A somatic proteoglycan controls Notch-directed germ cell fate

C. elegans Germline as Three Distinct Tumor Models

Tumor cells display abnormal growth and division, avoiding the natural process of cell death. These cells can be benign (non-cancerous growth) or malignant (cancerous growth). Over the past few decades, numerous in vitro or in vivo tumor models have been employed to understand the molecular mechanisms associated with tumorigenesis in diverse regards. However, our comprehension of how non-tumor cells transform into tumor cells at molecular and cellular levels remains incomplete. The nematode C. elegans has emerged as an excellent model organism for exploring various phenomena, including tumorigenesis. Although C. elegans does not naturally develop cancer, it serves as a valuable platform for identifying oncogenes and the underlying mechanisms within a live organism. In this review, we describe three distinct germline tumor models in C. elegans, highlighting their associated mechanisms and related regulators: (1) ectopic proliferation due to aberrant activation of GLP-1/Notch signaling, (2) meiotic entry failure resulting from the loss of GLD-1/STAR RNA-binding protein, (3) spermatogenic dedifferentiation caused by the loss of PUF-8/PUF RNA-binding protein. Each model requires the mutations of specific genes (glp-1, gld-1, and puf-8) and operates through distinct molecular mechanisms. Despite these differences in the origins of tumorigenesis, the internal regulatory networks within each tumor model display shared features. Given the conservation of many of the regulators implicated in C. elegans tumorigenesis, it is proposed that these unique models hold significant potential for enhancing our comprehension of the broader control mechanisms governing tumorigenesis.

The matrisome landscape controlling in vivo germ cell fates

The developmental fate of cells is regulated by intrinsic factors and the extracellular environment. The extracellular matrix (matrisome) delivers chemical and mechanical cues that can modify cellular development. However, comprehensive understanding of how matrisome factors control cells in vivo is lacking. Here we show that specific matrisome factors act individually and collectively to control germ cell development. Surveying development of undifferentiated germline stem cells through to mature oocytes in the Caenorhabditis elegans germ line enabled holistic functional analysis of 443 conserved matrisome-coding genes. Using high-content imaging, 3D reconstruction, and cell behavior analysis, we identify 321 matrisome genes that impact germ cell development, the majority of which (>80%) are undescribed. Our analysis identifies key matrisome networks acting autonomously and non-autonomously to coordinate germ cell behavior. Further, our results demonstrate that germ cell development requires continual remodeling of the matrisome landscape. Together, this study provides a comprehensive platform for deciphering how extracellular signaling controls cellular development and anticipate this will establish new opportunities for manipulating cell fates.

Nuclear factor Y is a pervasive regulator of neuronal gene expression

Nervous system function relies on the establishment of complex gene expression programs that provide neuron-type-specific and core pan-neuronal features. These complementary regulatory paradigms are controlled by terminal selector and parallel-acting transcription factors (TFs), respectively. Here, we identify the nuclear factor Y (NF-Y) TF as a pervasive direct and indirect regulator of both neuron-type-specific and pan-neuronal gene expression. Mapping global NF-Y targets reveals direct binding to the cis-regulatory regions of pan-neuronal genes and terminal selector TFs. We show that NFYA-1 controls pan-neuronal gene expression directly through binding to CCAAT boxes in target gene promoters and indirectly by regulating the expression of terminal selector TFs. Further, we find that NFYA-1 regulation of neuronal gene expression is important for neuronal activity and motor function. Thus, our research sheds light on how global neuronal gene expression programs are buffered through direct and indirect regulatory mechanisms.

Niche cells regulate primordial germ cell quiescence in response to basement membrane signaling

Stem cell quiescence, proliferation and differentiation are controlled by interactions with niche cells and a specialized extracellular matrix called basement membrane (BM). Direct interactions with adjacent BM are known to regulate stem cell quiescence; however, it is less clear how niche BM relays signals to stem cells that it does not contact. Here, we examine how niche BM regulates Caenorhabditis elegans primordial germ cells (PGCs). BM regulates PGC quiescence even though PGCs are enwrapped by somatic niche cells and do not contact the BM; this can be demonstrated by depleting laminin, which causes normally quiescent embryonic PGCs to proliferate. We show that following laminin depletion, niche cells relay proliferation-inducing signals from the gonadal BM to PGCs via integrin receptors. Disrupting the BM proteoglycan perlecan blocks PGC proliferation when laminin is depleted, indicating that laminin functions to inhibit a proliferation-inducing signal originating from perlecan. Reducing perlecan levels in fed larvae hampers germline growth, suggesting that BM signals regulate germ cell proliferation under physiological conditions. Our results reveal how BM signals can regulate stem cell quiescence indirectly, by activating niche cell integrin receptors.

Conformations, interactions and functions of intrinsically disordered syndecans

Syndecans are transmembrane heparan sulfate proteoglycans present on most mammalian cell surfaces. They have a long evolutionary history, a single syndecan gene being expressed in bilaterian invertebrates. Syndecans have attracted interest because of their potential roles in development and disease, including vascular diseases, inflammation and various cancers. Recent structural data is providing important insights into their functions, which are complex, involving both intrinsic signaling through cytoplasmic binding partners and co-operative mechanisms where syndecans form a signaling nexus with other receptors such as integrins and tyrosine kinase growth factor receptors. While the cytoplasmic domain of syndecan-4 has a well-defined dimeric structure, the syndecan ectodomains are intrinsically disordered, which is linked to a capacity to interact with multiple partners. However, it remains to fully establish the impact of glycanation and partner proteins on syndecan core protein conformations. Genetic models indicate that a conserved property of syndecans links the cytoskeleton to calcium channels of the transient receptor potential class, compatible with roles as mechanosensors. In turn, syndecans influence actin cytoskeleton organization to impact motility, adhesion and the extracellular matrix environment. Syndecan clustering with other cell surface receptors into signaling microdomains has relevance to tissue differentiation in development, for example in stem cells, but also in disease where syndecan expression can be markedly up-regulated. Since syndecans have potential as diagnostic and prognostic markers as well as possible targets in some forms of cancer, it remains important to unravel structure/function relationships in the four mammalian syndecans.

Higher-order epistasis shapes natural variation in germ stem cell niche activity

To study how natural allelic variation explains quantitative developmental system variation, we characterized natural differences in germ stem cell niche activity, measured as progenitor zone (PZ) size, between two Caenorhabditis elegans isolates. Linkage mapping yielded candidate loci on chromosomes II and V, and we found that the isolate with a smaller PZ size harbours a 148 bp promoter deletion in the Notch ligand, lag-2/Delta, a central signal promoting germ stem cell fate. As predicted, introducing this deletion into the isolate with a large PZ resulted in a smaller PZ size. Unexpectedly, restoring the deleted ancestral sequence in the isolate with a smaller PZ did not increase-but instead further reduced-PZ size. These seemingly contradictory phenotypic effects are explained by epistatic interactions between the lag-2/Delta promoter, the chromosome II locus, and additional background loci. These results provide first insights into the quantitative genetic architecture regulating an animal stem cell system.

The Matrix Reloaded—The Role of the Extracellular Matrix in Cancer

As the core component of all organs, the extracellular matrix (ECM) is an interlocking macromolecular meshwork of proteins, glycoproteins, and proteoglycans that provides mechanical support to cells and tissues. In cancer, the ECM can be remodelled in response to environmental cues, and it controls a plethora of cellular functions, including metabolism, cell polarity, migration, and proliferation, to sustain and support oncogenesis. The biophysical and biochemical properties of the ECM, such as its structural arrangement and being a reservoir for bioactive molecules, control several intra- and intercellular signalling pathways and induce cytoskeletal changes that alter cell shapes, behaviour, and viability. Desmoplasia is a major component of solid tumours. The abnormal deposition and composition of the tumour matrix lead to biochemical and biomechanical alterations that determine disease development and resistance to treatment. This review summarises the complex roles of ECM in cancer and highlights the possible therapeutic targets and how to potentially remodel the dysregulated ECM in the future. Furthering our understanding of the ECM in cancer is important as the modification of the ECM will probably become an important tool in the characterisation of individual tumours and personalised treatment options.

Analysis of the C. elegans Germline Stem Cell Pool

The Caenorhabditis elegans germline is an excellent model for studying the genetic and molecular regulation of stem cell self-renewal and progression of cells from a stem cell state to a differentiated state. The germline tissue is organized in an assembly line with the germline stem cell (GSC) pool at one end and differentiated gametes at the other. A simple mesenchymal niche caps the GSC pool and maintains GSCs in an undifferentiated state by signaling through the conserved Notch pathway. Notch signaling activates transcription of the key GSC regulators lst-1 and sygl-1 proteins in a gradient through the GSC pool. LST-1 and SYGL-1 proteins work with PUF RNA regulators in a self-renewal hub to maintain the GSC pool. In this chapter, we present methods for characterizing the C. elegans GSC pool and early stages of germ cell differentiation. The methods include examination of germlines in living and fixed worms, cell cycle analysis, and analysis of markers. We also discuss assays to separate mutant phenotypes that affect the stem cell vs. differentiation decision from those that affect germ cell processes more generally.

Formation of benign tumors by stem cell deregulation

Within living organisms, stem cells respond to various cues, including to niche signals and growth factors. Niche signals originate from the stem cell's microenvironment and promote the undifferentiated state by preventing differentiation, allowing for stem cell self-renewal. On the other hand, growth factors promote stem cell growth and proliferation, while their sources comprise of a systemic input reflecting the animal's nutritional and metabolic status, and a localized, homeostatic feedback signal from the tissue that the stem cells serve. That homeostatic signal prevents unnecessary stem cell proliferation when the corresponding differentiated tissues already have optimal cell contents. Here, we recapitulate progresses made in our understanding of in vivo stem cell regulation, largely using simple models, and draw the conclusion that 2 types of stem cell deregulations can provoke the formation of benign tumors. Namely, constitutive niche signaling promotes the formation of undifferentiated "stem cell" tumors, while defective homeostatic signaling leads to the formation of differentiated tumors. Finally, we provide evidence that these general principles may be conserved in mammals and as such, may underlie benign tumor formation in humans, while benign tumors can evolve into cancer.