Muscles are gaining stature of late, from being humble servants for getting around, to being a core metabolic organ and playing a part in mental health. They are one of the most interesting tissues from a cell biology and cytoarchitectural standpoint, with their electrically-activated activity, and their complex and intensely regimented organization. How does that organization happen? There has been a lot of progress on this question, one example of which is a recent paper on the regulation of Z-disc formation, using flies as a model system.
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| A section of muscle, showing its regimented structure. Wedged in the left middle is a cell nucleus. The rest of these cells are given over to sarcomeres- the repeating structure of muscles, with dark myosin central zones, and the sharp Z-lines in the light regions that anchor actin and separate adjacent sarcomeres. |
The basic repeating unit of muscle is the sarcomere, which occurs end-to-end within myofibrils, which are bundled together into muscle fibers, which constitute single muscle cells. Those cells are then in turn bundled into fascicles, bundles, and whole muscles. The sarcomere contains end-plates called the Z-disk, which attach actin filaments that travel lengthwise into the sarcomere (to variable distances depending on contraction). In the center of the sarcomere, interdigitated with the actin filaments, are myosin filaments, which look much thicker in the microscope. Myosin contains the ATP-driven motor which pulls along the actin, causing the whole sarcomere to contract. The two assemblies contact each other like two combs with interdigitated teeth.
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| Some molecular details of the sarcomere. Myosin is in green, actin in red. Titin is in blue, and nubulin in teal. The Z-disks are in light blue at the sides, where actin and titin attach. Note how the titin molecules extend from the Z-disks right through the myosin bundles, meet in the middle. Titin is highly elastic, unfolding like an accordion, and also has stress sensitivity, containing a protein kinase domain (located in the central M-band region) that can transmit mechanical stress signals. The diagram at bottom shows the domain structure of nebulin, which has the significant role of metering the length of the actin bundles. It is also typical in containing various domains that interact with numerous other proteins, in addition to repetitive elements that contribute to its length. |
There are over a hundred other molecules involved in this structure, but some of more notable ones are huge structural proteins, the biggest in the genome, which provide key guides for the sizes of some sarcomeric dimensions. Nubulin is a ~800 kDa protein that wraps around the actin filaments as they are assembled out from the Z-disk and sets the length of the actin polymer. The sizes of all the components of the sarcomere are critical, so that the actin filaments don't run into each other during contraction, the myosins don't run into the Z-disk wall, etc. Everything naturally has to be carefully engineered. Conversely, titin is a protein of ~4,000 kDa (over 34,000 amino acids long) that is highly elastic and spans from the Z-disk, through the myosin bundles, and to a pairing site at the M-line. In addition to forming the core around which the myosin motors cluster, thus determining the length of the myosin region, it appears to set the size of the whole sarcomere, and forms a spring that stores elastic force, among much else.
Many of these proteins come together at the Z-disk. Actin attaches to alpha-actinin there, and to numerous other proteins. One of these is ZASP, the subject of the current paper. ZASP joins the Z-disk very early, and contains domains (PDZ) that bind to alpha actinin, a key protein that anchors actin filaments, and other domains that bind to each other (ZM and LIM). To make things interesting, ZASP comes in several forms, from a couple of gene duplications and also from alternative splicing that includes or discards various exons during the processing of transcripts from these genes. In humans, ZASP has 14 exons and at least 12 differently spliced forms. Some of these forms include more or fewer of the self-interacting LIM domains. These authors figured that if the ZASP protein plays an early and guiding role in controlling Z-disk size, it may do so by arriving in its full-length, fully interlocking version early in development, and then later arriving in shorter "blocking" versions, lacking self-interacting domains, thereby terminating growth of the Z-disks.
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| Overexpression of the ZASP protein (bottom panels) causes visibly larger, yet also somewhat disorganized, Z-disks in fly muscles. Note how beautifully regular the control muscle tissue is, top. Left sides show fluorescence labels for both actin and ZASP, while right sides show fluorescence only from ZASP for the same field. |
The authors show (above) that overexpressing ZASP makes Z-disks grow larger and somewhat disorganized, while conversely, overexpressing truncated versions of ZASP leads to smaller Z-disks. They then show (below) that in the wild-type state, the truncated forms (from a couple of diverged gene duplicates) tend to reside at the outsides of the Z-disks, relative to the full length forms. They also show in connection with this that the truncated forms are also expressed later in development in flies, in concordance with the theory.
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| Images of Z-disks, end-on. These were not mutant, but are expressing fluorescently labelled ZASP proteins from the major full length form (Zasp52, c and d), or from endogenous gene duplicates that express "blocking" shortened forms (Zasp66 and Zasp67, panels in d). They claim by their merged image analysis (right) to find that full length ZASP resides with higher probability near the centers of the disks, while the shorter forms reside more towards the outsides. |
Compared with what else is known, (and unknown), this is a tiny step. It also begs a lot of questions- could gene expression be so finely controlled as to create the extremely regimented Z-disk pattern? (Unlikely) And if so, what controls all this gene expression and alternative splicing, both in normal development, and in wound repair and other times when muscle needs to be rebuilt, which can not be solely time-dependent, but appears, from the regularity of the pattern, to follow some independent metric of ideal Z-disk size? It is likely that there is far more to this story that will come out during further analysis.
It is notable that the Z-disk is a hotbed of genes that cause myopathies of various sorts when mutated. Thus the study of these structures, while fascinating in its own right and a window into the wonders of biology and our own bodies, is also informative in medical terms, and while unlikely to lead to significant treatments until the advent of gene therapy, may at least provide understanding of syndromes that might otherwise be though of as acts of a cruel god.
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