Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) Mutations: a Potential Common Cause of Alzheimer's Disease and Musculoskeletal Disorders

Abstract

Alzheimer's disease (AD), a worldwide growing health issue, is a purely neurodegenerative form of age‐induced dementia. Interestingly, weight loss is frequently associated with disease severity and clinical progression and has been shown to begin at least ten years before the diagnosis of the disease. Bone frailty and sarcopenia have each also been linked to the onset of this disease, although very little is known about the pathogenesis of these conditions in subjects with AD. Indeed, although a common link between dementia and osteoporosis has been suggested, it has never been conclusively demonstrated. The prevalence of both conditions increase with aging, and some genetic conditions can lead to both AD and osteoporosis. Examples for the potential common cause of the 2 conditions are mutations of the Triggering Receptor Expressed on Myeloid cells 2 (TREM2) gene, primarily expressed in myeloid cells or microglia in brain and in osteoclasts in bone, which result in AD and dementia, as well as in bone fragility. In a recent Genome Wide Association Study, a single nucleotide polymorphism (SNP) encoding the R47H variant in TREM2 was linked with strong single allele genetic risk factors for AD, as well as with increased risk for Parkinson's disease, amyotrophic lateral sclerosis and frontotemporal dementia. In addition, recent studies showed that mice expressing the R47H TREM2 variant exhibit loss of TREM2 function and neuritic dystrophy. In a similar manner, another TREM2 mutation, the Y38C variant, was previously described in the pathogenesis of Nasu‐Hakola disease (also known as PLOSL) which is characterized by extensive white matter loss and frontotemporal‐like dementia and polycystic lipomembranous osteodysplasia. To investigate whether mutations in the TREM2 gene, previously associated with AD and frontotemporal dementia, also affect the skeleton, we analyzed the musculoskeletal phenotype of mice in which the Y38C and R47H variants of the TREM2 gene are ubiquitously expressed, generated by Drs. Lamb and Landreth at the Stark Neuroscience Research Institute, Indiana University School of Medicine. Our studies showed decreased bone mass in mice expressing the TREM2 mutants at 6–8 months of age. Interestingly, even though TREM2 expression was absent in skeletal muscle of wild type mice, mice expressing the R47H TREM2 variant exhibit decrease muscle mass. In summary, our results suggest a common cause of cognitive disorders and bone loss, and support an indirect role of TREM2 mutations on the loss of muscle mass, secondary to the altered bone remodeling.Support or Funding InformationNIH/NIAMS R01AR067210ICMH Collaborative Pilot Project AwardThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.