 |
Mechanisms Underlying Development and Degeneration of the Nervous System
Summary: Li-Huei Tsai is interested in the mechanisms that underlie the production and positioning of neurons during brain development, signaling at synapses, and the demise of the nervous system in adult life.
Cyclin-dependent kinase 5 (Cdk5) was first isolated based on its homology to the mitotically active cyclin-dependent kinases (Cdc2/Cdk1). The principal associated kinase activity of Cdk5 is detected in the central nervous system, where the protein is enriched in postmitotic neurons. Monomeric forms of Cdks do not display any kinase activity; instead, association with an activator is required. Cdk5 activity is achieved through association with one of two activators, p35 or p39, two neuron-specific proteins sharing structural homology. Mice deficient in Cdk5 exhibit abnormalities in neuronal positioning and in axon and dendrite development, and they are not viable after birth. These mice show inverted laminar organization of pyramidal neurons in the cerebral cortex reminiscent of that of reeler mice, indicating that Cdk5 plays a role in neuronal migration and positioning. p35-knockout mice display a similar but less severe phenotype and survive into adulthood. Recent evidence indicates that Cdk5 also plays a central role in dendritic spine development, synaptogenesis, and memory formation.
Cdk5 and Neurodegeneration
Although Cdk5 plays an essential role in brain development, aberrant activation of Cdk5 is implicated in age-related neurodegenerative diseases such as Alzheimer's disease (AD). We have previously shown that cleavage of p35 by calpain under neurotoxic conditions results in the formation of p25, which hyperactivates Cdk5. In postmortem AD and ischemic brain samples, p25 is up-regulated.
The Inducible p25 Transgenic Mouse Model
We used a tetracycline-off system to create an inducible transgenic mouse model for p25. Upon doxycycline withdrawal, the CamKII-Tta and TetO-p25 (CK-p25) bitransgenic mice rapidly manifest signs of neurodegeneration, including neuronal loss, brain atrophy, reactive astrogliosis, and tau-associated pathology. Recently, we unexpectedly found that p25/Cdk5 also modulates β-amyloid (Aβ) pathology. Endogenous mouse Aβ peptides are significantly elevated in brains of CK-p25 mice. Intriguingly, elevation of Aβ is observed prior to the detection of any evidence of neuropathology in these mice. A pool of the elevated Aβ accumulates intraneuronally; this is closely associated with neuronal demise, as evidenced by the appearance of pyknotic nuclei in those neurons. Increased Aβ production is mediated in part by an up-regulation of the β-site APP-cleaving enzyme BACE1. These observations provide strong evidence that elevated p25 levels are sufficient to cause AD-like pathology in the mammalian brain.
Evidence for neuronal loss was first detected 6 weeks after the induction of p25. At this age, animals exhibit a profound impairment in learning and memory that is accompanied by synaptic loss and impaired long-term potentiation (LTP). Surprisingly, acute induction of p25 for 2 weeks significantly facilitated learning and enhanced LTP. Furthermore, if p25 is expressed for only 2 weeks, and then turned off for 4 weeks, there is no detectable neurodegeneration but enhanced LTP and learning ability is maintained. These results suggest a role for p25 in synaptic plasticity, learning, and memory, and provide a model where deregulation of a plasticity factor contributes to neurodegeneration.
Sirt1 Gain-of-Function Protects p25-Mediated Neurodegeneration
Given the severe and rapid onset of neurodegeneration and learning impairment exhibited by the CK-p25 mice, these animals appear to be an ideal tool for exploring therapeutic approaches that either ameliorate neurodegeneration or improve learning and memory. The Sir2 gene promotes longevity in a variety of organisms and may underlie the health benefits of caloric restriction, a diet that delays aging and neurodegeneration in mammals. Recently, we found that up-regulation of the activity of a mammalian Sir2 ortholog, Sirt1, can ameliorate p25/Cdk5-mediated neurodegeneration. Resveratrol, a Sirt1-activating molecule, promoted neuronal survival in a primary culture model of neurotoxicity including oxidative stress and p25-induced toxicity. In CK-p25 Tg mice, resveratrol reduced neurodegeneration in the hippocampus, prevented learning impairment, and decreased the acetylation of known Sirt1 substrates PGC-1α (PPARγ coactivator-1α) and p53. In addition, administration of a recombinant lentivirus expressing Sirt1 in the hippocampus of CK-p25 Tg mice conferred significant protection against neurodegeneration. Thus, Sirt1 is a unique molecular link between aging and human neurodegenerative disorders and a promising avenue for therapeutic intervention.
Recovery of Learning and Memory in CK-p25 Mice Is Associated with Chromatin Remodeling
To further explore new therapeutic strategies to improve learning and memory, we hypothesized that a potential mechanism to reinstate learning and memory in a degenerated brain would be to up-regulate the plasticity and function of the remaining neurons. Environmental enrichment (EE) was previously shown to facilitate memory formation and brain plasticity in wild-type rodents. To determine the effect of EE on learning behavior after neuronal loss had already occurred, we induced p25 in 11-month-old CK-p25 Tg mice for 6 weeks, followed by EE for 4 weeks. Despite a comparable extent of brain atrophy, EE-treated CK-p25 Tg mice showed marked increased associative and spatial learning when compared to the nonenriched CK-p25 Tg mice. Enhanced learning behavior was associated with increased dendritic branching and synapse number.
Although these results suggested that learning behavior could be improved by EE, we further investigated whether EE is beneficial for retrieving long-term memories. We applied a single contextual fear-conditioning training, which was previously shown to result in a stable long-term memory to 11-month-old, uninduced CK-p25 Tg mice. The mice were kept in the home cage for 4 weeks to allow the consolidation of hippocampus-independent long-term memories. Afterward, p25 expression was induced for either 3 or 6 weeks before the mice were subjected to the memory test. Mice with 3 weeks of p25 expression did not show overt pathology and displayed similar levels of freezing behavior indicative of learned fear, when compared to the control uninduced mice, demonstrating the retrieval of consolidated long-term memories in these mice. Conversely, little freezing was observed in 6-week-induced CK-p25 Tg mice. This suggests that the access to long-term memories has been lost. Remarkably, after 6 weeks of p25 induction, if the CK-p25 mice were kept in the enriched environment for 4 weeks, followed by memory test, freezing behavior was markedly increased, whereas mice kept in the home cage for 4 weeks did not show improved freezing behavior. These results suggest that EE facilitates the reaccess to long-term memories, presumably by reestablishing the synaptic network.
Histone acetylation, which has been implicated in transcriptional regulation via chromatin remodeling, has been shown to play a role in synaptic plasticity and learning behavior. We found that EE rapidly induces hippocampal and cortical acetylation of histones 3 and 4. The use of chemical inhibitors to inhibit histone deacetylases (HDACs) was previously shown to facilitate memory formation. Thus, we asked whether increased histone acetylation contributes to the beneficial effects of EE in learning behavior of CK-p25 mice. After 6 weeks of p25 induction in the 11-month-old CK-p25 mice, the general HDAC inhibitor sodium butyrate (SB) was administered daily for 4 weeks prior to contextual fear-conditioning training and memory test. SB elicited beneficial effects similar to those of EE in enhancing learning behavior. Moreover, similar to EE, SB treatment also facilitated the reaccess of long-term memories after severe neuronal loss had occurred. These results suggest that transcriptional activation of genes involved in synaptogenesis and synaptic plasticity mediated by increased histone acetylation and chromatin remodeling may contribute to the beneficial effects of EE in CK-p25 mice. Future work will aim to determine the specific HDACs that regulate distinct forms of synaptic plasticity, learning, and memory. This information may help to develop small molecules targeting specific HDACs that can be used in human trials.
Grants from the National Institutes of Health, the Riken Brain Scientific Institute, and the Stanley Center for Psychiatric Research provided partial support for these projects.
Last updated: June 13, 2008
|
|
|
|
