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Reproductive Cloning: They Want to Make a Baby



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BIOCHEMICAL READING

 

 

Manual

for

3rd year students,

Biochemical Department

 


 

М.Г. Попова. Biochemical Reading. Учебное пособие для студентов 3 курса.

 

Данное пособие предназначено для развития навыков перевода и пересказа научных текстов у студентов 3 курса и аспирантов биохимических специальностей биологических факультетов университетов. Пособие содержит оригинальные тексты на английском языке, взятые из научных журналов (Science и Scientific American), и упражнения к ним. Один текст рассчитан на 3 – 5 занятий в зависимости от уровня группы. В каждый раздел включены также два текста разного уровня сложности на русском языке для реферирования.

 

 

Автор благодарит Попову Н.С. за помощь в подборе материала.

 

 

Table of Contents

 

The First Human Cloned Embryo. 4

Moving On. 8

The Long Arm of the Immune System.. 13

The Cellular Chamber of Doom.. 18

License Withheld—Geminin Blocks DNA Replication. 23

Hear, Hear, for the Inner Ear 28

Sussing Out Stress. 32

Is the Genome the Secular Equivalent of the Soul?. 38

When Do Telomeres Matter?. 45

A Radical Proposal 50

 

 


UNIT 1

Pre-reading questions

1. How can people reproduce nowadays?

2. What is the attitude to cloning among scientists and the general public?

The First Human Cloned Embryo

Jose B.Cibelli, Robert P.Lanza and Michael D.West, with Carol Ezzell,

Scientific American, November 2001

 

Cloned early-stage human embryos and human embryos generated only from eggs, in a process called parthenogenesis now put therapeutic cloning within reach.

 

They were such tiny dots, yet they held such immense promise. After months of trying, on October 13, 2001, we came into our laboratory at Advanced Cell Technology to see under the microscope what we'd been striving for: little balls of dividing cells not even visible to the naked eye. Insignificant as they appeared, the specks were precious because they were, to our knowledge, the first human embryos produced using the technique of nuclear transplantation, otherwise known as cloning.

With a little luck, we hoped to coax the early embryos to divide into hollow spheres of 100 or so cells called blastocysts. We intended to isolate human stem cells from the blastocysts to serve as the starter stock for growing replacement nerve, muscle and other tissues that might one day be used to treat patients with a variety of diseases. Unfortunately, only one of the embryos progressed to the six-cell stage, at which point it stopped dividing. In a similar experiment, however, we succeeded in prompting human eggs on their own, with no sperm to fertilize them to develop parthenogenetically into blastocysts. We believe that together these achievements, the details of which we reported November 25 in the online journal e-biomed; The Journal of Regenerative Medicine, represent the dawn of a new age in medicine by demonstrating that the goal of therapeutic cloning is within reach.

 

Therapeutic cloning which seeks, for example, to use the genetic material from patient's own cells to generate pancreatic islets to treat diabetes or nerve cells to repair damaged spinal cords is distinct from reproductive cloning, which aims to implant a cloned embryo into a woman's uterus leading to the birth of a cloned baby. We believe that reproductive cloning has potential risks to both mother and fetus that make it unwarranted at this time, and we support a restriction on cloning for reproductive purposes until the safety and ethical issues surrounding it are resolved. Disturbingly, the proponents of reproductive cloning (see Reproductive Cloning: They Want to Make a Baby) are trying to co-opt the term “therapeutic cloning” by claiming that employing cloning techniques to create a child for a couple who cannot conceive through any other means treats the disorder of infertility. We object to this usage and feel that calling such a procedure “therapeutic” yields only confusion.

 

Reproductive Cloning: They Want to Make a Baby

Although therapeutic cloning seeks only to generate cells identical to a patient's that can be used to treat disease, several groups around the world have announced their intention to clone babies: Panos Zavos and Severino Antinori. Zavos is a professor of reproductive physiology at the University of Kentucky and co-founder of a fertility clinic in Lexington; Antinori is director of a Rome-based fertility clinic. Both have reputations as renegades: Antinori has helped many post-menopausal women become pregnant, including at least one woman who gave birth in her 60s. At a conference on human cloning at the National Academy of Sciences last August, Zavos said that he and Antinori would work to help couples in which the man did not produce viable sperm reproduce via cloning. The two announced they would have pregnancies by the end of this year. Their claim is credible: both have extensive expertise in fertility and access to potentially interested couples.

Clonaid/The RaKlians. The RaKlians are a religious group that believes that humans descended from extraterrestrials and that cloning can make people immortal. They have formed a company called Clonaid, whose efforts are led by Brigitte Boisselier, a chemist. Boisselier told the National Academy meeting last August that Clonaid had hundreds of women willing to contribute eggs for use in cloning. She argued that people should have the liberty to reproduce how they want, whether by combining their genetic material with another person's through sex or in vitro fertilization or by using only their own genetic material to create a clone.

Richard G. Seed. A physicist with an interest in embryology based near Chicago, Seed has been an advocate of cloning to treat severe infertility as well as to “replace a lost loved one with a twin”. He is known to have attracted a skilled reproductive scientist from China to aid in his efforts, but he does not appear to have the other resources he would need to succeed.

Answer the questions.

  1. What types of cloning are mentioned in the article? What is the difference between them? Are they both ethically justified? If yes, when can they be used?
  2. What do the authors of the article object to?
  3. Has R.G. Seed managed to fulfill his objective?
  4. What would happen if people could reproduce through parthenogenesis?

2. Find synonyms for the word cloning in the text. Is there any difference in their meaning?

3. Write a topic sentence for each paragraph and retell the text.

4. Scanning texts.

Ученые НИИ скотоводства японской префектуры Гифу успешно получили клон из замороженной клетки быка, умершего 16 лет назад.

Ученым удалось с помощью новой технологии вырастить клетку семенника, затем вычленить из нее ядро с носителем информации ДНК и заменить этим ядром ядро неоплодотворенной яйцеклетки коровы.

Первый бык-клон родился в ноябре 2007 года, и он, и родившиеся в прошлом году два его собрата живы и отличаются отменным здоровьем. “Факт создания здоровых животных из клеток, замороженных в эпоху несовершенства этой технологии, поистине впечатляет. Это дает надежду на возможность восстановления исчезнувших и истребленных видов животных”, - считают ученые института.

Успех японских ученых может иметь не только научное, но и промышленное значение, информирует РИА «Новости». Ведь до сих пор самой большой проблемой клонированных животных считалась их высокая смертность в первые месяцы после рождения.

А накануне рабочая группа правительственного комитета по безопасности мясопродуктов, рассматривающая вопрос безопасности использования мяса клонированных животных в пищу, сделала заключение о том, что “клоны ничем не отличаются от свиней и коров, рожденных естественным путем”. Основным выводом комиссии стало заключение, согласно которому если клонированное животное доживает до 6 месяцев, то его дальнейшее развитие и здоровье ничем не отличается от обычных коров и свиней.

Таким образом, если безопасность клонированной говядины и свинины будет подтверждена комитетом по безопасности продуктов питания, то уже в этом году можно ожидать поступления такого мяса в продажу. В настоящее время в Японии родилось 557 коров и быков-клонов, однако выжило всего 82.

(www.dni.ru)

Терапевтическое клонирование оказалось непригодно для получения стволовых клеток

 

Попытки создать гибриды человека и животного с целью получения эмбриональных стволовых клеток предпринимались учеными неоднократно. Поскольку извлечение яйцеклеток человека сопряжено с большими трудностями, некоторые группы исследователей обратились к идее использования яйцеклеток животных.

Реализовать перспективную технологию на практике, однако, оказалось невозможно. По сообщению Роберта Ланца (Robert Lanza), возглавлявшего работы по проекту в компании Advanced Cell Technology (США), опыты на яйцеклетках мышей, коров и кроликов не привели к желаемому результату — получению реального эмбриона. Исследователи отметили, что при смешении клеток человека и животного не происходит активации генов, необходимых для создания жизнеспособного зародыша.

Для проведения экспериментов ученые использовали технологию терапевтического клонирования. Суть ее сводится к тому, что ядро яйцеклетки животного заменяется ядром человеческой клетки (донора) другого типа. При удачном исходе начинается процесс деления и — как и в случае «обычного» оплодотворения — образуется эмбрион, генетически близкий к человеческому.

К сожалению, усилия исследователей ни к чему не привели. «За последние десять лет мы поставили сотни экспериментов, пытаясь получить стволовые клетки, генетически совместимые с донорским организмом, — говорит г-н Ланца. — Но всё впустую». Гибрид человека и мыши пережил лишь одно деление; опыты с яйцеклетками коровы и кролика прошли чуть успешнее, но и в этих случаях развитие прекратилось слишком рано. Ученые также предприняли неудачную попытку получения стволовых клеток путем фактического клонирования (т.e. создания «гибрида человека и человека»); впрочем, оказалось, что такой процесс теоретически возможен. «Наши наблюдения показывают, что при образовании обычного и клонированного эмбрионов активируется абсолютно одинаковый набор генов», — с надеждой отмечает доктор Ланца.

C сайта science.compulenta.ru, 4 февраля 2009 г

 

UNIT 2

Pre-reading questions

1. What does the central nervous system consist of?

2. What is the role of axons in the nervous activity?

Moving On

Barry J. Dickson

Science, March 2001

 

It is a feeling that many travelers will know: no sooner has a particular destination been reached, than it loses its charm and one begins to long for the next. For the traveler, this may make for an unsettling journey. But for axons navigating throughout the developing embryo in search of their synaptic partners, such restlessness is essential to keep them moving on from one intermediate target to the next, until they reach their final destination. This restlessness does, however, pose something of a paradox. How can an axon first be attracted to an intermediate target, but then lose interest as soon as it arrives, moving on to more enticing targets elsewhere?

 

For commissural axons - those axons that connect the two symmetrical halves of the central nervous system (CNS) - the midline of the body is a critical intermediate target. These axons grow first toward the midline, but when they reach it they then continue growing, moving across the mid-line into the opposite half of the body. Studies of axon outgrowth in mammalian CNS explant cultures have shown that commissural axons are directed by guidance molecules produced by a specialized group of midline cells that form a structure known as the floor plate. These guidance molecules include Netrin, an attractant, and Slit, a repellent. Remarkably, commissural axons switch their responsiveness to these cues as they cross the midline. Before crossing, they are attracted by Netrin but insensitive to Slit, whereas after crossing they no longer respond to Netrin (at least in the hindbrain) and are now repelled by Slit.

 

Switching sensitivity to these guidance cues seems like a good way to keep commissural axons moving through the midline, but how is attraction at the midline turned off and repulsion turned on? In Drosophila, where the activity of Slit in axon- guidance was first described, commissural axons increase their expression of the Slit receptor Robo as soon as they cross the midline. This explains how commissural axons acquire sensitivity to Slit. But how do they lose their sensitivity to Netrin?

 

Enter Stein and Tessier-Lavigne with their elegant study using the Xenopus spinal neuron turning assay pioneered by Poo and colleagues. In this assay, an isolated spinal axon growing in vitro is exposed to a gradient of a purified guidance factor released from a micropipette, and its growth rate and turning response are then monitored with time-lapse microscopy. This assay offers two important advantages over the more traditional explant assays. First, one can assess the response of a single axon to a single guidance cue. Second, by injecting messenger RNA into the blastomeres of early embryos, these axons can be forced to express any desired guidance receptor.

Stein and Tessier-Lavigne first noted that spinal axons from stage 22 Xenopus embryos are attracted by Netrin and insensitive to Slit, whereas stage 28 neurons are insensitive to Netrin but repelled by Slit. They next asked what would happen if stage 22 axons were exposed to both signals simultaneously, as is the case for commissural axons in vivo. Surprisingly, when confronted with the two cues together, these axons were neither attracted nor repelled! This cannot be just a case of repulsion and attraction canceling each other out, because these axons are not repelled by Slit at all, and Slit does not block the turning response to a different attractant. Somehow, Slit specifically silences the ability of Netrin to attract axons.

 

In a tour de force study with chimeric receptors and heterologous ligands in the Xenopus assay, Stein and Tessier-Lavigne go on to show that this silencing effect depends on a direct interaction between the cytoplasmic domains of Robo and the netrin receptor, DCC. This interaction is mediated by short conserved domains in each receptor - CCI in Robo and P3 in DCC. In a dramatic conclusion to their experiments, Stein and Tessier-Lavigne were able to reconstitute both the physical association and the silencing effect with a pair of chimeric receptors that were activated by two completely different ligands and associated through different interaction domains.

 

In a second paper Stein and colleagues use a similar series of assays to show that attraction by Netrin depends on the self-association of DCC receptors through the same P3 domain. They also make a compelling case that at least for Xenopus spinal axons and mammalian commissural axons Netrin signaling does not require the adenosin A2B receptor, as recently suggested. Previously Stein and co-workers demonstrated that DCC can also mediate repulsion by forming a complex with another Netrin receptor called UNC5. In this case, interaction depends on a different motif in the DCC cytoplasmic domain called PI. DCC can thus be switched from attraction to repulsion, or silenced completely, depending on its binding partner. The mechanism appears to be similar in each case - binding of a ligand to a co-receptor (Netrin to DCC for attraction, Netrin to UNC for repulsion, and Slit to Robo for silencing) allows the cytoplasmic domain of this co-receptor to bind to the cytoplasmic domain of DCC. Presumably, a different set of intracellular signaling proteins is then recruited to each receptor complex.

 

These new studies raise a number of important questions. Clearly, the most urgent need is to find out whether this silencing mechanism also operates in commissural axons in vivo. Fortunately, among their large collection of mutant and chimeric receptors, Stein and Tessier-Lavigne have generated a form of DCC that still mediates attraction by Netrin but cannot be silenced by Slit. Coaxing commissural axons to express this unsilenceable receptor in vivo should reveal what role, if any, silencing might play in forcing axons to move on once they have reached the midline.

 

If, as seems likely, silencing does indeed occur in vivo, then it will also be important to figure out how commissural axons regulate their response to Slit— perceiving it first as a silencer, then as a repellent, and later perhaps even as a branching and elongation factor. Regulation of Robo can only be part of the answer. In particular, Robo regulation cannot explain the different responses to Slit observed in stage 22 and stage 28 Xenopus spinal neurons. Perhaps commissural neurons change their responses to Slit according to some intrinsic program, in much the same way that they also change their neurotrophic requirements as they complete each leg of their journey.

 

The discovery of this silencing phenomenon also suggests an alternative explanation for the midline guidance errors observed in slit and robo fly embryo mutants. Could it be that axons stray across or linger at the midline in these mutants, in part because of a failure to silence attraction, rather than simply because of a loss of midline repulsion as previously thought? Separating the silencing and repellent functions of Slit would help to resolve this issue. Stein and Tessier-Lavigne suggest a way to do just this. In the Xenopus assays, a mutant form of Robo that lacks the CCI domain can still mediate repulsion but is unable to silence attraction. This mutant form of Robo has already been expressed in flies, and results in a low frequency of midline crossing errors. It is important to note, however, that Drosophila has two additional Slit receptors, Robo2 and RoboS. Both receptors contain the CCI domain and so may also contribute to silencing. It will be interesting to see whether deleting the CCI domains of all three Robo receptors leads to more severe midline crossing errors and, if so, whether these defects require netrin and DCC activity as predicted by the silencing model.

 

Finally, the studies of the Tessier-Lavigne laboratory force us to revise our view of how axons respond to multiple guidance cues. In vivo, axons are simultaneously exposed to a number of different attractive and repulsive forces. It has generally been thought that the axon integrates all of these signals in order to calculate its next move. But, as Stein and Tessier-Lavigne show, multiple guidance signals can also be combined in a hierarchical fashion, with one signal silencing the response to another. These two guidance strategies each make sense in different contexts. Integration, for example, has been most clearly demonstrated in the selection of different muscle targets by motor axons in Drosophila. Here, subtle differences in the way each axon responds to various muscle attractants and repellents may be an effective way to bias their preferences for specific muscle targets. In contrast, hierarchical guidance may be the better strategy at intermediate targets where axons must suddenly and drastically switch their preferences to ensure that they keep moving on toward their final destination.

 

Answer the questions.

  1. What is meant by axon navigation?
  2. What conclusion did Stein and Tessier-Lavigne make in the course of their experiments?
  3. Does receptor silencing occur in vivo?


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