Intracellular routing of wild-type and mutated polymeric immunoglobulin receptor in hippocampal neurons in culture

Transcytosis and trans-synaptic retention by postsynaptic ErbB4 underlie axonal accumulation of NRG3

Neuregulins (NRGs) are EGF-like ligands associated with cognitive disorders. Unprocessed proNRG3 is cleaved by BACE1 to generate the mature membrane-bound NRG3 ligand, but the subcellular site of proNRG3 cleavage, mechanisms underlying its transport into axons, and presynaptic accumulation remain unknown. Using an optogenetic proNRG3 cleavage reporter (LA143-NRG3), we investigate the spatial-temporal dynamics of NRG3 processing and sorting in neurons. In dark conditions, unprocessed LA143-NRG3 is retained in the trans-Golgi network but, upon photoactivation, is cleaved by BACE1 and released from the TGN. Mature NRG3 then emerges on the somatodendritic plasma membrane from where it is re-endocytosed and anterogradely transported on Rab4+ vesicles into axons via transcytosis. By contrast, the BACE1 substrate APP is sorted into axons on Rab11+ vesicles. Lastly, by a mechanism we denote "trans-synaptic retention," NRG3 accumulates at presynaptic terminals by stable interaction with its receptor ErbB4 on postsynaptic GABAergic interneurons. We propose that trans-synaptic retention may account for polarized expression of other neuronal transmembrane ligands and receptors.

NgCAM and VAMP2 reveal that direct delivery and dendritic degradation maintain axonal polarity

Neurons are polarized cells of extreme scale and compartmentalization. To fulfill their role in electrochemical signaling, axons must maintain a specific complement of membrane proteins. Despite being the subject of considerable attention, the trafficking pathway of axonal membrane proteins is not well understood. Two pathways, direct delivery and transcytosis, have been proposed. Previous studies reached contradictory conclusions about which of these mediates delivery of axonal membrane proteins to their destination, in part because they evaluated long-term distribution changes and not vesicle transport. We developed a novel strategy to selectively label vesicles in different trafficking pathways and determined the trafficking of two canonical axonal membrane proteins, neuron-glia cell adhesion molecule and vesicle-associated membrane protein-2. Results from detailed quantitative analyses of transporting vesicles differed substantially from previous studies and found that axonal membrane proteins overwhelmingly undergo direct delivery. Transcytosis plays only a minor role in axonal delivery of these proteins. In addition, we identified a novel pathway by which wayward axonal proteins that reach the dendritic plasma membrane are targeted to lysosomes. These results redefine how axonal proteins achieve their polarized distribution, a crucial requirement for elucidating the underlying molecular mechanisms.

Basolateral sorting and transcytosis define the Cu+-regulated translocation of ATP7B to the bile canaliculus

The Cu+ pump ATP7B plays an irreplaceable role in the elimination of excess Cu+ by the hepatocyte into the bile. The traffic and site of ATP7B action is a subject of controversy. One current proposal is that an increase in intracellular Cu+ results in the translocation of ATP7B to the lysosomes and excretion of excess Cu+ by lysosomal mediated exocytosis at the bile canaliculus. Here we show that ATP7B is transported from the trans-Golgi network to the bile canaliculus by basolateral sorting and endocytosis, and microtubule mediated transcytosis through the subapical compartment. Trafficking ATP7B is not incorporated into lysosomes and addition of Cu+ does not cause relocalization of lysosomes and the appearance of lysosome markers in the bile canaliculus. Our data describes the pathway of the Cu+-mediated transport of ATP7B from the TGN to the bile canaliculus and indicates that the bile canaliculus is the prime site of ATP7B action in the elimination of excess Cu+.

Significance of transcytosis in Alzheimer's disease: BACE1 takes the scenic route to axons

Neurons have developed elaborate mechanisms for sorting of proteins to their destination in dendrites and axons as well as dynamic local trafficking. Recent evidence suggests that polarized axonal sorting of β-site converting enzyme 1 (BACE1), a type I transmembrane aspartyl protease involved in Alzheimer's disease (AD) pathogenesis, entails an unusual journey. In hippocampal neurons, BACE1 internalized from dendrites is conveyed in recycling endosomes via unidirectional retrograde transport towards the soma and sorted to axons where BACE1 becomes enriched. In comparison to other transmembrane proteins that undergo transcytosis or elimination in somatodendritic compartment, vectorial transport of internalized BACE1 in dendrites is unique and intriguing. Dysfunction of protein transport contributes to pathogenesis of AD and other neurodegenerative diseases. Therefore, characterization of BACE1 transcytosis is an important addition to the multiple lines of evidence that highlight the crucial role played by endosomal trafficking pathway as well as axonal sorting mechanisms in AD pathogenesis.

Streptococcus pneumoniae Interacts with pIgR expressed by the brain microvascular endothelium but does not co-localize with PAF receptor

Streptococcus pneumoniae is thought to adhere to the blood-brain barrier (BBB) endothelium prior to causing meningitis. The platelet activating factor receptor (PAFR) has been implicated in this adhesion but there is a paucity of data demonstrating direct binding of the bacteria to PAFR. Additionally, studies that inhibit PAFR strongly suggest that alternative receptors for pneumococci are present on the endothelium. Therefore, we studied the roles of PAFR and pIgR, an established epithelial pneumococcal receptor, in pneumococcal adhesion to brain endothelial cells in vivo. Mice were intravenously infected with pneumococci and sacrificed at various time points before meningitis onset. Co-localization of bacteria with PAFR and pIgR was investigated using immunofluorescent analysis of the brain tissue. In vitro blocking with antibodies and incubation of pneumococci with endothelial cell lysates were used to further probe bacteria-receptor interaction. In vivo as well as in vitro pneumococci did not co-localize with PAFR. On the other hand the majority of S. pneumoniae co-localized with endothelial pIgR and pIgR blocking reduced pneumococcal adhesion to endothelial cells. Pneumococci physically interacted with pIgR in endothelial cell lysates. In conclusion, bacteria did not associate with PAFR, indicating an indirect role of PAFR in pneumococcal adhesion to endothelial cells. In contrast, pIgR on the BBB endothelium may represent a novel pneumococcal adhesion receptor.

Directional transneuronal spread of α-herpesvirus infection

Most α-herpesviruses are pantropic, neuroinvasive pathogens that establish a reactivateable, latent infection in the PNS of their natural hosts. Various manifestations of herpes disease rely on extent and direction of the spread of infection between the surface epithelia and the nervous system components that innervate that surface. One aspect of such controlled spread of infection is the capacity for synaptically defined, transneuronal spread, a property that makes α-herpesviruses useful tools for determining the connectivity of neural circuits. The current understanding of intra-axonal transport and transneuronal spread of α-herpesviruses is reviewed, focusing on work with herpes simplex virus and pseudorabies virus, the available in vitro technology used to study viral transport and spread is evaluated and how certain viral mutants can be used to examine neural circuit architecture is described in this article.

IgLON cell adhesion molecule Kilon is a crucial modulator for synapse number in hippocampal neurons

Kilon is a member of the IgLON family belonging to the immunoglobulin superfamily of cell adhesion molecules. In the present study, we investigated temporal and spatial changes of Kilon expression and its modulatory functions for synapse number using hippocampal cultured neurons. Kilon was observed to localize chiefly at axons and presynaptic terminals at early culture stage, however, it was seen mainly at dendritic postsynaptic spine of mature neurons at late culture stages. Kilon was solubilized with detergent treatment at early culture stages, while it resisted to extraction of the detergent in mature neurons. The overexpression of Kilon gene using a plasmid vector decreased the number of dendritic synapses at early culture stages, whereas the overexpression increased the number of dendritic synapses at late culture. These results demonstrate the alteration of modulatory function of Kilon for the number of dendritic synapses concomitant with changes in its localization and detergent solubility during neuronal culture development.

Pathway selection to the axon depends on multiple targeting signals in NgCAM

Similar to most differentiated cells, both neurons and epithelial cells elaborate distinct plasma membrane domains that contain different membrane proteins. We have previously shown that the axonal cell-adhesion molecule L1/NgCAM accumulates on the axonal surface by an indirect transcytotic pathway via somatodendritic endosomes. MDCK epithelial cells similarly traffic NgCAM to the apical surface by transcytosis. In this study, we map the signals in NgCAM required for routing via the multi-step transcytotic pathway. We identify both a previously mapped tyrosine-based signal as a sufficient somatodendritic targeting signal, as well as a novel axonal targeting signal in the cytoplasmic tail of NgCAM. The axonal signal is glycine and serine rich, but only the glycine residues are required for activity. The somatodendritic signal is cis-dominant and needs to be inactivated in order for the axonal signal to be executed. Additionally, we show that the axonal cytoplasmic signal promotes apical targeting in MDCK cells. Transcytosis of NgCAM to the axon thus requires the sequential regulated execution of multiple targeting signals.

beta-amyloid precursor protein can be transported independent of any sorting signal to the axonal and dendritic compartment

In neurons, amyloid precursor protein (APP) is localized to the dendritic and axonal compartment. Changes in subcellular localization affect secretase cleavage of APP, altering the generation of Abeta, and presumably also its pathogenic features. It was reported that APP is sorted initially to the axon and transcytosed subsequently to the somatodendritic compartment. This may be carried out by a recessive dendritic sorting signal in the cytoplasmic C-terminus, possibly the tyrosine based basolateral sorting signal (BaSS), and an axonal sorting motif within the extracellular juxtamembraneous domain. We investigated whether the C- or N-terminal domain of APP contains an independent dendritic or axonal sorting signal. We generated different APP deletion mutants, and produced chimeric proteins of APP and a non-related Type I transmembrane protein. Quantitative immunocytochemical analyses of transfected primary neurons showed that similar amounts of all APP mutants, lacking either the N- or C-terminus, were transported to the axonal and dendritic compartment. Investigations of the chimeric proteins showed that neither the N- nor the C-terminus of APP functions as independent sorting signal, whereas another tyrosine based dendritic sorting signal was sufficient to prevent axonal entry of APP. This data shows that, under steady state conditions, Heterologously expressed APP is transported equally to axons and dendrites irrespective of any putative sorting signal in its N- or C-terminus. This shows that APP can enter the axon in absence of the initial axonal sorting motif, indicating the existence of an alternative pathway allowing axonal entry of APP.

Retromer: multipurpose sorting and specialization in polarized transport

Retromer is an evolutionary conserved protein complex required for endosome-to-Golgi retrieval of lysosomal hydrolases' receptors. A dimer of two sorting nexins-typically, SNX1 and/or SNX2-deforms the membrane and thus cooperates with retromer to ensure cargo sorting. Research in various model organisms indicates that retromer participates in sorting of additional molecules whose proper transport has important repercussions in development and disease. The role of retromer as well as SNXs in endosomal protein (re)cycling and protein targeting to specialized plasma membrane domains in polarized cells adds further complexity and has implications in growth control, the establishment of developmental patterns, cell adhesion, and migration. This chapter will discuss the functions of retromer described in various model systems and will focus on relevant aspects in polarized transport.

Consequence of the presence of two different beta subunit isoforms in a GABAA receptor

The major isoforms of GABA(A) receptors are thought to be composed of two alpha, two beta and one gamma subunit(s). GABA(A) receptors containing two beta1 subunits respond differently to the anticonvulsive compound loreclezole and the general anaesthetic etomidate than receptors containing two beta2 subunits. Receptors containing beta2 subunits show a much larger allosteric stimulation by these agents than those containing beta1 subunits. We were interested to know how receptors containing both beta1 and beta2 subunits, in different positions respond to loreclezole and etomidate. To answer this question, subunits were fused at the DNA level to form dimeric and trimeric subunits. Concatenated receptors (alpha1-beta1-alpha1/gamma2-beta1, alpha1-beta2-alpha1/gamma2-beta1, alpha1-beta1-alpha1/gamma2-beta2 and alpha1-beta2-alpha1/gamma2-beta2) were expressed in Xenopus ooctyes and functionally compared in their response to the agonist GABA and to the positive allosteric modulators, loreclezole and etomidate. We have shown that (I) in the presence of both beta1 and beta2 subunits in the same pentamer (mixed receptors) direct gating by etomidate is similar to exclusively beta1 containing receptors; (II) In mixed receptors, stimulation by etomidate assumed characteristics intermediate to exclusively beta1 or beta2 containing receptors, but the values for the concentrations < 10 microM were always much closer to those observed in alpha1-beta1-alpha1/gamma2-beta1 receptors; and (III) mixed receptors show no positional effects.

Axon-dominant localization of cell-surface cholesterol in cultured hippocampal neurons and its disappearance in Niemann-Pick type C model cells

There is growing evidence showing the important role of cholesterol in maintaining neuronal function. In particular, much attention has been paid to the role of the cholesterol-rich microdomains called lipid rafts. However, the cholesterol distribution on neurons is not clear. Here, we investigated localization of cholesterol in cultured rat hippocampal neurons, using filipin and a novel cholesterol-binding reagent BCtheta. In our culture system, BCtheta detects only cell-surface cholesterol, whereas filipin stains both intracellular and cell-surface cholesterol. BCtheta staining appeared visible in a maturation-dependent manner and showed axon-dominant distribution of cell-surface cholesterol in fully matured neurons. A part of this cholesterol on axons was resistant to detergents at 4 degrees C, and thus might be involved in lipid rafts. Interestingly, Niemann-Pick type C model neurons induced by class 2 amphiphiles lost the cell-surface but not the intracellular cholesterol staining. Niemann-Pick type C disease is caused by the disruption of intracellular cholesterol transport and is known to induce neurodegeneration in brains accompanied by formation of neurofibrillary tangles. Our observations suggest the important role of cell-surface cholesterol in maintaining a functional axonal membrane and indicate that the observed defect in axonal surface cholesterol might lead to neurodegeneration.

Uncovering multiple axonal targeting pathways in hippocampal neurons

Neuronal polarity is, at least in part, mediated by the differential sorting of membrane proteins to distinct domains, such as axons and somata/dendrites. We investigated the pathways underlying the subcellular targeting of NgCAM, a cell adhesion molecule residing on the axonal plasma membrane. Following transport of NgCAM kinetically, surprisingly we observed a transient appearance of NgCAM on the somatodendritic plasma membrane. Down-regulation of endocytosis resulted in loss of axonal accumulation of NgCAM, indicating that the axonal localization of NgCAM was dependent on endocytosis. Our data suggest the existence of a dendrite-to-axon transcytotic pathway to achieve axonal accumulation. NgCAM mutants with a point mutation in a crucial cytoplasmic tail motif (YRSL) are unable to access the transcytotic route. Instead, they were found to travel to the axon on a direct route. Therefore, our results suggest that multiple distinct pathways operate in hippocampal neurons to achieve axonal accumulation of membrane proteins.

Neuronal Polarity and Trafficking

Among the most morphologically complex cells, neurons are masters of membrane specialization. Nowhere is this more striking than in the division of cellular labor between the axon and the dendrites. In morphology, signaling properties, cytoskeletal organization, and physiological function, axons and dendrites (or more properly, the somatodendritic compartment) are radically different. Such polarization of neurons into domains specialized for either receiving (dendrites) or transmitting (axons) cellular signals provides the underpinning for all neural circuitry. The initial specification of axonal and dendritic identity occurs early in neuronal life, persists for decades, and is manifested by the presence of very different sets of cell surface proteins. Yet, how neuronal polarity is established, how distinct axonal and somatodendritic domains are maintained, and how integral membrane proteins are directed to dendrites or accumulate in axons remain enduring and formidable questions in neuronal cell biology.

Two distinct mechanisms target membrane proteins to the axonal surface

We have investigated the trafficking of two endogenous axonal membrane proteins, VAMP2 and NgCAM, in order to elucidate the cellular events that underlie their polarization. We found that VAMP2 is delivered to the surface of both axons and dendrites, but preferentially endocytosed from the dendritic membrane. A mutation in the cytoplasmic domain of VAMP2 that inhibits endocytosis abolished its axonal polarization. In contrast, the targeting of NgCAM depends on sequences in its ectodomain, which mediate its sorting into carriers that preferentially deliver their cargo proteins to the axonal membrane. These observations show that neurons use two distinct mechanisms to polarize proteins to the axonal domain: selective retention in the case of VAMP2, selective delivery in the case of NgCAM.

Membrane and cytoskeleton dynamics during axonal elongation and stabilization

Proper nervous activities are gradually developing events. Reflecting this, embryonic neurons start differentiation by sprouting multiple extensions, neurites, which do not bear clear axonal or dendritic structural and molecular characteristics. Later in development one of these multiple neurites elongates further, generating a morphologically polarized neuron with a single long axon and many short dendrites. Still, despite such morphological differences these processes can switch destiny, further reflecting their immaturity. Final and irreversible axonal and dendritic commitment occurs after both axons and dendrites have elongated considerably. Recent evidence suggests that the transition from axonal immaturity to maturity reflects changes in the mechanisms used by neurons to control the precise membrane and cytoskeleton polarization. This chapter provides an overview of how these mechanisms contribute to the formation of an axon.

Alphaviral vectors for gene transfer into neurons

Alphaviruses are small, enveloped positive-strand RNA viruses that have been successfully transformed into expression vectors in the case of Semliki Forest virus (SFV), Sindbis virus (SIN), and Venezuelan equine encephalitis virus. Compared to other viral vectors, their advantages are easy and fast generation of recombinant viral particles, rapid onset, and high-level transgene expression. When applied to neuronal tissue, SFV and SIN vectors possess the additional advantage of efficiently and preferentially transducing neurons rather than non-neuronal cells. This article gives an overview of the biology of SFV and SIN, their generation into expression vectors, and their application in neurobiology, with particular emphasis on the transduction of hippocampal neurons. In addition, it describes the more recent development of alphaviral vectors with decreased or absent cytotoxicity and lowered transgene expression, temperature-controllable gene expression, and altered host-cell specificity in the central nervous system (CNS). Finally, the review evaluates the use of SFV and SIN vectors in hippocampal tissue cultures vs recombinant lentivirus, adenovirus type 5, adeno-associated virus type 2, and measles virus.

Intestinal dipeptidyl peptidase IV is efficiently sorted to the apical membrane through the concerted action of N- and O-glycans as well as association with lipid microdomains

The apical sorting of human intestinal dipeptidyl peptidase IV (DPPIV) occurs through complex N-linked and O-linked carbohydrates. Inhibition of O-linked glycosylation by benzyl-N-acetyl-alpha-d-galactosaminide affects significantly the sorting behavior of DPPIV in intestinal Caco-2 and HT-29 cells. However, random delivery to the apical and basolateral membranes and hence a more drastic effect on the sorting of DPPIV in both cell types is only observed when, in addition to O-glycans, the processing of N-glycans is affected by swainsonine, an inhibitor of mannosidase II. Together the data indicate that both types of glycosylation are critical components of the apical sorting signal of DPPIV. The sorting mechanism of DPPIV implicates its association with detergent-insoluble membrane microdomains containing cholesterol and sphingolipids, whereas an efficient association largely depends on the presence of a fully complex N- and O-linked glycosylated DPPIV. Interestingly, cholesterol is a more critical component in this context than sphingolipids, because cholesterol depletion by beta-cyclodextrin affects the detergent solubility and the sorting behavior of DPPIV more strongly than fumonisin, an inhibitor of sphingolipid synthesis.

Anterograde axonal transport, transcytosis, and recycling of neurotrophic factors: the concept of trophic currencies in neural networks

Traditional views of neurotrophic factor biology held that trophic factors are released from target cells, retrogradely transported along their axons, and rapidly degraded upon arrival in cell bodies. Increasing evidence indicates that several trophic factors such as brain-derived neurotrophic factor (BDNF), fibroblast growth factor (FGF-2), glial cell-line derived neurotrophic factor (GDNF), insulin-like growth factor (IGF-I), and neurotrophin-3 (NT-3), can move anterogradely along axons. They can escape the degradative pathway upon internalization and are recycled for future uses. Internalized ligands can move through intermediary cells by transcytosis, presumably by endocytosis via endosomes to the Golgi system, by trafficking of the factor to dendrites or by sorting into anterograde axonal transport with subsequent release from axon terminals and uptake by second- or third-order target neurons. Such data suggest the existence of multiple "trophic currencies," which may be used over several steps in neural networks to enable nurturing relationships between connected neurons or glial cells, not unlike currency exchanges between trading partners in the world economy. Functions of multistep transfer of trophic material through neural networks may include regulation of neuronal survival, differentiation of phenotypes and dendritic morphology, synapse plasticity, as well as excitatory neurotransmission. The molecular mechanisms of sorting, trafficking, and release of trophic factors from distinct neuronal compartments are important for an understanding of neurotrophism, but they present challenging tasks owing to the low levels of the endogenous factors.

Dynamics of glycine receptor insertion in the neuronal plasma membrane

The exocytosis site of newly synthesized glycine receptor was defined by means of a morphological assay to characterize its export from the trans-Golgi Network to the plasma membrane. This was achieved by expressing in transfected neurons an alpha1 subunit bearing an N-terminal tag selectively cleavable from outside the cell by thrombin. This was combined with a transient temperature-induced block of exocytic transport that creates a synchronized exocytic wave. Immunofluorescence microscopy analysis of the cell surface appearance of newly synthesized receptor revealed that exocytosis mainly occurred at nonsynaptic sites in the cell body and the initial portion of dendrites. At the time of cell surface insertion, the receptors existed as discrete clusters. Quantitative analysis showed that glycine receptor clusters are stable in size and subsequently appeared in more distal dendritic regions. This localization resulted from diffusion in the plasma membrane and not from exocytosis of transport vesicles directed to dendrites. Kinetic analysis established a direct substrate-product relationship between pools of somatic and dendritic receptors. This indicated that clusters represent intermediates between newly synthesized and synaptic receptors. These results support a diffusion-retention model for the formation of receptor-enriched postsynaptic domains and not that of a vectorial intracellular targeting to synapses.

Dynamics of glycine receptor insertion in the neuronal plasma membrane

The exocytosis site of newly synthesized glycine receptor was defined by means of a morphological assay to characterize its export from the trans-Golgi Network to the plasma membrane. This was achieved by expressing in transfected neurons an alpha1 subunit bearing an N-terminal tag selectively cleavable from outside the cell by thrombin. This was combined with a transient temperature-induced block of exocytic transport that creates a synchronized exocytic wave. Immunofluorescence microscopy analysis of the cell surface appearance of newly synthesized receptor revealed that exocytosis mainly occurred at nonsynaptic sites in the cell body and the initial portion of dendrites. At the time of cell surface insertion, the receptors existed as discrete clusters. Quantitative analysis showed that glycine receptor clusters are stable in size and subsequently appeared in more distal dendritic regions. This localization resulted from diffusion in the plasma membrane and not from exocytosis of transport vesicles directed to dendrites. Kinetic analysis established a direct substrate-product relationship between pools of somatic and dendritic receptors. This indicated that clusters represent intermediates between newly synthesized and synaptic receptors. These results support a diffusion-retention model for the formation of receptor-enriched postsynaptic domains and not that of a vectorial intracellular targeting to synapses.

Establishment of neuronal polarity: lessons from cultured hippocampal neurons

In recent years we have learned a great deal about the molecular mechanisms underlying axonal elongation and navigation and the manner in which extracellular signals modify a growth cone's course of action. Yet, the mechanisms responsible for the earlier events of axonal and dendritic generation are just beginning to be understood. The recent advances in this exciting field highlight the importance of studies of cell migration and axonal elongation for our current understanding of the establishment of neuronal polarity.

Large scale transient 5-HT3 receptor production with the Semliki Forest Virus Expression System

The expression of recombinant proteins with the Semliki Forest Virus (SFV) system has been scaled up to bioreactor scale. As a model protein for this study the human 5-HT(3) receptor was chosen. The gene for the receptor was subcloned into the SFV expression plasmid pSFV1. Virus production by in vivo packaging and production of the recombinant protein was scaled up, the latter to a reactor volume of 11.5 l. A Vibromix(TM) agitation system was chosen to overcome aggregation problems of BHK cells in suspension. In the process, cells were first grown to a density of 10(6) cells/ml, the medium was then exchanged with fresh medium and the culture was infected with the recombinant virus at an estimated multiplicity of infection of 30. 24 h post infection we measured an expression level of 3 million functional 5-HT(3) receptors per cell. For harvesting, the cells were pelleted by centrifugation. The receptor protein was purified in a single step (Hovius et al., 1998) by exploiting the hexa-His tag at minimal protein loss (51% yield). Experiments to optimise expression resulted in yields up to 8 million receptors per cell, when the pH of a suspension culture was controlled at pH 7.3. Rapid virus generation and protein production, high protein yields as well as successful large scale application have made the SFV expression system attractive to produce large quantities of recombinant protein in a very short time. After optimisation of the expression conditions (in particular by setting the pH at 7.3), yields were increased twofold.

A novel targeting signal for proximal clustering of the Kv2.1 K+ channel in hippocampal neurons

The discrete localization of ion channels is a critical determinant of neuronal excitability. We show here that the dendritic K+ channels Kv2.1 and Kv2.2 were differentially targeted in cultured hippocampal neurons. Kv2.1 was found in high-density clusters on the soma and proximal dendrites, while Kv2.2 was uniformly distributed throughout the soma and dendrites. Chimeras revealed a proximal restriction and clustering domain on the cytoplasmic tail of Kv2.1. Truncations and internal deletions revealed a 26-amino acid targeting signal within which four residues were critical for localization. This signal is not related to other known sequences for neuronal and epithelial membrane protein targeting and represents a novel cytoplasmic signal responsible for proximal restriction and clustering.

Subcellular localization of full-length and truncated Trk receptor isoforms in polarized neurons and epithelial cells

Neurotrophins affect neuronal development and plasticity via spatially localized effects, yet little is known about the subcellular distribution of the Trk neurotrophin receptors and the impact of this distribution on neurotrophin action. To address this, we examined the subcellular location of full-length TrkB and TrkC tyrosine kinase receptors and truncated TrkB isoforms after transfection of Madin-Darby canine kidney (MDCK) cells, dissociated primary hippocampal neurons, and cortical neurons within intact brain slices. Myc-, herpes virus glycoprotein (HVG)-, or FLAG-derived epitope-tagged receptor isoforms were created to allow their unambiguous identification and localization after transfection. All tagged receptors were appropriately synthesized, and full-length myc-TrkB and myc-TrkC mediated appropriate neurotrophin-signaling events. We found that full-length TrkB receptors were excluded from the apical domain of MDCK cells but that TrkC receptors were present in both apical and basolateral domains. Full-length TrkB and TrkC were found throughout transfected primary cultured hippocampal neurons and transfected neurons in neocortical brain slices and showed no evidence of vectorial sorting. Truncated forms of TrkB were also homogeneously distributed in MDCK cells, dissociated hippocampal neurons, and cortical neurons within slice preparations. Levels of full-length and truncated TrkB were examined in postsynaptic densities; both receptor isoforms were present but only moderately enriched in these structures. Together, these findings suggest that Trk receptors are uniformly distributed in both axonal and dendritic compartments and that local neurotrophin responses are controlled by other mechanisms.

Subcellular localization of full-length and truncated Trk receptor isoforms in polarized neurons and epithelial cells

Neurotrophins affect neuronal development and plasticity via spatially localized effects, yet little is known about the subcellular distribution of the Trk neurotrophin receptors and the impact of this distribution on neurotrophin action. To address this, we examined the subcellular location of full-length TrkB and TrkC tyrosine kinase receptors and truncated TrkB isoforms after transfection of Madin-Darby canine kidney (MDCK) cells, dissociated primary hippocampal neurons, and cortical neurons within intact brain slices. Myc-, herpes virus glycoprotein (HVG)-, or FLAG-derived epitope-tagged receptor isoforms were created to allow their unambiguous identification and localization after transfection. All tagged receptors were appropriately synthesized, and full-length myc-TrkB and myc-TrkC mediated appropriate neurotrophin-signaling events. We found that full-length TrkB receptors were excluded from the apical domain of MDCK cells but that TrkC receptors were present in both apical and basolateral domains. Full-length TrkB and TrkC were found throughout transfected primary cultured hippocampal neurons and transfected neurons in neocortical brain slices and showed no evidence of vectorial sorting. Truncated forms of TrkB were also homogeneously distributed in MDCK cells, dissociated hippocampal neurons, and cortical neurons within slice preparations. Levels of full-length and truncated TrkB were examined in postsynaptic densities; both receptor isoforms were present but only moderately enriched in these structures. Together, these findings suggest that Trk receptors are uniformly distributed in both axonal and dendritic compartments and that local neurotrophin responses are controlled by other mechanisms.

Hierarchy of sorting signals in chimeras of intestinal lactase-phlorizin hydrolase and the influenza virus hemagglutinin

Lactase-phlorizin hydrolase (LPH) is an apical protein in intestinal cells. The location of sorting signals in LPH was investigated by preparing a series of mutants that lacked the LPH cytoplasmic domain or had the cytoplasmic domain of LPH replaced by sequences that comprised basolateral targeting signals and overlapping internalization signals of various potency. These signals are mutants of the cytoplasmic domain of the influenza hemagglutinin (HA), which have been shown to be dominant in targeting HA to the basolateral membrane. The LPH-HA chimeras were expressed in Madin-Darby canine kidney (MDCK) and colon carcinoma (Caco-2) cells, and their transport to the cell surface was analyzed. All of the LPH mutants were targeted correctly to the apical membrane. Furthermore, the LPH-HA chimeras were internalized, indicating that the HA tails were available to interact with the cytoplasmic components of clathrin-coated pits. The introduction of a strong basolateral sorting signal into LPH was not sufficient to override the strong apical signals of the LPH external domain or transmembrane domains. These results show that basolateral sorting signals are not always dominant over apical sorting signals in proteins that contain each and suggest that sorting of basolateral from apical proteins occurs within a common compartment where competition for sorting signals can occur.

Axon/dendrite targeting of metabotropic glutamate receptors by their cytoplasmic carboxy-terminal domains

The subcellular targeting of neurotransmitter receptors is vital in controlling polarized information flow in the brain. We show here that metabotropic glutamate receptors are differentially targeted when expressed from defective viral vectors in cultured hippocampal neurons; mGluR1a and mGluR2 are targeted to dendrites and excluded from axons, whereas mGluR7 is targeted to axons and dendrites. Chimeras and deletions revealed that axon exclusion of mGluR2 versus axon targeting of mGluR7 is mediated by their 60 amino acid C-terminal cytoplasmic domains. Addition of the mGluR7 C-terminal sequence to mGluR2 or to the unrelated somatodendritic protein telencephalin (tln) induced axon targeting, indicating dominance of the axonal signal. These mGluR sorting signals represent novel plasma membrane axon/dendrite targeting signals.

Molecular cloning of a gene on chromosome 19q12 coding for a novel intracellular protein: analysis of expression in human and mouse tissues and in human tumor cells, particularly Reed-Sternberg cells in Hodgkin disease

A novel protein, named NNX3, was molecularly characterized by cloning its cDNA, and its gene was mapped to chromosome 19q12. The equivalent mouse cDNA and gene were also cloned to allow us to analyze expression in murine in addition to human cells and tissues. Human and mouse NNX3 genes are composed of nine exons coding for proteins that are unrelated to any known protein. Signal peptides and hydrophobic domains are absent, corroborating their localization in the cytoplasm in transfected Cos cells. In Western blotting and immunoprecipitation, human NNX3 appeared as a doublet of Mr 64K-66K, while mouse NNX3 was a 70-kDa protein, both apparently much larger than the predicted 50 kDa, due in part to a stretch of 16-18 acidic residues hinging two nearly equally sized domains. In addition, phosphorylation of serine residues was demonstrated. Putative nuclear targeting signals were predicted, but NNX3 protein and two truncated versions remained localized in the cytoplasm of transfected Cos cells. NNX3 was expressed in embryonic and adult mouse tissues, particularly in brain, muscle, and lung. The expression of human NNX3 was most notable in human skeletal muscle and in ganglion cells and was also evident in human tumors and derived cell lines. This was confirmed by entries appearing in the GenBank EST database during the later phase of this study, representing partial NNX3 cDNA isolated from diverse neoplastic and developing tissues. Surprisingly, NNX3 was immunochemically detected in Reed-Sternberg cells of Hodgkin disease, in parallel with restin, a cytoplasmic protein we previously characterized (J. Delabie et al., 1993, Leuk. Lymphoma 12, 21-26). The cloning and comprehensive molecular analysis of NNX3 as presented will form the basis for elucidating its function and, conversely, will constitute a marker for Reed-Sternberg cells in Hodgkin disease.

Membrane traffic in polarized neurons

The plasma membrane of neurons can be divided into two domains, the soma-dendritic and the axonal. These domains perform different functions: the dendritic surface receives and processes information while the axonal surface is specialized for the rapid transmission of electrical impulses. This functional specialization is generated by sorting and anchoring mechanisms that guarantee the correct delivery and retention of specific membrane proteins. Our understanding of neuronal membrane protein sorting is primarily based on studies of protein overexpression in cultured neurons. These studies revealed that newly synthesized membrane proteins are segregated in the Golgi apparatus in the cell body from where they are transported to the axonal or dendritic surface. Such segregation presumably depends on sorting motifs in the proteins' primary structure. They appear to be located in the cytoplasmic tail for dendritic proteins and in the transmembrane-ectodomain for axonal proteins. Recent studies on neurotransmitter segregation suggest that anchoring in the correct subdomain of the plasma membrane also requires cytoplasmic tail information for binding to the cytoskeleton either directly or by linker proteins. Both mechanisms, sorting and retention, gradually mature during neural development. Young neurons appear to develop initial polarity by other mechanisms, presumably analogous to the mechanisms used by migrating cells.

Mutations in the middle of the transmembrane domain reverse the polarity of transport of the influenza virus hemagglutinin in MDCK epithelial cells

The composition of the plasma membrane domains of epithelial cells is maintained by biosynthetic pathways that can sort both proteins and lipids into transport vesicles destined for either the apical or basolateral surface. In MDCK cells, the influenza virus hemagglutinin is sorted in the trans-Golgi network into detergent-insoluble, glycosphingolipid-enriched membrane domains that are proposed to be necessary for sorting hemagglutinin to the apical cell surface. Site- directed mutagenesis of the hemagglutinin transmembrane domain was used to test this proposal. The region of the transmembrane domain required for apical transport included the residues most conserved among hemagglutinin subtypes. Several mutants were found to enter detergent-insoluble membranes but were not properly sorted. Replacement of transmembrane residues 520 and 521 with alanines converted the 2A520 mutant hemagglutinin into a basolateral protein. Depleting cell cholesterol reduced the ability of wild-type hemagglutinin to partition into detergent-insoluble membranes but had no effect on apical or basolateral sorting. In contrast, cholesterol depletion allowed random transport of the 2A520 mutant. The mutant appeared to lack sorting information but was prevented from reaching the apical surface when detergent-insoluble membranes were present. Apical sorting of hemagglutinin may require binding of either protein or lipids at the middle of the transmembrane domain and this normally occurs in detergent-insoluble membrane domains. Entry into these domains appears necessary, but not sufficient, for apical sorting.

Rab17 localizes to recycling endosomes and regulates receptor-mediated transcytosis in epithelial cells

The small GTPase Rab17 is restricted to epithelial cells and its expression is induced during cell polarization. This observation has led to the suggestion that the protein may function in transcytosis, a pathway connecting the apical and basolateral endocytic systems. To analyze whether Rab17 plays a role in transcellular transport, we generated Madin-Darby canine kidney (MDCK) cell lines stably coexpressing wild-type or mutant Rab17 and the transcytotic polymeric immunoglobulin receptor (pIgR). Rab17 expressed in MDCK cells was found on small vesicles and tubules in the apical region of the cells. A significant fraction of the Rab17-positive structures was accessible to dimeric IgA internalized from the apical or basolateral cell surface via the pIgR. Furthermore, basolateral to apical transcytosis of dimeric IgA was impaired in MDCK cells overexpressing Rab17. Our data provides morphological and biochemical evidence for a role of Rab17 in the regulation of transcellular traffic through apical recycling endosomes in epithelial cells.

The polarized sorting of membrane proteins expressed in cultured hippocampal neurons using viral vectors

One model of neuronal polarity (Dotti and Simons, 1990) proposes that neurons and polarized epithelia use similar mechanisms to sort membrane proteins. To explore this hypothesis, we used viral vectors to express proteins in cultured neurons and assessed their distribution using quantitative immunofluorescence microscopy. Basolateral epithelial proteins were polarized to dendrites; more significantly, mutations of sequences required for their basolateral targeting in epithelia also disrupted dendritic targeting. Unexpectedly, apical proteins were not polarized to axons but were expressed at roughly equal amounts in dendrites and axons. These data provide strong evidence that targeting of basolateral and dendritic proteins depends on common mechanisms. In contrast, the sorting of proteins to the axon requires signals that are not present in apical proteins.

Neuronal polarity: essential role of protein-lipid complexes in axonal sorting

The viral glycoprotein hemagglutinin (HA) and the endogenous glycosylphosphatidylinositol-anchored protein Thy-1 are efficiently targeted to the axonal surface of fully polarized hippocampal neurons in culture. Here we have shown that in these cells HA and Thy-1 interact with sphingolipid-cholesterol rafts and are included in detergent-insoluble glycolipid-enriched complexes. Axonal HA and Thy-1, but not two dendritic membrane proteins, resisted extraction to detergents at 4 degrees C. Both HA and Thy-1 became detergent-soluble in neurons with reduced levels of cholesterol or sphingolipids. Missorting of the axonal Thy-1 but not of a dendritic membrane protein occurred in sphingolipid-deprived cells. These results indicate that neurons sort a subset of axolemmal proteins by a mechanism that requires the formation of protein-lipid rafts. The involvement of rafts in axonal membrane sorting may explain the neurological deficits observed in patients with certain types of Niemann-Pick disease.

Endocytosis and transcytosis

Vesicular coat proteins mediate the formation of nascent vesicles and select the cargo to be incorporated therein. As additional coat proteins are discovered that regulate vesicular traffic along very specific intracellular pathways, the possibility looms of regulating the intracellular trafficking and targeting of therapeutic agents by modulation of the action of vesicular coat proteins. Examples are provided of coat proteins thought to regulate the trafficking of pharmaceutically relevant molecules via clathrin-mediated endocytosis, caveolae-mediated endocytosis, and transcytosis.

Dendroaxonal transcytosis of transferrin in cultured hippocampal and sympathetic neurons

Previous studies using overexpressed polymeric immunoglobulin receptor in cultured neurons have suggested that these cells may use a dendroaxonal transcytotic pathway (Ikonen et al., 1993; de Hoop et al., 1995). By using a combination of semiquantitative light microscopy, video microscopy, and a biochemical assay, we show that this pathway is used by the endogenous ligand transferrin (Tf) and its receptor. Labeled Tf added to fully mature hippocampal neurons changes the intracellular distribution of its receptor from preferentially dendritic shortly after addition to dendritic and axonal at longer times. Incubation of living neurons with (caged)FITC-Tf followed by uncaging in the dendrites results in the later appearance of fluorescence in the axon of the same cell. In "chambered" sympathetic neurons in culture, 125I-Tf or iron as 55Fe-Tf added to the cell body/dendrite chamber is recovered in the axonal chamber, showing that internalized ligand from the cell body-dendrite area is released at the axonal end. Finally, we show that excitatory neurotransmitters increase Tf receptor transcytosis, whereas inhibitory neurotransmitters reduce it. The dendritic uptake, transcytotic transport, and axonal release of physiologically active Tf demonstrated here could be envisioned for other trophic factors and therefore have important consequences for neuronal anterograde target maturation. Moreover, the changes in transcytosis after neurotransmitter addition may be important in the cellular responses that follow electrical activation.

Inhibition of axonal growth by brefeldin A in hippocampal neurons in culture

The outgrowth of neuronal processes involves a great increase in the surface area of the cell. The supply of membrane material necessarily must be coordinated with the demands for neurite growth. The selective growth of only one or two neurites at any given time during the development of polarity raises the possibility that the production of materials by the soma is limiting for growth (Dotti and Banker, 1987; Dotti et al., Goslin and Banker, 1990). To examine the role of the availability of membrane components during the development of polarity and axonal elongation, we treated neurons with brefeldin A, an antibiotic that disrupts the trafficking of vesicles from the Golgi complex to the plasma membrane. Treatment with brefeldin A (1 microg/ml) inhibited axonal growth within 0.5 hr; in unpolarized cells it prevented the formation of an axon. These results indicate that the availability of membrane components of Golgi-derived vesicles is required for axonal growth and hence the development of polarity. Inhibitors of protein and RNA synthesis also blocked axonal growth and the development of polarity, but over a much slower time course. This suggests that the full complement of proteins and mRNAs required for the initial development of polarity is present for several hours before polarity is actually established.

Neuronal polarity: vectorial cytoplasmic flow precedes axon formation

Axon formation in multipolar neurons is believed to depend on the existence of precise sorting mechanisms for axonal membrane and membrane-associated proteins. Conclusive evidence in living neurons, however, is lacking. In the present study, we use light and video microscopy to address this issue directly. We show that axon formation is preceded by the appearance in one of the multiple neurites of (1) a larger growth cone, (2) a higher amount and greater transport of membrane organelles, (3) polarized delivery of TGN-derived vesicles, (4) a higher concentration of mitochondria and peroxisomes, (5) a higher concentration of a cytosolic protein, and (6) a higher concentration of ribosomes. These results provide evidence for the involvement of bulk cytoplasmic flow as an early determinant of neuronal morphological polarization. Molecular sorting events would later trigger the establishment of functional polarity.

Dendroaxonal transcytosis of transferrin in cultured hippocampal and sympathetic neurons

Previous studies using overexpressed polymeric immunoglobulin receptor in cultured neurons have suggested that these cells may use a dendroaxonal transcytotic pathway (Ikonen et al., 1993; de Hoop et al., 1995). By using a combination of semiquantitative light microscopy, video microscopy, and a biochemical assay, we show that this pathway is used by the endogenous ligand transferrin (Tf) and its receptor. Labeled Tf added to fully mature hippocampal neurons changes the intracellular distribution of its receptor from preferentially dendritic shortly after addition to dendritic and axonal at longer times. Incubation of living neurons with (caged)FITC-Tf followed by uncaging in the dendrites results in the later appearance of fluorescence in the axon of the same cell. In "chambered" sympathetic neurons in culture, 125I-Tf or iron as 55Fe-Tf added to the cell body/dendrite chamber is recovered in the axonal chamber, showing that internalized ligand from the cell body-dendrite area is released at the axonal end. Finally, we show that excitatory neurotransmitters increase Tf receptor transcytosis, whereas inhibitory neurotransmitters reduce it. The dendritic uptake, transcytotic transport, and axonal release of physiologically active Tf demonstrated here could be envisioned for other trophic factors and therefore have important consequences for neuronal anterograde target maturation. Moreover, the changes in transcytosis after neurotransmitter addition may be important in the cellular responses that follow electrical activation.

Inhibition of axonal growth by brefeldin A in hippocampal neurons in culture

The outgrowth of neuronal processes involves a great increase in the surface area of the cell. The supply of membrane material necessarily must be coordinated with the demands for neurite growth. The selective growth of only one or two neurites at any given time during the development of polarity raises the possibility that the production of materials by the soma is limiting for growth (Dotti and Banker, 1987; Dotti et al., Goslin and Banker, 1990). To examine the role of the availability of membrane components during the development of polarity and axonal elongation, we treated neurons with brefeldin A, an antibiotic that disrupts the trafficking of vesicles from the Golgi complex to the plasma membrane. Treatment with brefeldin A (1 microg/ml) inhibited axonal growth within 0.5 hr; in unpolarized cells it prevented the formation of an axon. These results indicate that the availability of membrane components of Golgi-derived vesicles is required for axonal growth and hence the development of polarity. Inhibitors of protein and RNA synthesis also blocked axonal growth and the development of polarity, but over a much slower time course. This suggests that the full complement of proteins and mRNAs required for the initial development of polarity is present for several hours before polarity is actually established.

Targeting of the synaptic vesicle protein synaptobrevin in the axon of cultured hippocampal neurons: evidence for two distinct sorting steps

Synaptic vesicles are concentrated in the distal axon, far from the site of protein synthesis. Integral membrane proteins destined for this organelle must therefore make complex targeting decisions. Short amino acid sequences have been shown to act as targeting signals directing proteins to a variety of intracellular locations. To identify synaptic vesicle targeting sequences and to follow the path that proteins travel en route to the synaptic vesicle, we have used a defective herpes virus amplicon expression system to study the targeting of a synaptobrevin-transferrin receptor (SB-TfR) chimera in cultured hippocampal neurons. Addition of the cytoplasmic domain of synaptobrevin onto human transferrin receptor was sufficient to retarget the transferrin receptor from the dendrites to presynaptic sites in the axon. At the synapse, the SB-TfR chimera did not localize to synaptic vesicles, but was instead found in an organelle with biochemical and functional characteristics of an endosome. The chimera recycled in parallel with synaptic vesicle proteins demonstrating that the nerve terminal efficiently sorts transmembrane proteins into different pathways. The synaptobrevin sequence that controls targeting to the presynaptic endosome was not localized to a single, 10- amino acid region of the molecule, indicating that this targeting signal may be encoded by a more distributed structural conformation. However, the chimera could be shifted to synaptic vesicles by deletion of amino acids 61-70 in synaptobrevin, suggesting that separate signals encode the localization of synaptobrevin to the synapse and to the synaptic vesicle.

Mechanisms of neuronal polarity

The mechanisms that permit neurons to establish axons and dendrites involve an interplay between a cell's genetic program and signals in its environment. Recent experiments have identified some of the important extracellular molecules that regulate dendritic development and have furthered our understanding of the endogenous cell biological mechanisms that underlie protein sorting. Some of the signaling pathways that allow extracellular cues to regulate neuronal morphogenesis are also being elucidated.

Assembly of GABAA receptors composed of alpha1 and beta2 subunits in both cultured neurons and fibroblasts

GABAA receptors are believed to be pentameric hetero-oligomers, which can be constructed from six subunits (alpha, beta, gamma, delta, epsilon, and rho) with multiple members, generating a large potential for receptor heterogeneity. The mechanisms used by neurons to control the assembly of these receptors, however, remain unresolved. Using Semliki Forest virus expression we have analyzed the assembly of 9E10 epitope-tagged receptors comprising alpha1 and beta2 subunits in baby hamster kidney cells and cultured superior cervical ganglia neurons. Homomeric subunits were retained within the endoplasmic reticulum, whereas heteromeric receptors were able to access the cell surface in both cell types. Sucrose density gradient fractionation demonstrated that the homomeric subunits were incapable of oligomerization, exhibiting 5 S sedimentation coefficients. Pulse-chase analysis revealed that homomers were degraded, with half-lives of approximately 2 hr for both the alpha1((9E10)) and beta2((9E10)) subunits. Oligomerization of the alpha1((9E10)) and beta2((9E10)) subunits was evident, as demonstrated by the formation of a stable 9 S complex, but this process seemed inefficient. Interestingly the appearance of cell surface receptors was slow, lagging up to 6 hr after the formation of the 9 S receptor complex. Using metabolic labeling a ratio of alpha1((9E10)):beta2((9E10)) of 1:1 was found in this 9 S fraction. Together the results suggest that GABAA receptor assembly occurs by similar mechanisms in both cell types, with retention in the endoplasmic reticulum featuring as a major control mechanism to prevent unassembled receptor subunits accessing the cell surface.

Assembly of GABAA receptors composed of alpha1 and beta2 subunits in both cultured neurons and fibroblasts

GABAA receptors are believed to be pentameric hetero-oligomers, which can be constructed from six subunits (alpha, beta, gamma, delta, epsilon, and rho) with multiple members, generating a large potential for receptor heterogeneity. The mechanisms used by neurons to control the assembly of these receptors, however, remain unresolved. Using Semliki Forest virus expression we have analyzed the assembly of 9E10 epitope-tagged receptors comprising alpha1 and beta2 subunits in baby hamster kidney cells and cultured superior cervical ganglia neurons. Homomeric subunits were retained within the endoplasmic reticulum, whereas heteromeric receptors were able to access the cell surface in both cell types. Sucrose density gradient fractionation demonstrated that the homomeric subunits were incapable of oligomerization, exhibiting 5 S sedimentation coefficients. Pulse-chase analysis revealed that homomers were degraded, with half-lives of approximately 2 hr for both the alpha1((9E10)) and beta2((9E10)) subunits. Oligomerization of the alpha1((9E10)) and beta2((9E10)) subunits was evident, as demonstrated by the formation of a stable 9 S complex, but this process seemed inefficient. Interestingly the appearance of cell surface receptors was slow, lagging up to 6 hr after the formation of the 9 S receptor complex. Using metabolic labeling a ratio of alpha1((9E10)):beta2((9E10)) of 1:1 was found in this 9 S fraction. Together the results suggest that GABAA receptor assembly occurs by similar mechanisms in both cell types, with retention in the endoplasmic reticulum featuring as a major control mechanism to prevent unassembled receptor subunits accessing the cell surface.

Identification of a somatodendritic targeting signal in the cytoplasmic domain of the transferrin receptor

Neurons are highly polarized cells that must sort proteins synthesized in the cell body for transport into the axon or the dendrites. Given the amount of time and energy needed to deliver proteins to the distal processes, neurons must have high fidelity mechanisms that ensure proper polarized protein trafficking. Although a variety of proteins are localized either to the somatodendritic domain or to the axon (), the question of whether there are signal-dependent mechanisms that sort proteins to distinct neuronal domains is only beginning to be addressed. To determine sequence requirements for the polarized sorting of transmembrane proteins into dendrites, we expressed mutant transferrin receptors in cultured rat hippocampal neurons, using a defective herpes virus vector. Wild-type human transferrin receptor colocalized with the endogenous protein in dendritic endosomes and was strictly excluded from axons, despite overexpression. Polarized targeting was abolished by deletion of cytoplasmic amino acids 7-10, 11-14, or 19-28, but not 29-42 or 43-58. These deletions also increased the appearance of transferrin receptor on the plasma membrane, implying that endocytosis and dendritic targeting are mediated by overlapping signals and similar molecular mechanisms. In addition, we have characterized a specialized para-Golgi endosome poised to play a critical role in the polarized recycling of transmembrane proteins.

Identification of a somatodendritic targeting signal in the cytoplasmic domain of the transferrin receptor

Neurons are highly polarized cells that must sort proteins synthesized in the cell body for transport into the axon or the dendrites. Given the amount of time and energy needed to deliver proteins to the distal processes, neurons must have high fidelity mechanisms that ensure proper polarized protein trafficking. Although a variety of proteins are localized either to the somatodendritic domain or to the axon (), the question of whether there are signal-dependent mechanisms that sort proteins to distinct neuronal domains is only beginning to be addressed. To determine sequence requirements for the polarized sorting of transmembrane proteins into dendrites, we expressed mutant transferrin receptors in cultured rat hippocampal neurons, using a defective herpes virus vector. Wild-type human transferrin receptor colocalized with the endogenous protein in dendritic endosomes and was strictly excluded from axons, despite overexpression. Polarized targeting was abolished by deletion of cytoplasmic amino acids 7-10, 11-14, or 19-28, but not 29-42 or 43-58. These deletions also increased the appearance of transferrin receptor on the plasma membrane, implying that endocytosis and dendritic targeting are mediated by overlapping signals and similar molecular mechanisms. In addition, we have characterized a specialized para-Golgi endosome poised to play a critical role in the polarized recycling of transmembrane proteins.

ADP ribosylation factor 1 is required for synaptic vesicle budding in PC12 cells

Carrier vesicle generation from donor membranes typically progresses through a GTP-dependent recruitment of coats to membranes. Here we explore the role of ADP ribosylation factor (ARF) 1, one of the GTP-binding proteins that recruit coats, in the production of neuroendocrine synaptic vesicles (SVs) from PC12 cell membranes. Brefeldin A (BFA) strongly and reversibly inhibited SV formation in vivo in three different PC12 cell lines expressing vesicle-associated membrane protein-T Antigen derivatives. Other membrane traffic events remained unaffected by the drug, and the BFA effects were not mimicked by drugs known to interfere with formation of other classes of vesicles. The involvement of ARF proteins in the budding of SVs was addressed in a cell-free reconstitution system (Desnos, C., L. Clift-O'Grady, and R.B. Kelly. 1995. J. Cell Biol. 130:1041-1049). A peptide spanning the effector domain of human ARF1 (2-17) and recombinant ARF1 mutated in its GTPase activity, both inhibited the formation of SVs of the correct size. During in vitro incubation in the presence of the mutant ARFs, the labeled precursor membranes acquired different densities, suggesting that the two ARF mutations block at different biosynthetic steps. Cell-free SV formation in the presence of a high molecular weight, ARF-depleted fraction from brain cytosol was significantly enhanced by the addition of recombinant myristoylated native ARF1. Thus, the generation of SVs from PC12 cell membranes requires ARF and uses its GTPase activity, probably to regulate coating phenomena.