Enhanced mitochondrial fusion during a critical period of synaptic plasticity in adult-born neurons.

Integration of new neurons into adult hippocampal circuits is a process coordinated by local and long-range synaptic inputs. To achieve stable integration and uniquely contribute to hippocampal function, immature neurons are endowed with a critical period of heightened synaptic plasticity, yet it remains unclear which mechanisms sustain this form of plasticity during neuronal maturation. We found that as new neurons enter their critical period, a transient surge in fusion dynamics stabilizes elongated mitochondrial morphologies in dendrites to fuel synaptic plasticity. Conditional ablation of fusion dynamics to prevent mitochondrial elongation selectively impaired spine plasticity and synaptic potentiation, disrupting neuronal competition for stable circuit integration, ultimately leading to decreased survival. Despite profuse mitochondrial fragmentation, manipulation of competition dynamics was sufficient to restore neuronal survival but left neurons poorly responsive to experience at the circuit level. Thus, by enabling synaptic plasticity during the critical period, mitochondrial fusion facilitates circuit remodeling by adult-born neurons.

Metabolic control of adult neural stem cell self-renewal by the mitochondrial protease YME1L.

The transition between quiescence and activation in neural stem and progenitor cells (NSPCs) is coupled with reversible changes in energy metabolism with key implications for lifelong NSPC self-renewal and neurogenesis. How this metabolic plasticity is ensured between NSPC activity states is unclear. We find that a state-specific rewiring of the mitochondrial proteome by the i-AAA peptidase YME1L is required to preserve NSPC self-renewal. YME1L controls the abundance of numerous mitochondrial substrates in quiescent NSPCs, and its deletion activates a differentiation program characterized by broad metabolic changes causing the irreversible shift away from a fatty-acid-oxidation-dependent state. Conditional Yme1l deletion in adult NSPCs in vivo results in defective self-renewal and premature differentiation, ultimately leading to NSPC pool depletion. Our results disclose an important role for YME1L in coordinating the switch between metabolic states of NSPCs and suggest that NSPC fate is regulated by compartmentalized changes in protein network dynamics.

Reweaving the Fabric of Mitochondrial Contact Sites in Astrocytes.

The endoplasmic reticulum (ER) and mitochondria are classically regarded as very dynamic organelles in cell lines. Their frequent morphological changes and repositioning underlie the transient generation of physical contact sites (so-called mitochondria-ER contacts, or MERCs) which are believed to support metabolic processes central for cellular signaling and function. The extent of regulation over these organelle dynamics has likely further achieved a higher level of complexity in polarized cells like neurons and astrocytes to match their elaborated geometries and specialized functions, thus ensuring the maintenance of MERCs at metabolically demanding locations far from the soma. Yet, live imaging of adult brain tissue has recently revealed that the true extent of mitochondrial dynamics in astrocytes is significantly lower than in cell culture settings. On one hand, this suggests that organelle dynamics in mature astroglia in vivo may be highly regulated and perhaps triggered only by defined physiological stimuli. On the other hand, this extent of control may greatly facilitate the stabilization of those MERCs required to maintain regionalized metabolic domains underlying key astrocytic functions. In this perspective, we review recent evidence suggesting that the resulting spatial distribution of mitochondria and ER in astrocytes in vivo may create the conditions for maintaining extensive MERCs within specialized territories - like perivascular endfeet - and discuss the possibility that their enrichment at these distal locations may facilitate specific forms of cellular plasticity relevant for physiology and disease.

Mitochondria-Endoplasmic Reticulum Contacts in Reactive Astrocytes Promote Vascular Remodeling.

Astrocytes have emerged for playing important roles in brain tissue repair; however, the underlying mechanisms remain poorly understood. We show that acute injury and blood-brain barrier disruption trigger the formation of a prominent mitochondrial-enriched compartment in astrocytic endfeet, which enables vascular remodeling. Integrated imaging approaches revealed that this mitochondrial clustering is part of an adaptive response regulated by fusion dynamics. Astrocyte-specific conditional deletion of Mitofusin 2 (Mfn2) suppressed perivascular mitochondrial clustering and disrupted mitochondria-endoplasmic reticulum (ER) contact sites. Functionally, two-photon imaging experiments showed that these structural changes were mirrored by impaired mitochondrial Ca2+ uptake leading to abnormal cytosolic transients within endfeet in vivo. At the tissue level, a compromised vascular complexity in the lesioned area was restored by boosting mitochondrial-ER perivascular tethering in MFN2-deficient astrocytes. These data unmask a crucial role for mitochondrial dynamics in coordinating astrocytic local domains and have important implications for repairing the injured brain.

Mitochondrial Dysfunction Combined with High Calcium Load Leads to Impaired Antioxidant Defense Underlying the Selective Loss of Nigral Dopaminergic Neurons.

Mitochondrial dysfunction is critically involved in Parkinson's disease, characterized by loss of dopaminergic neurons (DaNs) in the substantia nigra (SNc), whereas DaNs in the neighboring ventral tegmental area (VTA) are much less affected. In contrast to VTA, SNc DaNs engage calcium channels to generate action potentials, which lead to oxidant stress by yet unknown pathways. To determine the molecular mechanisms linking calcium load with selective cell death in the presence of mitochondrial deficiency, we analyzed the mitochondrial redox state and the mitochondrial membrane potential in mice of both sexes with genetically induced, severe mitochondrial dysfunction in DaNs (MitoPark mice), at the same time expressing a redox-sensitive GFP targeted to the mitochondrial matrix. Despite mitochondrial insufficiency in all DaNs, exclusively SNc neurons showed an oxidized redox-system, i.e., a low reduced/oxidized glutathione (GSH-GSSG) ratio. This was mimicked by cyanide, but not by rotenone or antimycin A, making the involvement of reactive oxygen species rather unlikely. Surprisingly, a high mitochondrial inner membrane potential was maintained in MitoPark SNc DaNs. Antagonizing calcium influx into the cell and into mitochondria, respectively, rescued the disturbed redox ratio and induced further hyperpolarization of the inner mitochondrial membrane. Our data therefore show that the constant calcium load in SNc DaNs is counterbalanced by a high mitochondrial inner membrane potential, even under conditions of severe mitochondrial dysfunction, but triggers a detrimental imbalance in the mitochondrial redox system, which will lead to neuron death. Our findings thus reveal a new mechanism, redox imbalance, which underlies the differential vulnerability of DaNs to mitochondrial defects.SIGNIFICANCE STATEMENT Parkinson's disease is characterized by the preferential degeneration of dopaminergic neurons (DaNs) of the substantia nigra pars compacta (SNc), resulting in the characteristic hypokinesia in patients. Ubiquitous pathological triggers cannot be responsible for the selective neuron loss. Here we show that mitochondrial impairment together with elevated calcium burden destabilize the mitochondrial antioxidant defense only in SNc DaNs, and thus promote the increased vulnerability of this neuron population.

Pleiotropic Mitochondria: The Influence of Mitochondria on Neuronal Development and Disease.

Mitochondria play many important biological roles, including ATP production, lipid biogenesis, ROS regulation, and calcium clearance. In neurons, the mitochondrion is an essential organelle for metabolism and calcium homeostasis. Moreover, mitochondria are extremely dynamic and able to divide, fuse, and move along microtubule tracks to ensure their distribution to the neuronal periphery. Mitochondrial dysfunction and altered mitochondrial dynamics are observed in a wide range of conditions, from impaired neuronal development to various neurodegenerative diseases. Novel imaging techniques and genetic tools provide unprecedented access to the physiological roles of mitochondria by visualizing mitochondrial trafficking, morphological dynamics, ATP generation, and ultrastructure. Recent studies using these new techniques have unveiled the influence of mitochondria on axon branching, synaptic function, calcium regulation with the ER, glial cell function, neurogenesis, and neuronal repair. This review provides an overview of the crucial roles played by mitochondria in the CNS in physiological and pathophysiological conditions.

Precise Long-Range Microcircuit-to-Microcircuit Communication Connects the Frontal and Sensory Cortices in the Mammalian Brain.

The frontal area of the cerebral cortex provides long-range inputs to sensory areas to modulate neuronal activity and information processing. These long-range circuits are crucial for accurate sensory perception and complex behavioral control; however, little is known about their precise circuit organization. Here we specifically identified the presynaptic input neurons to individual excitatory neuron clones as a unit that constitutes functional microcircuits in the mouse sensory cortex. Interestingly, the long-range input neurons in the frontal but not contralateral sensory area are spatially organized into discrete vertical clusters and preferentially form synapses with each other over nearby non-input neurons. Moreover, the assembly of distant presynaptic microcircuits in the frontal area depends on the selective synaptic communication of excitatory neuron clones in the sensory area that provide inputs to the frontal area. These findings suggest that highly precise long-range reciprocal microcircuit-to-microcircuit communication mediates frontal-sensory area interactions in the mammalian cortex.

Gad1-promotor-driven GFP expression in non-GABAergic neurons of the nucleus endopiriformis in a transgenic mouse line.

Transgenic animals have become a widely used model to identify and study specific cell types in whole organs. Promotor-driven reporter gene labeling of the cells under investigation has promoted experimental efficacy to a large degree. However, rigorous assessment of transgene expression specificity in these animal models is highly recommended to validate cellular identity and to isolate potentially mislabeled cell populations. Here, we report on one such mislabeled neuron population in a widely used transgenic mouse line in which GABAergic somatostatin-expressing interneurons (SOMpos INs) are labeled by eGFP (so-called GIN mouse, FVB-Tg(GadGFP)45704Swn/J). These neurons represent a subpopulation of all SOMpos INs. However, we report here on GFP labeling of non-GABAergic neurons in the nucleus endopiriformis of this mouse line.

Loss of the mitochondrial i-AAA protease YME1L leads to ocular dysfunction and spinal axonopathy.

Disturbances in the morphology and function of mitochondria cause neurological diseases, which can affect the central and peripheral nervous system. The i-AAA protease YME1L ensures mitochondrial proteostasis and regulates mitochondrial dynamics by processing of the dynamin-like GTPase OPA1. Mutations in YME1L cause a multi-systemic mitochondriopathy associated with neurological dysfunction and mitochondrial fragmentation but pathogenic mechanisms remained enigmatic. Here, we report on striking cell-type-specific defects in mice lacking YME1L in the nervous system. YME1L-deficient mice manifest ocular dysfunction with microphthalmia and cataracts and develop deficiencies in locomotor activity due to specific degeneration of spinal cord axons, which relay proprioceptive signals from the hind limbs to the cerebellum. Mitochondrial fragmentation occurs throughout the nervous system and does not correlate with the degenerative phenotype. Deletion of Oma1 restores tubular mitochondria but deteriorates axonal degeneration in the absence of YME1L, demonstrating that impaired mitochondrial proteostasis rather than mitochondrial fragmentation causes the observed neurological defects.

LincRNA H19 protects from dietary obesity by constraining expression of monoallelic genes in brown fat.

Increasing brown adipose tissue (BAT) thermogenesis in mice and humans improves metabolic health and understanding BAT function is of interest for novel approaches to counteract obesity. The role of long noncoding RNAs (lncRNAs) in these processes remains elusive. We observed maternally expressed, imprinted lncRNA H19 increased upon cold-activation and decreased in obesity in BAT. Inverse correlations of H19 with BMI were also observed in humans. H19 overexpression promoted, while silencing of H19 impaired adipogenesis, oxidative metabolism and mitochondrial respiration in brown but not white adipocytes. In vivo, H19 overexpression protected against DIO, improved insulin sensitivity and mitochondrial biogenesis, whereas fat H19 loss sensitized towards HFD weight gains. Strikingly, paternally expressed genes (PEG) were largely absent from BAT and we demonstrated that H19 recruits PEG-inactivating H19-MBD1 complexes and acts as BAT-selective PEG gatekeeper. This has implications for our understanding how monoallelic gene expression affects metabolism in rodents and, potentially, humans.

Critical periods regulating the circuit integration of adult-born hippocampal neurons.

The dentate gyrus (DG) in the adult brain maintains the capability to generate new granule neurons throughout life. Neural stem cell-derived new-born neurons emerge to play key functions in the way information is processed in the DG and then conveyed to the CA3 hippocampal area, yet accumulating evidence indicates that both the maturation process and the connectivity pattern of new granule neurons are not prefigured but can be modulated by the activity of local microcircuits and, on a network level, by experience. Although most of the activity- and experience-dependent changes described so far appear to be restricted to critical periods during the development of new granule neurons, it is becoming increasingly clear that the surrounding circuits may play equally key roles in accommodating and perhaps fostering, these changes. Here, we review some of the most recent insights into this almost unique form of plasticity in the adult brain by focusing on those critical periods marked by pronounced changes in structure and function of the new granule neurons and discuss how the activity of putative synaptic partners may contribute to shape the circuit module in which new neurons become finally integrated.

Spatiotemporal control of mitochondrial network dynamics in astroglial cells.

Mitochondria are increasingly recognized for playing important roles in regulating the evolving metabolic state of mammalian cells. This is particularly true for nerve cells, as dysregulation of mitochondrial dynamics is invariably associated with a number of neuropathies. Accumulating evidence now reveals that changes in mitochondrial dynamics and structure may play equally important roles also in the cell biology of astroglial cells. Astroglial cells display significant heterogeneity in their morphology and specialized functions across different brain regions, however besides fundamental differences they seem to share a surprisingly complex meshwork of mitochondria, which is highly suggestive of tightly regulated mechanisms that contribute to maintain this unique architecture. Here, we summarize recent work performed in astrocytes in situ indicating that this may indeed be the case, with astrocytic mitochondrial networks shown to experience rapid dynamic changes in response to defined external cues. Although the mechanisms underlying this degree of mitochondrial re-shaping are far from being understood, recent data suggest that they may contribute to demarcate astrocyte territories undergoing key signalling and metabolic functions.

Modulating Neuronal Competition Dynamics in the Dentate Gyrus to Rejuvenate Aging Memory Circuits.

The neural circuit mechanisms underlying the integration and functions of adult-born dentate granule cell (DGCs) are poorly understood. Adult-born DGCs are thought to compete with mature DGCs for inputs to integrate. Transient genetic overexpression of a negative regulator of dendritic spines, Kruppel-like factor 9 (Klf9), in mature DGCs enhanced integration of adult-born DGCs and increased NSC activation. Reversal of Klf9 overexpression in mature DGCs restored spines and activity and reset neuronal competition dynamics and NSC activation, leaving the DG modified by a functionally integrated, expanded cohort of age-matched adult-born DGCs. Spine elimination by inducible deletion of Rac1 in mature DGCs increased survival of adult-born DGCs without affecting proliferation or DGC activity. Enhanced integration of adult-born DGCs transiently reorganized adult-born DGC local afferent connectivity and promoted global remapping in the DG. Rejuvenation of the DG by enhancing integration of adult-born DGCs in adulthood, middle age, and aging enhanced memory precision.

Experience-dependent plasticity of adult-born neuron connectivity.

In contrast to most areas of the adult brain, the dentate gyrus (DG) of the hippocampus is endowed with the capability to generate new neurons life-long. While recent evidence suggests that these adult-born neurons exert specialized functions in information processing compared to pre-existing DG granule neurons, to which extent the establishment of their evolving connectivity may be regulated by experience has been elusive. We recently demonstrated that environmental enrichment (EE) induces a surprising input-specific reorganization of the presynaptic connectivity of adult-born neurons, and that this form of structural plasticity appears to large degree confined to a defined period of few weeks shortly after their generation. Here, I briefly discuss how these findings may uncover a previously unknown layer of complexity in the processes regulating the synaptic integration of adult-born neurons and propose that their circuit incorporation within the pre-existing hippocampal network is not prefigured but rather modulated by specific experiences.

A critical period for experience-dependent remodeling of adult-born neuron connectivity.

Neurogenesis in the dentate gyrus (DG) of the adult hippocampus is a process regulated by experience. To understand whether experience also modifies the connectivity of new neurons, we systematically investigated changes in their innervation following environmental enrichment (EE). We found that EE exposure between 2-6 weeks following neuron birth, rather than merely increasing the number of new neurons, profoundly affected their pattern of monosynaptic inputs. Both local innervation by interneurons and to even greater degree long-distance innervation by cortical neurons were markedly enhanced. Furthermore, following EE, new neurons received inputs from CA3 and CA1 inhibitory neurons that were rarely observed under control conditions. While EE-induced changes in inhibitory innervation were largely transient, cortical innervation remained increased after returning animals to control conditions. Our findings demonstrate an unprecedented experience-dependent reorganization of connections impinging onto adult-born neurons, which is likely to have important impact on their contribution to hippocampal information processing.

Pre-existing astrocytes form functional perisynaptic processes on neurons generated in the adult hippocampus.

The adult dentate gyrus produces new neurons that morphologically and functionally integrate into the hippocampal network. In the adult brain, most excitatory synapses are ensheathed by astrocytic perisynaptic processes that regulate synaptic structure and function. However, these processes are formed during embryonic or early postnatal development and it is unknown whether astrocytes can also ensheathe synapses of neurons born during adulthood and, if so, whether they play a role in their synaptic transmission. Here, we used a combination of serial-section immuno-electron microscopy, confocal microscopy, and electrophysiology to examine the formation of perisynaptic processes on adult-born neurons. We found that the afferent and efferent synapses of newborn neurons are ensheathed by astrocytic processes, irrespective of the age of the neurons or the size of their synapses. The quantification of gliogenesis and the distribution of astrocytic processes on synapses formed by adult-born neurons suggest that the majority of these processes are recruited from pre-existing astrocytes. Furthermore, the inhibition of astrocytic glutamate re-uptake significantly reduced postsynaptic currents and increased paired-pulse facilitation in adult-born neurons, suggesting that perisynaptic processes modulate synaptic transmission on these cells. Finally, some processes were found intercalated between newly formed dendritic spines and potential presynaptic partners, suggesting that they may also play a structural role in the connectivity of new spines. Together, these results indicate that pre-existing astrocytes remodel their processes to ensheathe synapses of adult-born neurons and participate to the functional and structural integration of these cells into the hippocampal network.

Sox2-mediated conversion of NG2 glia into induced neurons in the injured adult cerebral cortex.

The adult cerebral cortex lacks the capacity to replace degenerated neurons following traumatic injury. Conversion of nonneuronal cells into induced neurons has been proposed as an innovative strategy toward brain repair. Here, we show that retrovirus-mediated expression of the transcription factors Sox2 and Ascl1, but strikingly also Sox2 alone, can induce the conversion of genetically fate-mapped NG2 glia into induced doublecortin (DCX)(+) neurons in the adult mouse cerebral cortex following stab wound injury in vivo. In contrast, lentiviral expression of Sox2 in the unlesioned cortex failed to convert oligodendroglial and astroglial cells into DCX(+) cells. Neurons induced following injury mature morphologically and some acquire NeuN while losing DCX. Patch-clamp recording of slices containing Sox2- and/or Ascl1-transduced cells revealed that a substantial fraction of these cells receive synaptic inputs from neurons neighboring the injury site. Thus, NG2 glia represent a potential target for reprogramming strategies toward cortical repair.

Polarized expression of p75(NTR) specifies axons during development and adult neurogenesis.

VIDEO ABSTRACT: Newly generated neurons initiate polarizing signals that specify a single axon and multiple dendrites, a process critical for patterning neuronal circuits in vivo. Here, we report that the pan-neurotrophin receptor p75(NTR) is a polarity regulator that localizes asymmetrically in differentiating neurons in response to neurotrophins and is required for specification of the future axon. In cultured hippocampal neurons, local exposure to neurotrophins causes early accumulation of p75(NTR) into one undifferentiated neurite to specify axon fate. Moreover, knockout or knockdown of p75(NTR) results in failure to initiate an axon in newborn neurons upon cell-cycle exit in vitro and in the developing cortex, as well as during adult hippocampal neurogenesis in vivo. Hence, p75(NTR) governs neuronal polarity, determining pattern and assembly of neuronal circuits in adult hippocampus and cortical development.

Inflammation-induced alteration of astrocyte mitochondrial dynamics requires autophagy for mitochondrial network maintenance.

Accumulating evidence suggests that changes in the metabolic signature of astrocytes underlie their response to neuroinflammation, but how proinflammatory stimuli induce these changes is poorly understood. By monitoring astrocytes following acute cortical injury, we identified a differential and region-specific remodeling of their mitochondrial network: while astrocytes within the penumbra of the lesion undergo mitochondrial elongation, those located in the core-the area invaded by proinflammatory cells-experience transient mitochondrial fragmentation. In brain slices, proinflammatory stimuli reproduced localized changes in mitochondrial dynamics, favoring fission over fusion. This effect was triggered by Drp1 phosphorylation and ultimately resulted in reduced respiratory capacity. Furthermore, maintenance of the mitochondrial architecture critically depended on the induction of autophagy. Deletion of Atg7, required for autophagosome formation, prevented the reestablishment of tubular mitochondria, leading to marked reactive oxygen species accumulation and cell death. Thus, our data reveal autophagy to be essential for regenerating astrocyte mitochondrial networks during inflammation.