MMP-2 mediates Purkinje cell morphogenesis and spine development in the mouse cerebellum

Matrix metalloproteinase-2 (MMP-2) is a highly studied proteolytic enzyme, involved in many detrimental and beneficial functions throughout the body, and also active in the central nervous system (CNS). MMP-2 is profoundly expressed in the developing cerebellum and was recently reported to modulate granule cell proliferation by affecting cell cycle kinetics in cerebella of postnatal day 3 mouse pups. In this report, a two-dimensional difference gel electrophoresis proteomics study was implemented at this postnatal stage and revealed 16 differentially expressed proteins between MMP-2-deficient (MMP-2(-/-)) and wild-type cerebella. Among those, collapsin response mediator protein 1 (CRMP1) could be identified as the most significant differential protein between the two genotypes. Western blot experiments confirmed this finding and further disclosed a significant increase in phosphorylated CRMP1 expression in MMP-2(-/-) cerebella. Strikingly, subsequent immunohistochemical and microscopic analyses revealed an aberrant Purkinje cell (PC) dendritogenesis, possibly related to upregulated (phospho-) CRMP1 levels in these neonatal MMP-2(-/-) animals. Further, detailed morphometric analyses showed persistent PC morphological changes in MMP-2(-/-) mice, from the neonatal stage until adulthood. These were characterized by a reduced growth of PC somata, reduced dendritic tree sizes, and a decreased dendritic arborization. During development, the observed defects were accompanied by a temporarily disturbed parallel fiber and climbing fiber synaptic input on the PCs, while in adult MMP-2(-/-) animals, an increased PC spine density and reduced spine lengths were noted. The observed PC abnormalities might contribute to the mild defects in motor performance, i.e. balance and coordination, detected in adult MMP-2(-/-) mice. Overall, these findings indicate the importance of MMP-2 in CNS development and dendritogenesis, and highlight the importance of a correct developmental wiring for adult brain morphology and function.