1. Department of Medicine, Stanford University School of Medicine, Stanford, CA 94305, USA
2. Geriatric Research, Education and Clinical Center, VA Palo Alto Health Care System, Palo Alto, CA 94304, USA
3. These authors contributed independently to this work
4. Department of Molecular Gerontology, National Institute on Aging, NIH, Baltimore, MD 21224, USA
5. Department of Pharmaceutical Chemistry and Mass Spectrometry Facility, University of California, San Francisco, CA 94143, USA
6. Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA