An unscientific discussion on Telomeres and ageing

[Jason Taylor]

This post contains a series of informal e-mail exchanges that occured
in a nice "quite" listserve regarding the possible roles of telomeres
in aging.  I promised to the person who originally started the thread,
who doesn't have "news," to post them here on his behalf.  Enjoy! 

--Jason Taylor

From: "Rothstein, Mark" <mrothstein at mailcenter.tsmi.iitri.com>
Subject: Telomerase and ageing
Date: 13 Oct 1995 11:45:29 +0100

I'm presently taking a course on molecular biology of the cell, and we got to
the part describing DNA replication. The normal procedure of replication used
by the cell for copying the 6 x 10^7 base pairs (per chromosome) just doesn't
work on the ends (the telomeres). As a result, at each cell replication, 10
base pairs may go uncopied. Over time, some key gene may go unduplicated, and
a cell may die for lack of this critical information. To compensate, eucaryote
cells have an enzyme called telomerase which adds on one or more copies of a
non-coding DNA sequence to the telomere. This sequence then becomes a
"throw-away," and key information further up on the chromosome is saved.

Does anyone have any idea on what effects CR may have on this process? 

One effect comes to my mind right away--by limiting energy intake, CR folks
will have fewer cell divisions and thus fewer replications, and thus these
individuals will live longer. (Everything else being equal.) 

Does anyone know of other possible effects?
Would someone care to repost this to sci.life.extension? (I'm not on that
newslist.)

Thanks
Mark

Date: Sat, 14 Oct 1995 13:09:12 -0400 (EDT)
From: Ben Best <benbest at io.org>
Subject: Telomeres and CRAN

    Mark Rothstein seems to have a very confused understanding of 
the role of telomeres in aging. Telomeres offer an explanation as to
why dividing somatic cells undergo a limited number of divisions
(50-60 divisions per fibroblast in human beings). The maximum number
of divisions seen for a given species cell-type is called 
the *Hayflick Limit*.

    Telomeres are long non-functional strands of DNA at the end of 
chromosomes. Telomeres consist of a six-base repeating sequence of 
Thymine, Adenine and Guanine: TTAGGG. With each cell division, some
of the telomere is lost. The length of the remaining telomere is a
good indicator of how many divisions a dividing cell has left. Once
the telomere is gone, functional genetic DNA is lost with each cell
division -- and the cells are soon missing essential proteins. 
"Immortalized" cancer cells contain an enzyme called *telomerase*
that replaces lost telomeres, thus preventing them from experiencing
a Hayflick Limit. 

    Although there is a correlation between maximum lifespan and the
Hayflick Limit for a species, the Hayflick limit would usually predict
a lifespan 2-3 times longer than what is actually observed. Other 
factors must cause aging and death before the Hayflick Limit has a 
chance -- things like free-radical oxidation, non-enzymatic glycosylation,
hormone decline, etc. Neurons are non-dividing cells, so they do not
experience a Hayflick Limit. But they do age, and contribute to aging
in animals and humans.

    There is no evidence that Caloric Restriction with Adquate Nutrution 
(CRAN) has any effect on telomeres. Mark's statement that CRAN would 
result in "fewer cell divisions and thus fewer replications, and thus
these individuals will live longer" is an unproven hypothesis. CRAN
increases protein synthesis in some tissues and decreases it in others.
Dr. Walford seems to think that increased metabolic efficiency on a 
cellular level is the most important mechanism of action for CRAN, 
although there are others: such as reduced non-enzymatic glycosylation
secondary to a drop in blood glucose.

                     -- Ben Best (benbest at io.org)

Date: Sun, 15 Oct 1995 11:49:41 +1000
From: sj_skabo at postoffice.utas.edu.au (Stuart Skabo !!)
Subject: Re: Telomeres and CRAN

Hoi All, Stuart Skabo here for my first post, I originally followed this
list to gather info for an essay i had to do for my thesis, but have
continued out of an interest in using CRAN and mostly an interest in the
science behind it. As a molecular biologist i am very interested in
possible genetic mechanisms, and the actual genes that come into play
during a CR regime.

 Anyway this post is in reply to comments about telomeres by Ben Best and
Mark Rothstein;

We have recently had a seminar given by a world leader in the field of
Telomeres and Telomerase and comments about them on the list have been
accurate to an extent,
>    Telomeres are long non-functional strands of DNA at the end of
>chromosomes. Telomeres consist of a six-base repeating sequence of
>Thymine, Adenine and Guanine: TTAGGG. With each cell division, some
>of the telomere is lost.
[snip]
>Although there is a correlation between maximum lifespan and the
>Hayflick Limit for a species, the Hayflick limit would usually predict
>a lifespan 2-3 times longer than what is actually observed.

agreed. However the current evidence suggests that although telomeres do
shorten over the life time of an organism they are still long enough, by
far, at the end of that organisms life that there is no real correlation. I
have certainly not heard that _any_ functional genes are lost in this way
even in very proliferative tissues.

It is interesting to note for example that mice, having a relatively short
lifespan, have very long telomeres, much longer than humans, throughout
their lives. Many species including yeast have even longer ones based on
repeats of up to eighteen nucleotides.

A better way to look at senescence is through oxidative damage. I wrote an
essay 9the above mentioned one for my thesis) on Oxidative damage and aging
and would be willing to send it to interested parties.

A recent paper entitled "Oxidative DNA damage and senesence of human
diploid fibroblast cells" Proc. Natl. Acad Sci. USA, Vol 92, pp4337-4341
(May 1995) provides strong evidence for oxidations effect on the number of
divisions (the Hayflick limit)
In summary...
Cells grown at 3% (physiological oxygen) divide up to 50% more times and
reach higher cell densities than cells grown at atmospheric (20%) oxygen.


Thank you for your time, i hope you find this interesting, Stuart


-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
THESIS- the final Barrier......
MISSION- to boldly split infinitives that no person has split before......

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-| Stuart Skabo,
Email: sj_skabo at postoffice.sandybay.utas.edu.au    | Molecular Biology Unit
Phone:work (002) 20 2583 or home (002) 23 5543     | University of Tasmania
Snail: 3/14 Ashfield St.,Sandy Bay,Australia 7005  | Australia

From: Jason Taylor <taylor at twinkie.gsfc.nasa.gov>
Subject: Telomeres and CRAN (fwd)
Date: Sat, 14 Oct 1995 21:46:41 -0400 (EDT)

>     Telomeres are long non-functional strands of DNA at the end of
                         ^^^^^^^^^^^^^^   
> chromosomes. Telomeres consist of a six-base repeating sequence of 

Great post Ben, but surely you meant "non-expressed" here, as they may
serve several functions, least of which may be to permit the rest of
the DNA to be correctly reproduced.

> Thymine, Adenine and Guanine: TTAGGG. With each cell division, some

You may be correct, but I vagely recall that the exact makeup of
telomeres varies from species to species.

> of the telomere is lost. The length of the remaining telomere is a
> good indicator of how many divisions a dividing cell has left. Once
> the telomere is gone, functional genetic DNA is lost with each cell
> division -- and the cells are soon missing essential proteins. 

Note that these statements are logically incompatable with the ones
following it where it is stated that the "Hayflick lifespan" is much
longer than the actual one.  (Stuart Skabo already showed this going
from his own knowledge, but I couldn't resist an attempt to do
some theoretical biochemistry.)

..
> experience a Hayflick Limit. But they do age, and contribute to aging
> in animals and humans.

Yes, they do age, and in fact can be highly apoptotic, especially
without the appropriate nerve growth factors, which, as you state,
apparently would indicate that telomeres are unrelated to apoptosis.
Nevertheless, I have a pet theory that relates the two together
nicely.  (Details some other post perhaps.)

> 
>     There is no evidence that Caloric Restriction with Adquate Nutrution 
> (CRAN) has any effect on telomeres. Mark's statement that CRAN would 
> result in "fewer cell divisions and thus fewer replications, and thus
                                                                   ^^^^
> these individuals will live longer" is an unproven hypothesis. CRAN
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> increases protein synthesis in some tissues and decreases it in others.
> Dr. Walford seems to think that increased metabolic efficiency on a 
> cellular level is the most important mechanism of action for CRAN, 
> although there are others: such as reduced non-enzymatic glycosylation
> secondary to a drop in blood glucose.
> 

Well they do live longer with CR, but I agree that the number of
cellular divisions that occurs is at best only a small part of why.  I
personally think (another unpublished theory of mine) that the
mitocondrial OH- production rate is nonlinear to the ATP production
rate.  So high ATP generation rates (and high energy needs) would
imply extra radical production (even more than in a linear model).
Thus, if my theory is correct and we neglect other sources of
radicals, even with radical scavenging ability that is proportional to
caloric intake (which it is probably not), CR would still result in
lower cellular OH- levels.

Regards,
Jason
______________________________________________________________________________
Jason Taylor, Greenbelt, MD  USA|"Doctor, don't cut so deep!  That's the third 
http://www.wam.umd.edu/~theoblit| operating table you've ruined this week!"  

From: Ben Best <benbest at io.org>
Subject: Replies concerning telomeres and CRAN mechanisms

    I am surprised that this list has such sophisticate lurkers, especially
considering the low level of traffic here.

    "Old" tissues contain an increasing number of non-functional cells. 
Among the functional cells, the number of mitochondria per cell tends to
decline with age. This would tend to indicate a significant role for 
oxidative damage, but I had the impression that some non-functional cells
had become non-functional due to telomere loss. (Unfortunately, I can't
find a reference for this and it may be a figment of my imagination.) 
While the Hayflick Limit would not be a dominant factor in tissue aging,
all of the chromosomes in any cell do not have telomeres of the same length
and all of the proliferative cells in a given tissue would not be expected
to have the same average telomere length. Thus, it was my understanding 
that the effects of telomere loss were stochastic. Stuart Skabo's 
statement "I have certainly not heard that _any_ functional genes are 
lost in this way even in very proliferative tissues" sounds absolute rather
than probablistic. I would expect at least *a few* cells to run-out of 
telomeres even if this is not the major cause of non-functional cells in
a tissue.

     The fact that telomere loss is not the primary cause of aging does
not mean that the primary cause is to be found in non-dividing cells such
as neurons and heart muscle cells. Dr. Walford seems to think that the 
mechanisms by which CRAN works are found in the dividing cells. I quote 
from the beginning of the last paragraph of Chapter 5 of THE RETARDATION 
OF AGING AND DISEASE BY DIETARY RESTRICTION (1988, co-authored with 
Richard Weindruch, Ph.D):

         "Long-term dietary restriction of energy evokes an evolutionarily
   selected adaptive response whose main site of impact upon aging is at    
   the level of proliferative systems. This adaptation involves a selective
   upregulation in repair and protective processes, an increased metabolic 
   efficiency, a decreased production of damaging agents, and is accompanied
   by signals to the neuroendocrine network, particularly the hypothalamus."

   Both Stuart Skabo and Jason Taylor suggest that CRAN works through 
decreased oxidative damage, but Dr. Walford evidently believes that this
is only a part of the picture. My interpretation of his statement is that
CRAN turns some "genetic switches" that result in several alterations in
the function of dividing cells. One of these alterations is the increased
levels of the antioxidant catalase which is known to exist at higher 
levels in the cells of CRAN rodents.

   More research needs to be done to demonstrate the alterations of 
ATP-production systems in CRAN animals. Chapter 5 of Walford&Weindruch's
book also contains the statement:

     "We predict that future studies will establish that the energetic 
   efficiency of mitochondrial ATP production is subject to profound genetic
   variation and physiologic regulation, and that DR [Dietary Restriction]
   increases this efficiency. If coupling is loose, more energy is lost as
   heat and (perhaps) as free radicals, with less trapped in the high energy
   bond of ATP."

   So another "genetic switch" affected by CRAN may govern the 
efficiency of ATP production in the mitochondria. 

                     -- Ben Best (benbest at io.org)
______________________________________________________________________________
Jason Taylor, Greenbelt, MD  USA|"Doctor, don't cut so deep!  That's the third 
http://www.wam.umd.edu/~theoblit| operating table you've ruined this week!"