Microglia are the immune cells of the central nervous system (CNS) and play important roles under physiological and pathophysiological conditions. Activation of microglia has been reported for a variety of CNS diseases and is believed to be involved in inflammation-mediated neurodegeneration. Loss of TGFβ1 results in increased microgliosis and neurodegeneration in mice which indicates that TGFβ1 is an important regulator of microglial functions in vivo. Here, we addressed the role of endogenous TGFβ signaling for microglia in vitro. We clearly demonstrate active TGFβ signaling in primary microglia and further introduce Klf10 as a new TGFβ target gene in microglia. Moreover, we provide evidence that microglia express and release TGFβ1 that acts in an autocrine manner to activate microglial TGFβ/Smad signaling in vitro. Using microarrays, we identified TGFβ-regulated genes in microglia that are involved in TGFβ1 processing, its extracellular storage as well as activation of latent TGFβ. Finally, we demonstrate that pharmacological inhibition of microglial TGFβ signaling resulted in upregulation of the proinflammatory markers IL6 and iNOS and downregulation of the alternative activation markers Arg1 and Ym1 in vitro. Together, these data clearly show that endogenous TGFβ1 and autocrine TGFβ signaling is important for microglial quiescence in vitro and further suggest the upregulation of TGFβ1 in neurodegenerative diseases as a mechanism to regulate microglia functions and silence neuroinflammation.

Milk fat globule-epidermal growth factor-factor 8 (Mfge8) has been described as an essential molecule during microglia-mediated clearance of apoptotic cells via binding to phosphatidylserine residues and subsequent phagocytosis. Impaired uptake of apoptotic cells by microglia results in prolonged inflammatory responses and damage of healthy cells. Although the mechanisms of Mfge8-mediated engulfment of apoptotic cells are well understood, endogenous or exogenous factors that regulate Mfge8 expression remain elusive. Here, we describe that TGFβ1 increases the expression of Mfge8 and enhances the engulfment of apoptotic cells by primary mouse microglia in a Mfge8-dependent manner. Further, apoptotic cells are capable of increasing microglial TGFβ expression and release and shift the microglia phenotype toward alternative activation. Moreover, we provide evidence that Mfge8 expression is differentially regulated in microglia after classical and alternative activation and that Mfge8 is not able to exert direct antiinflammatory effects on LPS-treated primary microglia. Together, these results underline the importance of TGFβ1 as a regulatory factor for microglia and suggest that increased TGFβ1 expression in models of neurodegeneration might be involved in clearance of apoptotic cells via regulation of Mfge8 expression.

Microglia are involved in physiological as well as neuropathological processes in the central nervous system (CNS). Their functional states are often referred to as M1-like and M2-like activation, and are believed to contribute to neuroinflammation-mediated neurodegeneration or neuroprotection, respectively. Parkinson’s disease (PD) is one the most common neurodegenerative disease and is characterized by the progressive loss of midbrain dopaminergic (mDA) neurons in the substantia nigra resulting in bradykinesia, tremor, and rigidity. Interleukin 4 (IL4)-mediated M2-like activation of microglia, which is characterized by upregulation of alternative markers Arginase 1 (Arg1) and Chitinase 3 like 3 (Ym1) has been well studied in vitro but the role of endogenous IL4 during CNS pathologies in vivo is not well understood. Interestingly, microglia activation by IL4 has been described to promote neuroprotective and neurorestorative effects, which might be important to slow the progression of neurodegenerative diseases. In the present study, we addressed the role of endogenous and exogenous IL4 during MPP+-induced degeneration of mDA neurons in vitro and further addressed the impact of IL4-deficiency on neurodegeneration in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD in vivo. Our results clearly demonstrate that exogenous IL4 is important to protect mDA neurons in vitro, but endogenous IL4 seems to be dispensable for development and maintenance of the nigrostriatal system as well as MPTP-induced loss of TH+ neurons in vivo. These results underline the importance of IL4 in promoting a neuroprotective microglia activation state and strengthen the therapeutic potential of exogenous IL4 for protection of mDA neurons in PD models.