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[A1CR]

A highlight of the below pdf-availed paper may be: "In our
study, we have
... reported that the monkeys on CR display ... an emerging
survival
advantage compared to the age-matched controls (unpublished
observations)."

Anderson RM, Weindruch R.
Calorie restriction: Progress during mid-2005-mid-2006.
Exp Gerontol. 2006 Nov 22; [Epub ahead of print] No abstract
available.
PMID: 17125949

1. Overview
In keeping with the current trend, this year again has seen
a substantial
volume of literature on the topic of calorie restriction
(CR). Work in
widely diverse species has contributed to our understanding
of CR and
continues to provide important clues about the mechanism of
lifespan
extension by CR. Let us acknowledge at the outset that the
scope of this
review permits only a handful of these studies to be
described, for this we
apologize and list areas of progress that we view as most
important.

2. Towards a mechanistic understanding
A number of pathways originally identified in longevity
studies in
invertebrates have come under further scrutiny, requiring
some revision of
previous models. The role of the Sirtuin family of NAD
dependent
deacetylases in longevity and in the mechanism of CR has
drawn particular
attention (Longo and Kennedy, 2006). It remains to be seen
if the
contribution of Sirtuins to the mechanism of CR is conserved
in yeast, worms
and flies. As a result, data from these organisms may not
necessarily be
predictive of a role for Sirtuins in the mechanism of
lifespan extension by
CR in mammals. The role of mitochondrial respiration in CR
has also been
brought into question by a study that demonstrates lifespan
extension with
CR in a respiratory deficient yeast strain (Kaeberlein et
al., 2005a). The
fact that alternate pathways promoting longevity are induced
in strains
lacking respiratory capacity does not negate a role for
mitochondrial
metabolism where the organelles are functional. The
important finding from
these studies on strain dependent differences is that CR may
extend lifespan
by impacting multiple pathways and that there may be a
certain amount of
mechanistic plasticity.

The TOR pathway has long been suspected of providing a link
between
nutrition, metabolism and longevity, and deficiency in TOR
signaling extends
lifespan in worms and flies. A large-scale analysis of over
500 single gene
deletion strains in yeast has pointed to the involvement of
the nutrient
sensing TOR pathway in the mechanism of CR (Kaeberlein et
al., 2005b). There
is also evidence for down-regulation of the mTOR pathway in
the long-lived
Ames Dwarf mouse (Sharp and Bartke, 2005). The mammalian
mTOR pathway has
recently been shown to be involved in determining resting
oxygen consumption
and oxidative capacity in cultured cells (Schieke et al.,
2006). mTOR
affects mitochondrial function and the mitochondrial
phosphoproteome
independent of the regulation of ribosomal gene expression.
The influence of
TOR on mitochondrial function is of particular interest in
the context of
CR, because mammalian studies have demonstrated that
mitochondrial
metabolism and ROS generation are altered in tissues from
restricted animals
compared to controls (see below).

A significant challenge in the study of the mechanism of CR
in invertebrates
is presented by the lack of consensus in methodological
approach. In worms,
differences in the implementation of CR (bacterial
dilution/anexic media/use
of "genetic mimic") cause complications in determining key
factors involved.
As a result, the extent of crosstalk between the insulin
signaling pathway
and CR is controversial, and the involvement of TOR and
SIR-2.1 is still
under investigation (Houthoofd et al., 2005). Nonetheless,
the use of RNAi
screens proves invaluable in the identification of factors
that influence
longevity and continues to provide new and exciting leads
(Hansen et al.,
2005). An important issue relating to methodology has been
resolved in
Drosophila studies where the standard CR regimens reduce
calories by
dilution of either the yeast or sugar component of the diet.
By direct
comparison of isocaloric diet in terms of lifespan extension
and
reversibility of effect, it appears that all calories are
not created equal
and the yeast component is the key determinant of lifespan
in this organism
(Mair et al., 2005).

The extent of overlap between established regimens that
influence lifespan
in the mouse model is also unclear. In rodents, CR extends
the lifespan of
the Ames dwarf but does not further extend the lifespan of
the growth
hormone receptor knockout (GHRKO) mouse (Bonkowski et al.,
2006). The
inference is not that the GHRKO mouse and CR extend lifespan
by the same
mechanism, rather that elements required in the mechanism of
CR are absent
in the GHRKO mouse. This is based on differences in gene
expression profiles
in tissues from GHRKO and CR animals that do not support an
equivalent
mechanism of lifespan extension.

A number of studies have demonstrated an anti-inflammatory
effect of CR in
rodents. In rats, CR and exercise have beneficial effects on
circulating
levels of C reactive protein, a marker of inflammatory tone
(Kalani et al.,
2006). Transcriptional profiling of mouse adipose tissue
reveals a marked
reduction in the expression of genes involved in
inflammation with long term
CR (Higami et al., 2006). While we do not yet know how CR
brings about the
anti-inflammatory effect, this phenotype may be an important
contributor to
the delay in onset of age-associated diseases.

3. Mitochondria
The role of mitochondria in the mechanism of CR continues to
be an area of
active research. Differences in experimental approach have
yielded
interesting and sometimes apparently contradicting data. In
flies, CR does
not appear to affect mitochondrial numbers in muscle but
does appear to
alter mitochondrial morphology and in vitro enzyme
activities (Magwere et
al., 2006). In mammalian cells grown in serum derived from
CR rats,
mitochondrial biogenesis is increased and bioenergetic
efficiency is
improved with concomitant reduction in reactive oxygen
species (ROS)
generation (Lopez-Lluch et al., 2006). A separate study
reports increased
mitochondrial biogenesis with CR based on increased
transcription levels of
nuclear encoded mitochondrial genes and increased
mitochondrial DNA content
in tissues from restricted mice compared to controls (Nisoli
et al., 2005).
A role for nitric oxide in CR is suggested by the finding
that endothelial
nitric oxide synthase (eNOS) is induced by CR, and the
effect of CR on
mitochondrial markers is reduced in eNOS null mutant mice.
It will be
interesting to see if reactive species in general play a
signaling role in
the mechanism of CR.

Studies in isolated mitochondria from rat skeletal muscle
reveal a reduction
in ROS production with CR that is not due to changes in
proton leak
(Bevilacqua et al., 2005). Separate studies demonstrate that
CR opposes the
decline in oxidative capacity of skeletal muscle
mitochondria. This effect
is independent of mitochondrial DNA integrity suggesting
that there is a
difference in mitochondrial function with CR that cannot be
simply explained
by differences in mitochondrial DNA damage (Baker et al.,
2006). This same
study demonstrated reduced activities of both citrate
synthase and Complex
IV of the electron transport system in extracts from CR
muscle, however;
activities were maintained with age in CR tissues but
declined below the CR
levels in tissues from Control animals with age. Analysis of
Complex IV
activity in skeletal muscle in situ provides evidence that
mitochondrial
from CR animals have a higher affinity for oxygen (Hepple et
al., 2005).
This raises the possibility that data from isolated
mitochondria may not
reveal key differences in mitochondrial function with CR.

4. Studies in primates
A major goal of the field is to determine the potential of
CR in humans. On
route to this is to understand whether CR can slow the aging
process in
nonhuman primates that share close genetic makeup to humans.
There are two
studies (one at the USA's National Institute on Aging; the
other at the
University of Wisconsin) in rhesus monkeys that began in the
late 1980s,
which are examining the effects of CR on aging. In our
study, we have
previously reported that the monkeys on CR display signs of
improved health
(e.g. 70% less body fat, higher insulin sensitivity,
favorable changes in
circulating lipids). Further, CR imparts a complete
protection from Type 2
diabetes and an emerging survival advantage compared to the
age-matched
controls (unpublished observations). As the rhesus monkeys
at our Primate
Center have an average lifespan of 27 years and a maximum
lifespan 40 years,
it may be another 25 years before we obtain full survival
data from this
population.

Significant progress has also been made on the effects of
long-term CR in
humans. Direct evidence comes from studies of cardiovascular
aging in
long-term practitioners of CR who were reported in 2004 to
display markedly
improved risk factor profiles for protection against developing
cardiovascular disease, including core features of CR (e.g.,
reductions in
circulating insulin and glucose levels). These individuals
also display
fewer signs of aging in heart (diastolic) function (Meyer et
al., 2006).
Additional progress in human CR is occurring as result of
the USA's National
Institute on Aging funding to conduct CR investigations in
people. CR lowers
body temperature and insulin levels (both of which happen in
rodents on CR)
in overweight subjects (Heilbronn et al., 2006). In
addition, CR (as well as
exercise) lowers adipocyte size and some negative outcomes
such as lipid
deposition in visceral and hepatic tissues and insulin
resistance linked to
large adipocytes (Larson-Meyer et al., 2006).

5. Final comment
Clearly, significant advances in understanding the biology
of CR were gained
over the period reviewed. A glimpse at the publications
appearing subsequent
to this period suggests that next year's review will convey
even greater
insights.