Title

 

Authors

L. Lisi
Doctorate in Neuroscience.
Institute of Pharmacology, Catholic University Medical School, Rome Italy

 

Abstract

The mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase with a central role in the regulation of cell growth and proliferation, as well as of physiological processes such as transcription, mRNA turnover and translation, ribosomal biogenesis, vesicular trafficking, autophagy and cytoskeletal organization. The relevance of this pathway in the regulation of the immune system is well characterized. mTOR is essential for the proper activation and proliferation of effector T cells, it restricts the development of regulatory T cells, and it is critical in the downregulation of the innate immune responses. Within the cells, the kinase forms two different macromolecular complex, mTORC1 and mTORC2, with distinct functions. Rapamycin inhibits mTORC1 activity, via binding to the immunophilin FK-506-binding protein 1A, 12 kDa (FKBP12). However, some functions mediated by mTORC1 appear to be rapamycin insensitive. Moreover, rapamycin at higher concentrations and in chronic treatments interferes with mTORC2 regulatory functions in certain cell types. Nevertheless, the use of rapamycin has significantly contributed to elucidate the role of mTOR in the regulation of immune response, and it is now apparent that mTOR is key regulator of both the innate and the adaptive immune system. The present work aims to elucidate the role of mTOR in the regulation of glial inflammatory activation, thus in the regulation of the immune response within the central nervous system (CNS). In this regard we have observed that mTOR mediates cytokine dependent cell activation and proliferation in primary cultures of rat cortical microglia (1). In this experimental paradigm, mTOR inhibitors reduced nitric oxide synthase (NOS) activity and NOS2 expression induced by cytokines, but not those induced by the endotoxin lipopolysaccharide (LPS). The pharmacological inhibition of mTOR kinase activity also reduced cytokine dependent microglial proliferation and the intracellular levels of cyclooxygenase. Under basal conditions mTOR inhibitors significantly reduced microglial viability. Moreover, mTOR activation participates in processes involved in the upregulation of NOS2 in astrocytes (2). In primary cultures of rat cortical astrocytes, rapamycin significantly increased NOS2 mRNA levels after 4 h incubation in presence of pro-inflammatory stimuli. This stimulatory effect was transient, since no differences in either NOS2 mRNA or protein levels were detected after 24 h. Interestingly, reduced levels of NOS2 mRNA were detected after 48 hours, suggesting that rapamycin can modify NOS2 mRNA stability. In this regard, we found that rapamycin significantly reduced the half-life of NOS2 mRNA, from 4 h to 50 min when cells were co-incubated with cytokine mixture and 10 nM rapamycin. Similarly, rapamycin induced a significant up-regulation of tristetraprolin (TTP), a protein involved in the regulation of NOS2 mRNA stability. In conclusion, these results, obtained using primary cultures of rat glial cells (microglia and astrocytes) suggest that inhibition of mTOR kinase activity in glial cells results in antinflammatory actions. Therefore, mTOR inhibitors may have possible beneficial effects in the treatment of inflammatory-based CNS pathologies, like multiple sclerosis.

1. Dello Russo C, Lisi L et al, (2009). Biochemical Pharmacology 2009 Nov 1; 78(9):1242-51.
2. Lisi L et al, (2011). J Neuroinflammation. 2011 Jan 5;8(1):1