Region-specific regulation of stem cell-driven regeneration in tapeworms

Abstract

15Tapeworms grow at rates rivaling the fastest-growing metazoan tissues. To propagate 16 they shed large parts of their body; to replace these lost tissues they regenerate proglottids 17 (segments) as part of normal homeostasis. Their remarkable growth and regeneration are fueled 18 by adult somatic stem cells that have yet to be characterized molecularly. Using the rat intestinal 19 tapeworm, Hymenolepis diminuta, we find that regenerative potential is regionally limited to the 20 neck, where head-dependent extrinsic signals create a permissive microenvironment for stem 21 cell-driven regeneration. Using transcriptomic analyses and RNA interference, we characterize 22 and functionally validate regulators of tapeworm growth and regeneration. We find no evidence 23 that stem cells are restricted to the regeneration-competent neck. Instead, lethally irradiated 24 tapeworms can be rescued when cells from either regeneration-competent or regeneration-25 incompetent regions are transplanted into the neck. Together, the head and neck tissues provide 26 extrinsic cues that regulate stem cells, enabling region-specific regeneration in this parasite. 27 7 Previous in vivo studies have shown that H. diminuta can regenerate after serial rounds of 103 amputation and transplantation for over a decade 5 and perhaps indefinitely. Using in vitro culture, 104we confirmed that anterior fragments of H. diminuta can regenerate after at least four rounds of 105 serial amputation ( Fig. 1f-g). Decapitated (-head) fragments regenerated proglottids after the first 106 amputation; however, re-amputation abrogated regeneration (Fig. 1f-g). After decapitation, a 107 definitive neck could not be maintained and eventually, the whole tissue was comprised of 108 proglottids ( Fig. 1-figure supplement 2). Without the head, proglottid regeneration from the neck 109 is finite. Thus, while the neck is necessary and sufficient for proglottid regeneration, the head is 110 required to maintain an unsegmented neck and for persistent regeneration. 111If signals from the head regulate regeneration, is regenerative potential asymmetric across 112 the anterior-posterior (A-P) axis of the neck? We subdivided the neck into three 1 mm fragments 113 and found that the most-anterior neck fragments regenerated more proglottids than the middle or 114 posterior neck fragments ( Fig. 1h-i). Thus, regeneration potential is asymmetric across the neck 115

Screwed up: Spirality of segments and other iterated structures suggest an underlying principle of seriality in bilaterians

“…For instance, proglottids lack the set of characters that has been used to define segments (see Scholtz, 2002Scholtz, , 2010Scholtz, , 2020c. Furthermore, proglottids do not form by a combination of a pre-anal proliferation zone and an anteroposterior differentiation of segments, but rather by stem cells that form a growth zone in the neck area behind the anterior scolex; the differentiation follows a posteroanterior gradient (Koziol et al, 2016;Olson, 2008;Rozario et al, 2019). Likewise, the formation of the cuticular annuli in nematodes follows a different pathway compared with annelids and arthropods.…”