Ultrastructural localization of phospholipid synthesis in the rat trigeminal nerve during myelination

Annexin A6 controls neuronal membrane dynamics throughout chick cranial sensory gangliogenesis

Cranial sensory ganglia are components of the peripheral nervous system that possess a significant somatosensory role and include neurons within the trigeminal and epibranchial nerve bundles. Although it is well established that these ganglia arise from interactions between neural crest and neurogenic placode cells, the molecular basis of ganglia assembly is still poorly understood. Members of the Annexin protein superfamily play key roles in sensory nervous system development throughout metazoans. Annexin A6 is expressed in chick trigeminal and epibranchial placode cell-derived neuroblasts and neurons, but its function in cranial ganglia formation has not been elucidated. To this end, we interrogated the role of Annexin A6 using gene perturbation studies in the chick embryo. Our data reveal that placode cell-derived neuroblasts with reduced Annexin A6 levels ingress and migrate normally to the ganglionic anlage, where neural crest cell corridors correctly form around them. Strikingly, while Annexin A6-depleted placode cell-derived neurons still express mature neuronal markers, they fail to form two long processes, which are considered morphological features of mature neurons, and no longer innervate their designated targets due to the absence of this bipolar morphology. Moreover, overexpression of Annexin A6 causes some placode cell-derived neurons to form extra protrusions alongside these bipolar processes. These data demonstrate that the molecular program associated with neuronal maturation is distinct from that orchestrating changes in neuronal morphology, and, importantly, reveal Annexin A6 to be a key membrane scaffolding protein during sensory neuron membrane biogenesis. Collectively, our results provide novel insight into mechanisms underscoring morphological changes within placode cell-derived neurons that are essential for cranial gangliogenesis.

Developmental regulation of connexins 26, 32, 36, and 43 in trigeminal neurons

The transition from sucking to chewing during postnatal development is accompanied by changes in masticatory muscle activity patterns. We previously demonstrated that changes in numerous parameters of chemical synapses among neurons, and intrinsic membrane properties of neurons, comprising brainstem oral-motor circuits are coincident with changes in masticatory muscle activity patterns. Considering recent findings that implicate a role for gap junctions in early locomotor and respiratory behaviors, our present study focuses on the developmental regulation of connexin proteins in trigeminal neurons as a first step in understanding a role for gap junctions in developing oral-motor circuits used for ingestive behaviors. We conducted immunohistochemistry studies to examine connexin (Cx) 26, 32, 36, and 43 expression in trigeminal motor and mesencephalic trigeminal nuclei during postnatal development at the light and electron microscopic levels. Postnatal days (P) 1, 6, 14, 21, and adult mice were used. Cx32, 36, and 43 expression was developmentally regulated in the trigeminal motor nucleus, while Cx26 expression remained high throughout postnatal development. In the mesencephalic trigeminal nucleus, Cx26, 32, and 43 expression was intense throughout development, with only Cx36 showing a developmental regulation. Ultrastructural examination of neonatal trigeminal motoneurons and mesencephalic trigeminal neurons revealed connexin expression in cell membranes, cytoplasm, and cell nuclei (Cx43, Cx32). Our results show that connexin proteins are differentially regulated between trigeminal motoneurons and mesencephalic trigeminal neurons during development, and suggest a possible role for gap junctions in the development of trigeminal neurons and the function and maturation of oral-motor circuits.

Remodeling and sorting process of ethanolamine and choline glycerophospholipids during their axonal transport in the rabbit optic pathway

The existence of a mechanism by which the ester- and ether-linked aliphatic chains of the major phospholipids are retailored during their axonal transport and sorted to specific membrane systems along the optic nerve and tract was investigated. A mixture of [1-14C]hexadecanol and [3H]arachidonic acid was injected into the vitreous body of albino rabbits. At 24 h and 8 days later, the distribution (as measured by the 3H/14C ratio) and the positioning (as monitored by hydrolytic procedures) of radioactivity in the various phospholipid classes of retina, purified axons, and myelin of the optic nerve and tract were determined. At the two intervals after labeling, the 3H/14C ratios of each diradyl type of phosphatidylethanolamine and phosphatidylcholine were (a) substantially unchanged all along the axons within the optic nerve and tract and (b) markedly modified in comparison with those found in the retina and axons for molecular species selectively restricted to myelin sheath. Evidence is thus available that intraxonally moving ethanolamine and choline glycerophospholipids, among others, are added to axonal membranes most likely without extensive modifications. In contrast, they are transferred into myelin after retailoring. Through these two processes, the sorting and targeting of newly synthesized phospholipids to their correct membrane domains, such as axoplasmic organelles, axolemma, or periaxonal myelin, could be controlled.

Acyltransferase and acid hydrolase activities of the abalone photoreceptor cell

In order to study the synthesis and degradation processes of the photoreceptor membranes in the abalone, Nordotis discus, the localization of acyltransferase and acid hydrolase activities, respectively, were determined at the electron-microscopic level. Acyltransferase activity was localized on the cytoplasmic sides of thick (greater than 10 nm) membranes of the following organelles: a few cisternae at the trans (or concave) side of Golgi apparatus, Golgi and probably related vesicles, short tubules, curved pentalaminar disks and limiting membranes of the phagosomal multivesicular bodies; all organelles were scattered in the peri- to supranuclear cytoplasm. The phospholipids, which are major components of the photoreceptor membrane, are considered to be synthesized by these membranes. Acid phosphatase activity was localized in the lumina of Golgi cisternae and vesicles, lysosomes, and smaller multivesicular and related bodies, but not in multilamellar bodies. The matrices of the larger multivesicular bodies and of the pigment granule complexes showed arylsulfatase activity. Vesiculated and autophagocytosed photoreceptor microvilli seemed to be degraded by acid hydrolases, forming multivesicular and related bodies. Supporting cells also showed acyltransferase and acid hydrolase activities.

Phospholipid synthesis in the squid giant axon: enzymes of phosphatidylinositol metabolism

We examined the properties of several enzymes of phospholipid metabolism in axoplasm extruded from squid giant axons. The following synthetic enzymes, CDP-diglyceride: inositol transferase (EC 2.7.8.11), ATP:diglyceride phosphotransferase, diglyceride kinase (EC 2.7.2.-), and phosphatidylinositol kinase (EC 2.7.1.67), were all present in axoplasm. Phospholipid exchange proteins, which catalyzed the transfer of phosphatidylinositol and phosphatidylcholine between membrane preparations and unilamellar lipid vesicles, were also found. However, we did not find conditions under which the synthesis of CDP-diglyceride, phosphatidylserine, and phosphatidylinositol-4,5-diphosphate could be measured. Subcellular fractionation by differential centrifugation showed that the axoplasmic inositol transferase and phosphatidylinositol kinase activities were largely "microsomal," while the diglyceride kinase and exchange protein activities were primarily "cytosolic."

Ultrastructural localization of glycerolipid synthesis in rod cells of the isolated frog retina

The incorporation of two glycerolipid precursors, 3H-glycerol and 3H-choline, into rod cells of the isolated frog retina has been studied using quantitative electron microscope autoradiography. The results indicate that the endoplasmic reticulum (ER) is the major site of early incorporation of these precursors suggesting that the ER is the primary site of lipid synthesis. Of the different types of ER present in rod cells, the rough ER (RER) and nuclear envelope predominate in this activity. The organized region of smooth ER (SER) in the subellipsoid region does not appear to be of major quantitative importance, although SER closely intermingled with RER in the myoid region may be involved to some extent. We also compared the pattern of labelling observed at various incubation times in 3H-glycerol and 3H-choline with that observed with 3H-leucine. Differences were observed between the pattern of lipid and protein labelling, particularly in the labelling of the Golgi apparatus, mitochondria, plasma membrane, presynaptic terminals and outer segments. This suggests that lipids and proteins may differ in some aspects of the routes and mechanisms by which they are transported from their sites of synthesis to the membrane delimited compartments for which they are destined.

Acyltransferase activity and cytochemical localization in differentiating neuroblastoma

Acyltransferase (AT) enzyme activity was assayed biochemically and localized cytochemically in neuroblastoma monolayers to investigate the possible role of the enzyme in neuroblastoma differentiation and neurite extension. Treatment of cultures for 1 day with serum-free medium or Ro20-1724, a cyclic AMP phosphodiesterase inhibitor, induced neurite outgrowth but did not alter acyltransferase activity. Treatment for 4 days with dexamethasone or Ro20-1724 induced neurite outgrowth and a doubling of enzyme activity per cell. Chromatographic separation of lipid classes indicated that dexamethasone enhanced triacylglyceride synthesis. Acyltransferase was localized in mitochondria of neuroblastoma cells. The results show that 1) dexamethasone stimulates the storage lipid metabolic pathway in neuroblastoma cells and 2) increased acyltransferase activity is concomitant with dexamethasone-induced morphological differentiation. However, AT activity and neurite extension may not be causally related.

Membrane glycolipids: regional synthesis and axonal transport in a single identified neuron of Aplysia californica

Glycolipids moving along an identified axon Aplysia californica were synthesized and incorporated into intracytoplasmic membranes solely in the perikaryon: direct injection of tritiated sugar into the axon revealed no local synthesis or exchange. There was no indication for transfer into axoplasm from glia. Insertion of glycolipids into nascent membranes occurs coordinately with insertion of protein components in the cell body.

Structure of cell coats during initial stages of synapse formation on isolated cultured sympathetic neurons

The mordant tannic acid was applied after routine aldehyde and osmium fixation in order to study the ultrastructure of membrane coat material during synaptogenesis in combination cultures of superior cervical ganglion neurons and spinal cord explants. All contact between cultured neurons was mediated by coat material; apposition of outer membrane leaflets never occurred. In most areas, the contact found between membrane coat material lacked special arrangement, but in areas of contact between spinal cord growth cones and sympathetic ganglion neurons, distinct patches of bilateral membrane specialization were seen. Here apposed coat material interdigitated across the intracellular gap, outer membrane leaflets were parallel and dense material was evident in the adjacent cytoplasm. These 'differentiated' contacts were seen both prior to and simultaneously with the appearance in the same area of other synaptic organelles; they did not appear to be precursors of synaptic active zones. Findings suggest that a mosaic of specificities in neuronal coat material may determine the site of 'differentiated' contacts and that these contacts may be relevant to the subsequent formation of a synaptic active zone.

Incorporation of newly formed lecithin into peripheral nerve myelin

Radioactive choline was used to study the metabolism and movement of choline-containing phospholipids in peripheral nerve myelin of adult mice. Incorporation at various times after intraperitoneal injection was measured in serial segments of sciatic nerve as well as in myelin isolated from those segments. At no time (1 h to 35 days) could a proximal-distal difference in the extent of labeling be demonstrated. This finding suggests that incorporation of precursor choline phospholipids into nerve membranes is a local event, with little contribution from the neuronal perikaryon via axoplasmic transport. Autoradiographic investigations were undertaken to elucidate the pattern of movement of radioactive choline-labeled phospholipids, predominantly lecithin, into the myelin sheaths of the sciatic nerve. A sequence of autoradiographs was prepared from animals sacrificed between 20 min and 35 days after a microinjection of precursor directly into the nerve. Analysis of these autoradiograms revealed that labeling is initially concentrated in the Schwann cell cytoplasm. Later, the label moves first into the outer regions of the myelin sheaths and is eventually distributed evenly throughout the inner and outer layers of the sheath. At no time is there a build-up of label in the axon. The rate of uptake of precursor and subsequent redistribution of lecithin into the myelin were also examined in frog sciatic nerve (18 degrees C). Both uptake and redistribution processes were considerably slower in the cold-blooded animal.

Morphological changes in the neuritic growth cone and target neuron during synaptic junction development in culture

Our object was to characterize the morphological changes occurring in pre- and postsynaptic elements during their initial contact and subsequent maturation into typical synaptic profiles. Neurons from superior cervical ganglia (SCG) of perinatal rats were freed of their supporting cells and established as isolated cells in culture. To these were added explants of embryonic rat thoracic spinal cord to allow interaction between outgrowing cord neurites and the isolated autonomic neurons. Time of initial contact was assessed by light microscopy; at timed intervals thereafter, cultures were fixed for electron microscopy. Upon contact, growth cone filopodia became extensively applied to the SCG neuronal plasmalemma and manifested numerous punctate regions in which the apposing plasma membranes were separated by only 7-10 nm. The Golgi apparatus of the target neuron hypertrophied, and its production of coated vesicles increased. Similar vesicles were seen in continuity with the SCG plasmalemma near the close contact site; their apparent contribution of a region of postsynaptic membrane with undercoating was considered to be the first definitive sign of synapse formation. Tracer work with peroxidase and ferritin confirmed that the traffic of coated vesicles within the neuronal soma is largely from Golgi region to somal surface. Subsequent to the appearance of postsynaptic density, the form and content of the growth cone was altered by the loss of filopodia and the appearance of synaptic vesicles which gradually became clustered opposite the postsynaptic density. As the synapse matured, synaptic vesicles increased in number, cleft width and content increased, presynaptic density appeared, branched membranous reticulum became greatly diminished, and most lysosomal structures disappeared. Coated vesicles continued to be associated with the postsynaptic membrane at all stages of maturation. The incorporation of Golgi-derived vesicles into discrete regions of the cell membrane could provide the mechanism for confining specific characteristics of the neuronal membrane to the synaptic region.