Mitochondria In Axon Terminals Research Articles

Mitochondrial dysfunction in schizophrenia: With a focus on postmortem studies

Among the many brain abnormalities in schizophrenia are those related to mitochondrial functions such as oxidative stress, energy metabolism and synaptic efficacy. The aim of this paper is to provide a brief review of mitochondrial structure and function and then to present abnormalities in mitochondria in postmortem brain in schizophrenia with a focus on anatomy. Deficits in expression of various mitochondrial genes have been found in multiple schizophrenia cohorts. Decreased activity of complexes I and IV are prominent as well as abnormal levels of individual subunits that comprise the complexes of the electron transport chain. Ultrastructural studies have shown layer, input and cell specific decreases in mitochondria. In cortex, there are fewer mitochondria in axon terminals, neuronal somata of pyramidal neurons and oligodendrocytes in both grey and white matter. In the caudate and putamen mitochondrial number is linked with symptoms and symptom severity. While there is a decrease in the number of mitochondria in astrocytes, mitochondria are smaller in oligodendrocytes. In the nucleus accumbens and substantia nigra, mitochondria are similar in density, size and structural integrity in schizophrenia compared to controls. Mitochondrial production of ATP and calcium buffering are essential in maintaining synaptic strength and abnormalities in these processes could lead to decreased metabolism and defective synaptic activity. Abnormalities in mitochondria in oligodendrocytes might contribute to myelin pathology and underlie dysconnectivity in the brain. In schizophrenia, mitochondria are affected differentially depending on the brain region, cell type in which they reside, subcellular location, treatment status, treatment response and predominant symptoms.

Deletion of Neuronal GLT-1 in Mice Reveals Its Role in Synaptic Glutamate Homeostasis and Mitochondrial Function.

The glutamate transporter GLT-1 is highly expressed in astrocytes but also in neurons, primarily in axon terminals. We generated a conditional neuronal GLT-1 KO using synapsin 1-Cre (synGLT-1 KO) to elucidate the metabolic functions of GLT-1 expressed in neurons, here focusing on the cerebral cortex. Both synaptosomal uptake studies and electron microscopic immunocytochemistry demonstrated knockdown of GLT-1 in the cerebral cortex in the synGLT-1 KO mice. Aspartate content was significantly reduced in cerebral cortical extracts as well as synaptosomes from cerebral cortex of synGLT-1 KO compared with control littermates. 13C-Labeling of tricarboxylic acid cycle intermediates originating from metabolism of [U-13C]-glutamate was significantly reduced in synGLT-1 KO synaptosomes. The decreased aspartate content was due to diminished entry of glutamate into the tricarboxylic acid cycle. Pyruvate recycling, a pathway necessary for full glutamate oxidation, was also decreased. ATP production was significantly increased, despite unaltered oxygen consumption, in isolated mitochondria from the synGLT-1 KO. The density of mitochondria in axon terminals and perisynaptic astrocytes was increased in the synGLT-1 KO. Intramitochondrial cristae density of synGLT-1 KO mice was increased, suggesting increased mitochondrial efficiency, perhaps in compensation for reduced access to glutamate. SynGLT-1 KO synaptosomes exhibited an elevated oxygen consumption rate when stimulated with veratridine, despite a lower baseline oxygen consumption rate in the presence of glucose. GLT-1 expressed in neurons appears to be required to provide glutamate to synaptic mitochondria and is linked to neuronal energy metabolism and mitochondrial function.SIGNIFICANCE STATEMENT All synaptic transmitters need to be cleared from the extracellular space after release, and transporters are used to clear glutamate released from excitatory synapses. GLT-1 is the major glutamate transporter, and most GLT-1 is expressed in astrocytes. Only 5%-10% is expressed in neurons, primarily in axon terminals. The function of GLT-1 in axon terminals remains unknown. Here, we used a conditional KO approach to investigate the significance of the expression of GLT-1 in neurons. We found multiple abnormalities of mitochondrial function, suggesting impairment of glutamate utilization by synaptic mitochondria in the neuronal GLT-1 KO. These data suggest that GLT-1 expressed in axon terminals may be important in maintaining energy metabolism and biosynthetic activities mediated by presynaptic mitochondria.

Synaptic localization of the SUMOylation-regulating protease SENP5 in the adult mouse brain.

Covalent conjugation of small ubiquitin-like modifiers (SUMOs) or SUMOylation is a reversible post-translational modification that regulates the stability and function of target proteins. SUMOs are removed from substrate proteins by sentrin/SUMO-specific proteases (SENPs). Numerous studies have implicated SUMOylation in various physiological and pathological processes in neurons. To understand the functional roles of SUMOylation, it is necessary to determine the distribution of enzymes regulating SUMO conjugation and deconjugation; yet, the localization of SENPs has not been described in detail in intact brain tissue. Here, we report the distribution and subcellular localization of SENP3 and 5 in the adult murine brain. Immunohistochemical analyses revealed the ubiquitous distribution of both SENPs across different brain regions. Within individual cells, SENP3 was confined to the nucleus, consistent with the conventional view that SENPs regulate nuclear events. In contrast, SENP5 was detected in the neuropil but not in cell bodies. Moreover, strong SENP5 immunoreactivity was observed in regions with high numbers of synapses such as the cerebellar glomeruli, suggesting that SENP5 localizes to pre- and/or postsynaptic structures. We performed double immunolabeling in cultured neurons and found that SENP5 co-localized with pre- and post-synaptic markers, as well as a mitochondrial marker. Immunoelectron microscopy confirmed this finding and revealed that SENP5 was localized to presynaptic terminals, postsynaptic spines, and mitochondria in axon terminals. These findings advance the current understanding of the functional roles of SUMOylation in neurons, especially in synaptic regulation, and have implications for future therapeutic strategies in neurodegenerative disorders mediated by mitochondrial dysfunction.

Mitochondria, motor neurons and aging

While the role of mitochondria in aging has been well characterized, their involvement in motor neuron aging remains poorly understood. Thus, we performed an exhaustive ultrastructural study of mitochondria in motor neurons from aged rats that revealed dramatic alterations in the mitochondria of axon terminals at neuromuscular junctions, characterized by swelling, mitochondrial fusion and the presence of megamitochondria. These alterations were not observed in ventral horn motor neurons in the spinal cord of aged rats, which were only altered by the appearance of electron-dense bodies in the dilated matrix cristae. Using X-ray microanalytical techniques we demonstrated the presence of calcium in these bodies, suggesting Ca2+ overload. Moreover, in motor neurons from aged rats, cytochrome c and activated caspase 3 were detected in the cytoplasm of axon terminals at neuromuscular junctions, factors implicated in the apoptosis. Active caspase 3 was also found in the nucleus, soma and axons of aged alpha motor neuron neurons, where it mainly associated with microtubules. The colocalization of dynein and cleaved caspase 3 in neuromuscular junctions is strongly suggestive of the retrograde transport of apoptotic factors to the soma. These results are consistent with the early stages of degeneration in neuromuscular junctions during aging, which is followed by dying back. Given that aging is a key risk factor for neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), the identification of age-related motor neuron degeneration initiated at the distal end of the axon may provide a new therapeutic target for early intervention.

Endogenous peroxidase-like activity in the feline dorsal column nuclei and spinal cord.

Endogenous peroxidase-like activity was investigated with a combined light and electron microscopical technique in 15 cats. The lateral cervical nucleus, the dorsal column nuclei, and segments C6 and L5 of the spinal cord were incubated with diaminobenzidine-tetrahydrochloride (DAB) or tetramethylbenzidine (TMB). After histochemical reaction with DAB a considerable amount of activity was found in nerve cells, astrocytes and pericytes. The neuronal labelling was mainly located in mitochondria of axon terminals and in dendrites whereas the astrocytic and pericytic activity was found in cytoplasmic dense bodies. The quantity of stained structures differed considerably between the animals. In TMB reacted tissue endogenous peroxidase-like activity was only sparsely seen. It was found mainly in frozen sections, in which the neuropil and perivascular structures sometimes contained granules and irregular filaments. The significance of the findings is discussed in relation to observations in tracer studies using horseradish peroxidase.