Method for preparing cultures of central neurons: cytochemical and immunochemical studies

Targeting of endoplasmic reticulum-associated proteins to axons and dendrites in rotavirus-infected neurons

To analyze sorting and compartmentalization of molecules in neuronal endomembranes, the distribution of endogenous proteins associated with the endoplasmic reticulum (ER), intermediate compartment, the Golgi apparatus in cultures of dorsal root ganglion (DRG), and hippocampal neurons was compared with that of newly synthesized ER-associated rotavirus proteins. The endogenous ER-retained immunoglobulin heavy chain binding protein, protein disulfide isomerase, and a peptide containing the KDEL amino acid sequence appeared in the soma and dendrites up to their first branching, but not in axons. However, two other endogenous ER-associated proteins, calreticulin and calnexin, occurred in axons as well as in the somatodendritic domains. The ER-associated rotavirus proteins, VP7 and NSP4, were widely distributed in cell bodies and dendrites. The former appeared also in axons and its localization partially overlapped with that of calreticulin and calnexin. One intermediate compartment protein, ER-Golgi-intermediate compartment-protein-53 (ERGIC-53), extended beyond the first division of the dendrites and did not, as the small guanosine 5'-triphosphate (GTP)-binding protein rab2, appear in axons. The location of rab2 to small vesicles was distinct from that of rotavirus VP7. Cis/medial Golgi cistern proteins were restricted to the cell bodies and proximal dendrites. This study emphasizes the marked heterogeneity in the targeting to axons and dendrites of proteins associated with ER and intermediate compartments. Therefore, the composition of axonal ER-retained molecules differs from that in the soma and this variation may reflect differences in functions between the ER compartments. Viral proteins are useful reporters for such heterogeneities and rotavirus VP7 may be a tool to reveal sorting signals for targeting of vesicular proteins to axons via a nonclassical Golgi-independent mechanism. Such signals may also determine viral targeting to different regions of the brain.

Specific interactions between retrovirus Env and Gag proteins in rat neurons

In this work we have studied the intracellular localization properties of the Gag and Env proteins of Moloney murine leukemia virus (MLV) and human immunodeficiency virus (HIV) in dorsal root ganglion (DRG) neurons of rat. These neurons form thick bundles of axons, which facilitates protein localization studies by immunofluorescence analyses. When such neuron cultures were infected with recombinant Semliki Forest virus particles carrying the gag genes of either retrovirus, the expressed Gag proteins were localized to both the somatic and the axonal regions of the DRG neurons. In contrast, the Env proteins were confined only to the somatic region. When the Gag and Env proteins were coexpressed, the Gag proteins were also excluded from the axons. This effect of the Env proteins was shown to be dependent on the concentration of the Gag proteins in the neuron and also to be specific for homologous pairs of retrovirus proteins. Therefore, the results suggest that there are specific interactions between the Env and the Gag proteins of MLV and HIV in the DRG neurons.

Dengue virus infects mouse cultured neurons but not astrocytes

Cultures of mouse brain cells containing a high proportion either neurons or astrocytes were inoculated with various strains of dengue virus. At analysis by double immunofluorescence, all strains of dengue virus studied were tropic for neurons. In no case were the astrocytes infected. Only approximately 10% of all neurons were infected, suggesting that the virus may enter the cell through a membrane receptor present only in a subgroup of neurons.

Microtubule-associated protein 2 appears in axons of cultured dorsal root ganglia and spinal cord neurons after rotavirus infection

The immunohistochemical distribution of microtubule-associated protein 2 (MAP2), being normally restricted to nerve cell bodies and dendrites, became altered in rat dorsal root ganglia and spinal cord neurons in cultures infected with rhesus rotavirus. MAP2 appeared in axons of both sources of neurons as displayed with monoclonal antibodies to MAP2a + b and MAP2a + b + c at 48 hr post-infection (p.i.). Other cytoskeletal elements, i.e., tau, MAP1, MAP5, neurofilament, actin, and tubulin, did not reveal any alterations in the rotavirus-infected neurons. One of the rotavirus cytosolic proteins, the inner capsid protein vp6, was expressed in axons at 48 hr p.i. simultaneously with the appearance of MAP2, while two other viral proteins, vp4 and NS28, remained in the nerve cell bodies. By quantitative enzyme-linked immunosorbent assay (ELISA) a binding of single-shelled rotaviruses, which express vp6 on their surfaces, to purified MAP2 was found. There was no binding of these viral particles to tau or tubulin proteins. This study indicates that a selective interaction between certain viral and neuronal cytoskeletal proteins can occur and that a non-cytolytic viral infection can cause alterations in the polarized sorting of neuronal proteins.

The endoplasmic reticulum-associated VP7 of rotavirus is targeted to axons and dendrites in polarized neurons

Rotavirus, which matures and is retained in the endoplasmic reticulum, was used to examine how polarized dorsal root ganglion and spinal cord neurons distributed cytoplasmic and endoplasmic reticulum-associated proteins. A remarkable observation was that NS28, a trans-endoplasmic reticulum-membrane protein which functions as a receptor for budding particles, remained in the cell body during the whole course of infection (48 h) while the VP7 glycoprotein, which is endoplasmic reticulum associated and usually retained in the endoplasmic reticulum, was targeted to axons already 4 h post infection. VP7 was furthermore transported in an endo-beta-N-acetylglucosaminidase H sensitive form through the secretory pathway. The segregated appearances of NS28 and the endo-beta-N-acetylglucosaminidase H sensitive VP7 indicate that VP7 enters a transport compartment separate from NS28. Brefeldin A treatment rapidly disintegrated the Golgi apparatus of the neurons and rapidly blocked axonal transport of Sendai virus glycoproteins, while axonal transport of rotavirus VP7 was not blocked, suggesting that VP7 uses an intracellular pathway in neurons which does not involve the Golgi apparatus.

Interferon-gamma-like immunoreactivity in developing rat spinal ganglia neurons in vivo and in vitro

Interferon-gamma-like immunoreactivity was observed in a subpopulation of 16-day-old embryonic rat spinal ganglion neurons using two monoclonal antibodies directed against different epitopes of recombinant interferon-gamma. During ontogenesis both in vivo and in vitro, it was found that the strong immunoreactivity was confined to small neurons when neurons become morphologically distinct on the basis of size. In vivo, the interferon-gamma-immunoreactive neurons started to express major histocompatibility complex class I antigens after the first postnatal week, while in vitro no such antigen could be detected. A quantitative Elisa method was developed to determine the levels of major histocompatibility complex class I and interferon-gamma in vitro, whereby increased amounts of major histocompatibility complex class I antigen was detected after exposing the cultures to recombinant interferon-gamma and Sendai virus. Sendai virus also caused a small increase in interferon-gamma with a peak about 12 hours after infection. The in vitro system will be used to study further the role of the putative neuronal interferon-gamma-like molecule in the regulation of cell growth, for induction of major histocompatibility complex antigens and in virus infection of sensory neurons.

Segregation of viral structural proteins in cultured neurons of rat spinal ganglia and cord

Cultured spinal ganglion and spinal cord neurons were used to examine the intraneuronal distribution of five structural proteins of Sendai virus by immunohistochemistry. In spinal ganglion cells the internal, cytosolic viral proteins (the nucleocapsid, polymerase and matrix proteins) were confined to the perikarya, while the envelope glycoproteins (the haemagglutinin-neuraminidase and fusion proteins) also appeared in the axon-like processes. All five proteins occurred in the dendrite-like processes of spinal cord neurons. In both types of neuron the cytosolic viral proteins showed a pattern of distribution similar to that observed for the microtubule-associated protein MAP2. The segregated occurrence of the viral envelope and cytosolic proteins in axons may prevent virus assembly in axons and limit long-distance spread of paramyxoviruses in the nervous system.

Low serum levels of nerve growth factor in diabetic neuropathy

Several structural and functional similarities between nerve growth factor and insulin have been described. Diabetes mellitus, a disease with absolute or relative deficiency of insulin is frequently associated with peripheral neuropathy whose physiopathological mechanisms are obscure. In this study, we measured serum levels of NGF in 18 patients with diabetic neuropathy and 9 healthy controls; patients with diabetic neuropathy had lower levels of NGF than controls (p less than 0.01). When patients were separated in two groups according to degree of impairment of motor nerve conduction velocity, those with more than 10% of impairment, had lower levels of NGF than those with less than 10% of impairment, or controls. It was found a correlation between NGF levels and decrease of motor nerve conduction velocity; then, diabetic neuropathy seems to be associated to low serum levels of NGF, pointing out a possible role of NGF in the pathology of diabetic neuropathy.

A normal human brain cell aggregate model for neurobiological studies

A new in vitro model of normal human brain has been developed in which fetal human brain cells form three-dimensional aggregates that can be maintained for up to 60 days in culture. Cells appear fully differentiated at the time of initiation in culture; the predominant cells identified were astrocytes, neurons, and oligodendrocytes with myelin, with occasional ependymal cells and macrophages. The specific arrangement and numbers of neural cells within aggregates differed among brain specimens. Cell kinetics studies detected DNA synthesis throughout the culture interval. Aggregates cocultured with a human malignant glioma cell line (U251-MG) were progressively invaded by tumor cells. In aggregates infected with human cytomegalovirus (CMV), intracellular viral replication and morphologic changes characteristic of human brain infection with this pathogen were seen. This model of brain aggregates should prove valuable for multidisciplinary studies in human neurobiology, particularly in the fields of developmental neurobiology, neuro-oncogenesis, tumor cell invasion, and species-specific viral infection of the central nervous system.

Monoclonal antibody analysis of keratin expression in the central nervous system

A monoclonal antibody directed against a 65-kDa brain protein demonstrates an epitope found in keratin from human epidermis. By indirect immunofluorescence, the antibody decorates intracytoplasmic filaments in a subclass of astrocytes and Purkinje cells of adult hamster brain. Double-label immunofluorescence study using antibody to glial fibrillary acidic protein and this antibody reveals the 65-kDa protein to be closely associated with glial filaments in astrocytes of fetal mouse brain cultures. Immunoblot analysis of purified human epidermal keratin and hamster brain homogenate confirms the reactivity of this antibody to epidermal keratin polypeptides. All the major epidermal keratins were recognized by this antibody. It did not bind to the remaining major intermediate filament proteins. These findings suggest that monoclonal antibody 34C9 recognizes a cytoskeletal structure connected with intermediate filaments. In addition, the monoclonal antibody demonstrates that epidermal keratins share an epitope not only among themselves but also with a "neural keratin."

Influence of meningeal cells on the proliferation and maturation of rat neuroblasts in culture

Neuronal cells were obtained by dissociating cells from the cerebral hemispheres of rat embryos (10 to 17-day-old), either cleaned entirely or only partially of their meningeal membranes. These cells were seeded on poly-lysine-coated Petri dishes in serum-containing medium. The cultures most enriched in neuronal cells were obtained from brains of 13- to 15-day-old embryos and after 2 h, the culture medium was switched to Dulbecco's modified Eagle's medium, without serum, supplemented with the N1 supplements as described by Bottenstein et al. (1980). The proliferation of neuroblasts from 13-day-old embryos in the presence or absence of meningeal cells was studied by using a combination of tritiated thymidine autoradiography and immuno-staining against neurofilament proteins. The neuroblasts seem to proliferate during the first 3 days. The proliferative activity was further enhanced in the presence of meningeal cells. The glioblasts multiply only after a period of one week in culture conditions as observed here. The subsequent development of the neuroblasts was followed over a period of 4 weeks and the ultrastructural appearance of these cells was investigated at 2 and 3 weeks. In the presence of meningeal cells, many neurons, intensely stained for neurofilament proteins, survived for 21 days, while in control cultures they underwent massive degeneration after 2 weeks. Synapses with numerous clear vesicles were abundant in cultures grown under the influence of meningeal cells; they were rare and possessed few vesicles in control cultures. The data indicate that meningeal cells affect the proliferation and maturation of rat neuroblasts in culture.

Tissue culture analysis of neurite outgrowth in the presence and absence of serum: possible relevance for central nervous system regeneration

A tissue culture model has been developed to examine the hypothesis that axons can only regenerate when their growing tips are surrounded by extracellular fluid containing proteins derived from the blood. Fetal rat cerebral explants were cultured in serum medium for 10 days, followed by serum-free (SF) medium (from which serum had been removed) until 18 days in vitro (DIV). All explants cultured in serum medium for 0-10 DIV exhibited greater than 77% neurite viability (neurite viability ratio, NVR, 3.10). This degree of neurite viability was maintained for those explants exposed to serum until 18 DIV (NVR 2.82 at 18 DIV). By contrast, explants maintained in SF medium from 10-18 DIV had a much lower NVR, which, by 18 DIV, had declined to 0.30 (7.5% viability). Transmission electron microscopic analysis of explants fixed at 18 DIV confirmed these phase-contrast results and also showed a predominance of axonal profiles within the neurite population. In the center of explants, tissue viability was in excess of 75% in both the serum and SF media, suggesting that serum is of primary importance for axonal extension rather than neuronal survival. These data strengthen the hypothesis that blood-derived proteins may be needed for prolonged regeneration.

The 200- and 150-kDa neurofilament proteins react with IgG autoantibodies from chimpanzees with kuru or Creutzfeldt-Jakob disease; a 62-kDa neurofilament-associated protein reacts with sera from sheep with natural scrapie

Sera from 46 chimpanzees with spongiform encephalopathy (18 kuru, 28 Creutzfeldt-Jakob disease) and sera from 12 sheep with natural scrapie were tested for reactivity with immunoblots of neurofilament preparations obtained from mouse brain. The sera from the chimpanzees reacted mainly with the 200- and 150-kDa proteins of the neurofilament triplet and less frequently with the 70-kDa component of the triplet and with a 62-kDa neurofilament-associated protein. In contrast, the sera of sheep with natural scrapie reacted exclusively against the 62-kDa protein. The specificity of the reactions was established by comparison of sera reactivities with those of rabbit and mouse polyclonal antibodies and mouse monoclonal antibody to neurofilament proteins.

The 200- and 150-kDa neurofilament proteins react with IgG autoantibodies from patients with kuru, Creutzfeldt-Jakob disease, and other neurologic diseases

Sera from 65 patients with spongiform virus encephalopathies (29 with kuru, 36 with Creutzfeldt-Jakob disease), 79 with other neurologic diseases, and 65 control subjects were examined for reactivity in immunoblots of preparations of myelinated axons and neurofilaments from mouse brain. The sera reacted most frequently with the 200-kDa and 150-kDa neurofilament proteins and less frequently with the 70-kDa neurofilament protein and a 62-kDa neurofilament-associated protein. The sera reacted with the same proteins as those which reacted with rabbit and mouse polyclonal antibodies and mouse monoclonal antibody to neurofilament proteins. Serum reactions were also seen with Trixon X-100 extracts of chimpanzee brain and bovine spinal cord but not with Triton extracts of liver, kidney, and muscle.

Parvalbumin, a neuronal protein in brain cell cultures

Dissociated brain cell cultures were derived from 14-day-old embryonic as well as from newborn mice. The cells were grown in a medium containing 10% fetal calf serum. Indirect immunofluorescence was performed using antisera directed against the Ca2+-binding protein parvalbumin (Mr 12,000). In embryonic cultures a large proportion of cells was intensely stained by antiparvalbumin . In double-labelling experiments involving the simultaneous application of antisera against parvalbumin and the neuron-specific enolase, the enolase-containing cells were also parvalbumin-positive and both antisera revealed identical intracellular staining patterns. Conversely, almost no parvalbumin- and enolase-positive cells were present in cultures derived from newborn mice. However, in these cultures many cells were immunoreactive toward the myelin basic protein, an accepted marker for oligodendrocytes. The presence of parvalbumin within the embryonic brain cell cultures was confirmed by analyses of the culture extracts (4 mM EDTA, pH 7.5) by HPLC on reverse-phase supports, two-dimensional polyacrylamide gel electrophoresis, and immunoblotting. The present study suggests that in mouse brain cell cultures, parvalbumin is localized in neurons.

Proinsulin-like material in mouse foetal brain cell cultures

Two main forms of immunoreactive insulin have been identified in cultures of foetal mouse brain using HPLC and gel filtration. The major component which resembled proinsulin was converted by trypsin to the minor form which was similar to authentic pancreatic insulin in chromatographic behaviour. Both components showed immunological properties comparable to insulin and proinsulin including sensitivity of the former to reduction and alkylation.

Immunoreactive insulin from mouse brain cells in culture and whole rat brain

Foetal mouse brain cells were cultured as described previously [Sotelo, Gibbs, Gajdusek, Toh & Wurth (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 653-657] without added insulin and without foetal calf serum after 12 days in culture. Examination by phase-contrast microscopy showed that these modifications did not appear to affect growth and development of the cells adversely. Silver impregnation of the cultures and indirect immunofluorescence following reaction with tetanus toxin showed that a high proportion of the cells resembled neurones. Analysis of concentrated culture medium by radioimmunoassay and high-pressure liquid chromatography (h.p.l.c.) revealed that the cells produced two main forms of immunoreactive insulin which differed from authentic pancreatic insulin in retention time. Immunoreactive somatostatin was also produced in culture and this was resolved into at least three forms by h.p.l.c. Immunoreactive insulin was also extracted from whole rat brain by using two published procedures. The method of Havrankova, Schmechel, Roth & Brownstein [Proc. Natl. Acad. Sci. U.S.A. (1978) 75, 5737-5741] consistently gave greater yields of insulin than did that of Eng & Yalow [Diabetes (1980) 29, 105-109] and the concentration was about three times that of plasma. The extracted insulin was further characterized by h.p.l.c. in each case and was found to behave like authentic pancreatic insulin. The production of insulin and somatostatin by foetal mouse brain cells in culture suggests that they may be a useful model system for studies of neuropeptide biosynthesis.

Long-term culture of neurones from human cerebral cortex in serum-free medium

A method for cultivating neurones from the fetal human central nervous system in the absence of glial cells is described. Brain cells from 15-18-week-old human fetuses are plated on polylysine-coated surfaces and grown in a serum-free hormonally-defined medium. About 98% of the cells were identified as neurones using tetanus toxin as a marker. The cultures survive for up to 7 weeks and develop an extremely complex network of neurites.

Heterogeneic autoantibody against neurofilament protein in the sera of animals with experimental kuru and Creutzfeldt-Jakob disease and natural scrapie infection

Heterogeneic autoantibodies against axonal neurofilament proteins of mature mouse neurons grown in vitro were detected by the indirect immunofluorescence technique in 12.7% (9 of 71) of the sera from nonhuman primates infected with kuru, in 14.5% (17 of 117) and 4% (1 of 25), respectively, of the sera from nonhuman primates and laboratory rodents infected with Creutzfeldt-Jakob disease, and in 35% (7 of 20) of the sera from sheep naturally infected with scrapie. Autoantibody titers ranged from 1:16 to 1:512 in Creutzfeldt-Jakob disease-infected animals, 1:32 to 1:512 in kuru-infected animals, and 1:64 to 1:1,024 in sheep with natural scrapie. The sera from 11 monkeys and 17 hamsters infected with scrapie and from 19 chimpanzees inoculated with brain tissues from humans with other neurological diseases did not contain autoantibodies. Of the 41 chimpanzees with Creutzfeldt-Jakob disease, 6 had autoantibodies against neurofilament proteins before experimental inoculation, whereas 6 others developed autoantibodies after inoculation, 4 developed autoantibodies during the asymptomatic phase, and 2 developed autoantibodies during the terminal clinical phase. Of the 48 chimpanzees with kuru, 2 had autoantibodies before inoculation, 6 developed autoantibodies after inoculation, 3 developed autoantibodies during the asymptomatic phase, and 3 developed autoantibodies during the terminal clinical phase. Among the normal nonhuman primate controls, 4.6% (9 of 195) had autoantibodies. In contrast, no autoantibodies were detected in 49 control rodents and 13 control sheep. The increased incidence of autoantibodies against neurofilament proteins in animals with kuru, Creutzfeldt-Jakob disease, and scrapie constitutes the first evidence of an immunological reaction in this group of atypical infections caused by unconventional viruses and suggests that neurofilaments may be involved in pathogenesis.

Characterization of a factor that promotes neurite outgrowth: evidence linking activity to a heparan sulfate proteoglycan

Rat sympathetic neurons, plated onto extracellular matrix produced by cultured bovine corneal endothelial cells, rapidly extended neurites in the absence of nerve growth factor (NGF). The response was unaffected by antiserum to NGF. Rapid outgrowth also occurred when sympathetic neurons were plated onto polylysine-coated surfaces that had been exposed to serum-free medium conditioned by corneal endothelial cells (CMSF). A response was seen even when the neurons were cultured without serum. When plated onto a polylysine-coated dish treated with CMSF over half its surface, only the neurons on the treated half extended neurites. The active factor in CMSF was destroyed by trypsin, acid (pH 1.6), base (pH 12.7), or heating to 80 degrees C; it was stable to heating to 60 degrees C, collagenase, deoxyribonuclease, and neuraminidase. The factor elutes just after the void volume of a Sepharose 6B column. In associative cesium chloride gradients, it sediments as a peak centered at a density of 1.36-1.37, corresponding to a peak of material that can be biosynthetically labeled with [35S]sulfate or [3H]leucine. Material from this fraction was inactivated by heparinase, but not chondroitinase ABC, implying that a heparin sulfate proteoglycan is essential for the factor's activity. Inactivation by contaminants in the heparinase preparation was ruled out. Further purification indicated that the active factor may exist as an aggregate containing a heparin sulfate proteoglycan and other molecules. CMSF also promoted neurite outgrowth by other types of neurons. Furthermore, a variety of cell types were shown to produce factors similar to that in CMSF.

Growth of adult superior cervical ganglion explants in serum-free media

Neurite outgrowth from explants of superior cervical ganglion from adult rats can be achieved in a serum-free medium. Extensive neurite outgrowth occurred from ganglion explants maintained in Eagle's minimum essential medium supplemented with either 10% (V/V) fetal calf serum or 1% (W/V) bovine serum albumin and nerve growth factor. After one week in culture, the ATP content of explants maintained in the serum-free medium was slightly higher than that noted in explants cultured in the presence of fetal calf serum and amounts of phosphocreatine were significantly lower. Despite these differences in high energy phosphate content, the abundance and morphology of neuritic outgrowth were essentially the same from explants cultured in the two types of media. Comparable activities of a number of NADP+-dependent dehydrogenases were noted in explants maintained in the two types of media. Increases in the activities of the oxidative enzymes of the pentose pathway, which occur in axotomized ganglia in vivo, were observed in the cultured ganglion explants. NADP+-dependent isocitrate dehydrogenase activity remained constant in ganglion explants in vitro, and measurements of this activity were employed in a new method to quantitate neurite outgrowth. The activity of isocitrate dehydrogenase in lyophilized neurite processes that had grown out onto a Millipore filter substrate correlated well with visual estimates of neuritic outgrowth. Substitution of delipidated for normal bovine serum albumin in the growth medium resulted in a significant decrease in neuritic outgrowth from ganglion explants from both adult and weanling rats. Addition of fatty acids to media containing delipidated bovine serum albumin enhanced neuritic outgrowth in explants of weanling rats. Thus, lipophilic substances bound to bovine serum albumin including fatty acids appear necessary for optimal growth of neurites from explants of the rat superior cervical ganglion.

Autoantibodies against axonal neurofilaments in patients with Kuru and Creutzfeldt-Jakob disease

The serums of some patients with subacute spongiform encephalopathies contain an autoantibody in higher titer against a normal fibrillar protein within the axon of mature central neurons in culture. The morphological features of this neurofilament, as demonstrated by immunofluorescence and immunoperoxidase staining, and the partial characterization of the antibody are described. The detection of this hetero-specific autoantibody is the first evidence of an immune reaction in the spongiform encephalopathies.

Neurovirulence mutant of vesicular stomatitis virus with an altered target cell tropism in vivo

Intracerebral infection of weanling Swiss mice with a temperature-sensitive (ts) mutant of vesicular stomatitis virus (VSV), ts pi364, resulted in a unique neuropathological syndrome not previously described with other VSV mutants. Mice infected with wild-type VSV died from an acute encephalitis characterized by neuronal necrosis and efficient virus replication in both brain and spinal cord. In contrast, with VSV ts pi364, the most prominent histopathological feature was destruction of the ependyma of the lateral ventricles. Virus antigen was also limited to the leptomeninges and the lateral ventricles. Infected mice survived and developed hydrocephalus. Replication of ts pi364 in the brain was 10- to 100- fold less than that of wild-type VSV, and appearance of virus in the spinal cord was delayed. VSV ts pi364 was isolated from mouse cells persistently infected with VSV. Another VSV ts pi mutant, isolated from the same persistent infection, behaved in vivo like wild-type VSV, even though both mutants were very similar in plaque size, reversion frequency, cut-off temperature, and synthesis of virus-specific proteins at semipermissive temperature. These results strongly suggest that VSV ts pi364 has a second, non-ts mutation which results in a restricted target cell range in vivo; wild-type VSV can infect both neurons and ependymal cells, whereas ts pi364 does not replicate in neurons.