tweak ( tnfsf12 ) is a cell surface - associated type ii transmembrane protein ( 249 amino acids ) belonging to the tumor necrosis factor ( tnf ) superfamily . transmembrane tweak is processed into a secreted 156-amino - acid form , which adopts a homotrimeric conformation . tweak mrna has been reported to be expressed in several tissue types , such as heart , brain [ 3 , 4 ] , kidney [ 5 , 6 ] , and also in mononuclear blood cells . its protein product has multiple biological activities , including stimulation of cell growth and angiogenesis , induction of inflammatory cytokines [ 9 , 10 ] and stimulation of apoptosis [ 11 , 12 ] . it has been shown to be involved in the induction of cellular proliferation in liver cells , osteoblasts , astrocytes , synoviocytes , kidney cells [ 17 , 18 ] , and skeletal muscle . tweak may also play a role in the cellular differentiation of osteoclasts ; however it remains controversial whether this effect is direct   or indirect , via effects on the osteoblastic stromal cell expression of rankl ( tnfsf11 ) . tweak also plays a role in inducing glioma cell survival via imparting resistance to cytotoxic agents [ 3 , 22 ] . it induces the endothelial cell survival and can be a potential proangiogenic or antiangiogenic agent based upon the presence of angiogenic promoting cytokines [ 8 , 23 ] . additionally , an apoptotic effect of tweak has been observed in endometrial cancers   and peripheral blood monocytes [ 25 , 26 ] . the apoptotic function of tweak appears to be mediated via the induced secretion of tnf , with the tnf-tnf receptor complex , thereafter inducing autocrine cellular apoptosis by activating the ripk1-fadd - caspase-8 complex [ 11 , 27 ] . tweak was first described as an apoptotic factor by interacting with dr3 ( tnfrsf25 ) . however , there were conflicting reports to the tweak - dr3 interaction [ 28 , 29 ] . in addition , tweak has been reported to interact with cd163 ; however , the downstream effect of this interaction remains to be explored . tnfrsf12a ( tumor necrosis factor receptor super - family , member 12a ) , also known as fgf - inducible 14 ( fibroblast growth factor - inducible-14/fn14 ) , has been established to date to be the major , if not sole , receptor for tweak [ 12 , 31 , 32 ] . fn14 is the smallest member of the tnfr superfamily described so far , and it appears to signal via recruitment of several different tnfr - associated factors . this molecule has been reported to be expressed in variety of organs including the heart [ 34 , 35 ] , kidney [ 6 , 36 ] , and lung . the cytoplasmic domain of fn14 , like other members of the tnfr superfamily , does not contain consensus amino acid sequences characteristic of domains with enzymatic activity . this interaction can stimulate a variety of biological responses , depending on the cell type analyzed . ( 2008 ) hypothesized two modes of tweak - fn14 ( ligand - receptor ) interaction : ( i ) the ligand - dependent interaction , which involves the higher concentration of homotrimeric tweak , that binds to low concentration of fn14 in a heterohexameric complex [ 38 , 39 ] , and ( ii ) ligand - independent interaction when the ligand concentration is lower than the receptor concentration . here they are the canonical and noncanonical nf-b pathways [ 21 , 33 , 34 , 40 ] and the mapk pathway [ 4143 ] with possible binding to traf proteins . these diseases include autoimmune disorders [ 16 , 21 , 44 , 45 ] , neurological disorders [ 46 , 47 ] , periodontal disease , and cancers [ 3 , 22 , 24 , 4850 ] . because of its multifunctional properties , tweak is also being considered for use in therapeutics . it is also being considered as a potential early and prognostic biomarker for conditions such as kidney injury [ 52 , 53 ] , sle , atherosclerosis [ 55 , 56 ] , cardiovascular disorders [ 5759 ] , immune preconception marker , and abdominal aortic aneurysms . although the results obtained to date are captivating , it is clear that additional studies are required to determine whether tweak , and/or fn14 could be novel molecular targets for developing anticancer and antiautoimmune therapeutic agents in humans . thus , given its importance in the field of biomedical research , we carried out an extensive and iterative compilation of tweak - fn14 signaling pathway by literature mining . information gathered on protein - protein interactions , posttranslational modifications , protein transportation events , and regulation of gene expression , which are stimulated by tweak were compiled into a signaling pathway using a visualization tool , pathvisio . our compiled data will be useful for the scientific community to explore , further , the role of tweak in differential disease pathogenesis , in biomarker development . using similar approach , we have also developed signaling pathways on leptin   receptor activator for nuclear factor  b ligand ( rankl )   and follicle stimulating hormone ( fsh ) . in the current study , we have generated a reaction map of tweak signaling pathway , which is available for visualization at netslim   ( http://www.netpath.org/netslim/ ) , an accessory resource for visualization of netpath pathways . pubmed searches were performed using tweak or fn14 and their alternate names as keywords to retrieve relevant articles pertaining to tweak signaling . the articles were screened to capture molecular reactions stimulated by tweak in mammalian cells as compared to the corresponding unstimulated state . thereafter , with the use of an in - house developed software , pathbuilder   that enables conversion of pathway data into standard community formats , namely , psi - mi , biopax , and sbml formats , we annotated biological information and reactions pertaining to tweak signaling . these included protein - protein interactions , enzyme - substrate reactions , gene regulation events , and also various activation / inhibition reactions under a tweak stimulus . these data after manual revision were exported to the netpath database , ( http://www.netpath.org/ ) , a manually assembled resource for signaling pathways generated by our group   which provides the criteria for data compilation . the protein - protein interactions gathered from several experimental platforms were cataloged from literature into either binary or complex interactions . a binary interaction represents the interaction of two proteins either in homomeric or heteromeric form . a complex protein interaction comprises reactions involving more than two proteins , which again can be either homomeric or in heteromeric . for every protein - protein interaction , we documented information on subcellular localization , the experimental method used , the name and species of cell models , and finally , the hyperlinked pubmed identifier for the corresponding publication . we compiled the posttranslational modifications under tweak stimulus and mapped them to their corresponding protein sequences in the refseq database . further , activation or inhibition of the substrate in response to the stimulus was also compiled . the notable modifications chosen were phosphorylation , acetylation , ubiquitination , sumoylation , protein degradation , and methylation . direct included those reactions where the enzyme has been reported for the specific type of protein ( substrate ) modification . indirect reactions include those where the type of modification is experimentally proved ; however no information exists about its immediate upstream enzyme . the features added for the enzyme - substrate reactions include the type of posttranslational modification , the site and residue of each modification , the source of protein , the species used , and cellular localizations of the respective reaction . additionally , we have incorporated a pubmed identifier as a hyperlink pertaining to the reaction . several molecules , including the caspases-3 , -7 , and -8 ( casp3 , casp7 and casp8 ) , jun [ 20 , 29 , 69 , 70 ] , and nik ( map3k14 ) [ 52 , 71 ] , were activated , whereas stat1 was inhibited under tweak stimulus . these molecules do not abide by the enzyme - substrate reactions and protein - protein interaction parameters as described previously and thus can not be connected directly to the main frame of the tweak pathway and are referred to as orphan molecules . we have provided the source of protein , subcellular localization , species , and cell line in which the activation or inhibition event was reported . subcellular transportation events of proteins under the influence of tweak reported to date , with appropriate gene ontology terms , were added into the pathbuilder tool . these events were selected on the basis of the posttranslational modifications , physical interaction or regulatory events . a tweak stimulus resulting in subcellular relocalization of proteins was evidenced by fluorescent microscopy and immunohistochemical studies . in addition to a particular protein 's altered localization , we have also documented the source of protein and cell lines used . we have documented genes whose expressions are regulated by the tweak - fn14 signaling in humans . such genes that have been identified by various groups at the mrna level were catalogued from dna microarray and nonarray - based experiments such as northern blotting , quantitative rt - pcr , or sage . further , we have included transcription regulators ( transcription factors , or their coactivators / corepressors ) downstream of tweak - fn14 stimulus . some of these transcription regulators are involved in the regulation of the genes ( mentioned above ) upon tweak signaling . such transcriptional regulators have been identified by approaches such as chromatin immunoprecipitation assays , electrophoretic mobility shift assays , gene silencing , and promoter activity assays in tweak - fn14 signaling . data for protein - protein interactions , catalytic reactions , and transportation events were collected from diseased or normal mammalian sources that include humans and their orthologs . the manually assembled data in pathbuilder were compiled and imported into netpath ( explained under methodology ) . a composite map of pathway reactions pertaining to tweak signaling were generated using pathvisio   by following the netslim parameters as have been employed earlier by our group . netslim ( http://www.netpath.org/netslim/ ) is a tributary of netpath , which projects or summarizes only stringent reactions pertaining to the specific receptor - ligand complex compiled in a particular study , for example , tweak in this case . the criteria for selecting high confidence reactions for tweak pathway are provided in the netslim database ( http://www.netpath.org/netslim/criteria.html ) . we show here for the first time in any scientific repository a pathway illustration under tweak stimulus . given the multifunctional properties of tweak , we carried out a comprehensive literature search under tweak stimulus followed by manual amassment , thereafter reviewing and adding the data into netpath database . fifty - eight articles were found relevant to our amassment criteria from amongst 357 articles published between 1997 and 2011 . this study led to the documentation of 46 unique proteins amid which 17 were associated with protein - protein interactions , 20 involved in enzyme - substrate reactions , 13 involved in activation - inhibition reactions , and 8 were identified to be translocated from cytoplasm to nucleus . there were 28 genes identified to be differentially regulated under tweak stimulus in human systems . the data for visualization of tweak signaling pathways were obtained after filtering netpath data using netslim parameters . a total of 36 molecules involved in 42 reactions are visually depicted in the tweak pathway in netslim . the tweak data in netpath are available freely and can be used by the scientific community . the data are represented in various standard exchange formats that include biological pathway exchange ( biopax ) , systems biology markup language ( sbml )   and proteomics standards initiative molecular interaction ( psi - mi )   language formats . the psi - mi is a community standard language for molecular interaction data used for data comparison and exchange . biopax is another standard language that has features compatible with sbml and psi - mi formats . the tweak signaling representation can be downloaded from the netslim database in various formats , such as  gpml  ,  genmapp  ,  png  , and  pdf  . the gene regulation data are made available in tab - limited and microsoft excel formats . a pathway module is defined as an established cascade of events that takes place inside a cell that has no defined boundaries and is part of a generic network . some well - known modules are the nf-b , mapk , the jnk pathways and the pi3k / akt pathway modules . a schematic model of the tweak pathway with identified pathway modules is represented in figure 3 . the tweak - fn14 complex binds to the traf molecules , traf 1 , 2 , 3 , and 5 . however , the downstream signaling cascade(s ) that proceeds upon the association of tweak - fn14 complex and traf 1/3/5 ( traf1 , traf3 , traf5 ) is unavailable due to the lack of published studies to date . it was possible to decipher the downstream events following the formation of the traf2-ciap1 ( birc2 ) complex . the degradation of the traf2-ciap1 complex leads to the stabilization of nik and activation of the noncanonical nf-b pathway as represented in the model . the degradation of the traf2-ciap1 complex also leads to the activation of the caspase pathway resulting in the apoptosis of tumor cells [ 11 , 27 ] . ikner and ashkenazi   have shown that tweak activates apoptosis through the formation of a rip1-fadd - caspase8 complex by tnf mediated signaling , wherein ciap1 plays a crucial role . a possible role of tweak has been reported in bone and cartilage damage . in fibroblast - like synoviocytes , experimental evidence indicates that tweak - fn14 complex formation leads to the activation of p38 ( mapk14 ) , erk1/2 ( mapk3/mapk1 ) , jnk1/2 ( mapk8/mapk9 ) , and tak1 ( map3k7 ) . no evidence has been obtained from existing literature for further direct downstream targets of p38 and erk1/2 . however , the activation of tak1 leads further to the downstream activation of the nf-b / p65/p50 pathway . also , rac1 has been reported to interact directly with the tweak - fn14 complex leading to activation of the nf-b pathway . activation of akt via phosphorylation has been observed under tweak stimulus with an exception in the case of skeletal muscle . akt phosphorylation leads further to the inactivation of gsk3 resulting in an increase in levels of phospho - gsk3 and active ( dephosphorylated ) -catenin1 ( ctnnb1 ) . the cytoplasmic accumulation of active  -catenin1 results in its nuclear translocation . in addition to binding of tweak with fn14 , we have also documented the binding of cd163   and dr3   with tweak . since the interaction between tweak and dr3 remains controversial [ 28 , 29 ] and the downstream consequences of a possible tweak - cd163 interaction remain to be explored , the pathway illustration does not elaborate on the downstream events for these interactions . the ever increasing experimental data on the various molecular events taking place following ligand - receptor interactions , in this case between tweak and fn14 , make it essential to have a repository for the data and also to create a signaling pathway summary . our current work , which incorporates the tweak - signaling pathway data into  netpath  , would open avenues for further studies of tweak - associated proteins and related disorders , such as cancers and autoimmune diseases . to our understanding , this study compiles for the first time tweak induced signaling events ; these include ( i ) the inactivation of gsk3  followed by dissociation of  -catenin1 , ( ii ) the proapoptotic nature of tweak mediated through the expression of tnf , which further leads to the activation of caspase8 , and ( iii ) the association between tweak and ciap proteins ( 1 and 2 ) [ 11 , 76 ] . we believe that our data will be informative in therapeutic studies , in selecting / pathological events and the simultaneous production of blocking agents . repository is dynamic and will allow a progressive update of relevant data , as more published literature is introduced . in addition to the direct usage of the data stored in the  netpath  database , data can also be exported to other databases , enabling comparison and sharing amongst multiple databases , especially those which have compatible language formats , such as biopax . despite the minimal amount of data , ours can also be used in the overlay of various high - throughput data enabling pathway analysis and can be accessed by any pathway resource to generate a customized pathway . we are currently working on the features in  netpath  , which are incompatible with biopax , especially the ontology hierarchy that has been proposed by the biopax group . to our knowledge , our compilation of data in  netpath  will allow , for the first time for any available scientific repository , a comprehensive study of the tweak pathway and its potential biomedical applications .
tnf - related weak inducer of apoptosis ( tweak ) is a new member of the tnf superfamily . 
 it signals through tnfrsf12a , commonly known as fn14 . 
 the tweak - fn14 interaction regulates cellular activities including proliferation , migration , differentiation , apoptosis , angiogenesis , tissue remodeling and inflammation . 
 although tweak has been reported to be associated with autoimmune diseases , cancers , stroke , and kidney - related disorders , the downstream molecular events of tweak - fn14 signaling are yet not available in any signaling pathway repository . in this paper , we manually compiled from the literature , in particular those reported in human systems , the downstream reactions stimulated by tweak - fn14 interactions . 
 our manual amassment of the tweak - fn14 pathway has resulted in cataloging of 46 proteins involved in various biochemical reactions and tweak - fn14 induced expression of 28 genes . 
 we have enabled the availability of data in various standard exchange formats from netpath , a repository for signaling pathways . 
 we believe that this composite molecular interaction pathway will enable identification of new signaling components in tweak signaling pathway . 
 this in turn may lead to the identification of potential therapeutic targets in tweak - associated disorders .