Nonenzymatic and Trophic Activities of Carboxypeptidase E Regulate Bone Mass and Bioenergetics of Skeletal Stem Cells in Mice

Abstract

Bone and energy metabolism are integrated by common regulatory mechanisms. Carboxypeptidase E (CPE), also known as obesity susceptibility protein or neurotrophic factor‐α1, is recognized for its function in processing prohormones, including proinsulin and pro‐opiomelanocortin polypeptide. Independent of its enzymatic activity, CPE may also act as a secreted factor with divergent roles in neuroprotection and cancer growth; however, its role in the regulation of bone mass and skeletal cell differentiation is unknown. Male mice with global deficiency in CPE are characterized with profound visceral obesity, low bone mass in both appendicular and axial skeleton, and high volume of marrow fat. Interestingly, although metabolic deficit of CPE KO mice develops early in life, bone deficit develops in older age, suggesting that CPE bone‐specific activities differ from its enzymatic activities. Indeed, mutated CPE knockin (mCPE KI) mice ectopically expressing CPE‐E342Q, a mutated protein lacking enzymatic activity, develop the same obese phenotype and accumulate the same volume of marrow fat as CPE KO mice, but their bone mass is normal. In addition, differentiation of marrow hematopoietic cells toward tartrate‐resistant acid phosphatase‐positive multinucleated osteoclasts is highly increased in CPE KO mice, but normal in mCPE KI mice. Moreover, in murine skeletal stem cells, nonenzymatic trophic CPE has activated ERK signaling, increased cell proliferation and increased mitochondrial activity. Treatment of preosteoblastic cells with intact or mutated recombinant CPE led to a transient accumulation of small lipid droplets, increased oxidative phosphorylation, and increased cellular dependence on fatty acids as fuel for energy production. In human marrow aspirates, CPE expression increases up to 30‐fold in osteogenic conditions. These findings suggest that nonenzymatic and trophic activities of CPE regulate bone mass, whereas marrow adiposity is controlled by CPE enzymatic activity. Thus, CPE can be positioned as a factor regulating simultaneously bone and energy metabolism through a combination of shared and distinct mechanisms. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Novel interaction between neurotrophic factor-α1/carboxypeptidase E and serotonin receptor, 5-HTR1E, protects human neurons against oxidative/neuroexcitotoxic stress via β-arrestin/ERK signaling

“…NF-α1/CPE applied extracellularly protected rat hippocampal primary neurons against H 2 O 2 -induced cytotoxicity by activating ERK and AKT signaling cascades, which increased pro-survival protein, BCL2 and decreased caspase-3 activation [ 14 , 15 ]. In murine skeletal stem cells, treatment with NF-α1/CPE activated ERK signaling and increased proliferation [ 16 ]. However, the mechanism of action of extracellular NF-α1/CPE in neuroprotection remains elusive.…”

Hippocampal delivery of neurotrophic factor-α1/carboxypeptidase E gene prevents neurodegeneration, amyloidosis, memory loss in Alzheimer’s Disease male mice

“…Recently, Neurotrophic Factor-α1 (NF-α1), originally known as Carboxypeptidase E (CPE) was identified and shown to have a number of trophic effects [ 22 , 23 ], independent of its enzymatic activity; most notably in protecting neurons from stress-induced cell death in the hippocampal CA3 region in mice and was more critical than BDNF in this respect [ 24 ]. Studies using a mouse model expressing a non-enzymatic form of NF-α1/CPE demonstrated that it was as effective in neuroprotection as wild-type NF-α1/CPE [ 24 ].…”

CPE correlates with poor prognosis in gastric cancer by promoting tumourigenesis

“…CPE is also associated with many metabolic disorders in the human body, including infertility, diabetes mellitus, obesity, glucose homeostasis, and bone remodelling. It also plays a crucial role in psychiatric and mental processes, including emotional responses, memory, and Alzheimer's disease [ 3 , [7] , [8] , [9] , [10] ].…”