Defining the carbohydrate specificities of aplysia gonad lectin exhibiting a peculiar D-galacturonic acid affinity.
(65/1209)
Aplysia gonad lectin (AGL), which has been shown to stimulate mitogenesis in human peripheral lymphocytes, to suppress tumor cells, and to induce neurite outgrowth and improve cell viability in cultured Aplysia neurons, exhibits a peculiar galacturonic acid/galactose specificity. The carbohydrate binding site of this lectin was characterized by enzyme-linked lectino-sorbent assay and by inhibition of AGL-glycan interactions. Examination of the lectin binding with 34 glycans revealed that it reacted strongly with the following glycoforms: most human blood group precursor (equivalent) glycoproteins (gps), two Galalpha1-->4Gal-containing gps, and two d-galacturonic acid (GalUA)-containing polysaccharides (pectins from apple and citrus fruits), but poorly with most human blood group A and H active and sialylated gps. Among the GalUA and mammalian saccharides tested for inhibition of AGL-glycan binding, GalUA mono- to trisaccharides were the most potent ones. They were 8.5 x 10(4) times more active than Gal and about 1.5 x 10(3) more active than the human blood group P(k) active disaccharide (E, Galalpha1-->4Gal). This disaccharide was 6, 28, and 120 times more efficient than Galbeta1-->3GlcNAc(I), Galbeta1-->3GalNAc(T), and Galbeta1--> 4GlcNAc (II), respectively, and 35 and 80 times more active than melibiose (Galalpha1-->6Glc) and human blood group B active disaccharide (Galalpha1-->3Gal), respectively, showing that the decreasing order of the lectin affinity toward alpha-anomers of Gal is alpha1-->4 > alpha1-->6 > alpha1-->3. From the data provided, the carbohydrate specificity of AGL can be defined as GalUAalpha1-->4 trisaccharides to mono GalUA > branched or cluster forms of E, I, and II monomeric E, I, and II, whereas GalNAc is inactive. (+info)
Multifunctional neuron CC6 in Aplysia exerts actions opposite to those of multifunctional neuron CC5.
(66/1209)
The controls of somatic and autonomic functions often appear to be organized into antagonistic systems. This issue was explored in the bilaterally paired C cluster neuron, CC6, which was found to have properties that suggested that it might function antagonistically to the previously identified multiaction neuron, CC5. Similar to CC5, CC6 is an interganglionic neuron that sends its sole axon to the ipsilateral and contralateral pedal and pleural ganglia. Synaptic inputs to CC6 were opposite to those of CC5. For example, CC6 receives inhibitory inputs from mechanical touch to the lips and tentacles and is excited by firing of C-PR, a neuron involved in the control of a head extension response. Also during rhythmic buccal mass movements CC6 receives synaptic inputs that are out of phase with those received by CC5. CC6 is inhibited during a fictive locomotor program, whereas CC5 is excited, but unlike CC5, the inputs to CC6 are not rhythmic. CC6 has extensive mono- and polysynaptic outputs to many identified and unidentified neurons located in various central ganglia. Firing of CC6 evoked ipsilateral contraction of the transverse muscles of the neck, whereas CC5 contracts longitudinal neck muscles. CC6 monosynaptically inhibits the pedal artery shortener neuron, whereas CC5 monosynaptically excites the pedal artery shortener neuron. Specific motor neurons in the pedal ganglion receive synaptic inputs of opposite sign from CC5 and CC6. Although the inputs and most of the effects of CC6 were opposite to those of CC5, both cells were found to produce polysynaptic excitation of the abdominal ganglion neuron RBhe, a cell whose activity excites the heart. CC5 and CC6 appear to be multifunctional neurons that form an antagonist pair. (+info)
Unifying mechanism for Aplysia ADP-ribosyl cyclase and CD38/NAD(+) glycohydrolases.
(67/1209)
Highly purified Aplysia californica ADP-ribosyl cyclase was found to be a multifunctional enzyme. In addition to the known transformation of NAD(+) into cADP-ribose this enzyme is able to catalyse the solvolysis (hydrolysis and methanolysis) of cADP-ribose. This cADP-ribose hydrolase activity, which becomes detectable only at high concentrations of the enzyme, is amplified with analogues such as pyridine adenine dinucleotide, in which the cleavage rate of the pyridinium-ribose bond is much reduced compared with NAD(+). Although the specificity ratio V(max)/K(m) is in favour of NAD(+) by 4 orders of magnitude, this multifunctionality allowed us to propose a 'partitioning' reaction scheme for the Aplysia enzyme, similar to that established previously for mammalian CD38/NAD(+) glycohydrolases. This mechanism involves the formation of a single oxocarbenium-type intermediate that partitions to cADP-ribose and solvolytic products via competing pathways. In favour of this mechanism was the finding that the enzyme also catalysed the hydrolysis of NMN(+), a substrate that cannot undergo cyclization. The major difference between the mammalian and the invertebrate enzymes resides in their relative cyclization/hydrolysis rate-constant ratios, which dictate their respective yields of cADP-ribose (ADP-ribosyl cyclase activity) and ADP-ribose (NAD(+) glycohydrolase activity). For the Aplysia enzyme's catalysed transformation of NAD(+) we favour a mechanism where the formation of cADP-ribose precedes that of ADP-ribose; i.e. macroscopically the invertebrate ADP-ribosyl cyclase conforms to a sequential reaction pathway as a limiting form of the partitioning mechanism. (+info)
Presynaptic morphological changes associated with long-term synaptic facilitation are triggered by actin polymerization at preexisting varicositis.
(68/1209)
Morphological changes are thought to contribute to the expression of long-term synaptic plasticity, a cellular basis for learning and memory. The mechanisms mediating the initiation and maintenance of the morphological changes are poorly understood. We repeatedly imaged the axonal arbors of mechanosensory neurons of Aplysia as they formed new synaptic varicosities and axonal branches after applications of serotonin that cause long-term synaptic facilitation. New varicosities formed exclusively from preexisting varicosities, by splitting or branch outgrowth. These changes were prevented by cytochalasin D, which blocks actin polymerization and the turnover of actin filaments. The suppression of the morphological changes by cytochalasin D did not impair their expression when cytochalasin D was removed 24 hr after exposure to serotonin. These results imply that serotonin induces persistent effects at preexisting presynaptic varicosities, which enhance actin polymerization, and that this is essential for presynaptic morphological changes of long-term facilitation. (+info)
Probing f-actin flow by tracking shape fluctuations of radial bundles in lamellipodia of motile cells.
(69/1209)
We examined the dynamics of radial actin bundles based on time-lapse movies of polarized light images of living neuronal growth cones. Using a highly sensitive computer vision algorithm for tracking, we analyzed the small shape fluctuations of radial actin bundles that otherwise remained stationary in their positions in the growth cone lamellipodium. Using the tracking software, we selected target points on radial bundles and measured both the local bundle orientations and the lateral displacements between consecutive movie frames. We found that the local orientation and the lateral displacement of a target point are correlated. The correlation can be explained using a simple geometric relationship between the lateral travel of tilted actin bundles and the retrograde flow of f-actin structures. Once this relationship has been established, we have turned the table and used the radial bundles as probes to measure the velocity field of f-actin flow. We have generated a detailed map of the complex retrograde flow pattern throughout the lamellipodium. Such two-dimensional flow maps will give new insights into the mechanisms responsible for f-actin-mediated cell motility and growth. (+info)
Short-term synaptic enhancement modulates ingestion motor programs of aplysia.
(70/1209)
Activity-dependent synaptic plasticity regulates the flow of information in neuronal networks and has important implications for the expression of behavior. We find a functional role for short-term synaptic enhancement (STE) such as facilitation, augmentation, and post-tetanic potentiation at central synapses in the sea slug Aplysia californica. Consummatory feeding in Aplysia such as rhythmic biting is controlled by command-like cerebral-buccal interneurons (CBIs) that drive rhythmic motor output in the buccal ganglia. CBI interneuron-2 (CBI-2) makes monosynaptic connections onto buccal neurons, including premotor neurons B31/32 and B34 and motor neurons B61/62. Stimulating CBI-2 at a physiological firing frequency of 10 Hz for 30 sec causes these synapses to increase their EPSP amplitude by approximately 200%. This STE persists for nearly 2 min, during which time there is an increased cycle frequency of rhythmic ingestion buccal motor programs (iBMPs) elicited by CBI-2. This increase does not occur if the contralateral CBI-2 is trained and the test is performed with the ipsilateral CBI-2; therefore, the effect on motor programs only occurs in CBI-2 pathways in which STE is elicited. Furthermore, we find that STE elicited at CBI-2 buccal synapses permits iBMPs to be initiated at lower firing frequencies. Thus, STE of CBI-2 synapses appears to contribute to the initiation or modulation, or both, of buccal motor programs for rhythmic ingestion in Aplysia. (+info)
Multistability in recurrent neural loops arising from delay.
(71/1209)
The dynamics of a recurrent inhibitory neural loop composed of a periodically spiking Aplysia motoneuron reciprocally connected to a computer are investigated as a function of the time delay, tau, for propagation around the loop. It is shown that for certain choices of tau, multiple qualitatively different neural spike trains co-exist. A mathematical model is constructed for the dynamics of this pulsed-coupled recurrent loop in which all parameters are readily measured experimentally: the phase resetting curve of the neuron for a given simulated postsynaptic current and tau. For choices of the parameters for which multiple spiking patterns co-exist in the experimental paradigm, the model exhibits multistability. Numerical simulations suggest that qualitatively similar results will occur if the motoneuron is replaced by several other types of neurons and that once tau becomes sufficiently long, multistability will be the dominant form of dynamical behavior. These observations suggest that great care must be taken in determining the etiology of qualitative changes in neural spiking patterns, particularly when propagation times around polysynaptic loops are long. (+info)
Male-specific IDGF, a novel gene encoding a membrane-bound extracellular signaling molecule expressed exclusively in testis of Drosophila melanogaster.
(72/1209)
We identified a novel gene of Drosophila melanogaster, Male-specific IDGF (MSI), encoding a transmembrane signaling molecule with exclusive expression in the testis. This molecule (MSI) contains a single transmembrane domain and has 35% amino acid identity with insect-derived growth factor (IDGF), a soluble growth factor for embryonic cells of the flesh fly, Sarcophaga peregrina. When MSI was exogenously expressed in Schneiders's line 2 cells, it was shown to be localized on the cell surface and exhibits growth factor activity, suggesting that MSI is a membrane-bound extracellular signaling molecule. Gene expression studies revealed that MSI mRNA was restricted to mature primary spermatocytes, whereas MSI was detected in the cells at the later developmental stages. Analysis using four meiotic arrest mutants, aly, can, mia, and sa suggested that MSI is involved in spermiogenesis, the final differentiation step of spermatogenesis. These results suggest that MSI is an extracellular signaling molecule participating in spermatogenesis and is a new member of the IDGF family. (+info)