Transcript Lecture PPT - Carol Lee Lab

Temperature
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
Frog
22-28
Housefly
30-33
Tropical fish
20-28
Desert Iguana
36-41
Whale
Human
Rodent
Bat
Chicken
Dove
Bee
36
37
35-37
35-39
40
39-42
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
Endothermy
Buffers biochemical reactions against
temperature stress
Allows organisms to invade a broader range
of habitats
Thermoregulation:
evolutionary causes?
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
Endothermy
All animals produce heat, but endotherms
produce more
(4-8 times)
Metabolic rate is ~4-10 times higher
Largest component of energy budget
Where and How is Body Heat
Produced?
How is Body Heat Produced?
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
Cytochrome
oxidase activity
Mitochondrial
Surface Area
Lizard
100%
100%
Mouse
50%
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
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 coldadapted mammals,
hibernators, newborns
Response to High
Temperature 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?
Hsp70
Heat Shock
Proteins
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)
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
Unlike case of Drosophila,
diverse phenotypes here
were not “monstrous” but
potentially adaptive
Dependence on Hsp90 for
developmental stability
varied
Hsp90 inhibited
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
Vertebrates vs Invertebrates???
Because of their Freshwater origin
and osmotic properties, cold
temperatures pose problems for fishes


Cold temperatures pose problems for
fish in water because they are
hyposmotic
Freezing point is higher in their
extracellular fluids relative to ambient
seawater
Osmotic/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 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