Structural basis of nucleosome dynamics modulation by histone variants H2A.B and H2A.Z.2.2.

Nucleosomes are dynamic entities with wide-ranging compositional variations. Human histone variants H2A.B and H2A.Z.2.2 play critical roles in multiple biological processes by forming unstable nucleosomes and open chromatin structures, but how H2A.B and H2A.Z.2.2 confer these dynamic features to nucleosomes remains unclear. Here, we report cryo-EM structures of nucleosome core particles containing human H2A.B (H2A.B-NCP) at atomic resolution, identifying large-scale structural rearrangements in the histone octamer in H2A.B-NCP. H2A.B-NCP compacts approximately 103 bp of DNA wrapping around the core histones in approximately 1.2 left-handed superhelical turns, in sharp contrast to canonical nucleosome encompassing approximately 1.7 turns of DNA. Micrococcal nuclease digestion assay reveals that nineteen H2A.B-specific residues, including a ROF ("regulating-octamer-folding") sequence of six consecutive residues, are responsible for loosening of H2A.B-NCPs. Unlike H2A.B-NCP, the H2A.Z.2.2-containing nucleosome (Z.2.2-NCP) adopts a less-extended structure and compacts around 125 bp of DNA. Further investigation uncovers a crucial role for the H2A.Z.2.2-specific ROF in both H2A.Z.2.2-NCP opening and SWR1-dependent histone replacement. Taken together, these first high-resolution structure of unstable nucleosomes induced by histone H2A variants elucidate specific functions of H2A.B and H2A.Z.2.2 in enhancing chromatin dynamics.

Neural progenitor cells mediated by H2A.Z.2 regulate microglial development via Cxcl14 in the embryonic brain.

Microglia, the resident immune cells of the central nervous system, play an important role in the brain. Microglia have a special spatiotemporal distribution during the development of the cerebral cortex. Neural progenitor cells (NPCs) are the main source of neural-specific cells in the early brain. It is unclear whether NPCs affect microglial development and what molecular mechanisms control early microglial localization. H2A.Z.2, a histone variant of H2A, has a key role in gene expression regulation, genomic stability, and chromatin remodeling, but its function in brain development is not fully understood. Here, we found that the specific deletion of H2A.Z.2 in neural progenitor cells led to an abnormal increase in microglia in the ventricular zone/subventricular zone (VZ/SVZ) of the embryonic cortex. Mechanistically, H2A.Z.2 regulated microglial development by incorporating G9a into the promoter region of Cxcl14 and promoted H3k9me2 modification to inhibit the transcription of Cxcl14 in neural progenitor cells. Meanwhile, we found that the deletion of H2A.Z.2 in microglia itself had no significant effect on microglial development in the early cerebral cortex. Our findings demonstrate a key role of H2A.Z.2 in neural progenitor cells in controlling microglial development and broaden our knowledge of 2 different types of cells that may affect each other through crosstalk in the central nervous system.

Structural polymorphism in the L1 loop regions of human H2A.Z.1 and H2A.Z.2.

The histone H2A.Z variant is widely conserved among eukaryotes. Two isoforms, H2A.Z.1 and H2A.Z.2, have been identified in vertebrates and may have distinct functions in cell growth and gene expression. However, no structural differences between H2A.Z.1 and H2A.Z.2 have been reported. In the present study, the crystal structures of nucleosomes containing human H2A.Z.1 and H2A.Z.2 were determined. The structures of the L1 loop regions were found to clearly differ between H2A.Z.1 and H2A.Z.2, although their amino-acid sequences in this region are identical. This structural polymorphism may have been induced by a substitution that evolutionally occurred at the position of amino acid 38 and by the flexible nature of the L1 loops of H2A.Z.1 and H2A.Z.2. It was also found that in living cells nucleosomal H2A.Z.1 exchanges more rapidly than H2A.Z.2. A mutational analysis revealed that the amino-acid difference at position 38 is at least partially responsible for the distinctive dynamics of H2A.Z.1 and H2A.Z.2. These findings provide important new information for understanding the differences in the regulation and functions of H2A.Z.1 and H2A.Z.2 in cells.

GAS41 mediates proliferation and GEM chemoresistance via H2A.Z.2 and Notch1 in pancreatic cancer.

PURPOSE: GAS41 is a YEATS domain protein that binds to acetylated histone H3 to promote the chromatin deposition of H2A.Z in non-small cell lung cancer. The role of GAS41 in pancreatic cancer is still unknown. Here, we aimed to reveal this role. METHODS: GAS41 expression in pancreatic cancer tissues and cell lines was examined using qRT-PCR, Western blotting and immunohistochemistry. MTT, colony formation, spheroid formation and in vivo tumorigenesis assays were performed to assess the proliferation, tumorigenesis, stemness and gemcitabine (GEM) resistance of pancreatic cancer cells. Mechanistically, co-immunoprecipitation (co-IP) and chromatin immunoprecipitation (ChIP) assays were used to evaluate the roles of GAS41, H2A.Z.2 and Notch1 in pancreatic cancer. RESULTS: We found that GAS41 is overexpressed in human pancreatic cancer tissues and cell lines, and that its expression increases following the acquisition of GEM resistance. We also found that GAS41 up-regulates Notch, as well as pancreatic cancer cell stemness and GEM resistance in vitro and in vivo. We show that GAS41 binds to H2A.Z.2 and activates Notch and its downstream mediators, thereby regulating stemness and drug resistance. Depletion of GAS41 or H2A.Z.2 was found to down-regulate Notch and to sensitize pancreatic cancer cells to GEM. CONCLUSION: Our data indicate that GAS41 mediates proliferation and GEM resistance in pancreatic cancer cells via H2A.Z.2 and Notch1.

Deciphering the Enigma of the Histone H2A.Z-1/H2A.Z-2 Isoforms: Novel Insights and Remaining Questions.

The replication independent (RI) histone H2A.Z is one of the more extensively studied variant members of the core histone H2A family, which consists of many replication dependent (RD) members. The protein has been shown to be indispensable for survival, and involved in multiple roles from DNA damage to chromosome segregation, replication, and transcription. However, its functional involvement in gene expression is controversial. Moreover, the variant in several groups of metazoan organisms consists of two main isoforms (H2A.Z-1 and H2A.Z-2) that differ in a few (3-6) amino acids. They comprise the main topic of this review, starting from the events that led to their identification, what is currently known about them, followed by further experimental, structural, and functional insight into their roles. Despite their structural differences, a direct correlation to their functional variability remains enigmatic. As all of this is being elucidated, it appears that a strong functional involvement of isoform variability may be connected to development.

SUMO modification system facilitates the exchange of histone variant H2A.Z-2 at DNA damage sites.

Histone exchange and histone post-translational modifications play important roles in the regulation of DNA metabolism, by re-organizing the chromatin configuration. We previously demonstrated that the histone variant H2A.Z-2 is rapidly exchanged at damaged sites after DNA double strand break induction in human cells. In yeast, the small ubiquitin-like modifier (SUMO) modification of H2A.Z is involved in the DNA damage response. However, whether the SUMO modification regulates the exchange of human H2A.Z-2 at DNA damage sites remains unclear. Here, we show that H2A.Z-2 is SUMOylated in a damage-dependent manner, and the SUMOylation of H2A.Z-2 is suppressed by the depletion of the SUMO E3 ligase, PIAS4. Moreover, PIAS4 depletion represses the incorporation and eviction of H2A.Z-2 at damaged sites. These findings demonstrate that the PIAS4-mediated SUMOylation regulates the exchange of H2A.Z-2 at DNA damage sites.

Histone Hypervariants H2A.Z.1 and H2A.Z.2 Play Independent and Context-Specific Roles in Neuronal Activity-Induced Transcription of Arc/Arg3.1 and Other Immediate Early Genes.

The histone variant H2A.Z is an essential and conserved regulator of eukaryotic gene transcription. However, the exact role of this histone in the transcriptional process remains perplexing. In vertebrates, H2A.Z has two hypervariants, H2A.Z.1 and H2A.Z.2, that have almost identical sequences except for three amino acid residues. Due to such similarity, functional specificity of these hypervariants in neurobiological processes, if any, remain largely unknown. In this study with dissociated rat cortical neurons, we asked if H2A.Z hypervariants have distinct functions in regulating basal and activity-induced gene transcription. Hypervariant-specific RNAi and microarray analyses revealed that H2A.Z.1 and H2A.Z.2 regulate basal expression of largely nonoverlapping gene sets, including genes that code for several synaptic proteins. In response to neuronal activity, rapid transcription of our model gene Arc is impaired by depletion of H2A.Z.2, but not H2A.Z.1. This impairment is partially rescued by codepletion of the H2A.Z chaperone, ANP32E. In contrast, under a different context (after 48 h of tetrodotoxin, TTX), rapid transcription of Arc is impaired by depletion of either hypervariant. Such context-dependent roles of H2A.Z hypervariants, as revealed by our multiplexed gene expression assays, are also evident with several other immediate early genes, where regulatory roles of these hypervariants vary from gene to gene under different conditions. Together, our data suggest that H2A.Z hypervariants have context-specific roles that complement each other to mediate activity-induced neuronal gene transcription.