Modulation of Reelin signaling by Cyclin-dependent kinase 5

Physiological and pathological roles of the transcriptional kinases CDK12 and CDK13 in the central nervous system

The cyclin-dependent kinases 12 (CDK12) and 13 (CDK13) govern several steps of gene expression, including transcription, RNA processing and translation. The main target of CDK12/13 is the serine 2 residue of the carboxy-terminal domain of RNA polymerase II (RNAPII), thus influencing the directionality, elongation rate and processivity of the enzyme. The CDK12/13-dependent regulation of RNAPII activity influences the expression of selected target genes with important functional roles in the proliferation and viability of all eukaryotic cells. Neuronal cells are particularly affected by the loss of CDK12/13, as result of the high dependency of neuronal genes on RNAPII processivity for their expression. Deregulation of CDK12/13 activity strongly affects brain physiology by influencing the stemness potential and differentiation properties of neuronal precursor cells. Moreover, mounting evidence also suggest the involvement of CDK12/13 in brain tumours. Herein, we discuss the functional role(s) of CDK12 and CDK13 in gene expression regulation and highlight similarities and differences between these highly homologous kinases, with particular attention to their impact on brain physiology and pathology. Lastly, we provide an overview of CDK12/13 inhibitors and of their efficacy in brain tumours and other neoplastic diseases.

The 5-HT6 receptor interactome: New insight in receptor signaling and its impact on brain physiology and pathologies

The serotonin (5-HT)₆ receptor is a Gs-coupled receptor exclusively expressed in the central nervous system. Highest receptor densities are found in brain regions implicated in mnemonic functions where the receptor is primarily but not exclusively located in the primary cilium of neurons. The 5-HT₆ receptor continues to raise particular interest for neuropharmacologists, given the pro-cognitive effects of antagonists in a wide range of cognitive impairment paradigms in rodents and human. The 5-HT₆ receptor also finely controls key neuro-developmental processes including neuron migration and differentiation. However, its influence upon neurodevelopment and cognition is not solely mediated by its coupling to the Gs-adenylyl cyclase pathway, suggesting alternative signal transduction mechanisms. This prompted studies aimed at characterizing the receptor interactome that identified 125 candidate receptor partners, making the 5-HT₆ receptor one of the G protein-coupled receptors with the most extensively characterized interactome. These studies showed that the receptor localization at the plasma membrane and, consequently, its signal transduction, are finely modulated by several receptor partners. They demonstrated that prefrontal 5-HT₆ receptors engage the mTOR pathway to compromise cognition in neurodevelopmental models of schizophrenia, and a role of the 5-HT₆-mTOR pathway in temporal epilepsy. Finally, they revealed that the receptor activates Cdk5 signaling in an agonist-independent manner through a mechanism involving receptor phosphorylation by the associated Cdk5 and highlighted its key role in the migration of neurons and neurite growth. These new receptor-operated signaling mechanisms should be considered in the future development of drugs acting on 5-HT₆ receptors. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Canonical and Non-canonical Reelin Signaling

Reelin is a large secreted glycoprotein that is essential for correct neuronal positioning during neurodevelopment and is important for synaptic plasticity in the mature brain. Moreover, Reelin is expressed in many extraneuronal tissues; yet the roles of peripheral Reelin are largely unknown. In the brain, many of Reelin's functions are mediated by a molecular signaling cascade that involves two lipoprotein receptors, apolipoprotein E receptor-2 (Apoer2) and very low density-lipoprotein receptor (Vldlr), the neuronal phosphoprotein Disabled-1 (Dab1), and members of the Src family of protein tyrosine kinases as crucial elements. This core signaling pathway in turn modulates the activity of adaptor proteins and downstream protein kinase cascades, many of which target the neuronal cytoskeleton. However, additional Reelin-binding receptors have been postulated or described, either as coreceptors that are essential for the activation of the "canonical" Reelin signaling cascade involving Apoer2/Vldlr and Dab1, or as receptors that activate alternative or additional signaling pathways. Here we will give an overview of canonical and alternative Reelin signaling pathways, molecular mechanisms involved, and their potential physiological roles in the context of different biological settings.

Dab1-mediated colocalization of multi-adaptor protein CIN85 with Reelin receptors, ApoER2 and VLDLR, in neurons

Reelin-Dab1 signaling is indispensable for proper positioning of neurons in mammalian brain. Reelin is a glycoprotein secreted from Cajal-Reztuis cells in marginal zone of cerebral cortex, and its receptors are Apolipoprotein E receptor 2 (ApoER2) or very low density lipoprotein receptor (VLDLR) expressed on migrating neurons. When Reelin binds to ApoER2 or VLDLR, an adaptor protein Dab1 bound to the receptors undergoes Tyr phosphorylation that is essential for Reelin signaling. We reported previously that Cdk5-p35 phosphorylates Dab1 at Ser400 and Ser491 and the phosphorylation regulates its binding to CIN85, which is an SH3-containing multiadaptor protein involved in endocytic downregulation of receptor-tyrosine kinases. However, the interaction of CIN85 with Dab1 has not been addressed in neurons. We examined here a possibility that CIN85 has a role in Reelin signaling. We found nonpho-sphorylated Dab1-mediated colocalization of CIN85 with ApoER2. The colocalization of CIN85 with ApoER2 was increased in neurons stimulated with Reelin repeats 3-6, an active Reelin fragment. The stimulation recruited CIN85 to domains in plasma membrane where it colocalized with ApoER2 and Dab1 and then to EEA1-labeled early endosomes in the cytoplasm. In addition, Tyr phosphorylation of Dab1 strengthened the binding to CIN85. These results suggest that CIN85 participates in Reelin signaling through the binding to Dab1.

Abnormal activity of the MAPK- and cAMP-associated signaling pathways in frontal cortical areas in postmortem brain in schizophrenia

Recent evidence suggests that schizophrenia may result from alterations of integration of signaling mediated by multiple neurotransmitter systems. Abnormalities of associated intracellular signaling pathways may contribute to the pathophysiology of schizophrenia. Proteins and phospho-proteins comprising mitogen activated protein kinase (MAPK) and 3'-5'-cyclic adenosine monophosphate (cAMP)-associated signaling pathways may be abnormally expressed in the anterior cingulate (ACC) and dorsolateral prefrontal cortex (DLPFC) in schizophrenia. Using western blot analysis we examined proteins of the MAPK- and cAMP-associated pathways in these two brain regions. Postmortem samples were used from a well-characterized collection of elderly patients with schizophrenia (ACC=36, DLPFC=35) and a comparison (ACC=33, DLPFC=31) group. Near-infrared intensity of IR-dye labeled secondary antisera bound to targeted proteins of the MAPK- and cAMP-associated signaling pathways was measured using LiCor Odyssey imaging system. We found decreased expression of Rap2, JNK1, JNK2, PSD-95, and decreased phosphorylation of JNK1/2 at T183/Y185 and PSD-95 at S295 in the ACC in schizophrenia. In the DLPFC, we found increased expression of Rack1, Fyn, Cdk5, and increased phosphorylation of PSD-95 at S295 and NR2B at Y1336. MAPK- and cAMP-associated molecules constitute ubiquitous intracellular signaling pathways that integrate extracellular stimuli, modify receptor expression and function, and regulate cell survival and neuroplasticity. These data suggest abnormal activity of the MAPK- and cAMP-associated pathways in frontal cortical areas in schizophrenia. These alterations may underlie the hypothesized hypoglutamatergic function in this illness. Together with previous findings, these data suggest that abnormalities of intracellular signaling pathways may contribute to the pathophysiology of schizophrenia.

Collapsin response mediator proteins regulate neuronal development and plasticity by switching their phosphorylation status

Collapsin response mediator protein (CRMP) was originally identified as a molecule involved in semaphorin3A signaling. CRMPs are now known to consist of five homologous cytosolic proteins, CRMP1-5. All of them are phosphorylated and highly expressed in the developing and adult nervous system. In vitro experiments have clearly demonstrated that CRMPs play important roles in neuronal development and maturation through the regulation of their phosphorylation. Several recent knockout mice studies have revealed in vivo roles of CRMPs in neuronal migration, neuronal network formation, synapse formation, synaptic plasticity, and neuronal diseases. Dynamic spatiotemporal regulation of phosphorylation status of CRMPs is involved in many aspects of neuronal development.

ApoER2 and VLDLR in the developing human telencephalon

The Reelin-Dab1 signaling pathway plays a crucial role in regulating the migration and position of cortical neurons during the development of the cerebral cortex. Mutation in Reelin may result in severe developmental disorders such as autosomal recessive lissencephaly. Apolipoprotein E receptor type-2 (ApoER2) and very low-density lipoprotein receptor (VLDLR) are canonical receptors of Reelin, through which extracellular Reelin activates the intracellular adapter, Disabled1(Dab1), and subsequently interacts with other molecules. Although it is widely accepted that ApoER2 and VLDLR are indispensable components of the Reelin signaling pathway, little is known of their expression pattern in the laminated developing human brain. Here, we collected 18 cases of human fetal brains of 6-18 gestational weeks (GW) old and examined the expression of ApoER2 and VLDLR in the their telencephalon using immunocytochemical staining. We found that both receptors were absent in the preplate (PP) and the earliest stage of the cortical plate (CP). In later stages of CP development, ApoER2 was expressed earlier than VLDLR in the migrating neurons. Thus, the Reelin-Dab1 signaling pathway may not be involved in the formation of the preplate and deep layers of the CP. Instead, the pathway may act on neurons that are destined to form the more superficial layers of the CP. In addition, the pathway required ApoER2 only rather than both ApoER2 and VLDLR at the initiation of activity.

Serine phosphorylation regulates disabled-1 early isoform turnover independently of Reelin

The Reelin-Disabled 1 (Dab1) signaling pathway plays an important role in neuronal cell migration during brain development. Dab1, an intracellular adapter protein which is tyrosine phosphorylated upon Reelin stimulation, has been directly implicated in the transmission and termination of Reelin-mediated signaling. Two main forms of Dab1 have been identified in the developing chick retina, an early isoform (Dab1-E) expressed in progenitor cells and a late isoform (Dab1-L, a.k.a. Dab1) expressed in differentiated cells. Dab1-E is missing two Src family kinase (SFK) phosphorylation sites that are critical for Reelin-Dab1 signaling and is not tyrosine phosphorylated. We have recently demonstrated a role for Dab1-E in the maintenance of retinal progenitor cells. Here, we report that Dab1-E is phosphorylated at serine/threonine residues independent of Reelin. Cdk2, highly expressed in retinal progenitor cells, mediates Dab1-E phosphorylation at serine 475 which in turn promotes ubiquitination-triggered proteasome degradation of Dab1-E. Inhibition of protein phosphatase 1 and/or protein phosphatase 2A leads to increased Dab1-E instability. We propose that Dab1 turnover is regulated by both Reelin-independent serine/threonine phosphorylation and Reelin-dependent tyrosine phosphorylation.

M1 muscarinic receptor for the development of auditory cortical function

The sensory cortex is subject to continuous remodelling during early development and throughout adulthood. This process is important for establishing normal brain function and is dependent on cholinergic modulation via muscarinic receptors. Five muscarinic receptor genes encode five unique receptor subtypes (M_1-5). The distributions and functions of each subtype vary in central and peripheral systems. In the brain, the M₁ receptor is most abundant in the cerebral cortex, where its immunoreactivity peaks transiently during early development. This likely signifies the importance of M₁ receptor in the development and maintenance of normal cortical function. Several lines of study have outlined the roles of M₁ receptors in the development and plasticity of the auditory cortex. For example, M₁-knockout reduces experience-dependent plasticity and disrupts tonotopic mapping in the adult mouse auditory cortex. Further evidence demonstrates a role for M₁ in neurite outgrowth and hence determining the structure of cortical neurons. The disruption of tonotopic maps in M₁-knockout mice may be linked to alterations in thalamocortical connectivity, because the targets of thalamocortical afferents (layer IV cortical neurons) appear less mature in M₁ knockouts. Herein we review the literature to date concerning M₁ receptors in the auditory cortex and consider some future directions that will contribute to our understanding.

Phosphorylation of AATYK1 by Cdk5 suppresses its tyrosine phosphorylation

Apoptosis-associated tyrosine kinase 1 (AATYK1), a novel serine/threonine kinase that is highly expressed in the brain, is involved in neurite extension and apoptosis of cerebellar granule neurons; however, its precise function remains unknown. In this study, we investigated the interaction of AATYK1A with Cyclin-dependent kinase 5 (Cdk5)/p35, a proline-directed protein kinase that is predominantly expressed in neurons. AATYK1A bound to the p35 activation subunit of Cdk5 in cultured cells and in mouse brains and colocalized with p35 on endosomes in COS-7 cells. AATYK1A was phosphorylated at Ser34 by Cdk5/p35 in vitro, in cultured neurons and in mouse brain. In PC12D cells, Ser34 phosphorylation increased after treatment with nerve growth factor and phosphorylated AATYK1A accumulated in growth cones of PC12D cells. Ser34 phosphorylation suppressed the tyrosine phosphorylation of AATYK1A by Src family kinases. These results suggest a possibility that AATYK1A plays a role in early to recycling endosomes and its function is regulated by phosphorylation with Cdk5 or Src-family kinases.

Phospholipid transfer protein reduces phosphorylation of tau in human neuronal cells

Tau function is regulated by phosphorylation, and abnormal tau phosphorylation in neurons is one of the key processes associated with development of Alzheimer's disease and other tauopathies. In this study we provide evidence that phospholipid transfer protein (PLTP), one of the main lipid transfer proteins in the brain, significantly reduces levels of phosphorylated tau and increases levels of the inactive form of glycogen synthase kinase-3beta (GSK3 beta) in HCN2 cells. Furthermore, inhibition of phosphatidylinositol-3 kinase (PI3K) reversed the PLTP-induced increase in levels of GSK3 beta phosphorylated at serine 9 (pGSK3 beta(Ser9)) and partially reversed the PLTP-induced reduction in tau phosphorylation. We provide evidence that the PLTP-induced changes are not due to activation of Disabled-1 (Dab1), insofar as PLTP reduced levels of total and phosphorylated Dab1 in HCN2 cells. We have also shown that inhibition of tyrosine kinase activity of insulin receptor (IR) and/or insulin-like growth factor 1 (IGF1) receptor (IGFR) reverses the PLTP-induced increase in levels of phosphorylated Akt (pAkt(Thr308) and pAkt(Ser473)), suggesting that PLTP-mediated activation of the PI3K/Akt pathway is dependent on IR/IGFR receptor tyrosine kinase activity. Our study suggests that PLTP may be an important modulator of signal transduction pathways in human neurons.

Unravelling the complex role of Cdk5 in the developing cerebral cortex

The normal development of the mammalian CNS is entirely dependent on the coordinated behavior of its cellular components. Particular importance is attributed to the correct morphology, migration and communication of neurons. Recent years have seen the identification of many extracellular, cell surface and intracellular signaling molecules that are important for normal CNS development, consequently triggering huge progress in our understanding of the complex processes involved. A key molecule to emerge is Cdk5. To date, Cdk5 has been functionally linked with controlled neuronal morphology, migration, synaptic function, cognition, drug addiction, neuronal death and neurodegeneration. The complexity of its function has been confirmed by the ever increasing number of diverse upstream regulators, protein substrates and biological consequences of altered catalytic function. The aim of this review is to consolidate recent findings concerning the role of Cdk5 in the developing nervous system, particularly the cerebral cortex, where its importance is most clearly evidenced.

Suppression of mutant Huntingtin aggregate formation by Cdk5/p35 through the effect on microtubule stability

Huntington's disease (HD) is a polyglutamine [poly(Q)] disease with an expanded poly(Q) stretch in the N terminus of the huntingtin protein (htt). A major pathological feature of HD neurons is inclusion bodies, detergent-insoluble aggregates composed of poly(Q)-expanded mutant htt (mhtt). Misfolding of mhtt is thought to confer a toxic property via formation of aggregates. Although toxic molecular species are still debated, it is important to clarify the aggregation mechanism to understand the pathogenesis of mhtt. We show Cdk5/p35 suppresses the formation of mhtt inclusion bodies in cell lines and primary neurons. Although we expressed the N-terminal exon 1 fragment of htt lacking phosphorylation sites for Cdk5 in COS-7 cells, the kinase activity of Cdk5 was required for the suppression. Furthermore, Cdk5/p35 suppressed inclusion formation of atrophin-1, another poly(Q) protein, raising the possibility that Cdk5/p35 generally suppresses inclusion formation of poly(Q) proteins. Microtubules (MTs) were a downstream component of Cdk5/p35 in the suppression of inclusion formation; Cdk5/p35 disrupted MTs, which were required for the formation of inclusions. Moreover, stabilization of MTs by Taxol induced inclusions even with overexpression of Cdk5/p35. The formation of inclusions was also regulated by manipulating the Cdk5/p35 activity in primary rat or mouse cortical neuron cultures. These results indicate that Cdk5-dependent regulation of MT organization is involved in the development of aggregate formation and subsequent pathogenesis of poly(Q) diseases. This Cdk5 inhibition of htt aggregates is a novel mechanism different from htt phosphorylation and interaction with Cdk5 reported previously (Luo et al., 2005; Anne et al., 2007).