- Top of page
- THE MULTIFACETED NICHE
- A CONVERSATION WITH THE EXPERTS
A stem cell niche is a microenvironment that supports self-renewal of a population of stem cells, and their production of differentiated cells. While the definition evokes images of a stem cell Shangri-La—where a serene stem cell pool nestles within a niche that shelters and sustains it—the reality is much more tumultuous. Niches are subject to an ever-changing maelstrom of environmental factors, the ravages of old age, and the sly tactics of disease. Presented here is a basic overview of the different ways in which stem cell niches respond to local and systemic environments, and their impact on stem cell behavior. The primer culminates with a discussion of the topic with stem cell and niche biologists D. Leanne Jones, Ph.D., and Tudorita Tumbar, Ph.D. Developmental Dynamics 240:737–743, 2011. © 2011 Wiley-Liss, Inc.
A CONVERSATION WITH THE EXPERTS
- Top of page
- THE MULTIFACETED NICHE
- A CONVERSATION WITH THE EXPERTS
Featured here is an interview with Leanne Jones, Ph.D., and Tudorita Tumbar, Ph.D. (Fig. 1), who discuss current topics in stem cell/niche research and the future of the field.
Figure 1. (L) Leanne Jones, Ph.D., Assistant Professor, Salk Institute for Biological Studies. (R) Tudorita Tumbar, Assistant Professor, Department of Molecular Biology and Genetics, Cornell University.
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Developmental Dynamics: What is your lab's research focus?
Leanne Jones: My lab is primarily interested in the mechanisms regulating stem cell behavior, in particular the role of the local microenvironment (or niche), and how the relationship between stem cells and their environment changes during aging and disease progression.
Drosophila is emerging as an ideal system for studying the relationship between stem cell activity, tissue homeostasis, and longevity. In addition to established protocols for measuring lifespan, adult flies have several tissues that are maintained by resident stem cells. Genetic tools that allow inducible tissue and temporal specific gene expression and the ability to generate clones of stem cells that can be analyzed in vivo provide a distinct advantage in defining the effects of aging on stem cell behavior.
In an initial study to determine how aging affects stem cell behavior in the Drosophila testis, we found that the number and activity of germline stem cells decreases. However, one of the most striking observations we made was the decrease in the expression of a key self-renewal factor from adjacent niche support cells. So, one project in the lab is to investigate the mechanism by which this age-related decrease in self-renewal factors occurs. Behavior of stem cells in the Drosophila intestine also change with age and in response to chronic stress; therefore, we're also comparing and contrasting how somatic and germline stem cells respond to different types of environmental changes.
Tudorita Tumbar: Our research aims at understanding the molecular mechanisms that govern cell fate decisions in tissue stem cells. We use the mouse skin and hair follicle as a model system, and focus our experiments in vivo, trying to capture stem cell behavior in an unperturbed niche, in the absence of cell transplantation. While I was a postdoctoral fellow in Dr. Elaine Fuchs' laboratory, I invented a new system to target and specifically label cells with infrequent divisions based on histone H2B-GFP retention. Using this system we have characterized a putative population of hair follicle stem cells. Later on, in my laboratory at Cornell University, we were able to demonstrate in vivo at the single cell level that stem cells in the hair follicle are infrequently dividing and they behave as a population in choosing their fate.
Our working model is that some stem cells choose to differentiate without self-renewal, while other cells replenish the pool by creating more stem cells. We are currently asking how transcription factors that regulate self-renewal and differentiation in hair follicle stem cells interplay with epigenetic modifications to control tissue stem cell quiescence and cell fate decisions. Whenever possible, we try to link our findings in the mice with human disease. For example, we found that Runx1, a transcription factor essential for timely hair follicle stem cell activation from quiescence plays a role in skin squamous cell carcinoma.
Dev Dyn: What initially provoked your interest in this field?
LJ: My background is in cancer biology, so I'm fascinated by how a cell “decides” to keep proliferating or to initiate a differentiation program. Each time a stem cell divides, the daughter cells make the decision to self-renew (proliferate) or initiate differentiation; therefore, stem cells seemed to be the perfect system in which to study this question.
TT: There were two aspects that attracted me to stem cell research: first was the promise for therapy of currently un-curable diseases; second was the promise of a rich basic science field with potential for new cell and developmental biology mechanisms that would be responsible for determining unique stem cell properties and behavior.
Dev Dyn: Which papers have most impacted your research?
LJ: (1) This paper (Conboy et al., 2003) begins to tease apart the role of the systemic/local environments in contributing to the aging of tissues maintained by stem cells.
(2) Leatherman and Dinardo (2008) reinterprets data that I published as a postdoc. The results from our lab and the Matunis lab in 2001 indicated that JAK-STAT signaling acted autonomously in germ cells to regulate germline stem cell (GSC) proliferation. However, this study demonstrated that signaling from the somatic cells was sufficient to specify GSC proliferation in a nonautonomous manner. This study reminds me that (1) even when you think you know what is happening, you probably don't understand everything and (2) you should always be challenging the current model as you discover new genes and develop new tools.
(3) These back-to-back articles (Micchelli and Perrimon, 2006; Ohlstein and Spradling, 2006) first report of intestinal stem cells in flies. These papers have allowed my lab to take what we are doing in the germ line and directly compare and contrast those findings with another stem cell system. Because the two tissues are quite different, it allows us to explore both germline specific and stem cell generalizable phenomena.
(4) Two papers from the Kenyon lab (Hsin and Kenyon, 1999; Arantes-Oliveira et al., 2002) reveal that the germ line, specifically the proliferating germline stem cells, are a “pro-aging” tissue in worms whereas the somatic component of the gonad promotes longevity. How the germ line and/or reproduction influences aging and longevity is a fascinating question to me.
TT: (1) Thomson et al. (1998) was crucial for my career path. I read it at a time when I was completing my graduate studies in large-scale chromatin and looking for a postdoctoral position. I was impressed by the possibilities that this study opened up and by the stem cell field in general. I was irreversibly fascinated with cell fate acquisition and the complexity of cell intrinsic and cell extrinsic pathways that magically combine to create three-dimensional tissues.
(2) Work from Lavker's lab gave me the theoretical and experimental basis needed to design my own experimental system that allowed me, and later on other people in different areas of stem cell biology, to isolate and characterize a most quiescent population of tissue stem cells.
(3) This paper (Robinett et al., 1996) was published from my graduate study laboratory on work that was done before my joining this research team. Although it did not directly impact my current research, it did shape my thinking and approach in the most profound manner. This study inspired me to think out of the box and dare to try something different. I came up with a novel H2B-GFP pulse-chase system to mark cells in mouse tissues in vivo based on proliferation history and to quantify their divisions.
Dev Dyn: How has the study of stem cell niches in invertebrates informed vertebrate stem cell research? What are some important differences?
LJ: Stem cells are rare, and for a long time, specific markers and tools were lacking to identify stem cells in mammalian systems. In worms and flies, stem cells could be easily identified based on lineage-tracing experiments and specific markers, allowing them to be studied in vivo in the context of the tissues they sustain. Therefore, scientists using these systems were able to identify signaling pathways and niche components to establish paradigms for how stem cell behavior could be regulated that provided a platform for the scientists working in more complex stem cell systems.
An important difference stems from recent data indicating that one of the primary factors influencing stem cell behavior in mammalian systems is the vasculature (especially hematopoietic system, neural stem cells, and spermatogonial stem cells). Flies don't have blood or blood vessels, so this is one major difference. However, the tissues that serve similar functions, hemolymph and the trachea, respectively, could serve important stem cell support roles in flies.
TT: It is very interesting to see how work in invertebrates translates to vertebrates. Model systems such as Drosophila neuroblast and germ line, or C. elegans germ line have been very useful. Based on evidence at hand, it appears that the mouse nervous system and the muscle stem cells seem to follow a simple model of asymmetric cell fate decisions, similar with that proposed in Drosophila. However, recent work from my group (Zhang et al., 2009) and other groups (Clayton et al., 2007; Snippert et al., 2010) suggests that the long-lived progenitors (stem cells) in some tissues do not self-renew and differentiate by the well-accepted asymmetric cell fate decisions model. Instead the behavior of these stem cells can be described by a population deterministic model similar with that proposed by Judith Kimble for the C. elegans germ line, in which symmetric or unidirectional decisions might be the norm.
It seems important to keep an open mind and make as few assumptions as one can when starting to use a new system that has not been characterized in the enormous detail of the Drosophila. Major differences that stem cells face when in a mouse versus a fly is the life span, and the size of the tissues that they need to maintain. Although the evidence we have is slim, my impression is that in hair follicles, the short-lived progenitors located in the matrix might behave similarly with the stem cells of Drosophila. They would self-renew and differentiate continuously by asymmetric divisions, as long as they last, which is a couple of weeks, a life span closely approaching that of a fly. In contrast the hair follicle stem cells seems to act as a reservoir of senior (long-lived) citizens from which periodically some of them are killed to differentiate and maintain the tissue while the others compensate for the loss by symmetric expansion. This periodicity becomes obscured in rapidly regenerative tissues such as epidermis and intestine, where the stem cell behavior could be modeled as a stochastic and continuous process of stem cell loss and proliferation. An important aspect of this matter is that different vertebrate tissues and stem cells might behave very differently, a concept that is not yet engrained in the field as it is difficult to resist the tendency to generalize models that we often take for granted.
Dev Dyn: It has been shown that the niche and stem cells respond to changes in the larger systemic environment caused, for example, by aging or diet. What is your view of the role of the niche in interfacing with systemic cues?
TT: These are very exciting news, and it will be interesting to see how many tissues show similar responses. The data will allow us to better dissect what are stem cells intrinsic or specific characteristics and what is due to the environment. Right now the balance seems to shift toward a determinant role of the environment, although I predict that this will change in the future as we might become better at uncovering unique stem cell intrinsic properties. In that respect, the embryonic stem cells lead the way, and we have seen that these cells have special cell cycle properties and genome organization at the level of chromatin structure and epigenetic modifications. We only have a dim glimpse of these processes in tissue stem cells, mainly because it is much more difficult to do biochemistry with small populations of sorted tissue stem cells. It will also be interesting to see what these results really mean in the context of unperturbed tissues in higher organisms, when shifting the assays from in vitro and injury models such as transplantation, to a more natural setting. The in vivo data we have from Drosophila seem to be promising.
LJ: Ultimately, stem cell behavior is regulated by the integration of extrinsic cues with intrinsic factors. We are learning more and more about how changes in the environment lead to changes in stem cell behavior; however, our work has clearly demonstrated that simply manipulating the environment is not always sufficient to reverse or override intrinsic changes. Therefore, the identification and understanding of intrinsic, molecular responses to extrinsic changes, such as those that occur during aging or disease progression, will ultimately allow us to take the next step to develop strategies to manipulate stem cell behavior to counter negative or enhance positive influences from the environment.
Dev Dyn: Would you predict that systemic cues only impact niche/stem cell dynamics under extreme conditions, or do you expect that they are in constant flux throughout the lifetime of an animal?
LJ: Tissue homeostasis requires a precise balance between stem and progenitor cells, and the local microenvironment plays a major role in ensuring this balance. Therefore, there is constant communication between niche support cells and stem cells. With regards to how systemic signals could impact this communication, several labs have studied how signaling by means of the insulin/IGF pathway regulates stem cell behavior in the gonads and the intestine. Daniela Drummond-Barbosa's group, who work in the female germ line, has done some of the most elegant work in this area. As the levels of circulating Drosophila insulin-like peptides (dILPs) clearly fluctuate in response to nutritional conditions and with age, I would predict that systemic cues are constantly influencing local stem cell-niche cell interactions.
TT: The recent data from my laboratory and from other laboratories suggest that stem cells in mouse tissues such as hair follicle, epidermis, intestine, and testis, maintain their pool by a population deterministic model. In these models some stem cells are lost by differentiation or cell death while others that remain in the niche seem to subsequently compensate the loss by what appears to be a symmetric expansion. In light of these and other data, the field is now reconsidering asymmetric cell fate decisions as an inherent property of tissue stem cells during normal homeostasis. The stem cell loss could be modeled as a stochastic process in rapidly regenerative tissues with a constant need for cells. In the hair follicle, we showed that stem cell loss is tightly coupled with the initiation of a regenerative growth cycle, and is likely dependent upon both the localization of cells within their niche, and localization near the differentiating signals. I do think the flux of stem cells is continuous, or at least periodic, and that it is programmed into the normal tissue maintenance capacity.
Dev Dyn: Tudorita, the bulge niche is somewhat ill-defined. What do you think contributes to the bulge niche microenvironment?
TT: Clustering stem cells themselves seem to secrete molecules known to control cell proliferation, survival, and differentiation. It seems conceivable that these molecules create a protective “cloud” around the bulge, thus contributing to the “niche.” It is possible, although unlikely based on the evidence at hand, that some of these quiescent bulge cells are dedicated niche cells, or that some other cells in this rich bulge environment perform this function. It is also possible, and in my mind most likely, that there is a combination of several cell populations, including dermal cells, clustering of bulge cells themselves, blood vessels and nerves, which are rich in this zone, that all contribute to the creation of a specialized stem cell environment. This makes a challenging problem for tissue reconstruction in vitro, and speaks of the complexity of three-dimensional vertebrate tissues and their intimate connection with their environment.
Dev Dyn: Are there new techniques that you anticipate will help define niches and their properties?
TT: We need to identify the native niche components in vivo, in the unperturbed tissues, and be able to generate and characterize large amounts of mutants in higher organisms. What we would really benefit from is new imaging methods capable of generating large amounts (high throughput) of data characterizing and identifying different cell types and their arrangements within unprocessed tissues. Fluorescent immunolabeling techniques, the current workhorse of quantitative imaging technology, require extensive labeling steps coupled with a need to examine a multitude of markers that make meaningful high-throughput screening of phenotypes in complex vertebrate tissues a nearly impossible task. Especially promising is quantitative microscopy taking advantage of the natural interactions of transparent samples with light, mainly phase and polarization. To take advantage of these data we will need systematic and automated methods for storing, categorizing (understanding), and accessing it. The quantitative imaging data should generate feature-based searchable databases usable in direct quantitative comparisons across experiments performed by different investigators and in different laboratories. We need to be able to do “big science” by building synergies among smaller scale projects. Together, the aforementioned technologies will allow us to rapidly sift through vast amounts of data to identify subtle cell and tissue organization phenotypes, which would otherwise go unnoticed, although in fact they might be the most crucial for understanding stem cell and niche behavior.
LJ: I completely agree that being able to visualize stem cell activity in real time in vivo is an essential tool that will provide us with invaluable information about how stem cells behave during normal tissue homeostasis in wild type animals, in mutants, or in models of human disease. This information will allow us to build models to predict how transplanted stem cells may respond to different environments in the course of regenerative medicine. In addition, this information will help bioengineers generate niches, or even tissues, in vitro that can be used to maintain and/or expand stem cells for therapeutic uses.
Dev Dyn: What are some exciting ideas that are emerging in the field? What important questions remain to be answered?
TT: An important question is whether fate switches from stem to progenitor cells are reversible in normal homeostasis of adult vertebrate tissues. We have seen that just a couple of the right transcription factors can reverse the fate of a differentiated cell to embryonic stem cells. Does de-differentiation happen naturally during normal homeostasis and what is the extent of this phenomenon? When stem cell daughters undergo symmetric fate decisions to become stem cells and compensate for stem cell loss, is that a result of a symmetric or asymmetric stem cell division? The latter would be coupled with the ability of the more differentiated stem cell daughter to reverse back to the stem cell state when needed. Because divisions are rare within vertebrate stem cell pools, this has been difficult to monitor directly. In the Drosophila germ line there is evidence that such reversibility exists, although the reverted cells were deemed less potent and their accumulation in the stem cell pool was posed as the cause of tissue aging. It will be interesting to see how this model applies to vertebrates and how is it reconciled with the notion that stem cell pools can be rejuvenated by systemic cues.
LJ: I also look forward to seeing how universal “de-differentiation” within tissues will be as a mechanism to maintain stem cells, either as a basic mechanism underlying tissue homeostasis or in response to tissue damage.
In addition, I think that the role of local O2 levels, including ROS, in regulating stem cell behavior is quite interesting. This observation is tied to the fact that several stem cells reside within relatively hypoxic niches and that the vasculature is proving to be an integral component of the local niche in many systems.
One of the many fundamental questions that we are trying to tackle is how aging affects stem cells and the niche. Many patients that we hope to be able to treat with regenerative medicine and stem cell-based therapies will be older individuals. Data from our lab and others indicate that the niche also ages suggests it will likely be insufficient to supply these patients with “young, healthy” stem cells. Therefore, we must understand how the niche ages and consider ways to rejuvenate the niche or co-transplant new niche cells along with the stem cells. Furthermore, if we hope to recapitulate aging-related disease models in vitro, in the course of reprogramming patient fibroblasts, it will be essential to understand how the affected cell types age in normal individuals.