**2.5 Subfamily 5**

Until now the only two receptors of subfamily 5 characterized in parasitic Platyhelminthes are Ftz-F1 (Fushi Tarazu-factor 1) NRs from *S. mansoni*, one called SmFtz-F1 belonging to the NR5B1 group, and the other named SmFtz-F1α classified in the NR5A3 group [48, 49, 78, 79]. In addition, the previously mentioned sequence identified in *E. granulosus* genome Ftz-F1 (GenBank accession number: CDS15732) also belongs to this subfamily [60–62].

#### *Perspective Chapter: Parasitic Platyhelminthes Nuclear Receptors as Molecular Crossroads DOI: http://dx.doi.org/10.5772/intechopen.102648*

SmFtz-F1 was the first member of this subfamily to be characterized from a lophotrochozoan [48]. Subfamily 5 only contains orphan receptors that bind to their response element as monomers, the most studied members being mammalian SF-1 (steroidogenic factor-1) and LRH-1 (liver receptor homolog-1), both involved in embryonic development. The first member of the subfamily was isolated from *D. melanogaster* [80, 81]. SmFtz-F1 has a deduced amino acid sequence of 731 residues and an apparent molecular mass of 78 kDa (GenBank accession number AF158103), while SmFtz-F1α contains 1892 residues and an apparent mass of 207,402 kDa (GenBank accession number AY665680). The length of these receptors differs from orthologue members of the family, however, both proteins conserved the general structure of the nuclear receptors [48, 78]. The hinge region of SmFtz-F1α is particularly long (1027 amino acids). The DBDs of the two NRs share an identity of 55 to 75% to other members of the family, while LBDs are less conserved but contain the typical LBD signatures of the family as well as a high identity with the AF-2 sequence [48, 78]. Both receptors exhibit the expected monomeric DNA-binding ability since the DBD recognizes an SF1 response element-like sequence. However, SmFtz-F1 recognized this response element with a different binding affinity than SmFtz-F1α. The transactivation mechanism is also different between both receptors [79]. On the other side, as was previously mentioned, it was demonstrated that SmFtz-F1 dimerizes with SmRXR [69].

Although Ftz-F1 protein and mRNA expression are detected during all life cycles, expression levels differed according to the developmental stage. The higher expression of SmFtz-F1 was observed in the larval stages of miracidia, sporocysts, and cercaria, while the protein highest level was found in cercaria, schistosomula, and male adult work suggesting a role during host invasion and adaptation. The transcription behavior of SmFtz-F1α makes a difference between the two NRs since the higher mRNA level was detected in the schistosoma egg stage. A similar gonad distribution was also observed in several Ftz-F1 homologues [82, 83].

Taken together these events, it was hypothesized that target genes of both receptors exert different roles during the parasite development and these two receptors also have different ligands or co-activators. Co-activators characterization could start to decipher the transcriptional regulation complex formed by each nuclear receptor. In this sense, the search of transcription regulators of SmFtz-F1 was performed. The transcription co-activator CREB-binding protein (CBP) homologs from *S. mansoni*, named SmCBP1 and SmCBP2, were characterized. SmCBP1 can interact with SmFtz-F1 and activate the transcription of a reporter gene [84]. On the other side, a specific transcriptional co-repressor protein named SmFIP-1, which interacts with the AF2-AD motif of SmFtz-F1, was identified [85].

Finally, an interesting finding was the identification of the first target gene of SmFtz-F1, the micro-exon gene *meg-8.3* [86]. *meg-8.3* is expressed exclusively in the worm's esophageal gland, an enigmatic tissue that has recently been shown to play a critical role in defending the worm from host attack [87].

#### **2.6 New subfamily 7**

A very interesting finding for the biology of parasitic Platyhelminthes was the identification in *S. mansoni* of a new group of NRs that has two tandem DNA-binding domains and one LBD, named 2DBD, lacking in vertebrates [71]. Subsequently, members of this 2DBD subfamily have been identified in some mollusks, in *Echinococcus granulosus,* and other Platyhelminthes [21, 35, 36, 49]. *S. mansoni* expresses three

2DBD (Sm2DBDα, Sm2DBDβ, Sm2DBDγ) and homologous sequences were found in other parasitic Platyhelminthes including Monogenea, Cestoda, and Trematoda [21, 49]. 2DBD-NRs have the same P-box sequence (CEACKK) in the first DBD that is not present in another known NR [35, 71]. This characteristic P-box could determine a new target DNA binding specificity [88]. *In vitro* and *in vivo* studies show that Sm2DBDα could interact as a homodimer, not interacting with SmRXR or SmRXR1. Homodimer formation implies that four P-boxes may be involved in DNA binding. In addition, Wu and collaborators reported that the three Sm2DBDs are regulated during development and may have a differential role in the different stages [35]. Although the databases of *E. granulosus* (WormBase Parasite) report three Eg2DBD, our research group has cloned from protoscoleces of *E. g. sensu lato*, a coding sequence for an Eg2DBDα isoform (GenBank MH092994.2) not reported in existing databases. This transcript was probably originated through mRNA alternative splicing and was named Eg2DBDα.1 [36]. A bioinformatic description of this isoform, including domains structure, putative NLS signals, post-translational modifications, and a 3D model of the two DNA-binding domains, was performed [36].

Recently, molecular docking studies showed that unsaturated long-chain fatty acids, in particular oleic, linoleic, and arachidonic acids, are the Eg2DBDα.1 preferred ligands [89]. It is worth mentioning that this ligand's preference is similar to that of the EgFABP1 protein, previously characterized and studied by our research group [90, 91]. EgFABP1 is a fatty acid-binding protein which was localized in the nuclei of *E. granulosus* protoscoleces cells and other subcellular compartments [92]. Parasitic Platyhelminthes FABPs are considered essential proteins for these organisms since they are not able to synthesize fatty acids *de novo*, so these molecules could participate in host fatty acids uptake and distribution [93]. The interaction between vertebrate FABPs and PPAR nuclear receptors was demonstrated by several reports [94–97]. Taking into account the aforementioned, a model is proposed in **Figure 1**, where

#### **Figure 1.** *Schematic model of the putative Eg2DBDα.1 mechanism of action.*

*Perspective Chapter: Parasitic Platyhelminthes Nuclear Receptors as Molecular Crossroads DOI: http://dx.doi.org/10.5772/intechopen.102648*

EgFABP1 could transport host unsaturated long-chain fatty acids (FA) to the nucleus and transfer its ligand to Eg2DBDα.1. In this way, Eg2DBDα.1 could homodimerize or heterodimerize with other NR and bind to specific DNA response elements to regulate the gene expression of its target genes. Since, these fatty acids are probably acquired from the parasite–host, the signaling mechanism proposed involves a possible host–parasite communication mediated by Eg2DBDα.1 and EgFABP1. In addition, it is possible that co-activator and/or repressor proteins participate as part of the transcriptional regulatory complex.
