Protein degradation in the mouse visual system. I. Degradation of axonally transported and retinal proteins

Intra-axonal protein synthesis in development and beyond

Proteins can be locally produced in the periphery of a cell, allowing a rapid and spatially precise response to the changes in its environment. This process is especially relevant in highly polarized and morphologically complex cells such as neurons. The study of local translation in axons has evolved from being primarily focused on developing axons, to the notion that also mature axons can produce proteins. Axonal translation has been implied in several physiological and pathological conditions, and in all cases it shares common molecular actors and pathways as well as regulatory mechanisms. Here, we review the main findings in these fields, and attempt to highlight shared principles.

Does metabolic failure at the synapse cause Alzheimer's disease?

Alzheimer's disease (AD) a neurodegenerative disorder of widely distributed cortical networks evolves over years while A beta (Aβ) oligomer neurotoxicity occurs within seconds to minutes. This disparity combined with disappointing outcomes of anti-amyloid clinical trials challenges the centrality of Aβ as principal mediator of neurodegeneration. Reconsideration of late life AD as the end-product of intermittent regional failure of the neuronal support system to meet the needs of vulnerable brain areas offers an alternative point of view. This model introduces four ideas: (1) That Aβ is a synaptic signaling peptide that becomes toxic in circumstances of metabolic stress. (2) That intense synaptic energy and maintenance requirements of cortical hubs may exceed resources during peak demand initiating a neurotoxic cascade in these selectively vulnerable regions. (3) That axonal transport to and from neuron soma cannot account fully for high mitochondrial densities and other requirements of distant terminal axons. (4) That neurons as specialists in information management, delegate generic support functions to astrocytes and other cell types. Astrocytes use intercellular transport by exosomes and tunneling nanotubes (TNTs) to deliver mitochondria, substrates and protein reprocessing services to axonal sites distant from neuronal soma. This viewpoint implicates the brain's support system and its disruption by various age and disease-related insults as significant mediators of neurodegenerative disease. A better understanding of this system should broaden concepts of neurodegeneration and facilitate development of effective treatments.

The C-terminal domains of NF-H and NF-M subunits maintain axonal neurofilament content by blocking turnover of the stationary neurofilament network

Newly synthesized neurofilaments or protofilaments are incorporated into a highly stable stationary cytoskeleton network as they are transported along axons. Although the heavily phosphorylated carboxyl-terminal tail domains of the heavy and medium neurofilament (NF) subunits have been proposed to contribute to this process and particularly to stability of this structure, their function is still obscure. Here we show in NF-H/M tail deletion [NF-(H/M)(tailΔ)] mice that the deletion of both of these domains selectively lowers NF levels 3-6 fold along optic axons without altering either rates of subunit synthesis or the rate of slow axonal transport of NF. Pulse labeling studies carried out over 90 days revealed a significantly faster rate of disappearance of NF from the stationary NF network of optic axons in NF-(H/M)(tailΔ) mice. Faster NF disappearance was accompanied by elevated levels of NF-L proteolytic fragments in NF-(H/M)(tailΔ) axons. We conclude that NF-H and NF-M C-terminal domains do not normally regulate NF transport rates as previously proposed, but instead increase the proteolytic resistance of NF, thereby stabilizing the stationary neurofilament cytoskeleton along axons.

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.

The RNA binding and transport proteins staufen and fragile X mental retardation protein are expressed by rat primary afferent neurons and localize to peripheral and central axons

Neuronal proteins have been traditionally viewed as being derived solely from the soma; however, accumulating evidence indicates that dendritic and axonal sites are capable of a more autonomous role in terms of new protein synthesis. Such extra-somal translation allows for more rapid, on-demand regulation of neuronal structure and function than would otherwise be possible. While mechanisms of dendritic RNA transport have been elucidated, it remains unclear how RNA is trafficked into the axon for this purpose. Primary afferent neurons of the dorsal root (DRG) and trigeminal (TG) ganglia have among the longest axons in the neuraxis and such axonal protein synthesis would be advantageous, given the greater time involved for protein trafficking to occur via axonal transport. Therefore, we hypothesized that these primary sensory neurons might express proteins involved in RNA transport. Rat DRG and TG neurons expressed staufen (stau) 1 and 2 (detected at the mRNA level) and stau2 and fragile x mental retardation protein (FMRP; detected at the protein level). Stau2 mRNA was also detected in human TG neurons. Stau2 and FMRP protein were localized to the sciatic nerve and dorsal roots by immunohistochemistry and to dorsal roots by Western blot. Stau2 and FMRP immunoreactivities colocalized with transient receptor potential channel type 1 immunoreactivity in sensory axons of the sciatic nerve and dorsal root, suggesting that these proteins are being transported into the peripheral and central terminals of nociceptive sensory axons. Based on these findings, we propose that stau2 and FMRP proteins are attractive candidates to subserve RNA transport in sensory neurons, linking somal transcriptional events to axonal translation.

New insights into neuronal regeneration: the role of axonal protein synthesis in pathfinding and axonal extension

Protein synthesis in dendrites has become an accepted cellular mechanism that contributes to activity-dependent responses in the post-synaptic neuron. Although it was argued that protein synthesis does not occur in axons, early studies from a number of groups provided evidence for the presence of RNAs and active protein synthesis machinery in both invertebrate and vertebrate axons. Work over the past decade has confirmed these early findings and has proven the capability of axons to locally synthesize some of their own proteins. The functional significance of this localized protein synthesis remained largely unknown until recent years. Recent studies have shown that mRNA translation in developing and mature axons plays a role in axonal growth. In developing axons, protein synthesis allows the distal axon to autonomously respond to guidance cues by rapidly changing its direction of outgrowth. In addition, local proteolysis of axonal proteins contributes axonal guidance and growth cone initiation. This local synthesis and degradation of proteins are likely to provide novel insights into how growing axons navigate through their complex environment. In mature axons, injury triggers formation of a growth cone through localized protein synthesis, and moreover, in these injured axons locally synthesized proteins provide a retrogradely transported signal that can enhance regenerative responses. The intrinsic capability for axons to autonomously regulate local protein levels can be modulated by exogenous stimuli providing opportunities for enhancing regeneration. In this review, the concept of axonal protein synthesis is discussed from a historical perspective. Further, the implications of axonal protein synthesis and proteolysis for neural repair are considered.

RNA translation in axons

The cell body has classically been considered the exclusive source of axonal proteins. However, significant evidence has accumulated recently to support the view that protein synthesis can occur in axons themselves, remote from the cell body. Indeed, local translation in axons may be integral to aspects of synaptogenesis, long-term facilitation, and memory storage in invertebrate axons, and for growth cone navigation in response to environmental stimuli in developing vertebrate axons. Here we review the evidence supporting mRNA translation in axons and discuss the potential roles that local protein synthesis may play during development and subsequent neuronal function. We advance the view that local translation provides a rapid supply of nascent proteins in restricted axonal compartments that can potentially underlie long-term responses to transient stimuli.

The neurofilament middle molecular mass subunit carboxyl-terminal tail domains is essential for the radial growth and cytoskeletal architecture of axons but not for regulating neurofilament transport rate

The phosphorylated carboxyl-terminal "tail" domains of the neurofilament (NF) subunits, NF heavy (NF-H) and NF medium (NF-M) subunits, have been proposed to regulate axon radial growth, neurofilament spacing, and neurofilament transport rate, but direct in vivo evidence is lacking. Because deletion of the tail domain of NF-H did not alter these axonal properties (Rao, M.V., M.L. Garcia, Y. Miyazaki, T. Gotow, A. Yuan, S. Mattina, C.M. Ward, N.S. Calcutt, Y. Uchiyama, R.A. Nixon, and D.W. Cleveland. 2002. J. Cell Biol. 158:681-693), we investigated possible functions of the NF-M tail domain by constructing NF-M tail-deleted (NF-MtailDelta) mutant mice using an embryonic stem cell-mediated "gene knockin" approach that preserves normal ratios of the three neurofilament subunits. Mutant NF-MtailDelta mice exhibited severely inhibited radial growth of both motor and sensory axons. Caliber reduction was accompanied by reduced spacing between neurofilaments and loss of long cross-bridges with no change in neurofilament protein content. These observations define distinctive functions of the NF-M tail in regulating axon caliber by modulating the organization of the neurofilament network within axons. Surprisingly, the average rate of axonal transport of neurofilaments was unaltered despite these substantial effects on axon morphology. These results demonstrate that NF-M tail-mediated interactions of neurofilaments, independent of NF transport rate, are critical determinants of the size and cytoskeletal architecture of axons, and are mediated, in part, by the highly phosphorylated tail domain of NF-M.

Gene replacement in mice reveals that the heavily phosphorylated tail of neurofilament heavy subunit does not affect axonal caliber or the transit of cargoes in slow axonal transport

The COOH-terminal tail of mammalian neurofilament heavy subunit (NF-H), the largest neurofilament subunit, contains 44-51 lysine-serine-proline repeats that are nearly stoichiometrically phosphorylated after assembly into neurofilaments in axons. Phosphorylation of these repeats has been implicated in promotion of radial growth of axons, control of nearest neighbor distances between neurofilaments or from neurofilaments to other structural components in axons, and as a determinant of slow axonal transport. These roles have now been tested through analysis of mice in which the NF-H gene was replaced by one deleted in the NF-H tail. Loss of the NF-H tail and all of its phosphorylation sites does not affect the number of neurofilaments, alter the ratios of the three neurofilament subunits, or affect the number of microtubules in axons. Additionally, it does not reduce interfilament spacing of most neurofilaments, the speed of action potential propagation, or mature cross-sectional areas of large motor or sensory axons, although its absence slows the speed of acquisition of normal diameters. Most surprisingly, at least in optic nerve axons, loss of the NF-H tail does not affect the rate of transport of neurofilament subunits.

Cryptic peripheral ribosomal domains distributed intermittently along mammalian myelinated axons

A growing body of metabolic and molecular evidence of an endogenous protein-synthesizing machinery in the mature axon is a challenge to the prevailing dogma that the latter is dependent exclusively on slow axoplasmic transport to maintain protein mass in a steady state. However, evidence for a systematic occurrence of ribosomes in mature vertebrate axons has been lacking until recently, when restricted ribosomal domains, called "periaxoplasmic plaques," were described in goldfish CNS myelinated axons. Comparable restricted RNA/ribosomal "plaque" domains now have been identified in myelinated axons of lumbar spinal nerve roots in rabbit and rat on the basis of RNase sensitivity of YOYO-1-binding fluorescence, immunofluorescence of ribosome-specific antibodies, and ribosome phosphorus mapping by electron spectroscopic imaging (ESI). The findings were derived from examination of the axoplasm isolated from myelinated fibers as axoplasmic whole mounts and delipidated spinal nerve roots. Ribosomal periaxoplasmic plaque domains in rabbit axons were typically narrow ( approximately 2 microm), elongated ( approximately 10 microm) sites that frequently were marked by a protruding structure. The domain complexity included an apparent ribosome-binding matrix. The small size, random distribution, and variable intermittent axial spacing of plaques around the periphery of axoplasm near the axon-myelin border are likely reasons why their systematic occurrence has remained undetected in ensheathed axons. The periodic but regular incidence of ribosomal domains provides a structural basis for previous metabolic evidence of protein synthesis in myelinated axons.

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.

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.

Active polysomes are present in the large presynaptic endings of the synaptosomal fraction from squid brain

Previous data have suggested that the large nerve terminals present in the synaptosomal fraction from squid optic lobe are capable of protein synthesis (Crispino et al., 1993a,b). We have further examined this issue by comparing the translation products of synaptosomal and microsomal polysomes. Both preparations programmed an active process of translation, which was completely abolished by their previous treatment with EDTA. After immunoabsorption of the newly synthesized neurofilament (NF) proteins, the labeling ratio of the 60 and 70 kDa NF proteins was found to differ, in agreement with comparable differences obtained with intact synaptosomes. These observations indicate that the set of mRNAs translated by synaptosomes differs from that translated by nerve cell bodies. Hence, because NF proteins are neuron-specific, they support the view that the active synaptosomal polysomes are mostly localized in the large nerve terminals that represent the most abundant neuronal component of the fraction. This hypothesis was confirmed (1) by electron spectroscopic data demonstrating the presence of ribosomes and polysomes within the large nerve endings of the synaptosomal fraction, as well as in the carrot-like nerve endings of the retinal photoreceptors that constitute the only large terminals in the optic lobe, and (2) by light and high resolution autoradiography of synaptosomal samples incubated with [3H]leucine, showing that most labeled proteins are associated with the large nerve endings. This response was abolished by cycloheximide. Taken together, the data provide the first unequivocal demonstration that presynaptic nerve terminals are capable of protein synthesis.

Dynamic organization of endocytic pathways in axons of cultured sympathetic neurons

Despite the wealth of information about endocytic pathways in non-neuronal cells, little is known about these crucial sorting, recycling, and degradative pathways in neurons. In this report, we analyzed in detail the dynamic steady-state organization of endocytically derived organelles as they progress through the endosomal-lysosomal pathway in axons of live cultured sympathetic neurons. By ratiometric imaging of neurons endocytically labeled with the pH indicator 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS), we demonstrate a trimodal frequency distribution of endocytic organelle pH in axon shafts, indicating two rapid acidification steps in the progression from endocytosis to the lysosome. Axonal branch points display a unimodal organelle pH distribution (mean 6.4), implicating them as meeting places for endocytic organelles and Golgi-derived vesicles or as sorting sites. By following endocytic organelle traffic retrogradely from growth cone to soma, we identified significant transition points in the pathway. Growth cones exhibit a unimodal pH distribution comprised mainly of acidified recycling/sorting endosomes (mean 6.3). However, organelles in the axon shaft immediately adjacent to the growth cone display the distinct trimodal pH distribution of the axon, suggesting that important sorting events occur between these domains. An abrupt increase in organelle acidification occurs in the distal axon 50-150 microns from the growth cone, demonstrating a discontinuous spatial gradient of acidification along axons. Immunofluorescence microscopy reveals that the lysosomal glycoprotein LEP100 is present in axons and is concentrated in two important regions: the proximal axon where the endocytic organelle population is largely acidified, and the same region of the distal axon where substantial acidification occurs.

Alterations in the neutral proteinase activities of central and peripheral nervous systems of acrylamide-, carbon disulfide-, or 2,5-hexanedione-treated rats

Proteinases are widespread in neuronal or nonneuronal eukaryotic cells. They are suggested to play an important role in the turnover of proteins in neuronal perikaryon and axon, and digestion of the transported cytoskeletal proteins in synaptic terminals. We examined the effect of acrylamide (50 mg/kg, ip), carbon disulfide (700 ppm, 9 h, 7 d a week), and 2,5-hexanedione (2,5-HD) (1% in drinking water) treatment of rats on mCANP (2 mM Ca2+), microCANP (0.1 mM Ca2+), and CINP (Ca(2+)-independent) activity in telencephalon + diencephalon (FB), rhombencephalon + mesencephalon (LB), spinal cord (SC), and sciatic nerve (SN). The proteinase activity was determined in the 30,000g supernatant fraction of tissues using 14C-methylated casein as the substrate. mCANP activity in FB, LB, and SC was inhibited only by acrylamide. Acrylamide or 2,5-HD treatment had no effect on microCANP and CINP activities of SN, whereas carbon disulfide enhanced microCANP after 15 d and CINP activity after 10 d. It is suggested that alteration in in vitro calpain activity shown by these chemicals may not be directly related to their neurotoxic effect. However, calpain may still be playing a role in this polyneuropathy by alteration in activity through inflow of Ca2+, release of Ca2+ from intracellular organelles, or other factors. Modification of cytoskeletal proteins making them more susceptible to proteases and the role of some other proteinase is also possible.

Medium weight neurofilament mRNA in goldfish Mauthner axoplasm

Although axons are generally considered to lack the ability to synthesize proteins, the Mauthner axon (M-axon) of the goldfish has been reported to contain some of the basic components of the translational machinery, such as transfer RNA (tRNA), ribosomal RNA (rRNA), and ribosomes. To determine if the M-axon also contains mRNA, we isolated samples of M-axoplasm free of glial contamination as demonstrated by the absence of glial-specific mRNA and protein. Reverse transcription-polymerase chain reaction (RT-PCR) of M-axoplasmic cDNA in the presence of primers for the goldfish medium-weight neurofilament (NF-M) gene produced a single product of the expected length for RT-PCR amplification of goldfish NF-M mRNA. This mRNA might direct protein synthesis of NF-M within the M-axoplasm.

[32P]orthophosphate and [35S]methionine label separate pools of neurofilaments with markedly different axonal transport kinetics in mouse retinal ganglion cells in vivo

Newly synthesized neurofilament proteins become highly phosphorylated within axons. Within 2 days after intravitreously injecting normal adult mice with [32P]orthophosphate, we observed that neurofilaments along the entire length of optic axons were radiolabeled by a soluble 32P-carrier that was axonally transported faster than neurofilaments. 32P-incorporation into neurofilament proteins synthesized at the time of injection was comparatively low and minimally influenced the labeling pattern along axons. 32P-incorporation into axonal neurofilaments was considerably higher in the middle region of the optic axons. This characteristic non-uniform distribution of radiolabel remained nearly unchanged for at least 22 days. During this interval, less than 10% of the total 32P-labeled neurofilaments redistributed from the optic nerve to the optic tract. By contrast, newly synthesized neurofilaments were selectively pulse-labeled in ganglion cell bodies by intravitreous injection of [35S]methionine and about 60% of this pool translocated by slow axoplasmic transport to the optic tract during the same time interval. These findings indicate that the steady-state or resident pool of neurofilaments in axons is not identical to the newly synthesized neurofilament pool, the major portion of which moves at the slowest rate of axoplasmic transport. Taken together with earlier studies, these results support the idea that, depending in part on their phosphorylation state, transported neurofilaments can interact for short or very long periods with a stationary but dynamic neurofilament lattice in axons.

Phosphorylation on carboxyl terminus domains of neurofilament proteins in retinal ganglion cell neurons in vivo: influences on regional neurofilament accumulation, interneurofilament spacing, and axon caliber

The high molecular weight subunits of neurofilaments, NF-H and NF-M, have distinctively long carboxyl-terminal domains that become highly phosphorylated after newly formed neurofilaments enter the axon. We have investigated the functions of this process in normal, unperturbed retinal ganglion cell neurons of mature mice. Using in vivo pulse labeling with [35S]methionine or [32P]orthophosphate and immunocytochemistry with monoclonal antibodies to phosphorylation-dependent neurofilament epitopes, we showed that NF-H and NF-M subunits of transported neurofilaments begin to attain a mature state of phosphorylation within a discrete, very proximal region along optic axons starting 150 microns from the eye. Ultrastructural morphometry of 1,700-2,500 optic axons at each of seven levels proximal or distal to this transition zone demonstrated a threefold expansion of axon caliber at the 150-microns level, which then remained constant distally. The numbers of neurofilaments nearly doubled between the 100- and 150-microns level and further increased a total of threefold by the 1,200-microns level. Microtubule numbers rose only 30-35%. The minimum spacing between neurofilaments also nearly doubled and the average spacing increased from 30 nm to 55 nm. These results show that carboxyl-terminal phosphorylation expands axon caliber by initiating the local accumulation of neurofilaments within axons as well as by increasing the obligatory lateral spacing between neurofilaments. Myelination, which also began at the 150-microns level, may be an important influence on these events because no local neurofilament accumulation or caliber expansion occurred along unmyelinated optic axons. These findings provide evidence that carboxyl-terminal phosphorylation triggers the radial extension of neurofilament sidearms and is a key regulatory influence on neurofilament transport and on the local formation of a stationary but dynamic axonal cytoskeletal network.

Neurofilament proteins are synthesized in nerve endings from squid brain

It is generally believed that the proteins of the nerve endings are synthesized on perikaryal polysomes and are eventually delivered to the presynaptic domain by axoplasmic flow. At variance with this view, we have reported previously that a synaptosomal fraction from squid brain actively synthesizes proteins whose electrophoretic profile differs substantially from that of the proteins made in nerve cell bodies, axons, or glial cells, i.e., by the possible contaminants of the synaptosomal fraction. Using western analyses and immunoabsorption methods, we report now that (a) the translation products of the squid synaptosomal fraction include neurofilament (NF) proteins and (b) the electrophoretic pattern of the synaptosomal newly synthesized NF proteins is drastically different from that of the NF proteins synthesized by nerve cell bodies. The latter results exclude the possibility that NF proteins synthesized by the synaptosomal fraction originate in fragments of nerve cell bodies possibly contaminating the synaptosomal fraction. They rather indicate that in squid brain, nerve terminals synthesize NF proteins.

Slow axonal transport of soluble proteins and calpain in retinal ganglion cells of aged rabbits

The rate of slow axonal transport of soluble proteins in retinal ganglion cells of the rabbit decreased with approximately 25% in aged (6 years) compared to previous estimates in adult (2 years) animals. Immunobinding of calpain to microtiter plates coated with a monoclonal antibody to mu-calpain was used to isolate labelled axonally transported mu-calpain from the nerve extracts. It was found that the distribution of labelled mu-calpain in the retrobulbar optic pathway was similar to the distribution profile of the slowly migrating phase of soluble proteins.

Tumour necrosis factor causes an increase in axonal transport of protein and demyelination in the mouse optic nerve

An increase in fast axonal transport of protein by the optic nerve was found in mice following a single combined injection of human recombinant tumour necrosis factor alpha (rTNF) and [3H]proline into the vitreous chamber. Demyelination was observed in optic nerve fibres arising from the eyes of mice which received a single rTNF injection. No such changes were detected when heat-inactivated rTNF was injected with the label. The effects of intravitreal injection of rTNF on the pathophysiology of mouse optic nerve resembled those found in mice infected with Semliki Forest virus (SFV), an animal model of multiple sclerosis. We suggest that TNF could mediate at least some of the pathophysiological changes found in SFV-infected mice and may provide a clue concerning the disease mechanism in multiple sclerosis.

Proteolysis of microtubule-associated protein 2 and tubulin by cathepsin D

The in vitro degradation of microtubule-associated protein 2 (MAP-2) and tubulin by the lysosomal aspartyl endopeptidase cathepsin D was studied. MAP-2 was very sensitive to cathepsin D-induced hydrolysis in a relatively broad, acidic pH range (3.0-5.0). However, at a pH value of 5.5, cathepsin D-mediated hydrolysis of MAP-2 was significantly reduced and at pH 6.0 only a small amount of MAP-2 was degraded at 60 min. Interestingly, the two electrophoretic forms of MAP-2 showed different sensitivities to cathepsin D-induced degradation, with MAP-2b being significantly more resistant to hydrolysis than MAP-2a. To our knowledge, this is the first clear demonstration that MAP-2 is a substrate in vitro for cathepsin D. In contrast to MAP-2, tubulin was relatively resistant to cathepsin D-induced hydrolysis. At pH 3.5 and an enzyme-to-substrate ratio of 1: 20, only 35% of the tubulin was degraded by cathepsin D at 60 min. The cathepsin D-mediated hydrolysis of tubulin was optimal only at pH 4.5. These results demonstrate that MAP-2 and tubulin are unequally susceptible to degradation by cathepsin D. These data also imply a potential for rapid degradation of MAP-2 in vivo by cathepsin D either in lysosomes or perhaps autophagic vacuoles of the neuron.

Axons Sprout and Microtubules Increase After Local Inhibition of RNA Synthesis, and Microtubules Decrease after Inhibition of Protein Synthesis: A Morphometric Study of Rat Sural Nerves

Drugs that inhibit RNA or protein synthesis are known to affect some functional properties of axons. In this context, we studied the ultrastructural effects of actinomycin-D, an inhibitor of RNA synthesis, and cycloheximide and emetine, inhibitors of protein synthesis, in rat sural nerves. A silicone sleeve (4 mm long) loaded with drug was placed around the nerves and left for about a week. The ultrastructural alterations of axons and Schwann cells progressed over this period. After cycloheximide and emetine, the cytoplasm of Schwann cells was enlarged and the rough endoplasmic reticulum was prominent. After actinomycin-D, the Schwann cells reached the stage of lysis. Nonmedullated were more affected than myelinated axons. After cycloheximide and emetine, the axoplasmic matrix decreased substantially but reversibly. Microtubules of nonmedullated fibres decreased by about 50%. Actinomycin-D determined sprouting of axons and a rise of axonal microtubules; in nonmedullated axons, the normal inverse correlation between microtubular density and calibre gave way to a high and constant density for all axonal sizes. A few millimetres proximal and distal to the sleeve, the nerve tissue and the axonal microtubular content were close to normal, i.e. the effects of drugs were local. Present results suggest that the local turnover of amino acids in the axon is necessary to maintain the integrity of microtubule and neurofilament proteins. We propose that the Schwann cell down-regulates the axonal cytomatrix.

Calpain and calpastatin activity in the optic pathway

The levels of the neutral proteolytic enzymes calpains and their endogenous inhibitor calpastatin were determined in the retina and in the retrobulbar optic pathway in the albino rabbit. The highest level of calpains was observed in the optic nerve with decreasing levels in the optic tract and superior colliculus. The level of calpastatin in the retina was very low compared to that in the optic nerve and tract and other parts of the nervous system.

Synthesis, axonal transport, and turnover of the high molecular weight microtubule-associated protein MAP 1A in mouse retinal ganglion cells: tubulin and MAP 1A display distinct transport kinetics

Microtubule-associated proteins (MAPs) in neurons establish functional associations with microtubules, sometimes at considerable distances from their site of synthesis. In this study we identified MAP 1A in mouse retinal ganglion cells and characterized for the first time its in vivo dynamics in relation to axonally transported tubulin. A soluble 340-kD polypeptide was strongly radiolabeled in ganglion cells after intravitreal injection of [35S]methionine or [3H]proline. This polypeptide was identified as MAP 1A on the basis of its co-migration on SDS gels with MAP 1A from brain microtubules; its co-assembly with microtubules in the presence of taxol or during cycles of assembly-disassembly; and its cross-reaction with well-characterized antibodies against MAP 1A in immunoblotting and immunoprecipitation assays. Glial cells of the optic nerve synthesized considerably less MAP 1A than neurons. The axoplasmic transport of MAP 1A differed from that of tubulin. Using two separate methods, we observed that MAP 1A advanced along optic axons at a rate of 1.0-1.2 mm/d, a rate typical of the Group IV (SCb) phase of transport, while tubulin moved 0.1-0.2 mm/d, a group V (SCa) transport rate. At least 13% of the newly synthesized MAP 1A entering optic axons was incorporated uniformly along axons into stationary axonal structures. The half-residence time of stationary MAP 1A in axons (55-60 d) was 4.6 times longer than that of MAP 1A moving in Group IV, indicating that at least 44% of the total MAP 1A in axons is stationary. These results demonstrate that cytoskeletal proteins that become functionally associated with each other in axons may be delivered to these sites at different transport rates. Stable associations between axonal constituents moving at different velocities could develop when these elements leave the transport vector and incorporate into the stationary cytoskeleton.

Early posttranslational modifications of the three neurofilament subunits in mouse retinal ganglion cells: neuronal sites and time course in relation to subunit polymerization and axonal transport

We have characterized stages in the posttranslational processing of the three neurofilament subunits, High (NF-H), Middle (NF-M), and Low (NF-L), in retinal ganglion cells in vivo during the interval between synthesis in cell bodies within the retina and appearance of these polypeptides in axons at the level of the optic nerve (optic axons). Neurofilament proteins pulse-labeled by injecting mice intravitreally with [35S]methionine or [32P]orthophosphate, were isolated from Triton-soluble and Triton-insoluble fractions of the retina or optic axons by immunoprecipitation or immunoaffinity chromatography. Within 2 h after [35S]methionine injection, the retina contained neurofilament-immunoreactive radiolabeled proteins with apparent molecular weights of 160, 139, and 70 kDa, which co-migrated with subunits of axonal neurofilaments that were dephosphorylated in vitro with alkaline phosphatase. The two larger polypeptides were not labeled with [32P]orthophosphate, indicating that they were relatively unmodified forms of NF-H and NF-M. About 75% of the subunits were Triton-insoluble by 2 h after isotope injection, and this percentage increased to 98% by 6 h. Labeled neurofilament polypeptides appeared in optic axons as early as 2 h after injection. These subunits exhibited apparent molecular weights of 160, 139, and 70 kDa and were Triton-insoluble. The time of appearance of fully modified polypeptide forms differed for each subunit (2 h for NF-L, 6-18 h for NF-M, 18-24 h for NF-H) and was preceded by the transient appearance of intermediate forms. The modified radiolabeled subunits in optic axons 3 days after synthesis were heavily labeled with [32P]orthophosphate and exhibited the same apparent molecular weights as subunits of axonal neurofilaments (70 kDa, 145 and 140 kDa, and 195-210 kDa, respectively). Whole mounts of retina immunostained with monoclonal antibodies against NF-H in different states of phosphorylation demonstrated a transition from non-phosphorylated neurofilaments to predominantly phosphorylated ones within a region of the axon between 200 and 1000 microns downstream from the cell body. These experiments demonstrate that the addition of most phosphate groups to NF-M and NF-H takes place within a proximal region of the axon. The rapid appearance of modified forms of NF-L after synthesis may imply that processing of this subunit occurs at least partly in the cell body. The presence of a substantial pool of Triton-insoluble, unmodified subunits early after synthesis indicates that the heaviest incorporation of phosphate occurs after neurofilament proteins are polymerized.(ABSTRACT TRUNCATED AT 400 WORDS)

Cycloheximide-sensitive [35S]methionine labeling of proteins in goldfish retinal ganglion cell axons in vitro

Polypeptides of retinal ganglion cell axons of the goldfish, regenerating in culture, were labeled by [35S]methionine after decentralization from the explant. Microscopic samples, composed of isolated axonal fields, were analyzed by SDS ultramicroelectrophoresis and autoradiography. Of the several proteins exhibiting cycloheximide-sensitive labeling, beta-tubulin and actin were consistently and prominently labeled, although the possibility of a labeled alpha-isoform with a lower mol. wt. could not be ruled out.

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.

Neurofilament and tubulin transport slows along the course of mature motor axons

The rate of axonal transport of tubulin and of neurofilament proteins was measured in mature, non-growing, axons of rat lumbar ventral roots and sciatic nerves. A progressive decrease in the velocity of transport was observed along the course of the axons. The rate of advance of the 'front' of labeled neurofilaments decreased from 1.40 +/- 0.07 mm/day proximally to 0.64 +/- 0.04 mm/day distally. The rate for tubulin decreased from 3.10 +/- 0.13 proximally to 1.01 +/- 0.04 mm/day distally. These results demonstrate that a gradient of the velocity of cytoskeletal transport along axons is not a specific adaptation for radial growth of axons, as previously suggested. Local degradation of excess cytoskeletal proteins may permit axons to accommodate a net slowing of cytoskeletal transport without accumulation of neurofilaments or microtubules. Alternatively, the transport kinetics of radiolabeled proteins may be influenced by mixing of labeled and unlabeled cytoskeletal proteins within the axoplasm; in this case, a slowing of the movement of the labeled proteins may not reflect net slowing of transport of the total pool of cytoskeletal proteins.

Multiple phosphorylated variants of the high molecular mass subunit of neurofilaments in axons of retinal cell neurons: characterization and evidence for their differential association with stationary and moving neurofilaments

The 200-kD subunit of neurofilaments (NF-H) functions as a cross-bridge between neurofilaments and the neuronal cytoskeleton. In this study, four phosphorylated NF-H variants were identified as major constituents of axons from a single neuron type, the retinal ganglion cell, and were shown to have characteristics with different functional implications. We resolved four major Coomassie Blue-stained proteins with apparent molecular masses of 197, 200, 205, and 210 kD on high resolution one-dimensional SDS-polyacrylamide gels of mouse optic axons (optic nerve and optic tract). Proteins with the same electrophoretic mobilities were radiolabeled within retinal ganglion cells in vivo after injecting mice intravitreally with [35S]methionine or [3H]proline. Extraction of the radiolabeled protein fraction with 1% Triton X-100 distinguished four insoluble polypeptides (P197, P200, P205, P210) with expected characteristics of NF-H from two soluble neuronal polypeptides (S197, S200) with few properties of neurofilament proteins. The four Triton-insoluble polypeptides displayed greater than 90% structural homology by two-dimensional alpha-chymotryptic iodopeptide map analysis and cross-reacted with four different monoclonal and polyclonal antibodies to NF-H by immunoblot analysis. Each of these four polypeptides advanced along axons primarily in the Group V (SCa) phase of axoplasmic transport. By contrast, the two Triton-soluble polypeptides displayed only a minor degree of alpha-chymotryptic peptide homology with the Triton-insoluble NF-H forms, did not cross-react with NF-H antibodies, and moved primarily in the Group IV (SCb) wave of axoplasmic transport. The four NF-H variants were generated by phosphorylation of a single polypeptide. Each of these polypeptides incorporated 32P when retinal ganglion cells were radiolabeled in vivo with [32P]orthophosphate and each cross-reacted with monoclonal antibodies specifically directed against phosphorylated epitopes on NF-H. When dephosphorylated in vitro with alkaline phosphatase, the four variants disappeared, giving rise to a single polypeptide with the same apparent molecular mass (160 kD) as newly synthesized, unmodified NF-H. The NF-H variants distributed differently along optic axons. P197 predominated at proximal axonal levels; P200 displayed a relatively uniform distribution; and P205 and P210 became increasingly prominent at more distal axonal levels, paralleling the distribution of the stationary neurofilament network.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple fates of newly synthesized neurofilament proteins: evidence for a stationary neurofilament network distributed nonuniformly along axons of retinal ganglion cell neurons

We have studied the fate of neurofilament proteins (NFPs) in mouse retinal ganglion cell (RGC) neurons from 1 to 180 d after synthesis and examined the proximal-to-distal distribution of the newly synthesized 70-, 140-, and 200-kD subunits along RGC axons relative to the distribution of neurofilaments. Improved methodology for intravitreal delivery of [3H]proline enabled us to quantitate changes in the accumulation and subsequent decline of radiolabeled NFP subunits at various postinjection intervals and, for the first time, to estimate the steady state levels of NFPs in different pools within axons. Two pools of newly synthesized triplet NFPs were distinguished based on their kinetics of disappearance from a 9-mm "axonal window" comprising the optic nerve and tract and their temporal-spatial distribution pattern along axons. The first pool disappeared exponentially between 17 and 45 d after injection with a half-life of 20 d. Its radiolabeled wavefront advanced along axons at 0.5-0.7 mm/d before reaching the distal end of the axonal window at 17 d, indicating that this loss represented the exit of neurofilament proteins composing the slowest phase of axoplasmic transport (SCa or group V) from axons. About 32% of the total pool of radiolabeled neurofilament proteins, however, remained in axons after 45 d and disappeared exponentially at a much slower rate (t 1/2 = 55 d). This second NFP pool assumed a nonuniform distribution along axons that was characterized proximally to distally by a 2.5-fold gradient of increasing radioactivity. This distribution pattern did not change between 45 and 180 d indicating that neurofilament proteins in the second pool constitute a relatively stationary structure in axons. Based on the relative radioactivities and residence time (or turnover) of each neurofilament pool in axons, we estimate that, in the steady state, more neurofilament proteins in mouse RGC axons may be stationary than are undergoing continuous slow axoplasmic transport. This conclusion was supported by biochemical analyses of total NFP content and by electron microscopic morphometric studies of neurofilament distribution along RGC axons. The 70-, 140-, and 200-kD subunits displayed a 2.5-fold proximal to distal gradient of increasing content along RGC axons. Neurofilaments were more numerous at distal axonal levels, paralleling the increased content of NFP.(ABSTRACT TRUNCATED AT 400 WORDS)

In situ degradation of rapidly transported proteins in nerve terminals of retinal ganglion cells

The in situ degradation of rapidly transported proteins in nerve terminals of retinal ganglion cells was studied in pieces of the superior colliculus of the rabbit. Proteolytic degradation was found to be dependent upon extracellular calcium and intact calcium channels. Degradation was inhibited by leupeptin and SH-group blocking agents.

Ca2+-activated protease activity in frog sciatic nerve: characterization and effect on rapidly transported axonal proteins

Protease activity was studied in the frog sciatic nerve. The activity was measured as the release of TCA-soluble radioactivity from either 3H-labelled proteins transported by rapid axonal transport (AXT) or 3H-labelled ganglionic proteins. In nerve homogenates containing transported substrates, protease activity exhibited two peaks, one around pH 5 and one around pH 8. Ca2+ at 100 microM or higher concentrations only stimulated the latter, which was inhibited by 1 mM parachloromercuric benzoate, a sulphydryl reagent, but unaffected by ATP (1 mM). The proteolytic activity was recovered in the 10(5) g supernatant of the homogenate. In desheathed nerves containing 3H-labelled transported proteins, the protease activity could be activated by exposing the nerve to a Ca2+-ionophore, X-537 A, or to an elevated Ca2+-concentration (50 mM). These conditions were also shown to increase the influx and efflux of 45Ca2+ in the nerves. The results indicate the presence within axons of a Ca2+-activated soluble protease, which degrades rapidly transported proteins. The finding that the protease degraded ganglionic soluble proteins to about the same extent suggests a broad substrate specificity. The present system should be useful for further characterization of protease activity during various physiological conditions.

Slow axoplasmic transport: a fiction?

Ribosomes have not been observed in axoplasm. This had led to the notions that the perikaryon is the only source of neuronal proteins and that the axoplasm is supplied by a (slow) transport mechanism. However, we question these two notions because they are unable to give an account of real neurones in accordance with the body of biological knowledge. We point out, for example, that the synthetic rate of perikarya or the life span of axoplasmic proteins should be beyond known ranges for animal cells and that a uniform axon is unlikely to result if it is fed from one end. We propose an alternative view for the maintenance of the axon which accepts the controversial idea of axoplasmic synthesis of proteins; as a result, the slow transport becomes unnecessary. Our view gives a qualitative account of the observations dealing with the maintenance of the axoplasm. To account for the phenomenology in a more quantitative fashion, a computer simulation was carried out where the equations of the program provided only for axoplasmic synthesis of proteins; the set of curves retrieved were in good agreement with experimental findings believed so far to support the notion of slow transport. In conclusion, we think that the notion of "slow axoplasmic transport" has been a misinterpretation of good observations because the frame of reference was incomplete in not providing for axoplasmic synthesis of proteins.

Is protease activity involved in fast axonal transport?

N-alpha-p-Tosyl-L-Lysine Chloromethyl Ketone (TLCK), a protease inhibitor, was found to irreversibly inhibit rapid axonal transport of protein in vitro in the frog sciatic nerve. TLCK exerted its action at the axonal level and seemed to depress the rate rather than the amount of transported protein. The efficiency of TLCK as a protease inhibitor was demonstrated by polyacrylamide gel electrophoresis, which showed that degradation of high molecular weight proteins (presumably neurofilament subunits) into a 25000 dalton protein could be induced by exposing the frog nerves to triton-X and prevented by the presence of TLCK. Findings that TLCK, at a transport inhibiting concentration (0.1 mM), had little or no effects on either protein synthesis or ATP levels, suggest that TLCK did not affect transport due to general cytotoxic properties. The effects of TLCK is discussed in relation to possible roles of protease activity in axonal transport.

Cellular origin and biosynthesis of rat optic nerve proteins: a two-dimensional gel analysis

High resolution 2DGE (two-dimensional gel electrophoresis) was used to characterize neuronal and glial proteins of the rat optic nerve, to examine the phases of intraaxonal transport with which the neuronal proteins are associated, and to identify the ribosomal populations on which these proteins are synthesized. Neuronal proteins synthesized in the retinal ganglion cells were identified by injecting the eye with L-[35S]methionine, followed by 2DGE analysis of fast and slow axonally transported proteins in particulate and soluble fractions. Proteins synthesized by the glial cells were labeled by incubating isolated optic nerves in the presence of L-[35S]methionine and then analyzed by 2DGE. A number of differences were seen between filamentous proteins of neurons and glia. Most strikingly, proteins in the alpha- and beta-tubulin region of the 2D gels of glial proteins were distinctly different than was observed for axonal proteins. As expected, neurons but not glia expressed neurofilament proteins, which appeared among the slow axonally transported proteins in the particulate fraction; significant amounts of the glial filamentous protein, GFA, were also labeled under these conditions, which may have been due to transfer of amino acids from the axon to the glial compartment. The fast axonally transported proteins contained relatively large amounts of high-molecular-weight acidic proteins, two of which were shown to comigrate (on 2DGE) with proteins synthesized by rat CNS rough microsomes; this finding suggests that rough endoplasmic reticulum may be a major site of synthesis for fast transported proteins. In contrast, the free polysome population was shown to synthesize the principal components of slow axonal transport, including tubulin subunits, actin, and neurofilament proteins.

Degradation of neurofilament proteins by purified human brain cathepsin D

Cathepsin D (CD) was purified to homogeneity from postmortem human cerebral cortex. Incubation of CD with human neurofilament proteins (NFPs) prepared by axonal flotation led to the rapid degradation of the 200,000, 160,000, and 70,000 NFP subunits (200K, 160K, and 70K) which had been separated by one- or two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Degradation was appreciable at enzyme activity-to-substrate protein ratios that were two- to threefold lower than those in unfractionated homogenates from cerebral cortex. Quantitative measurements of NFPs separated by PAGE revealed that, at early stages of digestion, the 160K NFP was somewhat more rapidly degraded than the 70K subunit while the 200K NFP had an intermediate rate of degradation. At sufficiently high enzyme concentrations, all endogenous proteins in human NF preparations were susceptible to the action of CD. Human brain CD also degraded cytoskeletal proteins in NF preparations from mouse brain with a similar specificity. To identify specific NFP break-down products, antisera against each of the major NFPs were applied to nitrocellulose electroblots of NFPs separated by two-dimensional SDS-PAGE. In addition to detecting the 200K, 160K, and 70K NFP in human NF preparations, the antisera also detected nonoverlapping groups of polypeptides resembling those in NF preparations from fresh rat brain. When human NF preparations were incubated with CD, additional polypeptides were released in specific patterns from each NFP subunit. Some of the immuno-cross-reactive fragments generated from NFPs by CD comigrated on two-dimensional gels with polypeptides present in unincubated preparations. These results demonstrate that NFPs and other cytoskeletal proteins are substrates for CD. The physiological significance of these findings and the possible usefulness of analyzing protein degradation products for establishing the action of proteinases in vivo are discussed.

Limited proteolytic modification of a neurofilament protein involves a proteinase activated by endogenous levels of calcium

Posttranslational modification of a structural protein by limited proteolysis is demonstrated for the first time in the nervous system. The 145,000 dalton subunit of neurofilaments in mouse retinal ganglion cell (RGC) axons is selectively converted in vitro to the major 143,000 and 140,000 dalton neurofilament subunits by a neutral proteinase that is activated by endogenous levels of calcium and is distinguishable from other known brain proteinases. The close similarities between this in vitro process and the previously observed modification of the 145,000 dalton neurofilament protein during axoplasmic transport in vivo suggest that the same enzymatic mechanism is involved. These findings imply that limited proteolysis is an active process along central axons in vivo and that this enzyme may play a specific role in the function of the neuronal cytoskeleton.