Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase regulating diverse cellular functions including metabolism, transcription and cell survival. Numerous intracellular signalling pathways converge on GSK-3 and regulate its activity via inhibitory serine-phosphorylation. Recently, GSK-3 has been involved in learning and memory and in neurodegeneration. Here, we present evidence that implicates GSK-3 in synaptic plasticity. We show that phosphorylation at the inhibitory Ser9 site on GSK-3beta is increased upon induction of long-term potentiation (LTP) in both hippocampal subregions CA1 and the dentate gyrus (DG) in vivo. The increase in inhibitory GSK-3beta phosphorylation is robust and persists for at least one hour postinduction. Furthermore, we find that LTP is impaired in transgenic mice conditionally overexpressing GSK-3beta. The LTP deficits can be attenuated/rescued by chronic treatment with lithium, a GSK-3 inhibitor. These results suggest that the inhibition of GSK-3 facilitates the induction of LTP and this might explain some of the negative effects of GSK-3 on learning and memory. It follows that this role of GSK-3beta in LTP might underlie some of the cognitive dysfunction in diseases where GSK-3 dysfunction has been implicated, including Alzheimer's and other dementias.

One of important aspects of development of Alzheimer’s disease is degeneration of septal cholinergic neurons that innervate the hippocampus. We took advantage of widely used model of cholinergic deficit in the hippocampus, intracerebroventricular administration of 192IgG-saporin (Ig-saporin), to analyze the postponed consequences of cholinergic deficit in different parts of the hippocampus. We studied effects of the immunotoxin on the behavior of rats and gene expression in the dorsal and ventral hippocampus using RNA-seq approach. We found that under normal conditions dorsal and ventral parts of the hippocampus differ in the expression of 1129 protein-coding genes and 49 non-coding RNAs (ncRNAs) and do not differ in the expression of 10 microRNAs, which were detected in both parts of the hippocampus. Ig-saporin-induced degeneration of cholinergic septal neurons did not affect rat behavior in open field, T-maze, and passive avoidance task but impaired memory retention in Morris water maze. To analyze 192Ig-saporin-induced changes in the gene expression, we formed the following groups of genes: genes expressed exclusively in certain cell types (neurons, astrocytes, microglia, oligodendrocytes, and vascular cells) and, among universally expressed genes, a group of genes that encode ribosome-forming proteins. For all groups of genes, the alterations in the gene expression produced by the immunotoxin were stronger in the dorsal as compared to the ventral hippocampus. We found that, among groups of universally expressed genes, Ig-saporin increased the expression of ribosome-forming proteins in both dorsal and ventral hippocampus. Ig-saporin also strongly upregulated expression of microglia-specific genes only in the dorsal hippocampus. A subset of affected microglial genes comprised genes associated with inflammation, however, did not include genes related to acute inflammation such as interleukins-1b, -6, -15, and -18 as well as TNF. The expression of other cell-specific genes (genes specific for neurons, astrocytes, oligodendrocytes, and vascular cells) was unaffected. The data obtained suggest that disturbance of memory-associated behavior after administration of Ig-saporin is associated with upregulation of microglia-associated genes in the dorsal but not ventral hippocampus.

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Saporin, which is extracted from Saponaria officinalis, is a protein toxin that inactivates ribosomes. Saporin itself is non-selective toxin but acquires high specificity after conjugation with different ligands such as signaling peptides or antibodies to some surface proteins expressed in a chosen cell subpopulation. The saporin-based conjugated toxins were widely adopted in neuroscience as a convenient tool to induce highly selective degeneration of desired cell subpopulation. Induction of selective cell death is one of approaches used to model neurodegenerative diseases, study functions of certain cell subpopulations in the brain, and therapy. Here, we review studies where saporin-based conjugates were used to analyze cell mechanisms of sleep, general anesthesia, epilepsy, pain, and development of Parkinson’s and Alzheimer’s diseases. Limitations and future perspectives of use of saporin-based toxins in neuroscience are discussed.

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