Sequential binding of ezrin and moesin to L-selectin regulates monocyte protrusive behaviour during transendothelial migration.

Leukocyte transendothelial migration (TEM) is absolutely fundamental to the inflammatory response, and involves initial pseudopod protrusion and subsequent polarised migration across inflamed endothelium. Ezrin/radixin/moesin (ERM) proteins are expressed in leukocytes and mediate cell shape changes and polarity. The spatio-temporal organisation of ERM proteins with their targets, and their individual contribution to protrusion during TEM, has never been explored. Here, we show that blocking binding of moesin to phosphatidylinositol 4,5-bisphosphate (PIP2) reduces its C-terminal phosphorylation during monocyte TEM, and that on-off cycling of ERM activity is essential for pseudopod protrusion into the subendothelial space. Reactivation of ERM proteins within transmigrated pseudopods re-establishes their binding to targets, such as L-selectin. Knockdown of ezrin, but not moesin, severely impaired the recruitment of monocytes to activated endothelial monolayers under flow, suggesting that this protein plays a unique role in the early recruitment process. Ezrin binds preferentially to L-selectin in resting cells and during early TEM. The moesin-L-selectin interaction increases within transmigrated pseudopods as TEM proceeds, facilitating localised L-selectin ectodomain shedding. In contrast, a non-cleavable L-selectin mutant binds selectively to ezrin, driving multi-pseudopodial extensions. Taken together, these results show that ezrin and moesin play mutually exclusive roles in modulating L-selectin signalling and shedding to control protrusion dynamics and polarity during monocyte TEM.

"Young at heart": Regenerative potential linked to immature cardiac phenotypes.

The adult human myocardium is incapable of regeneration; yet, the zebrafish (Danio rerio) can regenerate damaged myocardium. Similar to the zebrafish heart, hearts of neonatal, but not adult mice are capable of myocardial regeneration. We performed a proteomics analysis of adult zebrafish hearts and compared their protein expression profile to hearts from neonatal and adult mice. Using difference in-gel electrophoresis (DIGE), there was little overlap between the proteome from adult mouse (>8weeks old) and adult zebrafish (18months old) hearts. Similarly, there was a significant degree of mismatch between the protein expression in neonatal and adult mouse hearts. Enrichment analysis of the selected proteins revealed over-expression of DNA synthesis-related proteins in the cardiac proteome of the adult zebrafish heart similar to neonatal and 4days old mice, whereas in hearts of adult mice there was a mitochondria-related predominance in protein expression. Importantly, we noted pronounced differences in the myofilament composition: the adult zebrafish heart lacks many of the myofilament proteins of differentiated adult cardiomyocytes such as the ventricular isoforms of myosin light chains and nebulette. Instead, troponin I and myozenin 1 were expressed as skeletal isoforms rather than cardiac isoforms. The relative immaturity of the adult zebrafish heart was further supported by cardiac microRNA data. Our assessment of zebrafish and mammalian hearts challenges the assertions on the translational potential of cardiac regeneration in the zebrafish model. The immature myofilament composition of the fish heart may explain why adult mouse and human cardiomyocytes lack this endogenous repair mechanism.