BASIC RESEARCH www.jasn.org Global Analysis Reveals the Complexity of the Human Glomerular Extracellular Matrix † † Rachel Lennon,* Adam Byron,* Jonathan D. Humphries,* Michael J. Randles,* † ‡ † | Alex Carisey,* Stephanie Murphy,* David Knight, Paul E. Brenchley, Roy Zent,§ and Martin J. Humphries* *Wellcome Trust Centre for Cell-Matrix Research and ‡Biological Mass Spectrometry Core Facility, Faculty of Life Sciences, and †Faculty of Medical and Human Sciences, University of Manchester, Manchester, United Kingdom; §Division of Nephrology, Department of Medicine, Vanderbilt University Medical Center, Nashville, Tennessee; and |Department of Medicine, Veterans Affairs Hospital, Nashville, Tennessee ABSTRACT The glomerulus contains unique cellular and extracellular matrix (ECM) components, which are required for intact barrier function. Studies of the cellular components have helped to build understanding of glomerular disease; however, the full composition and regulation of glomerular ECM remains poorly understood. We used mass spectrometry-based proteomics of enriched ECM extracts for a global analysis of human glomerular ECM in vivo and identified a tissue-specific proteome of 144 structural and regulatory ECM proteins. This catalog includes all previously identified glomerular components plus many new and abundant components. Relative protein quantification showed a dominance of collagen IV, collagen I, and laminin isoforms in the glomerular ECM together with abundant collagen VI and TINAGL1. Protein net- work analysis enabled the creation of a glomerular ECM interactome, which revealed a core of highly connected structural components. More than one half of the glomerular ECM proteome was validated using colocalization studies and data from the Human Protein Atlas. This study yields the greatest number of ECM proteins relative to previous investigations of whole glomerular extracts, highlighting the impor- tance of sample enrichment. It also shows that the composition of glomerular ECM is far more complex than previously appreciated and suggests that many more ECM components may contribute to glomerular development and disease processes. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium with the dataset identifier PXD000456. J Am Soc Nephrol 25: 939–951, 2014. doi: 10.1681/ASN.2013030233 The glomerulus is a sophisticated organelle com- In addition to operating as a signaling platform, prisinguniquecellularandextracellularmatrix(ECM) ECM provides a structural scaffold for adjacent cells components. Fenestrated capillary endothelial cells and has a tissue-specific molecular composition.3,4 and overlying podocytes are separated by a special- ized glomerular basement membrane (GBM), and Received March 11, 2013. Accepted September 10, 2013. these three components together form the filtration Published online ahead of print. Publication date available at barrier. Mesangial cells and their associated ECM, www.jasn.org. the mesangial matrix, exist between adjacent capil- Present address: Adam Byron, Edinburgh Cancer Research lary loops and maintain the three-dimensional or- United Kingdom Centre, Institute of Genetics and Molecular ganizationof the capillary bundle.Inturn,theparietal Medicine, University of Edinburgh, Edinburgh, United Kingdom. epithelial cells and ECM of Bowman’s capsule enclose Correspondence: Dr. Rachel Lennon, Wellcome Trust Centre for this network of capillaries. Cells adhere to ECM pro- Cell-Matrix Research, Michael Smith Building, University of teins by adhesion receptors, and these interactions are Manchester, Manchester M13 9PT, United Kingdom. Email: Rachel. required to maintain intact barrier function of the [email protected] glomerulus.1,2 Copyright © 2014 by the American Society of Nephrology J Am Soc Nephrol 25: 939–951, 2014 ISSN : 1046-6673/2505-939 939 BASIC RESEARCH www.jasn.org Candidate-based investigations of glomerular ECM have fo- Therefore, to interrogate this complexity effectively, a systems- cused on the GBM and shown that it resembles the typical level understanding of glomerular ECM is required. To address basal lamina found in multicellular organisms, containing a the need for a global analysis of the extracellular environment core of glycoproteins (collagen IV, laminins, and nidogens) within the glomerulus, we used mass spectrometry (MS)-based and heparan sulfate proteoglycans (agrin, perlecan, and colla- proteomics of glomerular ECM fractions to define the human gen XVIII).5 Mesangial and parietal cell ECMs have been less glomerular ECM proteome. well investigated; nonetheless, they are also thought to contain similar core components in addition to other glycoproteins, including fibronectin.6,7 Thus, the glomerulus consists of a RESULTS combination of condensed ECM within the GBM and Bow- man’s capsule and loose ECM supporting the mesangial cells. Isolation of Glomerular ECM The ECM compartments in the glomerulus are thought to Using purified isolates of human glomeruli (Figure 1A), we be distinct and exhibit different functional roles. The GBM is developed a fractionation approach to collect glomerular integral to the capillary wall and therefore, functionally linked ECM proteins (Figure 1B). Before homogenization and solu- to glomerular filtration.5 Mutations of tissue-restricted iso- bilization, extracted glomeruli appeared acellular, suggestive forms of collagen IV (COL4A3, COL4A4,andCOL4A5)and of successful ECM enrichment (Figure 1A). Western blotting laminin (LAMB2), which are found in the GBM, cause signif- confirmed enrichment of ECM proteins (collagen IVand lam- icant barrier dysfunction and ultimately, renal failure.8,9 Less inin) and depletion of cytoplasmic and nuclear proteins is understood about the functions of mesangial and parietal (nephrin, actin, and lamin B1) in the ECM fraction (Figure cell ECMs, although expansion of the mesangial compartment 1C). Glomerular ECM fractions from three adult human kid- is a histologic pattern seen across the spectrum of glomerular neys were then extracted and analyzed by MS. disease.10 Compositional investigation of the distinct glomerular Gene Ontology Enrichment Analysis of ECM Fractions ECM compartments is limited by the technical difficulties of All proteins identified by MS were allocated to categories separation. Early investigations of GBM constituents used the according to their gene ontology (GO) assignment, and the relative insolubility of ECM proteins to facilitate separation enrichment of GO terms in the dataset was assessed using GO from cellular proteins in the glomerulus but did not separate enrichment analysis, which is described in Supplemental the GBM from mesangial and parietal cells ECMs.11,12 Re- Methods. The network of enriched GO terms clustered into cently, studies incorporating laser microdissection of glomer- three subnetworks, and the largest, most confidently identified ular sections have been coupled with proteomic analyses.13,14 subnetwork was for ECM-related GO terms (Figure 2A). A These studies report both cellular and ECM components and large majority of other proteins were included in two addi- typically require pooled material from glomerular sections to tional subnetworks representing mitochondrial and cytoskel- improve protein identification. The ability of laser microdis- etal GO terms, and the presence of these proteins in the ECM section to separate glomerular ECM compartments has not yet fractions is likely to be caused by the strength of their inter- been tested; however, this approach will be limited by the molecular interactions with ECM proteins. Spectral counting amount of protein that is possible to retrieve. To achieve was used to determine the enrichment of ECM in each of the good coverage of ECM proteins within a tissue, proteomic four fractions collected from isolated human glomeruli. There studies need to combine a reduction in sample complexity was 38% enrichment of extracellular proteins in the glomer- with maximal protein quantity. Currently, the inability to sep- ular ECM fractions (Figure 2B), and this result compares arate glomerular ECM compartments in sufficient quantity favorably with the 12%–30% enrichment of ECM proteins re- is a limitation that prohibits proteomic studies of these struc- ported in comparable proteomic studies.15 tures; however, for other tissues, proteomic analysis of ECM has been achieved by enrichment of ECM combined with sam- Relative Abundance of Glomerular ECM Proteins ple fractionation.15 To determine the relative abundance of glomerular ECM Although the composition of the ECM in other tissues has proteins, we used MS to analyze additional glomerular ECM been addressed using proteomic approaches,15 studies of glo- fractions from three human kidneys, and we quantified the merular ECM to date have used candidate-based technologies. proteins using a peptide intensity approach,20 which is de- These studies have identified key molecular changes during scribed in Supplemental Methods. Using this analysis, the 10 development and disease and highlighted the compositional most abundant proteins were a combination of collagens, lam- and organizational dynamics of glomerular ECM. Nonethe- inin 521, and heparan sulfate proteoglycans (Figure 2C, Sup- less, the extracellular environment within the glomerulus is plemental Table 1), which have all previously been reported in the setting for a complex series of interactions between both the glomerulus. In addition, we found collagen VI and the structural ECM proteins and ECM-associated