Distribution of neuronal intermediate filament proteins in the developing mouse olfactory system

Distribution patterns of the zebrafish neuronal intermediate filaments inaa and inab

It has been reported that the neuronal intermediate filament (IF) α-internexin may plays a role in the formation of the neuronal cytoskeleton during mammalian development. From a phylogenetic viewpoint, zebrafish express inaa and inab as homologs of mammalian α-internexin. However, the distribution patterns of the inaa and inab proteins throughout zebrafish development have not been well-characterized. We generated antibodies specific for zebrafish inaa and inab and analyzed the distribution of these two proteins in developing zebrafish. Inaa was identified in the major subdivisions of embryonic and larval brains as early as 1 day postfertilization (dpf), including the telencephalon, optic tectum, and cerebellum, and inab was also detected in the same regions from 3 dpf to the adult stage. Moreover, we demonstrated for the first time that inaa was distinctively expressed in the photoreceptor-like cells of the pineal gland, where inab was sparsely detected. Besides, the expression of inaa in male adult fish was found to be stable under different photoperiod conditions. Thus, we suggest that inaa is one of useful markers for studies of zebrafish cone photoreceptors not only in the retina but also in the pineal gland. In conclusion, we report that the distribution patterns of inaa and inab are phylogenetically conserved in the telencephalon, optic tectum, and cerebellum. Moreover, inaa and inab had different expression patterns in the pineal gland and retina during zebrafish development. Both inaa and inab are neuronal IFs and their functional roles may be different in various aspects of zebrafish neuronal development.

The third wave: Intermediate filaments in the maturing nervous system

Intermediate filaments are critical for the extreme structural specialisations of neurons, providing integrity in dynamic environments and efficient communication along axons a metre or more in length. As neurons mature, an initial expression of nestin and vimentin gives way to the neurofilament triplet proteins and α-internexin, substituted by peripherin in axons outside the CNS, which physically consolidate axons as they elongate and find their targets. Once connection is established, these proteins are transported, assembled, stabilised and modified, structurally transforming axons and dendrites as they acquire their full function. The interaction between these neurons and myelinating glial cells optimises the structure of axons for peak functional efficiency, a property retained across their lifespan. This finely calibrated structural regulation allows the nervous system to maintain timing precision and efficient control across large distances throughout somatic growth and, in maturity, as a plasticity mechanism allowing functional adaptation.

Developmental pattern of the neuronal intermediate filament inaa in the zebrafish retina

α-Internexin is a member of the neuronal intermediate filament (nIF) protein family, which also includes peripherin and neurofilament (NF) triplet proteins. Previous studies found that expression of α-internexin precedes that of the NF triplet proteins in mammals and suggested that α-internexin plays a key role in the neuronal cytoskeleton network during development. In this study, we aimed to analyze the expression patterns and function of internexin neuronal intermediate filament protein-alpha a (inaa), the encoding gene of which is a homolog of the mammalian α-internexin, during retinal development in zebrafish. Via in vitro and in vivo studies, we demonstrated that zebrafish inaa is an α-internexin homolog that shares characteristics with nIFs. An immunohistochemical analysis of zebrafish revealed that inaa was distributed dynamically in the developing retina. It was widely localized in retinal neuroepithelial cells at 1 day postfertilization (dpf), and was mainly found in the ganglion cell layer (GCL) and inner part of the inner nuclear layer (INL) from 3-9 dpf; after 14 dpf, it was restricted to the outer nuclear layer (ONL). Moreover, we demonstrated for the first time that inaa acted distinctively from the cytoskeletal scaffold of zebrafish cone photoreceptors during development. In conclusion, we demonstrated the morphological features of a novel nIF, inaa, and illustrated its developmental expression pattern in the zebrafish retina. J. Comp. Neurol. 524:3810-3826, 2016. © 2016 Wiley Periodicals, Inc.

STOP proteins contribute to the maturation of the olfactory system

Regulation of microtubule dynamics is crucial for axon growth and guidance as well as for the establishment of synaptic connections. STOPs (Stable Tubule Only Polypeptides) are microtubule-associated proteins that regulate microtubule stabilization but are also able to interact with actin or Golgi membranes. Here, we have investigated the involvement of STOPs during the development of the olfactory system. We first describe the spatio-temporal expression patterns of N- and E-STOP, the two neuronal-specific isoforms of STOP. E- and N-STOP are expressed in the axonal compartment of olfactory sensory neurons, but are differentially regulated during development. Interestingly, each neuronal isoform displays a specific gradient distribution within the olfactory nerve layer. Then, we have examined the development of the olfactory system in the absence of STOPs. Olfactory axons display a normal outgrowth and targeting in STOP-null mice, but maturation of the synapses in the glomerular neuropil is altered.

Abnormal cellular translocation of alpha-internexin in spinal motor neurons of Dystonia musculorum mice

Dystonia musculorum (dt) is a mutant mouse with hereditary neuropathy where the dysfunction is mainly found in the dorsal root ganglia (DRG) neurons but not in the spinal motor neurons. However, the accumulation of intermediate filament (IF) proteins in the swelling axons of spinal motor neurons could be found in dt/dt mice. In order to understand the pathological role of neuronal IFs in the swelling axons of spinal motor neurons from dt/dt mice, we extensively examined the distribution of neuronal IF proteins. By immunofluorescence staining, our results indicated that alpha-internexin was a major component in the swelling axon and showed abnormal translocation in the nuclei of spinal motor neurons in dt/dt mice. This abnormal translocation of alpha-internexin in the nuclei of spinal motor neurons was also confirmed by Western blotting and immunoelectron microscopy. Instead of the 10-nm filamentous structure, a diffuse immunopositive pattern of alpha-internexin was observed in the nucleus of spinal motor neurons in dt/dt mutants. We further examined the cell death of spinal motor neurons by TUNEL assay, and no TUNEL-positive cells could be identified from spinal motor neurons in dt/dt mice. From these observations we suggest that abnormal accumulation of neuronal IFs in the swelling axons and abnormal translocation of alpha-internexin in the nuclei of the spinal motor neurons from dt/dt mice may not directly cause cell death of the spinal motor neurons.

The role of Dlx homeogenes in early development of the olfactory pathway

Development of the olfactory pathway requires interaction between cells and signals of different origin. Olfactory receptor neurons (ORN) in the olfactory placodes (OP) extend axons towards the forebrain (FB); with innervation taking place at a later time following degradation of the basement membrane. Cells from the OP migrate along ORN axons and differentiate into various elements, including ensheathing and Gonadotropin Releasing Hormone (GnRH)+ cells. The importance of the olfactory connection and migration is highlighted by the severe endocrine phenotype in Kallmann's patients who lack this migratory pathway. Little is known about the genetic control of intrinsic ORN properties. Inactivation of the distalless-related Dlx5 prevents connections between ORNs and FB. Using a grafting approach we show that misguidance and lack of connectivity is due to intrinsic defects in ORN neurites and migratory cells (MgC), and not to environmental factors. These data point to a cell-autonomous function of Dlx5 in providing ORN axons with their connectivity properties. Dlx5 also marks a population of early MgC that partly overlaps with the GnRH+ population. In the absence of Dlx5 MgCs of the Dlx5+ lineage migrate, associated with PSA-NCAM+ axons, but fail to reach the FB as a consequence of the lack of axonal connection and not an inability to migrate. These data suggests that Dlx5 is not required to initiate migration and differentiation of MgCs.

Neural expression of alpha-internexin promoter in vitro and in vivo

alpha-Internexin is a 66 kDa neuronal intermediate filament protein found most abundantly in the neurons of the nervous systems during early development. To characterize the function of mouse alpha-internexin promoter, we designed two different expression constructs driven by 0.7 kb or 1.3 kb of mouse alpha-internexin 5'-flanking sequences; one was the enhanced green fluorescent protein (EGFP) reporter for monitoring specific expression in vitro, and the other was the cre for studying the functional DNA recombinase in transgenic mice. After introducing DNA constructs into non-neuronal 3T3 fibroblasts and a neuronal Neuro2A cell line by lipofectamine transfection, we observed that the expression of EGFP with 1.3 kb mouse alpha-internexin promoter was in a neuron-dominant manner. To establish a tissue-specific pattern in the nervous system, we generated a transgenic mouse line expressing Cre DNA recombinase under the control of 1.3 kb alpha-Internexin promoter. The activity of the Cre recombinase at postnatal day 1 was examined by mating the cre transgenic mice to ROSA26 reporter (R26R) mice with knock-in Cre-mediated recombination. Analyses of postnatal day 1 (P1) newborns showed that beta-galactosidase activity was detected in the peripheral nervous system (PNS), such as cranial nerves innervating the tongue and the skin as well as spinal nerves to the body trunk. Furthermore, X-gal-labeled dorsal root ganglionic (DRG) neurons showed positive for alpha-Internexin in cell bodies but negative in their spinal nerves. The motor neurons in the spinal cord did not exhibit any beta-galactosidase activity. Therefore, the cre transgene driven by mouse alpha-internexin promoter, described here, provides a useful animal model to specifically manipulate genes in the developing nervous system.

A Possible Cellular Mechanism of Neuronal Loss in the Dorsal Root Ganglia of Dystonia musculorum (dt) Mice

Dystonia musculorum (dt) is a mutant mouse with hereditary sensory neuropathy. A defective bullous pemphigoid antigen 1 (BPAG1) gene is responsible for this mutation. In the present study, we examined the distribution of neuronal intermediate filament proteins in the central and peripheral processes of the dorsal root ganglia (DRG) in adult dt mice using different approaches. We found that not only BPAG1, but also alpha-internexin was absent in the DRG neurons in adult dt mice. To study the relationship between the absence of alpha-internexin and the progressive neuronal loss in the DRG of dt mice, we further cultured DRG neurons from embryonic dt mutants. Immunocytochemical assay of cultured DRG neurons from dt embryos revealed that alpha-internexin was aggregated in the proximal region of axons and juxtanuclear region of the cytoplasma, yet the other intermediate filament proteins were widely distributed in all processes. The active caspase-3 activity was observed in the dt neuron with massive accumulation of alpha-internexin. From our observations, we suggest that the interaction between BPAG1 and alpha-internexin may be one of the key factors involved in neuronal degeneration, and abnormal accumulation of alpha-internexin may impair the axonal transport and subsequently turns on the cascade of neuronal apoptosis in dt mice.

Morphological and molecular features of the mammalian olfactory sensory neuron axons: What makes these axons so special?

The main organization and gross morphology of the mammalian olfactory primary pathway, from the olfactory epithelium to the olfactory bulb, has been initially characterized using classical anatomical and ultrastructural approaches. During the last fifteen years, essentially thanks to the cloning of the odorant receptor genes, and to the characterization of a number of molecules expressed by the olfactory sensory neuron axons and their environment, significant new insights have been gained into the understanding of the development and adult functioning of this system. In the course of these genetic, biochemical and neuroanatomical studies, however, several molecular and structural features were uncovered that appear somehow to be unique to these axons. For example, these axons express odorant receptors in their terminal segment, and transport several mRNA species and at least two transcription factors. In the present paper, we review these unusual structural and molecular features and speculate about their possible functions in the development and maintenance of the olfactory system.

Cytoskeletal organization of the developing mouse olfactory nerve layer

Olfactory sensory neuron (OSN) axonal extension and targeting occur within the olfactory nerve layer (ONL) of the olfactory bulb (OB). The ONL can be differentiated into sublaminae: the outer (ONLo), where axons broadly target regions of the OB in tight fascicles, and inner (ONLi), where axons perform final targeting in loosely organized fascicles. During perinatal development, cadherin-2 and its binding partner, gamma-catenin, are preferentially expressed by OSN axons in the ONLo vs. the ONLi. Given the expression of these cytoskeleton-associated molecules, we hypothesized that cytoskeletal elements of OSN axons may be differentially expressed across the ONL. We therefore examined cytoskeletal organization of OSN axons in the ONL, focusing on the day of birth (P0). We show that microfilaments, microtubules, and the intermediate filament (IF) vimentin are homogeneously expressed across the ONL at P0. In contrast, the IFs peripherin and alpha-internexin are preferentially localized to the ONLo at P0, with alpha-internexin expressed by a restricted subset of OSNs. We also show that OSN axons in the ONLo are significantly smaller than those in the ONLi. The data demonstrate that, as OSN axons begin to exit the ONLo and target a specific region of the OB, there is a down-regulation of cytoskeletal elements and bound extracellular adhesion molecules. The increase in axon diameter may reflect additional mechanisms involved in glomerular targeting or the formation of the large terminal boutons of OSN axons within glomeruli.

The expression of alpha-internexin and peripherin in the developing mouse pineal gland

The mammalian pineal gland contains pinealocytes, interstitial glial cells, perivascular macrophages, neurons and neuron-like cells. The neuronal identity of neurons and neuron-like cells was an enigma. alpha-Internexin and peripherin are specific neuronal intermediate filament proteins and are expressed differentially in the CNS and PNS. We investigated the development of immunoreactivity and expression patterns of mRNAs for alpha-internexin and peripherin in the mouse pineal gland to determine the neuronal identity of these cells. Both alpha-internexin- and peripherin-immunoreactive cells were readily visualized only after birth. Both proteins were at the highest level on the postnatal day 7 (P7), rapidly declined at P14, and obtained their adult level at P21. Both protein and mRNA of alpha-internexin are expressed in some cells and nerve processes, but not all, of adult mouse pineal gland. Less number of peripherin immunoreactive or RNA-expressing cells and nerve processes were identified. Accumulations of alpha-internexin and peripherin proteins were also found in the cells from the aged pineal gland (P360). We concluded that some cells in the developing mouse pineal gland may differentiated into neurons and neuron-like cells expressing both alpha-internexin and/or peripherin only postnatally, and these cells possess dual properties of CNS and PNS neurons in nature. We suggested that they may act as interneurons between the pinealocyte and the distal neurons innervating the pinealocytes, or form a local circuitry with pinealocytes to play a role of paracrine regulatory function on the pinealocytes.

Timing of neuronal intermediate filament proteins expression in the mouse vomeronasal organ during pre- and postnatal development. An immunohistochemical study

Several types of intermediate filament proteins are expressed in developing and mature neurons; they cooperate with other cytoskeletal components to sustain neuronal function from early neurogenesis onward. In this work the timing of expression of nestin, peripherin, internexin, and the neuronal intermediate filament triplet [polypeptide subunits of low (NF-L), medium (NF-M), and high (NF-H) molecular weight] was investigated in the developing fetal and postnatal mouse vomeronasal organ (VNO) by means of immunohistochemistry. The results show that the sequence of expression of intermediate filament proteins is internexin, nestin, and NF-M in the developing vomeronasal sensory epithelium; internexin, peripherin, and NF-M in the developing vomeronasal nerve; and nestin, internexin and peripherin, NF-L, and NF-M in the nerve supply to accessory structures of the VNO. At sexual maturity (2 months) NF-M is only expressed in vomeronasal neurons and NF-M, NF-L and peripherin are expressed in extrinsic nerves supplying VNO structures. The differential distribution of intermediate filament proteins in the vomeronasal sensory epithelium and nerve is discussed in terms of the cell types present therein. It is concluded that several intermediate filament proteins are sequentially expressed during intrauterine development of the VNO neural structures in a different pattern according to the different components of the VNO.

Characterization of olfactory bulb glomeruli in schizophrenia

Olfactory deficits, observed in schizophrenia, may be associated with a disruption of synaptic transmission in the olfactory system. Using immunohistochemistry and optical densitometry, we assessed the integrity of the synaptic connection between olfactory receptor neurons and olfactory bulb target neurons in schizophrenia by comparing the level of eight proteins, expressed in the olfactory bulb glomeruli, among schizophrenia and control subjects. In schizophrenia, no change was observed in the levels of OMP, GAP43 and NCAM, proteins expressed by olfactory receptor neurons, suggesting an intact innervation of the olfactory bulb by these neurons. This was supported by the absence of change in calbindin level, which has been shown to decrease after the destruction of the olfactory epithelium. The level of synaptophysin, a pre-synaptic protein, was also unchanged. These findings suggested that axons of olfactory receptor neurons establish synapses with their olfactory bulb targets in schizophrenia. The absence of change in the level of poorly phosphorylated neurofilament of moderate and high molecular weight (NFM/HP) suggested no lack of dendritic innervation despite a previously seen reduction of glomerular MAP2 level in schizophrenia subjects. This and above findings were consistent with the absence of change in the level of beta-tubulin III, a protein expressed by neurons of both olfactory epithelium and bulb. Finally, we noted no significant decrease in trkB level, a neurotrophin receptor involved in the olfactory epithelium maintenance. This study showed no evidence of major structural alteration of the synapse between the olfactory epithelium and bulb in schizophrenia.

Expression of the neuronal calcium sensor protein NCS-1 in the developing mouse olfactory pathway

Neuron specific calcium sensor 1 (NCS-1) is widely expressed in the developing and adult nervous system. Like calmodulin, NCS-1 is a member of a family of calcium binding proteins that contain EF-hand motifs, which bind calcium and induce conformational changes in the protein. Their binding varies with calcium concentration, allowing them to act as true calcium sensors rather than just calcium binding proteins. This family of proteins has been implicated in important synaptic events including neurotransmitter release and synapse formation. We examined the expression of NCS-1 in the developing and mature olfactory system to determine whether this molecule may be playing a role in establishing and/or maintaining olfactory circuitry. During development, expression of NCS-1 in the olfactory epithelium was localized in the dendritic knobs and axons of olfactory sensory neurons. Axonal expression was down-regulated after synapse formation. In the developing olfactory bulb, NCS-1 was expressed in the processes of mitral/tufted and granule cells. However, in the adult olfactory bulb, strongest expression was found in a subset of periglomerular cells (PGCs). This subset of PGCs did not express other known markers of PGCs including tyrosine hydroxylase, glutamic acid decarboxylase, calbindin, or calretinin, and only partially overlapped with the subpopulation of PGCs that express parvalbumin. Together, these data suggest multiple and overlapping roles of NCS-1 in the developing and mature olfactory system.

Axonally transported peripheral signals regulate alpha-internexin expression in regenerating motoneurons

The class IV neuronal intermediate filament (IF) family proteins includes the neurofilament (NF) triplet proteins NF-L, NF-M, and NF-H and also the more recently characterized alpha-internexin-NF66. It is well established that NF-L, -M, and -H protein and mRNA are downregulated after peripheral nerve injury. We examined alpha-internexin protein expression after three facial nerve lesion paradigms: crush, transection, and resection. Alpha-internexin immunoreactivity was absent in the perikarya of uninjured facial motoneurons but increased dramatically in all three injury paradigms, with maximum immunoreactivity observed at 7 d after injury. Twenty-eight days after nerve crush or transection, there was a dramatic decrease in the number of alpha-internexin-positive cells. In contrast, alpha-internexin remained elevated 28 d after nerve resection, an injury that hinders regeneration and target reinnervation. In situ hybridization studies showed an increase in alpha-internexin mRNA expression in the facial nucleus at 7 and 14 d after injury. Retrograde transport of fluorogold from the whisker pads to the facial nucleus was seen only in motoneurons that lacked alpha-internexin immunoreactivity, supporting the idea that target reinnervation and inhibitory signals from the periphery regulate the expression of alpha-internexin. Blockage of axonal transport through local colchicine application induced strong immunoreactivity in motoneurons. Alpha-internexin expression was also examined after central axotomy of rubrospinal neurons, which constitutively show alpha-internexin immunoreactivity. After rubrospinal tractotomy, alpha-internexin immunoreactivity transiently increased by 7 d after injury but returned to control levels by 14 d. We conclude that alpha-internexin upregulation in injured motoneurons suggests a role for this IF protein in neuronal regeneration.

Development of the facial midline

"Intellectual excellence lies in having faith in the observation of apparently nontranscendental and unimportant facts. To observe an anatomic element calmly, with an open, analytical spirit, and with spiritual freedom, can lead to an explosive vortex of new knowledge."-Miguel Orticochea, M.D.(1) Traditional descriptive embryology based upon the interaction of frontonasal, lateral nasal, and medial nasal prominences is incapable of explaining the three-dimensional development of the facial midline. The internal structure of the nose and that of the oronasal midline can best be explained by the presence of paired A fields originating from the prechordal mesendoderm, associated with the nasal and optic placodes, supplied by the internal carotid artery, and sharing a common genetic coding with the prosomeres of the forebrain. Mesial drift of these fields leads to fusion of their medial walls; this in turn provides bilateral functional matrics within which form the orbits ethmoids, lacrimals, turbinates, premaxillae, vomerine bones, and the cartilages of the nose. This two-part paper reports six lines of evidence supporting the field theory model of facial development: (1) An apparent watershed exists in the midline of the base between the territories of the internal and external carotid systems. Isolation of the ICA in injected fetal specimens confirmed that the demarcation was distinct and restricted to the embryonic nasal capsule. (2) Field theory explains the developmental anatomy of the contents of the nasal capsule. (3) The neuromeric model of CNS development provides a genetic basis for the anatomy and behavior of fields. (4) Mutants for the Dlx5 gene demonstrate A field deletion patterns. These experiments relate the nasal placode to the structures of the A fields. (5) Separate regions of the original nasal placodes give rise to neurons, which are dedicated to separate sensory and endocrine systems. The A fields constitute the pathways by which these neurons reach the brain. (6) Non-cleft lip-related cleft palate, holoprosencephaly, and the Kallmann syndrome are clinical models that demonstrate the effects of anatomic disturbances within the A fields.

Expression of SCG10 and stathmin proteins in the rat olfactory system during development and axonal regeneration

The membrane-associated protein SCG10 is expressed specifically by neuronal cells. Recent experiments have suggested that it promotes neurite outgrowth by increasing microtubule dynamics in growth cones. SCG10 is related to the ubiquitous but neuron-enriched cytosolic protein stathmin. To better understand the role played by SCG10 and stathmin in vivo, we have analyzed the expression and localization of these proteins in both the olfactory epithelium and the olfactory bulb in developing and adult rats, as well as in adult bulbectomized rats. The olfactory epithelium is exceptional in that olfactory receptor neurons constantly regenerate and reinnervate the olfactory bulb throughout animal life-span. SCG10 and stathmin expression in the olfactory receptor neurons was found to be regulated during embryonic and postnatal development and to correlate with neuronal maturation. Whereas SCG10 expression was restricted to immature olfactory receptor neurons (GAP-43-positive, olfactory marker protein-negative), stathmin was also expressed by the basal cells. In the olfactory bulb of postnatal and adult rats, a moderate to strong SCG10 immunoreactivity was present in the olfactory nerve layer, whereas no labeling was detected in the glomerular layer. Olfactory glomeruli also showed no apparent immunoreactivity for several cytoskeletal proteins such as tubulin and microtubule-associated proteins. In unilaterally bulbectomized rats, SCG10 and stathmin were seen to be up-regulated in the regenerating olfactory epithelium at postsurgery stages corresponding to olfactory axon regeneration. Our data strongly suggest that, in vivo, both SCG10 and stathmin may play a role in axonal outgrowth during ontogenesis as well as during axonal regeneration.

Neuronal intermediate filament protein alpha-internexin facilitates axonal neurite elongation in neuroblastoma cells

We examined the localization and role of alpha-IN vs. other neuronal intermediate filaments before and during differentiation. Vimentin but not alpha-IN localized within filopodia-like neurites of undifferentiated cells. During differentiation, alpha-IN immunoreactivity accumulated within axonal neurites following vimentin but, as previously describe in neurons in situ, before the appearance of NF-L. We therefore manipulated alpha-IN synthesis, accumulation, and function in attempts to determine whether or not this intermediate filament species played a role in axonal development. Intracellular delivery of anti-alpha-IN antisense oligonucleotides and antibodies was permissive for neuritogenesis, yet compromised neurite elongation; this effect was further reflected in diminished levels of stabilized axonal microtubules. These data suggest that alpha-IN plays a role in the development of neuronal polarity. Relatively more alpha-IN than NF-L accumulated within the plastic axonal neurites induced following serum-deprivation, while stable, dbcAMP-induced neurites treatment contained equivalent levels of each. Protease inhibition increased NF-L and NF-H but not alpha-IN immunoreactivity within serum-deprived neurites, suggesting that proteolysis restricts NF-L accumulation pending neurite stabilization. To test the possibility that NF-H accumulation is dependent upon NF-L and cannot be mediated by alpha-IN, we examined levels of NF-H co-precipitated from cells with alpha-IN and NF-L. Virtually all newly synthesized NF-H co-precipitated with NF-L, while only a small percentage co-precipitated with alpha-IN. Finally, NF-H or NF-M were absent from the axon hillock or perikaryal area at the base of neurites, where alpha-IN immunoreactivity is prominent. These data extend earlier cell-free demonstrations that NF-H preferentially associates with NF-L rather than alpha-IN.