Download Berry Malynn Berry Dr. Bert Ely Genetics 303 6 November 2009

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Malynn Berry
Dr. Bert Ely
Genetics 303
6 November 2009
miRNAs Help in Cardiovascular Maintenance
Vascular injury may be caused from any number of incidents, from viral
meningitis to dislocated bones, but without the body’s post-injury reaction a small injury
could be fatal (Myelomonocytic cell recruitment, Management of Vascular Injuries). The
normal response in the body includes the transformation and translocation of smooth
muscle cells in vascular pathways to the injury site in order to rebuild the damaged
portion of blood vessel. This process is a naturally occurring response, but without the aid
of microRNAs, specifically miRNAs 143 and 145, the body would not effectively
perform this vital function (Xin).
miRNAs are sequences of around 22 nucleotides that do not code for a final
protein product. Instead, miRNAs work to repress the expression of specific target genes
by either degrading the mRNA target sequence or degrading the mRNA protein product.
SRF is a transcription factor involved in cell proliferation, transformation and migration
and controls these actions by regulating genes that code for various components that form
the actin cytoskeleton (Muscle Specific Signaling). Myocardin and myocardin related
transcription factors, cardiovascular coactivators, work with Serum Response Factor
when brought into the nucleus after stimulation, and this interaction is the cause for
smooth muscle cell differentiation. The normal phenotype of a smooth muscle cell is the
expression of mainly contractile and cytoskeletal proteins to maintain the structure of the
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blood vessels. When the smooth muscle cell is stimulated to differentiate in response to
vascular injury, the cell begins to express high levels of genes coding for growth factors
to quickly cover the damaged portion of blood vessel with neointima, the inner most cell
layer of the blood vessel (Parmacek). Smooth muscle cell differentiation, as shown by the
evidence from various studies, is a process that is also aided by multiple miRNAs
including miRNAs 143 and 145, as are many other functions within the heart. As the
cardiac crest develops in embryos, the presence of various miRNAs increases because
they are necessary for proper development of the heart. In fact, research performed by
Kurt Barringhaus and Phillip Zamore found that without certain miRNAs, the embryo
would self-destruct because of improper or inadequate heart development. There are also
many other miRNAs expressed in greater quantities within the cardiovascular system,
suggesting that these miRNAs play key roles in cardiac development and maintenance
(Barringhaus).
Previous studies of miRNA function in heart development and maintenance laid
the groundwork for this set of experiments performed to investigate the function of
miRNAs 143 and 145. Through the use of cloning vectors, the researchers replaced the
region coding for the miRNA with a neomycin resistance gene, effectively knocking out
either miRNA 143, 145 or both in the double knockout. This cloning vector with the
mutant gene was injected into blastocysts after first confirming that the neomycin
resistance gene was present in the place of miRNA 143, 145, or both by Southern
blotting. The resulting mutant mice were then crossbred to create mice with the single
knockout of one miRNA or the double knockout of both genes. Once the transgenic mice
were created and classified, the researchers examined the differences in smooth muscle
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cells, the overall cardiovascular health of the mutants, and effects of the knockouts on
neointimal formation in the mutants after vascular injury by carotid artery ligation. By
comparing vascular smooth muscle cells from mutant and from wild type mice in culture,
the researchers determined that the cells in the mutant mice were noticeably smaller than
those of wild type mice. The cells of the mutants also showed fewer actin fibers within
highly organized stress fibers. The smooth muscle cells of the mutants were smaller, less
healthy, and collectively built much thinner blood vessel walls in comparison to the cells
of wild type mice. After euthanizing the mice, the researchers determined the mass of the
heart and left ventricle of the miRNA 145 knockout mice in comparison to the wild type
mice and found that the mutants’ hearts were much smaller. Next, the researchers decided
to test the overall cardiovascular health of the mutants in comparison to the wild types.
They took the mean arterial and systolic blood pressures of healthy mice and miRNA 145
knockout mice and the results revealed that the mutants could be classified as
hypotensive because their unhealthy vascular walls were unable to stand up to the
pressure exerted on them by the heart. Next the researchers performed carotid artery
ligations on the transgenic mice to determine whether miRNAs 143 and 145 played a role
in the response of muscle cells to vascular injury. The mice with the double knockout and
miRNA 145 knockout had almost no post-injury reaction in comparison to the wild type
mice, and the mice with the miRNA 143 knockout had a diminished response in
comparison to the wild type (Xin et al.).
These results imply that miRNAs 143 and 145 are important for overall
cardiovascular health, and even though they are not necessary for life, their absence
greatly increases risk of mortality from cardiovascular complications. From this research,
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we can ascertain that miRNA 145 is more relevant or performs a larger role than miRNA
143 in neointima formation after vascular injury. Although this study was performed on
mice miRNAs, the same sequences are found within the human genome and are, in fact,
highly conserved across vertebrates. By applying the general knowledge about miRNAs
gained from this experiment, we may be soon able to control very minute actions within
our bodies by the augmentation or repression miRNAs. The identification of the genes
linked to specific occurrences within the body may allow us to routinely use miRNA
replacement or silencing to counteract the negative effects aging and diseases
(Barringhaus).
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Works Cited
Barringhaus, Kurt G., and Phillip D. Zamore. "MicroRNAs: Regulating a Change of
Heart." 119:16 (2009).
Kuwahara, Koichiro, Tomasa Barrientos, G. C. Teg Pipes, Shije Li, and Eric N. Olson.
"Muscle-Specific Signaling Mechanism That Links Actin Dynamics to Serum
Response Factor." Molecular and Cell Biology 25:8 (2005).
"Management of Vascular Injuries in Knee Dislocation - Wheeless' Textbook of
Orthopaedics."
<http://www.wheelessonline.com/ortho/management_of_vascular_injuries_in_kn
ee_dislocation>.
"Myelomonocytic cell recruitment causes fatal CNS vascular injury during acute viral
meningitis."
<http://pathology.med.nyu.edu/resources/publications/archives/pathologyfeatured-paper-15>.
Parmacek, Michael S. "Myocardin-Related Transcription Factors." 100:5 (2007).
Xin, Mei, Eric M. Small, Lillian M. Sutherland, Xiaoxia Oi, John McAnally, Craig F.
Plato, James A. Richardson, Rhonda Bassel-Duby, and Eric N. Olson.
“MicroRNAs miR-143 and miR145 modulate cyoskeletal dynamics and
responsiveness of smooth muscle cells to injury.” Genes & Development 23 (18 ):
2166-2178 (2009).