Tyrosine kinase regulation of a molluscan circadian clock

Tyrosine phosphorylation of cGMP-gated ion channels is under circadian control in chick retina photoreceptors

PURPOSE

To investigate the role of tyrosine phosphorylation in circadian regulation of cGMP-gated cation channels (CNGCs) of chicken cone photoreceptors.

METHODS

Chick retinas were studied on the second day of constant darkness (DD) after several days of entrainment to 12:12 hr light-dark (LD) cycles in vitro. Inside-out patch recordings were made during the subjective day and subjective night to quantify circadian changes in the sensitivity of CNGCs to activation by cGMP after treatment with various tyrosine kinase and tyrosine phosphatase inhibitors. Immunoprecipitation and immunoblot analysis were also used to examine tyrosine phosphorylation of CNGCs and closely associated proteins after separation by conventional and two-dimensional SDS-PAGE.

RESULTS

Treatment with tyrosine kinase inhibitors caused a significant decrease in K(1/2) for cGMP activation of CNGCs in patches excised from cones during the subjective day, but had no effect on K(1/2) during the subjective night. Conversely, treatment with a tyrosine phosphatase inhibitor caused a significant increase in the K(1/2) of CNGCs in patches excised during the subjective night but had no effect on channel K(1/2) during the subjective day. Broad spectrum serine-threonine phosphatase inhibitors had no effect. An 85-kDa tyrosine polypeptide that coimmunoprecipitated with CNGC alpha-subunits was detectable at higher levels during the subjective day than during the subjective night. CNGC alpha-subunits were not tyrosine phosphorylated as a function of the time of day.

CONCLUSIONS

Circadian control of cone CNGCs appears to entail elevated daytime tyrosine phosphorylation of an approximately 85-kDa auxiliary protein or another subunit of the CNGCs.

Novel effects on the Gonyaulax circadian system produced by the protein kinase inhibitor staurosporine

The protein kinase inhibitor staurosporine was found to cause a dramatic increase in the free-running period (FRP) of circadian rhythms in the dinoflagellate Gonyaulax polyedra, and its effect was similar when added at different phases of the circadian cycle. Chronic exposure to staurosporine lengthened the FRP by as much as 7 h without significantly affecting the amplitude or waveform of the bioluminescence rhythm. The effect on the length of the FRP occurred only above a threshold concentration, and it lasted for a limited number of cycles that depended on the dose of the drug. The FRP lengthening was not evident until 23 to 26 h after staurosporine addition, even though the drug entered Gonyaulax cells in 1 h or less. When tested in combination with bright light pulses, staurosporine was found to enhance both light-induced phase advances and delays, indicating that the drug acts on circadian phototransduction. At concentrations that alter the FRP and the response to light pulses, staurosporine appears to act on a small number of protein kinases, attenuating the activity of two individual protein kinases without affecting overall phosphate incorporation into proteins in vitro.

Effects of cyclin-dependent kinase inhibitors on transcription and ocular circadian rhythm of Aplysia

Cyclin-dependent kinases (CDKs) mediate cell-cycle phase transitions. Recently, CDKs have been associated with non-cell-cycle roles such as DNA repair, transcription, and phosphate metabolism in yeast. The cyclical processes, circadian rhythms and the eukaryotic cell cycle, are similar in many respects. It is possible that a kinase like CDK is involved in the control of circadian rhythms. In this study, the effects of CDK inhibitors (olomoucine, roscovitine, and butyrolactone I) on the Aplysia ocular circadian rhythm were investigated. Continuous treatments with olomoucine (10 microM) lengthened the free-running period of the rhythm, and pulse treatments of olomoucine (6 h, 100 microM) delayed the rhythm. The effects of olomoucine on the rhythm were qualitatively similar to those of a reversible inhibitor of transcription, 5,6-dichloro-beta-1-ribobenzimidazole. Subsequently, olomoucine was found to inhibit RNA synthesis in the eye of Aplysia and Bulla. All of the other CDK inhibitors used in this study also inhibited transcription in the eye of Aplysia, and their effects on transcription correlated with their effects on the circadian rhythm. This study adds substantial evidence to that previously obtained by using 5,6-dichloro-beta-1-ribobenzimidazole for a role of transcription in the mechanism responsible for circadian rhythmicity in the eye of Aplysia. Also, these results indicate that caution is warranted in interpreting results obtained by using CDK inhibitors, because these drugs appear to inhibit transcription as well as CDKs.

The SCN is comprised of a population of coupled oscillators

The circadian clock in the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus exhibits two necessary properties: (1) a mechanism for the generation of autonomous circadian rhythms in individual pacemaker cells, and (2) a means to synchronize the autonomous pacemaker cells. A variety of potential components of the endogenous pacemaker, including ion channels, second messengers, transcriptional factors, and the protein targets of kinases and transcription factors are reviewed. Similarly, reverse transmitter transport, extracellular ion fluxes, small membrane-diffusible molecules, glial regulation, and neural adhesion molecules are considered as possible synchronizing factors. Provisional criteria are suggested for empirical distinction of endogenous pacemaker versus synchronizing mechanisms.

The cyclin-dependent kinase (cdk) inhibitors, olomoucine and roscovitine, alter the expression of a molluscan circadian pacemaker

1. In this study, we determined the effects of the cyclin-dependent kinase (cdk) inhibitors, olomoucine and roscovitine, on the circadian rhythm of optic nerve impulse activity recorded from the eye of the marine snail Bulla gouldiana. 2. We found that olomoucine lengthened period and altered circadian phase in a dose-dependent manner without appreciably affecting gene transcription or translation. We also found that the more specific cdk inhibitor, roscovitine, was approximately 10-fold more effective in lengthening circadian period, while the inactive analogue, iso-olomoucine, was ineffective. 3. The current results, along with previous results from our laboratory, are consistent with the hypothesis that the biochemical mechanism responsible for generating the ocular circadian rhythm in B. gouldiana is related to the biochemical mechanism that regulates the eukaryotic cell division cycle, i.e., by modulation of the activity of protein kinases belonging to the cdk family.

Differential pre- and postsynaptic modulation of chemical transmission in the squid giant synapse by tyrosine phosphorylation

To assess the role of tyrosine phosphorylation/dephosphorylation balance in synaptic transmission, a set of studies was implemented at the squid giant synapse. Presynaptic induction of tyrosine phosphorylation, following administration of the tyrosine phosphatase inhibitor pervanadate, produced a sizable increase in presynaptic calcium current and a concomitant and paradoxical decrement of the postsynaptic potential amplitude. Presynaptic microinjection of an active protein tyrosine kinase dramatically increased calcium currents and incremented postsynaptic potential amplitude. By contrast, the same procedure at the postsynaptic terminal reduced the size of the postsynaptic potential. This differential effect may be prodromic to long-term plasticity, as postsynaptic sensitivity is momentarily deemphasized, whereas presynaptic second messenger cascades triggered by increased calcium currents are accentuated.

Identification of CDK- and cyclin-like proteins in the eye of Bulla gouldiana

The ocular circadian rhythm in the eye of Bulla gouldiana is generated by a rhythm in membrane potential of retinal neurons that is driven by alterations in potassium conductance. Since potassium conductance may be modulated by the phosphorylation of potassium channels, the circadian rhythm may reflect rhythmic changes in protein kinase activity. Furthermore, the circadian rhythm recorded from the Bulla eye can be phase shifted by agents that affect protein synthesis and protein phosphorylation on tyrosine residues. Interestingly, the eukaryotic cell division cycle is generated by similar processes. Rhythmic cell division is regulated by periodic synthesis and degradation of a protein, cyclin, and periodic tyrosine phosphorylation of a cyclin-dependent kinase (cdk), p34cdc2. The interaction between these two proteins results in rhythmic kinase activity of p34cdc2. Both cyclin and p34cdc2 are part of two diverse gene families, some of whose members have been localized to post-mitotic cell types with no function yet determined. In the current work, we identify proteins similar to the cdks and cyclin in the eye of Bulla. Neither of these ocular proteins are found in mitotic cells in Bulla, and the cdk-like protein (p40) is specific to the eye. Furthermore, the concentration of the cyclin-like protein (p66) is affected by treatments that phase shift the circadian rhythm. The identification of cdk and cyclin-like proteins in the Bulla eye is consistent with the hypothesis that the biochemical mechanism responsible for generating the ocular circadian rhythm in Bulla is related to the biochemical mechanism that regulates the eukaryotic cell division cycle.