Case 5 - BBS2042

Case 5 - What Happens When Signals Go Nuclear?

1. Learning Goals

• Understand the following topics related to nuclear receptors:

  • General Structure

  • Classes including:

  • Steroid Receptors

  • RXR Heterodimers

  • Orphan Receptors

  • Ligands

  • Activation of nuclear receptors

  • Influences on gene expression, focusing on:

  • Transactivation

  • Transrepression (inhibition)

  • Signaling pathways of nuclear receptors independent of DNA binding

  • Co-regulatory proteins in chromatin modification

  • The concept of a cistrome

  • Specific focus on Estrogen nuclear receptor, including:

  • Signaling mechanisms

  • Influence on gene expression

  • Functions in different tissues

2. What Are Nuclear Receptors?

• Nuclear Receptors (NRs):

  • Superfamily of intracellular, ligand-dependent transcription factors.

  • Functions as:

  • Sensors of small lipophilic molecules

  • Regulators of gene expression

• Key Differences from Membrane Receptors (e.g., GPCRs, RTKs):

  • Located in the cytoplasm and/or nucleus.

  • Bind hydrophobic ligands that diffuse across the plasma membrane.

  • Directly regulate transcription by binding to DNA.

• Count in Humans: 48 nuclear receptors, known collectively as the nuclear receptor superfamily.

2.1 General Structure of Nuclear Receptors

• 
  

• Architecture: Modular proteins with conserved structural domains despite ligand differences.

• 
  

• Functional Domains:
  

  Domain	Function	Conservation
  A/B	Transcription modulation (Activation Function-1, AF-1)	Low
  C	DNA binding (zinc fingers)	High
  D	Hinge; provides flexibility	Moderate
  E	Ligand binding (Activation Function-2, AF-2)	Moderate
  F	Variable function (often poorly characterized)	Low
  2.2 Functional Domains Detailed

  1. A/B Domain (N-terminal domain, NTD):

     • Highly variable among receptors.

     • Contains AF-1, allowing ligand-independent transcriptional activity.

     • Subject to post-translational modifications.

  2. C Domain – DNA-Binding Domain (DBD):

     • Most conserved region, containing two zinc finger motifs.

     • Responsible for:

       • Recognizing specific DNA sequences (response elements).

       • Receptor dimerization.

     • Binds to hormone response elements (HREs).

     • Zink fingers coordinate Zn²⁺ ions via cysteine residues.

  3. D Domain – Hinge Region:

     • Flexible linker containing:

       • Nuclear localization sequence (NLS).

       • Sites for post-translational modifications.

  4. E Domain – Ligand-Binding Domain (LBD):

     • Moderately conserved, binds specific ligands.

     • Contains:

       • Ligand-binding pocket.

       • Dimerization interface.

       • AF-2.

  5. F Domain (Optional):

     • Present in some receptors with variable function.

  3. Classification of Nuclear Receptors

  3.1 Type I – Steroid Hormone Receptors

  • Core Characteristics:

    • Bind steroid hormones and are normally located in the cytoplasm when not activated.

    • Form homodimers.

  • DNA Binding: Typically bind to inverted repeat DNA elements.

  • Evolutionary Family: Belong to the NR3 subfamily.

  • Typical Ligands: Derived from cholesterol, including:

    • Cortisol

    • Aldosterone

    • Testosterone

    • Estradiol

    • Progesterone

  • Structural Features:

    • Large ligand-binding pocket accommodating bulky steroid molecules.

    • Strong interaction with chaperone proteins (e.g., HSP90).

  3.2 Type II – RXR Heterodimeric Receptors

  • Core Characteristics:

    • Located primarily in the nucleus.

    • Form heterodimers with RXR, bind DNA as pre-formed dimers.

  • Regulation: Often regulate metabolism and development, mainly belonging to NR1 and NR2 subfamilies.

  • Types of Heterodimers:

    • Permissive: Activation possible via RXR ligand alone.

    • Non-permissive: Require binding of the partner ligand.

  3.3 Orphan Nuclear Receptors

  • Definition: Receptors that do not have an identified endogenous ligand or function constitutively (ligand-independent).

  • Characteristics:

    • Typically smaller or occluded ligand-binding pockets.

  • Examples:

    • NR4A family — involved in apoptosis, immune regulation, and neuronal development.

    • SF-1 (Steroidogenic Factor 1) involved in steroidogenesis.

    • DAX-1, transcriptional repressor significant in sex development.

  4. Activation of Nuclear Receptors

  • Key to Activation: Movement of Helix 12 in the Ligand-Binding Domain (LBD).

  • Without Ligand: Helix 12 blocks coactivator binding.

  • With Ligand: Helix 12 folds over the ligand pocket, creating a docking site for coactivators.

  4.1 Activation of Steroid Hormone Receptors (Type I)

  • Activation Examples:

    • Cortisol, Estradiol, Testosterone.

  • Step-by-Step Mechanism:

    1. Ligand Diffusion: Steroid hormones diffuse through the plasma membrane.

    2. Inactive Receptor: Binds to heat shock proteins (HSP90, HSP70) in the cytoplasm.

    3. Ligand Binding: Alters conformation, opens AF-2 region for coactivators.

    4. Chaperone Dissociation: Leads to receptor activation and NLS exposure.

    5. Homodimerization: Ligand-bound receptor forms a homodimer, increasing DNA affinity.

    6. Nuclear Translocation: Receptor transported into the nucleus via importin.

    7. DNA Binding: Homodimer binds to HREs on DNA.

    8. Coactivator Recruitment: Coactivators facilitate chromatin remodeling and RNA polymerase II recruitment.

  4.2 Activation of RXR Heterodimer Receptors (Type II)

  • Differences from Steroid Receptors: Typically, receptors are already located in the nucleus and bound to DNA.

  • Step-by-Step Mechanism:

    1. Basal State: Heterodimer with RXR bound to DNA.

    2. Corepressor Binding: Corepressors attached to inhibit transcription.

    3. Ligand Binding: Causes conformational change, releases corepressors.

    4. Coactivator Recruitment: Involves binding of coactivators leading to chromatin openness.

    5. Transcription Initiation: RNA polymerase II is recruited, gene transcription increases.

  4.3 Activation of Orphan Receptors

  • Varied mechanisms exist:

    • Constitutively Active

    • Ligand-Dependent (with unknown endogenous ligands)

    • Repressor-Type Orphans

  5. Nuclear Receptors as Gene Regulators

  • Once activated, nuclear receptors can influence gene expression by:

    1. Transactivation - Increasing transcription.

    2. Transrepression (Transinhibition) - Decreasing transcription.

  5.1 Transactivation

  • Definition: Activated nuclear receptors stimulate transcription of a target gene.

  • Functional Outcome: Genes like metabolic enzymes and anti-inflammatory proteins are often upregulated.

  5.2 Transrepression (Transinhibition)

  • Definition: The nuclear receptor suppresses gene expression indirectly through protein interactions.

    • Mechanisms include:

    1. Tethering: Binding to other TFs without direct binding to HRE.

       • Example: Glucocorticoid receptor interacts with NF-κB.

    2. Coactivator Competition: Receptor competes for coactivators with other TFs.

    3. Corepressor Recruitment: Binding of corepressors can lead to reduced transcription.

  5.3 Ligand-Dependent Derepression

  • Description: Binding of a ligand removes transcriptional repression.

  • Commonly occurs with Type II nuclear receptors.

  5.4 Signal-Dependent Derepression

  • Dependent on post-translational modifications in response to signaling pathways; involves cross-talk between cellular signals.

  6. What Are Co-Regulatory Proteins?

  • Proteins that do not bind DNA directly but interact with ligand-bound nuclear receptors to modify chromatin structure.

  6.1 Coactivators

  • Function: Promote gene expression when recruited by active receptors.

  • Act by modifying chromatin and facilitating assembly of transcription machinery.

  6.2 Corepressors

  • Role: Promote gene silencing, involved in repressing transcription through recruitment of HDACs.

  7. DNA-Binding–Independent (Non-Genomic) Downstream Effects of Nuclear Receptor Activation

  • Characteristics: Rapid responses occurring within seconds to minutes, not requiring direct DNA binding.

  7.1 Membrane-Associated Nuclear Receptors

  • Can localize to plasma membranes and interact with signaling proteins.

  • Example: Estrogen receptor activates Src kinase.

  7.2 Cytoplasmic Protein–Protein Interactions

  • Affect kinase activity and rapidly modulate signaling pathways.

  7.3 Modulation of Kinase Cascades

  • Influence pathways like MAPK and PI3K/Akt via direct interaction.

  8. What Is a Cistrome?

  • Defined as the complete set of DNA binding sites for a particular transcription factor across the genome.

  • Cistrome vs. Transcriptome: Cistrome pertains to binding sites while transcriptome involves expressed genes.

  8.1 Determinants of a Cistrome

  • Factors influencing cistromes include:

    • Chromatin Accessibility

    • Pioneer Factors (e.g., FOXA1)

    • DNA Sequence Context

    • Cell-Specific Expression of Co-Regulators

  9. The Estrogen Receptor: The Basics

  • Main Receptors: Estrogen receptor alpha (ERα) and beta (ERβ) with estradiol as the primary ligand.

  • Classical Signaling: Involves ligand diffusion, receptor binding, dimerization, DNA binding, and recruitment of coactivators leading to gene activation.

  • Transactivation and Transrepression: ER can recruit coactivators or tether to other transcription factors to modulate gene expressions.