The role of extracellular proteases, matrix metalloproteinase-2 and -9, in regulating the mammalian circadian clock

Molecules existing in the extracellular space (ECS) make up a complicated network of structural proteins, signaling molecules, cellular adhesion molecules and transmembrane receptors. Many ECS molecules are implicated as active regulators of structural and functional brain plasticity. When molecules in the ECS are improperly expressed or degraded, cellular communication becomes disrupted and can lead to a variety of disease pathologies. Thus, there are myriad overlapping regulatory mechanisms that determine and maintain the composition of the ECS. Research investigating extracellular molecule expression in the brain identified that some exhibit cyclic, near 24-hour rhythms and may be under circadian control. Further, extracellular molecules are hypothesized to help perpetuate the oscillations of the mammalian master clock located in the suprachiasmatic nucleus (SCN). Our goal was to expand upon past findings in our laboratory that implicate a pivotal role of extracellular proteases, namely those in the plasminogen activation cascade, in gating glutamate-induced phase shifts. Two proteases activated by this cascade are matrix metalloproteinase-2 and -9 (MMP-2/9). Across the brain, MMP-2/9 catalytically cleave extracellular scaffolding molecules like β-dystroglycan in response to increased neuronal activity. MMP-2/9-mediated degradation of extracellular molecules is also known to facilitate changes in neuronal and astrocytic morphology and alter N-methyl-D-aspartate receptor (NMDAR) activity. Interestingly, a previous study of MMP-2/9 in the hamster SCN found that MMP-9 activity exhibits a diurnal rhythm in activity, while MMP-2 does not. For this project, we aimed to elucidate the role of MMP-2/9 by further characterizing their expression and activity in mouse SCN tissue. We found that MMP-2/9 activity is highest during the day and that inhibiting their activity phase shifts neuronal activity rhythms in a time dependent manner. Further, we sought to better understand physiologic consequences of MMP-2/9 mediated cleavage of β-dystroglycan in regulating astrocytic diurnal morphologic rhythms. We confirmed that astrocyte morphology exhibits a day to night transition toward a highly ramified state. Although β-dystroglycan expression is constitutive across the day and potentially interacts with SCN astrocytes, their degradation does not appear to be regulated by MMP-2/9 activity.