The role of extracellular matrix in planarian regeneration

Abstract

Extracellular matrix (ECM) is an important niche component and its role in stem cell biology is well recognized; however, the role of ECM in tissue regeneration is not well understood. Recent findings suggest ECM synthesis and it’s remodeling by matrix metalloproteinases act as key regulatory systems in wound repair and tissue regeneration.

There are many animals with regenerative potentials, such as hydra, planarian and salamander. A common process in their tissue regeneration is the formation of a blastema. The planarians, however, have the most effective regenerative potential, being able to regenerate any missing parts of the organism and are considered “immortal” under the knife. This regenerative feat is thought to be by its large population of stem cells. Here, I use the planarian as a model system, studying the dynamic protein expression changes in the earliest events in tissue regeneration to gain insights into the role of ECM in regeneration.

Using a novel type of time-lapse microscopy, I showed that the blastema in the first 2 days of regeneration appears homogenous, containing mostly undifferentiated stem cells recruited from the old tissue. In contrast, major morphological changes could be observed between days 3 and 4, consistent with the rapid proliferation and or differentiation of stem cells, coordinated with reorganization of tissue components.

Proteome expression profiling of the blastema during the first 4 days of regeneration was performed that showed many distinctive expression pattern of proteins. Key changes included components of Wnt signaling pathways, which were up regulated in late stages of tail blastema, consistent with their roles in promoting tail formation. In contrast, negative regulators of Wnt were up regulated in the regenerating head ensuring the regeneration of anterior structures. Interestingly, many ECM proteins were also dynamically expressed suggesting the regulation of matrix assembly and remodeling is important for regeneration.

Expression analysis of two collagen chains, Dj-col1 and Dj-col2, showed that they were co-expressed with piwi-expressing stem cells in intact planarians; however, in blastema, their co-expression with piwi-positive cells was delinked. Knock down of Dj-col1 and Dj-col2 by RNAi in amputated planarians resulted in faster regeneration. Quantitative analysis indicated there was a general increase in the number of piwiexpressing stem cells following RNAi treatment, consistent with a negative effect on stem cell function and regeneration for these collagen chain proteins.

Expression of two putative matrilysins (Dj-mmp1 and Dj-mmp2) were found to be around the pharynx region in the intact planarian. In regeneration, their expressions decreased in regenerating blastema; however, the typical expression patterns of these genes were quickly reconstituted in the old tissue, consist with a role of these metalloproteinases (MMPs) in remodeling the old tissue for integration with the new tissues, or for the maintenance proportion and function of all the organs.

This study has provided the first insight into the role of ECM and MMPs in the regenerative processes of planarians. Additional analyses of other collagen chain proteins and more detailed functional studies are needed to better define the precise role of ECM molecules in regeneration.