Friday, January 5, 2018

Platelets vs. Bacteria

Platelets as potent scavengers of bacteria? Really?

Something like 750 billion tiny cell fragments called platelets circulate in the human blood stream. When an injury to a blood vessel occurs, they stick to the exposed collagen in groups—forming platelet plug. And trigger additional reactions that eventually result in a blood clot.

But did you know that they have other helpful jobs, too? Like rounding up bacteria and feeding them up to immune cells, which devour them to make us safe.

This innate immune function of platelets has recently been outlined by researchers, as the information below summarizes.

Read through the quick points below to get an overview of some immune functions of platelets. Then read the full articles if you want to know more about these discoveries—including some great diagrams, micrographs, and videos.

[A short item on this topic also appears in today's edition of my daily Nuzzel newsletter of curated headlines for A&P professors.]

Quick points about platelets as bacterial scavengers

  • At sites of vessel injury/inflammation, platelets that contact intact collagen stick together—but platelets that do not contact collagen are motile.

  • Motile platelets change shape from a "fried egg" to a polarized "half moon" to better navigate the shearing forces of blood flow.

    • They can even navigate "upstream" against the flow of blood.

  • Platelets can use mechanical force to pull particles—including bacteria—from surrounding substrates.

  • Platelets collect and bundle bacteria, which facilitates neutrophil activation and subsequent phagocytosis. 

Migration pattern of motile platelet (left). Platelets collecting bacteria into bundles.

What can we use from this in teaching undergraduate A&P?

  • Yeah, okay we don't have time to go into all the ins and outs of platelets in a typical A&P course, but we can mention that platelets are now known to have immune functions.

    • Consider circling back to this mention later, when (if) you cover innate immune mechanisms a bit later in the course.

  • Consider calling attention to the sensory functions needed for platelets to analyze their microenvironment within the bloodstream.

  • Consider pointing out the specialized structure and function of the platelet's plasma membrane.

    • Integrins (integral membrane proteins) have a role in detecting particles for adhesion, binding to them, and sorting them.

    • Invaginations of the plasma membrane facilitate bundling of bacteria.

  • The shape changes needed for migration and handling of bacteria require actin-myosin reactions to power them. As in muscle fibers, these contractions are triggered by influx of extracellular calcium. In case you want to circle back to that.

  • Perhaps we should make a stronger point in reminding students that although they are "cell fragments" without a nucleus, they're more than just bags of hemostatic chemicals.

  • All these opportunities to "circle back" to previously studied concepts helps students make connections in their developing conceptual framework. And help them form a better understanding of the "big picture."

Want to know more?

Platelets, On Your Marks, Get Set, Migrate!

  • Bambach S, Lämmermann T. Cell. 2017 vol: 171 (6) pp: 1256-1258
  • Introduction to the Gaertner, et. al., paper below—giving background and overview to enhance understanding of the new discoveries. Great diagram, too! Click "Supplemental information" in the article to access video clips. 

Migrating Platelets Are Mechano-scavengers that Collect and Bundle Bacteria.

  • Gaertner F et. al. Cell. 2017 vol: 171 (6) pp: 1368-1382.e23
  • Journal article describing the scavenger role of platelets. Includes a few very nice, simple diagrams—and some cool micrographs and data graphs. These can also be downloaded as PowerPoint slides. Click "Supplemental information" in the article to access video clips. 

Platelets Mediate Host Defense against Staphylococcus aureus through Direct Bactericidal Activity and by Enhancing Macrophage Activities.

  • Ali R et. al. Journal of immunology. 2017 vol: 198 (1) pp: 344-351
  • Journal article that supports the concept that platelets can kill MRSA bacteria and enhance their phagocytosis by macrophages.

[NOTE: If you can't access the full text of any resource, ask your school's reference librarian for help. If they can't provide direct access, they'll probably know how to get a copy of the resource for you. Quickly.]

Sketch: パタゴニア
Photos: LMU

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