Postdoctoral Fellow, Friedman Brain Institute, Icahn School of Medicine at Mount Sinai
Epigenetics: key to laying down spatial memories, providing possible new neurological medications
Understanding how memories are made, retrieved, and eventually fade over a lifetime is the stuff of poems and song. To medical researchers, solving the mysteries of memory is even more elusive. Researchers surmise that “laying down” a new memory and storing an old memory both involve making proteins at the space, or synapse, where one neuron meets another. But forming these also requires new gene expression in the cell nucleus, where DNA is stored and genes are “read” to establish cell-specific functions. In my work, I discovered that a key metabolic enzyme works directly within the nucleus of neurons to turn genes on or off when new memories are being established. This enzyme, called acetyl-CoA synthetase 2, or ACSS2, ‘fuels' a whole machinery of gene expression ‘on site’ in the nucleus of nerve cells to turn on key memory genes after learning. This discovery provides a new target for neuropsychiatric disorders, such as anxiety and depression, where neuro-epigenetic mechanisms are known to play key roles. How are memories stored in the brain? Forming memories involves a restructuring of the synapse, which relies on the coordinated expression of a group of memory genes. The addition of a chemical group, a process called acetylation, onto specific spots of the genome in neurons, opens up tightly-wound DNA. This allows genes involved in memory formation to be “read,” and eventually, for their encoded proteins to be made. Such epigenetic mechanisms are becoming better understood as important regulators of the many functions of neurons. And we discovered a new key player: the enzyme ACSS2 binds to memory genes directly to fuel their acetylation, which is ultimately controlling spatial memory in mice. ACSS2 ‘fuels’ the memory machine? We found that if the animals’ ACSS2 expression was blocked, long-term memory of where objects were placed in a study chamber was impaired. These mice did not investigate a moved object on the second day of a two-day trial, while control group mice did. This is because without ACSS2, the mice had no molecular path to engage memory genes to lock in where the objects were placed. In turn, this decrease in ACSS2 in specific brain regions impairs the read-out of key genes that function to form new memories or to update old ones. In the future, we hope to apply this newfound memory path to prevent the “laying-down” of traumatic memories -- or perhaps even erase them -- in people who suffer from post-traumatic stress disorder.
Abstract: Metabolic production of acetyl coenzyme A (acetyl-CoA) is linked to histone acetylation and gene regulation, but the precise mechanisms of this process are largely unknown. Here we show that the metabolic enzyme acetyl-CoA synthetase 2 (ACSS2) directly regulates histone acetylation in neurons and spatial memory in mammals. In a neuronal cell culture model, ACSS2 increases in the nuclei of differentiating neurons and localizes to upregulated neuronal genes near sites of elevated histone acetylation. A decrease in ACSS2 lowers nuclear acetyl-CoA levels, histone acetylation, and responsive expression of the cohort of neuronal genes. In adult mice, attenuation of hippocampal ACSS2 expression impairs long-term spatial memory, a cognitive process that relies on histone acetylation. A decrease in ACSS2 in the hippocampus also leads to defective upregulation of memory-related neuronal genes that are pre-bound by ACSS2. These results reveal a connection between cellular metabolism, gene regulation, and neural plasticity and establish a link between acetyl-CoA generation 'on-site' at chromatin for histone acetylation and the transcription of key neuronal genes.
Pub.: 01 Jun '17, Pinned: 10 Oct '17
Abstract: Understanding the cellular and molecular mechanisms underlying the formation and maintenance of memories is a central goal of the neuroscience community. It is well regarded that an organism's ability to lastingly adapt its behavior in response to a transient environmental stimulus relies on the central nervous system's capability for structural and functional plasticity. This plasticity is dependent on a well-regulated program of neurotransmitter release, post-synaptic receptor activation, intracellular signaling cascades, gene transcription, and subsequent protein synthesis. In the last decade, epigenetic markers like DNA methylation and post-translational modifications of histone tails have emerged as important regulators of the memory process. Their ability to regulate gene transcription dynamically in response to neuronal activation supports the consolidation of long-term memory. Furthermore, the persistent and self-propagating nature of these mechanisms, particularly DNA methylation, suggests a molecular mechanism for memory maintenance. In this review, we will examine the evidence that supports a role of epigenetic mechanisms in learning and memory. In doing so, we hope to emphasize (1) the widespread involvement of these mechanisms across different behavioral paradigms and distinct brain regions, (2) the temporal and genetic specificity of these mechanisms in response to upstream signaling cascades, and (3) the functional outcome these mechanisms may have on structural and functional plasticity. Finally, we consider the future directions of neuroepigenetic research as it relates to neuronal storage of information.
Pub.: 17 Jan '13, Pinned: 10 Oct '17
Abstract: Glioblastomas and brain metastases are highly proliferative brain tumors with short survival times. Previously, using (13)C-NMR analysis of brain tumors resected from patients during infusion of (13)C-glucose, we demonstrated that there is robust oxidation of glucose in the citric acid cycle, yet glucose contributes less than 50% of the carbons to the acetyl-CoA pool. Here, we show that primary and metastatic mouse orthotopic brain tumors have the capacity to oxidize [1,2-(13)C]acetate and can do so while simultaneously oxidizing [1,6-(13)C]glucose. The tumors do not oxidize [U-(13)C]glutamine. In vivo oxidation of [1,2-(13)C]acetate was validated in brain tumor patients and was correlated with expression of acetyl-CoA synthetase enzyme 2, ACSS2. Together, the data demonstrate a strikingly common metabolic phenotype in diverse brain tumors that includes the ability to oxidize acetate in the citric acid cycle. This adaptation may be important for meeting the high biosynthetic and bioenergetic demands of malignant growth.
Pub.: 20 Dec '14, Pinned: 10 Oct '17
Abstract: Overcoming metabolic stress is a critical step in tumor growth. Acetyl coenzyme A (acetyl-CoA) generated from glucose and acetate uptake is important for histone acetylation and gene expression. However, how acetyl-CoA is produced under nutritional stress is unclear. We demonstrate here that glucose deprivation results in AMP-activated protein kinase (AMPK)-mediated acetyl-CoA synthetase 2 (ACSS2) phosphorylation at S659, which exposed the nuclear localization signal of ACSS2 for importin α5 binding and nuclear translocation. In the nucleus, ACSS2 binds to transcription factor EB and translocates to lysosomal and autophagy gene promoter regions, where ACSS2 incorporates acetate generated from histone acetylation turnover to locally produce acetyl-CoA for histone H3 acetylation in these regions and promote lysosomal biogenesis, autophagy, cell survival, and brain tumorigenesis. In addition, ACSS2 S659 phosphorylation positively correlates with AMPK activity in glioma specimens and grades of glioma malignancy. These results underscore the significance of nuclear ACSS2-mediated histone acetylation in maintaining cell homeostasis and tumor development.
Pub.: 30 May '17, Pinned: 10 Oct '17
Abstract: Acetyl-coenzyme A (acetyl-CoA) is a central metabolic intermediate. The abundance of acetyl-CoA in distinct subcellular compartments reflects the general energetic state of the cell. Moreover, acetyl-CoA concentrations influence the activity or specificity of multiple enzymes, either in an allosteric manner or by altering substrate availability. Finally, by influencing the acetylation profile of several proteins, including histones, acetyl-CoA controls key cellular processes, including energy metabolism, mitosis, and autophagy, both directly and via the epigenetic regulation of gene expression. Thus, acetyl-CoA determines the balance between cellular catabolism and anabolism by simultaneously operating as a metabolic intermediate and as a second messenger.
Pub.: 04 Jun '15, Pinned: 10 Oct '17
Abstract: Learning and memory are two of the most magical capabilities of our mind. Learning is the biological process of acquiring new knowledge about the world, and memory is the process of retaining and reconstructing that knowledge over time. Most of our knowledge of the world and most of our skills are not innate but learned. Thus, we are who we are in large part because of what we have learned and what we remember and forget. In this Review, we examine the molecular, cellular, and circuit mechanisms that underlie how memories are made, stored, retrieved, and lost.
Pub.: 01 Apr '14, Pinned: 10 Oct '17
Abstract: Over the past 25 years, the broad field of epigenetics and, over the past decade in particular, the emerging field of neuroepigenetics have begun to have tremendous impact in the areas of learned behavior, neurotoxicology, CNS development, cognition, addiction, and psychopathology. However, epigenetics is such a new field that in most of these areas the impact is more in the category of fascinating implications as opposed to established facts. In this brief commentary, I will attempt to address and delineate some of the open questions and areas of opportunity that discoveries in epigenetics are providing to the discipline of neuroscience.
Pub.: 05 Nov '13, Pinned: 10 Oct '17
Abstract: Long-lasting memories require specific gene expression programmes that are, in part, orchestrated by epigenetic mechanisms. Of the epigenetic modifications identified in cognitive processes, histone acetylation has spurred considerable interest. Whereas increments in histone acetylation have consistently been shown to favour learning and memory, a lack thereof has been causally implicated in cognitive impairments in neurodevelopmental disorders, neurodegeneration and ageing. As histone acetylation and cognitive functions can be pharmacologically restored by histone deacetylase inhibitors, this epigenetic modification might constitute a molecular memory aid on the chromatin and, by extension, a new template for therapeutic interventions against cognitive frailty.
Pub.: 18 Jan '13, Pinned: 10 Oct '17