The more involved I become with stem cells and
the field of Regenerative Medicine, the more convinced I become of the
importance of the mitochondria. Many of us in clinical medicine seem to brush
over mitochondria. We now realize that many diseases are related in some way to
deficiencies of the mitochondria. Success in stem cell procedures may depend on
the health of the mitochondria. The above illustration shows the structure of
the mitochondria. Mitochondria
are rod-shaped organelles that can be considered the power generators of the
cell, converting oxygen and nutrients into adenosine triphosphate (ATP).
ATP is the chemical energy "currency" of the cell that powers the
cell's metabolic activities. Mitochondria
are often referred to as the powerhouses of the cell. They help turn the energy
we take from food into energy that the cell can use. But, there is more to
mitochondria than energy production. In
fact, only about 3 percent of the genes needed to make a mitochondrion go
into its energy production equipment. The vast majority are involved in other
jobs that are specific to the cell type where they are found. Here is another illustration of the inner
workings of the mitochondria
The mitochondria have two
membranes, an outer one and an inner one. Each membrane has different
functions. The Outer membrane allows small molecules to pass freely through the
outer membrane. This outer portion includes proteins called porins, which form
channels that allow proteins to cross. Most cellular stress
responses converge on the mitochondria. Consequently, the mitochondria must
rapidly respond to maintain cellular homeostasis and physiological demands by
fine-tuning a plethora of mitochondria-associated processes. The outer
mitochondrial membrane proteins are central to mediating mitochondrial
dynamics, coupled with continuous fission and fusion. These proteins also have
vital roles in controlling mitochondrial quality. When
cellular components like mitochondria become damaged or defective, they can be
recycled by cells through a process called autophagy, which literally means
self-eating. When mitochondria are degraded by autophagy, the process is
specifically referred to as mitophagy. Mitophagy often
occurs in defective mitochondria following damage or stress. This is
actually one of the important aspects of aging. As we age, mitophagy will
diminish resulting in increased damaged mitochondria. This has a snowball effect
in that it leads to increased reactive oxygen species (ROS), decreased
bioenergetics, and many age-related diseases. Mitochondrial damage may be the
seminal event in many different diseases. If we increase mitophagy we will slow
down aging. The following illustration shows the consequences of accumulated
mitochondrial damage.
The next structure to discuss is the inner mitochondrial membrane. It is
extensively folded and compartmentalized. The numerous invaginations of the
membrane are called cristae. Which are separated by crista
junctions from the inner boundary membrane juxtaposed to the outer membrane.
Cristae significantly increases the total membrane surface area compared to a
smooth inner membrane and thereby the available working space. The inner membrane is also loaded with proteins involved in electron transport and
ATP synthesis. This membrane surrounds the mitochondrial matrix, where the citric
acid cycle produces the electrons that travel from one protein complex to the
next in the inner membrane. The crista membranes contain most, if not all, of
the fully assembled complexes of the electron transport chain and the ATP
synthase. The following illustration demonstrates this concept. We see the two
membranes and subsequent ATP production. In review, at the inner mitochondrial
membrane a high energy electron is passed along the electron
transport chain.
The energy
released pumps hydrogen out of the matrix
space. The gradient created by this drives hydrogen back through the membrane,
through ATP synthase. As this happens, the enzymatic activity of ATP synthase
synthesizes ATP from ADP. This whole process is called oxidative
phosphorylation (OXPHOS), which is the main method and most efficient method
the body uses to make ATP. The more efficient this process the better in shape
one is.
Another structure present is the
mitochondrial ribosomes. Mitochondrial
ribosomes (mitoribosomes) perform protein synthesis inside mitochondria.
Throughout evolution, mitoribosomes have become functionally specialized for
synthesizing mitochondrial membrane proteins. Mitochondrial ribosomes resemble bacterial ribosomes and both bacteria
and mitochondria ribosomes share a slightly different genetic code from that in
the nucleus. Actually, we see that ribosomes have two parts, a large and a
small subunit.
Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA. This genetic material is known as mitochondrial DNA or mtDNA. Mitochondria are a trans-kingdom enigma. At the molecular level, the components of Human mitochondria are assembled from viruses, bacteria, and other organisms. As such, the organelle we see in human cells today is called a trans-kingdom mixture that doesn't fully resemble any of its ancestors.
The mitochondrial
genome is built of
16,569 DNA base pairs, whereas the nuclear genome is made of 3.3 billion DNA base pairs. In keeping with its bacterial ancestry, mtDNA
is also circular and multicopy with hundreds to thousands of copies present in
every cell. mtDNA is very genetically compact and encodes only 13 proteins, all
of which are core subunits of the oxidative phosphorylation (OXPHOS) complexes.
These OXPHOS complexes, found only within mitochondria, are unique in human
biology as they are the only cellular structures formed of proteins encoded by
genes from the two separate genomes. The nuclear DNA provides around 90% of the
required proteins for OXPHOS, and the mtDNA provides the remaining 10%.
Remember that the OXPHOS complexes are responsible for ATP production.
Mitochondria are the only organelle to have their own DNA. Mitochondrial
DNA (mtDNA) is more susceptible to damage (including mutations) than nuclear
DNA. The reason for this is many folds. Most likely this is due to a lack
of histones to protect the DNA from damage. The below diagram gives a brief
explanation of histones. Histones package and order the DNA into structural units called nucleosomes. They
act as spools around which the DNA gets coiled and thus a very long strand of
DNA can be fit into a much smaller space. This is demonstrated in the
following:
DNA damage is also caused by the proximity of mtDNA to
Reactive Oxygen Species (ROS) production. We must remember that the
mitochondria are engaged in oxidative phosphorylation which means that they are
using oxygen to produce energy. The by-product of the energy production is the
ROS. Also, mtDNA has limited DNA repair systems and limited proofreading
capacity during replication all of which can lead to accumulated mitochondrial
DNA damage. Furthermore, the mitochondrial DNA is ever changing. When a cell divides, its
mitochondria are partitioned between the two daughter cells. However, the
process of mitochondrial segregation occurs in a random manner and is much
less organized than the highly accurate process involved in nuclear DNA
division during cell replication commonly called cell mitosis. As a result,
daughter cells receive similar, but not identical, copies of their
mitochondrial DNA.
WHAT REGULATES THE
MITOCHONDRIA? THE SIRTUIN FAMILY OF PROTEINS
Sirtuins are a
family of proteins that regulate cellular health. Sirtuins play a key role in
regulating cellular homeostasis. Homeostasis involves keeping the cell in
balance. Sirtuins can only function in the presence of NAD+,
nicotinamide adenine dinucleotide, a coenzyme found in all living cells. NAD+
is vital to cellular metabolism and hundreds of other biological
processes. Humans contain
seven sirtuins (SIRT1-7) that modulate distinct metabolic and stress response
pathways. Three sirtuins, SIRT3, SIRT4 and SIRT5, are located in the mitochondrion.
The others are found in the nucleus and one in the cytoplasm. The basic role of sirtuins, however, is that they remove
acetyl groups from other proteins. Acetyl groups control specific reactions.
They are physical tags on proteins that other proteins recognize will react
with them. Sirtuins work with acetyl groups by doing whats called
deacetylation. This means they recognize theres an acetyl group on a molecule
then remove the acetyl group, which tees up the molecule for its job. One way
that sirtuins work is by removing acetyl groups (deacetylating) biological
proteins such as histones. When the histones have an acetyl group, the chromatin is
open, or unwound. When the histones
are deacetylated by sirtuins, the chromatin is closed, or tightly and neatly
wound, meaning gene expression is stopped, or silenced. This is not that common
for the Sirtuins in the mitochondria.
Mitochondria regulation is where things get interesting. If we start
manipulating the regulation of the mitochondria then there are a whole host of
conditions from aging to chronic neuro-degenerative conditions which we might
be able to impact. Recent findings have shed light on how the mitochondrial
Sirtuin functions in the control of basic mitochondrial biology, including
energy production, metabolism, apoptosis, intracellular signaling and perhaps
most importantly mitochondrial genesis. The following diagram shows some of
these aspects:
What these Sirtuins
do is help in the generation of cellular energy. As high-energy
electrons derived from glucose, amino acids or fatty acids fuels are passed
through a series of protein complexes (I-IV), their energy is used to pump
protons from the mitochondrial matrix through the inner membrane into the
inner-membrane space. This is referred to as the electron transport chain.
Ultimately, the electrons reduce oxygen to form water, and the protons flow
down their gradient through ATP synthase, driving the formation of ATP from
ADP. Reactive oxygen species (ROS) are a normal side-product of the respiration
process. ROS are essentially free radicals. During cellular stress or damage,
mitochondria release a variety of signals to the cytoplasm and the nucleus to
alert the cell of changes in mitochondrial function. In response, the nucleus
generates transcriptional changes (stimulates certain genes) to activate a
stress response or repair the damage. The main function of mitochondria is to metabolize or
break down carbohydrates and fatty acids in order to generate energy.
In review, ATP
generation occurs within the mitochondrial matrix, though the initial steps of
carbohydrate (glucose) metabolism occur outside the organelle. Glucose is first
converted into pyruvate and then transported into the matrix. Fatty acids on
the other hand, enter the mitochondria as is.
ATP is produced
through the course of three linked steps. First, using enzymes present in the
matrix, pyruvate and fatty acids are converted into a molecule known as
acetyl-CoA. This then becomes the starting material for a second chemical
reaction known as the citric acid cycle or Krebs Cycle. This step produces
plenty of carbon dioxide and two additional molecules, NADH and FADH2,
which are rich in electrons. The two molecules move to the inner mitochondrial
membrane and begin the third step: oxidative phosphorylation. In this last
chemical reaction, NADH and FADH2 donate their electrons to
oxygen, which leads to conditions suitable for the formation of ATP. As an
interesting aside, the optimal ratio of NAD+ /NADH is 700/1. Greater
amounts of NADH lead to aging. NADH is considered a marker of aging. A
secondary function of mitochondria is to synthesize proteins for their own use.
They work independently, and execute the transcription of DNA to RNA, and translation
of RNA to amino acids (the building blocks of protein), without using any
components of the cell.
Another aspect that the Sirtuins control is the control of
Apoptosis. Apoptosis is a cellular process of programmed cell death. This
occurs when the mitochondrial outer membrane allows much more permeability than
normal. This will ultimately commit the
cell to death. Mitochondrial
sirtuins act in synergistic or antagonistic ways to promote respiratory
function, antioxidant defense, insulin response and adipogenesis all of which
can protect individuals from aging and aging-related metabolic abnormalities.
If these cells are not dealt with they might become senescent cells. A
senescent cell is one that should have died but continues to remain alive. The problem
with the senescent cells is that they will release a number of inflammatory
growth factors which can cause havoc in the body.
HOW DO WE KEEP OUR
MITOCHONDRIA HEALTHY?
We have seen the ins and outs of the mitochondrial structure and
function. The question that begs is how do we keep the mitochondria healthy? More and more research
articles demonstrate the foundational importance of optimal mitochondrial
function for health. There is a growing body of research
showing that mitochondrial dysfunction is surprisingly common and associated
with most chronic diseases. The above and below illustrations give us an idea
of how to keep our mitochondria running smoothly. The first illustration shows
some supplements which keep things running smoothly:
The second illustration shows not only specific supplements but also
classes of supplements such as polyphenols (Polyphenols are micronutrients that we get through certain
plant-based foods) and proanthocyanidins (these are chemical
compounds that give the fruit or flowers of many plants their red, blue, or purple
colors). It also stresses some lifestyle factors that can increase mitochondrial
efficiency. The specific supplements that enhance mitochondria function are
evident in the list. Let us talk specifically about some of the polyphenols. They are included in many supplements, though they're also
easy to get in your diet from foods like fruits, vegetables, teas, and spices.
There are more than 8,000 types of polyphenols. A lack of polyphenols isnt associated with specific
side effects. However, they are regarded as lifespan essentials'' for
their potential to reduce the risk of chronic diseases. This is especially true
based on their effects on the mitochondria. Research suggests that supplementation with
pyrroloquinoline quinone, also known as PQQ, can improve the number of
mitochondria in the body while enhancing their functionality. This research
also suggests that effective treatment for many diseases caused
by mitochondrial dysfunction may rest at least partly in this
coenzyme. PQQ is readily found in the soil, so it
makes sense that the best dietary sources are fruits and vegetables grown in
that soil. Fermented foods are rich in these molecules. One of the best sources of PQQ is very dark chocolate.
MITOCHONDRIAL PEPTIDES
The above illustration shows some of the main peptides produced by the
mitochondria. Mitochondria derived peptides (MDPs) are a series of peptides encoded by mitochondrial DNA, and have similar
functions to mitochondria. They are new metabolic regulators of human body, and play a
cytoprotective role in maintaining mitochondrial function and cell viability
under pressure. Peptides
are biomolecules comprised of amino acids which play an important role in
modulating many physiological processes in our body. Peptides are
short strings of amino acids, typically comprising 250 amino acids. Amino
acids are also the building blocks of proteins, but proteins contain
more. Peptides may
be easier for the body to absorb than proteins because they are smaller and
more broken down than proteins.
Mitochondria produce numerous small polypeptides from their short open
reading frame (sORF) regions of mtDNA that have significant biological
activity. These include humanin, six small-humanin like peptides, and MOTS-c
(mitochondrial open reading frame of the 12S rRNA type-c), together termed
mitochondrial derived peptides (MDP). MOTS-c is a peptide which is called an exercise mimetic. Exercise
Mimetics are novel ways
of getting the benefits of exercising, without having to exercise. Multiple
studies have demonstrated MOTSc's ability to enhance lipid
beta-oxidation, increase thermogenic brown fat, decrease fat gain on a high-fat
diet, and improve glucose uptake during glycolysis. Various mitochondrial
peptides are produced but their use is not allowed in the USA under the current
regulations. Hopefully, this will change with time.
As time goes on we are discovering more and more about the importance of
the mitochondria and their ramifications to our health lifespan. We see that
methods to boost mitochondria efficiency are varied. But when all is said and
done. Some of the most important factors are exercise especially intermittent
high intensity training, intermittent fasting, a variety of supplements
including NAD. Low levels of oxidative stress such as is produced by
intravenous ozone therapy are also important in the proper function of the
mitochondria. We must remember that mitochondrial decay is inevitable; it cannot be prevented, at least with todays technology.
What is not inevitable is the rate of decay. The mitochondrial rate of
decay is determined by one thing: oxygen efficiency. Perhaps the following
diagram sums it all up:
We see many bad things happen when our mitochondria are not working
properly.
Thanks,Dr. P
