Differential localization of the high- and low-molecular weight variants of MAP2 in the developing retina

Actin Dynamics, Regulated by RhoA-LIMK-Cofilin Signaling, Mediates Rod Photoreceptor Axonal Retraction After Retinal Injury

Purpose

Retraction of the axon terminals of rod photoreceptors after retinal detachment breaks the first synapse in the visual pathway, resulting in visual impairment. Previous work showed that the mechanism of axonal retraction involves RhoA signaling and its downstream effector LIM Kinase (LIMK) activation. We examined the response of the downstream component cofilin, a direct binding protein of actin filaments, as well as the regulation by RhoA-LIMK-Cofilin signaling of actin assembly/disassembly, in the presynaptic ribbon terminal of injured rod cells.

Methods

Injury was produced by retinal detachment or rod cell isolation. Detached porcine retina was probed for levels and localization of phosphorylated cofilin with Western blots and confocal microscopy, whereas rod cell cultures of dissociated salamander retina were examined for filamentous actin assembly/disassembly with a barbed end assay and phalloidin staining.

Results

A detachment increased phosphorylation of cofilin in retinal explants; phosphorylation occurred in rod terminals in sections of detached retina. Isolation of rod cells resulted in axon retraction accompanied by an increase in actin barbed ends and a decrease in net filament labeling. All changes were significantly reduced by either Rho kinase (ROCK) or LIMK inhibition, using Y27632 or BMS-5, respectively. Cytochalasin D also reduced retraction and stabilized filaments in isolated rod cells.

Conclusions

These results indicate that actin depolymerization via activation of RhoA downstream kinases and cofilin contributes to axon retraction. Preventing depolymerization, in addition to actomyosin contraction, may stabilize ribbon synapses after trauma.

The Alzheimer's-related amyloid beta peptide is internalised by R28 neuroretinal cells and disrupts the microtubule associated protein 2 (MAP-2)

Age-related Macular Degeneration (AMD) is a common, irreversible blinding condition that leads to the loss of central vision. AMD has a complex aetiology with both genetic as well as environmental risks factors, and share many similarities with Alzheimer's disease. Recent findings have contributed significantly to unravelling its genetic architecture that is yet to be matched by molecular insights. Studies are made more challenging by observations that aged and AMD retinas accumulate the highly pathogenic Alzheimer's-related Amyloid beta (Aβ) group of peptides, for which there appears to be no clear genetic basis. Analyses of human donor and animal eyes have identified retinal Aβ aggregates in retinal ganglion cells (RGC), the inner nuclear layer, photoreceptors as well as the retinal pigment epithelium. Aβ is also a major drusen constituent; found correlated with elevated drusen-load and age, with a propensity to aggregate in retinas of advanced AMD. Despite this evidence, how such a potent driver of neurodegeneration might impair the neuroretina remains incompletely understood, and studies into this important aspect of retinopathy remains limited. In order to address this we exploited R28 rat retinal cells which due to its heterogeneous nature, offers diverse neuroretinal cell-types in which to study the molecular pathology of Aβ. R28 cells are also unaffected by problems associated with the commonly used RGC-5 immortalised cell-line, thus providing a well-established model in which to study dynamic Aβ effects at single-cell resolution. Our findings show that R28 cells express key neuronal markers calbindin, protein kinase C and the microtubule associated protein-2 (MAP-2) by confocal immunofluorescence which has not been shown before, but also calretinin which has not been reported previously. For the first time, we reveal that retinal neurons rapidly internalised Aβ_1-42, the most cytotoxic and aggregate-prone amongst the Aβ family. Furthermore, exposure to physiological amounts of Aβ_1-42 for 24 h correlated with impairment to neuronal MAP-2, a cytoskeletal protein which regulates microtubule dynamics in axons and dendrites. Disruption to MAP-2 was transient, and had recovered by 48 h, although internalised Aβ persisted as discrete puncta for as long as 72 h. To assess whether Aβ could realistically localise to living retinas to mediate such effects, we subretinally injected nanomolar levels of oligomeric Aβ_1-42 into wildtype mice. Confocal microscopy revealed the presence of focal Aβ deposits in RGC, the inner nuclear and the outer plexiform layers 8 days later, recapitulating naturally-occurring patterns of Aβ aggregation in aged retinas. Our novel findings describe how retinal neurons internalise Aβ to transiently impair MAP-2 in a hitherto unreported manner. MAP-2 dysfunction is reported in AMD retinas, and is thought to be involved in remodelling and plasticity of post-mitotic neurons. Our insights suggest a molecular pathway by which this could occur in the senescent eye leading to complex diseases such as AMD.

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Molecular basis of complex retinopathies such as AMD is incompletely understood.

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The Alzheimer's-related Aβ peptides are rapidly internalised by retinal neurons.

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Internalised Aβ is retained within neurons and transiently impairs MAP-2.

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Subretinally injected Aβ mimics its naturally-occurring distribution in aged retinas.

Enhanced metabolic activity coincides with survival and differentiation of cultured rat retinal ganglion cells exposed to glutamate

Neurotransmitters are prominent candidates for trans-cellular signals that influence the development of the CNS. The present study has examined the effect of glutamate on survival, differentiation and metabolic activity of cultured rat retinal ganglion cells at 3 days in vitro. Retinal cultures from neonatal Wistar rats were treated with glutamate for 48 h. The metabolic activity was markedly increased in the retinal ganglion cells exposed to 20 microM glutamate. This was accompanied by an enhanced survival of these neurons. The number of differentiated retinal ganglion cells as determined by microtubule-associated protein-2 labeling was significantly increased following exposure to low but not higher doses of glutamate. The effect of glutamate on the metabolic activity and differentiation was blocked by tetrodotoxin. The results of the present study shows that glutamate has a significant effect on survival, differentiation and metabolic activity. An increase in the metabolic activity indicates an enhancement in the electrical activity. Thus, our results are consistent with the hypothesis that glutamate is critically involved in the regulation of electrical activity in developing rat retinal ganglion cells.

Changes in MAP2 and tyrosinated alpha-tubulin expression in cochlear inner hair cells after amikacin treatment in the rat

The expression of MAP2 (microtubule-associated protein 2) and of tyrosinated alpha-tubulin was investigated immunocytochemically in the cochleas of normal and amikacin-treated rats. For MAP2, two different antibodies were used: anti-MAP2ab, against the high molecular weight forms, and anti-MAP2abc, additionally against the embryonic form c. In the cochlea of the normal rat, the outer (OHCs) and inner (IHCs) hair cells were labeled for MAP2abc. The labeling was weaker in IHCs than in OHCs. The hair cells were rarely labeled for MAPab. Both OHCs and IHCs were labeled for tyrosinated alpha-tubulin. In the cochlea of the amikacin-treated rat, aggregates of anti-MAP2abc and anti-tyrosinated alpha-tubulin antibodies were seen in the apical region of the IHCs as early as the end of the antibiotic treatment. In rats investigated during the following week, the cell body of most of the surviving IHCs were not labeled for MAP2abc and tyrosinated alpha-tubulin. Then, labeling for these two antibodies reappeared in the surviving IHCs, including their giant stereocilia. Fewer surviving IHCs were labeled for tyrosinated alpha-tubulin than for MAP2abc. The amikacin-poisoned IHCs were rarely labeled for MAP2ab. These results suggest that cochlear hair cells essentially express form c of MAP2. In the amikacin-damaged cochlea, the apical aggregation of MAP2c and tyrosinated alpha-tubulin within the poisoned IHCs could be implicated in a cell degenerative process. By contrast, the extinction and recovery of MAP2c and tyrosinated alpha-tubulin labeling in the remaining IHCs suggest the occurrence of a limited repair process. A possible role of MAP2 and tubulin in hair cell survival is discussed.

Differential localization of high- and low-molecular-weight variants of microtubule-associated protein 2 in the developing rat telencephalon

Microtubule-associated protein 2 (MAP2) occurs in developing mammalian neuronal tissue as both high- and low-molecular-weight forms with temporally regulated expression. We studied the MAP2 expression in the developing rat telencephalon with monoclonal antibodies that recognized both the high- and low-molecular-weight forms of MAP2 variants or that specifically recognized high-molecular-weight forms of MAP2 variants. Differences in the staining patterns of these antibodies reflected differences in the distribution of the high- and low-molecular-weight MAP2s. The immunoreactive sites of high- and low-molecular-weight MAP2 had a more widespread distribution in the embryonic telencephalon than those of high-molecular-weight MAP2. Many bipolar cells in the ganglionic eminence (GE) and in the intermediate zone (IZ) of the neocortex showed low-molecular-weight MAP2 immunoreactivity, but they showed weak or no high-molecular-weight MAP2 immunoreactivity. Expression of mRNA containing exons common to high- and low-molecular-weight MAP2 was detected in the tangentially ellipsoidal cells in the IZ, but expression of mRNA containing an exon specific to high-molecular-weight MAP2 was not detected in these cells by in situ hybridization. We interpreted these observations as indicating that the bipolar cells contained MAP2c preferentially, but contained MAP2a and MAP2b (MAP2a/b) at a very low or negligible level. The cells that expressed MAP2c preferentially among the MAP2 splicing variants composed 50% of the preplate cells, most of the MAP2-positive cells in the hippocampus and the corpus callosum. Double labeling by DiI staining and Dlx2 immunohistochemistry, or by Dlx2 and MAP2 immunohistochemistry, revealed that most of the Dlx2-positive cells in the IZ expressed MAP2c preferentially at embryonic day 16. Another double-labeling study revealed that most GAD-positive cells in the preplate were MAP2a/b positive, whereas most GAD-positive cells in the IZ expressed MAP2c preferentially, with only a negligible level of MAP2a/b immunoreactivity. We conclude that MAP2 immunoreactivity in the IZ was localized in the tangentially migrating neurons. The tangentially migrating neurons seemed to acquire MAP2a/b immunoreactivity as they entered the preplate or cortical plate and developed into mature neurons. Radially migrating neurons in the IZ were MAP2 negative. After entering to the preplate or the cortical plate, they became MAP2a/b positive as they developed into mature neurons.

Effect of antenatal betamethasone treatment on microtubule-associated proteins MAP1B and MAP2 in fetal sheep

Betamethasone has been used extensively to accelerate fetal lung maturation, yet little is known of its effects on neuronal morphogenesis in the developing fetus. Microtubule-associated proteins (MAPs) are a diverse family of cytoskeletal proteins that are important for brain development and the maintenance of neuroarchitecture. Vehicle (n = 7) or betamethasone (10 ug h-1, n = 7) was infused I.V. to fetal sheep over 48 h beginning at 0.87 of gestation (128 days of gestation), producing fetal plasma betamethasone concentrations resembling those to which the human fetus is exposed during antenatal glucocorticoid therapy. Paraffin sections of the left hemisphere were stained with monoclonal antibodies against MAP1B and the MAP2 isoforms MAP2a,b,c and MAP2a,b. The level of the juvenile isoform MAP2c was determined by comparison of the two MAP2 immunostainings. We were able to detect MAP1B and MAP2 immunoreactivity (IR) in the fetal sheep brain. MAP2c was the major MAP2, constituting 90.2 % of the total MAPBetamethasone exposure diminished MAP1B IR in the frontal cortex and caudate putamen (P < 0.05) but not in the hippocampus. A decrease of MAP2 IR was found in the frontal cortex, hippocampus and caudate putamen (P < 0.05). Loss of MAP2 IR was mainly due to the loss of MAP2c IR. Haematoxylin-eosin staining did not demonstrate irreversible neuronal damage. Regional cerebral blood flow determined using coloured microspheres was significantly decreased by 28 % in the frontal cortex and by 36 % in the caudate putamen but not in the hippocampus 24 h after the onset of betamethasone exposure (P < 0.05). The loss of MAP1B and MAP2a,b,c IR showed a significant correlation to the cerebral blood flow decrease only in the frontal cortex (P < 0.05). These data suggest that mechanisms other than metabolic insufficiency caused by the decreased cerebral blood flow may contribute to the loss of MAPs. The results suggest that clinical doses of betamethasone may have acute effects on cytoskeletal proteins in the fetal brain.

Angiotensin II and NGF differentially influence microtubule proteins in PC12W cells: role of the AT2 receptor

Angiotensin AT2 receptors have been shown to play a role in cell differentiation characterized by neurite outgrowth in neuronal cells of different origin. To further investigate AT2 receptor-mediated events leading to neurite formation, we examined the effect of AT2 receptor stimulation on the microtubule components, beta-tubulin, MAP1B and MAP2, by Western blot analysis and immunofluorescence in quiescent and nerve growth factor (NGF)-differentiated PC12W cells. These proteins are involved in neurite extension and neuronal maturation. Whereas NGF (0.5, 10, and 50 ng/ml) up-regulated these proteins after 3 days of stimulation, angiotensin II (ANG II; 10(-7) M) induced a different pattern. In quiescent PC12W cells, AT2 receptor stimulation up-regulated polymerized beta-tubulin and MAP2 but down-regulated MAP1B protein levels. In PC12W cells, differentiated by NGF (0.5 ng/ml), ANG II elevated polymerized beta-tubulin and reduced MAP1B. All ANG II effects were abolished by the AT2 receptor antagonist PD123177 (10(-5) M) but not affected by the AT1 receptor antagonist losartan (10(-5) M). These results implicate a specific role of AT2 receptors in cell differentiation and nerve regeneration via regulation of the cytoskeleton.

Continuous observation of multipotential retinal progenitor cells in clonal density culture

All neural cell types in the vertebrate retina, except astrocytes, have been shown to develop from multipotential progenitor cells. It is not known, however, to what extent the progenitor cells are heterogeneous in their developmental potential or to what extent cell-cell interactions versus cell-autonomous factors influence the types of cells they become. To address these issues we developed a clonal-density cell culture system where mouse retinal progenitor cells can survive, divide, and differentiate. We followed the development of clones both by continuous time-lapse video microscopy and by daily microscopic observation. We show that even when cultured at clonal density in a homogeneous general environment, where they cannot contact cells outside their own clone, the retinal progenitor cells vary in proliferative capacity, cell cycle time, and in the cell types that they generate. In addition, we show that under these conditions single progenitor cells can generate both neurons and glia, in which case the neurons almost always develop before glial cells, as is the case in vivo.

Dlx-2 homeobox gene controls neuronal differentiation in primary cultures of developing basal ganglia

Homeodomain-containing genes of the Dlx family are expressed in the developing basal ganglia. To investigate the role of Dlx genes during development, we studied their cellular localization in primary cultures of embryonic basal telencephalon, and examined the changes in cellular phenotypes resulting from blockade of Dlx-2 expression. Cells containing Dlx-1, Dlx-2, and Dlx-5 mRNAs are immature cells of the neuronal lineage expressing the microtubule-associated proteins (MAPs) MAP1B and MAP2, but not glial fibrillary acidic protein (GFAP). Treatment of these cells with antisense oligonucleotides targeted to Dlx-2 caused a specific decrease of Dlx-2 mRNA and protein. This decrease in the Dlx-2 gene product was associated with a decrease in the expression of MAP2, a protein localized in neuronal dendrites, along with a smaller decrease in the 200-kDa neurofilament subunit (NF-H). Proteins expressed preferentially in axons were unchanged. This reduction in MAP2 expression was associated with a decrease in dendrite outgrowth and an increased level of cell proliferation. None of these changes were elicited by antisense oligonucleotides targeted to Dlx-1. We suggest that the Dlx-2 gene product regulates two interrelated aspects of neuronal differentiation: the exit from the mitotic cycle and the capability to grow MAP2-positive dendrites. As such, this gene product may be important for the establishment of neuronal polarity, setting the stage for afferent synaptic connectivity.

Characterization of developmental stage and neuronal potential of the rat PNS-derived stem cell line, RT4-AC

RT4 is a family of cell lines derived from a rat peripheral neurotumor and consists of a multipotential stem cell line that spontaneously gives rise to three derivative cell types: one glial-like and two neuronal-like. Previous studies have established that the RT4 glial derivative expresses many properties of Schwann cells; however, the neuronal designation of the other RT4 derivatives is less well substantiated. To further characterize the developmental stage and lineages represented by the RT4 stem cell and its derivatives we examined the expression of 16 marker genes whose expression is either specific to neurons or in some cases, neural tissue. Taken together our results indicate that (i) the RT4 neuronal-like derivatives express only immature neuronal properties, (ii) the RT4 cell lines most closely resemble neural crest derivatives from embryonic day 10 to 12 in the rat, (iii) treatment with cAMP and steroids, although capable of promoting process extension by the RT4 neuronal-like derivatives, did not affect the expression of any of the 16 marker genes examined, and (iv) when compared to other neural stem cell systems, RT4-AC generates the most immature neuronal derivatives.

Graded distribution of the neural 2A10 antigen in the developing chicken retina

During retinal histogenesis, post-mitotic cells become located in different tissue layers, where they differentiate into distinct cell types. In an attempt to elucidate mechanisms of cell differentiation, we have employed hybridoma technology in conjunction with various in vitro techniques. Here, we present monoclonal antibody 2A10, which binds specifically to the cell surface of neurons and outgrowing neurites. Within the retina 2A10 antigen expression is developmentally regulated being most pronounced during the period of tissue layer formation. Elevated antigen expression is limited to post-mitotic neurons as revealed by labeling with bromodeoxyuridine. Retinal ganglion cells, which are the first neurons to develop, appear not to influence the overall developmental regulation of the antigen in the retina, since elimination of these cells by virtue of optic nerve transection in ovo did not alter the antigen expression. The antigen is distributed in a graded fashion in the radial axis of the retina. Maximal immunoreactivity was found at the inner surface of the retina (optic fiber layer), whereas only minute reactivity was detected in the outermost layer. This graded distribution could possibly be involved in a topographic system providing positional information for differentiating neurons. Operationally, MAb 2A10 is a useful marker for retinal neurons, and provides a tool for establishing pure Müller glia cultures by complement-mediated cytolysis of retinal neurons.

An isoform of microtubule-associated protein 2 (MAP2) containing four repeats of the tubulin-binding motif

Microtubule-associated protein 2 (MAP2) exists in both high- and low-molecular mass isoforms, each of which has a tubulin-binding domain consisting of 3 imperfect tandem repeats of 31 amino acids containing a more highly conserved 18 amino acid ‘core’ sequence. We describe here a novel form of low molecular mass MAP2 (MAP2c) that contains an additional 4th repeat of this tubulin-binding motif. Like the 3 previously known repeat sequences, this 4th copy is highly conserved between MAP2 and the two other known members of the same gene family, tau and MAP4. In each of these three genes the additional 4th repeat is inserted between the 1st and 2nd repeats of the 3-repeat form of the molecule. Experiments with brain cell cultures, in which the relative proportions of neurons and glia had been manipulated by drug treatment, showed that 4-repeat MAP2c is associated with glial cells whereas 3-repeat MAP2c is expressed in neurons. Whereas 3-repeat MAP2c is expressed early in development and then declines, the level of 4-repeat MAP2c increases later in development, corresponding to the relatively late differentiation of glial cells compared to neurons. When transfected into non-neuronal cells, the 4-repeat version of MAP2c behaved indistinguishably from the 3-repeat form in stabilising and rearranging cellular microtubules. The presence of an additional 4th repeat of the tubulin-binding motif in all three members of the MAP2 gene family suggests that this variant arose prior to their differentiation from an ancestral gene.

What do retinal müller (glial) cells do for their neuronal 'small siblings'?

Müller (radial glial) cells are the predominant glia of the vertebrate retina. They arise, together with rod photoreceptor cells, bipolar cells, and a subset of amacrine cells, from common precursor cells during a late proliferative phase. One Müller cell and a species-specific number of such neurons seem to form a columnar unit within the retinal tissue. In contrast, 'extracolumnar neurons' (ganglion cells, cone photoreceptor cells, horizontal cells, and another subset of amacrine cells) are born and start differentiation before most Müller cells are generated. It may be essential for such neurons to develop metabolic capacities sufficient to support their own survival, whereas late-born ('columnar') neurons seem to depend on a nursing function of their 'sisterly' Müller cell. Thus, out of the cell types within a retinal column it is exclusively the Müller cell that possesses the enzymes for glycogen metabolism. We present evidence that Müller cells express functional insulin receptors. Furthermore, isolated Müller cells rapidly hydrolyse glycogen when they are exposed to an elevated extracellular K+ ion concentration, a signal that is involved in the regulation of neuronal-glial metabolic cooperation in the brain. Müller cells are also thought to be essential for rapid and effective retinal K+ homeostasis. We present patch-clamp measurements on Müller cells of various vertebrate species that all demonstrate inwardly rectifying K+ channels; this type of channel is well-suited to mediate spatial buffering currents. A mathematical model is presented that allows estimation of Müller cell-mediated K+ currents. A simulation analysis shows that these currents greatly limit lateral spread of excitation beyond the borders of light-stimulated retinal columns, and thus help to maintain visual acuity.

Immunocytochemical localization of 205 kDa microtubule-associated protein (205 kDa MAP) in the guinea pig organ of Corti

We have studied the immunocytochemical localization of microtubule-associated proteins (MAPs) in the guinea pig organ of Corti. Using immunological methods with antibodies against MAP1A, MAP1B, MAP2, tau and 205 kDa MAP, we have identified 205 kDa MAP as a major MAP of the sensory epithelium in the organ of Corti. Immunoperoxidase microscopic study has shown that both cochlear hair cells and supporting cells reacted with anti-205 kDa MAP antibody. Immunoelectron microscopy revealed that 205 kDa MAP was associated with most microtubules in the sensory epithelial cells. It was also associated with the microtubules of bundle structures within supporting cells, suggesting that this MAP might form a part of cross-bridges between microtubules and between microtubules and actin filaments in the bundle structure. In contrast, MAP1A, MAP1B and tau, which are known to be expressed in neuronal tissue, were localized only in nerve fibers in the organ of Corti, not in the sensory epithelium. MAP2, which is known to be localized in dendrites and soma of nerve cells, was not distributed in nerve fibers in the organ of Corti. These results suggest possible roles of the 205 kDa MAP in the formation and maintenance of the highly polarized morphology of the epithelial cells of the organ of Corti, through stabilization and modulation of microtubule networks of these cells.

Murine BC1 RNA in dendritic fields of the retinal inner plexiform layer

Rodent BC1 RNA is a non-messenger RNA polymerase III transcript that is almost exclusively expressed in nerve cells. BC1 RNA has been localised in somatic and dendritic domains of neurons, and its location has been interpreted to indicate a functional role in extrasomatic postsynaptic protein synthesis. In previous in situ hybridisation experiments, it has been demonstrated that in the retina most of the BC1 labelling signal was confined to the ganglion cell layer and the inner plexiform layer. Dendritic processes of several types of neurons form the neuritic plexus of the inner plexiform layer, and in order to determine the contribution of ganglion cells to the BC1 labelling signal, we eliminated this cell type by transecting the optic nerve unilaterally in newborn mice. Deletion of the ganglion cells resulted in a significant reduction although not a complete elimination of the BC1 signal in the inner plexiform layer. These data indicate that dendritic processes of both ganglion cells and amacrine cells contain BC1 RNA.

Microtubule-associated protein 2 (MAP2)-immunoreactive neurons in the retina of Bufo marinus: colocalisation with tyrosine hydroxylase and serotonin in amacrine cells

Neuron populations in the retina of the toad, Bufo marinus, were labelled with a monoclonal antibody raised against microtubule-associated protein 2 (MAP2). A subpopulation of cones, probably corresponding to the blue-sensitive small single cones, large diameter amacrine cells in the most proximal row of the inner nuclear layer and some large ganglion cells in the ganglion cell layer were labelled. Double labelling experiments were carried out to establish the colocalisation of MAP2 with known putative transmitter substances of the anuran amacrine cells. MAP2 was colocalised in a subpopulation of serotonin-immunoreactive and in all tyrosine hydroxylase-immunoreactive amacrine cells. The results indicate, that the MAP2 content in the neurons of the anuran retina can be correlated with other well-defined neurochemical and/or physiological properties.

The 72-kDa microtubule-associated protein from porcine brain

A microtubule-associated protein (MAP) with a molecular mass of 72-kDa that was purified from porcine brain by using its property of heat stability in a low pH buffer was characterized. Low-angle rotary shadowing revealed that the 72-kDa protein was a rodlike protein approximately 55-75 nm long. The 72-kDa protein bound to microtubules polymerized from phosphocellulose column-purified tubulin (PC-tubulin) with taxol and promoted the polymerization of PC-tubulin in the absence of taxol. Microtubules polymerized by the 72-kDa protein showed a tendency to form bundles of several microtubules. Quick-freeze, deep-etch electron microscopy revealed that the 72-kDa protein formed short crossbridges between microtubules. We performed peptide mapping to analyze the relationship of the 72-kDa protein to other heat-stable MAPs, and the results showed some resemblance of the 72-kDa protein to MAP2. Cross-reactivity with a monoclonal anti-MAP2 antibody further suggested that the 72-kDa protein and MAP2 are immunologically related. To study the relationship between the 72-kDa protein and MAP2C, a smaller molecular form of MAP2 identified in juvenile rat brain, we prepared the 72-kDa protein from rat brain by the same method as that used for porcine brain. The fact that the 72-kDa protein from juvenile rat brain was also stained with our monoclonal anti-MAP2 antibody also suggested that the 72-kDa protein is an MAP2C homologue of the porcine brain.

Organization and development of horizontal cells in the goldfish retina, I: The use of monoclonal antibody AT101

We have produced and characterized a monoclonal antibody, AT101, which selectively labels both viable and formaldehyde-fixed horizontal cell axon terminals, but not their somas or axons, of the goldfish (Carassius auratus) retina. The antigen recognized by AT101 appears to be a cell surface glycoprotein with a molecular weight of about 35,000 Daltons, and is present exclusively or predominantly in nervous tissues of all vertebrate species examined. We have used AT101 as a probe to analyze immunocytochemically the organization of horizontal cell axon terminals (HCATs) in the adult goldfish retina, and the emergence and maturation of these terminals during retinal development. Because of continued growth at the retinal margin in adult goldfish, there is a peripheral-to-central gradient in the age of cells, with the most mature in the center and the youngest in the periphery. In the center and near periphery of the adult retina, HCATs have a fusiform morphology and form a dense network in the middle and proximal part of the inner nuclear layer. In the far peripheral retina, the axon terminals appear round or ellipsoid. The retina closest to the retinal margin is devoid of AT101 staining, indicating that either HCATs are absent or the antigen recognized by AT101 is not present on HCATs at this stage. A similar sequence of changes in staining pattern is seen during development. Although AT101 staining can first be demonstrated in the larval retina at 1 month after hatching, it appears mostly as punctate structures. At a later stage, there are round or ellipsoid structures that resemble in morphology and location (in the inner nuclear layer) those found in the far peripheral adult retina. Double-labeling experiments with AT101 and antiserum against tubulin also indicate that AT101 labels the HCATs when they first appear during development. These data suggest that the emergence and maturation of HCAT is a late event in retinal development.

The roles of microtubule-associated proteins in brain morphogenesis: a review

Microtubule-associated proteins (MAPs) are a diverse family of cytoskeletal proteins that copurify with tubulin in vitro. Recently a number of novel approaches have been used to learn more about the functions of MAPs during brain development, including: localization of MAPs and their mRNA in the developing brain, comparisons of MAPs between species to learn potential fundamental characteristics, biochemical analysis of changes in MAPs in process-bearing cell lines, and sequence analysis of MAP cDNAs and cDNA transfection studies. Taken together, these data allow us to assign roles to MAPs which are abundant in the developing brain, and to develop models for future studies. Four MAPs are particularly abundant in the developing brain: MAP1B, the high and low-molecular weight forms of MAP2, and juvenile tau. MAP1B is the only MAP to be found consistently in extending processes in both the developing and adult brain, making it a likely regulator of neurite outgrowth. High-molecular weight MAP2 and tau crosslink microtubules in dendrites and axons, respectively. Low-molecular weight MAP2 may be able to regulate MAP2-mediated crosslinking to make processes more labile during development and in adult brain regions where synaptogenesis is active. Tau-mediated crosslinking may be regulated by temporal regulation of the expression of tau forms with different binding affinities to tubulin. High-molecular weight MAP2 is sequestered into dendrites by the selective transport of its mRNA. This allows rapid and local regulation of MAP2 synthesis.

Immunocytochemical localization of microtubule-associated proteins 1A and 2 in the rat retina

We have studied the immunocytochemical localization of microtubule-associated proteins (MAPs) in rat retinal cells. Using biochemical and immunochemical methods we have identified microtubule-associated protein 1 (MAP1) and microtubule-associated protein 2 (MAP2) as major MAPs in the rat retina. With indirect immunofluorescence microscopy, the inner plexiform layer and the ganglion cell layer were stained with both anti-MAP1A antibody and anti-MAP2 antibody. Cells at the inner margin of the inner nuclear layer were prominently stained with anti-MAP2, but not with anti-MAP1A. Thin section-immunoelectron microscopy using colloidal gold-labeled secondary antibodies revealed MAP1A and MAP2 staining in the neuronal processes of the inner plexiform layer. A filamentous network between the microtubules in the neurites was stained with both antibodies. The developmental course of expression of MAP1A and MAP2 in rat retina was studied by indirect immunofluorescence microscopy. Although MAP2 was already present at 1 day postnatal, MAP1A was not detected until 7 days postnatal. These results indicate that: (1) retinal neurons are heterogeneous in their expression of MAPs; (2) retinal ganglion cells show the same intracellular distribution of MAP1A and MAP2 as typical nerve cells such as motor neurons; and (3) MAP1A and MAP2 are differentially expressed in developing rat retina.

Microtubule-associated protein 1A (MAP 1A) is a ganglion cell marker in adult rat retina

We have used antibodies raised against a cytoskeletal protein, microtubule-associated protein 1A (MAP 1A), to stain adult rat retina. In cryostat and polyethylene glycol-embedded radial sections, the fiber layer, ganglion cell layer, and inner plexiform layer were highly immunoreactive, a finding that suggested that the ganglion cell somata, axons, and dendrites were recognized by these antibodies. Retrograde labeling of retinal cell somata from the superior colliculus and dorso-lateral geniculate nucleus to identify ganglion cells showed colocalization of the tracer and immunoreactive cells. Double labeling with nuclear stains revealed that many cells in the ganglion cell layer, which are likely displaced amacrine cells, were not recognized by these antibodies. Furthermore, transection of ganglion cell axons, a procedure that causes retrograde degeneration of many of the axotomized ganglion cells, led to a decrease in the number of anti-MAP 1A immunoreactive cells in retinal wholemounts. Thus, MAP 1A antibodies preferentially stain ganglion cell somata and dendrites but not amacrine cells. These antibodies should be useful ganglion cell markers.

Microtubule-associated proteins characteristic of embryonic brain are found in the adult mammalian retina

We have used monoclonal antibodies against each of the major mammalian brain microtubule-associated proteins (MAPs), MAP1, MAP2, MAP3, MAP5, and tau, to study the timing of appearance and the cytological distribution of these proteins during the development of the rat retina. Western blots of adult rat retina reveal MAPs that are characteristic of embryonic brain, i.e., MAP5 and the low-molecular-weight forms of MAP2 (MAP2c) and tau (juvenile tau). At the onset of neuronal differentiation within the embryonic retina, MAP5, MAP3, MAP2c, and tau are found in the perikarya or extending axons of ganglion cells. High-molecular-weight MAP2, a dendrite marker, does not appear in the retina until the second day of postnatal development, when ganglion cell dendrites ramify within the inner plexiform layer. MAP1, which is characteristic of adult brain, does not appear in the retina until 1 week after birth, and is limited to ganglion cells and their processes. In the adult retina, MAP5 and MAP2c are concentrated within the inner segments and cell bodies of photosensitive cells, whereas tau is found in horizontal cells and more internal cell layers. Since photosensitive cells are unique among retinal neurons in their constant regeneration of their primary processes, the photoreceptive outer segments, both MAP5 and MAP2c appear not only to be involved in events associated with the embryonic differentiation and growth of neurites, but also in process regeneration in adult neurons that maintain some embryonic characteristics.

Phylogenetic conservation of brain microtubule-associated proteins MAP2 and tau

The major rat brain microtubule-associated proteins, MAP2 and tau, exhibit various properties that implicate them in the mechanisms underlying the growth of axons and dendrites during neuronal development. To determine if these properties represent fundamental morphogenetic mechanisms, we have examined the phylogenetic conservation of these proteins in Xenopus laevis, quail and rat with respect to their molecular form, cytological distribution and developmental expression. In all three species, the high-molecular weight form of MAP2 migrates as a pair of polypeptides (MAP2a and MAP2b); this doublet as well as the low-molecular weight form of MAP2 (MAP2c) and the tau proteins are markedly similar in size in the different classes of vertebrates. Immunohistochemical staining of the Xenopus and quail cerebellum showed that MAP2 is highly concentrated in dendrites whereas the tau proteins are predominantly confined to axons, exactly as they are in rat. The developmental regulation of these proteins in Xenopus and rat is also conserved. Between the larva and the adult (i.e. during metamorphosis) MAP2c undergoes a marked decrease while MAP2a undergoes a large increase. Thus, in both classes of vertebrates the timing of changes in MAP2 expression coincides with the maturation of neuronal morphology. Taken together, these conserved properties of MAP2 and tau in three phylogenetically divergent classes of vertebrates suggest that these proteins serve fundamental functions during neuronal morphogenesis.