foxF-1 Controls Specification of Non-body Wall Muscle and Phagocytic Cells in Planarians

Abstract

Summary Planarians are flatworms capable of regenerating any missing body part in a process requiring stem cells and positional information. Muscle is a major source of planarian positional information and consists of several types of fibers with distinct regulatory roles in regeneration. The transcriptional regulatory programs used to specify different muscle fibers are poorly characterized. Using single-cell RNA sequencing, we define the transcriptomes of planarian dorsal-ventral muscle (DVM), intestinal muscle (IM), and pharynx muscle. This analysis identifies foxF-1, which encodes a broadly conserved Fox-family transcription factor, as a master transcriptional regulator of all non-body wall muscle. The transcription factor genes nk4 and gata4/5/6–2 specify two different subsets of DVM, lateral and medial, respectively, whereas gata4/5/6–3 specifies IM. These muscle types all express planarian patterning genes. Both lateral and medial DVM are required for medial-lateral patterning in regeneration whereas medial DVM and IM have a role in maintaining and regenerating intestine morphology. In addition to the role in muscle, foxF-1 is required for the specification of multiple cell types with transcriptome similarities, including high expression levels of cathepsin genes. These cells include pigment cells, glia, and several other cells with unknown function. cathepsin+ cells phagocytose E. coli, suggesting these are phagocytic cells. In conclusion, we describe a regulatory program for planarian muscle cell subsets and phagocytic cells both driven by foxF-1. FoxF proteins specify different mesoderm-derived tissues in other organisms, suggesting that FoxF regulates formation of an ancient and broadly conserved subset of mesoderm derivatives in the Bilateria.

Cell-type diversity and regionalized gene expression in the planarian intestine revealed by laser-capture microdissection transcriptome profiling

“…Goblet cell-enriched npc2 was expressed by cells with minimal slc22a6 expression (Fig 5C), further reinforcing that npc2+ goblet cells are distinct from ctsla+ phagocytes as well as slc22a6+ basal cells. We also found that basal cells were distinct from numerous visceral muscle fibers that surround intestinal branches, occupying basal regions around digestive cells (109, 110), consistent with another study (28). Basal-specific slc22a6 mRNA did not colocalize with either a muscle-specific mRNA ( troponin I 4 (tni-4), Fig 5D) (5), or with labeling by an antibody that recognizes a muscle-specific antigen (mAb 6G10, Fig 5E) (53).…”

“…Growth and regeneration of intestinal branches require considerable remodeling of pre-existing tissue (19). Remodeling is governed by axial polarity cues (16, 20), ERK and EGFR signaling pathways (21–23), cytoskeletal regulators (24), and interactions with muscle (25–28). However, the mechanisms by which post-mitotic intestinal cells sense and respond to extrinsic signals are only superficially understood.…”

Expression of smooth muscle-like effectors and core cardiomyocyte regulators in the contractile papillae of Ciona

“…It should be noted that Foxf, but not Foxc, is expressed in the cardiopharyngeal lineage that gives rise to cardiomyocytes in Ciona [ 5 ], as well as in the heart/kidney complex of hemichordates [ 33 ], and the dorsal–ventral muscles of planaria that co-express Nk4 and GATA4/5/6 [ 90 ]. Similarly, Foxc has not been implicated (to our knowledge) in cardiomyocyte development outside of vertebrates.…”

Combinatorial chromatin dynamics foster accurate cardiopharyngeal fate choices

“…Similarly to other Forkhead factors 55 , Ciona Foxf is required to open cardiopharyngeal elements for either immediate or later activation in multipotent or fate-restricted progenitors, respectively. Notably Foxf homologs play deeply conserved roles in visceral muscles specification [68][69][70] , including during heart development in mammals 71 . Moreover, GATA motifs are over-represented among cardiopharyngeal-specific elements, also consistent with a conserved role for GATA homologs in heart development [72][73][74][75][76][77] .…”