Publications

Quantitative analysis of proteomes across multiple time points, organelles, and perturbations is essential for understanding both fundamental biology and disease states. The development of isobaric tags (e.g. TMT) have enabled the simultaneous measurement of peptide abundances across several different conditions. These multiplexed approaches are promising in principle because of advantages in throughput and measurement quality. However, in practice existing multiplexing approaches suffer from key limitations. In its simple implementation (TMT-MS2), measurements are distorted by chemical noise leading to poor measurement accuracy. The current state-ofthe-art (TMT-MS3) addresses this, but requires specialized quadrupole-iontrap-Orbitrap instrumentation. The complement reporter ion approach (TMTc) produces high accuracy measurements and is compatible with many more instruments, like quadrupoleOrbitraps. However, the required deconvolution of the TMTc cluster leads to poor measurement precision. Here, we introduce TMTc+, which adds the modeling of the MS2- isolation step into the deconvolution algorithm. The resulting measurements are comparable in precision to TMT-MS3/MS2. The improved duty cycle, and lower filtering requirements make TMTc+ more sensitive than TMT-MS3 and comparable with TMT-MS2. At the same time, unlike TMT-MS2, TMTc+ is exquisitely able to distinguish signal from chemical noise even outperforming TMT-MS3. Lastly, we compare TMTc+ to quantitative label-free proteomics of total HeLa lysate and find that TMTc+ quantifies 7.8k versus 3.9k proteins in a 5-plex sample. At the same time the median coefficient of variation improves from 13% to 4%. Thus, TMTc+ advances quantitative proteomics by enabling accurate, sensitive, and precise multiplexed experiments on more commonly used instruments.

Fertilization triggers release from meiotic arrest and initiates events that prepare for the ensuing developmental program. Protein degradation and phosphorylation are known to regulate protein activity during this process. However, the full extent of protein loss and phospho-regulation is still unknown. We examined absolute protein and phospho-site dynamics after fertilization by mass spectrometry-based proteomics. To do this, we developed a new approach for calculating the stoichiometry of phospho-sites from multiplexed proteomics that is compatible with dynamic, stable and multi-site phosphorylation. Overall, the data suggest that degradation is limited to a few low abundance proteins. However, this degradation promotes extensive dephosphorylation that occurs over a wide range of abundances during meiotic exit. We also show that eggs release a large amount of protein into the medium just after fertilization, most likely related to the blocks to polyspermy. Concomitantly, there is a substantial increase in phosphorylation likely tied to calcium activated kinases. We identify putative degradation targets as well as new components of the block to polyspermy. The analytical approaches demonstrated here are broadly applicable to studies of dynamic biological systems.

The bromodomain-containing protein BRD9, a subunit of the human BAF (SWI/SNF) nucleosome remodeling complex, has emerged as an attractive therapeutic target in cancer. Despite the development of chemical probes targeting the BRD9 bromodomain, there is a limited understanding of BRD9 function beyond acetyl-lysine recognition. We have therefore created the first BRD9-directed chemical degraders, through iterative design and testing of heterobifunctional ligands that bridge the BRD9 bromodomain and the cereblon E3 ubiquitin ligase complex. Degraders of BRD9 exhibit markedly enhanced potency compared to parental ligands (10- to 100-fold). Parallel study of degraders with divergent BRD9-binding chemotypes in models of acute myeloid leukemia resolves bromodomain polypharmacology in this emerging drug class. Together, these findings reveal the tractability of non-BET bromodomain containing proteins to chemical degradation, and highlight lead compound dBRD9 as a tool for the study of BRD9.

Asymmetric cell divisions produce two daughter cells with distinct fate. During embryogenesis, this mechanism is fundamental to build tissues and organs because it generates cell diversity. In adults, it remains crucial to maintain stem cells. The enthusiasm for asymmetric cell division is not only motivated by the beauty of the mechanism and the fundamental questions it raises, but has also very pragmatic reasons. Indeed, misregulation of asymmetric cell divisions is believed to have dramatic consequences potentially leading to pathogenesis such as cancers. In diverse model organisms, asymmetric cell divisions result in two daughter cells, which differ not only by their fate but also in size. This is the case for the early Xenopus laevis embryo, in which the two first embryonic divisions are perpendicular to each other and generate two pairs of blastomeres, which usually differ in size: one pair of blastomeres is smaller than the other. Small blastomeres will produce embryonic dorsal structures, whereas the larger pair will evolve into ventral structures. Here, we present a speculative model on the origin of the asymmetry of this cell division in the Xenopus embryo. We also discuss the apparently coincident asymmetric distribution of cell fate determinants and cell-size asymmetry of the 4-cell stage embryo. Finally, we discuss the asymmetric furrowing during epithelial cell cytokinesis occurring later during Xenopus laevis embryo development.

Targeted mass spectrometry assays for protein quantitation monitor peptide surrogates, which are easily multiplexed to target many peptides in a single assay. However, these assays have generally not taken advantage of sample multiplexing, which allows up to ten analyses to occur in parallel. We present a two-dimensional multiplexing workflow that utilizes synthetic peptides for each protein to prompt the simultaneous quantification of >100 peptides from up to ten mixed sample conditions. We demonstrate that targeted analysis of unfractionated lysates (2 hr) accurately reproduces the quantification of fractionated lysates (72 hr analysis) while obviating the need for peptide detection prior to quantification. We targeted 131 peptides corresponding to 69 proteins across all 60 National Cancer Institute cell lines in biological triplicate, analyzing 180 samples in only 48 hr (the equivalent of 16 min/sample). These data further elucidated a correlation between the expression of key proteins and their cellular response to drug treatment.

The pattern of the earliest cell divisions in a vertebrate embryo lays the groundwork for later developmental events such as gastrulation, organogenesis, and overall body plan establishment. Understanding these early cleavage patterns and the mechanisms that create them is thus crucial for the study of vertebrate development. This chapter describes the early cleavage stages for species representing ray-finned fish, amphibians, birds, reptiles, mammals, and proto-vertebrate ascidians and summarizes current understanding of the mechanisms that govern these patterns. The nearly universal influence of cell shape on orientation and positioning of spindles and cleavage furrows and the mechanisms that mediate this influence are discussed. We discuss in particular models of aster and spindle centering and orientation in large embryonic blastomeres that rely on asymmetric internal pulling forces generated by the cleavage furrow for the previous cell cycle. Also explored are mechanisms that integrate cell division given the limited supply of cellular building blocks in the egg and several-fold changes of cell size during early development, as well as cytoskeletal specializations specific to early blastomeres including processes leading to blastomere cohesion. Finally, we discuss evolutionary conclusions beginning to emerge from the contemporary analysis of the phylogenetic distributions of cleavage patterns. In sum, this chapter seeks to summarize our current understanding of vertebrate early embryonic cleavage patterns and their control and evolution.