Project description:The use of single-cell technologies for clinical applications requires disconnecting sampling from downstream processing steps. Early sample preservation can further increase robustness and reproducibility by avoiding artifacts introduced during specimen handling. We present FixNCut, a methodology for the reversible fixation of tissue followed by dissociation that overcomes current limitations. We applied FixNCut to human and mouse tissues to demonstrate the preservation of RNA integrity, sequencing library complexity, and cellular composition, while diminishing stress-related artifacts. Besides single-cell RNA sequencing, FixNCut is compatible with multiple single-cell and spatial technologies, making it a versatile tool for robust and flexible study designs.
Project description:The use of single-cell technologies for clinical applications requires disconnecting sampling from downstream processing steps. Early sample preservation can further increase robustness and reproducibility by avoiding artifacts introduced during specimen handling. We present FixNCut, a methodology for the reversible fixation of tissue followed by dissociation that overcomes current limitations. We applied FixNCut to human and mouse tissues to demonstrate the preservation of RNA integrity, sequencing library complexity, and cellular composition, while diminishing stress-related artifacts. Besides single-cell RNA sequencing, FixNCut is compatible with multiple single-cell and spatial technologies, making it a versatile tool for robust and flexible study designs.
Project description:Single-cell sequencing technologies are revolutionizing biology, but are limited by the need of dissociating fresh samples that can only be fixed at later stages. We present ACME (ACetic-MEthanol) dissociation, a species-versatile cell dissociation approach that fixes cells as they are being dissociated. ACME-dissociated cells have high RNA integrity, can be cryopreserved multiple times, can be sorted by Fluorescence-Activated Cell Sorting (FACS) and are permeable, enabling combinatorial approaches of single-cell transcriptomics. ACME is based on cheap reagents and it can be done in most labs and even sampling trips. As a proof of principle, we have performed SPLiT-seq with ACME cells to obtain around ~35K cells from two planarian species and identified all previously described cell types in similar proportions. This technique allows fixed, dissociated cells to be obtained from diverse organisms that can be cryopreserved and subjected to combinatorial barcoding methods for single-cell transcriptomics and thus will accelerate our knowledge of cell types across the tree of life.
Project description:To study how methanol fixation affects single-cell transcriptomic measurement, two cerebral organoids were dissociated. Cell suspension of each organoid was split into two aliquots. Methanol fixation was applied to one of the two aliquots. Single-cell RNA-seq with 10x Genomics was applied to the two aliquots separately.
Project description:Archival formalin-fixed paraffin-embedded (FFPE) tissue samples hold a wealth of transcriptomic information; however, little is known about potential artifacts. Previously, we identified a consistent shift in global RNA-sequencing profiles between matching frozen and FFPE samples. We hypothesized that this shift was from fixing fresh tissue in formalin. To test this idea, RNA-sequencing was performed on liver samples collected from male mice treated with 600 ppm of a reference chemical (phenobarbital, 600 ppm phenobarbital) or vehicle control for 7 days. Samples were divided into: (1) fresh-frozen (FR); (2) directly fixed in 10% buffered formalin for 18 hours followed by paraffin embedding (FFPE); (3) frozen then fixed as FFPE (FR>FFPE); or (4) frozen then fixed in 70% ethanol followed by paraffin embedding (FR>OH) (n=6/group/condition). Direct fixation resulted in 2946 differentially expressed genes (DEGs), 98% of which were down-regulated. Freezing prior to fixation resulted in ≥95% fewer DEGs vs. FR, indicating that most formalin-derived transcriptional effects occurred with fixation. This was supported by follow-up studies, which identified consistent enrichment in oxidative stress, mitochondrial dysfunction, and transcription elongation pathways with formalin fixation. Notably, formalin fixation in the parent study did not significantly impact chemical response profiles, which were consistent with CAR/PXR activation and 600 ppm phenobarbital exposure. Our results demonstrate distinct transcriptional effects of formalin fixation that could impact gene expression studies using FFPE samples.
Project description:Formaldehyde fixation is widely used for long-term maintenance of tissue. However, due to formaldehyde-induced cross-links, fixated tissue proteins are difficult to extract hampering the performance of mass spectrometry (MS)-proteomic analysis on formaldehyde-fixated tissue. Recent years saw the use of different combinations of high-temperature and solubilizing agents (usually derived from antigen retrieval techniques) to unravel formaldehyde-fixated paraffin-embedded tissue proteomes. However, in order to achieve protein extraction yields similar to fresh-frozen tissue high-temperature heating is necessary. Such harsh extraction conditions may affect, labile post-translational modifications such as phosphorylations resulting in the loss of important protein information. The objective of the present work is to assess cleavable fixative reagents that allow tissue preservation as well as efficient protein extraction from fixated tissue for MS-proteomics under mild conditions. To this regard, we investigated disuccinimidyl tartrate (DST) and dithiobis[succinimidylpropionate] (DSP) as cleavable fixating reagents. These compounds crosslink proteins by reacting with amino groups leading to amide bond formation. Linkers can be cleaved with sodium metaperiodate (cis-diols) or reducing agents (disulfide bonds), respectively. Our results show that reversible protein crosslinking allows tissue fixation with morphology preservation comparable to formalin. In addition, cleavage of DSP improves protein recovery from fixated tissue by a factor of 18 and increases the number of identified proteins by approximately 20% under mild extraction conditions avoiding the need for sample boiling, which could affect labile post-translational modifications. A major advantage of DSP over formaldehyde is the introduction of well-defined protein modifications that can be taken into account during database searching. In contrast to DSP fixation, DST fixation followed by periodic cleavage, while effective, resulted in side reactions that prevented effective protein extraction and subsequent protein identification. Protein crosslinkers, which can be cleaved under mild conditions, are thus viable alternatives to formaldehyde as tissue fixatives facilitating protein analysis from paraffin embedded, fixated tissue.
Project description:Background: Single-cell RNA sequencing has been widely adopted to estimate the cellular composition of heterogeneous tissues and obtain transcriptional profiles of individual cells. Multiple approaches for optimal sample dissociation and storage of single cells have been proposed as have single-nuclei profiling methods. What has been lacking is a systematic comparison of their relative biases and benefits. Results: Here, we compare gene expression and cellular composition of single cell suspensions prepared from adult mouse kidney using two tissue dissociation protocols. For each sample we also compare fresh cells to cryopreserved and methanol-fixed cells. Lastly, we compare this single-cell data to that generated using three single-nucleus RNA sequencing workflows. Our data confirms prior reports that digestion on ice avoids the stress response observed with 37°C dissociation. It also reveals cell types more abundant either in the cold or warm dissociations that may represent populations that require gentler or harsher conditions to be released intact. For cell storage, cryopreservation of dissociated cells results in a major loss of epithelial cell types; in contrast, methanol fixation maintains the cellular composition but suffers from ambient RNA leakage. Finally, cell type composition differences are observed between single-cell and single-nucleus RNA sequencing libraries. In particular, we note an under-representation of T, B and NK lymphocytes in the single-nucleus libraries. Conclusions: Systematic comparison of recovered cell types and their transcriptional profiles across the workflows has highlighted protocol-specific biases and thus enables researchers starting single cell experiments to make an informed choice.