Adhesion to the bone marrow niche - podosome-like structures in hematopoietic stem cells

Abstract

Maintenance and homeostasis of the hematopoietic system is enabled by hematopoietic stem and progenitor cells (HSPC). These multipotent cells ensure the replenishment of all hematopoietic cells by giving rise to progeny with the ability to differentiate into respective cells while also self-renewing themselves. A crucial factor to regulate this essential feature of HSPC is their interaction with a specific microenvironment – the hematopoietic stem cell niche in the bone marrow (BM). The interaction of HSPC with their niche provides the signals necessary for the cells to maintain their stemness and to sustain the HSPC population. In order to provide these signals constantly, adhesion of the stem cells to the extracellular matrix (ECM) of the BM niche is of the utmost importance. Yet, the way in which HSPC anchor to their specific microenvironment is not well understood. In the here presented work, the adhesion structures established by HSPC upon contact with fibronectin are investigated in regard to their appearance, maturity state and composition of cell adhesion molecules (CAM). By investigating the spatial distribution of native and mature adhesion structure markers alongside the actin cytoskeleton in adherent HSPC using super resolution microscopy it was found, that HSPC adhere in a dynamic manner and display varying morphologies that can contain protrusions and/or structures resembling podosomes. Further investigation of these podosome-like structures (PLS) revealed that these structures are defined by an F-actin spot strongly co-localizing with phosphorylated PYK2 (pPYK2). Assessment of the spatial distribution of pPYK2, alongside the classic podosome proteins vinculin and paxillin, showed a distribution of these molecules towards the periphery of the F-actin spot. This distribution pattern became more pronounced - resembling classic podosomes more - in cells with round morphology and increased number of PLS. Additionally, examining the differentiation state of HSPC with different morphologies, it was found that the round, PLS containing cells were less differentiated compared to cells with deviating observed morphologies. Together, these findings indicate a highly dynamic adhesion behavior of HSPC, involving PLS. Given the importance of proper stem cell retention in the BM niche environment, for full function of the hematopoietic system, understanding the adhesion processes involved is an essential step in gaining insight into hematopoiesis in health and disease.