Single-cell profiling of brain pericyte heterogeneity following ischemic stroke unveils distinct pericyte subtype-targeted neural reprogramming potential and its underlying mechanisms.

Rationale: Brain pericytes can acquire multipotency to produce multi-lineage cells following injury. However, pericytes are a heterogenous population and it remains unknown whether there are different potencies from different subsets of pericytes in response to injury. Methods: We used an ischemic stroke model combined with pericyte lineage tracing animal models to investigate brain pericyte heterogeneity under both naïve and brain injury conditions via single-cell RNA-sequencing and immunohistochemistry analysis. In addition, we developed an NG2+ pericyte neural reprogramming culture model from both murine and humans to unveil the role of energy sensor, AMP-dependent kinase (AMPK), activity in modulating the reprogramming/differentiation process to convert pericytes to functional neurons by targeting a Ser 436 phosphorylation on CREB-binding protein (CBP), a histone acetyltransferase. Results: We showed that two distinct pericyte subpopulations, marked by NG2+ and Tbx18+, had different potency following brain injury. NG2+ pericytes expressed dominant neural reprogramming potential to produce newborn neurons, while Tbx18+ pericytes displayed dominant multipotency to produce endothelial cells, fibroblasts, and microglia following ischemic stroke. In addition, we discovered that AMPK modulators facilitated pericyte-to-neuron conversion by modulating Ser436 phosphorylation status of CBP, to coordinate an acetylation shift between Sox2 and histone H2B, and to regulate Sox2 nuclear-cytoplasmic trafficking during the reprogramming/differentiation process. Finally, we showed that sequential treatment of compound C (CpdC) and metformin, AMPK inhibitor and activator respectively, robustly facilitated the conversion of human pericytes into functional neurons. Conclusion: We revealed that two distinct subtypes of pericytes possess different reprogramming potencies in response to physical and ischemic injuries. We also developed a genomic integration-free methodology to reprogram human pericytes into functional neurons by targeting NG2+ pericytes.

Regulation of hypothalamic reactive oxygen species and feeding behavior by phosphorylation of the beta 2 thyroid hormone receptor isoform.

Unlike other thyroid hormone receptors (THRs), the beta 2 isoform (THRB2) has a restricted expression pattern and is uniquely and abundantly phosphorylated at a conserved serine residue S101 (S102 in humans). Using tagged and or phosphorylation-defective (S101A) THRB2 mutant mice, we show that THRB2 is present in a large subset of POMC neurons and mitigates ROS accumulation during ROS-triggering events, such as fasting/refeeding or high fat diet (HFD). Excessive ROS accumulation in mutant POMC neurons was accompanied by a skewed production of orexigenic/anorexigenic hormones, resulting in elevated food intake. The prolonged exposure to pathogenic hypothalamic ROS levels during HFD feeding lead to a significant loss of POMC neurons in mutant versus wild-type (WT) mice. In cultured cells, the presence of WT THRB2 isoform, but not other THRs, or THRB2S101A, reduced ROS accumulation upon exogenous induction of oxidative stress by tert-butyl hydroperoxide. The protective function of phospho-THRB2 (pTHRB2) did not require thyroid hormone (TH), suggesting a TH-independent role of the THRB2 isoform, and phospho-S101 in particular, in regulating oxidative stress. We propose that pTHRB2 has a fundamental role in neuronal protection against ROS cellular damage, and mitigates hypothalamic pathological changes found in diet-induced obesity.

Leukemia inhibitory factor suppresses hepatic de novo lipogenesis and induces cachexia in mice.

Cancer cachexia is a systemic metabolic syndrome characterized by involuntary weight loss, and muscle and adipose tissue wasting. Mechanisms underlying cachexia remain poorly understood. Leukemia inhibitory factor (LIF), a multi-functional cytokine, has been suggested as a cachexia-inducing factor. In a transgenic mouse model with conditional LIF expression, systemic elevation of LIF induces cachexia. LIF overexpression decreases de novo lipogenesis and disrupts lipid homeostasis in the liver. Liver-specific LIF receptor knockout attenuates LIF-induced cachexia, suggesting that LIF-induced functional changes in the liver contribute to cachexia. Mechanistically, LIF overexpression activates STAT3 to downregulate PPARα, a master regulator of lipid metabolism, leading to the downregulation of a group of PPARα target genes involved in lipogenesis and decreased lipogenesis in the liver. Activating PPARα by fenofibrate, a PPARα agonist, restores lipid homeostasis in the liver and inhibits LIF-induced cachexia. These results provide valuable insights into cachexia, which may help develop strategies to treat cancer cachexia.

Protocol for measuring mitochondrial size in mouse and human liver tissues.

Mitochondrial dynamic process is important for cell viability, metabolic activity, and mitochondria health. Here, we present a protocol for measuring mitochondrial size through immunofluorescence staining, confocal imaging, and analysis in ImageJ. We describe the steps for tissue processing, antigen retrieval, mitochondrial staining using an integrating immunofluorescence assay, and computerized image analysis to measure each mitochondrial size in mouse and human liver tissues. This protocol reduces tissue sample volume and processing time for the preparation of primary cells. For complete details on the use and execution of this protocol, please refer to Pearah et al.1.

Blocking AMPKαS496 phosphorylation improves mitochondrial dynamics and hyperglycemia in aging and obesity.

Impaired mitochondrial dynamics causes aging-related or metabolic diseases. Yet, the molecular mechanism responsible for the impairment of mitochondrial dynamics is still not well understood. Here, we report that elevated blood insulin and/or glucagon levels downregulate mitochondrial fission through directly phosphorylating AMPKα at S496 by AKT or PKA, resulting in the impairment of AMPK-MFF-DRP1 signaling and mitochondrial dynamics and activity. Since there are significantly increased AMPKα1 phosphorylation at S496 in the liver of elderly mice, obese mice, and obese patients, we, therefore, designed AMPK-specific targeting peptides (Pa496m and Pa496h) to block AMPKα1S496 phosphorylation and found that these targeting peptides can increase AMPK kinase activity, augment mitochondrial fission and oxidation, and reduce ROS, leading to the rejuvenation of mitochondria. Furthermore, these AMPK targeting peptides robustly suppress liver glucose production in obese mice. Our data suggest these targeting peptides are promising therapeutic agents for improving mitochondrial dynamics and activity and alleviating hyperglycemia in elderly and obese patients.

Gluconeogenesis Flux in Metabolic Disease.

Gluconeogenesis is a critical biosynthetic process that helps maintain whole-body glucose homeostasis and becomes altered in certain medical diseases. We review gluconeogenic flux in various medical diseases, including common metabolic disorders, hormonal imbalances, specific inborn genetic errors, and cancer. We discuss how the altered gluconeogenic activity contributes to disease pathogenesis using data from experiments using isotopic tracer and spectroscopy methodologies. These in vitro, animal, and human studies provide insights into the changes in circulating levels of available gluconeogenesis substrates and the efficiency of converting those substrates to glucose by gluconeogenic organs. We highlight ongoing knowledge gaps, discuss emerging research areas, and suggest future investigations. A better understanding of altered gluconeogenesis flux may ultimately identify novel and targeted treatment strategies for such diseases.

Glucagon changes substrate preference in gluconeogenesis.

Fasting hyperglycemia in diabetes mellitus is caused by unregulated glucagon secretion that activates gluconeogenesis (GNG) and increases the use of pyruvate, lactate, amino acids, and glycerol. Studies of GNG in hepatocytes, however, tend to test a limited number of substrates at nonphysiologic concentrations. Therefore, we treated cultured primary hepatocytes with three identical substrate mixtures of pyruvate/lactate, glutamine, and glycerol at serum fasting concentrations, where a different U-13C- or 2-13C-labeled substrate was substituted in each mix. In the absence of glucagon stimulation, 80% of the glucose produced in primary hepatocytes incorporated either one or two 13C-labeled glycerol molecules in a 1:1 ratio, reflecting the high overall activity of this pathway. In contrast, glucose produced from 13C-labeled pyruvate/lactate or glutamine rarely incorporated two labeled molecules. While glucagon increased the glycerol and pyruvate/lactate contributions to glucose carbon by 1.6- and 1.8-fold, respectively, the glutamine contribution to glucose carbon was increased 6.4-fold in primary hepatocytes. To account for substrate 13C carbon loss during metabolism, we also performed a metabolic flux analysis, which confirmed that the majority of glucose carbon produced by primary hepatocytes was from glycerol. In vivo studies using a PKA-activation mouse model that represents elevated glucagon activity confirmed that most circulating lactate carbons originated from glycerol, but very little glycerol was derived from lactate carbons, reflecting glycerol's importance as a carbon donor to GNG. Given the diverse entry points for GNG substrates, hepatic glucagon action is unlikely to be due to a single mechanism.

Differential Metabolism of Glycerol Based on Oral versus Intravenous Administration in Humans.

Glycerol can be metabolized to glucose via gluconeogenesis or lactate via glycolysis. It is unknown if glycerol is metabolized similarly in the portal and systemic circulations in humans. Eight metabolically healthy overnight-fasted individuals received equimolar amounts of 13C3-glycerol orally and intravenously on two separate occasions with serial blood draws over four hours. Serum samples underwent liquid chromatography-mass spectrometry analysis. Oral 13C3-glycerol administration led to higher average serum glucose enrichment than intravenous administration (5.02 ± 1.43 versus 4.07 ± 0.79%, p = 0.009). In contrast, intravenous 13C3-glycerol administration yielded higher average serum lactate enrichment than oral administration (5.67 ± 0.80 versus 4.85 ± 1.30%, p = 0.032). Peak serum glucose enrichment was also higher with oral administration (9.37 ± 2.93 versus 7.12 ± 1.28%, p = 0.010). Glycerol metabolism across the portal and systemic circulations is not congruent. Orally administered labeled glycerol led to greater labeled glucose production, while intravenously administration yielded greater lactate production. These data support direct glycerol to lactate conversion in humans.

Activation of the canonical ER stress IRE1-XBP1 pathway by insulin regulates glucose and lipid metabolism.

Knockout of the transcription factor X-box binding protein (XBP1) is known to decrease liver glucose production and lipogenesis. However, whether insulin can regulate gluconeogenesis and lipogenesis through XBP1 and how insulin activates the inositol-requiring enzyme-XBP1 ER stress pathway remains unexplored. Here, we report that in the fed state, insulin-activated kinase AKT directly phosphorylates inositol-requiring enzyme 1 at S724, which in turn mediates the splicing of XBP1u mRNA, thus favoring the generation of the spliced form, XBP1s, in the liver of mice. Subsequently, XBP1s stimulate the expression of lipogenic genes and upregulates liver lipogenesis as previously reported. Intriguingly, we find that fasting leads to an increase in XBP1u along with a drastic decrease in XBP1s in the liver of mice, and XBP1u, not XBP1s, significantly increases PKA-stimulated CRE reporter activity in cultured hepatocytes. Furthermore, we demonstrate that overexpression of XBP1u significantly increases cAMP-stimulated expression of rate-limiting gluconeogenic genes, G6pc and Pck1, and glucose production in primary hepatocytes. Reexpression of XBP1u in the liver of mice with XBP1 depletion significantly increases fasting blood glucose levels and gluconeogenic gene expression. These data support an important role of XBP1u in upregulating gluconeogenesis in the fasted state. Taken together, we reveal that insulin signaling via AKT controls the expression of XBP1 isoforms and that XBP1u and XBP1s function in different nutritional states to regulate liver gluconeogenesis and lipogenesis, respectively.

Glycerol's contribution to lactate production outside of a glucose intermediate in fasting humans.

BACKGROUND: Glycerol is a well-recognized substrate for new glucose production via gluconeogenesis in the liver. However, its carbon contribution to the glycolytic intermediate lactate is not known in humans. METHODS: Here we infused stable isotope tracers 13C3-glycerol and 6,6-D2-glucose into six metabolically healthy individuals after an overnight fast to study glycerol metabolism and measure glucose rate of appearance. Serum samples underwent liquid chromatography-mass spectrometry analysis. RESULTS: Glycerol and glucose rates of appearance were 2.21 ± 1.42 µmol/kg/min and 7.81 ± 1.15 µmol/kg/min, respectively. Under steady-state conditions, the 13C enrichment for lactate was significantly higher than that of glucose (2.90 ± 0.52% versus 1.53 ± 0.78%, p = 0.017), suggesting direct glycerol to lactate metabolism. The percentage of lactate derived from glycerol was also significantly higher than the percentage of glucose (13.88 ± 2.69% versus 6.50 ± 2.59%, p = 0.005). CONCLUSION: Given that lactate itself is a carbon source for gluconeogenesis and tricycarboxylic cycle intermediates, glycerol's ability to donate carbons to lactate may make it quantitatively more important to intermediary metabolism than currently appreciated.

Targeting hepatic kisspeptin receptor ameliorates nonalcoholic fatty liver disease in a mouse model.

Nonalcoholic fatty liver disease (NAFLD), the most common liver disease, has become a silent worldwide pandemic. The incidence of NAFLD correlates with the rise in obesity, type 2 diabetes, and metabolic syndrome. A hallmark featureof NAFLD is excessive hepatic fat accumulation or steatosis, due to dysregulated hepatic fat metabolism, which can progress to nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis. Currently, there are no approved pharmacotherapies to treat this disease. Here, we have found that activation of the kisspeptin 1 receptor (KISS1R) signaling pathway has therapeutic effects in NAFLD. Using high-fat diet-fed mice, we demonstrated that a deletion of hepatic Kiss1r exacerbated hepatic steatosis. In contrast, enhanced stimulation of KISS1R protected against steatosis in wild-type C57BL/6J mice and decreased fibrosis using a diet-induced mouse model of NASH. Mechanistically, we found that hepatic KISS1R signaling activates the master energy regulator, AMPK, to thereby decrease lipogenesis and progression to NASH. In patients with NAFLD and in high-fat diet-fed mice, hepatic KISS1/KISS1R expression and plasma kisspeptin levels were elevated, suggesting a compensatory mechanism to reduce triglyceride synthesis. These findings establish KISS1R as a therapeutic target to treat NASH.

Optimized 3D Culture of Hepatic Cells for Liver Organoid Metabolic Assays.

The liver is among the principal organs for glucose homeostasis and metabolism. Studies of liver metabolism are limited by the inability to expand primary hepatocytes in vitro while maintaining their metabolic functions. Human hepatic three-dimensional (3D) organoids have been established using defined factors, yet hepatic organoids from adult donors showed impaired expansion. We examined conditions to facilitate the expansion of adult donor-derived hepatic organoids (HepAOs) and HepG2 cells in organoid cultures (HepGOs) using combinations of growth factors and small molecules. The expansion dynamics, gluconeogenic and HNF4α expression, and albumin secretion are assessed. The conditions tested allow the generation of HepAOs and HepGOs in 3D cultures. Nevertheless, gluconeogenic gene expression varies greatly between conditions. The organoid expansion rates are limited when including the TGFß inhibitor A8301, while are relatively higher with Forskolin (FSK) and Oncostatin M (OSM). Notably, expanded HepGOs grown in the optimized condition maintain detectable gluconeogenic expression in a spatiotemporal distribution at 8 weeks. We present optimized conditions by limiting A8301 and incorporating FSK and OSM to allow the expansion of HepAOs from adult donors and HepGOs with gluconeogenic competence. These models increase the repertoire of human hepatic cellular tools available for use in liver metabolic assays.

Thyroid hormone receptor phosphorylation regulates acute fasting-induced suppression of the hypothalamic-pituitary-thyroid axis.

Fasting induces profound changes in the hypothalamic-pituitary-thyroid (HPT) axis. After binding thyroid hormone (TH), the TH receptor beta 2 isoform (THRB2) represses Trh and Tsh subunit genes and is the principle negative regulator of the HPT axis. Using mass spectrometry, we identified a major phosphorylation site in the AF-1 domain of THRB2 (serine 101, S101), which is conserved among many members of the nuclear hormone receptor superfamily. More than 50% of THRB2 is phosphorylated at S101 in cultured thyrotrophs (TαT1.1) and in the mouse pituitary. All other THR isoforms lack this site and exhibit limited overall levels of phosphorylation. To determine the importance of THRB2 S101 phosphorylation, we used the TαT1.1 cell line and S101A mutant knock-in mice (Thrb2S101A ). We found that TH promoted S101 THRB2 phosphorylation and was essential for repression of the axis at physiologic TH concentrations. In mice, THRB2 phosphorylation was also increased by fasting and mimicked Trh and Tshb repression by TH. In vitro studies demonstrated that a master metabolic sensor, AMP-activated kinase (AMPK) induced phosphorylation at the same site and caused Tshb repression independent of TH. Furthermore, we identified cyclin-dependent kinase 2 (CDK2) as a direct kinase phosphorylating THRB2 S101 and propose that AMPK or TH increase S101 phosphorylation through the activity of CDK2. This study provides a physiologically relevant function for THR phosphorylation, which permits nutritional deprivation and TH to use a common mechanism for acute suppression of the HPT axis.

Utilizing tandem mass spectrometry for metabolic flux analysis.

Metabolic flux analysis (MFA) aims at revealing the metabolic reaction rates in a complex biochemical network. To do so, MFA uses the input of stable isotope labeling patterns of the intracellular metabolites. Elementary metabolic unit (EMU) is the computational framework to simulate the metabolite labeling patterns in a network, which was originally designed for simulating mass isotopomer distributions (MIDs) at the MS1 level. Recently, the EMU framework is expanded to simulate tandem mass spectrometry data. Tandem mass spectrometry has emerged as a new experimental approach to provide information on the positional isotope labeling of metabolites and therefore greatly improves the precision of MFA. In this review, we will discuss the new EMU framework that can accommodate the tandem mass isotopomer distributions (TMIDs) data. We will also analyze the improvement on the MFA precision by using TMID. Our analysis shows that combining the MIDs of the parent and daughter ions and the TMID for the MFA is more powerful than using TMID alone.

Metabolic Flux Analysis-Linking Isotope Labeling and Metabolic Fluxes.

Metabolic flux analysis (MFA) is an increasingly important tool to study metabolism quantitatively. Unlike the concentrations of metabolites, the fluxes, which are the rates at which intracellular metabolites interconvert, are not directly measurable. MFA uses stable isotope labeled tracers to reveal information related to the fluxes. The conceptual idea of MFA is that in tracer experiments the isotope labeling patterns of intracellular metabolites are determined by the fluxes, therefore by measuring the labeling patterns we can infer the fluxes in the network. In this review, we will discuss the basic concept of MFA using a simplified upper glycolysis network as an example. We will show how the fluxes are reflected in the isotope labeling patterns. The central idea we wish to deliver is that under metabolic and isotopic steady-state the labeling pattern of a metabolite is the flux-weighted average of the substrates' labeling patterns. As a result, MFA can tell the relative contributions of converging metabolic pathways only when these pathways make substrates in different labeling patterns for the shared product. This is the fundamental principle guiding the design of isotope labeling experiment for MFA including tracer selection. In addition, we will also discuss the basic biochemical assumptions of MFA, and we will show the flux-solving procedure and result evaluation. Finally, we will highlight the link between isotopically stationary and nonstationary flux analysis.

Metformin promotes CNS remyelination and improves social interaction following focal demyelination through CBP Ser436 phosphorylation.

Individuals with demyelinating diseases often experience difficulties during social interactions that are not well studied in preclinical models. Here, we describe a novel juvenile focal corpus callosum demyelination murine model exhibiting a social interaction deficit. Using this preclinical murine demyelination model, we discover that application of metformin, an FDA-approved drug, in this model promotes oligodendrocyte regeneration and remyelination and improves the social interaction. This beneficial effect of metformin acts through stimulating Ser436 phosphorylation in CBP, a histone acetyltransferase. In addition, we found that metformin acts through two distinct molecular pathways to enhance oligodendrocyte precursor (OPC) proliferation and differentiation, respectively. Metformin enhances OPC proliferation through early-stage autophagy inhibition, while metformin promotes OPC differentiation into mature oligodendrocytes through activating CBP Ser436 phosphorylation. In summary, we identify that metformin is a promising remyelinating agent to improve juvenile demyelination-associated social interaction deficits by promoting oligodendrocyte regeneration and remyelination.

Tracking the carbons supplying gluconeogenesis.

As the burden of type 2 diabetes mellitus (T2DM) grows in the 21st century, the need to understand glucose metabolism heightens. Increased gluconeogenesis is a major contributor to the hyperglycemia seen in T2DM. Isotope tracer experiments in humans and animals over several decades have offered insights into gluconeogenesis under euglycemic and diabetic conditions. This review focuses on the current understanding of carbon flux in gluconeogenesis, including substrate contribution of various gluconeogenic precursors to glucose production. Alterations of gluconeogenic metabolites and fluxes in T2DM are discussed. We also highlight ongoing knowledge gaps in the literature that require further investigation. A comprehensive analysis of gluconeogenesis may enable a better understanding of T2DM pathophysiology and identification of novel targets for treating hyperglycemia.

Dysregulated expression of monoacylglycerol lipase is a marker for anti-diabetic drug metformin-targeted therapy to correct impaired neurogenesis and spatial memory in Alzheimer's disease.

Rationale: Monoacylglycerol lipase (Mgll), a hydrolase that breaks down the endocannabinoid 2-arachidonoyl glycerol (2-AG) to produce arachidonic acid (ARA), is a potential target for neurodegenerative diseases, such as Alzheimer's disease (AD). Increasing evidence shows that impairment of adult neurogenesis by perturbed lipid metabolism predisposes patients to AD. However, it remains unknown what causes aberrant expression of Mgll in AD and how Mgll-regulated lipid metabolism impacts adult neurogenesis, thus predisposing to AD during aging. Here, we identify Mgll as an aging-induced factor that impairs adult neurogenesis and spatial memory in AD, and show that metformin, an FDA-approved anti-diabetic drug, can reduce the expression of Mgll to reverse impaired adult neurogenesis, prevent spatial memory decline and reduce ß-amyloid accumulation. Methods: Mgll expression was assessed in both human AD patient post-mortem hippocampal tissues and 3xTg-AD mouse model. In addition, we used both the 3xTg-AD animal model and the CbpS436A genetic knock-in mouse model to identify that elevated Mgll expression is caused by the attenuation of the aPKC-CBP pathway, involving atypical protein kinase C (aPKC)-stimulated Ser436 phosphorylation of histone acetyltransferase CBP through biochemical methods. Furthermore, we performed in vivo adult neurogenesis assay with BrdU/EdU labelling and Morris water maze task in both animal models following pharmacological treatments to show the key role of Mgll in metformin-corrected neurogenesis and spatial memory deficits of AD through reactivating the aPKC-CBP pathway. Finally, we performed in vitro adult neurosphere assays using both animal models to study the role of the aPKC-CBP mediated Mgll repression in determining adult neural stem/progenitor cell (NPC) fate. Results: Here, we demonstrate that aging-dependent induction of Mgll is observed in the 3xTg-AD model and human AD patient post-mortem hippocampal tissues. Importantly, we discover that elevated Mgll expression is caused by the attenuation of the aPKC-CBP pathway. The accumulation of Mgll in the 3xTg-AD mice reduces the genesis of newborn neurons and perturbs spatial memory. However, we find that metformin-stimulated aPKC-CBP pathway decreases Mgll expression to recover these deficits in 3xTg-AD. In addition, we reveal that elevated Mgll levels in cultured adult NPCs from both 3xTg-AD and CbpS436A animal models are responsible for their NPC neuronal differentiation deficits. Conclusion: Our findings set the stage for development of a clinical protocol where Mgll would serve as a biomarker in early stages of AD to identify potential metformin-responsive AD patients to restore their neurogenesis and spatial memory.

A Direct Comparison of Thyroid Hormone Receptor Protein Levels in Mice Provides Unexpected Insights into Thyroid Hormone Action.

Background: Thyroid hormone (TH) action is mediated by three major thyroid hormone receptor (THR) isoforms α1, ß1, and ß2 (THRA1, THRB1, and THRB2). These THRs and a fourth major but non-TH binding isoform, THRA2, are encoded by two genes Thra and Thrb. Reliable antibodies against all THR isoforms are not available, and THR isoform protein levels in mammalian tissues are often inferred from messenger RNA (mRNA) levels. Methods: We generated knock-in mouse models expressing endogenously and identically 2X hemagglutenin epitope (HA)-tagged THRs (THRA1/2, THRB1, and THRB2), which could then be detected by commercially available anti-HA antibodies. Using nuclear enrichment, immunoprecipitation, and Western blotting, we determined relative THR protein expression in 16 mouse organs. Results: In all peripheral organs tested except the liver, the predominant THR isoform was THRA1. Surprisingly, in metabolically active organs such as fat and muscle, THRB1 protein levels were up to 10 times lower than that of THRA1, while their mRNA levels appeared similar. In contrast to peripheral organs, the central nervous system (CNS) had a unique pattern with relatively low levels of both THRB1 and THRA1, and high levels of THRA2 expression. As expected, THRB2 was highly expressed in the pituitary, but a previously unknown sex-specific difference in THRB2 expression was found (female mice having higher pituitary expression than male mice). Higher THRB2 expression appears to make the central axis more sensitive to TH as both serum thyrotropin and Tshb mRNA levels were lower in female mice. Conclusions: Direct comparison of THR protein abundance in different organs using endogenously tagged HA-THR mouse lines shows that expression of THR isoforms is regulated at transcriptional and posttranscriptional levels, and in organ-specific manner. The prevalence of THRA1 and low abundance of THRB1 in majority of peripheral tissues suggest that peripheral actions of these isoforms should be revisited. A unique pattern of high THRA2 in CNS warrants further exploration of this non-TH binding isoform in brain development. Finally, THRB2, in addition to cell-specific control, is also regulated in a sex-specific manner, which may change the hypothalamus-pituitary-thyroid axis set point and perhaps metabolism in males and females.