Stem Cells and Hematopoeisis

Question 1

Where are blood cells developed from? How are adult stem cells collected?

Answer 1

• One way to classify stem cells is based on their origin
• ES cells are derived from the inner cell mass of an embryo, specifically the blastocyst
• Adult stem cells can be collected from various parts of the body
  
  • For example, hematopoietic stem cells can be isolated from the bone marrow

Question 2

What is iPS, and what are some pertinent features?

Answer 2

• A relatively new type of stem cell is the “induced pluripotent stem (iPS) cell.”
  
  • iPS cells are very similar to ES cells in that they can produce cells from all three germ layers
• iPS cells are made by introducing reprogramming genes (MYC, SOX2, KLF4, and OCT4) into somatic cells following by extended in vitro culture

Question 3

What are the pros and cons of iPSCs?

Answer 3

• A benefit of iPS cells is that they are derived from adult tissue, so there are fewer ethical issues than are associated with ES cells
• Caveats to iPS cells are that they are not fully characterized and it remains possible that they have reduced potency

Question 4

How are iPS cells delivered? What are some important features of treatment delivery?

Answer 4

• Hematopoietic stem cell transplantation is widely used and is the best example of a stem cell therapy
• Despite success with this tissue, there has been little progress in transplantation of other adult stem cells
• One key to using stem cells in regenerative medicine is the potential to generate patient matched stem cell lines
  
  • For example, the nucleus of a patient’s skin cell can be injected into an enucleated oocyte (a technique known as nuclear reprogramming) to generate autologous ES cells
• Alternatively, patient matched iPS cells could be made
• In either case, these stem cells could either be differentiated in culture to:
  
  • fully mature cell type, such as a neuron, and injected into the patient
  • partially differentiated to a progenitor cell that could be engrafted in the patient
    
    • A key step is to ensure that all the cells have differentiated, because the injection of pluripotent stem cells leads to tumor formation in recipients

Question 5

What are some landmark studies that introduced the concept of iPSCs?

Answer 5

• Several studies have provided proof of concept for the use of pluripotent stem cells as a therapy
• An elegant study published in Science in 2007 reported the creation of genetically matched iPS cells from a mouse with sickle cell anemia
  
  • Using homologous recombination, the researchers corrected the sickle cell mutation in the iPS line
  • Next, they derived hematopoietic stem cell from the iPS line (by expression of the HOXB4 gene) and engrafted these cells into mice with sickle cell anemia
  • Shortly after the transplant, the disease in the mice was significantly ameliorated
• Although this has been demonstrated for mice, there remain several hurdles for use of this strategy in humans, including the inability of generate bona fide HSCs from ES and iPS cells

Question 6

What are some key proprties of hematopoietic stem cells?

Answer 6

• Hematopoiesis is the process by which all blood cells are derived from a common precursor, the hematopoietic stem cell
• HSCs have the remarkable ability to give rise to billions of blood cells throughout the life of an individual
• HSCs must balance the need to expand with the need to maintain the stem cell pool by controlling their degree of apoptosis, self renewal, and differentiation
• The stem cell niche provides a supportive environment which enables HSCs to maintain a quiescent state and lay dormant until needed

Question 7

What is a key transcription factor for the differentiation of HSCs? What happens when this gene is mutated?

Answer 7

• The differentiation of HSCs into the mature cells is governed by a wide array of transcription factors and cytokines
• One key transcription factor is GATA1
  
  • Mutations in this gene are associated with rare forms of inherited anemia as well as pediatric acute myeloid leukemia

Question 8

Describe the key steps in the ontogeny of HSCs.

Answer 8

• To keep up with the rapidly increasing need for oxygen, the hematopoietic system has evolved to have multiple stages:
  
  • At the earliest time point, primitive HSCs, which primarily produce red blood cells, are found in the yolk sac (mammals) or in the intermediate cell mass (zebrafish) - ​derived from a common hematopoietic endothelial precursor named the hemangioblast
  • As the embryo matures, definitive HSCs emerge from new sources including the aorta gonad mesonephros (AGM) region and hemogenic endothelium and then colonize the fetal liver.
  • At this stage, HSCs are defined as definitive cells, which give rise to all blood cell types
• Zebrafish, mice and humans share a step-wise development of HSCs

Question 9

What is a major studies regarding the regeneration of HSCs? What is a major downside?

Answer 9

• Transplant of bone marrow harvested from the recipient (autologous) or from a matched or unmatched donor (allogeneic) is a common procedure to treat cancer or blood disorders
• In the absence of a suitable donor, an emerging alternative is the transplant of umbilical cord stem cells
  
  • Although this holds great promise, success is limited by the low numbers of HSCs in the sample and a consequential delay in engraftment
  • Using a zebrafish screen, Zon and colleagues discovered that prostaglandin E2 (PGE2) enhances the expansion of HSCs in vitro
    
    • Facilitates HSC reconstitution in animal models
• Clinical trials to test the activity of PGE2 and related compounds in umbilical cord transplants are underway