Molecular Biology

Utilizingresources available from the mother's body to guarantee healthy offspring growth is the fundamental reproductive strategy. Recently, we showed that a class of the largest extracellular vesicles known as exophers, which are responsible for the removal of neurotoxic components from neurons (Melentijevic et al., 2017) and damaged mitochondria from cardiomyocytes (Nicolás-Ávila et al., 2020), are released by the Caenorhabditis elegans hermaphrodite body wall muscles (BWM), to support embryonic growth (Turek et al., 2021). Employing worms expressing fluorescent reporters in BWM cells, we found that exopher formation (exophergenesis) is sex-specific and fertility-dependent. Moreover, exophergenesis is regulated by the developing embryo in utero, and exophers serve as transporters for muscle-generated yolk proteins, which can be used to nourish the next generation. Given the specific regulation of muscular exophergenesis, and the fact that muscle-generated exophers are much larger than neuronal ones and have different targeting, their identification and quantification required a modified approach from that designed for neuronal-derived exophers (Arnold et al., 2020). Here, we present a methodology for assessing and quantifying muscle-derived exophers that can be easily extended to determine their function and regulation in various biological contexts.

Graphical abstract

Nucleocytoplasmic transport deficits are suggested to play a role in neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). Given the importance and complexity of this process, understanding when these aberrations occur and which pathways are involved is of great importance. Here, we make use of CRISPR-Cas9 technology to design cell lines stably expressing fluorophore proteins shuttling between the nucleus and cytoplasm by karyopherins of choice. To validate this protocol, we measured an ALS-associated nucleocytoplasmic transport pathway in the presence of the disease-associated peptide poly-PR. This technique allows measuring a particular active nucleocytoplasmic transport pathway in intact cells in a neurodegenerative disease-associated context. Moreover, these experiments can be performed without the need for expensive equipment and have the potential to be upscaled for high-throughput screening purposes.

Many proteins appear exclusively nuclear at steady-state but in fact shuttle continuously back and forth between the nucleus and the cytoplasm. For example, nuclear RNA-binding proteins (RBPs) often accompany mRNAs to the cytoplasm, where they can regulate subcellular localization, translation and/or decay of their cargos before shuttling back to the nucleus. Nucleocytoplasmic shuttling must be tightly regulated, as mislocalization of several RBPs with prion-like domains such as FUS and TDP-43 causes the cytoplasmic accumulation of solid pathological aggregates that have been implicated in neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Traditionally, interspecies heterokaryon assays have been used to determine whether a nuclear protein of interest shuttles; those assays are based on the fusion between donor and recipient cells from two different species (e.g., mouse and human), which can be distinguished based on different chromatin staining patterns, and detecting the appearance of the protein in the recipient nucleus. However, identification of heterokaryons requires experience and is prone to error, which makes it difficult to obtain high-quality data for quantitative studies. Moreover, transient overexpression of fluorescently tagged RBPs in donor cells often leads to their aberrant subcellular localization. Here, we present a quantitative assay where stable donor cell lines expressing near-physiological levels of eGFP-tagged RBPs are fused to recipient cells expressing the membrane marker CAAX-mCherry, allowing to readily identify and image a large number of high-confidence heterokaryons. Our assay can be used to measure the shuttling activity of any nuclear protein of interest in different cell types, under different cellular conditions or between mutant proteins.