Correlative nano-imaging of metals and proteins in primary neurons by synchrotron X-ray fluorescence and STED super resolution microscopy: Experimental validation

Subcellular localisation and identification of single atoms using quantitative scanning transmission electron microscopy

Determining the concentration of elements in subcellular structures poses a significant challenge. By locating an elemental species at high spatial resolution and with subcellular context, and subsequently quantifying it on an absolute scale, new information about cellular function can be revealed. Such measurements have not as yet been realised with existing techniques due to limitations on spatial resolution and inherent difficulties in detecting elements present in low concentrations. In this paper, we use scanning transmission electron microscopy (STEM) to establish a methodology for localising and quantifying high-Z elements in a biological setting by measuring elastic electron scattering. We demonstrate platinum (Pt) deposition within neuronal cell bodies following in vivo administration of the Pt-based chemotherapeutic oxaliplatin to validate this novel methodology. For the first time, individual Pt atoms and nanoscale Pt clusters are shown within subcellular structures. Quantitative measurements of elastic electron scattering are used to determine absolute numbers of Pt atoms in each cluster. Cluster density is calculated on an atoms-per-cubic-nanometre scale, and used to show clusters form with densities below that of metallic Pt. By considering STEM partial scattering cross-sections, we determine that this new approach to subcellular elemental detection may be applicable to elements as light as sodium. LAY DESCRIPTION: Heterogeneous elemental distributions drive fundamental biological processes within cells. While carbon, hydrogen, oxygen and nitrogen comprise by far the majority of living matter, concentrations and locations of more than a dozen other species must also be tightly controlled to ensure normal cell function. Oxaliplatin is a first-line and adjuvant treatment for colorectal cancer. However, pain in the body's extremities (fingers and toes) significantly impairs clinical usage as this serious and persistent side effect impacts on both patient cancer care and quality of life. Annular dark-field (ADF) imaging in the scanning transmission electron microscope (STEM) provides an image with strong atom-number contrast and is sufficient to distinguish between different cell types and different organelles within the cells of the DRG. We also show that Pt may be imaged at the single atom level and be localised at very high resolution while still preserving a degree of ultrastructural context. The intrinsic image contrast generated is sufficient to identify these features without the need for heavy metal stains and other extensive processing steps which risk disturbing native platinum distributions within the tissue. We subsequently demonstrate that by considering the total elastic scattering intensity generated by nanometre-sized Pt aggregations within the cell, the ADF STEM may be used to make a measurement of local concentration of Pt in units of atoms per cubic nanometre. We further estimate the minimum atomic number required to visualise single atoms in this setting, concluding that in similar samples it may be possible to detect species as light as sodium with atomic sensitivity.

Synchrotron-based correlative imaging of metals and proteins in neuronal cells: state of the art and future challenges in neurometallomics

Metal homeostasis in the nervous system is subtly regulated and changes in metal distribution or content, either increases or decreases, are associated with neurodegeneration or cognitive impairment. Determining the localization and quantification of metals in different types of neurons is important information for understanding their role in neurobiology. Synchrotron X-ray fluorescence imaging is a powerful technique that provides very high sensitivity and high spatial resolution for imaging metals in cells. However, additional biological information is often required to correlate the subcellular localization of metals with specific proteins or organelles. The purpose of this article is to review the studies in neuroscience that correlate metal imaging by synchrotron X-ray fluorescence with protein localization by other techniques. This article highlights the diversity of correlative modalities that have been used, from fluorescence to super-resolution and infrared microscopy, and the wealth of information that has been extracted, but also discusses some current limitations. Future developments are needed, particularly for direct imaging of metals and proteins with a single instrument.

Native Cryo-Correlative Light and Synchrotron X-ray Fluorescence Imaging of Proteins and Essential Metals in Subcellular Neuronal Compartments

Essential metals such as iron, copper, and zinc are required for a wide variety of biological processes. For example, they act as cofactors in many proteins, conferring enzymatic activity or structural stability. Interactions between metals and proteins are often difficult to characterize due to the low concentration of metals in biological tissues and the sometimes labile nature of the chemical bonds involved. To better understand the cellular functions of essential metals, we correlate protein localization, using fluorescence light microscopy (FLM), and metal distribution with synchrotron X-ray fluorescence (SXRF), a high-sensitivity and high-spatial-resolution technique for metal imaging. Both chemical imaging modalities are implemented under cryogenic conditions to preserve native cell structure and chemical element distribution. As a proof of concept, we applied cryo-FLM and cryo-SXRF correlative imaging to cultured primary hippocampal neurons. Neurons were labeled under live conditions with fluorescent F-actin and tubulin dyes, then samples were flash-frozen and observed in a frozen hydrated state. This methodology, cryo-FLM combined to cryo-SXRF, revealed the distribution of iron, copper and zinc relative to F-actin and tubulin in the growth cones, dendrites, axons, and axonal en passant boutons of developing neurons.

TauSTED super-resolution imaging of labile iron in primary hippocampal neurons

Iron dyshomeostasis is involved in many neurological disorders, particularly neurodegenerative diseases where iron accumulates in various brain regions. Identifying mechanisms of iron transport in the brain is crucial for understanding the role of iron in healthy and pathological states. In neurons, it has been suggested that iron can be transported by the axon to different brain regions in the form of labile iron; a pool of reactive and exchangeable intracellular iron. Here we report a novel approach to imaging labile ferrous iron, Fe(II), in live primary hippocampal neurons using confocal and TauSTED (stimulated emission depletion) microscopy. TauSTED is based on super-resolution STED nanoscopy, which combines high spatial resolution imaging (<40 nm) with fluorescence lifetime information, thus reducing background noise and improving image quality. We applied TauSTED imaging utilizing biotracker FerroFarRed Fe(II) and found that labile iron was present as submicrometric puncta in dendrites and axons. Some of these iron-rich structures are mobile and move along neuritic pathways, arguing for a labile iron transport mechanism in neurons. This super-resolution imaging approach offers a new perspective for studying the dynamic mechanisms of axonal and dendritic transport of iron at high spatial resolution in living neurons. In addition, this methodology could be transposed to the imaging of other fluorescent metal sensors.

Multimodal and multiscale correlative elemental imaging: From whole tissues down to organelles

Chemical elements, especially metals, play very specific roles in the life sciences. The implementation of correlative imaging methods, of elements on the one hand and of molecules or biological structures on the other hand, is the subject of recent developments. The most commonly used spectro-imaging techniques for metals are synchrotron-induced X-ray fluorescence, mass spectrometry and fluorescence imaging of metal molecular sensors. These imaging methods can be correlated with a wide variety of other analytical techniques used for structural imaging (e.g., electron microscopy), small molecule imaging (e.g., molecular mass spectrometry) or protein imaging (e.g., fluorescence microscopy). The resulting correlative imaging is developed at different scales, from biological tissue to the subcellular level. The fields of application are varied, with some major research topics, the role of metals in the aetiology of neurodegenerative diseases and the use of metals for medical imaging or cancer treatment.