In the postnatal forebrain, the extracellular matrix protein reelin is expressed and secreted by subsets of GABAergic neurons, whereas in the cerebellum reelin is detected in glutamatergic cells of the granule cell layer. Thus, various regions of the postnatal brain present different patterns of reelin expression, whose significance remains unknown. We combined immunocytochemical and pharmacological approaches to characterize the phenotypic and temporal profiles of reelin expression in dissociated cultures of cerebellar granule neurons. A single type of reelin immunoreactivity, identified by a punctate labelling, was present in the somata of the majority of neurons. This immunoreactivity was observed throughout maturation and was exclusively present in glutamatergic neurons expressing the vesicular glutamate transporter 1. Neurons containing the reelin receptors apolipoprotein E receptor 2 (Apoer2) and very low-density lipoprotein receptor (Vldlr) represented about 80% of cerebellar neurons. The vast majority of reelin-positive neurons coexpressed Apoer2, suggesting that reelin immunoreactivity resulted in part from receptor-bound reelin. Inhibition of protein synthesis with cycloheximide completely abolished reelin immunoreactivity. In contrast, blocking protein secretion with brefeldin A did not affect the proportion of punctate neurons but revealed a subpopulation of neurons characterized by a solid reelin staining. These data show for the first time that a homogeneous population of glutamatergic neurons can synthesize and secrete reelin in cerebellar granule cells in vitro.

Low-density-lipoprotein receptors (LDLRs) are an evolutionarily ancient surface protein family with the ability to activate a diversity of extracellular signals across the cellular membrane in the adult central nervous system (CNS). Their intimate roles in modulating synaptic plasticity and their necessity in hippocampal-dependent learning and memory have only recently come to light. Two known LDLR ligands, specifically apolipoprotein E (apoE) and reelin, have been the most widely investigated in this regard. Most of our understanding of synaptic plasticity comes from investigation of both pre- and postsynaptic alterations. Therefore, it is interesting to note that neurons and glia that do not contribute to the synaptic junction in question can secrete signaling molecules that affect synaptic plasticity. Notably, reelin and apoE have been shown to modulate hippocampal long-term potentiation in general, and affect NMDA receptor and AMPA receptor regulation specifically. Furthermore, these receptors and signaling molecules have significant roles in neuronal degenerative diseases such as Alzheimer's disease. The recent production of recombinant proteins, knockout and transgenic mice for receptors and ligands and the development of human ApoE targeted replacement mice have significantly expanded our understanding of the roles LDLRs and their ligands have in certain disease states and the accompanying initiation of specific signaling pathways. This review describes the role LDLRs, apoE and reelin have in the regulation of hippocampal synaptic plasticity.