Adult hypothalamic neurogenesis has been recently reported, but the cell of origin and function of these newborn neurons are unknown. We utilize genetic fate mapping to show that median eminence tanycytes generate newborn neurons; blocking this neurogenesis alters weight and metabolic activity in adult mice. These findings describe a previously unreported neurogenic niche within the mammalian hypothalamus with important implications for metabolism.

Adult tissues undergo continuous cell turnover in response to stress, damage, or physiological demand. New differentiated cells are generated from dedicated or facultative stem cells or from self-renewing differentiated cells. Here we describe a different stem cell strategy for tissue maintenance, distinct from that observed for dedicated or facultative stem cells. We report the presence of nestin-expressing adult stem cells in the perilumenal region of the mature anterior pituitary and, using genetic inducible fate mapping, demonstrate that they serve to generate subsets of all six terminally differentiated endocrine cell types of the pituitary gland. These stem cells, while not playing a significant role in organogenesis, undergo postnatal expansion and start producing differentiated progeny, which colonize the organ that initially entirely consisted of differentiated cells derived from embryonic precursors. This generates a mosaic organ with two phenotypically similar subsets of endocrine cells that have different origins and different life histories. These parallel but distinct lineages of differentiated cells in the gland may help the maturing organism adapt to changes in the metabolic regulatory landscape.

The Smoothened gene is necessary for cells to transduce hedgehog signaling. Although we and others have previously shown that embryonic removal of Smoothened in the neural tube results in a loss of stem cells from the postnatal subventricular zone, it was unclear whether this reflected a requirement for hedgehog signaling in the establishment or maintenance of the adult niche. Here, we have examined the consequences of conditional removal of Smoothened gene function within the subventricular zone of the adult neural stem cell niche. We observe that both proliferation and neurogenesis are compromised when hedgehog signaling is removed from subventricular zone stem cells. Moreover, even after a 10 month survival period, the stem cell niche fails to recover. It has been reported that the adult subventricular zone quickly rebounds from an antimitotic insult by increasing proliferation and replenishing the niche. During this recovery, it has been reported that hedgehog signaling appears to be upregulated. When mice in which hedgehog signaling in the subventricular zone has been strongly attenuated are given a similar antimitotic treatment, recovery is limited to the reduced level of proliferation and neurogenesis observed before the mitotic insult. Furthermore, the limited recovery that is observed appears to be largely restricted to the minority of neural stem cells that escape the conditional inactivation of Smoothened gene function. These results demonstrate that ongoing hedgehog signaling is required to maintain adult neural stem cells and that their ability to self-renew is limited.

In the brain, astrocytes are multifunctional cells that react to insults and contain damage. However, excessive or sustained reactive astrocytes can be deleterious to functional recovery or contribute to chronic inflammation and neuronal dysfunction. Therefore, astrocyte activation in response to damage is likely to be tightly regulated. Although factors that activate astrocytes have been identified, whether factors also exist that maintain astrocytes as nonreactive or reestablish their nonreactive state after containing damage remains unclear. By using loss-and gainof-function genetic approaches, we show that, in the unperturbed adult neocortex, FGF signaling is required in astrocytes to maintain their nonreactive state. Similarly, after injury, FGF signaling delays the response of astrocytes and accelerates their deactivation. In addition, disrupting astrocytic FGF receptors results in reduced scar size without affecting neuronal survival. Overall, this study reveals that the activation of astrocytes in the normal and injured neocortex is not only regulated by proinflammatory factors, but also by factors such as FGFs that suppress activation, providing alternative therapeutic targets.brain damage | astrogliosis A strocytes are the most abundant cell type in the mammalian brain. The thin processes of protoplasmic astrocytes (graymatter astrocytes) canvas the neural parenchyma and make contact with several other cell types. Protoplasmic astrocytes carry out a variety of functions, including maintaining the bloodbrain barrier, ion homeostasis, neurotransmitter turnover, and synapse formation (1). Another major function of these astrocytes involves their activation in response to damage. Astrocyte activation, or astrogliosis, plays a central role in the response to most or all neurological insults including trauma, infections, stroke, tumorigenesis, neurodegeneration, and epilepsy. The extent of astrogliosis can influence long-term recovery, and the response of astrocytes to different insults is likely to be graded and complex (2, 3). Nevertheless, in most cases, astrocytes transiently become hypertrophic and express high levels of intermediate filaments such as GFAP, vimentin, tenascin C, and nestin, and, in cases of severe damage, astrocytes can also become proliferative and form a scar.Astrogliosis can have beneficial and detrimental effects on recovery. Astrogliosis is essential for minimizing the spread of damage and inflammation, but it is also inhibitory for axonal and cellular regeneration. For example, in transgenic mice in which reactive astrocytes are ablated or disabled, traumatic injury leads to the lack of normal scar formation, prolonged and more widespread inflammation, and a failure to reconstruct the bloodbrain barrier and maintain tissue integrity (4, 5). However, ablation or impairment of reactive astrocytes also leads to increased nerve fiber growth in the immediate vicinity of the injury site, which could improve axonal regeneration and functional recovery (4, 5). Therefore, levels of astrocy...

We examined the postnatal consequences of removing Hedgehog signaling within the adult stem cell niche. Although at birth the subventricular zone appears normal in mice lacking Hedgehog signaling, by postnatal day 8 it is greatly impaired, and cell death is increased. In addition, both the quiescent B stem cell population and transit-amplifying C cells become depleted postnatally. In contrast, the A cell population expands precociously, mostly fails to migrate to the olfactory bulbs, and is ultimately also depleted by postnatal day 30. In vitro and in vivo analyses demonstrate that this failure in migration is a result of nonautonomous signaling, possibly caused by a reduction in Slit1 ligand in A cells. These results suggest that Hedgehog signaling is required for the maintenance of the B and C cell populations and indirectly for the migration of the neurons that are generated from the adult stem cell niche.