myd88 was first described in 1990 as a gene upregulated during il-6-induced myeloid differentiation , but its homology to the cytosolic domains of drosophila toll and mammalian il-1rs ( whose homology had already been noted ) was not appreciated for another 4 years .
eventually , myd88 was implicated in signaling downstream of il-1r and mammalian tlrs [ 57 ] .
the c - terminal tir ( toll il-1r ) domain mediates the interaction with other tir domain - containing proteins ( receptors or adaptors ) ; the n - terminal death domain ( dd ) associates with the irak family members through homotypic dd interactions ( figure 1 ) .
consistent with these roles , overexpression of the dd of myd88 leads to spontaneous activation of nfb and c - jun n - terminal kinase ( jnk ) , whereas the tir domain can act as a dominant negative .
although this domain does not appear to be involved in the direct interactions described above , it is necessary for irak4 activation .
in fact , myd88s , a splice variant of myd88 lacking the int domain , is induced upon activation and acts as a dominant negative form of myd88 .
myd88 is composed of three main domains : a death domain ( dd ) ( 54 to 109 ) , intermediate domain ( int ) ( 110 to 155 ) , and toll - interleukin-1 receptor domain ( tir ) ( 159 to 296 ) . although the dd is annotated as 54 to 109 , proper folding of the dd seems to require amino acids 110 to 117 .
point mutations inducing a loss ( blue ) or gain ( red ) of function are indicated by amino acid number .
the site implicated in interferon regulatory factor 7 ( irf-7 ) binding ( 1 to 59 ) and the domain lost in the splice variant myd88s ( 110 to 155 ) are indicated in black .
mice lacking myd88 were reported in 1998 , and the initial analysis of these mice confirmed the importance of this adaptor downstream of the il-1r family . in the following year , myd88-deficient mice were shown to lack responsiveness to lipopolysaccharide ( lps ) , the ligand for tlr4 .
although this muted response protected myd88-deficient mice from endotoxic shock , myd88-deficient cells retained some residual signaling when stimulated with lps , suggesting myd88-independent signaling mechanisms downstream of tlr4 . ultimately , this observation led to the discovery of tir domain - containing adapter - inducing interferon - beta ( trif ) as an adaptor for tlr3 and 4 .
myd88-deficient mice have been used extensively as a model for tlr deficiency and are susceptible to a large variety of bacterial pathogens or parasites .
comparisons between myd88- and caspase-1/11-deficient mice have been used to control for lack of il-1r and il-18r signaling in myd88-deficient animals , but mice deficient for all tlr functions were recently described and may prove useful for evaluating the role of tlrs independently of receptors for il-1 family cytokines .
several inactivating mutations in myd88 have also been identified in humans with recurrent infections with pyogenic bacteria .
these mutations , as well as some rare missense polymorphisms , are associated with reduced irak4 activation , leading to impaired responses through tlrs and il-1 family members .
myd88-deficient humans do not appear abnormally susceptible to many viral , fungal , or parasitic infections . whether myd88 is somehow less central to human immunity is complicated by the vaccinations and antibiotic regimens received by these patients .
as our understanding of innate immunity has grown , it has become clear that other myd88-independent signals can mediate innate and adaptive immune responses to many pathogens , especially viruses .
the first structure of a tir domain was elucidated shortly after the first descriptions of the function of myd88 as a signaling adaptor .
this first study solved the structure of the human tlr1 tir domain and demonstrated that tir domains can oligomerize , but with an affinity in the millimolar range . of note ,
additional studies showed potentially significant differences in the structures of other tir domains , in particular the bb loop of the tir domain of myd88 .
this structural difference results in the inability of the tir domain of myd88 to dimerize with itself , which may be critical to prevent ligand - independent activation .
because of the relative weakness of tir - tir interactions , it has been postulated that conformational changes upon tlr ligand recognition bring tir domains in close proximity .
this change of avidity would allow for the initiation of myd88 signaling . in the case of tlr9
, it has indeed been shown that the receptor occurs as a homodimer and that ligand binding brings the cytoplasmic tails containing the tir domains in close proximity .
the structure of the extracellular domain of tlr3 has been determined in the absence   and presence   of ligand . although the tir domain was not part of the crystallized protein , modeling shows that ligand binding by a tlr3 homodimer brings the tir domains in close proximity .
similarly , the structure of the extracellular domains of a tlr1/2 heterodimer formed in the presence of ligand suggests a close association between the tir domains of each tlr .
the recently solved structure of the extracellular domain of tlr8   also supports this idea .
indeed , binding of a tlr8 homodimer to its ligand induces a conformational change in the extracellular domains , bringing the membrane - proximal regions together .
this conformational change most likely promotes the association of the transmembrane domains and cytoplasmic tails that contain the tir domain , although those were not included in the structure .
together , all of the structures of tlr and their ligands solved so far support the model outlined above .
myd88 can associate directly with receptor tir domains , so it was unexpected when the tir - domain - containing adaptor protein ( tirap [ also named mal ] ) , which contains its own tir domain and a lipid - binding motif , was shown to facilitate the association of myd88 and tlr4 .
initial studies showed that tirap was required for tlr4 and tlr2 signaling but was dispensable for tlr3 , 7 , or 9 signaling .
of note , the requirement for tirap does not appear absolute and can depend on ligand concentration as well as cell type .
however , recent evidence suggests that tirap can also be involved in tlr9 signaling . because the requirement for tirap during tlr9 signaling was evident only with particular ligands ( viruses but not the more stable and commonly used phosphorothioate - linked cpg dna ) and cell types ( immortalized macrophages but not the more phagocytic bone marrow - derived macrophages ) , it is possible that tirap is necessary only when few tlrs are engaged , suggesting that a certain threshold of tir domains must be achieved to trigger downstream myd88 signaling .
it is also possible that different types of ligands induce different degrees of conformational changes and tir clustering in tlr9 homodimers . in the case of tlr4 ,
where the requirement for tirap is most evident , several non - exclusive hypotheses have been proposed to explain the role of tirap .
tirap appears to be necessary because of its membrane localization and can be bypassed by targeting myd88 directly to the membrane . on the other hand
, there is evidence for a preferential interaction between tirap and myd88 tirs but not tlr4 and myd88 tirs .
clustering of tir domains is thought to provide a stable platform for myd88 binding and oligomerization .
this oligomerization of myd88 was apparent in early overexpression studies , but the complex formed by myd88 dds and irak dd was crystallized only recently .
this structure , termed the myddosome , is composed of a ring of six myd88 dds that serve as a seed for assembly of a ring of four irak4 dds and ultimately four irak2 dds in a helix .
this helical assembly was observed in other dd - based oligomeric complexes , such as the piddosome   and the death - inducing signaling complex ( disc ) .
all assemblies rely on three distinct types of interactions between dds , allowing sequential assembly of a large complex .
interestingly , the myddosome structure was obtained in the absence of most of the int domain ( residues 1 to 117 , the annotated int domain starts at residue 110 ) , but a similar stoichiometry was observed in the presence of the full int domain ( residues 1 to 157 ) .
however , because the structure was crystallized in the absence of tir domains , several questions remain unanswered .
for example , it is unclear whether all myd88 molecules in the myddosome are bound to a tlr or il-1r through their tir domain or whether the clustering of two tir domains is enough to allow stable dd association that seeds binding of additional myd88 dds and assembly of a full myddosome .
it will also be interesting to investigate whether myddosomes can connect multiple dimerized tlrs , as suggested by recent work on tumor necrosis factor ( tnf)-induced dd complexes .
in addition to showing myd88 functions induced by tir - containing receptors , a few studies have shown a role for myd88 downstream of receptors that do not contain tir domains , like interferon gamma receptor ( ifnr )   and tnf receptor superfamily member 13b ( also named taci ) . in those cases ,
it is unclear whether the same molecular mechanisms are involved in myd88 signaling , as taci but not ifnr appears to signal through the usual partners used during tlr / il-1r signaling . on the other hand , although most of the myd88-dependent signaling is mediated by dd - dependent irak recruitment , myd88 can also directly bind to interferon regulatory factor 7 ( irf-7 ) to promote type i ifn production .
this association is mediated by the n - terminal portion of myd88 but is not dependent on a dd fold ( as a myd88 60 - 296 can still interact with irf-7 ) .
it is interesting to note that a gain - of - function variant of myd88 ( l256p ) has been described in some classes of b - cell lymphomas .
this mutant can spontaneously assemble and lead to persistent nfb and signal transducer and activator of transcription 3 ( stat3 ) activation .
the mutation lies within the tir domain of myd88 , suggesting that myddosome assembly and signal initiation may be affected .
at least two models could explain the oncogenic potential of this mutation : either the mutation promotes the direct binding of myd88 tir homodimers , allowing efficient self - assembly , or the mutation enhances the affinity of myd88 for tlr or il-1r tir domains .
importantly , this mutation was observed in the absence of well - characterized oncogenic events ( for example , myc translocation ) , suggesting that this mutant of myd88 can act as an oncogenic driver . of note
, it is also possible that the overexpression of wildtype myd88 is sufficient to promote tumor growth in other cell types . in light of the oncogenic potential of myd88 , it may be interesting to revisit its role during tumorigenesis   ( for example , by comparing myd88-deficient mice to tlr - deficient mice ) .
myd88 acts as a central hub in inflammatory responses   and can induce signaling from several receptors , located either at the plasma membrane or in endosomes .
additionally , myd88 signaling can lead to the production of pro- or anti - inflammatory cytokines as well as type i ifns .
two tnf receptor - associated factors ( trafs)traf3 and traf6can be activated by irak family members and lead to the production of type i ifn or pro - inflammatory cytokines , respectively .
although the myd88-irak4-irak2 axis of the pathway can theoretically interact with both traf3 and traf6 complexes , it is likely that only some receptors , adaptors , or signaling molecules are available at given subcellular locations or in specific cell types . here
, we will discuss some established examples as well as new research avenues in the interplay between the initiation and localization of myd88 signaling .
tlr4 is unique among tlrs because it can signal both through myd88 to induce pro - inflammatory cytokines via nfb and through trif to induce type i ifn through irf3 .
interestingly , it was found that those signaling events are spatially separated , with tirap / myd88 signaling occurring from the surface whereas signaling from trif - related adaptor molecule / trif ( tram / trif ) occurs from endosomes . in that specific case ,
receptor internalization induces a switch from myd88 to trif signaling because of a change of adaptors , from tirap to the other tir - containing adaptor tram . because of the distinct lipid composition of the plasma membrane and endosomes , and of the affinity of tirap for the plasma membrane phospholipid phosphatidylinositol 4,5-bisphosphate ( pip2 ) , internalization limits tirap affinity and myd88 activation .
given the promiscuity of tirap binding to lipids , it is unclear whether the difference in affinities of tirap for pi(4,5)p2 and phosphatidylinositol ( 3,4,5)-trisphosphate ( pip3 ) is sufficient to mediate the switch from tirap to tram or whether other mechanisms can act simultaneously to promote tram association . aside from the case of tlr4
, there is evidence that the site of ligand recognition influences the signaling outcome even for receptors that rely solely on myd88 .
the existence of distinct signaling pathways under the control of a single receptor is evident in the case of tlr9 , which recognizes cpg motifs and can trigger the production of pro - inflammatory cytokines or type i ifn .
it has been appreciated for quite some time that different classes of cpg motifs ( a or b ) favor type i ifn or pro - inflammatory cytokine production , respectively .
analysis of plasmacytoid dendritic cells from adaptor protein 3 ( ap-3)-deficient mice revealed that this adaptor complex is critical for type i ifn production but dispensable or detrimental for pro - inflammatory cytokine production .
similarly , a localization motif in the cytoplasmic tail of tlr9 is required for pro - inflammatory cytokine but not type i ifn production .
again , lipid composition of the signaling compartment appears to be critical for specificity , as fusion of traf3 ( a critical signaling node downstream of irak kinases ) to a pleckstrin homology ( ph ) domain that allows localization to phosphatidylinositol 3,5-bisphosphate ( pip2)-containing vesicles can bypass the requirement for ap-3 . although these bifurcation pathways have mostly been identified in the case of tlr9 , they may occur during signaling through other receptors and may be underlying the ability of tlr2 to induce type i ifn during viral infection in inflammatory monocytes .
in addition to the spatial regulation of adaptors ( such as tirap ) and signaling molecules ( such as traf3 ) , it is clear that tlrs themselves are spatially regulated . the chaperone unc93b is responsible for the trafficking of all endosomal tlrs , but some tlrs can also use their own localization motifs for trafficking ( such as tlr7 and ap-4 ) , providing a basis for differential trafficking of distinct receptors .
ligands themselves can also be trafficked through different cellular compartments depending on their nature , as is the case with dna in the form of immune complexes , for example .
a key challenge for future years is to understand the interplay between the trafficking of receptors , adaptors , and ligands and how localization impacts signaling initiation and the interaction between myd88 and other specialized signaling components .
overall , since its discovery about 20 years ago , myd88 has emerged as a central and well - conserved node of innate immune responses through its role as a bridge between tir - containing receptors and dd - containing kinases .
the characterization of the myddosome suggests a clear molecular mechanism for signal transduction upon the local clustering of tir domain - containing receptors .
myd88 signaling can lead to distinct outputs depending on the context , usually leading to pro - inflammatory cytokine or type i ifn production .
distinct pathways downstream of irak family members regulate these outputs , and the outcome of signaling can be influenced by the cell type and location of signal initiation . in the years
ahead , a more dynamic view of myd88 interaction networks , from ligands to receptors to downstream signaling components , may provide us with additional insight into the regulation of diverse myd88 functions .
 myd88 is the canonical adaptor for inflammatory signaling pathways downstream of members of the toll - like receptor ( tlr ) and interleukin-1 ( il-1 ) receptor families . 
 myd88 links il-1 receptor ( il-1r ) or tlr family members to il-1r - associated kinase ( irak ) family kinases via homotypic protein - protein interaction . 
 activation of irak family kinases leads to a variety of functional outputs , including the activation of nuclear factor - kappa b ( nfb ) , mitogen - activated protein kinases , and activator protein 1 , making myd88 a central node of inflammatory pathways . as more details of myd88-dependent signaling 
 have been elucidated , it has become clear that the functions of this critical signaling component can be influenced by multiple interaction partners in distinct subcellular compartments . in this review , we will focus on recent developments in the understanding of the assembly of myd88 signaling complexes and the mechanisms leading to the diversification of myd88-based signaling . 