Structure of Inclusions of Huntington’s Disease Brain Revealed by Synchrotron Infrared Microspectroscopy: Polymorphism and Relevance to Cytotoxicity

Huntington’s disease is caused by a polyglutamine expansion in huntingtin. Affected brain regions contain characteristic aggregates of the misfolded expanded protein. Studies in cells and animals show that aggregates are polymorphic and that the secondary structure of the aggregates is likely to condition their cytotoxicity. Therefore knowing the structure of aggregates is important as neurotoxic secondary structures may be specifically targeted during the search for prophylactic or therapeutic drugs. The structure of aggregates in the brain of patients is still unknown. Using synchrotron based infrared microspectroscopy we demonstrate that the brains of patients with Huntington disease contain putative oligomers and various kinds of microscopic aggregates (inclusions) that can be distinguished by their differential absorbance at 1627 cm^–1 (amyloid β sheets) and 1639 cm^–1 (β sheets/unordered). We also describe the parallel/antiparallel organization of the β strands. As the inclusions enriched in both β sheets and β sheets/unordered structures are characteristic of severely affected brain regions, we conclude that this kind of amyloid inclusions is likely to be particularly toxic to neurons

Selected examples of infrared spectra from biopsies.

<p>a) Amorphous silica identified by a band at 1102 cm⁻¹, b) sodium hydrogen urate monohydrate identified by specific bands at 3600 and 1004 cm⁻¹, c) several calcium phosphates including whitlockite (peaks at 1080, 1025 cm⁻¹ and associated shoulders, d) octacalcium phosphate and carbapatite, identified by a shoulder at 1119 cm⁻¹, e) normal tissue, with signal of water (3300 cm⁻¹ and peaks around 1600 cm⁻¹) and proteins (peaks at 2900 cm⁻¹).</p

Optical image and mapping of BR165 biopsy (scale from blue to red with increasing concentration), and FT-IR spectra of crystals.

<p>a) Optical image of BR165, b) carbapatite map (done at 1030 cm^-1), c) sodium hydrogen urate monohydrate map (done at 3600 cm^-1), d) FT-IR spectra of those compounds.</p

Mapping and optical image of a birefringent structure.

<p>The poorly soluble foscarnet can be detected and quantified after a mapping of the biopsy thanks to the characteristic peak at 936 cm^-1.</p

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In Situ Chemical Composition Analysis of Cirrhosis by Combining Synchrotron Fourier Transform Infrared and Synchrotron X‑ray Fluorescence Microspectroscopies on the Same Tissue Section

Liver is subject to various chronic pathologies, progressively leading to cirrhosis, which is associated with an increased risk of hepatocellular carcinoma. There is an urgent need for diagnostic and prognostic markers of chronic liver diseases and liver cancer. Spectroscopy-based approaches can provide an overview of the chemical composition of a tissue sample offering the possibility of investigating in depth the subtle chemical changes associated with pathological states. In this study, we have addressed the composition of cirrhotic liver tissue by combining synchrotron Fourier transform infrared (FTIR) microspectroscopy and synchrotron micro-X-ray fluorescence (XRF) on the same tissue section using a single sample holder in copper. This allowed investigation of the in situ biochemical as well as elemental composition of cells and tissues at high spatial resolution. Cirrhosis is characterized by regeneration nodules surrounded by annular fibrosis. Hepatocytes within cirrhotic nodules were characterized by high content in esters and sugars as well as in phosphorus and iron compared with fibrotic septa. A high heterogeneity was observed between cirrhotic nodules in their content in sugars and iron. On fibrosis, synchrotron XRF revealed enrichment in calcium compared to cirrhotic hepatocytes. Careful scrutiny of tissue sections led to detection of the presence of microcrystals that were demonstrated as precipitates of calcite using synchrotron FTIR. These results demonstrated that synchrotron FTIR and synchrotron XRF microspectroscopies provide complementary information on the chemical composition of cirrhotic hepatocytes and fibrotic septa in cirrhosis

Second derivatives of IR spectra.

<p>Spectra recorded on steatosis or non-steatotic hepatocytes were superimposed (upper panel). Second derivatives of the spectra were calculated and superimposed in the frequency domain 2600–3200 cm⁻¹ (lower panel).</p

Spectroscopic analysis of non-steatotic hepatocytes on fatty liver.

<p>Spectroscopic analyses were performed on periportal hepatocytes on tissue section from normal or fatty liver. The video image is shown (left panel) with the corresponding averaged IR spectra (right panel) and the chemical imaging of the sum of DAG (middle panel).</p

History of patients and origin of samples.

<p>History of patients and origin of samples.</p

Analysis of steatosis using synchrotron FTIR microspectroscopy.

<p>A) Optical image of steatotic hepatocytes containing steatotic vesicles (white star) and non-steatotic hepatocytes (black star). B) Averaged IR spectra recorded inside steatotic vesicles (upper spectrum in blue) or on non-steatotic hepatocytes (lower spectrum in red). The band corresponding to olefin (3000–3060 cm⁻¹) is labelled by a black arrow. C) Chemical imaging of some bands on the tissue section.</p