There is no simple way to make a brain, even in a creature as small as a fruit fly. As an embryonic fly develops into adulthood, its central nervous system (CNS) expands almost 100-fold in mass. Neuronal, glial, immune, and vascular cells—in both the CNS and the peripheral nervous system (PNS)—must work in harmony to build the structures responsible for controlling movement and behavior. Since structure dictates function, the size and shape of the CNS must be tightly regulated, but the genes and pathways involved in the process have yet to be fully described.

In a recent study published in the September issue of G3: Genes|Genomes|Genetics, Lacin et al. use the power of forward genetics in Drosophila larvae to identify genes controlling nervous system shape. Using the robust genetic manipulation toolkit available in Drosophila, they further identify a glial subtype-specific molecular profile that functionally subdivides glia along the peripheral-central axis.

Their screen used the classic mutagenesis agent ethyl methanesulfonate (EMS) to randomly introduce mutations, generating more than 12,000 mutant lines that carried mutations specifically on the second chromosome. The authors screened for larval mutants with dramatically altered CNS shapes, sorting them into three categories: widened, elongated, or misshapen. Through a combination of genetic mapping, complementation analysis, and whole genome sequencing, they identified 50 mutant alleles across 17 genes that encode transcription factors, enzymes, signaling receptors, tumor suppressors, and basement membrane proteins.

Four of the mutant alleles were found in the senseless-2 (sens-2) gene, which encodes a zinc-finger domain transcription factor; these alleles caused massive elongation of the ventral nerve cord (the Drosophila equivalent to the spinal cord) that manifested very early in the first-instar larvae (see Figure 1). To understand the cellular basis for the mutant sens-2 CNS elongation phenotype, the authors generated an antibody against the Sens-2 protein and found it localized to most glia on peripheral nerves—but not in any CNS glial cells.

To determine whether sens-2’s role in determining ventral nerve cord length was specific to its presence in peripheral glia, the authors selectively knocked down its expression in those cells using the Gal4-UAS system. They found that sens-2 expression in peripheral glia is necessary to control CNS structure, and loss in those cells accounted for the observed elongation phenotype. Restoration of sens-2 expression rescued the elongation phenotype.

Lacin et al. were able to establish sens-2 as a marker distinguishing specific glial subtypes along the CNS-PNS axis with a profound impact on gross nervous system structure. In the future, the authors aim to investigate transcriptional targets of sens-2, which could help illuminate mechanisms governing glial development and differentiation in the PNS.

In recent years, the use of expensive -omics technologies to discover cellular heterogeneity at scale has become quite popular in neuroscience research, and the genes identified in these studies need validation and characterization. Here, Lacin et al. present a powerful demonstration that classical genetic studies in invertebrate model systems are still effective at powering neurogenetics and cellular heterogeneity research—at a fraction of the cost.

We’re taking time to get to know the members of the GSA’s Early Career Scientist Committees. Join us to learn more about our early career scientist advocates.

Daniela C. Soto
Communication and Outreach Subcommittee
University of California, Los Angeles

Research Interest

What genetic changes underlie our uniquely human traits and behaviors? In the last couple million years of evolution, some fascinating changes took place that make us who we are. My quest as a scientist is to uncover the genes responsible for these phenotypic changes, including their functions, their regulation, and their history. I am a bioinformatician that analyzes tons and tons of data and uses computation, statistics, and biology to make sense of it. During my PhD, I analyzed thousands of human and great ape genomes in search of neurodevelopmental genes that underwent dramatic or subtle changes during the last six million years of evolution, leading to the expansion of the neocortex. This research not only sheds light on our evolutionary history but also has clinical and therapeutical implications. Some neurodevelopmental genes are associated with neurodiversity traits, and their characterization will help us better understand the underlying architecture of the neurodiverse brain, leading to more effective medical and societal approaches.

But we’ve learned many of our human characteristics are not so unique. Some complex behaviors emerged a long time ago and are shared with our fellow mammals. We can learn a lot, for example, about human attachment from the prairie vole, a rodent that has “pair-bonding,” a scientific term akin to love. During my PhD, I was part of the reconstruction of the prairie vole genome, which will be used to look for the genomic changes that led to pair-bonding. Not only can rodents teach us about love, but they can also help us learn about our minds too. In my incoming postdoctoral position, I will use the mouse as a model organism to study depression, one of the most complex and prevalent neuropsychiatric disorders in our modern society. Depression research has the potential to impact millions of lives down the line by enabling better diagnosis and novel therapeutics. Considering the influx of data enabled by the ever-evolving sequencing technologies, there is no better time to interrogate our genomes.

As a PhD-trained scientist, you have many career options. What interests you the most?

My dream is to lead my own research group. Per Richard Feynman’s advice, I have a series of favorite questions always in my mind that I want to tackle. I am especially interested in the workings of our brain and the interplay between genetics and complex human traits and behaviors. My focus is on humans and other great apes, but I believe in the power of animal models and “natural laboratories” to deepen our knowledge of our mammal brains. I am also interested in leveraging the newest technologies; I want to use state-of-the-art genomic sequencing to explore the darker regions of the genome, including structurally variant loci and repeats that have been overlooked before due to technical limitations.

I have a deep admiration for academia, the pursuit of knowledge, the development of innovations, and the training of new generations. That being said, I am always amazed by the wide array of biotechnological and biomedical research taking place in the United States and its tangible impact in society. We saw it firsthand, for example, with the development of the mRNA vaccines during the COVID pandemic. Considering the cutting-edge research occurring in the private sector, I can see myself answering my favorite questions in that context as well.

In addition to your research, how do you want to advance the scientific enterprise?

For me, pursuing an academic career has a scope beyond science; it is also a matter of representation. I identify as a Latino woman. (I am Chilean!) Unfortunately, women are underrepresented in bioinformatics and Latino women even more so. In my journey to become a principal investigator, I want to openly advocate for a more welcoming field for young women and other underrepresented groups. I am deeply thankful for the role models that have cleared the path for me in this field, and I strive to pay it back by advocating for the next generations.

Besides my advocacy within academia, I also believe it is important to make science (and genetics) approachable and entertaining for broader audiences. One of my hobbies is listening and reading pop-science books and podcasts. This type of content has tremendous potential to introduce scientific ideas and discoveries to people who otherwise would not have that opportunity. During my academic career, I aim to become an excellent science communicator and writer, using approachable language and entertaining narratives to drive passions for science in young minds of diverse backgrounds. In the long run, I believe this simple approach can attract a more diverse pool of people to our field.

As a leader within the Genetics Society of America, what do you hope to accomplish?

I applied to the ECLP because I admire how the program provides complimentary training that scientists might not get otherwise. Academic research often keeps us extremely busy, and we might neglect to develop soft skills that would help our career and enable us to self-actualize. When joining the program, I had the simplest of goals: meet other people passionate about science communication and learn from them. I was not wrong. This was the right place. In my team, there is a group of people generating social media science content about the many funny little details of the wet lab, providing entertainment while demystifying science research for the general public. Others are writing blog posts or generating databases with home experiments for everyone to try from the comfort of their homes.

My own passions align with the team. I want to share the awe of science with general audiences. I believe that if we share science broadly, it will reach the ears of curious kids from historically marginalized groups who might see for the first time a place for themselves in STEM. But science, especially genetics, is hard to share with general audiences, let alone kids! How can we make genetics approachable and fun for kids and teenagers? My goal as a member of the Outreach and Communication Subcommittee is to develop content and material to tackle this issue. I am generating educational science content for families and kids to introduce them to genetics concepts, like illustrations and coloring pages, to provide as resources for the community.  

Previous leadership experience

• Student representative in the Integrative Genetics and Genomics Graduate Group at UC Davis as a vice-chair and mentoring coordinator, as well as member of the Diversity, Equity, and Inclusion committee, where we developed and analyzed a survey assessing diversity and climate in our graduate group.
• Mentor at the UC Davis Biochemistry and Molecular Medicine-Sacramento Charter High School summer research program.
• Volunteer in charge of generating graphics material for the Chilean Bioinformatics Symposium, the Northern California Computational Biology Symposium, the Chilean Society of Plant Genetics, and several other conferences and scientific communities.
• Instructor and panelist in initiatives to provide bioinformatics training to students in the United States and abroad, such as the Central Asia Pacific Genomics Workshop and the California Undergraduate Bioinformatics Virtual Conference.
• Volunteer in the Chilean chapter of Girls in Tech.

A new associate editor is joining GENETICS. We’re excited to welcome Xiajing Tong to the editorial team.

Xiajing Tong
Associate Editor

Xiajing Tong obtained her BS from the University of Science and Technology of China and earned her PhD from the Chinese Academy of Sciences. She conducted her postdoctoral training with Joshua Kaplan at Massachusetts General Hospital and Harvard Medical School, focusing on the regulation of synaptic transmission by autism-associated genes using C. elegans as a model organism. Currently, she is an Associate Professor of Biology at ShanghaiTech University. Her lab utilizes C. elegans as a model organism, along with studies in mammals, to investigate how sex-specific synaptic transmission and neural circuits mediate sex-differential physiology and behaviors.

AP-2 transcription factors, which control sleep in flies and worms, are confirmed to do the same in mammals.

Humans are not alone in their deep need for sleep. Almost all animals, even tiny nematode worms and fruit flies, suffer when deprived of their Z’s, but little is known about how sleep is controlled. New work published in GENETICS advances our understanding of this mysterious physiological state by pinpointing a key gene family that affects sleep architecture.

Two research groups independently conducted parallel studies on neural crest-derived AP-2 transcription factors in mice. Their work demonstrates that the associated genes play a conserved role in mammalian sleep, though there is some evolutionary divergence and added complexity in mammals compared to invertebrates.

From Worms to Mice

Henrik Bringmann leads the Max Planck Research Group on Sleeping and Waking. In 2013, his laboratory screened C. elegans worms for mutations affecting sleep. AP-2 deletion mutants showed no detectable sleep, suggesting these transcription factors are important regulators of the process. In 2016, Bringmann led another study showing that AP-2 transcription factors are also needed for sleep in Drosophila. 

“Sleep appears to have evolved at least 600 million years ago and has been conserved since then, which suggests that many fundamental principles of sleep regulation are conserved,” said Bringmann. “This said, sleep is more complex in humans than in worms or flies, so we need mammalian models to understand additional levels of complexity.”

The researchers’ next step was to establish whether a similar sleep-regulating role is played by related genes in mammals, the TFAP2 genes. In their most recent study, the team generated mice that were heterozygous knockouts for either Tfap2a or Tfap2b and compared each mutant to wild-type littermates. In addition to analyzing sleep duration and brain waves, they examined symptoms of sleep deprivation such as memory loss and stress resistance.

Since the AP-2 transcription factors promote sleep in invertebrates, researchers initially expected that the equivalent genes would also promote sleep in mice. They were therefore surprised to find that while losing Tfap2bfunction reduced both sleep quality and quantity, Tfap2a mutants slept for the typical amount of time and the quality of their sleep was actually higher than wild-type. This result suggests that the function of AP-2 transcription factors has diverged over the course of evolution, perhaps to allow sleep quality to be fine-tuned in either direction.

Working Backwards from Human Disorders

Meanwhile, another research group also published an article in the same issue of GENETICS examining the effects of two specific TFAP2B mutations in mice. Yu Hayashi of Kyoto University  and University of Tsukuba said his lab was inspired to pursue this avenue of research by a paper in PNAS. That study described multiple human families with mutations in TFAP2B that are associated with a rare disorder known as Char syndrome. Two of these families exhibited symptoms of disordered sleep, including sleepwalking and extremely shortened sleep.

“We were astonished to read about a family that slept just two to three hours per night and had no signs of fatigue. We thought that maybe analyzing this gene can help us understand what sleep is for in the first place,” said Hayashi. “We even thought that maybe this mutation might somehow substitute for the function of sleep. Could there be a way to help humans need less sleep?”

To answer these questions, Hayashi’s team set out to replicate those families’ specific TFAP2B point mutations in mouse models. They measured both how long the mice stayed awake and the duration of different sleep phases compared to wild-type mice and heterozygous knockouts. The results showed that TFAP2B helps determine the amount of nonrapid eye movement sleep (NREMS). However, the effects of the point mutations in mice did not match the symptoms observed in humans—the mice did not sleepwalk or show dramatically shortened sleep.

In the mice that carried the same mutation as the human family with short sleep, female mice showed fragmented sleep, while male mice did not. This result was unforeseen because there was no reported gender difference in the human family. “It was surprising for me to see the gender difference,” said lead author Ayaka Nakai, a graduate student in the Hayashi lab. It is possible that future research may uncover sex differences in sleep regulation.

In terms of generating a model of fatigue-free short sleep, the results did not match their initial hopes—sleep was reduced overall in heterozygous mutant mice, but it was generally fragmented rather than shortened. However, the results clearly established TFAP2B’s important role in sleep architecture and laid the groundwork for learning more about how sleep works.

Next Steps

Ultimately, said Bringmann, “it was satisfying to hear that both approaches converged on the same conclusions regarding TFAP2B’s role in sleep.” Together, these two papers establish that AP-2 transcription factors contribute to sleep control in mammals—just as they do in flies and worms.

The gene’s evolutionary conservation is a key lesson from the study, said Nakai.

For Nakai and Yu, the next research step is creating knockdown mice that have Tfap2b downregulated only in the nervous system. This will allow them to observe the neural effect of a homozygous loss-of-function mutation, which is developmentally lethal if the knockout is genome-wide. Studying a homozygous knockdown may offer clearer insights into how the gene affects neuron specification and activity.

Next, Bringmann is interested in examining other mammalian AP-2 paralogs, but he says invertebrates will continue to be important for identifying other genes that contribute to sleep regulation. “Going back and forth between different models will be the future of sleep research for the next years, and this will be facilitated by looking at homologous genes and conserved principles.”

CITATION:

Functional Divergence of Mammalian TFAP2a and TFAP2b Transcription Factors for Bidirectional Sleep Control

Yang Hu, Alejandra Korovaichuk,  Mariana Astiz, Henning Schroeder, Rezaul Islam, Jon Barrenetxea, Andre Fischer, Henrik Oster and Henrik Bringmann

GENETICS 2020 216: 735-752. 

https://doi.org/10.1534/genetics.120.303533

Sleep Architecture in Mice Is Shaped by the Transcription Factor AP-2β

Ayaka Nakai, Tomoyuki Fujiyama, Nanae Nagata, Mitsuaki Kashiwagi, Aya Ikkyu, Marina Takagi, Chika Tatsuzawa, Kaeko Tanaka, Miyo Kakizaki, Mika Kanuka, Taizo Kawano, Seiya Mizuno, Fumihiro Sugiyama, Satoru Takahashi, Hiromasa Funato, Takeshi Sakurai, Masashi Yanagisawa and Yu Hayashi

GENETICS 2020 216:753-764

https://doi.org/10.1534/genetics.120.303435

NUN bodies in C. elegans offer clues about nervous system differentiation.

One of the most active areas of research today explores the differences between individual cell types and how cells become differentiated. This specialization of cell types relies on changes in gene expression, but how those changes are orchestrated remains unknown.

Now, research published in GENETICS has identified a new organelle inside the nucleus of nerve cells in the nematode worm, C. elegans. Named “nuclear nervous system-specific bodies,” or NUN bodies, these organelles only occur in the cells of the nervous system, report Pham et al. The exact function of the NUN bodies isn’t known yet, but because these bodies are highly specific to nervous system cells, they may lead researchers to a better understanding of how these important cell types differentiate.

In their quest to unravel the mechanisms behind neuronal differentiation, the team of researchers, based at Columbia University and led by Oliver Hobert, take advantage of the simplicity of the C. elegans experimental system. The developmental fate of each of the worm’s cells is known, and the total number of distinct cell types in C. elegans is small, making it ideal for investigating tissue-specific features.

During the process of differentiation, the cells of the nervous system, known as neurons and glia, develop an unusually compact nucleus with a “speckly” appearance. This so-called granular nucleoplasm intrigued the team because there are very few morphological features that are common to all nervous system cells but absent from any other cell types.

“Why is there this nervous system specificity that’s so striking?” says first author Kenneth Pham, who was a post-baccalaureate student working in Hobert’s lab at the time. “That was the diving off point.”

Neuronal nucleus with multiple NUN bodies, as visualized with differential interference contrast microscopy. Image courtesy of the Hobert lab.‌

Inside the nucleus is the “control center” of the cell, where genes are turned on and off, determining the fate of the cell. Various organelles help keep things running smoothly, and it stands to reason that cells headed for different fates might have different nuclear bodies managing their gene expression patterns. To investigate, Pham set out to characterize the granules present in the nuclei of C. elegans nervous system cells.

To start with, he tested whether the granules were indeed phase-separated membraneless organelles by showing that, chemically, they behaved the same way as nucleoli. Like these known membraneless organelles, the nuclear granules dissolved in a hypertonic salt solution, and heat caused them to grow larger and decrease in number.

To make sure they weren’t just extra nucleoli, he tagged nucleolus-specific markers with fluorescent dye, revealing that each neuron contained only one nucleolus. The newly identified bodies were not nucleoli.

Pham set to work, searching for what else the granules might be. Although none are as cell type-specific as NUN bodies, a number of nuclear bodies have been described previously, such as splicing speckles, paraspeckles, PcG bodies, PML bodies, gems, stress-induced nuclear bodies, and clastosomes. He combed through the C. elegans genome, looking for any homologs of the proteins that make up these nuclear organelles in mammalian cells. By attaching a fluorescent tag to each protein, he could track whether the tag followed the NUN bodies in worm neurons. None of the six organelle components that he found in the worm genome appeared to colocalize with NUN bodies. Pham then used loss-of-function mutations in all of the organelle constituent genes that he could find, and none of them had any effect on the NUN bodies.

So, these new structures seem to be unrelated to known nuclear bodies, and what NUN bodies are made of remains a mystery. But Pham took a stab at identifying their function. In a type of cell called “canal associated neurons,” or CANs, the NUN bodies change over the course of development. At first, there may be five or six NUN bodies in the nucleus. As the worms develop, that drops to only two or three NUN bodies per nucleus, and they are enlarged.

To find out how this was happening, Pham zeroed in on genes that play a role in CAN maturation, knocking them out and looking for changes in NUN body behavior. A mutation that reduced the activity of a transcription factor, ceh-10, stopped the rearrangement of NUN bodies normally seen during development.

Ultimately, NUN bodies offer a tantalizing hint to how nervous system cells might differentiate, with lots of possible avenues for learning more. For instance, Pham began screening a mutant library to find genes needed for making NUN bodies, but so far, nothing has turned up. He’s moved on from Columbia now, beginning an MD/PhD program at the University of Pennsylvania, but he hopes someone picks up the project where he left off. “Please work on the mystery, because I really want to know.”

CITATION
A nervous system-specific subnuclear organelle in Caenorhabditis elegans
Kenneth Pham, Neda Masoudi, Eduardo Leyva-Díaz, Oliver Hobert
GENETICS, Volume 217, Issue 1, January 2021, iyaa016,

New Faculty Profiles allow GSA members who are establishing their first labs to introduce themselves to our wider community. If you’d like to submit your profile, please complete this form.

Aakanksha Singhvi

Assistant Member
Division of Basic Sciences
Fred Hutchinson Cancer Research Center
Lab website

Briefly describe the ongoing and expected research projects as your lab gets up and running.

In a nutshell, “Glia-Neuron interactions,” in health and disease. My laboratory wants to decode the molecular conversations between glia and neurons, the two major cell types of our nervous system. We know that disrupted interactions between glia and neurons are an underlying factor in many neurological disorders of development (e.g. Autism), function (e.g. sensory or cognitive impairments) and aging (e.g. Alzheimer’s). However, mechanisms underlying these interactions are still not well-defined at molecular resolution. We are excited to explore how glia regulate sensory perception, neuronal physiology, neural circuit activity, memory formation, and animal behavior. We use C. elegans as a genetic model because its nervous system has some unique features that make it an especially powerful system to address these questions in vivo.

How has being a member of GSA helped you advance in your career? Why do you think societies like GSA are important? 

My association with GSA started as a graduate student and has helped me grow as a scientist throughout my career, including now in setting up as a new PI. My first C. elegans paper was in GENETICS, and my professional networks have grown through GSA-sponsored worm meetings. As a postdoc, I served as the first Trainee Representative on the GSA publication committee and the TAGC 2016 organizing committee. I have also known of GSA’s support for trainees and advocacy for basic research. As I start a lab, I realize that these experiences have influenced my perspectives on many facets of science—research, administration, mentoring, publishing, and career paths. As a new PI, I am now organizing a Glia workshop at the 2019 C. elegans meeting, and I look forward to continued association with GSA members and trainees.

Are you looking to recruit students and/or postdocs? If so, please describe and be sure to also post the opportunity to GeneticsCareers.org. 
I am always excited to meet students and postdocs who share our wonder for glia-neuron interactions in the nervous system. My lab accepts graduate students from the fantastic inter-departmental programs between Fred Hutch and the University of Washington. Anyone who is interested, please email me! By the way, Seattle is a gorgeous city with an awesome and diverse scientific community.

What is your favorite thing about science or about your work? 
Everything about biology is so cool! As experiences, the thrill of thinking, creating, and exploring this wonder freely with fellow scientists, as well as the “aha” moment when a puzzle clicks into place, are perhaps my favorite things about this job. Every new data point or mutant analysis is a dopamine fix.
As a biological question, I cannot stop being in awe of the nervous system! How do we sense the world around us, make memories of this rich and complex information? How do cells molecularly talk to each other; what do they say? What goes wrong in neural disease or aging? There are so many awesome puzzles to solve that one lifetime feels too little. So, for now, we are focused on finding everything we can about the most mysterious of cells in our brain—glia.

What do you like to do when you’re not at work? 
My other passion is dancing, lindy-hop to dandiya! Throughout my postdoc I actively performed Bharatanatyam, an Indian classical dance form I have trained in, with a dance group in NYC. In a way, I find it parallels biology research. One spends long years passionately mastering the technique, the beauty of its structure and rules, and existing knowledge. Then, you start thinking outside the box, pushing boundaries with your creativity and unique thinking. Each performance takes practice and has a presentation structure and a story to tell. And it is pure joy when audiences engage with your story or findings!

Previous training experiences:

• BS, Gujarat University, India
• MS Fellow, National Center for Biological Sciences, India
• PhD, University of California Berkeley (with Dr. Gian Garriga)
• Postdoctorate, The Rockefeller University (with Dr. Shai Shaham)

Apoptotic pathway promotes asymmetric cell division during C. elegans development.

Cell division doesn’t always produce identical daughter cells; often, the demands of multicellular development require cells to split into two quite different daughters with quite different fates. These “asymmetric” divisions are needed so that cells can differentiate and specialize, and some cells are even programmed to die shortly after their creation to ensure the proper function of the organism as a whole. In GENETICS, Mishra et al. found that the apoptotic cell death pathway regulates asymmetric division in the nematode worm Caenorhabditis elegans.

C. elegans is an exceptionally useful model organism for studying development because the fate of each of its relatively few cells can be precisely mapped. Many of the cells destined for death in the worm are actually the product of unequal division into a larger cell that differentiates and a smaller cell that undergoes apoptosis. The authors of the new report had previously studied the parent of one such uneven division, a cell known as the embryonic neurosecretory motor neuron neuroblast. They found that in the parental neuroblast, there is a gradient of activated CED-3 caspase, an executioner of apoptosis. This gradient leads to more active CED-3 caspase in the smaller daughter cell, which helps facilitate its death.

The authors wondered whether this CED-3 caspase gradient might be a general phenomenon in asymmetric divisions, so in the GENETICS report they studied another cell that divides into a large cell that survives and a smaller cell that dies: the QL.p neuroblast. The authors identified a similar CED-3 caspase gradient in these cells, showing that the phenomenon is indeed somewhat general.

Then, the authors used loss-of-function mutants to explore the role of the CED-3 caspase and its related pathways in the asymmetric division of QL.p. They found that disrupting the cell death pathway impaired the ability of QL.p to divide asymmetrically and could impact the fate of the daughter cells—often giving rise to two living cells, rather than one that lives and one that dies. Mutations in other genes associated with asymmetric division, like pig-1, also affected the fate of the daughter cells but did not change the CED-3 caspase gradient.

The authors explain that, in QL.p, two molecular gradients are simultaneously created: one of “mitotic potential,” which is normally passed on to the larger daughter to facilitate its differentiation, and one of “apoptotic potential,” which is passed on to the smaller daughter and promotes its death. Although the details of these “potentials” are not yet understood, this separation within the parental cell seems crucial for ensuring that each cell reaches its proper endpoint.

Although caspases are well-known for their role in apoptosis, it is particularly noteworthy that mutations in CED-3 caspase do not only affect the ability of the small daughter cell to die. CED-3 caspase also appears to function in the division of the parental cell, suggesting a more complicated role of this molecular executioner during development.

CITATION:

Caenorhabditis elegans ced-3 Caspase Is Required for Asymmetric Divisions That Generate Cells Programmed To Die

Nikhil Mishra, Hai Wei, Barbara Conradt

GENETICS November 1, 2018 vol. 210 no. 3 983-998; https://doi.org/10.1534/genetics.118.301500

Mitochondria cell-autonomously regulate the secretion of neuropeptides in C. elegans.

Neurons are hard-working cells that need a lot of energy to do their jobs, so it’s no surprise that they are highly dependent on their mitochondria to function properly. Yet these organelles do much more for cells than simply produce energy. In GENETICS, Zhao et al. report on how mitochondria are directly involved in regulating the secretion of neuropeptides.

In a previous paper, the authors found that disruption of the gene ric-7 caused decreased neuropeptide secretion and locomotion defects in C. elegans—but the mechanism underlying these phenotypes was unclear because little was known about the function of ric-7. After another group demonstrated that ric-7 is required for the long-distance transportation of mitochondria from the neuron’s cell body into its axons, Zhao and colleagues hypothesized that disrupting mitochondrial transport might be the mechanism by which ric-7 defects cause neuronal phenotypes.

To test this, the authors expressed a chimeric kinesin construct, kin-Tom7, in ric-7 mutant axons. This chimera is a kinesin protein fused to a mitochondrial membrane protein. A prior study showed that kin-Tom7 restored transport of mitochondria to the axons of ric-7 mutants but did not affect other cellular functions. The authors showed that kin-Tom7 also rescues the ric-7 mutation-impaired neuropeptide secretion and locomotion defects, suggesting that improper mitochondrial transport was indeed the cause of neuronal defects in ric-7 mutants.

Because mitochondria are involved in so many cellular processes, the authors wondered which function(s) of axonal mitochondria might be necessary for neuropeptide secretion. Using selected mutant worms, they found that disrupting oxidative phosphorylation decreased neuropeptide secretion—but impairing mitochondrial calcium uptake didn’t.

Impaired oxidative phosphorylation can cause increased levels of reactive oxygen species (ROS) and hypoxia, so the authors suspected that these stress states might be involved in neuropeptide secretion. Indeed, they found that impairing the function of ROS detoxification enzymes—an alternative way to increase ROS—and growth in hypoxic conditions both led to decreased neuropeptide secretion.

Further investigation showed that the effects of axonal mitochondria on neuropeptide secretion were mediated by the hypoxia-inducible factor HIF-1, which is central to the response to hypoxia in C. elegans. Worms with constitutively active HIF-1 had lower secretion of neuropeptides, but this could be reversed by turning HIF-1 “off” again through other transgenic manipulations. Crucially, combining the constitutively active HIF-1 with the ric-7 defect in a double mutant had no additional phenotypic effects, suggesting the two proteins act in the same pathway. Consistent with this, hif-1 null mutations restored neuropeptide secretion in ric-7 mutants.

Together, these results support the idea that mitochondria regulate neuropeptide secretion in part by modulating ROS production and the hypoxic stress response. These findings could provide a mechanism by which the biochemical conditions within a neuron alter communication between neurons to trigger more widespread changes in the nervous system.

CITATION:

Axonal Mitochondria Modulate Neuropeptide Secretion Through the Hypoxic Stress Response in Caenorhabditis elegans

Tongtong Zhao, Yingsong Hao, Joshua M. Kaplan

Genetics September 2018 210: 275-285; https://doi.org/10.1534/genetics.118.301014

http://www.genetics.org/content/210/1/275

Food-deprivation inhibits the stress-induced sleep response in C. elegans.

For many animals, the essential physiological drives of sleep and food are intimately linked. You might have noticed this if you’ve ever stayed up far too late and found yourself craving a snack. Yet because it’s impossible for most animals to eat and sleep at the same time, these two biological necessities must compete with each other. In GENETICS, Goetting et al. report what happens when the need for sleep and the need for food come into direct conflict in the model nematode Caenorhabditis elegans.

When C. elegans is exposed to a stressful condition like high heat or UV radiation, it goes into a quiescent state called stress-induced sleep (SIS). Sleeping worms cease moving and feeding, and they barely respond to normally unpleasant stimuli—unless they are rudely awoken by a sharp poke.

In contrast, the authors found that food-deprived worms were less prone to this stress-induced snooze than their well-fed counterparts. This was true regardless of whether there was more food around to forage for; that is, the worms tended to remain active after stress whether or not they were provided with additional food after being starved.

The authors also found that starvation-induced sleep-suppression was enhanced when there were more worms on the plate. This makes sense, because higher population densities mean fewer resources, so seeking food instead of going to sleep is likely the better option. Interestingly, crowding only inhibited SIS when worms were starved; well-fed worms in crowded plates showed no change in SIS after stress treatment.

Insulin signaling and TGF-β signaling play major roles in sensing food availability in C. elegans, so the authors asked whether these pathways play a role in SIS under normal conditions. They found that worms with mutations in a TGF-β ligand, but not the insulin receptor, had impairments in SIS. Further investigation revealed that TGF-β involvement in SIS is downstream of the ALA neuron (a master neural regulator of SIS) and dependent on the gene DAF-3, which is involved in TGF-β signaling.

If this DAF-3-dependent pathway was necessary and sufficient for food-deprivation-induced inhibition of SIS, it would follow that worms lacking DAF-3 would have no trouble falling asleep after being starved and stressed—but when the authors tested this, they instead found that the DAF-3 mutants had the same response as wild-types: they stayed awake. Therefore, the authors conclude that although the TGF-β pathway plays a role in normal SIS, it is not solely responsible for the lack of SIS when worms are starved. Consistent with this notion, the authors uncovered evidence that the TOR signaling pathway, which is active when nutrients are available and promotes growth and protein synthesis, plays a role in starvation-induced sleep suppression as well.  

Since sleep deprivation can be deadly, the authors tested whether food deprivation affected how much the worms needed to sleep to stay alive. They found that worms that were deprived of food, stressed, and then kept awake survived better than worms that were just stressed and kept awake. Thus, starving the worms allowed them to stay awake, at least in part, because they needed their sleep less.

CITATION:

Food-Dependent Plasticity in Caenorhabditis elegans Stress-Induced Sleep Is Mediated by TOR–FOXA and TGF-b Signaling

Desiree L. Goetting, Rony Soto, Cheryl Van Buskirk

Genetics August 2018 209: 1183-1195; https://doi.org/10.1534/genetics.118.301204

There’s no doubt that an extra copy of chromosome 21 is what causes Down syndrome. There’s a lot of doubt, however, over which particular gene—or combination of genes—on chromosome 21 is the actual cause of its symptoms. To flesh out our understanding, geneticists must grapple with this large chunk of the genome that includes more than 200 genes. How many of these genes contribute to the Down phenotype? Which are the most important? What are their roles? Studies in mice and other animals have uncovered clues about the function of a few of the genes on chromosome 21, but many remain understudied.

In G3: Genes|Genomes|Genetics, Nordquist et al. used Caenorhabditis elegans to systematically investigate the function of these genes and identified some that could play previously overlooked roles in nervous system function. Because C. elegans has a simple nervous system and can be genetically manipulated quickly, the authors were able to screen dozens of genes—many more than were practical in previous studies using mice.

After investigating which human chromosome 21 orthologues may be essential for C. elegans survival, they studied 27 nonessential genes using existing mutant strains. Because people with Down Syndrome experience a variety of neurological and neuromuscular symptoms, the authors evaluated the worm mutants through a battery of tests for muscular and neurological function.

Ten C. elegans orthologs, when mutated, impaired the worms’ locomotion and nervous system. Several of these are known to be important for neural development in worms—for example, the gene cle-1 is needed for axon guidance. Three of the genes the authors identified had not been previously linked to the nervous system, and little to nothing is known about these genes’ functions.

These results are just the beginning. To reveal their role—if any—in human disease, the genes identified will need to be studied further. But our best bet for understanding the complex genetics underlying Down syndrome is to use a wide variety of tools, including the lowly but powerful worm.

CITATION:

Systematic Functional Characterization of Human 21st Chromosome Orthologs in Caenorhabditis elegans