Meiotic crossovers are produced when programmed double-strand breaks (DSBs) are repaired by recombination from homologous chromosomes (homologues). In a wide variety of organisms, meiotic HORMA-domain proteins are required to direct DSB repair towards homologues. This inter-homologue bias is required for efficient homology search, homologue alignment, and crossover formation. HORMA-domain proteins are also implicated in other processes related to crossover formation, including DSB formation, inhibition of promiscuous formation of the synaptonemal complex (SC), and the meiotic prophase checkpoint that monitors both DSB processing and SCs. We examined the behavior of two previously uncharacterized meiosis-specific mouse HORMA-domain proteins—HORMAD1 and HORMAD2—in wild-type mice and in mutants defective in DSB processing or SC formation. HORMADs are preferentially associated with unsynapsed chromosome axes throughout meiotic prophase. We observe a strong negative correlation between SC formation and presence of HORMADs on axes, and a positive correlation between the presumptive sites of high checkpoint-kinase ATR activity and hyper-accumulation of HORMADs on axes. HORMADs are not depleted from chromosomes in mutants that lack SCs. In contrast, DSB formation and DSB repair are not absolutely required for depletion of HORMADs from synapsed axes. A simple interpretation of these findings is that SC formation directly or indirectly promotes depletion of HORMADs from chromosome axes. We also find that TRIP13 protein is required for reciprocal distribution of HORMADs and the SYCP1/SC-component along chromosome axes. Similarities in mouse and budding yeast meiosis suggest that TRIP13/Pch2 proteins have a conserved role in establishing mutually exclusive HORMAD-rich and synapsed chromatin domains in both mouse and yeast. Taken together, our observations raise the possibility that involvement of meiotic HORMA-domain proteins in the regulation of homologue interactions is conserved in mammals.

Skeletal muscle stem cells (MuSCs) are required for regeneration of adult muscle following injury, a response that demands activation of mainly quiescent MuSCs. Despite the need for dynamic regulation of MuSC quiescence, relatively little is known about the determinants of this property. Here, we show that Suv4-20h1, an H4K20 dimethyltransferase, controls MuSC quiescence by promoting formation of facultative heterochromatin (fHC). Deletion of Suv4-20h1 reduces fHC and induces transcriptional activation and repositioning of the MyoD locus away from the heterochromatic nuclear periphery. These effects promote MuSC activation, resulting in stem cell depletion and impaired long-term muscle regeneration. Genetic reduction of MyoD expression rescues fHC formation and lost MuSC quiescence, restoring muscle regeneration capacity in Suv4-20h1 mutants. Together, these findings reveal that Suv4-20h1 actively regulates MuSC quiescence via fHC formation and control of the MyoD locus, thereby guarding and preserving the stem cell pool over a lifetime.

Objectives: Although dengue is one of the most common mosquito-borne viral diseases, few studies have investigated the relationship between the dengue virus and mosquito cells, and this study sought to describe the binding and propagation of the dengue viruses in C6/36 cells. Methods: The internalization and production of the dengue virus was assayed by standard plaque assay methodologies, while dengue virus receptor proteins were examined by a virus overlay protein-binding assay and candidate gene analysis coupled with virus inhibition studies. Results: All four serotypes were internalized linearly, and de novo virus production occurred 14–19 h postinfection. Virus overlay protein-binding assay identified a band of 50 kDa for dengue serotypes 2, 3 and 4 which comigrated with a protein that reacts with antibodies directed against the human 37/67-kDa high-affinity laminin receptor. Both antibodies directed against the human 37/67-kDa high-affinity laminin receptor protein and soluble laminin inhibited the binding and internalization of serotypes 3 and 4, but not serotypes 1 and 2. Conclusions: The results suggest that multiple receptors may be used by the dengue virus to enter into insect cells, and that one of these proteins may be a laminin-binding protein.

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Objectives: The mosquito-borne Japanese encephalitis virus is a leading cause of encephalitis worldwide. However, few studies have investigated the kinetics of Japanese encephalitis virus internalization and production in mosquito cells, and fewer still have investigated the nature of the molecules involved in the binding of the virus to mosquito cells. Methods: Using the Aedes albopictus/Stegomyia albopicta-derived C6/36 cell line, Japanese encephalitis virus internalization and production were assayed by standard plaque assay, and virus binding was investigated through preinfection enzymatic treatment of cells and virus overlay protein-binding assay of membrane fractions in native and denaturing gels. Results: The internalization of the virus was nonlinear, and intracellular infective viruses were detected 8 h after infection and exported to the medium 10 h after infection. The internalization of the virus was primarily mediated by protein elements, and several bands were observed after overlay assay to membrane proteins, although mass spectroscopy was unable to identify candidate proteins. Soluble laminin produced a marginal, but dose-dependent inhibition of infection. Conclusions: These results suggest that the mechanism of Japanese encephalitis entry, production, attachment and receptor usage are distinct from those employed by the dengue viruses.

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