Lecture 4: Comprehensive Guide to Stem Cells, Potency, and Cloning

Recapping Cell Potency and Developmental Pathways\n\nDevelopmental biology involves a steady loss of cellular potency as cells differentiate along specific pathways. The levels of potency are defined as follows:\n\n* Totipotent Cells: Exemplified by the zygote, these cells have the capacity to produce any other sort of cell in the organism.\n* Pluripotent Cells: These include embryonic stem cells ($ES$ cells) derived from the inner cell mass ($ICM$) of the blastocyst. They can differentiate into any tissue derived from the three germ layers ($ectoderm$, $mesoderm$, and $endoderm$) and the germline, though they traditionally were thought unable to form the trophoblast (extra-embryonic tissues). Note: Recent research from scientists at Monash University suggests $ES$ cells might actually be able to form trophoblasts as well.\n* Multipotent Cells: Found in both the embryo and the adult brain, these cells have a more restricted potential but can still form multiple related cell types.\n* Unipotent Cells: Differentiated cells that have reached the final stage of the pathway and have limited or no further differentiation potential, such as non-mitotic neuronal cells in the brain.\n\n# Definition and Core Properties of Stem Cells\n\nA stem cell is defined as an undifferentiated cell that possesses two fundamental abilities:\n\n1. Self-Renewal: The ability to divide and produce more cells of the same undifferentiated type.\n2. Differentiation: The ability to differentiate into tissue- or organ-specific cell types.\n\nThere are two primary broad categories of stem cells:\n\n* Embryonic Stem Cells ($ES$ cells).\n* Adult Stem Cells.\n\n# Potential Applications and Uses of Stem Cells\n\nStem cells offer profound therapeutic potential for modeling and treating human diseases. Their uses are categorized into three main areas:\n\n1. Regenerative Medicine: Replacing diseased or lost tissues and body parts. Examples include:\n * Bone marrow for leukemia treatment.\n * Nerve cells for Parkinson's disease or Alzheimer's disease.\n * Heart muscle for treating heart disease or myocardial infarction.\n * Pancreatic beta islet cells for Diabetes.\n * Other applications: spinal cord regeneration, osteoarthritis, traumatic brain injury, deafness, blindness, and baldness.\n2. Modeling and Treating Human Diseases: Stem cells can be taken from a human patient or a mouse model. By inducing the specific affected cell type using growth factors, researchers can chart the disease's progression in a petri dish and test new drugs or therapies to see if they alleviate the disease state.\n3. Understanding Normal Development: By observing genes turned on during the differentiation of stem cells into various cell types, scientists can study normal cell differentiation and ultimately understand or treat birth defects.\n\n# Characteristics and Propagation of Embryonic Stem Cells ($ES$ Cells)\n\n$ES$ cells are pluripotent cells derived from the Inner Cell Mass ($ICM$) of the blastocyst. Their characteristics include:\n\n* Expression of Pluripotency Markers: They express markers such as $Oct4$, which is required for the formation of the $ICM$.\n* In Vivo Differentiation: When transplanted into an immune-deficient mouse, they can differentiate into a variety of cell types.\n* Long Telomeres: $ES$ cells possess long, intact telomere sequences ($GGG-ATT$) at the ends of their chromosomes. While telomeres in normal somatic cells shorten with age and division, $ES$ cells maintain them to protect structural integrity.\n* Propagation in Vitro:\n * Mouse $ES$ cells: Propagated using the cytokine $LIF$ (Leukemia Inhibitory Factor) to keep them proliferating without differentiating.\n * Human $ES$ cells: Propagated using a cocktail of growth factors including $FGFs$ and $Wnts$. Human $ES$ cells were first isolated around the turn of the century ($1998$ - $2000$).\n\n# Differentiation Pathways and In Vitro Induction\n\nThe fate of $ES$ cells in culture is dictated by the specific growth or signaling factors added to the medium:\n\n* Glial Cells: If $ES$ cells are grown on a fibroblast feeder layer in medium containing Fibroblast Growth Factor ($FGF$) and Platelet Derived Growth Factor ($PDGF$), they are induced to form glial stem cells. Glial cells are non-neuronal cells that maintain homeostasis, form myelin, and provide support/protection for neurons.\n* Neuronal Cells: If $ES$ cells are exposed to Retinoic Acid ($RA$), they are induced to form neural stem cells, which can become functional neurons in vivo.\n* Note on Lateral Inhibition: This process (discussed in previous lectures) involves inducing one neuronal cell while preventing neighboring cells from doing the same, leading those neighbors to become glial cells instead.\n\n# Case Study: Diabetes Mellitus and $ES$ Cells\n\nDiabetes Mellitus (specifically Type $1$) involves the autoimmune destruction of insulin-producing beta islet cells in the pancreas. These cells are located in the Islets of Langerhans.\n\n* Induction Process: Researchers start with $ES$ cells expressing $Oct4$ or $Nanog$. Using transcription factors identified through developmental biology (including $SOX$ genes and $NKX$ genes), they induce the cells to progress toward a beta cell fate.\n* Markers of Success: Successful differentiation is marked by the expression of $Nestin$ (an early marker) and, ultimately, the production of insulin (stained green in images) and glucagon (stained red).\n* Rescue in Mice: $ES$-derived beta cells can rescue diabetic mice. When challenged with glucose in vitro, these cells show elevated insulin levels. When transplanted into diabetic knockout mice, they restore insulin production.\n\n# Limitations and Ethical Challenges of $ES$ Cells\n\nDespite their potential, $ES$ cells face several hurdles:\n\n* Heterogeneity: Current methods often produce heterogeneous populations at different stages of differentiation rather than a pure population of the desired cell type.\n* Tumorigenic Potential: Due to their pluripotency, any remaining undifferentiated $ES$ cells can form teratomas (tumors comprising all three germ layers) when transplanted.\n* Immune Rejection: $ES$ cells are not genetically matched to the patient.\n* Ethical Controversy: Creating human $ES$ cells necessitates the destruction of early human embryos (blastocysts).\n* Environmental Influence: The behavior of $ES$ cells is modified by their surroundings. Put in an $ICM$, they form a normal chimeric mouse; put under the skin, they form a teratoma.\n\n# Adult Somatic Stem Cells and the Stem Cell Niche\n\nAdult (somatic) stem cells are multipotent and derived from mature organs. They play a role in homeostasis, replenishing old or aged cells. They are found in Stem Cell Niches, which are specialized microenvironments where surrounding cells release specific factors to maintain them.\n\nExamples include:\n* Skin Stem Cells: Found in the epidermal layer; they divide to produce one stem cell and one differentiating keratinocyte.\n* Hematopoietic Stem Cells ($HSCs$): Found in the bone marrow, they give rise to all blood types. In $1961$, Till and McCulloch proved their existence by injecting bone marrow into irradiated mice, detecting the recovery of red cells, white cells, and platelets.\n * Myeloid Progenitor Lineage: Produces white blood cells and red blood cells.\n * Lymphoid Progenitor Lineage: Produces $T$ and $B$ lymphocytes and killer cells.\n\n# Mesenchymal Stem Cells ($MSCs$) and Osteoporosis Case Study\n\n$MSCs$ are multipotent stem cells found in fat, muscle, thymus, dental pulp, umbilical cord, and placenta. They differentiate into bone, cartilage, or muscle based on environmental cues (e.g., matrix elasticity, paracrine factors like $MyoD$ and $Pax7$).\n\nOsteoporosis Rescue:\n* Researchers used $Sca-1$ knockout mice ($Sca-1$ null/null), which are depleted of $MSCs$ and develop osteoporosis.\n* Injecting $MSCs$ from normal mice into these deficient mice increased bone deposition, as measured by calcium green fluorescent marking in the tibia.\n* Treatment restored mineralization from lower levels back toward the $40\, \%$ seen in wild-type mice.\n\n# Comparison Table: Embryonic vs. Adult Stem Cells\n\n| Feature | Embryonic Stem Cells | Adult Stem Cells |\n| :--- | :--- | :--- |\n| Potency | Pluripotent | Multipotent (Restricted) |\n| Harvesting | Large numbers from embryos | Limited numbers; difficult to isolate |\n| Immune Response | Potential rejection | Less likely (patient's own cells) |\n| Tumor Potential | High (Teratomas) | Lower |\n| Ethics | High controversy | Low moral/legal issues |\n\n# Induced Pluripotent Stem Cells ($iPS$ Cells)\n\n$iPS$ cells are adult differentiated cells (like skin fibroblasts) that have been genetically reprogrammed back into a stem cell state. This discovery by Shinya Yamanaka won the Nobel Prize in Medicine in $2012$.\n\nThe Four Yamanaka Factors:\n1. $Oct4$: Transcription factor to establish pluripotency.\n2. $Sox2$: Transcription factor to establish pluripotency; activates $Nanog$.\n3. $c-Myc$: Opens chromatin, making genes accessible to other factors.\n4. $Klf4$: Prevents cell death during the reprogramming process.\n\nAdvantages over $ES$ cells:\n* Patient Specific: Derived from the patient's own cells (e.g., skin scrape), preventing immune rejection.\n* No Ethical Issues: Does not involve the destruction of embryos.\n* Scalability: Large numbers can be generated from simple tissue samples.\n\n# Disease Modeling and Treatment with $iPS$ Cells\n\n* Alzheimer's Disease: Fibroblasts from an Alzheimer's patient can be turned into $iPS$ cells, then into neurons. This allows researchers to \"go back in time\" to study the progression of the disease in a petri dish and screen for drugs for customized medical treatment.\n* Sickle Cell Anemia: Proof of concept in mice showed that $iPS$ cells derived from tail tip fibroblasts could have their sickle cell mutation corrected via gene targeting. Once transplanted back, they rescued the anemia.\n* Parkinson's Disease Trial ($2015/2018$): In Japan, Takahashi and Yamanaka initiated clinical trials injecting dopamine-producing neuronal progenitor cells into the striatum (the brain region implicated in the disease) of patients. In one study, approximately $5,000,000$ cells were injected.\n\n# Reproductive and Therapeutic Cloning\n\nCloning involves generating genetically identical copies. Both types typically use Somatic Cell Nuclear Transfer (SCNT).\n\nSomatic Cell Nuclear Transfer (SCNT):\n1. Take an egg cell (ovum) and remove its nucleus (enucleation).\n2. Take the nucleus from a somatic body cell.\n3. Insert the somatic nucleus into the enucleated egg.\n\nTwo Types of Cloning:\n1. Reproductive Cloning: Creating a duplicate of an entire organism. \n * Dolly the Sheep ($1996$): The first cloned mammal. A nucleus from an adult sheep's udder was transferred into an enucleated egg, zapped with electricity to start development, and implanted into a foster mother.\n * Applications: Endangered species, elite livestock, or pets (cats/dogs). Forbidden in humans.\n2. Therapeutic Cloning: Generating stem cells for disease therapy. SCNT is used to create an embryo from which $ES$ cells are harvested. Today, $iPS$ technology has largely superseded SCNT for therapeutic purposes because it is easier and skips the egg/embryo stage entirely.\n\n# Questions & Discussion\n\nMid-Lecture Question:\n* Question: Imagine you've isolated what you believe to be pancreatic beta stem cells. What evidence would you need?\n* Answer: They should self-renew in culture, express pancreatic markers (like insulin) in culture, and behave as functional beta cells in vivo (producing insulin in response to glucose).\n\nEnd-Lecture Quiz:\n* Question: Which statement is true? (A) iPS derived from ICM, (B) Yamanaka factors are growth factors, (C) Therapeutic cloning transplants embryos into hosts, (D) iPS are totipotent.\n* Answer: (E) None of the above. Reason: iPS are from adult cells; Yamanaka factors are transcription factors; reproductive cloning (not therapeutic) involves host transplantation; iPS are pluripotent, not totipotent.\n\nPublic Perspective ($2012$ US Data):\n* $27\, \%$: Strongly in favor of expanding federal funding for $ES$ cell research.\n* $11\, \%$: Somewhat opposed.\n* $13\, \%$: Strongly opposed.\n* $15\, \%$: Not sure.\n\nFinal Consideration: Which would you choose for treatment: $ES$, $MSC$, or $iPS$ cells?", "title": "Lecture 4: Comprehensive Guide to Stem Cells, Potency, and Cloning"}