Fasting Activates Fatty Acid Oxidation to Enhance Intestinal Stem Cell Function during Homeostasis and Aging

Challenges in Studying Stem Cell Metabolism

The expanding field of stem cell metabolism has been supported by technical advances in metabolite profiling and novel functional analyses. While use of these methodologies has been fruitful, many challenges are posed by the intricacies of culturing stem cells in vitro, along with the distinctive scarcity of adult tissue stem cells and the complexities of their niches in vivo. This review provides an examination of the methodologies used to characterize stem cell metabolism, highlighting their utility while placing a sharper focus on their limitations and hurdles the field needs to overcome for the optimal study of stem cell metabolic networks.

Metabolic Regulation of Stem Cells and Differentiation: A Forkhead Box O Transcription Factor Perspective

Significance: Stem cell activation and differentiation occur along changes in cellular metabolism. Metabolic transitions translate into changes in redox balance, cell signaling, and epigenetics, thereby regulating these processes. Metabolic transitions are key regulators of cell fate and exemplify the moonlighting nature of many metabolic enzymes and their associated metabolites. Recent Advances: Forkhead box O transcription factors (FOXOs) are bona fide regulators of cellular homeostasis. FOXOs are multitasking proteins able to regulate cell cycle, cellular metabolism, and redox state. Recent and ongoing research poses FOXOs as key factors in stem cell maintenance and differentiation in several tissues. Critical Issues: The multitasking nature of FOXOs and their tissue-specific expression patterns hinders to disclose a possible conserved mechanism of regulation of stem cell maintenance and differentiation. Moreover, cellular metabolism, cell signaling, and epigenetics establish complex regulatory interactions, which challenge the establishment of the causal/temporal nature of metabolic changes and stem cell activation and differentiation. Future Directions: The development of single-cell technologies and in vitro models able to reproduce the dynamics of stem cell differentiation are actively contributing to define the role of metabolism in this process. This knowledge is key to understanding and designing therapies for those pathologies where the balance between proliferation and differentiation is lost. Importantly, metabolic interventions could be applied to optimize stem cell cultures meant for therapeutical applications, such as transplantations, to treat autoimmune and degenerative disorders. Antioxid. Redox Signal. 34, 1004-1024.

Reactive Oxygen Species in intestinal stem cell metabolism, fate and function

Long dismissed as merely harmful respiratory by-products, Reactive Oxygen Species (ROS) have emerged as critical intracellular messengers during cell growth and differentiation. ROS's signaling roles are particularly prominent within the intestine, whose high regenerative capacity is maintained by Intestinal Stem Cells (ISCs). In this review, we outline roles for ROS in ISCs as revealed by studies using Drosophila and mouse model systems. We focus particularly on recent studies highlighting how ROS ties to metabolic adaptations, which ensure energy supply matches demand during ISC activation and differentiation. We describe how declines in these adaptive mechanisms, through aging or pathology, promote reciprocal changes in ISC metabolism and ROS signaling. These changes ultimately contribute to aberrant ISC function, a loss of tissue homeostasis, and a shortened lifespan.

Renal tubule Cpt1a overexpression protects from kidney fibrosis by restoring mitochondrial homeostasis

Chronic kidney disease (CKD) remains a major epidemiological, clinical, and biomedical challenge. During CKD, renal tubular epithelial cells (TECs) present a persistent inflammatory and profibrotic response. Fatty acid oxidation (FAO), the main source of energy for TECs, is reduced in kidney fibrosis and contributes to its pathogenesis. To determine whether gain of function in FAO (FAO-GOF) could protect from fibrosis, we generated a conditional transgenic mouse model with overexpression of the fatty acid shuttling enzyme carnitine palmitoyl-transferase 1A (CPT1A) in TECs. Cpt1a-knockin (CPT1A-KI) mice subjected to 3 models of renal fibrosis (unilateral ureteral obstruction, folic acid nephropathy [FAN], and adenine-induced nephrotoxicity) exhibited decreased expression of fibrotic markers, a blunted proinflammatory response, and reduced epithelial cell damage and macrophage influx. Protection from fibrosis was also observed when Cpt1a overexpression was induced after FAN. FAO-GOF restored oxidative metabolism and mitochondrial number and enhanced bioenergetics, increasing palmitate oxidation and ATP levels, changes that were also recapitulated in TECs exposed to profibrotic stimuli. Studies in patients showed decreased CPT1 levels and increased accumulation of short- and middle-chain acylcarnitines, reflecting impaired FAO in human CKD. We propose that strategies based on FAO-GOF may constitute powerful alternatives to combat fibrosis inherent to CKD.

Subversion of Niche-Signalling Pathways in Colorectal Cancer: What Makes and Breaks the Intestinal Stem Cell

The intestinal epithelium fulfils pleiotropic functions in nutrient uptake, waste elimination, and immune surveillance while also forming a barrier against luminal toxins and gut-resident microbiota. Incessantly barraged by extraneous stresses, the intestine must continuously replenish its epithelial lining and regenerate the full gamut of specialized cell types that underpin its functions. Homeostatic remodelling is orchestrated by the intestinal stem cell (ISC) niche: a convergence of epithelial- and stromal-derived cues, which maintains ISCs in a multipotent state. Following demise of homeostatic ISCs post injury, plasticity is pervasive among multiple populations of reserve stem-like cells, lineage-committed progenitors, and/or fully differentiated cell types, all of which can contribute to regeneration and repair. Failure to restore the epithelial barrier risks seepage of toxic luminal contents, resulting in inflammation and likely predisposing to tumour formation. Here, we explore how homeostatic niche-signalling pathways are subverted in tumorigenesis, enabling ISCs to gain autonomy from niche restraints ("ISC emancipation") and transform into cancer stem cells capable of driving tumour initiation, progression, and therapy resistance. We further consider the implications of the pervasive plasticity of the intestinal epithelium for the trajectory of colorectal cancer, the emergence of distinct molecular subtypes, the propensity to metastasize, and the development of effective therapeutic strategies.

Metabolic Regulation of Intestinal Stem Cell Homeostasis

The balance between self-renewal and differentiation of intestinal stem cells is essential for intestinal epithelial homeostasis, which can be regulated by dietary cues. Recent evidences indicate that metabolic pathways sense changes in nutritional status to control stem cell fate, which may provide new clues for the prevention of intestinal diseases.

Aspirin in the Prevention of Colorectal Neoplasia

High-quality evidence indicates that regular use of aspirin is effective in reducing the risk for precancerous colorectal neoplasia and colorectal cancer (CRC). This has led to US and international guidelines recommending aspirin for the primary prevention of CRC in specific populations. In this review, we summarize key questions that require addressing prior to broader adoption of aspirin-based chemoprevention, review recent evidence related to the benefits and harms of aspirin use among specific populations, and offer a rationale for precision prevention approaches. We specifically consider the mechanistic implications of evidence showing differences in aspirin's effects according to age, the potential role of modifiable mechanistic biomarkers for personalizing prevention, and emerging evidence that the gut microbiota may offer novel aspirin-associated preventive targets to reduce high-risk neoplasia.

A diet-microbial metabolism feedforward loop modulates intestinal stem cell renewal in the stressed gut

Dietary patterns and psychosocial factors, ubiquitous part of modern lifestyle, critically shape the gut microbiota and human health. However, it remains obscure how dietary and psychosocial inputs coordinately modulate the gut microbiota and host impact. Here, we show that dietary raffinose metabolism to fructose couples stress-induced gut microbial remodeling to intestinal stem cells (ISC) renewal and epithelial homeostasis. Chow diet (CD) and purified diet (PD) confer distinct vulnerability to gut epithelial injury, microbial alternation and ISC dysfunction in chronically restrained mice. CD preferably enriches Lactobacillus reuteri, and its colonization is sufficient to rescue stress-triggered epithelial injury. Mechanistically, dietary raffinose sustains Lactobacillus reuteri growth, which in turn metabolizes raffinose to fructose and thereby constituting a feedforward metabolic loop favoring ISC maintenance during stress. Fructose augments and engages glycolysis to fuel ISC proliferation. Our data reveal a diet-stress interplay that dictates microbial metabolism-shaped ISC turnover and is exploitable for alleviating gut disorders.

Nutritional Control of Intestinal Stem Cells in Homeostasis and Tumorigenesis

Food and nutrition have a profound impact on organismal health and diseases, and tissue-specific adult stem cells play a crucial role in coordinating tissue maintenance by responding to dietary cues. Emerging evidence indicates that adult intestinal stem cells (ISCs) actively adjust their fate decisions in response to diets and nutritional states to drive intestinal adaptation. Here, we review the signaling mechanisms mediating the dietary responses imposed by caloric intake and nutritional composition (i.e., macronutrients and micronutrients), fasting-feeding patterns, diet-induced growth factors, and microbiota on ISCs and their relevance to the beginnings of intestinal tumors.

A bioengineering perspective on modelling the intestinal epithelial physiology in vitro

The small intestine is a specialised organ, essential for nutrient digestion and absorption. It is lined with a complex epithelial cell layer. Intestinal epithelial cells can be cultured in three-dimensional (3D) scaffolds as self-organising entities with distinct domains containing stem cells and differentiated cells. Recent developments in bioengineering provide new possibilities for directing the organisation of cells in vitro. In this Perspective, focusing on the small intestine, we discuss how studies at the interface between bioengineering and intestinal biology provide new insights into organ function. Specifically, we focus on engineered biomaterials, complex 3D structures resembling the intestinal architecture, and micro-physiological systems.

Stem Cell Metabolism and Diet

PURPOSE OF REVIEW

Diet has profound impacts on health and longevity. Evidence is emerging to suggest that diet impinges upon the metabolic pathways in tissue-specific stem cells to influence health and disease. Here, we review the similarities and differences in the metabolism of stem cells from several tissues, and highlight the mitochondrial metabolic checkpoint in stem cell maintenance and aging. We discuss how diet engages the nutrient sensing metabolic pathways and impacts stem cell maintenance. Finally, we explore the therapeutic implications of dietary and metabolic regulation of stem cells.

RECENT FINDINGS

Stem Cell transition from quiescence to proliferation is associated with a metabolic switch from glycolysis to mitochondrial OXPHOS and the mitochondrial metabolic checkpoint is critically controlled by the nutrient sensors SIRT2, SIRT3, and SIRT7 in hematopoietic stem cells. Intestine stem cell homeostasis during aging and in response to diet is critically dependent on fatty acid metabolism and ketone bodies and is influenced by the niche mediated by the nutrient sensor mTOR.

SUMMARY

Nutrient sensing metabolic pathways critically regulate stem cell maintenance during aging and in response to diet. Elucidating the molecular mechanisms underlying dietary and metabolic regulation of stem cells provides novel insights for stem cell biology and may be targeted therapeutically to reverse stem cell aging and tissue degeneration.

Effects of caloric restriction on immunosurveillance, microbiota and cancer cell phenotype: Possible implications for cancer treatment

Fasting, caloric restriction and foods or compounds mimicking the biological effects of caloric restriction, known as caloric restriction mimetics, have been associated with a lower risk of age-related diseases, including cardiovascular diseases, cancer and cognitive decline, and a longer lifespan. Reduced calorie intake has been shown to stimulate cancer immunosurveillance, reducing the migration of immunosuppressive regulatory T cells towards the tumor bulk. Autophagy stimulation via reduction of lysine acetylation, increased sensitivity to chemo- and immunotherapy, along with a reduction of insulin-like growth factor 1 and reactive oxygen species have been described as some of the major effects triggered by caloric restriction. Fasting and caloric restriction have also been shown to beneficially influence gut microbiota composition, modify host metabolism, reduce total cholesterol and triglyceride levels, lower diastolic blood pressure and elevate morning cortisol level, with beneficial modulatory effects on cardiopulmonary fitness, body fat and weight, fatigue and weakness, and general quality of life. Moreover, caloric restriction may reduce the carcinogenic and metastatic potential of cancer stem cells, which are generally considered responsible of tumor formation and relapse. Here, we reviewed in vitro and in vivo studies describing the effects of fasting, caloric restriction and some caloric restriction mimetics on immunosurveillance, gut microbiota, metabolism, and cancer stem cell growth, highlighting the molecular and cellular mechanisms underlying these effects. Additionally, studies on caloric restriction interventions in cancer patients or cancer risk subjects are discussed. Considering the promising effects associated with caloric restriction and caloric restriction mimetics, we think that controlled-randomized large clinical trials are warranted to evaluate the inclusion of these non-pharmacological approaches in clinical practice.

Signaling Network Centered on mTORC1 Dominates Mammalian Intestinal Stem Cell Ageing

The intestine integrates the function of digestion, absorption, and barrier, which is easily damaged by the external factors upon ageing. The intestinal stem cells (ISCs) exist at the intestinal crypt base and play an indispensable role in intestinal homeostasis and regeneration. The intestine ageing contributes to malabsorption and other associated illnesses, which were considered to be related to ISCs. Here, we summarize the current research progress of mammalian ISCs ageing and pay more attention to the central regulatory role of the mTORC1 signaling pathway in regulating mammalian ISCs ageing, and its related AMPK, FOXO, Wnt signaling pathways. Furthermore, we also discuss the interventions aimed at mTORC1 and its associated signaling pathways, which may provide potential strategies for rejuvenating aged ISCs and the therapy of age-related intestinal diseases. Graphical abstract Many signaling pathways are altered in the ageing ISCs, thereby inducing the decrease of ISC self-renewal, differentiation, and regeneration, an increasing of oxidative stress may contribute to damage to the ISCs. Interventions such as calorie restriction, fasting and so on can effectively alleviate these adverse effects.

The duration of food withdrawal affects the intestinal structure, nutrients absorption, and utilization in broiler chicken

Food withdrawal is usually used for accurate feed metabolizable energy (ME) assessment in poultry, but its effects on intestinal structure and the absorption of nutrients are unclear. In this study, broilers were fed ad libitum (CT) or withdrew food for 12 (FH12), 24 (FH24), 36 (FH36), or 48 hours (FH48). We showed that food withdrawal increased the energy assimilation when compared with the CT. Food withdrawal improved the digestibility of ether extract and the level of lipid substances and fatty acid-derived β-hydroxybutyrate in serum. Compared to the CT, food withdrawal did not influence the digestibility of starch. Due to 12 hours or longer food withdrawal duration increased glutamate oxidation and uric acid excretion, the analyzed digestibility of crude protein was underestimated, although the upregulated amino acid transporter genes. In addition, histological analysis showed that short-term food withdrawal (12 hours) increased intestinal villus height, crypt depth, and proliferative cell, whereas prolonged food withdrawal (more than 24 hours) impaired villus structure due to the decreased cell proliferation. Moreover, proteomics analysis revealed upregulated pathways in birds withdrawn food for 36 hours involved in nutrient absorption and amino acid oxidation. In conclusion, food withdrawal changes nutrient absorption and utilization, especially for amino acid and ether extract, and results in increased ME. Both glutamate oxidation and fatty acid incomplete oxidation are involved in energy supply after refeeding. In contrast to short-term food withdrawal, prolonged food withdrawal impairs the intestinal structure and villus renewal. Our findings deserve attention from nutritionists who are analyzing food digestibility.

Microbiome in cancer progression and therapy

A myriad of microbes living together with the host constitute microbiota, which possesses very diverse functions in regulation of host physiology. Recently, it has been unequivocally demonstrated that microbiota regulates cancer initiation, progression and responses to therapy. Here we review known pro-tumorigenic and anti-tumorigenic function of microbiota and mechanisms how microbes can regulate cancer cells and immune and stromal cells within the tumor microenvironment.

Therapeutic strategies targeting somatic stem cells: Chemical approaches

Therapeutic modulation of fate and behavior of somatic stem cells can generate safe and functional cells ex vivo for cell-based therapy, or to repair and regenerate damaged tissues in vivo. Chemical approaches involving small molecules have provided promising approaches for modulating cellular fate and function. These strategies offer opportunities that support regenerative medicine. Here, we discuss strategies targeting somatic stem cells through chemical approaches, highlighting their progression as well as future prospects.

Human Milk Oligosaccharides Protect against Necrotizing Enterocolitis by Activating Intestinal Cell Differentiation

SCOPE

Necrotizing enterocolitis (NEC) is a devastating gastrointestinal emergency and currently the leading cause of mortality in preterm infants. Recent studies show that human milk oligosaccharides (HMOs) reduce the frequency and incidence of NEC; however, the molecular mechanisms for their protection are largely unexplored.

METHODS AND RESULTS

To address this gap, a genome-wide profiling of the intestinal epithelial transcriptome in response to HMOs using RNA-sequencing is performed. It is found that HMOs alter the host transcriptome in 225 unique target genes pertaining to cell proliferation and differentiation, including upregulation of stem cell differentiation marker HMGCS2. To validate these results, differentiation in Caco-2Bbe1 (Caco-2) intestinal cells is verified by Alcian Blue staining and transepithelial electrical resistance (TER) recordings. Furthermore, an in vivo model of NEC is also employed whereby neonatal pups are gavage fed HMOs. Interestingly, HMOs-fed pups show enhanced cell MUC2 differentiation and HMGCS2 expression.

CONCLUSIONS

These findings demonstrate HMOs protect against NEC in part by altering the differentiation of the crypt-villus axis. In addition, this study suggests that pooled HMOs directly induce a series of biological processes, which provide mechanistic insights to how HMOs protect the host intestine.

The mitochondria regulation of stem cell aging

Mitochondrial dysfunction and stem cell exhaustion are among the nine separate hallmarks of aging. Emerging evidence however suggests that mitochondrial activity can have a profound influence on the self-renewal and function of stem cells, thus mechanistically linking mitochondrial function and stem cell decline. In this review, we discuss how accumulation of mtDNA mutations or alterations in mitochondrial dynamics, turnover, and signaling can modulate age-dependent stem cell function. Finally, we also describe how mitochondrial substrate utilization influences stem and progenitor activity. Together, this growing body of evidence suggests that modulation of mitochondrial activity might provide a strategy to slow or reverse age-dependent stem cell decline, and potentially, slow or reverse human aging.

SIRT7 Facilitates CENP-A Nucleosome Assembly and Suppresses Intestinal Tumorigenesis

SIRT7 is a member of the mammalian sirtuins and functions as an NAD+-dependent deacylase. Here we show that SIRT7 deficiency leads to a lowered histone acetyltransferase 1 (HAT1) activity and therefore decreased histone H4K5 and H4K12 acetylation. This in turn causes CENP-A dislocation at the centromere, which further affects chromatin assembly. SIRT7 ablation results in aneuploidy and aging phenotypes, including senescence and nucleolar expansion. Moreover, SIRT7 knockout mice are susceptible to DSS-induced colitis and alcohol-derived epithelial disturbance, revealing a disrupted intestinal epithelial homeostasis. Notably, absence of SIRT7 aggravates the susceptibility of colorectal cancer incidence in APCMin/+ mouse model and elicits further the Wnt signaling. Our findings indicate a tumor suppressive role of SIRT7 in the case of colorectal cancer. Together with the activities in maintaining genome integrity and intestinal homeostasis, activating SIRT7 may serve as a strategy to treat bowel diseases and colorectal cancer.

Upregulation of CPT1A is essential for the tumor-promoting effect of adipocytes in colon cancer

Colon tumors grow in an adipose tissue-enriched microenvironment. Locally advanced colon cancers often invade into surrounding adipose tissue with a direct contact with adipocytes. We have previously shown that adipocytes promote tumor growth by modulating cellular metabolism. Here we demonstrate that carnitine palmitoyltransferase I (CPT1A), a key enzyme controlling fatty acid oxidation (FAO), was upregulated in colon cancer cells upon exposure to adipocytes or fatty acids. In addition, CPT1A expression was increased in invasive tumor cells within the adipose tissue compared to tumors without direct contact with adipocytes. Silencing CPT1A abolished the protective effect provided by fatty acids against nutrient deprivation and reduced tumor organoid formation in 3D culture and the expression of genes associated with cancer stem cells downstream of Wnt/β-catenin. Mechanistically, CPT1A-dependent FAO promoted the acetylation and nuclear translocation of β-catenin. Furthermore, knockdown of CPT1A blocked the tumor-promoting effect of adipocytes in vivo and inhibited xenograft tumor initiation. Taken together, our findings identify CPT1A-depedent FAO as an essential metabolic pathway that enables the interaction between adipocytes and colon cancer cells.

Restoring aged stem cell functionality: Current progress and future directions

Stem cell dysfunction is a hallmark of aging, associated with the decline of physical and cognitive abilities of humans and other mammals [Cell 2013;153:1194]. Therefore, it has become an active area of research within the aging and stem cell fields, and various techniques have been employed to mitigate the decline of stem cell function both in vitro and in vivo. While some techniques developed in model organisms are not directly translatable to humans, others show promise in becoming clinically relevant to delay or even mitigate negative phenotypes associated with aging. This review focuses on diet, treatment, and small molecule interventions that provide evidence of functional improvement in at least one type of aged adult stem cell.

Intestinal epithelial plasticity and regeneration via cell dedifferentiation

The intestinal epithelium possesses a great capacity of self-renewal under normal homeostatic conditions and of regeneration upon damages. The renewal and regenerative processes are driven by intestinal stem cells (ISCs), which reside at the base of crypts and are marked by Lgr5. As Lgr5⁺ ISCs undergo fast cycling and are vulnerable to damages, there must be other types of cells that can replenish the lost Lgr5⁺ ISCs and then regenerate the damage epithelium. In addition to Lgr5⁺ ISCs, quiescent ISCs at the + 4 position in the crypt have been proposed to convert to Lgr5⁺ ISCs during regeneration. However, this “reserve stem cell” model still remains controversial. Different from the traditional view of a hierarchical organization of the intestinal epithelium, recent works support the dynamic “dedifferentiation” model, in which various cell types within the epithelium can de-differentiate to revert to the stem cell state and then regenerate the epithelium upon tissue injury. Here, we provide an overview of the cell identity and features of two distinct models and discuss the possible mechanisms underlying the intestinal epithelial plasticity.

Bile Acids Signal via TGR5 to Activate Intestinal Stem Cells and Epithelial Regeneration

BACKGROUND & AIMS

Renewal and patterning of the intestinal epithelium is coordinated by intestinal stem cells (ISCs); dietary and metabolic factors provide signals to the niche that control ISC activity. Bile acids (BAs), metabolites in the gut, signal nutrient availability by activating the G protein-coupled bile acid receptor 1 (GPBAR1, also called TGR5). TGR5 is expressed in the intestinal epithelium, but it is not clear how its activation affects ISCs and regeneration of the intestinal epithelium. We studied the role of BAs and TGR5 in intestinal renewal, and regulation of ISC function in mice and intestinal organoids.

METHODS

We derived intestinal organoids from wild-type mice and Tgr5^-/- mice, incubated them with BAs or the TGR5 agonist INT-777, and monitored ISC function by morphologic analyses and colony-forming assays. We disrupted Tgr5 specifically in Lgr5-positive ISCs in mice (Tgr5^ISC-/- mice) and analyzed ISC number, proliferation, and differentiation by flow cytometry, immunofluorescence, and organoid assays. Tgr5^ISC-/- mice were given cholecystokinin; we measured the effects of BA release into the intestinal lumen and on cell renewal. We induced colitis in Tgr5^ISC-/- mice by administration of dextran sulfate sodium; disease severity was determined based on body weight, colon length, and histopathology analysis of colon biopsies.

RESULTS

BAs and TGR5 agonists promoted growth of intestinal organoids. Administration of cholecystokinin to mice resulted in acute release of BAs into the intestinal lumen and increased proliferation of the intestinal epithelium. BAs and Tgr5 expression in ISCs were required for homeostatic intestinal epithelial renewal and fate specification, and for regeneration after colitis induction. Tgr5^ISC-/- mice developed more severe colitis than mice without Tgr5 disruption in ISCs. ISCs incubated with INT-777 increased activation of yes-associated protein 1 (YAP1) and of its upstream regulator SRC. Inhibitors of YAP1 and SRC prevented organoid growth induced by TGR5 activation.

CONCLUSIONS

BAs promote regeneration of the intestinal epithelium via activation of TGR5 in ISCs, resulting in activation of SRC and YAP and activation of their target genes. Release of endogenous BAs in the intestinal lumen is sufficient to promote ISC renewal and drives regeneration in response to injury.

A Single Metabolite which Modulates Lipid Metabolism Alters Hematopoietic Stem/Progenitor Cell Behavior and Promotes Lymphoid Reconstitution

Fatty acid β-oxidation (FAO), the breakdown of lipids, is a metabolic pathway used by various stem cells. FAO levels are generally high during quiescence and downregulated with proliferation. The endogenous metabolite malonyl-CoA modulates lipid metabolism as a reversible FAO inhibitor and as a substrate for de novo lipogenesis. Here we assessed whether malonyl-CoA can be exploited to steer the behavior of hematopoietic stem/progenitor cells (HSPCs), quiescent stem cells of clinical relevance. Treatment of mouse HSPCs in vitro with malonyl-CoA increases HSPC numbers compared with nontreated controls and ameliorates blood reconstitution capacity when transplanted in vivo, mainly through enhanced lymphoid reconstitution. Similarly, human HSPC numbers also increase upon malonyl-CoA treatment in vitro. These data corroborate that lipid metabolism can be targeted to direct cell fate and stem cell proliferation. Physiological modulation of metabolic pathways, rather than genetic or pharmacological inhibition, provides unique perspectives for stem cell manipulations in health and disease.

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Modulating lipid metabolism with malonyl-CoA increases murine HSPCs in vitro

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Malonyl-CoA promotes lymphoid cell fate of HSPCs in vitro and in vivo

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The myeloid bias in aged HSPCs is partially recovered with malonyl-CoA exposure

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Malonyl-CoA increases the numbers of human HSPCs in vitro

The eroding chromatin landscape of aging stem cells

Adult stem cells undergo both replicative and chronological aging in their niches, with catastrophic declines in regenerative potential with age. Due to repeated environmental insults during aging, the chromatin landscape of stem cells erodes, with changes in both DNA and histone modifications, accumulation of damage, and altered transcriptional response. A body of work has shown that altered chromatin is a driver of cell fate changes and a regulator of self-renewal in stem cells and therefore a prime target for juvenescence therapeutics. This review focuses on chromatin changes in stem cell aging and provides a composite view of both common and unique epigenetic themes apparent from the studies of multiple stem cell types.

Plasticity of Paneth cells and their ability to regulate intestinal stem cells

Paneth cells (PCs) are located at the bottom of small intestinal crypts and play an important role in maintaining the stability of the intestinal tract. Previous studies reported on how PCs shape the intestinal microbiota or the response to the immune system. Recent studies have determined that PCs play an important role in the regulation of the homeostasis of intestinal epithelial cells. PCs can regulate the function and homeostasis of intestinal stem cells through several mechanisms. On the one hand, under pathological conditions, PCs can be dedifferentiated into stem cells to promote the repair of intestinal tissues. On the other hand, PCs can regulate stem cell proliferation by secreting a variety of hormones (such as wnt3a) or metabolic intermediates. In addition, we summarise key signalling pathways that affect PC differentiation and mutual effect with intestinal stem cells. In this review, we introduce the diverse functions of PCs in the intestine.

Plasticity of Paneth cells and their ability to regulate intestinal stem cells

Paneth cells (PCs) are located at the bottom of small intestinal crypts and play an important role in maintaining the stability of the intestinal tract. Previous studies reported on how PCs shape the intestinal microbiota or the response to the immune system. Recent studies have determined that PCs play an important role in the regulation of the homeostasis of intestinal epithelial cells. PCs can regulate the function and homeostasis of intestinal stem cells through several mechanisms. On the one hand, under pathological conditions, PCs can be dedifferentiated into stem cells to promote the repair of intestinal tissues. On the other hand, PCs can regulate stem cell proliferation by secreting a variety of hormones (such as wnt3a) or metabolic intermediates. In addition, we summarise key signalling pathways that affect PC differentiation and mutual effect with intestinal stem cells. In this review, we introduce the diverse functions of PCs in the intestine.

A Dedicated Evolutionarily Conserved Molecular Network Licenses Differentiated Cells to Return to the Cell Cycle

Differentiated cells can re-enter the cell cycle to repair tissue damage via a series of discrete morphological and molecular stages coordinated by the cellular energetics regulator mTORC1. We previously proposed the term "paligenosis" to describe this conserved cellular regeneration program. Here, we detail a molecular network regulating mTORC1 during paligenosis in both mouse pancreatic acinar and gastric chief cells. DDIT4 initially suppresses mTORC1 to induce autodegradation of differentiated cell components and damaged organelles. Later in paligenosis, IFRD1 suppresses p53 accumulation. Ifrd1^-/- cells do not complete paligenosis because persistent p53 prevents mTORC1 reactivation and cell proliferation. Ddit4^-/- cells never suppress mTORC1 and bypass the IFRD1 checkpoint on proliferation. Previous reports and our current data implicate DDIT4/IFRD1 in governing paligenosis in multiple organs and species. Thus, we propose that an evolutionarily conserved, dedicated molecular network has evolved to allow differentiated cells to re-enter the cell cycle (i.e., undergo paligenosis) after tissue injury. VIDEO ABSTRACT.

Hallmarks of intestinal stem cells

Intestinal stem cells (ISCs) are highly proliferative cells that fuel the continuous renewal of the intestinal epithelium. Understanding their regulatory mechanisms during tissue homeostasis is key to delineating their roles in development and regeneration, as well as diseases such as bowel cancer and inflammatory bowel disease. Previous studies of ISCs focused mainly on the position of these cells along the intestinal crypt and their capacity for multipotency. However, evidence increasingly suggests that ISCs also exist in distinct cellular states, which can be an acquired rather than a hardwired intrinsic property. In this Review, we summarise the recent findings into how ISC identity can be defined by proliferation state, signalling crosstalk, epigenetics and metabolism, and propose an update on the hallmarks of ISCs. We further discuss how these properties contribute to intestinal development and the dynamics of injury-induced regeneration.

Aging and Rejuvenation of Neural Stem Cells and Their Niches

Aging has a profound and devastating effect on the brain. Old age is accompanied by declining cognitive function and enhanced risk of brain diseases, including cancer and neurodegenerative disorders. A key question is whether cells with regenerative potential contribute to brain health and even brain "rejuvenation." This review discusses mechanisms that regulate neural stem cells (NSCs) during aging, focusing on the effect of metabolism, genetic regulation, and the surrounding niche. We also explore emerging rejuvenating strategies for old NSCs. Finally, we consider how new technologies may help harness NSCs' potential to restore healthy brain function during physiological and pathological aging.

Mesenchymal Niche-Derived Neuregulin-1 Drives Intestinal Stem Cell Proliferation and Regeneration of Damaged Epithelium

Epidermal growth factor (EGF) maintains intestinal stem cell (ISC) proliferation and is a key component of organoid growth media yet is dispensable for intestinal homeostasis, suggesting roles for multiple EGF family ligands in ISC function. Here, we identified neuregulin 1 (NRG1) as a key EGF family ligand that drives tissue repair following injury. NRG1, but not EGF, is upregulated upon damage and is expressed in mesenchymal stromal cells, macrophages, and Paneth cells. NRG1 deletion reduces proliferation in intestinal crypts and compromises regeneration capacity. NRG1 robustly stimulates proliferation in crypts and induces budding in organoids, in part through elevated and sustained activation of mitogen-activated protein kinase (MAPK) and AKT. Consistently, NRG1 treatment induces a proliferative gene signature and promotes organoid formation from progenitor cells and enhances regeneration following injury. These data suggest mesenchymal-derived NRG1 is a potent mediator of tissue regeneration and may inform the development of therapies for enhancing intestinal repair after injury.

Behavioral Responses and Immune and Cytogenetic Parameters of Food-Restricted Mice

The experiments on mice showed that subchronic food restriction to 40 and 8% of unrestricted ration is a strong stressor inducing devastation of lymphoid organs, primarily the thymus and spleen. The mice in the group with severe food restriction (8% of normal ration) demonstrated increased front paw grip force. We also observed an increase in spontaneous motor activity in these animals correlated with food restriction. Food deprivation led to inhibition of proliferative activity of the bone marrow cells and suppression of erythropoiesis. Moreover, severe food restriction was accompanied by a decrease in the number of double-strand DNA breaks evaluated by the release of γH2AX⁺-cells and the ratio of polychromatophilic erythrocytes.

Histone Acetyltransferase MOF Blocks Acquisition of Quiescence in Ground-State ESCs through Activating Fatty Acid Oxidation

Self-renewing embryonic stem cells (ESCs) respond to environmental cues by exiting pluripotency or entering a quiescent state. The molecular basis underlying this fate choice remains unclear. Here, we show that histone acetyltransferase MOF plays a critical role in this process through directly activating fatty acid oxidation (FAO) in the ground-state ESCs. We further show that the ground-state ESCs particularly rely on elevated FAO for oxidative phosphorylation (OXPHOS) and energy production. Mof deletion or FAO inhibition induces bona fide quiescent ground-state ESCs with an intact core pluripotency network and transcriptome signatures akin to the diapaused epiblasts in vivo. Mechanistically, MOF/FAO inhibition acts through reducing mitochondrial respiration (i.e., OXPHOS), which in turn triggers reversible pluripotent quiescence specifically in the ground-state ESCs. The inhibition of FAO/OXPHOS also induces quiescence in naive human ESCs. Our study suggests a general function of the MOF/FAO/OXPHOS axis in regulating cell fate determination in stem cells.

Ageing, metabolism and the intestine

The intestinal epithelium serves as a dynamic barrier to the environment and integrates a variety of signals, including those from metabolites, commensal microbiota, immune responses and stressors upon ageing. The intestine is constantly challenged and requires a high renewal rate to replace damaged cells in order to maintain its barrier function. Essential for its renewal capacity are intestinal stem cells, which constantly give rise to progenitor cells that differentiate into the multiple cell types present in the epithelium. Here, we review the current state of research of how metabolism and ageing control intestinal stem cell function and epithelial homeostasis. We focus on recent insights gained from model organisms that indicate how changes in metabolic signalling during ageing are a major driver for the loss of stem cell plasticity and epithelial homeostasis, ultimately affecting the resilience of an organism and limiting its lifespan. We compare findings made in mouse and Drosophila and discuss differences and commonalities in the underlying signalling pathways and mechanisms in the context of ageing.

The impact of ageing on lipid-mediated regulation of adult stem cell behavior and tissue homeostasis

Adult stem cells sustain tissue homeostasis throughout life and provide an important reservoir of cells capable of tissue repair in response to stress and tissue damage. Age-related changes to stem cells and/or the specialized niches that house them have been shown to negatively impact stem cell maintenance and activity. In addition, metabolic inputs have surfaced as another crucial layer in the control of stem cell behavior (Chandel et al., 2016; Folmes and Terzic, 2016; Ito and Suda, 2014; Mana et al., 2017; Shyh-Chang and Ng, 2017). Here, we will present a brief review of how lipid metabolism influences adult stem cell behavior under homeostatic conditions and speculate on how changes in lipid metabolism may impact stem cell ageing. This review considers the future of lipid metabolism research in stem cells, with the long-term goal of identifying mechanisms that could be targeted to counter or slow the age-related decline in stem cell function.

Oxygen battle in the gut: Hypoxia and hypoxia-inducible factors in metabolic and inflammatory responses in the intestine

The gastrointestinal tract is a highly proliferative and regenerative tissue. The intestine also harbors a large and diverse microbial population collectively called the gut microbiome (microbiota). The microbiome-intestine cross-talk includes a dynamic exchange of gaseous signaling mediators generated by bacterial and intestinal metabolisms. Moreover, the microbiome initiates and maintains the hypoxic environment of the intestine that is critical for nutrient absorption, intestinal barrier function, and innate and adaptive immune responses in the mucosal cells of the intestine. The response to hypoxia is mediated by hypoxia-inducible factors (HIFs). In hypoxic conditions, the HIF activation regulates the expression of a cohort of genes that promote adaptation to hypoxia. Physiologically, HIF-dependent genes contribute to the aforementioned maintenance of epithelial barrier function, nutrient absorption, and immune regulation. However, chronic HIF activation exacerbates disease conditions, leading to intestinal injury, inflammation, and colorectal cancer. In this review, we aim to outline the major roles of physiological and pathological hypoxic conditions in the maintenance of intestinal homeostasis and in the onset and progression of disease with a major focus on understanding the complex pathophysiology of the intestine.

Ramadan Fasting and Risk of Covid-19

Almost all religions recommend periods of fasting. Many adult Muslims fast during the holy month of Ramadan each year. Ramadan fasting as a type of intermittent fasting is a non-pharmacological intervention refining the overall health. This year, Ramadan is coincided with coronavirus disease 2019 (COVID-19) outbreak making it one of the most challenging fasting periods for Muslims in the world. There is no solid direct evidence to suggest any adverse effect of Ramadan fasting during the COVID-19 pandemic in healthy individuals. However, there are exemptions in Ramadan Fasting and those at risk of health issues should not fast. COVID-19 is a new disease and there is limited studies concerning its risk factors. The purpose of this review was shedding more light on the potential mechanisms involved in influence of practice of fasting in all forms, including Ramadan fasting on the vulnerability to infection.

FASN-Dependent Lipid Metabolism Links Neurogenic Stem/Progenitor Cell Activity to Learning and Memory Deficits

Altered neural stem/progenitor cell (NSPC) activity and neurodevelopmental defects are linked to intellectual disability. However, it remains unclear whether altered metabolism, a key regulator of NSPC activity, disrupts human neurogenesis and potentially contributes to cognitive defects. We investigated links between lipid metabolism and cognitive function in mice and human embryonic stem cells (hESCs) expressing mutant fatty acid synthase (FASN; R1819W), a metabolic regulator of rodent NSPC activity recently identified in humans with intellectual disability. Mice homozygous for the FASN R1812W variant have impaired adult hippocampal NSPC activity and cognitive defects because of lipid accumulation in NSPCs and subsequent lipogenic ER stress. Homozygous FASN R1819W hESC-derived NSPCs show reduced rates of proliferation in embryonic 2D cultures and 3D forebrain regionalized organoids, consistent with a developmental phenotype. These data from adult mouse models and in vitro models of human brain development suggest that altered lipid metabolism contributes to intellectual disability.

Intermittent Fasting: Physiological Implications on Outcomes in Mice and Men

Intermittent fasting (IF) is a widely practiced dietary method that encompasses periodic restriction of food consumption. Due to its protective benefits against metabolic diseases, aging, and cardiovascular and neurodegenerative diseases, IF continues to gain attention as a preventative and therapeutic intervention to counteract these chronic diseases. Although numerous animal studies have reported positive health benefits of IF, its feasibility and efficacy in clinical settings remain controversial. Importantly, since dietary interventions such as IF have systemic effects, thoroughly investigating the tissue-specific changes in animal models is crucial to identify IF's mechanism and evaluate its potential adverse effects in humans. As such, we will review and compare the outcomes and underlying mechanisms of IF in both animal and human studies. Moreover, the limitations of IF and inconsistencies between preclinical and clinical studies will be discussed to provide insight into the gaps between translating research from bench to bedside.

Metabolic Regulation of Tissue Stem Cells

Adult tissue stem cells mediate organ homeostasis and regeneration and thus are continually making decisions about whether to remain quiescent, proliferate, or differentiate into mature cell types. These decisions often integrate external cues, such as energy balance and the nutritional status of the organism. Metabolic substrates and byproducts that regulate epigenetic and signaling pathways are now appreciated to have instructive rather than bystander roles in regulating cell fate decisions. In this review, we highlight recent literature focused on how metabolites and dietary manipulations can impact cell fate decisions, with a focus on the regulation of adult tissue stem cells.

Region-Specific Proteome Changes of the Intestinal Epithelium during Aging and Dietary Restriction

The small intestine is responsible for nutrient absorption and one of the most important interfaces between the environment and the body. During aging, changes of the epithelium lead to food malabsorption and reduced barrier function, thus increasing disease risk. The drivers of these alterations remain poorly understood. Here, we compare the proteomes of intestinal crypts from mice across different anatomical regions and ages. We find that aging alters epithelial immunity, metabolism, and cell proliferation and is accompanied by region-dependent skewing in the cellular composition of the epithelium. Of note, short-term dietary restriction followed by refeeding partially restores the epithelium by promoting stem cell differentiation toward the secretory lineage. We identify Hmgcs2 (3-hydroxy-3-methylglutaryl-coenzyme A [CoA] synthetase 2), the rate-limiting enzyme for ketogenesis, as a modulator of stem cell differentiation that responds to dietary changes, and we provide an atlas of region- and age-dependent proteome changes of the small intestine.

Roads to the Fountain of Youth? Rejuvenating Intestinal Stem Cells

The intestinal stem cells (ISCs) of old mice and humans exhibit a reduced capacity for regeneration and repair. Compromised intestinal function may play a key role in systemic aging-related changes: not only in the affected gut, but also in the nervous and cardiovascular systems. For example, progression of age-related neurodegenerative diseases such as Alzheimer's and Parkinson's has been linked to increased inflammation from gut microbiota in old mammals, which, in turn, may be linked bidirectionally with reduced ISC function. Intestinal organoid formation has been used to dissect the mechanisms of decline of ISC function. Alterations of the Wnt pathway, including downregulation of Wnt ligands in ISCs and upregulation of Wnt ligand inhibitor Notum in Paneth cells, and dysregulation of mTORC1 contribute to the observed age-related decline. Short-term fasting, caloric restriction, and peroxisome proliferator-activated receptor delta agonists have been reported to increase ISC function in adult mice. Moreover, the mTOR inhibitor rapamycin, NAD+ precursor nicotinamide riboside, and ABC99, a small molecule Notum inhibitor, have all been reported to rejuvenate ISC function in old mice and thus may have promise in humans. However, there is some controversy over the key mechanisms involved in loss of function of ISCs, which likely results, in part, from differences in how the in vitro organoid assays are performed. Moreover, how the microbiome modulates the function of ISCs and vice versa remains to be elucidated.

Vitamin D and the nutritional environment in functions of intestinal stem cells: Implications for tumorigenesis and prevention

Sporadic colon cancer accounts for ∼80% of CRC, with high incidence in western societies strongly linked to dietary patterns. The only mouse model for sporadic CRC results from feeding mice a purified rodent western-style diet (NWD1), establishing mouse intake of several common nutrients that mimic for each its level consumed in western populations at higher risk for colon cancer (higher fat, lower vitamin D3, calcium, methyl donors and fiber). This causes sporadic colon and small intestinal tumors at an incidence and frequency similar to that of humans. NWD1 perturbs intestinal cell maturation and Wnt signaling throughout villi and colonic crypts before tumors are detected. Surprisingly, feeding NWD1 decreases mouse Lgr5hi intestinal stem cell contribution to homeostasis and tumorigenesis, associated with extensive Lgr5hi cell transcriptional reprogramming, with nutrient levels interactive in these effects. There is a key impact of the lower vitamin D3 in NWD1 and its signaling through the Vdr. The DNA mismatch repair pathway is elevated in Lgr5hi cells by lower vitamin D3 and/or calcium in NWD1, reducing accumulation of relevant somatic mutations detected by single cell exome sequencing. There are also alterations in metabolic pathways, including down-regulation of oxidative phosphorylation. In compensation for compromise of Lgr5hi cells, NWD1 also reprograms cells derived from the Bmi1+ population, defined as those cells marked in Bmi1creERT2, Rosa26tom mice following tamoxifen injection, and at least a portion of these cells then function and persist as stem-like cells in mucosal homeostasis and tumorigenesis. The data establish a key role of the nutrient environment, and vitamin D signaling, in defining contribution of at least two different stem cell populations to mucosal homeostasis and tumorigenesis. This raises significant questions regarding impact of variable human diets on which and how multiple potential intestinal stem cell populations function in the human and give rise to tumors. Moreover, genetic and epigenetic changes in long-lived stem cells have important implications for understanding the effects of vitamin D and other nutrients on intestinal homeostasis and on intervention strategies for altering probability of tumor development.

From gut to glutes: The critical role of niche signals in the maintenance and renewal of adult stem cells

Stem cell behavior is tightly regulated by spatiotemporal signaling from the niche, which is a four-dimensional microenvironment that can instruct stem cells to remain quiescent, self-renew, proliferate, or differentiate. In this review, we discuss recent advances in understanding the signaling cues provided by the stem cell niche in two contrasting adult tissues, the rapidly cycling intestinal epithelium and the slowly renewing skeletal muscle. Drawing comparisons between these two systems, we discuss the effects of niche-derived growth factors and signaling molecules, metabolic cues, the extracellular matrix and biomechanical cues, and immune signals on stem cells. We also discuss the influence of the niche in defining stem cell identity and function in both normal and pathophysiologic states.

Autophagy Reprograms Alveolar Progenitor Cell Metabolism in Response to Lung Injury

Autophagy is a protective cellular mechanism in response to stress conditions. However, whether autophagy is required for maintenance of the alveolar epithelium is unknown. Here, we report that the loss of autophagy-related 5 (Atg5) in AT2 cells worsened bleomycin-induced lung injury. Mechanistically, during bleomycin injury, autophagy downregulated lipid metabolism but upregulated glucose metabolism in AT2 cells for alveolar repair. Chemical blockade of fatty acid synthesis promoted organoid growth of AT2 cells and counteracted the effects of autophagy loss on bleomycin injury. However, genetic loss of glucose transporter 1, interference with glycolysis, or interference with the pentose phosphate pathway reduced the proliferation of AT2 cells. Inhibition of glucose metabolism exacerbated the effects of bleomycin injury. Failure of autophagy generated additional hydrogen peroxide, which reduced AT2 cell proliferation. These data highlight an essential role for autophagy in reprogramming the metabolism of alveolar progenitor cells to meet energy needs for alveolar epithelial regeneration.

Metabolic Regulation of Cell Fate and Function

Increasing evidence implicates metabolic pathways as key regulators of cell fate and function. Although the metabolism of glucose, amino acids, and fatty acids is essential to maintain overall energy homeostasis, the choice of a given metabolic pathway and the levels of particular substrates and intermediates increasingly appear to modulate specific cellular activities. This connection is likely related to the growing appreciation that molecules such as acetyl-CoA act as a shared currency between metabolic flux and chromatin modification. We review recent evidence for a role of metabolism in modulating cellular function in four distinct contexts. These areas include the immune system, the tumor microenvironment, the fibrotic response, and stem cell function. Together, these examples suggest that metabolic pathways do not simply provide the fuel that powers cellular activities but instead help to shape and determine cellular identity.

HNF4 Regulates Fatty Acid Oxidation and Is Required for Renewal of Intestinal Stem Cells in Mice

BACKGROUND & AIMS

Functions of intestinal stem cells (ISCs) are regulated by diet and metabolic pathways. Hepatocyte nuclear factor 4 (HNF4) family are transcription factors that bind fatty acids. We investigated how HNF4 transcription factors regulate metabolism and their functions in ISCs in mice.

METHODS

We performed studies with Villin-Cre^ERT2;Lgr5-EGFP-IRES-Cre^ERT2;Hnf4α^f/f;Hnf4γ^{Crispr/Crispr} mice, hereafter referred to Hnf4αγ^DKO. Mice were given tamoxifen to induce Cre recombinase. Mice transgenic with only Cre alleles (Villin-Cre^ERT2, Lgr5-EGFP-IRES-Cre^ERT2, Hnf4α^+/+, and Hnf4γ^+/+) or mice given vehicle were used as controls. Crypt and villus cells were isolated, incubated with fluorescently labeled fatty acids or glucose analog, and analyzed by confocal microscopy. Fatty acid oxidation activity and tricarboxylic acid (TCA) cycle metabolites were measured in cells collected from the proximal half of the small intestine of Hnf4αγ^DKO and control mice. We performed chromatin immunoprecipitation and gene expression profiling analyses to identify genes regulated by HNF4 factors. We established organoids from duodenal crypts, incubated them with labeled palmitate or acetate, and measured production of TCA cycle metabolites or fatty acids. Acetate, a precursor of acetyl coenzyme A (CoA) (a product of fatty acid β-oxidation [FAO]), or dichloroacetate, a compound that promotes pyruvate oxidation and generation of mitochondrial acetyl-CoA, were used for metabolic intervention.

RESULTS

Crypt cells rapidly absorbed labeled fatty acids, and messenger RNA levels of Lgr5⁺ stem cell markers (Lgr5, Olfm4, Smoc2, Msi1, and Ascl2) were down-regulated in organoids incubated with etomoxir, an inhibitor of FAO, indicating that FAO was required for renewal of ISCs. HNF4A and HNF4G were expressed in ISCs and throughout the intestinal epithelium. Single knockout of either HNF4A or HNF4G did not affect maintenance of ISCs, but double-knockout of HNF4A and HNF4G resulted in ISC loss; stem cells failed to renew. FAO supports ISC renewal, and HNF4 transcription factors directly activate FAO genes, including Acsl5 and Acsf2 (encode regulators of acyl-CoA synthesis), Slc27a2 (encodes a fatty acid transporter), Fabp2 (encodes fatty acid binding protein), and Hadh (encodes hydroxyacyl-CoA dehydrogenase). In the intestinal epithelium of Hnf4αγ^DKO mice, expression levels of FAO genes, FAO activity, and metabolites of TCA cycle were all significantly decreased, but fatty acid synthesis transcripts were increased, compared with control mice. The contribution of labeled palmitate or acetate to the TCA cycle was reduced in organoids derived from Hnf4αγ^DKO mice, compared with control mice. Incubation of organoids derived from double-knockout mice with acetate or dichloroacetate restored stem cells.

CONCLUSIONS

In mice, the transcription factors HNF4A and HNF4G regulate the expression of genes required for FAO and are required for renewal of ISCs.