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- Reviewing Work on CISD2, a Mammalian Longevity Gene
- Investigating the Role of Microglia in Zebrafish Neural Regeneration
- Autophagy in Tauopathies Such as Alzheimer's Disease
- Assessing Mitochondrial Transfer Into Senescent Cells In Vitro
- A Long Discussion of the Role of Senescent Cells in Idiopathic Pulmonary Fibrosis
- Cellular Enlargement in Aging, a Poorly Studied Topic
- Cellular Senescence in Vascular Smooth Muscle Accelerates Medin Aggregation
- Intermittent Fasting is Protective Against the Effects of Vascular Aging in the Brain
- Increased Expression of Lysosomal Enzyme NAGLU Improves Health in Flies
- Wasteosomes as a Marker for Age-Related Impairment of Cerebrospinal Fluid Drainage
- Testing a Glutaminase Inhibitor to Clear Senescent Cells in Skin Models
- Non-Dividing Neurons Do In Fact Become Senescent, Impairing Brain Function
- Thymic Macrophage Populations Change with Age
- Carotid Artery Stiffness Correlates with Age-Related Damage and Dysfunction in the Brain
- The Gut Microbiome in Alzheimer's Disease
Reviewing Work on CISD2, a Mammalian Longevity Gene
Few genes have been shown to robustly alter mammalian longevity as a result of altered expression, with data obtained primarily in mice. Klotho is perhaps the most well known and well studied of that small but steadily growing portfolio. The topic of today's open access paper is another of these longevity genes, CISD2. Loss of CISD2 shortens lifespan, while increased expression extends life span in mice. CISD2 is upregulated after exercise, and may act through autophagy, a common factor in many approaches shown to modestly slow aging in laboratory species. Like other approaches to upregulation of autophagy, increased CISD2 expression improves liver function in mice. Recently researchers have extended mouse life span by a small degree via pharmacological approaches to upregulation of CISD2.
The authors of the this paper overstate, I think, the level of interest we should have in CISD2 upregulation as a basis for therapy. Any form of upregulation of autophagy might be described as a calorie restriction mimetic strategy, given that increased autophagy appears to be the primary means by which calorie restriction produces benefits to health and longevity. While calorie restriction improves health in humans, it certainly does not move the needle on life span in long-lived mammals the same way it does in short-lived mammals. The underlying reasons for this difference have yet to be established in any detail, but this is why we should be skeptical of most of the methods of slowing aging demonstrated in mice to date. They largely function through stress response pathways that converge on increased autophagy.
Rejuvenation: Turning Back Time by Enhancing CISD2
Currently, only eight genes (BUB1B, CISD2, KLOTHO, PAWR, PPARG, PTEN, SIRT1, and SIRT6) are listed as pro-longevity genes by the Human Aging Genomic Resources, which means that they have been experimentally demonstrated to mediate lifespan in mammals. The aging human population with age-associated diseases has become a problem worldwide. By 2050, the global population of those who are aged 65 years and older will have tripled. In this context, delaying age-associated diseases and increasing the healthy lifespan of the aged population has become an important issue for geriatric medicine.
CDGSH iron-sulfur domain 2 (CISD2), the causative gene for Wolfram syndrome 2 (WFS2), plays a pivotal role in mediating lifespan and healthspan by maintaining mitochondrial function, endoplasmic reticulum integrity, intracellular Ca2+ homeostasis, and redox status. Here, we summarize the most up-to-date publications on CISD2 and discuss the crucial role that this gene plays in aging and age-associated diseases. This review highlights the urgent need for CISD2-based pharmaceutical development to be used as a potential therapeutic strategy for aging and age-associated diseases.
This review mainly focuses on the following topics: (1) CISD2 is one of the few pro-longevity genes identified in mammals. Genetic evidence from loss-of-function (knockout mice) and gain-of-function (transgenic mice) studies have demonstrated that CISD2 is essential to lifespan control. (2) CISD2 alleviates age-associated disorders. A higher level of CISD2 during natural aging, when achieved by transgenic overexpression, improves Alzheimer's disease, ameliorates non-alcoholic fatty liver disease and steatohepatitis, and maintains corneal epithelial homeostasis.
(3) CISD2, the expression of which otherwise decreases during natural aging, can be pharmaceutically activated at a late-life stage of aged mice. As a proof-of-concept, we have provided evidence that hesperetin is a promising CISD2 activator that is able to enhance CISD2 expression, thus slowing down aging and promoting longevity. (4) The anti-aging effect of hesperetin is mainly dependent on CISD2 because transcriptomic analysis of the skeletal muscle reveals that most of the differentially expressed genes linked to hesperetin are regulated by hesperetin in a CISD2-dependent manner. Furthermore, three major metabolic pathways that are affected by hesperetin have been identified in skeletal muscle, namely lipid metabolism, protein homeostasis, and nitrogen and amino acid metabolism.
Investigating the Role of Microglia in Zebrafish Neural Regeneration
Some species, such as salamanders and zebrafish, exhibit proficient regeneration, in that individuals are capable of regrowing lost tissue in organs and limbs and central nervous system without scarring. In adult mammals, the result of injury is scarring rather than regrowth, even though embryos are capable of both initial growth and later complete regeneration. Following the hypothesis that mechanisms of proficient regeneration do in fact exist in adult mammals, but are in some way silenced, the research community is engaged in trying to identify the specific differences between mammals and highly regenerative species that determine whether regrowth or scarring takes place following injury.
One of the more interesting discoveries of recent years is that differences in the behavior of the innate immune cells known as macrophages appear important. In the central nervous system, microglia are the analogous, very similar cell type. Macrophages and microglia involve themselves in the interactions between various types of somatic cells, stem cells, and progenitor cells that take place during regeneration.
In today's research materials, researchers investigate some of the proteins that appear to change the behavior of microglia to allow for regeneration in the brains of zebrafish. Interestingly, one of them is TDP-43, which is implicated in neurodegeneration in humans due to its ability to misfold and form aggregates. The other, granulin, mediates clearance of TDP-43 aggregates. The work may prove to have relevance not just to inducing regeneration in the central nervous system, but also to addressing forms of neurodegeneration in which TDP-43 plays a prominent role.
Key factors identified for regeneration of brain tissue
In contrast to mammals, the central nervous system (CNS) of zebrafish has exceptional regenerative powers. In the case of injury, neural stem cells generate long-lived neurons, among other responses. Furthermore, CNS injuries prompt merely transitory reactivity of glial cells in zebrafish, which facilitates the integration of nerve cells into injured regions of the tissue.
The scientists deliberately inflicted CNS lesions in zebrafish, prompting the activation of microglia. At the same time, the researchers found an accumulation of lipid droplets and TDP-43 condensates in the lesions. To date, the protein TDP-43 has been primarily associated with neurodegenerative diseases. Granulin also played an important role in the zebrafish model. This protein contributed to the removal of the lipid droplets and TDP-43 condensates, whereupon the microglia transitioned from their activated to their resting form. The unscarred regeneration of the injury was the outcome. Zebrafish with experimentally induced granulin deficiency, by contrast, exhibited poor regeneration of the injury similar to what we see in mammals.
TDP-43 condensates and lipid droplets regulate the reactivity of microglia and regeneration after traumatic brain injury
Decreasing the activation of pathology-activated microglia is crucial to prevent chronic inflammation and tissue scarring. In this study, we used a stab wound injury model in zebrafish and identified an injury-induced microglial state characterized by the accumulation of lipid droplets and TDP-43+ condensates. Granulin-mediated clearance of both lipid droplets and TDP-43+ condensates was necessary and sufficient to promote the return of microglia back to the basal state and achieve scarless regeneration.
Moreover, in postmortem cortical brain tissues from patients with traumatic brain injury, the extent of microglial activation correlated with the accumulation of lipid droplets and TDP-43+ condensates. Together, our results reveal a mechanism required for restoring microglia to a nonactivated state after injury, which has potential for new therapeutic applications in humans.
Autophagy in Tauopathies Such as Alzheimer's Disease
Autophagy is the name given to a collection of maintenance processes responsible for tagging and recycling damaged, excess, or harmful proteins and structures in the cell. Better maintenance of molecular machinery means a better operation of cells and tissues. Upregulation of autophagy is a feature of many of the approaches shown to modestly slow aging in animal studies, those that mimic some of the biochemistry of calorie restriction. Calorie restriction itself is thought to improve health and longevity primarily through autophagy.
Here, researchers look at autophagy in the context of neurodegenerative conditions. Autophagy targeted at mitochondria, mitophagy, is particularly important to cell function given the importance of mitochondrial production of chemical energy store molecules, ATP, in energy-hungry tissues such as the brain. Further, given that toxic protein aggregates feature prominently in neurodegenerative conditions, and autophagy assists in clearing aggregates, this is another reason to study autophagy in this context.
That said, near all of the presently available approaches to upregulate autophagy, such as pharmacological means of increasing NAD levels via derivatives of vitamin B3, are not as good as either exercise or calorie restriction. Animal studies show that mTOR inhibition via rapamycin is in fact better than exercise (but not calorie restriction) when it comes to beneficial outcomes on health and life span, but rapamycin has downsides - it is an immunosuppressant. Efforts to produce drugs that inhibit mTOR without these undesirable side-effects are still underway.
The emerging role of autophagy and mitophagy in tauopathies: From pathogenesis to translational implications in Alzheimer's disease
Alzheimer's disease (AD) is the most prevalent neurodegenerative disease, affecting more than 55 million individuals worldwide in 2021. In addition to the "amyloid hypothesis," an increasing number of studies have demonstrated that phosphorylated tau plays an important role in AD pathogenesis. Both soluble tau oligomers and insoluble tau aggregates in the brain can induce structural and functional neuronal damage through multiple pathways, eventually leading to memory deficits and neurodegeneration.
Autophagy is an important cellular response to various stress stimuli and can generally be categorized into non-selective and selective autophagy. Recent studies have indicated that both types of autophagy are involved in AD pathology. Among the several subtypes of selective autophagy, mitophagy, which mediates the selective removal of mitochondria, has attracted increasing attention because dysfunctional mitochondria have been suggested to contribute to tauopathies.
In this review, we summarize the latest findings on the bidirectional association between abnormal tau proteins and defective autophagy, as well as mitophagy, which might constitute a vicious cycle in the induction of neurodegeneration. Neuroinflammation, another important feature in the pathogenesis and progression of AD, has been shown to crosstalk with autophagy and mitophagy. Additionally, we comprehensively discuss the relationship between neuroinflammation, autophagy, and mitophagy. By elucidating the underlying molecular mechanisms governing these pathologies, we highlight novel therapeutic strategies targeting autophagy, mitophagy and neuroinflammation, such as those using rapamycin, urolithin A, spermidine, curcumin, nicotinamide, and actinonin, for the prevention and treatment of AD.
Assessing Mitochondrial Transfer Into Senescent Cells In Vitro
Researchers here report on in vitro experiments to show that introducing functional mitochondria into a cell culture containing senescent cells reduces markers of senescence. It is an interesting question as to how this would work in living tissue, where the numbers of senescent cells are low, and mitochondria will be introduced into all cells. Since several companies are developing mitochondrial transfer as a therapy to treat the loss of mitochondrial function that is characteristic of age-related disease, we'll find out in the years ahead. Those groups are not specifically targeting cellular senescence, but can hardly avoid having senescent cells taking up their therapeutic mitochondria.
For all strategies that might leave senescent cells intact but modulate their harmful signaling, the question is whether or not this is a good idea. This particularly the case for strategies that might allow senescent cells to re-enter the cell cycle and replicate again. Some fraction of senescent cells become senescent for good reasons, such as potentially cancerous mutations or other forms of damage that produce dysfunction. Senolytics that destroy senescent cells seem a safer proposal, and efficient senolytics may turn out to be required in advance of some of the other rejuvenation therapies on the horizon, such as partial reprogramming and mitochondrial transfer.
Inhibition of cellular senescence hallmarks by mitochondrial transplantation in senescence-induced ARPE-19 cells
Retinal pigment epithelium (RPE) damage is a major factor in age-related macular degeneration (AMD). The RPE in AMD shows mitochondrial dysfunction suggesting an association of AMD with mitochondrial function. Mitochondrial transplantation into damaged cells or injured tissues is considered a novel cell-based therapeutic strategy. Delivery of mitochondria isolated from mesenchymal stem cells (MSCs) has the advantage of supplying the required number of mitochondria through rapid replication and multilineage differentiation compared with other cells; further, stem cells have low immunogenicity because of lower levels of surface antigens. A previous study has reported that MSC-derived mitochondrial transplantation protects the cornea against oxidative stress-induced mitochondrial damage.
Here, we investigated the effects of extrinsic mitochondrial transplantation on senescence-induced ARPE-19 cells, an RPE cell line. We demonstrated mitochondrial dysfunction in replicative senescence-induced ARPE-19 cells after repeated passage. Imbalanced mitophagy and mitochondrial dynamics resulted in increased mitochondrial numbers and elevated levels of mitochondrial and intracellular reactive oxygen species.
Exogenous mitochondrial transplantation can improve mitochondrial dysfunction and alleviate cellular senescence hallmarks, such as increased cell size, increased senescence-associated β-galactosidase activity, augmented NF-κB activity, increased inflammatory cytokines, and upregulated the cyclin-dependent kinase inhibitors p21 and p16. Further, cellular senescence properties were improved by exogenous mitochondrial transplantation in oxidative stress-induced senescent ARPE-19 cells. These results indicate that exogenous mitochondrial transplantation can modulate cellular senescence and may be considered a novel therapeutic strategy for AMD.
A Long Discussion of the Role of Senescent Cells in Idiopathic Pulmonary Fibrosis
Senescent cells are constantly created and destroyed throughout life, largely as a result of the replicative senescence that marks the end of life for a somatic cell, the Hayflick limit on cell division. With age, the pace of creation and destruction is disrupted, perhaps largely because the immune system ages to the point at which it falters in all of its tasks, clearance of senescent cells included. Senescent cells accumulate, and while never making up more than a small fraction of all somatic cells in any given tissue, the pro-growth, pro-inflammatory signaling generated by senescent cells is highly disruptive to organ structure and function.
Fibrosis is one of the more noteworthy manifestations of the presence of too many lingering senescent cells. Fibrosis is a malfunction of the normal processes of tissue maintenance, in which excessive collagen is deposited to form scar-like fibrils, disruptive of tissue function. It occurs in the aged kidney, heart, liver, and lungs, and is presently largely irreversible.
A greater degree of fibrosis and consequence loss of function in a given organ passes the threshold to be named as a medical condition, such as the idiopathic pulmonary fibrosis in the lung that is the topic of today's open access paper. It is a long and detailed discussion of what is known of the role of senescent cells in producing lung fibrosis; following a promising clinical trial testing a first generation senolytic therapy in patients with idiopathic pulmonary fibrosis, there is some hope that clearing senescent cells will prove to be a way to reverse fibrosis both in the lung and more generally.
Senescent AECII and the implication for idiopathic pulmonary fibrosis treatment
Idiopathic pulmonary fibrosis (IPF) is an irreversible fibrotic disease in the lungs and is the most common form of idiopathic interstitial pneumonia and idiopathic fibrotic lung disorder. Its biological process is defined as an abnormal repair response to repeated alveolar epithelial cells (AEC) damage and fibroblast-to-myofibroblast differentiation and characterized by the excessive disordered deposition of collagen in the extra- and intra-cellular matrix. Several potential risk factors, such as aging, genetic predisposition, chemical, environmental exposure, and bioenvironmental factor (bacteria and virus), can act on various types of lung cells and enhance the risk of developing IPF.
Of these risk factors, aging is considered an independent risk factor. Even in patients with a genetic predisposition, the onset of IPF seldom occurs before the sixth decade, and the incidence increases exponentially with advancing age. A longitudinal cohort study identifying independent risk factors for the progression of interstitial lung disease has shown that the risk of IPF in people aged 70 or over is 6.9 times that in people aged over 40, confirming that IPF is an age-related disease.
Cell senescence and stem cell exhaustion are the hallmarks of all age-related diseases, as in IPF. Alveolar type II epithelial cells (AECIIs) are the stem cells for the lungs and play a role in maintaining intrapulmonary homeostasis, immunity, and regeneration in the alveoli. Senescent AECIIs secrete high levels of interleukin, interferon, tumor necrosis factor, colony-stimulating factors, growth factors, and chemotactic cytokines, which promote fibroblast-to-myofibroblast differentiation and persistent tissue remodeling.
A recent study has uncovered that pulmonary fibrosis after coronavirus disease 2019 (COVID-19) may be caused by virus-induced AECII senescence. Preventing AECII senescence or targeting senescent cells in patients with COVID-19 can reduce the risk of pulmonary fibrosis. Researchers detected that AECII exhibited high levels of the senescence markers p21 and p16 from patients with IPF. Other numerous studies have shown that AECII senescence promotes the occurrence of IPF. However, pathological mechanisms underlying AECII senescence and specific effects of targeting senescent AECIIs on IPF remain unclear. This review will discuss the mechanism of AECII senescence, which drives the onset and progression of IPF, and highlights the advantages and disadvantages of targeting senescent AECIIs for IPF.
Cellular Enlargement in Aging, a Poorly Studied Topic
Some cells are small, others large. Cell size is connected to cell function, and different varieties of cell maintain tight control over their various different sizes. Senescent cells are known to become much larger than their origin cell type, and one effort to detect senescent cells in blood samples made use of this feature. Do non-senescent cells lose control of size in old tissues, however? To what degree is this a feature of aging that produces further downstream issues, versus being a consequence of other problematic changes in cell behavior that occur with age? These are not well-studied questions.
A large body of literature highlights two important findings: 1) Different cell types display different average sizes and 2) cells maintain a uniform size by using several regulatory pathways. This raises the question of why cells invest in maintaining their size. Therefore, understanding what happens when cells fail to regulate their size is important. While the first findings around this topic led to controversial conclusions, budding yeast has been a key model organism to provide the first evidence that cellular enlargement could be directly linked to cellular dysfunction during aging. It is known that budding yeast cells enlarge during aging. Preventing this enlargement with drugs preserves their replicative age. Similarly, preventing cellular enlargement in vitro in primary human cells has been shown to maintain their capacity to enter the cell cycle thereby avoiding cellular senescence.
Our recent publication dissected whether the role of cell size on cell function is based on correlation or causation. An intrinsic challenge was to manipulate cell size without targeting other pathways, and to delineate that the observed changes are causal and not correlative. This hurdle was tackled using hematopoietic stem cells (HSCs) in vivo. Six orthogonal approaches were examined under which HSCs became larger. In each of these conditions, HSC function was also compromised. HSC function was determined as their ability to form a blood system after transplantation into recipient mice. While it could be argued that each single manipulation affected HSC function unrelated to cell size, together these experiments suggest that the dysfunction was not driven by an unaccounted variable. Furthermore, alternate causes were excluded by analyzing other parameters of the hematopoietic system: homing, stem cell identity, differentiation potential and cell cycle state. Therefore, the simplest explanation is that enlargement of HSCs reduces their functionality.
Interestingly, numerous other cell types have also been observed to enlarge during aging. This raises the possibility that cellular enlargement contributes to aging in other cell types. Moving forward, the research community will benefit from further experiments providing critical evidence of whether cellular enlargement is cause or consequence of aging in other stem cell types and differentiating cells.
Cellular Senescence in Vascular Smooth Muscle Accelerates Medin Aggregation
Medin is one of a number of different amyloids that form in aging tissue, each a protein that can misfold in ways that encourage other molecules of the same protein to do the same, aggregating together to form solid deposits. Some amyloids are evidently toxic and disease-associated, while others, like medin, originally appeared more innocuous. It isn't harmless, however, just more subtle. Recent research suggested a pathological role for medin amyloid in Alzheimer's disease, in that it accelerates the aggregation of amyloid-β. Further, there is evidence for medin aggregation to contribute to cerebrovascular dysfunction. On that topic, researchers here note that cellular senescence in the vascular smooth muscle of blood vessel walls can provoke greater medin aggregation in that tissue, providing a link between those two distinct mechanisms of aging.
Vascular amyloidosis, caused when peptide monomers aggregate into insoluble amyloid, is a prevalent age-associated pathology. Aortic medial amyloid (AMA) is the most common human amyloid and is composed of medin, a 50-amino acid peptide. Emerging evidence has implicated extracellular vesicles (EVs) as mediators of pathological amyloid accumulation in the extracellular matrix (ECM). To determine the mechanisms of AMA formation with age, we explored the impact of vascular smooth muscle cell (VSMC) senescence, EV secretion, and ECM remodeling on medin accumulation.
Medin was detected in EVs secreted from primary VSMCs. Small, round medin aggregates colocalized with EV markers in decellularized ECM in vitro and medin was shown on the surface of EVs deposited in the ECM. Decreasing EV secretion with an inhibitor attenuated aggregation and deposition of medin in the ECM. Medin accumulation in the aortic wall of human subjects was strongly correlated with age and VSMC senescence increased EV secretion, increased EV medin loading, and triggered deposition of fibril-like medin.
Proteomic analysis showed VSMC senescence induced changes in EV cargo and ECM composition, which led to enhanced EV-ECM binding and accelerated medin aggregation. Abundance of the proteoglycan, HSPG2, was increased in the senescent ECM and colocalized with EVs and medin. Isolated EVs selectively bound to HSPG2 in the ECM and its knock-down decreased formation of fibril-like medin structures. These data identify VSMC-derived EVs and HSPG2 in the ECM as key mediators of medin accumulation, contributing to age-associated AMA development.
Intermittent Fasting is Protective Against the Effects of Vascular Aging in the Brain
Researchers here show that, in mice, intermittent fasting can protect against the damage done by cardiovascular aging that leads to a reduced blood supply to the brain. Forms of calorie restriction, such as intermittent fasting, are generally beneficial for long term health. This is well demonstrated in both mice and humans, though only short-lived species exhibit significant gains in life span as a result. It is interesting to note the opinion that intermittent fasting may be better than straight calorie restriction when it comes to mitigation of pathological mechanisms in neurodegenerative conditions.
Vascular cognitive impairment (VCI) embodies a spectrum of cognitive deficits that range from mild cognitive impairment to vascular dementia (VaD). VCI is associated with cerebrovascular diseases that arise from vascular pathological processes such as atherosclerosis, microvascular protein deposits, haemorrhages and microbleeds. These vascular pathologies lead to a state of reduced blood flow to the brain that is referred to as chronic cerebral hypoperfusion (CCH). Decreased cerebral perfusion has been reported to correlate with dementia severity, and has shown to be a predictive marker to identify individuals with mild cognitive impairment who develop dementia. CCH induces a cascade of cellular and molecular mechanisms that contributes to the pathogenesis of VCI - including oxidative stress and inflammation.
Intermittent fasting (IF) is defined as an eating pattern that cycles between periods of eating and fasting. IF has been extensively reported to extend both health and lifespan, and decrease the development of age-related disorders including cardiovascular, metabolic, and neurodegenerative diseases. Recently, IF has gained much interest as being more effective than caloric restriction for inducing neuroprotective effects in the brain.
In this study, we demonstrate for the first time that IF promotes neuroprotective effects in a model of VaD by maintaining the integrity of the neurovascular structures in the brain. We specifically show that IF attenuated vascular pathology by reducing microvascular leakage and blood-brain barrier dysfunction, while maintaining the expression of tight junction (TJ) proteins. IF was also effective in decreasing white matter lesion formation, hippocampal neuronal cell death and cell death markers, while maintaining myelin basic protein levels. Our data suggest that the effects of IF on the structural integrity of the neurovasculature may be mediated through mechanisms that decrease oxidative stress and matrix metalloproteinase expression. Overall, our findings indicate that prophylactic IF may be a potential therapy in reducing and preventing neurovascular pathology associated with VaD.
Increased Expression of Lysosomal Enzyme NAGLU Improves Health in Flies
Lysosomes are the destination for excess and damaged proteins and structures in the cell. A lysosome is a membrane-wrapped collection of enzymes capable of breaking down near everything it encounters into the raw materials a cell uses to manufacture new molecular machinery. Lysosomal function is known to decline with age, and improved lysosomal function might be expected to improve long-term health prospects. Here, researchers look at one specific lysosomal enzyme that is known to exhibit higher levels in human centenarians, and find that it improves health in flies. There is extension of life, but the effect size is so small, a few percentage points, that it shouldn't be taken seriously.
To identify new factors that promote longevity and healthy aging, we studied Drosophila CG13397, an ortholog of the human NAGLU gene, a lysosomal enzyme overexpressed in centenarians. We found that the overexpression of CG13397 (dNAGLU) ubiquitously, or tissue specifically, in the nervous system or fat body could extend fly life span. It also extended the life span of flies overexpressing human Aβ42, in a Drosophila Alzheimer's disease (AD) model.
To investigate whether dNAGLU could influence health span, we analyzed the effect of its overexpression on AD flies and found that it improved the climbing ability and stress resistance, including desiccation and hunger, suggesting that dNAGLU improved fly healthspan. We found that the deposition of Aβ42 in the mushroom body, which is the fly central nervous system, was reduced, and the lysosomal activity in the intestine was increased in dNAGLU over-expressing flies.
When NAGLU was overexpressed in human U251-APP cells, which expresses a mutant form of the Aβ-precursor protein (APP), these cells exhibited stronger lysosomal activity and and enhanced expression of lysosomal pathway genes. The concentration of Aβ42 was reduced, and the growth arrest caused by APP expression was reversed, suggesting that NAGLU could play a wider role beyond its catalytic activity to enhance lysosomal activity.
These results also suggest that NAGLU overexpression could be explored to promote healthy aging and to prevent the onset of neurodegenerative diseases, including AD.
Wasteosomes as a Marker for Age-Related Impairment of Cerebrospinal Fluid Drainage
There is a growing awareness that removal of metabolic waste from the brain is impaired with age, and that this contributes to the onset of neurodegenerative conditions characterized by rising levels of protein aggregates of various sorts in the brain. All of that waste might be better removed from the brain were paths through the cribriform plate and glymphatic system maintained at youthful efficiency. Leucadia Therapeutics is working on a safe way to open a new passage through the cribriform plate, but the glymphatic system is a more challenging prospect. It isn't fully understood how best to intervene in what is most likely a multifaceted degeneration connected to most or all of the fundamental mechanisms of aging.
The glymphatic system is responsible for clearing the brain parenchyma. To date, the terms glymphatic system failure or glymphatic system dysfunction have been used to define its malfunction. In a new paper, the concept of glymphatic insufficiency is defined as the inability of the glymphatic system to properly perform the brain's cleaning function. This makes it possible to describe that the failure can be acute or chronic, depending on the duration of the process, and to specify that the failure can be caused by a failure of the glymphatic system itself or by an overproduction of waste substances that exceeds the clearing capacity of this system.
The wasteosomes or amylase bodies of the human brain were first described in 1837. Researchers have shown that amylase bodies act as containers for waste substances from the brain and can be expelled by astrocytes into the cerebrospinal fluid. A new paper now provides evidence that increased wasteosomes or starch bodies in the human brain are a manifestation of chronic glymphatic system insufficiency. The first indication of this relationship is that most factors that are associated with large amounts of wasteosomes, such as ageing, certain cardiovascular disorders, and poor sleep quality, are also associated with disruptions of the glymphatic system.
The study also mentions several elements and evidence that suggest that chronic lymphatic insufficiency is a risk factor for neurodegenerative diseases, especially neurodegenerative diseases that involve the aggregation of certain fibrillar proteins, such as β-amyloid protein in Alzheimer's disease, phosphorylated tau in frontotemporal dementia and Alzheimer's disease, or α-synuclein in Parkinson's disease. "In case of lymphatic insufficiency, the elimination of these proteins is restricted, and all indications are that this contributes to the development of these diseases."
Testing a Glutaminase Inhibitor to Clear Senescent Cells in Skin Models
Glutaminase inhibitors can potentially eliminate senescent cells from aged tissues, another in the growing list of categories of senolytic compound. Researchers here test the proposition in various skin models, some involving immunodeficient mice hosting human skin grafts. At this point there are so many different senolytics that studies should start to focus on comparing efficacy. Where there is data to compare directly, development efforts conducted over the last decade have so far failed to greatly improve on the dasatinib and quercetin approach, the first senolytic treatment tested in mice and humans. It is likely that some of the companies developing novel senolytics can do considerably better, but that data is not yet available for review.
Skin aging caused by various endogenous and exogenous factors results in structural and functional changes to skin components. However, the role of senescent cells in skin aging has not been clarified. To elucidate the function of senescent cells in skin aging, we evaluated the effects of the glutaminase inhibitor BPTES (bis-2-(5-phenylacetamido-1, 3, 4-thiadiazol-2-yl)ethyl sulfide) on human senescent dermal fibroblasts and aged human skin. Here, primary human dermal fibroblasts (HDFs) were induced to senescence by long-term passaging, ionizing radiation, and treatment with doxorubicin, an anticancer drug. Cell viability of HDFs was assessed after BPTES treatment.
A mouse/human chimeric model was created by subcutaneously transplanting whole skin grafts from aged humans into nude mice. The model was treated intraperitoneally with BPTES or vehicle for 30 days. Skin samples were collected and subjected to reverse transcription-quantitative polymerase chain reaction (RT-qPCR), western blotting, and histological analysis. BPTES selectively eliminated senescent dermal fibroblasts regardless of the method used to induce senescence; aged human skin grafts treated with BPTES exhibited increased collagen density, increased cell proliferation in the dermis, and decreased aging-related secretory phenotypes, such as matrix metalloprotease and interleukin. These effects were maintained in the grafts 1 month after termination of the treatment.
In conclusion, selective removal of senescent dermal fibroblasts can improve the skin aging phenotype, indicating that BPTES may be an effective novel therapeutic agent for skin aging.
Non-Dividing Neurons Do In Fact Become Senescent, Impairing Brain Function
Cellular senescence is generally thought of as a characteristic of replicating cells; it is an end state reached when telomeres, reduced in length with each cell division, become too short. This is followed by programmed cell death or destruction by immune cells. When senescent cells linger, as is increasingly the case with age, they contribute to degenerative aging via their pro-growth, pro-inflammatory signaling, disruptive of tissue structure and function. Researchers have suggested that non-dividing, post-mitotic cells such as neurons can also exhibit a form of senescence, and here evidence is provided for this to be the case. Senescence in supporting cells in the brain, such as microglia and astrocytes, is known to contribute to neurodegeneration. If some neurons are also senescent, producing similar harmful signaling, then these cells will also contribute to the aging of the brain.
As cells age, they can undergo cellular senescence, which contributes to tissue dysfunction and age-related disorders. Senescence is also thought to play a role in cellular stress, molecular damage, and cancer initiation. However, scientists previously believed that senescence primarily occurred in dividing cells, not in neurons. Little was known about the senescence-like state of aging human neurons.
In this study, researchers took skin samples from people with Alzheimer's disease and converted those cells directly into neurons in the lab. They tested these neurons to see if they undergo senescence and examined the mechanisms involved in the process. They also explored senescence markers and gene expression of post-mortem brains from 20 people with Alzheimer's disease and matched healthy controls. This allowed the team to confirm that their results from the lab held true in actual human brain tissue.
The team found that senescent neurons are a source of late-life brain inflammation observed in Alzheimer's disease. As the neurons deteriorate, they release inflammatory factors that trigger a cascade of brain inflammation and cause other brain cells to run haywire. Additionally, the gene KRAS, which is commonly involved in cancer, could activate the senescent response. The consequences of even a small number of senescent neurons in the aging brain could have a significant impact on brain function. This is because a single neuron can make more than 1,000 connections with other neurons, affecting the brain's communication system.
In addition to these findings, the authors also administered a therapeutic (a cocktail of Dasatinib + Quercetin) to the patient neurons in a dish. Both drugs are used to remove senescent cells in the body in conditions such as osteoarthritis, so the authors wanted to see if they were effective in senescent cells in the central nervous system as well. They found that the drug cocktail reduced the number of senescent neurons to normal levels. Targeting senescent cells could thus be a useful approach for slowing neuroinflammation and neurodegeneration in Alzheimer's disease.
Thymic Macrophage Populations Change with Age
The thymus atrophies with age, limiting the supply of new T cells to support the adaptive immune system. This is an important aspect of immune aging. Digging deeper into the mechanisms of thymus aging is of interest to the extent that it might reveal practical approaches to intervention. The challenge of the thymus is near entirely its inaccessible location, making it hard to deliver the known factors that can induce thymic regrowth without side-effects in the rest of the body. Here, researchers find that one population of macrophages characteristic of thymic tissue diminishes with age, while another population expands. The researchers theorize that it might be possible to adjust these cell proportions to provoke thymic regrowth, but at this stage that proposal is quite theoretical. More research would be needed to validate the underlying hypothesis regarding how these macrophages are involved in either supporting thymic tissue or encouraging atrophy.
Tissue-resident macrophages are essential to protect from pathogen invasion and maintain organ homeostasis. The ability of thymic macrophages to engulf apoptotic thymocytes is well appreciated, but little is known about their ontogeny, maintenance, and diversity. Here, we characterized the surface phenotype and transcriptional profile of these cells and defined their expression signature. Thymic macrophages were most closely related to spleen red pulp macrophages and Kupffer cells and shared the expression of the transcription factor SpiC with these cells.
Single-cell RNA sequencing showed that the macrophages in the adult thymus are composed of two populations distinguished by the expression of Timd4 and Cx3cr1. Remarkably, Timd4+ cells were located in the cortex, while Cx3cr1+ macrophages were restricted to the medulla and the cortico-medullary junction. Using chimeras, transplantation of embryonic thymuses, and genetic fate mapping, we found that the two populations have distinct origins. Timd4+ thymic macrophages are of embryonic origin, while Cx3cr1+ macrophages are derived from adult hematopoietic stem cells. Aging has a profound effect on the macrophages in the thymus. Timd4+ cells underwent gradual attrition, while Cx3cr1+ cells slowly accumulated with age and, in older mice, were the dominant macrophage population in the thymus.
The clear correlation between the accumulation of Cx3cr1+ thymic macrophages and thymic involution suggests that some factors produced exclusively by these cells are relevant. For example, Cx3cr1+ thymic macrophages are the predominant producer of the growth factor PDGFα that is required for the maintenance of adipocyte stem cells and can stimulate tissue fibrosis. The gradual accumulation of Cx3cr1+ macrophages could increase the availability of PDGFα in the aging thymus stimulating extracellular matrix production and differentiation of precursors into adipocytes. This model predicts that limiting the influx of Cx3cr1+ macrophage precursors could delay thymus involution.
Carotid Artery Stiffness Correlates with Age-Related Damage and Dysfunction in the Brain
Researchers here note the clear correlation between a measure of arterial stiffness and various assessments of damage and dysfunction to the brain. Arterial stiffness leads to hypertension via its disruption of the feedback systems needed to control blood pressure. That in turn leads to pressure damage to delicate tissues, such as those of the brain. It is associated with dysfunction of the blood-brain barrier, allowing leakage of normally restricted cells and molecules that provoke inflammation in brain tissues, as well as other vascular issues that cause harm to the brain.
We examined the associations of carotid artery stiffness with cerebral small-vessel disease markers, cognition, and dementia subtypes in a memory clinic cohort. A total of 272 participants underwent carotid ultrasonography, brain magnetic resonance imaging, and neuropsychological assessment. Carotid ultrasonography was used to assess β-index, pressure-strain elastic modulus, and pulse-wave velocity-β.
Brain magnetic resonance images were graded for cerebral small-vessel disease markers, including white matter hyperintensities, lacunes, and cerebral microbleeds. Participants were classified as having no cognitive impairment, cognitive impairment and no dementia, or dementia subtyped as Alzheimer disease and vascular dementia. Cognition was assessed using National Institute of Neurological Disorders and Stroke / Canadian Stroke Network harmonization battery.
After adjusting for age, sex, cardiovascular risk factors, and diseases, multivariable models showed that β-index, elastic modulus, and pulse-wave velocity-β were associated with white matter hyperintensities, and elastic modulus (odds ratio [OR] 1.39) and pulse-wave velocity-β (OR 1.47) were independently associated with lacunes. Similarly, β-index (OR 2.04), elastic modulus (OR 2.22), and pulse-wave velocity-β (OR 2.50) were independently associated with vascular dementia. Carotid stiffness measures were independently associated with worse performance in global cognition, visuomotor speed, visuospatial function, and executive function. These associations became largely nonsignificant after further adjusting for cerebral small-vessel disease markers.
Thus in memory clinic patients, carotid artery stiffness was associated with white matter hyperintensities and lacunes, impairment in global and domain-specific cognition, and causative subtypes of dementia, particularly vascular. The effects of carotid stiffness on cognition were not independent of, and were partially mediated by, cerebral small-vessel disease.
The Gut Microbiome in Alzheimer's Disease
The gut microbiome changes with age in ways that provoke greater chronic inflammation throughout the body. Unresolved inflammation is a feature of aging with numerous contributing causes, such as the growing burden of senescent cells and molecular debris from stressed and damaged cells. Inflammation in brain tissue is a feature of all of the common neurodegenerative conditions, and ever more researchers are beginning to consider that it may occupy a central position in the pathology of Alzheimer's disease.
Several studies investigating the pathogenesis of Alzheimer's disease have identified various interdependent constituents contributing to the exacerbation of the disease, including amyloid-β plaque formation, tau protein hyperphosphorylation, neurofibrillary tangle accumulation, glial inflammation, and the eventual loss of proper neural plasticity. Recently, using various models and human patients, another key factor has been established as an influential determinant in brain homeostasis: the gut-brain axis.
The implications of a rapidly aging population and the absence of a definitive cure for Alzheimer's disease have prompted a search for non-pharmaceutical tools, of which gut-modulatory therapies targeting the gut-brain axis have shown promise. Yet multiple recent studies examining changes in human gut flora in response to various probiotics and environmental factors are limited and difficult to generalize; whether the state of the gut microbiota in Alzheimer's disease is a cause of the disease, a result of the disease, or both through numerous feedback loops in the gut-brain axis, remains unclear.
However, preliminary findings of longitudinal studies conducted over the past decades have highlighted dietary interventions, especially Mediterranean diets, as preventative measures for Alzheimer's disease by reversing neuroinflammation, modifying the intestinal barrier and blood-brain barrier (BBB), and addressing gut dysbiosis. Conversely, the consumption of Western diets intensifies the progression of Alzheimer's disease through genetic alterations, impaired barrier function, and chronic inflammation. This review aims to support the growing body of experimental and clinical data highlighting specific probiotic strains and particular dietary components in preventing Alzheimer's disease via the gut-brain axis.