Karin Klauke

Physical exercise leads to rapid adaptations in hippocampal vasculature: Temporal dynamics and relationship to cell proliferation and neurogenesis

Increased levels of angiogenesis and neurogenesis possibly mediate the beneficial effects of physical activity on hippocampal plasticity. This study was designed to investigate the temporal dynamics of exercise‐induced changes in hippocampal angiogenesis and cell proliferation. Mice were housed with a running wheel for 1, 3, or 10 days. Analysis of glucose transporter Glut1‐positive vessel density showed a significant increase after 3 days of wheel running. Cell proliferation in the dentate gyrus showed a trend towards an increase after 3 days of running and was significantly elevated after 10 days of physical exercise. Ten days of wheel running resulted in a near‐significant increase in the number of immature neurons, as determined by a doublecortin (DCX) staining. In the second part of the study, the persistence of the exercise‐induced changes in angiogenesis and cell proliferation was determined. The running wheel was removed from the cage after 10 days of physical activity. Glut‐1 positive vessel density and hippocampal cell proliferation were determined 1 and 6 days after removal of the wheel. Both parameters had returned to baseline 24 h after cessation of physical activity. The near‐significant increase in the number of DCX‐positive immature neurons persisted for at least 6 days, indicating that new neurons formed during the period of increased physical activity had survived. Together these experiments show that the hippocampus displays a remarkable angiogenic and neurogenic plasticity and rapidly responds to changes in physical activity.

Polycomb Cbx family members mediate the balance between haematopoietic stem cell self-renewal and differentiation

The balance between self-renewal and differentiation of adult stem cells is essential for tissue homeostasis. Here we show that in the haematopoietic system this process is governed by polycomb chromobox (Cbx) proteins. Cbx7 is specifically expressed in haematopoietic stem cells (HSCs), and its overexpression enhances self-renewal and induces leukaemia. This effect is dependent on integration into polycomb repressive complex-1 (PRC1) and requires H3K27me3 binding. In contrast, overexpression of Cbx2, Cbx4 or Cbx8 results in differentiation and exhaustion of HSCs. ChIP-sequencing analysis shows that Cbx7 and Cbx8 share most of their targets; we identified approximately 200 differential targets. Whereas genes targeted by Cbx8 are highly expressed in HSCs and become repressed in progenitors, Cbx7 targets show the opposite expression pattern. Thus, Cbx7 preserves HSC self-renewal by repressing progenitor-specific genes. Taken together, the presence of distinct Cbx proteins confers target selectivity to PRC1 and provides a molecular balance between self-renewal and differentiation of HSCs.

Hippocampal cell proliferation across the day: Increase by running wheel activity, but no effect of sleep and wakefulness

The present study investigated whether proliferation of hippocampal progenitors is subject to circadian modulation. Mice were perfused using 3h intervals throughout the light-dark cycle and brains were stained for Ki-67. Since Ki-67 is not expressed during the G0 phase of the cell cycle, we expected a decline in Ki-67 expression at the moment cells synchronously exit the cell cycle. However, despite the fact that various hippocampal factors fluctuate across the day, the number of dividing cells remained constant. In a second experiment, we studied whether disturbance of normal sleep affected the stable rate in cell proliferation. Our data show that 12h of sleep deprivation during the light phase did not influence proliferating cell number. A third experiment investigated whether physical activity, a condition known to enhance hippocampal cell proliferation, caused an elevation of the steady baseline number of proliferating progenitors, or a peak directly following the active phase of the animals. Mice were housed with a running wheel for 9 days. On the last day, animals were sacrificed either directly before or directly after the active phase. Exercise significantly promoted cell proliferation and this effect appeared to be strongest directly after the active period and to disappear during the resting phase. Our data suggest that hippocampal cell proliferation is not synchronized under basal conditions and is unchanged by sleep deprivation. However, running affected cell proliferation differentially at two times of day. These data demonstrate that the steady rate in cell proliferation is not indispensable, but can be changed by behavioral activity.

Polycomb-group proteins in hematopoietic stem cell regulation and hematopoietic neoplasms.

The equilibrium between self-renewal and differentiation of hematopoietic stem cells is regulated by epigenetic mechanisms. In particular, Polycomb-group (PcG) proteins have been shown to be involved in this process by repressing genes involved in cell-cycle regulation and differentiation. PcGs are histone modifiers that reside in two multi-protein complexes: Polycomb Repressive Complex 1 and 2 (PRC1 and PRC2). The existence of multiple orthologs for each Polycomb gene allows the formation of a multitude of distinct PRC1 and PRC2 sub-complexes. Changes in the expression of individual PcG genes are likely to cause perturbations in the composition of the PRC, which affect PRC enzymatic activity and target selectivity. An interesting recent development is that aberrant expression of, and mutations in, PcG genes have been shown to occur in hematopoietic neoplasms, where they display both tumor-suppressor and oncogenic activities. We therefore comprehensively reviewed the latest research on the role of PcG genes in normal and malignant blood cell development. We conclude that future research to elucidate the compositional changes of the PRCs and methods to intervene in PRC assembly will be of great therapeutic relevance to combat hematological malignancies.

Specific protein homeostatic functions of small heat‐shock proteins increase lifespan

During aging, oxidized, misfolded, and aggregated proteins accumulate in cells, while the capacity to deal with protein damage declines severely. To cope with the toxicity of damaged proteins, cells rely on protein quality control networks, in particular proteins belonging to the family of heat‐shock proteins (HSPs). As safeguards of the cellular proteome, HSPs assist in protein folding and prevent accumulation of damaged, misfolded proteins. Here, we compared the capacity of all Drosophila melanogaster small HSP family members for their ability to assist in refolding stress‐denatured substrates and/or to prevent aggregation of disease‐associated misfolded proteins. We identified CG14207 as a novel and potent small HSP member that exclusively assisted in HSP70‐dependent refolding of stress‐denatured proteins. Furthermore, we report that HSP67BC, which has no role in protein refolding, was the most effective small HSP preventing toxic protein aggregation in an HSP70‐independent manner. Importantly, overexpression of both CG14207 and HSP67BC in Drosophila leads to a mild increase in lifespan, demonstrating that increased levels of functionally diverse small HSPs can promote longevity in vivo.

Tracing dynamics and clonal heterogeneity of Cbx7-induced leukemic stem cells by cellular barcoding.

Summary Accurate monitoring of tumor dynamics and leukemic stem cell (LSC) heterogeneity is important for the development of personalized cancer therapies. In this study, we experimentally induced distinct types of leukemia in mice by enforced expression of Cbx7. Simultaneous cellular barcoding allowed for thorough analysis of leukemias at the clonal level and revealed high and unpredictable tumor complexity. Multiple LSC clones with distinct leukemic properties coexisted. Some of these clones remained dormant but bore leukemic potential, as they progressed to full-blown leukemia after challenge. LSC clones could retain multilineage differentiation capacities, where one clone induced phenotypically distinct leukemias. Beyond a detailed insight into CBX7-driven leukemic biology, our model is of general relevance for the understanding of tumor dynamics and clonal evolution.

Polycomb group proteins in hematopoietic stem cell aging and malignancies

Protection of the transcriptional “stemness” network is important to maintain a healthy hematopoietic stem cells (HSCs) compartment during the lifetime of the organism. Recent evidence shows that fundamental changes in the epigenetic status of HSCs might be one of the driving forces behind many age-related HSC changes and might pave the way for HSC malignant transformation and subsequent leukemia development, the incidence of which increases exponentially with age. Polycomb group (PcG) proteins are key epigenetic regulators of HSC cellular fate decisions and are often found to be misregulated in human hematopoietic malignancies. In this review, we speculate that PcG proteins balance HSC aging against the risk of developing cancer, since a disturbance in PcG genes and proteins affects several important cellular processes such as cell fate decisions, senescence, apoptosis, and DNA damage repair.