Phosphoinositide (PI) 3-kinase enhancer (PIKE) is a brain-specific GTPase that binds to PI 3-kinase and stimulates its lipid kinase activity. It exists in two forms: the first to be identified, PIKE-S, is shorter and exclusively nuclear; by contrast, the longer form, PIKE-L, resides in multiple intracellular compartments. Nerve growth factor treatment leads to PIKE-S activation by triggering the nuclear translocation of phospholipase C (PLC)-γ1, which acts as a physiological guanine nucleotide exchange factor (GEF) for PIKE-S through its Src-homlogy 3 (SH3) domain. Cytoplasmic PI 3-kinase and its lipid product phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3] regulate the membrane translocation and activation of many signaling molecules by binding to their pleckstrin homology (PH) domains. However, little is known about the physiological roles of their nuclear counterparts. The nuclear PLC-γ1/PIKE-S/PI 3-kinase signaling pathway seems to be an extension of the crosstalk between cytoplasmic PLC-γ1 and PI 3-kinase. PIKE-L contains a C-terminal extension consisting of an ADP ribosylation-GTPase-activating protein (ArfGAP) domain and two ankyrin repeats in addition to the N-terminal GTPase domain. PIKE-L could have additional, extranuclear functions, including regulation of postsynaptic signaling by metabotropic glutamate receptors.

Small GTP-binding proteins of the Ras superfamily, including Ras, Rho, Ran, Rab and Arf, function as molecular switches in a diverse array of biological processes, including cell proliferation and differentiation (Ras), vesicular transport (Rab and Arf), cytoskeletal organization and free radical generation (Rho), and nuclear transport and mitosis (Ran). GTPases cycle between the active, GTP-bound and inactive, GDP-bound states (Boguski and McCormick, 1993; Quilliam et al., 1995). Guanine nucleotide exchange factors (GEFs) act as positive regulators that promote release of GDP and consequent formation of the active GTP-bound state, whereas GTPase-activating proteins (GAPs) act as negative regulators, stimulating the intrinsic GTPase activity to generate the inactive GDP-bound form. Although the various GEFs have very similar substrates (small GTP-binding proteins) and functions (the dissociation of GDP), their primary sequences, tertiary structures and subcellular localizations are markedly divergent (Cherfils and Chardin, 1999; Pan and Wessling-Resnick, 1998). Signal transduction pathways involving small GTPases such as Ras have been well defined in the cytoplasm. However, little is known about signaling through nuclear GTPases. A recently identified brain-specific GTPase PIKE has been shown to reside in the nucleus and binds to phosphoinositide 3-kinase (PI 3-kinase) and enhances its kinase activity (Ye et al., 2000). PIKE exists in two forms: PIKE-S and PIKE-L. PIKE-S also interacts with phospholipase C (PLC)-γ1, which acts as a PIKE-S GEF, and with 4.1N, a neuronal isoform of the erythrocyte membrane cytoskeletal protein 4.1R that translocates to the nucleus following nerve growth factor (NGF) treatment, where it binds to PIKE-S. PIKE-L is linked by the scaffolding protein Homer to metabotropic glutamate receptors and mediates their anti-apoptotic actions by stimulating PI 3-kinase. Below, we review what is known about the PIKE proteins and discuss their potential roles in nuclear and cytoplasmic signaling.

PIKE-S was originally identified in a yeast two-hybrid screen searching for binding partners of the C-terminal domain (CTD) of protein 4.1N (Fig. 1). It comprises 753 residues, containing three proline-rich domains (PRDs) in the N-terminus, followed by a GTPase domain and a partial pleckstrin homology (PH) domain in the C-terminus. PIKE-S is brain specific and localizes exclusively to the nucleus. A longer isoform of PIKE (PIKE-L) and its alternatively spliced isoform contain a C-terminal extension consisting of an ADP ribosylation-GAP (ArfGAP) domain and two ankyrin repeats. The C-terminus resembles that of members of the centaurin family, which are GAPs for Arfs or share sequence similarity with ArfGAPs and Arf effectors (Cukierman et al., 1995; Donaldson, 2000).

Fig. 1.

PIKE-binding proteins. Protein 4.1N binds to the extreme N-terminal 23 amino acids of PIKE through its C-terminal domain. Both subunits (p85 and p110) of PI 3-kinase directly interact with PIKE. Interestingly, the p85 subunit and protein 4.1N bind to the same region of PIKE. However, the structural requirements for PIKE binding to p110 differ somewhat from binding to p85, the N-terminal fragment from amino acids 24-262 being critical for p110 to bind to PIKE. In addition, the mGluR1- and mGluR5-associated protein Homer 1 binds to the proline-rich domain of PIKE (amino acids 187-190).

Fig. 1.

PIKE-binding proteins. Protein 4.1N binds to the extreme N-terminal 23 amino acids of PIKE through its C-terminal domain. Both subunits (p85 and p110) of PI 3-kinase directly interact with PIKE. Interestingly, the p85 subunit and protein 4.1N bind to the same region of PIKE. However, the structural requirements for PIKE binding to p110 differ somewhat from binding to p85, the N-terminal fragment from amino acids 24-262 being critical for p110 to bind to PIKE. In addition, the mGluR1- and mGluR5-associated protein Homer 1 binds to the proline-rich domain of PIKE (amino acids 187-190).

The interaction between PIKE-S GTPase and PI 3-kinase is GTP dependent, like that between Ras and PI 3-kinase (Rodriguez-Viciana et al., 1994; Ye et al., 2000). Cytoplasmic PI 3-kinase activators, such as receptor tyrosine kinases (RTKs) and Ras, bind to one of the two PI 3-kinase subunits, but PIKE-S directly associates with both p85 and p110 (Ye et al., 2000). The interaction between PIKE-S and p85 is dependent on the N-terminal 23 residues, to which protein 4.1N also binds, but this region is not necessary for p110 to bind to PIKE-S. Thus, 4.1N competes with PI 3-kinase for binding to PIKE-S. Co-transfection of 4.1N into cells with wild-type PIKE-S abolishes the PIKE-S-induced activation of PI 3-kinase activity. This loss of activation is associated with the failure of PIKE-S to co-precipitate with PI 3-kinase in cells that have been co-transfected with 4.1N.

In addition to binding protein 4.1N and PI 3-kinase, PIKES also robustly associates with PLC-γ1. The third proline-rich domain of PIKE-S binds directly to the Src-homology 3 (SH3) domain of PLC-γ1 (Ye et al., 2002), which is a physiological GEF for PIKE-S. This GEF activity is mediated by the SH3 domain of PLC-γ1; its phospholipase catalytic activity is not required. The SH3 domain but not the phospholipase catalytic activity of PLC-γ1 is necessary for its mitogenic actions, which probably involve activation of nuclear PI 3-kinase (Ye et al., 2002). The finding that the PLC-γ1 SH3 domain is a physiological GEF for PIKE-S indicates that this domain can display biological activity, which might account for its mitogenic activity. This system responds to signaling from the plasma membrane, since NGF triggers nuclear translocation of PLC-γ and its binding to PIKE-S. These findings are consistent with numerous reports of nuclear roles for PLC-γ (Bertagnolo et al., 1998; Bertagnolo et al., 1995; Diakonova et al., 1997; Marmiroli et al., 1994; Martelli et al., 1994; Neri et al., 1998; Zini et al., 1995).

Almost all of the phosphoinositides and enzymes responsible for the metabolism of inositol lipids occur in nuclei as well as in extranuclear sites (Bacqueville et al., 2001; Cocco et al., 2001; D'Santos et al., 1998). Nuclear phosphoinositid lipids appear to regulate cell proliferation and differentiation (Matteucci et al., 1998; York and Majerus, 1994). Thus, in Swiss 3T3 cells, insulin-like growth factor (IGF)-I causes a rapid decrease in nuclear levels of polyphosphoinositol lipids [phosphatidylinositol 4-phosphate and phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P2)] with a concomitant increase in nuclear diacylglycerol (DAG) levels and nuclear translocation of protein kinase C (Divecha et al., 1991). During S-phase, nuclear levels of polyphosphoinositol lipids decrease by more than 50%, whereas cytoplasmic levels remain constant; this has suggested that nuclear turnover of these lipids has a role in DNA synthesis (York and Majerus, 1994).

PI 3-kinase is a key regulator of many cellular processes, including cell proliferation, survival, motility, vesicular trafficking and carbohydrate metabolism. It is activated by both RTKs and membrane-associated GTPases. Activated PI 3-kinase generates critical second messengers: the D3-position-phosphorylated phosphoinositides, which bind to the PH domains of numerous signaling molecules, including 3-phosphoinositide-dependent protein kinase 1 (PDK1), Akt/PKB, SGK and PLC-γ1. Although much has been learned about PI 3-kinase signal transduction in the cytoplasm, almost nothing is known about the role of this enzyme in the nucleus, despite previous studies showing that it is present there (Bacqueville et al., 2001; Bavelloni et al., 1999; Kim, 1998; Lu et al., 1998; Neri et al., 1994; Tanaka et al., 1999; Ye et al., 2000). Cytoplasmic PI 3-kinase activation requires activated RTKs [e.g. platelet-derived growth factor receptor (PDGFR), epidermal growth factor receptor (EGFR), CD28, etc.] or GTPase proteins such as Ras. However, none of these known PI 3-kinase activators is present in the nucleus. Stimulation of cells with NGF nevertheless activates nuclear PI 3-kinase and consequently leads to nuclear accumulation of 3-phosphorylated phosphoinositide lipids (Neri et al., 1999; Tanaka et al., 1999). PI 3-kinase translocates to the nucleus following NGF treatment with a time course that resembles the activation of PIKE-S (Ye et al., 2000). NGF also elicits nuclear translocation of 4.1N over a period of an hour; 4.1N thus lags behind PI 3-kinase and the peak activation of PIKE-S elicited by NGF. The decline of activated nuclear PI 3-kinase, which coincides with the appearance of nuclear 4.1N, might involve 4.1N sequestering nuclear PIKE-S away from PI 3-kinase. The decline in NGF-induced stimulation of PIKE-S GTPase activity takes place at about the same time and so might also contribute to the decline in nuclear PI 3-kinase activity.

Nuclear PI 3-kinase localizes to both the nuclear envelope and the internal nuclear matrix, which fits with the localization of its substrate PtdIns(4,5)P2. The tumor suppressor PTEN, a phosphatidylinositol phosphatase specific for the 3-position of the ring that inhibits PI 3-kinase)/Akt signaling (Yamada and Araki, 2001) also occurs in the nucleus.

Targets of cytoplasmic PI 3-kinase have been studied extensively. The lipid products of PI 3-kinase in the cytoplasm activate a variety of kinases, including Akt and PDK1 (Alessi et al., 1997; Frech et al., 1997). These activities influence cytoskeletal rearrangements, vesicle transport and apoptosis, which are largely cytoplasmic events. Some cytoplasmic targets of PI 3-kinase, such as Akt, translocate to the nucleus, but whether they function as downstream targets of PI 3-kinase in the nucleus is unknown. Thus, the nuclear processes regulated by the PI 3-kinase signaling system have not yet been established. Note that nuclear phosphatidylinositol (3,4,5)trisphosphate [PtdIns(3,4,5)P3] facilitates nuclear translocation of proteins with a PtdIns(3,4,5)P3-binding motif (the PH domain). Both nuclear PI 3-kinase and PtdIns(3,4,5)P3 are necessary for the nuclear translocation of PKC-ζ (Neri et al., 1999). Furthermore, a PtdIns(3,4,5)P3-binding protein, PIP3BP, is exported out of the nucleus by constitutively activated PI 3-kinase (Tanaka et al., 1999). The interaction of the PH domain and PtdIns(3,4,5)P3 is responsible for the relocation of PIP3BP. These results suggest that PIP3BP can shuttle between the nucleus and the cytoplasm, and this is dependent on the activity of PI 3-kinase, indicating that there may be an unknown function of PI 3-kinase in the nucleus.

PLC-γ1 is a tyrosine kinase substrate for many RTKs and non-RTKs and is essential for cell proliferation and differentiation (Bae et al., 1998; Huang et al., 1995; Smith et al., 1994; Smith et al., 1989; Wang et al., 1998). Once activated, PLC-γ1 translocates to the plasma membrane and triggers the hydrolysis of PtdIns(4,5)P2 into two second messengers -inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] and DAG - which regulate intracellular Ca2+ levels and activation of protein kinase C, respectively. PLC-γ1 does not possess any known nuclear localization signal and predominantly localizes to the cytoplasm. Nevertheless, it has been detected in the nucleus (Diakonova et al., 1997; Martelli et al., 1994; Neri et al., 1998; Zini et al., 1995) and localizes to the nucleus in highly transformed and proliferating cell lines (e.g. A431, HeLa, mouse hepatoma MH 22A and rat Zajdela ascitic hepatoma cells) but not in primary embryo skin or lung fibroblasts. The differential subcellular localizations in normal or highly transformed cell lines might reflect the degree of transformation of the cell type or phase of the cell cycle (Diakonova et al., 1997). This idea is consistent with the observation of an increased amount of PLC-γ1 in the nuclei of regenerating rat liver at 22 hours, which suggests that there is a relationship between the S-phase of the cell cycle and intranuclear localization of PLC-γ1 (Neri et al., 1997). These results are supported by the findings that the levels of nuclear PtdIns lipids decrease by over 50% at 2 hours and 4 hours after release from the G1/S boundary (S-phase of the cell cycle) and return to their original levels by 9 hours, whereas the levels of the cytoplasmic PtdIns lipids remain constant throughout this period. However, the levels of total cellular PtdIns, PtdIns(4)P, and PtdIns(4,5)P2 relative to total cellular phospholipid do not vary throughout the cell cycle. This indicates that there is specific nuclear PtdIns turnover that is activated during DNA synthesis (York and Majerus, 1994). Note also that nuclear, but not cytoplasmic, PLC-β1 inhibits IGF-regulated differentiation of erythroleukemia cells (Matteucci et al., 1998).

The mitogenic activity of PLC-γ1 is not dependent on its phospholipase activity, but requires its SH3 domain (Huang et al., 1995; Smith et al., 1994; Ye et al., 2002). Several studies have suggested cross-talk between PLC-γ1 and PI 3-kinase in the cytoplasm. For example, the PtdIns(3,4,5)P3 generated by PI 3-kinase influences PLC-γ1 membrane translocation and activation by binding to its PH domain and a C-terminal SH2 domain (Bae et al., 1998; Carpenter and Ji, 1999), and activation of PLC downregulates PI 3-kinase by at least two mechanisms: (1) inhibition of insulin receptor substrate (IRS)-1-associated PI 3-kinase; and (2) acute activation of a PtdIns(3,4,5)P3 5-phosphatase. To what extent might PLC-γ1 and PI 3-kinase engage in intranuclear cross-talk? These two enzymes use the same substrate, PtdIns(4,5)P2; their reciprocal activation might thus influence the availability of PtdIns(4,5)P2 for various signaling pathways (Batty et al., 1997). In addition, as described above, PLC-γ1 functions as a GEF for PIKE-S and thereby activates PI 3-kinase. Nuclear phosphoinositide signaling, exemplified by PLC-γ1-PI 3-kinase cross-talk, might regulate cell-cycle progression and differentiation independently of cell-surface phosphoinositides (Fig. 2) (Maraldi et al., 1999; Payrastre et al., 1992).

Fig. 2.

PLC-γ1 and PI 3-kinase signaling cross-talk. Activated PI 3-kinase-generated D3-phosphatidylinositol lipids mediate a variety of cellular processes, including cell survival, cell proliferation, vesicle trafficking, motility and carbohydrate metabolism. PtdIns(3,4,5)P3 binds to PLC-γ1 and is implicated in the cross-talk between PLC-γ1 and PI 3-kinase. Moreover, it might also bind to the PH domain of PIKE and regulate its GTPase activity through a negative-feedback mechanism.

Fig. 2.

PLC-γ1 and PI 3-kinase signaling cross-talk. Activated PI 3-kinase-generated D3-phosphatidylinositol lipids mediate a variety of cellular processes, including cell survival, cell proliferation, vesicle trafficking, motility and carbohydrate metabolism. PtdIns(3,4,5)P3 binds to PLC-γ1 and is implicated in the cross-talk between PLC-γ1 and PI 3-kinase. Moreover, it might also bind to the PH domain of PIKE and regulate its GTPase activity through a negative-feedback mechanism.

Band 4.1 proteins (4.1R, 4.1G, 4.1N and 4.1B) are multifunctional cytoskeletal proteins derived from four related genes, each of which is expressed in the nervous system. 4.1R-knockout mice exhibit multiple disturbances in behavior, which correlate with the absence of 4.1R in cerebellar granule cells and the hippocampal dentate gyrus (Walensky et al., 1998). The prototype of this family is band 4.1R, an erythrocyte-membrane-associated protein that confers stability and flexibility on erythrocytes by interacting with the cytoskeletal proteins spectrin, F-actin and band 3 and glycophorin C membrane proteins. Proteins 4.1 serve as anchors coupling the cytoskeletal matrix to the plasma membrane in both erythrocytes and nerve cells through their FERM (4.1-ezrinradixin-moesin) domains. This conserved domain is present in several protein families, including the tumor suppressor schwannomin/merlin/talin, unconventional myosins X and VIIa, several non-receptor phosphotyrosine phosphatases (PTPs) and tyrosine kinases such as Janus tyrosine kinase (JAKs) and focal adhesion kinases (FAKs) (Girault et al., 1999).

Protein 4.1 occurs in both presynaptic sites and postsynaptic densities (Biederer and Sudhof, 2001; Ohara et al., 1999; Scott et al., 2001; Shen et al., 2000; Walensky et al., 1999). Immunohistochemical studies reveal several patterns of neuronal staining, with localization in neuronal cell bodies, dendrites and axons. In certain neuronal locations, including the granule cell layers of the cerebellum and dentate gyrus, a distinct punctate-staining pattern is observed, which is consistent with synaptic localization. In primary hippocampal cultures, mouse 4.1N is enriched at discrete sites of synaptic contact, colocalizing with the postsynaptic density protein of 95 kDa (PSD-95; a postsynaptic marker) and glutamate receptor type 1 (GluR1), a member of the AMPA family of ionotropic, excitatory postsynaptic glutamate receptors (Walensky et al., 1999). CASK, a member of the MAGUK family involved in the organization of membrane-associated protein complexes, localizes at both the presynaptic and postsynaptic excitatory synapses (Hsueh and Sheng, 1999), where it binds to a neuronal cell-surface protein, neurexin, through its PDZ domain and HOOK region for protein 4.1N (Biederer and Sudhof, 2001). 4.1N also binds to the membrane-proximal region of GluR1 in vivo and is implicated in AMPA receptor clustering and association with the actin cytoskeleton (Shen et al., 2000).

The 4.1 protein family members contain, in addition to FERM and spectrin/actin-binding domains, CTDs that bind to several other proteins. These include nuclear mitotic apparatus protein (NuMA) (Mattagajasingh et al., 1999; Ye et al., 1999), the AMPA receptor GluR1 (Shen et al., 2000) and PIKE-S (Ye et al., 2000). Although 4.1N binds to PIKE-S, it does not affect its GTPase activity and presumably acts as an anchoring protein. One possibility is that 4.1N links PIKE-S to the nuclear matrix. This idea is consistent with the finding that PI 3-kinase is also detected tightly bound to nuclear matrices of HL-60 cells isolated by nuclease treatment and high salt extraction. Four days of ATRA (all-trans retinoic acid) treatment induces a striking increase in the level of nuclear-matrix-bound PI 3-kinase (Marchisio et al., 1998). Alternatively, 4.1N could link PIKE to postsynaptic receptors such as GluR1, which would indicate that it also functions at synapses (Fig. 3) (Shen et al., 2000).

Fig. 3.

PIKE-L association with glutamate receptors. Homer dimers couple Ins(1,4,5)P3 receptor to mGluR1,5, and mediate intracellular Ca2+ signaling. Shank/Homer interactions link mGluR1,5 to NMDA receptors through PSD-95 and GKAP. PIKE-L/Homer interactions might link mGluR1,5 to the AMPA receptor GluR1 through the association of the N-terminus of PIKE and the C-terminal domain of 4.1N. This postsynaptic complex might regulate the communication between AMPA receptors and mGluRs.

Fig. 3.

PIKE-L association with glutamate receptors. Homer dimers couple Ins(1,4,5)P3 receptor to mGluR1,5, and mediate intracellular Ca2+ signaling. Shank/Homer interactions link mGluR1,5 to NMDA receptors through PSD-95 and GKAP. PIKE-L/Homer interactions might link mGluR1,5 to the AMPA receptor GluR1 through the association of the N-terminus of PIKE and the C-terminal domain of 4.1N. This postsynaptic complex might regulate the communication between AMPA receptors and mGluRs.

A database search led to the identification of PIKE-L, which differs from PIKE-S in containing a 40 kDa C-terminal extension that includes an ArfGAP and two ankyrin repeat domains (Rong et al., 2003). PIKE-L and PIKE-S are alternatively spliced isoforms. Both PIKE-L and PIKE-S are brain specific. However, whereas PIKE-S occurs in all brain regions examined, PIKE-L is uniquely absent from the cerebellum. The subcellular localization of the two proteins differs. PIKE-S is exclusively nuclear, whereas PIKE-L occurs in multiple subcellular fractions and, by immunohistochemistry, is observed throughout the cell body and all neuronal processes.

Sequence analysis led to the discovery that PIKE-L binds to the adaptor protein Homer 1c. Residues 187-190 of PIKE-L have the sequence PKPF, which fit the consensus motif (PxxF) present in proteins that bind to the EVH1 domain of Homer (Xiao et al., 2000). PIKE-L and Homer 1C coprecipitate robustly, and the interaction is dependent upon the portion of PIKE-L that contains the PKPF sequence. Mutation of P187 of PIKE-L abolishes binding of PIKE-L to Homer 1C and provides a useful tool to analyze the importance of this binding in various signaling cascades.

What signaling cascades are associated with Homer proteins? This family of adaptor proteins is localized to postsynaptic densities, serving to crosslink cytoplasmic regions of group I metabotropic glutamate receptors (mGluRs) with Ins(1,4,5)P3 receptors as well as SHANK proteins (Xiao et al., 2000). The mGluRs comprise three groups. Group I receptors (mGluR1 and mGluR5) stimulate PLC-β through G proteins, leading to the formation of Ins(1,4,5)P3 and associated calcium mobilization. By contrast, group II and group III receptors are negatively coupled to adenylyl cyclase (De Blasi et al., 2001). One particularly prominent action of group I mGluRs is to protect neurons from apoptotic death (Copani et al., 1995). Binding of mGluRI to Homer and thereby PIKE-L occurs only in the case of the group I class of mGluRs, which are the only forms that contain the Homer-ligand PxxF motif.

The association of mGluR1 with PIKE-L via Homer 1C suggests that mGluR1 might activate PI 3-kinase separately from its well-known activation of PLC. This pathway has been verified by the demonstration that transfection of HEK293 cells with mGluR5 stimulates PI 3-kinase activity but mutants of mGluR5 that do not bind Homer fail to activate PI 3-kinase (Rong et al., 2003). Moreover, Homer mutants that do not bind GluR5 block PI 3-kinase activation. In hippocampal cultures, mGluR activation increases PI 3-kinase activity, whereas infection with an adenovirus containing a dominant-negative form of PIKE-L blocks such activation. In hippocampal cultures, delivery of the Homer-binding motif of PIKE as a dominant-negative construct blocks mGluR stimulation of PI 3-kinase (mutant peptides that do not bind Homer have no effect). Finally, mGluR activation fails to stimulate PI 3-kinase in the cerebellum, a brain region that is devoid of PIKE-L.

A major action of group I mGluRs is prevention of apoptosis, and this appears to be mediated by PIKE-L and PI 3-kinase (Rong et al., 2003). In hippocampal cultures, mGluR activation and PIKE-L transfection block the apoptotic effects of staurosporine and other agents. By contrast, a PIKE-L dominant-negative construct augments apoptosis, and a PIKE construct that cannot bind Homer prevents the anti-apoptotic actions of mGluR activation. Additionally, mGluR activation does not block apoptosis in the cerebellum, which lacks PIKE-L.

PIKE-L may have other activities besides mediating the effects of mGluR on PI 3-kinase and apoptosis (Fig. 4). Some possibilities are suggested by the structure of PIKE-L. Its C-terminal region contains an ArfGAP domain and two ankyrin repeats. GTPases are typically activated by GAP proteins, but PIKE-L appears to be the first GTPase that contains a GAP sequence within its own structure. Whether the ArfGAP domain functions as an internal GAP for the GTPase activity of PIKE-L remains to be determined, although there are instances of ArfGAP domains exerting such activity (Xia et al., 2003). An alternatively spliced form of PIKE-L that lacks the N-terminal domain and its three proline-rich areas has been identified as centaurin-γ (Xia et al., 2003).

Fig. 4.

The PIKE-L/Homer complex couples mGluR to PI 3-kinase and mediates neuronal survival. Through its EVH1 domain, dimerized Homer binds PIKE-L and mGluR1,5 via the `Homer-binding motif'. mGluR1,5 agonists trigger the formation of the PIKE-L/Homer complex and activate PI 3-kinase, enhancing neuronal survival.

Fig. 4.

The PIKE-L/Homer complex couples mGluR to PI 3-kinase and mediates neuronal survival. Through its EVH1 domain, dimerized Homer binds PIKE-L and mGluR1,5 via the `Homer-binding motif'. mGluR1,5 agonists trigger the formation of the PIKE-L/Homer complex and activate PI 3-kinase, enhancing neuronal survival.

Mediation of anti-apoptotic actions is the only mGluR function thus far examined in connection with PIKE. Group I mGluRs exert a variety of neuronal activities, influencing motor learning and coordination via cerebellar Purkinje cells (Aiba et al., 1994) and influencing long-term potentiation in the hippocampus (Lu et al., 1997). Conceivably, PIKE-L also participates in these activities.

NGF promotes nuclear translocation of PLC-γ1, which in turn acts as a GEF for PIKE-S. PIKE-S enhances nuclear PI 3-kinase activity and hence may be a nuclear counterpart of Ras in the absence of other known PI 3-kinase activators. This PLC-γ1/PIKE/PI 3-kinase signaling pathway extends the crosstalk between PI 3-kinase and PLC-γ1 from the cytoplasm to the nucleus. Identification of the biological targets of the nuclear PtdIns(3,4,5)P3 will provide insight into the roles of nuclear PI 3-kinase in nuclear signal transduction and should expand our understanding of the functions of nuclear phospholipid metabolism.

PIKE-L may have even broader relevance, displaying a breadth of influences in mGluR signaling. Stimulation of mGluRI by agonists increases neuronal survival and prevents apoptosis. The recent work indicates that a complex containing mGluRI, Homer 1c, PIKE-L and PI 3-kinase is responsible for the anti-apoptotic effects of mGluRI activation. PIKE-L could also have additional functions beside its anti-apoptotic role. For example, it might regulate synaptic plasticity by coupling mGluRI to the AMAP receptor.

This work is supported by grants from the National Institutes of Health (RO1, NS045627) to K.Y., and a UDPHS grant (DA-00266) and a Research Scientist Award (DA-00074) to S.H.S.; the authors are indebted to the Executive Editor Dr Richard Sever for critical reading of the manuscript.

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