Hierarchy of molecular-pathway usage in bone marrow homing and its shift by cytokines

Abstract

Efficient bone marrow (BM) homing is a prerequisite for successful engraftment of transplanted hematopoietic cells (HPCs).Contradictory conclusions about the contribution of SDF-1/CXCR4 have clouded our understanding of its role within the molecular pathway cooperation needed for BM homing, particularly with the welldefined hierarchic network of adhesion molecules. In the present study we sought to unravel cooperative and compensatory molecular pathways guiding BM homing. Fresh BM-HPCs, rendered either SDF-1 unresponsive or Gi-signaling refractory, homed quite efficiently, because of compensation by ␣4-integrin interacting with VCAM-1. The contribution of SDF-1/CXCR4-or Gi-protein-mediated signals to BM homing became apparent after their blockade was combined with deletion of ␣4-integrin, leading to dramatic reduction in BM homing. Similar conclusions were revealed when VCAM-1-deficient hosts were used. IntroductionBone marrow (BM) homing and engraftment are sequential steps in hematopoietic reconstitution after transplantation. The molecular mechanisms of BM homing have been under investigation for several years, and a host of contributing molecular players have been described, including integrins and endothelial selectins with their respective ligands, CD44, complement, certain lipid mediators, and intracellular signaling molecules. [1][2][3][4][5][6][7][8][9] Recently, the chemokine SDF-1 and its receptor CXCR4 have garnered much interest because of their dominant role as a homing-defining molecule in several tissues. 10,11 SDF-1 is expressed by a wide variety of cells, including hematopoietic cells (HPCs) and BM stromal cells, and is displayed as cell-bound, extracellular matrix-bound, or in soluble form. It is greatly up-regulated in many tissues, including BM, by "stress," such as injury or irradiation. CXCR4 is expressed predominantly intracellularly in immature hematopoietic cells, where it can be rapidly surface-expressed via clathrin-coated pits, to function as SDF-1 receptor. 12 All known functions of SDF-1 (antiapoptosis, proliferation, adhesion, and migration) are mediated through binding to its receptor, CXCR4, which signals through Gi-protein second messengers. Downstream signals involve IP3 and PKC, 13 and, although a Gi-independent contribution of the Jak/Stat pathway to IP3 activation has been shown, in the absence of Gi-signaling Jak/Stat activation could not elicit functional SDF-1 downstream signaling events. [14][15][16] In contrast to its wellestablished and generally accepted role in BM retention of HPCs and in long-term maintenance of hematopoiesis, evidence of a role of SDF-1/CXCR4/Gi-protein-dependent signaling in BM homing is unsettled because of discrepant data. [17][18][19][20][21][22][23][24][25] The purpose of the experiments described here was to explore possible reasons for disparity in findings about the role of Gi-signals in BM homing and to further define hierarchic pathway usage in homing. Our data suggest that BM homing of immature HPCs is guided by cooperating molecules w...

Chemokine-mobilized adult stem cells; defining a better hematopoietic graft

“…Moreover, homing of HPC refractory to SDF-1 migration is blocked in the absence of a4-integrins, indicating that homing via SDF-1a/CXCR4 can be fully compensated by a functional a4-integrin/VCAM-1 pathway. 94 The enhanced adhesion properties displayed by these GROb-mobilized cells would be consistent with primary reliance on the integrin pathway for homing and engraftment.…”

GSK3β regulates physiological migration of stem/progenitor cells via cytoskeletal rearrangement

“…The preferential effect of GSK3β inhibition on immature versus mature hematopoietic cells is intriguing and led us to search for a unique GSK3β-mediated signaling pathway involved in the motility of immature hematopoietic cells. In line with reports on enhanced migration and homing potential following exposure to SCF (28)(29)(30) and the reduced migratory potential of cKit-deficient immature hematopoietic cells (27), we studied cKit signaling as a potential upstream regulator of GSK3β activity. Indeed, we observed that exposure to SCF enhanced the in vitro migration of HSPCs, especially the more primitive phenotypic HSCs, toward CXCL12 in a GSK3β-dependent manner.…”

“…Catecholamines by themselves directly augment the motility of human HSPCs (25). Interestingly, cKit (also termed CD117), which serves as a marker for HSPCs, and its ligand SCF (also termed Kit ligand), are involved in human and murine HSPC motility (26)(27)(28)(29)(30), pointing to a signaling pathway that uniquely regulates the migration of immature hematopoietic cells over mature leukocytes. Moreover, mutant mice with low serum levels of IGF-1 exhibit markedly increased numbers of circulating HSPCs with normal numbers of mature white blood cells (WBCs) (31), pointing to another mechanism by which the egress of immature hematopoietic cells is preferentially regulated.…”

Stem cells: novel players in the treatment of erectile dysfunction

“…61 Recent data indicate that CXCL12 may play a role in trafficking ADSCs towards the major pelvic ganglion in rats with an injured cavernous nerve. 62 In our laboratory, we have performed a comparative analysis between ADSCs and BMSCs and were able to show that these two types of MSCs express a similar panel of chemokine receptors. We identified expression of mRNA for 12 of the currently known 21 chemokine receptors, and found that these receptors were located both on the cell surface as well as intracellularly, suggesting that chemokine receptors are recycled in these stem cells, as has been previously shown in various immune cells (unpublished data).…”

“…[58][59][60][61][62][63] Rolling and firm adhesion is followed by transendothelial migration across the physical endothelium/extracellular matrix barrier.…”