Essential role of proteasomes in maintaining self-renewal in neural progenitor cells

Metabolic and proteostatic differences in quiescent and active neural stem cells

Adult neural stem cells are neurogenesis progenitor cells that play an important role in neurogenesis. Therefore, neural regeneration may be a promising target for treatment of many neurological illnesses. The regenerative capacity of adult neural stem cells can be characterized by two states: quiescent and active. Quiescent adult neural stem cells are more stable and guarantee the quantity and quality of the adult neural stem cell pool. Active adult neural stem cells are characterized by rapid proliferation and differentiation into neurons which allow for integration into neural circuits. This review focuses on differences between quiescent and active adult neural stem cells in nutrition metabolism and protein homeostasis. Furthermore, we discuss the physiological significance and underlying advantages of these differences. Due to the limited number of adult neural stem cells studies, we referred to studies of embryonic adult neural stem cells or non-mammalian adult neural stem cells to evaluate specific mechanisms.

Molecular basis of retinal remodeling in a zebrafish model of retinitis pigmentosa

A hallmark of inherited retinal degenerative diseases such as retinitis pigmentosa (RP) is progressive structural and functional remodeling of the remaining retinal cells as photoreceptors degenerate. Extensive remodeling of the retina stands as a barrier for the successful implementation of strategies to restore vision. To understand the molecular basis of remodeling, we performed analyses of single-cell transcriptome data from adult zebrafish retina of wild type AB strain (WT) and a P23H mutant rhodopsin transgenic model of RP with continuous degeneration and regeneration. Retinas from both female and male fish were pooled to generate each library, combining data from both sexes. We provide a benchmark atlas of retinal cell type transcriptomes in zebrafish and insight into how each retinal cell type is affected in the P23H model. Oxidative stress is found throughout the retina, with increases in reliance on oxidative metabolism and glycolysis in the affected rods as well as cones, bipolar cells, and retinal ganglion cells. There is also transcriptional evidence for widespread synaptic remodeling and enhancement of glutamatergic transmission in the inner retina. Notably, changes in circadian rhythm regulation are detected in cones, bipolar cells, and retinal pigmented epithelium. We also identify the transcriptomic signatures of retinal progenitor cells and newly formed rods essential for the regenerative process. This comprehensive transcriptomic analysis provides a molecular road map to understand how the retina remodels in the context of chronic retinal degeneration with ongoing regeneration.

Adipose transplantation improves olfactory function and neurogenesis via PKCα-involved lipid metabolism in Seipin Knockout mice

BACKGROUND

Lipodystrophy-associated metabolic disorders caused by Seipin deficiency lead to not only severe lipodystrophy but also neurological disorders. However, the underlying mechanism of Seipin deficiency-induced neuropathy is not well elucidated, and the possible restorative strategy needs to be explored.

METHODS

In the present study, we used Seipin knockout (KO) mice, combined with transcriptome analysis, mass spectrometry imaging, neurobehavior test, and cellular and molecular assay to investigate the systemic lipid metabolic abnormalities in lipodystrophic mice model and their effects on adult neurogenesis in the subventricular zone (SVZ) and olfactory function. After subcutaneous adipose tissue (AT) transplantation, metabolic and neurological function was measured in Seipin KO mice to clarify whether restoring lipid metabolic homeostasis would improve neurobehavior.

RESULTS

It was found that Seipin KO mice presented the ectopic accumulation of lipids in the lateral ventricle, accompanied by decreased neurogenesis in adult SVZ, diminished new neuron formation in the olfactory bulb, and impaired olfactory-related memory. Transcriptome analysis showed that the differentially expressed genes (DEGs) in SVZ of adult Seipin KO mice were significantly enriched in lipid metabolism. Mass spectrometry imaging showed that the levels of glycerophospholipid and diglyceride (DG) were significantly increased. Furthermore, we found that AT transplantation rescued the abnormality of peripheral metabolism in Seipin KO mice and ameliorated the ectopic lipid accumulation, concomitant with restoration of the SVZ neurogenesis and olfactory function. Mechanistically, PKCα expression was up-regulated in SVZ tissues of Seipin KO mice, which may be a potential mediator between lipid dysregulation and neurological disorder. DG analogue (Dic8) can up-regulate PKCα and inhibit the proliferation and differentiation of neural stem cells (NSCs) in vitro, while PKCα inhibitor can block this effect.

CONCLUSION

This study demonstrates that Seipin deficiency can lead to systemic lipid disorder with concomitant SVZ neurogenesis and impaired olfactory memory. However, AT restores lipid homeostasis and neurogenesis. PKCα is a key mediator mediating Seipin KO-induced abnormal lipid metabolism and impaired neurogenesis in the SVZ, and inhibition of PKCα can restore the impaired neurogenesis. This work reveals the underlying mechanism of Seipin deficiency-induced neurological dysfunction and provides new ideas for the treatment of neurological dysfunction caused by metabolic disorders.

Analysis of Brain Protein Stability Changes in Mouse Models of Normal Aging and α-Synucleinopathy Reveals Age- and Disease-Related Differences

Here, we utilize the stability of proteins from rates of oxidation (SPROX) technique, to profile the thermodynamic stabilities of proteins in brain tissue cell lysates from Huα-Syn(A53T) transgenic mice at three time points including at 1 month (n = 9), at 6 months (n = 7), and at the time (between 9 and 16 months) a mouse became symptomatic (n = 8). The thermodynamic stability profiles generated here on 332 proteins were compared to thermodynamic stability profiles generated on the same proteins from similarly aged wild-type mice using a two-way unbalanced analysis of variance (ANOVA) analysis. This analysis identified a group of 22 proteins with age-related protein stability changes and a group of 11 proteins that were differentially stabilized in the Huα-Syn(A53T) transgenic mouse model. A total of 9 of the 11 proteins identified here with disease-related stability changes have been previously detected in human cerebral spinal fluid and thus have potential utility as biomarkers of Parkinson's disease (PD). The differential stability observed for one protein, glutamate decarboxylase 2 (Gad2), with an age-related change in stability, was consistent with the differential presence of a known, age-related truncation product of this protein, which is shown here to have a higher folding stability than full-length Gad2. Mass spectrometry data were deposited at ProteomeXchange (PXD016985).

Changing and stable chromatin accessibility supports transcriptional overhaul during neural stem cell activation and is altered with age

Neural stem cells (NSCs) in the adult and aged brain are largely quiescent, and require transcriptional reprogramming to re-enter the cell cycle. However, the mechanisms underlying these changes and how they are altered with age remain undefined. Here, we identify the chromatin accessibility differences between primary neural stem/progenitor cells in quiescent and activated states. These distinct cellular states exhibit shared and unique chromatin profiles, both associated with gene regulation. Accessible chromatin states specific to activation or quiescence are active enhancers bound by key pro-neurogenic and quiescence factors. In contrast, shared sites are enriched for core promoter elements associated with translation and metabolism. Unexpectedly, through integrated analysis, we find that many sites that become accessible during NSC activation are linked to gene repression and associated with pro-quiescence factors, revealing a novel mechanism that may preserve quiescence re-entry. Furthermore, we report that in aged NSCs, chromatin regions associated with metabolic and transcriptional functions bound by key pro-quiescence transcription factors lose accessibility, suggesting a novel mechanism of age-associated NSC dysfunction. Together, our findings reveal how accessible chromatin states regulate the transcriptional switch between NSC quiescence and activation, and how this switch is affected with age.

Coordinated regulation of the ribosome and proteasome by PRMT1 in the maintenance of neural stemness in cancer cells and neural stem cells

Previous studies suggested that cancer cells resemble neural stem/progenitor cells in regulatory network, tumorigenicity, and differentiation potential, and that neural stemness might represent the ground or basal state of differentiation and tumorigenicity. The neural ground state is reflected in the upregulation and enrichment of basic cell machineries and developmental programs, such as cell cycle, ribosomes, proteasomes, and epigenetic factors, in cancers and in embryonic neural or neural stem cells. However, how these machineries are concertedly regulated is unclear. Here, we show that loss of neural stemness in cancer or neural stem cells via muscle-like differentiation or neuronal differentiation, respectively, caused downregulation of ribosome and proteasome components and major epigenetic factors, including PRMT1, EZH2, and LSD1. Furthermore, inhibition of PRMT1, an oncoprotein that is enriched in neural cells during embryogenesis, caused neuronal-like differentiation, downregulation of a similar set of proteins downregulated by differentiation, and alteration of subcellular distribution of ribosome and proteasome components. By contrast, PRMT1 overexpression led to an upregulation of these proteins. PRMT1 interacted with these components and protected them from degradation via recruitment of the deubiquitinase USP7, also known to promote cancer and enriched in embryonic neural cells, thereby maintaining a high level of epigenetic factors that maintain neural stemness, such as EZH2 and LSD1. Taken together, our data indicate that PRMT1 inhibition resulted in repression of cell tumorigenicity. We conclude that PRMT1 coordinates ribosome and proteasome activity to match the needs for high production and homeostasis of proteins that maintain stemness in cancer and neural stem cells.

Activation of Autophagy Ameliorates Age-Related Neurogenesis Decline and Neurodysfunction in Adult Mice

Adult neurogenesis is the ongoing generation of functional new neurons from neural progenitor cells (NPCs) in the mammalian brain. However, this process declines with aging, which is implicated in the recession of brain function and neurodegeneration. Understanding the mechanism of adult neurogenesis and stimulating neurogenesis will benefit the mitigation of neurodegenerative diseases. Autophagy, a highly conserved process of cellular degradation, is essential for maintaining cellular homeostasis and normal function. Whether and how autophagy affects adult neurogenesis remains poorly understood. In present study, we revealed a close connection between impaired autophagy and adult neurogenetic decline. Expression of autophagy-related genes and autophagic activity were significantly declined in the middle-adult subventricular/subgranular zone (SVZ/SGZ) homogenates and cultured NPCs, and inhibiting autophagy by siRNA interference resulted in impaired proliferation and differentiation of NPCs. Conversely, stimulating autophagy by rapamycin not only revitalized the viability of middle-adult NPCs, but also facilitated the neurogenesis in middle-adult SVZ/SGZ. More importantly, autophagic activation by rapamycin also ameliorated the olfactory sensitivity and cognitional capacities in middle-adult mice. Taken together, our results reveal that compromised autophagy is involved in the decline of adult neurogenesis, which could be reversed by autophagy activation. It also shed light on the regulation of adult neurogenesis and paves the way for developing a therapeutic strategy for aging and neurodegenerative diseases.

The impact of physical exercise on hippocampus, in physiological condition and ageing-related decline: current evidence from animal and human studies

Physical exercise (PE), notoriously, promotes a state of general well-being, throughout the entire human lifespan. Moreover, maintaining an adequate and regular PE habit results to be a powerful preventive factor towards many diseases and may also help in managing existing pathological conditions. PE induces structural and functional changes in various districts of the body, determining biological and psychological benefits. Additionally, in elderly, PE might represent a remarkable tool reducing cognitive impairments related to the normal aging processes and it has also been found to have an impact in neurodegenerative diseases such as Alzheimer's disease. The present review aims to provide an overview about PE effects on hippocampus, since it is one of the brain regions most susceptible to aging and, therefore, involved in diseases characterized by cognitive impairment.

An NRF2 Perspective on Stem Cells and Ageing

Redox and metabolic mechanisms lie at the heart of stem cell survival and regenerative activity. NRF2 is a major transcriptional controller of cellular redox and metabolic homeostasis, which has also been implicated in ageing and lifespan regulation. However, NRF2's role in stem cells and their functioning with age is only just emerging. Here, focusing mainly on neural stem cells, which are core to adult brain plasticity and function, we review recent findings that identify NRF2 as a fundamental player in stem cell biology and ageing. We also discuss NRF2-based molecular programs that may govern stem cell state and function with age, and implications of this for age-related pathologies.

Proteasome Inhibitor MG132 is Toxic and Inhibits the Proliferation of Rat Neural Stem Cells but Increases BDNF Expression to Protect Neurons

Regulation of protein expression is essential for maintaining normal cell function. Proteasomes play important roles in protein degradation and dysregulation of proteasomes is implicated in neurodegenerative disorders. In this study, using a proteasome inhibitor MG132, we showed that proteasome inhibition reduces neural stem cell (NSC) proliferation and is toxic to NSCs. Interestingly, MG132 treatment increased the percentage of neurons in both proliferation and differentiation culture conditions of NSCs. Proteasome inhibition reduced B-cell lymphoma 2 (Bcl-2)/Bcl-2 associated X protein ratio. In addition, MG132 treatment induced cAMP response element-binding protein phosphorylation and increased the expression of brain-derived neurotrophic factor transcripts and proteins. These data suggest that proteasome function is important for NSC survival and differentiation. Moreover, although MG132 is toxic to NSCs, it may increase neurogenesis. Therefore, by modifying MG132 chemical structure and developing none toxic proteasome inhibitors, neurogenic chemicals can be developed to control NSC cell fate.

Protein Aggregation Triggered by Rewired Protein Homeostasis during Neuronal Differentiation

In this issue of Molecular Cell, Vonk et al. (2020) and Thiruvalluvan et al. (2020) identify key chaperones that confer resistance to protein aggregation in neural stem cells and become reduced upon differentiation.

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.

Differentiation Drives Widespread Rewiring of the Neural Stem Cell Chaperone Network

Neural stem and progenitor cells (NSPCs) are critical for continued cellular replacement in the adult brain. Lifelong maintenance of a functional NSPC pool necessitates stringent mechanisms to preserve a pristine proteome. We find that the NSPC chaperone network robustly maintains misfolded protein solubility and stress resilience through high levels of the ATP-dependent chaperonin TRiC/CCT. Strikingly, NSPC differentiation rewires the cellular chaperone network, reducing TRiC/CCT levels and inducing those of the ATP-independent small heat shock proteins (sHSPs). This switches the proteostasis strategy in neural progeny cells to promote sequestration of misfolded proteins into protective inclusions. The chaperone network of NSPCs is more effective than that of differentiated cells, leading to improved management of proteotoxic stress and amyloidogenic proteins. However, NSPC proteostasis is impaired by brain aging. The less efficient chaperone network of differentiated neural progeny may contribute to their enhanced susceptibility to neurodegenerative diseases characterized by aberrant protein misfolding and aggregation.

Post-Transcriptional Regulation of Homeostatic, Stressed, and Malignant Stem Cells

Cellular identity is not driven by differences in genomic content but rather by epigenomic, transcriptomic, and proteomic heterogeneity. Although regulation of the epigenome plays a key role in shaping stem cell hierarchies, differential expression of transcripts only partially explains protein abundance. The epitranscriptome, translational control, and protein degradation have emerged as fundamental regulators of proteome complexity that regulate stem cell identity and function. Here, we discuss how post-transcriptional mechanisms enable stem cell homeostasis and responsiveness to developmental cues and environmental stressors by rapidly shaping the content of their proteome and how these processes are disrupted in pre-malignant and malignant states.

Proteasome activation by insulin-like growth factor-1/nuclear factor erythroid 2-related factor 2 signaling promotes exercise-induced neurogenesis

Physical exercise-induced enhancement of learning and memory and alleviation of age-related cognitive decline in humans have been widely acknowledged. However, the mechanistic relationship between exercise and cognitive improvement remains largely unknown. In this study, we found that exercise-elicited cognitive benefits were accompanied by adaptive hippocampal proteasome activation. Voluntary wheel running increased hippocampal proteasome activity in adult and middle-aged mice, contributing to an acceleration of neurogenesis that could be reversed by intrahippocampal injection of the proteasome inhibitor MG132. We further found that increased levels of insulin-like growth factor-1 (IGF-1) in both serum and hippocampus may be essential for exercise-induced proteasome activation. Our in vitro study demonstrated that IGF-1 stimulated proteasome activity in cultured adult neural progenitor cells (NPCs) by promoting nuclear translocation of nuclear factor erythroid 2-related factor 2 (Nrf2), followed by elevated expressions of proteasome subunits such as PSMB5. In contrast, pretreating adult mice with the selective IGF-1R inhibitor picropodophyllin diminished exercise-induced neurogenesis, concurrent with reduced Nrf2 nuclear translocation and proteasome activity. Likewise, lowering Nrf2 expression by RNA interference with bilateral intrahippocampal injections of recombinant adeno-associated viral particles significantly suppressed exercise-induced proteasome activation and attenuated cognitive function. Collectively, our work demonstrates that proteasome activation in hippocampus through IGF-1/Nrf2 signaling is a key adaptive mechanism underlying exercise-related neurogenesis, which may serve as a potential targetable pathway in neurodegeneration.

Primary cilium and brain aging: role in neural stem cells, neurodegenerative diseases and glioblastoma

Brain aging is characterized by a progressive loss of tissue integrity and function as a consequence of impaired homeostasis and regeneration capacities. The primary cilium is a highly conserved organelle that projects from the cell surface in a single copy in virtually all mammalian cell types including neural stem/progenitors cells and neurons. Increasing evidence in the last decade points out that primary cilium could be a relevant mediator of neural stem cell activity, neurogenesis, neuronal maturation and maintenance, and brain tumorigenesis. In this review, we summarize the current knowledge about primary cilia roles in these processes. There is currently sufficient background to propose that defective primary cilia contribute to age-related cognitive decline and brain tumor development due to their critical roles in cell cycle control and signaling transduction. This might have potential applications on therapy against age-associated brain diseases.

Heterogeneity of Human Breast Stem and Progenitor Cells as Revealed by Transcriptional Profiling

During development, the mammary gland undergoes extensive remodeling driven by stem cells. Breast cancers are also hierarchically organized and driven by cancer stem cells characterized by CD44⁺CD24^low/− or aldehyde dehydrogenase (ALDH) expression. These markers identify mesenchymal and epithelial populations both capable of tumor initiation. Less is known about these populations in non-cancerous mammary glands. From RNA sequencing, ALDH⁺ and ALDH⁻CD44⁺CD24⁻ human mammary cells have epithelial-like and mesenchymal-like characteristics, respectively, with some co-expressing ALDH⁺ and CD44⁺CD24⁻ by flow cytometry. At the single-cell level, these cells have the greatest mammosphere-forming capacity and express high levels of stemness and epithelial-to-mesenchymal transition-associated genes including ID1, SOX2, TWIST1, and ZEB2. We further identify single ALDH⁺ cells with a hybrid epithelial/mesenchymal phenotype that express genes associated with aggressive triple-negative breast cancers. These results highlight single-cell analyses to characterize tissue heterogeneity, even in marker-enriched populations, and identify genes and pathways that define this heterogeneity.

•

Isolation and RNA-seq of ALDH⁺ and CD44⁺CD24⁻ breast cells

•

Unlike in cancer, there is substantial overlap in ALDH⁺ and CD44⁺CD24⁻ populations

•

Single-cell analysis of ALDH⁺ cells identifies unexpected subpopulation structure

•

Hybrid epithelial/mesenchymal ALDH⁺ cells have a cancer-like expression signature

Cancer stem cells don't waste their time cleaning-low proteasome activity, a marker for cancer stem cell function

A population of stem-like cells in tumors, the so-called cancer stem cells (CSCs), are being held responsible for therapy resistance and tumor recurrence. In analogy with normal stem cells, CSCs possess the capacity of long term self-renewal and multilineage differentiation. CSCs are believed to be more resistant to various therapies compared to their differentiated offspring and therefore the cause of tumor relapse. Markers for CSCs have been identified using xenograft transplantation assays and lineage tracing in mouse models, however the specificity and validity of many of these markers is under debate. Recently, low proteasome activity has been postulated as a novel CSC marker. In several solid malignancies a small subset of low proteasomal activity cells with CSC characteristics were identified, suggesting that proteasomal activity might be a functional marker for CSCs. In this perspective, we will discuss a recent study by Munakata et al., describing a population of colorectal cancer cells with CSC properties, characterized by low proteasome activity and treatment resistance. We will put this finding in a broader view by discussing the challenges and issues inherent with CSC identification, as well as some emerging insights in the CSC concept.