Extracellular Matrix Protein Matrilin-4 Regulates HSC Stress Response
Authors: Uckelmann Hannah , Blaszkiewicz Sandra , Essers Marieke 
Published: October 2019

Abstract
The life-long maintenance of the blood system is accomplished by a pool of self-renewing multipotent hematopoietic stem cells (HSCs). Adult HSCs are found in a dormant state for most of their lifetime, entering cell cycle only to maintain homeostatic blood supply. Under stress conditions such as infection or chemotherapy, the loss of mature blood cells leads to an activation of dormant HSCs to replenish the blood system. Gene expression analysis performed by our group now revealed that Matrilin-4 is highly expressed in long-term HSCs (LT-HSCs) compared to short-term HSCs or committed progenitors, suggesting a potential role of Matrilin-4 in HSC function. Matrilin-4 is a member of the von Willebrand factor A-containing family of extracellular adapter proteins, which form filamentous structures outside of cells.
Using mice lacking the entire family of Matrilins (1-4) we have investigated the role of Matrilins in HSC function. Constitutive Matrilin 1-4 KO mice exhibit normal hematopoiesis with a mild reduction in bone marrow cellularity and LSK numbers. However, when Matrilin KO bone marrow cells are pushed to proliferate in competitive transplantation assays with wildtype (WT) cells, they show a striking growth advantage. In a competitive transplant setting, where bone marrow cells of Matrilin KO versus WT mice are transplanted in a 1:1 ratio, the KO cells outcompete WT cells within four weeks, reaching a 90% chimerism at 16 weeks. This competitive advantage of Matrilin KO cells is evident in the long-term stem cell level as well as progenitors and is consistent in secondary transplants. To explore this remarkable phenotype, we performed single cell transplantation experiments of LT-HSCs and observed a more rapid reconstitution of peripheral blood cell levels of KO HSCs compared to WT controls. Confirming this growth advantage, Matrilin KO LSK cells show higher colony forming and serial replating potential in vitro, which can be rescued by the addition of recombinant or overexpressed Matrilin-4.
While Matrilin-4 is highly expressed in homeostatic HSCs, in vivo treatment with IFNα or other inflammatory agents, such as LPS or G-CSF result in a dramatic downregulation (25-fold) of Matrilin-4 on the transcript as well as the protein level. Moreover, Matrilin KO HSCs are more sensitive to inflammatory stress, as they show a 2-fold stronger cell cycle activation in response to IFNα in vivo. Critically, Matrilin-4 KO HSCs return to the G0 state of the cell cycle normally after stress-induced activation and transplantation, thereby preventing their exhaustion.
In summary, we show that the extracellular matrix protein Matrilin-4 is a novel component of the HSC niche, regulating HSC stress response. Surprisingly, HSCs lacking this extracellular matrix protein show a higher HSC potential due to an accelerated response to stress. Our data suggest that high expression of Matrilin-4 in LT-HSCs confers a resistance to stress stimuli. In situations of acute stress such as infection or transplantation however, this protection is rapidly lost to allow HSCs to efficiently replenish the blood system.

Disclosures
No relevant conflicts of interest to declare.