Deregulation of Drosha in the pathogenesis of hereditary hemorrhagic telangiectasia.

PURPOSE OF REVIEW: The TGFß (transforming growth factor ß) superfamily - a large group of structurally related and evolutionarily conserved proteins - profoundly shapes and organizes the vasculature during normal development and adult homeostasis. Mutations inactivating several of its ligands, receptors, or signal transducers set off hereditary hemorrhagic telangiectasia (HHT), a disorder that causes capillary networks to form incorrectly. Drosha, an essential microRNA-processing enzyme, also interfaces with TGFß signal transducers, but its involvement in vascular conditions had not been tested until recently. This review summarizes current evidence that links mutations of Drosha to HHT. RECENT FINDINGS: Genetic studies have revealed that rare missense mutations in the Drosha gene occur more commonly among HHT patients than in healthy people. Molecular analyses also indicated that Drosha enzymes with HHT-associated mutations generate microRNAs less efficiently than their wild-type counterpart when stimulated by TGFß ligands. In zebrafish or mouse, mutant Drosha proteins cause the formation of dilated, leaky blood vessels deprived of capillaries, similar to those typically found in patients with HHT. SUMMARY: Recent evidence suggests that Drosha-mediated microRNA biogenesis contributes significantly to the control of vascular development and homeostasis by TGFß. Loss or reduction of Drosha function may predispose carriers to HHT and possibly other vascular diseases.

Smad proteins bind a conserved RNA sequence to promote microRNA maturation by Drosha.

The signal transducers of the transforming growth factor beta (TGFbeta)/bone morphogenetic protein (BMP), the Smads, promote the expression of a subset of miRNAs by facilitating the cleavage reaction by Drosha. The mechanism that limits Smad-mediated processing to a selective group of miRNAs remained hitherto unexplored. In this study, we expand the number of TGFbeta/BMP-regulated miRNAs (T/B-miRs) to 20. Of interest, a majority of T/B-miRs contain a consensus sequence (R-SBE) within the stem region of the primary transcripts of T/B-miRs (pri-T/B-miRs). Here, we demonstrate that Smads directly bind the R-SBE. Mutation of the R-SBE abrogates TGFbeta/BMP-induced recruitment of Smads, Drosha, and DGCR8 to pri-T/B-miRs and impairs their processing, whereas introduction of R-SBE to unregulated pri-miRNAs is sufficient to recruit Smads and to allow regulation by TGFbeta/BMP. Thus, Smads are multifunctional proteins that modulate gene expression transcriptionally through DNA binding and posttranscriptionally through pri-miRNA binding and regulation of miRNA processing.

Acetylation of p53 stimulates miRNA processing and determines cell survival following genotoxic stress.

It is widely accepted that different forms of stress activate a common target, p53, yet different outcomes are triggered in a stress-specific manner. For example, activation of p53 by genotoxic agents, such as camptothecin (CPT), triggers apoptosis, while non-genotoxic activation of p53 by Nutlin-3 (Nut3) leads to cell-cycle arrest without significant apoptosis. Such stimulus-specific responses are attributed to differential transcriptional activation of various promoters by p53. In this study, we demonstrate that CPT, but not Nut3, induces miR-203, which downregulates anti-apoptotic bcl-w and promotes cell death in a p53-dependent manner. We find that acetylation of K120 in the DNA-binding domain of p53 augments its association with the Drosha microprocessor and promotes nuclear primary miRNA processing. Knockdown of human orthologue of Males absent On the First (hMOF), the acetyltransferase that targets K120 in p53, abolishes induction of miR-203 and cell death mediated by CPT. Thus, this study reveals that p53 acetylation at K120 plays a critical role in the regulation of the Drosha microprocessor and that post-transcriptional regulation of gene expression by p53 via miRNAs plays a role in determining stress-specific cellular outcomes.

Molecular basis for antagonism between PDGF and the TGFbeta family of signalling pathways by control of miR-24 expression.

Modulation of the vascular smooth-muscle-cell (vSMC) phenotype from a quiescent 'contractile' phenotype to a proliferative 'synthetic' phenotype has been implicated in vascular injury repair, as well as pathogenesis of vascular proliferative diseases. Both bone morphogenetic protein (BMP) and transforming growth factor-beta (TGFbeta)-signalling pathways promote a contractile phenotype, while the platelet-derived growth factor-BB (PDGF-BB)-signalling pathway promotes a switch to the synthetic phenotype. Here we show that PDGF-BB induces microRNA-24 (miR-24), which in turn leads to downregulation of Tribbles-like protein-3 (Trb3). Repression of Trb3 coincides with reduced expression of Smad proteins and decrease in BMP and TGFbeta signalling, promoting a synthetic phenotype in vSMCs. Inhibition of miR-24 by antisense oligonuclotides abrogates the downregulation of Trb3 as well as pro-synthetic activity of the PDGF-signalling pathway. Thus, this study provides a molecular basis for the antagonism between the PDGF and TGFbeta pathways, and its effect on the control of the vSMC phenotype.

SMAD proteins control DROSHA-mediated microRNA maturation.

MicroRNAs (miRNAs) are small non-coding RNAs that participate in the spatiotemporal regulation of messenger RNA and protein synthesis. Aberrant miRNA expression leads to developmental abnormalities and diseases, such as cardiovascular disorders and cancer; however, the stimuli and processes regulating miRNA biogenesis are largely unknown. The transforming growth factor beta (TGF-beta) and bone morphogenetic protein (BMP) family of growth factors orchestrates fundamental biological processes in development and in the homeostasis of adult tissues, including the vasculature. Here we show that induction of a contractile phenotype in human vascular smooth muscle cells by TGF-beta and BMPs is mediated by miR-21. miR-21 downregulates PDCD4 (programmed cell death 4), which in turn acts as a negative regulator of smooth muscle contractile genes. Surprisingly, TGF-beta and BMP signalling promotes a rapid increase in expression of mature miR-21 through a post-transcriptional step, promoting the processing of primary transcripts of miR-21 (pri-miR-21) into precursor miR-21 (pre-miR-21) by the DROSHA (also known as RNASEN) complex. TGF-beta- and BMP-specific SMAD signal transducers are recruited to pri-miR-21 in a complex with the RNA helicase p68 (also known as DDX5), a component of the DROSHA microprocessor complex. The shared cofactor SMAD4 is not required for this process. Thus, regulation of miRNA biogenesis by ligand-specific SMAD proteins is critical for control of the vascular smooth muscle cell phenotype and potentially for SMAD4-independent responses mediated by the TGF-beta and BMP signalling pathways.

Inhibition of microRNA-302 (miR-302) by bone morphogenetic protein 4 (BMP4) facilitates the BMP signaling pathway.

The signaling pathway mediated by BMPs plays an essential role during development as well as the maintenance of homeostasis in adult. Aberrant activation or inactivation of BMP signaling can lead to developmental defects and various human disorders. To fine-tune its activity, BMP signaling is regulated both positively and negatively by extrinsic and intrinsic regulatory factors that modulate binding of ligand to the receptors, and the activity of receptors and their dedicated signal transducers, the Smad proteins. Upon BMP binding to the receptor complex, Smad proteins translocate to the nucleus and modulate gene expression transcriptionally by directly associating with the promoter region of target genes, or post-transcriptionally through modulation of microRNA (miRNA) synthesis. In this study, we demonstrate that BMP signaling down-regulates transcription of the miRNA-302∼367 gene cluster. We show that the type II BMP receptor (BMPRII) is a novel target of miR-302. Upon overexpression, miR-302 targets a partially complementary sequence localized in the 3'-untranslated region (UTR) of BMPRII transcripts and leads to destabilization of the transcripts and down-regulation of BMP signaling. We propose that the negative regulatory loop of BMP4-miR-302-BMPRII is a potential mechanism for the maintenance and fine-tuning of the BMP signaling pathway in various systems.

Bone morphogenetic protein 4 promotes vascular smooth muscle contractility by activating microRNA-21 (miR-21), which down-regulates expression of family of dedicator of cytokinesis (DOCK) proteins.

The bone morphogenetic protein 4 (BMP4) signaling pathway plays a critical role in the promotion and maintenance of the contractile phenotype in vascular smooth muscle cell (vSMC). Misexpression or inactivating mutations of the BMP receptor gene can lead to dedifferentiation of vSMC characterized by increased migration and proliferation that is linked to vascular proliferative disorders. Previously we demonstrated that vSMCs increase microRNA-21 (miR-21) biogenesis upon BMP4 treatment, which induces contractile gene expression by targeting programmed cell death 4 (PDCD4). To identify novel targets of miR-21 that are critical for induction of the contractile phenotype by BMP4, biotinylated miR-21 was expressed in vSMCs followed by an affinity purification of mRNAs associated with miR-21. Nearly all members of the dedicator of cytokinesis (DOCK) 180-related protein superfamily were identified as targets of miR-21. Down-regulation of DOCK4, -5, and -7 by miR-21 inhibited cell migration and promoted cytoskeletal organization by modulating an activity of small GTPase. Thus, this study uncovers a regulatory mechanism of the vSMC phenotype by the BMP4-miR-21 axis through DOCK family proteins.

Bone morphogenetic protein signaling in vascular disease: anti-inflammatory action through myocardin-related transcription factor A.

Pulmonary artery hypertension (PAH) patients exhibit elevated levels of inflammatory cytokines and infiltration of inflammatory cells in the lung. Concurrently, mutations of bmpr2, the gene encoding the type II receptor of bone morphogenetic proteins (BMP), are found in ∼75% of patients with familial PAH, but a possible nexus between increased inflammation and diminished BMP signaling has hitherto remained elusive. We previously showed that BMP4 triggers nuclear localization of the Myocardin-related transcription factor A (MRTF-A) in human pulmonary artery smooth muscle cells (PASMC), resulting in the induction of contractile proteins. Here we report the BMPR2-dependent repression of a set of inflammatory mediators in response to BMP4 stimulation of PASMC. Forced expression of MRTF-A precisely emulates the anti-inflammatory effect of BMP4, while MRTF-A depletion precludes BMP4-mediated cytokine inhibition. BMP4 and MRTF-A block signaling through NF-κB, the keystone of most pathways leading to inflammatory responses, at the level of chromatin recruitment and promoter activation. Moreover, MRTF-A physically interacts with RelA/p65, the NF-κB subunit endowed with a transcription activation domain. Interestingly, the MRTF-A-NF-κB interaction is mutually antagonistic: stimulation of NF-κB signaling by TNFα, as well as p65 overexpression, hinders MRTF-A activity and the expression of contractile genes. Thus, a molecular inhibitory pathway linking BMP4 signaling, activation of MRTF-A, and inhibition of NF-κB provides insights into the etiology of PAH and a potential focus of therapeutic intervention.

Essential role of the amino-terminal region of Drosha for the Microprocessor function.

Drosha is a core component of the Microprocessor complex that cleaves primary-microRNAs (pri-miRNAs) to generate precursor-miRNA and regulates the expression of ∼80 ribosomal protein (RP) genes. Despite the fact that mutations in the amino-terminal region of Drosha (Drosha-NTR) are associated with a vascular disorder, hereditary hemorrhagic telangiectasia, the precise function of Drosha-NTR remains unclear. By deleting exon 5 from the Drosha gene and generating a Drosha mutant lacking the NTR (ΔN), we demonstrate that ΔN is unable to process pri-miRNAs, which leads to a global miRNA depletion, except for the miR-183/96/182 cluster. We find that Argonaute 2 facilitates the processing of the pri-miR-183/96/182 in ΔN cells. Unlike full-length Drosha, ΔN is not degraded under serum starvation, resulting in unregulated RP biogenesis and protein synthesis in ΔN cells, allowing them to evade growth arrest. This study reveals the essential role of Drosha-NTR in miRNA production and nutrient-dependent translational control.

Reversal of pulmonary veno-occlusive disease phenotypes by inhibition of the integrated stress response.

Pulmonary veno-occlusive disease (PVOD) is a rare form of pulmonary hypertension arising from EIF2AK4 gene mutations or mitomycin C (MMC) administration. The lack of effective PVOD therapies is compounded by a limited understanding of the mechanisms driving the vascular remodeling in PVOD. We show that the administration of MMC in rats mediates the activation of protein kinase R (PKR) and the integrated stress response (ISR), which lead to the release of the endothelial adhesion molecule VE-Cadherin in the complex with Rad51 to the circulation, disruption of endothelial barrier, and vascular remodeling. Pharmacological inhibition of PKR or ISR attenuates the depletion of VE-Cadherin, elevation of vascular permeability, and vascular remodeling instigated by MMC, suggesting potential clinical intervention for PVOD. Finally, the severity of PVOD phenotypes was increased by a heterozygous BMPR2 mutation that truncates the carboxyl tail of BMPR2, underscoring the role of deregulated BMP signal in the development of PVOD.

down-regulation of Kruppel-like factor-4 (KLF4) by microRNA-143/145 is critical for modulation of vascular smooth muscle cell phenotype by transforming growth factor-beta and bone morphogenetic protein 4.

In the postnatal vasculature, fully differentiated and quiescent vascular smooth muscle cells (VSMCs) in a "contractile" phenotype are required for the normal regulation of vascular tone. The transforming growth factor-ß (TGF-ß) superfamily of growth factors (TGF-ßs and bone morphogenetic proteins (BMPs)) are potent inducers of contractile phenotype and mediate (i) induction of contractile genes, and (ii) inhibition of VSMC growth and migration. Transcription of contractile genes is positively regulated by a regulatory DNA element called a CArG box. The CArG box is activated by the binding of serum response factor and its coactivators, myocardin (Myocd) or Myocd-related transcription factors (MRTFs). Krüppel-like factor-4 (KLF4) is known to inhibit activation of the CArG box. However, the potential role of KLF4 in the contractile activities of TGF-ß or BMP has not been explored. Here, we demonstrate that TGF-ß and BMP4 rapidly down-regulate KLF4 through induction of microRNA-143 (miR-143) and miR-145, which leads to a reduction of KLF4 transcripts and decreased KLF4 protein expression. Inhibition of miR-145 prevents down-regulation of KLF4 and activation of contractile genes by TGF-ß or BMP4, suggesting that modulation of KLF4 is a prerequisite for induction of contractile genes by TGF-ß and BMP4. Interestingly, both TGF-ß and BMP4 activate transcription of the miR-143/145 gene cluster through the CArG box, however, TGF-ß mediates this effect through induction of Myocd expression, whereas BMP4 utilizes nuclear translocation of MRTF-A. Thus, this study sheds light on both the similarities and the differences of TGF-ß and BMP4 signaling in the regulation of KLF4 and contractile genes.

How do you mend inactive tumor suppressor mutants? You glue them!

SMAD4 mutations that disrupt its interaction with SMAD3 and attenuate tumor suppression by TGF-ß are major oncogenic drivers. Tang et al. (2020) report the discovery of small molecules that restore the SMAD4:SMAD3 complex and its cytostatic activity, exemplifying the therapeutic potential of fixing tumor suppressor mutants using molecular glues.

Control of ribosomal protein synthesis by the Microprocessor complex.

Ribosome biogenesis in eukaryotes requires the coordinated production and assembly of 80 ribosomal proteins and four ribosomal RNAs (rRNAs), and its rate must be synchronized with cellular growth. Here, we showed that the Microprocessor complex, which mediates the first step of microRNA processing, potentiated the transcription of ribosomal protein genes by eliminating DNA/RNA hybrids known as R-loops. Nutrient deprivation triggered the nuclear export of Drosha, a key component of the Microprocessor complex, and its subsequent degradation by the E3 ubiquitin ligase Nedd4, thereby reducing ribosomal protein production and protein synthesis. In mouse erythroid progenitors, conditional deletion of Drosha led to the reduced production of ribosomal proteins, translational inhibition of the mRNA encoding the erythroid transcription factor Gata1, and impaired erythropoiesis. This phenotype mirrored the clinical presentation of human "ribosomopathies." Thus, the Microprocessor complex plays a pivotal role in synchronizing protein synthesis capacity with cellular growth rate and is a potential drug target for anemias caused by ribosomal insufficiency.

A novel regulatory mechanism of the bone morphogenetic protein (BMP) signaling pathway involving the carboxyl-terminal tail domain of BMP type II receptor.

Bone morphogenetic protein (BMP) signaling regulates many different biological processes, including cell growth, differentiation, and embryogenesis. BMPs bind to heterogeneous complexes of transmembrane serine/threonine (Ser/Thr) kinase receptors known as the BMP type I and II receptors (BMPRI and BMPRII). BMPRII phosphorylates and activates the BMPRI kinase, which in turn activates the Smad proteins. The cytoplasmic region of BMPRII contains a "tail" domain (BMPRII-TD) with no enzymatic activity or known regulatory function. The discovery of mutations associated with idiopathic pulmonary artery hypertension mapping to BMPRII-TD underscores its importance. Here, we report that Tribbles-like protein 3 (Trb3) is a novel BMPRII-TD-interacting protein. Upon BMP stimulation, Trb3 dissociates from BMPRII-TD and triggers degradation of Smad ubiquitin regulatory factor 1 (Smurf1), which results in the stabilization of BMP receptor-regulated Smads and potentiation of the Smad pathway. Downregulation of Trb3 inhibits BMP-mediated cellular responses, including osteoblast differentiation of C2C12 cells and maintenance of the smooth muscle phenotype of pulmonary artery smooth muscle cells. Thus, Trb3 is a critical component of a novel mechanism for regulation of the BMP pathway by BMPRII.

Bone morphogenetic protein signaling is required for RAD51-mediated maintenance of genome integrity in vascular endothelial cells.

The integrity of blood vessels is fundamental to vascular homeostasis. Inactivating mutations in the bone morphogenetic protein (BMP) receptor type II (BMPR2) gene cause hereditary vascular disorders, including pulmonary arterial hypertension and hereditary hemorrhagic telangiectasia, suggesting that BMPR2 and its downstream signaling pathway are pivotal to the maintenance of vascular integrity through an unknown molecular mechanism. Here we report that inactivation of BMPR2 in pulmonary vascular endothelial cells results in a deficit of RAD51, an enzyme essential for DNA repair and replication. Loss of RAD51, which causes DNA damage and cell death, is also detected in animal models and human patients with pulmonary arterial hypertension. Restoration of BMPR2 or activation of the BMP signaling pathway rescues RAD51 and prevents DNA damage. This is an unexpected role of BMP signaling in preventing the accumulation of DNA damage and the concomitant loss of endothelial integrity and vascular remodeling associated with vascular disorders.

Inactivating mutations in Drosha mediate vascular abnormalities similar to hereditary hemorrhagic telangiectasia.

The transforming growth factor-ß (TGF-ß) and bone morphogenetic protein (BMP) family of cytokines critically regulates vascular morphogenesis and homeostasis. Impairment of TGF-ß or BMP signaling leads to heritable vascular disorders, including hereditary hemorrhagic telangiectasia (HHT). Drosha, a key enzyme for microRNA (miRNA) biogenesis, also regulates the TGF-ß and BMP pathway through interaction with Smads and their joint control of gene expression through miRNAs. We report that mice lacking Drosha in the vascular endothelium developed a vascular phenotype resembling HHT that included dilated and disorganized vasculature, arteriovenous fistulae, and hemorrhages. Exome sequencing of HHT patients who lacked known pathogenic mutations revealed an overrepresentation of rare nonsynonymous variants of DROSHA Two of these DROSHA variants (P100L and R279L) did not interact with Smads and were partially catalytically active. In zebrafish, expression of these mutants or morpholino-directed knockdown of Drosha resulted in angiogenesis defects and abnormal vascular permeability. Together, our studies point to an essential role of Drosha in vascular development and the maintenance of vascular integrity, and reveal a previously unappreciated link between Drosha dysfunction and HHT.

Hyperactive locomotion in a Drosophila model is a functional readout for the synaptic abnormalities underlying fragile X syndrome.

Fragile X syndrome (FXS) is the most common cause of heritable intellectual disability and autism and affects ~1 in 4000 males and 1 in 8000 females. The discovery of effective treatments for FXS has been hampered by the lack of effective animal models and phenotypic readouts for drug screening. FXS ensues from the epigenetic silencing or loss-of-function mutation of the fragile X mental retardation 1 (FMR1) gene, which encodes an RNA binding protein that associates with and represses the translation of target mRNAs. We previously found that the activation of LIM kinase 1 (LIMK1) downstream of augmented synthesis of bone morphogenetic protein (BMP) type 2 receptor (BMPR2) promotes aberrant synaptic development in mouse and Drosophila models of FXS and that these molecular and cellular markers were correlated in patients with FXS. We report that larval locomotion is augmented in a Drosophila FXS model. Genetic or pharmacological intervention on the BMPR2-LIMK pathway ameliorated the synaptic abnormality and locomotion phenotypes of FXS larvae, as well as hyperactivity in an FXS mouse model. Our study demonstrates that (i) the BMPR2-LIMK pathway is a promising therapeutic target for FXS and (ii) the locomotion phenotype of FXS larvae is a quantitative functional readout for the neuromorphological phenotype associated with FXS and is amenable to the screening novel FXS therapeutics.

Let-7 microRNA-dependent control of leukotriene signaling regulates the transition of hematopoietic niche in mice.

Hematopoietic stem and progenitor cells arise from the vascular endothelium of the dorsal aorta and subsequently switch niche to the fetal liver through unknown mechanisms. Here we report that vascular endothelium-specific deletion of mouse Drosha (Drosha cKO), an enzyme essential for microRNA biogenesis, leads to anemia and death. A similar number of hematopoietic stem and progenitor cells emerge from Drosha-deficient and control vascular endothelium, but Drosha cKO-derived hematopoietic stem and progenitor cells accumulate in the dorsal aorta and fail to colonize the fetal liver. Depletion of the let-7 family of microRNAs is a primary cause of this defect, as it leads to activation of leukotriene B4 signaling and induction of the α4ß1 integrin cell adhesion complex in hematopoietic stem and progenitor cells. Inhibition of leukotriene B4 or integrin rescues maturation and migration of Drosha cKO hematopoietic stem and progenitor cells to the fetal liver, while it hampers hematopoiesis in wild-type animals. Our study uncovers a previously undefined role of innate leukotriene B4 signaling as a gatekeeper of the hematopoietic niche transition.Hematopoietic stem and progenitor cells are generated first from the vascular endothelium of the dorsal aorta and then the fetal liver but what regulates this switch is unknown. Here, the authors show that changing miRNA biogenesis and leukotriene B4 signaling in mice modulates this switch in the niche.

Augmented noncanonical BMP type II receptor signaling mediates the synaptic abnormality of fragile X syndrome.

Epigenetic silencing of fragile X mental retardation 1 (FMR1) causes fragile X syndrome (FXS), a common inherited form of intellectual disability and autism. FXS correlates with abnormal synapse and dendritic spine development, but the molecular link between the absence of the FMR1 product FMRP, an RNA binding protein, and the neuropathology is unclear. We found that the messenger RNA encoding bone morphogenetic protein type II receptor (BMPR2) is a target of FMRP. Depletion of FMRP increased BMPR2 abundance, especially that of the full-length isoform that bound and activated LIM domain kinase 1 (LIMK1), a component of the noncanonical BMP signal transduction pathway that stimulates actin reorganization to promote neurite outgrowth and synapse formation. Heterozygosity for BMPR2 rescued the morphological abnormalities in neurons both in Drosophila and in mouse models of FXS, as did the postnatal pharmacological inhibition of LIMK1 activity. Compared with postmortem prefrontal cortex tissue from healthy subjects, the amount of full-length BMPR2 and of a marker of LIMK1 activity was increased in this brain region from FXS patients. These findings suggest that increased BMPR2 signal transduction is linked to FXS and that the BMPR2-LIMK1 pathway is a putative therapeutic target in patients with FXS and possibly other forms of autism.

Hypoxia potentiates microRNA-mediated gene silencing through posttranslational modification of Argonaute2.

Hypoxia contributes to the pathogenesis of various human diseases, including pulmonary artery hypertension (PAH), stroke, myocardial or cerebral infarction, and cancer. For example, acute hypoxia causes selective pulmonary artery (PA) constriction and elevation of pulmonary artery pressure. Chronic hypoxia induces structural and functional changes to the pulmonary vasculature, which resembles the phenotype of human PAH and is commonly used as an animal model of this disease. The mechanisms that lead to hypoxia-induced phenotypic changes have not been fully elucidated. Here, we show that hypoxia increases type I collagen prolyl-4-hydroxylase [C-P4H(I)], which leads to prolyl-hydroxylation and accumulation of Argonaute2 (Ago2), a critical component of the RNA-induced silencing complex (RISC). Hydroxylation of Ago2 is required for the association of Ago2 with heat shock protein 90 (Hsp90), which is necessary for the loading of microRNAs (miRNAs) into the RISC, and translocation to stress granules (SGs). We demonstrate that hydroxylation of Ago2 increases the level of miRNAs and increases the endonuclease activity of Ago2. In summary, this study identifies hypoxia as a mediator of the miRNA-dependent gene silencing pathway through posttranslational modification of Ago2, which might be responsible for cell survival or pathological responses under low oxygen stress.