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. J. Comp. Neurol. 481:403–415, 2005. © 2004 Wiley‐Liss, Inc.