Macrophage phagocytosis and membrane fluidity in mice: the effect of age and dietary protein

Markers of the ageing macrophage: a systematic review and meta-analysis

Introduction

Ageing research is establishing macrophages as key immune system regulators that undergo functional decline. Due to heterogeneity between species and tissue populations, a plethora of data exist and the power of scientific conclusions can vary substantially. This meta-analysis by information content (MAIC) and systematic literature review (SLR) aims to determine overall changes in macrophage gene and protein expression, as well as function, with age.

Methods

PubMed was utilized to collate peer-reviewed literature relating to macrophage ageing. Primary studies comparing macrophages in at least two age groups were included. Data pertaining to gene or protein expression alongside method used were extracted for MAIC analysis. For SLR analysis, data included all macrophage-specific changes with age, as well as species, ontogeny and age of groups assessed.

Results

A total of 240 studies were included; 122 of which qualified for MAIC. The majority of papers focussed on changes in macrophage count/infiltration as a function of age, followed by gene and protein expression. The MAIC found iNOS and TNF to be the most commonly investigated entities, with 328 genes and 175 proteins showing consistent dysregulation with age across the literature. Overall findings indicate that cytokine secretion and phagocytosis are reduced and reactive oxygen species production is increased in the ageing macrophage.

Discussion

Collectively, our analysis identifies critical regulators in macrophage ageing that are consistently dysregulated, highlighting a plethora of targets for further investigation. Consistent functional changes with age found here can be used to confirm an ageing macrophage phenotype in specific studies and experimental models.

Association of age-related decrease in platelet membrane fluidity with platelet lipid peroxide

The influence of age on platelet lipid peroxide (LPO), platelet membrane fluidity and the composition of fatty acid was investigated in female Wistar rats widely ranging in age from 14 to 720 days old. LPO levels were significantly higher (p<0.05) in the platelets of upper age groups than in those of lower age groups, showing a significantly positive correlation with age (r=0.84, p<0.0001). Membrane fluidity, assessed by 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence polarization, was significantly reduced with age. The composition of fatty acid demonstrated an age-related elevation (p<0.05) in the unsaturation index. The rises in the LPO levels revealed a significantly positive correlation with DPH-polarization (r=0.73, p<0.0001). Thus our results suggested that the age-related deterioration of platelet membrane fluidity, despite a significant elevation in the unsaturation index, was due to the age-related higher basal levels of LPO in platelets.

Mitochondrial membrane fluidity and oxidative damage to mitochondrial DNA in aged and AD human brain

Oxidative damage on biological molecules has been proposed as a major cause of alterations observed in aging brain as well as in neurodegenerative diseases. In this study, we measured membrane fluidity in mitochondria extracted from three cerebral regions and cerebellum of Alzheimer disease (AD) patients and age-matched controls by means of fluorescence polarization technique. A significant reduction of mitochondrial membrane fluidity was found in AD, except in cerebellum. In controls, a decrease of membrane fluidity was observed along with age, and it was also related to the content of the oxidized nucleoside 8-hydroxy-2'-deoxyguanosine (OH8dG) in mitochondrial DNA (mtDNA). Alteration in membrane fluidity seems to be a result of lipid peroxidation, since it dramatically decreased when mitochondria were exposed to FeCl2 and H2O2. The parallel increase of viscosity in mitochondrial membrane and the amount of OH8dG in mtDNA is suggestive of a relationship between these biological markers of oxidative stress. These results provide further evidence that oxidative stress may play a role in the pathogenesis of AD.

Alterations in mitochondrial membrane fluidity by lipid peroxidation products

Age-related damage to the mitochondrial membrane, including decreased membrane fluidity, has been attributed to free radical reactions. Our previous studies point to lipid peroxidation as a primary cause in age-related changes in membrane fluidity. This report offers new evidence that lipid peroxidation-modulated decreases in membrane fluidity are mediated through two aldehydic lipid peroxidation products, 4-hydroxynonenal (HNE) and malondialdehyde (MDA). Hepatic mitochondria were isolated from both ad libitum fed (AL) and dietary restricted (DR) rats of different ages. Introduction of the aldehydes was found to decrease mitochondrial membrane fluidity, although the fluidity decrease induced by HNE was more pronounced than that induced by MDA. It seems likely that HNE modifies membrane fluidity by direct interaction with membrane phospholipids, as shown by the generation of a fluorescent complex between HNE and membrane phospholipids. Finally, HNE and MDA were isolated and quantitated in mitochondria. Their levels clearly differentiated between animals of different age and dietary groups. These data indicate that the reactive products of lipid peroxidation, especially HNE, may play an important role in mediating the decreased mitochondrial membrane fluidity observed in aging animals.

The effects of aging on the function of alveolar macrophages in mice

In order to determine whether the function of alveolar macrophages (AM) is modulated by aging, we measured the TNF-alpha production, phagocytic function, and surface antigen expression of AM from young and old mice. When AM were primed by IFN-gamma (500 units/ml) and triggered by LPS (100 micrograms/ml), TNF-alpha production by AM was significantly smaller in old mice as compared with young mice (young mice: 161.7 +/- 28.2 units/ml; old mice: 89.3 +/- 13.6 units/ml, P < 0.05). The percentage of AM which phagocytosed latex particles (more than one particle) in old mice was significantly lower than in young mice (young: 78.1 +/- 2.5%; old: 62.8 +/- 3.4%, P < 0.05). Ia antigen expression of the AM was significantly higher and asialo-GM1 antigen expression was significantly lower in old mice than in young mice (Ia: young, 0.030 +/- 0.005; old, 0.092 +/- 0.024, P < 0.05; asialo-GM1: young, 0.-9 +/- 0.01; old, 0.75 +/- 0.07, P < 0.01). These results suggest that alveolar macrophage function is at least decreased in part with aging in mice.

Age and membrane fluidity

Male rats aged 1, 9 and 19 months were used to study changes in membrane fluidity with age, employing the fluorescence polarization technique with 1,6-diphenyl-1,3,5-hexatriene (DPH) as the fluorescent probe. The intestinal microvillus membranes derived from the 19-month-old rats were found to possess lower fluidity than that observed with the membranes derived from the younger animals. The decrease in fluidity with age was also reflected in a corresponding increase in the gel-to-liquid crystalline transition temperature. Only small insignificant changes with age, were observed in the fluidity of the red blood cell membrane.

Age-related differences in the induction of 2-5A synthetase and 2-5A dependent binding protein activities by interferon in guinea pig peritoneal macrophages

2-5A synthetase and binding protein activities in peritoneal macrophages have been compared between young (6 month) and old (22-24 month) guinea pigs. Enzyme activities are lower in aged animals with a 17% and a 31% reduction in synthetase and binding protein activities, respectively. In addition, the response to the addition of mouse fibroblast interferon by macrophages from these two age groups is also substantially different. Whereas addition of interferon to young guinea pig macrophages elicits a 3.8- and a 1.7-fold increase in the synthetase and binding protein activities, only a marginal elevation in these two enzyme activities is found with interferon-treated old guinea pig macrophages. Analysis by thin layer chromatography demonstrates a marked difference in the relative distribution of the various oligomeric forms of 2-5A synthesized by young or old guinea pig macrophages. The binding protein in old animals appears to be significantly more thermolabile than the corresponding activity from young animals. The altered response to interferon and the difference in enzymatic properties in aged animals may represent part of the mechanisms involved in the progressive loss of the adaptative ability of an organism to environmental changes during senescence.