**4. Signalling cascades regulating NPC fate following neural damage**

Neurogenesis and gliogenesis are known to be initiated following brain damage, such as is‐ chemia, seizures, traumatic injury and neurodegenerative diseases [90-92]. However, these new neurons and glia usually do not effectively replenish those that were lost. Many of the normal signalling cascades are altered following injury. Below is a discussion of the major changes in these cascades that influence neuronal fate of the NSCs generated in the SVZ and SGZ following injury or disease.

#### **4.1. Brain injury**

catenin [53]. Interferon-γ, which signals via STAT1, and interferon-β which does not, both inhibit cultured adult NPC proliferation, but only interferon-γ promotes neuronal differen‐

Migration from the SVZ along the RMS involves long distances and multiple pathways [56] For example, it is dependent on Shh signalling, as evident by a decrease of neuroblasts in the olfactory bulb following Hedgehog signalling interruption [11]. Shh is a chemoattractant cue extrinsic to the neuroblast that guides migration to the olfactory bulb. Neurotrophic growth factor signalling is also important for migration, in particular insulin-like growth factor (IGF-1) null mice show an abundance of neuroblasts in the SVZ that have failed to mi‐ grate to the olfactory bulb [57]. Guidance cues from EphB2/ephrin-B2 pathways also enable formation of the chain migration from the SVZ to the olfactory bulb [58]. Recently, endocan‐ nabinoid signalling has been shown to regulate migration and neurogenesis in both the SVZ and dentate gyrus [59, 60]. Other molecules involved in this migration include polysialated neural cell adhesion molecule (PSA-NCAM) [61-63], Slit-Robo [64] and integrins [65, 66]. Many of these factors signal via the Rho kinase pathway, which is a downstream regulator of NPC migration [67]. In addition adult NPCs express a range of chemokine receptors and chemokines are expressed in different brain regions, with the highest levels in the olfactory bulb, suggesting an as yet largely unexplored role for chemokines in regulating basal adult

The migration distance for new neurons from the SGZ is relatively short as they travel into the granular layer above the SGZ, where guidance molecules may control this movement. NMDA receptor signalling is required for the proper migration of newborn granular cells in the dentate gyrus [69]. This is achieved through the activation of Disrupted-in-schizophre‐ nia (DISC1), as neurons without DISC1 migrate further into granular layer and into the mo‐ lecular layer [69, 70]. DISC1 also controls the dendritic maturation of newborn granule cells through GABA depolarization of NKCC1 and activation of the Akt-mTOR pathway [70, 71]. New neurons must integrate into existing circuitry or they will not survive. The vast majority of new neurons do not survive past 4 weeks. Interestingly, NMDA receptors expressed in neu‐ roblasts along the RMS are crucial to the integration of these neurons in existing olfactory bulb circuitry [72]. Glutamate is released from astrocyte-like cells that surround the neuroblasts. NMDA receptor activation in newly-born dentate gyrus granule cells also increases survival. Initial GABA depolarization plays a role in the maturation of neurons in the dentate gyrus and olfactory bulb [73, 74]. This depolarization and subsequent Ca2+ influx are required for den‐ drite initiation and elongation [75]. This process involves coordinated expression of the GABA receptor subunit alpha2 that controls the maturation of the new neurons [76]. In addition, ag‐ rin signalling is necessary for integration and survival of newborn neurons in the olfactory bulb, as demonstrated by a loss of agrin leading to improper synapse formation while an over‐

Neurotrophin signalling has important role in the survival and integration of new neurons. Brain-derived growth factor (BDNF) binding to the TrkB receptor tyrosine kinases increases

expression of agrin results in an increase in dendritic spines [77].

tiation [54, 55].

NPC migration [68].

**3.2. Migration and integration**

244 Trends in Cell Signaling Pathways in Neuronal Fate Decision

A traumatic lesion to the brain cortex results in an increase in proliferation of NSCs in the SVZ, although varied locations and degrees of injury have resulted in an incongruity of re‐ sults across the literature [93-98]. Nonetheless, it is generally agreed that the increase in pro‐ liferation results in an increase in neurogenesis at the SVZ [99]. Expression of growth factors such as BDNF, FGF2, GDNF, IGF-1 and VEGF are increased following ischemia and exoge‐ nous application further augments NSC proliferation and survival [100-105]. Shh expression is also upregulated in the SVZ following ischemia, potentially playing a role in the increase of proliferation, while Wnt expression does not change [106, 107]. Phosphorylated CREB is upregulated following ischemia and induces hippocampal neurogenesis [108].

Following proliferation these cells must migrate and integrate to damaged cortical tissue. The majority of research on ectopic migration from the SVZ has been performed following an ischemic insult and has demonstrated that cells do reach the injured striatum [90, 109-114]. It appears that the cells no longer migrate in a chain formation and carry on indi‐ vidually, interestingly, at the expense of the RMS population [109, 115]. This change in mi‐ gration is the direct result of chemoattractive cues expressed from the injury site. Chemokines and their receptors can attract neuroblasts from the RMS, for example it has been shown that Stromal cell-derived factor-1 (CXCL12) and its receptor CXCR4 are upregu‐ lated at the injury site [116, 117]. Expression of several chemokines and their receptors is up‐ regulated on adult NPCs by inflammatory cytokines, such as interferon-γ and TNF-α [68].

generation of immature neurons from NPCs. However, reduced concentrations of multiple neurotrophic factors and higher levels of FGF2 seem to induce a developmental arrest of newly generated neurons. Further, there is a down-regulation of *Olig2* and over-expression of *Ascl1* caused by amyloid-β that switches the cell fate to death [134, 135]. Generally, there is a decrease in proliferation and survival of NSCs in the dentate gyrus and SVZ with AD. A better understanding on the effects of amyloid-β on NSC proliferation and maturation is

Regulation of Basal and Injury-Induced Fate Decisions of Adult Neural Precursor Cells: Focus on SOCS2 and Related

Signalling Pathways

247

http://dx.doi.org/10.5772/53268

Parkinson's disease (PD) is the outcome of the loss of dopaminergic neurons in the substan‐ tia nigra of the midbrain (reviewed in [136]). In transgenic mouse models, there is a decrease in newly generated neurons in both the dentate gyrus and olfactory bulb [137, 138]. Altera‐ tions in neurogenesis have been linked to a decrease in *Notch1* and *Hes5* expression [138]. Lack of proliferation could be the explanation for a lack of migration of NSCs to the dam‐ aged regions in PD and AD [139]. Along these lines, manipulations that increase prolifera‐ tion also demonstrate migration, for example intraventricular injection of clustering ephrin-A1-Fc increased proliferation in the SVZ, followed by migration to the striatum and differentiation into dopaminergic neurons in a rodent model of Parkinson's disease [140]. Furthermore, exogenous application of EGF and FGF2 showed similar results [141]. Exciting research in salamanders has shown regeneration of dopamine neurons following ablation involving neurogenesis in quiescent cells. This activation is due to the loss of dopamine, demonstrating a control of dopamine signalling maintaining homeostasis [142]. Replace‐ ment of dopaminergic neurons relies on NSC differentiation into the proper neuronal fate. Recent studies have elucidated the transcription factors necessary to produce dopaminergic neurons. The combination of *Ascl1/Mash1*, *Nurr1* and *Lmx1a* result in the generation of func‐ tional dopaminergic neurons from mouse and human fibroblasts [143]. Other studies have shown that *Foxa2* in combination with *Nurr1* can also induce the production of nigral (A9)-

Other neurodegenerative diseases such as Huntington's disease have shown a decrease in neurogenesis. NPC proliferation is decreased in Huntington's disease in both the SGZ and SVZ, with some reports of reduced numbers of newly born neurons (reviewed in [132]). In a rat model of Huntington's disease, SGZ progenitor cell proliferation is decreased due to an increase in *Sox2*-positive quiescent stem cells and a decrease in CREB signalling [145].

Overall, further investigation is needed to clarify the changes in signalling pathways follow‐ ing neurodegenerative disease. One pathway that has been extensively studied both in basal neurogenesis and after injury is the suppressor of cytokine signalling (SOCS) family of pro‐ teins. The following section will discuss research involving the SOCS proteins and related

As discussed in the previous section, a diversity of signalling cascades are involved in regu‐ lating neuronal cell proliferation, differentiation and survival. However, JAK-STAT signal‐

**5. SOCS molecules and cytokine signalling pathways**

needed to improve this decrease in neurogenesis.

type midbrain neurons from NPCs [144].

pathways.

Migration is also altered following an epileptic seizure: the NSCs migrate along the RMS more quickly, while in the dentate gyrus there is faster integration and maturation [118, 119]. There are morphological changes to the hippocampal region including mossy fibre sprouting, dispersion of the granular cell layer, and ectopically migrated dentate granule cells in the hilus (reviewed in [120]).

When cells do migrate to the correct location they must differentiate into neurons to recover function of neurons lost. Unfortunately, this does not appear to be consistent. Recent work on ischemia has demonstrated that new neurons from the SVZ are found in the cortex near the le‐ sioned area, while injury of the somatosensory cortex showed the generation of astrocytes and microglia/macrophages without any new neurons [98, 121]. Other work has found the produc‐ tion of astrocytes and oligodendrocytes near the injury site as a result of expression of repress‐ ors of neuronal fate [122, 123]. For example, the BMP antagonist, chordin, and the transcription factor Olig2 both induce glial expression in neuroblasts at the injury site [124, 125]. However, following ischemia, pro-neuronal transcription factors are expressed in primate progenitors in the SGZ, including *Emx2*, *Pax6* and *Ngn2* [126]. Recently it has been shown that following thir‐ ty and sixty days after stroke, *Ascl1/Mash1* expressing cells in the ischemic striatum gave rise to GABAergic neurons and mature oligodendrocytes [127]. Even when a NSC differentiates into a neuron, the survival of these neurons is very low. Recent work has demonstrated that the Ras-related GTPase, Rit, is an important component in the survival of young granular cells in the dentate gyrus following a brain injury. Rit-/- mice show a marked increase in new neuron death following injury [128]. Recently, the small non-coding RNA molecule, miR-124a, was shown to be altered following stroke. Interestingly, it can mediate stroke induced neurogene‐ sis via the Notch signalling pathway [129]. Inhibition of the Notch pathway increases neuro‐ genesis after spinal cord injury in zebrafish resulting in higher proliferation and more motor neurons [130]. Lentiviral expression of Wnt3 increased neurogenesis following focal ischemia and improved functional recovery [131].

#### **4.2. Neurodegenerative diseases**

Reports on neurogenesis in neurodegenerative diseases are highly dependent on the disease model used. Variations in transgenic mice and other drug induced models are the most probable cause for the conflicting results. In many models of Alzheimer's disease, Parkin‐ son's disease and Huntington's disease there is impaired neurogenesis (reviewed in [132]). Alzheimer's disease (AD) is characterized by degeneration of basal forebrain cholinergic neurons in the cortex and hippocampus from the deposition of neurofibrillary tangles and amyloid-β plaques [133]. The neuropathologic hallmark of AD is the amyloid-β plaques; however small oligomeric amyloid-β appears to be the noxious component. Neurogenesis can be both increased and decreased in AD, depending on the transgenic model used (re‐ viewed in [132]). Early in the disease, oligomeric amyloid-β may transiently promote the generation of immature neurons from NPCs. However, reduced concentrations of multiple neurotrophic factors and higher levels of FGF2 seem to induce a developmental arrest of newly generated neurons. Further, there is a down-regulation of *Olig2* and over-expression of *Ascl1* caused by amyloid-β that switches the cell fate to death [134, 135]. Generally, there is a decrease in proliferation and survival of NSCs in the dentate gyrus and SVZ with AD. A better understanding on the effects of amyloid-β on NSC proliferation and maturation is needed to improve this decrease in neurogenesis.

Chemokines and their receptors can attract neuroblasts from the RMS, for example it has been shown that Stromal cell-derived factor-1 (CXCL12) and its receptor CXCR4 are upregu‐ lated at the injury site [116, 117]. Expression of several chemokines and their receptors is up‐ regulated on adult NPCs by inflammatory cytokines, such as interferon-γ and TNF-α [68].

Migration is also altered following an epileptic seizure: the NSCs migrate along the RMS more quickly, while in the dentate gyrus there is faster integration and maturation [118, 119]. There are morphological changes to the hippocampal region including mossy fibre sprouting, dispersion of the granular cell layer, and ectopically migrated dentate granule

When cells do migrate to the correct location they must differentiate into neurons to recover function of neurons lost. Unfortunately, this does not appear to be consistent. Recent work on ischemia has demonstrated that new neurons from the SVZ are found in the cortex near the le‐ sioned area, while injury of the somatosensory cortex showed the generation of astrocytes and microglia/macrophages without any new neurons [98, 121]. Other work has found the produc‐ tion of astrocytes and oligodendrocytes near the injury site as a result of expression of repress‐ ors of neuronal fate [122, 123]. For example, the BMP antagonist, chordin, and the transcription factor Olig2 both induce glial expression in neuroblasts at the injury site [124, 125]. However, following ischemia, pro-neuronal transcription factors are expressed in primate progenitors in the SGZ, including *Emx2*, *Pax6* and *Ngn2* [126]. Recently it has been shown that following thir‐ ty and sixty days after stroke, *Ascl1/Mash1* expressing cells in the ischemic striatum gave rise to GABAergic neurons and mature oligodendrocytes [127]. Even when a NSC differentiates into a neuron, the survival of these neurons is very low. Recent work has demonstrated that the Ras-related GTPase, Rit, is an important component in the survival of young granular cells in the dentate gyrus following a brain injury. Rit-/- mice show a marked increase in new neuron death following injury [128]. Recently, the small non-coding RNA molecule, miR-124a, was shown to be altered following stroke. Interestingly, it can mediate stroke induced neurogene‐ sis via the Notch signalling pathway [129]. Inhibition of the Notch pathway increases neuro‐ genesis after spinal cord injury in zebrafish resulting in higher proliferation and more motor neurons [130]. Lentiviral expression of Wnt3 increased neurogenesis following focal ischemia

Reports on neurogenesis in neurodegenerative diseases are highly dependent on the disease model used. Variations in transgenic mice and other drug induced models are the most probable cause for the conflicting results. In many models of Alzheimer's disease, Parkin‐ son's disease and Huntington's disease there is impaired neurogenesis (reviewed in [132]). Alzheimer's disease (AD) is characterized by degeneration of basal forebrain cholinergic neurons in the cortex and hippocampus from the deposition of neurofibrillary tangles and amyloid-β plaques [133]. The neuropathologic hallmark of AD is the amyloid-β plaques; however small oligomeric amyloid-β appears to be the noxious component. Neurogenesis can be both increased and decreased in AD, depending on the transgenic model used (re‐ viewed in [132]). Early in the disease, oligomeric amyloid-β may transiently promote the

cells in the hilus (reviewed in [120]).

246 Trends in Cell Signaling Pathways in Neuronal Fate Decision

and improved functional recovery [131].

**4.2. Neurodegenerative diseases**

Parkinson's disease (PD) is the outcome of the loss of dopaminergic neurons in the substan‐ tia nigra of the midbrain (reviewed in [136]). In transgenic mouse models, there is a decrease in newly generated neurons in both the dentate gyrus and olfactory bulb [137, 138]. Altera‐ tions in neurogenesis have been linked to a decrease in *Notch1* and *Hes5* expression [138]. Lack of proliferation could be the explanation for a lack of migration of NSCs to the dam‐ aged regions in PD and AD [139]. Along these lines, manipulations that increase prolifera‐ tion also demonstrate migration, for example intraventricular injection of clustering ephrin-A1-Fc increased proliferation in the SVZ, followed by migration to the striatum and differentiation into dopaminergic neurons in a rodent model of Parkinson's disease [140]. Furthermore, exogenous application of EGF and FGF2 showed similar results [141]. Exciting research in salamanders has shown regeneration of dopamine neurons following ablation involving neurogenesis in quiescent cells. This activation is due to the loss of dopamine, demonstrating a control of dopamine signalling maintaining homeostasis [142]. Replace‐ ment of dopaminergic neurons relies on NSC differentiation into the proper neuronal fate. Recent studies have elucidated the transcription factors necessary to produce dopaminergic neurons. The combination of *Ascl1/Mash1*, *Nurr1* and *Lmx1a* result in the generation of func‐ tional dopaminergic neurons from mouse and human fibroblasts [143]. Other studies have shown that *Foxa2* in combination with *Nurr1* can also induce the production of nigral (A9) type midbrain neurons from NPCs [144].

Other neurodegenerative diseases such as Huntington's disease have shown a decrease in neurogenesis. NPC proliferation is decreased in Huntington's disease in both the SGZ and SVZ, with some reports of reduced numbers of newly born neurons (reviewed in [132]). In a rat model of Huntington's disease, SGZ progenitor cell proliferation is decreased due to an increase in *Sox2*-positive quiescent stem cells and a decrease in CREB signalling [145].

Overall, further investigation is needed to clarify the changes in signalling pathways follow‐ ing neurodegenerative disease. One pathway that has been extensively studied both in basal neurogenesis and after injury is the suppressor of cytokine signalling (SOCS) family of pro‐ teins. The following section will discuss research involving the SOCS proteins and related pathways.
