Antibodies to heavy neurofilament subunit detect a subpopulation of damaged ganglion cells in retina

Pan-retinal ganglion cell markers in mice, rats, and rhesus macaques

Univocal identification of retinal ganglion cells (RGCs) is an essential prerequisite for studying their degeneration and neuroprotection. Before the advent of phenotypic markers, RGCs were normally identified using retrograde tracing of retinorecipient areas. This is an invasive technique, and its use is precluded in higher mammals such as monkeys. In the past decade, several RGC markers have been described. Here, we reviewed and analyzed the specificity of nine markers used to identify all or most RGCs, i.e., pan-RGC markers, in rats, mice, and macaques. The best markers in the three species in terms of specificity, proportion of RGCs labeled, and indicators of viability were BRN3A, expressed by vision-forming RGCs, and RBPMS, expressed by vision- and non-vision-forming RGCs. NEUN, often used to identify RGCs, was expressed by non-RGCs in the ganglion cell layer, and therefore was not RGC-specific. γ-SYN, TUJ1, and NF-L labeled the RGC axons, which impaired the detection of their somas in the central retina but would be good for studying RGC morphology. In rats, TUJ1 and NF-L were also expressed by non-RGCs. BM88, ERRβ, and PGP9.5 are rarely used as markers, but they identified most RGCs in the rats and macaques and ERRβ in mice. However, PGP9.5 was also expressed by non-RGCs in rats and macaques and BM88 and ERRβ were not suitable markers of viability.

Nerve fibre layer degeneration and retinal ganglion cell loss long term after optic nerve crush or transection in adult mice

We have investigated the long term effects of two different models of unilateral optic nerve (ON) lesion on retinal ganglion cells (RGCs) and their axons, in the injured and contralateral retinas of adult albino mice. Intact animals were used as controls. The left ON was intraorbitally crushed or transected at 0.5 mm from the optic disk and both retinas were analyzed at 2, 3, 5, 7, 14, 30, 45 or 90 days after injury. RGCs were immunoidentified with anti-Brn3a, and their axons with anti-highly phosphorylated axonal neurofilament subunit H (pNFH). After both lesions, RGC death in the injured retinas is first significant at day 3, and progresses quickly up to 7 days slowing down till 90 days. In the same retinas, the anatomical loss of RGC axons is not evident until day 30. However, by two days after both lesions there are changes in the expression pattern of pNFH: axonal beads, axonal club- or bulb-like formations, and pNFH⁺RGC somas. The number of pNFH⁺RGC somata peak at day 5 after either lesion and is significantly higher than in intact retinas at all time points. pNFH⁺RGC somata are distributed across the retina, in accordance with the pattern of RGC death which is diffuse and homogenous. In the contralateral retinas there is no RGC loss, but there are few pNFH⁺RGCs from day 2 to day 90. In conclusion, in albino mice, axotomy-induced RGC death precedes the loss of their intraretinal axons and occurs in two phases, a rapid and a slower, but steady, one. Injured retinas show similar changes in the pattern of pNFH expression and a comparable course of RGC loss.

Monoclonal antibody Py recognizes neurofilament heavy chain and is a selective marker for large diameter neurons in the brain

Almost 30 years ago, the monoclonal antibody Py was developed to detect pyramidal neurons in the CA3 region of the rat hippocampus. The utility of this antibody quickly expanded when several groups discovered that it could be used to identify very specific populations of neurons in the normal, developing, and diseased or injured central nervous system. Despite this body of literature, the identity of the antigen that the Py antibody recognizes remained elusive. Here, immunoprecipitation experiments from the adult rat cortex identified the Py antigen as neurofilament heavy chain (NF-H). Double immunolabeling of sections through the rat brain using Py and NF-H antibodies confirmed the identity of the Py antigen, and reveal that Py/NF-H+ neurons appear to share the feature of being particularly large in diameter. These include the neurons of the gigantocellular reticular formation, pyramidal neurons of layers II/III and V of the cortex, cerebellar Purkinje neurons as well as CA3 pyramidal neurons. Taken together, this finding gives clarity to past work using the monoclonal Py antibody, and immediately expands our understanding of the importance of NF-H in neural development, functioning, and disease.

The RNA binding protein RBPMS is a selective marker of ganglion cells in the mammalian retina

There are few neurochemical markers that reliably identify retinal ganglion cells (RGCs), which are a heterogeneous population of cells that integrate and transmit the visual signal from the retina to the central visual nuclei. We have developed and characterized a new set of affinity-purified guinea pig and rabbit antibodies against RNA-binding protein with multiple splicing (RBPMS). On western blots these antibodies recognize a single band at 〜24 kDa, corresponding to RBPMS, and they strongly label RGC and displaced RGC (dRGC) somata in mouse, rat, guinea pig, rabbit, and monkey retina. RBPMS-immunoreactive cells and RGCs identified by other techniques have a similar range of somal diameters and areas. The density of RBPMS cells in mouse and rat retina is comparable to earlier semiquantitative estimates of RGCs. RBPMS is mainly expressed in medium and large DAPI-, DRAQ5-, NeuroTrace- and NeuN-stained cells in the ganglion cell layer (GCL), and RBPMS is not expressed in syntaxin (HPC-1)-immunoreactive cells in the inner nuclear layer (INL) and GCL, consistent with their identity as RGCs, and not displaced amacrine cells. In mouse and rat retina, most RBPMS cells are lost following optic nerve crush or transection at 3 weeks, and all Brn3a-, SMI-32-, and melanopsin-immunoreactive RGCs also express RBPMS immunoreactivity. RBPMS immunoreactivity is localized to cyan fluorescent protein (CFP)-fluorescent RGCs in the B6.Cg-Tg(Thy1-CFP)23Jrs/J mouse line. These findings show that antibodies against RBPMS are robust reagents that exclusively identify RGCs and dRGCs in multiple mammalian species, and they will be especially useful for quantification of RGCs.

Melanopsin ganglion cells are the most resistant retinal ganglion cell type to axonal injury in the rat retina

We report that the most common retinal ganglion cell type that remains after optic nerve transection is the M1 melanopsin ganglion cell. M1 ganglion cells are members of the intrinsically photosensitive retinal ganglion cell population that mediates non-image-forming vision, comprising ∼2.5% of all ganglion cells in the rat retina. In the present study, M1 ganglion cells comprised 1.7±1%, 28±14%, 55±13% and 82±8% of the surviving ganglion cells 7, 14, 21 and 60 days after optic nerve transection, respectively. Average M1 ganglion cell somal diameter and overall morphological appearance remained unchanged in non-injured and injured retinas, suggesting a lack of injury-induced degeneration. Average M1 dendritic field size increased at 7 and 60 days following optic nerve transection, while average dendritic field size remained similar in non-injured retinas and in retinas at 14 and 21 days after optic nerve transection. These findings demonstrate that M1 ganglion cells are more resistant to injury than other ganglion cell types following optic nerve injury, and provide an opportunity to develop pharmacological or genetic therapeutic approaches to mitigate ganglion cell death and save vision following optic nerve injury.

IOP induces upregulation of GFAP and MHC-II and microglia reactivity in mice retina contralateral to experimental glaucoma

Background

Ocular hypertension is a major risk factor for glaucoma, a neurodegenerative disease characterized by an irreversible decrease in ganglion cells and their axons. Macroglial and microglial cells appear to play an important role in the pathogenic mechanisms of the disease. Here, we study the effects of laser-induced ocular hypertension (OHT) in the macroglia, microglia and retinal ganglion cells (RGCs) of eyes with OHT (OHT-eyes) and contralateral eyes two weeks after lasering.

Methods

Two groups of adult Swiss mice were used: age-matched control (naïve, n = 9); and lasered (n = 9). In the lasered animals, both OHT-eyes and contralateral eyes were analyzed. Retinal whole-mounts were immunostained with antibodies against glial fibrillary acid protein (GFAP), neurofilament of 200kD (NF-200), ionized calcium binding adaptor molecule (Iba-1) and major histocompatibility complex class II molecule (MHC-II). The GFAP-labeled retinal area (GFAP-RA), the intensity of GFAP immunoreaction (GFAP-IR), and the number of astrocytes and NF-200 + RGCs were quantified.

Results

In comparison with naïve: i) astrocytes were more robust in contralateral eyes. In OHT-eyes, the astrocyte population was not homogeneous, given that astrocytes displaying only primary processes coexisted with astrocytes in which primary and secondary processes could be recognized, the former having less intense GFAP-IR (P < 0.001); ii) GFAP-RA was increased in contralateral (P <0.05) and decreased in OHT-eyes (P <0.001); iii) the mean intensity of GFAP-IR was higher in OHT-eyes (P < 0.01), and the percentage of the retinal area occupied by GFAP+ cells with higher intensity levels was increased in contralateral (P = 0.05) and in OHT-eyes (P < 0.01); iv) both in contralateral and in OHT-eyes, GFAP was upregulated in Müller cells and microglia was activated; v) MHC-II was upregulated on macroglia and microglia. In microglia, it was similarly expressed in contralateral and OHT-eyes. By contrast, in macroglia, MHC-II upregulation was observed mainly in astrocytes in contralateral eyes and in Müller cells in OHT-eyes; vi) NF-200+RGCs (degenerated cells) appeared in OHT-eyes with a trend for the GFAP-RA to decrease and for the NF-200+RGC number to increase from the center to the periphery (r = −0.45).

Conclusion

The use of the contralateral eye as an internal control in experimental induction of unilateral IOP should be reconsidered. The gliotic behavior in contralateral eyes could be related to the immune response. The absence of NF-200+RGCs (sign of RGC degeneration) leads us to postulate that the MHC-II upregulation in contralateral eyes could favor neuroprotection.

Glaucoma-induced degeneration of retinal ganglion cells prevented by hypoxic preconditioning: a model of glaucoma tolerance

Like all cells, neurons adapt to stress by transient alterations in phenotype, an epigenetic response that forms the basis for preconditioning against acute ischemic injury in the central nervous system. We recently showed that a modified repetitive hypoxic preconditioning (RHP) regimen significantly extends the window of ischemic tolerance to acute retinal ischemic injury from days to months. The present study was undertaken to determine if this uniquely protracted neuroprotective phenotype would also confer resistance to glaucomatous neurodegeneration. Retinal ganglion cell death at somatic and axonal levels was assessed after both 3 and 10 wks of sustained intraocular hypertension in an adult mouse model of inducible, open-angle glaucoma, with or without RHP before intraocular pressure elevation. Loss of brn3-positive ganglion cell soma after 3 wks of experimental glaucoma, along with increases in several apoptotic endpoints, were all significantly and robustly attenuated in mice subjected to RHP. Soma protection by RHP was also confirmed after 10 wks of intraocular hypertension by brn3 and SMI32 immunostaining. In addition, quantification of axon density in the postlaminar optic nerve documented robust preservation in RHP-treated mice, and neurofilament immunostaining also revealed preconditioning-induced improvements in axon integrity/survival in both retina and optic nerve after 10 wks of experimental glaucoma. This uniquely protracted period of phenotypic change, established in retinal ganglion cells by the activation of latent antiapoptotic, prosurvival mechanisms at both somatic and axonal levels, reflects a novel form of inducible neuronal plasticity that may provide innovative therapeutic targets for preventing and treating glaucoma and other neurodegenerative diseases.

Retinal ganglion cell loss in a rat ocular hypertension model is sectorial and involves early optic nerve axon loss

PURPOSE

Previous analyses of the DBA/2J mouse glaucoma model show a sectorial degeneration pattern suggestive of an optic nerve head insult. In addition, there are large numbers of retinal ganglion cells (RGCs) that cannot be retrogradely labeled but maintain RGC gene expression, and many of these have somatic phosphorylated neurofilament labeling. Here the authors further elucidate these features of glaucomatous degeneration in a rat ocular hypertension model.

METHODS

IOP was elevated in Wistar rats by translimbal laser photocoagulation. Retina whole mounts were analyzed for Sncg mRNA in situ hybridization, fluorogold (FG) retrograde labeling, and immunohistochemistry for phosphorylated neurofilaments (pNF) at 10 and 29 days after IOP increase. A novel automatic method was used to estimate axon numbers in plastic sections of optic nerves.

RESULTS

Sncg mRNA was confirmed as a specific marker for RGCs in rat. Loss of RGCs after IOP elevation occurred in sectorial patterns. Sectors amid degeneration contained RGCs that were likely disconnected because these had pNF in their somas and dendrites, were not labeled by FG, and were associated with reactive plasticity within the retina. Most of the axon loss within the optic nerve already occurred by 10 days after the onset of IOP elevation.

CONCLUSIONS

These data demonstrate that the pattern of RGC loss after laser-induced ocular hypertension in rats is similar to that previously reported in DBA/2J mice. The results support the view that in glaucoma RGC axons are damaged at the optic nerve head and degenerate within the optic nerve before there is loss of RGC somas.

A Thy1-CFP DBA/2J mouse line with cyan fluorescent protein expression in retinal ganglion cells

A DBA/2J (D2) transgenic mouse line with cyan fluorescent protein (CFP) reporter expression in ganglion cells was developed for the analysis of ganglion cells during progressive glaucoma. The Thy1-CFP D2 (CFP-D2) line was created by congenically breeding the D2 line, which develops pigmentary glaucoma, and the Thy1-CFP line, which expresses CFP in ganglion cells. Microsatellite marker analysis of CFP-D2 progeny verified the genetic inclusion of the D2 isa and ipd loci. Specific mutations within these loci lead to dysfunctional melanosomal proteins and glaucomatous phenotype in D2 mice. Polymerase chain reaction analysis confirmed the inclusion of the Thy1-CFP transgene. CFP-fluorescent ganglion cells, 6-20 microm in diameter, were distributed in all retinal regions, CFP processes were throughout the inner plexiform layer, and CFP-fluorescent axons were in the fiber layer and optic nerve head. Immunohistochemistry with antibodies to ganglion cell markers NF-L, NeuN, Brn3a, and SMI32 was used to confirm CFP expression in ganglion cells. Immunohistochemistry with antibodies to amacrine cell markers HPC-1 and ChAT was used to confirm weak CFP expression in cholinergic amacrine cells. CFP-D2 mice developed a glaucomatous phenotype, including iris disease, ganglion cell loss, attrition of the fiber layer, and elevated intraocular pressure. A CFP-D2 transgenic line with CFP-expressing ganglion cells was developed, which has (1) a predominantly D2 genetic background, (2) CFP-expressing ganglion cells, and (3) age-related progressive glaucoma. This line will be of value for experimental studies investigating ganglion cells and their axons in vivo and in vitro during the progressive development of glaucoma.

Ocular hypertension impairs optic nerve axonal transport leading to progressive retinal ganglion cell degeneration

Ocular hypertension (OHT) is the main risk factor of glaucoma, a neuropathy leading to blindness. Here we have investigated the effects of laser photocoagulation (LP)-induced OHT, on the survival and retrograde axonal transport (RAT) of adult rat retinal ganglion cells (RGC) from 1 to 12 wks. Active RAT was examined with fluorogold (FG) applied to both superior colliculi (SCi) 1 wk before processing and passive axonal diffusion with dextran tetramethylrhodamine (DTMR) applied to the optic nerve (ON) 2 d prior to sacrifice. Surviving RGCs were identified with FG applied 1 wk pre-LP or by Brn3a immunodetection. The ON and retinal nerve fiber layer were examined by RT97-neurofibrillar staining. RGCs were counted automatically and color-coded density maps were generated. OHT retinas showed absence of FG+ or DTMR+RGCs in focal, pie-shaped and diffuse regions of the retina which, by two weeks, amounted to, approximately, an 80% of RGC loss without further increase. At this time, there was a discrepancy between the total number of surviving FG-prelabelled RGCs and of DMTR+RGCs, suggesting that a large proportion of RGCs had their RAT impaired. This was further confirmed identifying surviving RGCs by their Brn3a expression. From 3 weeks onwards, there was a close correspondence of DTMR+RGCs and FG+RGCs in the same retinal regions, suggesting axonal constriction at the ON head. Neurofibrillar staining revealed, in ONs, focal degeneration of axonal bundles and, in the retinal areas lacking backlabeled RGCs, aberrant staining of RT97 characteristic of axotomy. LP-induced OHT results in a crush-like injury to ON axons leading to the anterograde and protracted retrograde degeneration of the intraocular axons and RGCs.

Immunohistochemical localization of CNTFRalpha in adult mouse retina and optic nerve following intraorbital nerve crush: evidence for the axonal loss of a trophic factor receptor after injury

Ciliary neurotrophic factor (CNTF) is important for the survival and outgrowth of retinal ganglion cells (RGCs) in vitro. However, in vivo adult RGCs fail to regenerate and subsequently die following axotomy, even though there are high levels of CNTF in the optic nerve. To address this discrepancy, we used immunohistochemistry to analyze the expression of CNTF receptor alpha (CNTFRalpha) in mouse retina and optic nerve following intraorbital nerve crush. In normal mice, RGC perikarya and axons were intensely labeled for CNTFRalpha. At 24 hours after crush, the immunoreactivity normally seen on axons in the nerve was lost near the lesion. This loss radiated from the crush site with time. At 2 days postlesion, labeled axons were not detected in the proximal nerve, and at 2 weeks were barely detectable in the retina. In the distal nerve, loss of axonal staining progressed to the optic chiasm by 7 days and remained undetectable at 2 weeks. Interfascicular glia in the normal optic nerve were faintly labeled, but by 24 hours after crush they became intensely labeled near the lesion. Double labeling showed these to be both astrocytes and oligodendrocytes. At 7 days postlesion, darkly labeled glia were seen throughout the optic nerve, but at 14 days labeling returned to normal. It is suggested that the loss of CNTFRalpha from axons renders RGCs unresponsive to CNTF, thereby contributing to regenerative failure and death, while its appearance on glia may promote glial scarring.

Gene therapy and transplantation in CNS repair: The visual system

Normal visual function in humans is compromised by a range of inherited and acquired degenerative conditions, many of which affect photoreceptors and/or retinal pigment epithelium. As a consequence the majority of experimental gene- and cell-based therapies are aimed at rescuing or replacing these cells. We provide a brief overview of these studies, but the major focus of this review is on the inner retina, in particular how gene therapy and transplantation can improve the viability and regenerative capacity of retinal ganglion cells (RGCs). Such studies are relevant to the development of new treatments for ocular conditions that cause RGC loss or dysfunction, for example glaucoma, diabetes, ischaemia, and various inflammatory and neurodegenerative diseases. However, RGCs and associated central visual pathways also serve as an excellent experimental model of the adult central nervous system (CNS) in which it is possible to study the molecular and cellular mechanisms associated with neuroprotection and axonal regeneration after neurotrauma. In this review we present the current state of knowledge pertaining to RGC responses to injury, neurotrophic and gene therapy strategies aimed at promoting RGC survival, and how best to promote the regeneration of RGC axons after optic nerve or optic tract injury. We also describe transplantation methods being used in attempts to replace lost RGCs or encourage the regrowth of RGC axons back into visual centres in the brain via peripheral nerve bridges. Cooperative approaches including novel combinations of transplantation, gene therapy and pharmacotherapy are discussed. Finally, we consider a number of caveats and future directions, such as problems associated with compensatory sprouting and the reformation of visuotopic maps, the need to develop efficient, regulatable viral vectors, and the need to develop different but sequential strategies that target the cell body and/or the growth cone at appropriate times during the repair process.

Differentiation of human retinal pigment epithelial cells into neuronal phenotype by N-(4-hydroxyphenyl)retinamide

ARPE-19, a human retinal pigment epithelial (RPE) cell line, has been widely used in studies of RPE function as well as gene expression. Here, we report the novel finding that N-(4-hydroxyphenyl)retinamide (fenretinide), a synthetic retinoic acid derivative and a potential chemopreventive agent against cancer, induced the differentiation of ARPE-19 cells into a neuronal phenotype. The treated cells lost their epithelial phenotype and exhibited a typical neuronal shape with long processes (four to five times longer than the cell body). The onset of fenretinide-induced neuronal differentiation was dose and time dependent, started within 1-2 days, and lasted at least 4 weeks. Immunohistochemical studies indicated that the expression of neurofilament proteins (NF160 and NF200), calretinin and neural cell adhesion molecule was increased in these differentiated cells. Western blot analysis indicated that cellular retinaldehyde-binding protein, which is normally expressed in RPE cells, was decreased in treated cells. Protein analysis on a two-dimensional gel followed by matrix-assisted laser desorption ionization-time of flight mass spectrometric analysis demonstrated that heat-shock protein 70 was increased after fenretinide treatment. Thus, fenretinide, a synthetic retinoid, is able to induce neuronal differentiation of human RPE cells in culture.

Effect of propionic and methylmalonic acids on the high molecular weight neurofilament subunit (NF-H) in rat cerebral cortex

Propionic and methylmalonic acidemias are inherited neurometabolic disorders biochemically characterized by tissue accumulation of propionic (PA) and methylmalonic (MMA) acids, respectively. Neurofilaments (NF) are important cytoskeletal proteins and phosphorylation/dephosphorylation of NF is important to stabilize the cytoskeleton. We investigated the effects of PA and MMA on the high molecular weight neurofilament subunit associated with the cytoskeletal fraction of rat cerebral cortex along development. Cortical slices from 9- to 60-day-old rats were incubated with 2.5 mM PA or MMA. The cytoskeletal fraction was extracted and the immunoreactivity for phosphorylated or total NF-H was analyzed by immunoblotting using specific antibodies. Results showed that treatment of tissue slices with the acids induced an increased Triton-insoluble phosphorylated NF-H immunoreactivity in up to 17-day-old rats. Furthermore, treatments significantly increased the total amount of NF-H in 12-day-old rats. These findings indicate that PA and MMA alter the dynamic regulation of NF-H assembly in the cytoskeletal fraction.

Expression of Human Neurofilament-light Transgene in Mouse Neurons Transplanted into the Brain of Adult Rats

To investigate the expression of nerve cell-specific transgene products in neural transplants, we implanted into the hippocampus of immunosuppressed adult Sprague - Dawley rats cell suspensions obtained from the septal region of the fetal brain of mice that carry the human neurofilament-light (hNF-L) gene. In grafts examined between 3 weeks and 7 months after transplantation, axons and nerve cell somata immunoreacted to antibodies specific to the human NF-L subunit. Thus, the hNF-L protein appears to be a suitable marker of these grafted neurons. Transgenic mice bearing the hNF-L gene may be a convenient source of donor tissue or be used as hosts for neural transplantation studies. Furthermore, the hNF-L promoter/enhancer elements in this transgene may help direct neuronal expression of heterologous genes that could influence nerve cell responses in either the transplant or host tissues.

Effect of the branched-chain alpha-ketoacids accumulating in maple syrup urine disease on the high molecular weight neurofilament subunit (NF-H) in rat cerebral cortex

In this study we investigated the effects of the branched chain alpha-ketoacids accumulating in maple syrup urine disease (MSUD) on the concentrations of the high molecular weight neurofilament subunit (NF-H) associated with the cytoskeletal fraction of the cerebral cortex of 12-day-old rats. Cortical slices were incubated with alpha-ketoisocaproic acid (KIC), alpha-keto beta-methylvaleric acid (KMV) and alpha-ketoisovaleric acid (KIV) at concentrations ranging from 0.5 to 1.0 mM. The cytoskeletal fraction was extracted and the immunoreactivity for phosphorylated and total NF-H was analyzed by immunoblotting. The in vitro 32P incorporation into NF-H was also determined. Results showed that treatment of tissue slices induced with KMV increased Triton-insoluble phosphorylated NF-H immunoreactivity, with no alteration in total NF-H immunoreactivity. Furthermore, KIC treatment drastically increased the total amount of NF-H, whereas KIV did not change either phosphorylated or total NF-H immunoreactivity. KMV also increased the in vitro 32P incorporation into NF-H, confirming the highly phosphorylated NF-H levels detected in the immunoblot. These findings demonstrate that KIC and KMV alter the dynamic regulation of NF-H assembly in the cytoskeletal fraction. Therefore we may suggest that cytoskeletal disorganization may be one of the factors associated with the neurodegeneration characteristic of MSUD disease.

Partial regeneration and long-term survival of rat retinal ganglion cells after optic nerve crush is accompanied by altered expression, phosphorylation and distribution of cytoskeletal proteins

In a screen to identify genes that are expressed differentially in the retina after partial optic nerve crush, we identified MAP1B as an up-regulated transcript. Western blot analysis of inner retina protein preparations confirmed changes in the protein composition of the microtubule-associated cytoskeleton of crushed vs. uncrushed nerve. MAP1B immunoreactivity and transcript levels were elevated for two weeks after crush. Immunostaining and Western blots with monoclonal antibodies directed against developmentally regulated phosphorylation sites on MAP1B revealed a gradient of MAP1B phosphorylation from the proximal optic nerve stump to the soma of retinal ganglion cells. Most interestingly, using antibodies directed against developmentally regulated phosphorylation sites on MAP1B, we observed that a significant number of crushed optic nerve axons develop MAP1B-immunopositive growth cones, which cross the crush site and migrate along the distal nerve fragment. In parallel, an abnormal distribution of highly phosphorylated neurofilament protein (pNF-H) in the cell soma and dendrites of presumably axotomized retinal ganglion cells was observed following partial nerve crush. This redistribution is present for the period between day 7 and 28 postcrush and is not seen in cells that stay connected to the superior colliculus. Axotomized ganglion cells, which contain pNF-H in soma and dendrites appear to have been disconnected from the colliculus at an early stage but survive axonal trauma for long periods.

CNTF promotes the regrowth of retinal ganglion cell axons into murine peripheral nerve grafts

Autologous peripheral nerves were transplanted onto transected optic nerves of adult mice. We examined whether intraocular CNTF injections increased retinal ganglion cell (RGC) axon regeneration, and what types of RGCs regrew axons into grafts. After temporal CNTF eye injections there were more fluorogold-labelled regenerating RGCs (mean +/- s.e.m. 342+/-113.1; n=6) than in sham eye-injected mice (133+/-27.6; n=8). Greater numbers of regenerating RGCs (1198+/-367.6; n=6) were seen in mice receiving both nasal and temporal CNTF injections. The range of soma areas in regenerate and normal retinas was similar but the average size of regenerating RGCs was greater (212 microm2 vs 111 microm2). Most regenerating RGCs had large dendritic fields. The data suggest a heterogeneous response to axotomy in adult mice, large RGCs preferentially regrowing axons into PN grafts.

Survival of adult rat retinal ganglion cells with regrown axons in peripheral nerve grafts: a comparison of graft attachment with suture of fibrin glue

OBJECT

The goal of this study was to examine whether the method of attachment of a peripheral nerve graft would have an effect on retinal ganglion cell (RGC) regeneration.

METHODS

The number of adult rat RGCs with regrown axons in a peripheral nerve graft was compared under two grafting conditions: 1) attachment of the graft to the optic nerve stump made using a suture; and 2) attachment made using fibrin glue. Counts of RGCs retrogradely labeled with FluoroGold from the grafts 1 month after attachment revealed approximately seven times the number of RGCs in the fibrin-glue group compared with the suture group.

CONCLUSIONS

The use of fibrin glue may be a useful tool for enhancing the regrowth of central nervous system neuron axons into peripheral nervous system grafts.

Selective solubilization of high-molecular-mass neurofilament subunit during nerve regeneration

A reduction in neurofilament (NF) protein synthesis and changes in their phosphorylation state are observed during nerve regeneration. To investigate how such metabolic changes are involved in the reorganization of the axonal cytoskeleton, we studied the injury-induced changes in the solubility and axonal transport of NF proteins as well as their phosphorylation states in the rat sciatic nerve. In the control nerve, 15-25% of high-molecular-mass NF subunit (NF-H) was recovered in the 1% Triton-soluble fraction when fractionated in the presence of phosphatase inhibitors. After a complete loss of NF proteins distal to the injury site (70-75 mm from the spinal cord) 1 week after injury, NF-H detected in the regenerating sprouts at 2 weeks or later exhibited higher solubility (>50%) and lower C-terminal phosphorylation level than NF-H in the control nerve. Solubility increase was also apparent with L-[35S]methionine-labeled NF-H that was in transit in the proximal axon at the time of injury. The low-molecular-mass subunit remained in the insoluble fraction in both the normal and the regenerating nerves, indicating that selective solubilization of NF-H rather than total filament disassembly occurs during regeneration.

Antibodies against neurofilament subunits label retinal ganglion cells but not displaced amacrine cells of hamsters

Although neurofilament (NF) antibodies have been used to visualize ganglion cells and their axons in the retina, it is not known, however, how many ganglion cells contain NF, and how the various NF subunits are distributed in the ganglion cells. Moreover, it is not known whether displaced amacrine cells in the ganglion cell layer are also labelled. In order to see whether NF antibodies can be used as a specific marker for ganglion cells, antibodies raised against the low (NF-L), middle (NF-M) and high (NF-H) molecular weight subunits of NF were employed to stain retinal whole-mounts of adult hamsters after pre-labelling the ganglion cells with Granular Blue. It was found that NF-L and NF-H antibodies labelled 38,777 and 17,750 cells in the ganglion cell layer respectively. By co-localization with GB-labelled cells, 88% of NF-L positive cells and 91% of NF-H positive cells were found to be ganglion cells. In contrast, the NF-M antibody labelled only very few ganglion cells (418 per retina) although robust staining of axonal bundles was observed. Thus, NF antibodies may prove useful in studying this population of ganglion cells.

Inflammatory response and retinal ganglion cell degeneration following intraocular injection of ME7

Scrapie is a prion disease which occurs naturally in sheep and which can be transmitted experimentally to rodents. After intracerebral injection of ME7 into mouse, an atypical inflammatory response, characterized by T-lymphocytes and activated microglia is present early in the course of the disease. In the present work, we have investigated the relationship between this inflammatory response, astrocytosis and neuronal loss along the visual pathway after intraocular injection (intraocular) of ME7 in C57BL/6J mice. We have demonstrated that microglia activation and T-lymphocyte recruitment accompanies the spread of prion pathology along the visual pathway and in the early stages of the disease is restricted to the subcortical visual pathway. Inflammation was also present in non-visual areas in association with PrPsc deposition at late stages of the disease, possibily indicating that diffusion of the scrapie agent also contributes to the spread of the disease. After intraocular injection of the prion agent, the disease is believed to be transported into the brain via axons of retinal ganglion cells (RGCs). Despite the high levels of infectivity reported to be present in the retina early in the disease after intraocular injection of ME7, retinal pathology has not been extensively investigated. We have studied the RGCs response in whole mount retinas after intraocular injection of ME7. We have shown that RGCs degenerate after intraocular injection of ME7 whereas amacrine cells, retinal interneurones, are more resistant. Our results suggest that two distinct population of neurones, exposed in vivo at the same time to the same agent scrapie strain, show different susceptibility to the toxic effects of PrPsc.

Postnatal expressions of non-phosphorylated and phosphorylated neurofilament proteins in the rat hippocampus and the Timm-stained mossy fiber pathway

Neurofilament proteins (NFPs), the cytoskeletal proteins that are essential for axogenesis and maintenance of neuron shape in the nervous system, were studied for their spatial distributions at nine postnatal days (PN 3, 5, 7, 10, 14, 17, 21, 28, and 120). Simultaneously non-phosphorylated (SMI-32; 150/200 kDa; Sternberger) and phosphorylated (SMI-31; 200 kDa) NFP immunoreactivity in the entire developing rat hippocampus was studied, quantified, and compared to that of mossy fiber (MF) axons and terminals using Neo-Timm's histochemistry, the most selective, sensitive, and reproducible technique. Differential developmental expressions were observed between the two NFP states. SMI-32 was initially expressed on PN 3 only in the perikarya of pyramidal neurons in CA3. As early as PN 5, SMI-31 appeared in the MF pathway, in parallel to the growth of MF axons. By contrast, SMI-32 did not appear at any age in the MF pathway, including the MF terminal zone of stratum lucidum. At PN 14, the distribution of both NFPs in the MF system (MFs and their target neurons, i.e., CA3/CA4 pyramidal neurons and hilar neurons) was nearly complete; however, the peak densities of SMI-32 and SMI-31 were later at PN 21 and statistically equal to the most adult level (PN 120). The temporal regulation and maximal levels of SMI-32 and SMI-31 expressions on MF target neurons (CA3: SMI-32) and in the MF terminal zone (stratum lucidum: SMI-31) were nearly parallel to the progressive and rapid PN growth of the MF axons and terminals occurring between PN 14 and PN 17, suggesting that the mechanisms for maturation of MF synaptogenesis occur after PN 17.

Temporal and regional profiles of cytoskeletal protein accumulation in the rat brain following traumatic brain injury

To characterize the cytoskeletal aberration due to traumatic injury, temporal and regional profiles of changes in immunoreactivity of microtubule-associated protein 2 (MAP2), neurofilament heavy subunit protein (NFH) and heat shock protein 72 (HSP72) were investigated after different magnitudes of traumatic brain injury by fluid percussion. The experimental rat brain was perfusion-fixed at 1, 6 and 24 hours after traumatic brain injury. Conventional histological staining has demonstrated that the mildest traumatic brain injury (1.0 atm) induced no neuronal loss at the impact site and that neuron loss was apparent when traumatic brain injury was increased to 4.3 atm. The mildest traumatic brain injury, however, caused a significant increase in HSP72 immunoreactivity in the superficial cortical layers at the impact site as early as 1 hour after the injury. In the case of severe traumatic brain injury (4.3 atm), neuron loss was apparent in the area at the impact site, but the increase in HSP72 immunoreactivity was moderate, and it was observed only after 6 hours in the deep cortical layers under the necrotic area. The increased immunostaining of MAP2 was demonstrated in damaged axons and neuronal perikarya in the wider area surrounding the impact site at 6 and 24 hours after the injury. Six and 24 hours after the injury, perikaryal accumulation of neurofilament was observed, and the accumulated neurofilament was mostly phosphorylated. These results indicate that the severe traumatic brain injury of 4.3 atm triggers the abnormal accumulation of cytoskeletal proteins in neuronal perikarya, most probably due to an impairment of axonal transport. It is implied that the increased expression of HSP72 may be involved in the protective process of neurons after traumatic brain injury.

Acetylcholinesterase inhibitor treatment delays recovery from axotomy in cultured dorsal root ganglion neurons

We have previously reported that dorsal root ganglion neurons cultured in the presence of the highly specific, reversible acetylcholinesterase inhibitor 1,5-bis-(4-allyldimethylammoniumphenyl) pentan-3-one dibromide (BW284c51), showed significantly reduced neurite outgrowth and contained massive perikaryal inclusions of neurofilaments. In the present report we have more closely examined these changes in a time course study over a 21-day culture period using a combined morphological, immunocytochemical and enzymatic approach and additionally, describe, the effects of acetylcholinesterase inhibitor treatment on the state of neurofilament phosphorylation. Finally, we have examined the effects of co-administration of N6,2'-0-dibutyryladenosine 3':5'-cyclic monophosphate (dbcAMP) with BW284c51. At 1 day in culture, both control and treated cells displayed eccentrically located nuclei, numerous polysomes and perikaryal accumulations of neurofilaments which were immunoreactive with both phosphorylation- and nonphosphorylation-dependent neurofilament antibodies. These cytological changes, which are common features of the chromatolytic reaction following axotomy in vivo, rapidly resolved in the control neurons, where by 7 days in culture, the neurofilament accumulations had completely disappeared and neurite outgrowth was robust. In contrast, inhibitor-treated neurons retained the post-axotomy features up to 21 days and had significantly reduced neurite outgrowth. In addition, we have investigated a possible role of cyclic adenosine monophosphate (cAMP) in the recovery process since it has been shown to enhance neuritic outgrowth in cultured neurons. Our results demonstrate that the addition of dbcAMP, a membrane permeable analog of cAMP, significantly enhanced neuritic outgrowth and accelerated the recovery of BW284c51-treated dorsal root ganglion cells, as gauged by the disappearance of the axotomy-related cytological changes. Treatment with dbcAMP also increased acetylcholinesterase activity which has been positively correlated with neurite outgrowth both in vivo and in vitro. Together, these observations suggest that acetylcholinesterase has a non-cholinolytic, neurotrophic role in neuronal regeneration and development.

Influence of the postsynaptic target on the functional properties of neurons in the adult mammalian central nervous system

In this review we have attempted to summarize present knowledge concerning the regulatory role of target cells on the expression and maintenance of the neuronal phenotype during adulthood. It is well known that in early developmental stages the survival of neurons is maintained by specific neurotrophic factors derived from their target tissues. Neuronal survival is not the only phenotype that is regulated by target-derived neurotrophic factors since the expression of electrophysiological and cytochemical properties of neurons is also affected. However, a good deal of evidence indicates that the survival of neurons becomes less dependent on their targets in the adult stage. The question is to what extent are target cells still required for the maintenance of the pre-existing or programmed state of the neuron; i.e., what is the functional significance of target-derived factors during maturity? Studies addressing this question comprise a variety of neuronal systems and technical approaches and they indicate that trophic interactions, although less apparent, persist in maturity and are most easily revealed by experimental manipulation. In this respect, research has been directed to analyzing the consequences of disconnecting a group of neurons from their target-by either axotomy or selective target removal using different neurotoxins-and followed (or not) by the implant of a novel target, usually a piece of embryonic tissue. Numerous alterations have been described as taking place in neurons following axotomy, affecting their morphology, physiology and metabolism. All these neuronal properties return to normal values when regeneration is successful and reinnervation of the target is achieved. Nevertheless, most of the changes persist if reinnervation is prevented by any procedure. Although axotomy may represent, besides target disconnection, a cellular lesion, alternative approaches (e.g., blockade of either the axoplasmic transport or the conduction of action potentials) have been used yielding similar results. Moreover, in the adult mammalian central nervous system, neurotoxins have been used to eliminate a particular target selectively and to study the consequences on the intact but target-deprived presynaptic neurons. Target depletion performed by excitotoxic lesions is not followed by retrograde cell death, but targetless neurons exhibit several modifications such as reduction in soma size and in the staining intensity for neurotransmitter-synthesizing enzymes. Recently, the oculomotor system has been used as an experimental model for evaluating the functional effects of target removal on the premotor abducens internuclear neurons whose motoneuronal target is destroyed following the injection of toxic ricin into the extraocular medial rectus muscle. The functional characteristics of these abducens neurons recorded under alert conditions simultaneously with eye movements show noticeable changes after target loss, such as a general reduction in firing frequency and a loss of the discharge signals related to eye position and velocity. Nevertheless, the firing pattern of these targetless abducens internuclear neurons recovers in parallel with the establishment of synaptic contacts on a presumptive new target: the small oculomotor internuclear neurons located in proximity to the disappeared target motoneurons. The possibility that a new target may restore neuronal properties towards a normal state has been observed in other systems after axotomy and is also evident from experiments of transplantation of immature neurons into the lesioned central nervous system of adult mammals. It can be concluded that although target-derived factors may not control neuronal survival in the adult nervous system, they are required for the maintenance of the functional state of neurons, regulating numerous aspects of neuronal structure, chemistry and electro-physiology.(ABSTRUCT TRUNCATED)

Intracerebroventricular kainic acid administration in adult rat alters hippocampal calbindin and non-phosphorylated neurofilament expression

Calbindin and non-phosphorylated neurofilament proteins were assessed in hippocampus following a unilateral intracerebroventricular kainic acid injection at 4, 26, and 60 days post-lesion, using immunocytochemical expression. The density of calbindin-positive non-pyramidal neurons throughout the hippocampus showed no significant alteration at 4 days post-lesion, a significant decrease at 26 days post-lesion, and a partial recovery at 60 days post-lesion. In addition, calbindin immunoreactivity was dramatically reduced at 26 days post-lesion in the CA1 pyramidal and dentate granule cell layers and the mossy fibers, bilaterally. Although not significant statistically, most of these reductions showed signs of reversal at 60 days post-lesion except the CA1 pyramidal cell layer where the dramatic reductions persisted. Neurofilaments were also altered throughout the post-lesion period, particularly in abnormal expression of non-phosphorylated neurofilament proteins in mossy fibers. The apparent return of calbindin immunoreactivity in non-pyramidal neurons by 60 days post-lesion suggests that recovery from the lesion may involve remaining neuronal elements which either become reactivated with time or have the capability to express normal levels of calbindin with re-innervation. On the other hand, prolonged calbindin reductions in superficial CA1 pyramidal cells suggest sustained down-regulation of calbindin expression owing to persistent reductions in the activity of these neurons. The temporal correlation of the expression of non-phosphorylated neurofilaments in mossy fibers with their sprouting response following target loss suggests a potential role for non-phosphorylated neurofilaments in neuronal plasticity involving axonal sprouting. Alternatively, it may also suggest that injury-induced neurofilament modifications are either conducive or permissive for axonal sprouting.

NADPH diaphorase expression in the rat retina after axotomy--a supportive role for nitric oxide

The large majority of mammalian retinal ganglion cells degenerate following section of their axons in the optic nerve. It has been suggested that some axotomized retina ganglion cells die because of toxic agents produced within their immediate environment. Our hypothesis was that nitric oxide might be one of the toxic factors implicated in the death of adult retinal ganglion cells post-axotomy. In the first instance, we determined whether there were any changes in the retinal expression of NADPH diaphorase both 3 and 14 days following intraorbital section of the optic nerve in adult rats. Secondly, if nitric oxide was indeed implicated in the death of ganglion cells, then trophic factors which rescue these neurons might do so by decreasing the expression of nitric oxide synthase. Recently, we found that a collicular proteoglycan purified from the major target of retinal ganglion cells, the superior colliculus, rescued a greater proportion of adult ganglion cells from axotomy-induced death than most other known trophic factors. We thus injected this proteoglycan intraocularly after section of the optic nerve and examined its effect on the expression of NADPH diaphorase in the retina. Thirdly, an inhibitor of nitric oxide synthetase was repeatedly injected into the eye following the section of the optic nerve in order to determine if such a treatment might improve the survival of retinal ganglion cells. The present results indicate that section of the optic nerve does not alter the overall levels of NADPH diaphorase within the adult rat retina. Intraocular injections of the collicular proteoglycan actually increased the number of neurons expressing NADPH diaphorase, particularly in the ganglion cell layer. Finally, inhibition of nitric oxide synthetase following axotomy resulted in increased loss of retinal ganglion cells over a 2 week period when compared with controls. Our findings indicate that, rather than being toxic, small amounts of nitric oxide may be important for the survival of a proportion of injured retina ganglion cells.

Morphological classification of retinal ganglion cells in mice

Mice have been used for extensive studies on optic nerves and retinal ganglion cells, but mouse retinal ganglion cells have not been classified morphologically. In the present study, normally placed retinal ganglion cells and displaced retinal ganglion cells in pigmented and albino mice were classified morphologically using horseradish peroxidase. These cells were classified into three types according to the sizes of the soma and the dendritic field: type I cells, large soma and large dendritic field; type II cells, small-to-medium soma and small dendritic field; and type III cells, small-to-medium soma and large dendritic field. Some ganglion cells had both symmetric and asymmetric cells. Each type was further subdivided according to the termination level of dendrites in the inner plexiform layer and the dendritic branching pattern. Except for type III displaced ganglion cells, dendrites of the normally placed ganglion cells and the displaced ganglion cells ramify in the outer two-fifths of the inner plexiform layer (sublamina a) or the inner three-fifths of the inner plexiform layer (sublamina b). Type III displaced ganglion cells ramify only in sublamina a. Dendrites of some normally placed type I ganglion cells ramify in both sublaminae. Displaced biplexiform cells were observed, the dendrites of which ramify in both the inner and the outer plexiform layers. All cell types were found in both mouse strains.

Non-phosphorylated neurofilament protein immunoreactivity in adult and developing rat hippocampus: specificity and application in grafting studies

Neurofilament proteins are critical to the development and maintenance of neuronal shape in the nervous system. These proteins are developmentally regulated and several transition forms are expressed, prior to full neuronal stabilization. We have studied the spatial distribution and time course of expression of non-phosphorylated neurofilament protein (NPNFP) immunoreactivity in several preparations of rat hippocampus, using a mixture (SMI 311) of several monoclonal antibodies directed against NPNFP epitopes. Differential staining was observed in young and adult hippocampus. Large pyramidal neurons in CA3 and CA4 subfields were strongly immunoreactive in adult hippocampus whereas the smaller CA1 pyramidal neurons, most interneurons and dentate granule cells were immunonegative. SMI 311 staining initially appeared at postnatal day (P) 5 with positive staining in apical dendrites and soma in a few pyramidal neurons in CA3, but almost reached the adult pattern by P10. Compared to adult hippocampus, the number of immunoreactive interneurons in all subfields appeared increased at P10 and P15. In cultures of embryonic hippocampus, all neurons, regardless of their morphology, were SMI 311 positive, suggesting loss of differential expression in tissue culture conditions. However, SMI 311 expression in fetal hippocampal neurons grafted to adult hippocampus was similar to hippocampal neurons which had developed in situ. These results suggest that SMI 311 antibody identifies a distinct group of primarily CA3 and CA4 pyramidal cells in adult hippocampus. The application of SMI 311 immunostaining appears suitable for identification of large CA3 and CA4 pyramidal neurons within hippocampal transplants grafted to adult CNS but not in tissue culture.

Differential synthesis and cytoskeletal deposition of neurofilament subunits before and during axonal outgrowth in NB2a/d1 cells: evidence that segregation of phosphorylated subunits within the axonal cytoskeleton involves selective deposition

NB2a/d1 cells constitutively express and extensively phosphorylate neurofilament (NF) triplet proteins. However, only hypophosphorylated NFs are observed within the Triton-insoluble perikaryal cytoskeletons of undifferentiated and differentiated cells, while phosphorylated NF isoforms accumulate exclusively within the axonal neurites elaborated following treatment with dbcAMP. We examined NF synthesis and distribution of newly synthesized subunits by immunoprecipitation from 35S-methionine-radiolabeled undifferentiated and dbcAMP-treated differentiated cells. Following a 15 min pulse radiolabeling, NF-H isoforms migrating from approximately 160-200 kDa, NF-M isoforms migrating from approximately 97 k-145 Da, and a single 70 kDa NF-L isoform were readily detectable within Triton-soluble fractions from both undifferentiated and differentiated cells. During chase analyses in the absence of radiolabel, the entire spectrum of isoforms was present in Triton-soluble and -insoluble fractions from both undifferentiated and differentiated cells. However, differentiated cells displayed a significant increase in radiolabel associated with each subunit and isoform. Normalization of their NF synthesis levels to those of undifferentiated cells revealed that differentiated cells deposited 10-fold more radiolabeled subunits into the Triton-insoluble cytoskeleton as compared to undifferentiated cells. Similar levels of radiolabeled subunits were observed throughout the 2 hr period in dbcAMP-treated cells. By contrast, radiolabeled subunits and isoforms increased in undifferentiated cytoskeletons during the chase period, although final levels remained substantially lower than those observed in cytoskeletons of dbcAMP-treated cells. These data were considered with respect to potential mechanisms by which the phosphorylated NFs are normally excluded from perikaryal cytoskeletons. The presence of extensively phosphorylated subunits within perikarya indicates the presence of necessary NF kinases.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of collicular proteoglycan on the survival of adult rat retinal ganglion cells following axotomy

Consistent with numerous previous studies, we have found that in adult rats 29% of cells retrogradely prelabelled by injections into retino-recipient nuclei are lost 1 week after intraorbital section of the optic nerve. This figure increases to 76% 2 weeks after axotomy. Intraocular injections of 150 ng of 480 kDa chondroitin sulphate proteoglycan purified from the superior colliculi of neonatal rats were performed every third day after axotomy. This procedure resulted in the loss of only 3 and 28% of the axotomized retinal ganglion cells 7 and 14 days respectively after optic nerve section. Intraocular injections of chondroitin sulphate type C, one of the sugar types present on the collicular proteoglycan, also resulted in a significant saving of axotomized ganglion cells (with the loss of only 48% 14 days after optic nerve lesion). These findings suggest that the collicular proteoglycan, and to a lesser extent its sugar moieties, substantially slows down the degeneration of adult retinal ganglion cells following axotomy.

Involvement of clathrin light chains in the pathology of Pick's disease; implication for impairment of axonal transport

Clathrin, which constitutes coated vesicles and plays important roles in neuronal functions, has been reported to be involved in the pathology of Alzheimer's disease. In the brains of the patients with Pick's disease, distribution of clathrin was immunohistochemically investigated using monoclonal antibodies binding to different epitopes of clathrin light chain a and b. All the antibodies intensely labeled Pick's body and some perikarya of neurons, indicating impairment of slow axonal transport b (SCb). Antibodies against neurofilament, kinesin and synaptophysin also labeled Pick's body. These observations suggested impairment of axonal transport in the brains with Pick's disease, and might contribute to elucidating the pathology of Pick's body forming. It is implied that common pathological processes might lie in Alzheimer's disease and Pick's disease.

Immediate early gene expression in axotomized and regenerating retinal ganglion cells of the adult rat

To determine if axotomy-induced immediate early gene (IEG) expression accompanies regenerative efforts in central nervous system (CNS) neurons, immunohistochemistry using antibodies to c-Jun, JunD, JunB, c-Fos, FosB and Krox-24 proteins was used to examine gene expression in identified adult rat retinal ganglion cells (RGCs) under two conditions: (1) after axotomy alone, and (2) 1 month after replacement of the optic nerve with an autologous peripheral nerve graft to allow axonal regrowth. Strong RGC c-Jun expression was induced 1 day, but not 3 h, after axotomy in most RGCs and was maintained in surviving cells throughout the 3-week study period. Axotomy also induced a limited number of RGCs to express Krox-24, but only transiently. c-Fos expression was also seen in a limited number of control RGCs, however, it was not induced by axotomy. Nucleolar FosB immunoreactivity in axotomized RGCs persisted 1 day after axotomy, but was subsequently lost. One month after axotomy and peripheral nerve graft placement, identified RGCs with regrown axons showed only nuclear c-Jun and nucleolar FosB expression. These findings support a role for IEG expression in the regeneration process of CNS neurons.

Transient immunohistochemical labelling of rat retinal axons during Wallerian degeneration by a monoclonal antibody to neurofilaments

Immunohistochemical labelling with the monoclonal antibody SMI32 to non-phosphorylated epitopes on neurofilament proteins of high molecular weight class was low in rat central optic fibers of controls. After unilateral transection of optic nerve, a strong, transient increase of labelling with SMI32 occurred in degenerating fibers of optic tract at 2 and 4 days, which then declined at 8 and remained low at 21 days. Consequently, immunostaining with SMI32 may serve as a positive marker for degenerating fibers in rat optic system.

The ganglion cell response to optic nerve injury in the cat: differential responses revealed by neurofibrillar staining

The early responses of cat retinal ganglion cells to axotomy have been examined using neurofibrillar and Nissl-stained wholemounts. We were interested to learn whether the enhanced neurofilament expression, seen in a number of neuronal systems, was also present in different neuronal populations of the cat retina and could be used to study the distribution of these cells. We found that beta ganglion cells degenerate very rapidly after axotomy with the nuclei becoming pyknotic within a few days. Few beta cells showed increased neurofibrillar staining of the dendrites. The cell body degenerated prior to any visible degenerative changes in the axon. A proportion of the alpha and gamma ganglion cells degenerated in the first two to three weeks after axotomy. The alpha cells underwent markedly enhanced neurofibrillar staining of their dendrites prior to degeneration. The Nissl material of the cell bodies diminished as the cells degenerated but we have not observed pyknotic nuclei. The dendritic trees of some axotomised gamma cells were also revealed by the neurofibrillar stain three weeks after axotomy. These results show that retinal ganglion cells do not degenerate by a dying back process. We suggest that the rapid degeneration of the beta ganglion cell population comes about by excitotoxic cell death, a consequence of their large glutamatergic input from bipolar cells. The degenerating beta ganglion cells have the morphological appearance of cells undergoing apoptosis.

Effects of target depletion on adult mammalian central neurons: functional correlates

The physiological signals and patterns of synaptic connectivity that CNS neurons display after the loss of their target cells were evaluated in adult cats for one year. Abducens internuclear neurons were chosen as the experimental model because of their highly specific projection onto the medial rectus motoneurons of the oculomotor nucleus. Selective death of medial rectus motoneurons was induced by the injection into the medial rectus muscle of ricin, a potent cytotoxic lectin that leaves the presynaptic axons intact. The electrical activity of antidromically identified abducens internuclear neurons was recorded in chronic alert animals, during both spontaneous and vestibularly induced eye movements, before and after target removal. During the three weeks that followed ricin injection, abducens internuclear neurons exhibited several firing-related abnormal properties. There was an overall reduction in firing rate with a corresponding increase in the eye position threshold for recruitment. In addition, neuronal sensitivities to eye position and velocity were significantly decreased with respect to control data. Bursting activity was also altered since low-frequency delayed burst accompanied the saccades in the on-direction and, occasionally, internuclear neurons exhibited low-frequency discharges associated with off-directed saccades. Intracellular recordings carried out seven and 15 days after ricin injection demonstrated no significant changes in their electrical properties, although a marked depression of synaptic transmission was evident. The amplitude of both excitatory and inhibitory postsynaptic potentials of vestibular origin was reduced by 60-85% with respect to controls. However, postsynaptic potentials recorded one month after ricin injection showed normal amplitude values which persisted unaltered one year after target loss. Recovery of synaptic transmission occurred at the same time as the re-establishment of normal eye-related signals in the discharge pattern of abducens internuclear neurons recorded in alert cats from days 25-30 post lesion. The functional restoration of firing properties was maintained in the long term (one year). Conversely, abducens motoneurons showed normal firing and synaptic patterns at all time intervals analysed. These results demonstrate that, after an initial period of altered physiological properties, abducens internuclear neurons survive the loss of their target motoneurons and regain a normal discharge pattern and afferent synaptic connections.

Probing modifications of the neuronal cytoskeleton

The prominent death of central neurons in Alzheimer's and Parkinson's is reflected by changes in cell shape and by the formation of characteristic cytoskeletal inclusions (neurofibrillary tangles, Lewy bodies). This review focuses on the biology of neurofilaments and microtubule-associated proteins and identifies changes that can occur to these elements from basic and clinical research perspectives. Attention is directed at certain advances in neurobiology that have been especially integral to the identification of epitope domains, protein isoforms, and posttranslational (phosphorylation) events related to the composition, development, and structure of the common cytoskeletal modifications. Recently, a number of experimental strategies have emerged to simulate the aberrant changes in neurodegenerative disorders and gain insight into possible molecular events that contribute to alterations of the cytoskeleton. Descriptions of specific systems used to induce modifications are presented. In particular, unique neural transplantation methods in animals have been used to probe possible molecular and cellular conditions concerned with abnormal cytoskeletal changes in neurons.

Immunohistochemical studies with antibodies to neurofilament proteins on axonal damage in experimental focal lesions in rat

Immunohistochemistry with monoclonal antibodies against neurofilament (NF) proteins of middle and high molecular weight class, NF-M and NF-H, was used to study axonal injury in the borderzone of focal lesions in rats. Focal injury in the cortex was produced by infusion of lactate at acid pH or by stab caused by needle insertion. Infarcts in substantia nigra pars reticulata were evoked by prolonged pilocarpine-induced status epilepticus. Immunohistochemical staining for NFs showed characteristic terminal clubs of axons in the borderzone of lesions. Differences in the labelling pattern occurred with different antibodies which apparently depended on molecular weight class of NFs and phosphorylation state. These immunohistochemical changes of NFs can serve as a marker for axonal damage in various experimental traumatic or ischemic lesions.

Different stability of neurofilaments for trypsin treatment after axotomy in the dorsal motor nucleus of the vagal nerve and the hypoglossal nucleus

In an attempt to obtain information about changes of neurofilaments in motor neurons after axotomy, we immuno-histochemically investigated the accumulated neurofilaments in the dorsal motor nucleus of the vagal nerve, which shows nerve cell loss and degenerative changes after axotomy, and in the hypoglossal nucleus, which shows regenerative changes. Affected neurons in the hypoglossal nucleus showed intensified immunoreactivities for neurofilament antibodies phosphorylated at the carboxy-terminal, and these reactions disappeared with trypsin treatment. Accumulated neurofilaments in the neuronal perikarya in the dorsal motor nucleus of the vagal nerve and axons in brain stem also showed intensified immunoreactivities for the same antibodies, and these reactions remained positive after trypsin treatment. Anti-ubiquitin antibody preferentially stained accumulated neurofilaments in the affected vagal neurons, while no reaction was found in the affected hypoglossal neurons. Phosphorylated neurofilaments in hypoglossal neurons are vulnerable to trypsin treatment probably because of the blocking of polymerization or the disassembly of neurofilaments due to amino-terminal phosphorylation. In vagal neurons, the deteriorated amino-terminal phosphorylation or hyperphosphorylation at the carboxy-terminal seems to cause the cross-linkage and polymerization of neurofilaments, and densely packed polymerized neurofilaments probably fail in axonal transport resulting in nerve cell degeneration and death in the dorsal motor nucleus.

Immunohistochemical studies on neurofilamentous hypertrophy in degenerating retinal terminals of the olivary pretectal nucleus in the rat

Following section of the optic nerve, degenerating retinal terminals reveal an accumulation of neurofilaments (neurofilamentous hypertrophy) as demonstrated by silver impregnation techniques or electron microscopy. The present study examined degenerating retinal terminals by means of immunohistochemistry and antibodies specific for the triplet of neurofilament proteins of low (NF-L), medium (NF-M), and high (NF-H) molecular weight class. Following unilateral optic nerve section in the rat and survival of 1, 2, 4, 8, and 21 days, brains were perfused with aldehyde fixative, sliced on a vibratome and stained for neurofilaments by using the peroxidase-antiperoxidase technique. Other brains were frozen, cut in the native state, and slide-mounted sections were fixed by acetone. Side comparisons in visual pathways were made in frontal sections, taking advantage of the near complete crossing of retinal fibers in the rat. Anterograde degeneration of axons occurred in the optic tract and branchium colliculi. Changes of terminals were investigated in the olivary pretectal nucleus, which contains a dense aggregation of retinal terminals in the core region. The optic tract and branchium colliculi showed a reduction in immunostaining for neurofilament proteins following axotomy. Within the core region of the olivary pretectal nucleus, strong increases of immunoreactivity of NF-L and NF-M were detected beginning at 2 days postlesion and persisting at 8 days. No changes in NF-H proteins were found in the terminal regions with three different antibody probes. The increase in immunostaining reflects the accumulation of neurofilament proteins in the degenerating retinal terminals, i.e., neurofilamentous hypertrophy.(ABSTRACT TRUNCATED AT 250 WORDS)

The development of retinal ganglion cell decussation patterns in postnatal pigmented and albino ferrets

The decussation patterns of retinal ganglion cells in postnatal pigmented and albino ferrets were examined by using retrograde axonal tracers. Following unilateral injections into the optic pathway of newborn pigmented ferrets, approximately 13,000 cells were labelled in the ipsilateral retina. The majority (11,500) of these were located in temporal retina. Postnatally, the numbers of cells projecting ipsilaterally from temporal retina fell by 49%. High rates of loss were observed in both the smaller uncrossed projection from nasal retina (92%) and also in the crossed projection from temporal retina (84%). After injections on the day of birth, a decussation line was not obvious in the crossed projection: > or = 14,000 labelled cells were found in temporal retina. Double tracer studies showed that very few of these cells had axons which projected bilaterally. The numbers of ipsilaterally projecting cells labelled in neonatal albino ferrets was dramatically reduced. Only approximately 2500 were labelled in temporal retina following injections at birth. As in pigmented ferrets, about half of these cells subsequently died. The reduced uncrossed projection in albino neonates was associated with an increase in the crossed projection from temporal retina, in which approximately 21,000 cells were labelled following injections at birth. These results suggest that differential postnatal ganglion cell death establishes the adult decussation pattern in the contralateral retinal projection but merely refines the pattern already established in the uncrossed projection. Postnatal ganglion cell death plays no significant role in generating the abnormal projections found in albino ferrets.