Targeting nuclear receptor corepressors for reversible male contraception.

Despite numerous female contraceptive options, nearly half of all pregnancies are unintended. Family planning choices for men are currently limited to unreliable condoms and invasive vasectomies with questionable reversibility. Here, we report the development of an oral contraceptive approach based on transcriptional disruption of cyclical gene expression patterns during spermatogenesis. Spermatogenesis involves a continuous series of self-renewal and differentiation programs of spermatogonial stem cells (SSCs) that is regulated by retinoic acid (RA)-dependent activation of receptors (RARs), which control target gene expression through association with corepressor proteins. We have found that the interaction between RAR and the corepressor silencing mediator of retinoid and thyroid hormone receptors (SMRT) is essential for spermatogenesis. In a genetically engineered mouse model that negates SMRT-RAR binding (SMRTmRID mice), the synchronized, cyclic expression of RAR-dependent genes along the seminiferous tubules is disrupted. Notably, the presence of an RA-resistant SSC population that survives RAR de-repression suggests that the infertility attributed to the loss of SMRT-mediated repression is reversible. Supporting this notion, we show that inhibiting the action of the SMRT complex with chronic, low-dose oral administration of a histone deacetylase inhibitor reversibly blocks spermatogenesis and fertility without affecting libido. This demonstration validates pharmacologic targeting of the SMRT repressor complex for non-hormonal male contraception.

Corepressor SMRT is required to maintain Hox transcriptional memory during somitogenesis.

Nuclear hormone receptors (NRs), such as retinoic acid receptors (RARs), play critical roles in vertebrate development and homeostasis by regulating target gene transcription. Their activity is controlled by ligand-dependent release of corepressors and subsequent recruitment of coactivators, but how these individual receptor modes contribute to development are unknown. Here, we show that mice carrying targeted knockin mutations in the corepressor Silencing Mediator of Retinoid and Thyroid hormone receptor (SMRT) that specifically disable SMRT function in NR signaling (SMRTmRID), display defects in cranial neural crest cell-derived structures and posterior homeotic transformations of axial vertebrae. SMRTmRID embryos show enhanced transcription of RAR targets including Hox loci, resulting in respecification of vertebral identities. Up-regulated histone acetylation and decreased H3K27 methylation are evident in the Hox loci whose somitic expression boundaries are rostrally shifted. Furthermore, enhanced recruitment of super elongation complex is evident in rapidly induced non-Pol II-paused targets in SMRTmRID embryonic stem cells. These results demonstrate that SMRT-dependent repression of RAR is critical to establish and maintain the somitic Hox code and segmental identity during fetal development via epigenetic marking of target loci.

Rescue of a primary myelofibrosis model by retinoid-antagonist therapy.

Molecular targeting of the two receptor interaction domains of the epigenetic repressor silencing mediator of retinoid and thyroid hormone receptors (SMRT(mRID)) produced a transplantable skeletal syndrome that reduced radial bone growth, increased numbers of bone-resorbing periosteal osteoclasts, and increased bone fracture risk. Furthermore, SMRT(mRID) mice develop spontaneous primary myelofibrosis, a chronic, usually idiopathic disorder characterized by progressive bone marrow fibrosis. Frequently linked to polycythemia vera and chronic myeloid leukemia, myelofibrosis displays high patient morbidity and mortality, and current treatment is mostly palliative. To decipher the etiology of this disease, we identified the thrombopoietin (Tpo) gene as a target of the SMRT-retinoic acid receptor signaling pathway in bone marrow stromal cells. Chronic induction of Tpo in SMRT(mRID) mice results in up-regulation of TGF-ß and PDGF in megakaryocytes, uncontrolled proliferation of bone marrow reticular cells, and fibrosis of the marrow compartment. Of therapeutic relevance, we show that this syndrome can be rescued by retinoid antagonists, demonstrating that the physical interface between SMRT and retinoic acid receptor can be a potential therapeutic target to block primary myelofibrosis disease progression.

Corepressor SMRT promotes oxidative phosphorylation in adipose tissue and protects against diet-induced obesity and insulin resistance.

The ligand-dependent competing actions of nuclear receptor (NR)-associated transcriptional corepressor and coactivator complexes allow for the precise regulation of NR-dependent gene expression in response to both temporal and environmental cues. Here we report the mouse model termed silencing mediator of retinoid and thyroid hormone receptors (SMRT)(mRID1) in which targeted disruption of the first receptor interaction domain (RID) of the nuclear corepressor SMRT disrupts interactions with a subset of NRs and leads to diet-induced superobesity associated with a depressed respiratory exchange ratio, decreased ambulatory activity, and insulin resistance. Although apparently normal when chow fed, SMRT(mRID1) mice develop multiple metabolic dysfunctions when challenged by a high-fat diet, manifested by marked lipid accumulation in white and brown adipose tissue and the liver. The increased weight gain of SMRT(mRID1) mice on a high-fat diet occurs predominantly in fat with adipocyte hypertrophy evident in both visceral and s.c. depots. Importantly, increased inflammatory gene expression was detected only in the visceral depots. SMRT(mRID1) mice are both insulin-insensitive and refractory to the glucose-lowering effects of TZD and AICAR. Increased serum cholesterol and triglyceride levels were observed, accompanied by increased leptin and decreased adiponectin levels. Aberrant storage of lipids in the liver occurred as triglycerides and cholesterol significantly compromised hepatic function. Lipid accumulation in brown adipose tissue was associated with reduced thermogenic capacity and mitochondrial biogenesis. Collectively, these studies highlight the essential role of NR corepressors in maintaining metabolic homeostasis and describe an essential role for SMRT in regulating the progression, severity, and therapeutic outcome of metabolic diseases.

Rapid genomic sequencing for genetic disease diagnosis and therapy in intensive care units: a review.

Single locus (Mendelian) diseases are a leading cause of childhood hospitalization, intensive care unit (ICU) admission, mortality, and healthcare cost. Rapid genome sequencing (RGS), ultra-rapid genome sequencing (URGS), and rapid exome sequencing (RES) are diagnostic tests for genetic diseases for ICU patients. In 44 studies of children in ICUs with diseases of unknown etiology, 37% received a genetic diagnosis, 26% had consequent changes in management, and net healthcare costs were reduced by $14,265 per child tested by URGS, RGS, or RES. URGS outperformed RGS and RES with faster time to diagnosis, and higher rate of diagnosis and clinical utility. Diagnostic and clinical outcomes will improve as methods evolve, costs decrease, and testing is implemented within precision medicine delivery systems attuned to ICU needs. URGS, RGS, and RES are currently performed in <5% of the ~200,000 children likely to benefit annually due to lack of payor coverage, inadequate reimbursement, hospital policies, hospitalist unfamiliarity, under-recognition of possible genetic diseases, and current formatting as tests rather than as a rapid precision medicine delivery system. The gap between actual and optimal outcomes in children in ICUs is currently increasing since expanded use of URGS, RGS, and RES lags growth in those likely to benefit through new therapies. There is sufficient evidence to conclude that URGS, RGS, or RES should be considered in all children with diseases of uncertain etiology at ICU admission. Minimally, diagnostic URGS, RGS, or RES should be ordered early during admissions of critically ill infants and children with suspected genetic diseases.

SMRT repression of nuclear receptors controls the adipogenic set point and metabolic homeostasis.

The nuclear receptor corepressor, silencing mediator of retinoid and thyroid hormone receptors (SMRT), is recruited by a plethora of transcription factors to mediate lineage and signal-dependent transcriptional repression. We generated a knockin mutation in the receptor interaction domain (RID) of SMRT (SMRT(mRID)) that solely disrupts its interaction with nuclear hormone receptors (NHRs). SMRT(mRID) mice are viable and exhibit no gross developmental abnormalities, demonstrating that the reported lethality of SMRT knockouts is determined by non-NHR transcription factors. However, SMRT(mRID) mice exhibit widespread metabolic defects including reduced respiration, altered insulin sensitivity, and 70% increased adiposity. The latter phenotype is illustrated by the observation that SMRT(mRID)-derived MEFs display a dramatically increased adipogenic capacity and accelerated differentiation rate. Collectively, our results demonstrate that SMRT-RID-dependent repression is a key determinant of the adipogenic set point as well as an integrator of glucose metabolism and whole-body metabolic homeostasis.

Thyroid hormone receptor repression is linked to type I pneumocyte-associated respiratory distress syndrome.

Although the lung is a defining feature of air-breathing animals, the pathway controlling the formation of type I pneumocytes, the cells that mediate gas exchange, is poorly understood. In contrast, the glucocorticoid receptor and its cognate ligand have long been known to promote type II pneumocyte maturation; prenatal administration of glucocorticoids is commonly used to attenuate the severity of infant respiratory distress syndrome (RDS). Here we show that knock-in mutations of the nuclear co-repressor SMRT (silencing mediator of retinoid and thyroid hormone receptors) in C57BL/6 mice (SMRTmRID) produces a previously unidentified respiratory distress syndrome caused by prematurity of the type I pneumocyte. Though unresponsive to glucocorticoids, treatment with anti-thyroid hormone drugs (propylthiouracil or methimazole) completely rescues SMRT-induced RDS, suggesting an unrecognized and essential role for the thyroid hormone receptor (TR) in lung development. We show that TR and SMRT control type I pneumocyte differentiation through Klf2, which, in turn, seems to directly activate the type I pneumocyte gene program. Conversely, mice without lung Klf2 lack mature type I pneumocytes and die shortly after birth, closely recapitulating the SMRTmRID phenotype. These results identify TR as a second nuclear receptor involved in lung development, specifically type I pneumocyte differentiation, and suggest a possible new type of therapeutic option in the treatment of RDS that is unresponsive to glucocorticoids.

Nuclear receptor corepressor SMRT regulates mitochondrial oxidative metabolism and mediates aging-related metabolic deterioration.

The transcriptional corepressor SMRT utilizes two major receptor-interacting domains (RID1 and RID2) to mediate nuclear receptor (NR) signaling through epigenetic modification. The physiological significance of such interaction remains unclear. We find SMRT expression and its occupancy on peroxisome proliferator-activated receptor (PPAR) target gene promoters are increased with age in major metabolic tissues. Genetic manipulations to selectively disable RID1 (SMRT(mRID1)) demonstrate that shifting SMRT repression to RID2-associated NRs, notably PPARs, causes premature aging and related metabolic diseases accompanied by reduced mitochondrial function and antioxidant gene expression. SMRT(mRID1) cells exhibit increased susceptibility to oxidative damage, which could be rescued by PPAR activation or antioxidant treatment. In concert, several human Smrt gene polymorphisms are found to nominally associate with type 2 diabetes and adiponectin levels. These data uncover a role for SMRT in mitochondrial oxidative metabolism and the aging process, which may serve as a drug target to improve health span.

PGC-1beta controls mitochondrial metabolism to modulate circadian activity, adaptive thermogenesis, and hepatic steatosis.

The transcriptional coactivator peroxisome proliferator-activated receptor-gamma coactivator 1beta (PGC-1beta) is believed to control mitochondrial oxidative energy metabolism by activating specific target transcription factors including estrogen-related receptors and nuclear respiratory factor 1, yet its physiological role is not yet clearly understood. To define its function in vivo, we generated and characterized mice lacking the functional PGC-1beta protein [PGC-1beta knockout (KO) mice]. PGC-1beta KO mice are viable and fertile and show no overt phenotype under normal laboratory conditions. However, the KO mice displayed an altered expression in a large number of nuclear-encoded genes governing mitochondrial and metabolic functions in multiple tissues including heart, skeletal muscle, brain, brown adipose tissue, and liver. In contrast to PGC-1alpha KO mice that are reportedly hyperactive, PGC-1beta KO mice show greatly decreased activity during the dark cycle. When acutely exposed to cold, the KO mice developed abnormal hypothermia and morbidity. Furthermore, high-fat feeding induced hepatic steatosis and increased serum triglyceride and cholesterol levels in the KO mice. These results suggest that PGC-1beta in mouse plays a nonredundant role in controlling mitochondrial oxidative energy metabolism.

Peroxisome proliferator-activated receptor delta promotes very low-density lipoprotein-derived fatty acid catabolism in the macrophage.

Significant attention has focused on the role of low-density lipoprotein (LDL) in the pathogenesis of atherosclerosis. However, recent advances have identified triglyceride-rich lipoproteins [e.g., very LDL (VLDL)] as independent risk predictors for this disease. We have previously demonstrated peroxisome proliferator-activated receptor (PPAR)delta, but not PPARgamma, is the major nuclear VLDL sensor in the macrophage, which is a crucial component of the atherosclerotic lesion. Here, we show that, in addition to beta-oxidation and energy dissipation, activation of PPARdelta by VLDL particles induces key genes involved in carnitine biosynthesis and lipid mobilization mediated by a recently identified TG lipase, transport secretion protein 2 (also named desnutrin, iPLA2zeta, and adipose triglyceride lipase), resulting in increased fatty acid catabolism. Unexpectedly, deletion of PPARdelta results in derepression of target gene expression, a phenotype similar to that of ligand activation, suggesting that unliganded PPARdelta suppresses fatty acid utilization through active repression, which is reversed upon ligand binding. This unique transcriptional mechanism assures a tight control of the homeostasis of VLDL-derived fatty acid and provides a therapeutic target for other lipid-related disorders, including dyslipidemia and diabetes, in addition to coronary artery disease.