Beclin 1 of megakaryocytic lineage cells is locally dispensable for platelet hemostasis but functions distally in bone homeostasis

The crosstalk between megakaryocytic lineage cells and the skeletal system has just begun to be explored but remains largely elusive. Using conditional gene knockout mouse models, we demonstrated that loss of Beclin 1 (Becn1), a major regulator of mammalian autophagy, exclusively in the megakaryocytic lineage disrupted autophagy in platelets but did not compromise megakaryopoiesis or the formation and function of platelets. Unexpectedly, conditional Becn1 deletion in male mice led to a remarkable increase in bone mass with improved bone quality, in association with a decrease in sex hormone binding globulin (SHBG) and an increase in free testosterone (FT). In vivo Becn1 overexpression in megakaryocytic lineage-specific cells reduced bone mass and quality, along with an increase in SHBG and a decrease in FT. Transplantation of wild-type bone marrow cells into megakaryocytic lineage Becn1-deficient male mice restored bone mass and normalized SHBG and FT. Furthermore, bilateral orchiectomy of Becn1^f/f;Pf4-iCre mice, which are crippled with the production of testosterone, resulted in a reduction in bone mass and quality, whereas in vivo overexpression of SHBG, specifically in the liver of Becn1^f/f;Pf4-iCre mice, decreased FT and reduced bone mass and quality. In addition, metformin treatment, which induces SHBG expression, reduced FT and normalized bone mass in Becn1^f/f;Pf4-iCre mice. We thus concluded that Becn1 of the megakaryocytic lineage is dispensable locally for platelet hemostasis but limits bone mass by increasing SHBG, which in turn reduces the FT of male mice. Our findings highlight a mechanism by which Becn1 from megakaryocytic lineage cells distally balances bone growth.