nature of the response through gene expression analysis of isolated microglia.

IMPORTANCE

Inflammation has been proposed as a major component of neurodegenerative diseases. We have examined the role of key contributors to this inflammatory process, microglia, the major resident immune cell population of the brain, in a murine prion disease model of chronic neurodegeneration. Our study demonstrates that genes upregulated throughout the disease process are expressed predominantly by microglia. A disease-specific, highly proinflammatory signature was observed in addition to an upregulation of genes associated with metabolism and respiratory stress. This study strongly supports the growing recognition of the important contribution of microglia to a chronic neurodegenerative disease process.

Insights into Mechanisms of Chronic Neurodegeneration

Chronic neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and prion diseases are characterised by the accumulation of abnormal conformers of a host encoded protein in the central nervous system. The process leading to neurodegeneration is still poorly defined and thus development of early intervention strategies is challenging. Unique amongst these diseases are Transmissible Spongiform Encephalopathies (TSEs) or prion diseases, which have the ability to transmit between individuals. The infectious nature of these diseases has permitted in vivo and in vitro modelling of the time course of the disease process in a highly reproducible manner, thus early events can be defined. Recent evidence has demonstrated that the cell-to-cell spread of protein aggregates by a “prion-like mechanism” is common among the protein misfolding diseases. Thus, the TSE models may provide insights into disease mechanisms and testable hypotheses for disease intervention, applicable to a number of these chronic neurodegenerative diseases.

Classic Studies on the Potential of Stem Cell Neuroregeneration

The 1990s and 2000s were the beginning of an exciting time period for developmental neuroscience and neural stem cell research. By better understanding brain plasticity and the birth of new neurons in the adult brain, contrary to established dogma, hope for therapy from devastating neurological diseases was generated. The potential for stem cells to provide functional recovery in humans remains to be further tested and to further move into the clinical trial realm. The future certainly has great promise on stem cells to assist in alleviation of difficult-to-treat neurologic disorders. This article reviews classic studies of the 1990s and 2000s that paved the way for the advances of today, which can in turn lead to tomorrow's therapies.

Dab2 attenuates brain injury in APP/PS1 mice via targeting transforming growth factor-beta/SMAD signaling

Transforming growth factor-beta (TGF-β) type II receptor (TβRII) levels are extremely low in the brain tissue of patients with Alzheimer's disease. This receptor inhibits TGF-β1/SMAD signaling and thereby aggravates amyolid-beta deposition and neuronal injury. Dab2, a specific adapter protein, protects TβRII from degradation and ensures the effective conduction of TGF-β1/SMAD signaling. In this study, we used an adenoviral vector to overexpress the Dab2 gene in the mouse hippocampus and investigated the regulatory effect of Dab2 protein on TGF-β1/SMAD signaling in a mouse model of Alzheimer's disease, and the potential neuroprotective effect. The results showed that the TβRII level was lower in APP/PS1 mouse hippocampus than in normal mouse hippocampus. After Dab2 expression, hippocampal TβRII and p-SMAD2/3 levels were significantly increased, while amyloid-beta deposition, microglia activation, tumor necrosis factor-α and interleulin-6 levels and neuronal loss were significantly attenuated in APP/PS1 mouse brain tissue. These results suggest that Dab2 can exhibit neuroprotective effects in Alzheimer's disease by regulating TGF-β1/SMAD signaling.

Characterisation of temporal microglia and astrocyte immune responses in bile duct-ligated rat models of cirrhosis

BACKGROUND & AIMS

Microglia and astrocyte related pro-inflammatory responses are thought to underpin cerebral sequelae of acute liver failure. Conversely, despite background pro-inflammatory responses in cirrhosis, overt brain swelling and coma associated with acute-on-chronic liver failure, is infrequent unless precipitated (e.g. sepsis). Moreover in other chronic neurodegenerative disorders and sepsis, the brain is protected from recurrent microbial insults by compensatory microglial-associated immune responses. To characterise longitudinal cerebral immune responses in a bile duct-ligated (BDL) rat model of cirrhosis.

METHOD

Rats underwent BDL or sham operation before sacrifice at either 1-day, 1, 2 and 4 weeks post-surgery. We analysed consciousness, brain water, biochemistry and immunohistochemistry to assess activation of microglia (ED-1, OX6 and Iba-1), astrocytes (Glial fibrillary acidic protein - GFAP), cellular stress (Heat shock protein - Hsp 25) and pro-inflammatory mediator expression (inducible nitric oxide synthase (iNOS), interleukin-1beta (IL-1β) and tumour growth factor-beta (TGF-β)).

RESULTS

BDL significantly increased ammonia and bilirubin (P < 0.01 respectively). The classical microglial markers OX6, ED1 and Iba-1 and pro-inflammatory IL-1β and iNOS were not significantly increased. However, the alternative microglial marker and regulatory cytokine TGF-β was elevated from day 1 to 4 weeks post-BDL. GFAP expression was significantly increased in corpus callosum in all groups. In BDL rats, Hsp 25 was also increased in the corpus callosum, peaking at 2 weeks.

CONCLUSION

BDL triggers early alternative, but not classical, microglial activation. There was a correlation between astrocyte activation and cellular stress. These findings indicate early cerebral immune responses, which may be associated with immune tolerance to further challenge.

Neuronal and astroglial TGFβ-Smad3 signaling pathways differentially regulate dendrite growth and synaptogenesis

To address the role of the transforming growth factor beta (TGFβ)-Smad3 signaling pathway in dendrite growth and associated synaptogenesis, we used small inhibitory RNA to knockdown the Smad3 gene in either cultured neurons and or primary astrocytes. We found that TGFβ1 treatment of primary neurons increased dendrite extensions and the number of synapsin-1-positive synapses. When Smad3 was knockdown in primary neurons, dendrite growth was inhibited and the number of synapsin-1-positive synapses reduced even with TGFβ1 treatment. When astrocyte-conditioned medium (ACM), collected from TGFβ1-treated astrocytes (TGFβ1-stimulated ACM), was added to cultured neurons, dendritic growth was inhibited and the number of synapsin-1-positive puncta reduced. When TGFβ1-stimulated ACM was collected from astrocytes with Smad3 knocked down, this conditioned media promoted the growth of dendrites and the number of synapsin-1-positive puncta in cultured neurons. We further found that TGFβ1 signaling through Smad3 increased the expression of chondroitin sulfate proteoglycans, neurocan, and phosphacan in ACM. Application of chondroitinase ABC to the TGFβ1-stimulated ACM reversed its inhibitory effects on the dendrite growth and the number of synapsin-1-positive puncta. On the other hand, we found that TGFβ1 treatment caused a facilitation of Smad3 phosphorylation and translocation to the nucleus induced by status epilepticus (SE) in wild-type (Smad3(+/+)) mice, and this treatment also caused a promotion of γ-aminobutyric acid-ergic synaptogenesis impaired by SE in Smad3(+/+) as well as in Smad3(-/-) mice, but more dramatic promotion in Smad3(+/+) mice. Thus, we provide evidence for the first time that TGFβ-Smad3 signaling pathways within neuron and astrocyte differentially regulate dendrite growth and synaptogenesis, and this pathway may be involved in the pathogenesis of some central nervous system diseases, such as epilepsy.

The good and the bad of neuroinflammation in multiple sclerosis

Multiple sclerosis (MS) is the most common inflammatory, demyelinating, neurodegenerative disorder of the central nervous system (CNS). It is widely considered a T-cell mediated autoimmune disease that develops in genetically susceptible individuals, possibly under the influence of certain environmental trigger factors. The invasion of autoreactive CD4+ T-cells into the CNS is thought to be a central step that initiates the disease. Several other cell types, including CD8+ T-cells, B-cells and phagocytes appear to be involved in causing inflammation and eventually neurodegeneration. But inflammation is not entirely deleterious in MS. Evidence has accumulated in the recent years that show the importance of regulatory immune mechanisms which restrain tissue damage and initiate regeneration. More insight into the beneficial aspects of neuroinflammation might allow us to develop new treatment strategies for this enigmatic disease.

Decorin blocks scarring and cystic cavitation in acute and induces scar dissolution in chronic spinal cord wounds

In the injured central nervous system (CNS), transforming growth factor (TGF)-β1/2-induced scarring and wound cavitation impede axon regeneration implying that a combination of both scar suppression and axogenic treatments is required to achieve functional recovery. After treating acute and chronic dorsal funicular spinal cord lesions (DFL) in adult rats with the pan-TGF-β1/2 antagonist Decorin, we report that in: (1), acute DFL, the development of all injury parameters was significantly retarded e.g., wound cavity area by 68%, encapsulation of the wound by a glia limitans accessoria (GLA) by 65%, GLA basal lamina thickness by 94%, fibronectin, NG2 and Sema-3A deposition by 87%, 48% and 48%, respectively, and both macrophage and reactive microglia accumulations by 60%; and (2), chronic DFL, all the above parameters were attenuated to a lesser extent e.g., wound cavity area by 11%, GLA encapsulation by 25%, GLA basal lamina thickness by 31%, extracellular fibronectin, NG2 and Sema-3A deposition by 58%, 22% and 29%, respectively, and macrophage and reactive microglia accumulations by 44%. Moreover, in acute and chronic DFL, levels of tissue plasminogen activator (tPA) were raised (by 236% and 482%, respectively), as were active-MMP-2 (by 64% and 91%, respectively) and active-MMP-9 (by 122% and 18%, respectively), while plasminogen activator inhibitor-1 (PAI-1) was suppressed (by 56% and 23%, respectively) and active-TIMP-1 and active TIMP-2 were both lower but only significantly suppressed in acute DFL (by 56 and 21%, respectively). These findings demonstrate that both scar tissue mass and cavitation are attenuated in acute and chronic spinal cord wounds by Decorin treatment and suggest that the dominant effect of Decorin during acute scarring is anti-fibrogenic through suppression of inflammatory fibrosis by neutralisation of TGF-β1/2 whereas, in chronic lesions, Decorin-induction of tPA and MMP (concomitant with reduced complimentary levels of TIMP and PAI-1) leads to dissolution of the mature established scar by fibrolysis. Decorin also promoted the regeneration of similar numbers of axons through acute and chronic wounds. Accordingly, intrathecal delivery of Decorin offers a potential translatable treatment for scar tissue attenuation in patients with spinal cord injury.

Changes of peripheral TGF-β1 depend on monocytes-derived macrophages in Huntington disease

BACKGROUND

Huntington Disease (HD) is a neurodegenerative disorder resulting from the expansion of polyglutamine stretch in the huntingtin protein (Htt). Mutant HTT (mHtt) leads to progressive impairment of several molecular pathways that have been linked to disease pathogenesis. Defects in the production of a number of neurotrophic factors have been described as important determinants contributing to the development of HD. We have previously demonstrated that production of transforming growth factor-β1 (TGF-β1) is also deregulated in HD. Peripheral levels of TGF-β1 were markedly reduced early in the disease and returned to normal levels with disease severity. However, the cause and the biochemical origin of such abnormalities are still unclear.

RESULTS

We report here that the abnormal production of peripheral TGF-β1 depends on the changes in the percentage of TGF-β1-producing macrophages along disease course. Variation in the number of TGF-β1-producing macrophages resulted from differential activation state of the same cells, which displayed phenotypic and functional heterogeneity throughout the clinical course of HD. We further demonstrated that, similar to the periphery, the number of TGF-β1-immunoreactive cells in human post-mortem brain with HD, varied with neuropathological changes.

CONCLUSIONS

Our data indicate that reduced bioavailability of TGF-β1 in the serum of HD subjects is attributable to the variation of the number of TGF-β1-producing macrophages. Macrophages display a differential ability to produce TGF-β1, which reflects diversity in cells polarization throughout the disease course. Besides elucidating the biochemical origin of TGF-β1 fluctuations in HD, our study highlights an interesting parallelism between periphery and central compartment and underlines the potential of TGF-β1 as a possible indicator suitable for prediction of disease onset in HD.

Microglia and synapse: interactions in health and neurodegeneration

A series of discoveries spanning for the last few years has challenged our view of microglial function, the main form of immune defense in the brain. The surveillance of neuronal circuits executed by each microglial cell overseeing its territory occurs in the form of regular, dynamic interactions. Microglial contacts with individual neuronal compartments, such as dendritic spines and axonal terminals, ensure that redundant or dysfunctional elements are recognized and eliminated from the brain. Microglia take on a new shape that is large and amoeboid when a threat to brain integrity is detected. In this defensive form, they migrate to the endangered sites, where they help to minimize the extent of the brain insult. However, in neurodegenerative diseases that are associated with misfolding and aggregation of synaptic proteins, these vital defensive functions appear to be compromised. Many microglial functions, such as phagocytosis, might be overwhelmed during exposure to the abnormal levels of misfolded proteins in their proximity. This might prevent them from attending to their normal duties, such as the stripping of degenerating synaptic terminals, before neuronal function is irreparably impaired. In these conditions microglia become chronically activated and appear to take on new, destructive roles by direct or indirect inflammatory attack.

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.

Regulation of microglial proliferation during chronic neurodegeneration

An important component of chronic neurodegenerative diseases is the generation of an innate inflammatory response within the CNS. Microglial and astroglial cells play a key role in the development and maintenance of this inflammatory response, showing enhanced proliferation and activation. We studied the time course and regulation of microglial proliferation, using a mouse model of prion disease. Our results show that the proliferation of resident microglial cells accounts for the expansion of the population during the development of the disease. We identify the pathway regulated by the activation of CSF1R and the transcription factors PU.1 and C/EBPα as the molecular regulators of the proliferative response, correlating with the chronic human neurodegenerative conditions variant Creutzfeldt-Jakob disease and Alzheimer's disease. We show that targeting the activity of CSF1R inhibits microglial proliferation and slows neuronal damage and disease progression. Our results demonstrate that microglial proliferation is a major component in the evolution of chronic neurodegeneration, with direct implications for understanding the contribution of the CNS innate immune response to disease progression.

Regulation of microglia activity by glaucocalyxin-A: attenuation of lipopolysaccharide-stimulated neuroinflammation through NF-κB and p38 MAPK signaling pathways

Microglial cells are the resident macrophages and intrinsic arm of the central nervous system innate immune defense. Microglial cells become activated in response to injury, infection, environmental toxins, and other stimuli that threaten neuronal survival. Therefore, regulating microglial activation may have therapeutic benefits that lead to alleviating the progression of inflammatory-mediated neurodegeneration. In the present study, we investigated the effect of glaucocalyxin A (GLA) isolated from Rabdosia japonica on the production of pro-inflammatory mediators in lipopolysaccharide (LPS)-stimulated primary microglia and BV-2 cells. GLA significantly inhibited LPS-induced production of nitric oxide and reversed the morphological changes in primary microglia. Further, GLA suppressed expression of inducible nitric oxide synthase and cyclooxygenase-2 dose-dependently at the mRNA and protein levels. The production of proinflammatory cytokines such as tumor necrosis factor-α, interleukin-1β (IL)-1β, and IL-6 were inhibited by suppressing their transcriptional activity. Furthermore, GLA suppressed nuclear factor-κB activation by blocking degradation of IκB-α and inhibited the induction of lipocalin-2 expression in LPS-stimulated BV-2 cells. Mechanistic study revealed that the inhibitory effects of GLA were accompanied by blocking the p38 mitogen activated protein kinase signaling pathway in activated microglia. In conclusion, given that microglial activation contributes to the pathogenesis of neurodegenerative diseases, GLA could be developed as a potential therapeutic agent for treating microglia-mediated neuroinflammatory diseases.

Resection of the epileptogenic lesion abolishes seizures and reduces inflammatory cytokines of patients with temporal lobe epilepsy

Persistent neuroinflammation is implicated in the pathogenesis of seizures and neuronal degeneration of temporal lobe epilepsy (TLE). Circulating level of inflammatory cytokines was determined during inter-ictal period of 25 non-operated and 10 patients (OP) submitted to anterior temporal lobectomy. OP patients showed marked reduction of IL-1β, TNFα, MIP-1α, but not IL-6 and TGF-β1. Paired analysis done before and after lobectomy showed reduction of inflammatory cytokines but increased TGF-β1 levels, and lack of seizures for more than 6 months. Maintenance of high TGF-β1 and IL-6 cytokines in both groups suggests a role in down-regulation of neuroinflammation and promotion of brain tissue remodeling for neuronal reorganization.

Neuroimmune crosstalk in the central nervous system and its significance for neurological diseases

The central nervous system (CNS) is now known to actively communicate with the immune system to control immune responses both centrally and peripherally. Within the CNS, while studies on glial cells, especially microglia, have highlighted the importance of this cell type in innate immune responses of the CNS, the immune regulatory functions of other cell types, especially neurons, are largely unknown. How neuroimmune cross-talk is homeostatically maintained in neurodevelopment and adult plasticity is even more elusive. Inspiringly, accumulating evidence suggests that neurons may also actively participate in immune responses by controlling glial cells and infiltrated T cells. The potential clinical application of this knowledge warrants a deeper understanding of the mutual interactions between neurons and other types of cells during neurological and immunological processes within the CNS, which will help advance diagnosis, prevention, and intervention of various neurological diseases. The aim of this review is to address the immune function of both glial cells and neurons, and the roles they play in regulating inflammatory processes and maintaining homeostasis of the CNS.

Anti-prion activity of Brilliant Blue G

Background

Prion diseases are fatal neurodegenerative disorders with no effective therapy currently available. Accumulating evidence has implicated over-activation of P2X7 ionotropic purinergic receptor (P2X7R) in the progression of neuronal loss in several neurodegenerative diseases. This has led to the speculation that simultaneous blockade of this receptor and prion replication can be an effective therapeutic strategy for prion diseases. We have focused on Brilliant Blue G (BBG), a well-known P2X7R antagonist, possessing a chemical structure expected to confer anti-prion activity and examined its inhibitory effect on the accumulation of pathogenic isoforms of prion protein (PrPres) in a cellular and a mouse model of prion disease in order to determine its therapeutic potential.

Principal Findings

BBG prevented PrPres accumulation in infected MG20 microglial and N2a neural cells at 50% inhibitory concentrations of 14.6 and 3.2 µM, respectively. Administration of BBG in vivo also reduced PrPres accumulation in the brains of mice with prion disease. However, it did not appear to alleviate the disease progression compared to the vehicle-treated controls, implying a complex role of P2X7R on the neuronal degeneration in prion diseases.

Significance

These results provide novel insights into the pathophysiology of prion diseases and have important implications for the treatment.

The diverse roles of mononuclear phagocytes in prion disease pathogenesis

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are neurological diseases that can be transmitted through a number of different routes. A wide range of mammalian species are affected by the disease. After peripheral exposure, some TSE agents accumulate in lymphoid tissues at an early stage of disease prior to spreading to the nerves and the brain. Much research has focused on identifying the cells and molecules involved in the transmission of TSE agents from the site of exposure to the brain and several crucial cell types have been associated with this process. The identification of the key cells that influence the different stages of disease transmission might identify targets for therapeutic intervention. This review highlights the involvement of mononuclear phagocytes in TSE disease. Current data suggest these cells may exhibit a diverse range of roles in TSE disease from the transport or destruction of TSE agents in lymphoid tissues, to mediators or protectors of neuropathology in the brain.

Effect of compound of gelatin hydrogel microsphere incorporated with platelet-rich-plasma and alginate on sole defect in cattle

Platelet-rich plasma (PRP) was administered to cattle with hoof defects (10-mm drill hole) to promote sole defect healing. Two types of gels were prepared. One was a mixed gel [compound of gelatin microspheres (GM) incorporated with PRP and alginate] and was used for the test group, while the other was an alginate gel used for the control group. Each hole was filled with one of these gels. The drill hole depth, pressure pain, and hardness of the regenerated tissue were measured for 3 weeks after treatment. These measurements improved significantly in the test group than in the control group. The results suggest that a compound of GM incorporated with PRP and alginate had a hoof-regenerating effect in cattle.

Alzheimer's disease: pathological mechanisms and the beneficial role of melatonin

Alzheimer's disease (AD) is a highly complex neurodegenerative disorder of the aged that has multiple factors which contribute to its etiology in terms of initiation and progression. This review summarizes these diverse aspects of this form of dementia. Several hypotheses, often with overlapping features, have been formulated to explain this debilitating condition. Perhaps the best-known hypothesis to explain AD is that which involves the role of the accumulation of amyloid-β peptide in the brain. Other theories that have been invoked to explain AD and summarized in this review include the cholinergic hypothesis, the role of neuroinflammation, the calcium hypothesis, the insulin resistance hypothesis, and the association of AD with peroxidation of brain lipids. In addition to summarizing each of the theories that have been used to explain the structural neural changes and the pathophysiology of AD, the potential role of melatonin in influencing each of the theoretical processes involved is discussed. Melatonin is an endogenously produced and multifunctioning molecule that could theoretically intervene at any of a number of sites to abate the changes associated with the development of AD. Production of this indoleamine diminishes with increasing age, coincident with the onset of AD. In addition to its potent antioxidant and anti-inflammatory activities, melatonin has a multitude of other functions that could assist in explaining each of the hypotheses summarized above. The intent of this review is to stimulate interest in melatonin as a potentially useful agent in attenuating and/or delaying AD.

Systemic inflammation and Alzheimer's disease

A number of studies demonstrate disturbances of the central innate immune system in AD (Alzheimer's disease). In animal and human studies, there is evidence of close communication between systemic and central innate immune systems. Animal models of neurodegeneration show evidence of an exaggerated central innate immune response following systemic inflammation. Clinical studies of AD show evidence of increased cognitive decline and exaggerated sickness behaviour in response to systemic inflammation. Recognition of this communication pathway offers alternative explanations for a number of recognized risk factors in the development and progression of AD and highlights the potential of the manipulation of systemic innate immunity as a novel therapeutic approach.

Dysfunction of TGF-β1 signaling in Alzheimer's disease: perspectives for neuroprotection

Alzheimer's disease (AD) is a neurodegenerative disorder that affects about 35 million people worldwide. Current drugs for AD only treat the symptoms and do not interfere with the underlying pathogenic mechanisms of the disease. AD is characterized by the presence of β-amyloid (Aβ) plaques, neurofibrillary tangles, and neuronal loss. Identification of the molecular determinants underlying Aβ-induced neurodegeneration is an essential step for the development of disease-modifying drugs. Recently, an impairment of the transforming growth factor-β1 (TGF-β1) signaling pathway has been demonstrated to be specific to the AD brain and, particularly, to the early phase of the disease. TGF-β1 is a neurotrophic factor responsible for the initiation and maintenance of neuronal differentiation and synaptic plasticity. The deficiency of TGF-β1 signaling is associated with Aβ pathology and neurofibrillary tangle formation in AD animal models. Reduced TGF-β1 signaling seems to contribute both to microglial activation and to ectopic cell-cycle re-activation in neurons, two events that contribute to neurodegeneration in the AD brain. The neuroprotective features of TGF-β1 indicate the advantage of rescuing TGF-β1 signaling as a means to slow down the neurodegenerative process in AD.

The yin and yang of microglia

Microglia, the resident immune cells of the mammalian central nervous system (CNS), play a pivotal role in both physiological and pathological conditions such as the restoration of CNS integrity and the progression of neurodegenerative disorders. Extensive data have been published that describe neuroinflammation by microglial activation to have detrimental consequences on the developing and mature brain. On the other hand, a properly directed and limited inflammatory response is known to be a natural healing process after an insult in several other tissues. Thus, it is not surprising that research results illustrating benefits of neuroinflammation have been emerging over the past decade. Inflammation-mediated benefits for CNS outcomes include mechanisms such as neuroprotection, mobilization of neural precursors for repair, remyelination and axonal regeneration. Here, we review data that highlight the dual aspects of microglia with a focus on the developing brain, i.e. as aggressors potentiating damage and as helpers in the recovery process following CNS damage.

Systemic and central immunity in Alzheimer's disease: therapeutic implications

Clinical pharmaceutical trials aimed at modulating the immune system in Alzheimer's Disease have largely focused on either dampening down central proinflammatory innate immunity or have manipulated adaptive immunity to facilitate the removal of centrally deposited beta amyloid. To date, these trials have had mixed clinical therapeutic effects. However, a number of clinical studies have demonstrated disturbances of both systemic and central innate immunity in Alzheimer's Disease and attention has been drawn to the close communication pathways between central and systemic immunity. This paper highlights the need to take into account the potential systemic effects of drugs aimed at modulating central immunity and the possibility of developing novel therapeutic approaches based on the manipulation of systemic immunity and its communication with the central nervous system.

Rat neurosphere cells protect axotomized rat retinal ganglion cells and facilitate their regeneration

We investigated the ability of a population of rat neural stem and precursor cells derived from rat embryonic spinal cord to protect injured neurons in the rat central nervous system (CNS). The neonatal rat optic pathway was used as a model of CNS injury, whereby retinal ganglion cells (RGCs) were axotomized by lesion of the lateral geniculate nucleus one day after birth. Neural stem and precursor cells derived from expanded neurospheres (NS) were transplanted into the lesion site at the time of injury. Application of Fast Blue tracer dye to the lesion site demonstrated that significant numbers of RGCs survived at 4 and 8 weeks in animals that received a transplant, with an average of 28% survival, though in some individual cases survival was greater than 50%. No RGCs survived in animals that received a lesion alone. Furthermore, labeled RGCs were also observed when Fast Blue was applied to the superior colliculus (SC) at 4 weeks, suggesting that neurosphere cells also facilitated RGC to regenerate to their normal target. Transplanted cells did not migrate or express neural markers after transplantation, and secreted several neurotrophic factors in vitro. We conclude that NS cells can protect injured CNS neurons and promote their regeneration. These effects are not attributable to cell replacement, and may be mediated via secretion of neurotrophic factors. Thus, neuroprotection by stem cell populations may be a more viable approach for treatment of CNS disorders than cell replacement therapy.

Contribution of systemic inflammation to chronic neurodegeneration

Systemic infection or inflammation gives rise to signals that communicate with the brain and leads to changes in metabolism and behaviour collectively known as sickness behaviour. In healthy young individuals, these changes are normally transient with no long-term consequences. The microglia are involved in the immune to brain signalling pathways. In the aged or diseased brain, the microglia have a primed phenotype as a consequence of changes in their local microenvironment. Systemic inflammation impacts on these primed microglia and switches them from a relatively benign to an aggressive phenotype with the enhanced synthesis of pro-inflammatory mediators. Recent evidence suggests that systemic inflammation contributes to the exacerbation of acute symptoms of chronic neurodegenerative disease and may accelerate disease progression. The normal homeostatic role that microglia play in signalling about systemic infections and inflammation becomes maladaptive in the aged and diseased brain and this offers a route to therapeutic intervention. Prompt treatment of systemic inflammation or blockade of signalling pathways from the periphery to the brain may help to slow neurodegeneration and improve the quality of life for individuals suffering from chronic neurodegenerative disease.