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Anorexia nervosa (AN) is an eating disorder that is mediated by psychological and metabolic factors, yet it is unclear how these factors interact. The NAMA trial objective is to clarify the metabo-psychiatric interaction and identify how it affects AN-patients' behavior. This randomized trial will recruit 36 treatment-naïve female AN patients, 13-18 years of age, and 36 matched healthy controls. Participants will undergo psychiatric assessments followed by 12-hour overnight fasting. In the next morning baseline assessments of outcomes will be performed. Patients will be randomly allocated 1:1 to receive a mixture with calories or receive a mixture without calories. Healthy controls will also be allocated to receive mixtures with/without calories. Mixtures will be standardized for taste and appearance and allocation will be masked. Primary outcome measure is resting-state functional magnetic resonance brain imaging 60 minutes post-consumption of the mixture. Secondary outcomes include: 1. Blood samples to study markers reflecting metabolic states, hunger/satiety, and stress responses, 2. psychometric evaluations of subjective experiences, and 3. In a second meal 3 hours later, effects of previous calorie intake on subsequent food consumption will be assessed. This article describes the study protocol, including analysis plan, for a randomized controlled trial to comprehensively evaluate the effects of calorie intake in AN. The trial will distinguish psychological and metabolic neuronal networks associated with food intake and uncover how their integration affects food intake and other hallmark symptoms in AN. The aim is to accelerate treatment development by identifying brain mechanisms that drive AN. Clinicaltrials.gov identifier: NCT06814002.

Brain development and neural circuit function depend on the formation and termination of excitatory synapses. The regulation of excitatory synapse plasticity has long been associated with neuronal activity. In addition to neuronal activity, emerging data show that body metabolism affects synaptic plasticity. However, it is unclear how neuronal activity and metabolic signaling may interact to control the number and function of excitatory synapses. The nutrient sensor O-GlcNAc transferase (OGT), an enzyme that catalyzes O-GlcNAcylation of cytoplasmic and nuclear proteins depending on the metabolic state of the body, has been implicated in excitatory synapse maturation, but its activity-dependent roles and underlying mechanisms are unclear. Here, we investigated how OGT regulates excitatory synapse structure, number and AMPA-type glutamate receptors (AMPARs) in cultured hippocampal neurons under normal and activity-suppressed conditions. We show that OGT overexpression selectively enhances accumulation of the AMPARs subunit GluA1 in dendritic spines at a mature developmental stage (DIV14), but not during early development (DIV7). Chronic suppression of neuronal activity with tetrodotoxin (TTX) abolished the OGT-dependent increase in GluA1 expression, indicating that OGT-mediated regulation of AMPARs is activity-dependent. In parallel, OGT overexpression promoted coordinated growth and maturation of excitatory synapses, increasing the size and intensity of postsynaptic PSD-95 and presynaptic vGluT1 puncta, particularly at colocalized synaptic sites. These structural effects, as well as OGT-induced increases in excitatory synapse number, were eliminated by activity blockade. Together, our findings identify the nutrient sensor OGT as an activity-dependent regulator of excitatory synapse maturation and AMPARs accumulation, revealing a molecular mechanism by which neuronal activity and metabolic signaling can be integrated to shape synaptic connectivity and function.

Satiation is essential for energy homeostasis and is dysregulated in metabolic disorders like obesity and eating disorders such as anorexia nervosa. While satiation engages a large neural network across brain regions, how the communication within this network depends on metabolic fluctuations is unclear. This study shows that nutrient access can affect neuron-to-neuron communication in this network by regulating excitatory synaptic plasticity through O-GlcNAc transferase (OGT) in αCaMKII satiation neurons in the paraventricular nucleus (PVN). Using cell-specific knockout mice and electrophysiological recordings, we demonstrate that OGT deletion in PVNαCaMKII neurons increases input resistance and neuronal excitability while preserving basic membrane electrical properties. Strikingly, feeding triggered a robust 3.8-fold increase in excitatory synaptic input in wild-type neurons, whereas OGT-knockout neurons failed to exhibit this feeding-induced synaptic activation and instead displayed a paradoxical trend towards decreased synaptic activity upon food intake. Furthermore, OGT deletion destabilized glucose-dependent synaptic responses, with knockout neurons displaying maladaptive depression of excitatory transmission in conditions where stability is normally preserved. These findings establish OGT as a nutrient-sensitive modulator of synaptic plasticity that ensures appropriate satiation signaling by coupling metabolic state to synaptic plasticity.

Dendritic spines are essential for synaptic transmission, neural circuit organization, and cognitive function. Their morphology and density influence synaptic plasticity, learning, and memory. Many proteins in dendritic spines are modified with O-GlcNAc, a monosaccharide that can be attached and removed from serines and threonines. O-GlcNAc has been implicated in multiple brain disorders, yet the role of O-GlcNAcase (OGA), the enzyme that removes O-GlcNAc modification from proteins, in dendritic spine regulation remains unclear. This study examines the role of OGA in spine and synapse morphogenesis. Immunohistochemical and biochemical analyses reveal that OGA is present in dendritic spines. Functional assays show that OGA promotes spine maturation, increases spine density, and alters synapse size. Additionally, OGA modulates the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), down-regulating GluA2-containing receptors in developing and mature neurons. These findings highlight OGA as a key regulator of excitatory synaptic remodeling and a therapeutic target for synapse-related pathologies such as Alzheimer's disease and autism.

O-GlcNAc is the attachment of β-N-acetylglucosamine to the hydroxyl group of serine and threonine in nuclear and cytoplasmic proteins. It is generally not further elongated but exists as a monosaccharide that can be rapidly added or removed. Thousands of proteins involved in gene transcription, protein translation and degradation as well as the regulation of signal transduction contain O-GlcNAc. Brain is one of the tissues where O-GlcNAc is the most highly expressed and deletion of neuronal O-GlcNAc leads to death early in development. O-GlcNAc is also important for normal adult brain function, where dynamic processes like learning and memory at least in part depend on the modification of specific proteins by O-GlcNAc. Conversely, too much or too little O-GlcNAc in the brain contributes to several disorders including obesity, intellectual disability and Alzheimer's disease. In this chapter, we describe the expression and regulation of O-GlcNAc in the nervous system.

Caloric need has long been thought a major driver of appetite. However, it is unclear whether caloric need regulates appetite in environments offered by many societies today where there is no shortage of food. Here we observed that wildtype mice with free access to food did not match calorie intake to calorie expenditure. While the size of a meal affected subsequent intake, there was no compensation for earlier under- or over-consumption. To test how spontaneous eating is subject to caloric control, we manipulated O-linked β-N-acetylglucosamine (O-GlcNAc), an energy signal inside cells dependent on nutrient access and metabolic hormones. Genetic and pharmacological manipulation in mice increasing or decreasing O-GlcNAcylation regulated daily intake by controlling meal size. Meal size was affected at least in part due to faster eating speed. Without affecting meal frequency, O-GlcNAc disrupted the effect of caloric consumption on future intake. Across days, energy balance was improved upon increased O-GlcNAc levels and impaired upon removal of O-GlcNAcylation. Rather than affecting a perceived need for calories, O-GlcNAc regulates how a meal affects future intake, suggesting that O-GlcNAc mediates a caloric memory and subsequently energy balance.

Hundreds of proteins in the nervous system are modified by the monosaccharide O-GlcNAc. A single protein is often O-GlcNAcylated on several amino acids and the modification of a single site can play a crucial role for the function of the protein. Despite its complexity, only two enzymes add and remove O-GlcNAc from proteins, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). Global and local regulation of these enzymes make it possible for O-GlcNAc to coordinate multiple cellular functions at the same time as regulating specific pathways independently from each other. If O-GlcNAcylation is disrupted, metabolic disorder or intellectual disability may ensue, depending on what neurons are affected. O-GlcNAc's promise as a clinical target for developing drugs against neurodegenerative diseases has been recognized for many years. Recent literature puts O-GlcNAc in the forefront among mechanisms that can help us better understand how neuronal circuits integrate diverse incoming stimuli such as fluctuations in nutrient supply, metabolic hormones, neuronal activity and cellular stress. Here the functions of O-GlcNAc in the nervous system are reviewed.

Experience-driven synaptic plasticity is believed to underlie adaptive behavior by rearranging the way neuronal circuits process information. We have previously discovered that O-GlcNAc transferase (OGT), an enzyme that modifies protein function by attaching β-N-acetylglucosamine (GlcNAc) to serine and threonine residues of intracellular proteins (O-GlcNAc), regulates food intake by modulating excitatory synaptic function in neurons in the hypothalamus. However, how OGT regulates excitatory synapse function is largely unknown. Here we demonstrate that OGT is enriched in the postsynaptic density of excitatory synapses. In the postsynaptic density, O-GlcNAcylation on multiple proteins increased upon neuronal stimulation. Knockout of the OGT gene decreased the synaptic expression of the AMPA receptor GluA2 and GluA3 subunits, but not the GluA1 subunit. The number of opposed excitatory presynaptic terminals was sharply reduced upon postsynaptic knockout of OGT. There were also fewer and less mature dendritic spines on OGT knockout neurons. These data identify OGT as a molecular mechanism that regulates synapse maturity.

O-GlcNAcylation, a dynamic nutrient and stress sensitive post-translational modification, occurs on myriad proteins in the cell nucleus, cytoplasm and mitochondria. O-GlcNAcylation serves as a nutrient sensor to regulate signaling, transcription, translation, cell division, metabolism, and stress sensitivity in all cells. Aberrant protein O-GlcNAcylation plays a critical role both in the development, as well as in the progression of a variety of age related diseases. O-GlcNAcylation underlies the etiology of diabetes, and changes in specific protein O-GlcNAc levels and sites are responsible for insulin expression and sensitivity and glucose toxicity. Abnormal O-GlcNAcylation contributes directly to diabetes related dysfunction of the heart, kidney and eyes and affects progression of cardiomyopathy, nephropathy and retinopathy. O-GlcNAcylation is a critical modification in the brain and plays a role in both plaque and tangle formation, thus making its study important in neurodegenerative disorders. O-GlcNAcylation also affects cellular growth and metabolism during the development and metastasis of cancer. Finally, alterations in O-GlcNAcylation of transcription factors in macrophages and lymphocytes affect inflammation and cytokine production. Thus, O-GlcNAcylation plays key roles in many of the major diseases associated with aging. Elucidation of its specific functions in both normal and diseased tissues is likely to uncover totally novel avenues for therapeutic intervention.

Maintaining energy homeostasis is crucial for the survival and health of organisms. The brain regulates feeding by responding to dietary factors and metabolic signals from peripheral organs. It is unclear how the brain interprets these signals. O-GlcNAc transferase (OGT) catalyzes the posttranslational modification of proteins by O-GlcNAc and is regulated by nutrient access. Here, we show that acute deletion of OGT from αCaMKII-positive neurons in adult mice caused obesity from overeating. The hyperphagia derived from the paraventricular nucleus (PVN) of the hypothalamus, where loss of OGT was associated with impaired satiety. These results identify O-GlcNAcylation in αCaMKII neurons of the PVN as an important molecular mechanism that regulates feeding behavior.