Takayuki Suzuki

Half a century ago, thalidomide was widely prescribed to pregnant women as a sedative but was found to be teratogenic, causing multiple birth defects. Today, thalidomide is still used in the treatment of leprosy and multiple myeloma, although how it causes limb malformation and other developmental defects is unknown. Here, we identified cereblon (CRBN) as a thalidomide-binding protein. CRBN forms an E3 ubiquitin ligase complex with damaged DNA binding protein 1 (DDB1) and Cul4A that is important for limb outgrowth and expression of the fibroblast growth factor Fgf8 in zebrafish and chicks. Thalidomide initiates its teratogenic effects by binding to CRBN and inhibiting the associated ubiquitin ligase activity. This study reveals a basis for thalidomide teratogenicity and may contribute to the development of new thalidomide derivatives without teratogenic activity.

To elucidate the genetic mechanism of flowering in wheat, we performed expression, mutant and transgenic studies of flowering-time genes. A diurnal expression analysis revealed that a flowering activator VRN1, an APETALA1/FRUITFULL homolog in wheat, was expressed in a rhythmic manner in leaves under both long-day (LD) and short-day (SD) conditions. Under LD conditions, the upregulation of VRN1 during the light period was followed by the accumulation of FLOWERING LOCUS T (FT) transcripts. Furthermore, FT was not expressed in a maintained vegetative phase (mvp) mutant of einkorn wheat (Triticum monococcum), which has null alleles of VRN1, and never transits from the vegetative to the reproductive phase. These results suggest that VRN1 is upstream of FT and upregulates the FT expression under LD conditions. The overexpression of FT in a transgenic bread wheat (Triticum aestivum) caused extremely early heading with the upregulation of VRN1 and the downregulation of VRN2, a putative repressor gene of VRN1. These results suggest that in the transgenic plant, FT suppresses VRN2 expression, leading to an increase in VRN1 expression. Based on these results, we present a model for a genetic network of flowering-time genes in wheat leaves, in which VRN1 is upstream of FT with a positive feedback loop through VRN2. The mvp mutant has a null allele of VRN2, as well as of VRN1, because it was obtained from a spring einkorn wheat strain lacking VRN2. The fact that FT is not expressed in the mvp mutant supports the present model.

A tight loop between members of the fibroblast growth factor and the Wnt families plays a key role in the initiation of vertebrate limb development. We show for the first time that Tbx5 and Tbx4 are directly involved in this process. When dominant-negative forms of these Tbx genes were misexpressed in the chick prospective limb fields, a limbless phenotype arose with repression of both Wnt and Fgf genes By contrast, when Tbx5 and Tbx4 were misexpressed in the flank, an additional wing-like and an additional leg-like limbs were induced, respectively. This additional limb formation was accompanied by the induction of both Wnt and Fgf genes These results highlight the pivotal roles of Tbx5 and Tbx4 during limb initiation, specification of forelimb/hindlimb and evolution of tetrapod limbs, placing Tbx genes at the center of a highly conserved genetic program.

The zone of polarizing activity is the primary signaling center controlling anterior-posterior patterning of the amniote limb bud. The autopodial interdigits (IDs) are secondary signaling centers proposed to determine digit identity by acting on the cells of the digital ray. Here, we focus on events accompanying digital fate determination and define a region of the digital ray that expresses Sox9 and Bmpr1b and is phosphorylated-SMAD1/5/8 (p-SMAD1/5/8) positive. We name this region the phalanx-forming region (PFR), and show that the PFR cells arise from the distal subridge mesenchyme of digital ray. This phalanx-forming cell lineage is subsequently committed to the cartilage lineage; the fate of these cells is initially labile but becomes fixed as they are incorporated into the condensed cartilage of the digit primordium. Using an in vivo reporter assay, we establish that each digital PFR has a unique p-SMAD1/5/8 activity signature. In addition, we show that changes in this activity correlate with the identity of the digit that forms after experimental manipulation, supporting the idea that threshold signaling levels can lead to different developmental outcomes in a morphogenetic field. Our data define the molecular profile of the PFR, and we propose a model for understanding formation and variation of digits during autopodial development.