Isolation and characterization of detergent-resistant microdomains responsive to NCAM-mediated signaling from growth cones

Prion protein oligomers cause neuronal cytoskeletal damage in rapidly progressive Alzheimer's disease

Background

High-density oligomers of the prion protein (HDPs) have previously been identified in brain tissues of patients with rapidly progressive Alzheimer’s disease (rpAD). The current investigation aims at identifying interacting partners of HDPs in the rpAD brains to unravel the pathological involvement of HDPs in the rapid progression.

Methods

HDPs from the frontal cortex tissues of rpAD brains were isolated using sucrose density gradient centrifugation. Proteins interacting with HDPs were identified by co-immunoprecipitation coupled with mass spectrometry. Further verifications were carried out using proteomic tools, immunoblotting, and confocal laser scanning microscopy.

Results

We identified rpAD-specific HDP-interactors, including the growth arrest specific 2-like 2 protein (G2L2). Intriguingly, rpAD-specific disturbances were found in the localization of G2L2 and its associated proteins i.e., the end binding protein 1, α-tubulin, and β-actin.

Discussion

The results show the involvement of HDPs in the destabilization of the neuronal actin/tubulin infrastructure. We consider this disturbance to be a contributing factor for the rapid progression in rpAD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00422-x.

Cell Adhesion Molecules and Ubiquitination-Functions and Significance

Cell adhesion molecules of the immunoglobulin (Ig) superfamily represent the biggest group of cell adhesion molecules. They have been analyzed since approximately 40 years ago and most of them have been shown to play a role in tumor progression and in the nervous system. All members of the Ig superfamily are intensively posttranslationally modified. However, many aspects of their cellular functions are not yet known. Since a few years ago it is known that some of the Ig superfamily members are modified by ubiquitin. Ubiquitination has classically been described as a proteasomal degradation signal but during the last years it became obvious that it can regulate many other processes including internalization of cell surface molecules and lysosomal sorting. The purpose of this review is to summarize the current knowledge about the ubiquitination of cell adhesion molecules of the Ig superfamily and to discuss its potential physiological roles in tumorigenesis and in the nervous system.

Cytoplasmic domain of NCAM140 interacts with ubiquitin-fold modifier-conjugating enzyme-1 (Ufc1)

The neural cell adhesion molecule NCAM is implicated in different neurodevelopmental processes and in synaptic plasticity in adult brain. The cytoplasmic domain of NCAM interacts with several cytoskeletal proteins and signaling molecules. To identify novel interaction partners of the cytosolic domain of NCAM a protein macroarray has been performed. We identified the ubiquitin-fold modifier-conjugating enzyme-1 (Ufc1) as an interaction partner of NCAM140. Ufc1 is one of the enzymes involved in modification of proteins with the ubiquitin-like molecule ubiquitin-fold modifier-1 (Ufm1). We also observed a partial co-localization of NCAM140 with Ufc1 and Ufm1 and increased endocytosis of NCAM140 in the presence of Ufm1 suggesting a possible ufmylation of NCAM140 and a potential novel function of Ufm1 for cell surface proteins.

Neural cell adhesion molecule-mediated Fyn activation promotes GABAergic synapse maturation in postnatal mouse cortex

GABAergic basket interneurons form perisomatic synapses, which are essential for regulating neural networks, and their alterations are linked to various cognitive dysfunction. Maturation of basket synapses in postnatal cortex is activity dependent. In particular, activity-dependent downregulation of polysialiac acid carried by the neural cell adhesion molecule (NCAM) regulates the timing of their maturation. Whether and how NCAM per se affects GABAergic synapse development is unknown. Using single-cell genetics to knock out NCAM in individual basket interneurons in mouse cortical slice cultures, at specific developmental time periods, we found that NCAM loss during perisomatic synapse formation impairs the process of basket cell axonal branching and bouton formation. However, loss of NCAM once the synapses are already formed did not show any effect. We further show that NCAM120 and NCAM140, but not the NCAM180 isoform, rescue the phenotype. Finally, we demonstrate that a dominant-negative form of Fyn kinase mimics, whereas a constitutively active form of Fyn kinase rescues, the effects of NCAM knockdown. Altogether, our data suggest that NCAM120/NCAM140-mediated Fyn activation promotes GABAergic synapse maturation in postnatal cortex.

UCHL1 regulates ubiquitination and recycling of the neural cell adhesion molecule NCAM

The neural cell adhesion molecule (NCAM) is involved in neural development and in plasticity in the adult brain. NCAM140 and NCAM180 isoforms are transmembrane proteins with cytoplasmic domains that differ only in an alternatively spliced exon in the NCAM180 isoform. Both isoforms can interact with several extracellular and cytoplasmic molecules mediating NCAM-dependent functions. Most identified intracellular interaction partners bind to both isoforms, NCAM140 and NCAM180. To identify further intracellular interaction partners specifically binding to NCAM180 the cytosolic domain of human NCAM180 was recombinantly expressed and applied onto a protein macroarray containing the protein library from human fetal brain. We identified the ubiquitin C-terminal hydrolase (UCHL1) which has been described as a de-ubiquitinating enzyme as a potential interaction partner of NCAM180. Since NCAM180 and NCAM140 are ubiquitinated, NCAM140 was included in the subsequent experiments. A partial colocalization of both NCAM isoforms and UCHL1 was observed in primary neurons and the B35 neuroblastoma cell line. Overexpression of UCHL1 significantly decreased constitutive ubiquitination of NCAM180 and NCAM140 whereas inhibition of endogenous UCHL1 increased NCAM's ubiquitination. Furthermore, lysosomal localization of NCAM180 and NCAM140 was significantly reduced after overexpression of UCHL1 consistent with a partial colocalization of internalized NCAM with UCHL1. These data indicate that UCHL1 is a novel interaction partner of both NCAM isoforms that regulates their ubiquitination and intracellular trafficking.

GAP-43 dependency defines distinct effects of netrin-1 on cortical and spinal neurite outgrowth and directional guidance

Growth-associated protein-43 (GAP-43) is a major nervous system protein whose phosphorylation by protein kinase C regulates growth cone responses to extracellular guidance cues via F-actin. GAP-43 is essential for axon pathfinding in both cortical afferents and efferents: when it is genetically deleted, somatosensory, auditory and visual somatotopic maps fail to form, and telencephalic commissural axons fail to cross the midline. Here we investigated whether the midline guidance cue netrin-1 depends on GAP-43 for its functions in neurite growth and guidance. We used 3-dimensional collagen gel co-cultures to show that both endogenous netrin-1, expressed by the spinal cord floor plate, and recombinant netrin-1, expressed by transfected COS7 cells, stimulate neurite outgrowth and chemotropic guidance of neocortical callosal axons. In contrast both were significantly inhibited in GAP-43 (-/-) neocortical callosal axons, mimicking the in vivo phenotype. Conversely, neither netrin-1-stimulated neurite outgrowth nor guidance of dorsal spinal cord commissure axons were affected when GAP-43 was absent, again consistent with in vivo phenotype but suggesting fundamental differences in how neocortical and spinal cord axons respond to netrin-1. In addition, differences in GAP-43 dependency also distinguished how ventrolateral cortical efferents respond to netrin-1: in contrast to callosal neurites, in which netrin-1 required GAP-43 in order to stimulate both outgrowth and guidance, in ventrolateral efferents, netrin-1 required GAP-43 only to stimulate outgrowth, but not guidance. Moreover, netrin-1 increased the numbers of both types of cortical, but not spinal neurites. The results demonstrate previously unappreciated diversity in how different classes of neurons respond to the same guidance cue.

Expression of glycogenes in differentiating human NT2N neurons. Downregulation of fucosyltransferase 9 leads to decreased Lewis(x) levels and impaired neurite outgrowth

BACKGROUND

Several glycan structures are functionally relevant in biological events associated with differentiation and regeneration which occur in the central nervous system. Here we have analysed the glycogene expression and glycosylation patterns during human NT2N neuron differentiation. We have further studied the impact of downregulating fucosyltransferase 9 (FUT9) on neurite outgrowth.

METHODS

The expression of glycogenes in human NT2N neurons differentiating from teratocarcinoma NTERA-2/cl.D1 cells has been analysed using the GlycoV4 GeneChip expression microarray. Changes in glycosylation have been monitored by immunoblot, immunofluorescence microscopy, HPLC and MALDI-TOF MS. Peptide mass fingerprinting and immunoprecipitation have been used for protein identification. FUT9 was downregulated using silencing RNA.

RESULTS AND CONCLUSIONS

One hundred twelve mRNA transcripts showed statistically significant up-regulation, including the genes coding for proteins involved in the synthesis of the Lewis(x) motif (FUT9), polysialic acid (ST8SIA2 and ST8SIA4) and HNK-1 (B3GAT2). Accordingly, increased levels of the corresponding carbohydrate epitopes have been observed. The Lewis(x) structure was found in a carrier glycoprotein that was identified as the CRA-a isoform of human neural cell adhesion molecule 1. Downregulation of FUT9 caused significant decreases in the levels of Lewis(x), as well as GAP-43, a marker of neurite outgrowth. Concomitantly, a reduction in neurite formation and outgrowth has been observed that was reversed by FUT9 overexpression.

GENERAL SIGNIFICANCE

These results provided information about the regulation of glycogenes during neuron differentiation and they showed that the Lewis(x) motif plays a functional role in neurite outgrowth from human neurons.

Ectopic growth of hippocampal mossy fibers in a mutated GAP-43 transgenic mouse with impaired spatial memory retention

In a previous study, it was shown that transgenic mice, designated G-NonP, forget the location of a water maze hidden platform when tested 7 days after the last training day (Holahan and Routtenberg (2008) Hippocampus 18:1099-1102). The memory loss in G-NonP mice might be related to altered hippocampal architecture suggested by the fact that in the rat, 7 days after water maze training, there is discernible mossy fiber (MF) growth (Holahan et al. (2006) Hippocampus 16:560-570; Rekart et al. (2007) Learn Mem 14:416-421). In the present report, we studied the distribution of the MF system within the hippocampus of naïve, untrained, G-NonP mouse. In WT mice, the MF projection was restricted to the stratum lucidum of CA3 with no detectable MF innervation in distal stratum oriens (dSO). In G-NonP mice, in contrast, there was an ectopic projection terminating in the CA3 dSO. Unexpectedly, there was nearly a complete loss of immunostaining for the axonal marker Tau1 in the G-NonP transgenic mice in the MF terminal fields indicating that transgenesis itself leads to off-target consequences (Routtenberg (1996) Trends Neurosci 19:471-472). Because transgenic mice overexpressing nonmutated, wild type GAP-43 do not show this ectopic growth (Rekart et al., in press) and the G-NonP mice overexpress a mutated form of GAP-43 precluding its phosphorylation by protein kinase C (PKC), the possibility exists that permanently dephosphorylated GAP-43 disrupts normal axonal fasciculation which gives rise to the ectopic growth into dSO.

NCAM-mimetic, FGL peptide, restores disrupted fibroblast growth factor receptor (FGFR) phosphorylation and FGFR mediated signaling in neural cell adhesion molecule (NCAM)-deficient mice

Neural cell adhesion molecule (NCAM) is a membrane-bound glycoprotein expressed on the surface of neuronal and glial cells. Previous in vitro studies have demonstrated that NCAM promotes neuronal functions largely via three main interaction partners: the fibroblast growth factor receptor (FGFR), a member of Src family of tyrosine kinases, Fyn and Raf1 kinase which all activate different intracellular signaling pathways. The objective was to clarify, which signaling pathways are being disrupted in NCAM knockout mice and whether FGL peptide is able to restore observed disruptions. Therefore we compared the levels of phosphorylation of FGFR1, Src kinase Fyn, Raf1 kinase, MAP kinases, Akt kinase and calcium/calmodulin-dependent kinases II and IV (CaMKII and CaMKIV) in the hippocampus of NCAM knockout mice to their wild-type littermates. The data of our study show that mice constitutively deficient in all isoforms of NCAM have decreased basal phosphorylation levels of FGFR1 and CaMKII and CaMKIV. Furthermore, NCAM-mimetic, FGL peptide, is found to be able to restore FGFR1, CaMKII and CaMKIV phosphorylation levels and thereby mimic the interactions of NCAM at this receptor in NCAM deficient mice. Also, we found that Fyn(Tyr530), Raf1, MAP kinases and Akt kinase phosphorylation in adult animals is not affected by NCAM deficiency but interestingly, we found an over-expression of another cell adhesion molecule L1. We conclude that in NCAM deficient mice FGFR1-dependent signaling is disrupted and it can be restored by FGL peptide.

Regulation of GAP-43 at serine 41 acts as a switch to modulate both intrinsic and extrinsic behaviors of growing neurons, via altered membrane distribution

GAP-43 is the major neuronal substrate of protein kinase C (PKC). Its phosphorylation status dictates the severity of pathfinding errors by GAP-43 (+/-) growth cones in vivo, as well as its modulation of actin dynamics in vitro. These experiments show that stably overexpressing cDNAs mutant at its single PKC phosphorylation site at serine41 in retinoic acid treated SH-Sy5Y neuroblastoma cells regulates intrinsic and extrinsic behaviors of growing neurons. Intrinsically, only Wt and pseudophosphorylated GAP-43Ser41Asp precipitated with F-actin and potentiated F-actin - regulated filopodia formation. GAP-43Ser41Asp inhibited neurite outgrowth whereas only unphosphorylatable GAP-43Ser41Ala precipitated neurotubulin, potentiated neurotubulin accumulation in neurites and increased outgrowth. When PI3-kinase was inhibited GAP-43Ser41Asp-mediated filopodia formation was inhibited whereas GAP-43Ser41Ala-mediated neurite extension was potentiated. Extrinsically, only Wt and GAP-43Ser41Asp potentiated both homotypic adhesion and neurite outgrowth on NCAM-expressing monolayers and promoted NCAM stability. With respect to the underlying mechanism, more F-actin and NCAM colocalized with Wt and GAP-43Ser41Asp in detergent resistant membranes (DRMs) isolated from live cells and GAP-43Ser41Asp-mediated functions were insensitive to cholesterol depletion. In contrast, GAP-43Ser41Ala-mediated functions were sensitive to cholesterol depletion. Neither GAP-43Ser41Asp nor GAP-43Ser41Ala was able to protect against growth cone collapse mediated by PIP2 inhibitors. The results show that modification of GAP-43 at its PKC phosphorylation site directs its distribution to different membrane microdomains that have distinct roles in the regulation of intrinsic and extrinsic behaviors in growing neurons.

GAP-43 is essential for the neurotrophic effects of BDNF and positive AMPA receptor modulator S18986

Positive alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor modulators include benzamide compounds that allosterically modulate AMPA glutamate receptors. These small molecules that cross the blood-brain barrier have been shown to act as a neuroprotectant by increasing the levels of endogenous brain-derived neurotrophic factor (BDNF). Positive AMPA receptor modulators have also been shown to increase the levels of growth-associated protein-43 (GAP-43). GAP-43 plays a major role in many aspects of neuronal function in vertebrates. The goal of this study was to determine whether GAP-43 was important in mediating the actions of positive AMPA receptor modulator (S18986) and BDNF. Using cortical cultures from GAP-43 knockout and control mice, we show that (1) GAP-43 is upregulated in response to S18986 and BDNF in control cultures; (2) this upregulation of GAP-43 is essential for mediating the neuroprotective effects of S18986 and BDNF; (3) administration of S18986 and BDNF leads to an increase in the expression of the glutamate transporters GLT-1 and GLAST that are key to limiting excitotoxic cell death and this increase in GLT-1 and GLAST expression is completely blocked in the absence of GAP-43. Taken together this study concludes that GAP-43 is an important mediator of the neurotrophic effects of S18986 and BDNF on neuronal survival and plasticity, and is essential for the success of positive AMPA receptor modulator-BDNF-based neurotrophin therapy.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

Polysialic acid–neural cell adhesion molecule in brain plasticity: From synapses to integration of new neurons

Isoforms of the neuronal cell adhesion molecule (NCAM) carrying the linear homopolymer of alpha 2,8-linked sialic acid (polysialic acid, PSA) have emerged as particularly attractive candidates for promoting plasticity in the nervous system. The large negatively charged PSA chain of NCAM is postulated to be a spacer that reduces adhesion forces between cells allowing dynamic changes in membrane contacts. Accumulating evidence also suggests that PSA-NCAM-mediated interactions lead to activation of intracellular signaling cascades that are fundamental to the biological functions of the molecule. An important role of PSA-NCAM appears to be during development, when its expression level is high and where it contributes to the regulation of cell shape, growth or migration. However, PSA-NCAM does persist in adult brain structures such as the hippocampus that display a high degree of plasticity where it is involved in activity-induced synaptic plasticity. Recent advances in the field of PSA-NCAM research have not only consolidated the importance of this molecule in plasticity processes but also suggest a role for PSA-NCAM in the regulation of higher cognitive functions and psychiatric disorders. In this review, we discuss the role and mode of actions of PSA-NCAM in structural plasticity as well as its potential link to cognitive processes.

Identification of NCAM-binding peptides promoting neurite outgrowth via a heterotrimeric G-protein-coupled pathway

A combinatorial library of undecapeptides was produced and utilized for the isolation of peptide binding to the fibronectin type 3 modules (F3I-F3II) of the neural cell adhesion molecule (NCAM). The isolated peptides were sequenced and produced as dendrimers. Two of the peptides (denoted ENFIN2 and ENFIN11) were confirmed to bind to F3I-F3II of NCAM by surface plasmon resonance. The peptides induced neurite outgrowth in primary cerebellar neurons and PC12E2 cells, but had no apparent neuroprotective properties. NCAM is known to activate different intracellular pathways, including signaling through the fibroblast growth factor receptor, the Src-related non-receptor tyrosine kinase Fyn, and heterotrimeric G-proteins. Interestingly, neurite outgrowth stimulated by ENFIN2 and ENFIN11 was independent of signaling through fibroblast growth factor receptor and Fyn, but could be inhibited with pertussis toxin, an inhibitor of certain heterotrimeric G-proteins. Neurite outgrowth induced by trans-homophilic NCAM was unaffected by the peptides, whereas knockdown of NCAM completely abrogated ENFIN2- and ENFIN11-induced neuritogenesis. These observations suggest that ENFIN2 and ENFIN11 induce neurite outgrowth in an NCAM-dependent manner through G-protein-coupled signal transduction pathways. Thus, ENFIN2 and ENFIN11 may be valuable for exploring this particular type of NCAM-mediated signaling.

Translocation of activated heterotrimeric G protein Galpha(o) to ganglioside-enriched detergent-resistant membrane rafts in developing cerebellum

The association of gangliosides with specific proteins in the central nervous system was examined by co-immunoprecipitation with an anti-ganglioside antibody. The monoclonal antibody to the ganglioside GD3 immunoprecipitated phosphoproteins of 40, 53, 56, and 80 kDa from the rat cerebellum. Of these proteins, the 40-kDa protein was identified as the alpha-subunit of a heterotrimeric G protein, G(o) (Galpha(o)). Using sucrose density gradient analysis of cerebellar membranes, Galpha(o), but not Gbetagamma, was observed in detergent-resistant membrane (DRM) raft fractions in which GD3 was abundant after the addition of guanosine 5'-O-(thiotriphosphate) (GTPgammaS), which stabilizes G(o) in its active form. On the other hand, both Galpha(o) and Gbetagamma were excluded from the DRM raft fractions in the presence of guanyl-5'-yl thiophosphate, which stabilizes G(o) in its inactive form. Only Galpha(o) was observed in the DRM fractions from the cerebellum on postnatal day 7, but not from that in adult. After pertussis toxin treatment, Galpha(o) was not observed in the DRM fractions, even from the cerebellum on postnatal day 7. These results indicate the activation-dependent translocation of Galpha(o) into the DRM rafts. Furthermore, Galpha(o) was concentrated in the neuronal growth cones. Treatment with stromal cell-derived factor-1alpha, a physiological ligand for the G protein-coupled receptor, stimulated [(35)S]GTPgammaS binding to Galpha(o) and caused Galpha(o) translocation to the DRM fractions and RhoA translocation to the membrane fraction, leading to the growth cone collapse of cerebellar granule neurons. The collapse was partly prevented by pretreatment with the cholesterol-sequestering and raft-disrupting agent methyl-beta-cyclodextrin. These results demonstrate the involvement of signal-dependent Galpha(o) translocation to the DRM in the growth cone behavior of cerebellar granule neurons.

Cochlear damage induces GAP-43 expression in cholinergic synapses of the cochlear nucleus in the adult rat: a light and electron microscopic study

Recent studies suggest a potential for activity-dependent reconstruction in the adult mammalian brainstem that exceeds previous expectations. We found that a unilateral cochlear lesion led within 1 week to a rise of choline acetyltransferase (ChAT) immunoreactivity in the ventral cochlear nucleus of the affected side, matching the lesion-induced expression of growth-associated protein 43 (GAP-43) previously described. The rise of both ChAT and GAP-43 immunoreactivity was reflected in the average density of the staining. Moreover, the number of light-microscopically identifiable boutons increased in both stains. GAP-43-positive boutons could, by distinct ultrastructural features, regularly be identified as presynaptic endings. However, GAP-43 immunoreactivity was not only found in presynaptic endings with a classical morphology, but also in profiles that suggest morphological dynamic structures by showing filopodia, assemblages of pleomorphic vesicles, large vesicles (diameter up to 200 nm) fusing with the presynaptic plasma membrane close to synaptic contacts, small dense-core vesicles (diameter about 80 nm) and presynaptic ribosomes. Moreover, we observed perforated synapses as well as GAP-43 immunoreactivity condensed in rafts, both indicative of growing or changing neuronal connections. Classical and untypical ultrastructural profiles that contained GAP-43 also contained ChAT. We conclude that there is extensive deafness-induced GAP-43-mediated synaptic plasticity in the cochlear nucleus, and that this plasticity is predominantly, if not exclusively, based on cholinergic afferents.

Changes in the composition of detergent-resistant membrane domains of cultured neurons following protein kinase C activation

Changes in the composition of cell fractions, and in particular of detergent-resistant membranes (DRM) isolated from cultured rat cerebellar granule cells, were taken as possible changes in lipid raft composition during a signal transduction event. After activation of protein kinase C (PKC) with phorbol esters (PMA) or glutamate, the content of PKC and of proteins highly enriched (GAP43, Fyn, and PrP(c)) or not (MARCKS) in DRM was followed. PKC activation strongly increased its association with membranes (from 2% to 75%), causing its enrichment within DRM; the substrate GAP43, enriched in DRM, remained membrane associated, but its proportion in DRM dramatically decreased (from about 40% to 2.5%), suggesting its shift from raft to nonraft membranes, possibly as a consequence of phosphorylation by PKC. The distribution of Fyn and PrP(c) (DRM-enriched) and of MARCKS (present mainly outside DRM) did not change. PKC activation was followed by an increase of GAP43 and MARCKS phosphorylation (about 7- and 8-fold, respectively). Noteworthy was that, after cell treatment with the lipid raft-disrupting drug methyl-beta-cyclodextrin, PKC activation occurred normally, followed by MARCKS phosphorylation, but GAP43 phosphorylation did not occur. Taken altogether, these data suggest that the integrity of lipid rafts is necessary for PKC to affect GAP43 and catalyze its phosphorylation.

Membrane lipid rafts and their role in axon guidance

The plasma membrane of cells contains a variety of lipid and protein molecules that are often segregated and heterogeneously distributed in microdomains. Lipid rafts represent a generalized concept of membrane microdomains that are enriched in cholesterol and sphingolipids and, characteristically, resistant to cold detergent extraction. Lipid rafts have recently received considerable attention because they are thought to be involved in many cellular functions, in particular, signal transduction for extracellular stimuli. Many of these functions are also intimately related to the processes involved in neural development, including neurotrophic factor signaling and synaptic plasticity. Recent studies from our lab and others have indicated an important role for lipid rafts in axonal growth and guidance. Specifically, our data show that lipid rafts on the plasma membrane provide platforms for spatial and temporal control of guidance signaling by extracellular cues. In addition, lipid rafts may also function in other aspects of axonal growth and guidance, including spatial and temporal regulation of adhesion, cytoskeletal dynamics, and growth cone motility. Further elucidating how membrane rafts are involved in guided axonal growth would provide important insights into the intricate signaling mechanisms underlying neuronal wiring, which is fundamental for normal brain development and functional recovery after injury and diseases.

GAP-43 regulates NCAM-180-mediated neurite outgrowth

The neural cell adhesion molecule (NCAM), and the growth-associated protein (GAP-43), play pivotal roles in neuronal development and plasticity and possess interdependent functions. However, the mechanisms underlying the functional association of GAP-43 and NCAM have not been elucidated. In this study we show that (over)expression of GAP-43 in PC12E2 cells and hippocampal neurons strongly potentiates neurite extension, both in the absence and in the presence of homophilic NCAM binding. This potentiation is crucially dependent on the membrane association of GAP-43. We demonstrate that phosphorylation of GAP-43 by protein kinase C (PKC) as well as by casein kinase II (CKII) is important for the NCAM-induced neurite outgrowth. Moreover, our results indicate that in the presence of GAP-43, NCAM-induced neurite outgrowth requires functional association of NCAM-180/spectrin/GAP-43, whereas in the absence of GAP-43, the NCAM-140/non-receptor tyrosine kinase (Fyn)-associated signaling pathway is pivotal. Thus, expression of GAP-43 presumably acts as a functional switch for NCAM-180-induced signaling. This suggests that under physiological conditions, spatial and/or temporal changes of the localization of GAP-43 and NCAM on the cell membrane may determine the predominant signaling mechanism triggered by homophilic NCAM binding: NCAM-180/spectrin-mediated modulation of the actin cytoskeleton, NCAM-140-mediated activation of Fyn, or both.

The docking protein Cas links tyrosine phosphorylation signaling to elongation of cerebellar granule cell axons

Crk-associated substrate (Cas) is a tyrosine-phosphorylated docking protein that is indispensable for the regulation of the actin cytoskeletal organization and cell migration in fibroblasts. The function of Cas in neurons, however, is poorly understood. Here we report that Cas is dominantly enriched in the brain, especially the cerebellum, of postnatal mice. During cerebellar development, Cas is highly tyrosine phosphorylated and is concentrated in the neurites and growth cones of granule cells. Cas coimmunoprecipitates with Src family protein tyrosine kinases, Crk, and cell adhesion molecules and colocalizes with these proteins in granule cells. The axon extension of granule cells is inhibited by either RNA interference knockdown of Cas or overexpression of the Cas mutant lacking the YDxP motifs, which are tyrosine phosphorylated and thereby interact with Crk. These findings demonstrate that Cas acts as a key scaffold that links the proteins associated with tyrosine phosphorylation signaling pathways to the granule cell axon elongation.

Lipid rafts and regulation of the cytoskeleton during T cell activation

The ability of polarized cells to initiate and sustain directional responses to extracellular signals is critically dependent on direct communication between spatially organized signalling modules in the membrane and the underlying cytoskeleton. Pioneering work in T cells has shown that the assembly of signalling modules critically depends on the functional compartmentalization of membrane lipids into ordered microdomains or lipid rafts. The significance of rafts in T cell activation lies not only in their ability to recruit the signalling partners that eventually assemble into a mature immunological synapse but also in their ability to regulate actin dynamics and recruit cytoskeletal associated proteins, thereby achieving the structural polarization underlying stability of the synapse-a critical prerequisite for activation to be sustained. Lipid rafts vary quite considerably in size and visualizing the smallest of them in vivo has been challenging. Nonetheless it is now been shown quite convincingly that a surprisingly large proportion-in the order of 50%-of external membrane lipids (chiefly cholesterol and glycosphingolipids) can be dynamically localized in these liquid ordered rafts. Complementary inner leaflet rafts are less well characterized, but contain phosphoinositides as an important functional component that is crucial for regulating the behaviour of the actin cytoskeleton. This paper provides an overview of the interdependency between signalling and cytoskeletal polarization, and in particular considers how regulation of the cytoskeleton plays a crucial role in the consolidation of rafts and their stabilization into the immunological synapse.

Two types of detergent-insoluble, glycosphingolipid/cholesterol-rich membrane domains from isolated myelin

Two different types of low-density detergent-insoluble glycosphingolipid-enriched membrane domain (DIG) fractions were isolated from myelin by extraction with Triton X-100 (TX-100) in 50 mM sodium phosphate buffer at room temperature (20 degrees C) (procedure 1), in contrast to a single low-density fraction obtained by extraction with TX-100 in Tris buffer containing 150 mM NaCl and 5 mM EDTA at 4 degrees C (procedure 2). Procedure 1 has been used in the past by others for myelin extraction to preserve the cytoskeleton and/or radial component of oligodendrocytes and myelin, whereas procedure 2 is now more commonly used to isolate myelin DIG fractions. The two DIG fractions obtained by procedure 1 gave opaque bands, B1 and B2, at somewhat lower and higher sucrose density respectively than myelin itself. The single DIG fraction obtained by procedure 2 gave a single opaque band at a similar sucrose density to B1. Both B1 and B2 had characteristics of lipid rafts, i.e. high galactosylceramide and cholesterol content and enrichment in GPI-linked 120-kDa neural cell adhesion molecule (NCAM)120, as found by others for the single low-density DIG fraction obtained by procedure 2. However, B2 had most of the myelin GM1 and more of the sulfatide than B1, and they differed significantly in their protein composition. B2 contained 41% of the actin, 100% of the tubulin, and most of the flotillin-1 and caveolin in myelin, whereas B1 contained more NCAM120 and other proteins than B2. The single low-density DIG fraction obtained by procedure 2 contained only low amounts of actin and tubulin. B1 and B2 also had size-isoform selectivity for some proteins, suggesting specific interactions and different functions of the two membrane domains. We propose that B1 may come from non-caveolar raft domains whereas B2 may derive from caveolin-containing raft domains associated with cytoskeletal proteins. Some kinases present were active on myelin basic protein suggesting that the DIGs may come from signaling domains.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

RPTPalpha is essential for NCAM-mediated p59fyn activation and neurite elongation

The neural cell adhesion molecule (NCAM) forms a complex with p59^fyn kinase and activates it via a mechanism that has remained unknown. We show that the NCAM140 isoform directly interacts with the intracellular domain of the receptor-like protein tyrosine phosphatase RPTPα, a known activator of p59^fyn. Whereas this direct interaction is Ca²⁺ independent, formation of the complex is enhanced by Ca²⁺-dependent spectrin cytoskeleton–mediated cross-linking of NCAM and RPTPα in response to NCAM activation and is accompanied by redistribution of the complex to lipid rafts. Association between NCAM and p59^fyn is lost in RPTPα-deficient brains and is disrupted by dominant-negative RPTPα mutants, demonstrating that RPTPα is a link between NCAM and p59^fyn. NCAM-mediated p59^fyn activation is abolished in RPTPα-deficient neurons, and disruption of the NCAM–p59^fyn complex in RPTPα-deficient neurons or with dominant-negative RPTPα mutants blocks NCAM-dependent neurite outgrowth, implicating RPTPα as a major phosphatase involved in NCAM-mediated signaling.

Spatial and temporal control of signaling through lipid rafts

Sphingolipid- and cholesterol-dependent microdomains (rafts) order proteins at biological membranes and have been implicated in most signaling processes at the cell surface, but the principles and mechanisms through which lipid rafts influence signaling are not well understood. Recent studies have revealed how lipid rafts are rapidly redistributed and assembled locally in response to extracellular signals, and how components of raft-based signaling domains undergo rapid and regulated rearrangements influencing signal quality, duration, and strength. These findings highlight the exquisitely dynamic properties of signaling domains based on lipid rafts, and suggest that processes of raft trafficking and assembly take central roles in mediating spatial and temporal control of signaling.

Immunoseparation of Prion protein-enriched domains from other detergent-resistant membrane fractions, isolated from neuronal cells

The possibility of coexistence of different subtypes of membrane lipid rafts has been investigated in cerebellar granule cells, by submitting detergent-resistant membrane fractions to immunoprecipitation. Among the proteins and lipids present in detergent-resistant fractions, almost all Prion protein, GAP43 and PKC were present in the immunoprecipitate obtained with anti-GAP43 or anti-Prion protein antibody at 4 degrees C, together with a small fraction of cholesterol and sphingolipids, suggesting that they belong to a distinct subset of membranes. On the contrary, all Fyn and almost all MARCKS remained in the supernatant. Fluorescence microscopy experiments showed that Fyn and Prion protein were mostly not colocalized within a single neuron. Our results suggest that granule cells membranes contains different subtypes of detergent-resistant fractions, possibly deriving from different lipid rafts.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

Membrane/cytoskeleton communication

Accumulations of particular lipids in ordered arrays in the membrane (termed microdomains or lipid rafts) can attract proteins with specific targeting domains. Both the lipid and protein components of rafts communicate with the cytoskeleton directly thereby regulating cellular responses. Recent evidence implicating phosphoinositide 1,5 bisphosphate (PIP2) in cytoskeletal regulation shows that agonist sensitive regulation of PIP2 homoeostasis occurs specifically rafts, which appear to provide a major structural substrate for its function. The crucial role of PIP2 in generating cytoskeletal responses is chiefly achieved by regulating proteins that control actin dynamics directly. Many of these regulatory proteins are also specifically enriched in rafts either directly (by insertion into the lipid bilayer via acetylation motifs), or indirectly via interactions with other raft components. The notion that rafts form membrane platforms or modules that mediate signaling responses has been most extensively demonstrated in the immune synapse (IS) of T cells, a complex assemblage of rafts that integrates signaling cascades originating from the simultaneous activation of a wide variety of receptors. The IS is essential for both the amplification and maintenance of T-cell activation, and its assembly at the antigen presenting site depends on the interactions between rafts and the actin cytoskeleton that regulates coalescence of smaller raft components into the larger IS complex. Likewise the neuron, which represents the most highly polarized cell in the body, utilizes the regulation of actin dynamics in response to a plethora of extracellular signals to control axon pathfinding thereby sculpting nervous system cytoarchitecture with utmost precision. It is now becoming clear, that as in the T-cell, lipid rafts in the growing axon can assemble into highly specific, yet malleable and dynamic, signaling modules that regulate actin dynamics in a fashion that is also PIP2-dependent and that utilizes both familiar and novel regulatory mechanisms. It seems clear that raft mediated cytoskeletal regulation represents a highly conserved mechanism to integrate cellular responses to diverse signals.

Neural cell adhesion molecule (NCAM) association with PKCbeta2 via betaI spectrin is implicated in NCAM-mediated neurite outgrowth

In hippocampal neurons and transfected CHO cells, neural cell adhesion molecule (NCAM) 120, NCAM140, and NCAM180 form Triton X-100-insoluble complexes with betaI spectrin. Heteromeric spectrin (alphaIbetaI) binds to the intracellular domain of NCAM180, and isolated spectrin subunits bind to both NCAM180 and NCAM140, as does the betaI spectrin fragment encompassing second and third spectrin repeats (betaI2-3). In NCAM120-transfected cells, betaI spectrin is detectable predominantly in lipid rafts. Treatment of cells with methyl-beta-cyclodextrin disrupts the NCAM120-spectrin complex, implicating lipid rafts as a platform linking NCAM120 and spectrin. NCAM140/NCAM180-betaI spectrin complexes do not depend on raft integrity and are located both in rafts and raft-free membrane domains. PKCbeta2 forms detergent-insoluble complexes with NCAM140/NCAM180 and spectrin. Activation of NCAM enhances the formation of NCAM140/NCAM180-spectrin-PKCbeta2 complexes and results in their redistribution to lipid rafts. The complex is disrupted by the expression of dominant-negative betaI2-3, which impairs binding of spectrin to NCAM, implicating spectrin as the bridge between PKCbeta2 and NCAM140 or NCAM180. Redistribution of PKCbeta2 to NCAM-spectrin complexes is also blocked by a specific fibroblast growth factor receptor inhibitor. Furthermore, transfection with betaI2-3 inhibits NCAM-induced neurite outgrowth, showing that formation of the NCAM-spectrin-PKCbeta2 complex is necessary for NCAM-mediated neurite outgrowth.

Role of cholesterol in synapse formation and function

Cholesterol is a multifaceted molecule, which serves as essential membrane component, as cofactor for signaling molecules and as precursor for steroid hormones. Consequently, defects in cholesterol metabolism cause devastating diseases. So far, the role of cholesterol in the nervous system is less well understood. Recent studies showed that cultured neurons from the mammalian central nervous system (CNS) require glia-derived cholesterol to form numerous and efficient synapses. This suggests that the availability of cholesterol in neurons limits the extent of synaptogenesis. Here, I will summarize the experimental evidence for this hypothesis, describe what is known about the structural and functional role of cholesterol at synapses, and discuss how cholesterol may influence synapse development and stability.

Intracellular signaling by the neural cell adhesion molecule

Cell adhesion molecules are known to play far more complex roles than mechanically attaching one cell to an adjacent cell or to components of the extracellular matrix. Thus, important roles for cell adhesion molecules in the regulation of intracellular signaling pathways have been revealed. In this review, we discuss the present knowledge about signaling pathways activated upon homophilic binding of the neural cell adhesion molecule (NCAM). Homophilic NCAM binding leads to activation of a signal transduction pathway involving Ca2+ through activation of the fibroblast growth factor receptor, and to activation of the mitogen-activated protein kinase pathway. In addition, cyclic adenosine monophosphate and protein kinase A are involved in NCAM-mediated signaling. Among these pathways the possibility exists of cross talk or convergence, of which different possible mediators have been suggested. Finally, several downstream effector molecules leading to NCAM-mediated cellular endpoints have been demonstrated, including transcription factors and regulators of the cytoskeleton.

Migration of nerve growth cones requires detergent-resistant membranes in a spatially defined and substrate-dependent manner

Motility of nerve growth cones (GCs) is regulated by region-specific activities of cell adhesion molecules (CAMs). CAM activities could be modified by their localization to detergent-resistant membranes (DRMs), specialized microdomains enriched in signaling molecules. This paper deals with a question of whether DRMs are involved in GC migration stimulated by three CAMs; L1, N-cadherin (Ncad), and beta1 integrin. We demonstrate that L1 and Ncad are present in DRMs, whereas beta1 integrin is exclusively detected in non-DRMs of neurons and that localization of L1 and Ncad to DRMs is developmentally regulated. GC migration mediated by L1 and Ncad but not by beta1 integrin is inhibited after DRM disruption by micro-scale chromophore-assisted laser inactivation (micro-CALI) of GM1 gangliosides or by pharmacological treatments that deplete cellular cholesterol or sphingolipids, essential components for DRMs. Characteristic morphology of GCs induced by L1 and Ncad is also affected by micro-CALI-mediated DRM disruption. Micro-CALI within the peripheral domain of GCs, or even within smaller areas such as the filopodia and the lamellipodia, is sufficient to impair their migration. However, micro-CALI within the central domain does not affect GC migration. These results demonstrate the region-specific involvement of DRMs in CAM-dependent GC behavior.