Co-option of the limb patterning program in cephalopod eye development

Background Across the Metazoa, similar genetic programs are found in the development of analogous, independently evolved, morphological features. The functional significance of this reuse and the underlying mechanisms of co-option remain unclear. Cephalopods have evolved a highly acute visual system with a cup-shaped retina and a novel refractive lens in the anterior, important for a number of sophisticated behaviors including predation, mating, and camouflage. Almost nothing is known about the molecular-genetics of lens development in the cephalopod. Results Here we identify the co-option of the canonical bilaterian limb patterning program during cephalopod lens development, a functionally unrelated structure. We show radial expression of transcription factors SP6-9/sp1, Dlx/dll, Pbx/exd, Meis/hth, and a Prdl homolog in the squid Doryteuthis pealeii, similar to expression required in Drosophila limb development. We assess the role of Wnt signaling in the cephalopod lens, a positive regulator in the developing Drosophila limb, and find the regulatory relationship reversed, with ectopic Wnt signaling leading to lens loss. Conclusion This regulatory divergence suggests that duplication of SP6-9 in cephalopods may mediate the co-option of the limb patterning program. Thus, our study suggests that this program could perform a more universal developmental function in radial patterning and highlights how canonical genetic programs are repurposed in novel structures.

Cellular Senescence: Mechanisms and Therapeutic Potential

Cellular senescence is a complex and multistep biological process which cells can undergo in response to different stresses. Referring to a highly stable cell cycle arrest, cellular senescence can influence a multitude of biological processes—both physiologically and pathologically. While phenotypically diverse, characteristics of senescence include the expression of the senescence-associated secretory phenotype, cell cycle arrest factors, senescence-associated β-galactosidase, morphogenesis, and chromatin remodelling. Persistent senescence is associated with pathologies such as aging, while transient senescence is associated with beneficial programmes, such as limb patterning. With these implications, senescence-based translational studies, namely senotherapy and pro-senescence therapy, are well underway to find the cure to complicated diseases such as cancer and atherosclerosis. Being a subject of major interest only in the recent decades, much remains to be studied, such as regarding the identification of unique biomarkers of senescent cells. This review attempts to provide a comprehensive understanding of the diverse literature on senescence, and discuss the knowledge we have on senescence thus far.

Co-option of the Limb Patterning Program in Cephalopod Lens Development

Across the Metazoa, similar genetic programs are found in the development of analogous, independently evolved, morphological features. The functional significance of this reuse and the underlying mechanisms of co-option remain unclear. Here we identify the co-option of the canonical bilaterian limb pattering program redeployed during cephalopod lens development, a functionally unrelated structure. We show radial expression of transcription factorsSP6-9/sp1, Dlx/dll, Pbx/exd, Meis/hth, and aPrdlhomolog in the squidDoryteuthis pealeii, similar to expression required inDrosophilalimb development. We assess the role of Wnt signaling in the cephalopod lens, a positive regulator in the developing limb, and find the regulatory relationship reversed, with ectopic Wnt signaling leading to lens loss. This regulatory divergence suggests that duplication of SP6-9 in cephalopods may mediate this co-option. These results suggest that the limb network does not exclusively pattern appendage outgrowth but is performing a more universal developmental function: radial patterning.

Role of Retinoic Acid Signaling, FGF Signaling and Meis Genes in Control of Limb Development

The function of retinoic acid (RA) during limb development is still debated, as loss and gain of function studies led to opposite conclusions. With regard to limb initiation, genetic studies demonstrated that activation of FGF10 signaling is required for the emergence of limb buds from the trunk, with Tbx5 and RA signaling acting upstream in the forelimb field, whereas Tbx4 and Pitx1 act upstream in the hindlimb field. Early studies in chick embryos suggested that RA as well as Meis1 and Meis2 (Meis1/2) are required for subsequent proximodistal patterning of both forelimbs and hindlimbs, with RA diffusing from the trunk, functioning to activate Meis1/2 specifically in the proximal limb bud mesoderm. However, genetic loss of RA signaling does not result in loss of limb Meis1/2 expression and limb patterning is normal, although Meis1/2 expression is reduced in trunk somitic mesoderm. More recent studies demonstrated that global genetic loss of Meis1/2 results in a somite defect and failure of limb bud initiation. Other new studies reported that conditional genetic loss of Meis1/2 in the limb results in proximodistal patterning defects, and distal FGF8 signaling represses Meis1/2 to constrain its expression to the proximal limb. In this review, we hypothesize that RA and Meis1/2 both function in the trunk to initiate forelimb bud initiation, but that limb Meis1/2 expression is activated proximally by a factor other than RA and repressed distally by FGF8 to generate proximodistal patterning.

Transcriptional Trajectories in Mouse Limb Buds Reveal the Transition from Anterior-Posterior to Proximal-Distal Patterning at Early Limb Bud Stage

Limb patterning relies in large part on the function of the Hox family of developmental genes. While the differential expression of Hox genes shifts from the anterior–posterior (A–P) to the proximal–distal (P–D) axis around embryonic day 11 (E11), whether this shift coincides with a more global change of A–P to P–D patterning program remains unclear. By performing and analyzing the transcriptome of the developing limb bud from E10.5 to E12.5, at single-cell resolution, we have uncovered transcriptional trajectories that revealed a general switch from A–P to P–D genetic program between E10.5 and E11.5. Interestingly, all the transcriptional trajectories at E10.5 end with cells expressing either proximal or distal markers suggesting a progressive acquisition of P–D identity. Moreover, we identified three categories of genes expressed in the distal limb mesenchyme characterized by distinct temporal expression dynamics. Among these are Hoxa13 and Hoxd13 (Hox13 hereafter), which start to be expressed around E10.5, and importantly the binding of the HOX13 factors was observed within or in the neighborhood of several of the distal limb genes. Our data are consistent with previous evidence suggesting that the transition from the early/proximal to the late/distal transcriptome of the limb mesenchyme largely relies on HOX13 function. Based on these results and the evidence that HOX13 factors restrict Hoxa11 expression to the proximal limb, in progenitor cells of the zeugopod, we propose that HOX13 act as a key determinant of P–D patterning.

Minor spliceosome disruption causes limb growth defects without altering patterning

Disruption of the minor spliceosome causes primordial dwarfism in microcephalic osteodysplastic primordial dwarfism type 1. Similarly, primordial dwarfism in domesticated animals is linked to positive selection in minor spliceosome components. Despite the importance of minor intron splicing in limb size regulation, its role in limb development remains unexplored. Here we show that loss of U11 small nuclear RNA, an essential minor spliceosome component, results in stunted limbs that maintain patterning. Notably, earlier loss of U11 corresponded to increased severity. We find that limb size is reduced due to elevated minor intron retention in minor intron-containing genes that regulate cell cycle. Limb progenitor cells experience delayed prometaphase to metaphase transition and prolonged S-phase, resulting in death of rapidly dividing, distally located progenitors. Consequently, crucial limb patterning genes are upregulated and their expression is maintained spatially to achieve basic patterning. Overall, these findings reveal a potential mechanism shared in disease and domestication.

FoxB, a new and highly conserved key factor in arthropod dorsal–ventral (DV) limb patterning

Forkhead box (Fox) transcription factors evolved early in animal evolution and represent important components of conserved gene regulatory networks (GRNs) during animal development. Most of the researches concerning Fox genes, however, are on vertebrates and only a relatively low number of studies investigate Fox gene function in invertebrates. In addition to this shortcoming, the focus of attention is often restricted to a few well-characterized Fox genes such as FoxA (forkhead), FoxC (crocodile) and FoxQ2. Although arthropods represent the largest and most diverse animal group, most other Fox genes have not been investigated in detail, not even in the arthropod model species Drosophila melanogaster. In a general gene expression pattern screen for panarthropod Fox genes including the red flour beetle Tribolium castaneum, the pill millipede Glomeris marginata, the common house spider Parasteatoda tepidariorum, and the velvet worm Euperipatoides kanangrensis, we identified a Fox gene with a highly conserved expression pattern along the ventral ectoderm of arthropod and onychophoran limbs. Functional investigation of FoxB in Parasteatoda reveals a hitherto unrecognized important function of FoxB upstream of wingless (wg) and decapentaplegic (dpp) in the GRN orchestrating dorsal–ventral limb patterning.

De-novovariants inXIRP1associated with polydactyly and polysyndactyly in Holstein cattle

ABSTRACTCongenital polydactylous cattle are sporadically observed. Impairment of the limb patterning process due to altered control of the zone of polarizing activity (ZPA) was associated in several species with preaxial polydactyly and syndactyly. In cattle, the role of ZPA and other genes involved in limb patterning for polydactyly was not yet elucidated. Herein, we report on a preaxial type II polydactyly and a praeaxial type II+V polysyndactyly in two Holstein calves and screen whole genome sequencing data for associated variants. Using whole genome sequencing data of both affected calves did not show mutations in the candidate regions of ZRS and pZRS or in candidate genes associated with polydactyly, syndactyly and polysyndactyly in other species. Two indels, which are located inXIRP1within a common haplotype, were highly associated with the two phenotypes. Bioinformatic analyses retrieved an interaction betweenXIRP1andFGFR1, CTNNB1andCTNND1supporting a link between theXIRP1variants and embryonic limb patterning. The heterozygous haplotype was highly associated with the present polydactylous phenotypes due to dominant mode of inheritance with an incomplete penetrance in Holstein cattle.