DYRKP Kinase Regulates Cell Wall Degradation in Chlamydomonas by Inducing Matrix Metalloproteinase Expression

The cell wall of plants and algae is an important cell structure that protects cells from changes in the external physical and chemical environment. This extracellular matrix composed of polysaccharides and glycoproteins, is needed to be remodeled continuously throughout the life cycle. However, compared to matrix polysaccharides, little is known about the mechanisms regulating the formation and degradation of matrix glycoproteins. We report here that a plant kinase belonging to the dual-specificity tyrosine phosphorylation-regulated kinase (DYRK) family present in all eukaryotes regulates cell wall degradation in the model microalga Chlamydomonas reinhardtii by inducing the expression of matrix metalloproteinases (MMPs). In the absence of DYRKP, daughter cells fail to degrade the parental cell wall, and form multicellular structures. On the other hand, the complementation line of DYRKP was shown to degrade the parental cell wall normally. Transcriptomic and proteomic analyses indicate a marked down-regulation of MMP expression in the dyrkp mutants. Additionally, the expression of MMP was confirmed to be consistent with the expression pattern of DYRKP. Our findings show that DYRKP, by ensuring timely MMP expression, enables the successful execution of the cell cycle. Altogether, this study provides new insight into the life cycle regulation in plants and algae.Background Plants and algae have different types of polysaccharides in their cell walls, but they have glycoproteins in common. Glycoprotein synthesis and degradation must be tightly regulated to ensure normal growth and differentiation. However, little is known about the regulatory mechanism of glycoprotein degradation in both plants and algae. The cell cycle of Chlamydomonas reinhardtii begins anew with the hatching of daughter cells, and the role of matrix metalloproteinases (MMPs) is known to be important in this process. In our previous study, we observed that a knockout mutant of the plant kinase belonging to the dual-specificity tyrosine phosphorylation-regulated kinase (DYRKP) formed a palmelloid structure and failed to hatch.Questions What is the role of DYRKP in microalgae? Specifically, why does the dyrkp mutant form a palmelloid structure? Palmelloid is usually observed in dividing cells or after exposure to stresses. We therefore hypothesized that the palmelloid phenotype observed in dyrkp mutant could either be due to a defect in cell hatching or due to an increased stress state in the mutant population.Findings We answered these questions by comparative studies in different culture conditions and by examining additional dyrkp knockout mutants generated by CRISPR-Cas9 in various background strains with more or less intact cell walls. Palmelloid formation in the dyrkp mutant was observed under optimal growth (mixo- or auto-trophic condition) and very low light conditions. Interestingly, unlike the parent strain, in which only cell wall fragments are observed in old cultures, the parental cell wall of the dyrkp mutant remained almost intact even after the release of daughter cells. Also, the cell division rate of the cell wall-less dyrkp mutants was similar to their background strain. These results suggest that dyrkp mutants have a problem in degrading the parental cell walls. Indeed, proteomic and transcriptomic analyses revealed reduced levels of protease families in the dyrkp mutant, and in particular with a significantly lower amount of several key members of the MMP family. Through the analysis of complementation lines, we confirmed that the DYRKP was required for strong and rapid expression of MMPs.Next steps We are pursuing research to understand what the phosphorylation clients of DYRKP are and how they regulate the expression of the MMPs identified in this study.One sentence summary The DYRKP kinase induces the expression of matrix metalloproteinases involved in the degradation of the parental cell wall, allowing prompt hatching of daughter cells after cell division.