**6. SOCS2 in the brain**

This interesting spatiotemporal expression of SOCS2 instigated further research into its pos‐ sible role in neuronal development. The generation of the SOCS2 knockout (SOCS2-/-) and SOCS2 overexpressing transgenic (SOCS2Tg) mice has been instrumental in the functional characterisation of SOCS2 [169, 170]. SOCS2-/- mice display an overgrowth phenotype where adult mice are up to 40% heavier than their wild-type counterparts, mainly attributed to an increase in organ size and bone length [170]. This phenotype suggested an involvement of SOCS2 in the negative regulation of GH, a regulator of postnatal growth. To address this hy‐ pothesis, SOCS2Tg mice were generated [169]. Interestingly, SOCS2Tg mice also display an enhanced growth phenotype, indicating a potential dual action of SOCS2 where at high lev‐ els it may enhance rather than inhibit growth hormone signalling [169].

In-vitro, neural stem cells from SOCS2-/- mice show a marked reduction in the number of neu‐ rons generated [171], as opposed to SOCS2Tg mice which show an increase in neuron number [172-174]. Additionally, PC12 cells and neural cells from SOCS2Tg mice demonstrate in‐ creased neurite outgrowth in tissue culture [171, 174-176]. GH is an inhibitor of neural differen‐ tiation and its negative regulation by SOCS2 is evident by the reduction in neuronal differentiation in neural stem cell cultures of SOCS2-/- mice [171, 174]. The importance of GH/ SOCS2 signalling in neuronal differentiation can be illustrated by their involvement in the reg‐ ulation of the Ngn1 basic helix-loop-helix transcription factor [171]. Ngn1 has an important role in promotion of neurogenesis by at the same time inhibiting glial differentiation [177]. Im‐ portantly, Ngn1 is subject to inhibition by GH and this inhibition is overcome by SOCS2 over‐ expression [171]. Thus, a model has been proposed where GH and SOCS2 regulate neural stem cell differentiation through the modulation of Ngn1 expression [178].

GH binds and signals through the GH receptor (GHR) which belongs to the class I super‐ family of cytokine receptors. Like the LIFRβ/gp130 complex, signal transduction is carried out through the JAK-STAT pathway. GH binding activates GHR resulting in JAK activation. JAK2 is the major contributor to GH signalling and it phosphorylates tyrosine residues on the GHR that become binding sites primarily for STAT5a or STAT5b. Activated STAT5 then induces SOCS gene expression [179]. JAK2 may also activate STAT1 and STAT3, however this can be cell type specific [180, 181]. One mechanism by which SOCS2 may block STAT5 activation is via its binding to phosphorylated tyrosine residues at the STAT5 binding site on the GHR [182].

were found to be expressed at all ages (E10 to P25) with a common peak in expression be‐ tween E14 and P8. However, the level of SOCS2 expression was much higher in comparison. The spatial pattern of SOCS2 expression also distinguished it from the other SOCS genes, with moderate to high levels of expression in neurogenic regions and in newborn neurons. In the adult, SOCS2 was maintained in the CA3 region of the hippocampus and at a moder‐ ate level in the dentate gyrus, compared to other SOCS genes whose expression was not lo‐ calized, if expressed at all under basal conditions. SOCS2 expression was also present in the cerebral cortex and other regions such as the olfactory bulb, forebrain and cerebellum. Inter‐ estingly, SOCS2 was first upregulated at the time of neuronal differentiation, which is be‐ tween the developmental stages E10 and E12, suggesting a role for SOCS2 in neural

This interesting spatiotemporal expression of SOCS2 instigated further research into its pos‐ sible role in neuronal development. The generation of the SOCS2 knockout (SOCS2-/-) and SOCS2 overexpressing transgenic (SOCS2Tg) mice has been instrumental in the functional characterisation of SOCS2 [169, 170]. SOCS2-/- mice display an overgrowth phenotype where adult mice are up to 40% heavier than their wild-type counterparts, mainly attributed to an increase in organ size and bone length [170]. This phenotype suggested an involvement of SOCS2 in the negative regulation of GH, a regulator of postnatal growth. To address this hy‐ pothesis, SOCS2Tg mice were generated [169]. Interestingly, SOCS2Tg mice also display an enhanced growth phenotype, indicating a potential dual action of SOCS2 where at high lev‐

In-vitro, neural stem cells from SOCS2-/- mice show a marked reduction in the number of neu‐ rons generated [171], as opposed to SOCS2Tg mice which show an increase in neuron number [172-174]. Additionally, PC12 cells and neural cells from SOCS2Tg mice demonstrate in‐ creased neurite outgrowth in tissue culture [171, 174-176]. GH is an inhibitor of neural differen‐ tiation and its negative regulation by SOCS2 is evident by the reduction in neuronal differentiation in neural stem cell cultures of SOCS2-/- mice [171, 174]. The importance of GH/ SOCS2 signalling in neuronal differentiation can be illustrated by their involvement in the reg‐ ulation of the Ngn1 basic helix-loop-helix transcription factor [171]. Ngn1 has an important role in promotion of neurogenesis by at the same time inhibiting glial differentiation [177]. Im‐ portantly, Ngn1 is subject to inhibition by GH and this inhibition is overcome by SOCS2 over‐ expression [171]. Thus, a model has been proposed where GH and SOCS2 regulate neural stem

GH binds and signals through the GH receptor (GHR) which belongs to the class I super‐ family of cytokine receptors. Like the LIFRβ/gp130 complex, signal transduction is carried out through the JAK-STAT pathway. GH binding activates GHR resulting in JAK activation. JAK2 is the major contributor to GH signalling and it phosphorylates tyrosine residues on the GHR that become binding sites primarily for STAT5a or STAT5b. Activated STAT5 then

els it may enhance rather than inhibit growth hormone signalling [169].

cell differentiation through the modulation of Ngn1 expression [178].

precursor differentiation [168].

250 Trends in Cell Signaling Pathways in Neuronal Fate Decision

**6. SOCS2 in the brain**

SOCS2 can also regulate signalling via the EGF receptor (EGFR) [175, 176]. The main physio‐ logical target for EGFR is EGF. EGFR primarily activates and signals through the Ras/MAPK pathway [183]. In terms of the neuronal effects of EGF, it has been shown to enhance neurite outgrowth and survival of different populations of cultured neurons [175, 183]. Relevant to this review, it also has an important role in neurogenesis. As described above, in the adult SVZ and dentate gyrus, EGF regulates neural precursor cell proliferation [37]. The impor‐ tance of this role is evident when, in response to brain injury, there is an expansion of neural stem cell numbers in the SVZ as a result of an increased responsiveness to EGF due to EGFR upregulation [184]. Important for SOCS2 interaction, EGF also activates STAT5, a process in‐ volving the Src tyrosine kinase [185-187]. Overexpression of SOCS2 in PC12 cells inhibited this EGF induced STAT5 phosphorylation [176]. The EGFR was also constitutively phos‐ phorylated at the Src binding site, Tyr845, in SOCS2 overexpressing PC12 cells. It was there‐ fore proposed that SOCS2 competitively bound to Tyr845 and blocked its dephosphorylation by the phosphatase SHP2 to allow prolonged Src activation and enhancement of neurite out‐ growth [176].

However, while SOCS2 regulated SVZ-derived neurogenesis in a GH dependent manner during development, in the adult SVZ it appears to regulate neurogenesis via regulation of erythropoietin signalling [188]. Further, the mechanism by which SOCS2 regulates adult hippocampal neurogenesis is different and does not appear to involve GH or erythropoietin, although Epo transiently enhanced SGZ NPC proliferation [189, 190]. Hippocampal neuro‐ genesis was studied under control and voluntary exercise conditions (to enhance basal hip‐ pocampal neurogenesis) in wildtype, SOCS2Tg and GHR-/- mice. Mice of all 3 genotypes had similar basal levels of neurogenesis and equivalently increased neurogenesis in response to exercise at early timepoints (8 days) aimed at measuring extent of NPC proliferation. How‐ ever, at later timepoints (35 days) aimed at examining newborn neuron survival, there was a 50% increase in the survival of adult hippocampal neurons in SOCS2Tg mice, under basal conditions and following voluntary exercise. Additionally, SOCS2Tg mice performed better than wildtype animals in the Morris Water Maze which probes hippocampal-dependent cognition [190]. This was an exciting result, as it identified SOCS2 as a potential therapeutic target that could enhance the survival of newly born neurons following brain injury. How‐ ever, given that GHR-/- mice showed no differences in adult hippocampal neurogenesis compared to wildtype, the mechanism by which SOCS2 promotes survival in this case re‐ mains to be determined. One possible explanation for this increase in neuronal survival in SOCS2Tg mice may be that the enhanced neurite outgrowth observed in SOCS2Tg neurons may aid functional integration into existing circuitry and the consequent maturation and survival of neurons.
