J. G. Sheng

Glial‐Neuronal Interactions in Alzheimer's Disease: The Potential Role of a ‘Cytokine Cycle’ in Disease Progression

The role of glial inflammatory processes in Alzheimers disease has been highlighted by recent epidemiological work establishing head trauma as an important risk factor, and the use of anti‐inflammatory agents as an important ameliorating factor, in this disease. This review advances the hypothesis that chronic activation of glial inflammatory processes, arising from genetic or environmental insults to neurons and accompanied by chronic elaboration of neuroactive glia‐derived cytokines and other proteins, sets in motion a cytokine cycle of cellular and molecular events with neurodegenerative consequences. In this cycle, interleukin‐1 is a key initiating and coordinating agent. Interleukin‐1 promotes neuronal synthesis and processing of the β‐amyloid precursor protein, thus favoring continuing deposition of β‐amyloid, and activates astrocytes and promotes astrocytic synthesis and release of a number of inflammatory and neuroactive molecules. One of these, S100β, is a neurite growth‐promoting cytokine that stresses neurons through its trophic actions and fosters neuronal cell dysfunction and death by raising intraneuronal free calcium concentrations. Neuronal injury arising from these cytokine‐induced neuronal insults can activate microglia with further overexpression of interleukin‐1, thus producing feedback amplification and self‐propagation of this cytokine cycle. Additional feedback amplification is provided through other elements of the cycle. Chronic propagation of this cytokine cycle represents a possible mechanism for progression of neurodegenerative changes culminating in Alzheimers disease.

Enlarged and phagocytic, but not primed, interleukin-1α-immunoreactive microglia increase with age in normal human brain

Abstract Microglia-mediated inflammatory responses have been implicated in the pathogenesis of neuritic plaques in Alzheimer’s disease. The strong age association of Alzheimer’s disease incidence suggests that events in normal aging may promote such responses. We used immunohistochemistry and computerized image analysis to determine the numbers, size, activation state, and immunoreactive intensity of interleukin-1α-immunoreactive (IL-1α+) microglia in mesial temporal lobe of 20 neurologically normal individuals, 2–80 years of age. We also used Northern analysis to determine tissue levels of IL-1α mRNA in an additional 11 neurologically normal individuals aged 1 day to 78 years. IL-1α+ microglia were characterized as primed, enlarged, or phagocytic (enlarged with heterogeneous cytoplasmic contents) based on morphology. These three microglial subtypes showed significant differences in size [27 ± 1 58 ± 2 114 ± 6 (mean ± SEM) μm2/cell, respectively, P < 0.001 for each comparison] and in immunoreactive intensity [60 ± 1 68 ± 2 79 ± 2 (arbitrary units), respectively, P < 0.001 or better for each comparison]. There were significant age-associated increases in the total numbers of activated IL-1α+ microglia. Among microglial subtypes, there were significant increases in the numbers of enlarged (threefold) and especially phagocytic (elevenfold), but not primed, microglia. Tissue IL-1α mRNA levels were higher in individuals over 60 than in those less than 60 (P < 0.05). The age-associated increases in microglial activation were independent of postmortem interval, patient sex, and the presence of Alzheimer-type ‘senile’ changes. Age-associated increases in microglial activation and IL-1 expression may contribute to the age-associated increased risk of Alzheimer’s disease.

Glial-Neuronal Interactions in Alzheimer Disease: Progressive Association of IL-1α+ Microglia and S100β+ Astrocytes with Neurofibrillary Tangle Stages

Activated microglia, overexpressing interleukin-1 (IL-1), and activated astrocytes, overexpressing S100β, have been implicated in the formation and evolution of tau2-immunoreactive (tau2+) neuritic plaques in Alzheimer disease. In this study, we assessed the role of IL-lα+ microglia and S100β+ astrocytes in the pathogenesis of another cardinal histopathological feature of Alzheimer disease: tau2+ neurofibrillary tangles. Four distinct stages of neurofibrillary tangle formation were identified: neurons with granular perikaryal tau2 immunoreactivity (stage 0): fibrillar neuronal inclusions (stage 1); dense, neuronal soma-filling inclusions (stage 2); and acellular, fibrillar deposits (stage 3. “ghost tangles”). The numbers of tangles in randomly selected fields of parahippocampal cortex from II Alzheimer patients correlated with both the numbers of IL-1α+ microglia and the numbers of S100β+ astrocytes in these fields (r = 0.72, p < 0.02; r = 0.73, p = 0.01, respectively). There were progressive increases in frequency of association between tangle stages and both IL-1α+ microglia and S100β+ astrocytes: 48. 56, 67, and 92% of stage 0–3 tangles, respectively, had associated IL-1α+ microglia and 21, 37, 55, and 91% of stage 0–3 tangles had associated S100β+ astrocytes. This progressive association of activated IL-1α+ microglia and activated S100β+ astrocytes with tau2+ tangle stages suggests a role for glial-neuronal interactions in the degeneration of tangle-bearing neurons in Alzheimer disease.

Increased Neuronal β-Amyloid Precursor Protein Expression in Human Temporal Lobe Epilepsy: Association with Interleukin-1α Immunoreactivity

Abstract: Levels of immunoreactive β‐amyloid precursor protein and interleukin‐1α were found to be elevated in surgically resected human temporal lobe tissue from patients with intractable epilepsy compared with postmortem tissue from neurologically unaffected patients (controls). In tissue from epileptics, the levels of the 135‐kDa β‐amyloid precursor protein isoform were elevated to fourfold (p < 0.05) those of controls and those of the 130‐kDa isoform to threefold (p < 0.05), whereas those of the 120‐kDa isoform (p > 0.05) were not different from control values. β‐Amyloid precursor protein‐immunoreactive neurons were 16 times more numerous, and their cytoplasm and proximal processes were more intensely immunoreactive in tissue sections from epileptics than controls (133 ± 12 vs. 8 ± 3/mm2; p < 0.001). However, neither β‐amyloid precursor protein‐immunoreactive dystrophic neurites nor β‐amyloid deposits were found in this tissue. Interleukin‐1α‐immunoreactive cells (microglia) were three times more numerous in epileptics than in controls (80 ± 8 vs. 25 ± 5/mm2; p < 0.001), and these cells were often found adjacent to β‐amyloid precursor protein‐immunoreactive neuronal cell bodies. Our findings, together with functions established in vitro for interleukin‐1, suggest that increased expression of this protein contributes to the increased levels of β‐amyloid precursor protein in epileptics, thus indicating a potential role for both of these proteins in the neuronal dysfunctions, e.g., hyperexcitability, characteristic of epilepsy.

Microglial inter leukin-1α expression in brain regions in Alzheimer's disease: correlation with neuritic plaque distribution

Interleukin‐1α‐immunoreactive (IL‐1α+) microglia are prominent components of neuritic plaques in Alzheimers disease, and may be important in the evolution of neuritic plaques from diffuse amyloid deposits. Neuritic plaques show a characteristic distribution across cerebral regions and are absent in the cerebellum of patients with Alzheimers disease. We used single‐ and dual‐immunohistochemical labelling to investigate the possibility that the expression of IL‐1α is correlated with this regional distribution of neuritic (tau 2‐immunoreactive, tau 2+) plaques. In Alzheimers disease, tau 2+ neuritic plaques occurred with increasing frequency in grey matter of frontal and occipital lobes, temporal lobe, and hippocampus. There were positive correlations between the regional patterns of distribution of activated IL‐1α+ microglia and tau2+ neuritic plaques as well as between activated IL‐1α+ microglia and activated astrocytes. No activated IL‐1α+ microglia, tau 2+ neuritic plaques, or activated astrocyies were observed in cerebellum of these Alzheimer patients. These regional relationships between activated IL‐1α+ microglia, tau 2+ neuritic plaques, and activated astrocytes, together with the established functions of IL‐1, support a causal association between the overexpression of IL‐1 and the evolution of β‐amyloid deposits into neuritic plaques in Alzheimers disease.

Human brain S100β and S100β mRNA expression increases with age: Pathogenic implications for Alzheimer's disease

S100 beta is a neurite extension factor that has been implicated in the development of neuritic plaques in Alzheimers disease. We analyzed the expression of S100 beta and its encoding mRNA, using immunohistochemistry, enzyme-linked immunosorbent assay, and Northern blot analysis, in postmortem brain tissue from 26 neurologically normal patients, aged 1-80 years. Tissue levels of S100 beta and S100 beta mRNA, as well as the number of S100 beta-immunoreactive (S100 beta +) astrocytes, increased with advancing age (r = 0.60, p = 0.008; r = 0.65, p = 0.007: and r = 0.73, p = 0.001, respectively). In patients more than 60 years old, the number of S100 beta + astrocytes and the tissue levels of S100 beta and S100 beta mRNA were significantly higher than those in patients less than 60 years of age (p = 0.001, p = 0.035, and p = 0.047, respectively). All of these values, however, were significantly less than those found in Alzheimer patients (p < 0.05 or better). Our findings, together with the known functions of S100 beta, suggest that age-related increases in S100 beta expression are important in the pathogenesis of Alzheimers disease and may explain in part the increased incidence of this disease with advancing age.

Neuritic plaque evolution in Alzheimer's disease is accompanied by transition of activated microglia from primed to enlarged to phagocytic forms

Abstract Activated microglia, overexpressing the potent neuroactive cytokine interleukin-1, have been implicated as a driving force in the evolution of diffuse amyloid deposits into diagnostic neuritic plaques in Alzheimer’s disease. To evaluate this role further, we used double-label immunohistochemistry to classify and quantify plaque-associated and non-plaque-associated activated interleukin-1-immunoreactive microglia in parahippocampal tissue from 11 patients with Alzheimer’s disease. These activated microglia were subclassified as primed (only slightly enlarged), enlarged, or phagocytic (enlarged with heterogeneous cytoplasmic contents). We further determined the distribution of these microglial subtypes among four defined plaque types. Most (84%) primed microglia were not plaque associated, although 13% were present in diffuse non-neuritic plaques and 3% were present in diffuse neuritic plaques. In contrast, most enlarged (55%) and phagocytic (91%) microglia were plaque associated. Of plaque-associated enlarged microglia, most (71%) were found in diffuse neuritic plaques with the remainder evenly distributed between diffuse non-neuritic and dense-core neuritic plaques (15% each). Of plaque-associated phagocytic microglia, a few were present in diffuse non-neuritic plaques (5%), but most were found in diffuse neuritic plaques (62%) and dense-core neuritic plaques (33%). These findings show preferential association of primed microglia with diffuse amyloid deposits and imply that microglial transformation from primed, to enlarged, to phagocytic types occurs in concert with the evolution of amyloid plaques from diffuse amyloid deposits to the neuritic β-amyloid plaque forms in Alzheimer’s disease. Microglial phagocytic activity in neuritic plaques may reflect involvement in the processing of diffuse amyloid into condensed β-amyloid, or in clearance of neuritic debris.

Interleukin-1 promotion of MAPK-p38 overexpression in experimental animals and in Alzheimer's disease: potential significance for tau protein phosphorylation.

Activated (phosphorylated) mitogen-activated protein kinase p38 (MAPK-p38) and interleukin-1 (IL-1) have both been implicated in the hyperphosphorylation of tau, a major component of the neurofibrillary tangles in Alzheimers disease. This, together with findings showing that IL-1 activates MAPK-p38 in vitro and is markedly overexpressed in Alzheimer brain, suggest a role for IL-1-induced MAPK-p38 activation in the genesis of neurofibrillary pathology in Alzheimers disease. We found frequent colocalization of hyperphosphorylated tau protein (AT8 antibody) and activated MAPK-p38 in neurons and in dystrophic neurites in Alzheimer brain, and frequent association of these structures with activated microglia overexpressing IL-1. Tissue levels of IL-1 mRNA as well as of both phosphorylated and non-phosphorylated isoforms of tau were elevated in these brains. Significant correlations were found between the numbers of AT8- and MAPK-p38-immunoreactive neurons, and between the numbers of activated microglia overexpressing IL-1 and the numbers of both AT8- and MAPK-p38-immunoreactive neurons. Furthermore, rats bearing IL-1-impregnated pellets showed a six- to seven-fold increase in the levels of MAPK-p38 mRNA, compared with rats with vehicle-only pellets (P<0.0001). These results suggest that microglial activation and IL-1 overexpression are part of a feedback cascade in which MAPK-p38 overexpression and activation leads to tau hyperphosphorylation and neurofibrillary pathology in Alzheimers disease.

Life-long overexpression of S100β in Down’s syndrome: implications for Alzheimer pathogenesis

Chronic overexpression of the neurite growth-promoting factor S100beta has been implicated in the pathogenesis of neuritic plaques in Alzheimers disease. Such plaques are virtually universal in middle-aged Downs syndrome, making Downs a natural model of Alzheimers disease. We determined numbers of astrocytes overexpressing S100beta, and of neurons overexpressing beta-amyloid precursor protein (beta-APP), and assayed for neurofibrillary tangles in neocortex of 20 Downs syndrome patients (17 weeks gestation to 68 years). Compared to controls, there were twice as many S100beta-immunoreactive (S100beta+) astrocytes in Downs patients at all ages: fetal, young, and adult (p = 0.01, or better, in each age group). These were activated (i.e., enlarged), and intensely immunoreactive, even in the fetal group. There were no neurofibrillary changes in fetal or young Downs patients. The numbers of S100beta+ astrocytes in young and adult Downs patients correlated with the numbers of neurons overexpressing beta-APP (p < 0.05). Our findings are consistent with the idea that conditions--including Downs syndrome--that promote chronic overexpression of S100beta may confer increased risk for later development of Alzheimers disease.

Distribution of interleukin-1-immunoreactive microglia in cerebral cortical layers: implications for neuritic plaque formation in Alzheimer’s disease

Activated microglia overexpressing interleukin‐1 (IL‐1) are prominent neuropathological features of Alzheimer’s disease. We used computerized image analysis to determine the number of IL‐1α‐immunoreactive (IL‐1α+ ) microglia in cytoarchitectonic layers of parahippocampal gyrus (Brodmann’s area 28) of Alzheimer and control patients. For cortical layers I and II, the numbers of IL‐1α+ microglia were similar in Alzheimer and control patients. For layers III–VI, the numbers of IL‐1α+ microglia were higher than that seen in layers I–II for both Alzheimer and control patients. Moreover, for layers III–VI, the number of IL‐1α+ microglia in Alzheimer patients was significantly greater than that in control patients (relative Alzheimer values of threefold for layer III–V and twofold for layer VI; P<0.05 in each case). The cortical laminar distribution of IL‐1α+ microglia in Alzheimer patients correlated with the cortical laminar distribution of β‐amyloid precursor protein‐immunoreactive (β‐APP+ ) neuritic plaques found in Alzheimer patients (r=0.99, P<0.005). Moreover, the cortical laminar distribution of IL‐1α+ microglia in control patients also correlated with the cortical laminar distribution of β‐APP+ neuritic plaques found in Alzheimer patients (r=0.91, P<0.05). These correlations suggest that pre‐existing laminar distribution patterns of IL‐1α+ microglia (i.e. that seen in control patients) are important in determining the observed laminar distribution of β‐APP+ neuritic plaques in Alzheimer patients. These findings provide further support for our hypothesis that IL‐1 is a key driving force in neuritic plaque formation in Alzheimer’s disease.