Glomeruli filter blood through the coordination of podocytes, mesangial cells, fenestrated endothelial cells, and the glomerular basement membrane. Cellular changes, such as podocyte loss, are associated with pathologies like diabetic kidney disease. However, little is known regarding the in situ molecular profiles of specific cell types and how these profiles change with disease. Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) is well-suited for untargeted tissue mapping of a wide range of molecular classes. Importantly, additional imaging modalities can be integrated with MALDI IMS to associate these biomolecular distributions to specific cell types. Here, we integrated workflow combining MALDI IMS and multiplexed immunofluorescence (MxIF) microscopy. High spatial resolution MALDI IMS (5 μm) was used to determine lipid distributions within human glomeruli from a normal portion of fresh-frozen kidney cancer nephrectomy tissue revealing intra-glomerular lipid heterogeneity. Mass spectrometric data were linked to specific glomerular cell types and substructures through new methods that enable MxIF microscopy to be performed on the same tissue section following MALDI IMS, without sacrificing signal quality from either modality. Machine learning approaches were combined enabling cell type segmentation and identification based on MxIF data. This was followed by mining of cell type or cluster-associated MALDI IMS signatures using classification and interpretable machine learning. This allowed automated discovery of spatially specific molecular markers for glomerular cell types and substructures as well as lipids correlated to deep and superficial glomeruli. Overall, our work establishes a toolbox for probing molecular signatures of glomerular cell types and substructures within tissue microenvironments providing a framework applicable to other kidney tissue features and organ systems.

Background: We have developed a new class of dual polarity molecules for matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) capable of acquiring 5 μm pixel sizes with high sensitivity toward polar lipids and metabolites. Aminated cinnamic acid analogs (ACAAs) are vacuum stable, have high extinction coefficients at 355 nm, are highly sensitive to polar lipids, have low toxicity, and are affordable. Current molecules used for high spatial resolution MALDI IMS of polar lipids have shown great success, but are plagued with issues such as low sensitivity at high spatial resolution, vacuum instability, and/or high toxicity. Results: ACAAs were evaluated as MALDI matrices, testing them for vacuum stability, absorption at 355 nm, crystal size, sensitivity, and molecular coverage. Among them, 4-aminocinnamic acid (ACA) and 4-(dimethylamino)cinnamic acid (DMACA) were found to perform better than conventional MALDI matrices for lipid IMS experiments. ACA generated fewer in-source fragments due to its high extinction coefficient at 355 nm. This leads to better discernment of thermally labile molecules such as gangliosides compared to typical ‘soft’ ionization matrices like DHA using murine brain tissue. On the other hand, DMACA showed better optical properties than ACA, giving it higher sensitivity from many lipid classes, such as phospholipids and sulfatides. DMACA outperformed DAN and DHA at their individually optimized laser power at small pixel sizes (≤10 μm). DMACA also allows for lower laser power to be used without compromising sensitivity, which reduced the laser spot size at the sample surface from ∼6 μm to ∼4.5 μm without hardware modifications. Significance: Improved sensitivity and absorption efficiency at 355 nm allow for 5 μm pixel size MALDI IMS without oversampling while maintaining high S/N on commercial mass spectrometry platforms. Performing MALDI experiments at reduced laser energies minimizes tissue damage, enabling advanced multimodal MALDI IMS studies to be performed on single tissue sections. Comparisons and optimized MALDI IMS methods were performed on murine tissues and human kidney samples as part of the Human Biomolecular Atlas Program.

The glomerulus is a multicellular functional tissue unit (FTU) of the nephron that is responsible for blood filtration. Each glomerulus contains multiple substructures and cell types that are crucial for their function. To understand normal aging and disease in kidneys, methods for high spatial resolution molecular imaging within these FTUs across whole slide images is required. Here we demonstrate a workflow using microscopy-driven selected sampling to enable 5 μm pixel size matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) of all glomeruli within whole slide human kidney tissues. Such high spatial resolution imaging entails large numbers of pixels, increasing the data acquisition times. Automating FTU-specific tissue sampling enables high-resolution analysis of critical tissue structures, while concurrently maintaining throughput. Glomeruli were automatically segmented using coregistered autofluorescence microscopy data, and these segmentations were translated into MALDI IMS measurement regions. This allowed high-throughput acquisition of 268 glomeruli from a single whole slide human kidney tissue section. Unsupervised machine learning methods were used to discover molecular profiles of glomerular subregions and differentiate between healthy and diseased glomeruli. Average spectra for each glomerulus were analyzed using Uniform Manifold Approximation and Projection (UMAP) and k-means clustering, yielding 7 distinct groups of differentiated healthy and diseased glomeruli. Pixel-wise k-means clustering was applied to all glomeruli, showing unique molecular profiles localized to subregions within each glomerulus. Automated microscopy-driven, FTU-targeted acquisition for high spatial resolution molecular imaging maintains high-throughput and enables rapid assessment of whole slide images at cellular resolution and identification of tissue features associated with normal aging and disease.