Interactions between stem cells and the microenvironment in vivo - Announcements: fellowships, grants, & awards

Environmental Health Perspectives, Nov 15, 2003

National Institute of Neurological Disorders and Stroke (NINDS), the National Institute on Drug Abuse (NIDA), the National Institute of Deafness and Other Communication Disorders (NIDCD) the National Institute of Alcohol Abuse and Alcoholism (NIAAA), and the National Institute on Aging (NIA) invite applications for studies on the cellular and molecular signaling between the local environment within organisms and stem and progenitor cells that are-either introduced as transplants or are normally resident within host tissues and organs. The objective of this initiative is to promote a thorough exploration and characterization of the bi-directional communication between multipotent cells and the three-dimensional local milieu or niche that they encounter in vivo under normal and compromised states, such as with aging or following injury, disease or drug exposure. Of particular interest is the rigorous characterization of how interactions with localized cues in space and time regulate stem cell survival, migration, replication and 'plasticity' in the nervous system and other parts of the body. Projects that address comparisons between the responses of stem cells within niches in the developing and mature or aging nervous system in vivo, or in host microenvironments modified by injury, disease, or by exposure to drugs and alcohol would also be directly relevant to this Program Announcement with Set-aside (PAS), as are studies to compare different classes of stem cells or progeny at progressively more advanced stages of differentiation when placed in the same sites in vivo.

Unlike organs such as the skin and the gut that self-renew throughout life, the nervous system in adult mammals is restricted in its ability to replace neurons and glia that have been lost through injury, disease, alcohol and drug abuse or even advancing age. Stem cell research offers enormous potential for treating many congenital, developmental, psychiatric or degenerative diseases of the nervous system for which there are no treatments or cures. Under the appropriate tissue culture conditions, a variety of multi-potent cells appear to acquire many properties of neurons and glia--a first step toward developing cell replacement therapies for neurological dysfunction. The discovery of endogenous stem cells, residing either within the nervous system or in other tissues raises the possibility that these intrinsic systems may be harnessed to restore defective cells and functions. In both cases the expectation is that, when exposed to the optimal microenvironment in vivo, endogenous or transplanted stem cells will differentiate in a manner appropriate to the local brain region, and integrate with the existing circuits in the nervous system. The past decade has seen enormous progress in our understanding of the specific requirements of stem cells to proliferate and differentiate along specified lineages. This progress has been made possible by the discovery of a myriad of growth factors and substrate conditions followed by careful testing in culture. Unfortunately, the behavior of cells in tissue culture does not adequately predict how these same cells will behave when transplanted into the living host where multiple known and unknown factors converge to influence the biological process. We do not know the whole spectrum of factors present in vivo that influence cell fate. Effective use of stem and progenitor cells for therapeutic purposes hinges on their ability to thrive, integrate, and function in a biologically meaningful manner in vivo without causing adverse events. Therefore the next stage in developing cell restoration therapy requires understanding how the newly generated cells will behave within the host.

Recent reports indicate that the "niche" or local microenvironment that a stem cell encounters governs its behavior and fate. For example, adult neural stein cells produced neurons when transplanted into the neurogenic zone of the hippocampus, but produced astrocytes in the environment of the spinal cord. Further investigation showed that a specific component of the local environment, the regional astrocytes from the hippocampus were capable of instructing these stem cells to adopt a neuronal fate in vitro. In addition to regional differences within the nervous system, the microenvironment encountered by a stem cell may vary as a function of age of the host organism. Similarly, alteration of the niche by injury, drugs or other circumstances is likely to affect the ability of transplanted stem cells to survive, differentiate and integrate into existing neural circuitry. Understanding these changes will be important in making decisions about the use of cell replacement therapies in very young or elderly patients, in patients with a history, of alcohol or drug usage, or suffering from injury or other neurological conditions.

Transplanted cells can act to influence and change host cells in their vicinity. Stem cells may release agents that alter the activity or resiliency of damaged host cells. These dynamic interactions are inevitable as living cells and tissue contact, react and respond to each other in time and space. Teasing out and understanding these interactions poses a major challenge that must be faced in order to develop realistic cell replacement therapies and enhance normal tissue regeneration.