The Emerging Role of the RBM20 and PTBP1 Ribonucleoproteins in Heart Development and Cardiovascular Diseases

Abstract

Alternative splicing is a regulatory mechanism essential for cell differentiation and tissue organization. More than 90% of human genes are regulated by alternative splicing events, which participate in cell fate determination. The general mechanisms of splicing events are well known, whereas only recently have deep-sequencing, high throughput analyses and animal models provided novel information on the network of functionally coordinated, tissue-specific, alternatively spliced exons. Heart development and cardiac tissue differentiation require thoroughly regulated splicing events. The ribonucleoprotein RBM20 is a key regulator of the alternative splicing events required for functional and structural heart properties, such as the expression of TTN isoforms. Recently, the polypyrimidine tract-binding protein PTBP1 has been demonstrated to participate with RBM20 in regulating splicing events. In this review, we summarize the updated knowledge relative to RBM20 and PTBP1 structure and molecular function; their role in alternative splicing mechanisms involved in the heart development and function; RBM20 mutations associated with idiopathic dilated cardiovascular disease (DCM); and the consequences of RBM20-altered expression or dysfunction. Furthermore, we discuss the possible application of targeting RBM20 in new approaches in heart therapies.

Keywords: DCM; PTBP1; RBM20; RNA binding proteins; RRM motif; alternative splicing; exon exclusion; heart development; ribonucleoproteins; titin.

Figure 1

Schematic representation of the RBM20 and PTBP protein structures and multi-alignment of the RRM domains. (a) Numbers indicate the position of the amino acid residues relative to the protein domains. E-rich, glutamate-rich region; L-rich, leucine-rich region; P-rich, proline-rich region. RS, arginine/serine-rich region; ZnF1-2, zinc finger domains; NLS, nuclear localization signal; NES, nuclear export signal; RRM1 to 4, RNA-recognition motif domains. Percentage of homology of PTBP proteins is indicated relative to PTBP1. (b) Structure-based sequence alignment of the PTBP and RBM20 RRM domains. The alignment was performed by Clustal Omega analysis and edited using Jalview software [37]. Secondary structure elements predicted by the JPRED tool are indicated below the alignment. The RNA-binding domain cores, RNP1and RNP2, are indicated.

Figure 2

Schematic representation of examples of pre-mRNAs regulated by RBM20 and PTBP1. Colored exons represent constitutively spliced exons, while white exons represent alternative exons. (a) TTN (titin), CACNA1C (Calcium Voltage-Gated Channel Subunit Alpha1 C) and CAMK2D (Calcium/Calmodulin Dependent Protein Kinase II Delta) are examples of pre-mRNAs regulated by RBM20. (b) TPM1 (α-tropomyosin), TNNT2 (Troponin T2, Cardiac Type) and ACTN1 (α-actinin) are examples of pre-mRNAs regulated by PTBP1. (c) TTN (titin) and FHOD3 (Formin Homology 2 Domain Containing 3) are examples of pre-mRNAs regulated by both PTBP1 and RBM20.

Figure 3

Schematic representation of RBM20 and PTBP1 temporal expression during heart development.