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The mammalian retina, brain, spinal cord, and peripheral ganglia are all heterogeneous tissues. Each is composed of neuronal and glial cell partners embedded in a connective tissue bed and supplied by vascular and immune cells. This complicated structure presents many challenges to neuroscientists and cell biologists, e.g., how to carry out a quantitative study of neurons in a mature animal surrounded by the hormonal and immune stimuli. A reductionist view leads investigators to study single neurons in vitro, subtracting the immune and vascular components and simplifying the problem. While this has advantages, it limits relevance of the study. We present a method for studying the axonal transport of Herpes simplex virus in mature neurons in situ. Using genetically identical mice and carefully controlling the delivery of virus, an investigator can obtain insight into the transport of virus to and from the neuron cell body within the cellular environment of an intact animal. [ABSTRACT FROM AUTHOR]

Axonal neuropathy, or axonopathy, is a major category of neuropathy in the central and peripheral nervous systems. Axonopathy is characterized by axonal degeneration and dysfunctional axonal transport. Peripheral axonopathies are more common than central axonopathies due to their lack of protection from the blood–brain barrier and resultant vulnerability to metabolic challenges. Although the pathogenic mechanisms of peripheral axonal neuropathy are still unclear, the dying-back pattern of the axonal damage suggests axons, rather than neuronal cell bodies, are the primary targets of the disease. Recent studies have revealed that defects of the cytoskeleton and axonal transport are associated with several types of peripheral neuropathy and some central neurological diseases. Direct evidence from genetic studies demonstrates that mutations in major components of the cytoskeleton and axonal transport result in axonal defects in several types of Charcot-Marie-Tooth disease, amyotrophic lateral sclerosis, Alzheimer disease, and other types of genetic neurological disorders. In addition, post-translational modifications of cytoskeleton proteins also result in axonal defects in metabolic diseases like diabetic neuropathy. In this condition, phosphorylation and excess glycation of the axonal cytoskeletal components induce abnormal axonal functions. Advanced glycation end products (AGEs) and their receptors are most likely responsible for the axonal dysfunction. Taken together, understanding the defects in the axonal cytoskeleton and transport mechanisms provides important information for developing new treatments to prevent cytoskeletal damage in axonal neuropathy. [ABSTRACT FROM AUTHOR]

Neurofilaments (NFs) belong to type 1 V family of intermediate filaments and are the most abundant proteins of the nervous system. Mammalian NF triplet proteins constitute three subunits, low molecular weight (NF-L), medium molecular weight (NF-M), and high molecular weight (NF-H) subunit proteins. The NF-M/H carboxy terminal tail domain has multiple KSP repeats (40≥100) depending upon species, the phosphorylation of which regulates axonal caliber and axonal transport. The NF tail domain contains phosphorylation sites for proline directed kinases such as mitogen activated protein kinases (MAPKs), cyclin dependent protein kinase 5 (Cdk5), c-Jun amino terminal kinase (JNKs), and glycogen synthase kinase-3 (GSK-3). The signaling cascades involved in NF phosphorylation like MAPKs and Cdk5 can also be affected by myelin associated glycoprotein (MAG). MAG activates the Erk1/2 and Cdk5 activities due to the glial/axon interaction and increases the proline directed Ser/Thr phosphorylation of C-terminal domain of NF-M/H. The NF phosphorylation is topographically regulated. In normal neurons, NFs are phosphorylated only in the axonal compartment. However, in degenerative neurons such as Alzheimer΄s disease (AD), spinal cord motor neuron inclusions of amyotrophic lateral sclerosis (ALS), Lewy bodies of Parkinson΄s disease (PD), and Pick΄s disease neurofilament proteins are aberrantly phosphorylated in the cell bodies. Aberrant phosphorylation of NFs in neurodegeneration is either due to the deregulation of proline directed kinases such as MAPKs and Cdk5 or the downregulation of protein phosphatases such as protein phosphatase 2A (PP2A), calcineurin (PP2B), or both. Recently, studies from our laboratory have shown that peptidyl prolyl isomerase 1 (Pin1) stabilizes the NF phosphorylation in normal and stressed neurons. Pin1 selectively binds to the phosphorylated Ser/Thr-Pro residues and converts the cis isomers to the more stable trans isomers. The multiple SP repeats of NF-M/H are stabilized by Pin1 in a phosphorylation specific manner. Pin1 modulates the excitotoxic and oxidative stress induced perikaryal phosphorylation of NF-M/H. Here, we have discussed the factors regulating the topographic phosphorylation of NFs in health and disease. [ABSTRACT FROM AUTHOR]

In neurons, the process of axonal transport helps maintain health and homeostasis for normal neuronal functions. Protein products, synthesized in the cell body and organelles such as mitochondria, are transported via molecular ˵motors″ to the synapse for incorporation into membranes or cytoskeletal architecture, release into the synaptic cleft, or use in synaptic functioning. Retrograde transport occurs when substances, such as trophic factors, are taken up from the synaptic cleft via endocytosis and are transported back to the cell body to modulate further protein synthesis. Although retrograde transport is an important neuronal process, this chapter will focus on imaging of anterograde axonal transport and the effects of aging and neurodegeneration. Because axonal transport mechanisms become even more critical to maintain neuronal function in neuronal populations with long axonal projections, perturbations of axonal transport may contribute to the selective vulnerability of cortical projection neurons in neurodegenerative processes such as Alzheimer΄s disease (AD). With the increasing longevity of the world΄s population, aging and age-related diseases are becoming a major healthcare issue. Research in the areas of brain aging is critical, not only to maintaining life, but also to maintaining quality of life and sustaining a fully functional, independent, elderly population. Aging is also known to be a major risk factor for AD. According to pathological observations, cortical projection neurons are particularly vulnerable to AD pathophysiology (Hof et al., 1990). In such neurons, the process of axonal transport is particularly crucial to sustaining neuronal homeostasis and functions. Certain systems can be challenging to investigate non-invasively due to the relative inaccessibility of some brain regions and the technical challenges inherent in studying dynamic, sub-cellular processes. Therefore, the methodology to study axonal transport in living brains has been limited. This chapter will present on-going efforts to investigate this critical process using recently developed in vivo imaging techniques. [ABSTRACT FROM AUTHOR]

Most axonal and synaptic proteins are synthesized within the cell body and must travel long distances along axons to reach their sites of function. It is widely accepted that membranous organelles are bidirectionally transported by motor proteins, including kinesins and dynein, at fast rates (50–400 mm/day) along axonal microtubules. The mechanisms underlying slow axonal transport of proteins, including components of the cytoskeleton, have been more elusive, although recent genetic and live-cell imaging approaches have yielded general principles about the dynamic behaviors of cytoskeletal elements and formation of the axonal cytoskeleton. Cytoskeletal components may undergo slow transport (0.1–10 mm/day in vivo) as short polymers or oligomeric assemblies of subunits that fully assemble during transport or after they incorporate into the axonal cytoskeleton. Slow transport rates for a particular cargo reflect a pattern of rapid movements along axons interrupted by pauses of varying short durations. In mature myelinated axons, proteins in transport represent a relatively small pool that serve as precursors to a large metabolically stable stationary cytoskeleton composed of neurofilaments networked together by cross bridging proteins along with microtubule and actin filaments. Cytoskeletal components may turn over by local subunit exchange or proteolysis or, alternatively, by detachment of a larger filament fragment that is translocated to a degradative site. Defects in specific aspects of this complex process are the basis for certain neurological disorders. [ABSTRACT FROM AUTHOR]

Transport of material between the cell body and neuronal processes is crucial to neuronal function and survival. Growing evidence shows that deficits in axonal transport contribute to the pathogenesis of multiple neurodegenerative diseases including amyotrophic lateral sclerosis (ALS). Here we review recent data indicating that defects in anterograde and retrograde axonal transport are involved in ALS etiology. We discuss how ALS-linked mutations of copper-zinc superoxide dismutase (SOD1) could affect the molecular transport motor proteins and how this might lead to decreased function and viability of motor neurons in ALS. [ABSTRACT FROM AUTHOR]

Multiple sclerosis is a disease of the central nervous system usually characterised by relapsing episodes of neurological dysfunction, often followed some years later by progressive and irreversible decline. Lesions of multiple sclerosis are characterised by varying degrees of inflammation, myelin and oligodendrocyte loss, astrogliosis and axonal pathology. Axonal loss is seen in progressive phases of the disease and appears to correlate well with clinical disability. Prior to loss of axons, pathological specimens have revealed changes in the immunohistochemical phenotype of axons. Specifically, there may be evidence of dephosphorylation of neurofilaments within axons and transection of axons leading to the formation of axonal spheroids which are rich in dephosphorylated neurofilaments. Evidence of axonal transport defects may also be found in lesions with accumulation of amyloid precursor protein within the axon. Mechanisms of axonal pathology in multiple sclerosis remain unknown, but inflammation in the acute disease phases is likely to cause significant damage. There is also evidence that axonal loss may continue even in the absence of inflammation. It has been postulated that axonal loss in this situation occurs due to loss of trophic support from myelin and oligodendrocytes. Oligodendrocytes and myelin are known to provide trophic support for axons and specifically can influence phosphorylated neurofilament levels. The precise mechanisms are unknown, but recent evidence suggests a combination of contact mediated and soluble factors may increase neurofilament phosphorylation and promote axonal protection. Knowledge of such mechanisms may lead to improved therapies to prevent progressive disease. This chapter will discuss axonal changes in multiple sclerosis, specifically alterations in neurofilament phosphorylation states, and potential mechanisms of axonal protection. [ABSTRACT FROM AUTHOR]

Neurofilaments (NFs) are neuronal intermediate filaments assembled mainly from three subunit proteins, NF-L, NF-M and NF-H. NFs are the major cytoskeletal element of axons, particularly large myelinated axons. Accumulation of NFs in the cell body or proximal axons is a hallmark of motor neuron diseases, of which aging is a risk factor. It is not known whether the accumulation of NFs is the primary causative factor of these diseases; however, it is feasible that accumulated NFs are obstacles for axonal trafficking, resulting in a decreased supply of proteins and organelles required for the maintenance of or activity in the distal axons. Axonal transport of NFs slows down with aging and the decreased rate of transport could be a risk factor for disease. Therefore, it is important to determine age-dependent changes in properties of NFs. In this chapter we review the characteristics of NFs in aged animals. The most striking morphological change in NFs is their density within axons. NFs are more than twice as densely packed in the proximal region of aged rat sciatic nerve axons compared with those in young adult rats. A remarkable biochemical change is the reduction of NF-M content in aged NFs. This is partly because of the reduced transcription of NF-M in aged rats. The relationship between NF packing and reduced NF-M is discussed in terms of the age-dependent decrease in axonal transport and neurodegenerative diseases. [ABSTRACT FROM AUTHOR]

Neurofilaments are the most plentiful intermediate filament proteins found in neuronal cells. They are important cytoskeletal proteins that are made in the soma and transported into axonal and dendritic processes. Neurofilaments are transported by the process of slow axonal transport, which arises from fast transport interrupted by extended pauses. Some of the mechanisms regulating neurofilament assembly and transport have been identified. It is thought that phosphorylation decreases the rate of neurofilament transport by increasing the time neurofilaments spend pausing. There is also increasing evidence for the involvement of neurofilaments in a number of neurodegenerative disorders such as amyotrophic lateral sclerosis, hereditary spastic paraplegia, Charcot–Marie–Tooth disease, and Alzheimer΄s disease. Disrupted neurofilament transport is thought to be a common mechanism in some or all of these disorders. [ABSTRACT FROM AUTHOR]

Because α-internexin is expressed in embryonic brain and can form homopolymers in vitro, it was originally considered to be an independent filament system critical mainly during brain development and distinct from that assembled in neurons from neurofilament (NF) triplet proteins. Analysis of mice engineered to eliminate individual or different combinations of NF triplet or α-internexin has proved that α-internexin and NF proteins are constituents of the same NF and functionally interdependent. α-Internexin satisfies all criteria previously used to establish the NF triplet proteins as subunits of a single intermediate filament and can now be considered to be a fourth subunit of NF in the adult CNS. Its role as a subunit of CNS NF explains the close relationship of α-internexin with the pathology of CNS diseases associated with NF accumulation. [ABSTRACT FROM AUTHOR]

An encyclopedia entry for "myelin basic protein," is presented. It refers to is a component of the myelin sheath surrounding the axons of nerve cells. The myelin sheath, which has additional compounds of phospholipids and cholesterol, constitutes an isolating barrier during electrophysiological axonal signaling.