Intrinsic genetic programmes as well as extracellular signals underlie stem cell fate choices. Whether a cell undergoes self-renewal or differentiation is the result of the spatial and temporal convergence of niche cues and the intrinsic state of the cell. The architectural elements of the niche have been discussed in detail above. Feedback signals from newly generated progeny can also regulate neural stem cells via either cell–cell contact or diffusible signals. When neuroblasts and transit-amplifying cells are depleted by an anti-mitotic treatment, SVZ astrocytes divide to rapidly regenerate the SVZ network of chains (Doetsch et al. 1999a
), perhaps reflecting loss of feedback inhibition from the neuroblasts onto their ancestors, such as loss of GABA signalling (Liu et al. 2005
). The first molecular cues that regulate stem cell lineages in the adult brain have begun to be identified ().
Figure 4 Neural stem cell regulation. Neural stem cells in the two adult neurogenic niches, the SVZ and SGZ, can be regulated by (1) diffusible factors (EGF, FGF, TGF-α, VEGF, PEDF, hormones, BMPs, ATP, Wnts and GABA) and their receptors, (2) cell–cell (more ...)
Notch is a transmembrane protein whose signalling regulates stem cell self-renewal in different niches (reviewed in Molofsky et al. 2004
). Notch is activated upon binding Delta or Jagged, both membrane ligands, which causes cleavage of the intracellular tail of Notch and its translocation to the nucleus (reviewed in Gaiano & Fishell 2002
). Over-expression of activated Notch1 in the embryonic brain or of activated Notch1 or 3 in adult cultured hippocampal progenitors leads to the generation of astrocytes (Gaiano et al. 2000
; Chambers et al. 2001
; Tanigaki et al. 2001
). This raises the possibility that Notch is also involved in stem cell/progenitor maintenance in adult neurogenic niches, where Notch1 and Jagged1 are expressed (Stump et al. 2002
; Irvin et al. 2004
; Nyfeler et al. 2005
). However, whether Notch activation promotes the acquisition of a stem cell astrocyte fate or differentiation into non-stem cell niche astrocytes awaits further evaluation.
Another molecule involved in proliferation in adult neurogenic regions is Shh. During development, Shh acts as a morphogen, playing a crucial role in ventral patterning along the entire extent of the neuraxis, and as a mitogen, stimulating granule cell precursor proliferation in the cerebellum (reviewed in Ruiz i Altaba et al. 2002
). In the adult SVZ, Shh regulates the proliferation of SVZ astrocytes (type B cells) and transit-amplifying type C cells (Machold et al. 2003
; Ahn & Joyner 2005
; Palma et al. 2005
). Shh also affects proliferation in the SGZ (Lai et al. 2003
; Machold et al. 2003
), although it is unknown which cell type Shh acts on. As mentioned above, conditional mice deficient in shh
signalling components first exhibit deficits in neural stem cell niches post-natally. The cellular source of Shh has not yet been identified, although for the hippocampal formation, it has been proposed that Shh is anterogradely transported to the SGZ via the fimbria–fornix (Lai et al. 2003
; Machold et al. 2003
). Importantly, the signals that trigger Shh release in both niches are unknown.
Members of the Wnt family of soluble ligands play critical roles in various physiological processes during development and in the adult. Wnt signalling regulates stem cell self-renewal in several stem cell niches (reviewed in Kleber & Sommer 2004
). In adult neurogenic niches, Wnt signalling has thus far been shown to regulate neurogenesis in the SGZ (Lie et al. 2005
). Increasing or decreasing Wnt activity in vivo
leads to an increase or decrease of SGZ neurogenesis, respectively. Interestingly, astrocytes are the source of Wnts, highlighting their multiple roles in the niche.
Two mitogens widely used to culture neural stem cells in vitro
either as neurospheres or as adherent monolayers are epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) (Reynolds & Weiss 1992
; Palmer et al. 1995
). These assays test the potential of a cell to act as a stem cell, but do not necessarily reflect its in vivo
behaviour. Although neurospheres were believed to arise from the relatively quiescent in vivo
SVZ stem cells, the majority of EGF-responsive neurospheres arise from the rapidly dividing transit-amplifying cells (Doetsch et al. 2002
), which retain the capacity to act as stem cells when exposed to exogenous growth factors. These findings emphasize the need to study stem cells and their progeny in vivo
to elucidate their roles in adult neurogenesis. Dissection of the EGF- and bFGF-responsive SVZ cell types has revealed that signalling through the EGF- and FGF-receptors occurs at distinct stages in the stem cell lineage. bFGF likely acts on more quiescent SVZ astrocytes (Zheng et al. 2004
) and has been proposed, based on analysis of bFGF null mice, to maintain the pool of neural stem cells (Zheng et al. 2004
). In contrast, EGF-responsive cells are transit-amplifying C cells and a subset of SVZ astrocytes (Doetsch et al. 2002
). This differential regulation may allow expansion of activated stem cells and transit-amplifying cells without depleting the more quiescent stem cells. The endogenous ligand for the EGF-R is likely TGF-α, which is expressed in the choroid plexus and striatum (Seroogy et al. 1993
). Consistent with this, TGF-α null mice exhibit decreased proliferation and neurogenesis in the SVZ (Tropepe et al. 1997
). Precise dissection of the roles of bFGF and TGF-α will require inducible genetic systems that allow one to circumvent possible developmental defects. Within adult neurogenic niches, other secreted factors, such as glycosylated cystatin which is expressed by some SGZ astrocytes, act synergistically with bFGF to influence neurogenesis (Taupin et al. 2000
). Another signal that may synergize with bFGF is ATP (Mishra et al. 2006
). The ecto-ATPase NTPDase2, which regulates ATP signalling, is present in both SVZ and SGZ astrocytes (Braun et al. 2003
; Shukla et al. 2005
). The in vivo
role of nucleotide signalling in adult neurogenesis is yet to be explored.
Interestingly, transit-amplifying cells are emerging as key nodes in the stem cell lineage. Many signals, including dopamine (Hoglinger et al. 2004
), soluble amyloid precursor protein (Caille et al. 2004
), nitric oxide (Estrada et al. 1997
; Matarredona et al. 2005
) and Shh (Ahn & Joyner 2005
; Palma et al. 2005
), converge on the transit-amplifying cells and act cooperatively with signalling through the EGF receptor. As transit-amplifying cells are rapidly dividing, it is critical to regulate their proliferation to prevent runaway growth. Furthermore, this mode of regulation allows the relatively quiescent stem cells to divide infrequently, stopping them from accumulating mutations with division.
The structural elements and molecules important for stem cell self-renewal and differentiation, as well as for embryonic stem cells, are rapidly being elucidated. It will be fascinating to see if these pathways are conserved in other niches and, importantly, in different species. Within adult neurogenic niches, astrocytes are stem cells in the adult SVZ in both rodents (reviewed in Doetsch 2003a
) and humans (Sanai et al. 2004
), yet there may be fundamental differences in how neural stem cells from the two species interpret these molecular signals and ultimately affect their output. Uncovering these pathways, as well as the complex interactions within the niche, will provide invaluable insight into stem cell biology and into the potential use of neural stem cells for restorative neurogenesis in disease or trauma.