Metabolic gatekeepers: Dynamic roles of sugar transporters in insect metabolism and physiology

Sugars play multiple critical roles in insects, serving as energy sources, carbon skeletons, osmolytes and signalling molecules. The transport of sugars from source to sink via membrane proteins is essential for the uptake, distribution and utilization of sugars across various tissues. Sugar supply and distribution are crucial for insect development, flight, diapause and reproduction. Insect sugar transporters (STs) share significant structural and functional similarities with those in mammals and other higher eukaryotes. However, they exhibit unique characteristics, including differential regulation, substrate selectivity and kinetics. Here, we have discussed structural diversity, evolutionary trends, expression dynamics, mechanisms of action and functional significance of insect STs. The sequence and structural diversity of insect STs, highlighted by the analysis of conserved domains and evolutionary patterns, underpins their functional differentiation and divergence. The review emphasizes the importance of STs in insect metabolism, physiology and stress tolerance. It also discusses how variations in transporter regulation, expression, selectivity and activity contribute to functional differences. Furthermore, we have underlined the potential and necessity of studying these mechanisms and roles to gain a deeper understanding of insect glycobiology. Understanding the regulation and function of sugar transporters is vital for comprehending insect metabolism and physiological potential. This review provides valuable insights into the diverse functionalities of insect STs and their significant roles in metabolism and physiology.

Lactate dehydrogenase-1 may play a key role in the brain energy disturbance caused by cryptococcal meningitis

BACKGROUND

Cryptococcal meningitis (CM) may affect the conversion of lactate to pyruvate in the brain, resulting in abnormal levels of adenosine triphosphate (ATP) throughout the brain. Lactate conversion to pyruvate is mainly caused by lactic dehydrogenase 1 (LDH1), which is composed of four LDHB subunits. However, the underlying mechanism of LDH1 in CM remains unclear.

METHODS

Cerebrospinal fluid (CSF) from 17 patients was collected, including eight patients with non-infectious diseases of the central nervous system and nine patients with CM. Based on clinical data and laboratory reports, data regarding intracranial pressure, CSF white cell counts, lactate dehydrogenase (LDH), adenosine deaminase, glucose, protein, and chloridion were collected. Meanwhile, LDH1, LDH5, lactate, pyruvate, and ATP levels were detected in CSF. Whereafter, the levels of lactate, pyruvate, ATP, and the amplitude and frequency of action potentials in the neurons with low expression of LDHB were explored.

RESULTS

Intracranial pressure and white cell count in CSF were significantly increased in patients with CM. In patients with CM, the LDH1, pyruvate, and ATP levels in the CSF were significantly decreased, and the levels of lactate were found to be increased. Furthermore, pyruvate and ATP levels were decreased, while lactate was increased in the neurons with low expression of LDHB. The amplitude and frequency of APs in the neurons with low expression of LDHB were significantly decreased.

CONCLUSION

Reduced levels of LDH1 in the brain of patients with CM may lead to increased lactate levels, decreased pyruvate and ATP levels, and negatively affect neuronal activity.

Neural ageing and synaptic plasticity: prioritizing brain health in healthy longevity

Ageing is characterized by a gradual decline in the efficiency of physiological functions and increased vulnerability to diseases. Ageing affects the entire body, including physical, mental, and social well-being, but its impact on the brain and cognition can have a particularly significant effect on an individual's overall quality of life. Therefore, enhancing lifespan and physical health in longevity studies will be incomplete if cognitive ageing is over looked. Promoting successful cognitive ageing encompasses the objectives of mitigating cognitive decline, as well as simultaneously enhancing brain function and cognitive reserve. Studies in both humans and animal models indicate that cognitive decline related to normal ageing and age-associated brain disorders are more likely linked to changes in synaptic connections that form the basis of learning and memory. This activity-dependent synaptic plasticity reorganises the structure and function of neurons not only to adapt to new environments, but also to remain robust and stable over time. Therefore, understanding the neural mechanisms that are responsible for age-related cognitive decline becomes increasingly important. In this review, we explore the multifaceted aspects of healthy brain ageing with emphasis on synaptic plasticity, its adaptive mechanisms and the various factors affecting the decline in cognitive functions during ageing. We will also explore the dynamic brain and neuroplasticity, and the role of lifestyle in shaping neuronal plasticity.

A Retrospective Analysis of Central and Peripheral Metabolic Characteristics in Patients with Cryptococcal Meningitis

INTRODUCTION

Most current treatment strategies and investigations on cryptococcal meningitis (CM) focus primarily on the central nervous system (CNS), often overlooking the complex interplay between the CNS and the peripheral system. This study aims to explore the characteristics of central and peripheral metabolism in patients with CM.

METHODS

Patients diagnosed with CM as per the hospital records of the Fourth People's Hospital of Nanning were retrospectively analyzed. Patients were divided into two groups, non-structural damage of the brain (NSDB) and structural damage of the brain (SDB), according to the presence of brain lesions as detected with imaging. Based on the presence of enlarged cerebral ventricles, the cases in the SDB group were classified into non-ventriculomegaly (NVM) and ventriculomegaly (VM). Various parameters of cerebrospinal fluid (CSF) and peripheral blood (PB) were analyzed.

RESULTS

A significant correlation was detected between CSF and PB parameters. The levels of CSF-adenosine dehydrogenase (ADA), CSF-protein, CSF-glucose, and CSF-chloride ions were significantly correlated with the levels of PB-aminotransferase, PB-bilirubin, PB-creatinine (Cr), PB-urea nitrogen, PB-electrolyte, PB-protein, and PB-lipid. Compared with NSDB, the levels of CSF-glucose were significantly decreased in the SDB group, while the levels of CSF-lactate dehydrogenase (LDH) and CSF-protein were significantly increased in the SDB group. In the SDB group, the levels of PB-potassium, PB-hemoglobin(Hb), and PB-albumin were significantly decreased in the patients with VM, while the level of PB-urea nitrogen was significantly increased in these patients.

CONCLUSION

Metabolic and structural alterations in the brain may be associated with peripheral metabolic changes.

Drug-drug interactions between propofol and ART drugs: Inhibiting neuronal activity by affecting glucose metabolism

BACKGROUND

The use of two or more drugs carries the potential risk of drug-drug interactions (DDIs), which may result in adverse reactions. Some human immunodeficiency virus (HIV)-infected patients who receive antiretroviral therapy (ART) may require general anesthesia with propofol (PRL) before undergoing surgical treatment. Both PRL and ART drugs may lead to neuronal dysfunction, which can be accompanied by energy metabolism disorders. Neurons take in glucose mainly through glucose transporter 3 (Glut3) which is specifically expressed on the cell membranes of neurons. However, to date, no study has examined whether the DDIs of PRL and ART drugs interfere with glucose metabolism and Glut3 expression in neurons.

METHODS

An in vitro model was constructed using the primary cultures of neurons. PRL and ART drugs (EFV, AZT, and 3TC), were added at different concentrations (low, medium, and high). The neurons were exposed to the drugs for 1, 4, 8, and 12 h. The optimal drug concentration and exposure time were selected. The cellular survival rate, glucose concentration, electrophysiology, and the expression of Glut3 were detected.

RESULTS

There were no significant changes in the cellular survival rates of the neurons that were exposed to both PRL and ART drugs at low concentrations for 1 h. However, the survival rates of the neurons decreased significantly as the drug concentrations and durations increased. The glucose concentration of the neurons that were exposed to both PRL and the ART drugs was significantly decreased. The glucose concentration of the neurons was not affected by any individual drug. The amplitude of the action potential and the expression of Glut3 were decreased in the neurons that were exposed to both PRL and ART drugs.

CONCLUSIONS

The main adverse reactions induced by the DDIs between PRL and the ART drugs were decreased glucose metabolism and neuronal damage, which were caused by inhibiting the expression of Glut3. More importantly, we found that decreases in glucose metabolism predated neuronal damage.

Implications of glial metabolic dysregulation in the pathophysiology of neurodegenerative diseases

Glial cells are the most abundant cells of the brain, outnumbering neurons. These multifunctional cells are crucial for maintaining brain homeostasis by providing trophic and nutritional support to neurons, sculpting synapses, and providing an immune defense. Glia are highly plastic and undergo both structural and functional alterations in response to changes in the brain microenvironment. Glial phenotypes are intimately regulated by underlying metabolic machinery, which dictates the effector functions of these cells. Altered brain energy metabolism and chronic neuroinflammation are common features of several neurodegenerative diseases. Microglia and astrocytes are the major glial cells fueling the ongoing neuroinflammatory process, exacerbating neurodegeneration. Distinct metabolic perturbations in microglia and astrocytes, including altered carbohydrate, lipid, and amino acid metabolism have been documented in neurodegenerative diseases. These disturbances aggravate the neurodegenerative process by potentiating the inflammatory activation of glial cells. This review covers the recent advances in the molecular aspects of glial metabolic changes in the pathophysiology of neurodegenerative diseases. Finally, we discuss studies exploiting glial metabolism as a potential therapeutic avenue in neurodegenerative diseases.

Inter-Species Rescue of Mutant Phenotype-The Standard for Genetic Analysis of Human Genetic Disorders in Drosophila melanogaster Model

Drosophila melanogaster (the fruit fly) is arguably a superstar of genetics, an astonishing versatile experimental model which fueled no less than six Nobel prizes in medicine. Nowadays, an evolving research endeavor is to simulate and investigate human genetic diseases in the powerful D. melanogaster platform. Such a translational experimental strategy is expected to allow scientists not only to understand the molecular mechanisms of the respective disorders but also to alleviate or even cure them. In this regard, functional gene orthology should be initially confirmed in vivo by transferring human or vertebrate orthologous transgenes in specific mutant backgrounds of D. melanogaster. If such a transgene rescues, at least partially, the mutant phenotype, then it qualifies as a strong candidate for modeling the respective genetic disorder in the fruit fly. Herein, we review various examples of inter-species rescue of relevant mutant phenotypes of the fruit fly and discuss how these results recommend several human genes as candidates to study and validate genetic variants associated with human diseases. We also consider that a wider implementation of this evolutionist exploratory approach as a standard for the medicine of genetic disorders would allow this particular field of human health to advance at a faster pace.

Energy Metabolism and Intracellular pH Alteration in Neural Spheroids Carrying Down Syndrome

Brain diseases including Down syndrome (DS/TS21) are known to be characterized by changes in cellular metabolism. To adequately assess such metabolic changes during pathological processes and to test drugs, methods are needed that allow monitoring of these changes in real time with minimally invasive effects. Thus, the aim of our work was to study the metabolic status and intracellular pH of spheroids carrying DS using fluorescence microscopy and FLIM. For metabolic analysis we measured the fluorescence intensities, fluorescence lifetimes and the contributions of the free and bound forms of NAD(P)H. For intracellular pH assay we measured the fluorescence intensities of SypHer-2 and BCECF. Data were processed with SPCImage and Fiji-ImageJ. We demonstrated the predominance of glycolysis in TS21 spheroids compared with normal karyotype (NK) spheroids. Assessment of the intracellular pH indicated a more alkaline intracellular pH in the TS21 spheroids compared to NK spheroids. Using fluorescence imaging, we performed a comprehensive comparative analysis of the metabolism and intracellular pH of TS21 spheroids and showed that fluorescence microscopy and FLIM make it possible to study living cells in 3D models in real time with minimally invasive effects.

Modulation of Huntington's disease in Drosophila

Huntington's disease (HD) is a progressive neurodegenerative disorder which deteriorates the physical and mental abilities of the patients. It is an autosomal dominant disorder and is mainly caused by the expansion of a repeating CAG triplet. A number of animal models ranging from worms, fruit flies, mice and rats to pigs, sheep and monkeys are available which have been helpful in understanding various pathways involved during the progression of the disease. Drosophila is one of the most commonly used model organisms for biomedical science, due to low cost maintenance, short life span and easily implications of genetic tools. The present review provides brief description of HD and the studies carried out for HD to date taking Drosophila as a model.

Genetic Screen in Adult Drosophila Reveals That dCBP Depletion in Glial Cells Mitigates Huntington Disease Pathology through a Foxo-Dependent Pathway

Huntington’s disease (HD) is a progressive and fatal autosomal dominant neurodegenerative disease caused by a CAG repeat expansion in the first exon of the huntingtin gene (HTT). In spite of considerable efforts, there is currently no treatment to stop or delay the disease. Although HTT is expressed ubiquitously, most of our knowledge has been obtained on neurons. More recently, the impact of mutant huntingtin (mHTT) on other cell types, including glial cells, has received growing interest. It is currently unclear whether new pathological pathways could be identified in these cells compared to neurons. To address this question, we performed an in vivo screen for modifiers of mutant huntingtin (HTT-548-128Q) induced pathology in Drosophila adult glial cells and identified several putative therapeutic targets. Among them, we discovered that partial nej/dCBP depletion in these cells was protective, as revealed by strongly increased lifespan and restored locomotor activity. Thus, dCBP promotes the HD pathology in glial cells, in contrast to previous opposite findings in neurons. Further investigations implicated the transcriptional activator Foxo as a critical downstream player in this glial protective pathway. Our data suggest that combinatorial approaches combined to specific tissue targeting may be required to uncover efficient therapies in HD.

Drosophila Glia: Models for Human Neurodevelopmental and Neurodegenerative Disorders

Glial cells are key players in the proper formation and maintenance of the nervous system, thus contributing to neuronal health and disease in humans. However, little is known about the molecular pathways that govern glia-neuron communications in the diseased brain. Drosophila provides a useful in vivo model to explore the conserved molecular details of glial cell biology and their contributions to brain function and disease susceptibility. Herein, we review recent studies that explore glial functions in normal neuronal development, along with Drosophila models that seek to identify the pathological implications of glial defects in the context of various central nervous system disorders.

The contribution of glial cells to Huntington's disease pathogenesis

Glial cells play critical roles in the normal development and function of neural circuits, but in many neurodegenerative diseases, they become dysregulated and may contribute to the development of brain pathology. In Huntington's disease (HD), glial cells both lose normal functions and gain neuropathic phenotypes. In addition, cell-autonomous dysfunction elicited by mutant huntingtin (mHTT) expression in specific glial cell types is sufficient to induce both pathology and Huntington's disease-related impairments in motor and cognitive performance, suggesting that these cells may drive the development of certain aspects of Huntington's disease pathogenesis. In support of this imaging studies in pre-symptomatic HD patients and work on mouse models have suggested that glial cell dysfunction occurs at a very early stage of the disease, prior to the onset of motor and cognitive deficits. Furthermore, selectively ablating mHTT from specific glial cells or correcting for HD-induced changes in their transcriptional profile rescues some HD-related phenotypes, demonstrating the potential of targeting these cells for therapeutic intervention. Here we review emerging research focused on understanding the involvement of different glial cell types in specific aspects of HD pathogenesis. This work is providing new insight into how HD impacts biological functions of glial cells in the healthy brain as well as how HD induced dysfunction in these cells might change the way they integrate into biological circuits.

Spatially-resolved metabolic profiling of living Drosophila in neurodegenerative conditions using 1H magic angle spinning NMR

Drosophila flies are versatile animal models for the study of gene mutations in neuronal pathologies. Their small size allows performing in vivo Magic Angle Spinning (MAS) experiments to obtain high-resolution 1H nuclear magnetic resonance (NMR) spectra. Here, we use spatially-resolved 1H high-resolution MAS NMR to investigate in vivo metabolite contents in different segments of the fly body. A comparative study of metabolic changes was performed for three neurodegenerative disorders: two cell-specific neuronal and glial models of Huntington disease (HD) and a model of glutamate excitotoxicity. It is shown that these pathologies are characterized by specific and sometimes anatomically localized variations in metabolite concentrations. In two cases, the modifications of 1H MAS NMR spectra localized in fly heads were significant enough to allow the creation of a predictive model.

Glucose, glycolysis, and neurodegenerative diseases

Prolonged survival of a typical postmitotic neuron hinges on a balance between multiple processes, among these are a sustenance of ATP production and protection against reactive oxygen species. In neuropathological conditions, mitochondrial defects often lead to both a drop in ATP levels, as well as increase reactive oxygen species production from inefficient electron transport processes and NADPH-oxidases activities. The former often resulted in the phenomenon of compensatory aerobic glycolysis. The latter stretches the capacity of the cell's redox buffering capacity, and may lead to damages of key enzymes involved in energy metabolism. Several recent reports have indicated that enhancing glucose availability and uptake, as well as increasing glycolytic flux via pharmacological or genetic manipulation of glycolytic enzymes, could be protective in animal models of several major neurodegenerative diseases, including Parkinson's disease, Huntington's disease, and Amyotrophic lateral sclerosis. Activation of canonical Wnt signaling, which improves disease symptoms in mouse models of Alzheimer's disease also appears to work via an elevation of glycolytic enzymes and enhance glucose metabolism. Here, I discuss these findings and the possible underlying mechanisms of how an increase in glucose uptake and glycolysis could be neuroprotective. Increased glycolytic production of ATP would help alleviate energy deficiency, and ATP's hydrotropic effect may enhance solubility and clearance of toxic aggregates prevalent in many neurodegenerative diseases. Furthermore, channeling of glucose into the Pentose Phosphate Pathway would increase the redox buffering capacity of the cell.

Expression of Human Mutant Huntingtin Protein in Drosophila Hemocytes Impairs Immune Responses

The pathogenic effect of mutant HTT (mHTT) which causes Huntington disease (HD) are not restricted to nervous system. Such phenotypes include aberrant immune responses observed in the HD models. However, it is still unclear how this immune dysregulation influences the innate immune response against pathogenic infection. In the present study, we used transgenic Drosophila melanogaster expressing mutant HTT protein (mHTT) with hemocyte-specific drivers and examined the immune responses and hemocyte function. We found that mHTT expression in the hemocytes did not affect fly viability, but the numbers of circulating hemocytes were significantly decreased. Consequently, we observed that the expression of mHTT in the hemocytes compromised the immune responses including clot formation and encapsulation which lead to the increased susceptibility to entomopathogenic nematode and parasitoid wasp infections. In addition, mHTT expression in Drosophila macrophage-like S2 cells in vitro reduced ATP levels, phagocytic activity and the induction of antimicrobial peptides. Further effects observed in mHTT-expressing cells included the altered production of cytokines and activation of JAK/STAT signaling. The present study shows that the expression of mHTT in Drosophila hemocytes causes deficient cellular and humoral immune responses against invading pathogens. Our findings provide the insight into the pathogenic effects of mHTT in the immune cells.

JNK modifies neuronal metabolism to promote proteostasis and longevity

Aging is associated with a progressive loss of tissue and metabolic homeostasis. This loss can be delayed by single-gene perturbations, increasing lifespan. How such perturbations affect metabolic and proteostatic networks to extend lifespan remains unclear. Here, we address this question by comprehensively characterizing age-related changes in protein turnover rates in the Drosophila brain, as well as changes in the neuronal metabolome, transcriptome, and carbon flux in long-lived animals with elevated Jun-N-terminal Kinase signaling. We find that these animals exhibit a delayed age-related decline in protein turnover rates, as well as decreased steady-state neuronal glucose-6-phosphate levels and elevated carbon flux into the pentose phosphate pathway due to the induction of glucose-6-phosphate dehydrogenase (G6PD). Over-expressing G6PD in neurons is sufficient to phenocopy these metabolic and proteostatic changes, as well as extend lifespan. Our study identifies a link between metabolic changes and improved proteostasis in neurons that contributes to the lifespan extension in long-lived mutants.

Glial expression of disease-associated poly-glutamine proteins impairs the blood-brain barrier in Drosophila

Expansion of poly-glutamine (polyQ) stretches in several proteins has been linked to neurodegenerative diseases. The effects of polyQ-expanded proteins on neurons have been extensively studied, but their effects on glia remain unclear. We found that expression of distinct polyQ proteins exclusively in all glia or specifically in the blood-brain barrier (BBB) and blood-retina barrier (BRB) glia caused cell-autonomous impairment of BBB/BRB integrity, suggesting that BBB/BRB glia are most vulnerable to polyQ-expanded proteins. Furthermore, we also found that BBB/BRB leakage in Drosophila is reflected in reversed waveform polarity on the basis of electroretinography (ERG), making ERG a sensitive method to detect BBB/BRB leakage. The polyQ-expanded protein Atxn3-84Q forms aggregates, induces BBB/BRB leakage, restricts Drosophila lifespan and reduces the level of Repo (a pan-glial transcriptional factor required for glial differentiation). Expression of Repo in BBB/BRB glia can rescue BBB/BRB leakage, suggesting that the reduced expression of Repo is important for the effect of polyQ on BBB/BRB impairment. Coexpression of the chaperon HSP40 and HSP70 effectively rescues the effects of Atxn3-84Q, indicating that polyQ protein aggregation in glia is deleterious. Intriguingly, coexpression of wild-type Atxn3-27Q can also rescue BBB/BRB impairment, suggesting that normal polyQ protein may have a protective function.

Warburg Effect Metabolism Drives Neoplasia in a Drosophila Genetic Model of Epithelial Cancer

Cancers develop in a complex mutational landscape. Genetic models of tumor formation have been used to explore how combinations of mutations cooperate to promote tumor formation in vivo. Here, we identify lactate dehydrogenase (LDH), a key enzyme in Warburg effect metabolism, as a cooperating factor that is both necessary and sufficient for epidermal growth factor receptor (EGFR)-driven epithelial neoplasia and metastasis in a Drosophila model. LDH is upregulated during the transition from hyperplasia to neoplasia, and neoplasia is prevented by LDH depletion. Elevated LDH is sufficient to drive this transition. Notably, genetic alterations that increase glucose flux, or a high-sugar diet, are also sufficient to promote EGFR-driven neoplasia, and this depends on LDH activity. We provide evidence that increased LDHA expression promotes a transformed phenotype in a human primary breast cell culture model. Furthermore, analysis of publically available cancer data showed evidence of synergy between elevated EGFR and LDHA activity linked to poor clinical outcome in a number of human cancers. Altered metabolism has generally been assumed to be an enabling feature that accelerates cancer cell proliferation. Our findings provide evidence that sugar metabolism may have a more profound role in driving neoplasia than previously appreciated.

The Tiny Drosophila Melanogaster for the Biggest Answers in Huntington's Disease

The average life expectancy for humans has increased over the last years. However, the quality of the later stages of life is low and is considered a public health issue of global importance. Late adulthood and the transition into the later stage of life occasionally leads to neurodegenerative diseases that selectively affect different types of neurons and brain regions, producing motor dysfunctions, cognitive impairment, and psychiatric disorders that are progressive, irreversible, without remission periods, and incurable. Huntington's disease (HD) is a common neurodegenerative disorder. In the 25 years since the mutation of the huntingtin (HTT) gene was identified as the molecule responsible for this neural disorder, a variety of animal models, including the fruit fly, have been used to study the disease. Here, we review recent research that used Drosophila as an experimental tool for improving knowledge about the molecular and cellular mechanisms underpinning HD.

Live imaging using a FRET glucose sensor reveals glucose delivery to all cell types in the Drosophila brain

All complex nervous systems are metabolically separated from circulation by a blood-brain barrier (BBB) that prevents uncontrolled leakage of solutes into the brain. Thus, all metabolites needed to sustain energy homeostasis must be transported across this BBB. In invertebrates, such as Drosophila, the major carbohydrate in circulation is the disaccharide trehalose and specific trehalose transporters are expressed by the glial BBB. Here we analyzed whether glucose is able to contribute to energy homeostasis in Drosophila. To study glucose influx into the brain we utilized a genetically encoded, FRET-based glucose sensor expressed in a cell type specific manner. When confronted with glucose all brain cells take up glucose within two minutes. In order to characterize the glucose transporter involved, we studied Drosophila Glut1, the homologue of which is primarily expressed by the BBB-forming endothelial cells and astrocytes in the mammalian nervous system. In Drosophila, however, Glut1 is expressed in neurons and is not found at the BBB. Thus, Glut1 cannot contribute to initial glucose uptake from the hemolymph. To test whether gap junctional coupling between the BBB forming cells and other neural cells contributes to glucose distribution we assayed these junctions using RNAi experiments and only found a minor contribution of gap junctions to glucose metabolism. Our results provide the entry point to further dissect the mechanisms underlying glucose distribution and offer new opportunities to understand brain metabolism.

Nonmammalian Models of Huntington's Disease

Flies, worms, yeast and more recently zebra fish have all been engineered to express expanded polyglutamine repeat versions of Huntingtin with various resulting pathologies including early death, neurodegeneration, and loss of motor function. Each of these models present particular features that make it useful in studying the mechanisms of polyglutamine pathology. However, one particular unbiased readout of mHTT pathology is functional loss of motor control. Loss of motor control is prominent in patients, but it remains unresolved whether pathogenic symptoms in patients result from overt degeneration and loss of neurons or from malfunctioning of surviving neurons as the pathogenic insult builds up. This is why a functional assay such as motor control can be uniquely powerful in revealing early as well as late neurological deficits and does not rely on assumptions such as that the level of inclusions or the degree of neuronal loss can be equated with the level of pathology. Drosophila is well suited for such assays because it contains a functioning nervous system with many parallels to the human condition. In addition, the ability to readily express mHTT transgenes in different tissues and subsets of neurons allows one the possibility of isolating a particular effect to a subset of neurons where one can correlate subcellular events in response to mHTT challenge with pathology at both the cellular and organismal levels. Here we describe methods to monitor the degree of motor function disruption in Drosophila models of HD and we include a brief summary of other nonmammalian models of HD and discussion of their unique strengths.

Transcriptional profiling of mitochondria associated genes in prefrontal cortex of subjects with major depressive disorder

OBJECTIVES

Recent evidences suggest that mitochondrial dysfunction maybe involved in the pathophysiology of major depressive disorder (MDD); however, the role of mitochondrial genes in this disorder has not been studied systematically. In the present study, we profiled expression of mitochondrial genes in dorsolateral prefrontal cortex (dlPFC) of MDD and non-psychiatric control subjects.

METHODS

Human mitochondrial RT² profile PCR array plates were used to examine differentially expressed genes in dlPFC of 11 MDD and 11 control subjects. Differentially expressed genes were validated independently by qRT-PCR. Biological relevance of differentially expressed genes was analysed by gene ontology (GO) and ingenuity pathways analysis (IPA).

RESULTS

We found that 16 genes were differentially expressed in the MDD group compared with control group. Among them, three genes were downregulated and 13 genes upregulated. None of these genes were affected by confounding variables, such as age, post-mortem interval, brain pH, and antidepressant toxicology. Seven differentially expressed genes were successfully validated in MDD subjects. GO and IPA analyses identified several new regulatory networks associated with mitochondrial dysfunctions in MDD.

CONCLUSIONS

Our findings suggest abnormal mitochondrial systems in the brain of MDD subjects which could be involved in the etiopathogenesis of this disorder.

Metabolic reprogramming by the pyruvate dehydrogenase kinase-lactic acid axis: Linking metabolism and diverse neuropathophysiologies

Emerging evidence indicates that there is a complex interplay between metabolism and chronic disorders in the nervous system. In particular, the pyruvate dehydrogenase (PDH) kinase (PDK)-lactic acid axis is a critical link that connects metabolic reprogramming and the pathophysiology of neurological disorders. PDKs, via regulation of PDH complex activity, orchestrate the conversion of pyruvate either aerobically to acetyl-CoA, or anaerobically to lactate. The kinases are also involved in neurometabolic dysregulation under pathological conditions. Lactate, an energy substrate for neurons, is also a recently acknowledged signaling molecule involved in neuronal plasticity, neuron-glia interactions, neuroimmune communication, and nociception. More recently, the PDK-lactic acid axis has been recognized to modulate neuronal and glial phenotypes and activities, contributing to the pathophysiologies of diverse neurological disorders. This review covers the recent advances that implicate the PDK-lactic acid axis as a novel linker of metabolism and diverse neuropathophysiologies. We finally explore the possibilities of employing the PDK-lactic acid axis and its downstream mediators as putative future therapeutic strategies aimed at prevention or treatment of neurological disorders.

Oncostatic-Cytoprotective Effect of Melatonin and Other Bioactive Molecules: A Common Target in Mitochondrial Respiration

For several years, oncostatic and antiproliferative properties, as well as thoses of cell death induction through 5-methoxy-N-acetiltryptamine or melatonin treatment, have been known. Paradoxically, its remarkable scavenger, cytoprotective and anti-apoptotic characteristics in neurodegeneration models, such as Alzheimer’s disease and Parkinson’s disease are known too. Analogous results have been confirmed by a large literature to be associated to the use of many other bioactive molecules such as resveratrol, tocopherol derivatives or vitamin E and others. It is interesting to note that the two opposite situations, namely the neoplastic pathology and the neurodegeneration, are characterized by deep alterations of the metabolome, of mitochondrial function and of oxygen consumption, so that the oncostatic and cytoprotective action can find a potential rationalization because of the different metabolic and mitochondrial situations, and in the effect that these molecules exercise on the mitochondrial function. In this review we discuss historical and general aspects of melatonin, relations between cancers and the metabolome and between neurodegeneration and the metabolome, and the possible effects of melatonin and of other bioactive molecules on metabolic and mitochondrial dynamics. Finally, we suggest a common general mechanism as responsible for the oncostatic/cytoprotective effect of melatonin and of other molecules examined.

Using Drosophila models of Huntington's disease as a translatable tool

The Huntingtin (Htt) protein is essential for a wealth of intracellular signaling cascades and when mutated, causes multifactorial dysregulation of basic cellular processes. Understanding the contribution to each of these intracellular pathways is essential for the elucidation of mechanisms that drive pathophysiology. Using appropriate models of Huntington's disease (HD) is key to finding the molecular mechanisms that contribute to neurodegeneration. While mouse models and cell lines expressing mutant Htt have been instrumental to HD research, there has been a significant contribution to our understating of the disease from studies utilizing Drosophila melanogaster. Flies have an Htt protein, so the endogenous pathways with which it interacts are likely conserved. Transgenic flies engineered to overexpress the human mutant HTT gene display protein aggregation, neurodegeneration, behavioral deficits and a reduced lifespan. The short life span of flies, low cost of maintaining stocks and genetic tools available for in vivo manipulation make them ideal for the discovery of new genes that are involved in HD pathology. It is possible to do rapid genome wide screens for enhancers or suppressors of the mutant Htt-mediated phenotype, expressed in specific tissues or neuronal subtypes. However, there likely remain many yet unknown genes that modify disease progression, which could be found through additional screening approaches using the fly. Importantly, there have been instances where genes discovered in Drosophila have been translated to HD mouse models.

Enhanced neuronal glucose transporter expression reveals metabolic choice in a HD Drosophila model

Huntington’s disease is a neurodegenerative disorder caused by toxic insertions of polyglutamine residues in the Huntingtin protein and characterized by progressive deterioration of cognitive and motor functions. Altered brain glucose metabolism has long been suggested and a possible link has been proposed in HD. However, the precise function of glucose transporters was not yet determined. Here, we report the effects of the specifically-neuronal human glucose transporter expression in neurons of a Drosophila model carrying the exon 1 of the human huntingtin gene with 93 glutamine repeats (HQ93). We demonstrated that overexpression of the human glucose transporter in neurons ameliorated significantly the status of HD flies by increasing their lifespan, reducing their locomotor deficits and rescuing eye neurodegeneration. Then, we investigated whether increasing the major pathways of glucose catabolism, glycolysis and pentose-phosphate pathway (PPP) impacts HD. To mimic increased glycolytic flux, we overexpressed phosphofructokinase (PFK) which catalyzes an irreversible step in glycolysis. Overexpression of PFK did not affect HQ93 fly survival, but protected from photoreceptor loss. Overexpression of glucose-6-phosphate dehydrogenase (G6PD), the key enzyme of the PPP, extended significantly the lifespan of HD flies and rescued eye neurodegeneration. Since G6PD is able to synthesize NADPH involved in cell survival by maintenance of the redox state, we showed that tolerance to experimental oxidative stress was enhanced in flies co-expressing HQ93 and G6PD. Additionally overexpressions of hGluT3, G6PD or PFK were able to circumvent mitochondrial deficits induced by specific silencing of genes necessary for mitochondrial homeostasis. Our study confirms the involvement of bioenergetic deficits in HD course; they can be rescued by specific expression of a glucose transporter in neurons. Finally, the PPP and, to a lesser extent, the glycolysis seem to mediate the hGluT3 protective effects, whereas, in addition, the PPP provides increased protection to oxidative stress.

The Drosophila blood-brain barrier: development and function of a glial endothelium

The efficacy of neuronal function requires a well-balanced extracellular ion homeostasis and a steady supply with nutrients and metabolites. Therefore, all organisms equipped with a complex nervous system developed a so-called blood-brain barrier, protecting it from an uncontrolled entry of solutes, metabolites or pathogens. In higher vertebrates, this diffusion barrier is established by polarized endothelial cells that form extensive tight junctions, whereas in lower vertebrates and invertebrates the blood-brain barrier is exclusively formed by glial cells. Here, we review the development and function of the glial blood-brain barrier of Drosophila melanogaster. In the Drosophila nervous system, at least seven morphologically distinct glial cell classes can be distinguished. Two of these glial classes form the blood-brain barrier. Perineurial glial cells participate in nutrient uptake and establish a first diffusion barrier. The subperineurial glial (SPG) cells form septate junctions, which block paracellular diffusion and thus seal the nervous system from the hemolymph. We summarize the molecular basis of septate junction formation and address the different transport systems expressed by the blood-brain barrier forming glial cells.

Glia in Drosophila behavior

Glial cells constitute about 10 % of the Drosophila nervous system. The development of genetic and molecular tools has helped greatly in defining different types of glia. Furthermore, considerable progress has been made in unraveling the mechanisms that control the development and differentiation of Drosophila glia. By contrast, the role of glia in adult Drosophila behavior is not well understood. We here summarize recent work describing the role of glia in normal behavior and in Drosophila models for neurological and behavioral disorders.

Do glial cells play an anti-oxidative role in Huntington's disease?

Oxidative stress is a condition of imbalance between reactive oxygen species (ROS) formation and antioxidant capacity as a result of dysfunction of the antioxidant system. ROS can be served as a second messenger at low or moderate concentration, while excessive amount of ROS under oxidative stress condition would destroy macromolecules like proteins, DNA, and lipids, finally leading to cell apoptosis or necrosis. Changes in these macromolecules are involved in various pathological changes and progression of diseases, especially neurodegenerative diseases. Neurodegenerative diseases are morphologically featured by progressive neuronal cell loss, accompanied with inclusions formed by protein aggregates in neurons or glial cells. Neurons have always received much more attention than glial cells in neurodegenerative diseases. Actually, glial cells might play a key role in the functioning of neurons and cellular survival through an antioxidant way. Additionally, neurons can modulate the activities of glia either. Herein, the main purposes of this review are to mention the connection between Huntington's disease (HD) and oxidative stress, to summarize the characteristics and functions of glial cells in HD, to state the cross talk between neurons and glial cells, and to emphasize the conclusive role of activation of Keap1-Nrf2-ARE pathway in glial cells against oxidative stress in HD.

Longevity pathways and memory aging

The aging process has been associated with numerous pathologies at the cellular, tissue, and organ level. Decline or loss of brain functions, including learning and memory, is one of the most devastating and feared aspects of aging. Learning and memory are fundamental processes by which animals adjust to environmental changes, evaluate various sensory signals based on context and experience, and make decisions to generate adaptive behaviors. Age-related memory impairment is an important phenotype of brain aging. Understanding the molecular mechanisms underlying age-related memory impairment is crucial for the development of therapeutic strategies that may eventually lead to the development of drugs to combat memory loss. Studies in invertebrate animal models have taught us much about the physiology of aging and its effects on learning and memory. In this review we survey recent progress relevant to conserved molecular pathways implicated in both aging and memory formation and consolidation.

Copy-number variation of the neuronal glucose transporter gene SLC2A3 and age of onset in Huntington's disease

Huntington's disease (HD) is a devastating neurodegenerative disorder which is inherited in an autosomal dominant manner. HD is caused by a trinucleotide CAG repeat expansion that encodes a polyglutamine stretch in the huntingtin (HTT) protein. Mutant HTT expression leads to a myriad of cellular dysfunctions culminating in neuronal loss and consequent motor, cognitive and psychiatric disturbances in HD patients. The length of the CAG repeat is inversely correlated with age of onset (AO) in HD patients, while environmental and genetic factors can further modulate this parameter. Here, we explored whether the recently described copy-number variation (CNV) of the gene SLC2A3-which encodes the neuronal glucose transporter GLUT3-could modulate AO in HD. Strikingly, we found that increased dosage of SLC2A3 delayed AO in an HD cohort of 987 individuals, and that this correlated with increased levels of GLUT3 in HD patient cells. To our knowledge this is the first time that CNV of a candidate gene has been found to modulate HD pathogenesis. Furthermore, we found that increasing dosage of Glut1-the Drosophila melanogaster homologue of this glucose transporter-ameliorated HD-relevant phenotypes in fruit flies, including neurodegeneration and life expectancy. As alterations in glucose metabolism have been implicated in HD pathogenesis, this study may have important therapeutic relevance for HD.

Alleviating neurodegeneration in Drosophila models of PolyQ diseases

Polyglutamine (polyQ) diseases are a group of neurodegenerative conditions, induced from CAG trinucleotide repeat expansion within causative gene respectively. Generation of toxic proteins, containing polyQ-expanded tract, is the key process to cause neurodegeneration. Till now, although polyQ diseases remain uncurable, numerous therapeutic strategies with great potential have been examined and have been proven to be effective against polyQ diseases, including diverse small biological molecules and many pharmacological compounds mainly through prevention on formation of aggregates and inclusions, acceleration on degradation of toxic proteins and regulation of cellular function. We review promising therapeutic strategies by using Drosophila models of polyQ diseases including HD, SCA1, SCA3 and SBMA.

Ageing as a risk factor for disease

Age is the main risk factor for the prevalent diseases of developed countries: cancer, cardiovascular disease and neurodegeneration. The ageing process is deleterious for fitness, but can nonetheless evolve as a consequence of the declining force of natural selection at later ages, attributable to extrinsic hazards to survival: ageing can then occur as a side-effect of accumulation of mutations that lower fitness at later ages, or of natural selection in favour of mutations that increase fitness of the young but at the cost of a higher subsequent rate of ageing. Once thought of as an inexorable, complex and lineage-specific process of accumulation of damage, ageing has turned out to be influenced by mechanisms that show strong evolutionary conservation. Lowered activity of the nutrient-sensing insulin/insulin-like growth factor/Target of Rapamycin signalling network can extend healthy lifespan in yeast, multicellular invertebrates, mice and, possibly, humans. Mitochondrial activity can also promote ageing, while genome maintenance and autophagy can protect against it. We discuss the relationship between evolutionarily conserved mechanisms of ageing and disease, and the associated scientific challenges and opportunities.

Probing mechanisms that underlie human neurodegenerative diseases in Drosophila

The fruit fly, Drosophila melanogaster, is an excellent organism for the study of the genetic and molecular basis of metazoan development. Drosophila provides numerous tools and reagents to unravel the molecular and cellular functions of genes that cause human disease, and the past decade has witnessed a significant expansion of the study of neurodegenerative disease mechanisms in flies. Here we review the interplay between oxidative stress and neuronal toxicity. We cover some of the studies that show how proteasome degradation of protein aggregates, autophagy, mitophagy, and lysosomal function affect the quality control mechanisms required for neuronal survival. We discuss how forward genetic screens in flies have led to the isolation of a few loci that cause neurodegeneration, paving the way for large-scale systematic screens to identify such loci in flies as well as promoting gene discovery in humans.

Protection by glia-conditioned medium in a cell model of Huntington disease

The physiological role of huntingtin and the pathogenic mechanisms that produce the disease are unknown. Mutant huntingtin changes its normal localization and produces cytoplasmic and intranuclear inclusions, changes gene transcription, alters synaptic transmission, impairs mitochondrial activity and activates caspases and other pro-apoptotic molecules, promotes excitotoxicity, energy deficits, synthesis and release reduction of neurotrophic factors and oxidative stress. Previous studies confirm that the mutant huntingtin difficult neurotrophic function of astrocytes leading to neuronal dysfunction in Huntington’s disease. Our objective was to study the neuroprotective potential role of glia-conditioned medium (GCM) in an in vitro model of Huntington’s disease. We used conditionally-immortalized striatal neuronal progenitor cell lines (STHdhQ7/Q7 and STHdhQ111/Q111) expressing endogenous levels of normal and mutant huntingtin with 7 and 111 glutamines, respectively. We studied the protection of fetal and postnatal glia conditioned medium (GCM) on H2O2 (2 µM), glutamate (5 mM) and 3-nitropropionic acid (2.5 mM) related toxicity. We also compared the neuroprotective effects of GCM versus that of the growth factors bFGF, BDNF and GDNF. Fetal GCM protects from every toxin, reducing the cell death and increasing the cell survival. Fetal GCM reduces the caspases fragmentation of the protein PARP, the expression of chaperone Hsp70 and the accumulation of ROS and polyubiquitinated proteins. In addition, in Q111 striatal cells treated with H2O2 (2 µM) for 24 hours, the intracellular GSH levels are higher in the presence of GCM. Notably, the 13-day and 2-month postnatal GCM, totally protects from H2O2 induced cell death in mutant striatal cells. GCM neuroprotective effects are more potent than those of the already identified neurotrophic factors. We conclude that GCM protects Q111 cells from neuronal neurotoxins and the effects of GCM are more potent than those of any known neurotrophic factor. GCM may contain new and more potent, as yet unidentified, neurotrophic molecules, potentially useful in patients with Huntington’s disease.

Human neuronal uncoupling proteins 4 and 5 (UCP4 and UCP5): structural properties, regulation, and physiological role in protection against oxidative stress and mitochondrial dysfunction

Uncoupling proteins (UCPs) belong to a large family of mitochondrial solute carriers 25 (SLC25s) localized at the inner mitochondrial membrane. UCPs transport protons directly from the intermembrane space to the matrix. Of five structural homologues (UCP1 to 5), UCP4 and 5 are principally expressed in the central nervous system (CNS). Neurons derived their energy in the form of ATP that is generated through oxidative phosphorylation carried out by five multiprotein complexes (Complexes I-V) embedded in the inner mitochondrial membrane. In oxidative phosphorylation, the flow of electrons generated by the oxidation of substrates through the electron transport chain to molecular oxygen at Complex IV leads to the transport of protons from the matrix to the intermembrane space by Complex I, III, and IV. This movement of protons to the intermembrane space generates a proton gradient (mitochondrial membrane potential; MMP) across the inner membrane. Complex V (ATP synthase) uses this MMP to drive the conversion of ADP to ATP. Some electrons escape to oxygen-forming harmful reactive oxygen species (ROS). Proton leakage back to the matrix which bypasses Complex V resulting in a major reduction in ROS formation while having a minimal effect on MMP and hence, ATP synthesis; a process termed "mild uncoupling." UCPs act to promote this proton leakage as means to prevent excessive build up of MMP and ROS formation. In this review, we discuss the structure and function of mitochondrial UCPs 4 and 5 and factors influencing their expression. Hypotheses concerning the evolution of the two proteins are examined. The protective mechanisms of the two proteins against neurotoxins and their possible role in regulating intracellular calcium movement, particularly with regard to the pathogenesis of Parkinson's disease are discussed.

A neuroprotective role of the human uncoupling protein 2 (hUCP2) in a Drosophila Parkinson's disease model

Parkinson's disease (PD), caused by selective loss of dopaminergic (DA) neurons in the substantia nigra pars compacta, is the most common movement disorder. While its etiology remains unknown, mitochondrial dysfunction is recognized as one of the major cellular defects contributing to PD pathogenesis. Mitochondrial uncoupling protein 2 (UCP2) has been implicated in neuroprotection in several neuronal injury models. Here we show that hucp2 expression in Drosophila DA neurons under the control of the tyrosine hydroxylase (TH) promoter protects those flies against the mitochondrial toxin rotenone-induced DA neuron death, head dopamine depletion, impaired locomotor activity and energy deficiency. Under normal conditions, hUCP2 flies maintain an enhanced locomotor activity and have higher steady-state ATP levels suggesting improved energy homeostasis. We show that while no increased mitochondrial DNA content or volume fraction is measured in hUCP2 flies, augmented mitochondrial complex I activity is detected. Those results suggest that it is increased mitochondrial function but not mitochondrial biogenesis that appears responsible for higher ATP levels in hUCP2 flies. Consistent with this notion, an up-regulation of Spargel, the Drosophila peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1) homologue is detected in hUCP2 flies. Furthermore, a Spargel target gene Tfam, the mitochondrial transcription factor A is up-regulated in hUCP2 flies. Taken together, our results demonstrate a neuroprotective effect of hUCP2 in DA neurons in a Drosophila sporadic PD model. Moreover, as the TH promoter activity is present in both DA neurons and epidermis, our results reveal that hucp2 expression in those tissues may act as a stress signal to trigger Spargel activation resulting in enhanced mitochondrial function and increased energy metabolism.