Astrocyte HIF-2α supports learning in a passive avoidance paradigm under hypoxic stress Cindy V Leiton,1,2 Elyssa Chen,2 Alissa Cutrone,3 Kristy Conn,2 Kennelia Mellanson,4 Dania M Malik,5 Michael Klingener,6 Ryan Lamm,7 Michael Cutrone,8 John Petrie IV,9 Joher Sheikh,10 Adriana DiBua,11 Betsy Cohen,12 Thomas F Floyd13–15 1Department of Pathology, Stony Brook University, Stony Brook, NY, USA; 2Department of Anesthesiology, Stony Brook University, Stony Brook, NY, USA; 3Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN, USA; 4Department of Pharmacological Sciences, Stony Brook University, Stony Brook, NY, USA; 5Department of Systems Pharmacology and Translational Therapeutics, University of Pennsylvania, Philadelphia, PA, USA; 6Department of Genetics, Stony Brook University, Stony Brook, NY, USA; 7Department of General Surgery, Thomas Jefferson University Hospital, Philadelphia, PA, USA; 8Department of Anesthesiology, Duke University Medical Center, Durham, NC, USA; 9Department of Biochemistry & Molecular Biology, Bloomberg School of Public health, Johns Hopkins University, Baltimore, MD, USA; 10Department of Physiology and Biophysics, Georgetown University, Washington, DC, USA; 11Department of Chemistry, Hofstra University, Hempstead, NY, USA; 12Computer Science Department, Swarthmore College, Swarthmore, PA, USA; 13Department of Anesthesiology and Pain Management, University of Texas Southwestern, Dallas, TX, USA; 14Department of Cardiothoracic Surgery, University of Texas Southwestern, Dallas, TX, USA; 15Department of Radiology, University of Texas Southwestern, Dallas, TX, USA Background: The brain is extensively vascularized, uses ~20% of the body’s oxygen, and is highly sensitive to changes in oxygen. While synaptic plasticity and memory are impaired in healthy individuals by exposure to mild hypoxia, aged individuals appear to be even more sensitive. Aging is associated with progressive failure in pulmonary and cardiovascular systems, exposing the aged to both chronic and superimposed acute hypoxia. The HIF proteins, the “master regulators” of the cellular response to hypoxia, are robustly expressed in neurons and astrocytes. Astrocytes support neurons and synaptic plasticity via complex metabolic and trophic mechanisms. The activity of HIF proteins in the brain is diminished with aging, and the increased exposure to chronic and acute hypoxia with aging combined with diminished HIF activity may impair synaptic plasticity. Purpose: Herein, we test the hypothesis that astrocyte HIF supports synaptic plasticity and learning upon hypoxia. Materials and Methods: An Astrocyte-specific HIF loss-of-function model was employed, where knock-out of HIF-1α or HIF-2α in GFAP expressing cells was accomplished by cre-mediated recombination. Animals were tested for behavioral (open field and rotarod), learning (passive avoidance paradigm), and electrophysiological (long term potentiation) responses to mild hypoxic challenge. Results: In an astrocyte-specific HIF loss-of-function model followed by mild hypoxia, we identified that the depletion of HIF-2α resulted in an impaired passive avoidance learning performance. This was accompanied by an attenuated response to induction in long-term potentiation (LTP), suggesting that the hippocampal circuitry was perturbed upon hypoxic exposure following HIF-2α loss in astrocytes, and not due to hippocampal cell death. We investigated HIF-regulated trophic and metabolic target genes and found that they were not regulated by HIF-2α, suggesting that these specific targets may not be involved in mediating the phenotypes observed. Conclusion: Together, these results point to a role for HIF-2α in the astrocyte’s regulatory role in synaptic plasticity and learning under hypoxia and suggest that even mild, acute hypoxic challenges can impair cognitive performance in the aged population who harbor impaired HIF function. Keywords: astrocyte, hypoxia, HIF, learning, LTP, tripartite synapse, memory, glia, cognitive function

Surgery and trauma commonly cause spinal cord ischemia. We have developed a catheter-based optical tool to continuously monitor spinal cord blood flow intraoperatively in a ovine model of spinal surgery.

Background/Objectives: Early allograft dysfunction (EAD) is a common complication after liver transplantation and is associated with inferior graft survival. While normothermic machine perfusion (NMP) has reduced EAD incidence, the prediction of early graft performance prior to implantation remains elusive. We aimed to correlate the peri-transplant energetic and metabolic profile of liver grafts with post-transplant outcome in a cohort that included grafts preserved with NMP. Methods: Sequential biopsies were taken from 20 transplanted livers (10 immediate graft function [IGF] and 10 EAD), preserved by either static cold storage or NMP. Samples were collected immediately prior to implantation and 30 min after hepatic arterial reperfusion. Untargeted liquid chromatography-mass spectrometry was performed, and energy charge was calculated as (ATP + 1/2 ADP)/(ATP + ADP + AMP). Univariate and receiver operating characteristic analysis identified metabolites correlated with EAD and assessed predictive accuracy. Results: Hepatic concentrations of adenine nucleotides and calculated energy charge did not differ between outcome groups either before implantation or after reperfusion. In contrast, trans-urocanate was significantly enriched in IGF livers across both time points, and additional histidine catabolism pathway metabolites were preferentially increased in IGF grafts. Trans-urocanate demonstrated discriminatory performance for EAD with 80% sensitivity and 80% specificity, confirmed as the single strongest predictive feature among >1600 detected metabolites. Conclusions: These data identify histidine catabolism as a novel metabolic pathway associated with early graft function and a potential indicator of allograft resilience to ischemia-reperfusion injury. Integration of histidine pathway metabolites into perfusion-era viability assessment may serve as a discriminative biomarker of EAD and support future metabolite-guided graft optimization strategies.

Background/Objectives: Early allograft dysfunction (EAD) is a common complication after liver transplantation and is associated with inferior graft survival While normothermic machine perfusion (NMP) has reduced EAD incidence, prediction of early graft performance prior to implantation remains elusive. We aimed to correlate peri-transplant energetic and metabolic profile of liver grafts with post-transplant outcome in a cohort that included grafts preserved with NMP. Methods: Sequential biopsies were taken from 20 transplanted livers (10 immediate graft function [IGF] and 10 EAD), preserved by either static cold storage or NMP. Samples were collected immediately prior to implantation and 30 minutes after hepatic arterial reperfusion. Untargeted liquid chromatography-mass spectrometry was performed, and energy charge was calculated as (ATP + ½ ADP)/(ATP + ADP + AMP). Univariate and receiver operating characteristic analysis identified metabolites correlated with EAD and assessed predictive accuracy. Results: Hepatic concentrations of adenine nucleotides and calculated energy charge did not differ between outcome groups either before implantation or after reperfusion. In contrast, trans-urocanate was significantly enriched in IGF livers across both time points, and additional histidine catabolism pathway metabolites were preferentially increased in IGF grafts. Trans-urocanate demonstrated discriminatory performance for EAD with 80% sensitivity and 80% specificity, confirmed as the single strongest predictive feature among >1,600 detected metabolites. Conclusions: These data identify histidine catabolism as a novel metabolic pathway associated with early graft function and a potential indicator of allograft resilience to ischemia-reperfusion injury. Integration of histidine-pathway metabolites into perfusion-era viability assessment may serve as a discriminative biomarker of EAD and support future metabolite-guided graft optimization strategies.