Greasing the wheels of information transfer in cells : NCBS (Bangalore), Babraham Institute (Cambridge, UK) and University College London (UK).

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Anusha Krishnan

Mealtimes at a café, a darshini or a dhaba are visible and audible scenes of seeming chaos. However, the storm of shouted orders and responses are actually a pretty good communication system that co-ordinate the serving of food from kitchens, removal of used cutlery and resetting of tables. The interior of a cell is much like a busy eatery in that it also needs seamless information exchange between its various parts – the nucleus that issues orders, the protein manufacturing ribosomes, the lipid factories of the endoplasmic reticulum and the plasma membrane that co-ordinates signalling between the cell and its environment.

New insights into the communication systems within cells have been provided by a group of scientists from the National Centre for Biological Sciences (NCBS, Bangalore), Babraham Institute (Cambridge, UK) and University College London (UK). Their work has uncovered a key factor in the communication network between the plasma membrane and the endoplasmic reticulum – one that is essential in resetting the chemical composition of the plasma membrane after a bout of signalling. The researchers used fruit flies as a model system to investigate the role of a protein named “RDGBα” in maintaining the signalling networks of the plasma membrane within the light sensitive cells of the eyes.

The streams of chemical reactions that occur when light falls on the light sensitive cells or photoreceptors in an eye alter the chemical properties of the plasma membrane. This occurs due to a sequence of events – referred to as the phosphatidylinositol or PI cycle – that occurs in the cell membrane in many other instances. For example, when neurons in the brain adjust their response to stimulation or the cells in our body respond to rising blood sugar after a meal, the PI cycle is set in motion. In all of these settings, a lipid component of the cell membrane, named phosphatidylinositol bisphosphate (PtdInsP2) is broken down enzymatically to release another lipid, phosphatidic acid (PtdOH) as a byproduct.  In order to maintain continued signalling, the plasma membrane must restore its chemical composition by continuously replenishing PtdInsP2 and removing the PtdOH produced.

The generation of PtdInsP2 depends on a lipid precursor molecule, phosphatidylinositol (PtdIns), that is produced in another distinct cellular compartment, the endoplasmic reticulum. Since PtdIns and PtdOH are lipids (which belong to the family of fats), they cannot move by simple diffusion through the watery interior of the cell. So, how does the plasma membrane replenish its PtdIns supply and remove the excess PtdOH to maintain homeostasis during this signalling process?

The answer to this lies in the protein RDGBα (retinal degeneration B), which aids in maintaining the chemical integrity and function of the plasma membrane. Fruit flies without RDGBα function have photoreceptors that are defective in responding to light and undergo retinal degeneration on continued exposure to light. Investigations revealed that RDGBα can act as a shuttle moving PtdIns and PtdOH between membranes and this function of the protein is critical for normal photoreceptor structure and function.

One of the salient points about this piece of research is the essential meshing of in vitro biochemistry, cell biology and functional analysis in an intact organism to understand the role of a single protein in supporting plasma membrane properties. Dr. Padinjat Raghu, one of the corresponding authors of the paper states that although previous studies have looked at the function of RDGBα as biochemical reactions in vitro, none have related the outcome of these reductionist analyses to the function of the protein within the complex and crowded confines of a living cell.

“There has been considerable interest in the sequencing of the human genome and unravelling how encoded genes function in in health and disease. For obvious reasons there are limitations to detailed experimental analysis in humans. A recent study on the human equivalent of RDGB in cultured cells has showed that the protein functions in a similar manner to Drosophila RDGB. Therefore, our work demonstrating the importance of this protein in intact functioning flies highlights  the value of genetic models such as Drosophila in dissecting human gene function.” says Dr. Padinjat.

The process under study is a widely conserved chemical reaction that helps in information transfer in animal cells, and is a part of pathways that are implicated in the mechanism of diseases such as diabetes, tumour development and neurodegeneration. This study helps in revealing how fundamental biochemical processes in cells operate and in the future may help in the developing therapeutic strategies for human or animal diseases.

 

About the Authors:

Shweta Yadav , Aniruddha Panda and Swarna Mathre are affiliated to the National Centre for Biological Sciences (NCBS), Bangalore.

Kathryn Garner, Michelle Li, Evelyn Gomez-Espinosa and Shamshad Cockcroft are affiliated to the Department of Neuroscience, Physiology and Pharmacology, University College, London, UK.

Plamen Georgiev, Max Planck Institute of Immunbiology and Epigenetics in Freiburg, Germany. Hanneke Okkenhaug, Imaging Facility, Babraham Institute, UK.

Dr. Raghu Padinjat is affiliated to National Centre for Biological Sciences (NCBS), Bangalore.

Contact Information:

Padinjat Raghu: praghu@ncbs.res.in

The paper was published in the Journal of Cell Science on the 1st of September and can be read here.

Anusha Krishnan is a writer with the Research Media Services Division of Gubbi Labs.

Original News Source : NCBS News.

iBiology ,Bringing the World’s Best Biology to You – Initiative by Professor Ron Vale (UCSF, HHMI)

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iBiology’s mission is to convey, in the form of open-access free videos, the excitement of modern biology and the process by which scientific discoveries are made. Our aim is to let you meet the leading scientists in biology, so that you can find out how they think about scientific questions and conduct their research, and can get a sense of their personalities, opinions, and perspectives. We also seek to support educators who want to incorporate materials that illustrate the process and practice of science into their curriculum. This project is made possible by the good will of many biologists who are committed to making their work broadly accessible and to conveying the excitement of biology to a worldwide audience.

iBiology.org (formerly ibioseminars.org and ibiomagazine.org) was developed to bring the best biology to people throughout the world for free. Started in 2006 by University of California, San Francisco and Howard Hughes Medical Institute investigator, Professor Ron Vale, iBiology has grown to include over 300 seminars and short talks by the world’s leading scientists. Our collection includes talks by many Nobel Laureates and members of the United States National Academy of Sciences.

Source : iBiology – http://www.ibiology.org/

Key players in transport pathways in melanin synthesis identified

 

SNARE

– Suneha Mohanty

Researchers from IISc identify and shed light on the role of protein complexes that play key roles in transporting synthesizing enzymes that aid melanin production in cells.

Delivery of cargo to the correct address requires accurate directions and a dependable machinery for delivery. Any defects in them affect timely delivery or cause the cargo to be misdelivered. This chain of events is not very different at a cellular level. Our cells also have their own transport pathways responsible for the cargo delivery at the right destination at the right time. Any variations to that system shows up as symptoms to fatal diseases. A team of researchers from the Indian Institute of Science (IISc), Bangalore unravels the nitty gritties of one such transport pathway in specialized animal cells where failure to deliver the cargo— melanin synthesizing enzymes in this case—could result in disorders like albinism.

Melanin pigments are responsible for the colour of our skin and also play an important role in protection against radiations and any other damage from light. They are produced by a series of organic chemical reactions that occur in cellular organelles called melanosomes. These cell compartments need melanin synthesizing enzymes transported from other organelles to non-pigmented premature melanosomes where pigment production is initiated. The cargo transport pathways are facilitated with the help of four multi subunit cytosolic protein complexes, BLOC-1, -2, -3 and adaptor protein (AP)-3 complex.

BLOC-1 consists of 8 subunits, functioning in the initial step of transport pathway, while BLOC-2, a 3-subunit protein complex, functions towards the end of the pathway. The team of researchers led by Subba Rao Gangi Setty from the Department of Microbiology and Cell Biology in IISc recently elucidated the role of BLOC-2 in directing the cargo containing vesicles towards maturing melanosomes for subsequent reactions. It does so by the specific method of “tethering” or by stabilizing the two different membranes between which the cargo is transferred through fusion and subsequent cargo delivery.

They went on to show that mutations in any protien subunit of BLOC-2 result in inefficient delivery of melanin synthesizing proteins to maturing melanosomes and thus failure in production of the melanin pigment. This malfunction manifests in the form of albinism of skin and eye, also referred to as occulocutaneous albinism. This is one of the primary clinical symptoms in Hermansky-Pudlak Syndrome (HPS). The other symptoms are lung fibrosis and bleeding. Out of the 16 possible genetic mutations that can result in HPS, only 9 are known in humans so far. Three out of those nine subtypes are a result of mutations in the BLOC-2 protein complex.

Even though it is now established that BLOC-1 and-2 play key respective roles in the overall cargo delivery, how they achieve this is not yet clear. In addition to these key proteins, other cellular machineries are also known to be responsible for membrane fusion and cargo delivery. These proteins are called Soluble NSF (N-ethylmaleimide-sensitive factor) Attachment Protein REceptor (SNARE). SNARE proteins, a family of about 38 proteins has been known for their role in membrane fusion during the delivery of cargo. For the first time, the researchers have identified two members from the SNARE family that are involved in the protein transport pathways to melanosomes. Loss in expression of these proteins mistarget and degarde the enzymes, subsequently affecting melanosome maturation. The team demonstrated that higher expression of a mutant form of SNARE increased cell pigmentation. Furthermore, their study has also shown how these molecules recycle and regulate each other for efficient cargo delivery and melanosome formation.

The team intends to further work on uncovering the details of the intramolecular interactions between the SNAREs, BLOC-1/-2 complexes and other key players involved in the transport pathway. It is important to understand the recruitment of these molecules onto the membranes during maturation of melanosomes. This would help in understanding the formation of other organelles such as dense granules in platelets and lamellar bodies in Type II lung epithelial cells, which are also defective in some HPS patients. Moreover, such studies will illustrate the mechanism of organelle formation in addition to understanding the etiology of an autosomal recessive disorder.

Original News : Indiabioscience

http://beta.indiabioscience.org/news/2015/key-players-in-transport-pathways-in-melanin-synthesis-identified