Bases moleculares de la comunicación extracelular e intracelular (2 parte)

Overview of Cell Communication and Receptor Types

Introduction to Receptors

• The presentation revisits the classification of receptors into membrane receptors and intracellular receptors, focusing on their roles in cellular communication.

• Membrane receptors are further divided into ionotropic receptors, which facilitate ion movement, and metabolotropic receptors that interact with enzymes.

Metabolotropic Receptors

• Discussion includes metabolotropic receptors with intrinsic enzymatic activity and those associated with G-protein coupled receptors (GPCRs).

• Emphasis is placed on G-proteins as crucial signaling couplers in various pathways.

G-Protein Coupled Receptor Signaling Pathway

Activation Mechanism

• When a ligand binds to GPCRs, it triggers the release of the alpha subunit from the beta-gamma dimer, leading to GDP-GTP exchange.

• The activated alpha subunit stimulates adenylate cyclase, converting ATP to cyclic AMP (cAMP), which acts as a second messenger.

Role of Protein Kinase A

• cAMP activates protein kinase A (PKA), which consists of regulatory and catalytic subunits; this activation leads to phosphorylation of target proteins.

• An example provided is CREB (cAMP response element-binding protein), a transcription factor that translocates to the nucleus upon phosphorylation.

Importance of Signal Termination

Inactivation Processes

• The presentation highlights that both activation and deactivation processes are vital for maintaining cellular homeostasis.

• Protein phosphatases play a key role in removing phosphate groups from proteins, reversing phosphorylation effects.

Phosphodiesterases Functionality

• Phosphodiesterases hydrolyze cAMP into AMP, effectively terminating its action as a second messenger.

• There are 11 isoenzymes differing by substrate specificity and regulatory mechanisms affecting their activity across different tissues.

Returning to Basal State

Ligand-Receptor Dynamics

• The alpha subunit has intrinsic GTPase activity allowing it to revert back to its inactive state by hydrolyzing GTP.

• Ligands bind reversibly; thus, their dissociation is essential for stopping signal transduction initiated by receptor activation.

Examples of GPCR Signaling

• Notable examples include glucagon receptor involved in metabolic processes during fasting and adrenergic receptors (beta1, beta2, beta3), among others related to hormonal signaling.

Hormonal Signaling and Calcium Dynamics

Hormonal Expression and Receptor Types

• The discussion begins with the expression and secretion of thyroid hormones, highlighting various receptors such as D1 and D5 for dopamine, olfaction, and taste that signal through cyclic AMP (cAMP) to maintain homeostasis.

• It is noted that some receptor families are linked to the MPC pathway, specifically mentioning G inhibitory proteins (GI) that inhibit certain activities when activated.

Inhibition Mechanisms in Signal Transduction

• When a GI protein binds to its receptor, it activates a cascade leading to GDP being exchanged for GTP, which inhibits adenylate cyclase activity, resulting in decreased cAMP production.

• Examples include alpha-2 adrenergic receptors that inhibit adenylate cyclase while beta receptors promote signaling through cAMP formation.

Calcium Signaling Pathways

• The text discusses calcium dynamics within cells, emphasizing how calcium is compartmentalized due to its tendency to precipitate with phosphates. This compartmentalization prevents unwanted reactions.

• Cells expend energy maintaining specific calcium gradients between extracellular (millimolar levels) and intracellular environments (nanomolar levels), utilizing organelles like the endoplasmic reticulum and mitochondria.

Phosphoinositide Pathway Activation

• The signaling pathways involving phosphoinositides are introduced; upon ligand binding to GQ-coupled receptors, GDP is exchanged for GTP activating phospholipase C.

• Phospholipase C hydrolyzes phosphatidylinositol bisphosphate into diacylglycerol (DAG) and inositol trisphosphate (IP3), where IP3 facilitates calcium release from intracellular stores.

Role of Calmodulin in Cellular Processes

• Calmodulin's role is highlighted as it binds calcium ions leading to activation of calmodulin-dependent kinases which phosphorylate target proteins affecting cellular functions.

• Both calcium and DAG activate protein kinase C (PKC), which translocates to the membrane where it can phosphorylate additional target proteins involved in various cellular processes.

Protein Phosphorylation and Calcium Signaling

Importance of Returning to Basal States

• The analysis of the MP signaling pathway highlights the significance of systems returning to their basal state. This is crucial for processes like calcium phosphate regulation, which involves protein phosphatases that dephosphorylate proteins to restore their previous states.

Calcium Pumps and Cytosolic Concentrations

• Calcium pumps are present in various cellular locations, such as the endoplasmic reticulum and plasma membrane, facilitating calcium transport from the cytosol either extracellularly or back into storage. This action helps maintain cytosolic calcium concentrations at baseline levels.

Role of Calmodulin and Inositol Trisphosphate

• As cytosolic calcium levels decrease, proteins that bind calcium lose affinity, leading to calmodulin returning to an inactive state. Inositol trisphosphate (IP3) acts as a substrate for inositol phosphatases, which hydrolyze phosphates and diminish stimulation on ionotropic receptors associated with IP3.

GTPase Activity Restoration

• The GTPase activity of the alpha subunit restores the G protein's inactive basal state by allowing ligands to dissociate from receptors, preventing further activation of signaling pathways. This mechanism is essential for regulating cellular responses effectively.

Examples of GQ Protein-Coupled Receptors

• Various receptors coupled with GQ proteins include adrenergic alpha 1 receptors (involved in adrenaline signaling), serotonin receptors, muscarinic acetylcholine receptors (M1, M3, M5), oxytocin receptors, and others that mediate diverse effects through calcium signaling cascades involving inositol phosphates.

Interaction Between GPCRs and Ion Channels

Ion Channel Interactions with GQ Proteins

• Some GQ protein-coupled receptors interact directly with ion channels; this interaction influences cellular responses based on ion movement across membranes. Understanding these interactions is critical for grasping how signals translate into physiological actions at the cellular level.

GS Protein-Coupled Receptors and cAMP Production

• GS protein-coupled receptors activate adenylate cyclase to produce cyclic AMP (cAMP), which can also modulate calcium channels' activity—allowing ions' entry or exit depending on specific contexts within cells. This pathway illustrates another layer of complexity in signal transduction mechanisms.

Complexity in Receptor Classification

Challenges in Classifying Metabotropic vs Ionotropic Receptors

• The classification between metabotropic (enzymatic involvement) and ionotropic (direct channel involvement) receptor types can be ambiguous due to overlapping functionalities; some may not fit neatly into established categories but still play significant roles in signal transduction processes. Understanding these nuances is vital for comprehending receptor dynamics fully.

Cyclic GMP Pathway Overview

Introduction to Cyclic GMP Signaling

• The cyclic GMP pathway serves as a second messenger system similar to cyclic AMP but operates through different mechanisms involving guanylate cyclase activity—either membrane-bound or soluble forms that convert GTP into cyclic GMP upon activation by nitric oxide gas diffusion across membranes without needing surface receptor localization.

Role of cGMP-dependent Protein Kinase

• cGMP activates specific protein kinases dependent on cyclic GMP; these kinases facilitate phosphorylation events akin to those mediated by cAMP pathways—demonstrating how different nucleotide-based signaling molecules can converge on similar downstream effects within cells while maintaining distinct regulatory mechanisms throughout various physiological contexts.

Understanding Intrinsic Enzymatic Activity in Receptors

Regulation of cAMP and GMP Levels

• Cells must regulate cyclic AMP (cAMP) levels through synthesis via PKA and degradation by specific phosphodiesterases.

• Similar regulation applies to cyclic GMP (cGMP), maintaining a balance between synthesis via guanylate cyclase receptors and degradation.

Tyrosine Kinase Receptors Overview

• All members of the tyrosine kinase receptor family undergo self-phosphorylation upon ligand binding, crucial for their signaling function.

• These receptors must exist in a dimeric form to facilitate cross-phosphorylation, which is essential for initiating downstream signaling pathways.

Insulin Receptor Activation

• The insulin receptor is pre-dimerized with two alpha and two beta chains, where cross-phosphorylation occurs on the intracellular domains upon insulin binding.

• Examples of intrinsic enzymatic activity receptors include the insulin receptor and various growth factor receptors like EGF and PDGF.

Signaling Cascades Initiated by Insulin Receptor

• Upon activation by insulin, the phosphorylated tyrosines recruit proteins that initiate further signaling cascades.

• Two main branches emerge from this activation: one involving recruitment of proteins that recognize phosphorylated tyrosines.

Recruitment of Proteins in Signaling Pathways

• Phosphorylated domains are recognized by proteins with SH2 domains, facilitating a sequence of protein recruitment leading to defined cellular effects.

• The complex formed allows GDP-GTP exchange in RAS proteins, activating MAP kinases involved in cell proliferation and differentiation.

MAP Kinase Cascade Details

• Activated RAS triggers a cascade known as MAP kinases or mitogen-activated protein kinases, regulating transcription factors linked to cell processes.

• This cascade includes key components such as Raf, MEK, and ERK that ultimately influence gene expression related to cell growth.

Role of Insulin Receptor Substrates

• Another branch involves recruiting insulin receptor substrates (IRS), which are also phosphorylated by the activated receptor.

• IRS proteins play a critical role in mediating signals from the insulin receptor beyond self-phosphorylation.

Insulin Signaling Pathways and Their Mechanisms

Overview of Insulin Receptor Activation

• The insulin receptor undergoes phosphorylation, activating a kinase known as phosphatidylinositol 3-kinase (PI3K).

• PI3K utilizes phosphatidylinositol bisphosphate in signaling cascades, particularly involving inositol pathways.

Role of Protein Kinase B (PKB/AKT)

• PKB, also referred to as AKT, is central to the signaling cascade activated by insulin.

• PKB phosphorylates transcription factor Foxo1, which when phosphorylated is excluded from the nucleus and influences lipid metabolism.

Metabolic Regulation by PKB

• PKB regulates glycogen metabolism through glycogen synthase kinase 3 and affects lipid synthesis processes.

• By activating phosphodiesterase 3, PKB reduces cyclic AMP levels, impacting various metabolic pathways.

Interaction Between Insulin and Glucagon

• There exists an antagonistic relationship between glucagon (a fasting hormone) and insulin (a satiety hormone), mediated by PKB's actions.

Vesicle Movement and Glucose Transport

• Insulin signaling via PKB controls intracellular vesicle movement containing glucose transporter GLUT4 towards the plasma membrane.

• The protein RAP is involved in this process; its activity is regulated by whether it binds GDP or GTP due to phosphorylation events initiated by PKB.

Calcium's Role in Vesicle Translocation

• Calcium release facilitated by phospholipase C gamma activation allows for GLUT4 translocation to the plasma membrane.

Tissue-Specific Glucose Uptake Mechanisms

• Insulin-dependent tissues such as adipose tissue and resting skeletal muscle require GLUT4 mobilization for glucose uptake.

• Skeletal muscle can become insulin-independent during contraction due to calcium-mediated mechanisms that mobilize vesicles without insulin.

Receptor Signaling Mechanisms

Metabotropic Receptors and Their Functions

• Discussion on the importance of insulin for signaling, highlighting metabotropic receptors such as G-protein coupled receptors and those with intrinsic enzymatic activity.

• Introduction to itinerant enzymes associated with these receptors, particularly kinases that phosphorylate target proteins upon ligand binding.

JAK-STAT Pathway

• Explanation of the JAK-STAT signaling pathway, where phosphorylated STAT proteins translocate to the nucleus to regulate gene expression.

• Mention of leptin and its receptor as a key example of JAK-STAT signaling in appetite control, along with other hormones like prolactin and growth hormone.

Intracellular Receptors Overview

Type 1 Intracellular Receptors

• Description of intracellular receptors' structure, emphasizing their ability to bind DNA and their association with heat shock proteins (HSP90).

• Mechanism by which ligand binding releases HSP90, allowing receptor activation and nuclear translocation for gene regulation.

Type 2 Intracellular Receptors

• Characteristics of type 2 intracellular receptors located in the nucleus, which are inactive due to co-repressor proteins until activated by hormone binding.

• The role of coactivators in recruiting RNA polymerase post-hormone binding for mRNA transcription.

Examples of Hormonal Receptor Types

Type 1 Receptors

• Examples include steroid hormone receptors (estrogen, progesterone), which operate through mechanisms involving HSP90.

Type 2 Receptors

• Discussion on various type 2 receptors including thyroid hormone receptors (T3/T4), retinoic acid receptor, vitamin D receptor, and PPAR family members involved in metabolic regulation.

Molecular Mechanisms of Cell Signaling

Cholesterol Metabolism and Receptor Interaction

• Derivatives from cholesterol metabolism and oxysterols can bind to type 2 receptors, specifically LXR (liver X receptor), influencing gene expression.

• Various substances, including fatty acids and vitamins, mediate cellular responses through receptors beyond hormones, indicating a broader signaling landscape.

Gene Expression Regulation

• Both type 1 and type 2 receptors regulate gene expression, leading to phenotypic and functional changes in cells.

• Membrane metabolotropic receptors may possess intrinsic enzymatic activity or associate with G-proteins for signal transduction.

Cyclic AMP (cAMP) Dynamics

• cAMP levels are regulated by the balance between its synthesis via GS protein-coupled receptor activation and degradation by phosphodiesterases.

• cAMP activates protein kinase A (PKA), which regulates cellular processes through phosphorylation mechanisms.

Calcium Signaling Pathways

• The release of cytosolic calcium is controlled by intracellular storage dynamics and cellular import/export mechanisms.

• Calcium binds to calmodulin, activating calcium-dependent protein kinases that modulate various cellular functions through phosphorylation.

Guanosine Monophosphate (GMP) Signaling

• GMP levels result from the activity of soluble guanylate cyclase activated by membrane-bound or cytosolic receptors; it also undergoes degradation by specific phosphodiesterases.

• Insulin signaling involves a receptor with intrinsic tyrosine kinase activity triggering two main pathways: MAP kinase pathway for cell cycle regulation and PI3K pathway affecting multiple metabolic processes.

Intracellular Receptor Activation

• Intracellular receptors bind ligands leading to dimerization and attachment to specific DNA sequences, modifying transcriptional activity. This highlights the importance of nuclear hormone receptors in gene regulation.