Inter- and trans-generational impacts of environmental exposures on the germline resolved at the single-cell level.

Reproduction is a remarkably intricate process involving the interaction of multiple cell types and organ systems unfolding over long periods of time and that culminates with the production of gametes. The initiation of germ cell development takes place during embryogenesis but only completes decades later in humans. The complexity inherent to reproduction and its study has long hampered our ability to decipher how environmental agents disrupt this process. Single-cell approaches provide an opportunity for a deeper understanding of the action of toxicants on germline function and analyze how the response to their exposure is differentially distributed across tissues and cell types. In addition to single-cell RNA sequencing, other single-cell or nucleus level approaches such as ATAC-sequencing and multi-omics have expanded the strategies that can be implemented in reproductive toxicological studies to include epigenomic and the nuclear transcriptomic data. Here we will discuss the current state of single-cell technologies and how they can best be utilized to advance reproductive toxicological studies. We will then discuss case studies in two model organisms (Caenorhabditis elegans and mouse) studying different environmental exposures (alcohol and e-cigarettes respectively) to highlight the value of single-cell and single-nucleus approaches for reproductive biology and reproductive toxicology.

Protocol for nuclear dissociation of the adult C. elegans for single-nucleus RNA sequencing and its application for mapping environmental responses.

Caenorhabditis elegans is a valuable model to study organ, tissue, and cell-type responses to external cues. However, the nematode comprises multiple syncytial tissues with spatial coordinates corresponding to distinct nuclear transcriptomes. Here, we present a single-nucleus RNA sequencing (snRNA-seq) protocol that aims to overcome difficulties encountered with single-cell RNA sequencing in C. elegans. We describe steps for isolating C. elegans nuclei for downstream applications including snRNA-seq applied to the context of alcohol exposure. For complete details on the use and execution of this protocol, please refer to Truong et al. (2023).1.

Single-nucleus resolution mapping of the adult C. elegans and its application to elucidate inter- and trans-generational response to alcohol.

Single-cell transcriptomic platforms provide an opportunity to map an organism's response to environmental cues with high resolution. Here, we applied single-nucleus RNA sequencing (snRNA-seq) to establish the tissue and cell type-resolved transcriptome of the adult C. elegans and characterize the inter- and trans-generational transcriptional impact of ethanol. We profiled the transcriptome of 41,749 nuclei resolving into 31 clusters, representing a diverse array of adult cell types including syncytial tissues. Following exposure to human-relevant doses of alcohol, several germline, striated muscle, and neuronal clusters were identified as being the most transcriptionally impacted at the F1 and F3 generations. The effect on germline clusters was confirmed by phenotypic enrichment analysis as well as by functional validation, which revealed a remarkable inter- and trans-generational increase in germline apoptosis, aneuploidy, and embryonic lethality. Together, snRNA-seq represents a valuable approach for the detailed examination of an adult organism's response to environmental exposures.

NHR-23 activity is necessary for C. elegans developmental progression and apical extracellular matrix structure and function.

Nematode molting is a remarkable process where animals must repeatedly build a new apical extracellular matrix (aECM) beneath a previously built aECM that is subsequently shed. The nuclear hormone receptor NHR-23 (also known as NR1F1) is an important regulator of C. elegans molting. NHR-23 expression oscillates in the epidermal epithelium, and soma-specific NHR-23 depletion causes severe developmental delay and death. Tissue-specific RNAi suggests that nhr-23 acts primarily in seam and hypodermal cells. NHR-23 coordinates the expression of factors involved in molting, lipid transport/metabolism and remodeling of the aECM. NHR-23 depletion causes dampened expression of a nas-37 promoter reporter and a loss of reporter oscillation. The cuticle collagen ROL-6 and zona pellucida protein NOAH-1 display aberrant annular localization and severe disorganization over the seam cells after NHR-23 depletion, while the expression of the adult-specific cuticle collagen BLI-1 is diminished and frequently found in patches. Consistent with these localization defects, the cuticle barrier is severely compromised when NHR-23 is depleted. Together, this work provides insight into how NHR-23 acts in the seam and hypodermal cells to coordinate aECM regeneration during development.

Experimental considerations for study of C. elegans lysosomal proteins.

Lysosomes are an important organelle required for the degradation of a range of cellular components. Lysosome function is critical for development and homeostasis as dysfunction can lead to inherited genetic disorders, cancer, and neurodegenerative and metabolic diseases. The acidic and protease-rich environment of lysosomes poses experimental challenges. Many fluorescent proteins are quenched or degraded, while specific red fluorescent proteins can be cleaved from translational fusion partners and accumulate. While studying MLT-11, a Caenorhabditis elegans molting factor that localizes to lysosomes and the cuticle, we sought to optimize several experimental parameters. We found that, in contrast to mNeonGreen fusions, mScarlet fusions to MLT-11 missed cuticular and rectal epithelial localization. Rapid sample lysis and denaturation were critical for preventing MLT-11 fragmentation while preparing lysates for western blots. Using a model lysosomal substrate (NUC-1), we found that rigid polyproline linkers and truncated mCherry constructs do not prevent cleavage of mCherry from NUC-1. We provide evidence that extended localization in lysosomal environments prevents the detection of FLAG epitopes in western blots. Finally, we optimize an acid-tolerant green fluorescent protein (Gamillus) for use in C. elegans. These experiments provide important experimental considerations and new reagents for the study of C. elegans lysosomal proteins.

Efficient generation of a single-copy eft-3p::TIR1::F2A:: BFP::AID*::NLS allele in the C. elegans ttTi5605 insertion site through recombination-mediated cassette exchange.

The auxin-inducible degron (AID) system is a widely used system to conditionally deplete proteins. Using CRISPR/Cas9-based genome editing in C. elegans, we recently generated a set of single-copy, tissue-specific and pan-somatic TIR1-expressing strains carrying a BFP reporter inserted in single-copy into two commonly used, well-characterized genetic loci. However, we were unable to obtain a strain carrying a pan-somatic eft-3p::TIR1::F2A::BFP::AID*::NLS transgene inserted into the chromosome II ttTi5605 insertion site. Using recombination-mediated cassette exchange (RMCE) we were able to efficiently obtain this knock-in. The resulting strain displayed equivalent depletion of an AID*::GFP reporter compared to our previously generated eft-3p::TIR1::F2A::BFP::AID*::NLS transgene knocked into the chromosome I ttTi4348 insertion site. This work highlights the power of RMCE for generating new reagents for the AID system and provides an eft-3p::TIR1::F2A::BFP::AID*::NLS allele on chromosome II which will simplify genetic crossing schemes when using the AID system.

An expanded auxin-inducible degron toolkit for Caenorhabditis elegans.

The auxin-inducible degron (AID) system has emerged as a powerful tool to conditionally deplete proteins in a range of organisms and cell types. Here, we describe a toolkit to augment the use of the AID system in Caenorhabditis elegans. We have generated a set of single-copy, tissue-specific (germline, intestine, neuron, muscle, pharynx, hypodermis, seam cell, anchor cell) and pan-somatic TIR1-expressing strains carrying a co-expressed blue fluorescent reporter to enable use of both red and green channels in experiments. These transgenes are inserted into commonly used, well-characterized genetic loci. We confirmed that our TIR1-expressing strains produce the expected depletion phenotype for several nuclear and cytoplasmic AID-tagged endogenous substrates. We have also constructed a set of plasmids for constructing repair templates to generate fluorescent protein::AID fusions through CRISPR/Cas9-mediated genome editing. These plasmids are compatible with commonly used genome editing approaches in the C. elegans community (Gibson or SapTrap assembly of plasmid repair templates or PCR-derived linear repair templates). Together these reagents will complement existing TIR1 strains and facilitate rapid and high-throughput fluorescent protein::AID tagging of genes. This battery of new TIR1-expressing strains and modular, efficient cloning vectors serves as a platform for straightforward assembly of CRISPR/Cas9 repair templates for conditional protein depletion.