TERT activation targets DNA methylation and multiple aging hallmarks.

Insufficient telomerase activity, stemming from low telomerase reverse transcriptase (TERT) gene transcription, contributes to telomere dysfunction and aging pathologies. Besides its traditional function in telomere synthesis, TERT acts as a transcriptional co-regulator of genes pivotal in aging and age-associated diseases. Here, we report the identification of a TERT activator compound (TAC) that upregulates TERT transcription via the MEK/ERK/AP-1 cascade. In primary human cells and naturally aged mice, TAC-induced elevation of TERT levels promotes telomere synthesis, blunts tissue aging hallmarks with reduced cellular senescence and inflammatory cytokines, and silences p16INK4a expression via upregulation of DNMT3B-mediated promoter hypermethylation. In the brain, TAC alleviates neuroinflammation, increases neurotrophic factors, stimulates adult neurogenesis, and preserves cognitive function without evident toxicity, including cancer risk. Together, these findings underscore TERT's critical role in aging processes and provide preclinical proof of concept for physiological TERT activation as a strategy to mitigate multiple aging hallmarks and associated pathologies.

Isthmus progenitor cells contribute to homeostatic cellular turnover and support regeneration following intestinal injury.

The currently accepted intestinal epithelial cell organization model proposes that Lgr5+ crypt-base columnar (CBC) cells represent the sole intestinal stem cell (ISC) compartment. However, previous studies have indicated that Lgr5+ cells are dispensable for intestinal regeneration, leading to two major hypotheses: one favoring the presence of a quiescent reserve ISC and the other calling for differentiated cell plasticity. To investigate these possibilities, we studied crypt epithelial cells in an unbiased fashion via high-resolution single-cell profiling. These studies, combined with in vivo lineage tracing, show that Lgr5 is not a specific ISC marker and that stemness potential exists beyond the crypt base and resides in the isthmus region, where undifferentiated cells participate in intestinal homeostasis and regeneration following irradiation (IR) injury. Our results provide an alternative model of intestinal epithelial cell organization, suggesting that stemness potential is not restricted to CBC cells, and neither de-differentiation nor reserve ISC are drivers of intestinal regeneration.

Long-Term Expansion of Pancreatic Islet Organoids from Resident Procr+ Progenitors.

It has generally proven challenging to produce functional ß cells in vitro. Here, we describe a previously unidentified protein C receptor positive (Procr+) cell population in adult mouse pancreas through single-cell RNA sequencing (scRNA-seq). The cells reside in islets, do not express differentiation markers, and feature epithelial-to-mesenchymal transition characteristics. By genetic lineage tracing, Procr+ islet cells undergo clonal expansion and generate all four endocrine cell types during adult homeostasis. Sorted Procr+ cells, representing ∼1% of islet cells, can robustly form islet-like organoids when cultured at clonal density. Exponential expansion can be maintained over long periods by serial passaging, while differentiation can be induced at any time point in culture. ß cells dominate in differentiated islet organoids, while α, δ, and PP cells occur at lower frequencies. The organoids are glucose-responsive and insulin-secreting. Upon transplantation in diabetic mice, these organoids reverse disease. These findings demonstrate that the adult mouse pancreatic islet contains a population of Procr+ endocrine progenitors.

The Source and Dynamics of Adult Hematopoiesis: Insights from Lineage Tracing.

The generation of all blood cell lineages (hematopoiesis) is sustained throughout the entire life span of adult mammals. Studies using cell transplantation identified the self-renewing, multipotent hematopoietic stem cells (HSCs) as the source of hematopoiesis in adoptive hosts and delineated a hierarchy of HSC-derived progenitors that ultimately yield mature blood cells. However, much less is known about adult hematopoiesis as it occurs in native hosts, i.e., without transplantation. Here we review recent advances in our understanding of native hematopoiesis, focusing in particular on the application of genetic lineage tracing in mice. The emerging evidence has established HSCs as the major source of native hematopoiesis, helped to define the kinetics of HSC differentiation, and begun exploring native hematopoiesis in stress conditions such as aging and inflammation. Major outstanding questions about native hematopoiesis still remain, such as its clonal composition, the nature of lineage commitment, and the dynamics of the process in humans.

Defining Adult Stem Cells by Function, not by Phenotype.

Central to the classical hematopoietic stem cell (HSC) paradigm is the concept that the maintenance of blood cell numbers is exclusively executed by a discrete physical entity: the transplantable HSC. The HSC paradigm has served as a stereotypic template in stem cell biology, yet the search for rare, hardwired professional stem cells has remained futile in most other tissues. In a more open approach, the focus on the search for stem cells as a physical entity may need to be replaced by the search for stem cell function, operationally defined as the ability of an organ to replace lost cells. The nature of such a cell may be different under steady state conditions and during tissue repair. We discuss emerging examples including the renewal strategies of the skin, gut epithelium, liver, lung, and mammary gland in comparison with those of the hematopoietic system. While certain key housekeeping and developmental signaling pathways are shared between different stem cell systems, there may be no general, deeper principles underlying the renewal mechanisms of the various individual tissues.

Specialized Intercellular Communications via Cytonemes and Nanotubes.

In recent years, thin membrane protrusions such as cytonemes and tunneling nanotubes have emerged as a novel mechanism of intercellular communication. Protrusion-based cellular interactions allow for specific communication between participating cells and have a distinct spectrum of advantages compared to secretion- and diffusion-based intercellular communication. Identification of protrusion-based signaling in diverse systems suggests that this mechanism is a ubiquitous and prevailing means of communication employed by many cell types. Moreover, accumulating evidence indicates that protrusion-based intercellular communication is often involved in pathogenesis, including cancers and infections. Here we review our current understanding of protrusion-based intercellular communication.

Cell type-specific role of CBX2 and its disordered region in spermatogenesis.

Polycomb group (PcG) proteins maintain the repressed state of lineage-inappropriate genes and are therefore essential for embryonic development and adult tissue homeostasis. One critical function of PcG complexes is modulating chromatin structure. Canonical Polycomb repressive complex 1 (cPRC1), particularly its component CBX2, can compact chromatin and phase-separate in vitro. These activities are hypothesized to be critical for forming a repressed physical environment in cells. While much has been learned by studying these PcG activities in cell culture models, it is largely unexplored how cPRC1 regulates adult stem cells and their subsequent differentiation in living animals. Here, we show in vivo evidence of a critical nonenzymatic repressive function of cPRC1 component CBX2 in the male germline. CBX2 is up-regulated as spermatogonial stem cells differentiate and is required to repress genes that were active in stem cells. CBX2 forms condensates (similar to previously described Polycomb bodies) that colocalize with target genes bound by CBX2 in differentiating spermatogonia. Single-cell analyses of mosaic Cbx2 mutant testes show that CBX2 is specifically required to produce differentiating A1 spermatogonia. Furthermore, the region of CBX2 responsible for compaction and phase separation is needed for the long-term maintenance of male germ cells in the animal. These results emphasize that the regulation of chromatin structure by CBX2 at a specific stage of spermatogenesis is critical, which distinguishes this from a mechanism that is reliant on histone modification.

Neurogenesis and Gliogenesis in Nervous System Plasticity and Repair.

The brain constantly changes to store memories and adapt to new conditions. One type of plasticity that has gained increasing interest during the last years is the generation of new cells. The generation of both new neurons and glial cells contributes to neural plasticity and to some neural repair. There are substantial differences between mammalian species with regard to the extent of and mechanisms behind cell exchange in neural plasticity. Both neurogenesis and gliogenesis have several specific features in humans, which may contribute to the unique plasticity of the human brain.

Histone methyltransferase DOT1L is essential for self-renewal of germline stem cells.

Self-renewal of spermatogonial stem cells is vital to lifelong production of male gametes and thus fertility. However, the underlying mechanisms remain enigmatic. Here, we show that DOT1L, the sole H3K79 methyltransferase, is required for spermatogonial stem cell self-renewal. Mice lacking DOT1L fail to maintain spermatogonial stem cells, characterized by a sequential loss of germ cells from spermatogonia to spermatids and ultimately a Sertoli cell only syndrome. Inhibition of DOT1L reduces the stem cell activity after transplantation. DOT1L promotes expression of the fate-determining HoxC transcription factors in spermatogonial stem cells. Furthermore, H3K79me2 accumulates at HoxC9 and HoxC10 genes. Our findings identify an essential function for DOT1L in adult stem cells and provide an epigenetic paradigm for regulation of spermatogonial stem cells.

Functional rejuvenation of aged neural stem cells by Plagl2 and anti-Dyrk1a activity.

The regenerative potential of neural stem cells (NSCs) declines during aging, leading to cognitive dysfunctions. This decline involves up-regulation of senescence-associated genes, but inactivation of such genes failed to reverse aging of hippocampal NSCs. Because many genes are up-regulated or down-regulated during aging, manipulation of single genes would be insufficient to reverse aging. Here we searched for a gene combination that can rejuvenate NSCs in the aged mouse brain from nuclear factors differentially expressed between embryonic and adult NSCs and their modulators. We found that a combination of inducing the zinc finger transcription factor gene Plagl2 and inhibiting Dyrk1a, a gene associated with Down syndrome (a genetic disorder known to accelerate aging), rejuvenated aged hippocampal NSCs, which already lost proliferative and neurogenic potential. Such rejuvenated NSCs proliferated and produced new neurons continuously at the level observed in juvenile hippocampi, leading to improved cognition. Epigenome, transcriptome, and live-imaging analyses indicated that this gene combination induces up-regulation of embryo-associated genes and down-regulation of age-associated genes by changing their chromatin accessibility, thereby rejuvenating aged dormant NSCs to function like juvenile active NSCs. Thus, aging of NSCs can be reversed to induce functional neurogenesis continuously, offering a way to treat age-related neurological disorders.

Brain Repair by Cell Replacement via In Situ Neuronal Reprogramming.

Neurodegenerative diseases, characterized by progressive neural loss, have been some of the most challenging medical problems in aging societies. Treatment strategies such as symptom management have little impact on disease progression, while intervention with specific disease mechanisms may only slow down disease progression. One therapeutic strategy that has the potential to reverse the disease phenotype is to replenish neurons and rebuild the pathway lost to degeneration. Although it is generally believed that the central nervous system has lost the capability to regenerate, increasing evidence indicates that the brain is more plastic than previously thought, containing perhaps the biggest repertoire of cells with latent neurogenic programs in the body. This review focuses on key advances in generating new neurons through in situ neuronal reprogramming, which is tied to fundamental questions regarding adult neurogenesis, cell source, and mechanisms for neuronal reprogramming, as well as the ability of new neurons to integrate into the existing circuitry.

Generation of adult hippocampal neural stem cells occurs in the early postnatal dentate gyrus and depends on cyclin D2.

Lifelong hippocampal neurogenesis is maintained by a pool of multipotent adult neural stem cells (aNSCs) residing in the subgranular zone of the dentate gyrus (DG). The mechanisms guiding transition of NSCs from the developmental to the adult state remain unclear. We show here, by using nestin-based reporter mice deficient for cyclin D2, that the aNSC pool is established through cyclin D2-dependent proliferation during the first two weeks of life. The absence of cyclin D2 does not affect normal development of the dentate gyrus until birth but prevents postnatal formation of radial glia-like aNSCs. Furthermore, retroviral fate mapping reveals that aNSCs are born on-site from precursors located in the dentate gyrus shortly after birth. Taken together, our data identify the critical time window and the spatial location of the precursor divisions that generate the persistent population of aNSCs and demonstrate the central role of cyclin D2 in this process.

Sexual health among adolescent and young adult cancer survivors: A scoping review from the Children's Oncology Group Adolescent and Young Adult Oncology Discipline Committee.

Sexual function is a vital aspect of quality of life among adolescent and young adult (AYA) (ages 15-39 years) cancer survivors. Sexual function encompasses physical, psychosocial, and developmental factors that contribute to sexual health, all of which may be negatively impacted by cancer and treatment. However, limited information is available to inform the care of AYA cancer survivors in this regard. This scoping review, conducted by the Children's Oncology Group AYA Oncology Discipline Committee, summarizes available literature regarding sexual function among AYA cancer survivors, including relevant psychosexual aspects of romantic relationships and body image. Results suggest that, overall, AYA cancer survivors experience a substantial burden of sexual dysfunction. Both physical and psychosocial sequelae influence survivors' sexual health. Interventions to support sexual health and psychosexual adjustment after cancer treatment are needed. Collaborations between the Children's Oncology Group and adult-focused cooperative groups within the National Cancer Institute's National Clinical Trials Network are warranted to advance prospective assessment of sexual dysfunction and test interventions to improve sexual health among AYA cancer survivors.

Sharpening the blades of the dentate gyrus: how adult-born neurons differentially modulate diverse aspects of hippocampal learning and memory.

For decades, the mammalian hippocampus has been the focus of cellular, anatomical, behavioral, and computational studies aimed at understanding the fundamental mechanisms underlying cognition. Long recognized as the brain's seat for learning and memory, a wealth of knowledge has been accumulated on how the hippocampus processes sensory input, builds complex associations between objects, events, and space, and stores this information in the form of memories to be retrieved later in life. However, despite major efforts, our understanding of hippocampal cognitive function remains fragmentary, and models trying to explain it are continually revisited. Here, we review the literature across all above-mentioned domains and offer a new perspective by bringing attention to the most distinctive, and generally neglected, feature of the mammalian hippocampal formation, namely, the structural separability of the two blades of the dentate gyrus into "supra-pyramidal" and "infra-pyramidal". Next, we discuss recent reports supporting differential effects of adult neurogenesis in the regulation of mature granule cell activity in these two blades. We propose a model for how differences in connectivity and adult neurogenesis in the two blades can potentially provide a substrate for subtly different cognitive functions.

L-arginine homeostasis governs adult neural stem cell activation by modulating energy metabolism in vivo.

Neurogenesis in the developing and adult brain is intimately linked to remodeling of cellular metabolism. However, it is still unclear how distinct metabolic programs and energy sources govern neural stem cell (NSC) behavior and subsequent neuronal differentiation. Here, we found that adult mice lacking the mitochondrial urea metabolism enzyme, Arginase-II (Arg-II), exhibited NSC overactivation, thereby leading to accelerated NSC pool depletion and decreased hippocampal neurogenesis over time. Mechanistically, Arg-II deficiency resulted in elevated L-arginine levels and induction of a metabolic shift from glycolysis to oxidative phosphorylation (OXPHOS) caused by impaired attachment of hexokinase-I to mitochondria. Notably, selective inhibition of OXPHOS ameliorated NSC overactivation and restored abnormal neurogenesis in Arg-II deficient mice. Therefore, Arg-II-mediated intracellular L-arginine homeostasis directly influences the metabolic fitness of neural stem cells that is essential to maintain neurogenesis with age.

Fluctuation of lysosomal protein degradation in neural stem cells of the postnatal mouse brain.

Lysosomes are intracellular organelles responsible for degrading diverse macromolecules delivered from several pathways, including the endo-lysosomal and autophagic pathways. Recent reports have suggested that lysosomes are essential for regulating neural stem cells in developing, adult and aged brains. However, the activity of these lysosomes has yet to be monitored in these brain tissues. Here, we report the development of a new probe to measure lysosomal protein degradation in brain tissue by immunostaining. Our results indicate that lysosomal protein degradation fluctuates in neural stem cells of the hippocampal dentate gyrus, depending on age and brain disorders. Neural stem cells increase their lysosomal activity during hippocampal development in the dentate gyrus, but aging and aging-related disease reduce lysosomal activity. In addition, physical exercise increases lysosomal activity in neural stem cells and astrocytes in the dentate gyrus. We therefore propose that three different stages of lysosomal activity exist: the state of increase during development, the stable state during adulthood and the state of reduction due to damage caused by either age or disease.

Stimulation of the muscarinic receptor M4 regulates neural precursor cell proliferation and promotes adult hippocampal neurogenesis.

Cholinergic signaling plays a crucial role in the regulation of adult hippocampal neurogenesis; however, the mechanisms by which acetylcholine mediates neurogenic effects are not completely understood. Here, we report the expression of muscarinic acetylcholine receptor subtype M4 (M4 mAChR) on a subpopulation of neural precursor cells (NPCs) in the adult mouse hippocampus, and demonstrate that its pharmacological stimulation promotes their proliferation, thereby enhancing the production of new neurons in vivo. Using a targeted ablation approach, we also show that medial septum (MS) and the diagonal band of Broca (DBB) cholinergic neurons support both the survival and morphological maturation of adult-born neurons in the mouse hippocampus. Although the systemic administration of an M4-selective allosteric potentiator fails to fully rescue the MS/DBB cholinergic lesion-induced decrease in hippocampal neurogenesis, it further exacerbates the impairment in the morphological maturation of adult-born neurons. Collectively, these findings reveal stage-specific roles of M4 mAChRs in regulating adult hippocampal neurogenesis, uncoupling their positive role in enhancing the production of new neurons from the M4-induced inhibition of their morphological maturation, at least in the context of cholinergic signaling dysfunction.

Adult neurogenesis is necessary for functional regeneration of a forebrain neural circuit.

In adult songbirds, new neurons are born in large numbers in the proliferative ventricular zone in the telencephalon and migrate to the adjacent song control region HVC (acronym used as proper name) [A. Reiner et al., J. Comp. Neurol. 473, 377-414 (2004)]. Many of these new neurons send long axonal projections to the robust nucleus of the arcopallium (RA). The HVC-RA circuit is essential for producing stereotyped learned song. The function of adult neurogenesis in this circuit has not been clear. A previous study suggested that it is important for the production of well-structured songs [R. E. Cohen, M. Macedo-Lima, K. E. Miller, E. A. Brenowitz, J. Neurosci. 36, 8947-8956 (2016)]. We tested this hypothesis by infusing the neuroblast migration inhibitor cyclopamine into HVC of male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) to block seasonal regeneration of the HVC-RA circuit. Decreasing the number of new neurons in HVC prevented both the increase in spontaneous electrical activity of RA neurons and the improved structure of songs that would normally occur as sparrows enter breeding condition. These results show that the incorporation of new neurons into the adult HVC is necessary for the recovery of both electrical activity and song behavior in breeding birds and demonstrate the value of the bird song system as a model for investigating adult neurogenesis at the level of long projection neural circuits.

The choroid plexus maintains adult brain ventricles and subventricular zone neuroblast pool, which facilitates poststroke neurogenesis.

The brain's neuroreparative capacity after injuries such as ischemic stroke is partly contained in the brain's neurogenic niches, primarily the subventricular zone (SVZ), which lies in close contact with the cerebrospinal fluid (CSF) produced by the choroid plexus (ChP). Despite the wide range of their proposed functions, the ChP/CSF remain among the most understudied compartments of the central nervous system (CNS). Here, we report a mouse genetic tool (the ROSA26iDTR mouse line) for noninvasive, specific, and temporally controllable ablation of CSF-producing ChP epithelial cells to assess the roles of the ChP and CSF in brain homeostasis and injury. Using this model, we demonstrate that ChP ablation causes rapid and permanent CSF volume loss in both aged and young adult brains, accompanied by disruption of ependymal cilia bundles. Surprisingly, ChP ablation did not result in overt neurological deficits at 1 mo postablation. However, we observed a pronounced decrease in the pool of SVZ neuroblasts (NBs) following ChP ablation, which occurs due to their enhanced migration into the olfactory bulb. In the middle cerebral artery occlusion model of ischemic stroke, NB migration into the lesion site was also reduced in the CSF-depleted mice. Thus, our study establishes an important role of ChP/CSF in regulating the regenerative capacity of the adult brain under normal conditions and after ischemic stroke.

The evolution of sex roles: The importance of ecology and social environment.

Males and females often have different roles in reproduction, although the origin of these differences has remained controversial. Explaining the enigmatic reversed sex roles where males sacrifice their mating potential and provide full parental care is a particularly long-standing challenge in evolutionary biology. While most studies focused on ecological factors as the drivers of sex roles, recent research highlights the significance of social factors such as the adult sex ratio. To disentangle these propositions, here, we investigate the additive and interactive effects of several ecological and social factors on sex role variation using shorebirds (sandpipers, plovers, and allies) as model organisms that provide the full spectrum of sex role variation including some of the best-known examples of sex-role reversal. Our results consistently show that social factors play a prominent role in driving sex roles. Importantly, we show that reversed sex roles are associated with both male-skewed adult sex ratios and high breeding densities. Furthermore, phylogenetic path analyses provide general support for sex ratios driving sex role variations rather than being a consequence of sex roles. Together, these important results open future research directions by showing that different mating opportunities of males and females play a major role in generating the evolutionary diversity of sex roles, mating system, and parental care.