Planarian EGF repeat-containing genes megf6 and hemicentin are required to restrict the stem cell compartment

Abstract

The extracellular matrix (ECM) is important for maintaining the boundaries between tissues. This role is particularly critical in the stem cell niche, as pre-neoplastic or cancerous stem cells must pass these boundaries in order to invade into the surrounding tissue. Here, we examine the role of the ECM as a regulator of the stem cell compartment in the planarian Schmidtea mediterranea, a highly regenerative, long-lived organism with a large population of adult stem cells. We identify two EGF repeat-containing genes, megf6 and hemicentin, with identical knockdown phenotypes. We find that megf6 and hemicentin are needed to maintain the structure of the basal lamina, and in the absence of either gene, pluripotent stem cells migrate ectopically outside of their compartment and hyper-proliferate, causing lesions in the body wall muscle. These muscle lesions and ectopic stem cells are also associated with ectopic gut branches, which protrude from the normal gut towards the dorsal side of the animal. Interestingly, both megf6 and hemicentin knockdown worms are capable of regenerating tissue free of both muscle lesions and ectopic cells, indicating that these genes are dispensable for regeneration. These results provide insight into the role of planarian ECM in restricting the stem cell compartment, and suggest that signals within the compartment may act to suppress stem cell hyperproliferation. expressed in different cell populations, we find that both are required to maintain the epithelial basal lamina. In the absence of either gene, stem cells can escape their compartment and migrate towards the skin of the animal, where they divide at an accelerated rate and cause lesions in the muscle. These results show that the extracellular matrix plays a role in limiting the boundaries of the stem cell compartment. megf6 and hemicentin restrict the planarian stem cell niche PLOS Genetics | https://doi.

A role for the MEGF6 gene in predisposition to osteoporosis

“…This phenotype is remarkably similar to the severe disruption of the fin fold and aberrant migration of melanocytes into the fin fold area in zebrafish embryos lacking the megf6a/6b function. Furthermore, loss of planarian megf6 may disrupt the composition of the extracellular matrix (ECM) (Lindsay‐Mosher, Chan, & Pearson, 2020). Given the similarities between planarian and zebrafish loss‐of‐function phenotypes, MEGF6 in vertebrates may be a key regulator of the ECM.…”

Decellularization enables functional analysis of ECM remodeling in planarian regeneration

“…The study of ECM in a highly regenerative animal model, such as planarians (Reddien and Sánchez Alvarado 2004), is currently limited, preventing us from understanding how the ECM may or may not contribute to tissue regeneration (Isolani et al 2013;Seebeck et al 2017;Lindsay-Mosher, Chan, and Pearson 2020). To resolve this limitation, we established an integrative workflow consisting of tissue decellularization, proteomics, and RNA mediated genetic interference (RNAi) for the purification and characterization of planarian ECM.…”

“…Because our current knowledge of ECM biology in planarians is limited (Isolani et al 2013;Seebeck et al 2017;Lindsay-Mosher, Chan, and Pearson 2020), it is first necessary to develop a comprehensive and optimized workflow to characterize and study the planarian ECM. A recent study has characterized the transcriptional landscape of ECM components in planarians by constructing an in silico matrisome (Cote, Simental, and Reddien 2019).…”

DDX24, a D-E-A-D box RNA helicase, is required for muscle fiber organization and anterior pole specification essential for head regeneration in planarians

“…Interestingly, in planarians, muscles have additional connective tissue type roles too. They express all the matrisome related genes-these ECM glycoproteins, perlecan, hemicentin-1, for example, are necessary for maintaining spatial positioning of different mesenchymal cell-types in these flatworms (27,28). However, the factors that underlie this tremendous functional versatility of planarian muscles remains poorly understood.…”

“…Planarian muscles, in addition to their canonical roles in locomotion and skeletal support, predominantly express all the positional-control genes and morphogens associated with positional information establishment and patterning (24,25). Furthermore, muscles in planarians also act as 'structural scaffolds' for organ regeneration and perform the role of connective tissue by expressing many of the matrisome related genes (26)(27)(28). Therefore, given the multi-faceted functional role of muscle in regeneration, defect in muscle organization or loss of muscle associated factors inadvertently leads to defective regeneration (26,(29)(30)(31)(32).…”