Mitochondrial sodium/calcium exchanger NCLX regulates glycolysis in astrocytes, impacting on cognitive performance.

Intracellular Ca2+ concentrations are strictly controlled by plasma membrane transporters, the endoplasmic reticulum, and mitochondria, in which Ca2+ uptake is mediated by the mitochondrial calcium uniporter complex (MCUc), while efflux occurs mainly through the mitochondrial Na+ /Ca2+ exchanger (NCLX). RNAseq database repository searches led us to identify the Nclx transcript as highly enriched in astrocytes when compared with neurons. To assess the role of NCLX in mouse primary culture astrocytes, we inhibited its function both pharmacologically or genetically. This resulted in re-shaping of cytosolic Ca2+ signaling and a metabolic shift that increased glycolytic flux and lactate secretion in a Ca2+ -dependent manner. Interestingly, in vivo genetic deletion of NCLX in hippocampal astrocytes improved cognitive performance in behavioral tasks, whereas hippocampal neuron-specific deletion of NCLX impaired cognitive performance. These results unveil a role for NCLX as a novel modulator of astrocytic glucose metabolism, impacting on cognition.

Aberrant upregulation of the glycolytic enzyme PFKFB3 in CLN7 neuronal ceroid lipofuscinosis.

CLN7 neuronal ceroid lipofuscinosis is an inherited lysosomal storage neurodegenerative disease highly prevalent in children. CLN7/MFSD8 gene encodes a lysosomal membrane glycoprotein, but the biochemical processes affected by CLN7-loss of function are unexplored thus preventing development of potential treatments. Here, we found, in the Cln7∆ex2 mouse model of CLN7 disease, that failure in autophagy causes accumulation of structurally and bioenergetically impaired neuronal mitochondria. In vivo genetic approach reveals elevated mitochondrial reactive oxygen species (mROS) in Cln7∆ex2 neurons that mediates glycolytic enzyme PFKFB3 activation and contributes to CLN7 pathogenesis. Mechanistically, mROS sustains a signaling cascade leading to protein stabilization of PFKFB3, normally unstable in healthy neurons. Administration of the highly selective PFKFB3 inhibitor AZ67 in Cln7∆ex2 mouse brain in vivo and in CLN7 patients-derived cells rectifies key disease hallmarks. Thus, aberrant upregulation of the glycolytic enzyme PFKFB3 in neurons may contribute to CLN7 pathogenesis and targeting PFKFB3 could alleviate this and other lysosomal storage diseases.

Opa1 relies on cristae preservation and ATP synthase to curtail reactive oxygen species accumulation in mitochondria.

Reactive oxygen species (ROS) are a common product of active mitochondrial respiration carried in mitochondrial cristae, but whether cristae shape influences ROS levels is unclear. Here we report that the mitochondrial fusion and cristae shape protein Opa1 requires mitochondrial ATP synthase oligomers to reduce ROS accumulation. In cells fueled with galactose to force ATP production by mitochondria, cristae are enlarged, ATP synthase oligomers destabilized, and ROS accumulate. Opa1 prevents both cristae remodeling and ROS generation, without impinging on levels of mitochondrial antioxidant defense enzymes that are unaffected by Opa1 overexpression. Genetic and pharmacologic experiments indicate that Opa1 requires ATP synthase oligomerization and activity to reduce ROS levels upon a blockage of the electron transport chain. Our results indicate that the converging effect of Opa1 and mitochondrial ATP synthase on mitochondrial ultrastructure regulate ROS abundance to sustain cell viability.

Abrogating mitochondrial ROS in neurons or astrocytes reveals cell-specific impact on mouse behaviour.

Cells naturally produce mitochondrial reactive oxygen species (mROS), but the in vivo pathophysiological significance has long remained controversial. Within the brain, astrocyte-derived mROS physiologically regulate behaviour and are produced at one order of magnitude faster than in neurons. However, whether neuronal mROS abundance differentially impacts on behaviour is unknown. To address this, we engineered genetically modified mice to down modulate mROS levels in neurons in vivo. Whilst no alterations in motor coordination were observed by down modulating mROS in neurons under healthy conditions, it prevented the motor discoordination caused by the pro-oxidant neurotoxin, 3-nitropropionic acid (3-NP). In contrast, abrogation of mROS in astrocytes showed no beneficial effect against the 3-NP insult. These data indicate that the impact of modifying mROS production on mouse behaviour critically depends on the specific cell-type where they are generated.

Intertwined ROS and Metabolic Signaling at the Neuron-Astrocyte Interface.

Metabolism and redox signalling share critical nodes in the nervous system. In the last years, a series of major findings have challenged the current vision on how neural reactive oxygen species (ROS) are produced and handled in the nervous system. Once regarded as deleterious by-products, ROS are now shown to be essential for a metabolic and redox crosstalk. In turn, this coupling defines neural viability and function to control behaviour or leading to neurodegeneration when compromised. Findings like a different assembly of mitochondrial respiratory supercomplexes in neurons and astrocytes stands behind a divergent production of ROS in either cell type, more prominent in astrocytes. ROS levels are however tightly controlled by an antioxidant machinery in astrocytes, assumed as more efficient than that of neurons, to regulate redox signalling. By exerting this control in ROS abundance, metabolic functions are finely tuned in both neural cells. Further, a higher engagement of mitochondrial respiration and oxidative function in neurons, underpinned by redox equivalents supplied from the pentose phosphate pathway and from glia, differs from the otherwise strong glycolytic capacity of astrocytes. Here, we recapitulate major findings on how ROS and metabolism differ between neural cells but merge to define reciprocal signalling pathways, ultimately defining neural function and fate.

Glucose metabolism links astroglial mitochondria to cannabinoid effects.

Astrocytes take up glucose from the bloodstream to provide energy to the brain, thereby allowing neuronal activity and behavioural responses1-5. By contrast, astrocytes are under neuronal control through specific neurotransmitter receptors5-7. However, whether the activation of astroglial receptors can directly regulate cellular glucose metabolism to eventually modulate behavioural responses is unclear. Here we show that activation of mouse astroglial type-1 cannabinoid receptors associated with mitochondrial membranes (mtCB1) hampers the metabolism of glucose and the production of lactate in the brain, resulting in altered neuronal functions and, in turn, impaired behavioural responses in social interaction assays. Specifically, activation of astroglial mtCB1 receptors reduces the phosphorylation of the mitochondrial complex I subunit NDUFS4, which decreases the stability and activity of complex I. This leads to a reduction in the generation of reactive oxygen species by astrocytes and affects the glycolytic production of lactate through the hypoxia-inducible factor 1 pathway, eventually resulting in neuronal redox stress and impairment of behavioural responses in social interaction assays. Genetic and pharmacological correction of each of these effects abolishes the effect of cannabinoid treatment on the observed behaviour. These findings suggest that mtCB1 receptor signalling can directly regulate astroglial glucose metabolism to fine-tune neuronal activity and behaviour in mice.

An ex vivo Approach to Assess Mitochondrial ROS by Flow Cytometry in AAV-tagged Astrocytes in Adult Mice.

Mitochondrial reactive oxygen species (mROS) are naturally produced signalling molecules extremely relevant for understanding both health- and disease-associated biological processes. The study of mROS in the brain is currently underway to decipher their physiopathological roles and contributions in neurological diseases. Recent advances in this field have highlighted the importance of studying mROS signalling and redox biology at the cellular level. Neurons are especially sensitive to the harmful effects of excess mROS while astrocytic mROS have been shown to play a relevant physiological role in cerebral homeostasis and behaviour. However, given the complexity of the brain, investigating mROS formation in a specific cell-type in adult animals is methodologically challenging. Here we propose an approach to specifically assess mROS abundance in astrocytes that combines i) a targeting strategy based on the use of adeno-associated virus (AAV) vectors expressing the green fluorescent protein (GFP) under an astrocyte (glial fibrillary acidic protein or GFAP) promoter, along with, ii) a robust and widely extended protocol for the measurement of mROS by flow cytometry using commercial probes. The significance of this work is that it allows the selective study of astrocytic mROS abundance by means of easily accessible technology.

Astrocytic mitochondrial ROS modulate brain metabolism and mouse behaviour.

To satisfy its high energetic demand1, the brain depends on the metabolic cooperation of various cell types2-4. For example, astrocytic-derived lactate sustains memory consolidation5 by serving both as an oxidizable energetic substrate for neurons6 and as a signalling molecule7,8. Astrocytes and neurons also differ in the regulation of glycolytic enzymes9 and in the organization of their mitochondrial respiratory chain10. Unlike neurons, astrocytes rely on glycolysis for energy generation9 and, as a consequence, have a loosely assembled mitochondrial respiratory chain that is associated with a higher generation of mitochondrial reactive oxygen species (ROS)10. However, whether this abundant natural source of mitochondrial ROS in astrocytes fulfils a specific physiological role is unknown. Here we show that astrocytic mitochondrial ROS are physiological regulators of brain metabolism and neuronal function. We generated mice that inducibly overexpress mitochondrial-tagged catalase in astrocytes and show that this overexpression decreases mitochondrial ROS production in these cells during adulthood. Transcriptomic, metabolomic, biochemical, immunohistochemical and behavioural analysis of these mice revealed alterations in brain redox, carbohydrate, lipid and amino acid metabolic pathways associated with altered neuronal function and mouse behaviour. We found that astrocytic mitochondrial ROS regulate glucose utilization via the pentose-phosphate pathway and glutathione metabolism, which modulates the redox status and potentially the survival of neurons. Our data provide further molecular insight into the metabolic cooperation between astrocytes and neurons and demonstrate that mitochondrial ROS are important regulators of organismal physiology in vivo.

MiRNA expression profile of chronic lymphocytic leukemia patients with 13q deletion.

Deletion 13q (13q-) is the most common cytogenetic aberration in chronic lymphocytic leukemia (CLL) and is associated with the most favorable prognosis as the sole cytogenetic abnormality. However, it is heterogeneous whereby CLL patients with higher percentages of 13q- cells (13q-H) have a more aggressive clinical course and a distinct gene expression profile. The microRNA (miRNA) expression profile of CLL gives additional biological and prognostic information, but its expression in 13q- CLL has not been examined in detail. The miRNA expression of clonal B cell lymphocytes (CD19+ cells) of 38 CLL patients and normal B cells of six healthy donors was analyzed. CLL patients with higher percentages of 13q- cells (≥80%) showed a different level of miRNA expression from patients with lower percentages (<80%). Interestingly, miR-143 was downregulated and miR-155 was overexpressed in 13q-H. This deregulation affected important validated target genes involved in apoptosis (BCL2, MDM2, TP53INP1) and proliferation (KRAS, PI3K-AKT signaling), that could lead to decreased apoptosis and increased proliferation in 13q-H patients. This study provides new evidence about the heterogeneity of the 13q deletion in CLL patients, showing that miRNA regulation could be involved in several significant pathways deregulated in CLL patients with a high number of losses in 13q.