Nuclear Pores Regulate Muscle Development and Maintenance by Assembling a Localized Mef2C Complex.

Nuclear pore complexes (NPCs) are multiprotein channels connecting the nucleus with the cytoplasm. NPCs have been shown to have tissue-specific composition, suggesting that their function can be specialized. However, the physiological roles of NPC composition changes and their impacts on cellular processes remain unclear. Here we show that the addition of the Nup210 nucleoporin to NPCs during myoblast differentiation results in assembly of an Mef2C transcriptional complex required for efficient expression of muscle structural genes and microRNAs. We show that this NPC-localized complex is essential for muscle growth, myofiber maturation, and muscle cell survival and that alterations in its activity result in muscle degeneration. Our findings suggest that NPCs regulate the activity of functional gene groups by acting as scaffolds that promote the local assembly of tissue-specific transcription complexes and show how nuclear pore composition changes can be exploited to regulate gene expression at the nuclear periphery.

An internal promoter underlies the difference in disease severity between N- and C-terminal truncation mutations of Titin in zebrafish.

Truncating mutations in the giant sarcomeric protein Titin result in dilated cardiomyopathy and skeletal myopathy. The most severely affected dilated cardiomyopathy patients harbor Titin truncations in the C-terminal two-thirds of the protein, suggesting that mutation position might influence disease mechanism. Using CRISPR/Cas9 technology, we generated six zebrafish lines with Titin truncations in the N-terminal and C-terminal regions. Although all exons were constitutive, C-terminal mutations caused severe myopathy whereas N-terminal mutations demonstrated mild phenotypes. Surprisingly, neither mutation type acted as a dominant negative. Instead, we found a conserved internal promoter at the precise position where divergence in disease severity occurs, with the resulting protein product partially rescuing N-terminal truncations. In addition to its clinical implications, our work may shed light on a long-standing mystery regarding the architecture of the sarcomere.

A mammalian siderophore synthesized by an enzyme with a bacterial homolog involved in enterobactin production.

Intracellular iron homeostasis is critical for survival and proliferation. Lipocalin 24p3 is an iron-trafficking protein that binds iron through association with a bacterial siderophore, such as enterobactin, or a postulated mammalian siderophore. Here, we show that the iron-binding moiety of the 24p3-associated mammalian siderophore is 2,5-dihydroxybenzoic acid (2,5-DHBA), which is similar to 2,3-DHBA, the iron-binding component of enterobactin. We find that the murine enzyme responsible for 2,5-DHBA synthesis, BDH2, is the homolog of bacterial EntA, which catalyzes 2,3-DHBA production during enterobactin biosynthesis. RNA interference-mediated knockdown of BDH2 results in siderophore depletion. Mammalian cells lacking the siderophore accumulate abnormally high amounts of cytoplasmic iron, resulting in elevated levels of reactive oxygen species, whereas the mitochondria are iron deficient. Siderophore-depleted mammalian cells and zebrafish embryos fail to synthesize heme, an iron-dependent mitochondrial process. Our results reveal features of intracellular iron homeostasis that are conserved from bacteria through humans.

Selective interaction between Trf3 and Taf3 required for early development and hematopoiesis.

In zebrafish, TATA-box-binding protein (TBP)-related factor 3, Trf3, is required for early development and initiation of hematopoiesis, and functions by promoting expression of a single target gene, mespa. Recent studies have shown that in murine muscle cells, TRF3 interacts with the TBP-associated factor TAF3. Here we investigate the role of Taf3 in zebrafish embryogenesis. We find that like Trf3-depleted zebrafish embryos, Taf3-depleted embryos exhibit multiple developmental defects and fail to undergo hematopoiesis. Both Trf3 and Taf3 are selectively bound to the mespa promoter and are required for mespa expression. Significantly, Taf3 interacts with Trf3 but not Tbp, and a Trf3 mutant that disrupts this interaction fails to support mespa transcription, early development, and hematopoiesis. Thus, a selective interaction between Trf3 and Taf3 is required for early zebrafish development and initiation of hematopoiesis. Finally, we provide evidence that TRF3 and TAF3 are also required for hematopoiesis initiation in the mouse.

Targeting a TAF to make muscle.

In a recent issue of Molecular Cell, Deato et al. (2008) elucidate the basis by which the muscle-specific activator MyoD recruits the core transcription machinery to the promoter of a key regulatory gene involved in myogenic differentiation.

Initiation of zebrafish haematopoiesis by the TATA-box-binding protein-related factor Trf3.

TATA-box-binding protein (TBP)-related factor 3, TRF3 (also called TBP2), is a vertebrate-specific member of the TBP family that has a conserved carboxy-terminal region and DNA-binding domain virtually identical to that of TBP (ref. 1). TRF3 is highly expressed during embryonic development, and studies in zebrafish and Xenopus have shown that it is required for normal embryogenesis. Here we show that zebrafish embryos depleted of Trf3 exhibit multiple developmental defects and, in particular, fail to undergo haematopoiesis. Expression profiling for Trf3-dependent genes identified mespa, which encodes a transcription factor whose murine orthologue is required for mesoderm specification, and chromatin immunoprecipitation verified that Trf3 binds to the mespa promoter. Depletion of Mespa resulted in developmental and haematopoietic defects markedly similar to those induced by Trf3 depletion. Injection of mespa messenger RNA (mRNA) restored normal development to a Trf3-depleted embryo, indicating mespa is the single Trf3 target gene required for zebrafish embryogenesis. Zebrafish embryos depleted of Trf3 or Mespa also failed to express cdx4, a caudal-related gene required for haematopoiesis. Mespa binds to the cdx4 promoter, and epistasis analysis revealed an ordered trf3-mespa-cdx4 pathway. Thus, in zebrafish, commitment of mesoderm to the haematopoietic lineage occurs through a transcription factor pathway initiated by a TBP-related factor.