Secreted frizzled related proteins (sFRPs) have emerged as key regulators of a wide range of developmental and disease processes, with virtually all known functions of mammalian sFRPs attributed to their ability to antagonize Wnt signaling. Recently however, the Xenopus and zebrafish sFRP, Sizzled, was shown to function as an antagonist of Chordin processing by Tolloid-like metalloproteinases, leading to the proposal that sFRPs may function as evolutionarily-conserved antagonists of the chordinase activities of this class of proteinases. Herein, in contrast to this proposal, we show that the mammalian sFRP, sFRP2, does not affect Chordin processing, but instead can serve as a direct enhancer of the procollagen C-proteinase activity of Tolloid-like metalloproteinases. We further show that the level of fibrosis, in which procollagen processing by Tolloid-like proteinases plays a rate-limiting role, is markedly reduced in sFRP2-null mice subjected to myocardial infarction. Importantly, this reduced level of fibrosis is accompanied by significantly improved cardiac function. This study thus uncovers a novel function for sFRP2 and a potential therapeutic application for sFRP2 antagonism in controlling fibrosis in the infarcted heart.

The serum response factor (SRF) binds to coactivators, such as myocardin-related transcription factor-A (MRTF-A), and mediates gene transcription elicited by diverse signaling pathways. SRF/MRTF-A-dependent gene transcription is activated when nuclear MRTF-A levels increase, enabling the formation of transcriptionally active SRF/MRTF-A complexes. The level of nuclear MRTF-A is regulated by nuclear G-actin, which binds to MRTF-A and promotes its nuclear export. However, pathways that regulate nuclear actin levels are poorly understood. Here we show that MICAL-2, an atypical actin-regulatory protein, mediates SRF/MRTF-A-dependent gene transcription elicited by nerve growth factor and serum. MICAL-2 induces redox-dependent depolymerization of nuclear actin, which decreases nuclear G-actin and increases MRTF-A in the nucleus. Furthermore, we show that MICAL-2 is a target of CCG-1423, a small molecule inhibitor of SRF/MRTF-A-dependent transcription that exhibits efficacy in various preclinical disease models. These data identify redox modification of nuclear actin as a regulatory switch that mediates SRF/MRTF-A-dependent gene transcription.

SUMMARYThe GATA4 transcription factor is implicated in promoting cardiogenesis in combination with other factors, including TBX5, MEF2C and BAF60C. However, when expressed in embryonic stem cells (ESCs), GATA4 was shown to promote endoderm, not cardiac mesoderm. The capacity of related GATA factors to promote cardiogenesis is untested. We found that expression of the highly related gene, Gata5, very efficiently promotes cardiomyocyte fate from murine ESCs. Gata5 directs development of beating sheets of cells that express cardiac troponin T and show a full range of action potential morphologies that are responsive to pharmacological stimulation. We discovered that by removing serum from the culture conditions, GATA4 and GATA6 are each also able to efficiently promote cardiogenesis in ESC derivatives, with some distinctions. Thus, GATA factors can function in ESC derivatives upstream of other cardiac transcription factors to direct the efficient generation of cardiomyocytes.

BackgroundRetinoids regulate key developmental pathways throughout life, and have potential uses for differentiation therapy. It should be possible to identify novel retinoids by coupling new chemical reactions with screens using the zebrafish embryonic model.Principal FindingsWe synthesized novel retinoid analogues and derivatives by amide coupling, obtaining 80–92% yields. A small library of these compounds was screened for bioactivity in living zebrafish embryos. We found that several structurally related compounds significantly affect development. Distinct phenotypes are generated depending on time of exposure, and we characterize one compound (BT10) that produces specific cardiovascular defects when added 1 day post fertilization. When compared to retinoic acid (ATRA), BT10 shows similar but not identical changes in the expression pattern of embryonic genes that are known targets of the retinoid pathway. Reporter assays determined that BT10 interacts with all three RAR receptor sub-types, but has no activity for RXR receptors, at all concentrations tested.ConclusionsOur screen has identified a novel retinoid with specificity for retinoid receptors. This lead compound may be useful for manipulating components of retinoid signaling networks, and may be further derivatized for enhanced activity.

Mammalian histone deacetylases (HDACs) undergo phosphorylation to regulate their localization, activity and function. However, little is known about the regulation of plant HDAC function and activity by phosphorylation. Here, we report the crystal structure of the Reduced Potassium Dependency3/Histone Deacetylase1 (RPD3/HDA1) type class II histone deacetylase HDA15 in Arabidopsis (Arabidopsis thaliana). The histone deacetylase domain of HDA15 (HDA15HD) assembles as tetrameric forms with each monomer composed of 12 α-helices and 9 β-sheets. The L1 loop and β2 sheet of HDA15HD are the essential interfaces for the tetramer formation. The N-terminal zinc finger domain enhances HDA15HD dimerization and increases its enzymatic activity. Furthermore, HDA15 can also be phosphorylated at Ser448 and Ser452 in etiolated seedlings. The HDA15 phosphorylation status determines its sub-nuclear localization and oligomerization. Phosphomimetics of HDA15 partially disrupt its oligomerization and cause loss of enzymatic activity and translocation from the nucleolus into nucleoplasm. Together, these data indicate that phosphorylation plays a critical role in regulating the structure and function of HDA15.