The Samd14-Capping Protein Complex Controls Cell Signaling in the Erythropoietic Stress Response

In anemia, restoring homeostatic levels of erythrocytes requires an erythropoietic regenerative response to accelerate red blood cell (RBC) production. Elucidating mechanisms that drive the process of erythropoiesis in the context of regeneration or "stress erythropoiesis" can reveal new strategies for targeting ineffective erythropoiesis. An important component of stress erythropoiesis involves stress-dependent activation of genes/proteins through transcriptional enhancers. We discovered an enhancer in intron 1 of the Sterile Alpha Motif (SAM) Domain 14 (Samd14) gene elevates Samd14 expression, facilitates SCF/c-Kit signaling, and is needed for survival in a hemolytic anemia model. However, it is dispensable for erythropoietic development. Our prior work demonstrated that the SAM domain of Samd14 promotes c-Kit-mediated cellular signaling to regulate progenitor function, and this SAM domain has functional attributes unique from those of structurally related SAM domains. Using immunoprecipitation-mass spectrometry, we determined that Samd14 interacts with the Capzβ protein. Capzβ is a component of the actin capping protein (CP) complex, which interact as α- and β heterodimers during actin filament assembly/disassembly. CP exerts diverse functions in cell motility, vesicular transport, cell signaling and cytokinesis. Using a series of Samd14 deletion constructs, we tested whether the Samd14-Capzβ interaction is important for Samd14 promotion of c-Kit signaling in stress erythroid progenitors. Our findings determined that the region of Samd14 required for binding to Capzβ (amino acids 38-54) were required to restore c-Kit signaling. Our ongoing studies are examining whether Samd14-Capzβ is similarly required for colony formation and cell survival of stress erythroid progenitors, and whether additional SAM domain-containing proteins have a similar role in regulating stress erythropoiesis. Understanding the fundamental drivers of regenerative erythropoiesis can lead to new therapeutic strategies and prognostic/diagnostic markers.