The Role of Cdk5 in Neurodegeneration and Normal Brain Function
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. We created an inducible mouse model of p25 accumulation (CK-p25 mouse) that 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 memory deficits in the CK-p25 mouse model, indicating that this event operates synergistically with p25, leading to the manifestation of neurodegeneration and memory impairment.
In addition to the harmful effects of p25-mediated Cdk5 activity, emerging evidence shows Cdk5 to be a critical regulation of synaptic homeostasis and plasticity. Work from our lab shows that Cdk5 phosphorylates a number of important presynaptic proteins, such as CASK, Shank3, and the N-type voltage gated calcium channel CaV2.2, to modulate presynaptic function and regulate synaptic transmission. Further work is exploring the role that p25 generation might play in normal brain physiology and synaptic function, by studying knock-in mice in which the p35 protein is resistant to cleavage and p25 production.
Chromatin Remodeling and Learning and Memory
Among 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 cognitive impairment in Alzheimer’s disease.
Specifically, our work has shown that the inhibition of HDAC2 by HDAC inhibitors is beneficial to learning and memory in both normal mice and in mouse models of neurodegeneration. We find that HDAC2 levels are increased in both mouse models of AD as well as in postmortem brain samples from AD patients. More importantly, we show that the normalization of HDAC2 levels in mouse models of AD restores cognitive function, even following severe neurodegeneration. These findings advocate for the development of selective inhibitors of HDAC2 and suggest that cognitive capacities following neurodegeneration are not entirely lost, but merely impaired by this epigenetic blockade. Our current work is directed towards better understanding the HDAC2-corepressor complexes that are relevant to memory and which may be altered in AD. In addition, we wish to determine the changes that occur in the epigenome during neurodegeneration using methods of next-generation sequencing
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). In addition, we showed previously that activation of the NAD+-dependent deacetylase SIRT1 is neuroprotective in the CK-p25 mouse. Exciting new evidence is emerging that suggests that SIRT1 and HDAC1 work together to protect the neuronal genome from DNA damage.
Neuronal Networks, Optogenetics, and Neurodegeneration
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. Current work is examining the effect of the stimulation or inhibition of specific cells within hippocampal, basal forebrain, and amygdalar circuits upon neurodegeneration and cognitive deficits. We continue 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. In addition to the manipulation of specific circuits, we wish to map out which neural circuits are first disrupted by the deposition of amyloid b (Ab) protein in AD, and how Ab pathology propagates throughout the brain.
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. Current work in the lab is elucidating the biological functions for genes that have been identified as either schizophrenia or autism risk genes, or both, in the development of the brain. For example, we have validated several mRNA targets for the microRNA MIR137 gene, which has been associated with a risk for schizophrenia and for autism in genome-wide association studies. Recent work has also focused upon the role of the autism risk gene TAOK2, which we show is critical for the normal development of dendritic arbors, and thus synaptic contacts, in cortical pyramidal neurons.