Cell-Cell Communication

In

Multicellular Organisms

Multicellular Organisms RequireCell-Cell Interactions

Volvox, an algae

Evolution of multicellularity

Cell-Cell Adhesion Defines Multicellularity

The first traces of life appear nearly 3.5 billion years ago, in the early Archaean. Clearly identifiable fossils remain rare until the late Archaean, when stromatolites, layered mounds produced by the growth of microbial mats, become common in the rock record.

Caulobacter crescentus

Saccharomyces cerevisiae (G . Fink)

More Complex Multicellular

Organisms haveSpecialized Cell

Types

Cells Can Interact

With Each Other

Indirectly Through

Protein-basedScaffolds

Microbial Mats = Biofilms

Attachment of a bacterium to a surface, or substratum, is the initial step in the formation of a biofilm

Cell-surface adhesins that mediate contact with the host matrix

The maturation of a biofilm community and their architecture can vary from flat, homogenous biofilms,

to highly structured biofilms,

connection between quorum sensing and biofilm formation

Bacterial and fungal pathogens form biofilms

Microbial Mats = Biofilms

Hypothetical signal gradients in a biofilm system. This schematic represents a side-view of (a) a flat and (b) a structured biofilm (s = substratum). This diagram represents speculation regarding potential signal gradients (indicated by the gray scale), with higher signal concentrations indicated by darker coloration. Factors, such as diffusion constants for the signal, mass transfer and non-uniform signal production, within different regions of the biofilm could all affect signal gradients. The two micrographs at the right of the figure represent side-views of confocal micrographs of P. aeruginosa PAO1 forming a flat and a structured biofilm.

Quorum Sensing

Bacteria and Fungal Species CommunicateBy Sending and Receiving Chemical

Signals

the accumulation of signaling molecules enable a single cell to sense the number of bacteria (cell density).

The marine bioluminescent bacteria Vibrio fischeri was grown in liquid cultures and it was observed that the cultures produced light only when large numbers of bacteria were present (Greenberg, 1997).

Quorum Sensing

An Australian pinecone fish (12 cm long). The red organ on the lower jaw is a light organ that contains 1010 V. fischeri cells per ml fluid. Australian pinecone fish are nocturnal reef dwellers and they use the light organ to search for prey at night.

A Hawaiian bobtail squid. This adult squid is 2 cm long. There is a V. fischeri light organ close to the ink sac within the mantle cavity of the animal. This light organ contains 1011 V. fischeri cells per ml. These nocturnal squid emit light downwards through the mantle cavity and, by matching the intensity of the moon- and starlight above, they become invisible to predators below them.

Quorum SensingThe lux operon contains luxI followed by five genes that are required for light production (luxCDABE) and an additional gene of unknown function (luxG). The luxC, luxD and luxE genes code for components of an acid reductase that converts the long-chain fatty acid tetradecanoic acid into the fatty-aldehyde substrate (tetradecanal) for the light-producing enzyme luciferase. The luxA and luxB genes encode subunits of luciferase. The luxI gene encodes the enzyme (autoinducer (AI) acyl-homoserine lactone synthase) that produces the quorum-sensing signal 3-oxo-C6-HSL. The single gene transcribed in the opposite direction, luxR, encodes the signal-responsive transcription activator of the lux operon.

Quorum Sensing

Quorum Sensing

Model of acyl-homoserine-lactone (acyl-HSL) quorum sensing in a bacterial cell.   Tentative mechanisms for acyl-HSL synthesis and acyl-HSL interaction with LuxR-type proteins are shown. Double arrows with filled yellow circles at the cell envelope indicate the potential two-way diffusion of acyl-HSLs into and out of the cell. The proposed dimerization of LuxR (red) is based on genetic evidence and biochemical analysis of TraR; other LuxR-type proteins might form higher-order multimers.

All C.elegans cells Are Formed in a Defined

Program (959 somatic)

Cells Stick Together by Tight Junctions

Prevent Membrane Protein and Lipid

Diffusion

Desmosomes Are Button-Like Points of IntercellularContact that Rivet Cells Together

GAP Junctions

Examples of Cell Adhesion Molecules

30nM

Integrins are Matrix ReceptorsBind ligand with low affinityMore of them on the cell surfaceAllowing Cell Separation from the Matrix8 integrins bind fibronectin

Inside-out signaling

focal adhesion kinase (FAK) in mediates signals from theextracellular matrix through integrin receptors.FAK and its interacting partners play a central role in propagatingsignals that regulate cell motility.

Non-adherent cells undergo programmed cell death: Apoptosis

-double stranded DNA cleavage1-2 breaks can cause a cell cycle checkpoint

-asymmetry lost in PMPhosphatidyl serine becomes exposed to WBC

-caspase productionProteases that are themselves activated by cleavageProteolytic cascade

-pores in the mitochondriaInvolved in procaspase activation

Growth Factors and DNA damage can cause apoptosis

Bloom and Cross Nature Reviews Molecular Cell Biology 8, 149–160 (February 2007) | doi:10.1038/nrm2105