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 important 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.
Psychiatric diseases feature substantial abnormalities of sensory perception and expression of realities and behavior. They are common, chronic, and disabling, with a prevalence of 1–3 percent in the general population. The neuropathology of these diseases is poorly defined, and the etiology is largely unknown. Recent studies have identified many risk genes associated with schizophrenia and bipolar disorder. The disrupted in schizophrenia 1 (DISC1) gene was identified as a chromosome (1;11) translocation breakpoint in a large Scottish pedigree, with the translocation allele closely segregating with the manifestation of schizophrenia, bipolar disorder, and recurrent major depression. DISC1 encodes a large scaffolding protein that interacts with a number of signaling molecules, including PDE4B, Ndel1/Lis1, and ATF4/5. The translocation event is speculated to result in either the expression of a truncated protein product or haploinsufficiency of DISC1. DISC1 is highly expressed during neural development, and much of the work to date has focused on the role of DISC1 in postmitotic neurons. DISC1 has been shown to be required for different aspects of neural development. In the embryonic brain, DISC1 regulates the migration and differentiation of cortical projection neurons, while in the adult, knockdown of DISC1 expression causes accelerated migration and maturation of adult-born neurons.
We recently reported that DISC1 also regulates the proliferation of neural progenitor cells in the embryonic and adult brain. The use of RNA interference (RNAi) to decrease DISC1 expression led to an inhibition of proliferation, premature cell cycle exit, and premature differentiation into neurons. We determined that DISC1 regulates these processes by functioning as a positive regulator of the canonical Wnt signaling pathway. Specifically, we discovered that DISC1 directly binds and inhibits the function of GSK3β, which leads to an increase in cellular β-catenin levels and Wnt-dependent gene transcription. Furthermore, the increase in GSK3β activity due to DISC1 loss of function could be rescued either by expression of stabilized β-catenin or by administration of a selective GSK3 chemical inhibitor. In adult mice, inhibition of DISC1 using RNAi in the dentate gyrus not only impaired neurogenesis but also caused behavioral abnormalities, including hyperlocomotion and increased immobility (depression-like behavior) in the forced swim test. Remarkably, treatment with a GSK3 chemical inhibitor normalized these behaviors. These results suggest that the cellular and behavioral phenotypes due to DISC1 loss of function can be alleviated by hyperactivating the Wnt pathway.
Recently, a large body of genetic evidence provides association of reduced neurogenesis and brain size with mental illnesses, including schizophrenia. The role of DISC1 in controlling the outputs of Wnt signaling and neurogenesis hints at the possibility of a more general involvement of Wnt signaling components in psychiatric disorders. Lithium chloride, the most commonly prescribed medication for bipolar disorder, is known to directly and indirectly inhibit GSK3. We are collaborating with the human genetics and chemical biology groups at the Stanley Center for Psychiatric Research to elucidate the potential role of dysregulated Wnt signaling in the etiology of psychiatric disorders.
Alzheimer's Disease
Alzheimer's disease 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 showed that hyperactivation of Cdk5 occurs when its regulatory subunit p35 is cleaved by the Ca2+-activated protease calpain in neurotoxic conditions to liberate the carboxyl-terminal fragment p25. We hypothesized that p25 generation and accumulation plays an important role in Alzheimer's-like neurodegeneration. Several lines of evidence support this hypothesis. First, p35 cleavage and p25 generation is induced by known risk factors of Alzheimer's, including excitotoxicity, oxidative stress, genotoxic agents, and excessive amounts of β-amyloid peptides. Furthermore, an inducible mouse model for p25 accumulation gives rise to the key pathological hallmarks of Alzheimer's disease, 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 p25 mouse model, indicating that this event operates synergistically with p25 accumulation that leads to the manifestation of neurodegeneration and memory impairment. Therefore, the p25 mouse is the only model whereby expression of a single transgene is sufficient for the development of all the hallmark lesions of Alzheimer's.
The robust neurodegeneration and cognitive phenotypes exhibited by the p25 mice render the model ideal for exploring novel approaches for amelioration of memory impairment. We recently showed that treating the p25 mice with chemical HDAC (histone deacetylase) inhibitors induced robust synaptogenesis and dendritic growth, restored learning, and recovered long-term memory—even after massive neuronal loss had occurred. 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.
The mammalian genome encodes 11 zinc-dependent HDAC isoforms. The HDAC inhibitors used in the previous studies are nonselective and target multiple HDAC isoforms. An important question to be addressed was whether any of the 11 HDACs function in the brain and regulate synaptic plasticity and memory. We used a combination of mouse genetic and chemical approaches to identify HDAC2 as a potent regulator of memory formation and synaptic plasticity. We showed that mice overexpressing HDAC2 in neurons had impaired memory formation in a number of long-term memory paradigms, whereas HDAC2-deficient mice exhibited facilitated learning and memory. Furthermore, HDAC2-deficient mice showed significantly greater spine density and enhanced long-term potentiation (LTP) in the hippocampus, although the opposite was observed in the mice overexpressing HDAC2. Treatment of a nonselective HDAC inhibitor, SAHA, completely ameliorated learning impairment and restored synapse number in mice overexpressing HDAC2. Conversely, HDAC2-deficient mice did 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 showed that HDAC2 associates with promoters of a number of activity-regulated, synapse formation, and synaptic plasticity-related genes. The expression of these genes was up-regulated 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.
