January 25, 2013

Neuromuscular junctions (NMJs) are considered by many to be the business end of our nervous system. NMJs connect nerves with muscle cells, stimulating the muscle contractions that allow you to run from the bear that you just spotted glaring at you with a fork and knife in his paws. A recent paper adds to our understanding of the signaling on both sides of the NMJ. 

The two sides of a NMJ, the presynaptic and postsynaptic structures, are highly coordinated for proper development and plasticity of the junction. As with many cell and developmental processes, this coordination relies on the Wnt signaling pathway. In fruit flies, Wnt/Wingless (Wg) functions both pre- and post-synaptically in larval muscle fibers, and results from a recent paper show how this bidirectional signaling is balanced and regulated. Kamimura and colleagues found that the gene trol, which encodes the protein perlecan (a secreted heparan sulfate proteoglycan, for those down with HSPGs), regulates Wg signaling in fruit fly NMJs. trol mutations causes postsynaptic defects and an overproduction of synaptic boutons, which are button-like presynaptic hotspots of neurotransmitter-containing vesicles. In the images above, normal (top) and trol mutant larval NMJs show immunostaining for a presynaptic marker (magenta) and a postsynaptic marker (green). In mutants, some synaptic boutons lack a postsynaptic structure nearby (“ghost boutons", arrowheads) while some NMJs showed an overproduction of small synaptic boutons (“satellite boutons”, arrow).

Kamimura, K., Ueno, K., Nakagawa, J., Hamada, R., Saitoe, M., & Maeda, N. (2013). Perlecan regulates bidirectional Wnt signaling at the Drosophila neuromuscular junction originally published in the Journal of Cell Biology, 200 (2), 219-233 DOI: 10.1083/jcb.201207036

October 30, 2012

I’ve posted a lot of images from papers on human disease models lately, and that’s a good thing. Despite what you may hear from politicians mocking research using fruit flies, worms, or tiny fish, the power of model systems to study a human problem is undeniable to anyone with half a clue. All science on the continuum from basic research all the way to clinical trials is valuable…so thank your favorite scientist today! Today’s image is from a zebrafish paper that identifies a new pathway that may be a good therapy target for muscular dystrophies.

Muscular dystrophies are fairly common diseases that result in the weakening of the protein complexes that connect muscles to their underlying environment, called the extracellular matrix (ECM). The degeneration of muscle-ECM attachment eventually leads to progressive loss of muscle movement and locomotion for the patient. A recent paper identifies a new pathway in muscle-ECM attachment using zebrafish as a model. Goody and colleagues found that biosynthesis of the small molecule NAD+ can reverse muscle degeneration in certain types of dystrophies in zebrafish and even improve swimming. This pathway improves the organization of laminin, an important ECM protein, suggesting the function of an additional laminin receptor complex and pathway from those already studied. Goody and colleagues suggest that this new pathway may serve as a focal point for new muscular dystrophy therapies. In the images above, normal ECM basement membrane tissue adhered normally to both normal muscle cells (blue) and cells with muscle degeneration mutations (red), even after the stress of swimming. This experiment shows the importance of a healthy ECM in restoring function for dystrophic muscles. 

Goody, M., Kelly, M., Reynolds, C., Khalil, A., Crawford, B., & Henry, C. (2012). NAD+ Biosynthesis Ameliorates a Zebrafish Model of Muscular Dystrophy PLoS Biology, 10 (10) DOI: 10.1371/journal.pbio.1001409

October 19, 2012

The next time you see a fruit fly buzzing around your kitchen, take a beat before you smack it with a swatter and remind yourself of the amazing discoveries due to organisms like the (not so) lowly fruit fly. Maybe you’ll offer a thank-you glass of grape juice instead and show the intruder back outside. Today’s image is from a paper describing a fly model of a serious human disease, and serves as a great example of the power in a model organism.

Spinal muscular atrophy (SMA) is a heritable disease that results in infant mortality due motor neuron dysfunction and rapid degeneration of muscle. SMA is caused by the depletion of the SMN (survival motor neuron) protein. A recent paper describes the use of fruit flies in studying SMA, and shows that flies lacking SMN have reduced muscle size and defective motor neuron neurotransmission similar to SMA patients. Imlach and colleagues found that replenishing SMN levels in motor neurons and muscles did not reverse the defects of SMN depletion, yet increasing SMN levels in partner cells (proprioceptive neurons and interneurons) can reverse the defects. These results suggest that SMN depletion primarily affects the sensory-motor network, with secondary effects seen in the motor circuit. In addition, Imlach and colleagues found that increasing motor neural circuit excitability, either genetically or with drugs, could relieve SMN-depletion defects. Using this fly model of SMA, these results suggest that SMA patients may improve by enhancing motor neural network activity. The images above show the differences in muscle size in wild-type (left) and SMN-depleted (right) flies.

Imlach WL, Beck ES, Choi BJ, Lotti F, Pellizzoni L, & McCabe BD (2012). SMN Is Required for Sensory-Motor Circuit Function in Drosophila. Cell, 151 (2), 427-39 PMID: 23063130
Copyright ©2012 Elsevier Ltd. All rights reserved.

 

September 11, 2012

If I had to describe myself as a cell, I would want to say something about being a fired-up neuron, or a nurturing nurse cell, or a chameleon stem cell. Realistically, though, I’d be a muscle fiber—painfully organized and precise. When things aren’t so organized I’d just be a twitching little muscle cell until my closet was organized by sleeve length, my pantry in alphabetical order, my various to-do lists organized by priority. A recent paper describes how the nuclei and organelles within a muscle fiber are so regularly spaced (but sadly does not describe any suggestions for my own personal organization).

Striated muscle fibers have densely packed myofibrils, which are the structures that make muscle contractions happen. The muscle fiber is a large multinucleated cell, meaning that there are more than one nucleus. These nuclei, as well as other organelles, must be evenly distributed along the length of the fiber. KASH domain proteins play a role in positioning nuclei and other organelles in various cell types, and a recent paper found that two KASH proteins play this role in muscle cells in the fruit fly. Elhanany-Tamir and colleagues found that these two proteins, called Klar and MSP-300, cooperate to ensure the even spacing of nuclei within a muscle fiber. These two proteins link a nuclear ring of MSP-300 to astral microtubules. Without either of these KASH proteins, the nuclear spacing is irregular. MSP-300 also is important in the spacing of other organelles—mitochondria and ER—within the muscle fiber. In the images above, muscle fibers from wild-type (left), msp-300 mutant, and klar mutant fruit fly larvae are stained for MSP-300 (red) and a nuclear marker (green). In the mutants, the MSP-300 nuclear ring (arrowheads) is dissociated from the nucleus.

Elhanany-Tamir H, Yu YV, Shnayder M, Jain A, Welte M, & Volk T (2012). Organelle positioning in muscles requires cooperation between two KASH proteins and microtubules. originally published in the Journal of Cell Biology, 198 (5), 833-46 PMID: 22927463

July 26, 2012

Ahhh…I remember the first time I saw worms under the microscope. I was an undergrad attending my future graduate school’s recruitment weekend, during which a kick-ass scientist showed me beautiful worms with glowing green seam cells down their bodies and matter-of-factly told me that “Worms rock.” I silently agreed with her, and found myself a few months later in that lab and spreading the gospel of the awesome rocking ability of worms. So, whenever I see a worm paper I feel like I’m part of the family…using genetic nomenclature that makes fly biologists roll their eyes (right back at you, sillies) and waxing nostalgic about beers in front of Royce Hall at the big worm meetings. Today, stunning images of worm muscles serve as a great example of the power of worms to show scientists some fascinating biology.

A sarcomere is the basic unit of muscle that contracts and relaxes. The fine balance of the proteins involved in a functional sarcomere is achieved by degradation of damaged proteins and production of new proteins. This balance is tipped during muscle atrophy in humans, caused by disease, disuse, starvation, or old age. The small nematode worm C. elegans has been a great model for muscle development and function, and a recent paper describes how protein degradation in muscle is regulated. Wilson and colleagues found that the sarcomeric protein UNC-89 (obscurin) binds to another protein called MEL-26 in C. elegans. MEL-26 is an adaptor protein that plays an important role in the ubiquitin proteasome system that degrades damaged or old proteins. Mutations in the mel-26 gene cause disorganization of the sarcomere structure, and some of this disorganization is due to an increase in the activity of a microtubule-severing protein called MEI-1 (katanin). These results suggest that normally UNC-89 inhibits the MEL-26 degradation complex toward MEI-1 in muscle. In the images above, adult body wall muscle from C. elegans is stained for UNC-89 (left, purple in merged) and MEL-26 (middle, green in merged). Some MEL-26 is found at the M-line of the sarcomere, where UNC-89 predominantly sits (arrow).

Kristy J. Wilson, Hiroshi Qadota, Paul E. Mains, & Guy M. Benian (2012). UNC-89 (obscurin) binds to MEL-26, a BTB-domain protein, and affects the function of MEI-1 (katanin) in striated muscle of Caenorhabditis elegans Molecular Biology of the Cell, 23 (14), 2623-2634 DOI: 10.1091/mbc.E12-01-0055

June 2, 2011

Alphabet soup is not tasty, but must be devoured by any researcher. Some of the acronyms in biology, though, stand out for their importance and FRAP is one of them. FRAP is a very handy technique for studying the dynamics of proteins in a cell, and the paper from today’s image is a great example of how elegant and informative FRAP can be.

Titin is an abundant muscle protein that provides structural support for the sarcomere, which is the basic contracting unit in muscle. There are many isoforms of titin that allow structural and mechanical changes of the muscle tissue throughout development and during disease. The sturdiness that titin provides muscles may give an impression of a lack of dynamics, but a recent paper shows exactly the opposite. The authors in this study use a technique called fluorescence recovery after photobleaching (FRAP). In this case, fluorescently labeled titin was photobleached using a high power laser. By watching if and how the photobleached region recovers new fluorescent titin over time, da Silva Lopes and colleagues concluded that titin is very mobile and dynamic. Titin maintains unrestricted movement around sarcomeres, and this movement is dependent on calcium. In the images above, two different sized regions (top, bottom) of fluorescently-tagged titin within sarcomeres were photobleached. In both cases, the bleached regions (arrowheads in middle images) quickly recovered the titin fluorescent label (right) to levels similar to pre-bleach images (left).

da Silva Lopes, K., Pietas, A., Radke, M., & Gotthardt, M. (2011). Titin visualization in real time reveals an unexpected level of mobility within and between sarcomeres originally published in The Journal of Cell Biology, 193 (4), 785-798 DOI: 10.1083/jcb.201010099

March 10, 2011

Think of the first time you kissed your partner…I’m sure it led to piloerection. Nerds, stop the giggling…I’m talking about goosebumps! The stunning image above is from a paper describing the relationship between hair follicles and the muscles that cause goosebumps.

We have epidermal stem cells in our hair follicles in a region called the bulge. The cells in the bulge are closely associated with the arrector pili muscle (APM), which is the muscle responsible for causing goosebumps. A recent study describes how bulge cells deposit a protein called nephronectin into the underlying basement membrane, and this provides a niche for APM differentiation during hair follicle development. Image above shows hair follicles (blue) and smooth muscle proteins found in the APM (red, green). The white brackets show groups of hair follicles that share APMs.

Fujiwara, H., Ferreira, M., Donati, G., Marciano, D., Linton, J., Sato, Y., Hartner, A., Sekiguchi, K., Reichardt, L., & Watt, F. (2011). The Basement Membrane of Hair Follicle Stem Cells Is a Muscle Cell Niche Cell, 144 (4), 577-589 DOI: 10.1016/j.cell.2011.01.014
Copyright ©2011 Elsevier Ltd. All rights reserved.

January 27, 2011

Just when you think you understand how a gene works…BAM! Alternative splicing shows up and reminds us that there is so much yet to learn, even about a gene as well-studied as formin.

Genes get transcribed into RNA, which gets translated into proteins. After transcription of a gene, different regions of RNA called exons and introns are either connected together (exons) or removed (introns) in a process called splicing. Some genes undergo a process called “alternative splicing” that allows one single gene the ability to splice, or connect, the exons in multiple ways that result in different protein isoforms. A recent paper describes a previously-unidentified isoform of FHOD3 formin, an actin-nucleating protein found at high levels in the heart. This new isoform includes an alternative exon in certain muscle tissues, and this exon contains a phosphorylation site that allows an additional level of regulation of FHOD3. Images above are of neonatal rat heart cells with or without this alternative exon. The presence of the alternative exon (bottom cell) directs formin (left images, green in merged) to myofibrils (middle images, red in merged), which are the repetitive and contractile structures in muscle cells. Without the alternative exon, formin is found in aggregates in the cytoplasm (top cell).

Iskratsch, T., Lange, S., Dwyer, J., Kho, A., Remedios, C., & Ehler, E. (2010). Formin follows function: a muscle-specific isoform of FHOD3 is regulated by CK2 phosphorylation and promotes myofibril maintenance originally published in The Journal of Cell Biology, 191 (6), 1159-1172 DOI: 10.1083/jcb.201005060

October 18, 2010

Whenever I see a paper with the word “sarcomere” in the title, I can be sure to see lovely images of these ordered structures found in muscle cells. In my type-A personality’s quest for order, these images are always soothing.

A sarcomere is the basic unit of a muscle and gives muscle its striated appearance. Sarcomeres contain overlapping actin and myosin motors that function in the contraction of each unit; multiply each sarcomere’s contraction many many times, and you’re able to lift your coffee mug. It was believed for a while that mature sarcomeres were very rigid structures, but recently that view has shifted. A recent paper describes the function of a sarcomere protein called Lmod in actin nucleation, suggesting its role in dynamic repair and remodeling of mature sarcomeres. Image above shows a chicken cardiac muscle cell with Lmod in green and Z-lines, which are the sarcomere's outside anchor points for actin, in red (α-actinin). Zoomed images of the boxed region show Lmod on either side of the center of the sarcomere, called the M-line, suggesting it is found at the ends of actin filaments.

Reference: Aneta Skwarek-Maruszewska, Malgorzata Boczkowska, Allison L. Zajac, Elena Kremneva, Tatyana Svitkina, Roberto Dominguez, and Pekka Lappalainen. Authors’ Molecular Biology of the Cell paper can be found here.

May 6, 2010

Skeletal muscles are precisely structured arrays of actin and myosin, and several additional proteins that ensure proper muscle organization and contraction. A recent paper looked at the roles of tropomodulin in skeletal muscle, and found that in the absence of one tropomodulin, two other tropomodulins were able to preserve correct muscle morphology but not correct muscle function. Image above shows a transmission electron micrograph of normal muscle.

Reference: David S. Gokhin, Raymond A. Lewis, Caroline R. McKeown, Roberta B. Nowak, Nancy E. Kim, Ryan S. Littlefield, Richard L. Lieber, and Velia M. Fowler, 2010. Originally published in Journal of Cell Bioloy. doi: 10.1083/jcb.201001125. Paper can be found here.

March 25, 2010

Assembly of the junction between muscle and tendon cells is important for proper development of muscle tissue. A recent study has identified a new player in this process, a gene named slowdown. Developing muscles that lack slowdown have torn muscle fibers, as well as disconnected and broken tendons. The image above shows muscle fibers (red) and tendon cells (blue) in normal (left) and slowdown mutant (right) larvae in the fruit fly Drosophila.

Reference: Eliezer Gilsohn and Talila Volk, 2010. Development: 137, 785-794. doi: 10.1242/dev.043703. Adapted with permission by Development. Paper can be found here.