Developmental expression of matrix metalloproteinases 2 and 9 and their potential role in the histogenesis of the cerebellar cortex

The development of the cerebellar cortex depends on intrinsic genetic programs and orchestrated cell-cell/cell-matrix interactions. Matrix metalloproteinases (MMPs) are proteolytic enzymes that play an important role in these interactions. MMP-2 and MMP-9 are involved in diverse neuronal functions including migration, process extension, and synaptic plasticity. We investigated the spatiotemporal pattern of expression/activity of MMP-2/MMP-9 in the developing cerebellum and their role in the histogenesis of the cerebellar cortex. The levels of transcripts of MMP-2/MMP-9 were measured with real-time quantitative polymerase chain reaction. An initial decrease in MMP-2/MMP-9 transcripts was observed between postnatal days 3 (PD3) and PD6, and the mRNA levels remained relatively constant thereafter. Zymographic analysis revealed that the expression/activity of MMP-2/MMP-9 persisted longer than their transcripts; the downregulation occurred around PD9, suggesting a mechanism of translational or post-translational regulation. The gelatinase activity was localized in the external granule layer (EGL) and the internal granule layer during PD3-PD12. The immunoreactivity of MMP-2 was mainly localized in the EGL, the Bergmann glial fibers, and the Purkinje cell layer (PCL), whereas MMP-9 immunoreactivity was detected intensively in the PCL and the extracellular space of the molecular layer. Expression of MMP-9 was relatively weak in the EGL. The immunoreactivity of MMP-2/MMP-9 became undetectable after PD21. A similar expression pattern of MMP-2/MMP-9 was observed in organotypic cerebellar slice cultures. Exposure of organotypic slices to a specific MMP-2/MMP-9 inhibitor significantly increased the thickness of the EGL and concurrently decreased the number of migrating granule neurons in the molecular layer. Thus, MMP-2 and MMP-9 play a role in the postnatal cerebellar morphogenesis.