ENHANCED IMMUNE RESPONSE TO MMP3 STIMULATION IN MICROGLIA EXPRESSING MUTANT HUNTINGTIN C. CONNOLLY, a A. MAGNUSSON-LIND, b G. LU, a P. K. WAGNER, a A. L. SOUTHWELL, a M. R. HAYDEN, a M. BJO ¨ RKQVIST b AND B. R. LEAVITT a * a Centre for Molecular Medicine and Therapeutics and Department of Medical Genetics, Child and Family Research Institute, University of British Columbia, Vancouver, BC V5Z 4H4, Canada b Brain Disease Biomarker Unit, Department of Experimental Medical Sciences, Wallenberg Neuroscience Center, Lund University, S-221 84 Lund, Sweden Abstract—Huntington’s Disease (HD) is an inherited neu- rodegenerative disease caused by a polyglutamine expan- sion in the huntingtin protein. The YAC128 mouse model of HD expresses the full-length human huntingtin protein with 128 CAG repeats and replicates the phenotype and neu- rodegeneration that occur in HD. Several studies have impli- cated a role for neuroinflammation in HD pathogenesis. Studies on presymptomatic HD patients have illustrated microgliosis (activated microglia) in brain regions affected in HD. Mutant huntingtin expressing isolated primary mono- cytes (human HD patients) and primary macrophages (YAC128) are overactive in response to lipopolysaccharide (LPS) stimulation. In this study we demonstrate that cul- tured primary microglia (the resident immune cells of the brain cells) from YAC128 mice differentially express a wide number of cytokines compared to wildtype microglia cul- tures in response to LPS. Furthermore, this study outlines a direct interaction between mutant huntingtin and cytokine secretion in HD microglia. Increased cytokine release in YAC128 microglia can be blocked by cannabinoid activation or by mutant huntingtin knockdown with anti-sense oligonu- cleotide treatment. Matrix metalloprotease 3 (MMP3), an endogenous neuronal activator of microglia, also induces increased cytokine release from YAC128 microglia com- pared to wildtype microglia. We found elevated MMP levels in HD CSF, and MMP levels correlate with disease severity in HD. These data support a novel role for MMPs and micro- glial activation in HD pathogenesis. With an improved understanding of the specific cellular processes involved in HD neuroinflammation, novel therapeutic agents target- ing these processes can be developed and hold great promise in the treatment of HD. Ó 2016 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: neuroinflammation, Huntington’s Disease, micro- glia, MMP3. INTRODUCTION Huntington’s Disease (HD) is a neurodegenerative disorder caused by a polyglutamine expansion in the huntingtin protein encoded by a CAG tri-nucleotide repeat expansion in the HTT gene. Abnormal immune cell activation has previously been suggested to play a pathogenic role in a number of neurodegenerative diseases, including HD (Bjorkqvist et al., 2008). Previ- ously, the brain was thought to be immunologically privi- leged, and unable to generate humoral or cellular immune responses. Microglia are macrophage-like cells of the myeloid lineage and the major immune cells in the brain. These cells are constantly surveying the microenvironment and producing factors that can influ- ence the surrounding astrocytes and neurons of the brain. Activated microglia change their morphology, express increased MHC class I and class II antigens, and secrete proteases and pro-inflammatory cytokines (Bonifati and Kishore, 2007). Cytokines are inflammatory proteins secreted in response to many different stimuli by cells of the monocyte lineage (Turnbull and Rivier, 1999). Up- regulation of cytokines like IL-6, IL-12 and TNF-a leads to a communication with neighboring microglia cells, astro- cytes, neurons and myeloid progenitor cells in vivo (Kim et al., 2005). Pro-inflammatory cytokine production can lead to a wide variety of processes involved in neurodegen- erative pathology, such as free radical production, NMDA- mediated excitotoxicity, and caspase activation. Microglial activation has been implicated in HD pathogenesis (Moller, 2010; reviewed here, Ellrichmann et al., 2013). Increased inflammatory cytokines, both IL-1b and TNFa, have been observed in the striatum of HD patients, while separately microglia activation was associated with the progression of the disease (Silvestroni et al., 2009). Positron emission tomography (PET) studies identified increased striatal microglia in HD patients, and have correlated striatal neu- ronal dysfunction with increased microglial signal in HD (Tai et al., 2007). There are a number of inflammatory linked molecules that may mediate these changes in HD http://dx.doi.org/10.1016/j.neuroscience.2016.03.031 0306-4522/Ó 2016 IBRO. Published by Elsevier Ltd. All rights reserved. * Corresponding author. Address: Centre for Molecular Medicine and Therapeutics, 950 West 28th Avenue, Room 2020, Vancouver, BC V5Z4H4, Canada. Tel: +1-(604)-875-3801; fax: +1-(604)-875-3840. E-mail address: bleavitt@cmmt.ubc.ca (B. R. Leavitt). Abbreviations: ASO, antisense oligonucleotide; CSE, control standard endotoxin; FAAH, fatty acid amide hydrolysis; FRET, Fo¨rster resonance energy transfer; HD, Huntington’s Disease; INF-c, Interferon-c; LPS, lipopolysaccharide; MMP3, Matrix metalloprotease 3; MMPs, Matrix metalloproteinases; MSD, Meso Scale Discovery; MSN, medium spiny neuron; PET, Positron emission tomography; RAC, raclopride; RT, room temperature; TB, terbium; TIMPs, tissue inhibitors of metalloproteinases. Neuroscience 325 (2016) 74–88 74