Ultramicroscopy reveals axonal transport impairments in cortical motor neurons at prion disease

Protein Misfolding, Signaling Abnormalities and Altered Fast Axonal Transport: Implications for Alzheimer and Prion Diseases

Histopathological studies revealed that progressive neuropathies including Alzheimer, and Prion diseases among others, include accumulations of misfolded proteins intracellularly, extracellularly, or both. Experimental evidence suggests that among the accumulated misfolded proteins, small soluble oligomeric conformers represent the most neurotoxic species. Concomitant phenomena shared by different protein misfolding diseases includes alterations in phosphorylation-based signaling pathways synaptic dysfunction, and axonal pathology, but mechanisms linking these pathogenic features to aggregated neuropathogenic proteins remain unknown. Relevant to this issue, results from recent work revealed inhibition of fast axonal transport (AT) as a novel toxic effect elicited by oligomeric forms of amyloid beta and cellular prion protein PrP^C, signature pathological proteins associated with Alzheimer and Prion diseases, respectively. Interestingly, the toxic effect of these oligomers was fully prevented by pharmacological inhibitors of casein kinase 2 (CK2), a remarkable discovery with major implications for the development of pharmacological target-driven therapeutic intervention for Alzheimer and Prion diseases.

Prion protein inhibits fast axonal transport through a mechanism involving casein kinase 2

Prion diseases include a number of progressive neuropathies involving conformational changes in cellular prion protein (PrP^c) that may be fatal sporadic, familial or infectious. Pathological evidence indicated that neurons affected in prion diseases follow a dying-back pattern of degeneration. However, specific cellular processes affected by PrP^c that explain such a pattern have not yet been identified. Results from cell biological and pharmacological experiments in isolated squid axoplasm and primary cultured neurons reveal inhibition of fast axonal transport (FAT) as a novel toxic effect elicited by PrP^c. Pharmacological, biochemical and cell biological experiments further indicate this toxic effect involves casein kinase 2 (CK2) activation, providing a molecular basis for the toxic effect of PrP^c on FAT. CK2 was found to phosphorylate and inhibit light chain subunits of the major motor protein conventional kinesin. Collectively, these findings suggest CK2 as a novel therapeutic target to prevent the gradual loss of neuronal connectivity that characterizes prion diseases.

Low-cost multimodal light sheet microscopy for optically cleared tissues and living specimens

Light sheet microscopy techniques have expanded with designs to address many new applications. Due to rapid advancements in computing power, camera/detector technologies, and tissue clearing techniques, light sheet methods are becoming increasingly popular for biomedical imaging applications at the cellular and tissue levels. Light sheet imaging modalities couple rapid imaging rates, low-levels of phototoxicity, and excellent signal to noise ratios, contributing to their popularity for experimental biology. However, the current major limitation of light sheet microscopy arises from optical aberrations, with the main drawback being the defocusing introduced by refractive index variations that accompany clearing techniques. Here, we propose an inexpensive and easy to build light sheet based instrumentation to overcome this limitation by optomechanically decoupling the sample scanning movement from the detection step. Our solution is relatively simple to implement and also provides increased modularity by using a swappable excitation arm. This expands the range of samples we can image on a single system, from high resolution for single cells at ? m spatial resolution, to tissues with mm spatial resolution. We demonstrate our approach, using the system to image iDISCO cleared embryos and sciatic nerves, and provide the full three-dimensional reconstruction of these objects in minutes.

Axial resolution beyond the diffraction limit of a sheet illumination microscope with stimulated emission depletion

Planar illumination imaging allows for illumination of the focal plane orthogonal to the imaging axis in various light forms and is advantageous for high optical sectioning, high imaging speed, low light exposure, and inherently deeper imaging penetration into small organisms and tissue sections. The drawback of the technique is the low inherent resolution, which can be overcome by the incorporation of a dual-sheet stimulated emission depletion (STED) beam to the planar illumination excitation. Our initiative is the implementation of STED into the planar illumination microscope for enhanced resolution. We demonstrate some of our implementations. The depletion of STED in the microscope follows an inverse square root saturation for up to 2.5-fold axial resolution improvements with both high and low numerical aperture imaging objectives.

Light sheet microscopy

This chapter introduces the concept of light sheet microscopy along with practical advice on how to design and build such an instrument. Selective plane illumination microscopy is presented as an alternative to confocal microscopy due to several superior features such as high-speed full-frame acquisition, minimal phototoxicity, and multiview sample rotation. Based on our experience over the last 10 years, we summarize the key concepts in light sheet microscopy, typical implementations, and successful applications. In particular, sample mounting for long time-lapse imaging and the resulting challenges in data processing are discussed in detail.

Mapping immune processes in intact tissues at cellular resolution

Understanding the spatiotemporal changes of cellular and molecular events within an organism is crucial to elucidate the complex immune processes involved in infections, autoimmune disorders, transplantation, and neoplastic transformation and metastasis. Here we introduce a novel multicolor light sheet fluorescence microscopy (LSFM) approach for deciphering immune processes in large tissue specimens on a single-cell level in 3 dimensions. We combined and optimized antibody penetration, tissue clearing, and triple-color illumination to create a method for analyzing intact mouse and human tissues. This approach allowed us to successfully quantify changes in expression patterns of mucosal vascular addressin cell adhesion molecule-1 (MAdCAM-1) and T cell responses in Peyer's patches following stimulation of the immune system. In addition, we employed LSFM to map individual T cell subsets after hematopoietic cell transplantation and detected rare cellular events. Thus, we present a versatile imaging technology that should be highly beneficial in biomedical research.

Prion protein impairs kinesin-driven transport

Our previous studies have demonstrated that prion protein (PrP) leads to disassembly of microtubular cytoskeleton through binding to tubulin and its oligomerization. Here we found that PrP-treated cells exhibited improper morphology of mitotic spindles. Formation of aberrant spindles may result not only from altered microtubule dynamics - as expected from PrP-induced tubulin oligomerization - but also from impairing the function of molecular motors. Therefore we checked whether binding of PrP to microtubules affected movement generated by Ncd - a kinesin responsible for the proper organization of division spindles. We found that PrP inhibited Ncd-driven transport of microtubules. Most probably, the inhibition of the microtubule movement resulted from PrP-induced changes in the microtubule structure since Ncd-microtubule binding was reduced already at low PrP to tubulin molar ratios. This study suggests another plausible mechanism of PrP cytotoxicity related to the interaction with tubulin, namely impeding microtubule-dependent transport.

Involvement of astrocytes in transmissible spongiform encephalopathies: a confocal microscopy study

Astroglial proliferation associated with pathological prion protein (PrPsc) deposition is widely described in Transmissible Spongiform Encephalopathies (TSEs). However, little is known of the actual role played by glia in their pathogenesis. The aim of the study has been to determine whether PrPsc is located exclusively in neurons or in both neurons and glial cells present in the central nervous system in a natural Scrapie model. Samples of cerebellum from 25 Scrapie sheep from various flocks were sectioned. Following epitope retrieval with formic acid, proteinase K and heat treatment, primary antibody L42 and primary antibodies against glial fibrillary acidic protein were applied as prion- and astrocytic-specific markers, respectively. For visualization, a suitable mixture of fluorochrome-conjugated secondary antibodies was used. Relevant controls were processed in the same manner. As determined by confocal microscopy, PrPsc deposits co-localized with glial cells in all samples. Our results suggest that these cells can sustain active prion propagation, in agreement with similar findings from other studies of primary cell cultures and inoculated mice. Furthermore, despite ongoing debate regarding whether varied TSE sources show differences in their tropism for different cell lineages in the brains of affected animals, no differences in co-localization results were seen.

The OPFOS Microscopy Family: High-Resolution Optical Sectioning of Biomedical Specimens

We report on the recently emerging (laser) light-sheet-based fluorescence microscopy field (LSFM). The techniques used in this field allow to study and visualize biomedical objects nondestructively in high resolution through virtual optical sectioning with sheets of laser light. Fluorescence originating in the cross-section of the sheet and sample is recorded orthogonally with a camera. In this paper, the first implementation of LSFM to image biomedical tissue in three dimensions-orthogonal-plane fluorescence optical sectioning microscopy (OPFOS)-is discussed. Since then many similar and derived methods have surfaced, (SPIM, ultramicroscopy, HR-OPFOS, mSPIM, DSLM, TSLIM, etc.) which we all briefly discuss. All these optical sectioning methods create images showing histological detail. We illustrate the applicability of LSFM on several specimen types with application in biomedical and life sciences.

STED-SPIM: Stimulated emission depletion improves sheet illumination microscopy resolution

We demonstrate the first, to our knowledge, integration of stimulated emission depletion (STED) with selective plane illumination microscopy (SPIM). Using this method, we were able to obtain up to 60% improvements in axial resolution with lateral resolution enhancements in control samples and zebrafish embryos. The integrated STED-SPIM method combines the advantages of SPIM with the resolution enhancement of STED, and thus provides a method for fast, high-resolution imaging with >100 μm deep penetration into biological tissue.

Molecular structure of amyloid fibrils controls the relationship between fibrillar size and toxicity

Background

According to the prevailing view, soluble oligomers or small fibrillar fragments are considered to be the most toxic species in prion diseases. To test this hypothesis, two conformationally different amyloid states were produced from the same highly pure recombinant full-length prion protein (rPrP). The cytotoxic potential of intact fibrils and fibrillar fragments generated by sonication from these two states was tested using cultured cells.

Methodology/Principal Findings

For one amyloid state, fibril fragmentation was found to enhance its cytotoxic potential, whereas for another amyloid state formed within the same amino acid sequence, the fragmented fibrils were found to be substantially less toxic than the intact fibrils. Consistent with the previous studies, the toxic effects were more pronounced for cell cultures expressing normal isoform of the prion protein (PrP^C) at high levels confirming that cytotoxicity was in part PrP^C-dependent. Silencing of PrP^C expression by small hairpin RNAs designed to silence expression of human PrP^C (shRNA-PrP^C) deminished the deleterious effects of the two amyloid states to a different extent, suggesting that the role of PrP^C-mediated and PrP^C-independent mechanisms depends on the structure of the aggregates.

Conclusions/Significance

This work provides a direct illustration that the relationship between an amyloid's physical dimension and its toxic potential is not unidirectional but is controlled by the molecular structure of prion protein (PrP) molecules within aggregated states. Depending on the structure, a decrease in size of amyloid fibrils can either enhance or abolish their cytotoxic effect. Regardless of the molecular structure or size of PrP aggregates, silencing of PrP^C expression can be exploited to reduce their deleterious effects.

Cytoplasmic dynein in neurodegeneration

Cytoplasmic dynein 1 (later referred to as dynein) is the major molecular motor moving cargoes such as mitochondria, organelles and proteins towards the minus end of microtubules. Dynein is involved in multiple basic cellular functions, such as mitosis, autophagy and structure of endoplasmic reticulum and Golgi, but also in neuron specific functions in particular retrograde axonal transport. Dynein is regulated by a number of protein complexes, notably by dynactin. Several studies have supported indirectly the involvement of dynein in neurodegeneration associated with Alzheimer's disease, Parkinson's disease, Huntington's disease and motor neuron diseases. First, axonal transport disruption represents a common feature occurring in neurodegenerative diseases. Second, a number of dynein-dependent processes, including autophagy or clearance of aggregation-prone proteins, are found defective in most of these diseases. Third, a number of mutant genes in various neurodegenerative diseases are involved in the regulation of dynein transport. This includes notably mutations in the P150Glued subunit of dynactin that are found in Perry syndrome and motor neuron diseases. Interestingly, gene products that are mutant in Huntington's disease, Parkinson's disease, motor neuron disease or spino-cerebellar ataxia are also involved in the regulation of dynein motor activity or of cargo binding. Despite a constellation of indirect evidence, direct links between the motor itself and neurodegeneration are few, and this might be due to the requirement of fully active dynein for development. Here, we critically review the evidence of dynein involvement in different neurodegenerative diseases and discuss potential underlying mechanisms.

Light sheet microscopy for single molecule tracking in living tissue

Single molecule observation in cells and tissue allows the analysis of physiological processes with molecular detail, but it still represents a major methodological challenge. Here we introduce a microscopic technique that combines light sheet optical sectioning microscopy and ultra sensitive high-speed imaging. By this approach it is possible to observe single fluorescent biomolecules in solution, living cells and even tissue with an unprecedented speed and signal-to-noise ratio deep within the sample. Thereby we could directly observe and track small and large tracer molecules in aqueous solution. Furthermore, we demonstrated the feasibility to visualize the dynamics of single tracer molecules and native messenger ribonucleoprotein particles (mRNPs) in salivary gland cell nuclei of Chironomus tentans larvae up to 200 µm within the specimen with an excellent signal quality. Thus single molecule light sheet based fluorescence microscopy allows analyzing molecular diffusion and interactions in complex biological systems.

Coinfecting prion strains compete for a limiting cellular resource

Prion strain interference can influence the emergence of a dominant strain from a mixture; however, the mechanisms underlying prion strain interference are poorly understood. In our model of strain interference, inoculation of the sciatic nerve with the drowsy (DY) strain of the transmissible mink encephalopathy (TME) agent prior to superinfection with the hyper (HY) strain of TME can completely block HY TME from causing disease. We show here that the deposition of PrP(Sc), in the absence of neuronal loss or spongiform change, in the central nervous system corresponds with the ability of DY TME to block HY TME infection. This suggests that DY TME agent-induced damage is not responsible for strain interference but rather prions compete for a cellular resource. We show that protein misfolding cyclic amplification (PMCA) of DY and HY TME maintains the strain-specific properties of PrP(Sc) and replicates infectious agent and that DY TME can interfere, or completely block, the emergence of HY TME. DY PrP(Sc) does not convert all of the available PrP(C) to PrP(Sc) in PMCA, suggesting the mechanism of prion strain interference is due to the sequestering of PrP(C) and/or other cellular components required for prion conversion. The emergence of HY TME in PMCA was controlled by the initial ratio of the TME agents. A higher ratio of DY to HY TME agent is required for complete blockage of HY TME in PMCA compared to several previous in vivo studies, suggesting that HY TME persists in animals coinfected with the two strains. This was confirmed by PMCA detection of HY PrP(Sc) in animals where DY TME had completely blocked HY TME from causing disease.

Impaired axonal transport in motor neurons correlates with clinical prion disease

Prion diseases are fatal neurodegenerative disorders causing motor dysfunctions, dementia and neuropathological changes such as spongiosis, astroglyosis and neuronal loss. The chain of events leading to the clinical disease and the role of distinct brain areas are still poorly understood. The role of nervous system integrity and axonal properties in prion pathology are still elusive. There is no evidence of both the functional axonal impairments in vivo and their connection with prion disease. We studied the functional axonal impairments in motor neurons at the onset of clinical prion disease using the combination of tracing as a functional assay for axonal transport with immunohistochemistry experiments. Well-established and novel confocal and ultramicroscopy techniques were used to image and quantify labeled neurons. Despite profound differences in the incubation times, 30% to 45% of neurons in the red nucleus of different mouse lines showed axonal transport impairments at the disease onset bilaterally after intracerebral prion inoculation and unilaterally—after inoculation into the right sciatic nerve. Up to 94% of motor cortex neurons also demonstrated transport defects upon analysis by alternative imaging methods. Our data connect axonal transport impairments with disease symptoms for different prion strains and inoculation routes and establish further insight on the development of prion pathology in vivo. The alterations in localization of the proteins involved in the retrograde axonal transport allow us to propose a mechanism of transport disruption, which involves Rab7-mediated cargo attachment to the dynein-dynactin pathway. These findings suggest novel targets for therapeutic and diagnostic approaches in the early stages of prion disease.

For almost a century, prion disease symptoms, such as dementia and motor system defects, have been accompanied with neuropathological hallmarks in the central nervous system. In past decades, discrepancies between neuropathological changes and clinical symptoms showed that the processes triggering the disease remain elusive. We concentrated on the hypothesis that nervous system integrity and axonal properties may play an important role in the disease development. Since motor system defects are typical for prion disease, we investigated the centers of the motor system, red nucleus and hindlimb area of motor cortex. Although intracerebral prion infection led to a 30% to 45% bilateral reduction of labeled neurons in the red nucleus, infection into the right sciatic nerve—the major hindlimb nerve—led to unilateral reduction of labeled neurons in the red nucleus. Up to 94% reduction was found in the neurons of motor cortex hindlimb area. This reduction is probably caused by functional axonal impairments in motor neurons. Prion-induced alterations in protein distribution implicate a mechanism of transport disruption at cargo attachment to the retrograde axonal transport complex. Our work reveals a connection between axonal transport impairments and disease symptoms in vivo, providing further insight in the development of prion pathology and suggesting novel targets for therapeutic and diagnostic approaches.