Cell Communication Overview The Cellular Internet Celltocell communication

Cell Communication

Overview: The Cellular Internet Cell-to-cell communication is absolutely essential for multicellular organisms Nerve cells must communicate pain signals to muscle cells (stimulus) in order for muscle cells to initiate a response to pain

Biologists have discovered some universal mechanisms of cellular regulation

External Signals Signal Transduction Pathway (Relay Molecules)

Yeast cells identify their mates by cell signaling (early evidence of signaling) 1 Exchange of mating factors. Each cell type secretes a mating factor that binds to receptors on the other cell type. 2 Mating. Binding of the factors to receptors induces changes in the cells that lead to their fusion. factor Receptor a Yeast cell, mating type a factor a 3 New a/ cell. The nucleus of the fused cell includes all the genes from the a and a cells. Yeast cell, mating type a/

Signal Transduction Pathways Convert signals on a cell’s surface into cellular responses Are similar in microbes and mammals, suggesting an early origin

Local and Long-Distance Signaling Cells in a multicellular organism (tissues, organs, systems) communicate via chemical messengers A hormone is a chemical released by a cell in one part of the body, that sends out messages that affect cells in other parts of the organism All multicellular organisms produce hormones Plant hormones are also called phytohormones Hormones in animals are often transported in the blood

Animal and plant cells Have cell junctions that directly connect the cytoplasm of adjacent cells Plasma membranes Gap junctions between animal cells Plasmodesmata between plant cells Figure 11. 3 (a) Cell junctions. Both animals and plants have cell junctions that allow molecules to pass readily between adjacent cells without crossing plasma membranes.

In local signaling, animal cells May communicate via direct contact Figure 11. 3(b) Cell-cell recognition. Two cells in an animal may communicate by interaction between molecules protruding from their surfaces.

In other cases, animal cells Communicate using local regulators Local signaling Target cell Electrical signal along nerve cell triggers release of neurotransmitter Neurotransmitter diffuses across synapse Secretory vesicle Local regulator diffuses through extracellular fluid (a) Paracrine signaling. A secreting cell acts on nearby target cells by discharging molecules of a local regulator (a growth factor, for example) into the extracellular fluid. Target cell is stimulated (b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell.

In long-distance signaling Both plants and animals use hormones (e. g. Insulin) Long-distance signaling Blood vessel Endocrine cell Hormone travels in bloodstream to target cells Target cell Figure 11. 4 C (c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood. Hormones may reach virtually all body cells.

The Three Stages of Cell Signaling Earl W. Sutherland Discovered how the hormone epinephrine acts on cells

Sutherland’s Three Steps Sutherland suggested that cells receiving signals went through three processes Reception Transduction Response

Overview EXTRACELLULAR FLUID 1 Reception of cell signaling Plasma membrane CYTOPLASM 2 Transduction 3 Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Signal molecule Figure 11. 5

Step One - Reception occurs when a signal molecule binds to a receptor protein, causing it to change shape Receptor protein is on the cell surface

The binding between signal molecule (ligand) and receptor is highly specific A conformational change in a receptor is often the initial transduction of the signal

Step Two - Transduction The binding of the signal molecule alters the receptor protein in some way The signal usually starts a cascade of reactions known as a signal transduction pathway Multistep pathways can amplify a signal

Step Three - Response Cell signaling leads to regulation of cytoplasmic activities or transcription Signaling pathways regulate a variety of cellular activities

1. 6 Example of Pathway Steroid Hormone (testosterone) hormones bind to intracellular receptors EXTRACELLULAR FLUID Plasma membrane Receptor protein Hormonereceptor complex 1 The steroid hormone testosterone passes through the plasma membrane. 2 Testosterone binds to a receptor protein in the cytoplasm, activating it. 3 The hormone- receptor complex enters the nucleus and binds to specific genes. DNA m. RNA NUCLEUS CYTOPLASM 4 New protein The bound protein stimulates the transcription of the gene into m. RNA. 5 The m. RNA is translated into a specific protein.

Other pathways regulate genes by activating transcription factors that turn genes on or off Growth factor Receptor Phosphorylation cascade Reception Transduction CYTOPLASM Inactive transcription Active transcription factor P Response Figure 11. 14 DNA Gene NUCLEUS m. RNA

Termination of the Signal response is terminated quickly by the reversal of ligand binding

Receptors in the Plasma Membrane There are three main types of membrane receptors: G-protein-linked Tyrosine kinases Ion channel

G-protein-linked receptors detect molecules outside the cell and activate internal signal transduction pathways Signal-binding site Figure 11. 7 Segment that interacts with G proteins G-protein-linked Receptor Plasma Membrane GDP CYTOPLASM G-protein (inactive) Enzyme Activated Receptor GDP Signal molecule GTP Activated enzyme GTP Cellular response GDP Pi Inactivate enzyme

Receptor tyrosine kinase enzyme that can transfer a phosphate group from ATP to a protein in a cell. It functions as an "on" or "off" switch in many cellular functions 2% Signal molecule Helix in the Membrane Signal-binding site Signal molecule Tyrosines Tyr Tyr CYTOPLASM Receptor tyrosine kinase proteins (inactive monomers) Tyr Tyr Tyr Tyr Dimer Figure 11. 7 Activated relay proteins Tyr Tyr Tyr 6 ATP Activated tyrosinekinase regions (unphosphorylated dimer) 6 ADP P Tyr P Tyr P Fully activated receptor tyrosine-kinase (phosphorylated dimer) P Tyr P Tyr P Inactive relay proteins Cellular response 1 Cellular response 2

Ion channel receptors Signal molecule (ligand) Pore-forming membrane proteins that allow ions to pass through the channel pore Gate closed Ligand-gated ion channel receptor Ions Plasma Membrane Gate open Cellular response Gate close Figure 11. 7