Lithium chloride inhibits the phosphorylation of newly synthesized neurofilament protein, NF-M, in cultured chick sensory neurons

Molecular targets of lithium action

Lithium is highly effective in the treatment of bipolar disorder and also has multiple effects on embryonic development, glycogen synthesis, hematopoiesis, and other processes. However, the mechanism of lithium action is still unclear. A number of enzymes have been proposed as potential targets of lithium action, including inositol monophosphatase, a family of structurally related phosphomonoesterases, and the protein kinase glycogen synthase kinase-3. These potential targets are widely expressed, require metal ions for catalysis, and are generally inhibited by lithium in an uncompetitive manner, most likely by displacing a divalent cation. Thus, the challenge is to determine which target, if any, is responsible for a given response to lithium in cells. Comparison of lithium effects with genetic disruption of putative target molecules has helped to validate these targets, and the use of alternative inhibitors of a given target can also lend strong support for or against a proposed mechanism of lithium action. In this review, lithium sensitive enzymes are discussed, and a number of criteria are proposed to evaluate which of these enzymes are involved in the response to lithium in a given setting.

Abnormalities of cAMP signaling in affective disorders: implication for pathophysiology and treatment

OBJECTIVE

During the last decade, much attention has been given to the role of signal transduction pathways in affective disorders. This review describes the possible role of the cAMP signaling in such disorders.

METHODS

Among the components of cAMP signaling, this review focuses on the cAMP-dependent phosphorylation system. We analyzed the basic components of the cAMP-dependent phosphorylation system and the preclinical evidence supporting their involvement in the biochemical action of antidepressants and mood stabilizers. The clinical data available until now, concerning the possible link between the cAMP-dependent phosphorylation system and the pathophysiology of affective disorders, are also reviewed.

RESULTS

The studies herein presented demonstrated that the levels and the activity of cAMP-dependent protein kinase are altered by antidepressants and mood stabilizers. Furthermore. these medications are able to modify the phosphorylation state, as well as the levels of some of the cAMP-dependent protein kinase substrates. More recently, clinical studies have reported abnormalities in the cAMP-dependent phosphorylation system in both peripheral cells and the postmortem brain of patients with affective disorders.

CONCLUSIONS

Overall, these studies support an involvement of cAMP signaling in affective disorders. The precise knowledge of the findings has the potential to improve the understanding of pharmacotherapy and to provide directions for the development of novel biochemical and genetic research strategies on the pathogenesis of affective disorders.

Insulin transiently increases tau phosphorylation: involvement of glycogen synthase kinase-3beta and Fyn tyrosine kinase

The modulation of tau phosphorylation in response to insulin was examined in human neuroblastoma SH-SY5Y cells. Insulin treatment resulted in a transient increase in tau phosphorylation followed by a decrease in tau phosphorylation that correlated directly with a sequential activation and deactivation of glycogen synthase kinase-3beta (GSK-3beta). The insulin-induced increase in tau phosphorylation and concurrent activation of GSK-3beta was rapid (<2 min) and transient, and was associated with increased tyrosine phosphorylation of GSK-3beta. The increase in GSK-3beta tyrosine phosphorylation corresponded directly to an increase in the association of Fyn tyrosine kinase with GSK-3beta, and Fyn immunoprecipitated from cells treated with insulin for 1 min phosphorylated GSK-3beta to a significantly greater extent than Fyn immunoprecipitated from control cells. Subsequent to the increase in GSK-3beta activation and tau phosphorylation, treatment of cells with insulin for 60 min resulted in a dephosphorylation of tau and a decrease in GSK-3beta activity. Thus, insulin rapidly and transiently activated GSK-3beta and modulated tau phosphorylation, alterations that may contribute to neuronal plasticity.

Identification of four alternatively spliced isoforms of chicken casein kinase I alpha that are all expressed in diverse cell types

Casein kinase I (CKI) is a family of widely expressed protein kinases. It is previously shown in mammalian tissues that CKIalpha exists as two or three alternatively spliced isoforms (Rowles et al.,1991; Zhang et al., 1996; Kuret et al., 1997). We now report that four alternatively spliced isoforms of CKIalpha are expressed in many chicken cells and tissues. A partial cDNA clone was isolated from a chicken brain library, using a probe derived from a bovine CKIalpha cDNA. The translated sequence of this clone was 100% identical to the bovine homolog containing the 'L' insert, with the addition of 12 amino acids just before the C terminus that had previously been reported in human and Xenopus CKIalpa. After completing the missing portion of the coding sequence by 5' RACE (rapid amplification of cDNA ends), full-length cDNA was PCR amplified from chicken brain cDNA, yielding four different products. These were cloned and sequenced and found to correspond to the four CKIalpha isoforms: CKIalpha, CKIalphaL, CKIalphaS and CKILalphaLS, where 'S' is the insert consisting of the 12 human/Xenopus C-terminal amino acids. Using reverse transcription and polymerase chain reaction (Rt-PCR), it was shown that the four isoforms are all expressed in neurons, fibroblasts and several tissues. This represents the first demonstration that four splice variants exist and are all expressed in a single type of cell.

Lithium reduces tau phosphorylation by inhibition of glycogen synthase kinase-3

Lithium is one of the most widely used drugs for treating bipolar (manic-depressive) disorder. Despite its efficacy, the molecular mechanism underlying its action has not been elucidated. One recent study has proposed that lithium inhibits glycogen synthase kinase-3 and thereby affects multiple cellular functions. Because glycogen synthase kinase-3 regulates the phosphorylation of tau (microtubule-binding protein that forms paired helical filaments in neurons of the Alzheimer's disease brain), we hypothesized that lithium could affect tau phosphorylation by inhibiting glycogen synthase kinase-3. Using cultured human NT2N neurons, we demonstrate that lithium reduces the phosphorylation of tau, enhances the binding of tau to microtubules, and promotes microtubule assembly through direct and reversible inhibition of glycogen synthase kinase-3. These results provide new insights into how lithium mediates its effects in the central nervous system, and these findings could be exploited to develop a novel intervention for Alzheimer's disease.

Methylmalonic acid reduces the in vitro phosphorylation of cytoskeletal proteins in the cerebral cortex of rats

The present work was undertaken to determine the action of methylmalonic acid (MMA), a metabolite, which accumulates in high amounts in methylmalonic acidemia, on the endogenous phosphorylating system associated with the cytoskeletal fraction proteins of cerebral cortex of young rats. We demonstrated that pre-treatment of cerebral cortex slices of young rats with 2.5 mM buffered methylmalonic acid (MMA) is effective in decreasing in vitro incorporation of [32P]ATP into neurofilament subunits (NF-M and NF-L) and alpha- and beta-tubulins. Based on the fact that this system contains cAMP-dependent protein kinase (PKA), Ca2+/calmodulin-dependent protein kinase II (CaMKII) and protein phosphatase 1 (PP1), we first tested the effect of MMA on the kinase activities by using the specific activators cAMP and Ca2+/calmodulin or the inhibitors PKAI or KN-93 for PKA and CaMKII, respectively. We observed that MMA totally inhibited the stimulatory effect of cAMP and interfered with the inhibitory effect of PKAI. In addition, the metabolite partially prevented the stimulatory effect of Ca2+/calmodulin and interfered with the effect of KN-93. Furthermore, in vitro dephosphorylation of neurofilament subunits and tubulins was totally inhibited in brain slices pre-treated with MMA. Taken together, these results suggest that MMA, at the same concentrations found in tissues of methylmalonic acidemic children, inhibits the in vitro activities of PKA, CaMKII and PP1 associated with the cytoskeletal fraction of the cerebral cortex of rats, a fact that may be involved with the pathogenesis of the neurological dysfunction characteristic of methylmalonic acidemia.

Lithium inhibits Alzheimer's disease-like tau protein phosphorylation in neurons

In Alzheimer's disease, tau protein becomes hyperphosporylated, which can contribute to neuronal degeneration. However, the implicated protein kinases are still unknown. Now we report that lithium (an inhibitor of glycogen synthase kinase-3) causes tau dephosphorylation at the sites recognized by antibodies Tau-1 and PHF-1 both in cultured neurons and in vivo in rat brain. This is consistent with a major role for glycogen synthase kinase-3 in modifying proline-directed sites on tau protein within living neurons under physiological conditions. Lithium also blocks the Alzheimer's disease-like proline-directed hyperphosphorylation of tau protein which is observed in neurons treated with a phosphatase inhibitor. These data raise the possibility of using lithium to prevent tau hyperphosphorylation in Alzheimer's disease.

The effect of chronic lithium treatment on the calibre of axons and nerve fibres in the rat sural nerve

Cases of peripheral neuropathy have been reported in humans receiving lithium therapy. However, no previous studies have addressed the question of whether chronic lithium treatment causes morphological changes in the peripheral nervous system in experimental animals. The present study used stereological methods to determine whether long-term administration of lithium affected the calibre of the axons or of the nerve fibres in the rat sural nerve. Twenty-two rats were divided into 2 groups and given either no treatment or lithium, with serum levels averaging from 0.5 to 0.7 mmol/l. After 30 weeks of treatment, the animals were killed and the sural nerve was isolated at the level of the knee and was removed. The cross-sectional area of axons and of nerve fibres was estimated by point counting. Compared with the controls, a strong tendency towards a reduced nerve fibre area in the lithium-treated animals was found, with a between-group difference of 1.79 microns 2 (P = 0.06). For the axon area, the difference was 0.73 micron 2 (P = 0.20).

Inhibitory effect of lithium on cAMP dependent phosphorylation system

The aim of the present study was to assess the direct effect of lithium on cAMP dependent phosphorylation. The results show that lithium, but not rubidium, at therapeutic and high concentrations significantly decreases the cAMP stimulated MAP2 endogenous phosphorylation in microtubule fraction. An inhibitory effect of lithium has also been found using purified heat stable microtubule proteins phosphorylated by the catalytic subunit of PKA. These data suggest a direct effect of lithium on the cAMP dependent protein kinase.

Cytoskeletal-associated protein kinase and phosphatase activities from cerebral cortex of young rats

We describe the phosphorylation system associated with the Triton-insoluble cytoskeletal fraction that phosphorylates in vitro the 150 kDa neurofilament subunit (NF-M) and alpha and beta tubulin from cerebral cortex of rats. The protein kinase activities were determined in the presence of 20 microM cyclic AMP (cAMP), 1 mM calcium and 1 microM calmodulin (Ca2+/calmodulin) or 1 mM calcium, 0.2 mM phosphatidylserine and 0.5 microM phorbol 12,13-dibutyrate (Ca2+/PS/PDBu). Phosphorylation of these cytoskeletal proteins increased approximately 35% and 65% in the presence of cAMP and Ca2+/calmodulin, respectively, but was unaffected in the presence of Ca2+/PS/PDBu. Basal phosphorylation of these proteins studied increased approximately 35% and 72% in the presence of 0.5 microM okadaic acid and 0.01 microM microcystin-LR, respectively, suggesting the presence of phosphatase type 1. Results suggest that at least two protein kinases and one protein phosphatase are associated with the Triton-insoluble cytoskeletal fraction from cerebral cortex of rats.

A neurofilament-associated kinase phosphorylates only a subset of sites in the tail of chicken midsize neurofilament protein

Although neurofilaments are among the most highly phosphorylated proteins extant, relatively little is known about the kinases involved in their phosphorylation. The majority of the phosphates present on the two higher-molecular-mass neurofilament subunits are added to multiply repeated sequence motifs in the tail. We have examined the specificity of a neurofilament-associated kinase (NFAK) partially purified from chicken spinal cord that selectively phosphorylates the middle-molecular-mass neurofilament subunit, NF-M. Two-dimensional phosphopeptide mapping of 32P-labeled NF-M shows that, in vitro, NFAK phosphorylates a subset of peptides phosphorylated in vivo in cultured neurons. The absence of a complete complement of labeled phosphopeptides following in vitro phosphorylation, compared with phosphorylation in vivo, is not due to a lack of availability of phosphorylation sites because the same maps are obtained when enzymatically dephosphorylated NF-M is used as an in vitro substrate. Phosphopeptide maps from in vitro-phosphorylated NF-M and those from a recombinant fusion protein containing only a segment of the tail piece of chicken NF-M reveal identical labeled peptides. The fusion protein lacks a segment containing 17 KXX(S/T)P putative phosphorylation sites contained in the tail of chicken NF-M but contains a segment that includes four KSPs and a KSD site also present in the intact tail. These results suggest (a) that NFAK mediates the phosphorylation of some, but not all, potential phosphorylation sites within the tail of NF-M and (b) that multiple kinases are necessary for complete phosphorylation of the NF-M tail.

Characterization of a neurofilament-associated kinase that phosphorylates the middle molecular mass component of chicken neurofilaments

We have examined the properties of a chicken neurofilament (NF) kinase partially purified from NF-enriched preparations. This kinase cosediments with NFs following extraction with Triton X-100 and can be separated in an active form from NFs by treatment with 0.8 M KCl. Sequential chromatography of the salt extract on DEAE-cellulose and phosphocellulose results in an approximately 500-fold increase in specific activity over endogenous NF preparations as measured by 32P-incorporation into the middle molecular mass component of NFs (NF-M). The kinase is Mg(2+)-dependent, second messenger-independent and inhibited by high concentrations of heparin. It shows selectivity for NF-M and evidence is presented that the kinase phosphorylates NF-M solely in the tail domain. The kinase can also phosphorylate the microtubule-associated proteins tau and MAP2 as well as mammalian NF-M, all of which share putative phosphorylation sequences with chicken NF-M.

Immunocytochemical localization of annexin V (CaBP33), a Ca(2+)-dependent phospholipid- and membrane-binding protein, in the rat nervous system and skeletal muscles and in the porcine heart

We investigated the ultrastructural localization of annexin V a Ca(2+)-dependent phospholipid- and membrane-binding protein in the nervous system, heart, and skeletal muscles. The results indicate that in the cerebellum the protein is restricted to glial cells, where it is found diffusely in the cytoplasm as well as associated with plasma membranes. Bergmann glial cell bodies and processes and astrocytes in the cerebellar cortex and oligodendrocytes in the cerebellar white matter displayed an intense immune reaction product. In sciatic nerves, the protein was exclusively found in Schwann cells with a subcellular localization similar to that seen in glial cells in the cerebellum. Pituicytes in the neurohypophysis were intensely immunostained, whereas axons were not. In the heart, annexin V was restricted to the sarcolemma, transverse tubules, and intercalated discs. In skeletal muscles the protein was localized to the sarcolemma and transverse tubules. No evidence for the presence of the protein in the sarcoplasm or in association with mitochondria, the sarcoplasmic reticulum, or contractile elements was obtained. The observation that plasma membranes in cells expressing annexin V have the protein associated with them is in agreement with previous data on Ca(2+)-dependent binding of the protein to brain and heart membranes, and on existence of both EGTA- and Triton X-100-extractable and resistant fractions of annexin V in these membranes. The present data support the hypothesis that annexin V might be involved in membrane trafficking and suggest a role for this protein in the regulation of cytoplasmic activities in glial cells.

Adenine and guanine nucleotide content of Triton-extracted cytoskeletal fractions of nonmuscle cells

Determination of the adenine and guanine nucleotides in Triton X-100-extracted cytoskeletal fractions was utilized to estimate the actin and tubulin content of the assembled cytoskeletons in nonmuscle cells. Results with stable cell lines (i.e., rat pheochromocytoma PC12 and neuroblastoma NB41A3) and with primary cultures (i.e., human foreskin fibroblasts and chick embryonic dorsal root ganglion neurons) exhibited levels of cytoskeletal fraction ADP and GDP consistent with their assembly-induced nucleoside-5'-triphosphatase activities only previously analyzed in vitro. Likewise, estimates of actin and tubulin content fall in the range of values obtained by other experimental approaches. In contrast, analysis of whole cell nucleotides showed high [ATP]/[ADP] and [GTP]/[GDP] ratios, suggesting there is little, if any, contamination of the cytoskeletal nucleotide pool by other cellular nucleotides.

Rapid degradation of newly synthesized tubulin in lithium-treated sensory neurons

When cultured chick sensory neurons were labeled with [35S]methionine for 1 h or longer in the presence of 5-25 mM LiCl, we found a dose-dependent reduction in the level of radiolabeled tubulin, to one third of control levels, with no noticeable effect on other proteins. The magnitude of this response was identical after a 1-h or 72-h preincubation in 25 mM LiCl and returned to control values within 1 h after removal of LiCl. Short (5-min) pulse-chase experiments revealed that tubulin synthesis was not affected by Li+, but that newly synthesized tubulin was rapidly degraded, such that 50% of the labeled beta-tubulin was lost within 5 min. There was no enhanced degradation of tubulin present before exposure to Li+. Addition of LiCl at various times before and after a 10-min pulse suggested that tubulin becomes completely refractory to Li(+)-induced degradation within 10 min after translation. Although Li+ treatment resulted in a decrease in the fraction of extant tubulin present in the unassembled form, the Li(+)-induced degradation of nascent tubulin is not a consequence of shifts in assembly state, because colcemid or taxol treatment did not lead to rapid degradation of newly synthesized tubulin, and neither drug altered the response to Li+. We suggest that Li+ interferes with the correct folding of tubulin polypeptides, exposing sites, normally hidden, to the action of a protease(s).