Twisted gastrulation (TSG), a TWSG1 homolog, regulates dorsoventral axis formation in Drosophila, fly, frog, and fish embryos ^[1][4]. The TSG gene is one of seven known zygotic patterning genes that specify the fate of dorsal cells in Drosophila embryos. Mutations at the twisted gastrulation (TSG) loci interfere with early morphogenetic movements in Drosophila. Because of the mutation, embryos can fully extend their germbands but they do not extend dorsally. As a result, when normal embryos have fully extended germbands, the germbands in mutant embryos are folded into the interior on the ventral side of the embryo. TSG-mutated embryos have abnormal deep dorsal folds during early gastrulation, resulting in the failure of dorsal cells to slip laterally to make way for the expanding germband. They could develop but end up forming disorganized first-instar larvae ^[2][3]. The combination of TSG and decapentaplegic (DPP), a BMP2/4 homolog, determines the dorsal midline cell fate in Drosophila ^[3][4][24,25]. Drosophila Sog, a Chordin homolog, is cleaved by the metalloprotease tolloid in the presence of TSG, leading to DPP signaling activation ^[5][26]. Xenopus TSG also disrupts the interaction between BMP4 and Chordin ^[6][22]. TSG also enhances BMP signaling in the absence of Chordin in Zebrafish embryos ^[7][27], suggesting an interaction between TSG and an unknown factor. Thus, TSG functions as a BMP agonist. In parallel, TSG acts as a BMP antagonist. Several groups have reported that TSG forms a ternary complex with BMP and Chordin, resulting in stabilizing the inhibitory complex ^[1][8][9][4,14,15]. The antagonistic activity of TSG toward BMP signaling is conserved between Drosophila and vertebrates ^[1][8][4,14]. However, the dual functions of TSG homologs have not yet been fully elucidated. Loss-of-function studies have revealed a variety of phenotypes. In Xenopus, TSG knockdown results in moderate head and forebrain defects, which can be rescued by Chordin overexpression and BMP depletion ^[10][11][28,29]. It also causes moderately strong dorsalization of the embryonic axis in Zebrafish ^[12][30].

TWSG1 has previously been reported to be expressed in multiple tissues during the gastrulation, neurulation, and organogenesis stages such as the neural plate, neural tube, somite, branchial arches, surface ectoderm, choroid plexus, hippocampus, and endoderm. TWSG1-mutant mice exhibit neural arch deletions in the cervical vertebrae and show a high mortality rate of up to about 50% ^[13][35]. TWSG1-null BMP4 homozygous mutant mice showed holoprosencephaly (HPE), characterized by the failure of the prosencephalon to undergo median cleavage into bilateral cerebral hemispheres ^[14][31]. TWSG1 has been said to enhance BMP signaling by promoting the degeneration of Chordin. Although 7% of wild-type mice exposed to retinoic acid developed HPE, all TWSG1-deficient mice exposed to retinoic acid had severe HPE ^[15][36]. HPE in humans has been associated with chromosomal deletions of 18p11.3, containing the TWSG1 gene ^[8][14]. However, TWSG1 mutations have rarely been detected in human HPE ^[16][37]. Among all other regions of the brain such as the hippocampus, the choroid plexus expresses the highest TWSG1 level in both mice and humans. It is believed that TWSG1 is the main regulator of BMP present in the choroid plexus, which plays an important role in neural progenitor differentiation and neuronal repair ^[17][38]. In addition, TWSG1-null mice have shown a higher frequency of hydrocephalus than wild-type mice ^[17][38], suggesting a neuroprotective function of TWSG1.

Because BMP signaling is indispensable for osteogenesis ^[18][39], TWSG1 may play an essential role in this process. In fact, TWSG1-null mice exhibit mild vertebral abnormalities and osteoporosis ^[14][31]. TWSG1 knockout mice had a significant delay in the ossification of cervical and coccygeal vertebrae in newborns. As result, they were smaller in size than the wild-type mice ^[13][35]. Interestingly, craniofacial defects in TWSG1-null mice were significantly rescued by p53 deletion ^[19][40]. The reduction in the absolute number of osteoblasts explains the decreased serum levels of osteocalcin observed in TSG-null mice. TWSG1-deficient mice also display dwarfism with delayed endochondral ossification ^[20][41]. Moreover, osteoblasts isolated from TWSG1-null mice show impaired BMP signaling ^[21][34]. Therefore, TWSG1 acts as a BMP agonist during skeletogenesis. Enhanced osteoclastogenesis was also observed in TWSG1-null mice. Whether TSG favors or opposes BMP activity in osteoblasts is related to its levels of expression, as has been reported for non-osteoblastic cells ^[21][34]. These results indicate that the TSG gene inactivation causes a transient decrease in bone volume, associated with a decrease in bone mineral content and serum osteocalcin levels, without changes in the osteoblast or osteoclast number per surface or activity ^[21][34]. In contrast to osteoblasts, osteoclasts isolated from TWSG1-null mice displayed BMP signaling activation ^[21][34], implying the BMP antagonistic activity of TWSG1 in osteoclastogenesis. Consistently, TWSG1 overexpression has resulted in the inhibition of osteoclastogenesis in primary osteoclasts ^[22][42], and this inhibition is dependent on BMP binding ^[23][43]. TWSG1 can also be found in cartilage, joints, and tendons ^[24][44]. TWSG1 inhibits BMP2-stimulated SMAD1 phosphorylation in chondrocytes, and TWSG1 overexpression suppresses cartilage development in vivo ^[25][45].

TWSG1 plays an important role in T-cell differentiation and erythropoiesis. Both T and B cells dynamically express TWSG1 ^[26][46]. TWSG1-deficient mice display an atrophic thymus ^[20][41], which cooperates with Chordin to inhibit the BMP4-mediated suppression of thymocyte growth and differentiation ^[27][47]. TWSG1 suppressed hepcidin indirectly by inhibiting the signaling effects and associated hepcidin upregulation by BMP2 and BMP4 ^[28][48]. TWSG1 mRNA expression is enhanced after T-cell activation, leading to strong suppression of proliferation and cytokine production in primed alloreactive CD4+ T cells ^[29][49]. Surprisingly, this suppression depends on TGFβ signaling but not BMP signaling ^[29][49]. TWSG1 also regulates B-cell function. B cells express the BMP modifier TWSG1 in an activation-dependent manner. B-cell-specific TWSG1 deficiency does not affect B-cell development but it does alter B-cell responses. TWSG1 is a modulator of BMP activity, and no function for TWSG1 without BMP interaction has been reported. In the search for a BMP signaling network that could be regulated by TWSG1 in situ, BMP2 was identified in the vicinity of the splenic marginal zone ring ^[30][50]. In thalassemic mice with ineffective erythropoiesis, increased TWSG1 expression was observed in the spleen, bone marrow, and liver compared to that in healthy mice ^[28][48]. TWSG1 repressed BMP-inducible hepcidin, which positively regulated erythropoiesis ^[28][48]. Therefore, TWSG1 inhibits erythropoiesis.

The previous report showed that the BMP endothelial cell precursor-derived regulator (BMPER), one of the extracellular BMP modulators, acts proangiogenically on endothelial cells in a concentration-dependent manner. In addition, TWSG1 enhances endothelial cell ingrowth, migration, and sprouting, as confirmed by a mouse Matrigel plug and human umbilical vein endothelial cells (HUVEC) sprouting assay ^[31][51]. Accordingly, TWSG1 secreted from papillary thyroid cells induces HUVEC sprouting ^[32][52]. The expression of a panel of angiogenic factors in the tumor cell culture supernatant in the absence of TWSG1 showed that vascular endothelial growth factor (VEGF) was significantly decreased, whereas endostatin, PAI-1, and thrombospondin-1 were increased in the absence of TWSG1 compared to the control ^[32][52].

TWSG1 induces the phosphorylation of SMAD1/5/8, AKT, and extracellular signal-regulated kinase (ERK) ^[32][52], indicating that TWSG1 is a BMP agonist in endothelial cells. Interestingly, TWSG1 silencing also augments HUVEC sprouting ^[31][51], suggesting a function other than that of a secreted protein. In addition, TWSG1 and BMPER interfere with each other to function as pro-angiogenic factors ^[31][51]. This interaction is also important for arterial-venous specification, caudal vein plexus (CVP) formation, and correct development in Zebrafish ^[33][53].

According to the Human Protein Atlas (https://www.proteinatlas.org/ accessed on 11 October 2022), TWSG mRNA levels are different in various cancers. The relationship between TWSG1 expression and cancer development has also been elucidated. A loss of heterozygosity and truncating variants of TWSG1 have been observed in familial colorectal cancer ^[34][54]. TWSG1 expression is upregulated in cholangiocellular carcinoma, hepatocellular carcinoma, papillary thyroid cancer (PTC), and glioblastoma but downregulated in gastric and endometrial cancers ^{[32][35][36][37][38]}[2,18,52,55,56]. In gastric cancer, TWSG1 expression was low, and the lower expression level showed a correlation with higher pathological grading or the clinical stage of patients ^[35][2]. Thus, it is possible that TWSG1 acts as a tumor suppressor gene in gastric cancer. In PTC, lymph node metastases are correlated with high TWSG1 mRNA expression ^[32][52]. Moreover, TWSG1 knockdown inhibits the proliferation, migration, and invasion of PTC cells. TWSG1 may promote the expression of matrix metalloproteinases (MMPs) and enhance the activity of the BMP pathway. Thus, TWSG1 could be a new diagnostic target for PTC ^[32][52].

TWSG1 expression was initially detected in adult sheep ovaries ^[39][58]. Juvenile mouse ovaries also exhibited consistently high TWSG1 expression, which was not affected by FSH exposure in the granulosa cells of developing follicles ^[40][59]. Wang et al. found that TWSG1 and Nbl1 were the most abundant BMP antagonists expressed in rodent and human ovaries ^[41][60]. TWSG1 has been reported to have synergistic action with the Chordin subfamily, including CHRD and CHRDL1, the genes of which also showed moderate expression in the mammalian ovary. Bioactivity tests indicated that TWSG1 alone can directly inhibit the signaling of BMP6 or BMP7 by masking the receptor binding sites of BMPs. BMP2, BMP4, BMP7, GDF5, and activin A have been said to have an antagonizing ability towards CHRD, which has synergic effects with TWSG1 ^[40][59]. TWSG1 is the most abundant BMP antagonist in mouse ovaries ^[41][60]. TWSG1 colocalizes with Chordin within the theca cells, whereas it colocalizes with Chordin-like 1 in the developing granulosa cells of mouse ovaries ^[41][60], suggesting the differential roles of TWSG1 in each ovarian compartment. Some data suggest that TWSG1 may work coordinately with CHRD within theca/interstitial shells and also with CHRDL1 in developing granulosa cells. These interactions would modulate the intraovarian functions of the TGFβ superfamily members such as the control of progesterone production ^[40][59].

TWSG1 is also indispensable for mammary gland development in mice. Regulation of BMP signaling by TWSG1 is required for normal ductal elongation, branching of the ductal tree, lumen formation, and myoepithelial compartmentalization in the postnatal mammary gland ^[42][61]. Understanding how TWSG1 regulates normal ductal maturation could provide insights into the role it may be playing during carcinogenesis. Moreover, TWSG1 modulates BMP signaling following kidney injury. BMP7 plays an important role in establishing embryonic nephrons, protecting them from acute renal injuries and preventing and reversing chronic kidney injuries. For example, BMP7 counteracts and reverses epithelial–mesenchymal transition in renal epithelia by regulating E-cadherin. BMP7 also reduces the proximal tubule expression of cytokines such as IL-6 and macrophage chemoattractant protein, modifying inflammatory infiltration. Healthy kidneys express BMP7-antagonistic CHRDL1 in the straight segment of the proximal tubule and demonstrate little active BMP signaling in this region ^[43][62]. When a renal injury occurs, the epithelium of the straight segment is sloughed off and CHRDL1 activity is lost, correlating with increased BMP’s recovering signal response ^[43][62]. Finally, CHRDL1 expression is regained upon differentiation of regenerated epithelia, and BMP signaling is again reduced to basal levels ^[43][62]. Similarly, kidney injury triggers TWSG1 downregulation, leading to BMP signaling activation and renal protection ^[44][63].