Neural stem/progenitor cells (NS/PCs), found in both the developing and the adult mammalian central nervous system (CNS), are a heterogeneous population of multipotent cells with the potential to self-renew by symmetric cell division or to differentiate into neurons, astrocytes and oligodendrocytes through asymmetric cell division (Gage, 2000; Alvarez-Buylla et al., 2001; Temple, 2001; Götz and Huttner, 2005). NS/PCs have been found in almost all regions of the developing mammalian CNS, including the basal forebrain, cerebral cortex, ganglionic eminence, hippocampus, cerebellum, neural crest and spinal cord (Temple, 2001). Throughout development, NS/PCs give rise to neurons and glial cell populations of the CNS. In the adult CNS, NS/PCs are mainly found in the subventricular zone (SVZ) and subgranular layer (SGL) of hippocampal dentate gyrus (DG) (Göritz and Frisén, 2012). The ependymal cells lining the central canal of spinal cord of the adult mouse could be another potential source of adult NS/PCs (Meletis et al., 2008). Because neurogenesis and gliogenesis occur during different stages of mammalian brain development, it was long assumed that neurons and glial cells in the CNS were generated from distinct precursor populations, known as early-embryonic, late-embryonic, and adult NS/PCs. However, abundant evidence has since now demonstrated that embryonic and adult NS/PCs are likely lineage-related. Neuroepithelial cells behaving as NS/PCs during very early developmental stages of the mammalian CNS give rise to radial glial cells around embryonic day 12 (E12). As the progeny of neuroepithelial cells, radial glial cells act as NS/PCs in the fetal and perinatal brain, and develop into astrocyte-like stem cells in the adult brains. Astrocyte-like adult stem cells function as stem cells to generate new nerve cells in the adult mammalian CNS. (Doetsch et al., 1999; Alvarez-Buylla et al., 2001; Merkle et al., 2004; Merkle and Alvarez-Buylla, 2006).
Evidence from recent studies indicates that NS/PC self-renewal and the time course of NSC fate determination are regulated by a combination of nuclear modifications, transcription factors and extrinsic signals from the stem cell microenvironment, also known as stem cell niche (Shi et al., 2008). Cell adhesion molecules (CAMs) located on the cell surface bind to the extracellular matrix (ECM) as well as other cells, thereby connecting cells with their surroundings. As important participants in cell-cell and cell-ECM interactions, CAMs have been shown to play essential roles in NS/PC development, including proliferation, differentiation, and migration, through extrinsic signals from the stem cell niche. Furthermore, many CAMs have been studied for the potential application in repair of CNS and peripheral nervous system (PNS) damage. Based on the role of CAMs in the NS/PC development and on the repair of nervous system, many researchers have investigated the possibility of combining CAMs and stem cell-based transplantation therapy to treat neural disorders and neural injury.
This chapter will discuss the most recent research in CAM functions in NS/PC development, as well as highlighting the applications and future potential of applying CAMs in stem cell-based therapy.
2. Cell adhesion molecules
CAMs are transmembrane proteins located on the cell surface through which cells bind or interact with other cells and the ECM surrounding them. CAMs normally contain three domains: an intracellular domain by which CAMs bind to cytoskeleton proteins, an extracellular domain that interacts with the ECM or other CAMs and a transmembrane domain (Chothia and Jones, 1997). Through these three domains, CAMs can transfer extrinsic signals from microenvironment or other neighboring cells and trigger diverse signaling pathways. Although CAMs can be classified by whether they are calcium-dependent or calcium-independent, most CAMs are categorized into four families: the immunoglobulin superfamily, the cadherins, the integrins, and the selectins (Brackenbury et al., 1981; Shapiro et al., 2007).
The complexity of structure and functions in the nervous system are achieved by the complex interconnections among its component cells. Many types of CAM interactions between various types of neural cells in the nervous system have been reported. Here, the classification and structure-function relationship of CAMs highly expressed in nervous system or play essential roles in nervous system will be introduced and briefly discussed.
2.1. Immunoglobulin superfamily cell adhesion molecules
Immunoglobulin superfamily cell adhesion molecules (IgSF CAMs) are a group of calcium-independent CAMs that share a common structural domain called immunoglobulin (Ig) domain. Members from IgSF CAMs have diverse binding activities and mechanisms of function. Certain IgSF CAMs interact through their extracellular domains with the same type of CAMs, a mechanism referred to as homophilic binding. Conversely, through heterophilic binding, other IgSF CAMs bind to different CAMs or to other cell surface proteins as well as soluble proteins in the ECM, Due to their structural and binding specificity, different IgSF CAMs have been shown to play distinct roles in the nervous system.
2.1.1. L1CAM subfamily
L1CAM subfamily proteins, which include L1, close homolog of L1 (CHL1), NrCAM, and Neurofascin, are one of the most well known groups of IgSF CAMs and are widely studied in the nervous system. L1CAM proteins can homophilically bind to themselves, and based on their structure, can heterophilically interact with many other IgSF CAMs (e.g. NCAM, TAG-1, and contractin), ECM molecules (e.g. laminian, tenascins, Src and extracellular signal-regulated kinases (Erk)), cytoplasmic proteins, including cytosketeton proteins such as ankyrins, and traffic proteins such as AP-2 (Maness and Schachner, 2007).
The extracellular domain of L1CAM proteins contains six Ig-like domains and four or five fibronectin type III (FNIII) repeats, followed by a transmembrane domain and cytoplasmic domain (Fig. 1A). The Ig1 domain of L1/CHL1 can bind neuropilin-1, an important component of the axonal guidance molecule semaphorin 3A (Sema3A) (Castellani et al., 2002). Similarly, NrCAM has been shown to bind neuropilin-2 to form a complex that mediates Sema3B and Sema3F signaling (Falk et al., 2005). Fluorescent bead aggregation and cell binding assays demonstrate the N-terminal of the L1 Ig1 domain is essential for L1 homophilic binding (Jacob et al., 2002). Through unique motifs in the Ig6 domains, L1 and CHL1 trigger Erk, mitogen-activated protein kinase kinase (MAP2K), phosphatidylinositol-3 kinase (PI3 kinase), and Src signaling pathway by interacting with integrins in some biological processes (Schaefer et al., 1999; Schmid et al., 2000). The intracellular domains of L1CAMs contain a highly conserved motif by which L1CAM proteins bind to ankyrins, a group of adaptor proteins linking integral membrane proteins to the cytoskeleton (Bennett and Baines, 2001). When the cytoplamic domain of Neurofascin is tyrosine phosphorylated, doublecortin (DCX) is recruited and mediates the binding of Neurofascin to microtubules (Kizhatil et al., 2002). An alternative splicing generates a motif YRSL in the cytoplasmic domain of L1 that mediate interactions between L1 and AP-2, a clathrin adaptor that triggers tyrosine-based signals for endocytosis (Kamiguchi et al., 1998). L1 was also shown to directly bind to membrane-cytoskeleton linking ezrin-radixin-moesin (ERM) proteins, through which L1 can bind to the actin cytosketeton and regulate axonal outgrowth and neuronal differentiation (Dickson et al., 2002). These structure-dependent interactions between L1CAMs and other ECM molecules, cytoskeleton proteins and cytoplasmic molecules suggests that are L1CAMs an essential component during the extrinsic signaling transduction regulating neurite outgrowth, axon growth, cell migration and differentiation (Maness and Schachner, 2007). Moreover, the intracellular domain of CHL1 directly interacts with synaptic chaperones Hsc70, CSP and alphaSGT, thereby regulating SNAP25 and VAMP2-induced exocytotic machinery (Andreyeva et al., 2010).
Neural cell adhesion molecule (NCAM), also known as CD56, is a glycoprotein expressed on the cell surface of various cell types, including neurons, glial cells, and natural killer cells. NCAM is a unique member of IgSF CAMs because of its 27 alternatively spliced mRNAs and its three major protein isoforms: NCAM-120, NCAM-140, and NCAM-180, so named due to their molecular weights (Reyes et al., 1991).
These three NCAM isoforms share the same extracellular domain, but vary in their transmembrane domains and cytoplasmic region. The extracellular domain of NCAM contains five Ig-like domains and two FNIII repeats, followed by a transmembrane domain in NCAM-140 and NCAM-180 isoforms, but a glycophosphatidyl (GPI) anchor linking to the cell membrane in NCAM-120 (Fig. 1A) (Chothia and Jones, 1997). NCAM-140 has a shorter intracellular domain compared to NCAM-180. The different domains of NCAMs have been reported to play distinct roles in binding activities and biological functions. The Ig-like domains have been shown to be essential for NCAM homophilic binding, and the FNIII repeats are involved in signaling that regulate neurite outgrowth.
2.1.3. Nectins and Nectin-like molecules
Nectins and Nectin-like molecules (Necls) form another large IgSF CAM family, including four Nectins (Nectin-1, 2, 3, 4) and five Necls (Necl-1, 2, 3, 4, 5), and exhibit cell-cell adhesive functions in a wide range of tissues, including epithelia and neuronal tissue (Takai et al., 2003). All Nectins and Necls share the same structure domains, containing an extracellular domain with three Ig-like repeats, through which they play their roles in cell-cell adhesion activity, a single transmembrane region, and a cytoplasmic domain (Fig. 1A) (Sakisaka and Takai, 2004). Nectins and Necls can form
TAG-1, also called TAX-1 (human) or axonin-1, a 135 kDa glycoprotein expressed on the developing axons, belongs to IgSF CAMs superfamily and plays important roles in neurite outgrowth and cell aggregation. TAG-1 has six Ig-like domains followed by four FNIII repeats and is anchored to the cell membrane by a GPI tail (Fig. 1A) (Furley et al., 1990). During development of the central and peripheral nervous system, TAG-1 is transiently expressed both as a soluble form and a GPI-anchored form (Karagogeos et al., 1991). Binding analysis revealed that FNIII repeats but not Ig domains are sufficient for homophilic binding, although both TAG-1 domains types can promote the neurite outgrowth (Tsiotra et al., 1996; Pavlou et al., 2002). The four amino-terminal Ig-like domains have been shown to be important for TAG-1 and neural glial cell adhesion molecule (NgCAM)
Myelin-associated glycoprotein (MAG) is expressed on the surface of oligodendrocytes in the CNS and Schwann cells in the PNS, and has been implicated in neuron-oligodendrocyte and oligodendrocyte-oligodendrocyte interactions in the CNS and glia-glia interaction in the PNS (Sternberger et al., 1979; Quarles, 1984; Martini and Schachner, 1986; Poltorak et al., 1987). Two MAG isoforms have been identified as small MAG (S-MAG) and large MAG (L-MAG). Similar to other IgSF CAMs, MAGs contain an identical extracellular domains composed of five Ig-like domains, a transmembrane segment, and two distinct cytoplasmic domains, which distinguish the S-MAG and L-MAG (Fig. 1A) (Salzer et al., 1987). MAG binds gangliosides, the most abundant sialylated glycoconjugates in the nervous system, which are important for neuron-oligodendrocyte interaction (Schnaar et al., 1998). MAG was shown to interact with the leucine-rich repeat (LRR)-containing GPI-linked Nogo-66 receptor (NgR), inducing the inhibition of neurite outgrowth (Fournier et al., 2001; Domeniconi et al., 2002).
There are many other IgSF CAMs, not to be introduced in this chapter, found in other mammalian body systems, such as intercellular cell adhesion molecule (ICAM-1, expressed on endothelial cells and cells of the immune system), vascular cell adhesion molecule (VCAM-1, expressed on blood vessels), platelet-endothelial cell adhesion molecule (PECAM-1, expressed on the surface of platelets, monocytes, neutrophils, and some types of T-cells).
Cadherins, a family of calcium-dependent CAMs, are the major architectural molecules mediating cell-cell adhesion of extracellular domains at intercellular junctions. The cadherin superfamily consists of four subgroups, including classic cadherins, protocadherins, desmosomal and unconventional cadherins, which share a similar structure with extracellular Ca2+-binding domains, known as cadherin repeats (Angst et al., 2001). Although the homophilic binding of specific cadherin subtypes has been described frequently, several cadherin subtypes were also found to interact heterophilically (Ahrens et al., 2002).
2.2.1. Classic cadherins
In vertebrates, classic cadherins, including epithelial cadherin, neural cadherin, and placental cadherin and so on, have five extracellular cadherin repeats, a transmembrane domain, and a highly conserved intracellular domain (Fig. 1B) (Tepass et al., 2000). By binding to Ca2+ on the boundary between cadherin repeats, the extracellular domain is stabilized and undergoes homophilic interactions, which is essential for the adhesion function of cadherins (Nagar et al., 1996). Both
With more than 100 having been identified in mammals, protocadherins are the largest subfamily of cadherins (Hirano and Takeichi, 2012). Protocadherins can be further sorted into two groups, based on their genomic distribution: clustered protocadherins, whose coding genes are located on human chromosome 5 in tandem order, and consist of three gene subclusters Pcdhα, Pcdhβ and Pcdhγ, and non-clustered protocadherins, whose genes are distributed among different chromosomes and are divided into several subgroups, incuding δ-protocadherins (Vanhalst et al., 2005). The extracellular domains of protocadherins contain more than five cadherin motifs, which differ from the characteristic features of classical cadherins (Fig. 1B) (Sano et al., 1993). Unlike the highly conserved cytoplasmic tails of classic cadherins, the intracellular domains of protocadherins are variable, suggesting their diverse functions. Although protocadherins appear to display weaker cell-cell adhesion than classic cadherins, protocadherins show diverse biological functions in the CNS, including roles in neuronal differentiation and synaptogenesis (Hirano and Takeichi, 2012).
Integrins are a group of cell surface receptors that form a large CAM subfamily. Integrins are heterodimeric glycoproteins consisting of two distinct subunits: an α-subunit and β-subunit that each penetrates the cell membrane once and has a short intracellular tail which is typically 40 to 70 amino acids long. Thus far, 18 α-subunits and 8 β-subunits have been identified and 24 αβ combinations have been observed (Hynes, 2002). The extracellular domain of α-subunits, which generally are larger than 900 amino acids, is divided into four subdomains containing a ligand-binding N-terminal region including a sevenfold repeat, among which repeats 5, 6, and 7 contain a Ca2+- binding structure. Half of the α-subunits contain an extra I-domain, which contributes a divalent cation-binding site and facilitates an interaction with IgSF CAMs (Fig. 1C) (Landis, et al., 1994). The ectodomains of integrin β-subunits are typically larger than 600 amino acids and contain 8 subdomains, including an N-terminal signal region as well as a metal-binding site that is directly involved in ligand-integrin interactions (Fig. 1C). The extracellular N-terminal of α-chain and β-chain form a ligand-binding αβ headpiece (Fig. 1C). Through interactions between their extracellular domain and ECM ligands and other CAM family members, integrins can perform outside-in signaling to mediate cell response to its surrounding environment. Moreover, through their intracellular tails, integrins can also perform inside-out signaling, thereby relaying the intercellular cell state to the extracellular environment. Using both outside-in and inside-out signal transduction models, integrins not only aid in facilitating cell-ECM interactions, but also play roles in many other biological activities, including cell cycling as well as cell growth, survival, and differentiation.
The selectin family of CAMs is a group of calcium-dependent transmembrane glycoproteins. Three selectins have been identified, including E-selectin (found on endothelial cells), L-selectin (found on lymphocytes), and P-selectin (found on platelets and endothelial cells). All identified selectins share a similar extracellular structure, composed of an N-terminal sequence, a calcium-binding lectin domain, an EGF-like domain, in addition to a non-conserved transmembrane domain, a non-conserved cytoplasmic tail and a differing number of consensus repeats (Fig. 1D). Selectins have been reported to play important roles in the immune system associated with inflammation and cancer progression (Barthel et al., 2007).
3. Cell adhesion molecules and neural stem/progenitor cell development
In the mammalian embryonic CNS, NS/PCs are found in almost all regions, and give rise to every type of neural cells, including various subtypes of neurons, astrocytes, and oligoden drocytes, forming the most complex and functional organ of the body (Temple, 2001). In the adult CNS, astrocyte-like type-B stem cells reside only in certain regions, namely as the subventricular zone (SVZ), from which stem cells migrate into olfactory bulb (OB) and differentiate into mature neurons, and the SGL of hippocampal DG, from which stem cells migrate and extend processes into the molecular layer of the hippocampus (Gage, 2000). Ependymal cells lining the central canal of adult mouse spinal cord can exhibit dramatic proliferation after spinal cord injury and give rise to newborn glial cells in the injured spinal cord (Göritz1 and Frisén, 2012). Many CAMs from different subfamilies are expressed in these neurogenic regions, although their functions have not been fully understood. Increasingly evidences suggest that CAMs are not only important for maintenance of the architecture and shape of stem cell niche, but also play essential roles in signaling transduction from the stem cell niche to regulate cell survival, proliferation, differentiation, and migration. Recent progress in NS/PC cell fate research indicates that stem cell maintenance and cell fate determination are regulated by a combination of epigenetic modulation, intrinsic transcription factors and extrinsic signals from the NS/PC niche (Shi et al., 2008). Transcriptional factors play essential roles in NS/PC fate determination by regulating multiple downstream gene expression in NS/PCs. However, the functions of these intrinsic factors are regulated by extrinsic cues through various signaling pathways mediated by cell surface receptors and down-stream cellular signaling elements. Modulation of cell fate by such extrinsic cues provides an excellent mechanism for neural progenitor cells to adapt to the environment. As bridging molecules between extrinsic signals from neighbor cells or the ECM and intracellular transcriptional regulation, CAMs play critical roles during these processes.
3.1. Cell adhesion molecules regulate neural stem/progenitor cell self-renewal and proliferation
Self-renewal is essential for NS/PCs to perpetuate and maintain their population in an undifferentiated state, which is critical for the CNS development, learning and memory-related plasticity in adult animals as well as replacement for dead cell following injury. Recent studies showed that the NS/PC niche, comprised of ECM molecules, soluble factors, and cell surface molecules, is essential for NS/PC identification, maintenance of stem cell population, and preventing contact with differentiation stimuli and cell apoptotic signals (Doetsch, 2003; Miller and Gauthier-Fisher, 2009; Moore and Lemischka, 2006). Among the NS/PC niche molecules, CAMs play a critical role as transducers that mediate signaling between stem cells and their niche.
IgSF CAMs play diverse roles in the regulation of NS/PC self-renewal and proliferation through specific signaling pathways. L1 is highly expressed in the developing neurons of the cerebral cortex, hippocampus, and corpus callosum, with expression declining to low levels with maturation (Demyanenko et al., 1999). Reduced hippocampal neurons were observed in L1-deficient mice, suggesting that L1 may have a potential role in neurogenesis (Demyanenko et al., 1999). Although undifferentiated NS/PCs do not express L1, substrate-coated L1 inhibits the proliferation of cultured NS/PCs
Neuroepithelial attachments at adherens junctions are critical for NS/PC maintenance and self-renewal. As essential molecules during the nervous system development, cadherins have been reported to play roles in neural tube, neuroepithelial layer and boundary formation, synaptogenesis, axon guidance and regulation of NS/PC behaviors. E-cadherin is expressed in the embryonic and adult ventricles, where NS/PCs reside, and in clonal stem cell colonies
Many integrins are expressed in neurogenic regions of the developing and adult mammalian CNS. Cultured neurospheres from newborn mouse forebrain express five major integrins, including α5β1-, α6Aβ1-, αvβ1-, αvβ5-, αvβ8-, and low levels of α6Bβ1-integrin. Antibody inhibition of α5β1- and αvβ1-integrins reduces NS/PC proliferation (Jacques et al., 1998). With the use of fluorescence-activated cell sorting, α5β1-integrin was shown to be highly expressed in multipotent NS/PCs and downregulated during neuronal differentiation, suggesting a role in maintenance of characteristic NS/PC features (Yoshida et al., 2003). α6β1-integrin, which is highly expressed in human NS/PCs, is used as a neural precursor cell marker, and was found to form a functional complex with laminin γ1, and netrin-4 on the NS/PC surface, that promotes cell proliferation (Hall et al., 2006; Staquicini et al., 2009). Antibody inhibition of β1-integrin signaling results in an increased population of abventricular dividing precursors, a phenomenon also observed in the neocortex of mice deficient in Laminin α2, a ligand of β1-integrin (Loulier et al., 2009). Although diminished proliferation was observed in cultured β1-integrin-deficient neural precursors, a small percentage of β1-integrin-negative cells survived and formed neurospheres normally, suggesting that β1-integrin is not required for maintenance of all NS/PCs (Leone et al., 2005). Further investigation has revealed that β1-integrin contributes to the maintenance of NS/PCs by activating MAPK signaling (Campos et al., 2004; Wang et al., 2011). β1-integrin interacts with Notch and EGFR signalling pathways, suggesting that ECM molecule or growth factor involvement may be important in β1-integrin-mediated regulation of NS/PCs (Campos et al., 2006). β1-integrin is also involved in basic FGF (bFGF) and EGF-mediated proliferation of neuroepithelial cells (Suzuki et al., 2010). Moreover, the carbohydrate-binding protein Galectin-1 regulates proliferation of adult NS/PC through interactions with β1-integrin (Sakaguchi et al., 2010). Additionally, the FNIII domain 6-8 of ECM molecule tenascin-R inhibits NS/PC proliferation through interactions with β1-integrin, an effect that is eliminated with antibody inhibition of β1-integrin (Liao et al., 2008). PDGF stimulates proliferation of oligodendrocyte precursor cells (OPCs) through activation of αvβ3-integrin, which in turn activates PI3 kinase-dependent signaling pathway (Baron et al., 2002). Neurovascularization, regulated by αvβ8-integrin, plays essential roles during the CNS development (Proctor et al., 2005). Analysis of adult β8-integrin-deficient mouse brains shows abnormalities in the SVZ and RMS, with a smaller OB. Reduced proliferation of cells in the SVZ and RMS in β8-integrin-deficient brains and smaller neurospheres formed by β8-integrin-deficient NS/PCs reveal the important role of β8-integrin on NS/PC maintenance, potentially through transforming growth factor β (TGFβ) signaling (Mobley et al., 2009).
3.2. Cell adhesion molecules regulate neural stem/progenitor cell differentiation
In the mammalian CNS, different neural cell types arise and migrate in a precise temporospatial manner. During mouse brain development, neurons first arise around embryonic day 12 (E12), with neurogenesis peaking at E14 and ceaseing around E18. Astrocytes appear at approximately E18, with their numbers peaking in the neonatal period, and oligodendrocytes are generated after birth when the neurogenesis has largely subsided (Bayer and Altman, 1991).
The roles of cadherins on neural differentiation are still not well understood. Although neural differentiation appears to occur normally in N-cadherin-deficient mice (Kadowaki et al., 2007), a recent study knocking down N-cadherin expression by
β4-integrin plays an essential role in NS/PC differentiation. Under
3.3. Cell adhesion molecules regulate neuronal migration
When NS/PCs give birth to neurons or glial cells, the differentiating or differentiated cells will migrate away from the stem cell niche to their appropriate location. In the developing CNS, specific neurons migrate in a specific pathway to reach their final destination in the brain. For example, in the developing cerebral cortex, the newborn neurons generated by radial glial cells in the embryonic VZ undergo radial migration along the long processes of radial glial cells, forming the different layers of cortical plate. Newborn interneurons generated from the ganglionic eminence also migrate in tangential path into the cortex without interacting with radial glial cells (Ghashghaei et al., 2007). In the adult CNS, NS/PCs from SVZ migrate along the rostral migratory stream towards the OB and from the SGL of hippocampus migrate a short distance into molecular layer (Ghashghaei et al., 2007). As cell surface molecules interacting with the surrounding environment, CAMs play essential roles in cell migration..
Overexpression of L1 using human GFAP promoter in NS/PCs promotes neuronal migration in
Recently it was demonstrated that N-cadherin is involved in radial neuronal migration during cortical development. In N-cadherin conditional knockout mice, neuroepithelial and radial glial cells can not expand their cell bodies and processes to span the distance from the ventricular surface to the pial surface, which is essential for neuronal migration and cortical lamination, resulting in disorganization of the entire cortex (Kadowaki et al., 2007). Knockdown of N-cadherin expression by
Integrins were also shown to control migration of neural precursors. During migration along the RMS, antibodies for specific integrins, such as α1-, β1-, and αv-integrins, inhibit neuronal migration during the stages at which they are expressed (Murase and Horwitz, 2002). β1-integrins-dificient progenitor cells exhibit impaired migration in different ECM substrates (Leone et al., 2005). Antibody inhibition of neurosphere-expressed α6β1-integrins results in inhibition of tangential chain migration of NS/PC
3.4. Cell adhesion molecules regulate cell survival
During embryonic and early postnatal development, approximately 50% of newborn neurons die due to apoptosis in almost every CNS regions. In adult the CNS, cell death mostly occurs in the regions of neurogenesis. For example, in the adult hippocampus, about 70% of newborn neurons die within three weeks after failing to form the functional connections with the existing neural circuits. CAMs have been shown to play essential roles during apoptosis signaling.
Cadherins, such N-cadherin and Pcdhγs, have also been found to be involved in neural cell survival. N-cadherin can enhance cell survival of both mouse spinal cord neurons and rat hippocampal neurons
Integrins also play a role regulation of cell survival. Cultured β1-integrin-deficient neural progenitors display high levels of cell death (Leone et al., 2005). Inhibition of phosphatiducholine-specific phospholipase C in cultured neural cells results in a reduction of cell survival, associated with upregulation of β4-integrin and Rb protein (Lv et al., 2006).
4. Applications of cell adhesion molecules in stem cell-based regenerative therapy
Many CAMs have been investigated for pre-clinical studies in the treatment of neural injury and neurodegenerative disorders as their roles in CNS development and regeneration after CNS injury have been revealed. L1 has been reported to play important roles in neuronal survival and migration as well as neurite outgrowth and extension, axonal guidance and synaptic plasticity
Polysialic acid (PSA) is a long linear homopolymer glycan carried by NCAMs that plays essential roles in PSA-NCAM-mediated activities, including cell-cell interaction and cell migration (Hu et al., 1996; Johnson et al., 2005). Numerous studies have used PSA to promote adult CNS or PNS repair. Expression of PSA at the lesion site can loosen scar tissue and reduce inhibitory interactions with growth cones. Thus, engineered overexpression of PSA on the astrocyte scar enhanced Purkinje cell axonal regeneration in the lesioned cerebellum of growth related genes L1/GAP-43 double transgenic mice (Zhang et al., 2007). Similarly, induced expression of PSA in the glial scar of injured spinal cords promoted regeneration of sensory axons (Zhang et al., 2007). PSA glycomimetics has also been reported to promote plasticity and functional recovery after spinal cord injury in mice (Mehanna, 2010). PSA was also applied in the study of regeneration following peripheral nerve injury. PSA glycomimetic promotes myelination and functional recovery after peripheral nerve injury (Mehanna et al., 2009). PSA-mimetic enhances Schwann cell proliferation and process elongation in vitro, which may be mediated by interaction with Schwann cell-expressed NCAM and FGFR (Mehanna et al., 2009).
Human natural killer cell glycan (HNK-1) is found on glycolipids and glycoproteins, including many CAMs, such as L1, NCAM, MAG, TAG-1 in the nervous system, and has been shown to play roles in cell recognition and adhesion (Morita et al., 2008). Application of HNK-1 mimic peptide in injured peripheral nerves resulted in larger motor neuron somata and enhanced axonal remyelination resulting in better functional recovery compared to the mice treated with a scrambled peptide (Simova et al., 2006).
Due to the role of CAMs in the NS/PC development and repair of nervous system injury, many researchers have investigated the possibility of combining CAMs and stem cell-based transplantation therapy to treat neural disorder and neural injury models. As the most widely studied CAM associated with neural regeneration, L1 has been investigated as a potential candidate for stem cell-based therapeutic strategy for treatment of neurodegenerative diseases or regeneration after neural injury. Overexpresion of L1 in NS/PCs results in a reduced proliferation, enhanced neuronal migration and differentiation, as well as decreased astrogenesis using an
Stem cell-based therapeutic applications have attracted a great deal of attention as multiple potential stem cell types were developed, particularly with the establishment of induced pluripotent stem cells (iPSCs). These stems cells seemed to provide near limitless potential in treating many human diseases. However, stem cell-based therapeutic applications for neurological disorders have faced many obstacles and setbacks, such as immunorejection, tumor formation, and low efficiency resulting from low cell survival, and failure to migrate and form functional neural connections with existing neural circuitry. Moreover, the molecular mechanisms that underlie NS/PC proliferation and differentiation into distinct cell types remain unclear. Nonetheless, advances in stem cell research and advantages of the combination of CAMs and stem cells in pre-clinical research, like enhanced cell survival, promoted cell migration, and increased neuronal differentiation, are encouraging. All these advantages suggest that CAMs have tremendous potential for application in stem cell-based cell replacement therapy for neurodegenerative diseases and spinal injuries.
We thank Dr. Tao Sun and Miss Aisha Abdullah for providing thoughtful comments. Owing to space limitations, I apologize for being unable to cite many excellent papers in this field. This work was supported by the Ellison Medical Foundation (T. S.), an award from the Hirschl/Weill-Caulier Trust (T. S.) and an R01-MH083680 grant from the NIH/NIMH (T. S.).