Rictor, an mTORC2 Protein, Regulates Murine Lymphatic Valve Formation Through the AKT-FOXO1 Signaling

Arterioscler Thromb Vasc Biol. 2024 Sep;44(9):2004-2023. doi: 10.1161/ATVBAHA.124.321164. Epub 2024 Aug 1.

Abstract

Background: Lymphatic valves are specialized structures in collecting lymphatic vessels and are crucial for preventing retrograde lymph flow. Mutations in valve-forming genes have been clinically implicated in the pathology of congenital lymphedema. Lymphatic valves form when oscillatory shear stress from lymph flow signals through the PI3K/AKT pathway to promote the transcription of valve-forming genes that trigger the growth and maintenance of lymphatic valves. Conventionally, in many cell types, AKT is phosphorylated at Ser473 by the mTORC2 (mammalian target of rapamycin complex 2). However, mTORC2 has not yet been implicated in lymphatic valve formation.

Methods: In vivo and in vitro techniques were used to investigate the role of Rictor, a critical component of mTORC2, in lymphatic endothelium.

Results: Here, we showed that embryonic and postnatal lymphatic deletion of Rictor, a critical component of mTORC2, led to a significant decrease in lymphatic valves and prevented the maturation of collecting lymphatic vessels. RICTOR knockdown in human dermal lymphatic endothelial cells not only reduced the level of activated AKT and the expression of valve-forming genes under no-flow conditions but also abolished the upregulation of AKT activity and valve-forming genes in response to oscillatory shear stress. We further showed that the AKT target, FOXO1 (forkhead box protein O1), a repressor of lymphatic valve formation, had increased nuclear activity in Rictor knockout mesenteric lymphatic endothelial cells in vivo. Deletion of Foxo1 in Rictor knockout mice restored the number of valves to control levels in lymphatic vessels of the ear and mesentery.

Conclusions: Our work identifies a novel role for RICTOR in the mechanotransduction signaling pathway, wherein it activates AKT and prevents the nuclear accumulation of the valve repressor, FOXO1, which ultimately enables the formation and maintenance of lymphatic valves.

Keywords: AKT1 protein; FOXO1 protein; RICTOR protein; cellular mechanotransduction; lymphatic system; mice.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Carrier Proteins* / genetics
  • Carrier Proteins* / metabolism
  • Cells, Cultured
  • Endothelial Cells / metabolism
  • Forkhead Box Protein O1* / genetics
  • Forkhead Box Protein O1* / metabolism
  • Forkhead Transcription Factors / genetics
  • Forkhead Transcription Factors / metabolism
  • Humans
  • Lymphangiogenesis*
  • Lymphatic Vessels* / metabolism
  • Mechanistic Target of Rapamycin Complex 2* / genetics
  • Mechanistic Target of Rapamycin Complex 2* / metabolism
  • Mechanotransduction, Cellular*
  • Mice
  • Mice, Inbred C57BL
  • Mice, Knockout*
  • Multiprotein Complexes / genetics
  • Multiprotein Complexes / metabolism
  • Phosphorylation
  • Proto-Oncogene Proteins c-akt* / metabolism
  • RNA Interference
  • Rapamycin-Insensitive Companion of mTOR Protein* / genetics
  • Rapamycin-Insensitive Companion of mTOR Protein* / metabolism
  • Signal Transduction*
  • TOR Serine-Threonine Kinases / metabolism
  • Transfection

Substances

  • Rapamycin-Insensitive Companion of mTOR Protein
  • rictor protein, mouse
  • Proto-Oncogene Proteins c-akt
  • Forkhead Box Protein O1
  • Foxo1 protein, mouse
  • Mechanistic Target of Rapamycin Complex 2
  • Carrier Proteins
  • TOR Serine-Threonine Kinases
  • RICTOR protein, human
  • Forkhead Transcription Factors
  • Multiprotein Complexes
  • FOXO1 protein, human