The annual ISSCR meeting took place in Boston from the 14-17 June 2017. As usual, it was well attended (around 4,000 participants) and featured a broad range of stem cell related topics. There has been an explosion in the types and use of stem cell-derived organoids, perhaps reflecting the realization that multiple cell types organized in 3-D are essential for functional insight. Worryingly (at least to me) the areas of disease modeling and cell therapy do not seem to have yet generated as much clinical traction as anticipated and I fear that in the future, reports on the clinical contributions of stem cells may migrate, as bit players, to conferences dedicated to specific disease groups. I think this would be damaging to the ethos behind the establishment and function of the ISSCR.
There seemed increased emphasis on skeletal muscle satellite cells (SatC). Human SatC are of great interest to Genea Biocells (GBC) because of their alleged participation in the pathology of many muscle-related diseases and their transient appearance during the in vitro differentiation of myotubes from human pluripotential stem cells (hPSC). So far, it has not proved possible to capture and expand populations of SatC whilst maintaining them in a pristine state such that they can engraft into and sustain, long term, skeletal muscle integrity. Many attempts have been made, but resulting populations have not demonstrated long term repopulating potential. Perhaps the fault lies in our failure to recognize the precise nature and in vitro instability of that “pristine” state . Several presentations during ISSCR 2017 used mouse muscle to circle this issue: Tom Rando’s (Stanford University, USA) presentation focused on the differences between mouse Sat C populations isolated from young and old animals; his previous pioneering work, using parabiosis of young and old animals, had shown that aged muscle could be rejuvenated by factors present in the circulation of younger animals. The transcriptomes of young and old SatC populations were found to be very similar before parabiosis and even more tightly aligned after parabiosis. He then asked the question-does the SatC isolation procedure affect the observed transcriptome? The answer was a resounding “yes”. He used two procedures to identify the transcriptome present prior to isolation. One method involved the labeling of nascent SatC RNA prior to isolation; the other method used chemical fixing of the cells prior to isolation. Both methods revealed extensive differences between the cell transcriptomes before and after isolation. Almada (Harvard University, USA) performed a time course of transcription in murine pax7+myoD+ satellite cells and found that the Fos gene was transiently turned on during the isolation-triggered activation of these cells. This expression appeared essential to the expansion of the cells in vitro. The Fos expression is lost during culture, but subsequent virally-mediated reintroduction of an active fos gene stimulates further cell proliferation. He also pointed out that in vivo, Fos expression is seen in some SatC following muscle injury. It was not clear from his talk whether the loss of Fos expression during in vitro culture simultaneously affected the long term viability of the cells post transplantation or only their proliferation in vitro. In addition, it is not known whether the condition of his SatC cells was impacted by the other transcriptional changes seen by Rando.
The amelioration of SatC potency seen after isolation has been attributed by others to suboptimal culture conditions. Blau (Stanford University, USA) has attributed one aspect of this shortcoming to the rigidity of standard culture matrices like plastic. In her talk, she followed up her published work on the superior qualities of 12 KPascal hydrogels as a support matrix for SatC growth, by reporting that p38MAP kinase inhibitors can rescue the engraftment of old murine SatC so long as these are cultured on hydrogels. Mourikis (IMRB Paris, France) noted that murine SatC cells in situ were in a quiescent state and that Notch expression was critical for this. In a screen for Notch targets, he reported that one target, collagen V, a1 , stabilized SatC in culture when it was added as a soluble factor, and that it performed this stabilization by binding to calcitonin receptors on the SatC cells. Like Rando, he also claimed a radical change in the transcriptome of Sat C cells wrought by the isolation procedure.
The take home message from all these talks is that, unsurprisingly, isolation wrecks the SatC niche and provokes many transcriptional changes. Whether supplements to the culture media or supporting matrix can “rescue” these changes and re-establish a supportive niche remains unclear. Ideally, such optimized conditions would allow for the in vitro expansion of the SatC population followed by a reversion to the quiescent state; alternatively, the isolation-induced loss of the pristine state may be irreversible.
Organoids. Organoids, Organoids!!!
PSC under certain conditions will form embryoid bodies (EBs), which then can differentiate into multiple cell types. However, these EBs differ from early embryos in the disorganized nature of the resulting tissues. Nevertheless, EBs have provided valuable information on the interactions of different cell types in organogenesis. Not to be outdone, the incoming President of the ISSCR, Hans Clevers (Hubrecht, Utrecht, Netherlands) , has pioneered a cottage industry which uses adult stem cells to generate an EB-like mass of cells arranged in a disorganized but organ-like array, termed organoid. He started this work with the Lgr5+ intestinal stem cells that are found in the crypts at the base of the intestinal villi. However, this original finding has spawned a host of different organoids, including stomach, liver lung and cerebral types. These organoids are being used to interrogate differentiation cues, drug testing and in vivo tissue and organ repair. As well as reviewing these developments, Clevers also reconciled some controversial aspects of intestinal stem cell biology by reporting that the enterocyte (a differentiated intestinal villus cell that originates from the stem cell) can revert to the stem cell state under specific conditions (e.g. when the stem cells are killed). Such examples of reprogramming in vivo are increasing in frequency.
Linus Pauling revisited
Linus Pauling was famous for his scientific prowess with regard to protein structure and also infamous for his pronouncements on dietary supplements. I remember well his endorsement of the merits of taking prodigious quantities of vitamin C. He recommended doses of between 1-40gm per person per day as opposed to the recommended Daily allowance (RDA) of 60mg per person per day!! He also advised that Vitamin C could prevent cancer. There was always no doubt that taking vitamin C was important, after all its ingestion is essential to avoid scurvy. However, many professionals criticized his cancer advice. An interesting talk by Sean Morrison (University of Texas, SouthWestern, USA) gives some credence to an anti-tumor contribution by Vitamin C although dose was not mentioned. Sean began by describing efforts to scale down metabolomics studies in order to measure specific metabolite concentrations in rare cell populations within tissues. His methods allowed the assessment of 60 different metabolites in 104 FACS-sorted cells fixed in methanol. He looked in particular at cell populations representing different stages of erythroid and myeloid development. He reports that hemopoietic cells are particularly high in ascorbate (Vit C) presumably because of high levels of ascorbate receptors on their cell membranes. The depletion of ascorbate in knockout mice unable to make their own ascorbate, leads to expansion of the HSC compartment and an increase in the functionality of these cells. This is the hallmark of the loss of a tumor-suppressor gene. He conjectures that loss of ascorbate limits the activity of the Tet 2 protein and by increasing HSC proliferation, promotes leukemogenesis (i.e. a blood cancer). Dietary ascorbate can rescue the Tet 2 activity in the knockout mice. He notes that ascorbate concentrations vary 10-fold amongst humans and that mutation of Tet 2 activity is relatively common in older people. So maybe Linus Pauling was not completely wrong after all! Take your daily Vit C tablets.
Alan Colman 21 June 2017