{"id":{"repo_id":"vt","oai_identifier":"oai:vtechworks.lib.vt.edu:10919/139692"},"canonical_url":"https://search.dev.ndltd.org/etd/vt/oai:vtechworks.lib.vt.edu:10919/139692","repository":{"repo_id":"vt","name":"Virginia Tech","base_url":"https://vtechworks.lib.vt.edu/oai/request"},"display":{"title":"Impaired Ulk1 Ser555 Phosphorylation Promotes Amino Acid Reliance and Links Mitochondrial Inefficiency to Systemic Metabolic Inflexibility","abstract":"Metabolic flexibility—the capacity and ability to switch between energetic substrates in response to nutrient availability—is essential for systemic energy balance and protection against metabolic disease. Loss of this adaptive capacity contributes to metabolic diseases marked by obesity, insulin resistance, and dyslipidemia, elevating the risk of type 2 diabetes and cardiovascular disease. Under nutrient stress, skeletal muscle typically adjusts substrate utilization from glucose to fatty acid oxidation, while the liver supports peripheral energy needs through glucose production and auxiliary amino acid metabolism. In insulin-resistant states, this inter-organ coordination deteriorates, resulting in reduced glucose utilization and increased catabolism of glucogenic amino acids such as alanine—leading to inefficient energy use and metabolic inflexibility. Unc-51-like autophagy activating kinase 1 (Ulk1) is a serine/threonine kinase best known for initiating autophagy, but also serves as a nutrient-sensing hub regulated by AMPK and mTORC1. AMPK activates Ulk1 via phosphorylation at serine 555 (S555) during energetic stress, while mTORC1 inhibits Ulk1 under nutrient-rich conditions. To investigate the specific role of Ulk1(S555), we used global Ulk1(S555A) knock-in mice, which prevent phosphorylation at this site, and examined responses to short-term caloric restriction (CR) and fasting. Ulk1(S555A) mice exhibited impaired glucose oxidative capacity in skeletal muscle and liver during nutrient stress, despite improved systemic glucose clearance. Metabolic cage analyses of WT and Ulk1(S555A) showed no significant differences in heat and energy expenditure, despite Ulk1(S555A) delayed substrate switching and reduced physical activity. U-14C-glucose tracing confirmed reduced glucose oxidation, while multi-omics analyses revealed a tissue-specific dependence on amino acids in response to nutrient stress. Ulk1(S555A) skeletal muscle mitochondria displayed increased basal oxidation of alanine and serine with blunted ADP-stimulated respiration, while Ulk1(S555A) hepatic mitochondria demonstrated an impairment of pyruvate entry into the TCA cycle. These findings identify Ulk1(S555) as a critical integrator of AMPK signaling, regulating substrate prioritization and metabolic communication between skeletal muscle and liver. Targeting this axis may offer new therapeutic avenues to restore metabolic flexibility in obesity, type 2 diabetes, and related disorders.","abstract_html":"Metabolic flexibility—the capacity and ability to switch between energetic substrates in response to nutrient availability—is essential for systemic energy balance and protection against metabolic disease. Loss of this adaptive capacity contributes to metabolic diseases marked by obesity, insulin resistance, and dyslipidemia, elevating the risk of type 2 diabetes and cardiovascular disease. Under nutrient stress, skeletal muscle typically adjusts substrate utilization from glucose to fatty acid oxidation, while the liver supports peripheral energy needs through glucose production and auxiliary amino acid metabolism. In insulin-resistant states, this inter-organ coordination deteriorates, resulting in reduced glucose utilization and increased catabolism of glucogenic amino acids such as alanine—leading to inefficient energy use and metabolic inflexibility. Unc-51-like autophagy activating kinase 1 (Ulk1) is a serine/threonine kinase best known for initiating autophagy, but also serves as a nutrient-sensing hub regulated by AMPK and mTORC1. AMPK activates Ulk1 via phosphorylation at serine 555 (S555) during energetic stress, while mTORC1 inhibits Ulk1 under nutrient-rich conditions. To investigate the specific role of Ulk1(S555), we used global Ulk1(S555A) knock-in mice, which prevent phosphorylation at this site, and examined responses to short-term caloric restriction (CR) and fasting. Ulk1(S555A) mice exhibited impaired glucose oxidative capacity in skeletal muscle and liver during nutrient stress, despite improved systemic glucose clearance. Metabolic cage analyses of WT and Ulk1(S555A) showed no significant differences in heat and energy expenditure, despite Ulk1(S555A) delayed substrate switching and reduced physical activity. U-14C-glucose tracing confirmed reduced glucose oxidation, while multi-omics analyses revealed a tissue-specific dependence on amino acids in response to nutrient stress. Ulk1(S555A) skeletal muscle mitochondria displayed increased basal oxidation of alanine and serine with blunted ADP-stimulated respiration, while Ulk1(S555A) hepatic mitochondria demonstrated an impairment of pyruvate entry into the TCA cycle. These findings identify Ulk1(S555) as a critical integrator of AMPK signaling, regulating substrate prioritization and metabolic communication between skeletal muscle and liver. Targeting this axis may offer new therapeutic avenues to restore metabolic flexibility in obesity, type 2 diabetes, and related disorders.","abstract_has_math":false,"creators":["Willoughby, Orion"],"institution":"Virginia Tech","degree_name":"Doctor of Philosophy","degree_level":"doctoral","degree_discipline":"Human Nutrition, Foods, and Exercise","degree_department":"Human Nutrition, Foods and Exercise","school":null,"contributors":[],"advisors":[],"committee_chairs":["Drake, Joshua Chadwick"],"committee_members":["Gilbert, Elizabeth Ruth","Warren, Junko","Najt, Charles Paul","Craige, Siobhan"],"year":2025,"date_issued":"2025-11-19","date_published":"2025-11-19","updated_at":"2026-07-22T22:18:47Z","subjects":["Caloric Restriction","Fasting","Nutrient Stress","Bioenergetics","Metabolism","Nutrient Sensing","Metabolic Flexibility","Autophagy","Mitophagy","Skeletal Muscle","Liver"],"languages":["en"],"rights":["In Copyright"],"rights_urls":["http://rightsstatements.org/vocab/InC/1.0/"],"identifier_entries":[{"key":"dc:identifier.other","label":"Dc Identifier Other","values":["vt_gsexam:44884"],"render_values":[{"text":"vt_gsexam:44884","href":null,"code":true}]}]},"links":{"outbound_url":"https://hdl.handle.net/10919/139692","outbound_label":"Handle","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.committeechair","label":"Committee Chair","values":["Drake, Joshua Chadwick"]},{"key":"dc:contributor.committeemember","label":"Committee Member","values":["Gilbert, Elizabeth Ruth","Warren, Junko","Najt, Charles Paul","Craige, Siobhan"]},{"key":"dc:contributor.department","label":"Department","values":["Human Nutrition, Foods and Exercise"]},{"key":"dc:creator","label":"Author","values":["Willoughby, Orion"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.accessioned","label":"Dc Date Accessioned","values":["2025-11-20T09:00:34Z"]},{"key":"dc:date.available","label":"Dc Date Available","values":["2025-11-20T09:00:34Z"]},{"key":"dc:date.issued","label":"Date","values":["2025-11-19"]},{"key":"dc:publisher","label":"Institution","values":["Virginia Tech"]},{"key":"dc:type","label":"Dc Type","values":["Dissertation"]},{"key":"thesis:degree_discipline","label":"Discipline","values":["Human Nutrition, Foods, and Exercise"]},{"key":"thesis:degree_level","label":"Degree Level","values":["doctoral"]},{"key":"thesis:degree_name","label":"Degree Name","values":["Doctor of Philosophy"]},{"key":"thesis:institution_name","label":"Thesis Institution Name","values":["Virginia Polytechnic Institute and State University"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Caloric Restriction","Fasting","Nutrient Stress","Bioenergetics","Metabolism","Nutrient Sensing","Metabolic Flexibility","Autophagy","Mitophagy","Skeletal Muscle","Liver"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language.iso","label":"Language (ISO)","values":["en"]},{"key":"dc:rights","label":"Dc Rights","values":["In Copyright"]},{"key":"dc:rights.uri","label":"Rights URI","values":["http://rightsstatements.org/vocab/InC/1.0/"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.other","label":"Dc Identifier Other","values":["vt_gsexam:44884"]},{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://hdl.handle.net/10919/139692"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["Metabolic flexibility—the capacity and ability to switch between energetic substrates in response to nutrient availability—is essential for systemic energy balance and protection against metabolic disease. Loss of this adaptive capacity contributes to metabolic diseases marked by obesity, insulin resistance, and dyslipidemia, elevating the risk of type 2 diabetes and cardiovascular disease. Under nutrient stress, skeletal muscle typically adjusts substrate utilization from glucose to fatty acid oxidation, while the liver supports peripheral energy needs through glucose production and auxiliary amino acid metabolism. In insulin-resistant states, this inter-organ coordination deteriorates, resulting in reduced glucose utilization and increased catabolism of glucogenic amino acids such as alanine—leading to inefficient energy use and metabolic inflexibility. Unc-51-like autophagy activating kinase 1 (Ulk1) is a serine/threonine kinase best known for initiating autophagy, but also serves as a nutrient-sensing hub regulated by AMPK and mTORC1. AMPK activates Ulk1 via phosphorylation at serine 555 (S555) during energetic stress, while mTORC1 inhibits Ulk1 under nutrient-rich conditions. To investigate the specific role of Ulk1(S555), we used global Ulk1(S555A) knock-in mice, which prevent phosphorylation at this site, and examined responses to short-term caloric restriction (CR) and fasting. Ulk1(S555A) mice exhibited impaired glucose oxidative capacity in skeletal muscle and liver during nutrient stress, despite improved systemic glucose clearance. Metabolic cage analyses of WT and Ulk1(S555A) showed no significant differences in heat and energy expenditure, despite Ulk1(S555A) delayed substrate switching and reduced physical activity. U-14C-glucose tracing confirmed reduced glucose oxidation, while multi-omics analyses revealed a tissue-specific dependence on amino acids in response to nutrient stress. Ulk1(S555A) skeletal muscle mitochondria displayed increased basal oxidation of alanine and serine with blunted ADP-stimulated respiration, while Ulk1(S555A) hepatic mitochondria demonstrated an impairment of pyruvate entry into the TCA cycle. These findings identify Ulk1(S555) as a critical integrator of AMPK signaling, regulating substrate prioritization and metabolic communication between skeletal muscle and liver. Targeting this axis may offer new therapeutic avenues to restore metabolic flexibility in obesity, type 2 diabetes, and related disorders."]},{"key":"dc:description.abstractgeneral","label":"General Abstract","values":["Our bodies constantly adjust how they use nutrients like sugars, fats, and amino acids to meet energy needs. This ability—known as metabolic flexibility—helps maintain energy balance and protects against diseases such as obesity and type 2 diabetes. When this flexibility is lost, the body struggles to switch efficiently between fuel sources, leading to poor energy use and an increased risk of metabolic disorders. A protein called Ulk1, which helps cells respond to changes in nutrient availability, contributes to this process. Ulk1 is activated or inhibited by two major cellular energy sensors, AMPK and mTORC1, depending on whether the body is under energetic stress or well-fed conditions. We focused on a single site on Ulk1—serine 555 (S555)—that determines how the protein responds during nutrient stress. Using genetically modified mice that lack this regulatory site (Ulk1(S555A) mice), we examined how the body responds to fasting and short-term caloric restriction. We found that these mice had difficulty efficiently using glucose and other energy sources I both muscle and liver, despite showing normal overall energy expenditure. Their muscles relied more on amino acids such as alanine and serine, while their livers showed reduced ability to convert nutrients into usable energy. Together, these results show that the S555 site on Ulk1 plays a key role in helping tissue coordinate which fuels to use during fasting or low-energy conditions. Understanding this mechanism may lead to new strategies for improving energy balance and treating metabolic diseases like obesity and diabetes."]},{"key":"dc:description.degree","label":"Dc Description Degree","values":["Doctor of Philosophy"]},{"key":"dc:format.medium","label":"Dc Format Medium","values":["ETD"]},{"key":"dc:title","label":"Title","values":["Impaired Ulk1 Ser555 Phosphorylation Promotes Amino Acid Reliance and Links Mitochondrial Inefficiency to Systemic Metabolic Inflexibility"]}]}],"canonical_facts":{"dc:contributor.committeechair":["Drake, Joshua Chadwick"],"dc:contributor.committeemember":["Gilbert, Elizabeth Ruth","Warren, Junko","Najt, Charles Paul","Craige, Siobhan"],"dc:contributor.department":["Human Nutrition, Foods and Exercise"],"dc:creator":["Willoughby, Orion"],"dc:date.accessioned":["2025-11-20T09:00:34Z"],"dc:date.available":["2025-11-20T09:00:34Z"],"dc:date.issued":["2025-11-19"],"dc:description.abstract":["Metabolic flexibility—the capacity and ability to switch between energetic substrates in response to nutrient availability—is essential for systemic energy balance and protection against metabolic disease. Loss of this adaptive capacity contributes to metabolic diseases marked by obesity, insulin resistance, and dyslipidemia, elevating the risk of type 2 diabetes and cardiovascular disease. Under nutrient stress, skeletal muscle typically adjusts substrate utilization from glucose to fatty acid oxidation, while the liver supports peripheral energy needs through glucose production and auxiliary amino acid metabolism. In insulin-resistant states, this inter-organ coordination deteriorates, resulting in reduced glucose utilization and increased catabolism of glucogenic amino acids such as alanine—leading to inefficient energy use and metabolic inflexibility. Unc-51-like autophagy activating kinase 1 (Ulk1) is a serine/threonine kinase best known for initiating autophagy, but also serves as a nutrient-sensing hub regulated by AMPK and mTORC1. AMPK activates Ulk1 via phosphorylation at serine 555 (S555) during energetic stress, while mTORC1 inhibits Ulk1 under nutrient-rich conditions. To investigate the specific role of Ulk1(S555), we used global Ulk1(S555A) knock-in mice, which prevent phosphorylation at this site, and examined responses to short-term caloric restriction (CR) and fasting. Ulk1(S555A) mice exhibited impaired glucose oxidative capacity in skeletal muscle and liver during nutrient stress, despite improved systemic glucose clearance. Metabolic cage analyses of WT and Ulk1(S555A) showed no significant differences in heat and energy expenditure, despite Ulk1(S555A) delayed substrate switching and reduced physical activity. U-14C-glucose tracing confirmed reduced glucose oxidation, while multi-omics analyses revealed a tissue-specific dependence on amino acids in response to nutrient stress. Ulk1(S555A) skeletal muscle mitochondria displayed increased basal oxidation of alanine and serine with blunted ADP-stimulated respiration, while Ulk1(S555A) hepatic mitochondria demonstrated an impairment of pyruvate entry into the TCA cycle. These findings identify Ulk1(S555) as a critical integrator of AMPK signaling, regulating substrate prioritization and metabolic communication between skeletal muscle and liver. Targeting this axis may offer new therapeutic avenues to restore metabolic flexibility in obesity, type 2 diabetes, and related disorders."],"dc:description.abstractgeneral":["Our bodies constantly adjust how they use nutrients like sugars, fats, and amino acids to meet energy needs. This ability—known as metabolic flexibility—helps maintain energy balance and protects against diseases such as obesity and type 2 diabetes. When this flexibility is lost, the body struggles to switch efficiently between fuel sources, leading to poor energy use and an increased risk of metabolic disorders. A protein called Ulk1, which helps cells respond to changes in nutrient availability, contributes to this process. Ulk1 is activated or inhibited by two major cellular energy sensors, AMPK and mTORC1, depending on whether the body is under energetic stress or well-fed conditions. We focused on a single site on Ulk1—serine 555 (S555)—that determines how the protein responds during nutrient stress. Using genetically modified mice that lack this regulatory site (Ulk1(S555A) mice), we examined how the body responds to fasting and short-term caloric restriction. We found that these mice had difficulty efficiently using glucose and other energy sources I both muscle and liver, despite showing normal overall energy expenditure. Their muscles relied more on amino acids such as alanine and serine, while their livers showed reduced ability to convert nutrients into usable energy. Together, these results show that the S555 site on Ulk1 plays a key role in helping tissue coordinate which fuels to use during fasting or low-energy conditions. Understanding this mechanism may lead to new strategies for improving energy balance and treating metabolic diseases like obesity and diabetes."],"dc:description.degree":["Doctor of Philosophy"],"dc:format.medium":["ETD"],"dc:identifier.other":["vt_gsexam:44884"],"dc:identifier.uri":["https://hdl.handle.net/10919/139692"],"dc:language.iso":["en"],"dc:publisher":["Virginia Tech"],"dc:rights":["In Copyright"],"dc:rights.uri":["http://rightsstatements.org/vocab/InC/1.0/"],"dc:subject":["Caloric Restriction","Fasting","Nutrient Stress","Bioenergetics","Metabolism","Nutrient Sensing","Metabolic Flexibility","Autophagy","Mitophagy","Skeletal Muscle","Liver"],"dc:title":["Impaired Ulk1 Ser555 Phosphorylation Promotes Amino Acid Reliance and Links Mitochondrial Inefficiency to Systemic Metabolic Inflexibility"],"dc:type":["Dissertation"],"thesis:degree_discipline":["Human Nutrition, Foods, and Exercise"],"thesis:degree_level":["doctoral"],"thesis:degree_name":["Doctor of Philosophy"],"thesis:institution_name":["Virginia Polytechnic Institute and State University"]},"updated_at":"2026-07-22T22:18:47Z"}