Loss of Hypoxia Signaling Limits Physiologic and Muscle Adaptations to Aerobic Exercise in Aging

To assess the differential effects of exercise with age, Young (Y, 10-12 weeks) and Old (O, 23-25 months) mice were subjected to regimented treadmill running or no regimented exercise. Y, trained mice experienced a significant increase in maximal distance running, maximal speed of running, and lean muscle mass in comparison to age-matched, untrained controls. O mice did not improve significantly in any of these measures following training. Transcriptome analysis of gastrocnemius from Y mice demonstrated differential regulation of 120 genes with exercise. None of these genes were similarly regulated in the O group. Genes most upregulated following exercise in Y mice were direct targets of the hypoxia signaling pathway. Immunoblotting demonstrated that aryl hydrocarbon receptor nuclear translocator (ARNT), a critical regulator of hypoxia signaling, increased 3-fold with exercise in Y mice, but this increase was absent in O mice following exercise. To assess whether this loss of ARNT in O muscle impaired the exercise response, we generated a mouse with inducible, skeletal muscle-specific knockout of ARNT (ARNT muscle (m) KO). Following regimented exercise, ARNT mKO mice did not improve maximal distance running, maximal running speed, or lean muscle mass in comparison to untrained ARNT mKO mice. Littermate, age-matched ARNT wild type mice increased significantly in all of these measures following training. Administration of ML228, an ARNT agonist, increased maximal running distance and speed in response to exercise training in O mice. These results suggest that restoration of ARNT and hypoxia signaling may restore the physiologic response to exercise in aging.

SARS-CoV-2 B.1.1.7 (alpha) and B.1.351 (beta) variants induce pathogenic patterns in K18-hACE2 transgenic mice distinct from early strains

SARS-CoV-2 variants of concern (VOC) B.1.1.7 (alpha) and B.1.351 (beta) show increased transmissibility and enhanced antibody neutralization resistance. Here we demonstrate in K18-hACE2 transgenic mice that B.1.1.7 and B.1.351 are 100-fold more lethal than the original SARS-CoV-2 bearing 614D. B.1.1.7 and B.1.351 cause more severe organ lesions in K18-hACE2 mice than early SARS-CoV-2 strains bearing 614D or 614G, with B.1.1.7 and B.1.351 infection resulting in distinct tissue-specific cytokine signatures, significant D-dimer depositions in vital organs and less pulmonary hypoxia signaling before death. However, K18-hACE2 mice with prior infection of early SARS-CoV-2 strains or intramuscular immunization of viral spike or receptor binding domain are resistant to the lethal reinfection of B.1.1.7 or B.1.351, despite having reduced neutralization titers against these VOC than early strains. Our results thus distinguish pathogenic patterns in K18-hACE2 mice caused by B.1.1.7 and B.1.351 infection from those induced by early SARS-CoV-2 strains, and help inform potential medical interventions for combating COVID-19.

CSIG-12. THE ROLE OF HYPOXIA SIGNALING AS AN ONCOGENIC EFFECTOR IN ACVR1-MUTANT DIFFUSE INTRINSIC PONTINE GLIOMA

Diffuse intrinsic pontine gliomas (DIPG) are aggressive and highly treatment resistant pediatric tumors of the brainstem. The majority of the DIPG tumors (60-80%) carry somatic histone H3 mutations resulting in a global reduction of H3K27 trimethylation and subsequently a unique epigenetic landscape. Moreover, activating mutations in ACVR1 – the gene encoding for the BMP receptor ALK2 – were recently described in a large subset of DIPGs, suggestive of an oncogenic role of signaling from this receptor. Potential mechanisms of ACVR1-driven oncogenesis, however, remain poorly understood. Similar and overlapping ACVR1 mutations have been described in the rare disorder Fibrodysplasia Ossificans Progressiva (FOP), and evidence from FOP models have suggested several links between hypoxia signaling, BMP signaling, and FOP pathogenesis. We used cultured DIPG cells and adult glioma cells respectively to test the activity of the hypoxia pathway using a hypoxia-responsive elements (HRE)-luciferase reporter assay. Intriguingly, DIPG but not glioma cells overexpressing a mutant ACVR1 construct significantly elevated hypoxia signaling at hypoxia. BMP ligand stimulation resulted in a dose-dependent increase of hypoxia signaling in DIPG cells. By querying clinical gene expression data, we found elevated hypoxia signature scores in pediatric gliomas with mutant ACVR1 as compared to those with wildtype ACVR1. Finally, ACVR1 mutations frequently co-occur with Histone 3.3 and 3.1 mutations, and the hypoxia signature score was significantly higher than baseline only in tumors also harboring these Histone 3.3 or 3.1 mutations. Together, these data support the notion that hypoxia signaling is elevated downstream of ACVR1 specifically in pediatric gliomas and prompts for evaluation of hypoxia signaling as potential therapeutical target in DIPG.

Hif1α-Dependent Hypoxia Signaling Contributes to the Survival of Deep-Layer Neurons and Cortex Formation in A Mouse Model

Hypoxia-inducible factor 1 a (Hif1α) plays a crucial role in brain development. To study the function of Hif1α in early brain development, we generated neuroepithelial cell-specific Hif1α-knockout mice. Hif1α-knockout mice died soon after birth; these mice exhibited an abnormal head shape, indicating the presence of brain defects. Morphological analysis revealed that Hif1α ablation reduced the overall size of the brain, especially affecting the telencephalon. Neuronal apoptosis predominantly occurred in deep-layer neurons, consequently the alignment of cortical layers was severely disorganized in Hif1α knockout mice. Furthermore, we demonstrated that Vegf signaling contributes to the survival of deep-layer neurons as a downstream effector of Hif1α-dependent hypoxia signaling. Taken together, our findings demonstrate that Hif1α plays a critical role in the early stages of telencephalon development.

D-2-Hydroxyglutarate in Glioma Biology

Isocitrate dehydrogenase (IDH) mutations are common genetic abnormalities in glioma, which result in the accumulation of an “oncometabolite”, D-2-hydroxyglutarate (D-2-HG). Abnormally elevated D-2-HG levels result in a distinctive pattern in cancer biology, through competitively inhibiting α-ketoglutarate (α-KG)/Fe(II)-dependent dioxgenases (α-KGDDs). Recent studies have revealed that D-2-HG affects DNA/histone methylation, hypoxia signaling, DNA repair, and redox homeostasis, which impacts the oncogenesis of IDH-mutated cancers. In this review, we will discuss the current understanding of D-2-HG in cancer biology, as well as the emerging opportunities in therapeutics in IDH-mutated glioma.