title: The catalytic subunit of the system L1 amino acid transporter (slc7a5) facilitates nutrient signalling in mouse skeletal muscle.
abstract: The System L1-type amino acid transporter mediates transport of large neutral amino acids (LNAA) in many mammalian cell-types. LNAA such as leucine are required for full activation of the mTOR-S6K signalling pathway promoting protein synthesis and cell growth. The SLC7A5 (LAT1) catalytic subunit of high-affinity System L1 functions as a glycoprotein-associated heterodimer with the multifunctional protein SLC3A2 (CD98). We generated a floxed Slc7a5 mouse strain which, when crossed with mice expressing Cre driven by a global promoter, produced Slc7a5 heterozygous knockout (Slc7a5+/-) animals with no overt phenotype, although homozygous global knockout of Slc7a5 was embryonically lethal. Muscle-specific (MCK Cre-mediated) Slc7a5 knockout (MS-Slc7a5-KO) mice were used to study the role of intracellular LNAA delivery by the SLC7A5 transporter for mTOR-S6K pathway activation in skeletal muscle. Activation of muscle mTOR-S6K (Thr389 phosphorylation) in vivo by intraperitoneal leucine injection was blunted in homozygous MS-Slc7a5-KO mice relative to wild-type animals. Dietary intake and growth rate were similar for MS-Slc7a5-KO mice and wild-type littermates fed for 10 weeks (to age 120 days) with diets containing 10%, 20% or 30% of protein. In MS-Slc7a5-KO mice, Leu and Ile concentrations in gastrocnemius muscle were reduced by ∼40% as dietary protein content was reduced from 30 to 10%. These changes were associated with >50% decrease in S6K Thr389 phosphorylation in muscles from MS-Slc7a5-KO mice, indicating reduced mTOR-S6K pathway activation, despite no significant differences in lean tissue mass between groups on the same diet. MS-Slc7a5-KO mice on 30% protein diet exhibited mild insulin resistance (e.g. reduced glucose clearance, larger gonadal adipose depots) relative to control animals. Thus, SLC7A5 modulates LNAA-dependent muscle mTOR-S6K signalling in mice, although it appears non-essential (or is sufficiently compensated by e.g. SLC7A8 (LAT2)) for maintenance of normal muscle mass.
authors.affiliation: Division of Cell Signalling and Immunology, College of Life Sciences, University of Dundee, Dundee, United Kingdom.
authors.forename: Nadège
authors.initials: N
authors.lastname: Poncet
authors.affiliation: Division of Cell Signalling and Immunology, College of Life Sciences, University of Dundee, Dundee, United Kingdom ; Section on Molecular Morphogenesis, Program in Cellular Regulation and Metabolism (PCRM), NICHD, NIH, Bethesda, Maryland, United States of America.
authors.forename: Fiona E
authors.initials: FE
authors.lastname: Mitchell
authors.affiliation: Medical Research Council Protein Phosphorylation and Ubiquitylation Unit, College of Life Sciences, University of Dundee, Dundee, United Kingdom.
authors.forename: Adel F M
authors.initials: AF
authors.lastname: Ibrahim
authors.affiliation: Division of Cell Signalling and Immunology, College of Life Sciences, University of Dundee, Dundee, United Kingdom.
authors.forename: Victoria A
authors.initials: VA
authors.lastname: McGuire
authors.affiliation: Division of Molecular Microbiology, College of Life Sciences, University of Dundee, Dundee, United Kingdom.
authors.forename: Grant
authors.initials: G
authors.lastname: English
authors.affiliation: Division of Cell Signalling and Immunology, College of Life Sciences, University of Dundee, Dundee, United Kingdom.
authors.forename: J Simon C
authors.initials: JS
authors.lastname: Arthur
authors.affiliation: Section on Molecular Morphogenesis, Program in Cellular Regulation and Metabolism (PCRM), NICHD, NIH, Bethesda, Maryland, United States of America.
authors.forename: Yun-Bo
authors.initials: YB
authors.lastname: Shi
authors.affiliation: Division of Cell Signalling and Immunology, College of Life Sciences, University of Dundee, Dundee, United Kingdom.
authors.forename: Peter M
authors.initials: PM
authors.lastname: Taylor
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002478:Cells, Cultured; D004044:Dietary Proteins / Q000008:administration & dosage*; D005951:Glucose Tolerance Test; D007328:Insulin / Q000378:metabolism*; D007333:Insulin Resistance; D019426:Integrases / Q000378:metabolism; D027283:Large Neutral Amino Acid-Transporter 1 / Q000502:physiology*; D007930:Leucine / Q000008:administration & dosage*; D051379:Mice; D018345:Mice, Knockout; D008822:Mice, Transgenic; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism*; D010766:Phosphorylation; D012333:RNA, Messenger / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*
chemical_list: D004044:Dietary Proteins; D007328:Insulin; D027283:Large Neutral Amino Acid-Transporter 1; D012333:RNA, Messenger; C546843:mTOR protein, mouse; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; C045073:Cre recombinase; D019426:Integrases; D007930:Leucine