Stem cells are rare precursor cells that enable both normal turnover and regenerative replacement of differentiated cells in many adult tissues. Tight regulation of stem cell number and function in vivo is essential to maintain healthy tissues, although in most cases the mechanisms by which this regulation is achieved remain mysterious. For these reasons, my lab focuses on identifying new intrinsic and extrinsic regulators of stem cell activity, with a particular emphasis on hematopoietic and skeletal muscle stem cells and the ways in which these stem cells are altered by disease and by the normal, physiological process of aging.
Migration and Microenvironment Control Hematopoietic Stem Cells
Hematopoietic stem cells (HSCs) in adult mice and humans reside predominantly in the bone marrow and produce billions of red and white blood cells each day. These cells enter the bloodstream and tissues to deliver oxygen, fight infection, and repair tissue damage. Because they provide constant, lifelong production of functional blood cells, HSCs are critical components of bone marrow transplantation procedures. The success of these procedures requires (1) collection of sufficient numbers of stem cells for transplant, (2) efficient migration of these cells after intravenous transplant to their proper locations within the bone marrow, and (3) proliferation of these cells in the body to replenish lost cells rapidly.
In light of these requirements, a major objective of our work in the blood system has been to identify mechanisms that control HSC migration and expansion, with the hope that this knowledge may suggest improved strategies for effective treatment of hematological disease. To this end, we used transplant and parabiosis models to demonstrate that, rather than a "special event" stimulated specifically during transplantation, migration between the blood and bone marrow is a normal, physiological activity of HSCs. These data provided a cellular mechanism explaining the success of bone marrow transplant and paved the way to our identification of a novel regulator of stem cell trafficking (the immediate early response transcription factor Egr1).
Our recent studies of Egr1-knockout mice indicate that this transcription factor coordinately controls the overall number of HSCs in the bone marrow, the rate of HSC proliferation, and the migration of HSCs between the marrow and the blood. These studies establish stem cell trafficking as a key regulatory axis controlling HSC proliferation and anatomical localization in order to maintain appropriate hematopoietic function in vivo. Moreover, as EGR1 represents the first identified transcriptional regulator of HSC migration, this gene provides a novel molecular handle with which we can identify new pathways controlling HSC localization in the body.
In addition to analysis of intrinsic determinants of HSC proliferation and migration (such as Egr1), we also have investigated extrinsic regulators of HSC function. This work has utilized novel flow cytometry–based methods to demonstrate that expansion and migration of blood-forming stem cells in vivo is associated with an expansion of osteoblasts, bone-lineage cells that provide a supportive niche for HSC functions. Our ongoing work indicates that discrete functional changes occur within the osteoblastic niche cell population in response to hematological disease, treatment with chemotherapy agents, and aging. These changes in niche cells directly influence their ability to support HSC proliferation, self-renewal, and maintenance of reconstituting potential. These studies provide new insights into the complex intrinsic and extrinsic factors that regulate stem cell activity, and will help to elucidate the microenvironmental controls that determine HSC number and function.
Targeting Adult Skeletal Muscle Stem Cells for Therapeutic Muscle Repair
Similar to our work in the blood system, a major goal of my lab's work on muscle-forming stem cells has been to develop robust strategies to purify, expand, and transplant these cells to support the regeneration of skeletal muscle. Because these cells could, in theory, enable enduring replacement of mutant muscle fibers with normal, healthy fibers, this work is particularly relevant for the development of cell-based strategies to treat a variety of muscle degenerative diseases.
To this end, we recently isolated from adult mouse skeletal muscle a specific population of cells that act as muscle stem cells. These cells are a subset of muscle satellite cells (mononuclear cells found beneath the basal lamina surrounding mature muscle fibers). When transferred into the muscle of injured or diseased mice (including mouse models of human muscular dystrophy), they can contribute to up to 94 percent of muscle fibers and restore physiologic muscle function. Strikingly, these muscle stem cells also reseed a reserve pool of muscle precursors within the transplanted muscle and can be reactivated after engraftment to participate in subsequent rounds of muscle repair. These findings demonstrate the remarkable regenerative potential of skeletal muscle stem cells.
Our ability to directly identify and isolate adult skeletal muscle stem cells provides us with a new opportunity to interrogate their function during injury, repair, and disease. These ongoing studies have revealed a striking impairment in muscle stem cell maintenance and function, associated with degenerative and malignant muscle disease and with normal aging. We are investigating the molecular causes of these defects in muscle stem cell function and extending our findings to the isolation and study of analogous populations of human skeletal muscle stem cells. These studies will identify new regulatory pathways that control the activation, self-renewal, and regenerative potential of skeletal muscle stem cells.
Aging Stem Cells
Aging of multicellular organisms typically involves progressive decline in the body's ability to maintain homeostatic cell replacement and to regenerate tissues and organs after injury. In both the blood and the skeletal muscle, aging significantly impairs regenerative activity and can dysregulate normal homeostatic production of mature cells. These age-acquired defects in tissue function profoundly impact the health of older individuals, as evidenced by the high incidence of age-associated muscle deterioration (sarcopenia), bone marrow failure, immune dysfunction, and blood cancers in the elderly.
How aging causes deterioration of tissue function is poorly understood, but several lines of evidence, including datafrom my lab, suggest that loss or functional impairment of tissue-specific stem cells directly contributes to age-dependent failures in repair. Our data also indicate that the effects of aging on tissue stem cell function arise at least in part from alterations in the aged environment that suppress stem cell activity in older animals and are regulated by factors that circulate naturally in the bloodstream. In a comparative biology approach, we are using our stem cell and niche cell isolation methods, as well as our parabiotic mouse system (which allows the generation of age-matched [isochronic] and mismatched [heterochronic] animals sharing a common blood circulation), to identify systemic factors that modulate stem cell function and tissue regeneration in aged animals. New insights gained from these studies may ultimately enable novel interventions to delay or reverse the onset of age-related disease and extend the healthful life of aging individuals.
