Role of Ras Expression in B-cell Development in Mice

Background

B-cells are lymphocytes produced in the bone marrow (hence “B” cell) that are primarily responsible for the production of antibodies as a means to fight an infection.  These cells contain receptors which are integral membrane proteins that bind to specific antigens.  Once they are bound to the antigens, the B-cells engulf and digest the antigen and are then stimulated to enter the cell cycle.  This entrance into the cell cycle (and subsequent division) leads to the production of more B-cells with identical specificity towards antigens or the differentiation into plasma cells which will produce antibodies.

Here is a short informational video on B-cell lymphocytes:

B-cells have the ability to form Memory B-cells, which can live for a very long time and respond very quickly to repeated infections, increasing the likelihood of the organism’s survival.  Previously, it was only known that B-cell development was heavily influenced by Ras proteins (a family of hydrolases), but it was not known how or at what stage the Ras proteins affected the lymphocyte development.  More information on the role of Ras in B-cell development and differentiation can provide insight that will be useful in further understanding the immune system and may also provide targets for research on lymphatic cancers.

Diagram of how B-cells work in the fight against infections taken from: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/B_and_Tcells.html

Ras Proteins

Ras proteins are a family of hydrolases that are very common in cells as effectors of cellular differentiation and growth.  However, the role of Ras proteins in lymphocyte differentiation has remained unclear.  Recently, researchers found that Ras affects pro-B-cell (progenitor) differentiation in bone marrow cells by enhancing IL-7 (Interleukin 7) receptor levels and suppressing SOCS genes (Suppressors of Cytokine Signalling)(Li).

Li Paper model for the role of Ras expression in effecting B-cell development through SOCS and IL-7 receptors

IL-7 induces proliferation of T-cells in mice and has been shown to have an intricate relationship with cell apoptosis (Kim).  It has also been shown that loss of Ras suppression on SOCS leads to T-cell lymphoma formation (Krebs).  Recent research with dominant-negative-Ras mutant mice showed much lower levels of all stages of B-cells than WT mutants do(Li).

In order to test the role of Ras in B-cell development in mice, the Li and co. researchers used a dominant-negative-Ras mutant strain of mice (that is, these mice were dominant for having nonfunctional Ras genes).  The B-cell lymphocytes of interest were extracted from these mice and the cells were analyzed using flow cytometry techniques.  The dominant-negative mice showed statistically significantly lower numbers of pre-pro-B-cells, pro-B-cells, and pre-B-cells than the wild-type mice did.  The results from this can be seen below:

Data from the Li paper showing the lower levels of pro-B-cells that were due to the lack of IL-7R signals in the domninant-negative-Ras mutants, from extracted bone marrow (left, B) and from fetal mice livers (right, D)

These lower levels of pro-B-cells were due to the lack of IL-7R signals in the dnRas mutants.  The IL-7R induces the phosphorylation of STAT5 (a differentiator of B-cells) in both dnRas and WT mice at artificially high concentrations; however, STAT5 levels were much higher in WT mice than dnRas mice.  When this experiment was done at lower (closer to physiological) concentrations of IL-7R, only WT mice and not the dnRas mice progenitor B-cells phosphorylated STAT5.

The Ras pathway also includes a protein called Erk, which affects B-cell receptors (BCRs, outer membrane-bound antibodies).  Low production of BCRs has been linked to a defect in the B-cell differentiation process, and complete BCR deletion can cause “back-differentiation” of immature B-cells into cells with a pre-B-cell phenotype (Rowland).  Erk also affects pro-B-cell to pre-B-cell differentiation by controlling the cells response to IL-7R, which has lead researchers to believe that Erk mediates differentiation of B-cells along with BCRs.   The activation of Erk through Ras propagates BCR signals in pro-B-cells, and allows the immature B-cells to be exported from bone marrow and into transitional-B-cells, and from there into mature lymphocytes.  Further research into the Ras-Erk pathway will be beneficial to furthering the understanding of Ras’s effects on lymphocyte development.

These findings by Li et al show significant evidence in favor of their proposed model for the role of Ras in B-cell differentiation, especially in respect to the IL-7R/STAT pathway.  That STAT5 induction by IL-7R can restore B-cell differentiation in dnRas mice provides a pathway that has possibility as an anti-lymphoma target for anti-cancer treatments.

Resources

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/B_and_Tcells.html

Li, LX et al A Flt3- and Ras-Dependent Pathway Primes B Cell Development by Inducing a State of IL-7 responsiveness. J Immunol. 2010. 184. 1728-1736

Rowland, SL et al. Ras activation of Erk restores impaired tonic BCR signaling and rescues immature B cell differentiation. J Experimental Medicine. 2010. 207.0607-621

Mandal, M et al Ras orchestrates exit from the cell cycle and light-chain recombination during early B cell development. Nature Immunology. 2009. 10. 1110-1117

Krebs, D et al. SOCS Proteins: Negative regulators of Cytokine Signalling. Stem Cells. 2001. 19. 378-387

Kim, K et al. The Trophic Action of IL-7 on Pro-T Cells: Inhibition of Apoptosis of Pro-T1, -T2, and -T3 Cells Correlates with Bcl-2 and Bax Levels and Is Independent of Fas and p53 Pathways. J. Immunology. 1998. 160. 5735-5741.

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