Ironing out the role of ferroptosis in immunity.

Ferroptosis is a type of regulated cell death that drives the pathophysiology of many diseases. Oxidative stress is detectable in many types of regulated cell death, but only ferroptosis involves lipid peroxidation and iron dependency. Ferroptosis originates and propagates from several organelles, including the mitochondria, endoplasmic reticulum, Golgi, and lysosomes. Recent data have revealed that immune cells can both induce and undergo ferroptosis. A mechanistic understanding of how ferroptosis regulates immunity is critical to understanding how ferroptosis controls immune responses and how this is dysregulated in disease. Translationally, more work is needed to produce ferroptosis-modulating immunotherapeutics. This review focuses on the role of ferroptosis in immune-related diseases, including infection, autoimmune diseases, and cancer. We discuss how ferroptosis is regulated in immunity, how this regulation contributes to disease pathogenesis, and how targeting ferroptosis may lead to novel therapies.

Recharacterization of RSL3 reveals that the selenoproteome is a druggable target in colorectal cancer.

Ferroptosis is an iron-dependent, non-apoptotic form of cell death resulting from the accumulation of lipid peroxides. Colorectal cancer (CRC) accumulates high levels of intracellular iron and reactive oxygen species (ROS), thereby sensitizing cells to ferroptosis. The selenoprotein glutathione peroxidase (GPx4) is a key enzyme in the detoxification of lipid peroxides and can be inhibited by the compound (S)-RSL3 ([1S,3R]-RSL3). However, the stereoisomer (R)-RSL3 ([1R,3R]-RSL3), which does not inhibit GPx4, exhibits equipotent activity to (S)-RSL3 across a panel of CRC cell lines. Utilizing CRC cell lines with an inducible knockdown of GPx4, we demonstrate that (S)-RSL3 sensitivity does not align with GPx4 dependency. Subsequently, a biotinylated (S)-RSL3 was then synthesized to perform affinity purification-mass spectrometry (AP-MS), revealing that (S)-RSL3 acts as a pan-inhibitor of the selenoproteome, targeting both the glutathione and thioredoxin peroxidase systems as well as multiple additional selenoproteins. To investigate the therapeutic potential of broadly disrupting the selenoproteome as a therapeutic strategy in CRC, we employed further chemical and genetic approaches to disrupt selenoprotein function. The findings demonstrate that the selenoprotein inhibitor Auranofin can induce ferroptosis and/or oxidative cell death both in-vitro and in-vivo. Consistent with this data we observe that AlkBH8, a tRNA-selenocysteine methyltransferase required for the translational incorporation of selenocysteine, is essential for CRC growth. In summary, our research elucidates the complex mechanisms underlying ferroptosis in CRC and reveals that modulation of the selenoproteome provides multiple new therapeutic targets and opportunities in CRC.

Sulfide oxidation promotes hypoxic angiogenesis and neovascularization.

Angiogenic programming in the vascular endothelium is a tightly regulated process for maintaining tissue homeostasis and is activated in tissue injury and the tumor microenvironment. The metabolic basis of how gas signaling molecules regulate angiogenesis is elusive. Here, we report that hypoxic upregulation of ·NO in endothelial cells reprograms the transsulfuration pathway to increase biogenesis of hydrogen sulfide (H2S), a proangiogenic metabolite. However, decreased H2S oxidation due to sulfide quinone oxidoreductase (SQOR) deficiency synergizes with hypoxia, inducing a reductive shift and limiting endothelial proliferation that is attenuated by dissipation of the mitochondrial NADH pool. Tumor xenografts in whole-body (WBCreSqorfl/fl) and endothelial-specific (VE-cadherinCre-ERT2Sqorfl/fl) Sqor-knockout mice exhibit lower mass and angiogenesis than control mice. WBCreSqorfl/fl mice also exhibit decreased muscle angiogenesis following femoral artery ligation compared to control mice. Collectively, our data reveal the molecular intersections between H2S, O2 and ·NO metabolism and identify SQOR inhibition as a metabolic vulnerability for endothelial cell proliferation and neovascularization.

Beyond the Barrier: Unraveling the Mechanisms of Immunotherapy Resistance.

Immune checkpoint blockade (ICB) induces a remarkable and durable response in a subset of cancer patients. However, most patients exhibit either primary or acquired resistance to ICB. This resistance arises from a complex interplay of diverse dynamic mechanisms within the tumor microenvironment (TME). These mechanisms include genetic, epigenetic, and metabolic alterations that prevent T cell trafficking to the tumor site, induce immune cell dysfunction, interfere with antigen presentation, drive heightened expression of coinhibitory molecules, and promote tumor survival after immune attack. The TME worsens ICB resistance through the formation of immunosuppressive networks via immune inhibition, regulatory metabolites, and abnormal resource consumption. Finally, patient lifestyle factors, including obesity and microbiome composition, influence ICB resistance. Understanding the heterogeneity of cellular, molecular, and environmental factors contributing to ICB resistance is crucial to develop targeted therapeutic interventions that enhance the clinical response. This comprehensive overview highlights key mechanisms of ICB resistance that may be clinically translatable. Expected final online publication date for the Annual Review of Immunology, Volume 42 is April 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Comprehensive genetic study of the insulin resistance marker TG:HDL-C in the UK Biobank.

Insulin resistance (IR) is a well-established risk factor for metabolic disease. The ratio of triglycerides to high-density lipoprotein cholesterol (TG:HDL-C) is a surrogate marker of IR. We conducted a genome-wide association study of the TG:HDL-C ratio in 402,398 Europeans within the UK Biobank. We identified 369 independent SNPs, of which 114 had a false discovery rate-adjusted P value < 0.05 in other genome-wide studies of IR making them high-confidence IR-associated loci. Seventy-two of these 114 loci have not been previously associated with IR. These 114 loci cluster into five groups upon phenome-wide analysis and are enriched for candidate genes important in insulin signaling, adipocyte physiology and protein metabolism. We created a polygenic-risk score from the high-confidence IR-associated loci using 51,550 European individuals in the Michigan Genomics Initiative. We identified associations with diabetes, hyperglyceridemia, hypertension, nonalcoholic fatty liver disease and ischemic heart disease. Collectively, this study provides insight into the genes, pathways, tissues and subtypes critical in IR.

Disruption of hypoxia-inducible factor-2α in neutrophils decreases colitis-associated colon cancer.

Neutrophils are abundant immune cells in the colon tumor microenvironment. Studies have shown that neutrophils are recruited into hypoxic foci in colon cancer. However, the impact of hypoxia signaling on neutrophil function and its involvement in colon tumorigenesis remain unclear. To address this, we generated mice with a deletion of hypoxia-inducible factor (HIF)-1α or HIF-2α in neutrophils driven by the MRP8Cre (HIF-1αΔNeu) or (HIF-2αΔNeu) and littermate controls. In an azoxymethane (AOM)/dextran sulfate sodium (DSS) model of colon cancer, the disruption of neutrophils-HIF-1α did not result in any significant changes in body weight, colon length, tumor size, proliferation, or burden. However, the disruption of HIF-2α in neutrophils led to a slight increase in body weight, a significant decrease in the number of tumors, and a reduction in tumor size and volume compared with their littermate controls. Histological analysis of colon tissue from mice with HIF-2α-deficient neutrophils revealed notable reductions in proliferation as compared with control mice. In addition, we observed reduced levels of proinflammatory cytokines, such as TNF-α and IL-1ß, in neutrophil-specific HIF-2α-deficient mice in both the tumor tissue as well as the neutrophils. Importantly, it is worth noting that the reduced tumorigenesis associated with HIF-2α deficiency in neutrophils was not evident in already established syngeneic tumors or a DSS-induced inflammation model, indicating a potential role of HIF-2α specifically in colon tumorigenesis. In conclusion, we found that the loss of neutrophil-specific HIF-2α slows colon tumor growth and progression by reducing the levels of inflammatory mediators.NEW & NOTEWORTHY Despite the importance of hypoxia and neutrophils in colorectal cancer (CRC), the contribution of neutrophil-specific HIFs to colon tumorigenesis is not known. We describe that neutrophil HIF-1α has no impact on colon cancer, whereas neutrophil HIF-2α loss reduces CRC growth by decreasing proinflammatory and immunosuppressive cytokines. Furthermore, neutrophil HIF-2α does not reduce preestablished tumor growth or inflammation-induced colitis. The present study offers novel potential of neutrophil HIF-2α as a therapeutic target in CRC.

IMC29 Plays an Important Role in Toxoplasma Endodyogeny and Reveals New Components of the Daughter-Enriched IMC Proteome.

The Toxoplasma inner membrane complex (IMC) is a unique organelle that plays critical roles in parasite motility, invasion, egress, and replication. The IMC is delineated into the apical, body, and basal regions, defined by proteins that localize to these distinct subcompartments. The IMC can be further segregated by proteins that localize specifically to the maternal IMC, the daughter bud IMC, or both. While the function of the maternal IMC has been better characterized, the precise roles of most daughter IMC components remain poorly understood. Here, we demonstrate that the daughter protein IMC29 plays an important role in parasite replication. We show that Δimc29 parasites exhibit severe replication defects, resulting in substantial growth defects and loss of virulence. Deletion analyses revealed that IMC29 localization is largely dependent on the N-terminal half of the protein containing four predicted coiled-coil domains while IMC29 function requires a short C-terminal helical region. Using proximity labeling, we identify eight novel IMC proteins enriched in daughter buds, significantly expanding the daughter IMC proteome. We additionally report four novel proteins with unique localizations to the interface between two parasites or to the outer face of the IMC, exposing new subregions of the organelle. Together, this work establishes IMC29 as an important early daughter bud component of replication and uncovers an array of new IMC proteins that provides important insights into this organelle. IMPORTANCE The inner membrane complex (IMC) is a conserved structure across the Apicomplexa phylum, which includes obligate intracellular parasites that cause toxoplasmosis, malaria, and cryptosporidiosis. The IMC is critical for the parasite to maintain its intracellular lifestyle, particularly in providing a scaffold for daughter bud formation during parasite replication. While many IMC proteins in the later stages of division have been identified, components of the early stages of division remain unknown. Here, we focus on the early daughter protein IMC29, demonstrating that it is crucial for faithful parasite replication and identifying specific regions of the protein that are important for its localization and function. We additionally use proximity labeling to reveal a suite of daughter-enriched IMC proteins, which represent promising candidates to further explore this IMC subcompartment.

Role of IL4 and GMCSF in Predicting Survival in Esophageal Cancer.

BACKGROUND: Esophageal cancer (EC) originates in the setting of chronic inflammation. Although previous studies have sought to understand the role of inflammatory signaling in EC, the effect of these immunologic changes on patient outcomes remains understudied. This study's objective was to identify relationships between cytokine levels and prognosis in a mixed cohort of esophageal adenocarcinoma (EAC) and esophageal squamous cell carcinoma (ESCC) patients. STUDY DESIGN: A total of 37 serum cytokines were profiled at the time of resection using multiplex ELISA in 47 patients (42 esophageal adenocarcinoma, 5 esophageal squamous cell carcinoma). Cytokine levels were median-binarized and assessed using Cox regression models. Findings were validated at the RNA level using The Cancer Genome Atlas EC cohort (81 esophageal adenocarcinoma, 81 esophageal squamous cell carcinoma). RESULTS: Univariable analysis revealed high serum interleukin 4 (IL4) and granulocyte-macrophage colony-stimulating factor (GMCSF) were negatively associated with overall survival (p = 0.046, p = 0.040). Multivariable analysis determined both high serum IL4 or high serum GMCSF were negatively associated with survival independent of important clinical factors (hazard ratio [HR] 7.55, p < 0.001; HR 5.24, p = 0.001). These findings were validated at the RNA level in The Cancer Genome Atlas EC cohort, where multivariable analysis identified high IL4 expression, high CSF2 expression (encodes GMCSF), and advanced pathologic stage as independent negative predictors of survival when controlled for clinical factors (HR 2.35, p = 0.012; HR 1.97, p = 0.040). CONCLUSIONS: These results show that high IL4/GMCSF levels are negatively associated with survival in EC. These relationships are independent of pathologic stage and are identified across modalities, histologic subtypes, and the presence/absence of neoadjuvant therapy.

Microenvironmental ammonia enhances T cell exhaustion in colorectal cancer.

Effective therapies are lacking for patients with advanced colorectal cancer (CRC). The CRC tumor microenvironment has elevated metabolic waste products due to altered metabolism and proximity to the microbiota. The role of metabolite waste in tumor development, progression, and treatment resistance is unclear. We generated an autochthonous metastatic mouse model of CRC and used unbiased multi-omic analyses to reveal a robust accumulation of tumoral ammonia. The high ammonia levels induce T cell metabolic reprogramming, increase exhaustion, and decrease proliferation. CRC patients have increased serum ammonia, and the ammonia-related gene signature correlates with altered T cell response, adverse patient outcomes, and lack of response to immune checkpoint blockade. We demonstrate that enhancing ammonia clearance reactivates T cells, decreases tumor growth, and extends survival. Moreover, decreasing tumor-associated ammonia enhances anti-PD-L1 efficacy. These findings indicate that enhancing ammonia detoxification can reactivate T cells, highlighting a new approach to enhance the efficacy of immunotherapies.

Dysregulated Amino Acid Sensing Drives Colorectal Cancer Growth and Metabolic Reprogramming Leading to Chemoresistance.

BACKGROUND & AIMS: Colorectal cancer (CRC) is a devastating disease that is highly modulated by dietary nutrients. Mechanistic target of rapamycin complex 1 (mTORC1) contributes to tumor growth and limits therapy responses. Growth factor signaling is a major mechanism of mTORC1 activation. However, compensatory pathways exist to sustain mTORC1 activity after therapies that target oncogenic growth factor signaling. Amino acids potently activate mTORC1 via amino acid-sensing GTPase activity towards Rags (GATOR). The role of amino acid-sensing pathways in CRC is unclear. METHODS: Human colon cancer cell lines, preclinical intestinal epithelial-specific GATOR1 and GATOR2 knockout mice subjected to colitis-induced or sporadic colon tumor models, small interfering RNA screening targeting regulators of mTORC1, and tissues of patients with CRC were used to assess the role of amino acid sensing in CRC. RESULTS: We identified loss-of-function mutations of the GATOR1 complex in CRC and showed that altered expression of amino acid-sensing pathways predicted poor patient outcomes. We showed that dysregulated amino acid-sensing induced mTORC1 activation drives colon tumorigenesis in multiple mouse models. We found amino acid-sensing pathways to be essential in the cellular reprogramming of chemoresistance, and chemotherapeutic-resistant patients with colon cancer exhibited de-regulated amino acid sensing. Limiting amino acids in in vitro and in vivo models (low-protein diet) reverted drug resistance, revealing a metabolic vulnerability. CONCLUSIONS: Our findings suggest a critical role for amino acid-sensing pathways in driving CRC and highlight the translational implications of dietary protein intervention in CRC.

Protocol for isolation and analysis of small volatile microbiome metabolites from human or mouse samples.

Metabolites are crucial for bidirectional communication between host and microbiome. We describe a protocol for the isolation of organic and aqueous metabolites from mucosal scrapes and feces from mouse and human samples. Although some of the most reactive organic compounds may be lost, this approach generates a functionally reproducible metabolic extract containing both host and microbial compounds appropriate for quantitative mass spectrometry and functional characterization. Our mass spectrometry approach identifies low-abundant and difficult to identify microbially derived metabolites. For complete details on the use and execution of this protocol, please refer to Bell et al. (2021) and Das et al. (2020).

Pathogenic ATM and BAP1 germline mutations in a case of early-onset, familial sarcomatoid renal cancer.

Metastatic renal cell carcinoma (RCC) remains an incurable malignancy, despite recent advances in systemic therapies. Genetic syndromes associated with kidney cancer account for only 5%-8% of all diagnosed kidney malignancies, and genetic predispositions to kidney cancer predisposition are still being studied. Genomic testing for kidney cancer is useful for disease molecular subtyping but provides minimal therapeutic information. Understanding how aberrations drive RCC development and how their contextual influences, such as chromosome loss, genome instability, and DNA methylation changes, may alter therapeutic response is of importance. We report the case of a 36-yr-old female with aggressive, metastatic RCC and a significant family history of cancer, including RCC. This patient harbors a novel, pathogenic, germline ATM mutation along with a rare germline variant of unknown significance in the BAP1 gene. In addition, somatic loss of heterozygosity (LOH) in BAP1 and ATM genes, somatic mutation and LOH in the VHL gene, copy losses in Chromosomes 9p and 14, and genome instability are also noted in the tumor, potentially dictating this patient's aggressive clinical course. Further investigation is warranted to evaluate the association of ATM and BAP1 germline mutations with increased risk of RCC and if these mutations should lead to enhanced and early screening.

Survival Analyses: A Statistical Review for Surgeons.

Survival analyses are a group of statistical principles applied to accurately analyze the length of time until a previously defined event occurs. Increasing survival is the underlying goal for most medical interventions, and is particularly critical in oncology-related surgical fields. In order to justify the application of a novel surgical intervention or other cancer therapy, one must first definitively show improvements in patient survival compared to the existing method of treatment. In order to definitively recommend one treatment option over another, it is paramount to design a study that addresses and minimizes sources of bias where possible. This can be challenging due to numerous factors including selecting an appropriate study design, dealing with censored data, obtaining an appropriate sample size, and performing robust statistical analysis. It is critical for surgeons in every stage of training to be able to both understand and apply these methods in order to improve patient care. In this review, we discuss approaches to design survival studies, relevant errors/biases, and how to account for them and cover several field-standard methods to analyze survival data including Kaplan-Meier plots, the log-rank test, and Cox Proportional Hazards Models. Importantly, this is accompanied by easily repurposable examples of how to implement these analyses in both R and GraphPad Prism using a publicly available survival dataset. This review will assist surgeons of all training levels in the design and analysis of survival studies and serve as a starting point for advancing patient care.

Reuterin in the healthy gut microbiome suppresses colorectal cancer growth through altering redox balance.

Microbial dysbiosis is a colorectal cancer (CRC) hallmark and contributes to inflammation, tumor growth, and therapy response. Gut microbes signal via metabolites, but how the metabolites impact CRC is largely unknown. We interrogated fecal metabolites associated with mouse models of colon tumorigenesis with varying mutational load. We find that microbial metabolites from healthy mice or humans are growth-repressive, and this response is attenuated in mice and patients with CRC. Microbial profiling reveals that Lactobacillus reuteri and its metabolite, reuterin, are downregulated in mouse and human CRC. Reuterin alters redox balance, and reduces proliferation and survival in colon cancer cells. Reuterin induces selective protein oxidation and inhibits ribosomal biogenesis and protein translation. Exogenous Lactobacillus reuteri restricts colon tumor growth, increases tumor reactive oxygen species, and decreases protein translation in vivo. Our findings indicate that a healthy microbiome and specifically, Lactobacillus reuteri, is protective against CRC through microbial metabolite exchange.

A temporal examination of calcium signaling in cancer- from tumorigenesis, to immune evasion, and metastasis.

BACKGROUND: Although the study of calcium (Ca2+) is classically associated with excitable cells such as myocytes or neurons, the ubiquity of this essential element in all cellular processes has led to interest in other cell types. The importance of Ca2+ to apoptosis, cell signaling, and immune activation is of special import in cancer. MAIN: Here we review the current understanding of Ca2+ in each of these processes vital to the initiation, spread, and drug resistance of malignancies. We describe the involvement of Ca2+, and Ca2+ related proteins in cell cycle checkpoints and Ca2+ dependent apoptosis and discuss their roles in cellular immortalization. The role of Ca2+ in inter-cellular communication is also discussed in relevance to tumor-stromal communication, angiogenesis, and tumor microinvasion. The role that Ca2+ plays in immune surveillance and evasion is also addressed. Finally, we discuss the possibility of targeting Ca2+ singling to address the most pressing topics of cancer treatment: metastatic disease and drug resistance. CONCLUSION: This review discusses the current understanding of Ca2+ in cancer. By addressing Ca2+ facilitated angiogenesis, immune evasion, metastasis, and drug resistance, we anticipate future avenues for development of Ca2+ as a nexus of therapy.

Novel insights into the composition and function of the Toxoplasma IMC sutures.

The Toxoplasma inner membrane complex (IMC) is a specialized organelle underlying the parasite's plasma membrane that consists of flattened rectangular membrane sacs that are sutured together and positioned atop a supportive cytoskeleton. We have previously identified a novel class of proteins localizing to the transverse and longitudinal sutures of the IMC, which we named IMC sutures components (ISCs). Here, we have used proximity-dependent biotin identification at the sutures to better define the composition of this IMC subcompartment. Using ISC4 as bait, we demonstrate biotin-dependent labeling of the sutures and have uncovered two new ISCs. We also identified five new proteins that exclusively localize to the transverse sutures that we named transverse sutures components (TSCs), demonstrating that components of the IMC sutures consist of two groups: those that localize to the transverse and longitudinal sutures (ISCs) and those residing only in the transverse sutures (TSCs). In addition, we functionally analyze the ISC protein ISC3 and demonstrate that ISC3-null parasites have morphological defects and reduced fitness in vitro. Most importantly, Δisc3 parasites exhibit a complete loss of virulence in vivo. These studies expand the known composition of the IMC sutures and highlight the contribution of ISCs to the ability of the parasite to proliferate and cause disease.

In Vivo Biotinylation of the Toxoplasma Parasitophorous Vacuole Reveals Novel Dense Granule Proteins Important for Parasite Growth and Pathogenesis.

UNLABELLED: Toxoplasma gondii is an obligate intracellular parasite that invades host cells and replicates within a unique parasitophorous vacuole. To maintain this intracellular niche, the parasite secretes an array of dense granule proteins (GRAs) into the nascent parasitophorous vacuole. These GRAs are believed to play key roles in vacuolar remodeling, nutrient uptake, and immune evasion while the parasite is replicating within the host cell. Despite the central role of GRAs in the Toxoplasma life cycle, only a subset of these proteins have been identified, and many of their roles have not been fully elucidated. In this report, we utilize the promiscuous biotin ligase BirA* to biotinylate GRA proteins secreted into the vacuole and then identify those proteins by affinity purification and mass spectrometry. Using GRA-BirA* fusion proteins as bait, we have identified a large number of known and candidate GRAs and verified localization of 13 novel GRA proteins by endogenous gene tagging. We proceeded to functionally characterize three related GRAs from this group (GRA38, GRA39, and GRA40) by gene knockout. While Δgra38 and Δgra40 parasites showed no altered phenotype, disruption of GRA39 results in slow-growing parasites that contain striking lipid deposits in the parasitophorous vacuole, suggesting a role in lipid regulation that is important for parasite growth. In addition, parasites lacking GRA39 showed dramatically reduced virulence and a lower tissue cyst burden in vivo Together, the findings from this work reveal a partial vacuolar proteome of T. gondii and identify a novel GRA that plays a key role in parasite replication and pathogenesis. IMPORTANCE: Most intracellular pathogens reside inside a membrane-bound vacuole within their host cell that is extensively modified by the pathogen to optimize intracellular growth and avoid host defenses. In Toxoplasma, this vacuole is modified by a host of secretory GRA proteins, many of which remain unidentified. Here we demonstrate that in vivo biotinylation of proximal and interacting proteins using the promiscuous biotin ligase BirA* is a powerful approach to rapidly identify vacuolar GRA proteins. We further demonstrate that one factor identified by this approach, GRA39, plays an important role in the ability of the parasite to replicate within its host cell and cause disease.

Novel components of the Toxoplasma inner membrane complex revealed by BioID.

UNLABELLED: The inner membrane complex (IMC) of Toxoplasma gondii is a peripheral membrane system that is composed of flattened alveolar sacs that underlie the plasma membrane, coupled to a supporting cytoskeletal network. The IMC plays important roles in parasite replication, motility, and host cell invasion. Despite these central roles in the biology of the parasite, the proteins that constitute the IMC are largely unknown. In this study, we have adapted a technique named proximity-dependent biotin identification (BioID) for use in T. gondii to identify novel components of the IMC. Using IMC proteins in both the alveoli and the cytoskeletal network as bait, we have uncovered a total of 19 new IMC proteins in both of these suborganellar compartments, two of which we functionally evaluate by gene knockout. Importantly, labeling of IMC proteins using this approach has revealed a group of proteins that localize to the sutures of the alveolar sacs that have been seen in their entirety in Toxoplasma species only by freeze fracture electron microscopy. Collectively, our study greatly expands the repertoire of known proteins in the IMC and experimentally validates BioID as a strategy for discovering novel constituents of specific cellular compartments of T. gondii. IMPORTANCE: The identification of binding partners is critical for determining protein function within cellular compartments. However, discovery of protein-protein interactions within membrane or cytoskeletal compartments is challenging, particularly for transient or unstable interactions that are often disrupted by experimental manipulation of these compartments. To circumvent these problems, we adapted an in vivo biotinylation technique called BioID for Toxoplasma species to identify binding partners and proximal proteins within native cellular environments. We used BioID to identify 19 novel proteins in the parasite IMC, an organelle consisting of fused membrane sacs and an underlying cytoskeleton, whose protein composition is largely unknown. We also demonstrate the power of BioID for targeted discovery of proteins within specific compartments, such as the IMC cytoskeleton. In addition, we uncovered a new group of proteins localizing to the alveolar sutures of the IMC. BioID promises to reveal new insights on protein constituents and interactions within cellular compartments of Toxoplasma.