TemperatureTemperature

Response to Temperature Stress

Outline

(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?

Temperature

Affects the rates of biochemical reactions, and of physical processes (diffusion, osmosis)

Protein conformation and enzyme function

Affects metabolic rate

Body Temperature

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 Endotherms

Marine Deep Sea fish 4-6 Whale 36Frog 22-28 Human 37Housefly 30-33 Rodent 35-37Tropical fish 20-28 Bat 35-39Desert Iguana 36-41 Chicken 40

Dove 39-42

Bee 35-42

Evolution of Body Temperature

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

EndothermyEndothermy

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

Thermoregulation: evolutionary causes?

EndothermyEndothermy

All animals produce heat, but endotherms All animals produce heat, but endotherms produce moreproduce more

(4-8 times)(4-8 times)

Metabolic rate is ~4-10 times higherMetabolic rate is ~4-10 times higherLargest component of energy budgetLargest component of energy budget

Where and How is Body Heat Where and How is Body Heat Produced?Produced?

How is Body Heat Produced?How is Body Heat Produced?

• ATP-producing reactionsATP-producing reactions• ATP-consuming reactionsATP-consuming reactions• Ion pumping (ATP hydrolysis) (~25%)Ion pumping (ATP hydrolysis) (~25%)• Mitochondrial proton leakMitochondrial proton leak• Urea production (~2%)Urea production (~2%)• Glycolysis (~5%)Glycolysis (~5%)• EtcEtc

All Metabolic Activity produces HeatAll Metabolic Activity produces Heat

Mostly in the mitochondria Mostly in the mitochondria 3/4 in abdominal organs (brain, gut, liver, kidney, 3/4 in abdominal organs (brain, gut, liver, kidney, heart, lungs)heart, lungs)some in musclessome in muscles

Cytochrome Cytochrome oxidase activityoxidase activity

50%

100%

Liver

Kidney

Brain

Heart

Mitochondrial Mitochondrial Surface AreaSurface Area

Lizard

Mouse

50%

100%

Liver

Kidney

Brain

Heart

So heat production is directly linked to metabolism…

And also oxygen consumption and food intake

Mechanism of heat production Mechanism of heat production through mitochondrial proton leakthrough 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

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

Box 6.1, p. 220

Nonshivering thermogenesis

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 cold-adapted mammals, hibernators, newborns

Response to High Response to High Temperature StressTemperature Stress

• 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

Hsp70Hsp70 Heat Shock Proteins

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 copiesExcision strain: 10 copies

Number of copies affects the degree of hsp expression(the amount of hsp transcribed)

Evolutionary tradeoffs of high Hsp expression?

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)

Canalization (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

A potential “plasticity gene”in response to environmental stress

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

Hsp90 inhibited

Unlike case of Drosophila, diverse phenotypes here were not “monstrous” but potentially adaptive

Dependence on Hsp90 for developmental stability varied

Potential Mechanism:

Under stress, Hsp90 is recruited to maintain protein folding and the function of proteins

Ability to maintain developmental pathway is exceeded

Adaptation to Cold

Differences between Aquatic Differences between Aquatic Vertebrates vs Invertebrates???Vertebrates vs Invertebrates???

Because of their Freshwater originBecause of their Freshwater originand osmotic properties, cold and osmotic properties, cold temperatures pose problems for fishestemperatures pose problems for fishes

Cold temperatures pose problems for Cold temperatures pose problems for fish in water because they are fish in water because they are hyposmotichyposmotic

Freezing point is higher in their Freezing point is higher in their extracellular fluids relative to ambient extracellular fluids relative to ambient seawaterseawater

Osmotic/Temperature InteractionsOsmotic/Temperature Interactions

Freshwater Freezes at 0°C

Seawater Freezes at -1.89°C

But hyposmotic fish might freeze at -0.7°C

Solutions

Cold water fish have more NaCl in extracellular fluids

Uses osmolytes such as glycerol (works better than NaCl)

Antifreeze proteins

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 glycoproteinSome 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