Polyamines and Aging: A CLEAR Connection?

Unveiling the novel role of spermidine in leaf senescence: A study of eukaryotic translation factor 5A-independent and dependent mechanisms

Senescence is a crucial and highly active process in plants, optimising resource allocation and promoting phenotypic plasticity under restricted conditions. It involves global metabolic reprogramming for the organised disintegration and remobilization of resources. Polyamines (PAs) are polycationic biogenic amines prevalent in all eukaryotes and are necessary for cell survival. The commonly used PAs in plants include putrescine, spermidine, and spermine. Notably, the leaf's expression of S-adenosylmethionine decarboxylase and spermidine synthase gene family transcripts significantly changes during senescence. This suggests these genes are critical in spermidine metabolism and may condition metabolic reprogramming. One key role of spermidine in eukaryotes is to provide the 4-aminobutyl group for the posttranslational modification of lysine in eukaryotic translation factor 5A (eIF5A). This modification is catalysed by two sequential enzymatic steps leading to the activation of eIF5A by converting lysine to the unusual amino acid hypusine. Although eIF5A is well characterised to be involved in the translation of proline-rich repeat proteins and other hard-to-read motifs, the biological role of eIF5A has recently been clarified only in mammals. It could be better described at the plant functional level. The expression patterns of eIF5A isoforms and genes encoding machinery responsible for hypusination, differ between induced and developmental leaf senescence. In this paper, we summarise the existing knowledge on spermidine-dependent senescence control mechanisms in plants, raising the possibility that spermidine could be an element of a biological switch controlling the onset of a different type of senescence in an eIF5A-independent and dependent manner.

Purification and characterization of tomato arginine decarboxylase and its inhibition by the bacterial small molecule phevamine A

Polyamines play essential roles in plant growth and survival. Arginine decarboxylase (ADC), which converts arginine to agmatine, catalyzes the first step in polyamine biosynthesis from arginine. However, few biochemical studies with purified plant ADCs have been conducted to evaluate their catalytic efficiency. Tomato genome encodes two arginine decarboxylases: SlADC1 and SlADC2, which are critical for growth, development, and immune responses against bacterial pathogens. We expressed and purified soluble SlADC1 as a recombinant protein fused with maltose-binding protein tag from E. coli Rosetta 2(DE3) cells. Using the purified fusion protein, we characterized the biochemical properties of SlADC1 in vitro and explored it as a target of the bacterial small molecule phevamine A. We confirmed that the activity of SlADC1 depends on the cofactor pyridoxal 5'-phosphate. SlADC1 is specific toward l-arginine and its kinetic parameters were measured using a liquid chromatography-mass spectrometry method. Phevamine A is a competitive inhibitor of SlADC1 and reduces the activity of SlADC1 at high micromolar concentrations. Our purification and biochemical characterization of SlADC1 sets the stage for inhibition studies of this enzyme.

Modulation of DNA methylation by one-carbon metabolism: a milestone for healthy aging

Healthy aging can be defined as an extended lifespan and health span. Nutrition has been regarded as an important factor in healthy aging, because nutrients, bioactive food components, and diets have demonstrated beneficial effects on aging hallmarks such as oxidative stress, mitochondrial function, apoptosis and autophagy, genomic stability, and immune function. Nutrition also plays a role in epigenetic regulation of gene expression, and DNA methylation is the most extensively investigated epigenetic phenomenon in aging. Interestingly, age-associated DNA methylation can be modulated by one-carbon metabolism or inhibition of DNA methyltransferases. One-carbon metabolism ultimately controls the balance between the universal methyl donor S-adenosylmethionine and the methyltransferase inhibitor S-adenosylhomocysteine. Water-soluble B-vitamins such as folate, vitamin B6, and vitamin B12 serve as coenzymes for multiple steps in one-carbon metabolism, whereas methionine, choline, betaine, and serine act as methyl donors. Thus, these one-carbon nutrients can modify age-associated DNA methylation and subsequently alter the age-associated physiologic and pathologic processes. We cannot elude aging per se but we may at least change age-associated DNA methylation, which could mitigate age-associated diseases and disorders.

Resolving Geroplasticity to the Balance of Rejuvenins and Geriatrins

According to the cell centric hypotheses, the deficits that drive aging occur within cells by age dependent progressive damage to organelles, telomeres, biologic signaling pathways, bioinformational molecules, and by exhaustion of stem cells. Here, we amend these hypotheses and propose an eco-centric model for geroplasticity (aging plasticity including aging reversal). According to this model, youth and aging are plastic and require constant maintenance, and, respectively, engage a host of endogenous rejuvenating (rejuvenins) and gero-inducing [geriatrin] factors. Aging in this model is akin to atrophy that occurs as a result of damage or withdrawal of trophic factors. Rejuvenins maintain and geriatrins adversely impact cellular homeostasis, cell fitness, and proliferation, stem cell pools, damage response and repair. Rejuvenins reduce and geriatrins increase the age-related disorders, inflammatory signaling, and senescence and adjust the epigenetic clock. When viewed through this perspective, aging can be successfully reversed by supplementation with rejuvenins and by reducing the levels of geriatrins.

Autophagy at the intersection of aging, senescence, and cancer

Autophagy is an evolutionarily conserved cellular process in which macromolecules undergo lysosomal degradation. It fulfills essential roles in quality controlling cellular constituents and in energy homeostasis. Basal autophagy is also widely accepted to provide a protective role in aging and aging-related disorders, and its decline with age might precipitate the onset of a variety of diseases. In this review, we discuss the role of basal autophagy in maintaining homeostasis, in part through the maintenance of stem cell populations and the prevention of cellular senescence. We also consider how stress-induced senescence, for example, during oncogene activation and in premalignant disease, might rely on autophagy, and the possibility that the age-associated decline in autophagy might promote tumour development through a variety of mechanisms. Ultimately, evidence suggests that autophagy is required for malignant cancer progression in a number of settings. Thus, autophagy appears to be tumour-suppressive during the early stages of tumorigenesis and tumour-promoting at later stages.

Polyamines: Α bioenergetic smart switch for plant protection and development

The present review highlights the bioenergetic role of polyamines in plant protection and development and proposes a universal model for describing polyamine-mediated stress responses. Any stress condition induces an excitation pressure on photosystem II by reforming the photosynthetic apparatus. To control this phenomenon, polyamines act directly on the molecular structure and function of the photosynthetic apparatus as well as on the components of the chemiosmotic proton-motive force (ΔpH/Δψ), thus regulating photochemical (qP) and non-photochemical quenching (NPQ) of energy. The review presents the mechanistic characteristics that underline the key role of polyamines in the structure, function, and bioenergetics of the photosynthetic apparatus upon light adaptation and/or under stress conditions. By following this mechanism, it is feasible to make stress-sensitive plants to be tolerant by simply altering their polyamine composition (especially the ratio of putrescine to spermine), either chemically or by light regulation.

Exogenous spermine attenuates diabetic kidney injury in rats by inhibiting AMPK/mTOR signaling pathway

Diabetic nephropathy (DN) is the primary cause of end‑stage renal disease, which is closely associated with dysfunction of the podocytes, the main component of the glomerular filtration membrane; however, the exact underlying mechanism is unknown. Polyamines, including spermine, spermidine and putrescine, have antioxidant and anti‑aging properties that are involved in the progression of numerous diseases, but their role in DN has not yet been reported. The present study aimed to explore the role of polyamines in DN, particularly in podocyte injury, and to reveal the molecular mechanism underlying the protective effect of exogenous spermine. Streptozotocin intraperitoneal injection‑induced type 1 diabetic (T1D) rat models and high glucose (HG)‑stimulated podocyte injury models were established. It was found that in T1D rat kidneys and HG‑induced podocytes, ornithine decarboxylase (a key enzyme for polyamine synthesis) was downregulated, while spermidine/spermine N1‑acetyltransferase (a key enzyme for polyamines degradation) was upregulated, which suggested that reduction of the polyamine metabolic pool particularly decreased spermine content, is a major factor in DN progression. In addition, hyperglycemia can induce an increased rat kidney weight ratio, serum creatinine, urea, urinary albumin excretion and glomerular cell matrix levels, and promote mesangial thickening and loss or fusion of podocytes. The expression levels of podocyte marker proteins (nephrin, CD2‑associated protein and podocin) and autophagy‑related proteins [autophagy protein 5, microtube‑associated proteins 1A/1B light chain 3 (LC3)II/LC3I, Beclin 1 and phosphorylated (p)‑AMPK] were downregulated, while cleaved caspase‑3, P62 and p‑mTOR were increased. These changes could be improved by pretreatment with exogenous spermine or rapamycin (autophagic agonist). In conclusion, spermine may have the potential to prevent diabetic kidney injury in rats by promoting autophagy via regulating the AMPK/mTOR signaling pathway.

Evidence and perspectives of cell senescence in neurodegenerative diseases

Increased life expectancies have significantly increased the number of individuals suffering from geriatric neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). The financial cost for current and future patients with these diseases is overwhelming, resulting in substantial economic and societal costs. Unfortunately, most recent high-profile clinical trials for neurodegenerative diseases have failed to obtain efficacious results, indicating that novel approaches are desperately needed to treat these pathologies. Cell senescence, characterized by permanent cell cycle arrest, resistance to apoptosis, mitochondrial alterations, and secretion of senescence-associated secretory phenotype (SASP) components, has been extensively studied in mitotic cells such as fibroblasts, which is considered a hallmark of aging. Furthermore, multiple cell types in the senescent state in the brain, including neurons, microglia, astrocytes, and neural stem cells, have recently been observed in the context of neurodegenerative diseases, suggesting that these senescent cells may play an essential role in the pathological processes of neurodegenerative diseases. Therefore, this review begins by outlining key aspects of cell senescence constitution followed by examining the evidence implicating senescent cells in neurodegenerative diseases. In the final section, we review how cell senescence may be targeted as novel therapeutics to treat pathologies associated with neurodegenerative diseases.