Long-lived Postmitotic Cells Research Articles

Ciliated, Mitochondria-Rich Postmitotic Cells are Immune-privileged, and Mimic Immunosuppressive Microenvironment of Tumor-Initiating Stem Cells: From Molecular Anatomy to Molecular Pathway

Cancer whose major problems are metastasis, treatment resistance, and recurrence is the leading cause of death worldwide. Tumor-initiating stem cells (TiSCs) are a subset of the tumor population responsible for tumor resistance and relapse. Understanding the characteristics and shared features between tumor-initiating stem cells (TiSCs) and long-lived postmitotic cells may hold a key to better understanding the biology of cancer. Postmitotic cells have exited the cell cycle and are transitioned into a non-dividing and terminally differentiated state with a specialized function within a tissue. Conversely, a cancer cell with TiSC feature can divide and produce a variety of progenies, and is responsible for disease progression, tumor resistance to therapy and immune system and disease relapse. Surprisingly, our comprehensive evaluation of TiSCs suggests common features with long-lived post-mitotic cells. They are similar in structure (primary cilia, high mitochondrial content, and being protected by a barrier), metabolism (autophagy and senescence), and function (immunoescape and/or immune-privileged by a blood barrier). In-depth exploration showed how mitochondrial metabolism contributes to these shared features, including high energy demands arising from ciliary and microtubular functionality, increased metabolic activity, and movement. These findings can assist in decoding the remaining properties which offer insights into the biology of TiSCs, with potential implications for enhancing cancer treatment strategies and patient prognosis.

Novel therapeutic strategies targeting mitochondria as a gateway in neurodegeneration.

In recent years, multiple disciplines have focused on mitochondrial biology and contributed to understanding its relevance towards adult-onset neurodegenerative disorders. These are complex dynamic organelles that have a variety of functions in ensuring cellular health and homeostasis. The plethora of mitochondrial functionalities confers them an intrinsic susceptibility to internal and external stressors (such as mutation accumulation or environmental toxins), particularly so in long-lived postmitotic cells such as neurons. Thus, it is reasonable to postulate an involvement of mitochondria in aging-associated neurological disorders, notably neurodegenerative pathologies including Alzheimer's disease and Parkinson's disease. On the other hand, biological effects resulting from neurodegeneration can in turn affect mitochondrial health and function, promoting a feedback loop further contributing to the progression of neuronal dysfunction and cellular death. This review examines state-of-the-art knowledge, focus on current research exploring mitochondrial health as a contributing factor to neuroregeneration, and the development of therapeutic approaches aimed at restoring mitochondrial homeostasis in a pathological setting.

Neuronal Senescence in the Aged Brain.

Cellular senescence is a highly complicated cellular state that occurs throughout the lifespan of an organism. It has been well-defined in mitotic cells by various senescent features. Neurons are long-lived post-mitotic cells with special structures and functions. With age, neurons display morphological and functional changes, accompanying alterations in proteostasis, redox balance, and Ca2+ dynamics; however, it is ambiguous whether these neuronal changes belong to the features of neuronal senescence. In this review, we strive to identify and classify changes that are relatively specific to neurons in the aging brain and define them as features of neuronal senescence through comparisons with common senescent features. We also associate them with the functional decline of multiple cellular homeostasis systems, proposing the possibility that these systems are the main drivers of neuronal senescence. We hope this summary will serve as a steppingstone for further inputs on a comprehensive but relatively specific list of phenotypes for neuronal senescence and in particular their underlying molecular events during aging. This will in turn shine light on the association between neuronal senescence and neurodegeneration and lead to the development of strategies to perturb the processes.

Polymerases and DNA Repair in Neurons: Implications in Neuronal Survival and Neurodegenerative Diseases.

Most of the neurodegenerative diseases and aging are associated with reactive oxygen species (ROS) or other intracellular damaging agents that challenge the genome integrity of the neurons. As most of the mature neurons stay in G0/G1 phase, replication-uncoupled DNA repair pathways including BER, NER, SSBR, and NHEJ, are pivotal, efficient, and economic mechanisms to maintain genomic stability without reactivating cell cycle. In these progresses, polymerases are prominent, not only because they are responsible for both sensing and repairing damages, but also for their more diversified roles depending on the cell cycle phase and damage types. In this review, we summarized recent knowledge on the structural and biochemical properties of distinct polymerases, including DNA and RNA polymerases, which are known to be expressed and active in nervous system; the biological relevance of these polymerases and their interactors with neuronal degeneration would be most graphically illustrated by the neurological abnormalities observed in patients with hereditary diseases associated with defects in DNA repair; furthermore, the vicious cycle of the trinucleotide repeat (TNR) and impaired DNA repair pathway is also discussed. Unraveling the mechanisms and contextual basis of the role of the polymerases in DNA damage response and repair will promote our understanding about how long-lived postmitotic cells cope with DNA lesions, and why disrupted DNA repair contributes to disease origin, despite the diversity of mutations in genes. This knowledge may lead to new insight into the development of targeted intervention for neurodegenerative diseases.

CPNE7-Induced Autophagy Restores the Physiological Function of Mature Odontoblasts.

Dentin, which composes most of the tooth structure, is formed by odontoblasts, long-lived post-mitotic cells maintained throughout the entire life of the tooth. In mature odontoblasts, however, cellular activity is significantly weakened. Therefore, it is important to augment the cellular activity of mature odontoblasts to regenerate physiological dentin; however, no molecule regulating the cellular activity of mature odontoblasts has yet been identified. Here, we suggest that copine-7 (CPNE7) can reactivate the lost functions of mature odontoblasts by inducing autophagy. CPNE7 was observed to elevate the expression of microtubule-associated protein light chain 3-II (LC3-II), an autophagy marker, and autophagosome formation in the pre-odontoblast and mature odontoblast stages of human dental pulp cells. CPNE7-induced autophagy upregulated DSP and DMP-1, odontoblast differentiation and mineralization markers, and augmented dentin formation in mature odontoblasts. Furthermore, CPNE7 also upregulated NESTIN and TAU, which are expressed in the physiological odontoblast process, and stimulated the elongation of the odontoblast process by inducing autophagy. Moreover, lipofuscin, which progressively accumulates in long-lived post-mitotic cells and hinders their proper functions, was observed to be removed in recombinant CPNE7-treated mature odontoblasts. Thus, CPNE7-induced autophagy reactivated the function of mature odontoblasts and promoted the formation of physiological dentin in vivo. On the other hand, the well-known autophagy inducer, rapamycin, promoted odontoblast differentiation in pre-odontoblasts but did not properly reactivate the function of mature odontoblasts. These findings provide evidence that CPNE7 functionally reactivates mature odontoblasts and introduce its potential for dentinal loss-targeted clinical applications.

Readily Releasable Stores of Calcium in Neuronal Endolysosomes: Physiological and Pathophysiological Relevance.

Neurons are long-lived post-mitotic cells that possess an elaborate system of endosomes and lysosomes (endolysosomes) for protein quality control. Relatively recently, endolysosomes were recognized to contain high concentrations (400-600μM) of readily releasable calcium. The release of calcium from this acidic organelle store contributes to calcium-dependent processes of fundamental physiological importance to neurons including neurotransmitter release, membrane excitability, neurite outgrowth, synaptic remodeling, and cell viability. Pathologically, disturbances of endolysosome structure and/or function have been noted in a variety of neurodegenerative disorders including Alzheimer's disease (AD) and HIV-1 associated neurocognitive disorder (HAND). And, dysregulation of intracellular calcium has been implicated in the neuropathogenesis of these same neurological disorders. Thus, it is important to better understand mechanisms by which calcium is released from endolysosomes as well as the consequences of such release to inter-organellar signaling, physiological functions of neurons, and possible pathological consequences. In doing so, a path forward towards new therapeutic modalities might be facilitated.

The Nucleotide Sensor ZBP1 and Kinase RIPK3 Induce the Enzyme IRG1 to Promote an Antiviral Metabolic State in Neurons

As long-lived post-mitotic cells, neurons employ unique strategies to resist pathogen infection while preserving cellular function. Here, using a murine model of Zika virus (ZIKV) infection, we identified an innate immune pathway that restricts ZIKV replication in neurons and is required for survival upon ZIKV infection of the central nervous system (CNS). We found that neuronal ZIKV infection activated the nucleotide sensor ZBP1 and the kinases RIPK1 and RIPK3, core components of virus-induced necroptotic cell death signaling. However, activation of this pathway in ZIKV-infected neurons did not induce cell death. Rather, RIPK signaling restricted viral replication by altering cellular metabolism via upregulation of the enzyme IRG1 and production of the metabolite itaconate. Itaconate inhibited the activity of succinate dehydrogenase, generating a metabolic state in neurons that suppresses replication of viral genomes. These findings demonstrate an immunometabolic mechanism of viral restriction during neuroinvasive infection.

Axin2-expressing cells differentiate into reparative odontoblasts via autocrine Wnt/\u03b2-catenin signaling in response to tooth damage

In non-growing teeth, such as mouse and human molars, primary odontoblasts are long-lived post-mitotic cells that secrete dentine throughout the life of the tooth. New odontoblast-like cells are only produced in response to a damage or trauma. Little is known about the molecular events that initiate mesenchymal stem cells to proliferate and differentiate into odontoblast-like cells in response to dentine damage. The reparative and regenerative capacity of multiple mammalian tissues depends on the activation of Wnt/β-catenin signaling pathway. In this study, we investigated the molecular role of Wnt/β-catenin signaling pathway in reparative dentinogenesis using an in vivo mouse tooth damage model. We found that Axin2 is rapidly upregulated in response to tooth damage and that these Axin2-expressing cells differentiate into new odontoblast-like cells that secrete reparative dentine. In addition, the Axin2-expressing cells produce a source of Wnt that acts in an autocrine manner to modulate reparative dentinogenesis.

Keeping up the balance: role of HDACs in cardiac proteostasis and therapeutic implications for atrial fibrillation.

Cardiomyocytes are long-lived post-mitotic cells with limited regenerative capacity. Proper cardiomyocyte function depends critically on the maintenance of a healthy homeostasis of protein expression, folding, assembly, trafficking, function, and degradation, together commonly referred to as proteostasis. Impairment of proteostasis has a prominent role in the pathophysiology of ageing-related neurodegenerative diseases including Huntington's, Parkinson's, and Alzheimer's disease. Emerging evidence reveals also a role for impaired proteostasis in the pathophysiology of common human cardiac diseases such as cardiac hypertrophy, dilated and ischaemic cardiomyopathies, and atrial fibrillation (AF). Histone deacetylases (HDACs) have recently been recognized as key modulators which control cardiac proteostasis by deacetylating various proteins. By deacetylating chromatin proteins, including histones, HDACs modulate epigenetic regulation of pathological gene expression. Also, HDACs exert a broad range of functions outside the nucleus by deacetylating structural and contractile proteins. The cytosolic actions of HDACs result in changed protein function through post-translational modifications and/or modulation of their degradation. This review describes the mechanisms underlying the derailment of proteostasis in AF and subsequently focuses on the role of HDACs herein. In addition, the therapeutic potential of HDAC inhibition to maintain a healthy proteostasis resulting in a delay in AF onset and progression is discussed.

Oxidative Status and Lipofuscin Accumulation in Urothelial Cells of Bladder in Aging Mice

Age-related changes in various tissues have been associated with the onset of a number of age-related diseases, including inflammation and cancer. Bladder cancer, for instance, is a disease that mainly afflicts middle-aged or elderly people and is mostly of urothelial origin. Although research on age-related changes of long-lived post-mitotic cells such as neurons is rapidly progressing, nothing is known about age-related changes in the urothelium of the urinary bladder, despite all the evidence confirming the important role of oxidative stress in urinary bladder pathology. The purpose of this study was thus to investigate the oxidative status and age-related changes in urothelial cells of the urinary bladder of young (2 months) and aging (20 months) mice by means of various methods. Our results demonstrated that healthy young urothelium possesses a powerful antioxidant defence system that functions as a strong defence barrier against reactive species. In contrast, urothelial cells of aging bladder show significantly decreased total antioxidant capacity and significantly increased levels of lipid peroxides (MDA) and iNOS, markers of oxidative stress. Our study demonstrates for the first time that ultrastructural alterations in mitochondria and accumulation of lipofuscin, known to be one of the aging pigments, can clearly be found in superficial urothelial cells of the urinary bladder in aging mice. Since the presence of lipofuscin in the urothelium has not yet been reported, we applied various methods to confirm our finding. Our results reveal changes in the oxidative status and structural alterations to superficial urothelial cells similar to those of other long-lived post-mitotic cells.

Protective Effects of Flavonoid Baicalein against Menadione-Induced Damage in SK-N-MC Cells

Oxidative damage and redox metal homeostasis loss are two contributing factors in brain aging and widely distributed neurodegenerative diseases. Oxidative species in company with excessive amounts of intracellular free iron result in Fenton-type reaction with subsequent production of highly reactive hydroxyl radicals which initiate peroxidation of biomolecules and further formation of non-degradable toxic pigments called lipofuscin that amasses in long-lived postmitotic cells such as neurons. Dietary flavonoid baicalein can counteract the detrimental consequences through exertion of a multiplicity of protective actions within the brain including direct ROS scavenging activity and iron chelation. In this study, we evaluated the neuroprotective effects of baicalein in menadione (superoxide radical generator)-treated SK-N-MC neuroblastoma cell line. Our results showed that treatment of cells with menadione led to lipofuscin formation due to elevated intracellular iron contents and accumulation of oxidative products such as MDA and PCO. Also, menadione caused apoptotic cell death in SK-N-MC cells. However, pretreatment with baicalein (40 μM) reversed the harmful effects by chelating free iron and preventing biomolecules peroxidations. Moreover, baicalein prevented cell death through modulation of key molecules in apoptotic pathways including suppression of Bax and caspase-9 activities and induction of bcl2 expression. Key structural features such as presence of hydroxyl groups and iron-binding motifs in baicalein make it the appropriate candidate in antioxidant-based therapy in age-related neurodegenerative diseases.

Mitochondrial Autophagy and Lipofuscin Accumulation in Aging Odontoblasts

Aging of long-lived post-mitotic cells is characterized as a progressive and irreversible reduction of functional activity. In such cells, mitochondria are organelles critical for bioenergetic supply, whose turnover is mediated by an autophagic-lysosomal pathway. In human teeth, odontoblasts are post-mitotic cells responsible for sensory function and dentin preservation. Here, human odontoblasts were processed for immunohistochemistry with antibodies against mitochondrial (MTCO2) and lysosomal (LAMP2) markers, and comparatively analyzed in two age groups (young-adult and adult) with light and electron microscopy. Selective engulfment of mitochondrial profiles into autophagic vacuoles is common in young-adult odontoblasts, suggesting a microautophagic pathway. With age, the odontoblast layer is reduced in width, and mitochondrial elements converge around large clusters of autofluorescent lipofuscin deposits. Age-related changes in odontoblasts are observed as a long-term process in which the progressive accumulation of intralysosomal debris limits the autophagic turnover of mitochondrial components, causing an eventual decline in physiological cell functions, which leads to increased vulnerability under stress conditions.

Age-related lysosomal dysfunction: an unrecognized roadblock for cobalamin trafficking?

Vitamin-B(12) is a generic term for corrinoid compounds that exhibit the biological activity of cyanocobalamin and are collectively referred to as cobalamins. Methylcobalamin and 5-deoxyadenosylcobalamin are the active cobalamins in human metabolism. Cobalamin plays a crucial role in the maintenance of homocysteine and methylmalonyl-CoA homeostasis and is required for erythrocyte formation and DNA synthesis. Data from human and animal studies indicate that cobalamin deficiency impairs neuronal function; a process that is thought to contribute to age-related cognitive decline and dementia. Cobalamin deficiency also results in dysfunction of the peripheral nervous system; among other disorders. Although there is a detailed understanding of the biochemical pathways that are perturbed in cobalamin deficiency, the mechanisms underlying age-related dyshomeostasis in such pathways remain to be addressed. Because cobalamin utilization is dependent on its efficient transit through lysosomes, and mounting evidence indicates that lysosomal function deteriorates in aging long-lived post-mitotic cells such as neurons, in the present article we review published data that supports the proposition that impaired lysosomal processing of cobalamin may play a significant role in age-related (neuro) degenerative diseases.

Therapeutic Intervention at Cellular Quality Control Systems in Alzheimers and Parkinsons Diseases

Cellular homeostasis relies on quality control systems so that damaged biologic structures are either repaired or degraded and entirely replaced by newly formed proteins or even organelles. The clearance of dysfunctional cellular structures in long-lived postmitotic cells, like neurons, is essential to eliminate, per example, defective mitochondria, lipofuscin-loaded lysosomes and oxidized proteins. Short-lived proteins are degraded mainly by proteases and proteasomes whether most long-lived proteins and all organelles are digested by autophagy in the lysosomes. Recently, it an interplay was established between the ubiquitin-proteasome system and macroautophagy, so that both degradative mechanisms compensate for each other. In this article we describe each of these clearance systems and their contribution to neuronal quality control. We will highlight some of the findings that provide evidence for the dysfunction of these systems in Alzheimer's and Parkinson's diseases. Ultimately, we provide an outline on potential therapeutic interventions based on the modulation of cellular degradative systems.

Autophagic Activity and Aging in Human Odontoblasts

Odontoblasts are long-lived post-mitotic cells in the dental pulp, whose function is to form and maintain dentin. The survival mechanisms that preserve the viability of terminally differentiated odontoblasts during the life of a healthy tooth have not been described. In the present study, we characterized the autophagic-lysosomal system of human odontoblasts with transmission electron microscopy and immunocytochemistry, to analyze the mechanisms that maintain the functional viability of these dentinogenic cells. Odontoblasts were found to develop an autophagic-lysosomal system organized mainly by large autophagic vacuoles that are acid-phosphatase-positive to various degrees. These vacuoles expressed the autophagosomal and lysosomal markers LC3 and LAMP2, respectively, in an age-related pattern indicating the organization of a dynamic autophagic machinery. Progressive accumulation of lipofuscin within lysosomes indicates reduced lysosomal activity as a function of odontoblast aging. Our results suggest that autophagic activity in odontoblasts is a fundamental mechanism to ensure turnover and degradation of subcellular components. A reduction in the efficacy of this system might compromise cell viability and dentinogenic secretory capacity. In adult teeth, this condition is described as an ‘old odontoblast’ stage.

Mitochondrial oxidative damage and apoptosis in age-related hearing loss

Age-related hearing loss (AHL) is a universal feature of mammalian aging and is the most common sensory disorder in the elderly population. Experimental evidence suggests that mitochondrial dysfunction associated with reactive oxygen species (ROS) plays a central role in the aging process of cochlear cells. Although it is well established that mitochondria are the major source of ROS in the cell, specific molecular mechanisms of aging induced by ROS remain poorly characterized. Here we review the evidence that supports a central role for Bak-mediated mitochondrial apoptosis in AHL. We also propose that this mechanism may be of general relevance to age-related cell death in long-lived post-mitotic cells of multiple tissues, providing an opportunity for a targeted therapeutic intervention in human aging.

Mitochondrial turnover and aging of long-lived postmitotic cells: the mitochondrial-lysosomal axis theory of aging.

It is now generally accepted that aging and eventual death of multicellular organisms is to a large extent related to macromolecular damage by mitochondrially produced reactive oxygen species, mostly affecting long-lived postmitotic cells, such as neurons and cardiac myocytes. These cells are rarely or not at all replaced during life and can be as old as the whole organism. The inherent inability of autophagy and other cellular-degradation mechanisms to remove damaged structures completely results in the progressive accumulation of garbage, including cytosolic protein aggregates, defective mitochondria, and lipofuscin, an intralysosomal indigestible material. In this review, we stress the importance of crosstalk between mitochondria and lysosomes in aging. The slow accumulation of lipofuscin within lysosomes seems to depress autophagy, resulting in reduced turnover of effective mitochondria. The latter not only are functionally deficient but also produce increased amounts of reactive oxygen species, prompting lipofuscinogenesis. Moreover, defective and enlarged mitochondria are poorly autophagocytosed and constitute a growing population of badly functioning organelles that do not fuse and exchange their contents with normal mitochondria. The progress of these changes seems to result in enhanced oxidative stress, decreased ATP production, and collapse of the cellular catabolic machinery, which eventually is incompatible with survival.

Redox Activity Within the Lysosomal Compartment: Implications for Aging and Apoptosis

The lysosome is a redox-active compartment containing low-mass iron and copper liberated by autophagic degradation of metalloproteins. The acidic milieu and high concentration of thiols within lysosomes will keep iron in a reduced (ferrous) state, which can react with endogenous or exogenous hydrogen peroxide. Consequent intralysosomal Fenton reactions may give rise to the formation of lipofuscin or "age pigment" that accumulates in long-lived postmitotic cells that cannot dilute it by division. Extensive accumulation of lipofuscin seems to hinder normal autophagy and may be an important factor behind aging and age-related pathologies. Enhanced oxidative stress causes lysosomal membrane permeabilization, with ensuing relocation to the cytosol of iron and lysosomal hydrolytic enzymes, with resulting apoptosis or necrosis. Lysosomal copper is normally not redox active because it will form non-redox-active complexes with various thiols. However, if cells are exposed to lysosomotropic chelators that do not bind all the copper coordinates, highly redox-active complexes may form, with ensuing extensive lysosomal Fenton-type reactions and loss of lysosomal stability. Because many malignancies seem to have increased amounts of copper-containing macromolecules that are turned over by autophagy, it is conceivable that lysosomotropic copper chelators may be used in the future in ROS-based anticancer therapies.

Effect of untreated and photobleached bovine RPE melanosomes on the photoinduced peroxidation of lipids

Melanin is usually considered a photoprotective pigment and antioxidant agent, but it is unclear how melanosomes protect pigmented cells against oxidative stress induced by light and whether aging modulates its photoprotective function, particularly in long-lived post-mitotic cells such as the retinal pigment epithelium (RPE). To address these issues, we analyzed the effects of untreated and experimentally photobleached melanosomes from cow RPE on the peroxidation of liposomal lipids induced by irradiation with intense visible light or by a rose Bengal photosensitized reaction. Photobleached melanosomes were used as a model of photo-aged pigment granules, and the progress of lipid peroxidation was monitored by electron spin resonance (EPR) oximetry and the iodometric determination of lipid hydroperoxides. We observed that while untreated melanosomes inhibited the rose Bengal induced peroxidation of lipids only moderately, partially photobleached melanosomes had very little effect on this process. Untreated melanosomes also inhibited peroxidation of liposomal lipids induced by intense visible light; however, the inhibitory effect markedly changed with the irradiation time. On the other hand, partially photobleached pigment granules accelerated the photoinduced peroxidation of lipids. The observed effects illustrate the limited efficiency of melanin within granules to scavenge and quench reactive oxygen species randomly generated by photosensitized reactions. The photosensitizing ability of photobleached melanosomes may arise from their capacity to photogenerate hydrogen peroxide. Collectively, our data indicate that natural melanin is only a moderately efficient photoprotective pigment, which upon photoaging may lose its antioxidant efficiency and even become a photosensitizer.

Molecular and Biochemical Changes of the Cardiovascular System due to Smoking Exposure

Cigarette smoking (CS) is a major health hazard particularly for the cardiovascular system and cancer. The mechanisms involved in CS-related cardiovascular dysfunction have been largely debated. CS increases inflammation, thrombosis, and oxidation of low-density lipoproteins. Recent experimental and clinical data support the hypothesis that cigarette smoke exposure increases oxidative stress as a potential mechanism for initiating cardiovascular dysfunction. Cardiac myocytes, as well as and other long-lived postmitotic cells show dramatic smoke-related alterations that mainly affect the mitochondria and lysosomal compartment. Mitochondria are primary sites of reactive oxygen species formation that cause progressive damage to mitochondrial DNA and proteins in parallel to intralysosomal lipofuscin accumulation. There is amassing evidence that various mechanisms may contribute to accumulation of damaged mitochondria following initial oxidative injury. Such mechanisms may include clonal expansion of defective mitochondria, decreased propensity of altered mitochondria to become autophagocytosed, suppressed autophagy because of heavy lipofuscin loading of lysosomes and decreased efficiency of specific proteases involved into mitochondrial degradation. A possible interplay between microtubule plasticity and oxidative stress also exists in cardiomyocytes, so this could represent another potential mechanism by which smoking induces/accelerates atherosclerosis.