Cancer and the breakdown of multicellularity: What Dictyostelium discoideum, a social amoeba, can teach us

The conserved cellular roles of CLN proteins: Novel insights from Dictyostelium discoideum

The neuronal ceroid lipofuscinoses (NCLs), collectively referred to as Batten disease, are a group of fatal neurodegenerative disorders that primarily affect children. The etiology of Batten disease is linked to mutations in 13 genes that encode distinct CLN proteins, whose functions have yet to be fully elucidated. The social amoeba Dictyostelium discoideum has been adopted as an efficient and powerful model system for studying the diverse cellular roles of CLN proteins. The genome of D. discoideum encodes several homologs of human CLN proteins, and a growing body of literature supports the conserved roles and networking of CLN proteins in D. discoideum and humans. In humans, CLN proteins have diverse cellular roles related to autophagy, signal transduction, lipid homeostasis, lysosomal ion homeostasis, and intracellular trafficking. Recent work also indicates that CLN proteins play an important role in protein secretion. Remarkably, many of these findings have found parallels in studies with D. discoideum. Accordingly, this review will highlight the translatable value of novel work with D. discoideum in the field of NCL research and propose further avenues of research using this biomedical model organism for studying the NCLs.

The first embryo, the origin of cancer and animal phylogeny. I. A presentation of the neoplastic process and its connection with cell fusion and germline formation

The decisive role of Embryology in understanding the evolution of animal forms is founded and deeply rooted in the history of science. It is recognized that the emergence of multicellularity would not have been possible without the formation of the first embryo. We speculate that biophysical phenomena and the surrounding environment of the Ediacaran ocean were instrumental in co-opting a neoplastic functional module (NFM) within the nucleus of the first zygote. Thus, the neoplastic process, understood here as a biological phenomenon with profound embryologic implications, served as the evolutionary engine that favored the formation of the first embryo and cancerous diseases and allowed to coherently create and recreate body shapes in different animal groups during evolution. In this article, we provide a deep reflection on the Physics of the first embryogenesis and its contribution to the exaptation of additional NFM components, such as the extracellular matrix. Knowledge of NFM components, structure, dynamics, and origin advances our understanding of the numerous possibilities and different innovations that embryos have undergone to create animal forms via Neoplasia during evolutionary radiation. The developmental pathways of Neoplasia have their origins in ctenophores and were consolidated in mammals and other apical groups.

Mfsd8 Modulates Growth and the Early Stages of Multicellular Development in Dictyostelium discoideum

MFSD8 is a transmembrane protein that has been reported to transport chloride ions across the lysosomal membrane. Mutations in MFSD8 are associated with a subtype of Batten disease called CLN7 disease. Batten disease encompasses a family of 13 inherited neurodegenerative lysosomal storage diseases collectively referred to as the neuronal ceroid lipofuscinoses (NCLs). Previous work identified an ortholog of human MFSD8 in the social amoeba D. discoideum (gene: mfsd8, protein: Mfsd8), reported its localization to endocytic compartments, and demonstrated its involvement in protein secretion. In this study, we further characterized the effects of mfsd8 loss during D. discoideum growth and early stages of multicellular development. During growth, mfsd8⁻ cells displayed increased rates of proliferation, pinocytosis, and expansion on bacterial lawns. Loss of mfsd8 also increased cell size, inhibited cytokinesis, affected the intracellular and extracellular levels of the quorum-sensing protein autocrine proliferation repressor A, and altered lysosomal enzyme activity. During the early stages of development, loss of mfsd8 delayed aggregation, which we determined was at least partly due to impaired cell-substrate adhesion, defects in protein secretion, and alterations in lysosomal enzyme activity. Overall, these results show that Mfsd8 plays an important role in modulating a variety of processes during the growth and early development of D. discoideum.

The Cellular and Developmental Roles of Cullins, Neddylation, and the COP9 Signalosome in Dictyostelium discoideum

Cullins (CULs) are a core component of cullin-RING E3 ubiquitin ligases (CRLs), which regulate the degradation, function, and subcellular trafficking of proteins. CULs are post-translationally regulated through neddylation, a process that conjugates the ubiquitin-like modifier protein neural precursor cell expressed developmentally downregulated protein 8 (NEDD8) to target cullins, as well as non-cullin proteins. Counteracting neddylation is the deneddylase, COP9 signalosome (CSN), which removes NEDD8 from target proteins. Recent comparative genomics studies revealed that CRLs and the CSN are highly conserved in Amoebozoa. A well-studied representative of Amoebozoa, the social amoeba Dictyostelium discoideum, has been used for close to 100 years as a model organism for studying conserved cellular and developmental processes owing to its unique life cycle comprised of unicellular and multicellular phases. The organism is also recognized as an exceptional model system for studying cellular processes impacted by human diseases, including but not limited to, cancer and neurodegeneration. Recent work shows that the neddylation inhibitor, MLN4924 (Pevonedistat), inhibits growth and multicellular development in D. discoideum, which supports previous work that revealed the cullin interactome in D. discoideum and the roles of cullins and the CSN in regulating cellular and developmental processes during the D. discoideum life cycle. Here, we review the roles of cullins, neddylation, and the CSN in D. discoideum to guide future work on using this biomedical model system to further explore the evolutionarily conserved functions of cullins and neddylation.

The Dynamics of Aerotaxis in a Simple Eukaryotic Model

In aerobic organisms, oxygen is essential for efficient energy production, and it acts as the last acceptor of the mitochondrial electron transport chain and as regulator of gene expression. However, excessive oxygen can lead to production of deleterious reactive oxygen species. Therefore, the directed migration of single cells or cell clumps from hypoxic areas toward a region of optimal oxygen concentration, named aerotaxis, can be considered an adaptive mechanism that plays a major role in biological and pathological processes. One relevant example is the development of O₂ gradients when tumors grow beyond their vascular supply, leading frequently to metastasis. In higher eukaryotic organisms, aerotaxis has only recently begun to be explored, but genetically amenable model organisms suitable to dissect this process remain an unmet need. In this regard, we sought to assess whether Dictyostelium cells, which are an established model for chemotaxis and other motility processes, could sense oxygen gradients and move directionally in their response. By assessing different physical parameters, our findings indicate that both growing and starving Dictyostelium cells under hypoxic conditions migrate directionally toward regions of higher O₂ concentration. This migration is characterized by a specific pattern of cell arrangement. A thickened circular front of high cell density (corona) forms in the cell cluster and persistently moves following the oxygen gradient. Cells in the colony center, where hypoxia is more severe, are less motile and display a rounded shape. Aggregation-competent cells forming streams by chemotaxis, when confined under hypoxic conditions, undergo stream or aggregate fragmentation, giving rise to multiple small loose aggregates that coordinately move toward regions of higher O₂ concentration. By testing a panel of mutants defective in chemotactic signaling, and a catalase-deficient strain, we found that the latter and the pkbR1^null exhibited altered migration patterns. Our results suggest that in Dictyostelium, like in mammalian cells, an intracellular accumulation of hydrogen peroxide favors the migration toward optimal oxygen concentration. Furthermore, differently from chemotaxis, this oxygen-driven migration is a G protein-independent process.

Altered protein secretion in Batten disease

The neuronal ceroid lipofuscinoses (NCLs), collectively known as Batten disease, are a group of neurological diseases that affect all ages and ethnicities worldwide. There are 13 different subtypes of NCL, each caused by a mutation in a distinct gene. The NCLs are characterized by the accumulation of undigestible lipids and proteins in various cell types. This leads to progressive neurodegeneration and clinical symptoms including vision loss, progressive motor and cognitive decline, seizures, and premature death. These diseases have commonly been characterized by lysosomal defects leading to the accumulation of undigestible material but further research on the NCLs suggests that altered protein secretion may also play an important role. This has been strengthened by recent work in biomedical model organisms, including Dictyostelium discoideum, mice, and sheep. Research in D. discoideum has reported the extracellular localization of some NCL-related proteins and the effects of NCL-related gene loss on protein secretion during unicellular growth and multicellular development. Aberrant protein secretion has also been observed in mammalian models of NCL, which has allowed examination of patient-derived cerebrospinal fluid and urine for potential diagnostic and prognostic biomarkers. Accumulated evidence links seven of the 13 known NCL-related genes to protein secretion, suggesting that altered secretion is a common hallmark of multiple NCL subtypes. This Review highlights the impact of altered protein secretion in the NCLs, identifies potential biomarkers of interest and suggests that future work in this area can provide new therapeutic insight.

Summary: This Review discusses work in different model systems and humans, examining the impact of altered protein secretion in the neuronal ceroid lipofuscinoses group of diseases to provide novel therapeutic insights.

Why have aggregative multicellular organisms stayed simple?

Multicellularity has evolved numerous times across the tree of life. One of the most fundamental distinctions among multicellular organisms is their developmental mode: whether they stay together during growth and develop clonally, or form a group through the aggregation of free-living cells. The five eukaryotic lineages to independently evolve complex multicellularity (animals, plants, red algae, brown algae, and fungi) all develop clonally. This fact has largely been explained through social evolutionary theory's lens of cooperation and conflict, where cheating within non-clonal groups has the potential to undermine multicellular adaptation. Multicellular organisms that form groups via aggregation could mitigate the costs of cheating by evolving kin recognition systems that prevent the formation of chimeric groups. However, recent work suggests that selection for the ability to aggregate quickly may constrain the evolution of highly specific kin recognition, sowing the seeds for persistent evolutionary conflict. Importantly, other features of aggregative multicellular life cycles may independently act to constrain the evolution of complex multicellularity. All known aggregative multicellular organisms are facultatively multicellular (as opposed to obligately multicellular), allowing unicellular-level adaptation to environmental selection. Because they primarily exist in a unicellular state, it may be difficult for aggregative multicellular organisms to evolve multicellular traits that carry pleiotropic cell-level fitness costs. Thus, even in the absence of social conflict, aggregative multicellular organisms may have limited potential for the evolution of complex multicellularity.