Earth System Analysis & 2nd Copernican Revolution, Nature Millenn. Suppl. 
The Copernican revolution in the middle of our outgoing
millennium put people's world -the Earth- into the proper context
of our solar system and the cosmos. Today, at the end of the
second millennium our "world" goes far beyond planet Earth: From
the Apollo missions to the moon thirty years ago we got an
impressive visual image of our planet as a limited space and
environment. The last few decades have brought an incredible
increase in Earth sensors and computing simulation powers so that
we can start to get a view of the complex and intricate workings
of the interwoven sub-systems of the terrestrial environment and
ecosystem. H.J. Schellnhuber, Director of the Potsdam Institute
for Climate Impact Research (PIK) compares that emerging view of
the Earth system with that of an organism, "Gaia's body" and
suggests that its consequences will be that of a second Copernican
revolution (see figure). 
He sees three different principles of how one can attempt to
get a holistic picture of the Earth system: the "bird's eye"
principle of remote sensors that are already providing a torrent
of geo-data. The second principle is that of "digital mimicry" or
simulation where different "what-if"-scenarios can be tested
without having to experiment with the real system, of which we
only have one sample. The third principle Schellnhuber calls
"Lilliputian" after the land of little people that Gulliver
encountered on his travels. Ecological laboratories like the
Biosphere-II project can build an Earth model in hardware and
teach the researchers important lessons about what real living
systems might do and that can a-priori not be captured in a
digital simulation (see the Jurassic Park syndrome).
The understanding of the Earth system will bring up the
challenge of a geo-cybernetic task: If we can see the consequences
of greenhouse gas emissions in a 'business-as-usual' policy, what
could be done to induce a transition to a more desirable future?
With his scientific background Schellnhuber cannot resist writing
down an 'Earth equation' where the 'Earth variable E' depends on
"natural, N" and 'human, H' subsystems. It is interesting that he
then continues to split the human 'H' into a physical
'antroposphere A' and a 'spiritual (?)' subsystem of an emerging
'global subject, S'. The global subject S manifests itself in
global actions such as international protocols for climate
protection that increase the 'evolutionary fitness' of the Earth
system and prevents it from terminating in a Martian or Venusian
regime.
By defining the global subject as subsystem of the human
subsystem one can expect that a sustainable solution where, say,
ants become the dominant species on this planet will be avoided.
It seems, on the other hand a little risky to entrust the global
subject to a species that was around for barely a few million
years and after that short time had only a single human species
left standing.
The author ends with presenting three geo-strategic approaches
to a sustainable future: Optimization by a more organic
distribution of global tasks of food and energy production. A
stabilization strategy to fix the ozone layer and atmospheric
climate problems by iron fertilization and injection of "designer"
greenhouse gases for preventing the next ice age. Of course it is
clear that the complexity of the problem is tremendous because of
the interconnectedness of the subsystems. For instance it is
feasible that the storms and mild and rainy winters that Europe
has been experiencing will contribute to the termination of the
North Atlantic Conveyor-Belt and thereby in fact facilitate a new
European ice age.
The third geo-strategy is that of pessimization and the
creation of minimal safety standards for operating the Earth
system by geo-engineering only as much as is necessary for
creating "'guardrails' for responsible planetary management".
'Earth
system' analysis and the second Copernican
revolution , H. J.
Schellnhuber, Millennium Supplement, Nature 402: 6761 C19
(1999)
-
Traditional, reductionist 20th century biology tries to
explain biological phenomena (mysteries) by finding molecules with
special (miracle) functions. According to Hartwell et al. this
will change dramatically in the biology of the 3rd millennium:
Most biological functions arise from the interactions among a
large number of different(!) components. The fact that the
components (subsystems) are themselves quite complex and not all
identical makes biology so much harder to understand than, e.g.
particle physics where all matter is built up of a few different
sub-atomic particle types (proton, electron, etc.). But we know
that already simple, identical rules such as in cellular automata
can produce structures of amazing complexity. How much more
difficult to analyze and understand must be a system with
non-uniform and complex subsystems. One other feature
discriminates today's biological systems from simple physical
systems: they have evolved for a couple of billion years and only
those biological systems are around today that participated in
evolutionary progress and selection. For the analysis evolutionary
fitness can be formulated as a purpose of the biological system:
Biological systems "want" to reproduce.
Hartwell et al. propose to study biological systems not only on
the genetic molecular level but also in the framework of "modules"
that enhance evolutionary fitness. The authors give a number of
example where an understanding of the function of modules leads to
prediction for the properties of molecules such as DNA.
In the analysis of modular functions complex computer
simulations ("in silico" reconstructions) have become an important
heuristic tool for biological research. Thus the biology of the
next millennium will benefit greatly from contributions from
"synthetic sciences" like computer science and engineering but
also from insights gained in the abstract research on complex
systems: Bifurcations and non-equilibrium phase transitions appear
to be ubiquitous in biological systems. Another property that can
follows naturally from the framework of modules is complexity as a
consequence of a sequence of random historical events: Once a
module is in place mutations are unlikely to change it, rather
lead to modifications in the arrangements that might not be
optimal in a global sense. An example of a recent historical
accident in human design is the QWERTY keyboard that the authors
view as a "living fossil".
They conclude with a discussion of some of the key questions
for modular biology: " A major challenge for science in the
twenty-first century is to develop an integrated understanding of
how cells and organisms survive and reproduce. Cell biology is in
transition from a science that was preoccupied with assigning
functions to individual proteins or genes, to one that is now
trying to cope with the complex sets of molecules that interact to
form functional modules. There are several questions that we want
to answer. What are the parts of modules, how does their
interaction produce a given function, and which of their
properties are robust and which are evolvable? How are modules
constructed during evolution and how can their functions change
under selective pressure? How do connections between modules
change during evolution to alter the behavior of cells and
organisms?"
From
Molecular To Modular Cell
Biology, Leland H.
Hartwell, John J. Hopfield, Stanislas Leibler &
Andrew W. Murray, Millennium Supplement, Nature 402: 6761
C47 (1999)
-
One of the big scientific challenges for the next
millennium will be what E.O. Wilson once called "consilience" or
'unity of knowledge' where scientific arguments between different
sciences and even political and religious organizations will be
resolved on a higher level. The question about the role of "nature
vs. nurture" influence of human development has played a central
role in the discussion. Specifically the issue of general
intelligence, how it is defined and to what degrees it has genetic
origins has sparked public discussion especially after the
publication of 'The Bell Curve'.
Plomin presents arguments for the genetic contribution to
behavioral disorders from autism to schizophrenia to the very
common reading disability. Strong evidence for genetic
contributions has been established through a large number of
statistical correlations discovered under diverse genetic
(siblings and twins) and environmental (up-bringing) conditions.
But instead of looking for individual genes that are responsible
for these disorders it seems more realistic (and complex) to try
to assume that there might be a whole number of genes whose
interactions can produce a given disorder. This view is known as
the quantitative trait locus (QTL) and has profoundly different
implications than the case where a single gene is the sole culprit
for a disorder. For instance already today in the US newborn
babies are routinely checked for phenylketonuria (PKU) a disorder
based on a defect in a single gene that can lead to mental
retardation. Early diagnosis and nutritional adjustment can
greatly ameliorate the effects of the disease.
Functional genomics will, according to Plomin, will be able to
define and predict traits as complex as general cognitive ability
"g", a variant of the traditional intelligence quotient (IQ). To
the science fiction scenario of classifying babies at birth into
alpha, beta and gamma humans he responds with a prediction of
early intervention for those children at risk of mental
retardation similar to the PKU example. He also expects a
beneficial impact of functional genomics when it helps parents to
recognize the genetic limitations (and talents) of their children
instead of developing unrealistic expectations based on family
history, social status, etc.
Genetics
And General Cognitive
Ability, Robert Plomin,
Millennium Supplement, Nature 402: 6761 C25 (1999)
The Future Of Evolutionary Developmental Biology, Nature Millennium Suppl.
"Combining fields as diverse as comparative embryology,
palaeontology, molecular phylogenetics and genome analysis, the
new discipline of evolutionary developmental biology aims at
explaining how developmental processes and mechanisms become
modified during evolution, and how these modifications produce
changes in animal morphology and body plans. In the next century
this should give us far greater mechanistic insight into how
evolution has produced the vast diversity of living organisms,
past and present."
The
Future Of Evolutionary Developmental
Biology, Peter W. H.
Holland, Millennium Supplement, Nature 402: 6761 C41
(1999)
-
Futurologists from half a century ago produced a major
howler: They predicted flying cars and rolling sidewalks
everywhere but completely missed how computers would change our
lives. Today we anticipate a significant increase in the role of
computers over the next decades but maybe their influence will be
replaced by something completely different. Perhaps that is the
reason why Butler only projects out to the year 2010. Like most
science fiction movies he predicts a strange blend of today's
habits with beefed-up technology: Are we really going to use ten
gigabyte lines in order to make conference calls where we can
"shake hands" with avatars of our collaborators?
It is more likely that increased compute power will be used to
make simulations more realistic. Everyone who has worked in the
field knows that one can always saturate the fastest machines by
just cranking up the resolution of a simulation (that is why
typical simulation jobs on supercomputers always took about one
hour and always will).
As the title indicates astrophysical and biological simulations
will be at the core of number crunching activities. The next
generation IBM supercomputer is even designed specifically to
simulate protein folding (see ComDig 1999.beta6.10). Other
complexity challenged computational goals are simulations of cells
and the eco system of planet Earth. Most of the computational
speed-up is gained by linking thousands and tens of thousands of
PC processors together either in a dedicated machine or even
across the Internet like in a Beowulf cluster of Linux machines.
The introduction of 64-bit machines later in 2000 will greatly
enhance the capabilities of such clusters.
One application of this concept has led to the largest
distributed computing project in history on the planet's largest
virtual supercomputer: using a downloadable screensaver SETI@home
more than a million PC users on the Internet donate unused CPU
cycles to search for extra-terrestrial life by scanning through 35
gigabytes of signals received daily at the Arecibo Radio Telescope
in Puerto Rico (see the movie "Contact" for details).
Problems like SETI are easy to process in parallel because the
data can be split up in chunks that can be processed without
having to communicate much with other processors. For most complex
simulations what is going on at one part of the system influences
most other parts so that fast communication between the processors
becomes critical. A new software toolkit available at
www.cactuscode.org significantly facilitates the process of
"parallelization" of computer jobs.
But even the fastest Internet connections would be too slow for
efficient parallel simulations. Therefore some researchers are
working on the next step taking advantage of a computational trick
that has evolved in nature: Brains don't split up into CPU and RAM
but the computation is done directly in the memory elements of
neurons. Therefore putting the central processing unit inside the
memory promises much more exciting developments than fancy avatars
with whom you can shake hands.
Computing
2010: From Black Holes To
Biology, Declan
Butler, Millennium Supplement,
Nature 402: 6761 C67 (1999)
-
Our brain, as one of our most complex organs, performs a
number of functions whose neurobiological mechanisms remain as a
challenge for science of the next millennium. Among the most
important functions are cognition (perception, learning, memory,
attention, decision-making, language and motor planning), and
emotion, both of them cannot be implemented in today's computers.
Nichols and Newsome expect significant progress in the next few
decades based on improved technology to monitor and measure
neuronal activity, they don't expect many changes in conceptual
understanding. They describe three levels of understanding:
localization, representation and micro circuitry. With the help of
PET scans and functional magnetic resonance imaging (fMRI) (both
measuring changes in blood flow to active brain areas) as well as
techniques measuring the electro magnetic fields of neuronal
activity directly much progress has been made in associating
different brain regions with different cognitive events. One
important insight, however, was that we cannot expect a one-to-one
map between events and brain regions: many events are mapped to
different regions simultaneously and the same region can respond
to different types of events. Because of the complexity of a
single neuron even dramatic improvements in the understanding of
neuronal micro circuitry will still leave a major challenge in
synthesizing the information to understand the coupling across
hierarchies of organization that lead to observed behavior. It is
not clear if the question about consciousness can be answered in a
scientific manner since there is no agreed upon definition of
consciousness that does not depend on subjective reports.
Conceptually easier are questions about how decisions are made.
In monkeys one was able to identify neurons whose activity
preceded the behavioral manifestation of a decision by several
seconds. Will it be possible with the help of smart brain
monitoring devices to see what decision a person will make before
that person is actually consciously making that decision? How
about if I can see what decision I am going to make in a few
second, can I still change my mind? We can anticipate that results
in that direction will revive philosophical discussions about
determinism and free will.
The
Neurobiology Of
Cognition , M.
James Nichols & William T. Newsome, Millennium
Supplement, Nature 402: 6761 C35 (1999)
New Theories Help Explain Mysteries of Autism, New York Times
Autism, the disorder illustrated by Dustin Hoffman in the
"Rainman" movie has a number of peculiar symptoms. Common to them
are inabilities to relate to others in social interactions and
peculiar focus on details that sometimes leads to the phenomenon
of "idiot savants", patients with extraordinary metal capabilities
e.g. in mental arithmetic and photo graphic memory. In the early
days of psychology traumatic childhood experiences and
deprivations have been postulated as the cause for the disorder.
More recent results point more towards genetic origins (involving
five or six genes) that lead to changes in the organization of
different brain regions. For instance it seems that specific
neurons respond selectively to actions of other individuals
independent to one's own action (see " Cortical Mechanisms of
Human Imitation", ComDig 1999.9.11). Other indications for genetic
origins are the first onset of clear symptoms happening in
specific developmental stages, often around an age of 14 to 22
months.
It seems that autistic brains are generally bigger and heavier
with abnormalities in three different regions, all of which are
relevant to control social behavior: Frontal lobes involved in
decision making and planning are thicker, cells in the limbic
system -where emotions are processed- are a third smaller and more
numerous but more immature. Finally, cells in the cerebellum
-involved in predicting the next movements, thoughts, and emotions
are reduced by up to 50%.
Some of the cells (in the amygdala) respond to faces, whereas
autistic children tend to ignore facial expressions. Neuronal
activity corresponding to arousal is several times higher than on
average. That might explain why details that most would ignore
don't bore them. Today the prognosis for therapy is good if the
disorder is diagnosed (a strong symptom is the inability of a two
year old to speak short sentences at age 2) by age 2 to 3
years.
New
Theories Help Explain Mysteries of
Autism, New York
Times
Transitions and Resonances in the Behavior of Heart Cells, Chaos
Heart rhythms have long been a prominent example of how
non-linear dynamics can describe biological oscillations that were
inaccessible to traditional, linear approaches. Yehia et al. show
in detail how already in single cells transitions to oscillations
can be induced that might lay the foundation to the understanding
of a number of heart diseases.
Abstract: The transmembrane potential of a single
quiescent cell isolated from rabbit ventricular muscle was
recorded using a suction electrode in whole-cell recording mode.
The cell was then driven with a periodic train of current pulses
injected into the cell through the same recording electrode. When
the interpulse interval or basic cycle length (BCL) was
sufficiently long, 1:1 rhythm resulted, with each stimulus pulse
producing an action potential. Gradual decrease in BCL invariably
resulted in loss of 1:1 synchronization at some point. When the
pulse amplitude was set to a fixed low level and BCL gradually
decreased, N + 1:N rhythms (N2) reminiscent of clinically observed
Wenckebach rhythms were seen. Further decrease in BCL then yielded
a 2:1 rhythm. In contrast, when the pulse amplitude was set to a
fixed high level, a period-doubled 2:2 rhythm resembling alternans
rhythm was seen before a 2:1 rhythm occurred. With the pulse
amplitude set to an intermediate level (i.e., to a level between
those at which Wenckebach and alternans rhythms were seen), there
was a direct transition from 1:1 to 2:1 rhythm as the BCL was
decreased: Wenckebach and alternans rhythms were not seen. When at
that point the BCL was increased, the transition back to 1:1
rhythm occurred at a longer BCL than that at which the {1:12:1}
transition had initially occurred, demonstrating hysteresis. With
the BCL set to a value within the hysteresis range, injection of a
single well-timed extrastimulus converted 1:1 rhythm into 2:1
rhythm or vice versa, providing incontrovertible evidence of
bistability (the coexistence of two different periodic rhythms at
a fixed set of stimulation parameters). Hysteresis between 1:1 and
2:1 rhythms was also seen when the stimulus amplitude, rather than
the BCL, was changed. Simulations using numerical integration of
an ionic model of a single ventricular cell formulated as a
nonlinear system of differential equations provided results that
were very similar to those found in the experiments. The
steady-state action potential duration restitution curve, which is
a plot of the duration of the action potential during 1:1 rhythm
as a function of the recovery time or diastolic interval
immediately preceding that action potential, was determined.
Iteration of a finite-difference equation derived using the
restitution curve predicted the direct {1:12:1} transition, as
well as bistability, in both the experimental and modeling work.
However, prediction of the action potential duration during 2:1
rhythm was not as accurate in the experiments as in the model.
Finally, we point out a few implications of our findings for
cardiac arrhythmias (e.g., Mobitz type II block, ischemic
alternans). ©1999 American Institute of Physics.
Hysteresis
and bistability in the direct transition from 1:1 to 2:1
rhythm in periodically driven single ventricular
cells, Ali R. Yehia, Dominique
Jeandupeux, Francisco Alonso, and Michael R. Guevara ,
Chaos, Volume 9, Issue 4, pp. 916-931
Why Do Microorganisms Survive Deep Underground?, Science Daily, INEEL
"Even Dante would blanch at the conditions
kilometers below the earth's surface. Temperatures climb past 100
degrees Celsius, pressures hundreds of times greater than
atmospheric pressure bear down, and space is so tight even
microorganisms can barely budge. Yet, even there life persists.
Now subsurface scientists have begun to identify the factors that
determine why microorganisms survive deep underground in some
places, but not others, report researchers from the Department of
Energy's Idaho National Engineering and Environmental Laboratory
and Princeton University. The INEEL specializes in subsurface
science as part of its environmental mission. High temperatures
ensure nothing can live too far below the earth's surface. But
pressure, the availability of water, the porosity of the
surrounding rock and the flow of chemical nutrients also limit
where extremophiles--microorganisms that relish harsh
conditions--can exist. (…)"We're at the point of recognizing
that microorganisms have remarkable abilities to colonize these
environments and trying to understand the parameters that control
that colonization," said INEEL microbiologist Rick Colwell, who
presented a synthesis of recent findings in the Biogeoscience:
Deep Biospheres: Where and How? poster session today at the
American Geophysical Society meeting in San Francisco. A better
understanding of how extremophiles survive deep underground may
shed light on how life endured the earth's violent youth, or show
scientists where to look for life on other planets, said Princeton
geochemist T.C. Onstott. Temperature appears to be the primary
factor in limiting how deep extremophiles can go. No known
microorganism can live for long at 120 degrees Celsius. Since the
surface temperature averages 15 degrees Celsius and the
temperature in the ground increases with depth about 19 degrees
Celsius every kilometer, extremophiles should die off between five
and six kilometers below the surface of dry land. (…)Pressure
limits the range of extremophiles less than temperature does. Most
microorganisms can survive pressure 600 times atmospheric
pressure, which corresponds to a depth of six kilometers. At that
depth in most locations the temperature likely exceeds 120 degrees
Celsius. Lack of water and chemical nutrients likely prohibits
deep subsurface life in arid, geologically stable regions. For
instance, little grows between the surface and the groundwater of
the Snake River Plain, on which INEEL sits. Conversely,
extremophiles may be more abundant deep in active geological
features, such as faults, mid-ocean ridges and salt deposits,
where fluids and nutrients flow more freely. Subsurface
environments are so austere some extremophiles live in a state of
nearly suspended animation. Microorganisms living on the surface
divide after hours or days. Those living deep underground may
divide only after hundreds of thousands of years. Life may have
persevered below the surface 4 billion years ago, when asteroids
routinely pelted the earth and caused the oceans to boil, Onstott
said, so microorganisms living deep underground may provide clues
about the emergence of life on the developing planet. "If you want
to understand primitive microbial ecosystems, the only place you
can go is into the subsurface," he said. If life exists elsewhere
in the solar system, it may be tucked beneath the surface of other
planets or their moons. By studying subsurface extremophiles on
earth, researchers may learn where to look in their search for
extraterrestrial life. (…)Onstott agrees. "In terms of
microbiology," he said, "I think the field is headed toward
identifying energy sources for these microorganisms, correlating
these sources to microbial activity and determining whether that
activity has changed the subsurface environment." "
Life
In The Inferno: Researchers Identify Factors That
Determine Where Microorganisms Can Survive In The Hellish
World Deep Underground ,
Science Daily 12/22/99,The original news release can be
found at: Idaho
National E & E Laboratory
Adaptive Agents and Collective Intelligence on the Internet?, arXiv
A major ingredient of the Internet are "routers" that
tell pieces of information where to go next in the network on the
way to their destination. The performance will degrade if some
routers attract too much traffic. Therefore routers can be viewed
as interacting agents in the complex system of the overall
network. Wolpert et al. observe that "Adaptivity, both of the
individual agents and of the interaction structure among the
agents, seems indispensable for scaling up multi­agent
systems (MAS's) in noisy environments. One important consideration
in designing adaptive agents is choosing their action spaces to be
as amenable as possible to machine learning techniques, especially
to reinforcement learning (RL) techniques."
They found " … the perhaps surprising fact that simply
changing the action space of the agents to be better suited to RL
can result in very large improvements in their potential
performance: at their best settings, our learning­amenable
router agents achieve throughputs up to three and one half times
better than that of the standard Bellman­Ford routing
algorithm, even when the Bellman­Ford protocol traffic is
maintained. We then demonstrate that much of that potential
improvement can be realized by having the agents learn their
settings when the agent interaction structure is itself adaptive.
"
In a related paper Kumer and Wolpert show that sub-optimal
routing can be caused by ""side-effects", in this case of current
routing decision on future traffic." They develop a theory that
addresses this problem: "The theory of COllective INtelligence
(COIN) is concerned precisely with the issue of avoiding such
deleterious side-effects. We present key concepts from that theory
and use them to derive an idealized algorithm whose performance is
better than that of the Ideal Shortest Path Algorithm (ISPA), even
in the infinitesimal limit. We present experiments verifying this,
and also showing that a machine-learning-based version of this
COIN algorithm in which costs are only imprecisely estimated (a
version potentially applicable in the real world) also outperforms
the ISPA, despite having access to less information than does the
ISPA."
Adaptivity
in Agent-Based Routing for Data
Networks, David H. Wolpert,
Sergey Kirshner, Chris J. Merz, Kagan Tumer, Report-no:
NASA-ARC-IC-99-122
Avoiding
Braess' Paradox through Collective
Intelligence, Kagan Tumer,
David H. Wolpert, Report-no:
NASA-ARC-IC-99-124
Cortical Mechanisms of Human Imitation, Science
The ability to imitate someone else is a major factor
that allows infants (already a few hours after birth) and small
children to learn from their parents and older siblings. It is
also a fundamental factor for social interactions for instance in
initiating a communication, a signal of understanding what the
other means. The capability seems to be a central part that is
missing in autistic children.
Imitation of behavior on the other hand is behavior not limited
to humans: dolphins can be very creative in imitating movements of
humans even if they are missing the body parts that they observe
moving. Monkeys also are known to imitate ("monkey") the movements
of others.
Iacoboni et al. conducted a series of careful experiments to
identify brain regions that specialize on the essence of
imitation. To demonstrate this effect beyond reasonable scientific
doubt they had to exclude alternative explanations that are based
on the observation of the body part itself etc. Their finding
indicates that there are two regions the "left inferior cortex
(opercular region) and the rostral-most region of the right
parietal lobule" that are selectively active during imitation,
regardless of how it is evoked. This distribution of activity
seems to help keeping the distinction between the "actor" and the
"imitator".
Cortical
Mechanisms of Human Imitation
, Marco Iacoboni,
Roger P. Woods, Marcel Brass, Harold Bekkering, John C.
Mazziotta, Giacomo Rizzolatti, Science
What Kills Motor Neurons in Locked-In Patients?, Science
Nitric oxide is a chemical that has powerful
cellular signaling capabilities (see, Ancient
origins of nitric oxide signaling in biological systems, ComDig
1999.beta6.11). Now it also seems to be involved in signaling
motor neurons that it is time to commit cellular suicide in
apoptosis. The large scale dying-off of motor neurons is happening
in amyotrophic lateral sclerosis (ALS), also called Lou Gehrig
disease which is not curable and leaves patients in a locked-in
state where their brain is fully functional but they are not able
to move a single muscle (see Turning
Thoughts Into Actions, ComDig 1999.beta1.4).
'Its cause in most cases is not known, but 2% of ALS patients
carry mutations in Cu,Zn superoxide dismutase (SOD), an enzyme
that scavenges the superoxide free radical. Estévez et al.
report that mutant SOD, which is unable to bind zinc (but still
binds copper), induces cultured motor neurons to undergo
apoptosis. If the wild-type SOD was forced to give up its zinc, it
also caused motor neurons to die. When both wild-type and mutant
SOD were replete with zinc, then both SODs protected motor neurons
from apoptosis upon removal of nurturing growth factors. The
authors propose that loss of zinc from SOD induces motor neuron
apoptosis through an oxidative mechanism that produces nitric
oxide. '
Induction
of Nitric Oxide -- Dependent Apoptosis in Motor Neurons
by Zinc-Deficient Superoxide
Dismutase, Alvaro G.
Estévez, John P. Crow, Jacinda B. Sampson,
Christopher Reiter, Yingxin Zhuang, Gloria J. Richardson,
Margaret M. Tarpey, Luis Barbeito, and Joseph S. Beckman
, Science 286: 2498-2500.
