Are megakaryocyte podosomes relevant targets in thrombocytopenia? – MegaPod

Background. Thrombocytopenia is defined as an abnormally low blood platelet count, exposing patients at high risks of hemorrhages. It is a common clinical problem in chemotherapy-treated cancer patients with varying incidence, severity and duration. Platelet are produced by megakaryocytes (MK) in the bone marrow (BM). During this process, MKs must detach and transit from the dense cellular BM microenvironment through the sinusoid vessel wall toward the circulation, a process called intravasation. Being a transient process by nature, its exploration in vivo is still challenging and remain poorly understood. Using high-resolution BM imaging, we recently have identified a new in vivo cellular meta-structure, called PodoPZ, composed of an actomyosin-based network of interconnected podosomes which is located in the peripheral zone of MKs. At this strategic location, the PodoPZ is a privileged site of interactions with the microenvironment, to provide trans-endothelium crossing and thereby ensure successful thrombopoiesis, without damaging the integrity of the sinusoidal wall.
Central hypothesis. In MegaPod project, we hypothesize that PodoPZ is a key player in the complex adaptation of MK to microenvironmental cues during its journey from the BM to the bloodstream across the endothelial barrier. By regulating MK intravasation and platelet release, it contributes to platelet recovery during homeostasis and after chemotherapy-induced thrombocytopenia (CIT).
Objectives and approaches.
The MegaPod program is built on a solid consortium with complementary expertise in megakaryocyte physiology, podosome signaling and mechanics, and state of the art imaging technologies. It will also benefit from a collaboration with the university of Michigan for genetic murine models.
Combining innovative genetic and super-resolution imaging approaches in mouse models of bone marrow regeneration, we will determine (1) how drug-induced changes in the BM environment affect PodoPZ organization and function in MK. Our preliminary data on a preclinical models of CIT indicate that the PodoPZ adapts its structure to the radical changes of the stressed BM environment. Comparing physiological situation with CIT mice models, we will provide an overall picture of the dynamic PodoPZ adaptation to the surrounding BM microenvironment, and we will identify specific sets of properties associated to drug-induced thrombocytopenia. (2) how the PodoPZ drives the passage of the MK through the endothelium. We will examine how MK intravasation rely to the coupling between force generation, adhesive or degradative functions of the PodoPZ. (3) what are the specific signaling pathways in the MK supporting the functional regulation of the PodoPZ. We will characterize a new in vitro 3D model mimicking the confined environment of BM in order to control, maintain and manipulate the dynamics of PodoPZ. This in vitro approach will be used as a platform for omics and biochemical approaches necessary to identify new molecular PodoPZ regulators.
Relevance and strategy. At the fundamental level, MegaPod project aims to advance our knowledge on the poorly characterized megakaryocyte intravasation in vivo, a process which is absolutely mandatory for efficient platelet release. Our findings are also expected to have an impact for a wider scientific community, as they will also bring new perspectives on how podosomes might function in vivo in other tissues. Indeed, different cell types, including macrophages and osteoclasts, elaborate interconnected pododomes, similar to PodoPZ,and, to date, only a few data have been published on podosome in vivo. At the clinical level, a novel application could stem from this work, whereby targeting megakaryocyte intravasation to ensuring platelet release, rather than stimulating megakaryocyte differentiation, would offer an alternative treatment method for better management of chemotherapy-induced thrombocytopenia in the future.