Response to Temperature Stress

(1) Endothermy in Mammals
(2) Response to Heat Stress
(3) Response to Cold

Geographic Distribution of species is determined, in part, by temperature

How do organisms deal with Temperature Stress?


Affects the rates of biochemical reactions, and of physical processes (diffusion, osmosis)

Protein conformation and enzyme function

Affects metabolic rate


Poikilotherm : body temperature is variable

Homeotherm : body temperature is constant

Ectotherm : regulate body temperature externally (behavior); most are poikilotherms

Endotherm : elevated body temperature using metabolic heat (mammals, birds, tuna, some insects); many are homeotherms

Variation in Body Temperature ° C
Ectotherms
Marine Deep Sea fish 4-6
Frog
Housefly
Tropical fish
Desert Iguana
22-28
30-33
20-28
36-41
Endotherms
Whale
Human
Rodent
Bat
Chicken
Dove
Bee
36
37
35-37
35-39
40
39-42
35-42


Why are endotherms 35-40 ° C?

Many ectotherms aim for these temperatures as well




Optimal for enzyme activity
Faster neuronal, hormonal function
Slight elevation: Easier to gain than lose heat
Physical properties of water are at an ideal balance of viscosity, specific heat, and ionization

Buffers biochemical reactions against temperature stress
Allows organisms to invade a broader range of habitats

Evolved independently multiple times WHY?
Two Hypotheses:
Thermoregulatory Advantage
•
•
Maintain constant body temperature
Easier to maintain a high body T than lower
Aerobic Capacity Advantage
•
•
Selection for enhanced physical performance
(endurance, locomotion)
With increased heat production as a secondary effect

All animals produce heat, but endotherms produce more
(4-8 times)
Metabolic rate is ~4-10 times higher
Largest component of energy budget

All Metabolic Activity produces Heat
• ATP-producing reactions
• ATP-consuming reactions
• Ion pumping (ATP hydrolysis) (~25%)
• Mitochondrial proton leak
• Urea production (~2%)
• Glycolysis (~5%)
• Etc
Mostly in the mitochondria
3/4 in abdominal organs (brain, gut, liver, kidney, heart, lungs) some in muscles

100%
Mitochondrial
Surface Area
Lizard
100%
Cytochrome oxidase activity
50%
Mouse
50%


So heat production is directly linked to metabolism…

And also oxygen consumption and food intake

Mechanism of heat production through mitochondrial proton leak

Typically, the Electron transport chain and oxidative phosphorylation (ATP production) is coupled.
When they are not, the energy released by electron transport is released as heat, rather than used to make ATP

ATP Synthase

In specialized cells of
Endotherms, protons leak across the membrane through uncoupling protein 1 (UCP1 = thermogenin)
Such proton diffusion generates heat

Box 6.1, p. 220
This uncoupled reaction occurs to a high degree in brown adipose tissue, which has large numbers of large mitochondria
Cold --> release Norepinephrine
Hydrolyzes triacylglycerols in BAT (Brown Adipose
Tissue) cells to release fuels for mitochondrial oxidation
Lipid oxidation proceeds with UCP1 activated

Increase rate of oxidation of stored lipids
Uncoupling of oxidative phosphorylation from electron transport in the mitochondria
Allows energy to be released as heat rather than stored as
ATP
More prominent in coldadapted mammals, hibernators, newborns

• Last time we discussed the structure and function of enzymes, which are proteins
• Protein folding depends on thermodynamics, and can be disrupted by high temperatures
• How is protein structure and function maintained under conditions of temperature (or other) stresses?

Ensure correct protein folding
Not only used for temperature stress, but also other stresses
(osmotic shock, etc)
Figure: silver staining of
Hsps in the cell

Hsp70

• The fruit fly, Drosophila melanogaster lay their eggs on rotting fruit
• The larvae can experience very high temperatures while growing on the fruit
• They use the enzyme alcohol dehydrogenase (ADH) to break down alcohol that accumulates in the rotting fruit
• They need to protect their proteins and enzymes such as
ADH against denaturing under heat stress

Inserting extra copies of Hsp 70 enhanced tolerance of high temperature in Drosophila melanogaster
Extra copy strain: 12 copies
Excision strain: 10 copies
Number of copies affects the degree of hsp expression
(the amount of hsp transcribed)

Cost to growth: Constant (constitutive) expression of hsp inhibits cell proliferation (would inhibit growth)
Cost to Reproduction: decreases rates of age-specific mortality during normal aging, while maternally experienced heat shock depresses the production of mature progeny (Silbermann and Tatar 2000)

Temperatures at which HSPs are induced have evolved to correspond to temperatures that are stressful for a given species or cell type.

Antarctic organisms begin to express HSPs at relatively low temperatures (< 10 ° C)
(Vayda and Yuan 1994)

Some hyperthermophiles do not express HSPs until temperatures exceed 60 ° C
(Trent, Osipiuk et al. 1990; Ohta,
Honda et al. 1993; Polla, Kantengwa et al. 1993; Trent, Gabrielsen et al.
1994)

Hypothermic regions of mammals (e.g., testis) express
HSPs at lower temperatures than normothermic organs
(Sarge 1995; Sarge, Bray et al. 1995)

(flip side of plasticity)

Influenced by developmental stability

Stress could disrupt canalization and lead to new phenotypes

Particular genes might be important for maintaining developmental stability and buffer against perturbations

Queitsch et al. 2002. Nature . 417:618-624


Heat-shock protein 90 (Hsp 90) chaperones the maturation of many regulatory proteins

In Drosophila melanogaster, buffers genetic variation in morphogenetic pathways

Reducing Hsp90 function in Drosophila or
Arabidopsis produces an array of morphological phenotypes, revealing hidden genetic variation

Development abnormalities in
HSP90 deficient
Drosophila
(Rutherford and
Lindquist. 1998.
Nature)
Study criticized because fitness consequences were not examined

Normal
Hsp90 inhibited

Normal
Unlike case of Drosophila , diverse phenotypes here were not “monstrous” but potentially adaptive
Dependence on Hsp90 for developmental stability varied
Hsp90 inhibited


Under stress, Hsp90 is recruited to maintain protein folding and the function of proteins

Ability to maintain developmental pathway is exceeded


Cold temperatures pose problems for fish in water because they are hyposmotic

Freezing point is higher in their extracellular fluids relative to ambient seawater


Freshwater Freezes at 0 ° C

Seawater Freezes at -1.89
° C

But hyposmotic fish might freeze at -0.7
° C


Cold water fish have more NaCl in extracellular fluids

Uses osmolytes such as glycerol (works better than NaCl)

Antifreeze proteins


200x more effective than NaCl

Not freeze until -6 ° C

Hot commodity these days
(cryopreservation, food preservation, health)

Antarctic Fish Pagothenia borchgrevinki

Molecular structure of an antifreeze glycoprotein
Some are multigene families that have experienced multiple gene duplications

Diagram of the adsorption-inhibition mechanism of
AFGPs (modified from Eastman, 1993)

J Mol Evol (2002) 54:403 –410

When mapped onto the three-dimensional structure of the fish antifreeze type III antifreeze structure, these codons correspond to amino acid positions that surround but do not interrupt the putative ice-binding surface.
The selective agent may be related to efficient binding to diverse ice surfaces or some other aspect of AFP function.

Most of the Amino Acid
Substitutions are at the Ice binding Surfaces of the
AntiFreeze Proteins