Podoplanin function is switched by partner proteins on fibroblastic reticular cells

Podoplanin is an inflammatory marker upregulated in many pathologies and correlated with invasive cell behaviour. Podoplanin is reported to facilitate both actomyosin contractility and formation of cell protrusions. However, how podoplanin can elicit these opposing phenotypes is unknown. We examined podoplanin functions in lymph node fibroblastic reticular cells (FRCs), with high endogenous podoplanin expression. We report that podoplanin expression, localisation and function are dependent on partner proteins. CLEC-2 binding up-regulates podoplanin transcription, and tetraspanin CD82 is essential for trafficking of podoplanin to the plasma membrane. At the cell surface, podoplanin regulates cytoskeletal dynamics, balanced by its membrane binding partners hyaluronan receptor CD44 and tetraspanin CD9. Both CD44 and CD9 dampen podoplanin-dependent actomyosin contractility, and in vitro , CD9/podoplanin promotes filopodia-like protrusions whereas CD44/podoplanin promotes lamellipodia formation. Both CD44 and CD9 are required to coordinate protrusion formation and spreading of FRCs in response to CLEC-2 + dendritic cells, a requirement for acute lymph node expansion. In vivo , surface expression levels of podoplanin, CD44 and CD9 are specifically upregulated on T-cell zone FRCs in the early phase of lymph node expansion. Our data support a model whereby podoplanin resides in distinct plasma membrane domains, and that CLEC-2 binding serves as a molecular switch to change podoplanin function.