Developmental changes in the expression of chemokine receptor CCR1 in the rat cerebellum

Chemokines are small, soluble proteins that regulate leukocyte migration, adhesion, and proliferation. Recent evidence suggests that chemokine receptors are expressed in the central nervous system and that their functions extend beyond their roles in inflammation. Specific chemokines and their receptors are implicated in cerebellar development. In this study, we evaluated the expression of beta-chemokine receptor CCR1 in the immature and adult rat cerebellum and report striking developmental changes in CCR1 expression. Reverse transcriptase polymerase chain reaction assays of cerebellum revealed moderate increases in CCR1 mRNA expression from postnatal day (P) 3 to adulthood. Light and confocal microscopy were used to evaluate developmental changes in the neuroanatomical and cell-specific distribution of CCR1 immunoreactivity. CCR1 immunoreactivity was detected as early as P3 and peaked between P7 and P21. The predominant CCR1-immunoreactive neuronal cell types included granule cells of the internal granular layer, Purkinje cells, Golgi cells, and molecular layer interneurons; Bergmann glia, astrocytes, and resting microglia also expressed CCR1. In contrast, granule cells in the external germinal layer, descending granule cells, and activated microglia rarely expressed CCR1. We also evaluated the expression of the CCR1 ligand macrophage inflammatory protein-1alpha (MIP-1alpha/CCL3). Two cell populations expressed MIP-1alpha: physiologically activated microglia in white matter (P7-P14) and Purkinje cells (P7-adult). MIP-1alpha-positive cells were frequently located near the processes and cell bodies of CCR1-immunoreactive cells, during times of neuronal and glial maturation (second and third postnatal weeks). These findings provide support for the hypothesis that CCR1 plays a role in postnatal cerebellar development.