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Mechanisms Underlying Memory Disorders
Summary: Li-Huei Tsai is interested in elucidating the pathogenic mechanisms underlying neurological disorders affecting cognition. The major research areas in her laboratory include neurodegenerative disease and psychiatric disorders.
Neurodegenerative Disease
The role of p25/Cdk5 in neurodegeneration. Alzheimer's disease (AD) is a devastating and irreversible brain disorder that eventually leads to dementia. Cyclin-dependent kinase 5 (Cdk5) is a brain-specific protein serine/threonine kinase essential for brain development, synaptic plasticity, learning, and memory. We have shown that the hyperactivation of Cdk5 occurs when its regulatory protein p35 is cleaved by the Ca2+-activated protease calpain, under neurotoxic conditions, to liberate the carboxyl-terminal fragment p25. We hypothesized that p25 generation and accumulation play important roles in AD-like neurodegeneration. Several lines of evidence support this hypothesis.
First, p35 cleavage and p25 generation are induced by known risk factors of AD, including excitotoxicity, oxidative stress, genotoxic agents, and excessive amounts of β-amyloid peptides. Furthermore, an inducible mouse model for p25 accumulation (CK-p25 mouse) displays key pathological hallmarks of AD, including profound neuronal loss in the forebrain, increased β-amyloid peptide production, tau pathology, and severe cognitive impairment. In this model, increased β-amyloid peptide levels are observed prior to neuronal loss; furthermore, reducing β-amyloid peptide production ameliorates neurodegeneration in the CK-p25 mouse model, indicating that this event operates synergistically with p25, leading to the manifestation of neurodegeneration and memory impairment. Therefore, the CK-p25 mouse is the only model whereby expression of a single transgene is sufficient for the development of all the hallmark lesions of AD.
Chromatin remodeling and learning and memory. Among all the available AD mouse models, the CK-p25 mouse uniquely exhibits the profound neuronal loss observed in more advanced stages of AD. Thus, we used this model to explore novel therapeutic approaches that may be beneficial to cognition even after profound synaptic loss and neuronal death. We show that treating CK-p25 mice with chemical histone deacetylase (HDAC) inhibitors induces robust synaptogenesis and dendritic growth, restores learning, and recovers long-term memory—even after massive neuronal loss has occurred. These findings demonstrate that an epigenetic mechanism involving increased histone acetylation and chromatin remodeling can be beneficial for learning and memory, even after prominent neuronal loss and neurodegeneration. These observations suggest that memory is not completely erased after neurodegeneration and provide compelling evidence for developing HDAC inhibitors to reverse late-stage Alzheimer's, where patients commonly exhibit dementia.
We used a combination of mouse genetic and chemical approaches to identify HDAC2 as a potent regulator of memory formation and synaptic plasticity. Mice overexpressing HDAC2 in neurons have impaired memory formation in a number of long-term memory paradigms, whereas HDAC2-deficient mice exhibit facilitated learning and memory. Furthermore, HDAC2-deficient mice show a significantly greater spine density and enhanced long-term potentiation (LTP) in the hippocampus, although the opposite is observed in the mice overexpressing HDAC2. Treatment with a nonselective HDAC inhibitor, SAHA, completely ameliorates learning impairments and restores synapse number in mice overexpressing HDAC2. Conversely, HDAC2-deficient mice do not show further improvement of either memory formation or synapse number upon SAHA treatment. These observations suggest that HDAC2 is a major target for the beneficial effects of chemical HDAC inhibitors on learning and memory.
We postulated that HDAC2 exerts its effect on learning and memory by repressing gene expression via chromatin remodeling. More specifically, we speculated that HDAC2 targets memory-associated genes and represses their expression by binding to their regulatory elements. This notion is supported by chromatin immunoprecipitation experiments, which show that HDAC2 associates with the promoters of a number of activity-regulated, synapse formation, and synaptic plasticity-related genes. The expression of these genes is upregulated in HDAC2-deficient mice. These results indicate that HDAC2 negatively regulates learning and memory and that selective HDAC2 inhibitors are desirable for treating human neurological disorders associated with cognitive impairments.
Loss of genome integrity in neurodegeneration; the interaction of p25/Cdk5 and HDAC1. We performed gene expression profiling in the brains of CK-p25 mice soon after the induction of p25 expression and prior to the manifestation of symptoms and pathology, with the hope of identifying early events induced by p25/Cdk5 that lead to neuronal demise. Unexpectedly, gene products implicated in the DNA damage response pathway and the cell division cycle were markedly upregulated following acute p25 induction. Immunohistochemistry in CK-p25 mice following acute p25 induction revealed extensive DNA double-strand break (DSB) damage in p25-expressing neurons of the hippocampus. Neurons suffering DSBs also express ectopic cell cycle markers. Although ectopic cell cycle reentry was previously reported in postmortem AD brain and in mouse models of AD, this is the first evidence for DSBs in neurodegeneration.
In examining the mechanism underlying p25-induced DNA damage, we showed that histone deacetylase 1 (HDAC1) activity is downregulated in the CK-p25 mouse. In cultured primary neurons, inactivation of HDAC1 results in DSBs, aberrant cell cycle protein expression, and neuronal death. Restoring HDAC1 activity by overexpressing wild-type HDAC1 rescued neurons from DSBs and cell death. These results indicate that certain chromatin enzymes, such as HDAC1, function to promote neuronal survival. Recent findings indicate that HDAC1 is recruited to sites of DSBs in neurons (see video). We are elucidating the mechanisms by which HDAC1 corroborates with other nuclear proteins to protect cells from DNA damage.
SIRT1 in neuroprotection and cognition. The NAD+-dependent deacetylase SIRT1 is involved in a variety of complex processes relevant to aging, including the regulation of oxidative stress, metabolism, and circadian rhythms, as well as in molecular pathways regulated by cocaine. We previously reported that SIRT1 is upregulated in mouse models of AD, in amyotrophic lateral sclerosis (ALS), and in primary neurons challenged with neurotoxic insults. In the CK-p25 mouse, resveratrol, a SIRT1 chemical activator, reduced neurodegeneration in the hippocampus, prevented learning impairment, and decreased the acetylation of the known SIRT1 substrates PGC-1α and p53. Furthermore, injection of SIRT1 lentivirus in the hippocampus of p25 transgenic mice conferred significant protection against neurodegeneration. Thus, SIRT1 constitutes an important molecular link between aging and human neurodegenerative disorders and provides a promising avenue for therapeutic intervention.
We recently reported that SIRT1 modulates synaptic plasticity and memory formation via a novel microRNA-mediated mechanism. Activation of SIRT1 enhances, whereas its loss of function impairs, synaptic plasticity. Surprisingly, these effects were mediated via post-transcriptional regulation of cAMP response element–binding protein (CREB) expression by a brain-specific microRNA, miR-134. SIRT1 normally functions to limit expression of miR-134 via a repressor complex containing the transcription factor YY1. Unchecked miR-134 expression following SIRT1 deficiency results in the downregulated expression of CREB and brain-derived neurotrophic factor (BDNF), thereby impairing synaptic plasticity. These findings demonstrate a new role for SIRT1 in cognition and a previously unknown microRNA-based mechanism by which SIRT1 regulates these processes. Furthermore, these results describe a separate branch of SIRT1 signaling, in which SIRT1 has a direct role in regulating normal brain function in a manner that is disparate from its cell survival functions.
The use of optogenetics in the study of brain function. To better understand AD, we have aspired to alter the activity of specific neuronal circuits and evaluate the consequences on pathology, network activity, and behavior. The use of optogenetics allows for the manipulation of specific populations of neurons. In collaboration with Christopher Moore (Massachusetts Institute of Technology) and Karl Deisseroth (HHMI, Stanford University), we assessed the role of the fast-spiking parvalbumin-positive (PV+) interneurons in network oscillations and coordinating the activity of large neuronal ensembles in the mouse sensory cortex. Cortical gamma oscillations (20–80 Hz) predict increases in focused attention. Failure in gamma regulation is a hallmark of neurological and psychiatric disease. We targeted channelrhodopsin (ChR2) specifically to PV+ cortical interneurons and found that light-driven activation of PV+ interneurons selectively amplifies gamma oscillations. These data support the hypothesis that gamma oscillations are generated by synchronous activity of fast-spiking inhibitory interneurons, with the resulting rhythmic inhibition producing neural ensemble synchrony by generating a narrow window for effective excitation. This initial collaborative effort encouraged us to take advantage of this technology to probe potential network dysfunction associated with neurodegeneration and other brain disorders, and to elucidate the contribution of various brain circuits in the early stage of pathogenesis in AD.
Psychiatric Disorders
The integrity of the structure and function of the central nervous system relies on the production of the correct number of neurons and their correct positioning throughout the mammalian brain. Our prior work demonstrated that alterations in the cell division plane of neural progenitors could markedly impact the size of the progenitor pool and the final number of neurons produced. We have also identified molecules that play essential roles in the migration and positioning of postmitotic neurons in the developing cerebral cortex. Abnormal architecture of the brain is implicated in other neurological disorders, including autism, epilepsy, and psychiatric diseases such as schizophrenia and bipolar disorder.
DISC1. We previously demonstrated that the disrupted in schizophrenia 1 (DISC1) protein, the product of a gene whose translocation strongly increases the risk for mental illnesses in a large Scottish pedigree, regulates neural progenitor proliferation by directly binding to and inhibiting GSK3β to modulate canonical Wnt signaling. We show that DISC1 binds and inhibits GSK3, which in turn maintains the stability of -catenin and -catenin–mediated signaling events. This is an exciting finding, as the most common medication for bipolar disease, lithium, is a known inhibitor of GSK3. Thus, DISC1 functions as an endogenous GSK3 inhibitor to maintain Wnt signaling. This work provided a framework for understanding how alterations in the GSK3/-catenin pathway may contribute to the etiology of psychiatric disorders.
Dixdc1. Our work on the role of DISC1 and Wnt signaling regulation in neural progenitor proliferation and psychiatric disease led us to the characterization of the DISC1-interacting protein Dixdc1. This work revealed that Dixdc1 interacts with DISC1 to regulate neural progenitor proliferation, but not migration, by comodulating Wnt-GSK3/-catenin signaling. We also showed that the phosphorylation of Dixdc1 by Cdk5 facilitates its interaction with the DISC1-binding partner Ndel1 and is necessary for normal neuronal migration. These results further delineate the mechanisms by which DISC1 regulates multiple processes during brain development and provide insight into how dysregulation of DISC1 may contribute to psychiatric disease.
Grants from the National Institutes of Health and the Stanley Foundation provided partial support for these projects.
As of May 30, 2012
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