Slow axoplasmic transport: a fiction?

The Trail of Axonal Protein Synthesis: Origins and Current Functional Landscapes

Local protein synthesis (LPS) in axons is now recognized as a physiological process, participating both in the maintenance of axonal function and diverse plastic phenomena. In the last decades of the 20th century, the existence and function of axonal LPS were topics of significant debate. Very early, axonal LPS was thought not to occur at all and was later accepted to play roles only during development or in response to specific conditions. However, compelling evidence supports its essential and pervasive role in axonal function in the mature nervous system. Remarkably, in the last five decades, Uruguayan neuroscientists have contributed significantly to demonstrating axonal LPS by studying motor and sensory axons of the peripheral nervous system of mammals, as well as giant axons of the squid and the Mauthner cell of fish. For LPS to occur, a highly regulated transport system must deliver the necessary macromolecules, such as mRNAs and ribosomes. This review discusses key findings related to the localization and abundance of axonal mRNAs and their translation levels, both in basal states and in response to physiological processes, such as learning and memory consolidation, as well as neurodevelopmental and neurodegenerative disorders, including Alzheimer's disease, autism spectrum disorder, and axonal injury. Moreover, we discuss the current understanding of axonal ribosomes, from their localization to the potential roles of locally translated ribosomal proteins, in the context of emerging research that highlights the regulatory roles of the ribosome in translation. Lastly, we address the main challenges and open questions for future studies.

Functional Genomics of Axons and Synapses to Understand Neurodegenerative Diseases

Functional genomics studies through transcriptomics, translatomics and proteomics have become increasingly important tools to understand the molecular basis of biological systems in the last decade. In most cases, when these approaches are applied to the nervous system, they are centered in cell bodies or somatodendritic compartments, as these are easier to isolate and, at least in vitro, contain most of the mRNA and proteins present in all neuronal compartments. However, key functional processes and many neuronal disorders are initiated by changes occurring far away from cell bodies, particularly in axons (axopathologies) and synapses (synaptopathies). Both neuronal compartments contain specific RNAs and proteins, which are known to vary depending on their anatomical distribution, developmental stage and function, and thus form the complex network of molecular pathways required for neuron connectivity. Modifications in these components due to metabolic, environmental, and/or genetic issues could trigger or exacerbate a neuronal disease. For this reason, detailed profiling and functional understanding of the precise changes in these compartments may thus yield new insights into the still intractable molecular basis of most neuronal disorders. In the case of synaptic dysfunctions or synaptopathies, they contribute to dozens of diseases in the human brain including neurodevelopmental (i.e., autism, Down syndrome, and epilepsy) as well as neurodegenerative disorders (i.e., Alzheimer's and Parkinson's diseases). Histological, biochemical, cellular, and general molecular biology techniques have been key in understanding these pathologies. Now, the growing number of omics approaches can add significant extra information at a high and wide resolution level and, used effectively, can lead to novel and insightful interpretations of the biological processes at play. This review describes current approaches that use transcriptomics, translatomics and proteomic related methods to analyze the axon and presynaptic elements, focusing on the relationship that axon and synapses have with neurodegenerative diseases.

Rat Sciatic Nerve Axoplasm Proteome Is Enriched with Ribosomal Proteins during Regeneration Processes

Axons are complex subcellular compartments that are extremely long in relation to cell bodies, especially in peripheral nerves. Many processes are required and regulated during axon injury, including anterograde and retrograde transport, glia-to-axon macromolecular transfer, and local axonal protein synthesis. Many in vitro omics approaches have been used to gain insight into these processes, but few have been applied in vivo. Here we adapted the osmotic ex vivo axoplasm isolation method and analyzed the adult rat sciatic-nerve-extruded axoplasm by label-free quantitative proteomics before and after injury. 2087 proteins groups were detected in the axoplasm, revealing translation machinery and microtubule-associated proteins as the most overrepresented biological processes. Ribosomal proteins (73) were detected in the uninjured axoplasm and increased their levels after injury but not within whole sciatic nerves. Meta-analysis showed that detected ribosomal proteins were present in in vitro axonal proteomes. Because local protein synthesis is important for protein localization, we were interested in detecting the most abundant newly synthesized axonal proteins in vivo. With an MS/MS-BONCAT approach, we detected 42 newly synthesized protein groups. Overall, our work indicates that proteomics profiling is useful for local axonal interrogation and suggests that ribosomal proteins may play an important role, especially during injury.

In the Right Place at the Right Time: miRNAs as Key Regulators in Developing Axons

During neuronal circuit formation, axons progressively develop into a presynaptic compartment aided by extracellular signals. Axons display a remarkably high degree of autonomy supported in part by a local translation machinery that permits the subcellular production of proteins required for their development. Here, we review the latest findings showing that microRNAs (miRNAs) are critical regulators of this machinery, orchestrating the spatiotemporal regulation of local translation in response to cues. We first survey the current efforts toward unraveling the axonal miRNA repertoire through miRNA profiling, and we reveal the presence of a putative axonal miRNA signature. We also provide an overview of the molecular underpinnings of miRNA action. Our review of the available experimental evidence delineates two broad paradigms: cue-induced relief of miRNA-mediated inhibition, leading to bursts of protein translation, and cue-induced miRNA activation, which results in reduced protein production. Overall, this review highlights how a decade of intense investigation has led to a new appreciation of miRNAs as key elements of the local translation regulatory network controlling axon development.

The role of translation elongation factor eEF1 subunits in neurodevelopmental disorders

The multi-subunit eEF1 complex plays a crucial role in de novo protein synthesis. The central functional component of the complex is eEF1A, which occurs as two independently encoded variants with reciprocal expression patterns: whilst eEF1A1 is widely expressed, eEF1A2 is found only in neurons and muscle. Heterozygous mutations in the gene encoding eEF1A2, EEF1A2, have recently been shown to cause epilepsy, autism, and intellectual disability. The remaining subunits of the eEF1 complex, eEF1Bα, eEF1Bδ, eEF1Bγ, and valyl-tRNA synthetase (VARS), together form the GTP exchange factor for eEF1A and are ubiquitously expressed, in keeping with their housekeeping role. However, mutations in the genes encoding these subunits EEF1B2 (eEF1Bα), EEF1D (eEF1Bδ), and VARS (valyl-tRNA synthetase) have also now been identified as causes of neurodevelopmental disorders. In this review, we describe the mutations identified so far in comparison with the degree of normal variation in each gene, and the predicted consequences of the mutations on the functions of the proteins and their isoforms. We discuss the likely effects of the mutations in the context of the role of protein synthesis in neuronal development.

Myosin Va but Not nNOSα is Significantly Reduced in Jejunal Musculomotor Nerve Terminals in Diabetes Mellitus

Nitric oxide (NO) mediated slow inhibitory junction potential and mechanical relaxation after electrical field stimulation (EFS) is impaired in diabetes mellitus. Externally added NO donor restore nitrergic function, indicating that this reduction result from diminution of NO synthesis within the pre-junctional nerve terminals. The present study aimed to investigate two specific aims that may potentially provide pathophysiological insights into diabetic nitrergic neuropathy. Specifically, alteration in nNOSα contents within jejunal nerve terminals and a local subcortical transporter myosin Va was tested 16 weeks after induction of diabetes by low dose streptozotocin (STZ) in male Wistar rats. The results show that diabetic rats, in contrast to vehicle treated animals, have: (a) nearly absent myosin Va expression in nerve terminals of axons innervating smooth muscles and (b) significant decrease of myosin Va in neuronal soma of myenteric plexus. In contrast, nNOSα staining in diabetic jejunum neuromuscular strips showed near intact expression in neuronal cell bodies. The space occupancy of nitrergic nerve fibers was comparable between groups. Normal concentration of nNOSα was visualized within a majority of nitrergic terminals in diabetes, suggesting intact axonal transport of nNOSα to distant nerve terminals. These results reveal the dissociation between presences of nNOSα in the nerve terminals but deficiency of its transporter myosin Va in the jejunum of diabetic rats. This significant observation of reduced motor protein myosin Va within jejunal nerve terminals may potentially explain impairment of pre-junctional NO synthesis during EFS of diabetic gut neuromuscular strips despite presence of the nitrergic synthetic enzyme nNOSα.

Axonal protein synthesis and the regulation of primary afferent function

Local protein synthesis has been demonstrated in the peripheral processes of sensory primary afferents and is thought to contribute to the maintenance of the neuron, to neuronal plasticity following injury and also to regeneration of the axon after damage to the nerve. The mammalian target of rapamycin (mTOR), a master regulator of protein synthesis, integrates a variety of cues that regulate cellular homeostasis and is thought to play a key role in coordinating the neuronal response to environmental challenges. Evidence suggests that activated mTOR is expressed by peripheral nerve fibers, principally by A-nociceptors that rapidly signal noxious stimulation to the central nervous system, but also by a subset of fibers that respond to cold and itch. Inhibition of mTOR complex 1 (mTORC1) has shown that while the acute response to noxious stimulation is unaffected, more complex aspects of pain processing including the setting up and maintenance of chronic pain states can be disrupted suggesting a route for the generation of new drugs for the control of chronic pain. Given the role of mTORC1 in cellular homeostasis, it seems that systemic changes in the physiological state of the body such as occur during illness are likely to modulate the sensitivity of peripheral sensory afferents through mTORC1 signaling pathways.

beta-Actin and beta-Tubulin are components of a heterogeneous mRNA population present in the squid giant axon

Previously, we have reported that the squid giant axon contains a heterogeneous population of polyadenylated mRNAs, as well as biologically active polyribosomes. To define the composition of this unique mRNA population, cDNA libraries were constructed to RNA obtained from the axoplasm of the squid giant axon and the parental cell bodies located in the giant fiber lobe. Here, we report that the giant axon contains mRNAs encoding beta-actin and beta-tubulin. The axonal location of these mRNA species was confirmed by in situ hybridization histochemistry, and their presence in the axoplasmic polyribosome fraction was demonstrated by polymerase chain reaction methodology. Taken together, these findings establish the identity of two relatively abundant members of the axonal mRNA population and suggest that key elements of the cytoskeleton are synthesized de novo in the squid giant axon.

Modeling of intracellular transport and compartmentation

The complexity and internal organization of mammalian cells as well as the regulation of intracellular transport processes has increasingly moved into the focus of investigation during the past two decades. Advanced staining and microscopy techniques help to shed light onto spatial cellular compartmentation and regulation, increasing the demand for improved modeling techniques. In this chapter, we summarize recent developments in the field of quantitative simulation approaches and frameworks for the description of intracellular transport processes. Special focus is therefore laid on compartmented and spatiotemporally resolved simulation approaches. The processes considered include free and facilitated diffusion of molecules, active transport via the microtubule and actin filament network, vesicle distribution, membrane transport, cell cycle-dependent cell growth and morphology variation, and protein production. Commercially and freely available simulation packages are summarized as well as model data exchange and harmonization issues.

Local Gene Expression in Axons and Nerve Endings: The Glia-Neuron Unit

Neurons have complex and often extensively elongated processes. This unique cell morphology raises the problem of how remote neuronal territories are replenished with proteins. For a long time, axonal and presynaptic proteins were thought to be exclusively synthesized in the cell body, which delivered them to peripheral sites by axoplasmic transport. Despite this early belief, protein has been shown to be synthesized in axons and nerve terminals, substantially alleviating the trophic burden of the perikaryon. This observation raised the question of the cellular origin of the peripheral RNAs involved in protein synthesis. The synthesis of these RNAs was initially attributed to the neuron soma almost by default. However, experimental data and theoretical considerations support the alternative view that axonal and presynaptic RNAs are also transcribed in the flanking glial cells and transferred to the axon domain of mature neurons. Altogether, these data suggest that axons and nerve terminals are served by a distinct gene expression system largely independent of the neuron cell body. Such a local system would allow the neuron periphery to respond promptly to environmental stimuli. This view has the theoretical merit of extending to axons and nerve terminals the marginalized concept of a glial supply of RNA (and protein) to the neuron cell body. Most long-term plastic changes requiring de novo gene expression occur in these domains, notably in presynaptic endings, despite their intrinsic lack of transcriptional capacity. This review enlightens novel perspectives on the biology and pathobiology of the neuron by critically reviewing these issues.

RNA trafficking in axons

A substantial number of studies over a period of four decades have indicated that axons contain mRNAs and ribosomes, and are metabolically active in synthesizing proteins locally. For the most part, little attention has been paid to these findings until recently when the concept of targeting of specific mRNAs and translation in subcellular domains in polarized cells emerged to contribute to the likelihood and acceptance of mRNA targeting to axons as well. Trans-acting factor proteins bind to cis-acting sequences in the untranslated region of mRNAs integrated in ribonucleoprotein (RNPs) complexes determine its targeting in neurons. In vitro studies in immature axons have shown that molecular motors proteins (kinesins and myosins) associate to RNPs suggesting they would participate in its transport to growth cones. Tau and actin mRNAs are transported as RNPs, and targeted to axons as well as ribosomes. Periaxoplasmic ribosomal plaques (PARPs), which are systematically distributed discrete peripheral ribosome-containing, actin-rich formations in myelinated axons, also are enriched with actin and myosin Va mRNAs and additional regulatory proteins. The localization of mRNAs in PARPs probably means that PARPs are local centers of translational activity, and that these domains are the final destination in the axon compartment for targeted macromolecular traffic originating in the cell body. The role of glial cells as a potentially complementary source of axonal mRNAs and ribosomes is discussed in light of early reports and recent ultrastructural observations related to the possibility of glial-axon trans-endocytosis.

Ribosomal distributions in axons of mammalian myelinated fibers

The distribution of ribosomes and polysomes in uninjured myelinated axons of rat sciatic nerve was analyzed. Ribosomes were identified by immunocytochemistry at the light and electron microscopic levels. A polyclonal antibody developed against ribosomes recognized both rRNA and ribosomal proteins. The distribution of the immunoreaction product was similar to that obtained with human anti-ribosomal P protein. The immunoreaction product distributions were of two types in axons: 1) periodic localization in the cortical region of axoplasm that appeared as a compact structural aggregate, consistent with that described as a periaxoplasmic ribosomal plaques (PARP) domain (Koenig et al. [2000] J. Neurosci. 20:8390-8400), and 2) scattered small immuno-reactive clusters of varying sizes (RNP) within the central core of the axon. The latter observation suggested the possibility that RNP-like particles could be associated with the axonal transport system and in transit. Immunoreaction product was also associated with a novel structural inclusion, possibly multi-vesicular in makeup that was located in the axon and at the myelin-axon interface, and visible at the light and EM levels. The potential significance of this structural peculiarity is considered.

Axonal and presynaptic protein synthesis: new insights into the biology of the neuron

The presence of a local mRNA translation system in axons and terminals was proposed almost 40 years ago. Over the ensuing period, an impressive body of evidence has grown to support this proposal -- yet the nerve cell body is still considered to be the only source of axonal and presynaptic proteins. To dispel this lingering neglect, we now present the wealth of recent observations bearing on this central idea, and consider their impact on our understanding of the biology of the neuron. We demonstrate that extrasomatic translation sites, which are now well recognized in dendrites, are also present in axonal and presynaptic compartments.

The Microtubular Pattern Changes at the Spinal Cord-Root Junction and Reverts at the Root-Peripheral Nerve Junction in Sensory and Motor Fibres of the Rat

In the rat, we studied the microtubular content of central nervous system (CNS) axons (pyramidal tract, dorsal funiculus, and intracord domain of motor axons), of radicular axons (ventral and dorsal roots), and of peripheral axons (sural and lateral gastrocnemius nerves). The microtubular density had an inverse relationship with the size of the axon. Within the CNS, values ranged from over 120 microtubules/microm2 for axons smaller than 0.1 microm2 of the pyramidal tract and dorsal funiculus to 24 for 3-microm motor axons (area, 7 microm2) in their spinal cord domain. Peripheral nerve and CNS axons of the same size had comparable microtubular densities. In contrast, the microtubular density of dorsal and ventral root axons was one half that of CNS or peripheral nerve axons of equal calibre. Considered along the axon, the microtubular density of motor and sensory fibres is high in the CNS domain, low in the root, and high again in the peripheral nerve domain. These observations are inconsistent with the notion that the cytoskeleton moves coherently away from the perikaryon. We conclude that the axonal microtubular content accords with the calibre of the fibre and with the anatomical region where it courses. We propose that axonal microtubules are regulated by local cues.

Protein synthesis in axons and terminals: significance for maintenance, plasticity and regulation of phenotype. With a critique of slow transport theory

This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and expression of plasticity in axons and terminals. Recent evidence of discrete ribosomal domains, subjacent to the axolemma, which are distributed at intermittent intervals along axons, are described. Studies of locally synthesized proteins, and proteins encoded by RNA transcripts in axons indicate that the latter comprise constituents of the so-called slow transport rate groups. A comprehensive review and analysis of published data on synaptosomes and identified presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modifiable synapses. The concept that all axonal proteins are supplied by slow transport after synthesis in the perikaryon is challenged because the underlying assumptions of the model are discordant with known metabolic principles. The flawed slow transport model is supplanted by a metabolic model that is supported by evidence of local synthesis and turnover of proteins in axons. A comparison of the relative strengths of the two models shows that, unlike the local synthesis model, the slow transport model fails as a credible theoretical construct to account for axons and terminals as we know them. Evidence for a dynamic anatomy of axons is presented. It is proposed that a distributed "sprouting program," which governs local plasticity of axons, is regulated by environmental cues, and ultimately depends on local synthesis. In this respect, nerve regeneration is treated as a special case of the sprouting program. The term merotrophism is proposed to denote a class of phenomena, in which regional phenotype changes are regulated locally without specific involvement of the neuronal nucleus.

Ribosomes and polyribosomes are present in the squid giant axon: an immunocytochemical study

Ribosomes and polyribosomes were detected by immuno-electron microscopy in the giant axon and small axons of the squid using a polyclonal antibody against rat brain ribosomes. The ribosomal fraction used as antigen was purified by ultracentrifugation on a sucrose density gradient and shown to contain ribosomal RNAs and native ribosomes. The polyclonal antibody raised in rabbits reacted with at least ten proteins on immunoblots of purified rat brain ribosomes as well as with a set of multiple ribosomal proteins prepared from the squid giant fiber lobe. Immunoreactions were performed on cryostat sections of the stellate nerve cut at a distance of more than 3 cm from the stellate ganglion, using pre-embedding techniques. Ribosomes and polyribosomes were identified within the giant axon and small axons using electron microscopic methods, following binding of peroxidase-conjugated anti-rabbit IgG secondary antibody. Polysomes were more frequently localized in peripheral axoplasm, including the cortical layer of the giant axon, and were generally associated with unidentified cytoskeletal filaments or with dense matrix material. The immunochemical demonstration of ribosomes and polyribosomes in the giant axon and small axons of the squid confirms similar observations in the squid and the goldfish obtained with the method of electron spectroscopic imaging, and strongly supports the view that a local system of protein synthesis is present in axons. The immunochemical method here described offers an alternative tool for the selective identification of ribosomes, and is likely to prove of value in the analyses of other axonal systems.

Protein synthesis in presynaptic endings from squid brain: modulation by calcium ions

Previous biochemical, autoradiographic, and ultrastructural data have shown that, in the synaptosomal fraction of the squid optic lobe, protein synthesis is largely due to the presynaptic terminals of the retinal photoreceptor neurons (Crispino et al. [1993a] Mol. Cell. Neurosci. 4:366-374; Crispino et al. [1993b] J. Neurochem. 61:1144-1146; Crispino et al. [1997] J. Neurosci. 17:7694-7702). We now report that this process is close to its maximum at the basal concentration of cytosolic Ca++, and is markedly inhibited when the concentration of this ion is either decreased or increased. This conclusion is supported by the results of experiments with: 1) compounds known to increase the level of cytosolic Ca++, such as A23187, ionomycin, thapsigargin, and caffeine; 2) compounds sequestering cytosolic calcium ions such as BAPTA-AM; and 3) agents that block the role of Ca++ as second messenger, such as TFP and W7, which inhibit calmodulin, and calphostin, which inhibits protein kinase C. We conclude that variations in the level of cytosolic Ca++ induced in presynaptic terminals by neuronal activity may contribute to the modulation of the local synthesis of protein.

Protein-synthesizing machinery in the axon compartment

Contrary to the prevailing view that the axon lacks the capacity to synthesize proteins, a substantial body of evidence points to the existence of a metabolically active endogenous translational machinery. The machinery appears to be largely localized in the cortical zone of the axon, where, in vertebrate axons, it is distributed longitudinally as intermittent, discrete domains, called periaxoplasmic plaques. Studies, based on translation assays and probes of RNA transcripts in axon models such as the squid giant axon and selected vertebrate axons, provide evidence of locally synthesized proteins, most of which appear to be constituents of the slow axoplasmic transport rate groups. Metabolic and molecular biological findings are consistent with the view that the synthesis of proteins undergoing local turnover in the axonal compartment of macroneurons depends on the activity of an endogenous translational machinery. The documented presence of a metabolically active machinery in presynaptic terminals of squid photoreceptor neurons is also described. Finally, potential sources of axoplasmic RNAs comprising the machinery, which may include the ensheathing cell of the axon, as well as the cognate cell body, are also discussed.

Regenerating axons of the rat require a local source of proteins

Regenerating axons need proteins to grow and we explored whether a local supply is necessary. Crushed peroneal nerves were entubulated with silicone sleeves, plain or loaded with cycloheximide (CHX); some nerves were frozen to kill resident cells. When a plain sleeve was placed distal to the crush, axons regrew 5.0 mm in 3 days (pinch test), and 4.6 mm when the sleeve was placed around a frozen nerve (n.s). CHX administered distal to the crush reduced the elongation by approximately 58% (P < 0.01) in unfrozen or frozen nerves whilst its administration central to the crush was ineffectual. Immunostaining of nerves with GAP-43 gave similar values. Under the electron microscope, axonal sprouts were less frequent when CHX was used irrespective of the cellular or acellular condition of the nerve. Therefore, an inhibitor of protein synthesis reduces axonal regrowth, an effect mediated neither by parent neurones nor by resident cells. We propose that axons synthesize proteins.

Cytoplasmic mechanisms of axonal and dendritic growth in neurons

The structural mechanisms responsible for the gradual elaboration of the cytoplasmic elongation of neurons are reviewed. In addition to discussing recent work, important older work is included to inform newcomers to the field how the current perspective arose. The highly specialized axon and the less exaggerated dendrite both result from the advance of the motile growth cone. In the area of physiology, studies in the last decade have directly confirmed the classic model of the growth cone pulling forward and the axon elongating from this tension. Particularly in the case of the axon, cytoplasmic elongation is closely linked to the formation of an axial microtubule bundle from behind the advancing growth cone. Substantial progress has been made in understanding the expression of microtubule-associated proteins during neuronal differentiation to stiffen and stabilize axonal microtubules, providing specialized structural support. Studies of membrane organelle transport along the axonal microtubules produced an explosion of knowledge about ATPase molecules serving as motors driving material along microtubule rails. However, most aspects of the cytoplasmic mechanisms responsible for neurogenesis remain poorly understood. There is little agreement on mechanisms for the addition of new plasma membrane or the addition of new cytoskeletal filaments in the growing axon. Also poorly understood are the mechanisms that couple the promiscuous motility of the growth cone to the addition of cytoplasmic elements.

Slow axonal transport mechanisms move neurofilaments relentlessly in mouse optic axons

Pulse-labeling studies of slow axonal transport in many kinds of axons (spinal motor, sensory ganglion, oculomotor, hypoglossal, and olfactory) have led to the inference that axonal transport mechanisms move neurofilaments (NFs) unidirectionally as a single continuous kinetic population with a diversity of individual transport rates. One study in mouse optic axons (Nixon, R. A., and K. B. Logvinenko. 1986. J. Cell Biol. 102:647-659) has given rise to the different suggestion that a significant and distinct population of NFs may be entirely stationary within axons. In mouse optic axons, there are relatively few NFs and the NF proteins are more lightly labeled than other slowly transported slow component b (SCb) proteins (which, however, move faster than the NFs); thus, in mouse optic axons, the radiolabel of some of these faster-moving SCb proteins may confuse NF protein analyses that use one dimensional (1-D) SDS-PAGE, which separates proteins by size only. To test this possibility, we used a 2-mm "window" (at 3-5 mm from the posterior of the eye) to compare NF kinetics obtained by 1-D SDS-PAGE and by the higher resolution two-dimensional (2-D) isoelectric focusing/SDS-PAGE, which separates proteins both by their net charge and by their size. We found that 1-D SDS-PAGE is insufficient for definitive NF kinetics in the mouse optic system. By contrast, 2-D SDS-PAGE provides essentially pure NF kinetics, and these indicate that in the NF-poor mouse optic axons, most NFs advance as they do in other, NF-rich axons. In mice, greater than 97% of the radiolabeled NFs were distributed in a unimodal wave that moved at a continuum of rates, between 3.0 and 0.3 mm/d, and less than 0.1% of the NF population traveled at the very slowest rates of less than 0.005 mm/d. These results are inconsistent with the proposal (Nixon and Logvinenko, 1986) that 32% of the transported NFs remain within optic axons in an entirely stationary state. As has been found in other axons, the axonal transport system of mouse optic axons moves NFs and other cytoskeletal elements relentlessly from the cell body to the axon tip.

Calibers and microtubules of nerve fibers: differential effect of undernutrition in developing and adult rats

Sural nerves of 9-week-old rats undernourished since birth, and of adult rats food-restricted for 27 and 48 days, were studied to explore the effect of severe undernutrition on the caliber and microtubules of axons in growing and non-growing animals. In 9-week-old undernourished rats, the number and caliber of myelinated fibers were normal while the cross-sectional area of non-medullated fibers was 29% smaller than controls. By contrast, in adult undernourished rats the cross-sectional area of myelinated fibers was affected sooner and to a greater extent (-28%) than that of non-medullated fibers (-23%). Regardless of age, in both controls and in undernourished rats non-medullated fibers of equal caliber had similar microtubular content. The same was found in 3-microns myelinated axons. These findings indicate that food restriction affects proportionately caliber and microtubules of axons. It is proposed that the anatomy of the axon is in a dynamic equilibrium and that microtubules participate in the specification of the axonal caliber.

Is the supply of axoplasmic proteins a burden for the cell body? Morphometry of sensory neurons and amino acid incorporation into their cell bodies

Since the perikaryon is considered to be the source of all axoplasmic proteins, we estimated the amount of protein synthesized in cell bodies and axoplasmic volumes of sensory neurons of two anuran species (Xenopus and Caudiverbera) to detect a correlation between these variables. The range of cell body volumes was 1:28 and 1:38 in Xenopus and Caudiverbera, respectively, while that of axoplasmic volumes was 1:5000-6000. The protein synthesis in glial and neuronal cell bodies was assessed with pulses of labeled amino acids followed by radioautography. No obvious correlation was found between axoplasmic volume and either rate or amount of amino acid incorporated into cell bodies. The rate of amino acid incorporation into glial and neuronal cell bodies was of the same order of magnitude. Results suggest that the maintenance of the axoplasm does not seem to be a burden for the perikaryon.

Calibre and Microtubule Content of the Non-Medullated and Myelinated Domains of Optic Nerve Axons of Rats

Calibres and microtubule contents of the non-medullated and myelinated domains of optic nerve axons of adult rats were studied with the electron microscope. The cross-sectional areas of the non-medullated domain was 0.25 microm2, and that of the myelinated domain 0.40 microm2, that is, greater by 59%. The increase in size was uneven across the axonal population; it was marked in fine and medium sized axons, and modest in the largest axons. The number of microtubules increased with axonal size; the density, however, decreased from 85 mirotubules/microm2 in 0.1 microm2 axons to about 20 in 1.2 microm2 axons. In axons of equal cross sectional area, the microtubular density of the myelinated and non-medullated domains was the same. Microtubular density values of optic axons resemble those of dorsal roots more than those of peripheral nerve axons of equal calibre. The facts that optic axons increase in size and gain microtubules behind the eyeball while the microtubular packing decreases suggest a local regulation of the axonal cytoskeleton.

Microtubules and calibers in normal and regenerating axons of the sural nerve of the rat

The calibers and microtubular content of axons were studied in normal and regenerating fibers of the sural nerve from 17 to 122 days after a lesion of the sciatic nerve of young adult rats. During this period (70-175 days of age), the cross-sectional area of control myelinated axons almost doubled but that of nonmedullated axons did not change. In regenerating nerves, after 122 days of recovery, the cross-sectional area of myelinated fibers was still 38% below that of the normal side. In contrast, the regenerating nonmedullated population was richer in fine (less than 0.2 micron2) and in coarse (greater than 0.9 micron2) fibers than on the control side; the cross-sectional area averages were 0.50 and 0.54-0.70 micron 2 for the normal and regenerating populations, respectively. The microtubular density of normal 3-micron myelinated fibers averaged 24.0 microtubules/micron2. In regenerating fibers of the same size the density varied between 19.2 and 23.2 microtubules/micron2. Microtubular density values of normal and regenerating fibers were not statistically different. In nonmedullated fibers, the microtubular content (expressed as microtubular density or number of microtubules per axon) correlated with the caliber of the fiber. In these correlations, only minor differences were observed between regenerating and uninjured fibers. Our results indicate that nonmedullated fibers terminate their radial growth well before myelinated fibers do, and that axonal microtubular content correlates with the local size of the fiber and is largely insensitive to regeneration.

Microtubules and calibers in developing axons

The caliber and microtubular content of growing axons were assessed with the electron microscope in the sural nerve of the rat from birth until the age of 63 days (young adult). The caliber of both nonmedullated and myelinated fibers increased throughout the period of observation. At birth, nonmedullated fibers smaller than 0.2 micron2 represented 69% of the population; at day 63 less than 7% were that small. Irrespective of the age of the rat, the number of microtubule profiles of nonmedullated fibers increased with the cross-sectional area of the axon although their packing decreased. In myelinated fibers of a given caliber, the packing of microtubules increased with time, and by day 63 the density had reached its final value. In nonmedullated fibers of a given caliber, a similar trend was observed after day 31; i.e., fibers showed a small but consistent increase in density. However, before that, and in contrast to myelinated fibers, nonmedullated fibers of defined calibers exhibited a transient increase in the microtubular density. Notwithstanding, during the history of an individual fiber the packing of its microtubules may decrease continuously until stabilizing, because the developing fiber is increasing its caliber and hence decreasing its microtubular density. In the 5-day-old rats, the caliber spectra of myelinated and nonmedullated axons overlapped in the 0.49-0.83 micron2 range and their microtubular densities were similar. We conclude that the microtubular content of developing axons correlates with caliber, an architectural feature, and is largely independent of growth and of its myelinated condition. We propose that the cytoskeletal rather than the transport function commands the organization of axonal microtubules.

Microtubules and caliber of central and peripheral processes of sensory axons

The microtubular content and caliber of sensory axons were studied in the L7 dorsal root, at the distal pole of the L7 spinal ganglion, and in the sural nerve of cats. Calibers of myelinated axons were symmetrical about the ganglion. In contrast, nonmedullated axons were strikingly different; 80% of the population at the root were smaller than 0.4 micron2, whilst just across the ganglion the same group was less than 20%. The microtubule densities of myelinated axons of the root were 11.8 and 6.1 microtubules/micron2 for 3- and 10 microns diameter axons, respectively. Across the ganglion the densities of myelinated axons of equal sizes were 24.2 and 14.4 microtubules/micron2, respectively. These values represent an approximate ratio of 1:2 between central and peripheral microtubule densities. Microtubule densities for nonmedullated axons also decreased with the increase in the cross-sectional area. The densities of root nonmedullated axons ranged between 90 and 10 microtubules/micron2; these were smaller, usually by a factor of three, than the densities of peripheral axons of a similar size (range: 367-44). Contrasting with the differences observed across the ganglion, the microtubular content and caliber of sensory axons seems to be quite uniform along their peripheral course. This is supported by the similar values found in the juxtaganglionic and sural nerves. It is estimated that an axon that contains 90 microtubules/micron2 has 26.7 mg of tubulin per ml of axoplasm in its assembled form, and 3.0 mg/ml if it contains 10 microtubules/micron2; these values are the practical limits of assembled tubulin in axoplasms.(ABSTRACT TRUNCATED AT 250 WORDS)