The Leucine Rich Repeat Kinase 2 (LRRK2) is one of causative genes of familial Parkinson’s disease (PD). The M2397T polymorphism in LRRK2 is genetically associated with sporadic Crohn’s disease (CD). LRRK2 is expressed in human CD14+ monocytes, induced by interferon-γ (IFN-γ) and suppresses inflammatory activation. We hypothesize that IFN-γ-induced LRRK2 and inflammatory gene expression is altered by LRRK2 genetic polymorphism found in CD and PD cases. A total of 46 CD and 51 control cases, and 16 PD cases and 16 PD-linked LRRK2 mutation cases were recruited. Live human CD14+ monocytes were isolated from donors for ex vivo IFN-γ stimulation and gene expression analysis. IFN-γ potently enhanced TNFA, IL12, HLADRA1 and LRRK2 expression, which was suppressed by FK506, a calcineurin-specific inhibitor, but further enhanced by LRRK2-specific kinase inhibitor (GSK2578215A). The 2397-M/M CD risk allele enhanced IFN-γ responses of CD14+ cells in CD but not in control group. CD14+ monocytes from G2019S and R1441C LRRK2 mutated PD cases and carriers show no changes in IFN-γ responses for TNFA or IL12, reduced response for HLADRA1, and enhanced responses for LRRK2 in FK506-sensitive manner. These data demonstrate that CD-associated LRRK2 mutations are significant modifiers of innate immune response in CD14+ monocytes, and PD-associated LRRK2 mutation may contribute to reduced antigen presentation response.

The accumulation of phosphorylated tau protein (pTau) in the entorhinal cortex (EC) is the earliest tau pathology in Alzheimer's disease (AD). Tau tubulin kinase-1 (TTBK1) is a neuron-specific tau kinase and expressed in the EC and hippocampal regions in both human and mouse brains. Here we report that collapsin response mediator protein-2 (CRMP2), a critical mediator of growth cone collapse, is a new downstream target of TTBK1 and is accumulated in the EC region of early stage AD brains. TTBK1 transgenic mice show severe axonal degeneration in the perforant path, which is exacerbated by crossing with Tg2576 mice expressing Swedish familial AD mutant of amyloid precursor protein (APP). TTBK1 mice show accumulation of phosphorylated CRMP2 (pCRMP2), in the EC at 10 months of age, whereas age-matched APP/TTBK1 bigenic mice show pCRMP2 accumulation in both the EC and hippocampal regions. Amyloid-β peptide (Aβ) and TTBK1 suppress the kinetics of microtubule polymerization and TTBK1 reduces the neurite length of primary cultured neurons in Rho kinase-dependent manner in vitro. Silencing of TTBK1 or expression of dominant-negative Rho kinase demonstrates that Aβ induces CRMP2 phosphorylation at threonine 514 in a TTBK1-dependent manner, and TTBK1 enhances Aβ-induced CRMP2 phosphorylation in Rho kinase-dependent manner in vitro. Furthermore, TTBK1 expression induces pCRMP2 complex formation with pTau in vitro, which is enhanced upon Aβ stimulation in vitro. Finally, pCRMP2 forms a complex with pTau in the EC tissue of TTBK1 mice in vivo, which is exacerbated in both the EC and hippocampal tissues in APP/TTBK1 mice. These results suggest that TTBK1 and Aβ induce phosphorylation of CRMP2, which may be causative for the neurite degeneration and somal accumulation of pTau in the EC neurons, indicating critical involvement of TTBK1 and pCRMP2 in the early AD pathology.

Triggering receptor expressed on myeloid cells 2 (TREM2) is a single transmembrane molecule uniquely expressed in microglia. TREM2 mutations are genetically linked to Nasu-Hakola disease and associated with multiple neurodegenerative disorders, including Alzheimer's disease. TREM2 may regulate microglial inflammation and phagocytosis through coupling to the adaptor protein TYRO protein-tyrosine kinase-binding protein (TYROBP). However, there is no functional system for monitoring this protein-protein interaction. We developed a luciferase-based modality for real-time monitoring of TREM2-TYROBP coupling in live cells that utilizes split-luciferase complementation technology based on TREM2 and TYROBP fusion to the C- or N-terminal portion of the luciferase gene. Transient transfection of human embryonic kidney 293 cells with this reporter vector increased luciferase activity upon stimulation with an anti-TREM2 antibody, which induces their homodimerization. This was confirmed by ELISA-based analysis of the TREM2-TYROBP interaction. Antibody-mediated TREM2 stimulation enhanced spleen tyrosine kinase (SYK) activity and uptake of in microglial cell line BV-2 in a kinase-dependent manner. Interestingly, the TREM2 T66M mutation significantly enhanced luciferase activity without stimulation, indicating constitutive coupling to TYROBP. Finally, flow cytometry analyses indicated significantly lower surface expression of T66M TREM2 variant than wild type or other TREM2 variants. These results demonstrate that our TREM2 reporter vector is a novel tool for monitoring the TREM2-TYROBP interaction in real time.

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