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

Proteomic Analysis of Retinal Tissue in an S100B Autoimmune Glaucoma Model

Glaucoma is a neurodegenerative disease that leads to damage of retinal ganglion cells and the optic nerve. Patients display altered antibody profiles and increased antibody titer, e.g., against S100B. To identify the meaning of these antibodies, animals were immunized with S100B. Retinal ganglion cell loss, optic nerve degeneration, and increased glial cell activity were noted. Here, we aimed to gain more insights into the pathophysiology from a proteomic point of view. Hence, rats were immunized with S100B, while controls received sodium chloride. After 7 and 14 days, retinae were analyzed through mass spectrometry and immunohistology. Using data-independent acquisition-based mass spectrometry, we identified more than 1700 proteins on a high confidence level for both study groups, respectively. Of these 1700, 43 proteins were significantly altered in retinae after 7 days and 67 proteins revealed significant alterations at 14 days. For example, α2-macroglobulin was found significantly increased not only by mass spectrometry analysis, but also with immunohistological staining in S100B retinae at 7 and 14 days. All in all, the identified proteins are often associated with the immune system, such as heat shock protein 60. Once more, these data underline the important role of immunological factors in glaucoma pathogenesis.

Proteomics and systems biology in optic nerve regeneration

We present an overview of current state of proteomic approaches as applied to optic nerve regeneration in the historical context of nerve regeneration particularly central nervous system neuronal regeneration. We present outlook pertaining to the optic nerve regeneration proteomics that the latter can extrapolate information from multi-systems level investigations. We present an account of the current need of systems level standardization for comparison of proteome from various models and across different pharmacological or biophysical treatments that promote adult neuron regeneration. We briefly overview the need for deriving knowledge from proteomics and integrating with other omics to obtain greater biological insight into process of adult neuron regeneration in the optic nerve and its potential applicability to other central nervous system neuron regeneration.

Alzheimer's Retinopathy: Seeing Disease in the Eyes

The neurosensory retina emerges as a prominent site of Alzheimer's disease (AD) pathology. As a CNS extension of the brain, the neuro retina is easily accessible for noninvasive, high-resolution imaging. Studies have shown that along with cognitive decline, patients with mild cognitive impairment (MCI) and AD often suffer from visual impairments, abnormal electroretinogram patterns, and circadian rhythm disturbances that can, at least in part, be attributed to retinal damage. Over a decade ago, our group identified the main pathological hallmark of AD, amyloid β-protein (Aβ) plaques, in the retina of patients including early-stage clinical cases. Subsequent histological, biochemical and in vivo retinal imaging studies in animal models and in humans corroborated these findings and further revealed other signs of AD neuropathology in the retina. Among these signs, hyperphosphorylated tau, neuronal degeneration, retinal thinning, vascular abnormalities and gliosis were documented. Further, linear correlations between the severity of retinal and brain Aβ concentrations and plaque pathology were described. More recently, extensive retinal pericyte loss along with vascular platelet-derived growth factor receptor-β deficiency were discovered in postmortem retinas of MCI and AD patients. This progressive loss was closely associated with increased retinal vascular amyloidosis and predicted cerebral amyloid angiopathy scores. These studies brought excitement to the field of retinal exploration in AD. Indeed, many questions still remain open, such as queries related to the temporal progression of AD-related pathology in the retina compared to the brain, the relations between retinal and cerebral changes and whether retinal signs can predict cognitive decline. The extent to which AD affects the retina, including the susceptibility of certain topographical regions and cell types, is currently under intense investigation. Advances in retinal amyloid imaging, hyperspectral imaging, optical coherence tomography, and OCT-angiography encourage the use of such modalities to achieve more accurate, patient- and user-friendly, noninvasive detection and monitoring of AD. In this review, we summarize the current status in the field while addressing the many unknowns regarding Alzheimer's retinopathy.

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.

PiT2 regulates neuronal outgrowth through interaction with microtubule-associated protein 1B

PiT2 is a member of the inorganic phosphate transporter family, and is extensively expressed in the nervous system. It was found that loop7 domain of PiT2 is not required for retroviral recognition and transport function. The exact functions of loop7 remain poorly understood. Here we show that loop7 of PiT2 is necessary for the transport of PiT2 protein to the cell surface. Further, loop7 is also related to the outgrowth of neurite in Neuro2A cells interacts with the light chain 1 of microtubule-associated protein 1B (MAP1B). PiT2 with mutated MAP1B binding sites affect neurite outgrowth whereas Pi transport function deficient mutants of PiT2 do not. We also show that Drosophila dPiT interacts with microtubule-associated protein Futsch, and dPiT is crucial for the normal development of neuromuscular junctions (NMJs). These results indicate that PiT2 might participate in the regulation of neuronal outgrowth by interacting with MAP1B and independently of its Pi transport function in the nervous system.

Rab35 Functions in Axon Elongation Are Regulated by P53-Related Protein Kinase in a Mechanism That Involves Rab35 Protein Degradation and the Microtubule-Associated Protein 1B

Rab35 is a key protein for cargo loading in the recycling endosome. In neuronal immortalized cells, Rab35 promotes neurite differentiation. Here we describe that Rab35 favors axon elongation in rat primary neurons in an activity-dependent manner. In addition, we show that the p53-related protein kinase (PRPK) negatively regulates axonal elongation by reducing Rab35 protein levels through the ubiquitin-proteasome degradation pathway. PRPK-induced Rab35 degradation is regulated by its interaction with microtubule-associated protein 1B (MAP1B), a microtubule stabilizing binding protein essential for axon elongation. Consistently, axon defects found in MAP1B knock-out neurons were reversed by Rab35 overexpression or PRPK inactivation suggesting an epistatic relationship among these proteins. These results define a novel mechanism to support axonal elongation, by which MAP1B prevents PRPK-induced Rab35 degradation. Such a mechanism allows Rab35-mediated axonal elongation and connects the regulation of actin dynamics with membrane trafficking. In addition, our study reveals for the first time that the ubiquitin-proteasome degradation pathway regulates a Rab GTPase.

SIGNIFICANCE STATEMENT

Rab35 is required for axonal outgrowth. We define that its protein levels are negatively regulated by p53-related protein kinase (PRPK). We show that microtubule-associated protein 1B (MAP1B) interacts with PRPK, preventing PRPK-dependent Rab35 proteasome degradation. We demonstrate that Rab35 regulates Cdc42 activity in neurons. This is the first evidence showing that a Rab protein is regulated by degradation dependent on the ubiquitin-proteasome system.

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.

Microtubule-associated protein 1B (MAP1B)-deficient neurons show structural presynaptic deficiencies in vitro and altered presynaptic physiology

Microtubule-associated protein 1B (MAP1B) is expressed predominantly during the early stages of development of the nervous system, where it regulates processes such as axonal guidance and elongation. Nevertheless, MAP1B expression in the brain persists in adult stages, where it participates in the regulation of the structure and physiology of dendritic spines in glutamatergic synapses. Moreover, MAP1B expression is also found in presynaptic synaptosomal preparations. In this work, we describe a presynaptic phenotype in mature neurons derived from MAP1B knockout (MAP1B KO) mice. Mature neurons express MAP1B, and its deficiency does not alter the expression levels of a subgroup of other synaptic proteins. MAP1B KO neurons display a decrease in the density of presynaptic and postsynaptic terminals, which involves a reduction in the density of synaptic contacts, and an increased proportion of orphan presynaptic terminals. Accordingly, MAP1B KO neurons present altered synaptic vesicle fusion events, as shown by FM4-64 release assay, and a decrease in the density of both synaptic vesicles and dense core vesicles at presynaptic terminals. Finally, an increased proportion of excitatory immature symmetrical synaptic contacts in MAP1B KO neurons was detected. Altogether these results suggest a novel role for MAP1B in presynaptic structure and physiology regulation in vitro.

The MAP1B case: an old MAP that is new again

The functions of microtubule-associated protein 1B (MAP1B) have historically been linked to the development of the nervous system, based on its very early expression in neurons and glial cells. Moreover, mice in which MAP1B is genetically inactivated have been used extensively to show its role in axonal elongation, neuronal migration, and axonal guidance. In the last few years, it has become apparent that MAP1B has other cellular and molecular functions that are not related to its microtubule-stabilizing properties in the embryonic and adult brain. In this review, we present a systematic review of the canonical and novel functions of MAP1B and propose that, in addition to regulating the polymerization of microtubule and actin microfilaments, MAP1B also acts as a signaling protein involved in normal physiology and pathological conditions in the nervous system.

Interaction of tubby-like protein-1 (Tulp1) and microtubule-associated protein (MAP) 1A and MAP1B in the mouse retina

Tubby-like protein-1 (Tulp1) is a photoreceptor-specific protein involved in the transport of specific proteins from the inner segment (IS) to the outer segment (OS) in photoreceptor cells. Mutations in the human TULP1 gene cause an early onset form of retinitis pigmentosa. Our previous work has shown an association between Tulp1 and the microtubule-associated protein, MAP1B. An allele of Mtap1a, which encodes the MAP1A protein, significantly delays photoreceptor degeneration in Tulp1 mutant mice. MAP1 proteins are important in stabilizing microtubules in neuronal cells, but their role in photoreceptors remains obscure. To investigate the relationship between Tulp1 and MAP1 proteins, we performed western blots, immunoprecipitations (IP), immunohistochemistry and proximity ligand assays (PLA) in wild-type and tulp1-/- mouse retinas. Our IP experiments provide evidence that Tulp1 and MAP1B interact while PLA experiments localize their interaction to the outer nuclear layer and IS of photoreceptors. Although MAP1A and MAP1B protein levels are not affected in the tulp1-/- retina, they are no longer localized to the OS of photoreceptors. This may be the cause for disorganized OSs in tulp1-/- mice, and indicate that their transport to the OS is Tulp1-dependent.

Heterogeneity of intrinsically photosensitive retinal ganglion cells in the mouse revealed by molecular phenotyping

Intrinsically photosensitive retinal ganglion cell (ipRGC) types can be distinguished by their dendritic tree stratification and intensity of melanopsin staining. We identified heavily stained melanopsin-positive M1 cells branching in the outermost part of the inner plexiform layer (IPL) and weakly melanopsin-positive M2 cells branching in the innermost layer of the IPL. A third type can be distinguished by the displacement of the soma to the inner nuclear layer and has morphological similarities with either M1 cells or M2 cells, and is termed here displaced or M-d cells. The aim of the present study was to examine the phenotypic traits of ipRGC types. Using whole retinae from adult mice, we performed immunohistochemistry using melanopsin immunostaining and a number of antibodies directed against proteins typically expressed in retinal ganglion cells. The majority of M1 and M2 ipRGCs expressed Isl-1, microtubule associated protein-2 (MAP2), γ-synuclein, and NeuN, whereas Brn3 transcription factor and the different neurofilaments (NF68, NF160, NF200) were able to discriminate between ipRGC subtypes. Brn3 was expressed preferentially in M2 cells and in a small subpopulation of weakly melanopsin-positive M-d cells with similarities to M2 cells. All three neurofilaments were primarily expressed in large M2 cells with similarities to the recently described alpha-like M4 cells, but not in M1 cells. Expression of NF68 and NF160 was also observed in a few large M-d ipRGCs. These findings show that ipRGCs are not a phenotypically homogenous population and that specific neuronal markers (Brn3 and neurofilament) can partly distinguish between different ipRGC subtypes.

P-Rex2, a Rac-guanine nucleotide exchange factor, is expressed selectively in ribbon synaptic terminals of the mouse retina

Background

Phosphatidylinositol (3,4,5)-trisphosphate-dependent Rac Exchanger 2 (P-Rex2) is a guanine nucleotide exchange factor (GEF) that specifically activates Rac GTPases, important regulators of actin cytoskeleton remodeling. P-Rex2 is known to modulate cerebellar Purkinje cell architecture and function, but P-Rex2 expression and function elsewhere in the central nervous system is unclear. To better understand potential roles for P-Rex2 in neuronal cytoskeletal remodeling and function, we performed widefield and confocal microscopy of specimens double immunolabeled for P-Rex2 and cell- and synapse-specific markers in the mouse retina.

Results

P-Rex2 was restricted to the plexiform layers of the retina and colocalized extensively with Vesicular Glutamate Transporter 1 (VGluT1), a specific marker for photoreceptor and bipolar cell terminals. Double labeling for P-Rex2 and peanut agglutinin, a cone terminal marker, confirmed that P-Rex2 was present in both rod and cone terminals. Double labeling with markers for specific bipolar cell types showed that P-Rex2 was present in the terminals of rod bipolar cells and multiple ON- and OFF-cone bipolar cell types. In contrast, P-Rex2 was not expressed in the processes or conventional synapses of amacrine or horizontal cells.

Conclusions

P-Rex2 is associated specifically with the glutamatergic ribbon synaptic terminals of photoreceptors and bipolar cells that transmit visual signals vertically through the retina. The Rac-GEF function of P-Rex2 implies a specific role for P-Rex2 and Rac-GTPases in regulating the actin cytoskeleton in glutamatergic ribbon synaptic terminals of retinal photoreceptors and bipolar cells and appears to be ideally positioned to modulate the adaptive plasticity of these terminals.

Tumorspheres but not adherent cells derived from retinoblastoma tumors are of malignant origin

Verification that cell lines used for cancer research are derived from malignant cells in primary tumors is imperative to avoid invalidation of study results. Retinoblastoma is a childhood ocular tumor that develops from loss of functional retinoblastoma protein (pRb) as a result of genetic or epigenetic changes that affect both alleles of the RB1 gene. These patients contain unique identifiable genetic signatures specifically present in malignant cells. Primary cultures derived from retinoblastoma tumors can be established as non-adherent tumorspheres when grown in defined media or as attached monolayers when grown in serum-containing media. While the RB1 genotypes of tumorspheres match those of the primary tumor, adherent cultures have the germline RB1 genotype. Tumorspheres derived from pRb-negative tumors do not express pRb and express the neuroendocrine tumor markers synaptophysin and microtubule-associated protein 2 (MAP2). Adherent cells are synaptophysin-negative and express pRb, the epithelial cell marker cytokeratin that is expressed in the retinal pigmented epithelium and the vascular endothelial cell marker CD34. While tumorspheres are of malignant origin, our results cast doubt on the assumption that adherent tumor-derived cultures are always valid in vitro models of malignant cells and emphasize the need for validation of primary tumor cultures.

Microtubule-associated protein tau in bovine retinal photoreceptor rod outer segments: comparison with brain tau

Recent studies have suggested a possible involvement of abnormal tau in some retinal degenerative diseases. The common view in these studies is that these retinal diseases share the mechanism of tau-mediated degenerative diseases in brain and that information about these brain diseases may be directly applied to explain these retinal diseases. Here we collectively examine this view by revealing three basic characteristics of tau in the rod outer segment (ROS) of bovine retinal photoreceptors, i.e., its isoforms, its phosphorylation mode and its interaction with microtubules, and by comparing them with those of brain tau. We find that ROS contains at least four isoforms: three are identical to those in brain and one is unique in ROS. All ROS isoforms, like brain isoforms, are modified with multiple phosphate molecules; however, ROS isoforms show their own specific phosphorylation pattern, and these phosphorylation patterns appear not to be identical to those of brain tau. Interestingly, some ROS isoforms, under the normal conditions, are phosphorylated at the sites identical to those in Alzheimer's patient isoforms. Surprisingly, a large portion of ROS isoforms tightly associates with a membranous component(s) other than microtubules, and this association is independent of their phosphorylation states. These observations strongly suggest that tau plays various roles in ROS and that some of these functions may not be comparable to those of brain tau. We believe that knowledge about tau in the entire retinal network and/or its individual cells are also essential for elucidation of tau-mediated retinal diseases, if any.

Association of microtubule-associated protein (MAP1B) with growing axons in cultured hippocampal neurons

Microtubule-associated protein 1B (MAP1B) is a major constituent of the neuronal cytoskeleton early in development. This protein is present in embryonic brain and is composed of two isoforms that are the result of differential phosphorylation. We examined the distribution of MAP1B during the differentiation of cultured hippocampal neurons and compared it to that of MAP2 and tubulin. We demonstrated by immunofluorescent doublestaining that MAP1B and MAP2 are colocalized in cell bodies and the minor processes of hippocampal neurons during the early stages of development, before the establishment of neuronal polarity. Later, when neurons acquire axonal and dendritic characteristics, MAP1B is sorted into growing axons, including the growth cone, whereas MAP2 is restricted to dendrites and cell bodies. Unlike tubulin, the localization of MAP1B in growing axons is not uniform. Rather, the protein is found concentrated in the distal portion. During later stages of development, the neurons extend a network of fasciculating axonal and dendritic neurites in which the segregation of MAP1B and MAP2 is maintained. However, the staining of MAP1B in mature neuronal cultures decreases in a pattern that resembles the decline of this protein during brain development. These results support the association of MAP1B with growing axons and its correct developmental regulation in the hippocampal culture system.

Mice devoid of Tau have increased susceptibility to neuronal damage in myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis

The abundant axonal microtubule-associated protein tau regulates microtubule and actin dynamics, thereby contributing to normal neuronal function. We examined whether mice deficient in tau (Tau(-/-)) or with high levels of human tau differ from wild-type (WT) mice in their susceptibility to neuroaxonal injury in experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis. After sensitization with MOG35-55, there was no difference in clinical disease course between human tau and WT mice, but Tau mice had more severe clinical disease and significantly more axonal damage in spinal cord white matter than those in WT mice. Axonal damage in gray matter correlated with clinical severity in individual mice. By immunoblot analysis, the early microtubule-associated protein-1b was increased 2-fold in the spinal cords of Tau mice with chronic experimental autoimmune encephalomyelitis versus naive Tau mice. This difference was not detected in comparable WT animals, which suggests that there was compensation for the loss of tau in the deficient mice. In addition, levels of the growth arrest-specific protein 7b, a tau-binding protein that is stabilized when bound to tau, were higher in WT than those in Tau(-/-) spinal cord samples. These data indicate that loss of tau exacerbates experimental autoimmune encephalomyelitis and suggest that maintaining tau integrity might reduce the axonal damage that occurs in inflammatory neurodegenerative diseases such as multiple sclerosis.

Blood-derived human iPS cells generate optic vesicle-like structures with the capacity to form retinal laminae and develop synapses

PURPOSE

We sought to determine if human induced pluripotent stem cells (iPSCs) derived from blood could produce optic vesicle-like structures (OVs) with the capacity to stratify and express markers of intercellular communication.

METHODS

Activated T-lymphocytes from a routine peripheral blood sample were reprogrammed by retroviral transduction to iPSCs. The T-lymphocyte-derived iPSCs (TiPSCs) were characterized for pluripotency and differentiated to OVs using our previously published protocol. TiPSC-OVs were then manually isolated, pooled, and cultured en masse to more mature stages of retinogenesis. Throughout this stepwise differentiation process, changes in anterior neural, retinal, and synaptic marker expression were monitored by PCR, immunocytochemistry, and/or flow cytometry.

RESULTS

TiPSCs generated abundant OVs, which contained a near homogeneous population of proliferating neuroretinal progenitor cells (NRPCs). These NRPCs differentiated into multiple neuroretinal cell types, similar to OV cultures from human embryonic stem cells and fibroblast-derived iPSCs. In addition, portions of some TiPSC-OVs maintained their distinctive neuroepithelial appearance and spontaneously formed primitive laminae, reminiscent of the developing retina. Retinal progeny from TiPSC-OV cultures expressed numerous genes and proteins critical for synaptogenesis and gap junction formation, concomitant with the emergence of glia and the upregulation of thrombospondins in culture.

CONCLUSIONS

We demonstrate for the first time that human blood-derived iPSCs can generate retinal cell types, providing a highly convenient donor cell source for iPSC-based retinal studies. We also show that cultured TiPSC-OVs have the capacity to self-assemble into rudimentary neuroretinal structures and express markers indicative of chemical and electrical synapses.

Microtubule-associated protein 1B (MAP1B) is required for dendritic spine development and synaptic maturation

Microtubule-associated protein 1B (MAP1B) is prominently expressed during early stages of neuronal development, and it has been implicated in axonal growth and guidance. MAP1B expression is also found in the adult brain in areas of significant synaptic plasticity. Here, we demonstrate that MAP1B is present in dendritic spines, and we describe a decrease in the density of mature dendritic spines in neurons of MAP1B-deficient mice that was accompanied by an increase in the number of immature filopodia-like protrusions. Although these neurons exhibited normal passive membrane properties and action potential firing, AMPA receptor-mediated synaptic currents were significantly diminished. Moreover, we observed a significant decrease in Rac1 activity and an increase in RhoA activity in the post-synaptic densities of adult MAP1B(+/-) mice when compared with wild type controls. MAP1B(+/-) fractions also exhibited a decrease in phosphorylated cofilin. Taken together, these results indicate a new and important role for MAP1B in the formation and maturation of dendritic spines, possibly through the regulation of the actin cytoskeleton. This activity of MAP1B could contribute to the regulation of synaptic activity and plasticity in the adult brain.

Characterization of the transcripts and protein isoforms for cytoplasmic polyadenylation element binding protein-3 (CPEB3) in the mouse retina

Background

Cytoplasmic polyadenylation element binding proteins (CPEBs) regulate translation by binding to regulatory motifs of defined mRNA targets. This translational mechanism has been shown to play a critical role in oocyte maturation, early development, and memory formation in the hippocampus. Little is known about the presence or functions of CPEBs in the retina. The purpose of the current study is to investigate the alternative splicing isoforms of a particular CPEB, CPEB3, based on current databases, and to characterize the expression of CPEB3 in the retina.

Results

In this study, we have characterized CPEB3, whose putative role is to regulate the translation of GluR2 mRNA. We identify the presence of multiple alternative splicing isoforms of CPEB3 transcripts and proteins in the current databases. We report the presence of eight alternative splicing patterns of CPEB3, including a novel one, in the mouse retina. All but one of the patterns appear to be ubiquitous in 13 types of tissue examined. The relative abundance of the patterns in the retina is demonstrated. Experimentally, we show that CPEB3 expression is increased in a time-dependent manner during the course of postnatal development, and CPEB3 is localized mostly in the inner retina, including retinal ganglion cells.

Conclusion

The level of CPEB3 was up-regulated in the retina during development. The presence of multiple CPEB3 isoforms indicates remarkable complexity in the regulation and function of CPEB3.

Plasma membrane expansion: a neuron's Herculean task

The formation of axons and dendrites and maintenance of the neuron's vastly expanded surface require the continuous addition of new membrane. This is achieved by membrane synthesis through the secretory pathway followed by regulated vesicle fusion with the plasma membrane, typically in the distal neurite. However, it is far from simple: multiple distinct membrane carriers are used to target specific membrane domains, dendrites seem to operate semi-autonomously from the rest of the neuron, and exocytosis for membrane expansion is different from that for release of synaptic vesicles. Current knowledge of this process and its implications for neuronal development, function and repair are reviewed.

Genesis of neuronal and glial progenitors in the cerebellar cortex of peripuberal and adult rabbits

Adult neurogenesis in mammals is restricted to some brain regions, in contrast with other vertebrates in which the genesis of new neurons is more widespread in different areas of the nervous system. In the mammalian cerebellum, neurogenesis is thought to be limited to the early postnatal period, coinciding with end of the granule cell genesis and disappearance of the external granule cell layer (EGL). We recently showed that in the rabbit cerebellum the EGL is replaced by a proliferative layer called ‘subpial layer’ (SPL) which persists beyond puberty on the cerebellar surface. Here we investigated what happens in the cerebellar cortex of peripuberal rabbits by using endogenous and exogenously-administered cell proliferation antigens in association with a cohort of typical markers for neurogenesis. We show that cortical cell progenitors extensively continue to be generated herein. Surprisingly, this neurogenic process continues to a lesser extent in the adult, even in the absence of a proliferative SPL. We describe two populations of newly generated cells, involving neuronal cells and multipolar, glia-like cells. The genesis of neuronal precursors is restricted to the molecular layer, giving rise to cells immunoreactive for GABA, and for the transcription factor Pax2, a marker for GABAergic cerebellar interneuronal precursors of neuroepithelial origin that ascend through the white matter during early postnatal development. The multipolar cells are Map5+, contain Olig2 and Sox2 transcription factors, and are detectable in all cerebellar layers. Some dividing Sox2+ cells are Bergmann glia cells. All the cortical newly generated cells are independent from the SPL and from granule cell genesis, the latter ending before puberty. This study reveals that adult cerebellar neurogenesis can exist in some mammals. Since rabbits have a longer lifespan than rodents, the protracted neurogenesis within its cerebellar parenchyma could be a suitable model for studying adult nervous tissue permissiveness in mammals.

Survival and remodeling of melanopsin cells during retinal dystrophy

The melanopsin positive, intrinsically photosensitive retinal ganglion cells (ipRGCs) of the inner retina have been shown to send wide-ranging projections throughout the brain. To investigate the response of this important cell type during retinal dystrophy, we use the Royal College of Surgeons (RCS) dystrophic rat, a major model of retinal degeneration. We find that ipRGCs exhibit a distinctive molecular profile that remains unaltered during early stages of outer retinal pathology (15 weeks of age). In particular, these cells express βIII tubulin, α-acetylated tubulin, and microtubule-associated proteins (MAPs), while remaining negative for other RGC markers such as neurofilaments, calretinin, and parvalbumin. By 14 months of age, melanopsin positive fibers invade ectopic locations in the dystrophic retina and ipRGC axons/dendrites become distorted (a process that may involve vascular remodeling). The morphological abnormalities in melanopsin processes are associated with elevated immunoreactivity for MAP1b and a reduction in α-acetylated tubulin. Quantification of ipRGCs in whole mounts reveals reduced melanopsin cell number with increasing age. Focusing on the retinal periphery, we find a significant decline in melanopsin cell density contrasted by a stability of melanopsin positive processes. In addition to these findings, we describe for the first time, a distinct plexus of melanopsin processes in the far peripheral retina, a structure that is coincident with a short wavelength opsin cone-enriched rim. We conclude that some ipRGCs are lost in RCS dystrophic rats as the disease progresses and that this loss may involve vascular remodeling. However, a significant number of melanopsin positive cells survive into advanced stages of retinal degeneration and show indications of remodeling in response to pathology. Our findings underline the importance of early intervention in human retinal disease in order to preserve integrity of the inner retinal photoreceptive network.

Retinal cell types are differentially affected in sheep with scrapie

Transmissible spongiform encephalopathies (TSEs) are a group of fatal neurodegenerative diseases characterized microscopically by spongiform lesions (vacuolation) in the neuropil, neuronal loss, and gliosis. Accumulation of the abnormal form of the prion protein (PrP(Sc)) has been demonstrated in the retina of natural and non-natural TSE-affected hosts, with or without evidence of microscopically detectable retinal pathology. This study was conducted to investigate the effect of PrP(Sc) accumulation on retinal neurons in a natural host lacking overt microscopical evidence of retinal degeneration by comparing the distribution of retinal cell type-specific markers in control and scrapie-affected sheep. In retinas with PrP(Sc)-immunoreactivity, there was disruption of the normal immunoreactivity patterns of the alpha isoform of protein kinase C (PKCalpha) and vesicular glutamate transporter 1 (VGLUT1), markers of retinal bipolar cells. Altered immunoreactivity was also observed for microtubule-associated protein 2 (MAP2), a marker of a subset of retinal ganglion cells, and glutamine synthetase (GS), a marker of Müller glia. These results demonstrate alterations of immunoreactivity patterns for proteins associated with specific cell types in retinas with PrP(Sc) accumulation, despite an absence of microscopical evidence of retinal degeneration.

A three-dimensional analysis of the development of the horizontal cell mosaic in the rat retina: implications for the mechanisms controlling pattern formation

The horizontal cells are known to form a mono-layered mosaic in the adult retina, but are scattered at different retinal depths in early development. To help clarifying when and which spatial constraints appear in the relative positioning of these cells, we have performed a quantitative analysis of the three-dimensional (3D) organization of the horizontal cell mosaic at different developmental stages in the postnatal rat retina. We first analyzed the two-dimensional (2D) distribution of the horizontal cell projections onto a plane parallel to the upper retinal surface in retinal flat-mounts, and thus to the future mature horizontal cell mosaic. We found that this 2D distribution was non random since postnatal day 1 (P1), and had a subsequent stepwise improvement in regularity. This preceded the alignment of cells in a single monolayer, which was observed on P6. We then computed true horizontal cell spacing in 3D, finding non-random 3D positioning already on P1. Simulation studies showed that this order might simply derive from the 2D order observed in the projections of the cells in flat-mount, combined with their limited spread in retinal depth. Throughout the period analyzed, the relative positions of horizontal cells are in good agreement with a minimal spacing rule in which the exclusion zone corresponds to the average size of the inner core of the cell dendritic tree estimated from P1 samples. These data indicate the existence of different phases in the process of horizontal cell 3D spatial ordering, supporting the view that multiple mechanisms are involved in the development of the horizontal cell mosaic.

Induction of axon and dendrite formation during early RGC-5 cell differentiation

The retinal ganglion cell (RGC)-like RGC-5 line can be differentiated with staurosporine to stop dividing, extend neurites, and increase levels of several ganglion cell markers. This allows study of regulation of neurite development on a single cell basis. However, it is unclear whether the neurites induced by differentiation have features characteristic of dendrites or axons. To address this question, RGC-5 cells were differentiated with staurosporine and then immunoblotted for microtubule-associated protein 2 (MAP2) and actin, or stained immunocytochemically for different MAP2 isoforms, tau, growth-associated protein 43 (GAP-43), or the neuronal marker beta-III-tubulin. We found that staurosporine-induced differentiation led to an upregulation of MAP2c, a MAP2 isoform expressed in developing neurons. Some neurites expressed MAP2c but not the dendritic markers MAP2a and MAP2b, consistent with an axonal phenotype. Some neurites expressed the axonal marker tau in a characteristic proximal-to-distal gradient, and had GAP-43 labeling characteristic of axonal growth cones. The presence of MAP2c in differentiated RGC-5 cells is indicative of RGC-like neurite development, and the pattern of staining for the different MAP2 isoforms, as well as positivity for tau and GAP-43, indicates that differentiation induces axon-like and dendrite-like neurites.

Microtubule-associated protein 1B, a growth-associated and phosphorylated scaffold protein

Microtubule-associated protein 1B, MAP1B, is one of the major growth associated and cytoskeletal proteins in neuronal and glial cells. It is present as a full length protein or may be fragmented into a heavy chain and a light chain. It is essential to stabilize microtubules during the elongation of dendrites and neurites and is involved in the dynamics of morphological structures such as microtubules, microfilaments and growth cones. MAP1B function is modulated by phosphorylation and influences microtubule stability, microfilaments and growth cone motility. Considering its large size, several interactions with a variety of other proteins have been reported and there is increasing evidence that MAP1B plays a crucial role in the stability of the cytoskeleton and may have other cellular functions. Here we review molecular and functional aspects of this protein, evoke its role as a scaffold protein and have a look at several pathologies where the protein may be involved.

Activity-dependent dendritic arborization mediated by CaM-kinase I activation and enhanced CREB-dependent transcription of Wnt-2

Members of the Wnt signaling family are important mediators of numerous developmental events, including activity-dependent dendrite development, but the pathways regulating expression and secretion of Wnt in response to neuronal activity are poorly defined. Here, we identify an NMDA receptor-mediated, Ca2+-dependent signaling pathway that couples neuronal activity to dendritic arborization through enhanced Wnt synthesis and secretion. Activity-dependent dendritic outgrowth and branching in cultured hippocampal neurons and slices is mediated through activation by CaM-dependent protein kinase kinase (CaMKK) of the membrane-associated gamma isoform of CaMKI. Downstream effectors of CaMKI include the MAP-kinase pathway of Ras/MEK/ERK and the transcription factor CREB. A serial analysis of chromatin occupancy screen identified Wnt-2 as an activity-dependent CREB-responsive gene. Neuronal activity enhances CREB-dependent transcription of Wnt-2, and expression of Wnt-2 stimulates dendritic arborization. This novel signaling pathway contributes to dynamic remodeling of the dendritic architecture in response to neuronal activity during development.

Role of MAP1B in axonal retrograde transport of mitochondria

The MAPs (microtubule-associated proteins) MAP1B and tau are well known for binding to microtubules and stabilizing these structures. An additional role for MAPs has emerged recently where they appear to participate in the regulation of transport of cargos on the microtubules found in axons. In this role, tau has been associated with the regulation of anterograde axonal transport. We now report that MAP1B is associated with the regulation of retrograde axonal transport of mitochondria. This finding potentially provides precise control of axonal transport by MAPs at several levels: controlling the anterograde or retrograde direction of transport depending on the type of MAP involved, controlling the speed of transport and controlling the stability of the microtubule tracks upon which transport occurs.

Cells positive for microtubule-associated protein 1B (MAP 1B) are present along rat and human efferent ductules and epididymis

Microtubule-associated protein 1B (MAP 1B) is a neuronal cytoskeleton marker with predominant expression in the developing nervous system. The present study provides evidence for the expression of this cytoskeleton protein in non-neuronal and neuronal cells along rat and human efferent ductules and epididymis (initial segment, caput, and cauda). Reverse transcription/polymerase chain reaction and Western blot analysis were used to confirm the presence of MAP 1B (mRNA and protein) in rat tissues. Immunohistochemical studies revealed MAP-1B-positive staining in columnar ciliated cells present in efferent ductules and in narrow cells located in the initial segment, in both rat and human. MAP-1B-positive basal cells, located underneath the columnar cells, were only identified in the initial segment and caput epididymidis of the rat. Qualitative analysis of tissues from 40-day-old and 120-day-old rats indicated that the number of MAP-1B-positive ciliated, narrow, and basal cells per tubule increased with sexual maturation. These immunoreactive cells did not stain for dopamine beta-hydroxylase or acetylcholinesterase, indicating that they were not adrenergic or cholinergic in nature. Immunohistochemical studies also revealed the presence of MAP-1B-positive staining in interstitial nerve fibers in caput and cauda epididymidis from both rat and human. Thus, the expression of MAP 1B is not confined to a specific cell type in rat and human efferent ductules and epididymis. The functional significance of this cytoskeleton protein in tissues from the male reproductive tract requires further investigation.

Glutamate-induced Ca2+ influx in third-order neurons of salamander retina is regulated by the actin cytoskeleton

Ligand-gated ion channels (ionotropic receptors) link to the cortical cytoskeleton via specialized scaffold proteins and thereby to appropriate signal transduction pathways in the cell. We studied the role of filamentous actin in the regulation of Ca influx through glutamate receptor-activated channels in third-order neurons of salamander retina. Staining by Alexa-Fluor 488-phalloidin, to visualize polymerized actin, we show localization of filamentous actin in neurites, and the membrane surrounding the cell soma. With Ca(2+) imaging we found that in dissociated neurons, depolymerization of filamentous actin by latrunculin A, or cytochalasin D significantly reduced glutamate-induced intracellular Ca(2+) accumulation to 53+/-7% of control value. Jasplakinolide, a stabilizer of filamentous actin, by itself slightly increased the glutamate-induced Ca(2+) signal and completely attenuated the inhibitory effect when applied in combination with actin depolymerizing agents. These results indicate that in salamander retinal neurons the actin cytoskeleton regulates Ca(2+) influx through ionotropic glutamate receptor-activated channels, suggesting regulatory roles for filamentous actin in a number of Ca(2+)-dependent physiological and pathological processes.

Distinct gene expression profiles and reduced JNK signaling in retinitis pigmentosa caused by RP1 mutations

To understand the mechanisms underlying autosomal dominant progressive retinitis pigmentosa (RP) caused by the mutations of the RP1 gene and to identify molecules that play roles in the early disease process, we used Affymetrix U74Av2 microarrays to compare the gene expression profiles of retinas from Rp1-/- and Rp1+/+ mice at postnatal days (P) 7, 10, 14, 18 and 21. These profiles were independently verified by comparison with results of retinal serial analysis of gene expression, U74Av2 array studies of mouse retinas, real-time PCR and in situ hybridization. We found that the disruption of Rp1 significantly affected the expression of multiple clusters of genes whose products were involved in diverse biological pathways. The molecular responses to the disruption of Rp1 changed dramatically during development and were distinct from responses to the disruption of photoreceptor transcription factors (Crx-/- or Nrl-/-) and a phototransduction molecule (Pde6brd1). We found specific alterations of gene expression in the c-Jun N-terminal kinase (JNK) signaling cascades. Western analysis confirmed that the phosphorylation of key members in the JNK signaling cascades (i.e. JNK1, JNK2, MAP2, MKK4 and c-Jun) is reduced, whereas phospho-ERK and phospho-p38 are unchanged, in Rp1-/- retinas at P18-21. Immunostaining demonstrated that, like Rp1, phospho-JNKs and phospho-MAP2 are present in outer segments of photoreceptors. Our studies reveal unique molecular phenotypes in multiple biological pathways and the specific reduction of JNK signaling cascades in RP1 diseases, and suggest that RP1, a doublecortin-containing microtubule associated protein, and JNK signaling cascades play integral roles in photoreceptor development and maintenance. Our studies further suggest JNK-related therapeutic strategies for RP1 diseases.

Microtubule-associated protein 1B controls directionality of growth cone migration and axonal branching in regeneration of adult dorsal root ganglia neurons

During development, microtubule-associated protein 1B (MAP1B) is one of the earliest MAPs, preferentially localized in axons and growth cones, and plays a role in axonal outgrowth. Although generally downregulated in the adult, we have shown that MAP1B is constitutively highly expressed in adult dorsal root ganglia (DRGs) and associated with central sprouting and peripheral regeneration of these neurons. Mutant mice with a complete MAP1B null allele that survive until adulthood exhibit a reduced myelin sheath diameter and conductance velocity of peripheral axons and lack of the corpus callosum. Here, to determine the function of MAP1B in axonal regeneration, we used cultures of adult DRG explants and/or dissociated neurons derived from this map1b-/- mouse line. Whereas the overall length of regenerating neurites lacking MAP1B was similar to wild-type controls, our analysis revealed two main defects. First, map1b-/- neurites exhibited significantly (twofold) higher terminal and collateral branching. Second, the turning capacity of growth cones (i.e., "choice" of a proper orientation) was impaired. In addition, lack of MAP1B may affect the post-translational modification of tubulin polymers: quantitative analysis showed a reduced amount of acetylated microtubules within growth cones, whereas the distribution of tyrosinated or detyrosinated microtubules was normal. Both growth cone turning and axonal branch formation are known to involve local regulation of the microtubule network. Our results demonstrate that MAP1B plays a role in these processes during plastic changes in the adult. In particular, the data suggest MAP1B implication in the locally coordinated assembly of cytoskeletal components required for branching and straight directional axon growth.

Vesicular glutamate transporter 3 expression identifies glutamatergic amacrine cells in the rodent retina

Synaptic transmission from glutamatergic neurons requires vesicular glutamate transporters (VGLUTs) to concentrate cytosolic glutamate in synaptic vesicles. In retina, glutamatergic photoreceptors and bipolar cells exclusively express the VGLUT1 isoform, whereas ganglion cells express VGLUT2. Surprisingly, the recently identified VGLUT3 isoform was found in presumed amacrine cells, generally considered to be inhibitory interneurons. To investigate the synaptic machinery and conceivable secondary neurotransmitter composition of VGLUT3 cells, and to determine a potential functional role, we further investigated these putative glutamatergic amacrine cells in adult and developing rodent retina. Reverse transcriptase-PCR substantiated VGLUT3 expression in mouse retina. VGLUT3 cells did not immunostain for ganglion or bipolar cell markers, providing evidence that they are amacrine cells. VGLUT3 colocalized with synaptic vesicle markers, and electron microscopy showed that VGLUT3 immunostained synaptic vesicles. VGLUT3 cells were not immunoreactive for amacrine cell markers gamma-aminobutyric acid, choline acetyltransferase, calretinin, or tyrosine hydroxylase, although they immunostain for glycine. VGLUT3 processes made synaptic contact with ganglion cell dendrites, suggesting input onto these cells. VGLUT3 immunostaining was closely associated with the metabotropic glutamate receptor 4, which is consistent with glutamatergic synaptic exocytosis by these cells. In the maturing mouse retina, Western blots showed VGLUT3 expression at postnatal day 7/8 (P7/8). VGLUT3 immunostaining in retinal sections was first observed at P8, achieving an adult pattern at P12. Thus, VGLUT3 function commences around the same time as VGLUT1-mediated glutamatergic transmission from bipolar cells. Furthermore, a subset of VGLUT3 cells expressed the circadian clock gene period 1, implicating VGLUT3 cells as part of the light-entrainable retina-based circadian system.

Bone morphogenetic protein-2 and -4 limit the number of enteric neurons but promote development of a TrkC-expressing neurotrophin-3-dependent subset

The hypothesis that BMPs (bone morphogenetic proteins), which act early in gut morphogenesis, also regulate specification and differentiation in the developing enteric nervous system (ENS) was tested. Expression of BMP-2 and BMP-4, BMPR-IA (BMP receptor subunit), BMPR-IB, and BMPR-II, and the BMP antagonists, noggin, gremlin, chordin, and follistatin was found when neurons first appear in the primordial bowel at embryonic day 12 (E12). Agonists, receptors, and antagonists were detected in separated populations of neural crest- and noncrest-derived cells. When applied to immunopurified E12 ENS precursors, BMP-2 and BMP-4 induced nuclear translocation of phosphorylated Smad-1 (Sma and Mad-related protein). The number of neurons developing from these cells was increased by low concentrations and decreased by high concentrations of BMP-2 or BMP-4. BMPs induced the precocious appearance of TrkC-expressing neurons and their dependence on neurotrophin-3 for survival. BMP-4 interacted with glial cell line-derived neurotrophic factor (GDNF) to enhance neuronal development but limited GDNF-driven expansion of the precursor pool. BMPs also promoted development of smooth muscle from mesenchymal cells immunopurified at E12. To determine the physiological significance of these observations, the BMP antagonist noggin was overexpressed in the developing ENS of transgenic mice under the control of the neuron-specific enolase promoter. Neuronal numbers in both enteric plexuses and smooth muscle were increased throughout the postnatal small intestine. These increases were already apparent by E18. In contrast, TrkC-expressing neurons decreased in both plexuses of postnatal noggin-overexpressing animals, again an effect detectable at E18. BMP-2 and/or BMP-4 thus limit the size of the ENS but promote the development of specific subsets of enteric neurons, including those that express TrkC.

Deletion of the N-terminus of murine map2 by gene targeting disrupts hippocampal ca1 neuron architecture and alters contextual memory

Microtubule-associated protein-2 (MAP2) is a brain specific A-kinase anchoring protein that targets the cyclic AMP-dependent protein kinase holoenzyme (PKA) to microtubules. Phosphorylation of MAP2 by different protein kinases is crucial for neuronal growth. The N-terminus of MAP2 contains the binding site for regulatory subunit II of cAMP-dependent protein kinase (PKA-RIIbeta). Using homologous recombination, we created a mutant line of mice (delta1-158) that express truncated MAP2 lacking the N-terminal peptide and the PKA binding site. Deletion of the PKA binding site from the MAP2 gene resulted in decreased efficiency of MAP2 phosphorylation. Biochemical and immunohistochemical studies demonstrate major changes in the morphology of hippocampal neurons in delta1-158 mice. Behavioral tests indicate that delta1-158 mice were impaired (exhibited less conditioned freezing) relative to Wild-Type (WT) controls during a test of contextual, but not during auditory cue, fear conditioning when tested at 8 weeks or 8 months of age. The delta1-158 mice displayed a heightened sensitivity to shock at 8 weeks, but not at 8 months of age. We conclude that PKA binding to MAP2 and MAP2 phosphorylation is essential for the selective development of contextual memory.

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.

Dynamic microtubule-dependent interactions position homotypic neurones in regular monolayered arrays during retinal development

In the vertebrate retina cell layers support serial processing, while monolayered arrays of homotypic neurones tile each layer to allow parallel processing. How neurones form layers and arrays is still largely unknown. We show that monolayered retinal arrays are dynamic structures based on dendritic interactions between the array cells. The analysis of three developing retinal arrays shows that these become regular as a net of dendritic processes links neighbouring array cells. Molecular or pharmacological perturbations of microtubules within dendrites lead to a stereotyped and reversible disruption of array organization: array cells lose their regular spacing and the arrangement in a monolayer. This leads to a micro-mechanical explanation of how monolayers of regularly spaced ‘like-cells’ are formed.

Microtubule-associated protein 1B is involved in the initial stages of axonogenesis in peripheral nervous system cultured neurons

Neuronal process extension is dependent on the reorganisation of the cytoskeleton, in particular microtubules and microfilaments, and one of the ways in which microtubules are regulated is by a group of microtubule-associated proteins called MAPs. MAP1B, the first MAP to be expressed in developing neurons, has been shown to play an important role during axonogenesis. Previously, we have shown that a phosphorylated isoform of MAP1B is involved in maintaining growth cone microtubules in a dynamically unstable state. In order to further investigate the role of MAP1B during axonogenesis we have cultured dorsal root ganglion (DRG) neurons from a MAP1B deficient mutant mouse. These mice express only trace amounts of MAP1B, have defects in the development of their nervous system and die perinatally. Cultured DRG neurons from MAP1B deficient mice show a reduction in axon elongation and an increase in growth cone area. The reduction in axon elongation is most likely to occur due to an inhibition in the early stages of axonogenesis. Using time-lapse video we have verified that during the first 2 h after plating, MAP1B deficient neurones extend their axons with an average speed that is half the speed of control neurones. These results support the participation of MAP1B during the initial stages of axonogenesis.

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.

Distribution of the phosphorylated form of microtubule associated protein 1B in the fish visual system during optic nerve regeneration

Microtubule associated proteins are a heterogeneous group of proteins that have been implicated in regulating microtubule stability. They play an important role in the organisation of the neuronal cytoskeleton during neurite outgrowth, plasticity and regeneration. The fish visual system presents a considerable degree of plasticity. Thus, the retina grows continually throughout life and the optic nerve regenerates after crush. In the present study, we compared the distribution of the microtubule associated protein 1B in its phosphorylated form (MAP1B-phos) in the normal adult fish visual system with that observed during optic nerve regeneration after adult optic nerve crush using a specific monoclonal antibody mAb-150. Expression of MAP1B-phos was observed in some ganglion cell somata and in developing, growing axons within the control optic nerve. Few immunoreactive terminals were seen in the control optic tectum. After optic nerve crush, we found additional MAP1B-phos expression in regenerating axons throughout the visual system. Our results demonstrate that MAP1B-phos is present in growing and regenerating axons of fish retinal ganglion cells, which suggests that the phosphorylated form of MAP1B may play an important role in developmental and regeneration processes within the fish central nervous system.

Regulation of the expression and phosphorylation of microtubule-associated protein 1B during regeneration of adult dorsal root ganglion neurons

Microtubule-associated protein 1B is a major constituent of the neuronal cytoskeleton during the early stages of development. This protein and its phosphorylated isoform, microtubule-associated protein 1B-P, defined by the monoclonal antibody 1B-P [Boyne L. J. et al. (1995) J. Neurosci. Res. 40, 439-450], are present in growing axons and concentrated in the distal end near the growth cone. In most regions of the central nervous system, microtubule-associated protein 1B and microtubule-associated protein 1B-P are developmentally down-regulated. They remain, however, at relatively high levels in the adult peripheral nervous system, where microtubule-associated protein 1B-P is localized exclusively in axons. The aim of this study was to examine the levels of microtubule-associated protein 1B and its phosphorylated isoform during regenerative growth of peripheral axons. Following transection and re-apposition of the sciatic nerve at midthigh, the levels of total microtubule-associated protein 1B, microtubule-associated protein 1B-P and microtubule-associated protein 1B messenger RNA were analysed in dorsal root ganglion neurons and sciatic nerve axons using western blots and RNase protection assays. After the lesion, there was a small decrease in the levels of microtubule-associated protein 1B and its messenger RNA in dorsal root ganglion neurons. The proximal axonal stump showed a similar decrease in the levels of microtubule-associated protein 1B 30days after lesion and returned to normal 60-90days post-lesion. In the distal stump of the sciatic nerve, the levels of microtubule-associated protein 1B increased dramatically and rapidly between three and 14days, but the protein was localized mainly in activated Schwann cells and myelin-like structures, and not in axons [Ma D. et al. (1999) Brain Res. 823, 141-153]. With the regeneration of axons into the distal stump, an intense expression of microtubule-associated protein 1B was observed in these axons. Microtubule-associated protein 1B-P, however, disappeared from the degenerated distal axonal stump as early as three days post-operation, and was absent in the regenerating axons and in Schwann cells between three and 14days. The levels of microtubule-associated protein 1B-P recovered slowly and did not reach the normal levels even after 90days post-operation. In contrast to the response following transection, the levels of microtubule-associated protein 1B and microtubule-associated protein 1B-P were much less affected after nerve crush. We propose that the relatively high levels of microtubule-associated protein 1B and its messenger RNA in adult dorsal root ganglions support peripheral neuron regeneration. The presence of microtubule-associated protein 1B in the regenerating axons suggests that microtubule-associated protein 1B is involved in axonal growth during peripheral nerve regeneration. However, the phosphorylated microtubule-associated protein 1B-P isoform, associated with growing axons during development, is not present in the regenerating axons after transection, presumably because of changes in the activities of kinases and phosphatases associated with the injury. These observations underscore the difference between axonal development and regeneration and the importance of injury-related effects that occur locally.

Perinatal lethality of microtubule-associated protein 1B-deficient mice expressing alternative isoforms of the protein at low levels

Microtubule-associated protein 1B (MAP1B) has been implicated in axogenesis in cultured cells. To gain insight into the functions that MAP1B plays in vivo, we analyzed a strain of Map1B mutant mice generated by a gene trapping approach. Homozygous mice die on the first day after birth, probably due to a severe abnormal development of the nervous system. They present alterations in the structure of several brain regions. The normal Map1B gene yields different protein isoforms from alternatively spliced transcripts. The smaller isoforms were present in wild type, hetero-, and homozygous mice, but their expression was higher in the mutants than in the wild-type. Moreover, trace amounts of MAP1B protein were also observed in Map1B homozygous mutants, indicating an alternative splicing around the gene trap insertion. Thus, the Map1B gene trapped mutation reported in this work did not generated a null mutant, but a mouse with a drastic deficiency in MAP1B expression. Analyses of these mice indicate the presence of several neural defects and suggest the participation of MAP1B in neuronal migration.

Tau-mediated process outgrowth is differentially altered by the expression of MAP2b and MAP2c in Sf9 cells

It is well documented that the MAPs, MAP2 and tau, play pivotal roles in neurite outgrowth. Several isoforms of MAP2 and tau are coexpressed in neurons, suggesting that the pattern of neurite outgrowth results from a functional equilibrium among these isoforms. In the present study, by coexpressing two of these MAPs at the same time in Sf9 cells, we demonstrated that tau-mediated process outgrowth is affected differently by MAP2b and MAP2c. MAP2b impairs tau ability to induce process outgrowth. Tau affects MAP2c capacity to induce the formation of multiple processes. There is evidence that actin microfilaments (F-actin) are involved in the elaboration of tau-mediated process outgrowth in Sf9 cells. We compared the effects of MAP2b and MAP2c with the effects of tau on F-actin distribution and stability in Sf9 cells. In MAP2b- and MAP2c-expressing cells with processes, F-actin was redistributed. However, in MAP2b-expressing cells without processes, the distribution of F-actin appears to be similar to the one in wild-type infected cells. Collectively, these results indicate that MAP2b could impair the ability of MAP2c and tau to redistribute F-actin in Sf9 cells, thereby decreasing their capacity to induce process formation. Furthermore, MAP2b and MAP2c patterns of process outgrowth were differentially modified by depolymerization of F-actin by cytochalasin D (CD). As previously reported for tau-expressing cells, the MAP2b-expressing cells developed a higher number of processes per cell and a higher number of cells presented processes in the presence of CD. However, the number of cells with multiple processes was lower in MAP2b-expressing cells than in tau-expressing cells treated with CD at 24 h postinfection. This suggests that MAP2b exerts an effect on F-actin stability at an earlier stage of infection than tau. MAP2c had also some stabilizing effects on F-actin at an early stage of infection, since the percentage of cells presenting one process was similar to the nontreated cells. Therefore, MAP2b seems to have less capacity than MAP2c to redistribute F-actin but, nonetheless, both of these MAP2 isoforms exert a stabilizing effect on F-actin at an early stage of infection. Finally, by modifying phosphorylation we showed that MAP2c capacity to induce multiple processes is related to protein phosphorylation in Sf9 cells. Therefore, the differential effect of MAP2c and MAP2b on process outgrowth seems also to depend on protein phosphorylation.

MAP1B expression and microtubule stability in growing and regenerating axons

MAP1B is a microtubule-associated phosphoprotein that is particularly highly expressed in developing neurons. There is experimental evidence that it plays an important role in neuronal differentiation, especially the extension of axons and dendrites, but exactly what role is unclear. Recent experiments have shed light on the gene structure of MAP1B and identified some of the kinases that phosphorylate the protein. Implicit in these findings is the idea that MAP1B regulates the organisation of microtubules in neurites and is itself regulated in a complex way and at a number of levels.

Novel low molecular weight microtubule-associated protein-2 isoforms contain a functional nuclear localization sequence

Known high and low molecular weight (LMW) MAP2 protein isoforms result from alternative splicing of the MAP2 gene. Contrary to previous reports that MAP2 is neural-specific, we recently identified MAP2 mRNA and protein in somatic and germ cells of rat testis, and showed the predominant testicular isoform is LMW. Although cytoplasmic in neural tissue, MAP2 appeared predominantly nuclear in germ cells using immunohistochemistry. We sought to determine whether this unexpected localization was due to the inclusion of exon 10 within novel LMW MAP2 isoforms. Normally excluded from the LMW MAP2c, exon 10 harbors a putative CcN motif, comprising a nuclear localization sequence (NLS) flanked by regulatory phosphorylation sites for protein kinase CK2 and cdc2 kinase. Characterization of MAP2 mRNA in adult and immature brain and testis, by reverse transcriptase-polymerase chain reaction/Southern analysis and Northern blot, identified novel LMW forms containing exons 10 and 11, previously detected only in high molecular weight MAP2a and 2b. The MAP2 NLS targeted a large heterologous protein to the nucleus, as demonstrated using bacterially expressed MAP2-CcN-beta-galactosidase fusion protein and an in vitro nuclear import assay. Antibodies raised against the fusion protein produced a testicular immunohistochemical staining pattern correlating with MAP2 protein distribution in the nucleus of most germ cells, and precipitated both approximately 70-kDa and >220-kDa proteins recognized by the commercial MAP2-specific HM2 monoclonal antibody, supporting our hypothesis of a novel LMW MAP2 isoform. These results demonstrate the presence of a functional NLS in MAP2 and indicate that novel LMW MAP2 isoforms may be targeted to the nucleus in both neural and non-neuronal tissues.

Identification and immunolocalization of actin cytoskeletal components in light- and dark-adapted octopus retinas

Photoreceptors in the octopus retina are of the rhabdomeric type, with rhabdomeres arising from the plasma membrane on opposite sides of the cylindrical outer segment. Each rhabdomere microvillus has an actin filament core, but other actin-binding proteins have not been identified. We used immunoblotting techniques to identify actin-binding proteins in octopus retinal extracts and immunofluorescence microscopy to localize the same proteins in fixed tissue. Antibodies directed against alpha-actinin and vinculin recognized single protein bands on immunoblots of octopus retinal extract with molecular weights comparable to the same proteins in other tissues. Anti-filamin identified two closely spaced bands similar in molecular weight to filamin in other species. Antibodies to the larger of the Drosophila ninaC gene products, p174, identified two bands lower in molecular weight than p174. Anti-villin localized a band that was significantly less in molecular weight than villin found in other cells. Epifluorescence and confocal microscopy were used to map the location of the same actin-binding proteins in dark- and light-adapted octopus photoreceptors and other retinal cells. Antibodies to most of the actin-binding proteins showed heavy staining of the photoreceptor proximal/supportive cell region accompanied by rhabdom membrane and rhabdom tip staining, although subtle differences were detected with individual antibodies. In dark-adapted retinas anti-alpha-actinin stained the photoreceptor proximal/supportive cell region where an extensive junctional complex joins these two cell types, but in the light, immunoreactivity extended above the junctional complex into the rhabdom bases. Most antibodies densely stained the rhabdom tips but anti-villin exhibited a striated pattern of localization at the tips. We believe that the actin-binding proteins identified in the octopus retina may play a significant role in the formation of new rhabdomere microvilli in the dark. We speculate that these proteins and actin remain associated with an avillar membrane that connects opposing sets of rhabdomeres in light-adapted retinas. Association of these cytoskeletal proteins with the avillar membrane would constitute a pool of proteins that could be recruited for rapid microvillus formation from the previously avillar region.