Redox‐Dependent Bone Alkaline Phosphatase Dysfunction Drives Part of the Complex Bone Phenotype in Mice Deficient for Memo1

Abstract

Mediator of ErbB2-driven cell Motility 1 (MEMO1) is an intracellular redox protein that integrates growth factors signaling with the intracellular redox state. We have previously reported that mice lacking Memo1 displayed higher plasma calcium levels and other alterations of mineral metabolism, but the underlying mechanism was unresolved and the bone phenotype was not described. Here, we show that Cre/lox-mediated MEMO1 deletion in the whole body of C57Bl/6 mice (Memo cKO) leads to severely altered trabecular bone and lower mineralization, with preserved osteoblast and osteoclast number and activity, but altered osteoblast response to epidermal growth factor (EGF) and FGF2. More strikingly, Memo cKO mice display decreased alkaline phosphatase (ALP) activity in serum and in bone, while ALPL expression level is unchanged. Bone intracellular redox state is significantly altered in Memo cKO mice and we inferred that ALP dimerization was reduced in Memo cKO mice. Indeed, despite similar ALP oxidation, we found increased ALP sensitivity to detergent in Memo cKO bone leading to lower ALP dimerization capability. Thus, we report a severe bone phenotype and dysfunctional bone ALP with local alteration of the redox state in Memo cKO mice that partially mimics hypophosphatasia, independent of ALPL mutations. These findings reveal Memo as a key player in bone homeostasis and underline a role of bone redox state in controlling ALP activity.

Renal Memo1 Differentially Regulates the Expression of Vitamin D-Dependent Distal Renal Tubular Calcium Transporters

“…Memo kKO mice have slightly different and more moderate alterations of mineral metabolism compared to whole body Memo KO mice. In particular, Memo kKO mice did not show elevated calcemia or osteopenia in contrast to whole body Memo null mice (Haenzi et al, 2014 ; Moor et al, 2018 ). This is suggestive of a possible role of bone and/or intestinal Memo in modulating calcium mobilization.…”

“…Memo kKO mice and whole body Memo null mouse models showed varying trends of increased FGF23 serum concentrations (Haenzi et al, 2014 ; Moor et al, 2018 ). Additionally, Memo kKO mice fed the experimental 500 IU/kg vitamin D diet displayed significantly increased serum FGF23 levels compared to controls.…”

“…However, phosphatemia was unchanged between Memo null mice and controls (Haenzi et al, 2014 ). Memo cKO mice display also a specific bone phenotype, resembling partially hypophosphatasia (Moor et al, 2018 ) and pointing to a role of Memo in the assembly of active alkaline phosphatase dimers.…”

Memo1gene expression in kidney and bone is unaffected by dietary mineral load and calciotropic hormones

“…Serum analyses of Klotho and Fgf23 ‐deficient mice showed excessive 1,25(OH) 2 ‐vitamin D 3 levels, a finding that has been variably found in Memo1 ‐deficient mice depending on the genetic background (Haenzi et al, 2014; Moor, Ramakrishnan, et al, 2018). The promoters of Fgf23 and Klotho both contain vitamin D response elements (VDRE) (Forster et al, 2011; Orfanidou, Malizos, Varitimidis, & Tsezou, 2012).…”

“…Memo1 is an evolutionary conserved protein in all kingdoms of life that has shown intracellular expression in cytoplasma and nucleus (Haenzi et al, 2014; Moor, Haenzi, et al, 2018; Schlatter et al, 2012). A conditional and inducible knockout mouse model with postnatal deletion of exon 2 of the Memo1 gene has resulted in a syndrome of aging and premature death with traits such as elevated calcemia, elevated FGF23 and 1,25(OH) 2 ‐vitamin D 3 , bone disease, lung emphysema, atrophy of subcutaneous fat, insulin hypersensitivity, and renal insufficiency (Haenzi et al, 2014; Moor, Ramakrishnan, et al, 2018). This phenotype significantly overlaps with phenotypes of mouse models deficient in KLOTHO (Kuro‐o et al, 1997) or FGF23 (Shimada et al, 2004), two proteins which have tremendously reshaped our understanding of the regulation of calcium and phosphate metabolism by the kidney and bone and to a lesser extent also the intestine (Hu, Shiizaki, Kuro‐O, & Moe, 2013; Moor & Bonny, 2016).…”