Physiological Gradients of Serum Albumin Regulate SDF-1/CXCR4 Dependent Migration.

Hematopoietic stem cell (HSC) egress to the circulation and homing to the bone marrow (BM) are regulated by interactions between CXCR4 and SDF-1. Serum albumin is the major protein component of plasma. Concentration gradients of albumin between plasma and interstitial fluid account for the colloid osmotic pressure, which is a central regulator of the intravasal blood volume. SDF-1/CXCR4 dependent migration of mobilized peripheral blood CD34+ hematopoietic stem cells (PBSC) was studied in transwell migration assays. We compared the effect of RPMI diluted Heparin-plasma (50% plasma, 50% RPMI) versus undiluted plasma as a migration medium. To our surprise the use of undiluted plasma in the upper chamber containing the cells and diluted plasma in the lower chamber containing 100ng/ml SDF-1, resulted in a 3 fold increase of migrating PBSCs compared to experiments without a gradient (undiluted versus undiluted plasma or diluted versus diluted plasma) while diluted plasma in the upper chamber and undiluted in the lower chamber inhibited the SDF-1 dependent migration 3,5 fold. To further characterize this observation we removed high molecular weight proteins (>50kd) by filtration of the plasma. The filtrate was used to dilute plasma to obtain samples with reduced protein contents. Transwell experiments showed that negative gradients of plasma proteins (high concentration in the upper chamber vs low concentration in the lower chamber) stimulate (up to 2 fold) while positive gradients inhibit SDF-1/CXCR4 dependent migration of PBSC (up to 3 fold). Migration experiments were repeated with RPMI, supplemented with varying concentrations of serum albumin as migration medium. Negative gradients of albumin stimulated migration while positive gradients were inhibiting. Interestingly, a gradient of 4% albumin in the upper chamber and 1% albumin in the lower chamber enabled migration in the absence of a SDF-1 gradient (100ng/ml SDF-1 in both upper and lower chamber). Albumin gradients did not stimulate migration in the absence of SDF-1. The absence of serum albumin in both chambers abolished the SDF-1/CXCR4 dependent migration of PBSCs. Our results show that gradients of serum albumin strongly influence the SDF-1/CXCR4 dependent migration of PBSCs. Negative gradients of albumin between blood and bone marrow might be supportive for the homing of PBSC to the stem cell niche, which suggests a new function of the multitask protein serum albumin.