N-Cadherin in Osteolineage Cells Is Not Required for Maintenance of Hematopoietic Stem Cells

Abstract 2390

Osteoblast lineage cells have been shown to play an important role in regulating hematopoietic stem cells (HSCs), and there is intense interest in identifying HSC regulatory molecules they produce. It has been reported that HSCs lie adjacent to spindle-shaped N-cadherin⁺ osteoblasts (SNO cells) and home to them in irradiated recipients, suggesting that N-cadherin may tether HSCs to their niche. Studies of N-cadherin expression in HSCs and its role in regulating HSC function have yielded conflicting results. Conditional deletion of Cdh2 (encoding N-cadherin) in HSCs had no affect on HSC number or function. On the other hand, silencing of N-cadherin using shRNA or expression of a dominant negative N-cadherin mutant resulted in the loss of HSC quiescence and repopulating activity. In addition to forming homodimers, N-cadherin is able to interact with other cadherins such E-cadherin, C-cadherin, and R-cadherin as well as non-cadherins such as KLRG1. Thus it is possible that expression of other cadherins in HSCs may compensate for the loss of N-cadherin. Rather than attempt to reconcile the conflicting results involving HSC production of N-cadherin, we chose to investigate what role that osteolineage production of N-cadherin plays in the regulation of hematopoiesis. Specifically, we conditionally deleted N-cadherin from osteoblast lineage cells using transgenic mice expressing Cre-recombinase under control of the osterix promoter (Osx-Cre mice). Our lineage mapping studies using the Osx-Cre mice demonstrated that this transgene directs recombination in SNO cells in the bone marrow. Accordingly, we intercrossed the Osx-Cre mice with Cdh2^flox mice to generate N-cadherin-deleted (Cdh2^flox/flox Osx-Cre) and control (Cdh2^flox/flox) mice. N-cadherin expression was efficiently ablated in osteoblast lineage cells as assessed by mRNA expression (20-fold lower than control mice) and immunostaining of bone sections. Blood counts, bone marrow and spleen cellularity, and leukocyte differentials in N-cadherin-deleted mice were no different from control mice, indicating that basal hematopoiesis is normal. Moreover, the number of phenotypic HSCs (defined as lin⁻ c-kit⁺ sca⁺ CD41⁻ CD48⁻ CD150⁺ cells) and their cell cycle status was normal. HSC long-term repopulating activity and self-renewal capacity were assessed by competitive repopulation assays and serial transplantation, respectively; we show that loss of osteoblast N-cadherin had no effect on these parameters. N-cadherin has been implicated in the homing and retention of HSCs to the bone marrow. However, we show that homing and engraftment of wildtype cells into N-cadherin-deleted recipients was normal. Finally, we tested the response to G-CSF, a potent HSC mobilizing stimulus, which leads to a profound loss of osteoblasts. N-cadherin-deleted mice showed normal mobilization of progenitors to the blood and spleen. Together, our data show that N-cadherin expression on SNO cells (and other osteoblast-lineage cells) is dispensable for HSC maintenance and trafficking.