Stem cells persist throughout life in many tissues including the central nervous system (CNS) and regenerate mature cells that are lost due to turnover, injury, and disease. However, the function of stem cells declines with age in diverse tissues including the hematopoietic system, muscle, and brain. Consistent with this, aging tissues have less repair capacity and an increased incidence of degenerative disease. These observations raise the possibility that declines in stem cell function during aging contribute to age-related morbidity and that by uncovering the mechanisms responsible for these declines we might uncover targets for therapeutic intervention. The forebrain lateral ventricle subventricular zone (SVZ) contains stem cells that engage in neurogenesis throughout adult life. The frequency of stem cells, their self-renewal potential, mitotic activity in vivo, and rate of neurogenesis all decline with age, but the physiological mechanisms responsible for these declines are only beginning to be identified. During the prior funding period we discovered that Ink4a expression increases with age in these stem cells. Ink4a encodes a cyclin-dependent kinase inhibitor, p16Ink4a, that impairs proliferation and promotes cellular senescence by activating Rb. Deletion of Ink4a partially rescues the decline in stem cell frequency, mitotic activity, and neurogenesis in aging mice without affecting young mice. In preliminary studies we have discovered that the Alternative reading frame (Arf) at the Ink4a/Arf locus is not detectable in fetal and young adult stem cells but increases in expression with age and negatively regulates stem cell frequency and function. Arf encodes the p19Arf tumor suppressor, which impairs proliferation and promotes cellular senescence by promoting p53 function. This raises the question of whether Arf contributes to the decline in stem cell function with age.
In Aim 1 we will test whether conditional deletion of Arf from aging neural stem cells partially rescues the decline in stem cell frequency and function during aging. A second question is what regulates the increase in Ink4a and Arf expression with age. We have discovered that the high mobility group transcriptional regulator, Hmga2, is expressed by fetal and young adult, but not old adult stem cells. Hmga2 increases the frequency and function of neural stem cells in fetal and young adult mice by negatively regulating Ink4a and Arf expression.
In Aim 2 we will test whether Hmga2 negatively regulates Ink4a/Arf expression by repressing the JunB transcription factor and whether changes in JunB expression also regulate neural stem cell aging. Finally, we have discovered that the microRNA let-7b, which inhibits Hmga2 expression, increases its expression with age in neural stem/progenitor cells.
In Aim 3 we will test whether the decline in Hmga2 expression during aging is caused by the increase in let-7 function. Our experiments offer the opportunity to unravel a novel pathway in which let-7b, Hmga2, JunB, p16Ink4a, and p19Arf regulate the age-related decline in stem cell function and neurogenesis in the mammalian CNS.
Aging tissues exhibit reduced stem cell function, reduced regenerative capacity, and an increased incidence of degenerative disease. We have identified a series of genes that function as part of a pathway that reduces stem cell function and tissue regenerative capacity during aging. By better understanding these mechanisms we will gain new insights into why aging tissues have reduced regenerative capacity as well as therapeutic strategies to enhance regeneration.
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