Single-cell Resolution Fate-mapping Of The Early Foregut Endoderm

Lineage tracing is one of the classical issues in developmental biology.The earliest lineage tracing study dated back to the 19 th century.The absolute transparent early embryos of zebrafish make it among the most convenient model for the lineage tracing studies.Endoderm is one of the most important progenitor pools.Cell descendants from endoderm contribute to digestive organs such as liver,pancreas,biliary system,lungs(gall bladder in zebrafish),thyroid,thymus,and the epithelial lining of the respiratory and digestive tracts.How gastrointestinal organs arise from the endoderm is a question of great importance in our understanding and treatment of relative diseases.In zebrafish,the single cell-layered endoderm sheet offers particularly easy access to the lineage tracing for each and every one of the endoderm cells.Our interest has been raised to push the limit to the foregut organ earliest progenitors after gastrulation and the monolayer structure of zebrafish endoderm at 3 to 4-somite enabled this goal to be achieved at the single-cell resolution.Despite that endoderm fate might still be labile around 3 to 4-somite stage,the future destiny of each endoderm cell corresponded very broadly to their antero-posterior and medio-lateral positions at this time.In order to perform single-cell lineage tracing of zebrafish early endoderm cells,we constructed Tg(sox17:h Kik GR1),marked all definitive endoderm(by promoter of the transcription factor SRY-box containing gene 17(sox17))cells with green fluorescence protein h Kik GR1.The fate map here showed lineage tracing result of in total 274 endoderm cells ranging between 4 rows anterior to somite 1 and 4 rows beneath somite 4.One at each time,a zebrafish endoderm cells labeled via h Kik GR1 within somite 1to somite 4 at 3 to 4-somite stage were photoconverted red by controlled UV light exposure.The red fluorescence labeled cells were followed at 48 hpf from a ventral view with scanned image by confocal microscopy to assess their fate.Statistic of its lineage specification tendency was obtained from at least 3 repeats.By performing and repeating this fate mapping strategy accurately on single 3-4s endoderm cell of each and every cell locus and recording results from over one thousand embryos,we presented here the single-cell resolution fate map of zebrafish early foregut endoderm at 3-4 somites.According to our map,endoderm cells located in the inner most a few tiers on both sides of the dorsal midline contributed to pancreas bud.Liver had two progenitor pools located more laterally and bi-laterally.Swim-bladder came from the top 2 rows of cells within the first somite.Progenitor pools of liver,pancreas,and gut were taking independent yet compensating areas in the endoderm sheet,indicating that before commitment,progenitors have already located in specific areas.The map portraying 32-36 hpf lineage specification status was drawn to understand the development of dorsal and ventral pancreatic bud using the same technique.There were already two separated pools for dorsal pancreatic bud and ventral pancreatic bud in the 3 to 4-somite endoderm.The dorsal pancreatic pool located closer to the midline,and have a wider span than the possible ventral pancreatic pool.Different progenitor domains in the endoderm may have variant regenerative or oncogenic capacities.With this single-cell resolution fate map and proper cell-labeling,cell sorting technique,single-cell RNA-seq applied to a few cells,we were able to collect organ specific progenitors and generate invaluable detailed transcriptome profiles of them.Analyzation showed over 20,000 gene were found expressing the 3 groups,which are pancreas group,liver group,and gut group.After differential expression analysis,there are over 400 significantly differentially expressed genes among the 3 groups.194 differentially expressed genes were found in liver vs.pancreas group;257 differentially expressed genes were found in pancreas vs.gut group.122 differentially expressed genes were found in liver vs.gut group.3 differentially expressed genes appeared in all of the 3comparing sets.By differential-expressed genes enrichment analysis,some differentially expressed signaling pathways were discovered.Differential-expressed genes and signaling pathways uncovered by transcriptome analysis provided important leads for our following research on endoderm organ progenitor identity and regulatory mechanism.Increasingly popular diabetes and other human pancreatic diseases pose a huge threat to human health,so a lot of research in the field of biology and medicine is devoted to understanding and curing such diseases.Whether the events in organ development are repeated during organ regeneration and whether they are regulated by the same mechanism is an important issue in the research of developmental biology.This question is not only important for understanding organ regeneration,but also for continuously improving induced cell reprogramming and differentiation to obtain functional,therapeutically suitable cells.Therefore,while tracking the endoderm cell lineage,we also took the pancreatic endocrine ?-cells as an example to study the regeneration of endoderm organs.We established a transgenic model Tg(ins:CFP-NTR)for the specific killing of zebrafish pancreatic endocrine ?-cells based on hybrid genetic-chemical ablation method.This model can achieve almost complete ablation of pancreatic endocrine ?-cells in the zebrafish embryos.Zebrafish embryos with almost complete ?-cell ablation can show nascent ?-cells in about 7 hours after ablation and continue to increase.We crossed this transgenic model with fluorescent reporter transgenic lines of other important ?-cell development regulators and observed the expression patterns of these regulators during ?-cell regeneration.It was found that pre-existing neurod,pdx1,mnx1,or nkx2.2a positive cells can contribute to regenerated ?-cells.Further statistical data show that the neonatal ?-cells generated by regeneration have different positive expression ratios of neurod,pdx1,mnx1,and nkx2.2a,and sometimes different expression patterns in the control group and the ablaltion group.Combined with our statistical data,the published information on the normal expression pattern of the four genes during development,and the existing reports on the source of regeneration of the zebrafish pancreatic endocrine ?-cells,we analyze and believe that The pdx1 + mnx1 +nkx2.2a + triple-positive nascent ?-cells do not fit in any existing ?-cell regeneration model,suggesting that there may be pancreatic progenitor cells as a source of nascent?-cells during regeneration.