Regulation of stem cells by extracellular matrix proteins in regenerative planarians

Abstract

Regulation of stem cell maintenance and differentiation is important for the development of an animal; its growth and tissue repair/regeneration. Impairment leads to unpredictable behavior of stem cells such as cellular senescence and cancer. The freshwater flatworm planarians can regenerate all missing tissues after amputation. This incredible regenerative ability is in part due to the maintenance of a large population of adult pluripotent stem cells (neoblasts) in their mesenchyme. These neoblasts respond to tissue wounding and form new tissues. A robust stem cell regulatory system must exist to control the behavior and function of the neoblasts. Despite a poor understanding of such regulatory system, the cellular microenvironment/niche is known to play a role, which includes the extracellular matrix (ECM). A functional stem cell niche should contain components which control stem cell’s proper behaviors, such as self-renewal, differentiation and migration. These behaviors of stem cell can be systemically analyzed through repopulation assay in planarian, where neoblasts are required to rapidly proliferate and migrate to regions where neoblasts were lost. Through RNAi screening of 161 ECM genes using a functional readout of stem cell repopulation, we provided insights into the potential function of netrins, collagens and laminins, and ECM receptors regulating neoblast behavior.

Compared to mammalian niche, planarian niche showed a high complexity of neoblast regulation, where different ECM components are collectively produced by CNS, muscles and intestine, which regulate distinct behaviors of neoblast in the niche. CNS-expressed ECM netrin is found to be required for neoblast migration, consistent to other systems where netrins are required for axon guidance. However, unlike in most cases where netrins only interact with neuronal cell receptor UNC5 and DCC, neoblasts do not express these receptors. Instead, planarian netrin interacts with integrins in neoblasts. As neoblasts are stem cells of all different lineages, this discovery showed how neural tissue can regulate stem cells of other lineages through netrins.

In many systems, collagens provide mechanical/structural attachment for cells to migrate, proliferate and differentiate. In planarian, role of collagens in controlling neoblast proliferation and differentiation, but not migration, is observed. Surprisingly, where in most systems type IV collagens are pro-mitotic, the opposite was found in planarian niche. Moreover, unlike the case in most systems, an opposing effect of proliferation rate is found between fibrillar/FACIT and type IV collagens. Importantly, these collagens are expressed in different regions of the niche, providing a novel mechanism to control stem cell proliferation rate as they migrate to different regions. High plasticity of ECM expression is also discovered, as the region of collagen expression changed rapidly in respond to amputation, such that neoblasts can hyperproliferate and differentiate in proper regions specifically during regeneration.

Overall, this study demonstrated how planarian utilizes conserved ECM components to build a niche that can dynamically control behavior of stem cell from multiple lineages, to perform tissue turnover and regeneration. By testing our findings in other animal models, insights could be given on how tissue regeneration and cancer prevention can be achieved in mammals and humans.