Chemical Sensing and Cell Signaling in Protists

4 November, 1998

Gordon Wolfe

I. What is a signal?

Kinds of signals
Terminology

II. Chemical signal mechanisms

Excreted vs. contact
Signal molecules and cell receptors
Physical constraints

III. Microbial responses

Developmental changes
Directed movements
Endocytosis

IV. Intraspecific examples: Life history

mating cues
aggregation and multicellularity

V. Interspecific examples

(Symbioses)
Phagocytosis/defense

information

 

environmental stimulus which can be perceived by sensory system and evoke an adaptive response

 

communication

 

stimulus is biological

 

"an action on the part of one organism (or cell) that alters the probability pattern of behavior in another organism (or cell) in an adaptive fashion", E.O. Wilson, 1970

 

signal

 

directed communication (sender & receiver)

Subject headings:

Chemical sensing and response

Chemical communication

Cell-cell interactions

Cell signaling

Signal vs. trophic communication

Definition by result in behavior

• attractant
• repellent

• stimulent
• deterrent
• arrestant

II. Chemical signal mechanisms

Excreted vs. contact mechanisms

Signal Molecules and Cell receptors

The mode of information transfer determines the physical and chemical characteristics of the signal molecules

Excreted signals in aqueous environments

  tend to be polar, hydrophilic molecules which are water-soluble

short-chain fatty acids, sugars, amino acids and derivatives, polar lipids, hydrophilic polypeptides and proteins

nonpolar compounds with low solubilities may occur if receptors are sensitive

• These compounds are often highly concentrated inside cells but concentrations in water are very low due to rapid consumption

turnover time («)

signal must 'decay' rapidly in order to produce gradients
biological removal, photooxidation, adsorption

Size, scaling, and physical constraints

molecular diffusion is major form of transmission of chemical information

characteristic diffusion length r = (4Dt)1/2

where D is the molecular diffusivity 10-9 m2/s

Reynolds number: ratio of inertial to viscous forces

R v L / x

v = velocity of fluid
L = characteristic length
x = kinematic viscosity 10-6 m/s

Most protists are 10 - 1000 µm in size L = 10-4 m
Most protists swim <<1000 µm/s v = 10-4 m/s

R = 0.01 - 0.1: viscous forces dominate

Limitation on chemical searching due to movement because swimming requires constant energy (Dusenbury & Snell, 1995)

Contact signals

• Once cells come into close proximity, direct contact is the most important form of chemical transmission

• Contact-mediated signaling mechanisms are usually extensions of membrane lipid or protein consituents:

glycolipids
glycoproteins

Lectins: proteins which bind to specific saccharide groups

multivalent
function universally for cell-cell recognition

• Agglutination / colony formation
  
• Mating interactions
  
• Intracellular defense systems
  
• Phagocytosis?

Why are sugars used as cell recognition systems?

• Many ways to link sugar monomers: create very large number of patterns from a small set of subunits

III. Microbial responses to stimuli

A) Movement

Chemotaxis: directed movements due to chemical gradient in space or time

orthokinesis: change in linear velocity
klinokinesis: change in rate of turning

Types of movements

swimming: undulipodia (flagella) or cilia

100 - 1000 µm s-1
protists are less affected by random thermal movement (Brownian movement) than bacteria and can orient and position
often swim in gentle helix

gliding on surfaces: benthic diatoms, desmids

crawling on surfaces: ameboid movement due to protoplasmic streaming

polarized cell growth: fungal hyphae

B) Intraspecific responses: developmental changes


Generalized life-cycle:

IV. Intraspecific Examples: Mating pheromones

Fertilization in algae and fungi is a contact-mediated recognition process, often mediated by lectin-like glycoconjugates

Chlamydomonas, Alexandrium

 

fertilizaton / mating well-studied in ciliates

gamone: chemical signal which induces conjugation

models: Paramecium, Tetrahymena, Blepharisma, Euplotes

gamones are polypeptides; complex signals

Euplotes octocarinatus:

ten mating types
4 codominant alleles mt1 - mt4; control excretion of gamones 1-4

Examples: aggregation and colony formation

The path to multicellularity:

single cells --> contact --> aggregation --> colony formation

Examples:

Phaeocystis: colonial haptophyte alga

'Dutchman's 'baccy juice"

major ecological problem in northern Europe

(induction mechanisms not yet known)

cellular slime mold Dictyostelium

solitary bacterivorous amoeboid stage

starvation induces colony formation which differentiates and moves via pulses of cyclic AMP

V. Interspecific examples:

Endocytosis

Phagocytosis

Examples:

Prey recognition by the ciliate Litonotus (Ricci 1996)

Phagocytosis by Dictyostelium: several receptors, identified by mutagenesis experiments:

1) lectin-like receptors which are specific for glucose moieties on bacterial cell walls

2) non-specific receptors (glycoproteins, 'contact site B') are activated following starvation and mediate cell aggregation

Both possibly active in phagocytosis and aggregation / colony formation

Defense

'Inducible' defenses: activation following signal from predator

Ciliate Euplotes octocarinatus responds to chemical cues from predatory ciliates (Stenostomum, Lembadion) by increasing length

The DMS-acrylate defense hypothesis

1. Protists respond to chemical informational cues

information --> communication --> signals

2. Signals classified by source (semiochemicals) or action

3. Two main chemical signal mechanisms: excreted vs. contact

4. Molecules depend on mechanism and physical environment

5. Microbial responses:

taxis - directed movement

intraspecific responses: mating, aggregation, encystment

interspecific responses: symbioses, endocytosis