Hematopoietic stem and progenitor cell (HSPC) transplantation is a vital tool
in the treatment of hematopoietic and other malignancies. HSPC reside in the
bone marrow (BM), and can be isolated directly from BM or from peripheral blood
after a process termed mobilization: molecular cues are used to coax the HSPC
out of their niche and into the circulation. A number of factors are known
to induce mobilization, including a variety of cytokines and chemokines. The
most commonly used clinical regimen is multi-day treatment with granulocyte-colony
stimulating factor (G-CSF). However, there is a wide range of efficacy among
different individuals, and the mechanism by which G-CSF induces mobilization
is not well understood.
Cell adhesion molecules, proteases, chemokines, and cytokines have all
been shown to play roles in regulating HSPC homing to, and egress from, the
BM niche.1,2 Specifically, a key role for the CXCL12/CXCR4 axis has been established
by a number of studies.3-6 CXCL12, also known as SDF-1, acts as a chemoattractant
for HSPCs and is downregulated in the BM environment in response to G-CSF
administration.3,7 In addition, studies have demonstrated that proteolytic
activity induced by G-CSF treatment degrades CXCL12, and this degradation correlates
with HSPC egress from BM.3,4 Cell adhesion molecules including integrins, VCAM-1,
and selectins have all been shown to influence HSPC mobilization and/or adhesion
to BM components.1,2 P/E-selectin double mutant mice exhibit defective homing
to BM and increased HSPC numbers in peripheral blood. Inhibitors of selectins,
including sulfated glycans such as fucoidan, a polysaccharide isolated from
brown seaweed, also promote mobilization of HSPC.8,9 While fucoidan isn’t
found in mammalian cells, a functionally similar molecule, sulfatide (sulfated
galactosylceramide), is produced by human myeloid cells.
Interest in whether sulfatide might play a role in normal HSPC trafficking
led Katayama et al. to examine mobilization in mice lacking UDP-galactose:
ceramide galactosyltransferase (Cgt), a gene required for sulfatide production.
Consistent with the authors’ hypothesis, Cgt-/- mice fail to undergo
significant mobilization of HSPC in response to G-CSF and fucoidan. However,
the reason for this result is surprising.
Although proteolysis was normal in Cgt-/- BM, differences in CXCL12 protein
and RNA levels in bone tissue were detected in Cgt-/- mice. The authors attributed
the failure of HSPC mobilization to the persistence of high levels of CXCL12
despite treatment with G-CSF. Osteoblast activity was also altered in Cgt-/-
mice: cells were flatter and had shorter cell protrusions into bone tissue.
Interestingly, G-CSF treatment was noted to result in similar changes
in osteoblasts of wild-type mice.
The novel result came when the authors considered a potential role for the
nervous system in the Cgt -/- mobilization phenotype. Besides sulfatide, Cgt
is essential for production of galactocerebrosides, major
components of the myelin sheaths that surround nerve fibers and facilitate
nerve impulse conduction. Indeed, Cgt-/- mice die early in post-natal life
due to severe tremor and ataxia.10,11 A series of experiments provided support
for the involvement of the sympathetic nervous system (SNS) in control
of HSPC mobilization. First, it was demonstrated that mice deficient in
SNS function due to knockout of dopamine β hydroxylase (Dβh), the enzyme required
for the production of norepinephrine, show a dramatic reduction in HSPC mobilization
by G-CSF. Furthermore, osteoblast morphology and CXCL12 levels were not affected
by G-CSF administration in the Dβh-/- mice, implying that G-CSF is unable
to exert its normal effects in the absence of adrenergic signals from the SNS.
Finally, treatment of wild-type mice with a β adrenergic antagonist, the
beta blocker propranolol, also reduced HSPC mobilization, while the β2 adrenergic
agonist clenbuterol partially rescued G-CSF induced mobilization in Dβh-/-
mice.
 |
| Figure 1. Model depicting the role of the sympathetic nervous
system (SNS) in HSPC mobilization from bone marrow. Prior to mobilization by G-CSF (left side) osteoblasts
display normal morphology and produce CXCL12, which retains HSPC in the bone marrow niche. G-CSF (right side)
decreases CXCL12 levels in bone marrow by causing release of proteases by neutrophils and decreased synthesis
of CXCL12 by osteoblasts. The effect of G-CSF on osteoblasts is mediated in part by adrenergic neurons, based
on the finding that knockout mice with inadequate myelination (Cgt-/- mice), or lack of
norepinephrine (Dbh-/- mice), fail to respond to G-CSF. |
Importantly, adrenergic signaling alone is not sufficient to induce HSPC egress
from the BM: treatment of wild-type mice with clenbuterol did not cause mobilization.
The authors suggest that G-CSF causes HSPC mobilization by inhibiting osteoblast
production of CXCL12 through both SNS-dependent and independent means
(Figure 1). Their results raise the interesting possibility that differences
in sympathetic tone may explain variable responses to G-CSF, and suggest a
potential clinical avenue for optimization of HSPC mobilization from patients
and donors.
References
- Lapidot, T. & I. Petit (2002) Exp. Hematol. 30:973.
- Papayannopoulou, T. (2004) Blood 103:1580.
- Petit, I. et al. (2002) Nat. Immunol. 3:687.
- Wright, D.E. et al. (2002) J. Exp. Med. 195:1145.
- Levesque, J.-P. et al. (2003) J. Clin. Invest. 111:187.
- Broxmeyer, H.E. et al. (2005) J. Exp. Med. 207:1307.
- Katayama, Y. et al. (2006) Cell 124:407.
- Sweeney, E.A. et al. (2000) Proc. Natl. Acad. Sci. USA 97:6544.
- Frenette, P.S. & L. Weiss (2000) Blood 96:2460.
- Bosio, A. et al. (1996) Proc. Natl. Acad. Sci. USA 93:13280.
- Coetzee, T. et al. (1996) Cell 86:209.
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