Stem cell heterogeneity: implications for aging and regenerative medicine

Retargeting vesicular stomatitis virus glycoprotein pseudotyped lentiviral vectors with enhanced stability by in situ synthesized polymer shell

The ability to introduce transgenes with precise specificity to the desired target cells or tissues is key to a more facile application of genetic therapy. Here, we describe a novel method using nanotechnology to generate lentiviral vectors with altered recognition of host cell receptor specificity. Briefly, the infectivity of the vesicular stomatitis virus glycoprotein (VSV-G) pseudotyped lentiviral vectors was shielded by a thin polymer shell synthesized in situ onto the viral envelope, and new binding ability was conferred to the shielded virus by introducing acrylamide-tailored cyclic arginine-glycine-aspartic acid (cRGD) peptide to the polymer shell. We termed the resulting virus "targeting nanovirus." The targeting nanovirus had similar titer with VSV-G pseudotypes and specifically transduced Hela cells with high transduction efficiency. In addition, the encapsulation of the VSV-G pseudotyped lentivirus by the polymer shell did not change the pathway that VSV-G pseudotypes enter and fuse with cells, as well as later events such as reverse transcription and gene expression. Furthermore, the targeting nanovirus possessed enhanced stability in the presence of human serum, indicating protection of the virus by the polymer shell from human serum complement inactivation. This novel use of nanotechnology demonstrates proof of concept for an approach that could be more generally applied for redirecting viral vectors for laboratory and clinical purposes.

A comparative review of aging and B cell function in mice and humans

Immune system function declines with age. Here we review and compare age-associated changes in murine and human B cell pools and humoral immune responses. We summarize changes in B cell generation and homeostasis, as well as notable changes at the subcellular level; then discuss how these changes help to explain alterations in immune responses across the adult lifespan of the animal. In each section we compare and contrast findings in the mouse, arguably the best animal model of the aging immune system, with current understanding of B cell immunity in humans.

Regenerative medicine: challenges and perspectives for successful therapies

Regenerative Medicine (RM) has the promise to revolutionize the treatment of many debilitating diseases for which the current therapies are inadequate. To realize the full potential of RM, a pragmatic approach needs to be taken by all stakeholders keeping in mind the lessons learnt from recombinant protein manufacturing, gene therapy trials, etc., to develop novel service delivery models for economic viability and regulatory processes in the absence of long-term data. In this chapter, we focus on the three main drivers of RM field and discuss the potential pitfalls and possible ways to mitigate them in order to move the field closer to clinical implementation.

Less is more: unveiling the functional core of hematopoietic stem cells through knockout mice

Hematopoietic stem cells (HSCs) represent one of the first recognized somatic stem cell types. As such, nearly 200 genes have been examined for roles in HSC function in knockout mice. In this review, we compile the majority of these reports to provide a broad overview of the functional modules revealed by these genetic analyses and highlight some key regulatory pathways involved, including cell cycle control, Tgf-β signaling, Pten/Akt signaling, Wnt signaling, and cytokine signaling. Finally, we propose recommendations for characterization of HSC function in knockout mice to facilitate cross-study comparisons that would generate a more cohesive picture of HSC biology.

Lifespan differences in hematopoietic stem cells are due to imperfect repair and unstable mean-reversion

The life-long supply of blood cells depends on the long-term function of hematopoietic stem cells (HSCs). HSCs are functionally defined by their multi-potency and self-renewal capacity. Because of their self-renewal capacity, HSCs were thought to have indefinite lifespans. However, there is increasing evidence that genetically identical HSCs differ in lifespan and that the lifespan of a HSC is predetermined and HSC-intrinsic. Lifespan is here defined as the time a HSC gives rise to all mature blood cells. This raises the intriguing question: what controls the lifespan of HSCs within the same animal, exposed to the same environment? We present here a new model based on reliability theory to account for the diversity of lifespans of HSCs. Using clonal repopulation experiments and computational-mathematical modeling, we tested how small-scale, molecular level, failures are dissipated at the HSC population level. We found that the best fit of the experimental data is provided by a model, where the repopulation failure kinetics of each HSC are largely anti-persistent, or mean-reverting, processes. Thus, failure rates repeatedly increase during population-wide division events and are counteracted and decreased by repair processes. In the long-run, a crossover from anti-persistent to persistent behavior occurs. The cross-over is due to a slow increase in the mean failure rate of self-renewal and leads to rapid clonal extinction. This suggests that the repair capacity of HSCs is self-limiting. Furthermore, we show that the lifespan of each HSC depends on the amplitudes and frequencies of fluctuations in the failure rate kinetics. Shorter and longer lived HSCs differ significantly in their pre-programmed ability to dissipate perturbations. A likely interpretation of these findings is that the lifespan of HSCs is determined by preprogrammed differences in repair capacity.

All hematopoietic stem cells (HSCs) are characterized by the capacities to produce all blood cell-types by differentiation and to maintain their own population through self-renewal divisions. Every individual HSC, therefore, can generate a complete blood system, or clone, conveying oxygenation and immune protection for a limited time. The time for which all mature blood cell-types can be found in a clone is called the lifespan. Interestingly, HSCs with different lifespans co-exist in the same host. We address the unresolved question: what controls the lifespan of HSCs of the same genotype exposed to the same environment? Here, we use a new approach to multi-scale modeling based on reliability theory and non-linear dynamics to address this question. Large-scale fluctuations in the experimental failure rate kinetics of HSC clones are identified to predict small-scale, genome level, events of deep penetrance, or magnitudes that approach population size. We broadly find that one condition explains our experimental data: repair mechanisms are a priori imperfect and do not improve, nor deteriorate, during the lifespan. As a result, progressively “worse-than-old” genome replicates are generated in self-renewal. A likely interpretation of our findings is that the lifespan of adult HSCs is determined by epigenetically pre-programmed differences in repair capacity.

In senescence, age-associated B cells secrete TNFα and inhibit survival of B-cell precursors

Aged mice exhibit ~ 5-10-fold increases in an ordinarily minor CD21/35(-) CD23(-) mature B-cell subset termed age-associated B cells (ABCs). ABCs from old, but not young, mice induce apoptosis in pro-B cells directly through secretion of TNFα. In addition, aged ABCs, via TNFα, stimulate bone marrow cells to suppress pro-B-cell growth. ABC effects can be prevented by the anti-inflammatory cytokine IL-10. Notably, CD21/35(+) CD23(+) follicular (FO) splenic and FO-like recirculating bone marrow B cells in both young and aged mice contain a subpopulation that produces IL-10. Unlike young adult FO B cells, old FO B cells also produce TNFα; however, secretion of IL-10 within this B-cell population ameliorates the TNFα-mediated effects on B-cell precursors. Loss of B-cell precursors in the bone marrow of old mice in vivo was significantly associated with increased ABC relative to recirculating FO-like B cells. Adoptive transfer of aged ABC into RAG-2 KO recipients resulted in significant losses of pro-B cells within the bone marrow. These results suggest that alterations in B-cell composition during old age, in particular, the increase in ABC within the B-cell compartments, contribute to a pro-inflammatory environment within the bone marrow. This provides a mechanism of inappropriate B-cell 'feedback' that promotes down-regulation of B lymphopoiesis in old age.

Clonal architecture of chronic myelomonocytic leukemias

Early clonal dominance may distinguish chronic myelomonocytic leukemia from other chronic myeloid neoplasms with similar gene mutations. Early dominance of TET2-mutated cells in the hematopoietic tissue promotes myeloid differentiation skewing toward the granulomonocytic line.

Immune cell dysfunction and inflammation in end-stage renal disease

Uraemia causes inflammation and reduces immune system function as evidenced by an increased risk of viral-associated cancers, increased susceptibility to infections and decreased vaccination responses in patients with end-stage renal disease (ESRD). The substantially increased risk of atherosclerosis in these patients is also probably related to uraemia-associated inflammation. Uraemia is associated with a reduction in the number and function of lymphoid cells, whereas numbers of myeloid cells in uraemic patients are normal or increased with increased production of inflammatory cytokines and reactive oxygen species. Similar to healthy elderly individuals, patients with ESRD have increased numbers of specific proinflammatory subsets of T cells and monocytes, suggesting the presence of premature immunological ageing in these patients. These cells might contribute to inflammation and destabilization of atherosclerotic plaques, and have, therefore, been identified as novel nonclassical cardiovascular risk factors. The cellular composition of the immune system does not normalize after successful kidney transplantation despite a rapid reduction in inflammation and oxidative stress. This finding suggests that premature ageing of the immune system in patients with ESRD might be related to a permanent skewing of the haematopoetic stem cell population towards myeloid-generating subsets, similar to that seen in healthy elderly individuals.

Neurorescue effects and stem properties of chorionic villi and amniotic progenitor cells

The capability to integrate into degenerative environment, release neurotrophic cytokines, contrast oxidative stress and an inherent differentiation potential towards siteappropriate phenotypes are considered crucial for the use of stem cells in tissue repair and regeneration. Naïve human chorial villi- (hCVCs) and amniotic fluid- (hAFCs) derived cells, whose properties and potentiality have not been extensively investigated, may represent two novel foetal cell sources for stem cell therapy. We previously described that long-term transplantation of hAFCs in the lateral ventricles of wobbler and healthy mice was feasible and safe. In the present study we examine the in vitro intrinsic stem potential of hCVCs and hAFCs for future therapeutic applications in neurodegenerative disorders. Presence of stem lineages was evaluated assessing the expression pattern of relevant candidate markers by flow cytometry, reverse transcription-polymerase chain reaction (RT-PCR) and immunocytochemistry. Release of cytokines that may potentialy sustain endogenous neurogenesis and/or activate neuroprotective pathways was quantified by enzyme-linked immunosorbent assays (ELISAs). We also performed an in vitro neurorescue assay, wherein a neuroblastoma cell line damaged by 6-hydroxydopamine (6-OHDA) was treated with hCVC/hAFC-derived conditioned medium (CM). Naïve hCVCs/hAFCs show a neurogenic/angiogenic predisposition. Both cell types express several specific neural stem/progenitor markers, such as nestin and connexin 43, and release significant amounts of brain-derived neurotrophic factor, as well as vascular endothelial growth factor. hCVC and hAFC populations comprise several interesting cell lineages, including mesenchymal stem cells (MSCs) and cells with neural-like phenotypes. Moreover, although CMs obtained from both cell cultures actively sustained metabolic activity in a 6-OHDA-induced Parkinson's disease (PD) cell model, only hCVC-derived CMs significantly reduced neurotoxin-induced apoptosis. In conclusion, this study demonstrates that naïve hAFCs and hCVCs may enhance cell-recovery following neuronal damage through multiple rescue mechanisms, and may provide a suitable means of stem cell therapy for neurodegenerative disorders including PD.

Application of Chinese herbal medicines to revitalize adult stem cells for tissue regeneration

It has been established in the recent several decades that adult stem cells play a crucial role in tissue renewal and regeneration. Adult stem cells locate in certain organs can differentiate into functional entities such as macrophages and bone cells. Hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs) are two of the most important populations of adult stem cells. The application of these stem cells offers a new insight in treating various pathological conditions, through replenishing cells of specific functions by turning on or off the differentiating program within quiescent stem cell niches. Apart from that, they are also capable to travel through the circulation, migrate to injury sites and differentiate to enhance regeneration process. Recently, Chinese medicine (CM) has shown to be potential candidates to activate adult stem cells for tissue regeneration. This review summarizes our own, as well as others' findings concerning the use of Chinese herbal medicine in the regulation processes of adult stem cells differentiation and their movement in tissue repair and rejuvenation. A number of Chinese herbs are used as therapeutic agents and presumably preventive agents on metabolic disorders. In our opinion, the activation of adult stem cells self-regeneration not only provides a novel way to repair tissue damage, but also reduces the use of targeted drug that adversely altering the normal metabolism of human subjects.

Current insights in the cellular and molecular biology of chronic myelomonocytic leukemia

SUMMARY Chronic myelomonocytic leukemia is a rare clonal myeloid disorder most often seen in the elderly that remains a virtually incurable disease. Chronic myelomonocytic leukemia has long been considered as a myelodysplastic syndrome by diagnostic classifications, but recent insights in the cellular and molecular biology of the disease has refined its identity. The malignant clone was shown to generate myeloid-derived suppressive cells that may contribute to disease expansion, whereas the role of progenitor hypersensitivity to granulomonocyte colony-stimulating factor probably defines two distinct subgroups. At least one gene mutation can now be identified in almost all the patients. The most frequently mutated genes are TET2, SRSF2 and ASXL1, with a frequent combination of mutations in the first two genes, whereas ASXL1 mutations define a poor prognostic subgroup of patients. A number of additional mutations have been identified that confer to the disease its phenotype specificity; for example, mutations in RUNX1 induce thrombocytopenia, those in SF3B1 can be associated with anemia, and those in signaling molecules including NRAS, KRAS, CBL, JAK2 and FLT3, characterize the proliferative forms of the disease. Based on these recent observations, new working models on disease pathogenesis are proposed and may serve as a basis for the search for alternative and more efficient therapeutic approaches.

Enabling stem cell therapies for tissue repair: current and future challenges

Stem cells embody the tremendous potential of the human body to develop, grow, and repair throughout life. Understanding the biologic mechanisms that underlie stem cell-mediated tissue regeneration is key to harnessing this potential. Recent advances in molecular biology, genetic engineering, and material science have broadened our understanding of stem cells and helped bring them closer to widespread clinical application. Specifically, innovative approaches to optimize how stem cells are identified, isolated, grown, and utilized will help translate these advances into effective clinical therapies. Although there is growing interest in stem cells worldwide, this enthusiasm must be tempered by the fact that these treatments remain for the most part clinically unproven. Future challenges include refining the therapeutic manipulation of stem cells, validating these technologies in randomized clinical trials, and regulating the global expansion of regenerative stem cell therapies.

Hematopoietic stem cell heterogeneity takes center stage

Over the past 10 years, increasing evidence has accumulated that heterogeneity is a feature of hematopoietic stem cell (HSC) proliferation, self-renewal, and differentiation based on examination of these properties at a clonal level. The heterogeneous behavior of HSCs reflects the operation of a complex interplay of intrinsic and extrinsic variables. In this review, we discuss key findings from the last 5 years that reveal new insights into the mechanisms involved.

Aging and neoteny in the B lineage

Aging and the physiologic decline of tissues and cells were once thought to be irreversible. However, recent studies suggest that various tissues, especially parts of the hematopoietic system, can be rejuvenated. Here we review potential mechanisms for this process and how they may be used to reverse age-related disorders and aging in general. We propose the novel hypothesis that altering the homeostatic process during cellular depletion can reverse aging in the hematopoietic system.

Leukemia and benzene

Excessive exposure to benzene has been known for more than a century to damage the bone marrow resulting in decreases in the numbers of circulating blood cells, and ultimately, aplastic anemia. Of more recent vintage has been the appreciation that an alternative outcome of benzene exposure has been the development of one or more types of leukemia. While many investigators agree that the array of toxic metabolites, generated in the liver or in the bone marrow, can lead to traumatic bone marrow injury, the more subtle mechanisms leading to leukemia have yet to be critically dissected. This problem appears to have more general interest because of the recognition that so-called "second cancer" that results from prior treatment with alkylating agents to yield tumor remissions, often results in a type of leukemia reminiscent of benzene-induced leukemia. Furthermore, there is a growing literature attempting to characterize the fine structure of the marrow and the identification of so called "niches" that house a variety of stem cells and other types of cells. Some of these "niches" may harbor cells capable of initiating leukemias. The control of stem cell differentiation and proliferation via both inter- and intra-cellular signaling will ultimately determine the fate of these transformed stem cells. The ability of these cells to avoid checkpoints that would prevent them from contributing to the leukemogenic response is an additional area for study. Much of the study of benzene-induced bone marrow damage has concentrated on determining which of the benzene metabolites lead to leukemogenesis. The emphasis now should be directed to understanding how benzene metabolites alter bone marrow cell biology.

Stem cell powwow in Squaw Valley

The Keystone Symposium entitled 'The Life of a Stem Cell: from Birth to Death' was held at Squaw Valley, CA, USA in March 2012. The meeting brought together researchers from across the world and showcased the most recent developments in stem cell research. Here, we review the proceedings at this meeting and discuss the major advances in fundamental and applied stem cell biology that emerged.

Chronic inflammation and aging: DNA damage tips the balance

The aged immune system, typically hyporesponsive to infection and vaccination, can be hyperresponsive in the context of inflammatory pathology. Here we review current work examining the mechanisms behind the amplified inflammatory profile of aged adaptive immunity, and the reciprocal relationship between chronic inflammation and immune aging. Aged hematopoietic stem cells are driven to differentiate following accumulated DNA damage, thus depleting the stem cell pool and increasing the number of damaged effector cells in the circulation. Chronic DNA damage responses in lymphocytes as well as senescent cells of other lineages initiate the production of inflammatory mediators. In addition, aged lymphocytes become less reliant on specific antigen for stimulation and more prone to activation through innate receptors. When these lymphocytes are exposed to inflammatory signals produced by senescent tissues, the bias toward inflammation exacerbates destruction without necessarily improving immunity.