衰老叶片中叶绿素的降解

综述了近年来关于衰老叶片中叶绿素降解的研究情况,包括叶绿素代谢的中间产物、终产物、主要代谢途径、代谢酶及代谢途径在细胞内的定位及代谢调节方面的研究进展。     

Cell-cell affinity of senescent human erythrocytes

During their 120-day life span, human red blood cells (RBC) undergo several physicochemical changes, including an increased tendency to aggregate in plasma or polymer solutions. This study was designed to examine potential associations between age-related differences in RBC mobility, aggregation, and membrane glycocalyx properties for cells suspended in buffer and in 3 g/dl solutions of 70.3 kDa dextran. A recent model for depletion-mediated RBC aggregation was employed to calculate the changes of glycocalyx properties that were consistent with experimental electrophoretic mobility (EPM) and aggregation data. Young and old cells were obtained by density separation, after which aggregation and EPM were determined versus ionic strength; old cells exhibited a two- to threefold greater aggregation in dextran. EPM of old cells was identical to young cells in polymer-free media yet was 4% greater in dextran. The greater EPM for old RBC indicates a larger polymer depletion layer, which could be explained either by a 10-15% decrease of their glycocalyx thickness or a similar percentage decrease of polymer penetration into their glycocalyx. The larger depletion layer leads to markedly elevated cell-cell affinities for old cells, with the computed affinity increases consistent with enhanced old RBC aggregation. These results provide a rational explanation for the aggregation and EPM behavior of old RBC, and raise the possibility of depletion-mediated interactions contributing to senescent cell removal from the circulation.     

Attempted hybridizations with senescent human fibroblasts

If the aging of diploid human fibroblasts reflects stable genetic or epigenetic changes which are few in number and different in different cells, then complementation could occur in hybrids between individual senescent cells. However neither pairs of aged fibroblasts nor even pairs of young and senescent fibroblasts produce viable hybrids under conditions known to promote cell fusion.     

Membrane proteins in senescent erythrocytes.

The examination of erythrocyte senescence has been facilitated by recent advances in techniques for the isolation of aged red cells. One of these methods, which uses biotinylated rabbit erythrocytes, has been used to examine the state of membrane proteins in effete cells. These aged red cells were found to have normal ratios of alpha-spectrin and beta-spectrin as well as normal levels of ankyrin. The observation concerning ankyrin is particularly important due to the sensitivity of this protein to proteolysis and the postulated action of proteinases in the aging process. The senescent erythrocytes were also found to have an altered ratio of bands 4.1a and 4.1b without any apparent change in the total level of 4.1. In addition, the analysis of the aged cell membranes did not show any large-molecular-mass aggregated protein at the origin of the SDS/polyacrylamide gels, indicating a lack of transglutaminase activity in the senescence process for rabbit erythrocytes. These results indicate that aging of the rabbit erythrocyte is not accompanied by gross proteolytic degradation or transglutaminase-catalysed cross-linking of membrane components.     

Pseudo-DNA damage response in senescent cells

Cellular senescence is currently viewed as a response to DNA damage. In this report, we showed that non-damaging agents such as sodium butyrate-induced p21 and ectopic expression of either p21 or p16 cause cellular senescence without detectable DNA breaks. Nevertheless, senescent cells displayed components of DNA damage response (DDR) such as γH2AX foci and uniform nuclear staining for p-ATM. Importantly, there was no accumulation of 53BP1 in γH2AX foci of senescent cells. Consistently, comet assay failed to detect DNA damage. Rapamycin, an inhibitor of mTOR, which was shown to suppress cellular senescence, decreased γH2AX foci formation. Thus, cellular senescence leads to activation of atypical DDR without detectable DNA damage. Pseudo-DDR may be a marker of general over-activation of senescent cells.     

Lipid crystallization in senescent membranes from cotyledons

Lipid transition temperatures for rough and smooth microsomal membranes isolated from bean (Phaseolus vulgaris) cotyledon tissue at various stages of germination were determined by wide angle x-ray diffraction. The transition temperatures were established by recording diffraction patterns through a temperature series until a sharp x-ray reflection centered at a Bragg spacing of 4.15 Å and denoting the presence of crystalline lipid was discernible. For rough and smooth microsomes from 2-day-old tissue, the transitions occurred at 0 C and 3 C, respectively, indicating that at this early stage in the germination sequence the membrane lipid is entirely liquid-crystalline at physiological temperature. By the 4th day of germination, the transition temperatures had increased to 32 C for smooth microsomes and 35 C for rough microsomes, indicating that at 29 C, which was the growth temperature, portions of the membrane lipid were crystalline. During the later stages of germination, the transition temperature for smooth microsomes continued to rise through 44 C at day 7 to 56 C at day 9, by which time the cotyledons were extensively senescent and beginning to abscise. There was also a dramatic increase in the proportion of membrane lipid in the crystalline phase at 29 C. By contrast, the rough microsomes showed little change in transition temperature and only a slight increase in the proportion of crystalline lipid during this late period in germination. The data indicate that substantial amounts of the lipid is senescing membranes are crystalline even at physiological temperature. Moreover, there is a temporal correlation between the appearance of this crystallinity and loss of membrane function, suggesting that the two may be causally related.     

Actin in young and senescent fibroblasts.

Long-term exposure of guinea pigs to a diet rich in maize oil caused an increase in adipose-tissue lipoprotein lipase activity. A similar diet rich in beef tallow had no such effect, and neither diet affected the enzyme activity in heart, lung, diaphragm or skeletal muscle.     

Mitochondrial DNA damage induces apoptosis in senescent cells

Senescence is a cellular response to damage and stress. The senescence response prevents cancer by suppressing the proliferation of cells with a compromised genome and contributes to optimal wound healing in normal tissues. Persistent senescent cells are also thought to drive aging and age-associated pathologies through their secretion of inflammatory factors that modify the tissue microenvironment and alter the function of nearby normal or transformed cells. Understanding how senescent cells alter the microenvironment would be aided by the ability to induce or eliminate senescent cells at will in vivo. Here, we combine the use of the synthetic nucleoside analog ganciclovir (GCV) with herpes simplex virus thymidine kinase (HSVtk) activity to create or eliminate senescent human cells. We show that low concentrations of GCV induce senescence through the accumulation of nuclear DNA damage while higher concentrations of GCV, similar to those used in vivo, kill non-dividing senescent cells via mitochondrial DNA (mtDNA) damage and caspase-dependent apoptosis. Using this system, we effectively eliminated xenografted normal human senescent fibroblasts or induced senescence in human breast cancer cells in vivo. Thus, cellular senescence and mtDNA damage are outcomes of synthetic nucleoside analog treatment, indicating that the GCV–HSVtk combination can be used effectively to promote the targeted formation or eradication of senescent cells.     

Induction of apoptosis in human replicative senescent fibroblasts

Cellular senescence is an irreversible growth phase characteristic of normal cells. We have found that human senescent fibroblasts can be induced to undergo programmed cell death (apoptosis) by ceramide, TNF-alpha, or okadaic acid. The most profound effects were induced by TNF-alpha and okadaic acid treatment. In the present study, we also evaluated the contribution of lysosomal activation as a possible mechanism underlying the induction of apoptosis. Four lysosomal enzyme activities were measured: beta-galactosidase, alpha-galactosidase A, beta-glucoronidase, and acid phosphatase. Using an in situ assay, we have found that the activity of beta-galactosidase, which is also a biochemical marker of senescence, is induced in young proliferating fibroblasts following exposure to all three apoptotic inducing agents. The other enzymes were not significantly induced in young fibroblasts following exposure to agents that induce apoptosis. During replicative senescence, three of the four lysosomal enzymes tested (beta-galactosidase, alpha-galactosidase A, and beta-glucoronidase) are constitutively expressed at high levels. TNF-alpha was the only agent that induced lysosomal activity in senescent fibroblasts, of which only alpha-galactosidase A activity was induced. Our studies show that senescent fibroblasts can be induced to undergo apoptosis in a signal-dependent manner. However, the lysosomal enzymes examined do not appear to be correlated with apoptotic induction.     

Role of senescent fibroblasts on alkali-induced corneal neovascularization

Cellular senescence acts as a potent regulator of tumor suppression and fibrosis limitation; however, its contribution and crosstalk with neovascularization during normal wound healing has not been examined. Here, we explored the role of senescent fibroblasts on neovascularization with a mouse model of alkali-induced corneal wound healing. Senescent cells accumulated in corneal stroma from day 7 to 27 after alkali burn and peaked on day 14, which was consistent with the development of corneal neovascularization (CNV). In vitro and in vivo assays confirmed that the senescent cells were derived primarily from activated corneal fibroblasts. Furthermore, senescent corneal fibroblasts exhibited enhanced synthesis and secretion of extracellular matrix-degrading enzymes (matrix metalloproteinases 2, 3, and 14 and tissue- and urokinase-type plasminogen activators) and angiogenic factors (vascular endothelial growth factor) and decreased expression of anti-angiogenic factors (pigment epithelium-derived factor and thrombospondins), which supported the proliferation, migration, and promotion of tube formation of vascular endothelial cells. Intrastromal injection of premature senescent fibroblasts induced CNV earlier than that of normal fibroblasts, while matrix metalloproteinase inhibitors blocked the early onset of senescent cell-induced CNV. Therefore, senescent fibroblasts promoted the alkali-induced CNV partially via the enhanced secretion of matrix metalloproteases.     

Genetic modulation of senescent phenotypes in Homo sapiens

Single-gene mutations can produce human progeroid syndromes--phenotypes that mimic usual or "normative" aging. These can be divided into two classes--those that have their impacts upon multiple organs and tissues (segmental progeroid syndromes) and those that have their major impacts upon a single organ or tissue (unimodal progeroid syndromes). The prototypic example of the former is the Werner syndrome, a condition caused by mutations of the RecQ family of DNA helicases. Research on the Werner syndrome and a surprising number of other progeroid syndromes support the importance of the maintenance of genomic stability as a partial antidote to aging. The prototypic examples of the latter are Alzheimer type dementias. The three gene products that cause rare autosomal-dominant early-onset varieties of these disorders all participate in the modulation of the beta amyloid precursor protein. They thus support the importance of the maintenance of proper protein processing and folding as a partial antidote to aging.     

Resistance of senescent keratinocytes to UV-induced apoptosis.

Irradiation with ultraviolet (UV) triggers programmed cell death (apoptosis) in keratinocytes. This process is believed to protect against skin carcinogenesis since the cells with damaged DNA are selectively removed, limiting the likelihood of the development of a malignant keratinocyte clone. The p53 protein is able to detect mutation-bearing DNA fragments and is thus indispensable for the UV-induced apoptosis in the epidermis. Since age is a risk factor for the development of skin tumors we investigated whether ultraviolet induces apoptosis and p53 activation in senescent keratinocytes. Cultured senescent keratinocytes were irradiated with broad-band ultraviolet, apoptosis was assessed using TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling) technique and the p53 activation pattern was determined with Western blotting and immunofluorescent staining with a panel of anti-p53 antibodies recognising different conformational forms of the protein (PAb 122, PAb 240, DO-7). In senescent keratinocytes arrested in the G1 phase of cell cycle, ultraviolet irradiation (100-2000 J/m2) caused accumulation and nuclear translocation of p53. However, in contrast to young cells where UV induces apoptotic cell death in G1, apoptosis was not detected in senescent cells. There were subtle differences in the p53 activation pattern between senescent keratinocytes and known patterns in young keratinocytes and other cell types. In senescent keratinocytes a constitutional nuclear expression of p53 (conformational form recognized by PAb 240) was present and the p53 induction in response to ultraviolet radiation was rapid. Suppression of apoptosis in senescent keratinocytes may be an important mechanism responsible for enhanced skin carcinogenesis in old age.     

Metabolism of radiolabelled galactolipids in senescent barley leaves

The metabolism of galactolipids in senescent primary leaves of barley was followed upon the incorporation of [1-¹⁴C]oleic acid during greening of etiolated shoots. In presenscent leaves the bulk of the label was present in apolar compounds of which 36% were galactolipids and 57% phospholipids. In segments induced to senesce in continouos darkness the loss of the predominant 18:3/18:3 molecular species of galactolipids was accompanied by the temporary accumulation of 18:3/18:3 phosphatidylcholine, a species which was almost absent in the presenescent leaves. A large proportion of radiocactivity lost in the galactolipids was eventually released as CO₂. Label was also transferred to sugars and other polar compounds. A pathway of galactolipid breakdown featuring the conversion of the diacylglycerol cores to phosphatidylcholine followed by the release of free fatty acids in glyoxisome-like microbodies and gluconeogenesis via β-oxidation and the glyoxylic acid cycle is proposed.     

The role of senescent T cells in immunopathology

The development of senescence in tissues of different organs and in the immune system are usually investigated independently of each other although during ageing, senescence in both cellular systems develop concurrently. Senescent T cells are highly inflammatory and secrete cytotoxic mediators and express natural killer cells receptors (NKR) that bypass their antigen specificity. Instead they recognize stress ligands that are induced by inflammation or infection of different cell types in tissues. In this article we discuss data on T cell senescence, how it is regulated and evidence for novel functional attributes of senescent T cells. We discuss an interactive loop between senescent T cells and senescent non‐lymphoid cells and conclude that in situations of intense inflammation, senescent cells may damage healthy tissue. While the example for immunopathology induced by senescent cells that we highlight is cutaneous leishmaniasis, this situation of organ damage may apply to other infections, including COVID‐19 and also rheumatoid arthritis, where ageing, inflammation and senescent cells are all part of the same equation.     

Aldolase B in the liver of senescent rats

The process of enzymatic aging was studied in livers of adult and senescent rats for aldolase B. No “cross-reacting material” was found in livers of 27 to 30-month-old rats, estimated by the ratio aldolase activity/antigen amount. The activity towards the two substrates of aldolase, fructose 1,6-diphosphate and fructose 1-phosphate did not vary in senescent animals. Moreover, other physico-chemical properties of the enzyme such as thermal inactivation, immunological reactivity and Michealis constant remain unchanged. These results provide arguments againt the occurence of errors in protein synthesis as a cause of aging.     

Protein turnover in senescent cultured chick embryo fibroblasts

The over-all rates of protein synthesis, degradation and net accumulation were estimated in rapidly growing young and slowly doubling old cultures of chick fibroblasts. We find that not only the rate of protein synthesis is reduced in senescent cultures, but the average rate of protein degradation is also slowed down considerably. This decrease in the rate of protein breakdown in aging cells stands in contrast with the previously observed acceleration of this process by other conditions (such as serum deprivation or overcrowding) that lead to the cessation of cellular growth. Though the retarded protein degradation may contribute to the acculation of abnormal proteins in senescent cells we find that the breakdown of grossly abnormal puromycin peptides proceeds equally rapidly in young and old cultures. The protein content of senescent cells increases by 1.8-fold as compared to young cells, while the average cell volume is increased even more (almost 5-fold). By contrast, consideration of the over-all balance of protein metabolism in these cells indicates that the average concentration of metabolically turning-over proteins is somewhat higher in senescent than in young fibroblasts.     

Investigations of proliferative and senescent callus of soybean

Different explant sources from several Giycine max (L.) Merr. cultivars and several G. soja F. J. Herm Plant Introductions were tested for calluas production. Embryo axis explants produced callus most readily; hence, they were selected for media response studies and microscopic evaluation. Two callus types, proliferative and senescing, were identified and characterized by light and scanning electron microscopy (SEM). Proliferative callus was composed of chloroplast-containing cells interspersed with colorless meristematic areas, while senescing callus contained mostly large, vacuolate cells without chloroplasts. In SEM preparations a furry, layered, mucilaginous coating was seen on the outer surface of the senescing callus, while proliferative callus cells had uncoated, smooth or wrinkled surfaces. The mucilage contained glucose and galactose.