Interleukin-7 signaling (Homo sapiens)
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Description
Interleukin-7 (IL7) is produced primarily by T zone fibroblastic reticular cells found in lymphoid organs, and also expressed by non-hematopoietic stromal cells present in other tissues including the skin, intestine and liver. It is an essential survival factor for lymphocytes, playing a key anti-apoptotic role in T-cell development, as well as mediating peripheral T-cell maintenance and proliferation. This dual function is reflected in a dose-response relationship that distinguishes the survival function from the proliferative activity; low doses of IL7 (<1 ng/ml) sustain only survival, higher doses (>1 ng/ml) promote survival and cell cycling (Kittipatarin et al. 2006, Swainson et al. 2007).
The IL7 receptor is a heterodimeric complex of the the common cytokine-receptor gamma chain (IL2RG, CD132, or Gc) and the IL7-receptor alpha chain (IL7R, IL7RA, CD127). Both chains are members of the type 1 cytokine family. Neither chain is unique to the IL7 receptor as IL7R is utilized by the receptor for thymic stromal lymphopoietin (TSLP) while IL2RG is shared with the receptors for IL2, IL4, IL9, IL15 and IL21. IL2RG consists of a single transmembrane region and a 240aa extracellular region that includes a fibronectin type III (FNIII) domain thought to be involved in receptor complex formation. It is expressed on most lymphocyte populations. Null mutations of IL2RG in humans cause X-linked severe combined immunodeficiency (X-SCID), which has a phenotype of severely reduced T-cell and natural killer (NK) cell populations, but normal numbers of B cells. In addition to reduced T- and NK-cell numbers, Il2rg knockout mice also have dramatically reduced B-cell populations suggesting that Il2rg is more critical for B-cell development in mice than in humans. Patients with severe combined immunodeficiency (SCID) phenotype due to IL7R mutations (see Puel & Leonard 2000), or a partial deficiency of IL7R (Roifman et al. 2000) have markedly reduced circulating T cells, but normal levels of peripheral blood B cells and NK cells, similar to the phenotype of IL2RG mutations, highlighting a requirement for IL7 in T cell lymphopoiesis. It has been suggested that IL7 is essential for murine, but not human B cell development, but recent studies indicate that IL7 is essential for human B cell production from adult bone marrow and that IL7-induced expansion of the progenitor B cell compartment is increasingly critical for human B cell production during later stages of development (Parrish et al. 2009).
IL7 has been shown to induce rapid and dose-dependent tyrosine phosphorylation of JAKs 1 and 3, and concomitantly tyrosine phosphorylation and DNA-binding activity of STAT5a/b (Foxwell et al. 1995). IL7R was shown to directly induce the activation of JAKs and STATs by van der Plas et al. (1996). Jak1 and Jak3 knockout mice displayed severely impaired thymic development, further supporting their importance in IL7 signaling (Rodig et al. 1998, Nosaka et al. 1995).
The role of STAT5 in IL7 signaling has been studied largely in mouse models. Tyr449 in the cytoplasmic domain of IL7RA is required for T-cell development in vivo and activation of JAK/STAT5 and PI3k/Akt pathways (Jiang et al. 2004, Pallard et al. 1999). T-cells from an IL7R Y449F knock-in mouse did not activate STAT5 (Osbourne et al. 2007), indicating that IL7 regulates STAT5 activity via this key tyrosine residue. STAT5 seems to enhance proliferation of multiple cell lineages in mouse models but it remains unclear whether STAT5 is required solely for survival signaling or also for the induction of proliferative activity (Kittipatarin & Khaled, 2007).
The model for IL7 receptor signaling is believed to resemble that of other Gc family cytokines, based on detailed studies of the IL2 receptor, where IL2RB binds constitutively to JAK1 while JAK3 is pre-associated uniquely with the IL2RG chain. Extending this model to IL7 suggests a similar series of events: IL7R constitutively associated with JAK1 binds IL7, the resulting trimer recruits IL2RG:JAK3, bringing JAK1 and JAK3 into proximity. The association of both chains of the IL7 receptor orients the cytoplasmic domains of the receptor chains so that their associated kinases (Janus and phosphatidylinositol 3-kinases) can phosphorylate sequence elements on the cytoplasmic domains (Jiang et al. 2005). JAKs have low intrinsic enzymatic activity, but after mutual phosphorylation acquire much higher activity, leading to phosphorylation of the critical Y449 site on IL7R. This site binds STAT5 and possibly other signaling adapters, they in turn become phosphorylated by JAK1 and/or JAK3. Phosphorylated STATs translocate to the nucleus and trigger the transcriptional events of their target genes.
The role of the PI3K/AKT pathway in IL7 signaling is controversial. It is a potential T-cell survival pathway because in many cell types PI3K signaling regulates diverse cellular functions such as cell cycle progression, transcription, and metabolism. The ERK/MAPK pathway does not appear to be involved in IL7 signaling (Crawley et al. 1996).
It is not clear how IL7 influences cell proliferation. In the absence of a proliferative signal such as IL7 or IL3, dependent lymphocytes arrest in the G0/G1 phase of the cell cycle. To exit this phase, cells typically activate specific G1 Cyclin-dependent kinases/cyclins and down regulate cell cycle inhibitors such as Cyclin-dependent kinase inhibitor 1B (Cdkn1b or p27kip1). There is indirect evidence suggesting a possible role for IL7 stimulated activation of PI3K/AKT signaling, obtained from transformed cell lines and thymocytes, but not confirmed by observations using primary T-cells (Kittipatarin & Khaled, 2007). IL7 withdrawal results in G1/S cell cycle arrest and is correlated with loss of cdk2 activity (Geiselhart et al. 2001), both events which are known to be regulated by the dephosphorylating activity of Cdc25A. Expression of a p38 MAPK-resistant Cdc25A mutant in an IL-7-dependent T-cell line as well as in peripheral, primary T-cells was sufficient to sustain cell survival and promote cell cycling for several days in the absence of IL7 (Khaled et al. 2005). Cdkn1b is a member of the CIP/KIP family of cyclin-dependent cell cycle inhibitors (CKIs) that negatively regulates the G1/S transition. In IL7 dependent T-cells, the expression of Cdkn1b was sufficient to cause G1 arrest in the presence of IL7. Withdrawal of IL7 induced the upregulation of Cdkn1b and arrested cells in G1 while siRNA knockout of Cdkn1b enhanced cell cycle progression. However, adoptive transfer of Cdkn1b-deficient lymphocytes into IL7 deficient mice indicated that loss of Cdkn1b could only partially compensate for the IL7 signal needed by T-cells to expand in a lymphopenic environment (Li et al. 2006), so though Cdkn1b may be involved in negative regulation of the cell cycle through an effect on cdk2 activity, its absence is not sufficient to fully induce cell cycling under lymphopenic conditions. View original pathway at Reactome.
The IL7 receptor is a heterodimeric complex of the the common cytokine-receptor gamma chain (IL2RG, CD132, or Gc) and the IL7-receptor alpha chain (IL7R, IL7RA, CD127). Both chains are members of the type 1 cytokine family. Neither chain is unique to the IL7 receptor as IL7R is utilized by the receptor for thymic stromal lymphopoietin (TSLP) while IL2RG is shared with the receptors for IL2, IL4, IL9, IL15 and IL21. IL2RG consists of a single transmembrane region and a 240aa extracellular region that includes a fibronectin type III (FNIII) domain thought to be involved in receptor complex formation. It is expressed on most lymphocyte populations. Null mutations of IL2RG in humans cause X-linked severe combined immunodeficiency (X-SCID), which has a phenotype of severely reduced T-cell and natural killer (NK) cell populations, but normal numbers of B cells. In addition to reduced T- and NK-cell numbers, Il2rg knockout mice also have dramatically reduced B-cell populations suggesting that Il2rg is more critical for B-cell development in mice than in humans. Patients with severe combined immunodeficiency (SCID) phenotype due to IL7R mutations (see Puel & Leonard 2000), or a partial deficiency of IL7R (Roifman et al. 2000) have markedly reduced circulating T cells, but normal levels of peripheral blood B cells and NK cells, similar to the phenotype of IL2RG mutations, highlighting a requirement for IL7 in T cell lymphopoiesis. It has been suggested that IL7 is essential for murine, but not human B cell development, but recent studies indicate that IL7 is essential for human B cell production from adult bone marrow and that IL7-induced expansion of the progenitor B cell compartment is increasingly critical for human B cell production during later stages of development (Parrish et al. 2009).
IL7 has been shown to induce rapid and dose-dependent tyrosine phosphorylation of JAKs 1 and 3, and concomitantly tyrosine phosphorylation and DNA-binding activity of STAT5a/b (Foxwell et al. 1995). IL7R was shown to directly induce the activation of JAKs and STATs by van der Plas et al. (1996). Jak1 and Jak3 knockout mice displayed severely impaired thymic development, further supporting their importance in IL7 signaling (Rodig et al. 1998, Nosaka et al. 1995).
The role of STAT5 in IL7 signaling has been studied largely in mouse models. Tyr449 in the cytoplasmic domain of IL7RA is required for T-cell development in vivo and activation of JAK/STAT5 and PI3k/Akt pathways (Jiang et al. 2004, Pallard et al. 1999). T-cells from an IL7R Y449F knock-in mouse did not activate STAT5 (Osbourne et al. 2007), indicating that IL7 regulates STAT5 activity via this key tyrosine residue. STAT5 seems to enhance proliferation of multiple cell lineages in mouse models but it remains unclear whether STAT5 is required solely for survival signaling or also for the induction of proliferative activity (Kittipatarin & Khaled, 2007).
The model for IL7 receptor signaling is believed to resemble that of other Gc family cytokines, based on detailed studies of the IL2 receptor, where IL2RB binds constitutively to JAK1 while JAK3 is pre-associated uniquely with the IL2RG chain. Extending this model to IL7 suggests a similar series of events: IL7R constitutively associated with JAK1 binds IL7, the resulting trimer recruits IL2RG:JAK3, bringing JAK1 and JAK3 into proximity. The association of both chains of the IL7 receptor orients the cytoplasmic domains of the receptor chains so that their associated kinases (Janus and phosphatidylinositol 3-kinases) can phosphorylate sequence elements on the cytoplasmic domains (Jiang et al. 2005). JAKs have low intrinsic enzymatic activity, but after mutual phosphorylation acquire much higher activity, leading to phosphorylation of the critical Y449 site on IL7R. This site binds STAT5 and possibly other signaling adapters, they in turn become phosphorylated by JAK1 and/or JAK3. Phosphorylated STATs translocate to the nucleus and trigger the transcriptional events of their target genes.
The role of the PI3K/AKT pathway in IL7 signaling is controversial. It is a potential T-cell survival pathway because in many cell types PI3K signaling regulates diverse cellular functions such as cell cycle progression, transcription, and metabolism. The ERK/MAPK pathway does not appear to be involved in IL7 signaling (Crawley et al. 1996).
It is not clear how IL7 influences cell proliferation. In the absence of a proliferative signal such as IL7 or IL3, dependent lymphocytes arrest in the G0/G1 phase of the cell cycle. To exit this phase, cells typically activate specific G1 Cyclin-dependent kinases/cyclins and down regulate cell cycle inhibitors such as Cyclin-dependent kinase inhibitor 1B (Cdkn1b or p27kip1). There is indirect evidence suggesting a possible role for IL7 stimulated activation of PI3K/AKT signaling, obtained from transformed cell lines and thymocytes, but not confirmed by observations using primary T-cells (Kittipatarin & Khaled, 2007). IL7 withdrawal results in G1/S cell cycle arrest and is correlated with loss of cdk2 activity (Geiselhart et al. 2001), both events which are known to be regulated by the dephosphorylating activity of Cdc25A. Expression of a p38 MAPK-resistant Cdc25A mutant in an IL-7-dependent T-cell line as well as in peripheral, primary T-cells was sufficient to sustain cell survival and promote cell cycling for several days in the absence of IL7 (Khaled et al. 2005). Cdkn1b is a member of the CIP/KIP family of cyclin-dependent cell cycle inhibitors (CKIs) that negatively regulates the G1/S transition. In IL7 dependent T-cells, the expression of Cdkn1b was sufficient to cause G1 arrest in the presence of IL7. Withdrawal of IL7 induced the upregulation of Cdkn1b and arrested cells in G1 while siRNA knockout of Cdkn1b enhanced cell cycle progression. However, adoptive transfer of Cdkn1b-deficient lymphocytes into IL7 deficient mice indicated that loss of Cdkn1b could only partially compensate for the IL7 signal needed by T-cells to expand in a lymphopenic environment (Li et al. 2006), so though Cdkn1b may be involved in negative regulation of the cell cycle through an effect on cdk2 activity, its absence is not sufficient to fully induce cell cycling under lymphopenic conditions. View original pathway at Reactome.
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gene, SOCS2
gene:p-STAT5 dimerAnnotated Interactions
gene, SOCS2
gene:p-STAT5 dimergene, SOCS2
gene:p-STAT5 dimerThe p85 subunit of PI3-kinase (PI3K) binds to phosphorylated Tyrosine-449 (Y449) on IL7R.
Y449F substitution inhibits PI3K-dependent proliferation of IL7-stimulated murine B-lineage cells (Venkitaraman & Cowling 1994). Stimulation of human lymphocyte precursor cells with IL7 induced tyrosine phosphorylation of the p85 subunit of PI3K and activation of PI3K kinase activity (Dadi et al. 1994). It is thought that, depending on species differences and stage of lymphocyte development, IL7 induced PI3K pathway can promote signals that are important for survival and proliferation of both T cells and B cells. Activation of PI3K leads to the generation of membrane associated PIP3 and membrane recruitment of AKT/PKB, the key downstream target of PI3K. AKT mediates phosphorylation of downstream substrates involved in regulation of cell survival and proliferation. IL7 induced activation of PI3K/AKT in human thymocytes has been reported (Pallard et al. 1999; Johnson et al. 2008). In mouse thymocytes IL7 stimulation resulted in the inactivation of BAD by serine phosphorylation; the PI3K/AKT pathway has been implicated in BAD phosphorylation. These results suggest that IL7 signaling via AKT inactivates the pro-apoptotic protein BAD promoting T cell survival (Li et al. 2004).
Rochman et al. (2009) suggested that IL7 promotes lymphocyte survival by activating the pro-survival PI3K/AKT signaling pathway and by increasing the expression of survival factors such as BCL2 and myeloid cell leukemia sequence 1 (MCL-1) while inhibiting the expression of pro-apoptotic factors BAX and BAD.
Interleukin-7 induced PI3K-dependent phosphorylation of AKT1 (Akt1 or PKB) and its downstream targets GSK-3, FOXO1, and FOXO3a (Barata et al.2004).
It has been sugggested that JAK1 phosphorylates IL7R (Jiang et al. 2004) and it is believed that JAK3, associated with IL2RG, phosphorylates the tyrosine residues in the cytoplasmic portion of IL7R that lead to recruitment of STATs (Fry & Mackall 2002). This is consistent with the lack of intrinsic tyrosine kinase activity in IL7R:JAK1 in the absence of IL2RG:JAK3 (Lai et al. 1996). Phosphorylated Y449 is believed to be the docking site for STAT5 and possibly PI3K, which are then activated by JAKs (Lin et al. 1995, Jiang et al. 2004). T-cells from IL7R Y449F knock-in mice did not activate Signal transducer and activator of transcription A or B (STAT5A, STAT5B) (Osbourne et al. 2007), indicating that IL7 regulates STAT5 activity via this key tyrosine.
STAT5 is activated in COS7 cells when co‑transfected with JAK3 (Lin et al. 1996), though this does not demonstrate that JAK3 phosphorylates STAT5 proteins in response to IL7 in vivo (Lin & Leonard, 2000). STAT5 phosphorylation is represented here as a black box event because it is not clear which JAK kinase phosphorylates STAT5 in vivo.
IL2RG interacts with the intermediate 1:1 IL7:IL7R complex, forming the active ternary complex, which binds Interleukin-7 (IL7) with a 3-fold higher affinity (Kd =80 pM).
Interleukin-7 (IL7)-activated Signal transducer and activator of transcription 5A or 5B (typically referred to as STAT5) translocates to the nucleus (Landires et al. 2011) and is recruited rapidly to the promoters of IL7-regulated genes (Ye et al. 2001, Stanton & Brodeur 2005).
This is a black box event because dissociation is inferred from the subsequent translocation of STAT5 to the nucleus.In pre-B cells, in contrast to pro-B cells, IL7 signaling is downregulated, which induces BRWD1 transcription. BRWD1 enhances the recruitment of the recombinase complex V(D)J recombination-activating protein 1: V(D)J recombination-activating protein 2 (RAG1:RAG2) to the immunoglobulin kappa locus, thus promoting V(D)J recombination - a key event in B lymphopoiesis (Mandal et al. 2015).
This is a black-box event since details of the mechanism of transcription and translation are omitted.
Following binding of TSLP to Cytokine receptor-like factor 2 (CRLF2, TSLPR) and Interleukin-7 receptor subunit alpha (IL7R), Signal transducer and activator of transcription 5A and 5B (STAT5A and STAT5B) and Signal transducer and activator of transcription 3 (STAT3) were phosphorylated in various Ba/F3 cell populations (Reche et al. 2001).
This is a black box event since the kinase responsible for STAT phosphorylation is unknown.
This is a black box event since it is not clear whether this binding event occurs in association with other IL7 receptor complexes or independently.
It has been sugggested that JAK1 phosphorylates IL7R (Jiang et al. 2004) and it is believed that JAK3, associated with IL2RG, phosphorylates the tyrosine residues in the cytoplasmic portion of IL7R that lead to recruitment of STATs (Fry & Mackall 2002). This is consistent with the lack of intrinsic tyrosine kinase activity in IL7R:JAK1 in the absence of IL2RG:JAK3 (Lai et al. 1996).
The receptor for Thymic stromal lymphopoietin (TSLP) consists of Cytokine receptor-like factor 2 (CRLF2, TSLPR) and Interleukin-7 receptor subunit alpha (IL7R). Ba/F3cells expressing human IL7R or human TSLPR, or both stimulated with TSLP induce phosphorylation of Signal transducer and activator of transcription 3 (STAT3) and Signal transducer and activator of transcription 5A or B (STAT5A, STAT5B or STAT5), only in cells expressing both receptors (Reche et al.2001).
This is a black box event since there is no experimental details confirming binding of STAT3 and STAT5 to the receptor complex.
Signal transducer and activator of transcription 5A, 5B (STAT5A and STAT5B) and Signal transducer and activator of transcription 3 (STAT3) are phosphorylated upon addition of human Thymic stromal lymphopoietin (TSLP) when both Cytokine receptor-like factor 2 (CRLF2) and Interleukin-7 Receptor (IL7R) are present (Reche et al. 2001). Subsequently the phosphorylated STATs dissociate.
This is black box event since as not been demonstrated experimentally.
Signal transducer and activator of transcription 5A (STAT5A) and Signal transducer and activator of transcription 5B (STAT5B) dimers bind to similar core gamma-interferon activated sequence (GAS) motifs (Soldaini et al., 2000). STAT5 also form homo- and hetero-tetramers with distinct or expanded DNA-binding properties. This is a black box event because apart of the mentioned here genes could be another subset of genes up or downregulated by STAT5. Genes that are regulated by STAT5 include Interleukin-2 receptor alpha (IL2RA) (John et al. 1996), TNFSF11 (RANKL), Connexin-26 (GJB2) and Cyclin D1 (Hennighausen & Robinson, 2005). A comprehensive listing of hepatic STAT5B regulated genes is available from microarray/STAT5B knockout mice (Clodfelter et al. 2006), and similarly for STAT5-dependent genes regulated by the GH receptor (Rowland et al. 2005, Barclay et al. 2011).
STAT5 is activated in COS7 cells when co‑transfected with JAK3 (Lin et al. 1996), though this does not demonstrate that JAK3 is responsible for STAT5 phosphorylation in vivo (Lin & Leonard, 2000). STAT5 binding is represented here as a black box event as it is inferred to be a prerequisite of IL7-induced STAT5 phosphorylation.