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Biomedical subjects

R N Cohen

Publications and source records attributed to R N Cohen.

10 recordsLinked to original sources
PubMed2001

An unliganded thyroid hormone receptor causes severe neurological dysfunction.

Congenital hypothyroidism and the thyroid hormone (T(3)) resistance syndrome are associated with severe central nervous system (CNS) dysfunction. Because thyroid hormones are thought to act principally by binding to their nuclear receptors (TRs), it is unexplained why TR knock-out animals are reported to have normal CNS structure and function. To investigate this discrepancy further, a T(3) binding mutation was introduced into the mouse TR-beta locus by homologous recombination. Because of this T(3) binding defect, the mutant TR constitutively interacts with corepressor proteins and mimics the hypothyroid state, regardless of the circulating thyroid hormone concentrations. Severe abnormalities in cerebellar development and function and abnormal hippocampal gene expression and learning were found. These findings demonstrate the specific and deleterious action of unliganded TR in the brain and suggest the importance of corepressors bound to TR in the pathogenesis of hypothyroidism.

Animals↗
PubMed2001

The specificity of interactions between nuclear hormone receptors and corepressors is mediated by distinct amino acid sequences within the interacting domains.

The thyroid hormone receptor (TR) and retinoic acid receptor (RAR) isoforms interact with the nuclear corepressors [NCoR (nuclear corepressor protein) and SMRT (silencing mediator for retinoid and thyroid hormone receptors)] in the absence of ligand to silence transcription. NCoR and SMRT contain C-terminal nuclear hormone receptor (NHR) interacting domains that each contain variations of the consensus sequence I/L-x-x-I/V-I (CoRNR box). We have previously demonstrated that TRbeta1 preferentially interacts with NCoR, whereas RARalpha prefers SMRT. Here, we demonstrate that this is due, in part, to the presence of a novel NCoR interacting domain, termed N3, upstream of the previously described domains. An analogous domain is not present in SMRT. This domain is specific for TR and interacts poorly with RAR. Our data suggest that the presence of two corepressor interacting domains are necessary for full interactions with nuclear receptors in cells. Interestingly, mutation of N3 alone specifically decreases binding of NCoR to TR in cells but does not decrease NCoR-RAR interactions. In addition, while the exact CoRNR box sequence of a SMRT interacting domain is critical for recruitment of SMRT by RAR, the CoRNR box sequences themselves do not explain the strong interaction of the N2 domain with TRbeta1. Additional regions distal to the CoRNR box sequence are needed for optimal binding. Thus, through sequence differences in known interacting domains and the presence of a newly identified interacting domain, NCoR is able to preferentially bind TRbeta1. These preferences are likely to be important in corepressor action in vivo.

Amino Acid Sequence↗
PubMed2000

Thyroid hormone-independent interaction between the thyroid hormone receptor beta2 amino terminus and coactivators.

Thyroid hormone receptors (TRs) mediate hormone action by binding to DNA response elements (TREs) and either activating or repressing gene expression in the presence of ligand, T(3). Coactivator recruitment to the AF-2 region of TR in the presence of T(3) is central to this process. The different TR isoforms, TR-beta1, TR-beta2, and TR-alpha1, share strong homology in their DNA- and ligand-binding domains but differ in their amino-terminal domains. Because TR-beta2 exhibits greater T(3)-independent activation on TREs than other TR isoforms, we wanted to determine whether coactivators bound to TR-beta2 in the absence of ligand. Our results show that TR-beta2, unlike TR-beta1 or TR-alpha1, is able to bind certain coactivators (CBP, SRC-1, and pCIP) in the absence of T(3) through a domain which maps to the amino-terminal half of its A/B domain. This interaction is specific for certain coactivators, as TR-beta2 does not interact with other co-factors (p120 or the CBP-associated factor (pCAF)) in the absence of T(3). The minimal TR-beta2 domain for coactivator binding is aa 21-50, although aa 1-50 are required for the full functional response. Thus, isoform-specific regulation by TRs may involve T(3)-independent coactivator recruitment to the transcription complex via the AF-1 domain.

Binding Sites↗
PubMed2000

The nuclear corepressors recognize distinct nuclear receptor complexes.

The thyroid hormone receptor (TR) and retinoic acid receptor (RAR) isoforms have the capacity to silence gene expression in the absence of their ligands on target response elements. This active repression is mediated by the ability of the corepressors, nuclear receptor corepressor (NCoR) and silencing mediator of retinoid and thyroid hormone receptors (SMRT), to recruit a complex containing histone deacetylase activity. Interestingly, NCoR and SMRT share significant differences in the their two nuclear receptor-interacting domains (IDs), suggesting that they may recruit receptors with different affinities. In addition, the role of the receptor complex bound to a response element has not been fully evaluated in its ability to recruit separate corepressors. We demonstrate in this report that the proximal ID in NCoR and SMRT, which share only 23% homology, allows preferential recognition of nuclear receptors, such that TR prefers to recruit NCoR, and RAR prefers to recruit SMRT, to DNA response elements. However, mutations in the TR found in the syndromes of resistance to thyroid hormone can change the corepressor recruited by changing the complex (homodimer or heterodimer) formed on the TRE. These results demonstrate that the corepressor complex recruited can be both nuclear receptor- and receptor complex-specific.

Binding Sites↗
PubMed1998

Defective release of corepressor by hinge mutants of the thyroid hormone receptor found in patients with resistance to thyroid hormone.

On positive thyroid hormone response elements (pTREs), thyroid hormone receptor (TR) binding to DNA in the absence of ligand (thyroid hormone, T3) decreases transcription (silencing). Silencing is due to a family of recently described nuclear corepressor proteins (NCoR and SMRT) which bind to the CoR box in the hinge region of TR. Ligand-dependent activation of TR is associated with displacement of corepressors and recruitment of coactivating proteins. Resistance to thyroid hormone (RTH) is due to mutations in the beta isoform of the thyroid hormone receptor (TR-beta). To date, three RTH mutations reportedly with near-normal T3 binding (A234T, R243Q, and R243W) have been described in or near the CoR box. To determine the mechanism of RTH caused by these mutants, the interaction of wild type (wt) and mutant TRs with the corepressor, NCoR, and the coactivator, SRC-1, was tested in gel-shift assays. As expected, NCoR bound wt TR in the absence of T3 and dissociated from TR with increasing T3 concentration. SRC-1 failed to bind wt TR in the absence of T3, but bound to TR with increasing avidity as T3 concentrations rose. At no T3 concentration did both NCoR and SRC-1 bind to wt TR, indicating that their binding to TR was mutually exclusive. Hinge mutants bound NCoR normally in the absence of T3; however, dissociation of NCoR and recruitment of SRC-1 was markedly impaired except at very high T3 concentrations. Importantly, hinge mutant TRs when complexed to DNA bound T3 poorly despite their near-normal T3 binding in solution. These binding studies correlated with functional assays showing defective transactivation of pTREs by hinge mutants except at high T3 concentrations. Thus, we describe a novel mechanism of RTH whereby TR hinge mutants selectively affect T3 binding when complexed to DNA, and prevent NCoR dissociation from TR. Our data also suggest that solution T3 binding by RTH mutants may not accurately reflect physiologically relevant T3 binding by TR when bound to DNA.

DNA↗
PubMed1998

Two separate NCoR (nuclear receptor corepressor) interaction domains mediate corepressor action on thyroid hormone response elements.

The nuclear corepressor (NCoR) binds to the thyroid hormone receptor (TR) in the absence of ligand. NCoR-TR interactions are mediated by two interaction domains in the C-terminal portion of NCoR. Binding of NCoR to TR results in ligand-independent repression on positive thyroid hormone response elements. The interactions between NCoR interaction domains and TR on DNA response elements, however, have not been well characterized. We have found that both interaction domains are capable of binding TR on thyroid hormone response elements. In addition, the NCoR interaction domains interact much more strongly with the TR than those present in the silencing mediator of retinoic acid and TRs (SMRT). Furthermore, deletion of either NCoR interaction domain does not significantly impair ligand-independent effects on positive or negative thyroid hormone response elements. Finally, both NCoR interaction domains appear to preferentially bind TR homodimer over TR-retinoid X receptor heterodimer in electrophoretic mobility shift assays. These data suggest that either NCoR interaction domain is capable of mediating the ligand-independent effects of TR on positive and negative thyroid hormone response elements.

Amino Acid Sequence↗
PubMed1994

Aplasia cutis congenita, elevated alpha-fetoprotein, and a distinct amniotic fluid acetylcholinesterase electrophoretic band.

Aplasia cutis congenita affecting the elbows, knees, hips, and gluteal area was observed in a female newborn, product of a twin pregnancy. One of the twins was a fetus papyraceous detected at 15 weeks of pregnancy. During the course of the pregnancy, maternal thrombocytosis was diagnosed and treated with aspirin. alpha-Fetoprotein was elevated in maternal serum and amniotic fluid, and a distinct electrophoretic acetylcholinesterase band was seen in amniotic fluid. These findings are in agreement with the classification of aplasia cutis congenita as proposed by Frieden et al in which type V is related to the presence of a fetus papyraceous or placental infarcts. The findings in the present case may be explained by the effect of the dead twin on the surviving fetus and the extensive denuded skin areas. Long-term follow-up of the infant showed that the lesions were cured, most of them with minimal scars. Increased risk for aplasia cutis congenita should be considered when elevated maternal and amniotic fluid alpha-fetoprotein and a distinct electrophoretic band of acetylcholinesterase are found. Especially when one of the twins is dead.

Acetylcholinesterase↗
PubMed1986

T cell lines derived from the spinal cords of mice with experimental allergic encephalomyelitis are self reactive.

Experimental allergic encephalomyelitis (EAE) is an animal model of T cell-mediated, central nervous system neuropathology that may be a relevant animal model for multiple sclerosis. EAE is usually induced by sensitization of animals with a xenogeneic myelin basic protein (MBP). Recently, MBP-reactive T cell lines and clones derived from lymphoid tissue of animals with EAE have proved very useful in elucidating certain aspects of the pathogenesis in EAE. However, questions relating to how T cells actually mediate the pathologic changes seen in EAE remain unresolved. We now report for the first time the derivation of long-term, interleukin 2-dependent T cell lines and sublines from a site of pathology in murine EAE--the spinal cord. All of the spinal cord-derived T cell lines and sublines were found to be "autoreactive" in that they responded to self (murine) MBP as well as to the xenogeneic immunogen, porcine MBP. The ability to derive T cell lines and sublines from the spinal cords of mice with EAE should now aid in the elucidation of pathogenetic mechanisms in EAE by allowing for a characterization of those T cells found at the site of pathology.

Animals↗