Development of Mice with Brain-Specific Deletion of Floxed Glud1 (Glutamate Dehydrogenase 1) Using Cre Recombinase Driven by the Nestin Promoter

A tribute to Leif Hertz: The historical context of his pioneering studies of the roles of astrocytes in brain energy metabolism, neurotransmission, cognitive functions, and pharmacology identifies important, unresolved topics for future studies

Leif Hertz, M.D., D.Sc. (honōris causā) (1930-2018), was one of the original and noteworthy participants in the International Conference on Brain Energy Metabolism (ICBEM) series since its inception in 1993. The biennial ICBEM conferences are organized by neuroscientists interested in energetics and metabolism underlying neural functions; they have had a high impact on conceptual and experimental advances in these fields and on promoting collaborative interactions among neuroscientists. Leif made major contributions to ICBEM discussions and understanding of metabolic and signaling characteristics of astrocytes and their roles in brain function. His studies ranged from uptake of K+ from extracellular fluid and its stimulation of astrocytic respiration, identification, and regulation of enzymes specifically or preferentially expressed in astrocytes in the glutamate-glutamine cycle of excitatory neurotransmission, a requirement for astrocytic glycogenolysis for fueling K+ uptake, involvement of glycogen in memory consolidation in the chick, and pharmacology of astrocytes. This tribute to Leif Hertz highlights his major discoveries, the high impact of his work on astrocyte-neuron interactions, and his unparalleled influence on understanding the cellular basis of brain energy metabolism. His work over six decades has helped integrate the roles of astrocytes into neurotransmission where oxidative and glycogenolytic metabolism during neurotransmitter glutamate turnover are key aspects of astrocytic energetics. Leif recognized that brain astrocytic metabolism is greatly underestimated unless the volume fraction of astrocytes is taken into account. Adjustment for pathway rates expressed per gram tissue for volume fraction indicates that astrocytes have much higher oxidative rates than neurons and astrocytic glycogen concentrations and glycogenolytic rates during sensory stimulation in vivo are similar to those in resting and exercising muscle, respectively. These novel insights are typical of Leif's astute contributions to the energy metabolism field, and his publications have identified unresolved topics that provide the neuroscience community with challenges and opportunities for future research.

Dynamin-like Protein 1 (DNML1) as a Molecular Target for Antibody-Based Immunotherapy to Treat Glaucoma

Slow and progressive loss of retinal ganglion cells (RGCs) is the main characteristic of glaucoma, the second leading cause of blindness worldwide. Previous studies have shown that impaired mitochondrial dynamics could facilitate retinal neurodegeneration. Mitochondrial dynamics are regulated directly (fission) or more indirectly (fusion) by dynamin-like protein 1 (DNML1). Therefore, DNM1L might be a promising target for an antibody-based approach to treat glaucoma. The consequences of targeting endogenous DNM1L by antibodies in a glaucoma animal model have not been investigated yet. Here, we show that the intravitreal application of an anti-DNM1L antibody showed protective effects regarding the survival of RGCs and their axons in the retinal nerve fiber layer (RNFL). Antibody treatment also improved retinal functionality, as observed by electroretinography (Ganzfeld ERG). Western blot analysis revealed altered DNM1L phosphorylation and altered expression of proteins related to apoptosis suggesting a decreased apoptosis rate. Mass spectrometry analysis revealed 28 up-regulated and 21 down-regulated proteins (p < 0.05) in both experimental groups. Protein pathway analysis showed that many proteins interacted directly with the target protein DNM1L and could be classified into three main protein clusters: Vesicle traffic-associated (NSF, SNCA, ARF1), mitochondrion-associated (HSP9A, SLC25A5/ANT2, GLUD1) and cytoskeleton-associated (MAP1A) signaling pathway. Our results demonstrate that DNM1L is a promising target for an antibody-based approach to glaucoma therapy.

Ccdc134 deficiency impairs cerebellar development and motor coordination

Coiled-coil domain containing 134 (CCDC134) has been shown to serve as an immune cytokine to exert antitumor effects and to act as a novel regulator of hADA2a to affect PCAF acetyltransferase activity. While Ccdc134 loss causes abnormal brain development in mice, the significance of CCDC134 in neuronal development in vivo is controversial. Here, we report that CCDC134 is highly expressed in Purkinje cells (PCs) at all developmental stages and regulates mammalian cerebellar development in a cell type-specific manner. Selective deletion of Ccdc134 in mouse neural stem cells (NSCs) caused defects in cerebellar morphogenesis, including a decrease in the number of PCs and impairment of PC dendritic growth, as well as abnormal granule cell development. Moreover, loss of Ccdc134 caused progressive motor dysfunction with deficits in motor coordination and motor learning. Finally, Ccdc134 deficiency inhibited Wnt signaling but increased Ataxin1 levels. Our findings provide evidence that CCDC134 plays an important role in cerebellar development, possibly through regulating Wnt signaling and Ataxin1 expression levels, and in controlling cerebellar function for motor coordination and motor learning, ultimately making it a potential contributor to cerebellar pathogenesis.

Glutamate Dehydrogenase Is Important for Ammonia Fixation and Amino Acid Homeostasis in Brain During Hyperammonemia

Impaired liver function may lead to hyperammonemia and risk for hepatic encephalopathy. In brain, detoxification of ammonia is mediated mainly by glutamine synthetase (GS) in astrocytes. This requires a continuous de novo synthesis of glutamate, likely involving the action of both pyruvate carboxylase (PC) and glutamate dehydrogenase (GDH). An increased PC activity upon ammonia exposure and the importance of PC activity for glutamine synthesis has previously been demonstrated while the importance of GDH for generation of glutamate as precursor for glutamine synthesis has received little attention. We therefore investigated the functional importance of GDH for brain metabolism during hyperammonemia. To this end, brain slices were acutely isolated from transgenic CNS-specific GDH null or litter mate control mice and incubated in aCSF containing [U-¹³C]glucose in the absence or presence of 1 or 5 mM ammonia. In another set of experiments, brain slices were incubated in aCSF containing 1 or 5 mM ¹⁵N-labeled NH₄Cl and 5 mM unlabeled glucose. Tissue extracts were analyzed for isotopic labeling in metabolites and for total amounts of amino acids. As a novel finding, we reveal a central importance of GDH function for cerebral ammonia fixation and as a prerequisite for de novo synthesis of glutamate and glutamine during hyperammonemia. Moreover, we demonstrated an important role of the concerted action of GDH and alanine aminotransferase in hyperammonemia; the products alanine and α-ketoglutarate serve as an ammonia sink and as a substrate for ammonia fixation via GDH, respectively. The role of this mechanism in human hyperammonemic states remains to be studied.

AMP-activated protein kinase (AMPK) regulates astrocyte oxidative metabolism by balancing TCA cycle dynamics

AMP-activated protein kinase (AMPK) is an important energy sensor located in cells throughout the human body. From the periphery, AMPK is known to be a metabolic master switch controlling the use of energy fuels. The energy sensor is activated when the energy status of the cell is low, initiating energy-producing pathways and deactivating energy-consuming pathways. All brain cells are crucially dependent on energy production for survival, and the availability of energy substrates must be closely regulated. Intriguingly, the role of AMPK in the regulation of brain cell metabolism has been sparsely investigated, particularly in astrocytes. By investigating metabolism of ¹³ C-labeled energy substrates in acutely isolated hippocampal slices and cultured astrocytes, with subsequent mass spectrometry analysis, we here show that activation of AMPK increases glycolysis as well as the capacity of the TCA cycle, that is, anaplerosis, through the activity of pyruvate carboxylase (PC) in astrocytes. In addition, we demonstrate that AMPK activation leads to augmented astrocytic glutamate oxidation via pyruvate recycling (i.e., cataplerosis). This regulatory mechanism induced by AMPK activation is mediated via glutamate dehydrogenase (GDH) shown in a CNS-specific GDH knockout mouse. Collectively, these findings demonstrate that AMPK regulates TCA cycle dynamics in astrocytes via PC and GDH activity. AMPK functionality has been shown to be hampered in Alzheimer's and Parkinson's disease and our findings may therefore add to the toolbox for discovery of new metabolic drug targets.

Comparative Quantitative Analysis of Porcine Optic Nerve Head and Retina Subproteomes

Optic nerve head (ONH) and retina (RET) are the main sites of damage in neurodegenerative optic neuropathies including glaucoma. Up to date, little is known about the molecular interplay between these two adjoining ocular components in terms of proteomics. To close this gap, we investigated ONH and RET protein extracts derived from porcine eyes (n = 12) (Sus scrofa domestica Linnaeus 1758) using semi-quantitative mass spectrometry (MS)-based proteomics comprising bottom-up LC–ESI MS/MS and targeted SPE-MALDI-TOF MS analysis. In summary, more than 1600 proteins could be identified from the ONH/RET tissue complex. Moreover, ONH and RET displayed tissue-specific characteristics regarding their qualitative and semi-quantitative protein compositions. Gene ontology (GO)-based functional and protein–protein interaction analyses supported a close functional connection between the metabolic-related RET and the structural-associated ONH subproteomes, which could be affected under disease conditions. Inferred from the MS findings, stress-associated proteins including clusterin, ceruloplasmin, and endoplasmin can be proposed as extracellular mediators of the ONH/ RET proteome interface. In conclusion, ONH and RET show obvious proteomic differences reflecting characteristic functional features which have to be considered for future protein biomarker profiling studies.

Glutamate Dehydrogenase-Deficient Mice Display Schizophrenia-Like Behavioral Abnormalities and CA1-Specific Hippocampal Dysfunction

Brain imaging has revealed that the CA1 subregion of the hippocampus is hyperactive in prodromal and diagnosed patients with schizophrenia (SCZ), and that glutamate is a driver of this hyperactivity. Strikingly, mice deficient in the glutamate synthetic enzyme glutaminase have CA1 hypoactivity and a SCZ-resilience profile, implicating glutamate-metabolizing enzymes. To address this further, we examined mice with a brain-wide deficit in the glutamate-metabolizing enzyme glutamate dehydrogenase (GDH), encoded by Glud1, which should lead to glutamate excess due to reduced glutamate metabolism in astrocytes. We found that Glud1-deficient mice have behavioral abnormalities in the 3 SCZ symptom domains, with increased baseline and amphetamine-induced hyperlocomotion as a positive symptom proxy, nest building and social preference as a negative symptom proxy, and reversal/extradimensional set shifting in the water T-maze and contextual fear conditioning as a cognitive symptom proxy. Neuroimaging of cerebral blood volume revealed hippocampal hyperactivity in CA1, which was associated with volume reduction. Parameters of hippocampal synaptic function revealed excess glutamate release and an elevated excitatory/inhibitory balance in CA1. Finally, in a direct clinical correlation using imaging-guided microarray, we found a significant SCZ-associated postmortem reduction in GLUD1 expression in CA1. These findings advance GLUD1 deficiency as a driver of excess hippocampal excitatory transmission and SCZ symptoms, and identify GDH as a target for glutamate modulation pharmacotherapy for SCZ. More broadly, these findings point to the likely involvement of alterations in glutamate metabolism in the pathophysiology of SCZ.

Glutamate dehydrogenase is essential to sustain neuronal oxidative energy metabolism during stimulation

The enzyme glutamate dehydrogenase (GDH; Glud1) catalyzes the (reversible) oxidative deamination of glutamate to α-ketoglutarate accompanied by a reduction of NAD⁺ to NADH. GDH connects amino acid, carbohydrate, neurotransmitter and oxidative energy metabolism. Glutamine is a neurotransmitter precursor used by neurons to sustain the pool of glutamate, but glutamine is also vividly oxidized for support of energy metabolism. This study investigates the role of GDH in neuronal metabolism by employing the Cns- Glud1^-/- mouse, lacking GDH in the brain (GDH KO) and metabolic mapping using ¹³C-labelled glutamine and glucose. We observed a severely reduced oxidative glutamine metabolism during glucose deprivation in synaptosomes and cultured neurons not expressing GDH. In contrast, in the presence of glucose, glutamine metabolism was not affected by the lack of GDH expression. Respiration fuelled by glutamate was significantly lower in brain mitochondria from GDH KO mice and synaptosomes were not able to increase their respiration upon an elevated energy demand. The role of GDH for metabolism of glutamine and the respiratory capacity underscore the importance of GDH for neurons particularly during an elevated energy demand, and it may reflect the large allosteric activation of GDH by ADP.

Ablation of the auditory cortex results in changes in the expression of neurotransmission-related mRNAs in the cochlea

The auditory cortex (AC) dynamically regulates responses of the Organ of Corti to sound through descending connections to both the medial (MOC) and lateral (LOC) olivocochlear efferent systems. We have recently provided evidence that AC has a reinforcement role in the responses to sound of the auditory brainstem nuclei. In a molecular level, we have shown that descending inputs from AC are needed to regulate the expression of molecules involved in outer hair cell (OHC) electromotility control, such as prestin and the α10 nicotinic acetylcholine receptor (nAchR). In this report, we show that descending connections from AC to olivocochlear neurons are necessary to regulate the expression of molecules involved in cochlear afferent signaling. RT-qPCR was performed in rats at 1, 7 and 15 days after unilateral ablation of the AC, and analyzed the time course changes in gene transcripts involved in neurotransmission at the first auditory synapse. This included the glutamate metabolism enzyme glutamate decarboxylase 1 (glud1) and AMPA glutamate receptor subunits GluA2-4. In addition, gene transcripts involved in efferent regulation of type I spiral ganglion neuron (SGN) excitability mediated by LOC, such as the α7 nAchR, the D2 dopamine receptor, and the α1, and γ2 GABAA receptor subunits, were also investigated. Unilateral AC ablation induced up-regulation of GluA3 receptor subunit transcripts, whereas both GluA2 and GluA4 mRNA receptors were down-regulated already at 1 day after the ablation. Unilateral removal of the AC also resulted in up-regulation of the transcripts for α7 nAchR subunit, D2 dopamine receptor, and α1 GABAA receptor subunit at 1 day after the ablation. Fifteen days after the injury, AC ablations induced an up-regulation of glud1 transcripts.

Analysis of CCN Protein Expression and Activities in Vasoproliferative Retinopathies

The retina is a complex neurovascular structure that conveys light/visual image through the optic nerve to the visual cortex of the brain. Neuronal and vascular activities in the retina are physically and functionally intertwined, and vascular alterations are consequential to the proper function of the entire visual system. In particular, alteration of the structure and barrier function of the retinal vasculature is commonly associated with the development of vasoproliferative ischemic retinopathy, a set of clinically well-defined chronic ocular microvascular complications causing blindness in all age groups. Experimentally, the retinal tissue provides researchers with a convenient, easily accessible, and directly observable model suitable to investigate whether and how newly identified genes regulate vascular development and regeneration. The six mammalian CCN gene-encoded proteins are part of an extracellular network of bioactive molecules that regulate various aspects of organ system development and diseases. Whether and how these molecules regulate the fundamental aspects of blood vessel development and pathology and subsequently the neurovascular link in the retina are open-ended questions. Sophisticated methods have been developed to gain insight into the pathogenesis of retinal vasculopathy. This chapter describes several useful methodologies and animal models to investigate the regulation and potential relevance of the CCN proteins in vasoproliferative diseases of the retina.

Visual reporters for study of the osteoblast lineage

Advancing our understanding of osteoblast biology and differentiation is critical to elucidate the pathological mechanisms responsible for skeletal diseases such as osteoporosis. Histology and histomorphometry, the classical methods to study osteoblast biology, identify osteoblasts based on their location and morphology and ability to mineralize matrix, but do not clearly define their stage of differentiation. Introduction of visual transgenes into the cells of osteoblast lineage has revolutionized the field and resulted in a paradigm shift that allowed for specific identification and isolation of subpopulations within the osteoblast lineage. Knowledge acquired from the studies based on GFP transgenes has allowed for more precise interpretation of studies analyzing targeted overexpression or deletion of genes in the osteoblast lineage. Here, we provide a condensed overview of the currently available promoter-fluorescent reporter transgenic mice that have been generated and evaluated to varying extents. We cover different stages of the lineage as transgenes have been utilized to identify osteoprogenitors, pre-osteoblasts, osteoblasts, or osteocytes. We show that each of these promoters present with advantages and disadvantages. The studies based on the use of these reporter mice have improved our understanding of bone biology. They constitute attractive models to target osteoblasts and help to understand their cell biology.

The Amplifying Pathway of the β-Cell Contributes to Diet-induced Obesity

Efficient energy storage in adipose tissues requires optimal function of the insulin-producing β-cell, whereas its dysfunction promotes diabetes. The associated paradox related to β-cell efficiency is that excessive accumulation of fat in adipose tissue predisposes for type 2 diabetes. Insulin exocytosis is regulated by intracellular metabolic signal transduction, with glutamate dehydrogenase playing a key role in the amplification of the secretory response. Here, we used mice with β-cell-selective glutamate dehydrogenase deletion (βGlud1(-/-)), lacking an amplifying pathway of insulin secretion. As opposed to control mice, βGlud1(-/-) animals fed a high calorie diet maintained glucose tolerance and did not develop diet-induced obesity. Islets of βGlud1(-/-) mice did not increase their secretory response upon high calorie feeding, as did islets of control mice. Inhibited adipose tissue expansion observed in knock-out mice correlated with lower expression of genes responsible for adipogenesis. Rather than being efficiently stored, lipids were consumed at a higher rate in βGlud1(-/-) mice compared with controls, in particular during food intake periods. These results show that reduced β-cell function prior to high calorie feeding prevented diet-induced obesity.

GDH-Dependent Glutamate Oxidation in the Brain Dictates Peripheral Energy Substrate Distribution

Glucose, the main energy substrate used in the CNS, is continuously supplied by the periphery. Glutamate, the major excitatory neurotransmitter, is foreseen as a complementary energy contributor in the brain. In particular, astrocytes actively take up glutamate and may use it through oxidative glutamate dehydrogenase (GDH) activity. Here, we investigated the significance of glutamate as energy substrate for the brain. Upon glutamate exposure, astrocytes generated ATP in a GDH-dependent way. The observed lack of glutamate oxidation in brain-specific GDH null CnsGlud1(-/-) mice resulted in a central energy-deprivation state with increased ADP/ATP ratios and phospho-AMPK in the hypothalamus. This induced changes in the autonomous nervous system balance, with increased sympathetic activity promoting hepatic glucose production and mobilization of substrates reshaping peripheral energy stores. Our data reveal the importance of glutamate as necessary energy substrate for the brain and the role of central GDH in the regulation of whole-body energy homeostasis.

Estrogens and the control of energy homeostasis: a brain perspective

Despite their prominent roles in the control of reproduction, estrogens pervade many other bodily functions. Key metabolic pathways display marked sexual differences, and estrogens are potent modulators of energy balance, as evidenced in extreme conditions of estrogen deficiency characterized by hyperphagia and decreased energy expenditure, and leading to obesity. Compelling evidence has recently demonstrated that, in addition to their peripheral effects, the actions of estrogens on energy homeostasis are exerted at central levels, to regulate almost every key aspect of metabolic homeostasis, from feeding to energy expenditure, to glucose and lipid metabolism. We review herein the state-of-the-art of the role of estrogens in the regulation of energy balance, with a focus on their central effects and modes of action.

Behavioral phenotyping of Nestin-Cre mice: implications for genetic mouse models of psychiatric disorders

Genetic mouse models based on the Cre-loxP system have been extensively used to explore the influence of specific gene deletions on different aspects of behavioral neurobiology. However, the interpretation of the effects attributed to the gene deletion might be obscured by potential side effects secondary to the Cre recombinase transgene insertion or Cre activity, usually neither controlled nor reported. Here, we performed a comprehensive behavioral analysis of endophenotypes of neuropsychiatric disorders in the extensively used Nestin(Cre) mouse line, commonly employed to restrict genetic modifications to the CNS. We observed no alterations in locomotion, general exploratory activity, learning and memory, sociability, startle response and sensorimotor gating. Although the overall response to stimuli triggering anxiety-like behaviors remained unaltered in Nestin(Cre) mice, a strong impairment in the acquisition of both contextual- and cued-conditioned fear was observed. These results underline the importance of adequately controlling the behavioral performance of the employed Cre-lines per-se in pre-clinical neurobehavioral research.

Deciphering direct and indirect influence of thyroid hormone with mouse genetics

T3, the active form of thyroid hormone, binds nuclear receptors that regulate the transcription of a large number of genes in many cell types. Unraveling the direct and indirect effect of this hormonal stimulation, and establishing links between these molecular events and the developmental and physiological functions of the hormone, is a major challenge. New mouse genetics tools, notably those based on Cre/loxP technology, are suitable to perform a multiscale analysis of T3 signaling and achieve this task.