Hematopoietic-Extrinsic Cues Dictate Circadian Redistribution of Mature and Immature Hematopoietic Cells in Blood and Spleen.

Miriam Stenzinger,Keven Hoerster,Erik Maronde,Darja Karpova,Martina Pfeffer,Christian Unterrainer,Eliza Wiercinska,Sabine Harenkamp,Halvard Bonig

doi:10.3390/cells8091033

Abstract

Circadian oscillations in circulating leukocyte subsets including immature hematopoietic cells have been appreciated; the origin and nature of these alterations remain elusive. Our analysis of wild-type C57BL/6 mice under constant darkness confirmed circadian fluctuations of circulating leukocytes and clonogenic cells in blood and spleen but not bone marrow. Clock gene deficient Bmal1−/− mice lacked this regulation. Cell cycle analyses in the different hematopoietic compartments excluded circadian changes in total cell numbers, rather favoring shifting hematopoietic cell redistribution as the underlying mechanism. Transplant chimeras demonstrate that circadian rhythms within the stroma mediate the oscillations independently of hematopoietic-intrinsic cues. We provide evidence of circadian CXCL12 regulation via clock genes in vitro and were able to confirm CXCL12 oscillation in bone marrow and blood in vivo. Our studies further implicate cortisol as the conveyor of circadian input to bone marrow stroma and mediator of the circadian leukocyte oscillation. In summary, we establish hematopoietic-extrinsic cues as causal for circadian redistribution of circulating mature/immature blood cells.

Highlights

Circadian rhythms serve to anticipate regularly occurring environmental changes, light–dark cycles, in order to prepare the organism for the consequences thereof, such as changes in physical activity, food availability, and pathogen exposure [1,2]
Even more pronounced, a significant sinusoidal function with 10-fold differences in circulating clonogenic cell (CFU-C) counts was observed over the course of the day under light/12 h darkness (LD) (Figure 1C) and circadian regulation of the colony-forming unit-culture (CFU-C) was assessed under darkness conditions (DD) conditions (Figure 1F)
The WT hematopoietic cells in Bmal1−/− hosts lacked sinusoidal white blood cell (WBC) (Figure 4C) and CFU-C oscillation (Figure 4E) and were completely arrhythmic, just as observed in pure clock gene deficient mice. These results demonstrate that the endogenous clock within the hematopoietic cells is entirely dispensable for the circadian blood cell oscillation as the clock gene deficient Bmal1−/− hematopoiesis adapts to the physiological circadian rhythm in a WT environment

Summary

Introduction

Circadian rhythms serve to anticipate regularly occurring environmental changes, light–dark cycles, in order to prepare the organism for the consequences thereof, such as changes in physical activity, food availability, and pathogen exposure [1,2]. The circadian clock is organized in a hierarchical fashion: the suprachiasmatic nucleus (SCN) in the hypothalamus, acting as the central pacemaker, receives light input from the retina and conveys this timing information to the entire organism via subordinate oscillators in peripheral tissues [3]. In the absence of external entraining cues, so called “zeitgebers”, the intrinsic circadian system is defined as self-sustainable and persists with a period length of about 24 h under constant conditions. This feature distinguishes it from diurnal rhythms, which are extinguished under constant darkness [3]. Peripheral oscillators exist in many organs and are synchronized by the SCN via direct and indirect routes. Direct synchronization is mediated through humoral or neuronal signals, Cells 2019, 8, 1033; doi:10.3390/cells8091033 www.mdpi.com/journal/cells

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