Stem Cells and Differentiation (Video 41) PDF

Video 41: Stem Cells and Differentiation, Part I Slide 3: Stem cell definition. Characteristics of stem cells: Can (potentially) continuously divide (self-renewal) Can differentiate into a variety of cells/tissues They are unspecialized (“blank slate”) Can divide/self-renew...

Video 41: Stem Cells and Differentiation, Part I Slide 3: Stem cell definition. Characteristics of stem cells: Can (potentially) continuously divide (self-renewal) Can differentiate into a variety of cells/tissues They are unspecialized (“blank slate”) Can divide/self-renew over long periods Can differentiate into more specialized cell types Adult stem cells are typically quiescent until needed, but some continuously proliferate, self-renew, differentiate, etc. (e.g., Lg5+ CBC cells in the intestine) Cellular differentiation is the process when cells change from one cell type to another, typically to a more specialized cell type. It is usually controlled by cell signaling pathways. In some contexts, dedifferentiation takes place; this is the process of change to a less specialized, more “stem-like” cell. Slide 4: Stem cell division and differentiation. On the schematic: A is a stem cell, B is a progenitor cell (a cell that has initiated differentiation), C is a differentiated cell. 1 is a symmetric stem cell division when a stem cell divides to form two new stem cells. 2 is an asymmetric stem cell division when the division results in a stem cell for maintenance and a cell that can differentiate. 3 is a progenitor division, which expands the progenitor cell population. 4 designates a terminal differentiation. Slide 5: Stem cell types Totipotent - each cell can form a new individual; for example, early human embryo (1-3 days) cells. Pluripotent - these cells can form any of over 200 cell types; example: embryonic stem cells [some human blastocyst (5-14 days) cells] Multipotent – these cells are somewhat differentiated, but can still form several tissues; examples: cells from fetal tissue, cord blood stem cells, and adult stem cells Unipotent: can only form only one lineage. Should these cells be classified as stem cells given their limited differentiation abilities? Examples are adult skin stem cells. We also differentiate professional vs. facultative stem cells. Professional designates permanent stem cells. The classic example is the Lg5+ CBC cells in the intestine. Facultative stem cells are the daughter cells that can dedifferentiate upon injury and become stem cells. A classic example is the dedifferentiation in the intestine. Slide 6: Classes of stem cells. Embryonic stem cells. For example, from an early embryo. These have a “blank slate” and can form many types of cells/tissues, typically these are pluripotent. Embryonic germ cells. These cells are derived from those parts of the (e.g., human) embryo/fetus that would eventually produce gametes (eggs or sperm). Adult stem cells. These are (relatively) undifferentiated cells that can be found among the more specialized differentiated cells in a tissue/organ (after birth). Adult stem cells typically exhibit a more restricted ability to produce various types of cells/tissues (multipotent, unipotent), as well as a somewhat more restricted ability to self-renew. Examples are skin, fat, bone marrow, intestine, brain, etc. Cord Blood Stem Cells (typically multipotent) Induced Pluripotent Stem Cells (artificial). These are adult cells made pluripotent, and functionally equivalent to embryonic stem cells. Slide 7: Induced Pluripotent Stem Cells are important since with them we can bypass the need to use embryonic stem cells. The use of embryonic stem cells is controversial because of ethical, social, and political reasons. Thus, we create pluripotent stem cells by introducing stemness-related gene expression into adult cells. Here is the scheme for the generation of induced pluripotent stem (IPS) cells: 1. Isolate and culture donor cells 2. Transduce stem cell-associated genes into the cells by viral vectors. Red cells indicate the cells express the exogenous genes 3. Harvest and culture the cells according to ES cell culture, using mitotically inactivated feeder cells (light gray) 4. A small subset of the transfected cells become IPS cells and generate ES-like colonies Slide 8: IPS advantages: bypass the controversies of using embryonic stem cells have pluripotent differentiation potential However, the process requires optimization (therefore, these are not yet a substitute for embryonic stem cells) Slide 9: The stem and progeny cell hierarchy. As stem cells begin the pathway to differentiation, at each step they become more differentiated and lose stemness and potency: this describes going to a common progenitor, a more differentiated restricted progenitor, and then to fully differentiated mature cells. Slide 10: The general model of adult stem cells. Typically, most adult multipotent stem cells are quiescent. Upon some sort of stimulus, the cells begin the process of differentiation to create more mature cells. You have the poised state when the transit stem cell prepares for differentiation. Upon cell cycle entry, this cell becomes an active stem cell, which can undergo asymmetric cell division to form another stem cell and a progenitor. The progenitor can differentiate into a mature cell. Dedifferentiation is also possible, when a progenitor becomes less differentiated, gains stemness, and transitions into the original multipotent stem cell. Slide 11: In the intestinal epithelium, the default situation is that of actively proliferating stem cells continuously replenishing the dying differentiated cells (i.e., the shedding cells). Slide 12: Let us consider an intestinal crypt. Mature cell types found in the intestine are listed at the top right. There are two types of stem cells. At the bottom of the crypt, near the stem cell niche are the CBC stem cells. These are proliferative, damage-sensitive, Lgr5-positive, AScl2-positive, Olfm4-positive. If these cells are lost, they can be replaced by +4 cells/de- differentiating TA (transit-amplifying) cells. The CBC central cells have a survival advantage (see the diagram). The CBC cells are probably the cells giving rise to colorectal cancer. Thus, the CBC cells are the actively dividing intestinal stem cells that proliferate and differentiate. Higher up in the crypt are the +4 stem cells. These are likely quiescent and damage-resistant. Characterized by the expression of BMI1, Hopx, and Tert. These cells may be quiescent secretory precursors that are recruited to become CBC stem cells upon damage to the intestine (functioning as a “reserve”). They are not stem cells in the strict sense and are short-lived under normal conditions. As the CBC stem cells differentiate, they move up the crypt, becoming proliferating transit-amplifying (TA) cells (also called progenitors). The TA cells differentiate into other cell types as they move up. Only the Paneth cells, found in the small, but not large, intestine, move down as they fully differentiate. The CBC stem cells also replace themselves through asymmetric proliferation that gives rise to more stem cells and progenitors. Slide 13: Intestinal Signaling. The action of stem cell proliferation and differentiation in the intestine is controlled by cell signaling. Wnt signaling and Bmp antagonists are higher at the bottom of the crypt and decrease going up; these tend to promote stem cell self-renewal. BMP, Hedgehog, and Hippo signaling increase going up the crypt and they promote differentiation. Notch signaling also helps control differentiation. Slide 14: Notch signaling In TA Compartment helps control intestinal cell differentiation via lateral inhibition. Notch-Dll interactions affect signaling in each cell. The increased expression of HES1 and decreased expression of Atoh1 and Dll in the Notch-high cells promote differentiation to the absorptive cell lineage. This reinforces the cell fate as a Notch-high and Dll-low cell). In the Notch-low cells, HES1 expression is decreased, leading to increased Atoh1 expression, which in turn upregulates Dll expression. This reinforces the Notch-low, Dll-high phenotype and promotes differentiation to a secretory cell lineage. TA compartment: transit-amplifying cell compartment Slide 15: Fundamental concepts. Stem cells have defined characteristics and are specific types. Stem cell types have different levels of potency. Stem cells are in a hierarchy leading to differentiation. Induced pluripotent stem cells have the potential to replace embryonic stem cells for experimentation and therapy. The general model of the stem cell lifecycle can be compared to the different example of the intestine. intestinal stem cells have their unique characteristics, cell signaling controls, and differentiation pathways.