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.
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.
Bacteria and Fungal Species CommunicateBy Sending and Receiving Chemical
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).
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.
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.
Integrins are Matrix ReceptorsBind ligand with low affinityMore of them on the cell surfaceAllowing Cell Separation from the Matrix8 integrins bind fibronectin
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