Mitochondrial DNA maintenance and expression

The mitochondrial genome is a vital component of animal metabolism, physiology, and development. C. elegans mitochondrial DNA (mtDNA) is typical of animal mitochondrial genomes in its size, 13,794 nucleotides in length, and gene content of 32 genes: 2 ribosomal RNAs, 22 transfer RNAs, and 12 protein subunits of the mitochondrial respiratory chain (MRC). Unlike nuclear DNA, mtDNA is maternally inherited and can be present at tens to tens of thousands of copies per cell. Its copy number is developmentally regulated, with mtDNA increasing about 30-fold between the L1 and the adult stages. Blocking mtDNA increase leads to larval arrest. Underlying its essential role in the biology of C. elegans, over 200 nuclear genes are needed to replicate, transcribe, and maintain the mitochondrial genome and to assemble the translation machinery required for expressing mitochondrial proteins. Disruptions in these processes have shown that the mitochondrion plays a critical role in aging, life span determination, reactive oxygen species response, the unfolded protein response, and apoptosis. Oddly, despite the essential role of mtDNA encoded genes in the cellular and organismal biology of C. elegans, mutations in mtDNA have not been reported. By contrast, over 300 lesions in human mtDNA have been described, many associated with neurological, endocrinological or muscle diseases.

Wnt signaling pathway

Wnt glycoproteins are signaling molecules that control a wide range of developmental processes and is a conserved feature of metazoan development. In C. elegans Wnt signaling has been shown to play a role in cell fate specification and determination of cell polarity, cell migration, and axis determination during axon outgrowth. A 'canonical' Wnt signaling pathway has been elucidated in vertebrate and invertebrate model systems where Wnt binding leads to the stabilization of the transcription factor beta-catenin, which then enters the nucleus to regulate Wnt pathway target genes. Like other species, the C. elegans genome encodes multiple genes for Wnt ligands, EGL-20, LIN-44, MOM-2, CWN-1, CWN-2) and Wnt receptors (LIN-17, MOM-5, MIG-1, CFZ-2, LIN-18). Canonical Wnt signaling in C. elegans, utilizes the beta-catenin BAR-1 to convert POP-1 into an activator and controls the expression of several homeobox genes. However, unlike vertebrates or Drosophila, the C. elegans genome encodes multiple beta-catenin genes (HMP-2, BAR-1, SYS-1, WRM-1), which give rise to noncanonical Wnt signalling pathways: for example, the endoderm induction pathway requires the beta-catenin WRM-1 and parallel input from a mitogen-activated kinase (MAPK) pathway to downregulate POP-1.

Mitosis

Mitosis is part of the eukaryotic cell cycle and results in the production of two daughter cells each with a copy of the genome. The cell cycle itself is comprised of an interphase (made up of three stages G1, S, and G2) and the M (mitotic) phase. Cell growth, active transcription and translation, and DNA replication occur during interphase. During M phase duplicated DNA (chromatin) condense into sister chromatids (prophase); the nuclear envelop breaks down, kinetochore microtubles attach to the chromosomes and centrosomes are pushed to the poles of the growing spindle (prometaphase); the chromosomes are lined up on the metaphase plate (metaphase); sister chromatids are pulled to spindle poles at opposite ends of the cell (anaphase); the nuclear envelop is reformed and the chromatids decondense to chromatin (telophase); and the cell is cleaved into two by a contractile ring and the resolution of a cleavage furrow (cytokinesis). In some variant cell cycles nuclear division may not be followed by cell division, or G1 and G2 phases may be absent.

Intestine development

The C. elegans intestine is attached to the posterior pharynx and extends the length of the worm, ending at the rectum. This major organ of the worm consists of 20 large, polyploid epithelial cells arranged in pairs, forming a tube. The intestine is responsible for food digestion, nutrient absorption, and synthesizing and storing macromomlecules such as fat droplets and birefringent gut granules. The intestine also plays major roles in the rhythmic behavior of the defecation cycle as well as stress responses and lifespan.

Anaphase

In C. elegans, anaphase is comprised of two separable components, anaphase A, where the chromosomes separate from each other before any chromosome to pole movement, and anaphase B, where the spindle poles move away from each other, with the concomitant movement of the chromosomes to the poles. During anaphase B, the movement of the spindles, which carry the chromosomes, occurs through a combination of pulling and pushing forces. Cortical forces attached at the centrosomes pull the microtubule asters away from one another, while central spindle forces from overlapping microtubule arrays that had formed between separating chromosomes, push the chromosomes away from one another. The holocentric nature of C. elegans chromosomes entails special consideration to ensure the forces at all of the microtubule attachment sites of the chromosome are coordinated so that shearing of the chromosome during segregation does not occur.

Reproduction

A fundamental biological process resulting in the production of offspring. In C. elegans, reproduction can result from self-fertilization within a hermaphrodite or by fertilization by mating of a male and hermaphrodite. In C. elegans, during the fourth larval stage of hermaphrodite development, the germline undergoes spermatogenesis to make functional sperm that are stored in the the gonad. In the adult stage, the germline ceases spermatogenesis and switches to oogenesis to produce oocytes that are fertilized by the previously generated spermatozoa or by spermatozoa deposited into the spermatheca during mating with a male. The embryos produced by self-fertilization are encased in an eggshell and initiate development within the uterus of the hermaphrodite. When they reach about the 30-cell stage, the egg-embryos are laid by the hermaphrodite through a vulva.

Oogenesis

Oogenesis is the process of generating functional oocytes from an undifferentiated germ cell. In most animal species, oocytes arrest during meiotic prophase. The completion of meiosis and the preparation of the oocyte for fertilization are triggered in response to intercellular signaling in a process called meiotic maturation. During meiotic maturation, the oocyte transitions to metaphase of meiosis I, the nuclear envelope breaks down, the cortical cytoskeleton undergoes rearrangement, and the meiotic spindle is assembled. By contrast, in C. elegans, the processes of meiotic maturation, ovulation, and fertilization are temporally coupled. Meiotic maturation is triggered by major sperm protein (MSP), which acts as a hormone. In turn the maturing oocyte signals its own ovulation. During ovulation the oocyte passes through the spermatheca becoming fertilized on the way to the uterus.

Defecation

In C. elegans the expulsion of intestinal contents occurs every 45-50 seconds. This cycle is characterized by a pattern of muscle contractions under both muscle and neuronal control. The steps of the defecation cycle are a posterior body contraction (pBoc), an anterior body contraction (aBoc), and the final expulsion step (Exp) where the enteric muscles contract, opening the anus and allowing the intestinal contents to be released. Each step is independently controlled as mutations exist that affect one step but do not alter the timing or occurrence of the other. Further, Ca++ oscillations in the intestine, rather than neuronal stimulation, have been shown to control the initiating pBoc step. The contractions of the enteric muscles are controlled by GABA motor neurons AVL and DVB through an excitatory GABA-gated cation channel. The periodicity of the cycle is influenced by the presence of food, is temperature compensated, and can be reset by mechanosensory input.

Locomotion

The movement of the animal in relation to its environment requires coordinating an awareness of environmental cues with the firing of neuronal circuitry affecting the simultaneous contraction and relaxation of opposing muscle groups. C. elegans exhibits many types of movement, the two major types are crawling and swimming. Each of these movements have been further characterized by dominant body shapes, trajectories, angles, speeds, etc., peculiar to the movement. Fundamental to survival of the worm is the ability to sense and move towards or away from different stimuli. Forward and backwards movements can be induced in the lab through the stimulation of the mechanosensory neural network.

Unfolded protein response

The unfolded protein response (UPR) is a stress response that is critical to maintaining protein homeostasis (proteostasis)- the functional concentration of properly folded protein concentration in an organism. The UPR in entirety involves stress signals in the endoplasmic reticulum the mitochondria and the cytoplasm that are activated by increases in misfolded proteins. The increase in misfolded proteins affect protein concentration and can result in the aggregation of protein species. To restore protein homeostasis, these stress signals up-regulate or down-regulate protein transcription as well as regulate protein translation. These systems also influence protein folding by increasing the concentration of chaperones to aid in the folding process. In addition, these systems can increase the activity of the endoplasmic reticulum-associated degradation (ERAD) pathway, to deal with the increase in misfolded proteins. Eventually, sustained activation of the UPR will lead to cellular apoptosis.