Concepts for regulation of axon integrity by enwrapping glia

Long axons and their enwrapping glia (Schwann cells and oligodendrocytes) form a unique compound structure that serves as conduit for transport of electric and chemical information in the nervous system. The peculiar cytoarchitecture over an enormous length as well as its substantial energetic requirements make this conduit particularly susceptible to detrimental alterations. Degeneration of long axons independent of neuronal cell bodies is observed comparatively early in a range of neurodegenerative conditions as a consequence of abnormalities in Schwann cells and oligodendrocytes. This leads to the most relevant disease symptoms and highlights the critical role that these glia have for axon integrity, but the underlying mechanisms remain elusive. The quest to understand why and how axons degenerate is now a crucial frontier in disease-oriented research. This challenge is most likely to lead to significant progress if the inextricable link between axons and their flanking glia in pathological situations is recognized. In this review I compile recent advances in our understanding of the molecular programs governing axon degeneration, and mechanisms of enwrapping glia’s non-cell autonomous impact on axon-integrity. A particular focus is placed on emerging evidence suggesting that enwrapping glia nurture long axons by virtue of their intimate association, release of trophic substances, and neurometabolic coupling. The correction of defects in these functions has the potential to stabilize axons in a variety of neuronal diseases in the peripheral and central nervous system

Targeting NMNAT1 to Axons and Synapses Transforms Its Neuroprotective Potency In Vivo

Axon and synapse degeneration are common components of many neurodegenerative diseases, and their rescue is essential for effective neuroprotection. The chimeric Wallerian degeneration slow protein (Wld(S)) protects axons dose dependently, but its mechanism is still elusive. We recently showed that Wld(S) acts at a non-nuclear location and is present in axons. This and other recent reports support a model in which Wld(S) protects by extranuclear redistribution of its nuclear NMNAT1 portion. However, it remains unclear whether cytoplasmic NMNAT1 acts locally in axons and synapses or at a non-nuclear site within cell bodies. The potency of axon protection by non-nuclear NMNAT1 relative to Wld(S) also needs to be established in vivo. Because the N-terminal portion of Wld(S) (N70) localized to axons, we hypothesized that it mediates the trafficking of the NMNAT1 portion. To test this, we substituted N70 with an axonal targeting peptide derived from amyloid precursor protein, and fused this to NMNAT1 with disrupted nuclear targeting. In transgenic mice, this transformed NMNAT1 from a molecule unable to inhibit Wallerian degeneration, even at high expression levels, into a protein more potent than Wld(S), able to preserve injured axons for several weeks at undetectable expression levels. Preventing NMNAT1 axonal delivery abolished its protective effect. Axonally targeted NMNAT1 localized to vesicular structures, colocalizing with extranuclear Wld(S), and was cotransported at least partially with mitochondria. We conclude that axonal targeting of NMNAT activity is both necessary and sufficient to delay Wallerian degeneration, and that promoting axonal and synaptic delivery greatly enhances the effectiveness

Using formalised floor-plans in the design and creation of virtual environments

Includes bibliographical references.Virtual Reality (VR) is an new and exciting medium waiting to be fully explored and capitalised by media experts. The advantages of the 3D graphics andinteractive nature of VR allows unique communication opportunities from a content expert to a target audience. Despite the attractiveness of VR as a communications medium, it has not yet reached its full potential in that role. We believe that this is because content experts do not have the necessary tools to create Virtual Environment (VE) applications to their specifications. The design of a VE is complex and tools to support every aspect of the design processes are few and far between. On the other hand, there has been much activity in the development of authoring tools in view of supporting content experts in the creation of VEs. Although these tools simplify the creation process, they still require some degree of programming. We believe that in order to allow a content expert to exploit the medium, support of the design phase is needed to bridge the gap between designers and creators of these environments

Target-dependence of sensory neurons: An ultrastructural comparison of axotomised dorsal root ganglion neurons with allowed or denied reinnervation of peripheral targets

Evidence is emerging for a role of rough endoplasmic reticulum (RER) in the form of stress granules, the unfolded protein response and protein bodies in the response of neurons to injury and in neurodegenerative diseases. Here, we have studied the role of the peripheral target in regulating the RER and polyribosomes of Nissl bodies in axotomised adult cat dorsal root ganglion (DRG) neurons where axonal regeneration and peripheral target reinnervation was either allowed or denied. Retrograde labelling with horseradish peroxidise was used as an independent marker to enable selection of only those DRG neuronal cell bodies with axons in the injured intercostal nerves. Indications of polyribosomal dispersal were seen by 6 h following axotomy, and by 24 h the normal orderly arrangement of lamellae of RER in Nissl bodies had become disorganised. These ultrastructural changes preceded light microscopical chromatolysis by 1–3 d. The retrograde response was maximal 8–32 d after axotomy. Clusters of debris-laden satellite cells/macrophages were present at this time but no ultrastructural evidence of neuronal apoptosis or necrosis was seen and there were no differences in the initial retrograde response according to the type of injury. By 64 d following axotomy with reinnervation, approximately half the labelled DRG neurons showed restoration of the orderly arrangement of RER and polyribosomes in their Nissl bodies. This was not seen after axotomy with reinnervation denied. We propose that the target-dependent changes in Nissl body ultrastructure described here are part of a continuum that can modify neuronal protein synthesis directed towards growth, maintenance or death of the neuron. This represents a possible structural basis for mediating the varied effects of neurotrophic interactions.I. P. Johnson and T. A. Sear

The progressive nature of Wallerian degeneration in wild-type and slow Wallerian degeneration (Wld(S)) nerves

BACKGROUND: The progressive nature of Wallerian degeneration has long been controversial. Conflicting reports that distal stumps of injured axons degenerate anterogradely, retrogradely, or simultaneously are based on statistical observations at discontinuous locations within the nerve, without observing any single axon at two distant points. As axon degeneration is asynchronous, there are clear advantages to longitudinal studies of individual degenerating axons. We recently validated the study of Wallerian degeneration using yellow fluorescent protein (YFP) in a small, representative population of axons, which greatly improves longitudinal imaging. Here, we apply this method to study the progressive nature of Wallerian degeneration in both wild-type and slow Wallerian degeneration (Wld(S)) mutant mice. RESULTS: In wild-type nerves, we directly observed partially fragmented axons (average 5.3%) among a majority of fully intact or degenerated axons 37–42 h after transection and 40–44 h after crush injury. Axons exist in this state only transiently, probably for less than one hour. Surprisingly, axons degenerated anterogradely after transection but retrogradely after a crush, but in both cases a sharp boundary separated intact and fragmented regions of individual axons, indicating that Wallerian degeneration progresses as a wave sequentially affecting adjacent regions of the axon. In contrast, most or all Wld(S )axons were partially fragmented 15–25 days after nerve lesion, Wld(S )axons degenerated anterogradely independent of lesion type, and signs of degeneration increased gradually along the nerve instead of abruptly. Furthermore, the first signs of degeneration were short constrictions, not complete breaks. CONCLUSIONS: We conclude that Wallerian degeneration progresses rapidly along individual wild-type axons after a heterogeneous latent phase. The speed of progression and its ability to travel in either direction challenges earlier models in which clearance of trophic or regulatory factors by axonal transport triggers degeneration. Wld(S )axons, once they finally degenerate, do so by a fundamentally different mechanism, indicated by differences in the rate, direction and abruptness of progression, and by different early morphological signs of degeneration. These observations suggest that Wld(S )axons undergo a slow anterograde decay as axonal components are gradually depleted, and do not simply follow the degeneration pathway of wild-type axons at a slower rate

Localization of Axonal Motor Molecules Machinery in Neurodegenerative Disorders

Axonal transport and neuronal survival depend critically on active transport and axon integrity both for supplying materials and communication to different domains of the cell body. All these actions are executed through cytoskeleton, transport and regulatory elements that appear to be disrupted in neurodegenerative diseases. Motor-driven transport both supplies and clears distal cellular portions with proteins and organelles. This transport is especially relevant in projection and motor neurons, which have long axons to reach the farthest nerve endings. Thus, any disturbance of axonal transport may have severe consequences for neuronal function and survival. A growing body of literature indicates the presence of alterations to the motor molecules machinery, not only in expression levels and phosphorylation, but also in their subcellular distribution within populations of neurons, which are selectively affected in the course of neurodegenerative diseases. The implications of this altered subcellular localization and how this affects axon survival and neuronal death still remain poorly understood, although several hypotheses have been suggested. Furthermore, cytoskeleton and transport element localization can be selectively disrupted in some disorders suggesting that specific loss of the axonal functionality could be a primary hallmark of the disorder. This can lead to axon degeneration and neuronal death either directly, through the functional absence of essential axonal proteins, or indirectly, through failures in communication among different cellular domains. This review compares the localization of cytoskeleton and transport elements in some neurodegenerative disorders to ask what aspects may be essential for axon survival and neuronal death

Prohibitin 1 is essential to preserve mitochondria and myelin integrity in Schwann cells

In peripheral nerves, Schwann cells form myelin and provide trophic support to axons. We previously showed that the mitochondrial protein prohibitin 2 can localize to the axon-Schwann-cell interface and is required for developmental myelination. Whether the homologous protein prohibitin 1 has a similar role, and whether prohibitins also play important roles in Schwann cell mitochondria is unknown. Here, we show that deletion of prohibitin 1 in Schwann cells minimally perturbs development, but later triggers a severe demyelinating peripheral neuropathy. Moreover, mitochondria are heavily affected by ablation of prohibitin 1 and demyelination occurs preferentially in cells with apparent mitochondrial loss. Furthermore, in response to mitochondrial damage, Schwann cells trigger the integrated stress response, but, contrary to what was previously suggested, this response is not detrimental in this context. These results identify a role for prohibitin 1 in myelin integrity and advance our understanding about the Schwann cell response to mitochondrial damage. Prohibitin 2 can localize to the axon-Schwann-cell interface and is required for myelin formation. Here, the authors show that deletion of prohibitin 1 in Schwann cells instead triggers severe myelin loss likely caused by mitochondrial dysfunction, and not rescued by inhibition of the ensuing integrated stress response

WldS protein requires Nmnat activity and a short N-terminal sequence to protect axons in mice

The slow Wallerian degeneration (WldS) protein protects injured axons from degeneration. This unusual chimeric protein fuses a 70–amino acid N-terminal sequence from the Ube4b multiubiquitination factor with the nicotinamide adenine dinucleotide–synthesizing enzyme nicotinamide mononucleotide adenylyl transferase 1. The requirement for these components and the mechanism of WldS-mediated neuroprotection remain highly controversial. The Ube4b domain is necessary for the protective phenotype in mice, but precisely which sequence is essential and why are unclear. Binding to the AAA adenosine triphosphatase valosin-containing protein (VCP)/p97 is the only known biochemical property of the Ube4b domain. Using an in vivo approach, we show that removing the VCP-binding sequence abolishes axon protection. Replacing the WldS VCP-binding domain with an alternative ataxin-3–derived VCP-binding sequence restores its protective function. Enzyme-dead WldS is unable to delay Wallerian degeneration in mice. Thus, neither domain is effective without the function of the other. WldS requires both of its components to protect axons from degeneration

Endogenous Nmnat2 Is an Essential Survival Factor for Maintenance of Healthy Axons

We conclude that endogenous Nmnat2 prevents spontaneous degeneration of healthy axons and propose that, when present, the more long-lived, functionally related WldS protein substitutes for Nmnat2 loss after axon injury. Endogenous Nmnat2 represents an exciting new therapeutic target for axonal disorders