Characterization of TBX20 isoforms and protein interactions in heart development

Proper cardiogenesis is critical for the development of vertebrates. Abnormalities in cardiogenesis can lead to congenital heart defects (CHDs), which occur in approximately 1% of live births. The cardiac transcription factor network contains different transcription factor families which direct the expression of critical cardiac genes. Determining how the cardiac transcription factors are regulated will provide insight in the mechanisms of cardiogenesis and CHDs. The T-box (TBX) transcription factor family is an ancient gene family important for development. Several TBX genes are expressed within the developing heart and play critical roles in differentiation, proliferation, and morphogenesis. One important TBX protein is TBX20, which is critical for cardiogenesis in mice. In humans, missense mutations in TBX20 have been found in patients with congenital heart defects.

Characterization of modifiers of TBX20 activity will help elucidate the genetic mechanisms of heart development and CHDs. A yeast two-hybrid screen using an embryonic mouse heart cDNA library and TBX20b as bait was used to identify protein interactions. This led to the identification of an interaction with muskelin (MKLN1), a primarily cytoplasmic protein with potential roles in scaffolding of signal transduction machinery and nucleocytoplasmic protein shuttling. MKLN1 directly binds to the T-box DNA-binding domain of only the TBX20b isoform by its kelch repeats domain. Immunostaining of transfected cells revealed colocalization within the cytoplasm; however, there was no change in subcellular localization of TBX20b by overexpression of MKLN. Immunohistochemistry staining of embryonic mouse hearts indicated coexpression in the endocardial valvular and myocardial interventricular cells. This study identified and characterized a novel protein interaction with the potential to regulate TBX20 activity.

The TBX transcription factor family includes many important regulators of vertebrate development. Currently, three TBX genes are alternatively spliced into different isoforms. This chapter reviews the importance of the cardiac transcription factor network and emphasizes the need for isoform-specific expression and functional analyses. Overall, this dissertation helps to define the cardiac transcription factor network and highlights the importance of isoforms and interacting proteins.

Keywords: heart development, T-box transcription factors, TBX20, MKLN1, proteinprotein interaction