Extracellular matrix (ECM) is an important component of stem cell niche. Remodelling of ECM mediated by ECM regulators such as MMPs plays a vital role in stem cell function. However, the mechanisms that modulate the function of ECM regulators in the stem cell niche are understudied. Here, we explored the role of the transcription factor (TF), ETS-1 expressed in the cathepsin+ cell population in regulating the expression of the ECM regulator, mt-mmpA, thereby modulating basement membrane thickness. In planarians, the basement membrane around the gut/inner parenchyma is thought to act as a niche for pluripotent stem cells. It has been shown that the early epidermal progenitors migrate outward from this region and progressively differentiate to maintain the terminal epidermis. Our data shows thickening of basement membrane in the absence of ets-1 results in defective migration of stem cells progeny. Furthermore, the absence of ets-1 led to a defective epidermal progenitor landscape, in spite of its lack of expression in those cell types. Together, our results demonstrate the active role of ECM remodelling in regulating tissue homeostasis and regeneration in planaria.

Planaria is an ideal system to study factors involved in regeneration and tissue homeostasis. Little is known about the role of metabolites and small molecules in stem cell maintenance and lineage specification in planarians. Using liquid chromatography and mass spectrometry (LC‐MS)‐based quantitative metabolomics, we determined the relative levels of metabolites in stem cells, progenitors, and differentiated cells of the planarian Schmidtea mediterranea. Tryptophan and its metabolic product serotonin are significantly enriched in stem cells and progenitor population. Serotonin biosynthesis in these cells is brought about by a noncanonical enzyme, phenylalanine hydroxylase. Knockdown of Smed‐pah leads to complete disappearance of eyes in regenerating planaria, while exogenous supply of serotonin and its precursor rescues the eyeless phenotype. Our results demonstrate a key role for serotonin in eye regeneration.

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