2024 Stem Cells, Cell Asymmetry, and Cell Death Notes (BIOL 374)

Burman University, BIOL 374 Cellular Biology, Stem Cells Stem Cells, Cell Asymmetry, and Cell Death (Topic 9) Learning Objectives: After completing this module, you will be able to 1. Define the following terms: apoptosis, apoptosome, asymmetric vs symmetric cell division, Bcl-2 family, ca...

Burman University, BIOL 374 Cellular Biology, Stem Cells Stem Cells, Cell Asymmetry, and Cell Death (Topic 9) Learning Objectives: After completing this module, you will be able to 1. Define the following terms: apoptosis, apoptosome, asymmetric vs symmetric cell division, Bcl-2 family, caspases, cell fate, cell lineage, embryonic stem cells, iPS cells, multipotent, pluripotent, progenitor cells, stem cell, stem-cell niche survival signals and totipotent. 2. Differentiate the different categories of stem cells used in biological research and therapy. 3. Discuss the key stem cells in the intestinal stem-cell niche. 4. Compare apoptosis to necrosis. Text: Lodish et al. (2021), Ch. 22 Verse: The sons of Noah who came out of the ark were Shem, Ham and Japheth. (Ham was the father of Canaan.) These were the three sons of Noah, and from them came the people who were scattered over the whole earth. Gen 9:18-19 (NIV) Are you affected by increasing divisiveness, whether it is political leanings, or races, or nationalities, or gender, or religion (Christian/Muslim), or vaccinated/unvaccinated, that exists in this world? The Bible reminds us that we are all related – brothers and sisters, family – with the same lineage. Let us do our part to break down these divisive barriers. Be kind and love one another! 9.1 Cell lineage § Developmental history of a cell from its birth until its final division and differentiation into a particular cell type is known as its cell fate. § A cell lineage traces the birth order of cells as they progressively become more restricted in their developmental potential and differentiate into specialized cell types [F22-1c]. § e.g., cell lineage in the nematode worm C. elegans is well defined for all cells of this organism (F22-26). LW2024 Burman University, BIOL 374 Cellular Biology, Stem Cells § Cell fate in the embryo can be determined by cell location experiments in a labelled cell and followed during development [F22-26bc]. 9.2 Stem cells § Unspecialized cells that can reproduce themselves (i.e., self-renewing) as well as generate specific types of more specialized cells are called stem cells (F22-11). § There are four main types of stem cells as categorized by the researchers working with them: 1. Adult stem cells are stem cells that are derived from a stem cell that is present in adult tissue. 2. Fetal stem cells are stem cells that are taken from fetuses; these are more powerful and have more differential potential then adult stem cells. 3. Embryonic stem cells are taken from blastocyst, an early-stage embryo usually at 4- 5 days after fertilization. Embryonic stem cells have the most potential for differentiation amongst them all. 4. Induced stem cells (or induced pluripotent stem cells; iPS) are adult cells that have been reprogrammed to give rise to stem cell capabilities. § Totipotent cells [F22-2] - Human embryos pass through an 8-cell stage in which each cell can still form every tissue, both embryonic and extraembryonic (e.g., placental tissue). - These 8 cells are totipotent. § ES cells - At 16-cell stage, some of the cells are committed to particular differentiation paths, hence are no longer totipotent. - At the blastocysts stage, a distinctive inner cell mass becomes separated from other cells. These inner cell mass become embryonic stem cells (ES cells) that can generate all embryonic tissues, but not extraembryonic tissues (i.e., placental tissues). - These ES cells are pluripotent. Pluripotent cells can give rise to all the cell types that make up the body. - The pluripotency of ES cells is controlled by multiple factors: [F22-5] 1. State of DNA methylation 2. Chromatin regulators 3. Certain miRNAs 4. Transcription factors like Oct4, Sox2 and Nanog. - ES cells can be grown indefinitely in culture (in defined media on a bed of fibroblast cells that nourish the stem cells by providing specific growth factors), LW2024 Burman University, BIOL 374 Cellular Biology, Stem Cells where they can divide symmetrically, so that each daughter cell remains pluripotent and can potentially give rise to all the tissues of an animal [F22-4a]. § Adult stem cells - Most cells have lifespans much shorter than that of the organism and so need to be constantly replaced, e.g., cells lining stomach and some immune cells live only a few days. - Adult stem cells are thus important in the replacement of these cells. - Unlike ES cells, adult stem cells are multipotent (except germ- line stem cells that are unipotent). - Multipotent cells can develop into more than one cell type but are more limited than pluripotent cells because they cannot generate all cell types. - The number of stem cells of a particular type stays constant (F22-12a) or increases (F22-12b) during an organism’s lifetime. - Other daughter cells, termed transiently amplifying cells, divide rapidly, and undergo limited numbers of self-renewal divisions, but ultimately produce lineage-restricted progenitor cells F22-11). § Hematopoietic stem cells replenish all the necessary blood cells. Progenitors and precursors proliferate under the control of distinct cytokines [F22-18]. LW2024 Burman University, BIOL 374 Cellular Biology, Stem Cells § Intestinal stem cells continuously generate all the cells of the intestinal epithelium. - The most abundant epithelial cells, the absorptive enterocytes, transport nutrients essential for survival from the intestinal lumen into the body. - The intestinal epithelium is the most rapidly self-renewing tissue in adult mammals, turning over every 5 days (in humans 300 million intestinal epithelium cells are lost daily). - The most differentiated cells reside at the tips of the villus, and are regenerated when damaged, sloughed, or aged. These cells are continually regenerated from a stem-cell population located deep in the intestinal wall in pits called crypts (F22-14). - Stem cells are formed in niches that provide signals to maintain a self-renewing population of undifferentiated stem cells. - The niche must maintain stem cells without allowing their excess proliferation and must block differentiation (via specific controls that involve signaling pathways, e.g., Wnt pathway in intestinal niches). - Intestinal stem cells reside at the base of the crypts express the Lgr5 receptor. Lgr5+ cells are the precursors of the transit amplifying cells, the terminal differentiating cells and several differentiated epithelial cells in the villus. - Lgr5+ stem cells begin to differentiate as they are pushed out of the niche at the bottom of the crypt when the Wnt signal is reduced. - If the Notch signal is present, Lgr5+ cells differentiate towards absorptive enterocytes; otherwise, they become secretory cells (Notch off). - These Lgr5+ cells are adjacent to the Paneth cells, which form part of the niche. Paneth cells secrete antimicrobial defense proteins. They are produced from the transit amplifying cells every 3-6 weeks. - The +4 “reserve” stem cells (which occupies the 4th position from the crypt base) can restore the Lgr5+ stem cells following injury. They are also generated from the Lgr5+ cells (F22-14b). - Telocytes are thin cells with long protrusions that form a 3-dimensional network underlying the epithelial cells throughout the intestines and are the major source of growth factors (Wnt, Notch, etc) necessary for stem-cell self-renewal. - Enteroendocrine cells are specialized cells that produce and release hormones in response to several stimuli. - Tuft cells, sometimes referred to as brush cells, are chemosensory cells in the epithelial lining of the intestines and respiratory tract. LW2024 Burman University, BIOL 374 Cellular Biology, Stem Cells - The main role of goblet cells is to secrete mucus to protect the mucous membranes. - M cells are highly specialized immune cells that take up intestinal microbial antigens. § iPS - Induced pluripotent stem (iPS) cells can be derived from somatic cells by the expression of combinations of key transcription factors, including KLF4, Sox2, Oct4 and Myc. - Differentiated cells produced in culture from human iPS cells can be used to understand the underlying cause of a disease as well as to screen drugs that can be used to treat the disease, e.g., ALS (F22-7). - b islet cells produced in culture from human iPS cells secrete insulin normally in response to an elevation of glucose in the media and reverse the high glucose levels in diabetic mice. 9.3 Cell Polarity and Asymmetric Cell Division § Symmetric cell divisions [F22-1a] - Many yeasts, fungi and other single-celled eukaryotes undergo symmetric cell division, where division gives rise to two daughter cells that look and function exactly like the parent cell. - Mature hepatocytes also divide symmetrically. § Asymmetric cell divisions [F22-1b] - In complex multicellular plants and animals, distinct developmental or environment signals result in daughter cells inheriting different components (e.g., mRNA, proteins) from the parental cells despite receiving the same genes. - Daughter cells may differ in size, shape, or protein composition, or their genes may be in different states of activity or potential activity. - Asymmetric cell division requires that the parental cell to become asymmetric, or polarized before cell division, so that the cell contents are unequally distributed to daughter cells. - Cells have an intrinsic program that can generate polarity using feedback loops. LW2024 Burman University, BIOL 374 Cellular Biology, Stem Cells § General steps in generating polarized cells: 1. To know in which orientation to polarize, cells must be exposed to a spatial clue. 2. Receptors (or other mechanisms) to sense the clue. 3. Signal transduction pathway initiated 4. to regulate the cytoskeleton (microtubules and/or microfilaments) to reorganize in the appropriated polarized manner. 5. Polarized cytoskeleton provides framework for the transport of fate/cell polarity determinants. 6. Reinforcement of cell polarity determinants (F22-24a). § Cell polarity determinants - Cell polarity requires specific determinants, including mRNAs, proteins, and lipids, to be asymmetrically localized in a cell. - If the mitotic spindle is positioned so that these determinants are segregated during cell division, the two daughter cells will have different cell fate determinants (F22-24b). - Otherwise, if mitotic spindle is not orientated appropriately, the determinants will not be segregates properly, and the daughter cells could have the same fate. 9.4 Cell death § Vertebrate cells require trophic factors (or survival signals) to survive. In the absence of these factors, cells commit suicide via a programmed cell death called apoptosis. § Conserved apoptotic pathways include 3 components - membrane-bound regulatory proteins, cytosolic regulatory proteins, and apoptotic proteases (called caspases in vertebrates). § Once activated, caspases cleave specific intracellular substrates, leading to the demise of the cell. § Other proteins, like CED-4 and Apaf-1, that bind the cytoplasmic regulatory proteins and caspases are required for caspase activation. § Apoptotic cells shrink, condense, and then fragment, releasing small membrane-bound apoptotic bodies, which are engulfed by neighbouring phagocytic cells (F22-34a). Unlike necrotic cells that swell and burst, releasing their intracellular content, which can damage surrounding cells and frequently cause inflammation. LW2024 Burman University, BIOL 374 Cellular Biology, Stem Cells § The Bcl-2 family contains pro- and anti-apoptotic proteins, where most are transmembrane proteins [F22-41]. § In mammals, apoptosis can be triggered by oligomerization of Bax or Bak proteins. These pro-apoptotic members of the Bcl-2 family form channels in the outer mitochondrial membrane, leading to efflux of cytochrome c and SMAC/DIABLO proteins to the cytosol. § These proteins then promote caspase activation and cell death (F22-36b, 22-42). § Bcl-2 (and Bcl-xL) can restrain the oligomerization of Bax and Bak by binding to them, thus blocking cell death. § Pro-apoptotic BH3-only proteins (e.g., Puma, Bad, Bim) activated by environmental stress bind directly to Bcl-2, causing Bax and Bak to dissociate from Bcl-2. This stimulates the oligomerization of Bax and Bak, allowing cytochrome c to escape into the cytosol. In the cytoplasm cytochrome c binds to Apaf-1 activating caspases (F22-42). § Direct interactions between pro- and anti-apoptotic proteins lead to cell death: 1. Binding of extracellular trophic factors can trigger changes in these interactions, resulting in cell survival (F22-42, step 1). A survival signal generated by trophic factor binding to their receptors trigger a phosphorylation relay that leads to the phosphorylation of Bad. Bad is sequesterd by cytosolic 14-3-3 protein upon phosphorylation and unable to bind Bcl-2. 2. DNA damage leads to the induction of Puma gene synthesis. Puma binds to Bcl-2. 3. Removal of cells from its substratum result in a disruption of integrin signaling leading to the release of Bim from the cytoskeleton. Bim also binds to Bcl-2 to promote Bak-Bax pore formation. LW2024 Burman University, BIOL 374 Cellular Biology, Stem Cells § Fas-mediated apoptosis - Binding of extracellular death signals, like tumor necrosis factor and Fas ligand, to their receptors oligomerizes an associated protein FADD, which in turn triggers the caspase cascade, leading to apoptosis [F22-44a]. LW2024