Protein phosphotyrosine in mouse brain: developmental changes and regulation by epidermal growth factor, type I insulin-like growth factor, and insulin

Differential effects of hypoxia-ischemia on subunit expression and tyrosine phosphorylation of the NMDA receptor in 7- and 21-day-old rats

The effect of cerebral hypoxia-ischemia (HI) on levels and tyrosine phosphorylation of the NMDA receptor was examined in 7- (P7) and 21 (P21)-day-old rats. Unilateral HI was administered by ligation of the right common carotid artery and exposure to an atmosphere of 8% O2/92% N2 for 2 (P7) or 1.5 (P21) h. This duration of HI produces significant infarction in nearly all of the survivors with damage being largely restricted to the cortex, striatum, and hippocampus of the hemisphere ipsilateral to the carotid artery ligation. NR2A levels in the right hemisphere of P7 pups were markedly reduced after 24 h of recovery, while NR1 and NR2B remained unchanged. In contrast, NR2B, but not NR2A, was reduced after HI at P21. At both ages, HI resulted in a transient increase in tyrosine phosphorylation of a number of forebrain proteins that peaked between 1 and 6 h of recovery. At both P7 and P21, tyrosine phosphorylation of NR2B was enhanced 1 h after HI and had returned to basal levels by 24 h. HI induced an increase in tyrosine phosphorylation of NR2A in 21 day, but not in 7-day-old animals. The differential effects of HI on the NMDA receptor at different post-natal ages may contribute to changing sensitivity to hypoxia-ischemia.

N-Methyl-D-aspartate receptor subunit proteins and their phosphorylation status are altered selectively in Alzheimer's disease

The N-methyl-D-aspartate (NMDA) receptor is a subtype of the ionotropic glutamate receptor that plays a pivotal role in synaptic mechanisms of learning and memory. We tested the hypothesis that NMDA receptor protein levels are abnormal in Alzheimer's disease (AD). By immunoblotting, we assessed levels of both non-phosphorylated and phosphorylated receptor subunit proteins from four separate regions of 16 post-mortem brains. Three patient groups with thorough pre-mortem neuropsychological testing were evaluated, including AD, early AD (p-AD), and control patients. Protein levels and phosphorylation status of NMDA receptor subunits NR1, NR2A and NR2B were correlated with measurements of cognitive performance. Selective regional reductions in NMDA receptor subunit protein levels were found in AD compared to controls, but protein levels in the p-AD group were similar to controls. Reductions of NR1 (53%, P<0.05) and NR2B (40%, P<0.05) were identified in hippocampus. Reductions of NR2A (39%, P<0.05) and NR2B (31%, P<0.01) were found in entorhinal cortex. No reductions were noted in occipital cortex and caudate. Phosphorylated NR2A (30%, P<0.05) and NR2B (56%, P<0.01) were selectively reduced in entorhinal cortex in AD when compared to controls. Both phosphorylated and non-phosphorylated NMDA receptor protein levels in entorhinal cortex correlated with Mini-Mental Status Examination (MMSE) and Blessed (BIMC) scores. The losses of phosphorylated and non-phosphorylated NMDA receptor subunit proteins correlated with changes in synaptobrevin levels (a presynaptic protein), but not with age or post-mortem interval. Our results demonstrate that NMDA receptor subunits are selectively and differentially reduced in areas of AD brain, and these abnormalities correlate with presynaptic alterations and cognitive deficits in AD.

Cyclic nucleotide phosphodiesterases: relating structure and function

Cyclic nucleotide phosphodiesterases (PDEs) comprise a superfamily of metallophosphohydrolases that specifically cleave the 3',5'-cyclic phosphate moiety of cAMP and/or cGMP to produce the corresponding 5'-nucleotide. PDEs are critical determinants for modulation of cellular levels of cAMP and/or cGMP by many stimuli. Eleven families of PDEs with varying selectivities for cAMP or cGMP have been identified in mammalian tissues. Within these families, multiple isoforms are expressed either as products of different genes or as products of the same gene through alternative splicing. Regulation of PDEs is important for controlling myriad physiological functions, including the visual response, smooth muscle relaxation, platelet aggregation, fluid homeostasis, immune responses, and cardiac contractility. PDEs are critically involved in feedback control of cellular cAMP and cGMP levels. Activities of the various PDEs are highly regulated by a panoply of processes, including phosphorylation events, interaction with small molecules such as cGMP or phosphatidic acid, subcellular localization, and association with specific protein partners. The PDE superfamily continues to be a major target for pharmacological intervention in a number of medically important maladies.

A possible role for tyrosine kinases in the regulation of the neuronal dopamine transporter in mouse striatum

The present investigation was undertaken to test the hypothesis that a reduction in the activity of protein tyrosine kinases would result in an alteration in dopamine transport. Genistein, a broad-spectrum inhibitor of protein tyrosine kinases, inhibited dopamine uptake into mouse striatal homogenates with an IC50 of 18 microM. The inhibition by genistein was rapid, reversible and somewhat selective, in that genistein did not inhibit the uptake of choline or GABA under similar conditions. Kinetic analyses indicated that genistein was a non-competitive inhibitor. Another protein tyrosine kinase inhibitor, tyrphostin 23, also inhibited transport but was significantly less potent than genistein. Tyrphostin 25 and lavendustin A were without major effect on dopamine uptake. In addition, the inactive structural analog of genistein, genistein, had no significant effect on dopamine uptake. The inhibition of dopamine transport by 50 microM genistein was accompanied by a reduction in the level of a 110-kDa band of tyrosine phosphoprotein. It is suggested that protein tyrosine kinases play a role in the cascade of events which ultimately lead to regulation of neuronal dopamine transport.

STEP61: a member of a family of brain-enriched PTPs is localized to the endoplasmic reticulum

The STEP family of protein tyrosine phosphatases is highly enriched within the CNS. Members of this family are alternatively spliced to produce both transmembrane and cytosolic variants. This manuscript describes the distinctive intracellular distribution and enzymatic activity of the membrane-associated isoform STEP61. Transfection experiments in fibroblasts, as well as subcellular fractionations, sucrose density gradients, immunocytochemical labeling, and electron microscopy in brain tissue, show that STEP61 is an intrinsic membrane protein of striatal neurons and is associated with the endoplasmic reticulum. In addition, structural analysis of the novel N-terminal region of STEP61 reveals several motifs not present in the cytosolic variant STEP46. These include two putative transmembrane domains, two sequences rich in Pro, Glu, Asp, Ser, and Thr (PEST sequences), and two polyproline-rich domains. Like STEP46, STEP61 is enriched in the brain, but the recombinant protein has less enzymatic activity than STEP46. Because STEP46 is contained in its entirety within STEP61 and differs only in the extended N terminus of STEP61, this amino acid sequence is responsible for the association of STEP61 with membrane compartments and may also regulate its enzymatic activity.

Tyrosine phosphorylation of myelin protein PO

Po (M(r) 30 kDa), the major protein component of peripheral nervous system (PNS) myelin, is known to be phosphorylated by protein kinase C on serine residues at multiple sites. This study was conducted to assess whether other amino acids might be phosphorylated in the protein. Segments of rat sciatic nerve were incubated with 32P in either the presence or absence of phorbol ester. Labeled Po was isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subjected to partial acid hydrolysis. Upon separation of the hydrolysis products by either thin-layer electrophoresis or thin-layer chromatography, a radioactive spot was detected which comigrated with authentic phosphotyrosine. In other experiments, nerves were incubated with the tyrosine phosphatase inhibitors vanadate or vanadyl hydroperoxide (pervanadate). When the nerve homogenate proteins were separated on gels and probed with a monoclonal antibody to phosphotyrosine on Western blots, a positive immune reaction was obtained for a protein species which migrated with the same mobility as PO on Coomassie Blue-stained gels. In the absence of 2-mercaptoethanol, this immunoreactive band displayed increased mobility on gels which is characteristic of the migration pattern of Po. The same immunostaining results were obtained using a purified peripheral myelin fraction prepared from nerve homogenates. Furthermore, the positions of immunoreactive bands produced by anti-Po and antiphosphotyrosine antibodies coincided on the same immunoblot of myelin proteins and purified Po. These data indicate that one or more tyrosyl residues in Po can be phosphorylated in intact sciatic nerve.

The early intracellular signaling pathway for the insulin/insulin-like growth factor receptor family in the mammalian central nervous system

Several studies support the idea that the polypeptides belonging to the family of insulin and insulin-like growth factors (IGFs) play an important role in brain development and continue to be produced in discrete areas of the adult brain. In numerous neuronal populations within the olfactory bulb, the cerebral and cerebellar cortex, the hippocampus, some diencephalic and brainstem nuclei, the spinal cord and the retina, specific insulin and IGF receptors, as well as crucial components of the intracellular receptor signaling pathway have been demonstrated. Thus, mature neurons are endowed with the cellular machinery to respond to insulin and IGF stimulation. Studies in vitro and in vivo, using normal and transgenic animals, have led to the hypothesis that, in the adult brain, IGF-I not only acts as a trophic factor, but also as a neuromodulator of some higher brain functions, such as long-term potentiation and depression. Furthermore, a trophic effect on certain neuronal populations becomes clearly evident in the ischemic brain or neurodegenerative disorders. Thus, the analysis of the early intracellular signaling pathway for the insulin/IGF receptor family in the brain is providing us with new intriguing findings on the way the mammalian brain is sculpted and operates.

Ethanol inhibits insulin receptor substrate-1 tyrosine phosphorylation and insulin-stimulated neuronal thread protein gene expression

Neuronal thread proteins (NTPs) are molecules that accumulate in the brains of patients with Alzheimer's disease, and may play a key role in both normal and neurodegenerative neuritic sprouting. In this investigation we determined whether NTP expression is up-regulated by insulin, an important neurotrophic factor that stimulates differentiation-associated neurite outgrowth, and studied the effects of ethanol, a known inhibitor of growth factor receptor tyrosine phosphorylation, on NTP expression and insulin-mediated signal transduction cascade in neuronal [primitive neuroectodermal tumour cell line 2; (PNET2)] cells. PNET2 cells were treated with 50 m-units/ml insulin in the presence or absence of 100 mM ethanol for 0.2-96 h, and cell proliferation and expression of NTP molecules were investigated by metabolic labelling, immunoprecipitation and immunohistochemical staining. Insulin stimulation resulted in an immediate increase in the levels of three (38, 18 and 15 kDa) of five NTP species (the others were of 26 and 21 kDa), followed by a decline in expression within 120 min; however, studies performed up to 96 h of culture demonstrated up-regulation by insulin of all five NTP species. Ethanol either abolished or severely muted the short- and long-term insulin-mediated upregulation of NTP expression, and substantially reduced insulin-mediated neuronal differentiation. The effects of ethanol on NTP gene expression were associated with impaired insulin-mediated tyrosine phosphorylation of both the insulin receptor beta subunit and the insulin receptor substrate-1 (IRS-1), resulting in decreased association of phosphatidylinositol 3-kinase with IRS-1. The findings suggest that ethanol may inhibit NTP expression associated with central nervous system neuronal differentiation by uncoupling the IRS-1-mediated insulin signal transduction pathway.

Focal adhesion kinase in rat central nervous system

Focal adhesion kinase (pp125FAK, FAK) is a 125 kDa non-receptor tyrosine kinase enriched in focal adhesions of various cell types, where it is thought to transduce signals triggered by contact with the extracellular matrix. We have studied the expression and localization of FAK in rat CNS. Immunoblotting, immunohistochemistry and in situ hybridization revealed the presence of FAK in all regions of the adult brain and demonstrated its enrichment in specific neuronal populations of the cerebral and cerebellar cortex, as well as in the hippocampus. During development, FAK protein levels were highest around birth in cerebral cortex and caudate putamen and decreased in the adult. In situ hybridization revealed enrichment of FAK mRNA in the ventricular germinative and external layers during the last period of embryonic growth. In primary cultures FAK immunoreactivity was localized in focal adhesions in astrocytes, whereas in developing neurons the highest levels were found in growth cones and perikarya. In the growth cone, FAK immunoreactivity colocalized with actin filaments. In mature neurons FAK appeared to be distributed in the whole cytoplasm, with no enrichment in any cellular compartment. Our results demonstrate the presence of high levels of FAK in rat CNS, maximal during development but persistent in the adult. Its enrichment in growth cones suggests that it may play a role in neurite outgrowth, as well as in plasticity in the adult.

Insulin-induced differentiation and modulation of neuronal thread protein expression in primitive neuroectodermal tumor cells is linked to phosphorylation of insulin receptor substrate-1

Neuronal thread proteins (NTPs) are a family of developmentally regulated molecules expressed in central nervous system (CNS) neurons and primitive neuroectodermal tumor (PNET) cell lines. NTP gene expression is modulated with DNA synthesis, neuritic sprouting, and neuronal differentiation. The present study explores the mechanism of insulin modulation of NTP gene expression during neuronal differentiation using PNET cell lines of CNS origin. PNET2 cells underwent neuronal differentiation with neurite outgrowth coupled with transient up-regulation of several species of NTP. In contrast, PNET1 cells failed to differentiate in response to insulin stimulation, although insulin receptors were more abundant than in PNET2 cells. Analysis of the insulin-mediated signal transduction pathway demonstrated that the lack of insulin responsiveness in PNET1 cells was primarily caused by impaired insulin-mediated tyrosyl phosphorylation of the insulin receptor substrate-1 (IRS-1). Correspondingly, the association between phosphatidyl-inositol 3 (PI3) kinase and phosphorylated IRS-1 was reduced in PNET1 compared with PNET2 cells. In contrast, the levels of IRS-1 protein were similar in PNET1 and PNET2 cells, and expression of the insulin receptor beta subunit (Ir beta) and insulin-mediated tyrosyl phosphorylation of the Ir beta were greater in PNET1 than PNET2 cells. The findings suggest that insulin effected neuronal differentiation and modulation of NTP gene expression in PNET cells utilizes a signal transduction cascade that requires tyrosyl phosphorylation of IRS-1.

Apoptosis induced by an anti-epidermal growth factor receptor monoclonal antibody in a human colorectal carcinoma cell line and its delay by insulin

Both EGF and insulin, or IGF, stimulate the growth of many cell types by activating receptors that contain tyrosine kinase activities. A monoclonal antibody (mAb 225) against the EGF receptor produced in this laboratory has been shown to competitively inhibit EGF binding and block activation of receptor tyrosine kinase. Here we report that a human colorectal carcinoma cell line, DiFi, which expresses high levels of EGF receptors on plasma membranes, can be induced to undergo G1 cell cycle arrest and programmed cell death (apoptosis) when cultured with mAb 225 at concentrations that saturate EGF receptors. Addition of IGF-1 or high concentrations of insulin can delay apoptosis induced by mAb 225, while the G1 arrest cannot be reversed by either IGF-1 or insulin. Insulin/IGF-1 cannot activate EGF receptor tyrosine kinase that has been inhibited by mAb 225. Moreover, an mAb against the IGF-1 receptor, which has little direct effect on DiFi cell growth, can block the capacity of insulin/IGF-1 to delay apoptosis induced by mAb 225, suggesting that the insulin/IGF-1-mediated delay of apoptosis is acting through the IGF-1 receptor. In contrast, insulin/IGF-1 cannot delay the apoptosis caused by the DNA damaging agent, cisplatin. The results indicate that EGF receptor activation is required both for cell cycle progression and for prevention of apoptosis in DiFi cells, and that a signal transduction pathway shared by receptors for insulin/IGF-1 and EGF may be involved in regulating apoptosis triggered by blockade of the EGF receptor.

Modulation of protein tyrosine phosphorylation in rat insular cortex after conditioned taste aversion training

Protein tyrosine phosphorylation is a major signal transduction pathway involved in cellular metabolism, growth, and differentiation. Recent data indicate that tyrosine phosphorylation also plays a role in neuronal plasticity. We are using conditioned taste aversion, a fast and robust associative learning paradigm subserved among other brain areas by the insular cortex, to investigate molecular correlates of learning and memory in the rat cortex. In conditioned taste aversion, rats learn to associate a novel taste (e.g., saccharin) with delayed poisoning (e.g., by LiCl injection). Here we report that after conditioned taste aversion training, there is a rapid and marked increase in tyrosine phosphorylation of a set of proteins in the insular cortex but not in other brain areas. A major protein so modulated, of 180 kDa, is abundant in a membrane fraction and remains modulated for more than an hour after training. Exposure of the rats to the novel taste alone results in only a small modulation of the aforementioned proteins whereas administration of the malaise-inducing agent per se has no effect. To the best of our knowledge, this is the first demonstration of modulation of protein tyrosine phosphorylation in the brain after a behavioral experience.

Decreased insulin-like growth factor I-mediated protein tyrosine phosphorylation in human olivopontocerebellar atrophy and lurcher mutant mouse

We examined insulin-like growth factor I (IGF-I)-dependent phosphorylation and protein tyrosine kinase (PTK) activity in cerebellar cortex of normal humans, patients with olivopontocerebellar atrophy (OPCA) ("C" kindred) and in lurcher mutant mouse, a suggested animal model for OPCA. PTK activity and IGF-I-dependent protein tyrosine phosphorylation was significantly reduced in cerebellar cortex of human OPCA patients as compared to the normal controls. Immunoblot analysis also demonstrated a decrease in cerebellar 80 kDa phosphotyrosine protein in these patients. By autoradiography, IGF-I receptors were localized in the molecular layer of 30-day-old control and lurcher mutant mice cerebella. However, the lurcher mutant mice showed a decrease in [125I]-IGF-I binding in the molecular layer as compared to the littermate controls. The IGF-I receptor autophosphorylation was also markedly reduced in 15-day- and 22-day-old lurcher cerebella. These results suggest that the process of cerebellar degeneration in human OPCA and lurcher mutant mouse may be associated with altered IGF-I receptor binding and protein tyrosine phosphorylation.

Depolarization and neurotransmitters increase neuronal protein tyrosine phosphorylation

In rat hippocampal slices and in neurons in primary culture, K(+)-induced depolarization increased markedly and rapidly tyrosine phosphorylation of a 110-kDa protein (pp110) and, to a lesser degree, of a 120-kDa protein (pp120), in a calcium-dependent fashion. Glutamate, 1-aminocyclopentane-trans-1,3-dicarboxylic acid (an agonist of metabotropic glutamate receptors), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (an agonist of ionotropic glutamate receptors) stimulated also tyrosine phosphorylation of pp110 and pp120. These effects were not observed in astrocytes in primary culture. In hippocampal slices tyrosine phosphorylation of pp110 and pp120 was stimulated by Ca(2+)-ionophores and by phorbol esters and antagonized by a chelator of intracellular Ca2+ and by drugs that inhibit protein kinase C. Stimulation of muscarinic and alpha 1-adrenergic receptors increased also tyrosine phosphorylation of pp110 and pp120. These results demonstrate that membrane depolarization and stimulation of neurotransmitter receptors activate a tyrosine phosphorylation pathway in neurons. This pathway involves an increase in intracellular Ca2+ concentrations and the activation of protein kinase C. It may provide a biochemical basis for some neurotrophic effects of electrical activity and neurotransmitters and may contribute to the role of tyrosine phosphorylation in long-term potentiation.

Stimulation of protein-tyrosine phosphorylation in rat striatum after lesion of dopamine neurons or chronic neuroleptic treatment

Even though the short-term actions of dopamine on postsynaptic receptors are well-characterized, the molecular bases for long-term trophic interactions between dopamine neurons and their targets remain unclear. Since protein-tyrosine phosphorylation plays a key role in the action of trophic factors, we have investigated its possible involvement in the interactions between dopamine neurons and their striatal targets. Lesioning rat nigrostriatal dopamine neurons by using 6-hydroxydopamine increased the phosphorylation on tyrosine of several proteins, including a major 180-kDa protein (pp180) in the ipsilateral striatum. Protein-tyrosine kinase activity was also increased in the striatum ipsilateral to the lesion, whereas no change in phosphotyrosine phosphatase activity was detected. The stimulation of pp180 phosphorylation was observed 1, 2, and 8 weeks after 6-hydroxydopamine lesion, was selective for the destruction of dopamine neurons, and was mimicked by chronic blockade of dopamine receptors with neuroleptics. Additional lesion experiments and subcellular fractionation showed that pp180 is located in neuronal postsynaptic densities, suggesting that pp180 is a postsynaptic component of corticostriatal synapses. Our results indicate that lesion of specific afferent fibers can activate tyrosine phosphorylation in central neurons and suggest that tyrosine phosphorylation is involved in the long-term consequences of dopamine deficiency and may play a role in synaptic plasticity.