Role of Cdk5 and Tau phosphorylation in heterotrimeric G protein-mediated retinal growth cone collapse

Differential expression of cyclin-dependent kinases in the adult human retina in relation to CDK inhibitor retinotoxicity

Cyclin-dependent kinases (CDKs) are a family of kinases associated predominantly with cell cycle control, making CDK inhibitors interesting candidates for anti-cancer therapeutics. However, retinal toxicity (loss of photoreceptors) has been associated with CDK inhibitors, including the pan-CDK inhibitor AG-012896. The purpose of this research was to use a novel planar sectioning technique to determine CDK expression profiles in the ex vivo human retina with the aim of identifying isoforms responsible for CDK retinotoxicity. Four CDK isoforms (CDK11, 16, 17 and 18) were selected as a result of IC₅₀ data comparing neurotoxic (AG-012986 and NVP-1) and non-neurotoxic (dinaciclib and NVP-2) CDK inhibitors, with IC₅₀s at CDK11 showing a clear difference between the neurotoxic and non-neurotoxic drugs. CDK11 was maximally expressed in the photoreceptor layer, whereas CDK16, 17 and 18 showed maximal expression in the inner nuclear layer. CDK5 (an isoform associated with retinal homeostasis) was maximally expressed in the retinal ganglion cell layer. Apart from CDK18, each isoform showed expression in the photoreceptor layer. The human Müller cell line MIO-M1 expressed CDK5, 11, 16 and 17 and AG-01298 (0.02-60 µM) caused a dose-dependent increase in MIO-M1 cell death. In conclusion, CDK11 appears the most likely candidate for mediation of photoreceptor toxicity. RNA profiling can be used to determine the distribution of genes of interest in relation to retinal toxicity in the human retina.

Expression of Serine/Threonine Protein-Kinases and Related Factors in Normal Monkey and Human Retinas: The Mechanistic Understanding of a CDK2 Inhibitor Induced Retinal Toxicity

Protein-kinase inhibitors are among the most advanced compounds in development using the new drug discovery paradigm of developing small-molecule drugs against specific molecular targets in cancer. After treatment with a cyclin dependent kinase CDK2 inhibitor in monkey, histopathological analysis of the eye showed specific cellular damage in the photoreceptor layer. Since this CDK2 inhibitor showed activity also on other CDKs, in order to investigate the mechanism of toxicity of this compound, we isolated cones and rods from the retina of normal monkey and humans by Laser Capture Microdissection. Using Real-Time PCR we first measured the expression of cyclin dependent protein-kinases (CDK)1, 2, 4, 5, Glycogen synthase kinase 3β (GSK3β) and microtubule associated protein TAU. We additionally verified the presence of these proteins in monkey eye sections by immunohistochemistry and immunofluorescence analysis and afterwards quantified GSK3β , phospho-GSK3β and TAU by Reverse Phase Protein Microarrays. With this work we demonstrate how complementary gene expression and protein-based technologies constitute a powerful tool for the understanding of the molecular mechanism of a CDK2 inhibitor induced toxicity. Moreover, this investigative approach is helpful to better understand and characterize the mechanism of species-specific toxicities and further support a rational, molecular mechanism-based safety assessment in humans.

Regulators of G-protein-signaling proteins: negative modulators of G-protein-coupled receptor signaling

Regulators of G-protein-signaling (RGS) proteins are a category of intracellular proteins that have an inhibitory effect on the intracellular signaling produced by G-protein-coupled receptors (GPCRs). RGS along with RGS-like proteins switch on through direct contact G-alpha subunits providing a variety of intracellular functions through intracellular signaling. RGS proteins have a common RGS domain that binds to G alpha. RGS proteins accelerate GTPase and thus enhance guanosine triphosphate hydrolysis through the alpha subunit of heterotrimeric G proteins. As a result, they inactivate the G protein and quickly turn off GPCR signaling thus terminating the resulting downstream signals. Activity and subcellular localization of RGS proteins can be changed through covalent molecular changes to the enzyme, differential gene splicing, and processing of the protein. Other roles of RGS proteins have shown them to not be solely committed to being inhibitors but behave more as modulators and integrators of signaling. RGS proteins modulate the duration and kinetics of slow calcium oscillations and rapid phototransduction and ion signaling events. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. Human and animal studies have revealed that RGS proteins play a vital role in physiology and can be ideal targets for diseases such as those related to addiction where receptor signaling seems continuously switched on.

G-protein-coupled receptor cell signaling pathways mediating embryonic chick retinal growth cone collapse induced by lysophosphatidic acid and sphingosine-1-phosphate

In the development of the nervous system, one of the critical aspects is the proper navigation of axons to their targets, i.e. the problem of axonal guidance. We used the chick visual system as a model to investigate the role of the lysophospholipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) as potential axon guidance cues. We showed that both LPA and S1P cause a specific, dose-dependent growth cone collapse of retinal neurons in vitro in the chick model system, with slight differences compared to the mouse but very similar to observations in Xenopus. Because LPA and S1P receptors are G-protein-coupled receptors, we analyzed the intracellular signaling pathways using pharmacological inhibitors in chick retinal neurons. Blocking rho kinase (ROCK) prevented growth cone collapse by LPA and S1P, while blocking PLC or chelating calcium had no effect on growth cone collapse. Inhibition of Gi/o with pertussis toxin resulted in a partial reduction of growth cone collapse, both with LPA and with S1P. Inhibition of p38 blocked growth cone collapse mediated by LPA but not S1P. Thus, in addition to the involvement of the G12/13-ROCK pathway, LPA- and S1P-induced collapse of chick retinal growth cones has a partial requirement for Gi/o.

Cdk5/p35 and Rho-kinase mediate ephrin-A5-induced signaling in retinal ganglion cells

Ephrin-As are repulsive axonal guidance cues that regulate retinotectal projection. EphA tyrosine kinases, which are the receptors of ephrin-As, activate signaling cascades leading to cytosckeleton reorganization. Here, we address the role of cyclin-dependent kinase (Cdk) 5 in Eph receptor signaling induced by ephrin-A5. Ephrin-A5 induced a cell morphological response in PC-3M cells that endogenously express Cdk5 and EphA2, a receptor for ephrin-A5. This response was augmented by the transfection of p35, which is a neuronal regulator of Cdk5. While the morphological response of native PC-3M cells was not affected by olomoucine, an inhibitor of Cdk, the response was inhibited in the p35-transfected cells. In retinal ganglion cells, either olomoucine at 20 microM or Y-27632 at 10 microM, an inhibitor of Rho-kinase/ROKalpha/ROCKII, showed maximum inhibitory effect against ephrin-A5 (10 microg/ml)-induced growth cone collapse. Combined application of olomoucine and Y-27632 further suppressed the ephrin-A5-induced response. Ephrin-A5 evoked phosphorylation of Cdk5 at Tyr15 and tau, a substrate of Cdk5 in retinal growth cones. Recombinant herpes simplex virus expressing Cdk5 mutant (kinase-negative or Tyr15 to Ala) showed a dominant-negative effect on the ephrin-A5-induced growth cone collapse. These findings demonstrate that both Cdk5 and the Rho kinase pathway independently contribute to the downstream of ephrin-A-induced signaling in retinal ganglion cells.

Eicosanoid activation of protein kinase C epsilon: involvement in growth cone repellent signaling

Exposure of growing neurons to thrombin or semaphorin 3A stimulates a receptor-mediated signaling cascade that results in collapse of their growth cones. This collapse response necessitates eicosanoid production, as we have shown earlier. The present report investigates whether and which protein kinase C (PKC) isoforms may be activated by such eicosanoids. To examine these questions, we isolated growth cones from fetal rat brain and tested whether thrombin or the eicosanoid, 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE), could activate endogenous growth cone PKC. We show that both thrombin and 12(S)-HETE stimulate the phosphorylation of the myristoylated alanine-rich protein kinase C substrate, an 87-kDa adhesion site protein. Furthermore, we show both with immunoprecipitated and with recombinant PKC that 12(S)-HETE activation is selective for the epsilon isoform and does not require accessory proteins. Last, we demonstrate that PKC activation is necessary for thrombin-induced growth cone collapse. These data indicate that eicosanoid-mediated repellent effects result from the direct and selective activation of PKCepsilon and suggest the involvement of myristoylated alanine-rich protein kinase C substrate phosphorylation in growth cone collapse.

Fyn and Cdk5 mediate semaphorin-3A signaling, which is involved in regulation of dendrite orientation in cerebral cortex

Semaphorin-3A (Sema3A), a member of class 3 semaphorins, regulates axon and dendrite guidance in the nervous system. How Sema3A and its receptors plexin-As and neuropilins regulate neuronal guidance is unknown. We observed that in fyn- and cdk5-deficient mice, Sema3A-induced growth cone collapse responses were attenuated compared to their heterologous controls. Cdk5 is associated with plexin-A2 through the active state of Fyn. Sema3A promotes Cdk5 activity through phosphorylation of Tyr15, a phosphorylation site with Fyn. A Cdk5 mutant (Tyr15 to Ala) shows a dominant-negative effect on the Sema3A-induced collapse response. The sema3A gene shows strong interaction with fyn for apical dendrite guidance in the cerebral cortex. We propose a signal transduction pathway in which Fyn and Cdk5 mediate neuronal guidance regulated by Sema3A.

Axon guidance: the cytoplasmic tail

Recent advances in the study of axon guidance have begun to clarify the intricate signalling mechanisms utilised by receptors that mediate path-finding. Many of these axon guidance receptors, including Plexin B, EphA, ephrin B and Robo, regulate the Rho family of GTPases, to effect changes in motility. Recent studies demonstrate a critical role for the cytoplasmic tails of guidance receptors in signalling and also reveal the potential for a great deal of crosstalk between the various receptor-signalling pathways.

The inverted neurogenetic gradient of the mammalian isocortex: development and evolution

In this paper we review recent evidence on the molecular control of cell migration in the isocortex, and present an hypothesis for the evolutionary origin of the inside-out neurogenetic gradient of this structure. We suggest that there are at least two key factors involved in the acquisition of the inside-out gradient: (i) the expression of the protein reelin, which arrests the migration of cortical plate cells by detaching them from the radial glial fiber. This permits younger neurons to use the same fiber to migrate past the previous neurons; and (ii) the second factor is an intracellular signaling pathway dependent on a cyclin-dependent protein kinase (Cdk5). Cdk5 may work by inhibiting N-cadherin mediated cell aggregation as young cells cross the cortical plate, permitting them to move to the more superficial layers. Interestingly, the mutation in Cdk5 affects the migration of only those cells belonging to superficial layers, which are considered to be an evolutionary acquisition of the mammalian isocortex.

Factors controlling axonal and dendritic arbors

The sculpting and maintenance of axonal and dendritic arbors is largely under the control of molecules external to the cell. These factors include both substratum-associated and soluble factors that can enhance or inhibit the outgrowth of axons and dendrites. A large number of factors that modulate axonal outgrowth have been identified, and the first stages of the intracellular signaling pathways by which they modify process outgrowth have been characterized. Relatively fewer factors and pathways that affect dendritic outgrowth have been described. The factors that affect axonal arbors form an incompletely overlapping set with those that affect dendritic arbors, allowing selective control of the development and maintenance of these critical aspects of neuronal morphology.

A mammalian homolog of yeast MOB1 is both a member and a putative substrate of striatin family-protein phosphatase 2A complexes

Striatin and S/G(2) nuclear autoantigen (SG2NA) are related proteins that contain membrane binding domains and associate with protein phosphatase 2A (PP2A) and many additional proteins that may be PP2A regulatory targets. Here we identify a major member of these complexes as class II mMOB1, a mammalian homolog of the yeast protein MOB1, and show that its phosphorylation appears to be regulated by PP2A. Yeast MOB1 is critical for cytoskeletal reorganization during cytokinesis and exit from mitosis. We show that mMOB1 associated with PP2A is not detectably phosphorylated in asynchronous murine fibroblasts. However, treatment with the PP2A inhibitor okadaic acid induces phosphorylation of PP2A-associated mMOB1 on serine. Moreover, specific inhibition of PP2A also results in hyperphosphorylation of striatin, SG2NA, and three unidentified proteins, suggesting that these proteins may also be regulated by PP2A. Indirect immunofluorescence produced highly similar staining patterns for striatin, SG2NA, and mMOB1, with the highest concentrations for each protein adjacent to the nuclear membrane. We also present evidence that these complexes may interact with each other. These data are consistent with a model in which PP2A may regulate mMOB1, striatin, and SG2NA to modulate changes in the cytoskeleton or interactions between the cytoskeleton and membrane structures.

Phosphorylation by cyclin-dependent protein kinase 5 of the regulatory subunit of retinal cGMP phosphodiesterase. I. Identification of the kinase and its role in the turnoff of phosphodiesterase in vitro

Cyclic GMP phosphodiesterase (PDE) is an essential component in retinal phototransduction. PDE is regulated by Pgamma, the regulatory subunit of PDE, and GTP/Talpha, the GTP-bound alpha subunit of transducin. In previous studies (Tsuboi, S., Matsumoto, H. , Jackson, K. W., Tsujimoto, K., Williamas, T., and Yamazaki, A. (1994) J. Biol. Chem. 269, 15016-15023; Tsuboi, S., Matsumoto, H., and Yamazaki, A. (1994) J. Biol. Chem. 269, 15024-15029), we showed that Pgamma is phosphorylated by a previously unknown kinase (Pgamma kinase) in a GTP-dependent manner in photoreceptor outer segment membranes. We also showed that phosphorylated Pgamma loses its ability to interact with GTP/Talpha, but gains a 10-15 times higher ability to inhibit GTP/Talpha-activated PDE than that of nonphosphorylated Pgamma. Thus, we propose that the Pgamma phosphorylation is probably involved in the recovery phase of phototransduction through shut off of GTP/Talpha-activated PDE. Here we demonstrate that all known Pgammas preserve a consensus motif for cyclin-dependent protein kinase 5 (Cdk5), a protein kinase believed to be involved in neuronal cell development, and that Pgamma kinase is Cdk5 complexed with p35, a neuronal Cdk5 activator. Mutational analysis of Pgamma indicates that all known Pgammas contain a P-X-T-P-R sequence and that this sequence is required for the Pgamma phosphorylation by Pgamma kinase. In three different column chromatographies of a cytosolic fraction of frog photoreceptor outer segments, the Pgamma kinase activity exactly coelutes with Cdk5 and p35. The Pgamma kinase activity ( approximately 85%) is also immunoprecipitated by a Cdk5-specific antibody, and the immunoprecipitate phosphorylates Pgamma. Finally, recombinant Cdk5/p35, which were expressed using clones from a bovine retina cDNA library, phosphorylates Pgamma in frog outer segment membranes in a GTP-dependent manner. These observations suggest that Cdk5 is probably involved in the recovery phase of phototransduction through phosphorylation of Pgamma complexed with GTP/Talpha in mature vertebrate retinal photoreceptors.

Functions of semaphorins in axon guidance and neuronal regeneration

The semaphorin family comprises secreted and transmembrane signaling proteins that function in the nervous, immune, respiratory and cardiovascular systems. Sema3A, a secreted type of semaphorin, is now recognized as the most potent repulsive molecule inhibiting or repelling neurite outgrowth. The biological actions of Sema3A are mediated via neuropilin (Npn)-1, a receptor or one of the components of a receptor complex for Sema3A. Although the molecular mechanisms of Sema3A-Npn-1 signaling are largely unknown, a pertussis toxin-sensitive trimeric G protein(s), Rac-1, collapsin response mediator protein (CRMP), cyclic nucleotides and tyrosine kinase(s) have been implicated as essential and/or modulatory components of these processes. As repulsive molecules could be impediments to axon outgrowth, determining how these repulsive molecules exert their actions has the potential of uncovering new therapeutic approaches to injury and/or degeneration of neuronal tissues.