1st CABD Workshop: Applied Technologies for Aquatic Vertebrates
Venue: Aquatic Vertebrate Platform
Centro Andaluz de Biología del Desarrollo-CSIC-UPO Registration Opens Date: Monday, April 1, 2013 Registration Deadline: Monday, April 22, 2013 Acceptance Notification Date: Wednesday, April 24, 2013 Participation: Limited number of participants. Date: 27 May to 6 June
Cell death happens all the time: during normal development to shape brains or fingers, or to remove the surplus of lymphocytes after a won battle, and often also during disease. Therefore cell death cannot be used too liberally. If a dying cell were just simply to die and release its content, many active enzymes could then chemically attack the healthy neighbors. It might also call for immune cells to mount an inflammatory response against the normally confined content of the expiring cell causing unnecessary havoc. However, physiological cell death occurs in a regulated, step-wise manner called apoptosis. During this process, the designated apoptotic cell triggers a cascade of proteolytic events that chops into pieces its components. These “death proteases” are called caspases, and are extremely voracious. So, then, how come that they do not devour the many proteins that help holding together the plasma membrane, causing the spilling out of the cell’s content –including the caspases themselves? The Sánchez-Alcázar lab, at the CABD, has discovered that in order to prevent caspase access to the membrane, the dying cells build a protecting shield just underneath the membrane made of microtubules –a sort of impermeable scaffolding. So, while the cell is swiftly disposing of its proteins and DNA, the membrane remains functional. This might help the removal of the apoptotic cell by macrophages and avoid what experts call necrosis –the uncontrolled and potentially pathogenic cell death. For more information, please check: Apoptotic microtubules delimit an active caspase free area in the cellular cortex during the execution phase of apoptosis. Cell Death and Disease (2013) 4, e527- (FC, CABD News).
A new patent describes a method for assessing drugs as new therapeutics for MELAS.[“Method for screening / assessment of the effectiveness of drugs for treatment of MELAS syndrome and mitochondrial diseases. PT 0036/2012”]
Cells in a developing tissue are like small computers, capable of processing the information coming from local neighbors and distant hormone-producing organs. One specially important is positional information: it allows cells to adopt particular fates in the right place within an organ. Positional information is conveyed by chemical signals, often called morphogens. These chemicals are produced by specialized groups of cells which, like beacons, are used as positional references. Hedgehog (Hh) molecules act as such signals. Hh molecules, which are conserved in evolution, play innumerous roles during development, from the generation of neuronal diversity across the neural tube, through the identity of fingers in a hand to the patterning of a fly retina. And often tumors show aberrant Hh signaling. However, it still poorly understood how the chemical gradient of Hh molecules is processed by cells to acquire different fates according to their position within this flow of Hh.
By using a simple and genetically tractable organ system, the simple eyes (or ocelli) of the fruitfly, Aguilar-Hidalgo, Domínguez-Cejudo and co-workers obtain a first answer to this problem: the architecture of the gene network of the cells is such that a continuous Hh gradient is translated into a binary cell fate choice: either to become photoreceptor, or ectodermal cell. This choice is made so that not only the two fates are realized, but the right number of each cell type is produced. Mathematical modeling reveals potential properties of such a system. First, it is resilient to noise (that is, biochemical fluctuations should be buffered, so that even if they existed, the final ocellar structure would not be perturbed). Second, after a massive parameter space exploration, the modeling suggests that continuous variations in reactions constants (as expected by slow, cumulative mutations during evolution) can give rise to discontinuous variations in morphologies –a bit like a jumping morphological evolution. In addition, the system described by the authors shows some tantalizing parallels with the Hh-controlled system that generates motorneuron diversity in the spinal cord of vertebrates. Therefore, the paper opens a number of questions that relate to the function of Hh morphogens in specifying cell types and the role played by the responding cellular networks in the stability and evolution of the organs these morphogens control.
More information can be found in: A Hh-driven gene network controls specification, pattern and size of the Drosophila simple eyes. Aguilar-Hidalgo D, Domínguez-Cejudo MA, Amore G, Brockmann A, Lemos MC, Córdoba A, Casares F. Development. 2013 Jan;140(1):82-92. doi: 10.1242/dev.082172. Epub 2012 Nov 15.
During the Devonian, and for a period that lasted some 10 million years, some fish-like creatures were lurking in murky, shallow waters. Some would make it out of the water, colonizing the land, back then a real land of (ecological) opportunities. These land-colonizers were our ancestors. Their evolutionary journey entailed numerous morphological and physiological adaptations, including using lungs for breathing or modifying their skin to prevent dessication. But one of the key innovations was the development of stronger limbs capable of sustaining their weight outside the water, allowing them to move around. This required the elaboration of the distal fin of these ancestral animals, an elaboration that, as eons passed, generated the hands and feet of extant tetrapods, with their great variety of forms and adaptations, including the human hand –perhaps the most sophisticated mechanical tool produced by evolution. How did this distal elaboration begin? Developmental Biology went in Paleontology´s aid. First, the hand in extant tetrapods requires a burst in expression of a set of Hox genes, including Hoxd13, in the tip of the developing limb bud. In the absence of this transcription factor, the hand/foot region (the so-called autopod) is missing. And this burst is tetrapod specific. Second, this tetrapod-specific Hoxd13 burst required tetrapod-specific DNA control regions (or CREs). Therefore, the hypothesis was that the acquisition of novel CREs might have increased hoxd13 expression in the tip of the fin bud of one of those ancestors, leading to the generation of new bone material. These new bones would have been useful to its bearer, leading to the further elaboration of the distal fin into a proto-hand and beyond through further variation and selection. But what about the functional proof? As going back to the Devonian is out of the question, Renata Freitas and co-workers used an evolutionary reasoning: if a character is present in two animal groups, it must have been present in the last common ancestor of both groups. In the fish-to-limb issue, this translates as: if an extant fish were able to generate an autopod like structure under certain condition, like a tetrapod, this capacity must have existed already in the last common ancestor of fish and tetrapods –our shallow water dweller ancestor. So Renata set to test this in the “handy “ zebrafish. First, she mimicked a burst of hoxd13 expression in the developing tip of a zebrafish fin. The resulting fins showed higher growth and the formation of new cartilage (that preceded bone) with distal (i.e. tip) identity, as shown by a number of molecular markers, very much like a developing mouse limb bud would do. This transformation ran parallel to the thinning of the ectodermal component of the fin (that gives rise to the radials in fish fins), recapitulating what paleontologists have observed in the fossil record. Second, one tetrapod-specific CRE, with known tip activity in the mouse limb, was introduced in the fish. This CRE controls the expression of the fluorescent protein GFP and the transgenic fish showed a green glow at the tip of their fins, again very much like this mouse CRE does in its normal context –the mouse limb. Therefore, even after so many years of evolution, an extant fish genome “knows” how to activate a tetrapod specific CRE in the right place and to orchestrate a response downstream of Hoxd13 to generate the “right” type of tissue. Quite something for a humble fish. But the key thing is that evolutionary reasoning: if today’s fishes can do it, our last common ancestor could do it too. So here you have the functional proof to the hypothesis, coarsely rerunning a 10 million years tape in a 2 day experiment!
For further information: Hoxd13 Contribution to the Evolution of Vertebrate Appendages. Renata Freitas, Carlos Gómez-Marín, Jonathan Mark Wilson, Fernando Casares, José Luis Gómez-Skarmeta. Developmental Cell. 11 December, 2012 Volume 23, Issue 6
Mechanisms controlling the cells’ grip to their substratum explains the origami of eye morphogenesis.
The eye balls derive from the lateral outpocketting of the embryonic anterior neural tube. However, this sac must fold to form a cup before becoming a real eye. Its inner surface will develop into the light-sensing retina and will hold inside the forming lens, very much like a baseball glove catching a ball. This act of epithelial origami is essential, as the shape of the eye determines its optical properties–and its function thereof. The gene ojoplano (Opo, “flateye” in its Spanish translation) was known to be required for the retinal folding. The work published now by the Martínez-Morales lab (CABD, Sevilla) throws light into the mechanisms controlled by Opo. Tissue movement often relies on the ability of the tissue to exert forces onto the extracellular matrix. Its grip is mediated by integrins, which link cells to their substrate matrix. Opo regulates the dynamics of attachment/detachment to the matrix by regulating the availability of integrins on the cells surface. The paper reveals that Opo regulates the activity Numb and Numbl which results into the slowing down of integrin removal from the cells’ basal surface, which takes place within specialized vesicles coated with the protein clathrin. This preferential action on the basal, versus apical cell sides is critical, as it generates asymmetric tensions within the epithelium which, ultimately, leads to its shaping into a cup, prefiguring the shape of the final, functional eye. The experiments were carried out in the Medaka fish (which, very conveniently, has enormous, bulging embryonic eyes) and in collaboration with groups at the CABIMER (Sevilla) and the University of Heidelberg. The paper is now published (on-line version) in Developmental Cell (Numb/Numbl-Opo Antagonism Controls Retinal Epithelium Morphogenesis by Regulating Integrin Endocytosis, by Ozren Bogdanović, Mariana Delfino-Machín, María Nicolás-Pérez, María P. Gavilán, Inês Gago-Rodrigues, Ana Fernández-Miñán, Concepción Lillo, Rosa M. Ríos, Joachim Wittbrodt and Juan R. Martínez-Morales).
Link between chromatin dance and gene expression control unveiled.
The DNA in the genome is wrapped in spirals around globules of proteins called histones. This spiraling allows the packing of a meter-long genome to fit within the tiny nucleus of each cell. Its degree can vary, though: regions with lose packing are “open for business” –that is, these DNA segments can be copied and thus genes can be expressed. However, other regions are very compact and remain silent. Since cell function requires a perfectly regulated gene expression control, any alteration in DNA packing can lead to abnormal gene expression and thus to disease. Interestingly, the active and inactive chromatin bits are segregated to different regions within the nucleus: active chromatin goes to the center, while the inactive or silent chromatin locates at the periphery, attached to the nucleus’ envelope. Compaction of the chromatin into silent DNA requires the chemical modification of the H3 histone on a key aminoacid residue into the H3K9me3 form. The Gasser (Friedrich Miescher Institute for Biomedical Research, Switzerland) and Askjaer (CABD, Spain) teams have identified two enzymes, MET-2 and SET-25, acting in successive steps, as the ones responsible for the modification of histone H3 at the nuclear envelop in the worm C. elegans. Thereby, these enzymes muzzle some regions of the genome and lock them in the nuclear envelop. These two processes are reversible, allowing for dynamic gene regulation. This study appears in the 31 August 2012 issue of the journal Cell. Step-wise methylation of histone H3K9 positions heterochromatin at the nuclear periphery. Towbin BD, González-Aguilera C, Sack R, Gaidatzis D, Kalck V, Meister P, Askjaer P, Gasser SM. (2012). Cell, 150, 934-47.