PROTEIN SYNTHESIS IN THE BRAIN OF OCTOPUS VULGARIS, LAM

Abstract— The process of protein synthesis in the brain of Octopus vulgaris Lam has been examined after systemic administration of [³H]leucine and upon incubation of the tissue in sea water containing the radioactive precursor. After injection of [³H]leucine in the branchial heart, the radioactivity of the TCA-soluble fractions of the three main brain divisions reached a maximum in about 30 min and decreased thereafter, while incorporation into the protein fractions was complete in approx. 2 h. Per unit wet weight the radioactivity of brain proteins was higher than that of most other organs. In vitro the rate of incorporation of [³H]leucine in the protein fraction of the optic lobe remained low for more than 1 h, but increased several fold thereafter. Preincubation of the tissue in sea water abolished the lag period. Similar effects were observed in the vertical lobe as well as in the optic lobe of young and adult octopuses but not in the white body, a non-nervous organ. The process of protein synthesis in the optic lobe is markedly inhibited by puromycin, cycloheximide and chloramphenicol. Electrophoretic analysis on polyacrylamide gels indicated that the soluble proteins labelled in vitro and in vivo are similar.     

SITE OF BIOSYNTHESIS OF BRAIN-SPECIFIC PROTEINS IN THE GIANT FIBRE SYSTEM OF THE SQUID

The synthesis of brain-specific proteins has been examined in perikaryal and axonal regions of the giant fibre system of the squid. After in vitro incubation of stellate ganglia, stellate nerves and isolated giant axons with radioactive amino acids, the labelled soluble proteins have been extracted from the giant fibre lobe, the axoplasm and the axonal sheath of the giant axon and have been separated by gel electrophoresis on a continuous system. In addition, they have been challenged with antisera prepared against the cephalopod brain-specific proteins L1 and L2 and the resulting precipitate has been resolved by sodium dodecyl sulphate-gel electrophoresis. Synthesis of these two proteins appears to be restricted to the giant fibre lobe, while an additional discrete protein band (L5) also becomes clearly labelled in the isolated giant axon. Radioactive components migrating in the region of the L1 and L2 proteins are synthesized in the isolated giant axon. They can be distinguished from tbese proteins on the basis of electrophoretic and immunochemical criteria.     

IDENTIFICATION OF A GROUP OF OCTOPUS BRAIN PROTEINS AS HISTONES

Abstract— We have investigated the subcellular localization of a group of proteins solubilized with SDS from saline-washed particulates of octopus brain and identified as the most rapidly moving species in SDS-gel electrophoresis. These proteins are several-fold more concentrated in the vertical and optic lobes than in the suboesophageal lobe. In the optic lobe their concentration decreases with increasing body weight (G iuditta et al , 1975).
Upon separation of hypertonic sucrose homogenates from the optic lobe in a nuclear pellet and in a floating layer containing cytoplasmic particulates, the proteins appear associated with the nuclear fraction. Differential extraction of the latter fraction shows that the proteins have the same solubility and electrophoretic mobilities as histones. Proteins with similar properties are present in other brain regions and in non-nervous organs.     

PROTEIN PATTERNS IN DIFFERENT LOBES AND DURING DEVELOPMENT OF OCTOPUS BRAIN

Abstract— Utilizing techniques of continuous and SDS-electrophoresis we have examined the saline-soluble and SDS-soluble (membrane-bound) proteins extracted from the main lobes of adult octopus brain and from the developing optic lobe of the same species. Several additional protein bands are present among the soluble and the membrane-bound proteins of the vertical lobe in comparison with the suboesophageal lobe. Since the former contains an essentially homogeneous population of small neurons, while the suboesophageal lobe is rich in large nerve cells, these protein bands have been attributed to the small neuronal type present in the vertical lobe.
In the course of a 10,000-fold increment in body weight, from 0.4 g to 4 kg, there is a significant increase in the concentration of several soluble proteins extracted from the optic lobe. Three of these proteins increase to a marked degree. Among the membrane-bound proteins some show a moderate increase with age while other protein components of smaller molecular weight undergo a moderate decrease. The overall tissue concentration of the membrane-bound proteins increases between 0.4 g and 50 g body weight, slightly declining in animals of larger size.     

Axonal transport of RNA during regeneration of the optic nerves of goldfish.

The distribution of radioactive RNA and RNA precursors in the goldfish optic tecta following intraocular injection of 3H-uridine has been studied during various stages of optic nerve regeneration. 3H-uridine was injected into the posterior chamber of the right eye 17, 30, or 60 days after both optic nerves were crushed. Five were sacrificed at time intervals ranging from 0.5 to 21 days after injection. One day prior to sacrificing, 14C-proline was also injected into the right eye as a marked of fast axonal protein transport. Seventeen to 23 days after crushing, the approximate time of nerve reconnection, the amount of radioactive RNA appearing in the left optic tectum was increased by more than ten times control values. Approximately 30 days after crushing the nerve, when the reconnected nerve is maturing, RNA values were still elevated, but significantly decreased from the earlier stage. By 60 days after crushing the optic nerve, the amounts of RNA in the left tectum was close to normal. Evidence suggesting that, at least, some of the radioactive RNA in the tectum originated from RNA transported along optic axons rather than from RNA synthesized locally in the tectum was provided by autoradiographic experiments. Autoradiograms of paraffin sections taken from the goldfish optic tecta after the intraocular injection of 3H-uridine showed a distribution of grains in a linear pattern, suggesting a distribution over the incoming fibers during the reconnection stage of regeneration. Electron microsocpic autoradiography of glutaraldehyde fixed epoxy sections confirmed that a significant number of grains (shown to be 3H-RNA) were, in fact, over regenerating optic axons. Intracranial injection of 3H-uridine, during the same stage of regeneration, on the other hand, resulted in a distribution of grains, specifically over cell perikaprya. These experiments suggest that during the reconnection phase of nerve regeneration, large amounts of RNA may be carried within regenerating optic axons as they enter the optic tectum.     

Axonal transport of RNA during regeneration of the optic nerves of goldfish

The distribution of radioactive RNA and RNA precursors in the goldfish optic tecta following intraocular injection of ³H-uridine has been studied during various stages of optic nerve regeneration. ³H-uridine was injected into the posterior chamber of the right eye 17, 30, or 60 days after both optic nerves were crushed. Fish were sacrificed at time intervals ranging from 0.5 to 21 days after injection. One day prior to sacrificing, ¹⁴C-proline was also injected into the right eye as a marker of fast axonal protein transport. Seventeen to 23 days after crushing, the approximate time of nerve reconnection, the amount of radioactive RNA appearing in the left optic tectum was increased by more than ten times control values. Approximately 30 days after crushing the nerve, when the reconnected nerve is maturing, RNA values were still elevated, but significantly decreased from the earlier stage. By 60 days after crushing the optic nerve, the amounts of RNA in the left tectum was close to normal. Evidence suggesting that, at least, some of the radioactive RNA in the tectum originated from RNA transported along optic axons rather than from RNA synthesized locally in the tectum was provided by autoradiographic experiments. Autoradiograms of paraffin sections taken from the goldfish optic tecta after the intraocular injection of ³H-uridine showed a distribution of grains in a linear pattern, suggesting a distribution over the incoming fibers during the reconnection stage of regeneration. Electron microscopic autoradiography of glutaraldehyde fixed epoxy sections confirmed that a significant number of grains (shown to be ³H-RNA) were, in fact, over regenerating optic axons. Intracranial injection of ³H-uridine, during the same stage of regeneration, on the other hand, resulted in a distribution of grains, specifically over cell perikarya. These experiments suggest that during the reconnection phase of nerve regeneration, large amounts of RNA may be carried within regenerating optic axons as they enter the optic tectum.     

Migration of ribosomes along the axons of the chick visual pathway.

The axoplasmic migration of ribosomes has been detected in the visual system of the chick. Monocular injection of radioactive uridine or an amino acid mixture was followed by sedimentation analysis in sucrose or cesium sulfate density gradients, of ribosomes prepared from the retinae of injected eyes and the left and right optic lobes. By this means both RNA and protein components of ribosomes were found to migrate from the retina to the innervated contralateral optic lobe. Following denervation of the distal nerve segment by eye removal, the stability of the transported RNA was reduced, suggesting its presynaptic location. The transport of RNA was not significantly imparied by intraocular injection of inhibitors of informational RNA or mitochondrial RNA synthesis prior to injection of radioactive uridine but was depressed by a low dose of actinomycin D.     

THE INCORPORATION OF GLUCOSE INTO BRAIN GLYCOGEN AND THE ACTIVITIES OF CEREBRAL GLYCOGEN PHOSPHORYLASE AND SYNTHETASE: SOME EFFECTS OF AMPHETAMINE

Abstract— The effects of amphetamine sulphate (5 mg/kg intraperitoneally) on the incorporation of radioactive carbon from [U-¹⁴C]glucose into the glycogen of mouse cerebral cortex, midbrain and hind-brain have been investigated. In all brain regions studied amphetamine induced a rapid decrease in glycogen followed by a slower return to control values. No significant alterations were observed in the steady state concentration of cerebral glucose. The initial fall in glycogen was associated with a fall in its specific radioactivity relative to that of cerebral glucose, whereas the resynthesis of the polysaccharide was associated with a marked increase in the relative specific radioactivity of glycogen. Other experiments demonstrated that amphetamine initially stimulates the breakdown of prelabelled glycogen and that the resulting molecule has fewer 1,4 linked glucose side chains.
Studies of the relative forms of the enzymes glycogen phosphorylase and glycogen synthetase suggested that rapid post mortem changes were less likely to occur if cerebral tissue was fixed by means of a freeze-blowing technique. Amphetamine administration resulted in a rapid though transient elevation of phosphorylase a activity in mouse forebrain. The level of glycogen synthetase I activity was unchanged initially but was markedly elevated during the period when there was a large increase in the rate of incorporation of glucose into glycogen. It is suggested that cerebral glycogen metabolism is controlled, at least in part, by the interconversion of the 'active' and 'inactive' forms of glycogen phosphorylase and synthetase.     

Axonal migration of various ribonucleic acid species along the optic tract of the chick

—The avian visual system has been used to study the axonal transport of RNA and protein. After monocular injection of radioactive uridine into 1-day-old chicks, a considerable amount of labelled RNA migrated along the optic tract to the optic tectum contralateral to the injected eye. This RNA was largely ribosomal, although it was contained in several subcellular fractions. The migration of RNA appeared to be a slow process. However, following monocular injection of radioactive proline, the migration of ribosomal protein was rapid. This discrepancy was resolved by examination of the kinetics of labelling of RNA and protein within the retina after intraocular injection of a mixture of labelled uridine and proline. Cytoplasmic RNA was labelled much more slowly than cytoplasmic protein. This lag in labelling of RNA could account for the delayed arrival of RNA at the contralateral optic lobe and suggests that ribosomes may travel rapidly along the axon. In other experiments, eyes were removed 4 days after the injection of labelled precursors. After a further 14 days, the remaining radioactivity in RNA and protein of contralateral optic lobes was 5–15% of that attributable to migration along the axon in control, unenucleated birds. Thus, the survival of the bulk of migrating macromolecules depends on the integrity of synaptic terminals. This observation suggests that both RNA and protein migrate within the axon rather than extra-axonally, and that they remain largely within the nerve cells along the axons of which they are transported.     

Different Polypeptides Are Rapidly Transported in Auditory and Optic Neurons

Abstract: Rapidly transported proteins and glycoproteins in the auditory and optic nerves of the guinea pig were analyzed by electrophoresis and two-dimensional electrofocusing/electrophoresis. Proteins transported in the auditory nerve were analyzed in the cochlear nucleus 3 h after cochlear injection of radioactive precursor, and proteins transported in the optic nerve were analyzed in the superior colliculus 6 h after intraocular injection of radioactive precursor. Two-dimensional analysis showed that several rapidly transported polypeptides were present in one system, but not in the other. By use of [³H]fucose as a precursor or by separating [³⁵S]methionine-labeled polypeptides on immobilized concanavalin A or wheat germ agglutinin, it was shown that most of the proteins transported in only one system are glycoproteins. As previously reported a polypeptide of molecular weight 140,000 was a major labeled species in the auditory nerve. This polypeptide was also found in the optic nerve, but only as a minor species. Two other polypeptides with molecular weights and isoelectric points similar to those of the 140,000 molecular weight polypeptide were present in both systems, but were much more abundant in the optic nerve. The major labeled polypeptide in both systems had a molecular weight of 25,000.     

SELECTIVE LABELLING OF TWO PHASES OF AXONAL TRANSPORT OF CHOLESTEROL IN THE CHICK OPTIC SYSTEM

Abstract— In the chick optic system cholesterol is axonally transported in two phases which appear to take their cholesterol from different cellular pools. The intraocular injection of radioactire cholesterol results in the specific labelling of the slow phase which carries cholesterol in the unesterificd form and appears to move at the same rate as the slow phase of protein transport (R ostas et al. , 1975). The intraocular injection of radioactive mevalonic acid, a metabolic precursor of cholesterol, results in the preferential labelling of a more rapid phase of axonal transport which also carries cholesterol in the unesterified form and is first detected at the optic tectum 10 h after the injection. It is likely that this rapid phase travels at the same rate as the rapid phase of protein transport and that the delayed arrival at the tectum is due to a lag time in the retina caused by the synthesis of cholesterol and its packaging for transport. Because the individual pools for the two transport phases can be selectively labelled, the retina and optic nerve provide a unique model system in which the metabolic turnover, intracellular compartmentalization and intracellular transport of cholesterol can be studied.     

Phosphorylation of Proteins in Normal and Regenerating Goldfish Optic Nerve

Within 6 h after radiolabeled phosphate was injected into the eye of goldfish, labeled acid-soluble and acid-precipitable material began to appear in the optic nerve and subsequently also in the lobe of the optic tectum, to which the optic axons project. From the rate of appearance of the acid-precipitable material, a maximal velocity of axonal transport of 13-21 mm/day could be calculated, consistent with fast axonal transport group II. Examination of individual proteins by two-dimensional gel electrophoresis revealed that approximately 20 proteins were phosphorylated in normal and regenerating nerves. These ranged in molecular weight from approximately 18,000 to 180,000 and in pI from 4.4 to 6.9. Among them were several fast transported proteins, including protein 4, which is the equivalent of the growth-associated protein GAP-43. In addition, there was phosphorylation of some recognizable constituents of slow axonal transport, including alpha-tubulin, a neurofilament constituent (NF), and another intermediate filament protein characteristic of goldfish optic axons (ON2). At least some axonal proteins, therefore, may become phosphorylated as a result of the axonal transport of a phosphate carrier. Some of the proteins labeled by intraocular injection of 32P showed changes in phosphorylation during regeneration of the optic axons. By 3-4 weeks after an optic tract lesion, five proteins, including protein 4, showed a significant increase in labeling in the intact segment of nerve between the eye and the lesion, whereas at least four others (including ON2) showed a significant decrease. When local incorporation of radiolabeled phosphate into the nerve was examined by incubating nerve segments in 32P-containing medium, there was little or no labeling of the proteins that showed changes in phosphorylation during regeneration. Segments of either normal or regenerating nerves showed strong labeling of several other proteins, particularly a group ranging in molecular weight from 46,000 to 58,000 and in pI from 4.9 to 6.4. These proteins were presumably primarily of nonneuronal origin. Nevertheless, if degeneration of the axons had been caused by removal of the eye 1 week earlier, most of the labeling of these proteins was abolished. This suggests that phosphorylation of these proteins depends on the integrity of the optic axons.     

Morphological changes of the optic lobe from late embryonic to adult stages in oval squids Sepioteuthis lessoniana

The optic lobe is the largest brain area within the central nervous system of cephalopods and it plays important roles in the processing of visual information, the regulation of body patterning, and locomotive behavior. The oval squid Sepioteuthis lessoniana has relatively large optic lobes that are responsible for visual communication via dynamic body patterning. It has been observed that the visual behaviors of oval squids change as the animals mature, yet little is known about how the structure of the optic lobes changes during development. The aim of the present study was to characterize the ontogenetic changes in neural organization of the optic lobes of S. lessoniana from late embryonic stage to adulthood. Magnetic resonance imaging and micro‐CT scans were acquired to reconstruct the 3D‐structure of the optic lobes and examine the external morphology at different developmental stages. In addition, optic lobe slices with nuclear staining were used to reveal changes in the internal morphology throughout development. As oval squids mature, the proportion of the brain making up the optic lobes increases continuously, and the optic lobes appear to have a prominent dent on the ventrolateral side. Inside the optic lobe, the cortex and the medulla expand steadily from the late embryonic stage to adulthood, but the cell islands in the tangential zone of the optic lobe decrease continuously in parallel. Interestingly, the size of the nuclei of cells within the medulla of the optic lobe increases throughout development. These findings suggest that the optic lobe undergoes continuous external morphological change and internal neural reorganization throughout the oval squid's development. These morphological changes in the optic lobe are likely to be responsible for changes in the visuomotor behavior of oval squids from hatching to adulthood.     

In Vivo Phosphorylation of Axonal Proteins in Goldfish Optic Nerve During Regeneration

In vivo phosphorylation of axonal proteins was investigated in normal and regenerating optic nerves of goldfish by two-dimensional gel electrophoresis. By 6-24 h after intraocular injection of H3(32)PO4, approximately 20 optic nerve proteins ranging in size from 19 to 180 kilodaltons and in pI from 4.4 to 6.8 were seen to have incorporated radiolabel. Five of these proteins showed a robust increase in incorporation of phosphate during regeneration. Among the latter was an acidic (pI 4.5) 45-kilodalton protein, which has previously been shown to be conveyed by fast axonal transport and to increase dramatically in its rate of synthesis during regeneration of goldfish optic axons.     

Optic nerve input‐dependent regulation of neural stem cell proliferation in the optic tectum of adult zebrafish

Adult neurogenesis attracts broad attention as a possible cure for neurological disorders. However, its regulatory mechanism is still unclear. Therefore, they have been studying the cell proliferation mechanisms of neural stem cells (NSCs) using zebrafish, which have high regenerative potential in the adult brain. The presence of neuroepithelial‐type NSCs in the optic tectum of adult zebrafish has been previously reported. In the present study, it was first confirmed that NSCs in the optic tectum decrease or increase in proportion to projection of the optic nerves from the retina. At 4 days after optic nerve crush (ONC), BrdU‐positive cells decreased in the optic tectum's operation side. In contrast, at 3 weeks after ONC, BrdU‐positive cells increased in the optic tectum's operation side. To study the regulatory mechanisms, they focused on the BDNF/TrkB system as a regulatory factor in the ONC model. It was found that bdnf was mainly expressed in the periventricular gray zone (PGZ) of the optic tectum by using in situ hybridization. Interestingly, expression level of bdnf significantly decreased in the optic tectum at 4 days after ONC, and its expression level tended to increase at 3 weeks after ONC. They conducted rescue experiments using a TrkB agonist and confirmed that decrease of NSC proliferation in the optic tectum by ONC was rescued by TrkB signal activation, suggesting stimuli‐dependent regulation of NSC proliferation in the optic tectum of adult zebrafish. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 419–437, 2017     

Developmental Changes in β-Citryl-L-G1utamate Concentration and Its Synthetic and Hydrolytic Activities in Neuronal Cells Cultured from Chick Embryo Optic Lobes

Developmental changes in the concentration of beta-citryl-L-glutamate(beta-CG) have been examined in the cerebrum and optic lobe of the developing chick brain and in primary cultured neuronal cells from the chick embryo optic lobes with gas chromatographic and HPLC methods originated in our studies. A sharp peak was shown by beta-CG, with a maximal concentration at 13 days of incubation in the optic lobe of the developing chick brain but decreasing markedly to adult levels. The developmental change in primary cultured neurons was similar to that in the optic lobe of the developing chick brain. Changes in synthetic and hydrolytic activities of beta-CG were studied during growth of primary cultured neurons. Incorporation of radioactivities from radiolabeled pyruvate and alanine into beta-CG increased significantly on day 3 of culture, reaching a plateau on day 6, whereas that from radioactive glutamine and glutamate increased gradually from day 3 to day 12 of culture. The hydrolyzing enzyme activity of beta-CG during neuron growth was low until day 3 of culture, when it increased significantly until day 12. Similar developmental changes were observed in the developing chick embryo optic lobes.     

Analysis of coupling between optic lobe circadian pacemakers in the cricket Gryllus bimaculatus

The coupling mechanism between weakly coupled two optic lobe circadian pacemakers in the cricket Gryllus bimaculatus was investigated by recording the locomotor activity, under light-dark cycles with various lengths, after the optic nerve was unilaterally severed. The activity rhythm split into two components under the light cycles different from 24 h: one was readily entrained to the light cycle and the other only loosely entrained or freeran. Additional removal of the optic lobe on the intact side resulted in a loss of the entrained component and that on the blinded side caused the reverse effect, indicating that the entrained component was driven by the pacemaker on the intact side and the other by the one on the blinded side. The synchronization between the two components was achieved only in light cycles with a limited length between 23 and 25 h. Without this range, the desynchronization of the components occurred. In the split rhythm, the phase-dependent modulation of the period of freerunning component and the mutual suppression of locomotor activity during the subjective day phase were clearly observed. The suppression was also evident in the lights-on peak that was the masking effect of light. The light cycle with dim light significantly reduced the ratio of animals with the pacemaker coupling as well as the magnitude of the period modulation. These results suggest (1) that the mutual coupling is achieved only when the difference in the periods between the two pacemakers is within an allowable range, (2) that the photic information is also involved in the mechanism of mutual coupling, and (3) that the suppression of activity occurs at the regulatory center for locomotion.Abbreviations CT circadian time - DD constant darkness - LL constant light - LD light to dark cycle - T length of light to dark cycle - freerunning period     

AXONAL TRANSPORT OF PROTEINS IN THE HYPOTHALAMO-NEUROHYPOPHYSIAL SYSTEM OF THE RAT

—Axonal transport of proteins in the hypothalamo-neurohypophysial system of the rat was studied after a local injection of [³⁵S]cysteine in the region of the supraoptic nucleus. The migration of labelled proteins was followed by measuring the specific radioactivity of the proteins in various parts of the hypothalamo-neurohypophysial tract. Between 2 and 4 h after the isotope injection there was a sharp increase in the protein-bound specific radioactivity of the posterior pituitary lobe, demonstrating that a transport of ³⁵S-labelled proteins had occurred from the supraoptic nucleus to the neurohypophysis. The rate of the transport was 2-3 mm/h. During the first 24 h after the injection a continuous accumulation of labelled material occurred in the neural lobe. Considerable radioactivity could still be recovered 6 days after the isotope injection. Fractionation of the neurohypophysial proteins by polyacrylamide gel electrophoresis revealed that approximately 90 per cent of the radioactivity of the soluble proteins was recovered in a single protein fraction. Labelling of this fraction was not observed until 2 h after isotope injection. The radioactivity increased markedly up to 4 h. It is suggested that this protein component is involved in the neurohypophysial response to osmotic stress since the protein disappeared from the posterior lobe upon dehydration of the rat.     

A COMPARISON OF THE AXONAL TRANSPORT OF TAURINE AND PROTEINS IN THE GOLDFISH VISUAL SYSTEM

Abstract— Radioactive cystathionine, a metabolic precursor of taurine, was injected into the right eye of goldfish. At various times after injection the retina and both optic tecta were extracted with trichloroacetic acid (TCA) and the amount and nature of the radioactivity was determined. Radioactive taurine and inorganic sulfate were present in the TCA-soluble extract of retina and radioactive taurine and a small amount of inorganic sulfate was found in the contralateral optic tectum. That taurine is migrating intraaxonally and is not diffusing in extraaxonal spaces is suggested from experiments in which the migration of taurine was compared with that of [¹⁴C]mannitol, used here as a marker of extracellular diffusion. In the time studied (up to 15 h) mannitol did not migrate to the tectum, whereas taurine was detectable in the tectum as early as 8 h after injection. Since intra-axonal diffusion of amino acids and other small molecules in this system has been ruled out, it is likely that taurine is being transported axonally. The axonal transport of taurine was found to be similar to the fast component of protein transport because: (1) their rates of transport are similar, (2) the transport of both is blocked by the protein synthesis inhibitor cycloheximide, (3) vinblastine, which disrupts neurotubules, appears to have similar effects on both protein and taurine transport, and (4) both rapidly transported proteins and taurine remain mostly intra-axonal once they have been transported to the tectum. Taurine and proteins differ in that rapidly transported proteins are primarily paniculate in nature and localized to a large extent in nerve endings, while taurine is primarily in a soluble fraction and is present in nerve endings only in trace amounts. We suggest that taurine may be loosely linked to a newly synthesized protein in the soma and is then transported along with that protein on a similar conveying mechanism in the axoplasm.     

Concomitant requirement for Notch and Jak/Stat signaling during neuro-epithelial differentiation in the Drosophila optic lobe

The optic lobe forms a prominent compartment of the Drosophila adult brain that processes visual input from the compound eye. Neurons of the optic lobe are produced during the larval period from two neuroepithelial layers called the outer and inner optic anlage (OOA, IOA). In the early larva, the optic anlagen grow as epithelia by symmetric cell division. Subsequently, neuroepithelial cells (NE) convert into neuroblasts (NB) in a tightly regulated spatio-temporal progression that starts at the edges of the epithelia and gradually move towards its centers. Neuroblasts divide at a much faster pace in an asymmetric mode, producing lineages of neurons that populate the different parts of the optic lobe. In this paper we have reconstructed the complex morphogenesis of the optic lobe during the larval period, and established a role for the Notch and Jak/Stat signaling pathways during the NE-NB conversion. After an early phase of complete overlap in the OOA, signaling activities sort out such that Jak/Stat is active in the lateral OOA which gives rise to the lamina, and Notch remains in the medial cells that form the medulla. During the third instar, a wave front of enhanced Notch activity progressing over the OOA from medial to lateral controls the gradual NE-NB conversion. Neuroepithelial cells at the medial edge of the OOA, shortly prior to becoming neuroblasts, express high levels of Delta, which activates the Notch pathway and thereby maintains the OOA in an epithelial state. Loss of Notch signaling, as well as Jak/Stat signaling, results in a premature NE-NB conversion of the OOA, which in turn has severe effects on optic lobe patterning. Our findings present the Drosophila optic lobe as a useful model to analyze the key signaling mechanisms controlling transitions of progenitor cells from symmetric (growth) to asymmetric (differentiative) divisions.