Nerve impulses increase the phosphorylation state of protein I in rabbit superior cervical ganglion

Intraterminal injection of synapsin I or calcium/calmodulin-dependent protein kinase II alters neurotransmitter release at the squid giant synapse

Synapsin I and calcium/calmodulin-dependent protein kinase II were pressure-injected into the preterminal digit of the squid giant synapse to test directly the possible regulation of neurotransmitter release by these substances. Neurotransmitter release was determined by measuring the amplitude, rate of rise, and latency of the postsynaptic potential generated in response to presynaptic depolarizing steps under voltage clamp conditions. Injection of dephosphosynapsin I decreased the amplitude and rate of rise of the postsynaptic potential, whereas injection of either phosphosynapsin I or heat-treated dephosphosynapsin I was without effect. Conversely, injection of calcium/calmodulin-dependent protein kinase II, which phosphorylates synapsin I on site II, increased the rate of rise and amplitude and decreased the latency of the postsynaptic potential. The effects of these proteins were observed without any detectable change in the initial phase of the presynaptic calcium current. A synapsin I-like protein and calcium/calmodulin-dependent protein kinase II were demonstrated by biochemical and immunochemical techniques to be present in squid nervous tissue. The data support the hypothesis that synapsin I regulates the availability of synaptic vesicles for release; we propose that calcium entry into the nerve terminal activates calcium/calmodulin-dependent protein kinase II, which phosphorylates synapsin I on site II, dissociating it from the vesicles and thereby removing a constraint in the release process.

Ankyrin and synapsin: spectrin-binding proteins associated with brain membranes

Brain membranes contain an actin-binding protein closely related in structure and function to erythrocyte spectrin. The proteins that attach brain spectrin to membranes are not established, but, by analogy with the erythrocyte membrane, may include ankyrin and protein 4.1. In support of this idea, proteins closely related to ankyrin and 4.1 have been purified from brain and have been demonstrated to associate with brain spectrin. Brain ankyrin binds with high affinity to the spectrin beta subunit at the midregion of spectrin tetramers. Brain ankyrin also has binding sites for the cytoplasmic domain of the erythrocyte anion channel (band 3), as well as for tubulin. Ankyrins from brain and erythrocytes have a similar domain structure with protease-resistant domains of Mr = 72,000 that contain spectrin-binding activity, and domains of Mr = 95,000 (brain ankyrin) or 90,000 (erythrocyte ankyrin) that contain binding sites for both tubulin and the anion channel. Brain ankyrin is present at about 100 pmol/mg membrane protein, or about twice the number of copies of spectrum beta chains. Brain ankyrin thus is present in sufficient amounts to attach spectrin to membranes, and it has the potential to attach microtubules to membranes as well as to interconnect microtubules with spectrin-associated actin filaments. Another spectrin-binding protein has been purified from brain membranes, and this protein cross-reacts with erythrocyte 4.1. Brain 4.1 is identical to the membrane protein synapsin, which is one of the brain's major substrates for cAMP-dependent and Ca/calmodulin-dependent protein kinases with equivalent physical properties, immunological cross-reaction, and peptide maps. Synapsin (4.1) is present at about 60 pmol/mg membrane protein, and thus is a logical candidate to regulate certain protein linkages involving spectrin.

Electrical stimulation increases phosphorylation of tyrosine hydroxylase in superior cervical ganglion of rat

Electrical stimulation of the superior cervical ganglion of the rat increased the phosphorylation of tyrosine hydroxylase (tyrosine 3-monooxygenase, EC 1.14.16.2) in this tissue. Ganglia were incubated with [32P]Pi for 90 min and were then electrically stimulated via the preganglionic nerve. Tyrosine hydroxylase was isolated from homogenates of the ganglia by immunoprecipitation followed by polyacrylamide gel electrophoresis. 32P-labeled tyrosine hydroxylase was visualized by radioautography, and the incorporation of 32P into the enzyme was quantitated by densitometry of the radioautograms. Stimulation of ganglia at 20 Hz for 5 min increased the incorporation of 32P into tyrosine hydroxylase to a level 5-fold that found in unstimulated control ganglia. The increase in phosphorylation of tyrosine hydroxylase was dependent on the duration and frequency of stimulation. Preganglionic stimulation did not increase the phosphorylation of tyrosine hydroxylase in a medium that contained low Ca2+ and high Mg2+. Increases in phosphorylation were reversible; within 30 min after the cessation of stimulation, the incorporation of 32P into tyrosine hydroxylase decreased to the level found in unstimulated ganglia. The nicotinic antagonist hexamethonium reduced the increase in 32P incorporation into tyrosine hydroxylase by about 50%, while the muscarinic antagonist atropine had no effect. Thus, preganglionic stimulation appeared to increase the phosphorylation of tyrosine hydroxylase in part by a nicotinic mechanism and in part by a noncholinergic mechanism. Antidromic stimulation of ganglia also increased the phosphorylation of tyrosine hydroxylase. Two-dimensional gel electrophoresis revealed that electrical stimulation also increased the incorporation of 32P into at least six other phosphoproteins in the ganglion.

Impulse conduction regulates myelin basic protein phosphorylation in rat optic nerve

The influence of action potential conduction in myelinated axons on the state of phosphorylation of myelin basic protein was studied in rat optic nerve incubated in vitro. For this purpose we used a technique that permits continuous recording of the responses of nerves to electrical stimulation together with the "back-phosphorylation" assay. Our results indicate that action potential conduction, but not electrical stimulation, increased the state of phosphorylation of myelin basic protein. The increment in basic protein phosphorylation was related to the number of impulses conducted, up to a maximal change which occurred after 12 X 10(3) impulses. Also, the effect of action potential conduction was reversible, since the state of myelin basic protein phosphorylation returned to control levels within 5 min of stopping stimulation. These findings raise the interesting possibility that myelin basic protein phosphorylation plays a role in some dynamic function of myelin, perhaps related to ion transport or to the process of recovery of ionic gradients.

Calcium uptake and calcium-dependent phosphorylation during development of rat brain neurons in culture

The emergence of the capacities for calcium uptake and calcium-regulated protein phosphorylation during the development of embryonic brain neurons in tissue culture was examined. In the maturing cells, the enhancement in 45Ca2+-uptake upon stimulation with high K+ increased by 3-4 fold during the second week in vitro, in parallel to an increase in the capacity for high K+-induced Ca2+-dependent release of prelabeled [3H]dopamine. The pattern of incorporation of [32Pi]phosphate into the major phosphoproteins in maturing cells under nonstimulating conditions also changed during cell development: the incorporation of 32Pi into two proteins of apparent molecular weights--55,000 and 43,000 dalton--increased, but decreased in a 45,000 dalton protein. Stimulation of mature cells (after 10-11 days in vitro) resulted in a Ca2+-dependent increase in the amount of 32Pi incorporated into the 43,000 dalton protein and a decrease in the amount incorporated into the 55,000 dalton protein. This calcium-regulated phosphorylation pattern was not observed until 6 days in vitro. Introduction of Ca2+ into the immature cells by means of the Ca2+ ionophore A23187 did not alter the phosphorylation pattern and did not cause neurotransmitter release. The amount of [35S]methionine incorporated into a 43,000 dalton protein which comigrated with the 43,000 dalton phosphoprotein also increased upon cell maturation. The results suggest that this phosphoprotein (which does not comigrate with nonphosphorylated actin on two-dimensional polyacrylamide gels) develops in the cells in parallel to the emerging processes of the stimulation-induced calcium entry and calcium-dependent neurosecretion.

Identification and comparison of protein I in chick and rat forebrain

Protein I has been identified and compared in membranes prepared from chick and rat forebrain. Based upon five criteria known to characterize protein I, namely, (1) its ability to serve as a substrate for both the cyclic AMP-dependent protein kinase and (2) the Ca2+- dependent, calmodulin-requiring protein kinase, (3) its ability to be extracted from membranes at low pH, (4) its characteristic pattern of digestion by collagenase, and (5) its existence as a basic protein, we have determined that although protein I of rat brain consists of the usual doublet polypeptides Ia and Ib, only a single chick forebrain polypeptide is detectable which possesses protein I-like properties.

Effect of inhibitors of protein synthesis on dopamine modulation of the slow-EPSP in rabbit superior cervical ganglion

The protein synthesis inhibitors, anisomycin and cycloheximide, were tested for their ability to prevent dopamine-induced long-term enhancement of the slow-EPSP in rabbit superior cervical ganglion. Exposure of ganglia to either inhibitor of protein synthesis, at a concentration that suppressed [3H]leucine incorporation into ganglionic protein by at least 95%, had no effect on the development of dopamine-induced enhancement of the slow-EPSP. Incubation of ganglia with dopamine, without an inhibitor of protein synthesis, was without effect on [3H]leucine incorporation into ganglionic protein. It is concluded that synthesis of new protein is not required for the development of long-term enhancement of the slow-EPSP induced by dopamine.

Protein phosphorylation in the brain

Protein phosphorylation represents an approach, sometimes the only approach available, to study the molecular basis for a wide variety of neurophysiological phenomena. The injection of protein kinases or protein kinase inhibitors into neurones has provided direct evidence that activation of protein kinases has an obligatory role in the mechanisms by which numerous extracellular signals produce specific physiological responses in neurones. A diversity of substrate proteins for the kinases have already been found. In several instances, the identity and functional role of these substrate proteins have been established.

Depolarisation-dependent protein phosphorylation in rat cortical synaptosomes: factors determining the magnitude of the response

The sequence of molecular events linking depolarisation-dependent calcium influx to the release of neurotransmitters from nerve terminals is unknown; however, calcium-stimulated protein phosphorylation may play a role. In this study the incorporation of phosphate into proteins was investigated using an intact postmitochondrial pellet isolated from rat cerebral cortex. The rate and relative incorporation of label into individual phosphoproteins depended on the prelabelling time and buffer concentrations of calcium and phosphate. After prelabelling for 45 min, depolarisation caused a greater than 20% increase in the labelling of 10 phosphoproteins, and this initial increase was maximal with 41 mM K+ for 5 s, or 30 microM veratridine for 15 s, in the presence of 1 mM calcium. Both agents also led to an initial dephosphorylation of four phosphoproteins. Depolarisation for 5 min led to a significant decrease in the labelling of all phosphoproteins. All of the depolarisation-stimulated changes in protein phosphorylation were calcium-dependent. The depolarisation conditions found to optimally alter the phosphorylation of synaptosomal proteins find many parallels in studies on calcium uptake and neurotransmitter release. However, the uniform responses of such a large number of phosphoproteins to the multitude of depolarisation conditions studied suggest that the changes could equally well relate to recovery events such as biosynthesis of neurotransmitters and regulation of intraterminal metabolic activity.

Depolarizing agents regulate the phosphorylation of myelin basic protein in rat optic nerves

The regulation of the state of phosphorylation of myelin basic protein has been studied in intact rat optic nerves incubated in vitro. For this purpose the endogenous state of phosphorylation was preserved and the "back-phosphorylation" technique was used to determine the amount of dephosphorylated protein present in extracts of the nerves. Our results indicate that when nerves were incubated in the presence of depolarizing agents, the state of phosphorylation of myelin basic protein was increased. This effect was calcium-dependent and was partly inhibited by chlorpromazine.

Calcium-dependent protein phosphorylation and dephosphorylation in intact brain neurons in culture

Preincubation of intact fetal rat brain neurons in culture with 32Pi results in the incorporation of 32Pi into about 20 specific proteins. Upon stimulation by electrical field stimulation or by K+-induced depolarization, highly significant calcium-dependent increase in phosphorylation of a protein of app. Mr 43 000 and decrease in phosphorylation of an app. Mr 55 000 protein occur. These changes can be attributed to the entry of Ca2+ into the cellular cytoplasm since they can occur upon selective permeabilization of the cell membrane to Ca2+ by the Ca2+-ionophore A23187 and are not observed upon stimulation of the cells in the presence of the Ca2+ channel blocker D-600. These data suggest that these phosphoproteins may be involved in the regulation of processes underlying neurotransmitter release.

Synapsin I (protein I), a nerve terminal-specific phosphoprotein. III. Its association with synaptic vesicles studied in a highly purified synaptic vesicle preparation

Synapsin I (protein I) is a neuron-specific phosphoprotein, which is a substrate for cAMP-dependent and Ca/calmodulin-dependent protein kinases. In two accompanying studies (De Camilli, P., R. Cameron, and P. Greengard, and De Camilli, P., S. M. Harris, Jr., W. B. Huttner, and P. Greengard, 1983, J. Cell Biol. 96:1337-1354 and 1355-1373) we have shown, by immunocytochemical techniques at the light microscopic and electron microscopic levels, that synapsin I is present in the majority of, and possibly in all, nerve terminals, where it is primarily associated with synaptic vesicles. In the present study we have prepared a highly purified synaptic vesicle fraction from rat brain by a procedure that involves permeation chromatography on controlled-pore glass as a final purification step. Using immunological methods, synapsin I concentrations were determined in various subcellular fractions obtained in the course of vesicle purification. Synapsin I was found to copurify with synaptic vesicles and to represent approximately 6% of the total protein in the highly purified synaptic vesicle fraction. The copurification of synapsin I with synaptic vesicles was dependent on the use of low ionic strength media throughout the purification. Synapsin I was released into the soluble phase by increased ionic strength at neutral pH, but not by nonionic detergents. The highly purified synaptic vesicle fraction contained a calcium-dependent protein kinase that phosphorylated endogenous synapsin I in its collagenase-sensitive tail region. The phosphorylation of this region appeared to facilitate the dissociation of synapsin I from synaptic vesicles under the experimental conditions used.

Regulation of phosphorylation of proteins I, IIIa, and IIIb in rat neurohypophysis in vitro by electrical stimulation and by neuroactive agents

The state of phosphorylation of proteins I, IIIa, and IIIb--neuron-specific phosphoproteins--was studied in neurosecretory endings of the neurohypophysis in vitro. Brief periods (a few seconds) of electrical stimulation caused large increases in the state of phosphorylation of all three proteins. The three proteins were dephosphorylated within 1 min after termination of the stimulation. High potassium, 8-bromo-cAMP, and dopamine also stimulated the phosphorylation of the three proteins. The effect of dopamine was blocked by the dopamine antagonist fluphenazine. Peptide mapping of protein I revealed that electrical stimulation or high potassium increased the state of phosphorylation of two regions of the molecule, whereas 8-bromo-cAMP and dopamine increased the state of phosphorylation of only one of these regions.