Biochemical and Biophysical Research Communications
Serum- and glucocorticoid-inducible kinases in microglia
Introduction
Brain inflammation has been suggested to be associated with various neurological disorders. The inflammatory responses are mediated mainly by non-neuronal cells such as microglia and astrocytes. Microglial cells are derived from myelogenous cells and have many characters of immune cells [1], [2]. They work to maintain brain homeostasis, for example, by removing dead cells under resting condition. When brain receives severe stresses such as bacterial infection, stroke and tramatic injury, microglial cells are activated. Once activated, they release inflammatory cytokines such as tumor necrosis factor α (TNFα) and produce reactive oxygen species via inducible nitric oxide synthase (iNOS) [3], [4]. As these events promote neuronal dysfunction, anti-inflammatory reagents appear to be beneficial for those disorders mentioned above [5], [6].
In addition to disorders that have apparent acute stressful causes, anti-inflammation therapy can also be applied to neuropsychiatric disorders such as schizophrenia and chronic neurodegenerative disorders such as amyotrophic lateral sclerosis [7], [8]. Therefore, further studies of microglial inflammatory responses and their modulators sheds light on comprehensive understanding and future therapeutic strategy of neurological disorders.
Serum- and glucocorticoid-inducible kinase 1 (SGK1) is a member of SGK family. Its expression is rapidly induced by serum and glucocorticoid [9]. Increasing evidence suggests that SGKs including SGK1 contribute to various physiological and pathophysiological processes [9], [10]. In particular, SGKs, especially SGK1, have recently been shown to play a role in immune cells. For example, pathogenic IL-17-producing T cells are induced under high salt condition in an SGK1-dependent manner, and the induction is anticipated to exacerbate autoimmune encephalomyelitis [11]. Also, inhibition of SGK1 affects neutrophil chemotaxis [12]. In addition, recent studies have found SGK1 to be implicated in NF-κB activity, and that the regulated NF-κB signaling is associated with immunological significances such as inflammatory responses in monocytes and differentiation of dendritic cells [13], [14].
Although all of the SGK family members are expressed in brain [15], the detailed distribution and function are not clear. There have been reports that describe apparent existence of SGK1 in neurons, astrocytes and oligodendrocytes [16], [17], [18]. In addition, the existence of SGK1 in a minor proportion of microglia in brain has been demonstrated by Wärntges et al. [18]. However, the presence of other SGKs and the roles of SGKs in microglial cells remain to be elucidated. This study examines the expression of SGK isoforms and explores the effects of SGK inhibition in multiple microglial cell lines.
Section snippets
Reagents and antibodies
The following reagents and antibodies were used: Isogen (Nippon Gene); lipopolysaccharide (LPS, Escherichia coli 111:B4, Sigma); gsk650394 (Selleckchem); protease inhibitor cocktail (Sigma); 2,3-diaminonaphthalene, tetracycline (Dojindo); 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT, Wako); fluorescein diacetate (FDA, Wako); propidium iodide (PI, Wako); Cytoplasmic & Nuclear Protein Extraction Kit (101Bio); rabbit polyclonal antibody against actin (Sigma); mouse monoclonal
Expression of SGK isoforms in microglial cells
SGK family members, including SGK1, SGK2, and SGK3, are expressed in brain [15]. Recent studies have disclosed their potential functions [9], [23]. For example, their roles in neuronal cells and oligodendrocytes have been documented [17], [24], [25], [26]. A study by Wärntges et al. also reported the presence of SGK1 in microglia in a minor proportion in brain [18]. However, their function in microglial cells remains unknown. In this study, we first examined the presence of SGK isoforms in a
Discussion
Early studies have revealed the pivotal function of SGK1 in renal salt reabsorption [29]. Since then, significances of SGKs in other systems such as reproduction and immunity have been demonstrated [11], [30]. Similarly, there have been some reports which propose the relationship between SGKs and neurological diseases such as psychiatric and neurodegenerative disorders. In the case of depression, physical stresses increase SGK1 in neuronal progenitor cells, which plays a role in depression [31]
Competing financial interests
The authors declare no competing financial interests.
Acknowledgments
This work was supported by Grant-in-Aid for Scientific Research No. 16K09520 from the Japan Society for the Promotion of Science, Japan (K.I.), Grant-in-Aid for Scientific Research on Innovative Areas “Glial Assembly” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (T.U.), and NIH R01NS066027 and NIHMD S21MD000101 (Z.-G.X).
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