Since MPN disease progression is in part driven by conversion of the malignant microenvironment (ME) into a malignant niche, which nurtures and protects leukemic stem cells (LSC) and their progeny, this may provide new options for MPN treatment. Whereas site and composition of the malignant niche is dictated by the individual driver mutation, extravasation and homing of leukemic cells and LSCs from the circulation to the niche and the long-term retention within the malignant niche requires chemokine-guided, integrin-mediated adhesion to extracellular matrix proteins and cell surface-presented ligands to ensure the activation of signaling pathways involved in cell growth, survival, and differentiation.

Integrins are a diverse family of transmembrane receptors that connect the extracellular matrix and cellular components to the intracellular cytoskeleton. They thereby play a crucial role in detecting and transmitting biochemical and mechanical signals from the cell’s direct ME. To fulfill this role, integrins must undergo a conformational change, transitioning from an inactive „bent-closed“ state to an „extended-open“ conformation capable of binding ligands. While chemokine receptors and antigen receptors kickstart integrin activation in hematopoietic cells via intracellular signaling cascades, which leads to the binding of Talin-1 (Tln1) and Kindlin-3 (K3) to the integrin beta subunit, the complete activation and long-lasting ligand binding of integrins occur when a signaling hub forms, centered around Tln1. This so-called adhesome is connected to the actin cytoskeleton, enabling the cell to engage in biomechanical probing of and react to its surrounding environment.

K3 is exclusively expressed in hematopoietic cells and in line with its restricted expression profile, mutations in K3 among patients with leukocyte adhesion deficiency type III lead to hematopoietic disorders and often result in a lethal phenotype. Similarly, the full body-knockout of K3 in mice results in perinatal lethality due to severe cell adhesion defects in hematopoietic cells and consequently anemia (dysfunctional erythrocytes), hemorrhage (dysfunctional platelets), and a compromised immune system (dysfunctional effector cells).

We recently investigated the role of K3-mediated adhesion and retention in the development and treatment of chronic myeloid leukemia (CML), a Bcr-Abl (BA)-driven MPN and we were able to show that K3- mediated integrin activation is essential for the retention of CML-LSCs within their supportive bone marrow niches, as the loss of K3 leads to the mobilization of LSCs into the PB and induces apoptosis in these cells. Furthermore, we could offer evidence that the transcriptional regulation of K3 and Tln1 is highly specific, depending on the oncogene driving the leukemic cells. Hence, understanding the precise downstream factors responsible for K3 and Tln1 transcription could lead to the development of more effective and applicable therapies, especially in the context of disease persistence and chemoresistance.

Interestingly, in JAK2V617F cells, neither increased K3 nor Tln1 mRNA levels were altered by Ruxolitinib (a frequently used JAK1 and JAK2 inhibitor) treatment. These observations suggest the involvement of additional pathways and mechanisms not targeted by the clinically used kinase inhibitors. Therefore, this projects aims:

To define integrin-ligand interactions in the bone marrow ME of stem cells in healthy and MPN mice and MPN mice after Ruxolitinib treatment failure.

To target K3-mediated integrin adhesion in leukemic cells to overcome Ruxolitinib treatment failure in vivo.

To characterize the transcriptional regulation of K3 and Tln1 in JAK2V617F expressing stem cells to identify novel therapies against MPN cells.

To define the expression and regulation of K3 and Tln1 in human MPN samples and therefrom-derived inducible pluripotent stem cells.

To assess the functional role of K3 in human MPN-derived cells.