Subcellular fractionation of rat cerebellum: separation of synaptosomal populations and heterogeneity of mitochondria

Exploring Subcellular Cerebellar Fractions with the Electron Microscope

Differential ultracentrifugation and subcellular fractionation historically helped to study the components of the cell, to discover new cellular organelles, and to decipher their morphological and molecular properties. In neuroscience, the technique has yielded important results on neuron biochemistry and the mechanisms of synaptic transmission. This Cerebellar Classic is devoted to the pioneering work of Manuel del Cerro, Ray S. Snider, and Mary Lou Oster-Granite, who isolated purified fractions after successive centrifugations of the rat cerebellum from birth to adulthood and studied them under the electron microscope.

Commentary on "Subcellular Fractions of Adult and Developing Rat Cerebellum, by M. del Cerro, R. S. Snider and M. L. Oster, July 1969"

Subcellular fractionation by differential ultracentrifugation has allowed the study of the cell and its organelles from a morphological, physiological, and biochemical perspective. Combined with electron microscopy, and by using animals at different stages of postnatal development, these methods yielded useful results concerning the ontogeny of synaptosomes, mitochondria, and myelin and broadened the possibilities to investigate the molecular underpinnings of cerebellar histogenesis.

Reconstruction of pathways associated with amino acid metabolism in human mitochondria

We have used a bioinformatics approach for the identification and reconstruction of metabolic pathways associated with amino acid metabolism in human mitochondria. Human mitochondrial proteins determined by experimental and computational methods have been superposed on the reference pathways from the KEGG database to identify mitochondrial pathways. Enzymes at the entry and exit points for each reconstructed pathway were identified, and mitochondrial solute carrier proteins were determined where applicable. Intermediate enzymes in the mitochondrial pathways were identified based on the annotations available from public databases, evidence in current literature, or our MITOPRED program, which predicts the mitochondrial localization of proteins. Through integration of the data derived from experimental, bibliographical, and computational sources, we reconstructed the amino acid metabolic pathways in human mitochondria, which could help better understand the mitochondrial metabolism and its role in human health.

Molecular characterization of mitocalcin, a novel mitochondrial Ca2+-binding protein with EF-hand and coiled-coil domains

Here we have identified and characterized a novel mitochondrial Ca2+-binding protein, mitocalcin. Western blot analysis demonstrated that mitocalcin was widely expressed in mouse tissues. The expression in brain was increased during post-natal to adult development. Further analyses were carried out in newly established neural cell lines. The protein was expressed specifically in neurons but not in glial cells. Double-labeling studies revealed that mitocalcin was colocalized with mitochondria in neurons differentiated from 2Y-3t cells. In addition, mitocalcin was enriched in the mitochondrial fraction purified from the cells. Immunohistochemical studies on mouse cerebellum revealed that the expression pattern of mitocalcin in glomeruli of the internal granular and molecular layers was well overlapped by the distribution pattern of mitochondria. Immunogold electron microscopy showed that mitocalcin was associated with mitochondrial inner membrane. Overexpression of mitocalcin in 2Y-3t cells resulted in neurite extension. Inhibition of the expression in 2Y-3t cells caused suppression of neurite outgrowth and then cell death. These findings suggest that mitocalcin may play roles in neuronal differentiation and function through the control of mitochondrial function.

Differences in some of the metabolic properties of mitochondria isolated from cerebral cortex and olfactory bulb of the rat

The metabolic properties of mitochondria from rat cerebral cortex and olfactory bulb were investigated. The pyruvate-supported oxygen uptake rates by olfactory bulb mitochondria were significantly lower than those by cerebrocortical mitochondria. This is consistent with the differences in pyruvate dehydrogenase complex activities between these mitochondrial preparations. Pyruvate dehydrogenase kinase, NAD-linked isocitrate dehydrogenase, and hexokinase activities in olfactory bulb mitochondria were significantly lower than those in cerebrocortical mitochondria. However, NADP-linked isocitrate dehydrogenase, and NAD-linked and NADP-linked glutamate dehydrogenase activities in olfactory bulb mitochondria were significantly higher than those in cerebrocortical mitochondria. The differences between these two mitochondrial preparations in terms of the activities of these energy-metabolizing enzymes reflect the differences detected in the homogenates of these regions.

Fine structure of squid (Loligo pealei) optic lobe synapses

Cephalopod optic lobes are a well-known source of cholinergic nerve endings [Dowdall and Whittaker (1973) J. Neurochem, 20, 921-935]. In order to utilize this property for subsequent analyses of cholinergic mechanisms of transmission in the CNS, we describe the ultrastructure of the entire optic lobe of the squid (Loligo pealei) and relate the morphology of synaptosomes to the intact tissue. In the cortex, chemical junctions were found showing two basic forms. The first was an invaginated synapse, appearing only between presynaptic bags and spines which may originate from the trunks of amacrine cells of the outer granule layer. The second was that of a typical synapse, found in almost all layers except the upper portion of the first radial layer. Synapses in the medulla were predominantly of the second type, although a few photoreceptor endings extended to this region as well. The different types of terminals observed in the intact squid optic lobe corresponded to the different types of endings recognized in a synaptosome fraction derived from these lobes. Because of its high content of cholinergic endings and distinct synaptic types, the squid optic lobe may contribute to the elucidation of the mechanisms of cholinergic transmission in the central nervous system. In addition, electrotonic synapses were found between photoreceptor processes in the cortex, as well as other elements of the neuropil.

The activities of some energy-metabolising enzymes in nonsynaptic (free) and synaptic mitochondria derived from selected brain regions

The enzyme complement of two different mitochondrial preparations from adult rat brain has been studied. One population of mitochondria (synaptic) is prepared by the lysis of synaptosomes, the other (non-synaptic or free) by separation from homogenates. These populations have been prepared from distinct regions of the brain: cortex, striatum, and pons and medulla oblongata. The following enzymes have been measured: pyruvate dehydrogenase (EC 1.2.4.1), citrate synthase (EC 4.1.3.7), NAD-linked isocitrate dehydrogenase (EC 1.1.1.41), NADP-linked isocitrate dehydrogenase (EC 1.1.1.42), fumarase (EC 4.2.1.2), NAD-linked malate dehydrogenase (EC 1.1.1.37), D-3-hydroxybutyrate dehydrogenase (EC 1.1.1.30), and mitochondrially bound hexokinase (EC 2.7.1.1) and creatine kinase (EC 2.7.3.2). The nonsynaptic (free) mitochondria show higher enzyme specific activities in the regions studied than the corresponding values recorded for the synaptic mitochondria. The significance of these observations is discussed in the light of the different metabolic activities of the two populations of mitochondria and the compartmentation of the metabolic activities of the brain.

MIT-23: a mitochondrial marker for terminal neuronal differentiation defined by a monoclonal antibody

We describe a monoclonal antibody directed against a neuron-specific mitochondrial protein from rat brain. On protein blots the antibody recognizes a single polypeptide of apparent molecular weight of 23,000. By solid-phase immunoassay the antigen was detected in nonneural tissues. Within neurons, the antibody stains cytoplasmic granules that immunoelectron microscopy shows are mitochondria, hence the designation MIT-23. Immunocytochemical staining of the cerebellar cortex showed that MIT-23 occurs in all the neuronal types but is absent from glial and other nonneuronal cells. During neonatal development of the cerebellum, MIT-23 appears in neurons after their final cell division or migration is completed, suggesting that specific proteins associated with mitochondria participate in neuronal maturation.

Acetyl-CoA synthesizing enzymes in cholinergic nerve terminals

The activities of five enzymes involved in acetyl-CoA synthesis, pyruvate dehydrogenase complex, ATP citrate lyase, carnitine acetyltransferase, acetyl-CoA synthetase, and citrate synthase, were determined in normal nucleus interpeduncularis and nucleus interpeduncularis in which cholinergic terminals were removed following lesion of the habenulointerpeduncular tract. The activities of aspartate transaminase, fumarase, and GABA transaminase also were determined to compare the effect of lesion on other mitochondrial enzymes which are not linked to the biosynthesis of ACh. In normal nucleus interpeduncularis the activities of carnitine acetyltransferase and pyruvate dehydrogenase complex were higher than the activity of ChAT (choline acetyltransferase), whereas the activities of acetyl-CoA synthetase and citrate synthase were considerably lower than that of ChAT. The effect of the lesion separated the enzymes into two groups: the activities of pyruvate dehydrogenase complex, carnitine acetyltransferase, fumarase and aspartate transaminase decreased by 30--40%, whereas the activities of the other enzymes descreased 5--15%. ChAT activity was in all cases less than 15% of normal. It could be concluded that none of the acetyl-CoA synthesizing enzymes decreased to the degree that ChAT did. Only pyruvate dehydrogenase complex and carnitine acetyltransferase seem to be localized in cholinergic terminals to a significant degree. ATP citrate lyase as well as acetyl-CoA synthetase seem to have less significance in supporting acetyl-CoA formation in cholinergic nerve terminals.