In Vivo Veritas
Message from Director Irv Weissman
In vivo veritas
How can we know whether our discoveries in stem cell research and regenerative medicine are on the right track? How can we know whether our discoveries will reveal how, in normal circumstances, the body maintains each tissue and organ optimally and how, in disease, tissues and organs embark on a path toward the disease?
In our field, these questions revolve around the stem cells that maintain and regenerate tissues in the bodyand how stem cells involved in disease (for example leukemia stem cells)actually develop and act to cause the disease--also in the body.
In my life in biomedical science at Stanford, I have witnessed the huge growth of fundamental discoveries coming from advances in molecular biology and immunology. These discoveries have provided ways to approach our science to allow these ‘Balkan states,’ which formerly spoke in scientific jargon only familiar to the few within their small discipline, to now speak to each other in common languages. My own lab had to develop the technique of tissue-section cell-surface immunohistochemistry to bring the knowledge of cell types studied mainly in test tubes to fundamental inquiries of organ and tissue ‘geography’. Now everyone seems to have moved this to the level of single cell ‘omics’ --called spatial transcriptomics or proteomics--wherein a wide variety of different cell types are mapped into the highly organized and often dynamic structures of tissues and organs in the body. What was formerly simply memorized is now becoming organized and understood by function or cell-cell interactions. Molecular biologists and geneticists now put up slides of a lymph node,lung or even parts of the brain and provide ideas how structure and function relate. And usually they also put up other maps of single-cell RNA sequences from those organs, and outline not only single cell states, but also the trajectories of cell differentiation from and between those states.
Where stem cell biology comes in is through the recognition that each stem cell is the irreducible unit of function in these biological systems. Stem cells, by maintaining organs and tissues through self-renewal and differentiation, create a dynamic system in which trajectories are tested directly, not declared by a static measurement of spatial ‘omics displayed as Umaps and Leiden Maps, etc.
The danger of these declared trajectories is that morphologies and their overinterpretation can lead to mistakes when those ideas are not test by cell lineage analyses---whether by transplantation or lineage tracing.
When I was a Stanford medical student spending the allowed research time between clinical years, I worked with Jim Gowans at Oxford. There, one day, I attended a lecture by a famous histologist who showed a Giemsa-stained lymphocyte, then on the next slide showed another cell almost indistinguishable from the first, then another cell similar to those two, and so on. He ended with neurons, and declared he had shown the trajectory of the origin of the brain from lymphocytes. In contrast to this method, Gowans had labeled pure lymphocytes from the thoracic duct and reinfused them into a vein at the physiological rate at which the duct naturally empties. He found the lymphocytes made immune responses, not thoughts. Similarly, I had shown that locally labeled cells in the thymus took particular emigration routes through the blood to regions of lymph nodes, spleen, gut associated lymphoid organs, etc. --but not the brain.
We had learned the principle of in vivo veritas—“in life, truth” (a play, of course on the famous latin adage in vino veritas—“in wine, truth”)., What you study in cell cultures or tissues must eventually be shown to operate in the body.
Genetics can reveal what can happen and what genes are involved, physiology how things happen normally, and pathology what happens when things go wrong. And in all of these, when things go wrong, the best description of diseases in humans provides a framework for understanding the underlying events leading to these diseases. This disease is trying to tell us how things happen, not how things might happen.
Not too many years ago, Shinya Yamanaka intuited that perhaps genes expressed in embryonic stem cells—themselves representative of the pre-implantation blastocyst’s inner cell mass—can carry the specification of pluripotency. Amazingly, he proved it with just 4 genes retrovirally transferred first into fibroblasts reprogramed them into pluripotent ES like cells. Briggs and King and Gurdon and Wilmut had provided the precedent that somatic cell nuclei placed into an enucleated egg could similarly be reprogrammed to pluripotency.
But neither gene transfer nor nuclear transfer were seemingly representative of anything happening normally in the body. It has puzzled me how Yamanaka’s seminal discovery could exist as an in vitro and invented phenomenon without also happening naturally in the body. But now Joanna Wysocka and her group have shown that Yamanaka had fortunately landed at genes whose expression does induce pluripotency in every vertebrate body. Wysocka and her group showed that these genes reprogram neural ectoderm to become another basic germ layer, mesoderm, in the generation of neural crest stem cells that make the underlying structures of the face and headThese experiments could not have been done without all of the tools I described above, but it solidified for me that I needed to follow the principle in vivo veritas to accept a process as not only possible to happen, but as actually happening in the body.
Joanna Wysocka, PhD
For Friends of the Institute, I apologize for this long prelude to Wysocka’s breakthrough discovery. I wanted you to know that even disparate steps to make an advance can eventually be understood by knowing that the living organism is the final step and can still be the valid benchmark for whether the advance (discovery) will have reproducibility, and eventually lead to clinical translations for the benefit of patients. For the students and trainees, I hope the examples I have described that lead up to the decisive experiments in the Wysocka et al paper will remind you also that in vivo veritas can help guide you in your own careers.
--Irv weissman