Biochemical composition and dynamics of the axonal cytoskeleton in the corticospinal system of the adult hamster

Processive flow by biased polymerization mediates the slow axonal transport of actin

Classic pulse-chase studies have shown that actin is conveyed in slow axonal transport, but the mechanistic basis for this movement is unknown. Recently, we reported that axonal actin was surprisingly dynamic, with focal assembly/disassembly events ("actin hotspots") and elongating polymers along the axon shaft ("actin trails"). Using a combination of live imaging, superresolution microscopy, and modeling, in this study, we explore how these dynamic structures can lead to processive transport of actin. We found relatively more actin trails elongated anterogradely as well as an overall slow, anterogradely biased flow of actin in axon shafts. Starting with first principles of monomer/filament assembly and incorporating imaging data, we generated a quantitative model simulating axonal hotspots and trails. Our simulations predict that the axonal actin dynamics indeed lead to a slow anterogradely biased flow of the population. Collectively, the data point to a surprising scenario where local assembly and biased polymerization generate the slow axonal transport of actin without involvement of microtubules (MTs) or MT-based motors. Mechanistically distinct from polymer sliding, this might be a general strategy to convey highly dynamic cytoskeletal cargoes.

Spinal shock revisited: a four-phase model

Spinal shock has been of interest to clinicians for over two centuries. Advances in our understanding of both the neurophysiology of the spinal cord and neuroplasticity following spinal cord injury have provided us with additional insight into the phenomena of spinal shock. In this review, we provide a historical background followed by a description of a novel four-phase model for understanding and describing spinal shock. Clinical implications of the model are discussed as well.

A mathematical model for assessing changes in neurofilament protein levels in neurites and cell bodies of differentiating neuroblastoma cells

A mathematical model which allows the calculation of the level of neurofilament protein in the cell body (x) and in the neurites (y) of differentiating SK-N-SH cells is presented. The model considers the changes in cell number (proliferating cells) and the number of cells with neurites (differentiating cells). It takes into account the fact that (i) when cells are cultured in differentiating conditions, an increase in cell number is initially observed and (ii) in a non-synchronized population of differentiating cells, the length of neurite extended by individual cells varies within the population. Total neurofilament protein levels in a population of cells were measured by enzyme-linked immunoabsorbant assay and application of the model to the data allowed values for x and y to be calculated. The validity of the model is supported by the fact that the predicted total neurofilament protein levels are highly correlated with the experimentally derived neurofilament protein levels. The model should be of use in temporal studies of cytoskeletal proteins involved in neuronal growth/differentiation and also in studies in which the system is a target of toxic insult.

Transport of cytoskeletal proteins in axons of hippocampal pyramidal cells

Axonal transport of cytoskeletal proteins has not yet been extensively studied in the brain proper, in contrast to the peripheral nerves and optic nerves. The authors have developed a means for the study of transport of cytoskeletal proteins in axons of hippocampal pyramidal cells. Proteins of intrinsic neurons of the dorsal hippocampus were labeled by microinjection of 35S methionine, and the subsequent transport of labeled proteins was characterized in the axons projecting into the fimbria-fornix. A peak of labeled proteins was present in the fimbria-fornix at 4-12 days after labeling, corresponding to transport rates 0.2-0.7 mm/day. The most abundant proteins at each time studied exhibited one-dimensional electrophoretic mobilities of actin and tubulin; neurofilaments were less intensely labeled. The observed specializations of cytoskeletal transport, especially the paucity of tubulin transport at rates of 2-4 mm/day, may predispose hippocampal pyramidal cells to accumulate tubulin and microtubule-associated proteins in their cell bodies in various disease states.

The regulation of neurofilament protein dynamics by phosphorylation: clues to neurofibrillary pathobiology

Neurofilament proteins are continuously modified during their lifetime by a succession of protein kinases and phosphatases. Site-specific phosphorylation or dephosphorylation within different polypeptide domains of each neurofilament subunit is now believed to regulate such behaviors of neurofilaments as subunit polymerization and exchange, axonal transport, interactions with other cytoskeletal proteins and degradation. Local regulation of phosphorylation events could account for variations in the size, morphology and dynamics of the neurofilament network in different regions of the neuron. The apparent greater plasticity of the neurofilament network in regions like the perikaryon, initial segment and nodes along the axon may provide some insight into the vulnerability of these regions in neurofibrillary disease.

Differential regulation of beta III and other tubulin genes during peripheral and central neuron development

Mammalian peripheral and central neurons differ considerably in the composition and properties of their axonal cytoskeletons. Recent reports of the selective expression of a high molecular weight (HMW) tau protein in neurons with peripherally projecting axons have furthered the idea that the microtubules in central and peripheral neurons are disparate. In the present study, we examined the possibility that the various tubulin genes are differentially expressed in central versus peripheral neurons. To examine this, we compared the expression of four of the beta-tubulin mRNAs (classes beta I, beta II, beta III, beta IV) and the alpha 1-tubulin mRNA in rat dorsal root ganglion (DRG) neurons with their expression in cerebral cortex during postnatal development (P5-90), using northern blots and in situ hybridization. We document both similarities and differences in tubulin gene expression in these two regions of the neuraxis during postnatal development. In both DRG and cortex, the expression of the class beta I- and beta II-tubulin mRNAs and the alpha 1-tubulin mRNA was higher at earlier stages of postnatal development than in the adult. However, class beta IV-tubulin mRNA levels increased during cortical development but decreased during DRG postnatal development. The opposite pattern was found for the neuron-specific class beta III-tubulin gene, the mRNA levels of which were high in cortex, at birth and then decreased with increasing postnatal development. In DRG, the beta III-tubulin mRNA levels generally increased during postnatal development. Beta III-tubulin protein levels were examined qualitatively at different developmental stages (5-90 days) by immunoblotting and immunocytochemistry.(ABSTRACT TRUNCATED AT 250 WORDS)

Slow transport of the cytoskeleton after axonal injury

The delivery of cytoskeletal proteins to the axon occurs by slow axonal transport. We examined how the rate of slow transport was altered after axonal injury. When retinal ganglion cell (RGC) axons regenerated through peripheral nerve grafts, an increase in the rate of slow transport occurred during regrowth of the injured axons. We compared these results to axonal injury in the optic nerve where no substantial regrowth occurs and found a completely different response. Slow transport was decreased approximately tenfold in rate in the proximal segment of crushed optic nerves. This decreased rate of slow transport was not induced immediately, but occurred about 1 week after injury. To explore whether a decrease in the rate of slow transport was induced when the regeneration of peripheral nerves was physically blocked, we examined slow transport in motor neurons after the sciatic nerve was transected and ligated. In this case, no change in the rate of the comigrating tubulin and neurofilament (NF) radioactive peaks were observed. We discuss how the changes in the rate of slow transport may reflect different neuronal responses to injury and speculate about the possible molecular changes in the expression of tubulin which may contribute to the observed changes.

Neuronal protein NP185 is developmentally regulated, initially expressed during synaptogenesis, and localized in synaptic terminals

Evidence is presented here that demonstrates the presence of NP185 (AP3) in neuronal cells, specifically within syn-aptic terminals of the central nervous system and in the peripheral nervous system, particularly in the neuro-muscular junction of adult chicken muscle. Biochemical results obtained in our laboratories indicate that NP185 is associated with brain synaptic vesicles, with clathrin-coated vesicles, and with the synaptosomal plasma membrane. Also, NP185 binds to tubulin and clathrin light chains and the binding is regulated by phosphorylation (Su et al., 1991). Based on these properties and the data reported here, we advance the postulate that NP185 fulfills multiple functions in synaptic terminals. One function is that of a plasma membrane docking or channel protein, another of a signaling molecule for brain vesicles to reach the synaptic terminal region, and a third is that of a recycling molecule by binding to protein components on the lipid bilayer of the synaptic plasma membrane during the process of endocytosis. In support of these premises, a thorough study of NP185 using the developing chick brain, adult mouse brain, and chicken straited muscle was begun by temporally and spatially mapping the expression and localization of NP185 in evolving and mature nerve endings. To achieve these objectives, monoclonal antibodies to NP185 were used for immunocytochemistry in tissue sections of chicken and mouse cerebella. The distribution of NP185 was compared with those of other cytoskeletal and cytoplasmic proteins of axons and synapses, namely synaptophysin, vimentin, neurofilament NF68, and the intermediate filaments of glial cells (GFAP). The data indicate that expression of NP185 temporally coincides with synaptogenesis, and that the distribution of this protein is specific for synaptic terminal buttons of the CNS and the PNS.

Changes in cytoskeletal protein synthesis following axon injury and during axon regeneration

Injury to the axons of facial motoneurons stimulates increases in the synthesis of actin, tubulins, and GAP-43, and decreases in the synthesis of neurofilament proteins: mRNA levels change correspondingly. In contrast to this robust response of peripheral neurons to axotomy, injured central nervous system neurons show either an attenuated response that is subsequently aborted (rubrospinal neurons) or overall decreases in cytoskeletal protein mRNA expression (corticospinal and retinal ganglion neurons). There is evidence that these changes in synthesis are regulated by a variety of factors, including loss of endoneurially or target-derived trophic factors, positive signals arising from the site of injury, changes in the intraaxonal turnover of proteins, and substitution of target-derived trophic support by factors produced by glial cells. It is concluded that there is, as yet, no coherent explanation for the upregulation or downregulation of any of the cytoskeletal proteins following axotomy or during regeneration. In considering the relevance of these changes in cytoskeletal protein synthesis to regeneration, it is emphasized that they are unlikely to be involved in the initial outgrowth of the injured axons, both because transit times between cell body and injury site are too long, and because sprouting can occur in isolated axons. Injury-induced acceleration of the axonal transport of tubulin and actin in the proximal axon is likely to be more important in providing the cytoskeletal protein required for initial axonal outgrowth. Subsequently, the increased synthesis and transport velocity for actin and tubulin increase the delivery of these proteins to support the increased volume of the maturing regenerating axons. Reduction in neurofilament synthesis and changes in neurofilament phosphorylation may permit the increased transport velocity of the other cytoskeletal proteins. There is little direct evidence that alterations in cytoskeletal protein synthesis are necessary for successful regeneration, nor are they sufficient in the absence of a supportive environment. Nevertheless, the correlation that exists between a robust cell body response and successful regeneration suggests that an understanding of the regulation of cytoskeletal protein synthesis following axon injury must be a part of any successful strategy to improve the regenerative capacity of the central nervous system.

Corticospinal neurons exhibit a novel pattern of cytoskeletal gene expression after injury

We examined changes in the expression of major cytoskeletal protein mRNAs in adult hamster corticospinal neurons after axotomy. While a number of studies had determined that peripheral neurons exhibit major alterations in cytoskeletal gene expression after axotomy, no previous studies had addressed the question of whether or not intrinsic mammalian CNS neurons, which do not have the ability to successfully regenerate axons after injury, alter their expression of tubulin and neurofilament genes after injury. In the present study we used in situ hybridization methods to examine this issue. 35S-labeled cDNA probes for the low molecular weight neurofilament protein (NF-L) mRNA and an alpha-tubulin mRNA species (M alpha 1) were used for in situ hybridizations of sections of the sensorimotor cortex obtained 2, 7, and 14 days after unilateral axotomy of the corticospinal tract in the caudal medulla. Film as well as emulsion autoradiography showed dramatic decreases in both alpha-tubulin and NF-L mRNA levels within axotomized neurons in layer Vb of the sensorimotor cortex. Tubulin mRNA levels were decreased as early as 2 days after injury whereas NF-L mRNA levels were not decreased until later times. Ribosomal RNA (rRNA) levels in axotomized corticospinal neurons were also examined using in situ hybridization with a 35S-labeled rDNA probe. These studies showed only a slight decrease in rRNA levels in corticospinal neurons at 14 days after axotomy. Immunoblotting experiments of total protein from corticospinal axons in the medulla were performed to assess whether the axonal composition immediately proximal to the injury site reflected changes in cell body gene expression. Both alpha-tubulin and NF-L levels were found to decrease in corticospinal axons by 28 days after injury. These findings, to our knowledge, are the first to demonstrate that a class of mammalian CNS neurons have an intrinsically different cytoskeletal response to axonal injury than do PNS neurons. The failure to upregulate tubulin gene expression following injury may contribute to the ineffective regenerative response of these long-tract CNS neurons.

Regional variation in the abundance of axonal cytoskeletal proteins

The relative abundance of several axonal cytoskeletal proteins was determined by immunoassay at various sites in the peripheral and central nervous systems of adult rats. Within the peripheral nervous system, the ratio of tubulin to neurofilaments was greatest for nerves composed of unmyelinated axons and least for nerves with large myelinated axons. MAP1 protein was more prominent in unmyelinated fibers; conversely tau proteins were relatively more abundant in large myelinated axons. An immunochemical index of neurofilament phosphorylation was less for unmyelinated fibers than for myelinated ones. In the fimbria-fornix, pyramidal tract, and superior cerebellar peduncle, similar trends were observed: small axons had more MAP1, less tau, and a greater ratio of tubulin to neurofilament proteins. The phosphorylation index was greatest for the superior cerebellar peduncle, the tract with the largest axons. The immunochemical index of neurofilament phosphorylation was greater for the optic nerve than for axonal tracts in the brain proper. These results suggest that development of large myelinated axons is associated with greater neurofilament content, neurofilament phosphorylation, and with greater abundance of tau proteins in the CNS and the PNS; however, quantitative aspects of these relationships differ in the PNS and the CNS.

Phosphorylation-dependent epitopes on neurofilament proteins and neurofilament densities differ in axons in the corticospinal and primary sensory dorsal column tracts in the rat spinal cord

The highest molecular weight neurofilament protein (NF-H) is multiply phosphorylated at epitopes which can be distinguished by specific monoclonal antibodies on Western blots. Eight characterized antibodies were used in immunocytochemistry to examine the tissue distributions of phosphorylated variants of NF-H in axons of the adult rat spinal cord. The most striking difference in staining was found between axons in the cuneate tract and those in the neighboring dorsal corticospinal tract. Axons in the cuneate tract reacted intensely with antibodies to phosphorylated epitopes of NF-H and poorly with antibodies to dephosphorylated epitopes of NF-H, whereas the reverse was the case for the axons of the dorsal corticospinal tract. These differences showed that systematic variations in the phosphorylation of NF-H in long-tract axons in the central nervous system occur as a function of cell type. When the cytoskeletons of these axons were compared by electron microscopy, the neurofilaments of the cuneate fibers were seen to be more abundant and formed a latticework, more compactly organized than the neurofilaments of the dorsal corticospinal axons. By comparison, the dorsal corticospinal axons were relatively richer in microtubules than the cuneate axons. Although the cuneate fiber tract contained many more large (greater than 2.0 microns 2 in cross section) axons than did the dorsal corticospinal tract, these differences in cytoskeletal organization were apparent even when myelinated axons of similar sizes (0.4 micron 2 to 2.0 microns 2) were compared. In addition, the number of neurofilaments in cuneate axons in the 0.4 to 2.0 microns 2 size range was significantly better correlated with axon size than was the case for this size range of dorsal corticospinal axons. Thus, the differences seen in the organization of the neurofilament latticework and the phosphorylation of NF-H between axons found in these two tracts both appeared to be correlated with cell type, and were independent of length or caliber of the axons.