Samantha Harris

Thin filament length (TFL) is an important determinant of the force-sarcomere length (SL) relation of cardiac muscle. However, the various mechanisms that control TFL are not well understood. Here we tested the previously proposed hypothesis that the actin-binding protein nebulin contributes to TFL regulation in the heart by using a cardiac-specific nebulin cKO mouse model (αMHC Cre Neb cKO). Atrial myocytes were studied because nebulin expression has been reported to be most prominent in this cell type. TFL was measured in right and left atrial myocytes using deconvolution optical microscopy and staining for filamentous actin with phalloidin and for the thin filament pointed-end with an antibody to the capping protein Tropomodulin-1 (Tmod1). Results showed that TFLs in Neb cKO and littermate control mice were not different. Thus, deletion of nebulin in the heart does not alter TFL. However, TFL was found to be ~0.05μm longer in the right than in the left atrium and Tmod1 expression was increased in the right atrium. We also tested the hypothesis that the length of titin's spring region is a factor controlling TFL by studying the Rbm20(ΔRRM) mouse which expresses titins that are ~500kDa (heterozygous mice) and ~1000kDa (homozygous mice) longer than in control mice. Results revealed that TFL was not different in Rbm20(ΔRRM) mice. An unexpected finding in all genotypes studied was that TFL increased as sarcomeres were stretched (~0.1μm per 0.35μm of SL increase). This apparent increase in TFL reached a maximum at a SL of ~3.0μm where TFL was ~1.05μm. The SL dependence of TFL was independent of chemical fixation or the presence of cardiac myosin-binding protein C (cMyBP-C). In summary, we found that in cardiac myocytes TFL varies with SL in a manner that is independent of the size of titin or the presence of nebulin.

The N-terminus of cMyBP-C can activate actomyosin interactions in the absence of Ca2+, but it is unclear which domains are necessary. Prior studies suggested that the Pro-Ala rich region of human cMyBP-C activated force in permeabilized human cardiomyocytes, whereas the C1 and M-domains of mouse cMyBP-C activated force in permeabilized rat cardiac trabeculae. Because the amino acid sequence of the P/A region differs between human and mouse cMyBP-C isoforms (46% identity), we investigated whether species-specific differences in the P/A region could account for differences in activating effects. Using chimeric fusion proteins containing combinations of human and mouse C0, Pro-Ala, and C1 domains, we demonstrate here that the human P/A and C1 domains activate actomyosin interactions, whereas the same regions of mouse cMyBP-C are less effective. These results suggest that species-specific differences between homologous cMyBP-C isoforms confer differential effects that could fine-tune cMyBP-C function in hearts of different species.

Cardiac myosin binding protein-C (cMyBP-C) is an accessory protein found in the A-bands of vertebrate sarcomeres and mutations in the cMyBP-C gene are a leading cause of familial hypertrophic cardiomyopathy. The regulatory functions of cMyBP-C have been attributed to the N-terminus of the protein, which is composed of tandem immunoglobulin (Ig)-like domains (C0, C1, and C2), a region rich in proline and alanine residues (the Pro-Ala rich region) that links C0 and C1, and a unique sequence referred to as the MyBP-C motif, or M-domain, that links C1 and C2. Recombinant proteins that contain various combinations of the N-terminal domains of cMyBP-C can activate actomyosin interactions in the absence of Ca(2+), but the specific sequences required for these effects differ between species; the Pro-Ala region has been implicated in human cMyBP-C whereas the C1 and M-domains appear important in mouse cMyBP-C. To investigate whether species-specific differences in sequence can account for the observed differences in function, we compared sequences of the Pro-Ala rich region in cMyBP-C isoforms from different species. Here we report that the number of proline and alanine residues in the Pro-Ala rich region varies significantly between different species and that the number correlates directly with mammalian body size and inversely with heart rate. Thus, systematic sequence differences in the Pro-Ala rich region of cMyBP-C may contribute to observed functional differences in human versus mouse cMyBP-C isoforms and suggest that the Pro-Ala region may be important in matching contractile speed to cardiac function across species.

Cardiac myosin-binding protein-C (cMyBP-C) is a thick-filament-associated protein that performs regulatory and structural roles within cardiac sarcomeres. It is a member of the immunoglobulin (Ig) superfamily of proteins consisting of eight Ig- and three fibronectin (FNIII)-like domains, along with a unique regulatory sequence referred to as the M-domain, whose structure is unknown. Domains near the C-terminus of cMyBP-C bind tightly to myosin and mediate the association of cMyBP-C with thick (myosin-containing) filaments, whereas N-terminal domains, including the regulatory M-domain, bind reversibly to myosin S2 and/or actin. The ability of MyBP-C to bind to both myosin and actin raises the possibility that cMyBP-C cross-links myosin molecules within the thick filament and/or cross-links myosin and thin (actin-containing) filaments together. In either scenario, cMyBP-C could be under mechanical strain. However, the physical properties of cMyBP-C and its behavior under load are completely unknown. Here, we investigated the mechanical properties of recombinant baculovirus-expressed cMyBP-C using atomic force microscopy to assess the stability of individual cMyBP-C molecules in response to stretch. Force-extension curves showed the presence of long extensible segment(s) that became stretched before the unfolding of individual Ig and FNIII domains, which were evident as sawtooth peaks in force spectra. The forces required to unfold the Ig/FNIII domains at a stretch rate of 500 nm/s increased monotonically from ∼30 to ∼150 pN, suggesting a mechanical hierarchy among the different Ig/FNIII domains. Additional experiments using smaller recombinant proteins showed that the regulatory M-domain lacks significant secondary or tertiary structure and is likely an intrinsically disordered region of cMyBP-C. Together, these data indicate that cMyBP-C exhibits complex mechanical behavior under load and contains multiple domains with distinct mechanical properties.