Chronically increased transforming growth factor-β1 strongly inhibits hippocampal neurogenesis in aged mice

There is increasing evidence that hippocampal learning correlates strongly with neurogenesis in the adult brain. increases in neurogenesis after brain injury also correlate with improved outcomes. With aging the capacity to generate new neurons decreases dramatically, both under normal conditions and after injury. How this decrease occurs is not fully understood, but we hypothesized that transforming growth factor (TGF)-beta 1, a cell cycle regulator that rapidly increases after injury and with age, might play a role. We found that chronic overproduction of TGF-beta 1 from astrocytes almost completely blocked the generation of new neurons in aged transgenic mice. Even young adult TGF-beta 1 mice had 60% fewer immature, doublecortin-positive, hippocampal neurons than wild-type littermate controls. Bromodeoxyuridine labeling of dividing cells in 2-month-old TGF-beta 1 mice confirmed this decrease in neurogenesis and revealed a similar decrease in astrogenesis. Treatment of early neural progenitor cells with TGF-beta 1 inhibited their proliferation. This strongly suggests that TGF-beta 1 directly affects these cells before their differentiation into neurons and astrocytes. Together, these data show that TGF-beta 1 is a potent inhibitor of hippocampal neural progenitor cell proliferation in adult mice and suggest that it plays a key role in limiting injury and age-related neurogenesis.