The majority of cells in the nervous system arise during the embryonic and early post

Transcription

1 Introduction

2 Introduction The majority of cells in the nervous system arise during the embryonic and early post natal period. These cells are derived from population of neural stem cells first shown by Reynolds and Weiss (1992). Neural stem cells are progenitor cells present in the central nervous system (CNS) that not only can self-renew but also can generate various CNS cell lineages. Neural stem cells have been isolated and cultured from nearly all regions of the embryonic mouse CNS including the septum, cortex, thalamus, ventral mesencephalon and spinal cord. In culture, stem cells exhibit very similar features - extensive proliferative ability, self-renewal and differentiation of the progeny into neurons, astrocytes and oligodendrocytes. Cells isolated from different regions of the rat and human CNS have also been shown to possess the ability to form neurospheres in serum-free culture conditions in the presence ofmitogens however, the rate of success of passaging neurospheres for extended periods of time has been variable (Louis et al., 2008). Nervous system function depends on a complex architecture of neuronal networks and this complexity arises from the staggering morphological intricacy that neurons acquire during the course of differentiation ( da Silva & Dotti, 2002). This neuritic outgrowth is regulated by a variety of signaling mechanisms, growth factors, cytokines, transcription factors and soluble as well as membrane-bound receptors (Goldshmit et al., 2004a). Though several molecules involved in this signaling are now known, how extracellular signals regulate changes in this cytoskeletal arrangement are just 1

3 beginning to be elucidated. One group of molecules that may play a role in this process is the suppressor of cytokine signaling (SOCS) proteins. SOCS belongs to the family of cytokine inducible proteins and comprises of molecules induced upon cytokine stimulation, which block further signaling in a classic negative feedback loop. It consists of eight family members, CIS and the SOCS 1-7 proteins (Hilton et al., 1998; Wormald & Hilton, 2004). The SOCS members are localized in the cytoplasm, where they interact with their target proteins (Hansen et al., 1999; Ram & Waxman, 1999). It has been shown that SOCS1, SOCS2 and SOCS3 are expressed in the nervous system throughout development in the developing and adult mouse nervous system (Polizzotto et al., 2000). SOCS 1 regulates the interferon gamma mediated sensory neuron survival (Turnley et al., 2001). SOCS2 is involved in the neuronal differentiation by inhibiting the growth hormone signaling and also induces neuritic outgrowth by regulation of epidermal growth factor receptor activation (Scott et al., 2006; Turnley et al., 2002). SOCS3 over-expression inhibits astrogliogenesis and promotes maintenance of neural stem cells ((Bjorbaek et al., 1999; Cao et al., 2006; Zhu et al., 2008). In a recent report it was shown that in SOCS-5 transgenic mice, Th2 differentiation is inhibited by inhibiting IL-4 signaling (Seki et al., 2002). However, since no data is available from SOCS-5-deficient mice, the in vivo functions of SOCS- 5 have not yet been clarified. SOCS6 protein has been less extensively studied and has been shown to induce insulin resistance in the retina and promotes survival of the retinal neurons (Liu et al., 2008). Though it has not been shown to inhibit signaling via growth hormone, leukaemia 2

4 inhibitory factor, or prolactin (Masuhara et al., 1997; Nicholson et al., 1999), it is known to impair the insulin receptor signaling (Howard & Flier, 2006; Krebs et al., 2002; Li et al., 2004; Mooney et al., 2001) and is involved in the proteasome mediated degradation (Bayle et al., 2006). Among SOCS family members, SOCS6 has a unique addition of 300 amino acids to its N-terminal region; though the role of this addition remains unclear. Therefore, the SOCS6 protein might be expected to function differently than the other SOCS members (Hwang et al., 2007a). In vivo functions of SOCS-6 have not yet been fully described. SOCS are also thought to be involved in determining cell fate and in regulating inflammatory process (Elliott et al., 2004). By targeting distinct signaling pathways, SOCS can alter gene expression and can cause diverse diseases such as inflammatory bowel disease, allergy, autoimmune diseases and diabetes (Fujimoto et al., 2003). Various SOCS genes are expressed in the brain (Polizzotto et al., 2000). SOCS2 and SOCS3 have been shown to promote differentiation in neuronal cells (Goldshmit et al., 2004b; Yadav et al., 2005). Modulation of SOCS3 expression by IGF-1 may lead to attenuation of strength and duration of cytokine signaling. Effect of SOCS on cell survival was observed in human neuroblastoma cells, where SOCS-3 blocked STAT-3- mediated prosurvival effect ofigf-1 (Yadav et al., 2005). Although steady progress has been made in the understanding of the functions of various SOCS molecules, very little evidence exists on SOCS signaling mechanisms, regulation and expression in response to cytokines and/or IGF-1. With the following - background the aims and objectives of my thesis are: 3

5 AIM The aim of this thesis is to identify and characterize SOCS molecules involved in neural cell differentiation. OBJECTIVES 1. Characterization of neural stem cells. 2. Investigating the role of various SOCS family members in neural stem cell survival and differentiation mediated by I GF Identifying the specific ST ATs involved in the up-regulation of SOCS. 4. Exploring the mechanism of association of SOCS with different signaling pathways in neuronal cells in response to IGF-1. 4