The functional role of phosphatase Ssu72 in alveolar macrophages

Abstract

Ssu72 phosphatase was originally identified for its role in regulation of RNA Polymerase II (RNAP-II) activity by directly regulating phosphorylation of serine and tyrosine residues of C-terminal domain of RANP-II. However, the function of Ssu72 phosphatase is expanding. Recent reports suggest different functional roles of Ssu72 in different cell types suggesting that Ssu72 might be involved in many cellular processes and molecular pathways. For example, Ssu72 regulates hepatocyte cell cycle by directly regulating phosphorylation of Retinoblastoma protein. Moreover, Ssu72 regulated development of arthritis by regulating Th17 cell differentiation and function. Based on these recent reports, Ssu72 might play key roles in other cells types including those of macrophages. Lysm-CreSsu72fl/fl mice was used to study the functional role of Ssu72 in macrophages, and surprisingly, Ssu72 was found to regulate lung resident alveolar macrophage (AM) development and maturation but not those of any other macrophages. Using Cd11c-creSsu72fl/fl mice, AM precursors were traced from embryonic stages, which showed that Ssu72 regulated development and maturation of AMs from neonatal stages of mice. Mechanism wise, Ssu72 was rapidly induced upon Granulocyte macrophage-colony stimulating factor (GM-CSF) stimulation in AMs and upon GM-CSF stimulation, Ssu72 directly bound to GM-CSF receptor  chain (GM-CSFR c). Moreover, GM-CSF increased phosphorylation of GM-CSFR c and downstream molecules, and induced dysregulations in cell cycle, death, and mitochondrial respiration in mature Ssu72-deficient AMs compared with those of mature WT AMs. The dysregulations found in mature Ssu72-deficient AMs were restored by treatment with AZD1480, which restored GM-CSFR c phosphorylation in mature Ssu72-deficient AMs like that of mature WT AMs. Furthermore, Lysm-CreSsu72fl/fl mice failed to develop allergic asthma, which Ssu72-deficient AMs were directly responsible for. Combined, these results demonstrate the critical role of Ssu72 in regulation of GM-CSF signaling by binding to and regulating phosphorylation of GM-CSFR c, thereby contributing to the development and maturation of AMs.

Ssu72는 탈인산가수분해효소로써, 리보핵산중합효소-II의 인산화 정도 및 활성화를 조절하는 단백으로 잘 알려져 있다. 하지만, Ssu72가 대식세포에 미치는 영향에 대한 보고는 전무하다. 이 연구는 Ssu72가 폐포대식세포의 GM-CSF 신호전달체계에 관여하여 폐포대식세포의 항상성 및 기능에 관여함을 규명하였다. 폐포대식세포에 GM-CSF 자극이 주어졌을 시, Ssu72는 GM-CSF 수용체 베타체인과 결합하고 그 인산화 정도를 조절하여 GM-CSF 신호전달 체계를 조절함을 확인하였고, 따라서 Ssu72가 결핍된 폐포대식세포는 정상 폐포대식세포에 비교해 세포주기, 사멸, 증식 그리고 미토콘드리아 호흡에 문제가 발생함을 확인하였다. 더 나아가, Ssu72가 결핍된 폐포대식세포에 GM-CSF 수용체의 베타체인의 인산화를 담당하고 있는 JAK2에 대한 억제제 처리를 통해 GM-CSF 신호전달 체계를 회복시켰을 시 비정상적인 세포 항상성이 정상화 되는 것을 확인하였다. 또한, Lysm-CreSsu72fl/fl 마우스는 생후직후부터 폐포대식세포의 발달 및 성장 그리고 기능에 문제가 발생하여 있음을 확인하였고, 이는 알레르기성 천식을 억제함을 확인하였다. 특히, Ssu72 결핍 폐포대식세포에 JAK2 억제제 처리 시 천식의 발병률이 다시 증가함을 확인하였다. 따라서, 이 연구 결과들은 GM-CSF 신호전달 체계가 Ssu72에 의해 미세조정 되고 있고, Ssu72가 GM-CSF 수용체 베타체인과 결합함으로써 폐포대식세포의 발달 및 성장 그리고 기능에 관여함을 규명하였다.