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Enteroendocrine cells (EECs) are rare sensory cells in the intestinal epithelium that coordinate digestive physiology by secreting a diverse repertoire of peptide hormones. These hormones are the main effectors of EEC function, and their characterization requires direct observation by mass spectrometry due to the specialized protein cleavage and posttranslational modifications that yield their mature forms. Based on the distinct subset of hormones they predominantly secrete, EECs can be categorized into subtypes. How each EEC subtype is specified, however, remains poorly understood. Here, we describe EEC subtype differentiation and hormone production in the zebrafish. Using single-cell RNA sequencing data, we identified EEC progenitors and six EEC subtypes in zebrafish and revealed that their expression profiles are consistent across larval and adult stages. Mass spectrometry analysis of isolated zebrafish EECs identified highly processed peptides derived from 19 of 23 hormone-coding genes expressed by EECs, including a previously undescribed zebrafishsecretinortholog. We assembled reporters for zebrafish EEC subtypes to test the lineage relationships between EEC subtypes and the EEC progenitor population, which expressesneurogenin 3 (neurog3). Despite its essential role in mammalian EEC differentiation, we found that selective cytotoxic ablation ofneurog3+ cells in zebrafish only reduced a subset of EEC subtypes and loss of theneurog3gene had no impact on EEC numbers. Finally, we discovered that selective ablation ofghrelin+ EECs reduced a different subset of EEC subtypes, together suggesting thatneurog3+ andghrelin+ cells serve as distinct precursors for separate EEC subtypes. We anticipate these observations and resources will facilitate future studies in the zebrafish to discern the developmental biology, physiology, and endocrinology of EEC subtypes. 

Secreted modular calcium-binding proteins (SMOCs) are conserved matricellular proteins found in organisms fromCaenorhabditis elegansto humans. SMOC homologs characteristically contain 1 or 2 extracellular calcium-binding (EC) domain(s) and 1 or 2 thyroglobulin type-1 (TY) domain(s). SMOC proteins inDrosophilaandXenopushave been found to interact with cell surface heparan sulfate proteoglycans (HSPGs) to exert both positive and negative influences on the conserved bone morphogenetic protein (BMP) signaling pathway. In this study, we used a combination of biochemical, structural modeling, and molecular genetic approaches to dissect the functions of the sole SMOC protein inC.elegans. We showed that CeSMOC-1 binds to the heparin sulfate proteoglycan GPC3 homolog LON-2/glypican, as well as the mature domain of the BMP2/4 homolog DBL-1. Moreover, CeSMOC-1 can simultaneously bind LON-2/glypican and DBL-1/BMP. The interaction between CeSMOC-1 and LON-2/glypican is mediated specifically by the EC domain of CeSMOC-1, while the full interaction between CeSMOC-1 and DBL-1/BMP requires full-length CeSMOC-1. We provide both in vitro biochemical and in vivo functional evidence demonstrating that CeSMOC-1 functions both negatively in a LON-2/glypican-dependent manner and positively in a DBL-1/BMP-dependent manner to regulate BMP signaling. We further showed that in silico,Drosophilaand vertebrate SMOC proteins can also bind to mature BMP dimers. Our work provides a mechanistic basis for how the evolutionarily conserved SMOC proteins regulate BMP signaling. 

In the frigid, oxygen-rich Southern Ocean (SO), Antarctic icefishes (Channichthyidae; Notothenioidei) evolved the ability to survive without producing erythrocytes and hemoglobin, the oxygen-transport system of virtually all vertebrates. Here, we integrate paleoclimate records with an extensive phylogenomic dataset of notothenioid fishes to understand the evolution of trait loss associated with climate change. In contrast to buoyancy adaptations in this clade, we find relaxed selection on the genetic regions controlling erythropoiesis evolved only after sustained cooling in the SO. This pattern is seen not only within icefishes but also occurred independently in other high-latitude notothenioids. We show that one species of the red-blooded dragonfish clade evolved a spherocytic anemia that phenocopies human patients with this disease via orthologous mutations. The genomic imprint of SO climate change is biased toward erythrocyte-associated conserved noncoding elements (CNEs) rather than to coding regions, which are largely preserved through pleiotropy. The drift in CNEs is specifically enriched near genes that are preferentially expressed late in erythropoiesis. Furthermore, we find that the hematopoietic marrow of icefish species retained proerythroblasts, which indicates that early erythroid development remains intact. Our results provide a framework for understanding the interactions between development and the genome in shaping the response of species to climate change.