How Neural Stem Cells Create New and Varied Neurons

A new study in fruit flies has elucidated a mechanism in neural stem cells by which these types of stem cells generate the wide range of neurons that they form.

Chris Doe, a professor of biology from the Institute of Neuroscience at the University of Oregon, and his co-authors have used the common fruit fly Drosophila melanogaster to investigate the cellular mechanism by which neural stem cells make their distinctive progeny.

As Doe put it, “The question we confronted was ‘How does a single kind of stem cell, like a neural stem cell, make all kinds of neurons?”

Researchers have known for some period of time that stem cells have the capacity to produce new cells, but the study by Doe’s group shows how a select group of stem cells can create progenitor cells that can generate numerous subtypes of cells.

Doe’s study builds on previous studies in which Doe and his colleagues identified the specific set of stem cells that generated neural precursors. These so-called “intermediate neural progenitors” or INPs can expand to form several different new cell types. However, this study did not account for the diversity of the cells generated even if it did account for the number of cells generated (see Boone JQ, Doe CQ, Dev Neurobiol. 2008 Aug;68(9):1185-95).

“While it’s been known that individual neural stem cells or progenitors could change over time to make different types of neurons and other types of cells in the nervous system, the full extent of this temporal patterning had not been described for large neural stem cell lineages, which contain several different kinds of neural progenitors,” according to this study’s first author Omar Bayraktar.

The cell types discovered in this study have analogs in the developing human brain and the research has potential applications for human biologists who want to know how neurons form in the human brain.

The paper from Doe’s lab was published along another study on the generation of diverse neurons by a group from New York University. These two papers provide new insight into the means by which neural stem cells generate the wide range of neurons found in the brains of fruit flies and humans.

In their study, Bayraktar and Doe specifically examined stem cells in fruit fly brains known as type II neuroblasts, which generate INPs. However, in this study, the type II neuroblasts were shown to generate INPs, which then go on to form distinct neural subtypes. Even though previous work showed that INPs went on to form about 100 new neurons, in this paper, the INPs were shown to make about 400-500 new neurons.

Another interesting finding was that the gene expression patterns of INPs, which began with three different transcription factors (Dichaete, Grainy Head, and Eyeless). These transcription factors lay the groundwork for INP differentiation, but once INP formation occurs, a new transcriptional program is extended that extends the types of neurons that INPs can form. Such nested transcriptional programs are also common during the specification of neural stem cell progeny in humans brains, with many of the same transcription factors playing a central role in neuron specification.

“If human biologists understand how the different types of neurons are made, if we can tell them ‘This is the pathway by which x, y, and Z neurons are made,’ then they may be able to reprogram and redirect stem cells to make these precise neurons,” Doe said.

However, the mechanism described in this paper has its limits. Eventually the process of generation new cells stops. One of the next questions to answer will be what makes the mechanism turn off, according to Doe.

“This vital research will no doubt capture the attention of human biologists,” said Kimberly Andrews Espy, who is vice-president for research and innovation and the dean of the UP graduate school. “Researchers at the University of Oregon continue to further our understanding of the processes that undergird development to improve the health and well-being of people throughout the world.”

See Bayraktar OA, Doe CQ. Combinatorial temporal patterning in progenitors expands neural diversity. Nature. 2013 Jun 27;498(7455):449-55. doi: 10.1038/nature12266.

Adult Stem Cell Possess Internal Machinery to Resist Ischemic Injury

Adult stem cells that have been transplantation into a sick patient are often faced with harsh conditions that lead to their untimely death before they can help the patient. Several different strategies have been applied to stem cells to “toughen them up” so that they can resist these conditions. Treating them with particular growth factors have proven effective in some experiments, as has oxygen or glucose deprivation. A recent paper in the journal ANTIOXIDANTS & REDOX SIGNALING by Gang Lu, Muhammad Ashraf, and Khawaja Husnain Haider from the Department of Pathology at the University of Cincinnati, Ohio has shown that treatment of stem cells with insulin-like growth factor-1 (IGF-1) and glucose and oxygen deprivation activate a biochemical pathway that seems to be common to many different types of stem cells that help cell resist ischemic (oxygen-poor) conditions.

This paper examined bone marrow stem cells. When the bone marrow stem cells were deprived of oxygen and glucose for 12 hours, they discovered that a well-known signaling molecule called ERK1/2 was activated. ERK1/2 stands for “extracellular signal-related kinases-1/2. These molecules are “kinases,” which simply means that they are enzymes that attach phosphate groups to other proteins. These phosphate groups change the 3-D structure of proteins can induce them to change they function. ERK1/2 are activated whenever the cell binds particular types of growth factors. Growth factor-binding sets a series of steps in motion that leads to the activation of ERK1/2. ERK1/2 phosphorylates its target, which sets a phosphorylation cascade into motion, and this changes the behavior of the cell.

There are ways to inhibit ERK1/2 activation, and when the University of Ohio team did just that (with a drug called PD98059), the bone marrow stem cells did not activate ERK1/2 when derived of oxygen and glucose and the cells died. This shows that activation of ERK1/2 occurs as a result of oxygen and glucose deprivation, and the activation of ERK1/2 is required for the cells to survive the harsh conditions.

Next, they discovered that ERK1/2 is also activated by treating the cells with IGF-1. Since IGF-1 treatment also helps stem cells adapt to harsh conditions, it is possible that these two treatments use the same internal mechanism to help stem cells adapt to harsh conditions.

What is a downstream target of ERK1/2 that throws the switch that allows cells to adapt the harsh conditions? The researchers received two clues when they discovered that drugs that prevent the release of calcium ion stores into the cell interior also prevent cells from adapting to harsh conditions. This tipped them off that a target of calcium-ion signaling was probably the downstream target of ERK1/2. That target is protein kinase C (PKC).

To shore up their hypothesis, they treated stem cells with a chemical that is known to activate PKC (phorbol esters). These chemicals completely acclimatized the cells to harsh conditions and when the bone marrow stem cells were grown after they had been engineered with a permanently active form of PKC, the stem cells did not require any preconditioning in order to resist harsh conditions.

These data are remarkable and since calcium signaling is a pathway that we know a great deal about and there are lots of chemicals available to manipulate it, it should be possible to precondition a whole host of stem cells to resist harsh conditions before they are ever used. These types of treatments should improve adult stem cell treatments for a variety of conditions.

Cultured Smooth Muscle Cells are Formed from Stem Cells

Laboratory research needs tissue as a model system. Smooth muscle is found in the urogenital system, circulatory system, digestive system, and respiratory systems of the human body. Various diseases affect smooth muscle and being able to work on cultured smooth muscle would greatly advance the ability of medical researchers to find treatments for smooth muscle disorders.

To address this need, Cambridge University scientists have devised a protocol for generating different types of vascular smooth muscle cells (SMCs) using cells from patients’ skin. This work could lead to new treatments and better screening for cardiovascular disease.

The Cambridge group used embryonic stem cells and reprogrammed skin cells. Skin cells were turned into induced pluripotent skin cells (iPSCs), which were then differentiated into SMCs. They found that they could create all the major vascular smooth muscle cells in high purity using iPSCs. This technique can also be scaled up to produce clinical-grade SMCs.

The scientists created three subtypes of SMCs from these different types of stem cells. They also showed that various SMC subtypes responded differently when exposed to substances that cause vascular diseases. They concluded that differences in the developmental origin play a role in the susceptibility of SMCs to various diseases. Furthermore, the developmental origin of specific SMCs might part some role in determining where and when common vascular diseases such as aortic aneurysms or atherosclerosis originate.

Alan Colman MD, Principle Investigator of the Institute of Medical Biology at Cambridge University, said: “This is a major advance in vascular disease modeling using patient-derived stem cells. The development of methods to make multiple, distinct smooth muscle subtypes provides tools for scientists to model and understand a greater range of vascular diseases in a culture dish than was previously available.”