Poster abstracts

Poster number 13 submitted by Andrea Ladd

CUG-BP1 and MBNL1 may developmentally regulate alternative splicing transitions during embryonic cardiac morphogenesis

Andrea Ladd (Department of Cell Biology, Lerner Research Institute, Cleveland Clinic), Kyle Brimacombe (Department of Cell Biology, Lerner Research Institute, Cleveland Clinic)

Abstract:
CUG binding protein 1 (CUG-BP1) and muscleblind-like protein 1 (MBNL1) have been shown to bind to discrete sites in several of the same pre-mRNAs and regulate cell-specific alternative splicing in an antagonistic manner. We previously proposed that a change in the balance between CUG-BP1 and MBNL1 activities in the developing heart drives a fetal-to-adult transition in cardiac troponin T (cTNT) alternative splicing. To investigate whether CUG-BP1 and MBNL1 also developmentally regulate alternative splicing during embryonic cardiogenesis, we examined CUG-BP1 and MBNL1 expression in the developing chick. Chicken CUG-BP1 and MBNL1 are 94% and 96% identical, respectively, to their human orthologs at the amino acid level. CUG-BP1 is first detected in the developing heart by in situ hybridization during fusion of the primitive heart tube (H&H st.8-10), concomitant with the onset of cardiac differentiation. Expression continues in the heart during cardiac morphogenesis, as the heart tube undergoes looping (st.10-14), the myocardium undergoes trabeculation and endocardial cushion outgrowth septates the heart into chambers (st.18-23). In contrast, MBNL1 is not detected until looping is well underway and induction of endocardial cushion outgrowth has begun (st.14). Western blot analyses revealed that although CUG-BP1 is continuously expressed in the embryonic heart (st.10-35), there is a strong, transient up-regulation of CUG-BP1 protein during later stages of cardiac morphogenesis (st.18-29). During this same time, MBNL1 undergoes a dramatic isoform transition without an appreciable change in the overall level of expression. These changes in CUG-BP1 and MBNL1 expression coincide with a change in the pattern of cTNT alternative splicing, a known pre-mRNA target, as well as several other alternative splicing transitions in the heart. Together these results suggest that CUG-BP1 and MBNL1 proteins not only mediate fetal-to-adult changes in splicing, but also influence developmentally regulated splicing events during embryonic cardiac morphogenesis.

Keywords: alternative splicing, CUG-BP1, MBNL1