Expression of neurofilament proteins during development of the nervous system in the squid Loligo pealei

Neuronal segmentation in cephalopod arms

The prehensile arms of the cephalopod are among these animals most remarkable features, but the neural circuitry governing arm and sucker movements remains largely unknown. We studied the neuronal organization of the adult axial nerve cord (ANC) of Octopus bimaculoides with molecular and cellular methods. The ANCs, which lie in the center of every arm, are the largest neuronal structures in the octopus, containing four times as many neurons as found in the central brain. In transverse cross section, the cell body layer (CBL) of the ANC wraps around its neuropil (NP) with little apparent segregation of sensory and motor neurons or nerve exits. Strikingly, when studied in longitudinal sections, the ANC is segmented. ANC neuronal cell bodies form columns separated by septa, with 15 segments overlying each pair of suckers. The segments underlie a modular organization to the ANC neuropil: neuronal cell bodies within each segment send the bulk of their processes directly into the adjoining neuropil, with some reaching the contralateral side. In addition, some nerve processes branch upon entering the NP, forming short-range projections to neighboring segments and mid-range projections to the ANC segments of adjoining suckers. The septa between the segments are employed as ANC nerve exits and as channels for ANC vasculature. Cellular analysis establishes that adjoining septa issue nerves with distinct fiber trajectories, which across two segments (or three septa) fully innervate the arm musculature. Sucker nerves also use the septa, setting up a nerve fiber "suckerotopy" in the sucker-side of the ANC. Comparative anatomy suggests a strong link between segmentation and flexible sucker-laden arms. In the squid Doryteuthis pealeii, the arms and the sucker-rich club of the tentacles have segments, but the sucker-poor stalk of the tentacles does not. The neural modules described here provide a new template for understanding the motor control of octopus soft tissues. In addition, this finding represents the first demonstration of nervous system segmentation in a mollusc.

Squid Giant Axon Contains Neurofilament Protein mRNA but does not Synthesize Neurofilament Proteins

When isolated squid giant axons are incubated in radioactive amino acids, abundant newly synthesized proteins are found in the axoplasm. These proteins are translated in the adaxonal Schwann cells and subsequently transferred into the giant axon. The question as to whether any de novo protein synthesis occurs in the giant axon itself is difficult to resolve because the small contribution of the proteins possibly synthesized intra-axonally is not easily distinguished from the large amounts of the proteins being supplied from the Schwann cells. In this paper, we reexamine this issue by studying the synthesis of endogenous neurofilament (NF) proteins in the axon. Our laboratory previously showed that NF mRNA and protein are present in the squid giant axon, but not in the surrounding adaxonal glia. Therefore, if the isolated squid axon could be shown to contain newly synthesized NF protein de novo, it could not arise from the adaxonal glia. The results of experiments in this paper show that abundant 3H-labeled NF protein is synthesized in the squid giant fiber lobe containing the giant axon's neuronal cell bodies, but despite the presence of NF mRNA in the giant axon no labeled NF protein is detected in the giant axon. This lends support to the glia-axon protein transfer hypothesis which posits that the squid giant axon obtains newly synthesized protein by Schwann cell transfer and not through intra-axonal protein synthesis, and further suggests that the NF mRNA in the axon is in a translationally repressed state.

Behavioral development in embryonic and early juvenile cuttlefish (Sepia officinalis)

Though a mollusc, the cuttlefish Sepia officinalis possesses a sophisticated brain, advanced sensory systems, and a large behavioral repertoire. Cuttlefish provide a unique perspective on animal behavior due to their phylogenic distance from more traditional (vertebrate) models. S. officinalis is well-suited to addressing questions of behavioral ontogeny. As embryos, they can perceive and learn from their environment and experience no direct parental care. A marked progression in learning and behavior is observed during late embryonic and early juvenile development. This improvement is concomitant with expansion and maturation of the vertical lobe, the cephalopod analog of the mammalian hippocampus. This review synthesizes existing knowledge regarding embryonic and juvenile development in this species in an effort to better understand cuttlefish behavior and animal behavior in general. It will serve as a guide to future researchers and encourage greater awareness of the utility of this species to behavioral science.

Compartment-Specific Phosphorylation of Squid Neurofilaments

Studies of the giant axon and synapse of third-order neurons in the squid stellate ganglion have provided a vast literature on neuronal physiology and axon transport. Large neuronal size also lends itself to comparative biochemical studies of cell body versus axon. These have focused on the regulation of synthesis, assembly, posttranslational modification and function of neuronal cytoskeletal proteins (microtubules (MTs) and neurofilaments (NFs)), the predominant proteins in axoplasm. These contribute to axonal organization, stability, transport, and impulse transmission responsible for rapid contractions of mantle muscles underlying jet propulsion. Studies of vertebrate NFs have established an extensive literature on NF structure, organization, and function; studies of squid NFs, however, have made it possible to compare compartment-specific regulation of NF synthesis, assembly, and function in soma versus axoplasm. Since NFs contain over 100 eligible sites for phosphorylation by protein kinases, the compartment-specific patterns of phosphorylation have been a primary focus of biochemical studies. We have learned that NF phosphorylation is tightly compartmentalized; extensive phosphorylation occurs only in the axonal compartment in squid and in vertebrate neurons. This extensive phosphorylation plays a key role in organizing NFs, in association with microtubules (MTs), into a stable, dynamic functional lattice that supports axon growth, diameter, impulse transmission, and synaptic activity. To understand how cytoskeletal phosphorylation is topographically regulated, the kinases and phosphatases, bound to NFs isolated from cell bodies and axoplasm, have also been studied.

The homeodomain transcription factor Phox2 in the stellate ganglion of the squidLoligo pealei

Homeodomain transcription factors regulate development of embryos and cellular physiology in adult systems. Paired-type homeodomain genes constitute a subclass that has been particularly implicated in establishment of neuronal identity in the mammalian nervous system. We isolated fragments of eight homeodomain genes of this subclass expressed in the stellate ganglion of the North Atlantic long finned squid Loligo pealei (lp) [Note: Loligo pealei has been officially renamed Doryteuthis pealei. For reasons of uniformity and clarity Loligo pealei (lp) is used here]. Of the most abundant ones, we cloned a full length cDNA which encoded the squid ortholog of the paired-type homeodomain proteins Phox2a/b. The homology of lpPhox2 to invertebrate and mammalian Phox2 was limited to the homeodomain. In contrast to mouse Phox2b, lpPhox2 was unable to transactivate the dopamine beta-hydroxylase (DBH) promoter in a heterologous mammalian transfection system. In vivo, lpPhox2 was expressed in the developing stellate ganglion of stage 27 squid embryos and continued to be expressed in the adult stellate neurons where expression was confined to the giant fiber lobe containing the neurons that form the giant axons. The expression of lpPhox was similarly timed and distributed as the Fmrf gene. Furthermore, the Fmrf upstream region contained putative Phox2a/b binding sites. These results suggest a role of lpPhox2 in the developmental specification of neuronal identity and regulation of neurons of the squid giant axon.

Differential expression of the FMRF gene in adult and hatchling stellate ganglia of the squid Loligo pealei

The giant fiber system of the squid Loligo pealei mediates the escape response and is an important neurobiological model. Here, we identified an abundant transcript in the stellate ganglion (SG) that encodes a FMRFamide precursor, and characterized FMRFamide and FI/LRF-amide peptides. To determine whether FMRFamide plays a role in the adult and hatchling giant fiber system, we studied the expression of the Fmrf gene and FMRFamide peptides. In stage 29 embryos and stage 30 hatchlings, Ffmr transcripts and FMRFamide peptide were low to undetectable in the SG, in contrast to groups of neurons intensely expressing the Fmrf gene in several brain lobes, including those that innervate the SG. In the adult SG the Fmrf gene was highly expressed, but the FMRFamide peptide was in low abundance. Intense staining for FMRFamide in the adult SG was confined to microneurons and fibers in the neuropil and to small fibers surrounding giant axons in stellar nerves. This shows that the Fmrf gene in the SG is strongly regulated post-hatching, and suggests that the FMRFamide precursor is incompletely processed in the adult SG. The data suggest that the SG only employs the Fmrf gene post-hatching and restricts the biosynthesis of FMRFamide, demonstrating that this peptide is not a major transmitter of the giant fiber system. This contrasts with brain lobes that engage FMRFamide embryonically as a regulatory peptide in multiple neuronal systems, including the afferent fibers that innervate the SG. The biological significance of these mechanisms may be to generate diversity within Fmrf-expressing systems in cephalopods.

Microscopic anatomy: normal structure

A peripheral nerve trunk is composed of nerve fascicles supported in a fibrous collagenous sheath and defined by concentric layers of cells (the perineurium) that separate the contents (the endoneurium) from its fibrous collagen support (the epineurium). In the endoneurium are myelinated and unmyelinated fibers that are axons combined with their supporting Schwann cells to provide physical and electrical connections with end-organs such as muscle fibers and sensory endings. Axons are tubular neuronal extensions with a cytoskeleton of neurotubules and tubulin along which organelles and proteins can travel between the neuronal cell body and the axon terminal. During development some axons enlarge and are covered by a chain of Schwann cells each associated with just one axon. As the axons grow in diameter, the Schwann cells wrap round them to produce a myelin sheath. This consists of many layers of compacted Schwann cell membrane plus some additional proteins. Adjacent myelin segments connect at highly specialized structures, the nodes of Ranvier. Myelin insulates the axon so that the nerve impulse can jump from one node to the next. The region adjacent to the node, the paranodal segment, is the site of myelin terminations on the axolemma. There are connections here between the Schwann cell and the axon via a complex chain of proteins. The Schwann cell cytoplasm in the adjacent segment, the juxtaparanode, contains most of the Schwann cell mitochondria. In addition to the node, continuity of myelin lamellae is broken at intervals along the internode by helical regions of decompaction known as Schmidt-Lanterman incisures; these are seen as paler conical segments in suitably stained microscopical preparations and provide a pathway between the adaxonal and abaxonal cytoplasm. Smaller axons without a myelin sheath conduct very much more slowly and have a more complex relationship with their supporting Schwann cells that has important implications for repair.

Intermediate filament genes as differentiation markers in the leech Helobdella

The intermediate filament (IF) cytoskeleton is a general feature of differentiated cells. Its molecular components, IF proteins, constitute a large family including the evolutionarily conserved nuclear lamins and the more diverse collection of cytoplasmic intermediate filament (CIF) proteins. In vertebrates, genes encoding CIFs exhibit cell/tissue type-specific expression profiles and are thus useful as differentiation markers. The expression of invertebrate CIFs, however, is not well documented. Here, we report a whole-genome survey of IF genes and their developmental expression patterns in the leech Helobdella, a lophotrochozoan model for developmental biology research. We found that, as in vertebrates, each of the leech CIF genes is expressed in a specific set of cell/tissue types. This allows us to detect earliest points of differentiation for multiple cell types in leech development and to use CIFs as molecular markers for studying cell fate specification in leech embryos. In addition, to determine the feasibility of using CIFs as universal metazoan differentiation markers, we examined phylogenetic relationships of IF genes from various species. Our results suggest that CIFs, and thus their cell/tissue-specific expression patterns, have expanded several times independently during metazoan evolution. Moreover, comparing the expression patterns of CIF orthologs between two leech species suggests that rapid evolutionary changes in the cell or tissue specificity of CIFs have occurred among leeches. Hence, CIFs are not suitable for identifying cell or tissue homology except among very closely related species, but they are nevertheless useful species-specific differentiation markers.

Identification of nerve plexi in connective tissues of the sea cucumber Holothuria glaberrima by using a novel nerve-specific antibody

The echinoderm nervous system is one of the least studied among invertebrates, partly because the tools available to study the neurobiology of this phylum are limited. We have now produced a monoclonal antibody (RN1) that labels a nervous system component of the sea cucumber Holothuria glaberrima. Western blots show that our antibody recognizes a major band of 66 kDa and a minor band of 53 kDa. Immunohistological experiments show that, in H. glaberrima, the antibody distinctly labels most of the known nervous system structures and some components that were previously unknown or little studied. A surprising finding was the labeling of nervous plexi within the connective tissue compartments of all organs studied. Double labeling with holothurian neuropeptides and an echinoderm synaptotagmin showed that RN1 labeled most, if not all, of the fibers labeled by these neuronal markers, but also a larger component of cells and fibers. The presence of a distinct connective tissue plexus in holothurians is highly significant since these organisms possess mutable connective tissues that change viscosity under the control of the nervous system. Therefore, the cells and fibers recognized by our monoclonal antibodies may be involved in controlling tensility changes in echinoderm connective tissue.

Trafficking of axonal K+ channels: potential role of Hsc70

Voltage-gated potassium ion channels in axons underlie the repolarization phase of the membrane action potential and help to set the resting potential. In addition to being present in the axolemma, they are also found in axoplasm in small vesicles, 30-50 nm in diameter, which may serve as a reserve pool of K+ channel protein (Clay and Kuzirian [2000] J Neurobiol 45:172-184). We have developed a novel technique for extracting these vesicles from axoplasm, which relies on the ability of Texas red to bind to them, thereby reducing their buoyancy so that they are amenable to pelleting by ultracentrifugation (Clay and Kuzirian [2000] J Neurobiol 45:172-184). The mechanism underlying this process may be binding of Texas red to Hsc70, which is primarily a cytosolic protein. However, a small portion of it is located on the surface of vesicles. Kinesin is also on the vesicle surface. This protein is membrane bound in our in vitro vesicle preparation when solutions that do not contain MgATP are added to extruded axoplasm. The addition of MgATP to the solution appears to release a significant amount of kinesin from the vesicles, possibly by the Hsc70-MgATP catalysis mechanism recently proposed by Tsai et al.

Association of a nonmuscle myosin II with axoplasmic organelles

Association of motor proteins with organelles is required for the motors to mediate transport. Because axoplasmic organelles move on actin filaments, they must have associated actin-based motors, most likely members of the myosin superfamily. To gain a better understanding of the roles of myosins in the axon we used the giant axon of the squid, a powerful model for studies of axonal physiology. First, a approximately 220 kDa protein was purified from squid optic lobe, using a biochemical protocol designed to isolate myosins. Peptide sequence analysis, followed by cloning and sequencing of the full-length cDNA, identified this approximately 220 kDa protein as a nonmuscle myosin II. This myosin is also present in axoplasm, as determined by two independent criteria. First, RT-PCR using sequence-specific primers detected the transcript in the stellate ganglion, which contains the cell bodies that give rise to the giant axon. Second, Western blot analysis using nonmuscle myosin II isotype-specific antibodies detected a single approximately 220 kDa band in axoplasm. Axoplasm was fractionated through a four-step sucrose gradient after 0.6 M KI treatment, which separates organelles from cytoskeletal components. Of the total nonmuscle myosin II in axoplasm, 43.2% copurified with organelles in the 15% sucrose fraction, while the remainder (56.8%) was soluble and found in the supernatant. This myosin decorates the cytoplasmic surface of 21% of the axoplasmic organelles, as demonstrated by immunogold electron-microscopy. Thus, nonmuscle myosin II is synthesized in the cell bodies of the giant axon, is present in the axon, and is associated with isolated axoplasmic organelles. Therefore, in addition to myosin V, this myosin is likely to be an axoplasmic organelle motor.

Differential expression and localization of neuronal intermediate filament proteins within newly developing neurites in dissociated cultures of Xenopus laevis embryonic spinal cord

The molecular subunit composition of neurofilaments (NFs) progressively changes during axon development. In developing Xenopus laevis spinal cord, peripherin emerges at the earliest stages of neurite outgrowth. NF-M and XNIF (an alpha-internexin-like protein) appear later, as axons continue to elongate, and NF-L is expressed after axons contact muscle. Because NFs are the most abundant component of the vertebrate axonal cytoskeleton, we must understand why these changes occur before we can fully comprehend how the cytoskeleton regulates axon growth and morphology. Knowing where these proteins are localized within developing neurites and how their expression changes with cell contact is essential for this understanding. Thus, we examined by immunofluorescence the expression and localization of these NF subunits within dissociated cultures of newly differentiating spinal cord neurons. In young neurites, peripherin was most abundant in distal neuritic segments, especially near branch points and extending into the central domain of the growth cone. In contrast, XNIF and NF-M were usually either absent from very young neurites or exhibited a proximal to distal gradient of decreasing intensity. In older neurites, XNIF and NF-M expression increased, whereas that of peripherin declined. All three of these proteins became more evenly distributed along the neurites, with some branches staining more intensely than others. At 24 h, NF-L appeared, and in 48-h cultures, its expression, along with that of NF-M, was greater in neurites contacting muscle cells, arguing that the upregulation of these two subunits is dependent on contact with target cells. Moreover, this contact had no effect on XNIF or peripherin expression. Our findings are consistent with a model in which peripherin plays an important structural role in growth cones, XNIF and NF-M help consolidate the intermediate filament cytoskeleton beginning in the proximal neurite, and increased levels of NF-L and NF-M help further solidify the cytoskeleton of axons that successfully reach their targets.

A role for the EphA family in the topographic targeting of vomeronasal axons

We have investigated the role of the Eph family of receptor tyrosine kinases and their ligands in the establishment of the vomeronasal projection in the mouse. Our data show intriguing differential expression patterns of ephrin-A5 on vomeronasal axons and of EphA6 in the accessory olfactory bulb (AOB), such that axons with high ligand concentration project onto regions of the AOB with high receptor concentration and vice versa. These data suggest a mechanism for development of this projection that is the opposite of the repellent interaction between Eph receptors and ligands observed in other systems. In support of this idea, when given the choice of whether to grow on lanes containing EphA-F(c)/laminin or F(c)/laminin protein (in the stripe assay), vomeronasal axons prefer to grow on EphA-F(c)/laminin. Analysis of ephrin-A5 mutant mice revealed a disturbance of the topographic targeting of vomeronasal axons to the AOB. In summary, these data, which are derived from in vitro and in vivo experiments, indicate an important role of the EphA family in setting up the vomeronasal projection.

Fast transport of neurofilament protein along microtubules in squid axoplasm

Using squid axoplasm as a model system, we have visualized the fast transport of non-filamentous neurofilament protein particles along axonal microtubules. This transport occurs at speeds of 0.5-1.0 microm/second and the majority of neurofilament particles stain with kinesin antibody. These observations demonstrate, for the first time, that fast (0.5-1.0 microm/second) transport of neurofilament proteins occurs along microtubules. In addition, our studies suggest that neurofilament protein can be transported as non-membrane bound, nonfilamentous subunits along axons, and that the transport is kinesin-dependent. Microtubule-based fast transport might therefore provide a mechanism for the distribution and turnover of neurofilament, and perhaps other cytoskeletal proteins, throughout neurons.

Immunoelectronmicroscopy of soluble and membrane proteins with a sensitive postembedding method

The application of immunoelectronmicroscopy to soluble proteins is limited because soluble proteins can redistribute during fixation. Fixation may also adversely affect the recognition of proteins associated with membranes. We show here how displacements of soluble proteins can be prevented and antigen sensitivity improved by freeze-substitution immunocytochemistry. The usefulness of this method for soluble cytoplasmic proteins is demonstrated for the twitchin protein in Aplysia muscle and the kinesin motor proteins in squid giant axons, in which the sizes of various cytoplasmic pools of kinesins are estimated. The utility for membrane proteins present in small numbers of copies is demonstrated by labeling a glutamate receptor subunit in mouse cerebellar cortex and the ZO-1 protein in tight junctions between MDCK cells. Thus, freeze-substitution immunocytochemistry can show the native distribution of both soluble and membrane proteins labeled with polyclonal antibodies and, at the same time, can reveal structural features comparable to those in chemically fixed or osmium freeze-substituted samples.

Neurofilament phosphorylation

Neurofilament proteins (NFPs) are highly phosphorylated molecules in the axonal compartment of the adult nervous system. The phosphorylation of NFP is considered an important determinant of filament caliber, plasticity, and stability. This process reflects the function of NFs during the lifetime of a neuron from differentiation in the embryo through long-term activity in the adult until aging and environmental insult leads to pathology and ultimately death. NF function is modulated by phosphorylation-dephosphorylation in each of these diverse neuronal states. In this review, we have summarized some of these properties of NFP in adult nervous tissue, mostly from work in our own laboratory. Identification of sites phosphorylated in vivo in high molecular weight NFP (NF-H) and properties of NF-associated and neural-specific kinases phosphorylating specific sites in NFP are described. A model to explain the role of NF phosphorylation in determining filament caliber, plasticity, and stability is proposed.