LIFE SCIENCES

Synthetic Life: Should We Do It?

The Ability Is Only Decades Away

In Mary Shelley’s classic tale, Dr. Victor Frankenstein assembled a human body from parts retrieved from corpses. The novel, first published nearly two hundred years ago, raised questions that we would now consider to fall within the realm of bioethics. In fact, if Frankenstein wanted to carry out his experiment today he would need to bring it to the attention of the Institutional Review Board at his university, which would doubtless reject it.

Yet a number of laboratories around the world are attempting to perform a reconstitution of life eerily similar to Frankenstein’s dream, but on a microscopic scale. There is even a name for such science: synthetic biology.

Without a grasp of the science itself any discussion of the policy implications of synthetic biology would be ill-informed.

The history of attempts to fabricate artificial cells that increasingly are approaching the definition of living organisms is complex, compelling, and most of all fast-moving. These efforts have not yet succeeded, but there is reason to believe that the goal may be achieved in the next decade. When someone somewhere on Earth announces that they have been successful, significant public concerns will arise in terms of safety issues, but also because success will challenge deeply held religious beliefs that the synthesis of living organisms should be reserved for a Creator.

Before delving into these serious safety and ethical issues, however, we first need a quick history of synthetic biology to date. The reason: without a grasp of the science itself any discussion of the policy implications of synthetic biology would be ill-informed.

Assembling a system of molecules capable of reproduction was first achieved in 1955 when it was demonstrated by University of California, Berkeley professors Heinz Fraenkel-Conrat and Robley Williams that tobacco mosaic virus could be separated into its coat protein and RNA. Neither component was active by itself, but when mixed together the two parts reassembled into the infectious agent.

Viruses are not considered to be alive in the usual sense of the word, because they can only reproduce themselves using the machinery of a living cell. But a successful reconstitution of viral functions encouraged other investigators to attempt the reassembly of more complex systems. In the 1970s, Cornell University biochemist Efraim Racker was the most successful practitioner of this art, employing detergents such as deoxycholic acid to disperse membranous components of cells. When the detergent was removed, small membranous vesicles formed that resembled microscopic balloons filled with water. Their membranes contained the original components of the cell membranes, albeit somewhat scrambled in their orientation.

Despite the scrambling, Racker and his colleagues were able to reconstitute electron transport reactions and ATP synthesis of mitochondrial and chloroplast membranes. All living cells use some form of electron transport to generate energy, either from nutrients or, in the case of the green chloroplasts of plants, from sunlight. The energy is stored as ATP, the universal energy currency of life. Racker’s success meant that two key activities of living cells had been reassembled in the lab.

The point of this brief history is that relatively complex biological functions can be reconstituted by the self-assembly of their dispersed components.

Similar techniques were soon applied to other structures. For instance, Walther Stoeckenius of the University of California, San Francisco and Dieter Oesterhelt, now at the Max Planck Institute, reconstituted the proton pump of purple membranes isolated from a halophilic bacterial species that uses the energy of a proton gradient to synthesize ATP. In a remarkable collaboration, Racker and Stoeckenius then teamed up to produce a membranous system containing both the proton pump of halobacteria and the ATP synthase of mitochondria, and demonstrated that the hybrid membrane structures could synthesize ATP using light as an energy source.

This paper has often been cited as the publication that finally confirmed chemiosmotic synthesis of ATP, leading to a Nobel Prize in Chemistry for Peter Mitchell of the Glynn Research Laboratories in 1978. Mitchell in 1961 proposed that the primary function of electron transport was to pump up a hydrogen ion gradient across membranes, and that the gradient’s energy was used to drive the synthesis of ATP by the ATP synthase enzyme present in mitochondria, chloroplasts, and bacterial membranes. Mitchell’s chemiosmotic theory is now found in every college textbook of biochemistry.

Later, University of California, Los Angeles biochemist Paul Boyer was awarded the 1997 Nobel Prize in Chemistry for his proposal that the hydrogen ion gradient actually caused part of the synthase to spin at several thousand revolutions per minute as ATP was synthesized, a truly amazing discovery.

The point of this brief history is that relatively complex biological functions can be reconstituted by the self-assembly of their dispersed components. So why not try reconstituting a whole cell? If this turns out to be possible, it will help us untangle what we mean by “life.” Perhaps we might even elucidate the major steps that led to the origin of cellular life nearly four billion years ago.

Let’s first consider what might happen if we disassembled a microscopic living organism and tried to put the pieces back together. We would not use a nucleated cell like an amoeba or a human lymphocyte. Too complicated. Too many moving parts.

Instead we should use a much simpler form of life, such as a tiny bacterium called Mycoplasma, a kind of microbial parasite that requires a relatively rich nutrient environment. Only 482 genes are present in its genome, while more complex bacteria such as E. coli have over 4,000 genes. The human genome, by way of contrast, has nearly 30,000 genes at last count.

The cells of Mycoplasma are bounded by a naked membrane composed of a mixture of lipids, or fatty substances in the form of a bimolecular layer common to all membranes, with a variety of functional proteins and enzymes integrated into the bilayer. Some of the enzymes are responsible for extracting energy from nutrients and using the energy to synthesize ATP, while others are essential transporters of nutrients from the external medium into the cell.

Examples of nutrients are glucose, as an energy source, amino acids as building blocks for proteins, and phosphate for nucleic acids. The interior contents of the cell include a circular strand of DNA having genes responsible for synthesizing proteins required for metabolism. A variety of structural components are also present, including thousands of ribosomes, the molecular machines that synthesize proteins, and hundreds of soluble enzymes involved in metabolism.

What happens when we add a detergent to Mycoplasma? The detergent penetrates the lipid bilayer, which becomes unstable and breaks up into smaller particles containing lipid, membrane proteins, and the detergent molecules. With the membrane gone, the interior components are released. What we see visually is that the slightly turbid suspension of bacteria becomes clear, and if examined with a microscope no cells would be visible. The resulting solution contains all the components of the original living cell, but they have become diluted and disorganized.

Now we can try to reassemble them by injecting a certain amount of order back into the system. This is done by a process called dialysis, in which smaller molecules like detergents leak through a porous membrane while larger molecules remain behind. The same process is used to treat patients with kidney disease. As the detergent leaves, the lipids self-assemble into bilayers that take the form of small vesicles. The membranous boundaries of the vesicles incorporate most of the functional enzymes and transport proteins that were present in the bacterial cell membranes.

Each vesicle contains a random sample of the original contents of the bacterial cell, with one exception: The circle of DNA, the genome that was originally packed tightly into the original living cells, has unraveled and is too large to be captured when the vesicles reassemble. Although there are ways to do it, getting a large chunk of DNA into an artificial cell is going to be a problem for anyone attempting to fabricate synthetic life.

We and others have carried out such experiments. The figure below shows an electron micrograph of one of our preparations, not from Mycoplasma but rather from the lab rat of microbiology, a bacterium called E. coli. Several dark particles can be seen within each vesicle, and these are ribosomes—the ubiquitous protein synthesis machinery present in all living cells and absolutely necessary for all living things.

Ribosomes in reconstituted E. Coli. SOURCE: David Deamer.

Well, is it that easy? Did we reassemble something that is alive?

Several of my colleagues have carried out experiments that bear on this question, including Luigi Luisi at ETH Zurich, also known as The Swiss Federal Institute of Technology; the Yomo research group at Osaka University; and Vincent Noireaux and Albert Libchaber at Rockefeller University in New York. They found that if amino acids and an energy source are supplied to the ribosomes in the vesicles, along with a specific gene for a marker protein called GFP, or green fluorescent protein, then the vesicles begin to glow green when illuminated with UV light.

This means that the ribosomes are functional, and that the vesicles have one of the most fundamental properties of life—they can use genetic information to synthesize a protein. In other words, in a very limited sense, they can grow.

But are they alive?

The answer is no. Even though they can grow, only a single protein is produced, and everything else is left behind. To be truly alive, the vesicles need most if not all of their original 400-plus genes in the form of a DNA strand containing the genetic information required to direct the synthesis of hundreds of different proteins and RNA species, nearly half of which are the components of the ribosomes themselves.

What’s more, the vesicle would need genes for polymerase enzymes so that the DNA could be replicated as part of the growth process, as well as a way for lipid to be synthesized because the membranous boundary must grow to accommodate the internal growth. Transport proteins must also be synthesized and incorporated into the lipid bilayer, otherwise the vesicles have no access to external sources of nutrients and energy.

What’s more, a whole set of regulatory processes must be in place so that all of this growth is coordinated. Finally, when the vesicles grow to approximately twice their original size, there must be a way for them to divide into daughter cells that share the original genetic information.

This description makes it clear why we still don’t understand how life began on Earth. The complexity of a living system is wonderfully intricate, and there is still much to learn about simpler versions of this process that must have occurred as a precursor to life as we know it today, which represents several billion years of evolution.

There is a certain danger in what I just said. Why? Because creationists and proponents of intelligent design seize upon such statements as a scientist’s confession that it is hopeless to try to understand the origin of life, or to make synthetic life. If it is beyond our current understanding, they argue, there must have been a supreme intelligence at work.

I disagree, of course. The creationist viewpoint typically arises from a snapshot of scientific progress, and creationists revel in scientific admissions that there are things we don’t yet understand, thinking that this must somehow be embarrassing for the scientist. Creationists, of course, do know how it all happened: God did it. End of story.

Now we return to the question in the title of this essay: Should we do it?

Scientists are not satisfied with answers that represent a statement of faith, but want to keep on asking questions. We are much more optimistic about the chances that we will be able to use our knowledge of physics, chemistry, and biology to assemble a living system. The reason is that we don’t see just a snapshot of our current state of knowledge with its unanswered questions. Instead, we perceive a movie of progress over the past half century. We see how vast dark tangles of ignorance have given way to a deep understanding of life’s mechanisms.

Just for starters, we now know the sequence of three billion bases in the human genome, an achievement that was undreamed of when James Watson and Francis Crick established the double helix structure of DNA in the early 1950s. We can manipulate bacterial genomes at will, cause stem cells to develop into specific tissues, even produce clones of sheep, dogs, cats, and other mammalian species. Given the momentum of scientific progress, it seems entirely plausible that in the next 50 years we will be able to reassemble bacterial cells from a parts list, and perhaps even produce a new form of life, a second origin of life, but this time under laboratory conditions.

Now we return to the question in the title of this essay: Should we do it?

The public has watched and been astonished by the powers unleashed by scientists. Virtually every revolutionary advance in recent history can be traced back to a few individual scientists following a trail of intuitive ideas invisible to others, driven by a passionate curiosity about how things work. Marie Curie and Henri Becquerel gave us radium, uranium, and atomic energy. William Shockley and his colleagues, all Nobelists, demonstrated the odd electrical properties of silicon that led to a vast semiconductor industry; Alexander Fleming noticed a mold that inhibited bacterial growth, giving us penicillin, the concept of antibiotics, and the pharmaceutical industry.

So who is going to give us synthetic life? Craig Venter is trying. He and his colleagues at the J. Craig Venter Institute in Maryland have already sequenced Venter’s DNA, and he is the first human being in history to reveal his genome, all three billion base pairs, for public scrutiny. They have discovered the minimal number of genes required for a simple bacterium to grow and reproduce, and recently transferred a complete genome from one bacterial species to another—equivalent to a microscopic brain transplant.

Is this an ethical pursuit? Should there be laws against tinkering with life this way?

The immediate concern, of course, is the balance between valuable new knowledge and public safety. On the plus side, we might be able to design forms of artificial life that can make vast supplies of inexpensive useful products available. In a primitive way, we already do this. Biotechnology industry giant Genentech Inc., for example, uses genetically modified bacteria to make human insulin and other proteins. Indeed, most of the major pharmaceutical industries brew up vats of microorganisms to produce antibiotics.

Other companies, including Venter’s, are trying to grow specialized bacteria that can produce energy sources such as hydrogen gas. But these are only slightly modified versions of existing forms of life, and it is a costly and complex process to isolate a small amount of desired product from a mass of bacterial protein.

The goal of synthetic biology is to design a simplified version of life that produces the desired product with high efficiency and uses an abundant energy source such as sunlight, or cellulose derived from agricultural waste. So far, there are virtually no restrictions on this kind of research, nor should there be, in my judgment. But bioethical principles require us to look further, to weigh risks and benefits.

So there is no way to control attempts to produce artificial life.

What are the risks of learning how to make artificial life? Given the example of HIV and the AIDS epidemic, it’s not difficult to look ahead to potential risks. The HIV virus is an accident of nature, and we can trace its origin back to the consumption of bush meat–most likely chimpanzees–in Africa, then to one or a few infected individuals traveling to Haiti who initially spread the virus in the 1960s, and finally to the outbreak among the gay community and drug users in the early 1980s.

The public—the taxpayers who support most research—will wonder about the chances that an accident might happen in the laboratory. After all, the primary property of life is reproduction and evolution. What if we make an artificial organism that escapes, evolves, and reproduces, using the human body as a source of energy and nutrients? As often happens, writers of science fiction have examined such possibilities, and Michael Crichton describes one such scenario in his novel Prey.

Even worse than an accidental release, what about the possibility that a terrorist group will design a synthetic bacterium that is passed between human beings, multiplies in the gut, and produces botulin toxin? Or a virus like HIV that can be transmitted like influenza, rather than as a sexually transmitted disease. Just imagine the public outrage if the AIDS virus were discovered to be someone’s invention, rather than an accident of viral evolution in the wild. It can even be argued that the Department of Homeland Security should carry out research in synthetic biology in order to find ways to defend against such threats.

The bottom line is that there are always risks associated with every benefit, and that global human society is too complex ever to exert absolute control. An example is stem cell research. For political reasons, bowing to far-right Christian fundamentalists, the Bush administration banned the use of federal funding to develop new embryonic stem cell lines. South Korea and Japan simply took this as an opportunity to jump ahead in the race of discovery, and Californians voted to provide $3 billion of state taxpayers’ money to support such research.

So there is no way to control attempts to produce artificial life. All we can do is look ahead to the potential dangers and try to provide thoughtful guidance to researchers and policymakers so that they will consider possible dangers and be aware of public concern. Most research in this area is carried out using grants from federal agencies such as the National Institutes of Health, the National Sciences Foundation, and the National Aeronautics and Space Administration, and all proposals undergo peer review.

We must make this process as transparent as possible, and we must have open discussions of risks and benefits so that the referees can make informed decisions about the value of the research, and potential dangers.

In my judgment, the search for a method to reassemble life is still in its initial curiosity-driven phase. Today, there is no reason to do anything but cheer on the few scientists who are pioneers in this unexplored territory. Risks are low and acceptable, primarily because evolution has had several million years to throw viral and bacterial pathogens at human existence. Even though our immune systems have lost some battles, we are still winning the war.

I would expect that a laboratory version of artificial life, if released into the wild, would be like a helpless young mouse in a world filled with hungry and ruthless predators. But that’s my judgment call, and now it’s time to hear from everyone else. Should we do it?

David Deamer’s primary research area concerns the manner in which linear macromolecules traverse nanoscopic channels. A second line of research concerns molecular self-assembly processes related to the structure and function of biological membranes, and particularly the origin and evolution of membrane structure.

Tags: synbio

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