MerTK activity is not necessary for the proliferation of glioblastoma stem cells.

MerTK has been identified as a promising target for therapeutic intervention in glioblastoma. Genetic studies documented a range of oncogenic processes that MerTK targeting could influence, however robust pharmacological validation has been missing. The aim of this study was to assess therapeutic potential of MerTK inhibitors in glioblastoma therapy. Unlike previous studies, our work provides several lines of evidence that MerTK activity is dispensable for glioblastoma growth. We observed heterogeneous responses to MerTK inhibitors that could not be correlated to MerTK inhibition or MerTK expression in cells. The more selective MerTK inhibitors UNC2250 and UNC2580A lack the anti-proliferative potency of less-selective inhibitors exemplified by UNC2025. Functional assays in MerTK-high and MerTK-deficient cells further demonstrate that the anti-cancer efficacy of UNC2025 is MerTK-independent. However, despite its efficacy in vitro, UNC2025 failed to attenuate glioblastoma growth in vivo. Gene expression analysis from cohorts of glioblastoma patients identified that MerTK expression correlates negatively with proliferation and positively with quiescence genes, suggesting that MerTK regulates dormancy rather than proliferation in glioblastoma. In summary, this study demonstrates the importance of orthogonal inhibitors and disease-relevant models in target validation studies and raises a possibility that MerTK inhibitors could be used to target dormant glioblastoma cells.

Mer regulates microglial/macrophage M1/M2 polarization and alleviates neuroinflammation following traumatic brain injury.

BACKGROUND: Traumatic brain injury (TBI) is a leading cause of death and disability worldwide. Microglial/macrophage activation and neuroinflammation are key cellular events following TBI, but the regulatory and functional mechanisms are still not well understood. Myeloid-epithelial-reproductive tyrosine kinase (Mer), a member of the Tyro-Axl-Mer (TAM) family of receptor tyrosine kinases, regulates multiple features of microglial/macrophage physiology. However, its function in regulating the innate immune response and microglial/macrophage M1/M2 polarization in TBI has not been addressed. The present study aimed to evaluate the role of Mer in regulating microglial/macrophage M1/M2 polarization and neuroinflammation following TBI. METHODS: The controlled cortical impact (CCI) mouse model was employed. Mer siRNA was intracerebroventricularly administered, and recombinant protein S (PS) was intravenously applied for intervention. The neurobehavioral assessments, RT-PCR, Western blot, magnetic-activated cell sorting, immunohistochemistry and confocal microscopy analysis, Nissl and Fluoro-Jade B staining, brain water content measurement, and contusion volume assessment were performed. RESULTS: Mer is upregulated and regulates microglial/macrophage M1/M2 polarization and neuroinflammation in the acute stage of TBI. Mechanistically, Mer activates the signal transducer and activator of transcription 1 (STAT1)/suppressor of cytokine signaling 1/3 (SOCS1/3) pathway. Inhibition of Mer markedly decreases microglial/macrophage M2-like polarization while increases M1-like polarization, which exacerbates the secondary brain damage and sensorimotor deficits after TBI. Recombinant PS exerts beneficial effects in TBI mice through Mer activation. CONCLUSIONS: Mer is an important regulator of microglial/macrophage M1/M2 polarization and neuroinflammation, and may be considered as a potential target for therapeutic intervention in TBI.

Apoptotic cell-mimicking gold nanocages loaded with LXR agonist for attenuating the progression of murine systemic lupus erythematosus.

Systemic lupus erythematosus (SLE) constitutes an autoimmune disease characterized by the breakdown of tolerance to self-antigens, sustained production of pathogenic autoantibodies, and damage to multiple organs and tissues. Nanoparticle (NP)-based therapeutics have demonstrated efficacy in attenuating the progression of SLE. However, investigations of nano-drugs that address the crucial initiating factor in the pathogenesis of SLE; e.g., inefficient clearance of apoptotic cells by phagocytes and consequent accumulation of self-antigens, have seldom been reported. Here, an apoptotic cell-mimicking gold nanocage (AuNC)-based nano drug carrier capable of correcting the impaired clearance of apoptotic cells in SLE was rationally designed and generated by conjugating phosphatidylserine (PS) on the surface of liposome-coated AuNCs for liver X receptor (LXR) agonist T0901317 delivery. Notably, PS-lipos-AuNC@T0901317 could efficiently enhance apoptotic cell clearance by elevating the expression of Mer, one of the pivotal phagocytosis-associated receptors on macrophages, resulting in decreased production of anti-dsDNA autoantibodies, reduced inflammatory response, and alleviation of kidney damage in lupus model mice. Additionally, PS-lipos-AuNC could be tracked by photoacoustic imaging for nano drug carrier biodistribution. By addressing the crucial pathogenic factor of SLE, the NP-based delivery system in this study is envisioned to provide a promising strategy to treat this complex and challenging disease.

Increased Mer and Axl receptor tyrosine kinase expression on glomeruli in lupus nephritis.

Mer and Axl receptor tyrosine kinases (MerTK and AxlTK) play important roles in the clearance of apoptotic cells and the inhibition of inflammatory responses. Previous studies demonstrated that they might participate in glomerular injury in mice model. This study aimed to elucidate the expression of MerTK and AxlTK on glomeruli and analyze their clinical significance in lupus nephritis (LN) patients. Twenty-nine LN and 10 primary nephrotic syndrome (NS) patients were recruited. The expression of MerTK and AxlTK on glomeruli was measured by immunohistochemistry. Correlations between the levels of MerTK and AxlTK and clinical data were investigated. Statistical differences in each group were calculated by one-way analysis of variance, t test, or Mann-Whitney U test. Correlations were evaluated with Pearson's or Spearman's correlation tests. Both MerTK and AxlTK were expressed mainly on mesangial cells. LN patients demonstrated more expression of MerTK and AxlTK than primary NS patients (1.19 ± 1.01 × 10-2 vs 0.21 ± 0.29 × 10-2, 7.25 ± 2.69 × 10-2 vs 3.10 ± 1.22 × 10-2, p < 0.01). In LN patients, MerTK expression correlated with AxlTK (r = 0.529, p < 0.01). LN patients with class IV expressed more MerTK and AxlTK (1.50 ± 1.03 × 10-2 and 7.56 ± 2.93 × 10-2). The expression of MerTK and AxlTK varied according to the deposition of immunoglobulin and complements on glomeruli. Both MerTK and AxlTK expressions were increased on glomeruli and varied according to pathological classifications. Thus, we assumed that both two subsets might participate in the pathogenesis of LN.