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Signals from the sympathetic nervous system regulate hematopoietic stem cell egress from bone marrow.

Katayama Y et al., Cell 2006;124:407-421
Summary by Steve Greco, Advance Stem Cell Class, Fall 2006

Figure 1. Schematic of G-CSF-induced mobilization of HSC. Noradrenergic fibers innervating the BM respond to G-CSF by releasing norepinephrine. In a normal mouse model, the neurotransmitter causes reduced osteoblast function and decreased bone SDF-1 levels; thus leading to HSC mobilization. When the nerve input is injured, via mutation or lesioning, or neurotransmitter release is blocked (antagonists), osteoblast integrity is maintained with no change in SDF-1 levels. As a result these cells are retained in their niche.

LAY SUMMARY

The bone marrow (BM) provides the home, or niche, for hematopoietic stem cells (HSC). HSC are the stem cells responsible for producing all of the immune and blood cells. When needed, HSC can exit the BM into the circulating blood and migrate into injured tissues for repair or cell replacement. The mechanism by which HSC leave the BM into the blood is not fully understood. The following study examines how mice expressing a mutant gene called Cgt could reveal details about this underlying mechanism.

The investigators first examined if there were differences in the stem cells between the normal and mutant mice. Results showed that administration of a factor called G-CSF caused the HSC in the normal mice to escape, but not in the mutants. Studies showed that these differences were not a result of the inherent properties of the stem cells themselves, but of the niches they resided in.

Additional investigation found that the mutant mice had defects in nervous system signal transmission. These defects unraveled a connection between nerve fibers within the BM and the migration of HSC out of the BM. The mutant mice were not secreting the neurotransmitter norepinephrine in response to G-CSF administration, and this is thought to be a crucial mechanism governing migration of HSC out of the BM.

SCIENTIFIC SUMMARY

Hematopoietic stem cells (HSC) and progenitor cells are found in distinct niches within the bone marrow (BM) which are conducive to maintaining stem cell function and homeostasis. The egression of these cells from their niche to the peripheral circulation is a process by which HSC can traffic the other blood compartments and monitor damaged or empty niches. While this course of action is primarily homeostatic, clinicians and researchers can artificially induce HSC mobilization by systemic administration of the cytokine, granulocyte-colony stimulationg factor (G-CSF). This inducible mobilization has served as a valuable tool for cancer therapeutics and bone marrow transplantation.

A central dogma within G-CSF administration has been the underlying cellular mechanisms that govern the release of HSC from the BM niche and into the peripheral blood. Previous studies have shown mobilization of HSC as a result of reduced osteoblast function and decreasing levels of BM stromal derived factor-1 (SDF-1). However, the complete mechanism has yet to be discovered. In this summary, Katayama et al., address this topic by investigating G-CSF-induced HSC and progenitor cell mobilization in a ceramide galactosyltransferase-deficient (Cgt -/-) transgenic mouse model. This model shows low G-CSF-induced HSC egress, and is characterized by reduced neuron neurotransmission.

The investigators began their studies by investigating the reduced HSC responsiveness to G-CSF in Cgt -/- mice. Examination into the number of HSC in the BM of Cgt -/- and Cgt +/+ mice revealed similar numbers, thus demonstrating that the mobilization phenomena was not a result of absolute starting HSC number. Investigation into other potential differences between the HSC of the +/+ and -/- mice revealed similar responsiveness to SDF-1 and mobilization post-transplant into irradiated normal mice. The two mouse phenotypes also showed similar BM protease activities. The next set of studies examined levels of SDF-1 in the bone marrow extracellular fluid (BMEF) versus bone. Interestingly, the double mutant mice did not have decreased levels of SDF-1 within bone in response to G-CSF administration. To address this issue, the investigators determined the effect of G-CSF on osteoblast function in the normal versus double mutant mice. Osteoblasts have been shown to be important in maintaining HSC within their BM niche. G-CSF has been thought to inhibit osteoblast function, and thereby allow escape of HSC into the periphery. By examing expression of the osteoblast-specific transcription factor, Runx2, osteoblast function was assessed in response to G-CSF in the normal versus mutant mice. As thought, normal mice had reduced osteoblast function in response to the cytokine, however mutant mice had a less marked effect.

Taken together, the mutant mice lacked a decrease in bone SDF-1 levels in response to G-CSF and their osteoblast function was less compromised. The question remaining was the role of Cgt in causing this phenomenon.

The Cgt double mutant mouse lifespan was 18-30 days, with the mice suffering from severe tremors and ataxia. Thus, the mutation had a neural element which the investigators deduced could be related to the differences in HSC egression. It has long been known that severing the innervating BM nerve fibers, specifically the adrenergic inputs, leads to rapid cellular depletion of the BM. To address this possibility, normal mice were injected with 6-hydroxydopamine (6OHDA) to induce catecholaminergic lesions. 6OHDA mice were found to have a marked reduction in mobilized HSC and progenitor cells in response to G-CSF administration. Similar results were observed with dopamine-β-hydroxylase double mutant (Dbh -/-) mice, which are incapable of synthesizing norepinephrine from dopamine. These results were repeatable in normal mice with addition of β-antagonists (β-blockers), and could be rescued with β2-adrenergic agonist.

In summary, noradrenergic fibers innervating the BM respond to G-CSF by releasing norepinephrine. In a normal mouse model, the neurotransmitter causes reduced osteoblast function and decreased bone SDF-1 levels; thus leading to HSC mobilization. When the nerve input is injured, via mutation or lesioning, or neurotransmitter release is blocked (antagonists), osteoblast integrity is maintained with no change in SDF-1 levels. As a result these cells are retained in their niche.

This seminal work has broad clinical implications in manipulating HSC egression through the use of previously FDA-approved compounds known to block adrenergic release, such as β-blockers. Knowledge of the neural input in HSC mobilization opens new areas of research to examine how the nervous system could be used to manipulated HSC niches and their homing properties.

A black box that this work alludes to is the mechanism through which G-CSF alters catecholamine release, whether directly acting on the neuron, on other glial cells or through some other indirect route.

Commentary
Other avenues of investigation pertaining to this research project include the role of the BM stromal fibroblasts. These cells are intricately involved in maintaining the HSC niche and have been shown to be intertwined with the innervating nerve fibers of the BM.