Neuropathology: many paths lead to hereditary spastic paraplegia

Autophagy in Myelinating Glia

Autophagy is the cellular process involved in transportation and degradation of membrane, proteins, pathogens, and organelles. This fundamental cellular process is vital in development, plasticity, and response to disease and injury. Compared with neurons, little information is available on autophagy in glia, but it is paramount for glia to perform their critical responses to nervous system disease and injury, including active tissue remodeling and phagocytosis. In myelinating glia, autophagy has expanded roles, particularly in phagocytosis of mature myelin and in generating the vast amounts of membrane proteins and lipids that must be transported to form new myelin. Notably, autophagy plays important roles in removing excess cytoplasm to promote myelin compaction and development of oligodendrocytes, as well as in remyelination by Schwann cells after nerve trauma. This review summarizes the cell biology of autophagy, detailing the major pathways and proteins involved, as well as the roles of autophagy in Schwann cells and oligodendrocytes in development, plasticity, and diseases in which myelin is affected. This includes traumatic brain injury, Alexander's disease, Alzheimer's disease, hypoxia, multiple sclerosis, hereditary spastic paraplegia, and others. Promising areas for future research are highlighted.

Muscle paresis and passive stiffness: key determinants in limiting function in Hereditary and Sporadic Spastic Paraparesis

BACKGROUND

People with Hereditary and Sporadic Spastic Parapresis (SP) walk with a stiff legged gait characterised by a lack of knee flexion.

OBJECTIVE

We investigated the relationship between lower limb strength and stiffness and knee flexion during swing phase while walking in 20 people with SP and 18 matched controls.

METHODS

Maximal isometric strength was measured using a dynamometer. Passive stiffness and spasticity was assessed during motor-driven slow (5°/s) and fast (60°/s) stretches at the ankle and knee while the subject was relaxed or preactivating the muscle. Walking was assessed using 3D motion analysis.

RESULTS

Isometric muscle strength was decreased in people with SP with over a 50% reduction in strength being found in the ankle dorsiflexors. Passive stiffness, assessed during slow stretches, was 35% higher in the plantarflexors in people with SP (P<0.05). Faster stretches induced large stretch evoked muscle activity and over a 110% increase in stiffness at the ankle and knee in people with SP reflecting the presence of spasticity (P<0.05). However, stretch reflex size and stiffness was similar between the groups following identical stretches of the pre-activated muscle (P>0.05). Lower knee flexion during swing phase was associated with reduced knee flexion velocity at the end of stance phase which in turn was associated with reduced plantarflexor strength and increased passive stiffness in the knee extensors.

CONCLUSIONS

The relative importance of muscle paresis and passive stiffness in limiting walking in SP suggests that these impairments should be the target of future therapies.

Effect of spastic paraplegia mutations in KIF5A kinesin on transport activity

Hereditary spastic paraplegia (HSP) is a neurodegenerative disease caused by motoneuron degeneration. It is linked to at least 30 loci, among them SPG10, which causes dominant forms and originates in point mutations in the neuronal Kinesin-1 gene (KIF5A). Here, we investigate the motility of KIF5A and four HSP mutants. All mutations are single amino-acid exchanges and located in kinesin's motor or neck domain. The mutation in the neck (A361V) did not change the gliding properties in vitro, the others either reduced microtubule affinity or gliding velocity or both. In laser-trapping assays, none of the mutants moved more than a few steps along microtubules. Motility assays with mixtures of homodimeric wild-type, homodimeric mutant and heterodimeric wild-type/mutant motors revealed that only one mutant (N256S) reduces the gliding velocity at ratios present in heterozygous patients, whereas the others (K253N, R280C) do not. Attached to quantum dots as artificial cargo, mixtures involving N256S mutants produced slower cargo populations lagging behind in transport, whereas mixtures with the other mutants led to populations of quantum dots that rarely bound to microtubules. These differences indicate that the dominant inheritance of SPG10 is caused by two different mechanisms that both reduce the gross cargo flux, leading to deficient supply of the synapse.

Impairments in fast axonal transport and motor neuron deficits in transgenic mice expressing familial Alzheimer's disease-linked mutant presenilin 1

Presenilins (PS) play a central role in gamma-secretase-mediated processing of beta-amyloid precursor protein (APP) and numerous type I transmembrane proteins. Expression of mutant PS1 variants causes familial forms of Alzheimer's disease (FAD). In cultured mammalian cells that express FAD-linked PS1 variants, the intracellular trafficking of several type 1 membrane proteins is altered. We now report that the anterograde fast axonal transport (FAT) of APP and Trk receptors is impaired in the sciatic nerves of transgenic mice expressing two independent FAD-linked PS1 variants. Furthermore, FAD-linked PS1 mice exhibit a significant increase in phosphorylation of the cytoskeletal proteins tau and neurofilaments in the spinal cord. Reductions in FAT and phosphorylation abnormalities correlated with motor neuron functional deficits. Together, our data suggests that defects in anterograde FAT may underlie FAD-linked PS1-mediated neurodegeneration through a mechanism involving impairments in neurotrophin signaling and synaptic dysfunction.

Visualization of degenerating axons in a dysmyelinating mouse mutant with axonal loss

Mice homozygously deficient for the myelin component P0 show loss of axons in peripheral nerves. In order to investigate the morphological characteristics of degenerating axons, we crossbred the myelin mutants with a transgenic mouse line expressing yellow fluorescent protein (YFP) in a small proportion of neurons. Peripheral nerves of the double mutants were prepared into small fiber bundles and investigated by fluorescence microscopy. We could identify the tips of degenerating axon as bulb-like structures. Additionally, by electron microscopy, these structures were characterized as axoplasmic extensions containing numerous membraneous compartments. By immunoelectron microscopy, the degenerating end bulbs were in contact with ensheathing Schwann cells that contained YFP-immunoreactivity possibly reflecting phagocytosis of axon material by these cells. Immunohistochemistry using antibodies against macrophages revealed that YFP-positive bulbs, but also other axonal swellings, were often associated with macrophages supporting our previous findings that myelin-related axonal loss is partially mediated by these cells.

Immune cells contribute to myelin degeneration and axonopathic changes in mice overexpressing proteolipid protein in oligodendrocytes

Overexpression of the major myelin protein of the CNS, proteolipid protein (PLP), leads to late-onset degeneration of myelin and pathological changes in axons. Based on the observation that in white matter tracts of these mutants both CD8+ T-lymphocytes and CD11b+ macrophage-like cells are numerically elevated, we tested the hypothesis that these cells are pathologically involved in the primarily genetically caused neuropathy. Using flow cytometry of mutant brains, CD8+ cells could be identified as activated effector cells, and confocal microscopy revealed a close association of the T-cells with MHC-I+ (major histocompatibility complex class I positive) oligodendrocytes. Crossbreeding the myelin mutants with mice deficient in the recombination activating gene-1 (RAG-1) lacking mature T- and B-lymphocytes led to a reduction of the number of CD11b+ cells and to a substantial alleviation of pathological changes. In accordance with these findings, magnetic resonance imaging revealed less ventricular enlargement in the double mutants, partially because of more preserved corpora callosa. To investigate the role of CD8+ versus CD4+ T-lymphocytes, we reconstituted the myelin-RAG-1 double mutants with bone marrow from either CD8-negative (CD4+) or CD4-negative (CD8+) mice. The severe ventricular enlargement was only found when the double mutants were reconstituted with bone marrow from CD8+ mice, suggesting that the CD8+ lymphocytes play a critical role in the immune-related component of myelin degeneration in the mutants. These findings provide strong evidence that a primary glial damage can cause secondary immune reactions of pathological significance as it has been suggested for some forms of multiple sclerosis and other leukodystrophies.

Hereditary spastic paraplegia

The hereditary spastic paraplegias (HSPs) comprise a large group of inherited neurologic disorders. HSP is classified according to the mode of inheritance, the HSP locus when known, and whether the spastic paraplegia syndrome occurs alone or is accompanied by additional neurologic or systemic abnormalities. Analysis of 11 recently discovered HSP genes provides insight into HSP pathogenesis. Hereditary spastic paraplegia is a clinical diagnosis for which laboratory confirmation is sometimes possible, and careful exclusion of alternate and co-existing disorders is an important element in HSP diagnosis. Treatment for HSP is presently limited to symptomatic reduction of muscle spasticity, reduction in urinary urgency, and strength and gait improvement through physical therapy. Prenatal genetic testing in HSP is possible for some individuals with the increasing availability of HSP gene analysis.