The extracellular matrix (ECM) is a dynamic complex of glycoproteins, proteoglycans, carbohydrates, and collagen that serves as an interface between mammalian cells and their extracellular environment. Essential for normal cellular homeostasis, physiology, and events that occur during development, it is also a key functionary in a number of human diseases, including cancer and neurodegeneration.
Dictyostelium secretes an ECM during development that regulates multicellularity, cell motility, cell differentiation, and morphogenesis, and provides structural support and protective layers to the resulting differentiated cell types. The Dictyostelium ECM shares many of the features of mammalian and plant ECM, and thus presents an excellent system for studying its structure and function. As a genetically tractable model organism, Dictyostelium offers the potential to further elucidate ECM functions and to possibly reveal previously unknown roles for the ECM.
Dictyostelium secretes an ECM during development that regulates multicellularity, cell motility, cell differentiation, and morphogenesis, and provides structural support and protective layers to the resulting differentiated cell types. The Dictyostelium ECM shares many of the features of mammalian and plant ECM, and thus presents an excellent system for studying its structure and function. As a genetically tractable model organism, Dictyostelium offers the potential to further elucidate ECM functions and to possibly reveal previously unknown roles for the ECM.
Summary of the known components and events mediated by the ECM of Dictyostelium. Like mammals, during multicellular development, Dictyostelium cells release precursor proteins that are proteolytically processed to release bioactive peptides that regulate cell motility and differentiation. Matricellular proteins have been identified that regulate cell motility. Structural and cell adhesion proteins maintain the multicellular status of the aggregate, while the functional roles of the many signalling proteins remain to be analyzed. Based on research in mammals, matricellular proteins could function to mediate these signalling events. Other known roles of mammalian ECM proteins (e.g., morphogenetic functions) have so far not been sufficiently analyzed in Dictyostelium. Figure taken from Huber and O'Day, 2017.
As the Dictyostelium slug migrates along the substratum, the ECM is left behind as a collapsed tube, which has facilitated analyses of the specific components that make up the ECM. Our recent work used LC-MS/MS to reveal over 300 proteins in the Dictyostelium ECM (Huber and O'Day, 2015). Proteomic profiling revealed proteins involved in metabolic processes (~50%), transport (~9%), fruiting body development (~7%), biological adhesion (~4%), proteolysis (~3%), and cell motility (~2.5%) (Huber and O’Day, 2015; Huber and O’Day, 2017). In addition, of the over 300 proteins identified, ~48% are involved in some sort of binding, while ~30% are homologous to known enzymes. Together, these findings have provided valuable new insight into the primary functions associated with ECM proteins in Dictyostelium.
GO annotation of proteins detected in the sheath ECM of two wild-type strains of Dictyostelium, NC4 and WS380B. Numbers represent the % of the total number of proteins detected. For a complete list of the proteins identified refer to Fig. S2 in Huber and O′Day, 2015. Table taken from Huber and O'Day, 2017.
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The ECM surrounding the multicellular slug and the cell types contained within. The sheath ECM, which is synthesized from the tip of the multicellular slug, is shed from the back of the slug as it migrates along the substratum. (Top panel) a diversity of processes occur in the ECM during slug migration. Cells secrete EcmA and EcmD which provide structure to the ECM. Secreted proteins such as AcbA and CyrA are processed into bioactive fragments that in turn bind to the cell surface to modulate cellular processes (e.g., spore differentiation and cell motility, respectively). (Bottom panel) different cell types within the slug sort to specific locations. Figure taken from Huber and O'Day, 2017.
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While the structural components of the mammalian ECM have been well defined, the functions of the non-structural components are still being revealed. Matricellular proteins belong to a family of non-structural ECM proteins that contain binding sites for both the ECM (i.e., matri-) and cell surface proteins (i.e., -cellular). Members of this protein family regulate a diversity of cellular processes and are linked to a number of human diseases, such as cancer and neurodegeneration. Some of the key features of matricellular proteins are that they associate with extracellular proteases and growth factors, are expressed at high levels during development, do not directly contribute directly to the organization or physical properties of extracellular structures, and modulate cellular processes by binding to the cell surface and initiating intracellular signal transduction. In mammals, the most well studied function for these proteins is their ability to modulate cell adhesion and migration through the interaction of their epidermal growth factor (EGF)-like repeats with the cell surface.
Our previous work characterized the cysteine-rich, calmodulin (CaM)-binding protein (CaMBP) CyrA as the first matricellular protein to be identified in a eukaryotic microbe (Suarez et al., 2011; Huber et al., 2012; Huber and O’Day, 2012b; O’Day and Huber, 2013). In keeping with its matricellular classification, CyrA contains four tandem C-terminal EGF-like repeats that bind to the cell surface and modulate cell movement during Dictyostelium development (Huber and O’Day, 2009; Huber and O’Day, 2011; Nikolaeva et al., 2012; Huber and O’Day, 2012a; Huber and O’Day, 2012b; O’Day and Huber, 2013).
We are currently engaged in studies that are characterizing a cyrA knockout cell line. Our long-term objective is to use Dictyostelium to gain fresh new insight into the structure and function of the mammalian ECM, including revealing new matricellular proteins, proteases that regulate the processing of ECM components, and identifying the functions of the many uncharacterized proteins in the Dictyostelium ECM.
Our previous work characterized the cysteine-rich, calmodulin (CaM)-binding protein (CaMBP) CyrA as the first matricellular protein to be identified in a eukaryotic microbe (Suarez et al., 2011; Huber et al., 2012; Huber and O’Day, 2012b; O’Day and Huber, 2013). In keeping with its matricellular classification, CyrA contains four tandem C-terminal EGF-like repeats that bind to the cell surface and modulate cell movement during Dictyostelium development (Huber and O’Day, 2009; Huber and O’Day, 2011; Nikolaeva et al., 2012; Huber and O’Day, 2012a; Huber and O’Day, 2012b; O’Day and Huber, 2013).
We are currently engaged in studies that are characterizing a cyrA knockout cell line. Our long-term objective is to use Dictyostelium to gain fresh new insight into the structure and function of the mammalian ECM, including revealing new matricellular proteins, proteases that regulate the processing of ECM components, and identifying the functions of the many uncharacterized proteins in the Dictyostelium ECM.