Harald Neumann

Integrated systems approach identifies genetic nodes and networks in late-onset Alzheimer's disease.

The genetics of complex disease produce alterations in the molecular interactions of cellular pathways whose collective effect may become clear through the organized structure of molecular networks. To characterize molecular systems associated with late-onset Alzheimers disease (LOAD), we constructed gene-regulatory networks in 1,647 postmortem brain tissues from LOAD patients and nondemented subjects, and we demonstrate that LOAD reconfigures specific portions of the molecular interaction structure. Through an integrative network-based approach, we rank-ordered these network structures for relevance to LOAD pathology, highlighting an immune- and microglia-specific module that is dominated by genes involved in pathogen phagocytosis, contains TYROBP as a key regulator, and is upregulated in LOAD. Mouse microglia cells overexpressing intact or truncated TYROBP revealed expression changes that significantly overlapped the human brain TYROBP network. Thus the causal network structure is a useful predictor of response to gene perturbations and presents a framework to test models of disease mechanisms underlying LOAD.

Debris clearance by microglia: an essential link between degeneration and regeneration

Microglia are cells of myeloid origin that populate the CNS during early development and form the brains innate immune cell type. They perform homoeostatic activity in the normal CNS, a function associated with high motility of their ramified processes and their constant phagocytic clearance of cell debris. This debris clearance role is amplified in CNS injury, where there is frank loss of tissue and recruitment of microglia to the injured area. Recent evidence suggests that this phagocytic clearance following injury is more than simply tidying up, but instead plays a fundamental role in facilitating the reorganization of neuronal circuits and triggering repair. Insufficient clearance by microglia, prevalent in several neurodegenerative diseases and declining with ageing, is associated with an inadequate regenerative response. Thus, understanding the mechanism and functional significance of microglial-mediated clearance of tissue debris following injury may open up exciting new therapeutic avenues.

Clearance of apoptotic neurons without inflammation by microglial triggering receptor expressed on myeloid cells-2

Elimination of apoptotic neurons without inflammation is crucial for brain tissue homeostasis, but the molecular mechanism has not been firmly established. Triggering receptor expressed on myeloid cells-2 (TREM2) is a recently identified innate immune receptor. Here, we show expression of TREM2 in microglia. TREM2 stimulation induced DAP12 phosphorylation, extracellular signal–regulated kinase phosphorylation, and cytoskeleton reorganization and increased phagocytosis. Knockdown of TREM2 in microglia inhibited phagocytosis of apoptotic neurons and increased gene transcription of tumor necrosis factor α and nitric oxide synthase-2, whereas overexpression of TREM2 increased phagocytosis and decreased microglial proinflammatory responses. Thus, TREM2 deficiency results in impaired clearance of apoptotic neurons and inflammation that might be responsible for the brain degeneration observed in patients with polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy/Nasu-Hakola disease.

Cytotoxic T lymphocytes in autoimmune and degenerative CNS diseases.

Cytotoxic T lymphocytes (CTLs) with a CD8(+) phenotype have the potential to recognize and attack major histocompatibility complex (MHC) class I-expressing brain cells. Most brain cells, including neurons, can be stimulated to present peptides to CD8(+) CTLs by MHC class I molecules, and are susceptible to CTL-mediated cytotoxicity in culture. In disease-affected brain parenchyma, CD8(+) CTLs outnumber other T-cell subtypes. They show clonal expansion in several inflammatory and degenerative CNS diseases, such as multiple sclerosis (MS), virus-induced inflammatory brain diseases and paraneoplastic neurological disorders. In MS, damage of axons is closely linked to the CD8(+) CTLs, and protection against CTL-mediated damage should be considered as a new therapeutic approach in MS and other neuroinflammatory diseases.

Control of glial immune function by neurons

The immune status of the central nervous system (CNS) is strictly regulated. In the healthy brain, immune responses are kept to a minimum. In contrast, in a variety of inflammatory and neurodegenerative diseases, including multiple sclerosis, infections, trauma, stroke, neoplasia, and Alzheimers disease, glial cells such as microglia gain antigen‐presenting capacity through the expression of major histocompatibility complex (MHC) molecules. Further, proinflammatory cytokines, such as tumor necrosis factor‐α (TNF), interleukin‐1β (IL‐1β), and interferon‐γ (IFN‐γ), as well as chemokines, are synthesized by resident brain cells and T lymphocytes invade the affected brain tissue. The proinflammatory cytokines stimulate microglial MHC expression in the lesioned CNS areas only. However, the induction of brain immunity is strongly counterregulated in intact CNS areas. For instance, recent work demonstrated that microglia are kept in a quiescent state in the intact CNS by local interactions between the microglia receptor CD200 and its ligand, which is expressed on neurons. Work done in our laboratory showed that neurons suppressed MHC expression in surrounding glial cells, in particular microglia and astrocytes. This control of MHC expression by neurons was dependent on their electrical activity. In brain tissue with intact neurons, the MHC class II inducibility of microglia and astrocytes by the proinflammatory cytokine IFN‐γ was reduced. Paralysis of neuronal electric activity by neurotoxins restored the induction of MHC molecules on microglia and astrocytes. Loss of neurons or their physiological activity would render the impaired CNS areas recognizable by invading T lymphocytes. Thus, immunity in the CNS is inhibited by the local microenvironment, in particular by physiologically active neurons, to prevent unwanted immune mediated damage of neurons. GLIA 36:191–199, 2001.

Tumor necrosis factor inhibits neurite outgrowth and branching of hippocampal neurons by a rho-dependent mechanism

In response to injury and inflammation of the CNS, brain cells including microglia and astrocytes secrete tumor necrosis factor-α (TNF). This pro-inflammatory cytokine has been implicated in both neuronal cell death and survival. We now provide evidence that TNF affects the formation of neurites. Neurons cultured on astrocytic glial cells exhibited reduced outgrowth and branching of neurites after addition of recombinant TNF or prestimulation of glial cells to secrete TNF. This effect was absent in neurons of TNF receptor-deficient mice cultured on prestimulated glia of wild-type mice and was reverted by blocking TNF with soluble TNF receptor IgG fusion protein. TNF activated in neurons the small GTPase RhoA. By inactivating Rho with C3 transferase, the inhibitory effect of TNF on neurite outgrowth and branching was abolished. These results suggest that glia-derived TNF, as part of an injury or inflammatory process, can inhibit neurite elongation and branching during development and regeneration.

Transection of Major Histocompatibility Complex Class I-Induced Neurites by Cytotoxic T Lymphocytes

Damage to neurites with transection of axons and spheroid formation is commonly noted in the central nervous system during viral and autoimmune diseases such as multiple sclerosis, but it remains open whether such changes are caused primarily by immune mechanisms or whether they are secondary to inflammation. The present experiments explored whether neurites can be directly attacked by cytotoxic T lymphocytes (CTLs). Cultured murine neurons induced by interferon-gamma and tetrodotoxin to express major histocompatibility complex class I were pulsed with a dominant peptide of the lymphochoriomeningitis virus envelope glycoprotein (GP33) and then confronted with GP33-specific CD8(+) CTLs. Within 3 hours the neurites developed cytoskeleton breaks with adjacent solitary neuritic spheroids, as documented by confocal examination of the cytoskeletal marker beta-tubulin III. At the same time cytoskeleton staining of the neuronal somata showed no damage. The CTLs selectively attacked neurites and induced segmental membrane disruption 5 to 30 minutes after the establishment of peptide-specific CTL-neurite contact, as directly visualized by live confocal imaging. Thus, major histocompatibility complex class I/peptide-restricted CD8(+) T lymphocytes can induce lesions to neurites, which might be responsible for axonal damage during neuroinflammatory diseases.

Microglial clearance function in health and disease

Microglial cells are of hematopoietic origin, populate the CNS during early development and form the brains innate immune cell type. Besides their well-known role in immune defense, microglia have an active and homeostatic function in the normal CNS based on high motility of their ramified processes and endocytic clearance of apoptotic vesicular material. During development microglia contribute to the reorganization of neuronal connections, however microglia have also pivotal roles during acute and chronic neurodegeneration. Microglia become attracted to site of injury by nucleotides released from damaged neurons. Scavenger receptors expressed on microglia bind to debris and microglial phagocytic receptors signal via immunoreceptor tyrosine-based activation motif (ITAM)--containing adaptor proteins to promote phagocytosis of extracellular material. Insufficient clearance by microglia appears to be prevalent in neurodegenerative diseases such as Alzheimers disease.

Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis

Microglia are the resident brain macrophages and they have been traditionally studied as orchestrators of the brain inflammatory response during infections and disease. In addition, microglia has a more benign, less explored role as the brain professional phagocytes. Phagocytosis is a term coined from the Greek to describe the receptor-mediated engulfment and degradation of dead cells and microbes. In addition, microglia phagocytoses brain-specific cargo, such as axonal and myelin debris in spinal cord injury or multiple sclerosis, amyloid-β deposits in Alzheimers disease, and supernumerary synapses in postnatal development. Common mechanisms of recognition, engulfment, and degradation of the different types of cargo are assumed, but very little is known about the shared and specific molecules involved in the phagocytosis of each target by microglia. More importantly, the functional consequences of microglial phagocytosis remain largely unexplored. Overall, phagocytosis is considered a beneficial phenomenon, since it eliminates dead cells and induces an anti-inflammatory response. However, phagocytosis can also activate the respiratory burst, which produces toxic reactive oxygen species (ROS). Phagocytosis has been traditionally studied in pathological conditions, leading to the assumption that microglia have to be activated in order to become efficient phagocytes. Recent data, however, has shown that unchallenged microglia phagocytose apoptotic cells during development and in adult neurogenic niches, suggesting an overlooked role in brain remodeling throughout the normal lifespan. The present review will summarize the current state of the literature regarding the role of microglial phagocytosis in maintaining tissue homeostasis in health as in disease.

Anti‐inflammatory activity of nerve growth factor in experimental autoimmune encephalomyelitis: inhibition of monocyte transendothelial migration

In order to analyze a putative immunomodulatory effect of NGF in experimental autoimmune encephalomyelitis (EAE) of the Lewis rat, we transduced myelin basic protein (MBP)‐specific CD4+ T cells with a recombinant retrovirus encoding NGF. These TMBPNGF cells secreted high levels of NGF, along with an unaltered Th1‐like cytokine pattern. Transfer studies showed that TMBPNGF cells were unable to mediate clinical EAE, when transferred alone, and, more important, they efficiently suppressed induction of clinical EAE by non‐transduced MBP‐specific T cells (TMBP cells). In contrast, NGF transduced ovalbumin‐specific T cells, which secreted high NGF levels, did not affect EAE induction. Suppression of clinical EAE by TMBPNGF cells was associated with a general reduction of inflammatory CNS infiltrates, with a most pronounced decrease of the monocyte/macrophage component. Using a culture model of the endothelial blood‐brain barrier (BBB), we found that NGF directly acts on blood‐derived monocytes via the p75 NGF receptor, thus interfering with monocyte migration through the activated BBB endothelium. Our data establish NGF as an anti‐inflammatory mediator interfering with T cell mediated autoimmune disease in the CNS. They further point to monocyte migration through blood vascular endothelium as one possible mechanism of NGF action.