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

Signal Transduction Pathway External Signals Signal Transduction Pathway

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

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

Long-distance signaling In long-distance signaling Both plants and animals use hormones (e.g. Insulin) Hormone travels in bloodstream to target cells (c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood. Hormones may reach virtually all body cells. Long-distance signaling Blood vessel Target cell Endocrine cell Figure 11.4 C

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

Relay molecules in a signal transduction pathway Overview of cell signaling EXTRACELLULAR FLUID Receptor Signal molecule Relay molecules in a signal transduction pathway Plasma membrane CYTOPLASM Activation of cellular response Figure 11.5 Reception 1 Transduction 2 Response 3

Step One - Reception Reception occurs when a signal molecule (ligand) 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

Example of Pathway Steroid hormones bind to intracellular receptors (testosterone) EXTRACELLULAR FLUID Receptor protein DNA mRNA NUCLEUS CYTOPLASM Plasma membrane Hormone- receptor complex New protein Figure 11.6 1 The steroid hormone testosterone passes through the plasma membrane. Testosterone binds to a receptor protein in the cytoplasm, activating it. 2 The hormone- receptor complex enters the nucleus and binds to specific genes. 3 The bound protein stimulates the transcription of the gene into mRNA. 4 The mRNA is translated into a specific protein. 5

Other pathways regulate genes by activating transcription factors that turn genes on or off Reception Transduction Response mRNA NUCLEUS Gene P Active transcription factor Inactive DNA Phosphorylation cascade CYTOPLASM Receptor Growth factor Figure 11.14

Termination of the Signal 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 Plasma Membrane Enzyme G-protein (inactive) CYTOPLASM Cellular response Activated enzyme Activated Receptor Signal molecule Inactivate Segment that interacts with G proteins GDP GTP P i Signal-binding site Figure 11.7

Receptor tyrosine kinases Signal molecule Signal-binding site CYTOPLASM Tyrosines Signal molecule Helix in the Membrane Tyr Dimer Receptor tyrosine kinase proteins (inactive monomers) P Cellular response 1 Inactive relay proteins Activated relay proteins Cellular response 2 Activated tyrosine- kinase regions (unphosphorylated dimer) Fully activated receptor tyrosine-kinase (phosphorylated 6 ATP 6 ADP Figure 11.7

Ion channel receptors Figure 11.7 Cellular response Gate open Gate close Ligand-gated ion channel receptor Plasma Membrane Signal molecule (ligand) Figure 11.7 Gate closed Ions