RNAi Screening Identifies A Novel Role for A-Kinase Anchoring Protein 12 (AKAP12) in B Cell Development and Function

Abstract 855

The cAMP signaling pathway has emerged as a key regulator of hematopoietic cell proliferation, differentiation, and apoptosis. Signal specificity is achieved through local activation of signaling enzymes that are anchored to subcellular organelles and membranes. In particular, A-kinase anchoring proteins (AKAPs) coordinate and control cAMP responsive events. AKAPs were originally classified based on their ability to bind cAMP-dependent protein kinase (protein kinase A; PKA). The activity of PKA is regulated by its two regulatory subunits, which from a dimer that binds to the two catalytic subunits. Binding of cAMP to the regulatory dimer dissociates the catalytic subunits and activates PKA. Anchoring of PKA by AKAPs constrains PKA activity to a relevant subset of potential substrates. Thus, AKAPs contribute to the precision of intracellular signaling events by directing anchored enzyme pools to a subset of their physiological substrates at specific subcellular localizations.

Using an in vitro short hairpin RNA (shRNA) screen against potentially druggable targets, we have uncovered a requirement for AKAP12 in the proliferation of a cultured pre-B cell leukemia cell line. In the hematopoietic system of mice and humans, expression of AKAP12 is tightly restricted to the pro/pre/immature stages of B lymphopoiesis, suggesting a potential role in pre-B cell receptor (pre-BCR) or BCR signaling. We find that retroviral knockdown or germline knockout of AKAP12 in mice leads to an increase in pre B and immature B cells in the bone marrow. In contrast, B cell numbers in the spleen are significantly reduced, as are recirculating B cells in the bone marrow. Transplantation of AKAP12 null hematopoietic stem and progenitor cells from fetal liver into wildtype recipients demonstrates an autonomous defect in the development of AKAP12−/− B cells. Competitive bone marrow transplantations confirm that this defect is cell autonomous and not due to a defective bone marrow environment or secretion of a B cell inhibitory factor.

To identify AKAP12 interaction partners, we overexpressed FLAG-epitope tagged AKAP12 in a pre-B cell leukemia cell line. Affinity purification of AKAP12 showed a repeated co-immunoprecipitation of poorly characterized RIO kinase 1 (RIOK1). Our current efforts are focused on investigating the interaction between RIOK1 and AKAP12 and their role in the control of B cell development and cell cycle progression. Further, we are focusing on a likely role for AKAP12 in the scaffolding of PKA, PKC and phosphodiesterases by analyzing the activation of signaling cascades in cultured primary wildtype and AKAP12−/− pre B cells. Additionally, we are investigating the role of the BCR in vivo by testing if enforced expression of BCR components rescue B cell development in a AKAP12−/− BCR transgenic mouse model (SW_HEL mouse). In summary, we have confirmed a novel role for AKAP12 in B lymphopoiesis.