Why do we need organoids?
Organs from a test tube
Alexandra Bruter, <url>
In the popular science section of the journal Nature, a review article was recently published on the topic of so-called organoids – primitive miniature analogues of real organs grown in the laboratory in test tubes (Cassandra Willyard, The boom in mini stomachs, brains, breasts, toddlers and more).
To be honest, there is no hope that such organoids will solve the problems of deficiency and tissue compatibility of donor organs in the near future. Attempts at therapeutic use of organoids are isolated and, except for one case, which we will talk about later, are limited to the transplantation of individual cells, sometimes tissues, and not organoids as a whole.
A significant part of the research using organoids is carried out for fundamental purposes, the rest – to study the effects of drugs. Many medicines act selectively on people: they act on some, they do not act on others. The reasons for this are not always known, they probably lie in the genetic differences that have not yet been identified. By creating an organoid corresponding to the disease from the patient's own cells (this usually takes 2-3 weeks), you can find out which drugs will be effective. In addition, the toxicity of drugs can be tested on organoids. Usually, before switching to humans, drugs are tested on animals whose metabolism may differ from that of humans. The results of experiments on organoids from human cells will have a better predictive ability and will be more ethical.
In some cases, organoids were formed as a by-product of experiments, researchers discovered this and began to look for applications.
In fact, there is nothing incomprehensible in the fact that organoids are formed by themselves. The human body, and the body of a representative of any other species, develops from a fertilized egg without prompting from the outside. This means that from the very beginning it contains all the information necessary for development. The mother's body provides the developing fetus with oxygen, food, partially protects against infections and helps to dispose of waste, but the embryo development plan is an integral part of the embryo itself. Some teratogenic substances, for example, thalidomide or retinoic acid, if taken by a pregnant woman, can disrupt this development plan and cause fetal abnormalities. But in general, if the embryo is not disturbed by teratogens, fed in time, supplied with oxygen and excreted waste products, it copes on its own.
The question is how the information controlling the development of the embryo is implemented, and whether we can decipher it so accurately and completely that we can grow the necessary organ from embryonic stem cells. Thanks to the technology of obtaining iPS (induced pluripotent stem) cells for each person, it is possible to individually obtain cells that will differ slightly in their properties from the cells of the embryo at the earliest stages of its development, when a separate organism can grow from each cell. If the signals governing embryogenesis were decoded, fully compatible organs could be grown from such cells for transplantation.
Decoding the signals that control embryogenesis is still in its infancy. But it is already clear that not all of these signals originate from inside the cell, whose fate is determined ("On the 15th day of development, turn on gene A. In three days, turn off and turn on gene B"). Some of the signals are chemical signals from neighboring cells ("If cell B is next to you, and it secretes protein G outwards, turn into cell D") and mechanical signals from the environment ("If the surface tension has reached the E value, turn into cell G"). It turns out that all tissues and organs develop interconnected, and it is difficult to grow any organ separately.
Another snag lies in the fact that mechanical signals have long remained undervalued. Standard laboratory practice is to grow cells attached to a special adhesive plastic. The cells spread out and form a layer one cell thick on the surface of the cup. These conditions are extremely far from those in which the cells are located inside the embryo that has begun crushing. In the embryo, the shape of the cells is closer to spherical and it has the same neighbors on all sides, forming dense contacts with it.
In 2011, Madeleine Lancaster managed to create a miniature brain in a laboratory in Vienna, although she did not plan it at all. She experimented with stem nerve cells that were supposed to grow in an attached state. For some reason, the cells did not want to attach (this can happen, for example, if you accidentally mix up the same-looking Petri dishes made of adhesive and non-adhesive plastic) and began to form spheres. The researcher suspected something was wrong when dark dots began to appear on the sides of the spheres. The fact is that at a certain stage of brain development, the rudiments of the eyes with retinal pigment cells begin to form on its sides by themselves. Taking a closer look at the spheres, the laboratory staff found that they contain neurons of various types and in general are very similar to the brain in the early stages of embryonic development.
Surprisingly, the cells organized themselves completely independently, as soon as they stopped being forced into a flattened state attached to the bottom. This, however, is not the most common case. Usually, you still have to select chemical factors (more often proteins, but not always) and apply them in a strictly defined sequence to get an organoid.
In other, mainly Japanese, studies, it was possible to separately create organoids resembling fragments of the nervous system: the cerebral cortex, the pituitary gland and the rudiment of the eye. The latter two may have therapeutic applications: to serve as a source of cells for transplantation to people with pituitary insufficiency and with retinopathy.
Scientists from Utrecht first discovered intestinal stem cells, and then with their help managed to create a mini-intestine. To do this, we also had to abandon the idea of cultivating cells in an attached state. Choosing optimal cultivation conditions, the scientists decided to grow them in matrigel, a soft jelly in which it is easy for cells to maintain a three–dimensional structure. This led to success: the cells began to form hollow spheres consisting of cells of different types, and inside they formed outgrowths similar to ordinary intestinal villi.
These spherical mini-intestines were adapted in order to test the effectiveness of the drug for patients with cystic fibrosis. In such patients, due to a malfunction of the ion channels, water is constantly distilled from the intercellular space into the cells, and the mucus filling the intercellular space becomes very viscous because of this. This phenomenon affects, first of all, the lungs and intestines. If you grow a mini-intestine from the cells of a cystic fibrosis patient, then water does not penetrate into the hollow intestine-sphere. If you add a drug effective for unblocking channels to the cultivated organoids, then water begins to penetrate inside, and the organoids swell. The increase in size is easy to notice. The researchers were able to test their observations on 100 patients and 6 medications. Some patients managed to find a really effective therapy.
Among other mini-organs, kidneys attract special attention. Taking on many blows, they often fail, and donor organs are in huge short supply and often poorly compatible. At the same time, the kidney is one of the most complex organs. They consist of 25-30 types of different cells and have a complex three-dimensional structure. In addition, the embryonic development of the kidneys is very difficult – a person is born with a third kidney, and the first two pairs of kidneys and ureters simply disappear in embryogenesis or turn into other organs.
Melissa Little managed to grow something vaguely resembling an embryonic kidney in her laboratory, consisting of nephrons and cells forming the ureter. She hopes that after some improvement, such a kidney will be able to be transplanted into mice. But they may have other uses. Such mini-kidneys can be useful to test drugs for nephrotoxicity (a common side effect). From some points of view, animal experiments are better in this sense, because they allow us to study the effect of drugs on different body systems, but mini-organs can be created from human cells, and the difference between humans and animals can be critical in some molecular biological issues.
One of the main successes of the idea of mini-organs today is associated with liver organoids. The liver also often fails, and there are also not enough donor organs. Although the liver is relatively homogeneous in its cellular composition, cell therapy is complicated by the fact that hepatocytes – adult liver cells – refuse to live and multiply in the laboratory. During embryonic development, the formation of the liver requires signals from neighboring cells of both mesodermal (middle germ leaf, forms muscles, bones and internal organs consisting of smooth muscles) origin, and ectodermal (outer germ leaf, forms skin cells, epithelium of internal organs and nerve tissue) origin. The liver itself is of endodermal origin. By adding these two types of cells to the hepatoblasts, scientists were able to make liver organoids the size of the liver of a six-week-old embryo form. But it turned out that if you introduce such organoids in large quantities into the failing liver of an adult animal, then its function is restored. The authors of the study hope to move on to human trials soon.
In addition to the organs mentioned here, we have already managed to get a mini-prostate, a mini-lung, and even a prototype of a breast. Restoring liver function with the help of appropriate organoids seems to be a fairly realistic task, but the liver is a relatively homogeneous organ, and there are few complex spatial structures in it. From the point of view of replacing a kidney or, for example, a heart – organs whose three-dimensional structure is very important, the technology of a decellularized (cell-free) matrix seems more promising. This technology consists in the fact that donor cells are washed out of the donor organ and only a framework of intercellular matter remains, which is populated by recipient cells. This intermediate technology allows you to preserve the three-dimensional structure and avoid rejection of the new organ by the immune system.
For organoids, other applications related to personalized medicine and drug screening are quite possible. In addition, they make an invaluable contribution to fundamental embryology, allowing you to determine the set of factors needed for the development of each organ and tissue.
Portal "Eternal youth" http://vechnayamolodost.ru
11.08.2015