Sympathetic Neuropathy Of The Hematopoietic Stem Cell Niche Is Essential For Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) are originated by mutations in a hematopoietic stem cell (HSC), frequently in the Janus kinase 2 (JAK2) gene. Different outcomes of this common event and limited efficacy of JAK2 inhibitors suggest the contribution of other factors. Additional HSC mutations and HSC-niche interaction might influence MPN progression, characterized by sequential expansion of HSCs, blood cells and megakaryocytes. Ensuing bone marrow (BM) fibrosis and osteosclerosis, which are contributed by osteoblastic lineage cells in a BCR/ABL CML model (Schepers et al Cell Stem Cell 2013), impede normal hematopoiesis. We have previously shown that BM nestin⁺ mesenchymal stem cells (MSCs) innervated by sympathetic nerve fibers regulate HSCs (Méndez-Ferrer et al Nature 2008 & 2010). Here we demonstrate that damage to this regulatory network is required for MPN manifestation. Nestin⁺ MSCs and NESTIN mRNA expression were rapidly reduced in the BM of MPN patients and mice expressing the human JAK2-V617F mutation. This reduction was not due to nestin⁺ MSC differentiation into fibroblasts or osteoblasts, as shown by 25-week lineage-tracing studies using Nes-CreERT2;RCE-loxP mice, but instead caused by early MSC apoptosis. In turn, nestin⁺ cell reduction stimulated MPN progression; selective nestin⁺ cell depletion using Nes-CreERT2;iDTA mice increased peripheral white and red blood cells, megakaryocytic invasion of spleen germinal centers and BM osteosclerosis. Our recent results indicate that the neural crest contributes during development to BM MSCs with specialized HSC niche function and that postnatal murine BM Nestin-GFP⁺ cells do not only contain MSCs but also Schwann cell precursor-like cells (Isern et al, ISSCR Annual Meeting 2013). BM Nestin-GFP⁺ cells from MPN mice showed reduced expression of HSC maintenance and mesenchymal genes, and increased expression of genes related to axon guidance and Schwann cell differentiation. Principal component analyses of independent biological samples further showed that control BM nestin⁺ cells clustered together with MSCs, whereas MPN BM nestin⁺ cells resembled Schwann cell precursors. These data suggested alterations to the neural component of the BM HSC niche in MPN. Indeed, BM sympathetic nerve fibers and Schwann cells, closely associated but different from Nestin-GFP⁺ cells, were rapidly reduced in the BM of diseased animals. Symptomatic MPN mice were treated with selective β₃-adrenergic agonists to compensate for the loss of sympathetic stimulation of nestin⁺ MSCs. Treatment with BRL37344 or the recently FDA approved drug Mirabegron prevented MPN-associated neutrophilia and thrombocytosis, while it did not affect peripheral blood counts of wild-type mice. While vehicle-injected animals showed severe BM fibrosis, long-term BRL37344 treatment led to virtual absence of focal reticulin deposits or excessive fibroblasts. To further confirm the contribution of BM neural damage to MPN pathogenesis, diseased mice were treated with a neuroprotective agent. Sympathetic nerve-ensheathing Schwann cells were strongly reduced in the BM of vehicle-injected animals but preserved in 4-methylcatechol-treated mice. Like in BRL37344-treated animals, this was associated with prevention of very early MPN events, including neutrophilia and BM overproduction of the pro-inflammatory cytokine interleukin-1β. Since MPN is originated by a mutant HSC, we reasoned that sympathetic neuropathy might contribute to MPN pathogenesis through early disruption of the HSC niche. The chemokine Cxcl12 regulates HSC migration and proliferation. At early MPN stage, HSC expansion and mobilization correlated with decreased BM Cxcl12 expression and protein levels. Concomitantly, BM nestin⁺ MSC number and their Cxcl12 expression were significantly reduced. BRL37344 treatment completely restored the number of BM nestin⁺ cells and improved Cxcl12 BM levels. Treatment with 4-methylcatechol or BRL37344 prevented the early expansion of mutant hematopoietic progenitors, whereas long-term BRL37344 treatment efficiently reduced mutant hematopoietic progenitor numbers in BM and peripheral blood. These results demonstrate that damage of the niche, induced by the mutated HSCs, critically contributes to JAK2-V617F⁺ MPN pathogenesis. They also unravel HSC niche-forming MSCs and their neural regulation as promising novel therapeutic targets in MPN.