The function of Retinoblastoma tumor suppressor (pRB) is greatly influenced by the cellular context, therefore the consequences of pRB inactivation are cell-type-specific. Here we employ single cell RNA-sequencing (scRNA-seq) to profile the impact of an Rbf mutation during Drosophila eye development. First, we build a catalogue of 11,500 wild type eye disc cells containing major known cell types. We find a transcriptional switch occurring in differentiating photoreceptors at the time of axonogenesis. Next, we map a cell landscape of Rbf mutant and identify a mutant-specific cell population that shows intracellular acidification due to increase in glycolytic activity. Genetic experiments demonstrate that such metabolic changes, restricted to this unique Rbf mutant population, sensitize cells to apoptosis and define the pattern of cell death in Rbf mutant eye disc. Thus, these results illustrate how scRNA-seq can be applied to dissect mutant phenotypes.

At the surface of many cells is a compendium of glycoconjugates that form an interface between the cell and its surroundings; the glycocalyx. The glycocalyx serves several functions that have captivated the interest of many groups. Given its privileged residence, this meshwork of sugar-rich biomolecules is poised to transmit signals across the cellular membrane, facilitating communication with the extracellular matrix and mediating important signalling cascades. As a product of the glycan biosynthetic machinery, the glycocalyx can serve as a partial mirror that reports on the cell's glycosylation status. The glycocalyx can also serve as an information-rich barrier, withholding the entry of pathogens into the underlying plasma membrane through glycan-rich molecular messages. In this review, we provide an overview of the different approaches devised to engineer glycans at the cell surface, highlighting considerations of each, as well as illuminating the grand challenges that face the next era of ‘glyco-engineers’. While we have learned much from these techniques, it is evident that much is left to be unearthed.

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Proteoglycans are heterogeneous macromolecular glycoconjugates that orchestrate many important cellular processes. While much attention has focused on the poly-sulfated glycosaminoglycan chains that decorate proteoglycans, other important elements of proteoglycan architecture, such as their core proteins and cell surface localization, have garnered less emphasis. Hence, comprehensive structure-function relationships that consider the replete proteoglycan architecture as glycoconjugates are limited. Here, we present a comprehensive approach to study proteoglycan structure and biology by fabricating defined semi-synthetic modular proteoglycans that can be tailored for cell surface display. To do so, we integrate amber codon reassignment in the expression of sequence-fined proteoglycan core proteins, metabolic oligosaccharide engineering to produce functionalizable glycosaminoglycans, and bioorthogonal click chemistry to covalently tether the two components. These materials permit the methodical dissection of the parameters required for optimal binding and function of various proteoglycanbinding proteins, and they can be modularly displayed on the surface of any living cell. We demonstrate that these sophisticated materials can recapitulate the functions of native proteoglycans in mouse embryonic stem cell differentiation and cancer cell spreading, while permitting the identification of the most important contributing elements of proteoglycan architecture toward function. This technology platform will confer structural resolution toward the investigation of proteoglycan structure-function relationships in cell biology.

Myogenic differentiation, the irreversible developmental process where precursor myoblast muscle stem cells become contractile myotubes, is heavily regulated by glycosylation and glycan–protein interactions at the cell surface and the extracellular matrix. The glycan-binding protein galectin-1 has been found to be a potent activator of myogenic differentiation. While it is being explored as a potential therapeutic for muscle repair, a precise understanding of its glycoprotein interactors is lacking. These gaps are due in part to the difficulties of capturing glycan–protein interactions in live cells. Here, we demonstrate the use of a proximity tagging strategy coupled with quantitative mass-spectrometry-based proteomics to capture, enrich, and identify the glycan-mediated glycoprotein interactors of galectin-1 in cultured live mouse myoblasts. Our interactome dataset can serve as a resource to aid the determination of mechanisms through which galectin-1 promotes myogenic differentiation. Moreover, it can also facilitate the determination of the physiological glycoprotein counter-receptors of galectin-1. Indeed, we identify several known and novel glycan-mediated ligands of galectin-1 as well as validate that galectin-1 binds the native CD44 glycoprotein in a glycan-mediated manner.

Proteoglycans are heterogeneous macromolecular glycoconjugates that orchestrate many important cellular processes. While much attention has focused on the poly-sulfated glycosaminoglycan chains that decorate proteoglycans, other important elements of proteoglycan architecture, such as their core proteins and cell surface localization, have garnered less emphasis. Hence, comprehensive structure-function relationships that consider the replete proteoglycan architecture as glycoconjugates are limited. Here, we present a comprehensive approach to study proteoglycan structure and biology by fabricating defined semi-synthetic modular proteoglycans that can be tailored for cell surface display. To do so, we integrate amber codon reassignment in the expression of sequence-fined proteoglycan core proteins, metabolic oligosaccharide engineering to produce functionalizable glycosaminoglycans, and bioorthogonal click chemistry to covalently tether the two components. These materials permit the methodical dissection of the parameters required for optimal binding and function of various proteoglycan-binding proteins, and they can be modularly displayed on the surface of any living cell. We demonstrate that these sophisticated materials can recapitulate the functions of native proteoglycans in mouse embryonic stem cell differentiation and cancer cell spreading, while permitting the identification of the most important contributing elements of proteoglycan architecture toward function. This technology platform will confer structural resolution toward the investigation of proteoglycan structure-function relationships in cell biology.

Proteoglycans are now well regarded as key facilitators of cell biology. While a majority of their interactions and functions are attributed to the decorating glycosaminoglycan chains, there is a growing appreciation for the roles of the proteoglycan core protein and for considering proteoglycans as replete protein-glycan conjugates. This appreciation, seeded by early work in proteoglycan biology, is now being advanced and exalted by modern approaches in chemical glycobiology. In this review, we discuss up-and-coming methods to unearth the fine-scale architecture of proteoglycans that modulate their functions and interactions. Crucial to these efforts is the production of chemically defined materials, including semi-synthetic proteoglycans and the in situ capture of interacting proteins. Together, the integration of chemical biology approaches promises to expedite the dissection of the structural heterogeneity of proteoglycans and deliver refined insight into their functions.

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