Planarian Epidermal Stem Cells Respond to Positional Cues to Promote Cell-Type Diversity

Successful regeneration requires that progenitors of different lineages form the appropriate missing cell types. However, simply generating lineages is not enough. Cells produced by a particular lineage often have distinct functions depending on their position within the organism. How this occurs in regeneration is largely unexplored. In planarian regeneration, new cells arise from a proliferative cell population (neoblasts). We used the planarian epidermal lineage to study how the location of adult progenitor cells results in their acquisition of distinct functional identities. Single-cell RNA sequencing of epidermal progenitors revealed the emergence of distinct spatial identities as early in the lineage as the epidermal neoblasts, with further pre-patterning occurring in their post-mitotic migratory progeny. Establishment of dorsal-ventral epidermal identities and functions, in response to BMP signaling, required neoblasts. Our work identified positional signals that activate regionalized transcriptional programs in the stem cell population and subsequently promote cell-type diversity in the epidermis.National Institutes of Health (U.S.) (Grant R01GM080639

Cellular and Molecular Responses Unique to Major Injury Are Dispensable for Planarian Regeneration

The fundamental requirements for regeneration are poorly understood. Planarians can robustly regenerate all tissues after injury, involving stem cells, positional information, and a set of cellular and molecular responses collectively called the “missing tissue” or “regenerative” response. follistatin, which encodes an extracellular Activin inhibitor, is required for the missing tissue response after head amputation and for subsequent regeneration. We found that follistatin is required for the missing tissue response regardless of the wound context, but causes regeneration failure only after head amputation. This head regeneration failure involves follistatin-mediated regulation of Wnt signaling at wounds and is not a consequence of a diminished missing tissue response. All tested contexts of regeneration, including head regeneration, could occur with a defective missing tissue response, but at a slower pace. Our findings suggest that major cellular and molecular programs induced specifically by large injuries function to accelerate regeneration but are dispensable for regeneration itself. In regenerative organisms, a large array of cellular responses are triggered at major injuries. However, which of these responses are fundamentally required for regeneration to occur remains unknown. Tewari et al. find that hallmark cellular and molecular responses induced uniquely at large injuries are dispensable for planarian regeneration. Keywords: regeneration; wound response; WnT signaling; TGF-β signaling; planarians; follistatinNational Institutes of Health (U.S.) (Grant R01GM080639

Landmarks in Existing Tissue at Wounds Are Utilized to Generate Pattern in Regenerating Tissue

Regeneration in many organisms involves the formation of a blastema, which differentiates and organizes into the appropriate missing tissues. How blastema pattern is generated and integrated with pre-existing tissues is a central question in the field of regeneration. Planarians are free-living flatworms capable of rapidly regenerating from small body fragments [1]. A cell cluster at the anterior tip of planarian head blastemas (the anterior pole) is required for anterior-posterior (AP) and medial-lateral (ML) blastema patterning [2–4]. Transplantation of the head tip into tails induced host tissues to grow patterned head-like outgrowths containing a midline. Given the important patterning role of the anterior pole, understanding how it becomes localized during regeneration would help explain how wounds establish pattern in new tissue. Anterior pole progenitors were specified at the pre-existing midline of regenerating fragments, even when this location deviated from the ML median plane of the wound face. Anterior pole progenitors were specified broadly on the dorsal-ventral (DV) axis and subsequently formed a cluster at the DV boundary of the animal. We propose that three landmarks of pre-existing tissue at wounds set the location of anterior pole formation: a polarized AP axis, the pre-existing midline, and the dorsal-ventral median plane. Subsequently, blastema pattern is organized around the anterior pole. This process, utilizing positional information in existing tissue at unpredictably shaped wounds, can influence the patterning of new tissue in a manner that facilitates integration with pre-existing tissue in regeneration.National Institute of General Medical Sciences (U.S.) (Award T32GM007753)National Institutes of Health (U.S.) (Grant R01GM080639

Assembly of Drosophila Centromeric Chromatin Proteins during Mitosis

Semi-conservative segregation of nucleosomes to sister chromatids during DNA replication creates gaps that must be filled by new nucleosome assembly. We analyzed the cell-cycle timing of centromeric chromatin assembly in Drosophila, which contains the H3 variant CID (CENP-A in humans), as well as CENP-C and CAL1, which are required for CID localization. Pulse-chase experiments show that CID and CENP-C levels decrease by 50% at each cell division, as predicted for semi-conservative segregation and inheritance, whereas CAL1 displays higher turnover. Quench-chase-pulse experiments demonstrate that there is a significant lag between replication and replenishment of centromeric chromatin. Surprisingly, new CID is recruited to centromeres in metaphase, by a mechanism that does not require an intact mitotic spindle, but does require proteasome activity. Interestingly, new CAL1 is recruited to centromeres before CID in prophase. Furthermore, CAL1, but not CENP-C, is found in complex with pre-nucleosomal CID. Finally, CENP-C displays yet a different pattern of incorporation, during both interphase and mitosis. The unusual timing of CID recruitment and unique dynamics of CAL1 identify a distinct centromere assembly pathway in Drosophila and suggest that CAL1 is a key regulator of centromere propagation

Organizer regeneration and patterning of stem cell lineages in planarians

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Biology, 2017.Cataloged from PDF version of thesis.Includes bibliographical references.Planarians are freshwater flatworms capable of whole-body regeneration. Like development, regeneration requires the establishment of tissue patterns and the specification of appropriate cell types. However, regeneration has the additional challenges of performing these tasks in the absence of developmental cues, and in the presence of pre-existing, differentiated tissues. Planarian head regeneration involves the anterior pole, which is a cluster of cells in the tip of head required for proper head patterning. We used transplantation to show that the head tip region, containing the anterior pole, has organizing activity. We sought to establish how the anterior pole is placed during regeneration. Anterior pole progenitors are specified medially and accumulate to form a cluster at the DV median plane. Pole progenitors are specified at the pre-existing midline, and coalesce to the DV boundary in the blastema. These findings demonstrate that during its formation, the anterior pole integrates positional information from the pre-existing tissues. This process places the anterior pole in the appropriate location to organize patterning during head regeneration. Regeneration in planarians involves two important components. Neoblasts are pluripotent stem cells that are the source of all new cells during regeneration. Position control genes (PCGs) are developmental signaling genes that are constitutively expressed in adult planarian muscle, and are thought to provide the molecular instructions for regeneration. How neoblasts respond to PCGs to produce regionally appropriate cells types remains unknown. To better understand this process, we characterized the planarian epidermis. Bulk RNA-sequencing of mature epidermal cells and subsequent in situ validation identified numerous spatial patterns. To understand when these patterns arose during differentiation, we performed single-cell sequencing (SCS) of epidermal progenitors. Positional identities present in the mature epidermis were also present in progenitors, with dorsal-ventral identity present in spatially distant neoblasts. Epidermal neoblasts were able change their DV identity upon inhibition of Bmp signaling. This demonstrates that neoblasts can respond to changes in their signaling environment, linking positional information from muscle to the generation of regionally-appropriate cells types.by Isaac M. Oderberg.Ph. D

activin-2 is required for regeneration of polarity on the planarian anterior-posterior axis

Planarians are flatworms and can perform whole-body regeneration. This ability involves a mechanism to distinguish between anterior-facing wounds that require head regeneration and posterior-facing wounds that require tail regeneration. How this head-tail regeneration polarity decision is made is studied to identify principles underlying tissue-identity specification in regeneration. We report that inhibition ofactivin-2, which encodes an Activin-like signaling ligand, resulted in the regeneration of ectopic posterior-facing heads following amputation. During tissue turnover in uninjured planarians, positional information is constitutively expressed in muscle to maintain proper patterning. Positional information includes Wnts expressed in the posterior and Wnt antagonists expressed in the anterior. Upon amputation, several wound-induced genes promote re-establishment of positional information. The head-versus-tail regeneration decision involves preferential wound induction of the Wnt antagonistnotumat anterior-facing over posterior-facing wounds. Asymmetric activation ofnotumrepresents the earliest known molecular distinction between head and tail regeneration, yet how it occurs is unknown.activin-2RNAi animals displayed symmetric wound-induced activation ofnotumat anterior- and posterior-facing wounds, providing a molecular explanation for their ectopic posterior-head phenotype.activin-2RNAi animals also displayed anterior-posterior (AP) axis splitting, with two heads appearing in anterior blastemas, and various combinations of heads and tails appearing in posterior blastemas. This was associated with ectopic nucleation of anterior poles, which are head-tip muscle cells that facilitate AP and medial-lateral (ML) pattern at posterior-facing wounds. These findings reveal a role for Activin signaling in determining the outcome of AP-axis-patterning events that are specific to regeneration.</jats:p

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

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 factors encoded by 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.National Institutes of Health (U.S.) (Grant R01GM080639