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Expression of a negative regulator of human erythropoiesis by fluidized lymphocyte plasma membranes
1Department of Medicine, University of Connecticut Health Center, Farmington 06030.
Insights
Accessory cells regulate erythroid progenitor growth via membrane-bound factors. Modifying B cell membrane fluidity altered factor release and unexpectedly inhibited burst-forming unit-erythroid (BFU-E) proliferation.
Area of Science:
- Hematology
- Cell Biology
- Biochemistry
Background:
- Paracrine factors regulate hematopoiesis, with some localized to accessory cell plasma membranes.
- A unique integral membrane glycoprotein from human B cells acts as a paracrine factor for erythroid progenitor cells.
- This factor is released via extracellular vesicles, which are distinct from parent cell membranes.
Purpose of the Study:
- To investigate the role of B cell plasma membrane fluidity in regulating the release of erythroid-modulating factors.
- To determine if altering membrane fluidity affects the activity of these paracrine factors.
- To explore the potential for positive or negative regulation of erythroid differentiation by accessory cell membranes.
Main Methods:
- Purification of an integral membrane glycoprotein from human B cells.
- Analysis of extracellular vesicle composition and membrane fluidity.
- Fluidization of B cell plasma membranes using lipid emulsions (Liposin II, phosphatidylcholine, phosphatidylethanolamine).
- Assessment of membrane fluidity using steady-state fluorescence polarization.
- Quantification of extracellular vesicle exfoliation.
- Evaluation of effects on burst-forming unit-erythroid (BFU-E) proliferation.
Main Results:
- Extracellular vesicles were biochemically distinct and more fluid than parent B cell membranes.
- Lipid treatment significantly increased vesicle exfoliation (3-4 fold).
- Unexpectedly, fluidized B cells, their shed vesicles, and conditioned media exhibited an inhibitory effect on BFU-E proliferation.
- Membrane fluidity was reduced in lipid-treated cells.
Conclusions:
- Accessory cell plasma membrane composition and fluidity can modulate the release and activity of erythroid regulatory factors.
- Altering membrane fluidity can lead to both increased factor release and potent inhibition of erythroid progenitor proliferation.
- These findings suggest a dynamic mechanism where accessory cells can switch between promoting and inhibiting erythroid differentiation based on membrane lipid exchange.
Abstract:
Regulation of hematopoiesis by paracrine molecules occurs in vitro. In some cases, hematopoietic paracrine factors have been localized to the plasma membrane of accessory cells. We have purified a unique integral membrane glycoprotein from normal human B cells that functions in vitro as a paracrine factor whose activity is directed toward erythroid progenitor cells. This factor is also spontaneously exfoliated from the cell surface as a component of extracellular vesicles. Analysis of the lipid and protein compositions and membrane lipid order of these extracellular vesicles reveals them to be biochemically distinct and more fluid than their parent membranes. Evidence in nonhematopoietic culture systems indicates that cell membrane function may be altered by modifying membrane fluidity. In an effort to accelerate growth factor release, plasma membranes of B cells were fluidized by incubation with an emulsion of Liposin II, phosphatidylcholine, and phosphatidylethanolamine. Fluidity assessed by steady-state fluorescence polarization was reduced in lipid-treated cells. Exfoliation was 3-4-fold higher from lipid-treated cells relative to untreated cells. Unexpectedly, a negative signal for burst-forming unit-erythroid (BFU-E) proliferation was expressed in membranes, in shed extracellular vesicles, and in supernatants of medium conditioned by the fluidized cells. Purification of the inhibitor is under way. The data are consistent with the hypothesis that accessory cell plasma membranes may positively or negatively regulate erythroid differentiation, depending upon the exchange of cholesterol and phospholipids between plasma membrane and ambient lipid pools.