Lampron et al. use a cuprizone mouse model of demyelination/remyelination to show that in CX3CR1-deficient mice, the clearance of myelin debris by microglia is impaired, affecting the integrity of axon and myelin sheaths.

Microglia are the immune cells of the brain, they are activated in the brain of Alzheimer's disease (AD) patients and mouse models of AD, and they express the innate immune receptor toll-like receptor 2 (TLR2). The present study investigated role of this receptor in the progression of AD-like pathologies. Here we show that amyloid ␤ (A␤) stimulates TLR2 expression in a small proportion of microglia. We then generated triple transgenic mice that are deficient in TLR2 from mice that harbor a mutant human presenelin 1 and a chimeric mouse/human amyloid precursor protein (APP) genes. TLR2 deficiency accelerated spatial and contextual memory impairments, which correlated with increased levels of A␤ 1-42 and transforming growth factor ␤1 in the brain. NMDA receptors 1 and 2A expression levels were also lower in the hippocampus of APP-TLR2 Ϫ/Ϫ mice. Gene therapy in cells of the bone marrow using lentivirus constructs expressing TLR2 rescued the cognitive impairment of APP-TLR2 Ϫ/Ϫ mice. Indeed, lenti-green fluorescent protein/TLR2 treatment had beneficial effects by restoring the memory consolidation process disrupted by TLR2 deficiency in APP mice. These data suggest that TLR2 acts as an endogenous receptor for the clearance of toxic A␤ by bone-marrow-derived immune cells. The cognitive decline is markedly accelerated in a context of TLR2 deficiency. Upregulating this innate immune receptor may then be considered as a potential new powerful therapeutic approach for AD.Key words: bone marrow stem cells; inflammation; innate immunity microglia; neuroprotection; postsynaptic receptors; transforming growth factor ␤1 IntroductionAlzheimer's disease (AD) is characterized by memory loss and increasingly severe dementia (Hardy and Selkoe, 2002). The neuropathological changes are manifested by accumulation of plaques containing the amyloid ␤ (A␤) protein, intracellular neurofibrillary tangles, activated microglia and astrocytes, and degenerating neurons. Early in the disease process, A␤ accumulation reduces synapse density of cortical and hippocampal neurons, which strongly correlates with memory impairments (Selkoe, 2002). A␤ of 40 and 42 amino acids are highly toxic for the synaptic connections, and their levels are elevated in the brain of AD patients. These peptides are produced from the cleavage of amyloid precursor protein (APP) by enzymatic complexes known as ␤-and ␥-secretase. Many mutations in APP and presenelin (PS1) genes are believed to increase A␤ in the brain of patients (Hardy and Selkoe, 2002), and expression of these mutated genes induces AD-like pathologies in mice (Hsiao et al., 1996;Borchelt et al., 1997).Microglia are the immune cells of the brain, and they are attracted to amyloid deposits both in human samples and in rodent transgenic models that develop this disease. The precise role of microglia in AD is still under intensive debate. Microglia are activated by A␤ and secrete neurotoxic molecules, but they have neuroprotective actions by secreting neurotrophic agents and eliminating toxic A␤ by phagocyt...

Alzheimer's disease (AD) is characterized by the accumulation of amyloid beta (Aβ) that is assumed to result from impaired elimination of this neurotoxic peptide. Most patients with AD also exhibit cerebral amyloid angiopathy, which consists of Aβ deposition within the cerebral vasculature. The contribution of monocytes in AD has so far been limited to macrophage precursors. In this study, we aimed to investigate whether circulating monocytes could play a role in the elimination of Aβ. With live intravital two-photon microscopy, we demonstrate that patrolling monocytes are attracted to and crawl onto the luminal walls of Aβ-positive veins, but not on Aβ-positive arteries or Aβ-free blood vessels. Additionally, we report the presence of crawling monocytes carrying Aβ in veins and their ability to circulate back into the bloodstream. Selective removal of Ly6C(lo) monocytes in APP/PS1 mice induced a significant increase of Aβ load in the cortex and hippocampus. These data uncover the ability of Ly6C(lo) monocytes to naturally target and eliminate Aβ within the lumen of veins and constitute a potential therapeutic target in AD.

Alzheimer's disease (AD) is the most common cause of dementia worldwide. The pathogenesis of this neurodegenerative disease, currently without curative treatment, is associated with the accumulation of amyloid β (Aβ) in brain parenchyma and cerebral vasculature. AD patients are unable to clear this toxic peptide, leading to Aβ accumulation in their brains and, presumably, the pathology associated with this devastating disease. Compounds that stimulate the immune system to clear Aβ may therefore have great therapeutic potential in AD patients. Monophosphoryl lipid A (MPL) is an LPS-derived Toll-like receptor 4 agonist that exhibits unique immunomodulatory properties at doses that are nonpyrogenic. We show here that repeated systemic injections of MPL, but not LPS, significantly improved AD-related pathology in APP swe /PS1 mice. MPL treatment led to a significant reduction in Aβ load in the brain of these mice, as well as enhanced cognitive function. MPL induced a potent phagocytic response by microglia while triggering a moderate inflammatory reaction. Our data suggest that the Toll-like receptor 4 agonist MPL may be a treatment for AD.lzheimer's disease (AD) is a neurodegenerative pathology characterized by the accumulation of amyloid beta (Aβ) and neurofibrillary tangles in the brain parenchyma (1). Inflammation, which occurs in parallel with the progression of the disease, is featured by the production of cytokines by activated microglia. The role of these cells in the pathogenesis of AD remains unclear and is an area of active investigation. Whereas chronic activation of microglial cells by Aβ can trigger the exaggerated release of cytokines and neurotoxic mediators that could be detrimental to neurons, microglia can also clear Aβ via increased phagocytosis and proteolytic degradation, which may be neuroprotective (2).Toll-like receptors (TLRs) on the surface of microglial cells have been shown to bind Aβ, which triggers downstream intracellular signaling cascades (3, 4). Microglia deficient in TLR2, TLR4, or the coreceptor CD14 are not activated by Aβ and do not exhibit a phagocytic response (5). Transgenic AD mice lacking TLR4 have markedly elevated levels of diffuse and fibrillar Aβ (3). Furthermore, stimulation of microglial cells with TLR2-, TLR4-, or TLR9-specific agonists accelerates Aβ clearance both in vitro and in vivo (3, 6, 7).Monophosphoryl lipid A (MPL) is a chemically detoxified lipid A moiety derived from Salmonella minnesota R595 LPS (8). This TLR4 ligand is at least 100-fold less pyrogenic than LPS yet maintains many of the immunomodulatory properties of LPS (9). Importantly, MPL is safe in humans and has been administered to millions of patients as a component of several vaccine formulations such as the Cervarix vaccine (10). We investigated herein the chronic use of the nonpyrogenic TLR4 agonist MPL and compared it with a strong TLR4 ligand (LPS) in a mouse model of AD.Although the therapeutic potential of innate immune activation for AD is being evaluated in preclinical models, this conc...

The signals that regulate the fate of circulating monocytes remain unknown. In the present study, we demonstrate that triggering of the NOD2 receptor by muramyl dipeptide (MDP) converts inflammatory Ly6C monocytes into patrolling Ly6C monocytes. Administration of MDP to Nr4a1 mice, which lack Ly6C monocytes, or to Ly6C-depleted mice led to the emergence of blood-patrolling monocytes with a profile similar to that of Ly6C monocytes, including high expression of CX3CR1 and LFA1. Using intravital microscopy in animal models of inflammatory diseases, we also found that converted Ly6C monocytes patrol the endothelium of blood vessels and that their presence contributes to a reduction in the inflammatory response following MDP injection. Our results demonstrate that NOD2 contributes to the regulation of blood monocytes and suggest that it could be therapeutically targeted to treat inflammatory diseases.

BackgroundThe Blood–brain barrier (BBB) controls brain supply with oxygen and nutrients, and protects the brain from toxic metabolites, such as beta-amyloid (Aβ) peptides. The neurovascular unit (NVU) couples vascular and neuronal functions by controlling BBB parameters based on brain needs. As such, NVU/BBB dysfunction, associated to irregularities in cerebral blood flow (CBF), has been proposed to contribute in the pathogenesis of Alzheimer’s disease (AD), mainly by impairing cerebral Aβ clearance. However, the spatiotemporal contribution of the NVU/BBB in the neurodegenerative cascades remains elusive.ResultsBy using C57BL/6J mice subjected to right common carotid artery (rCCA) permanent ligation in order to induce mild chronic cerebral hypoperfusion, we show here that cerebral hypoperfusion induced NVU dysfunction by reducing ABCB1 protein expression in brain capillaries. ABCB1 reduction was mainly triggered by an enhanced Glycogen Synthase Kinase 3 (GSK3β) activation, which decreased β-catenin nuclear abundance. Moreover, cerebral hypoperfusion triggered early vascular deposition of peripherally applied human Aβ1-42 peptides, which has shifted from highly vascular to the parenchyma 6 weeks later, forming small stable Aβ deposits. Hypoperfusion induced a deregulation in glucose metabolism, as brain reperfusion, or the administration of a high dose of glucose, diminished GSK3β activation, recuperated β-catenin nuclear abundance, reestablished ABCB1 protein expression, and prevented Aβ vascular early deposition. These results demonstrate that mild chronic cerebral hypoperfusion creates a metabolically deregulated microenvironment, thus triggering the brain entry and aggregation of peripherally applied human Aβ1-42 peptides.ConclusionOur study offers new insights on the initiation of the neurodegenerative cascades observed in AD, which could be valuable in developing adequate treatment strategies.

A pathological hallmark of multiple sclerosis (MS) is myelin loss in brain white matter accompanied by compromised remyelination. Demyelinated lesions are deeply associated with oligodendrocyte apoptosis and a robust inflammatory response. Although various studies point towards a noxious role of inflammation in MS, others emphasize a positive role for the innate immune cells in disease progression. A cytokine well-known to stimulate cell survival, proliferation and differentiation of myeloid cells, macrophage colony-stimulating factor (mCSF), was administered to mice during a 5 week-long cuprizone diet. Treated mice exhibited reduced myelin loss during the demyelination phase, together with an increased number of microglia and oligodendrocyte precursor cells in lesion sites. Tamoxifen-induced conditional deletion of the mCSF receptor in microglia from cuprizone-fed mice caused aberrant myelin debris accumulation in the corpus callosum and reduced microglial phagocytic response. mCSF therefore plays a key role in stimulating myelin clearance by the brain innate immune cells, which is a prerequisite for proper remyelination and myelin repair processes.

Microglia are the innate immune cells of the CNS and their proliferation, activation, and survival have previously been shown to be highly dependent on macrophage colony-stimulating factor receptor (CSF1R). Here we investigated the impact of the receptor in such processes using two different models of nerve injuries, namely hypoglossal axotomy and cuprizone-induced demyelination. Both models are associated with a robust microgliosis. The role of CSF1R was investigated using the gene deletion Cre/Lox system, which allows the conditional knockout following tamoxifen administration. We found that after 5 weeks of cuprizone diet that CSF1R suppression caused a significant impairment of microglia function. A reduced microgliosis was detected in the corpus collosum of CSF1R knockout mice compared to controls. In contrast to cuprizone model, the overall number of Iba1 cells was unchanged at all the times evaluated following hypoglossal axotomy in WT and cKO conditions. After nerve lesion, a tremendous proliferation was noticed in the ipsilateral hypoglossal nucleus to a similar level in both knockout and wild-type groups. We also observed infiltration of bone-marrow derived cells specifically in CSF1R-deficient mice, these cells tend to compensate the CSF1R signaling pathway suppression in resident microglia. Taking together our results suggest a different role of CSF1R in microglia depending on the model. In the pathologic context of cuprizone-induced demyelination CSF1R signaling pathway is essential to trigger proliferation and survival of microglia, while this is not the case in a model of systemic nerve injury. M-CSF/CSF1R is consequently not the unique system involved in microgliosis following nerve damages.