DFG project - Jörg Müller I

In vivo role of FLT3-CD45 signalling for bone remodelling

The aim of the previous project was to assess the role of the two transmembrane, receptor-like protein tyrosine phosphatases PTPRJ/DEP-1 and PTPRC/CD45 in regulating transformation of myeloid cells by the acute myeloid leukaemia (AML)-related oncoprotein FLT3-ITD in vivo. Using a disease-specific mouse model (FLT3-ITD), we found that loss of either of these PTPs aggravated a myeloproliferative disease, supporting their tumour-suppressor role. These studies have been published in Hematologica and Oncogene. In addition to aberrant haematopoiesis, the inactivation of the Ptprc gene in FLT3-ITD mice resulted in a marked and unexpected bone phenotype. Comprehensive analysis revealed that a similar, but milder phenotype was already present in the FLT3-ITD knock-in background. Our pilot studies revealed severe abnormalities of bone architecture, cell biology, and remodelling in the latter mice, which were even more pronounced with inactivation of Ptprc. The significant alteration of parameters of bone formation or resorption indicated potentially altered osteoblast (OB) and osteoclast (OC) function. These data indicated a previously unrecognized role of FLT3 in regulation of bone metabolism, which is affected by Ptprc.

The aim of this proposal is to unravel the mechanisms underlying the FLT3/ FLT3-ITD-Ptprc-mediated signalling axis to control bone formation and remodelling. Molecular analysis of the relevant bone cell types, their stem cell biology and repopulation capacity and the involved signalling mechanisms are the focus of our studies to understand how FLT3 and Ptprc regulate bone metabolism. Our previously established FLT3-ITD/ Ptprc-/-mouse strain and the corresponding WT, FLT3-ITD and Ptprc-/- control strains represent the technical basis for this proposal.

We will address the following objectives:

1. Characterization of the bone phenotype of FLT3-ITD expressing mice in detail,
2. Determination of the role of FLT3-ITD and CD45 in bone cell differentiation and function,
3. Analysis of the underlying pathways of the altered bone cell function in FLT3-ITD and FLT3-ITD Ptprc-/- mice and
4. Elucidation of the origin of ectopic bone formation and aberrant HSC homing.

Taken together, detailed characterization of the phenotype of the FLT3-ITD Ptprc-/- mice in comparison to wild-type mice and mice carrying single mutations provides mechanistic insights, how FLT3 and Ptprc control bone remodelling, the HSC niche, and mutual interactions of both. Detailed understanding of these processes for normal and pathological haematopoiesis and bone remodelling offers the potential for targeted intervention for bone and hematologic disorders and may translate into improved outcome for affected patients.

Involved people: Carolin Lossius (PhD student), Jörg Müller (project leader)

Cooperation: The project will be realized in close collaboration with Prof. Lorenz C. Hofbauer, Universitätsklinikum Dresden der Technischen Universität Dresden, Bereich Endokrinologie, Diabetes & Osteologie and Prof. Martina Rauner, Group Leader and Scientific Direktor of the „Bone Lab“, Universitätsklinikum Carl Gustav Carus, TU Dresden. Local close collaboration with the Department of Haematology and Oncology of the Jena University Hospital.

Funding: DFG Mu955/14-1, 2020-2023