Triggering OspC production in Borrelia burgdorferi during tick feeding: Is temperature the real signal?

The Ixodes tick, the vector of the Lyme disease spirochete, goes months without a meal.  During this time, the Borrelia burgdorferi spirochetes living in its midgut live quiet lives, sipping on the tick's antifreeze to sustain themselves. When the tick finally takes a blood meal from a warm-blooded victim, B. burgdorferi responds by producing a number of new proteins, some of which are needed for transmission to and infection of the mammalian host.  Among these proteins is the outer surface lipoprotein OspC, whose function involves capture of tick (see this post) and mammalian host proteins.  How does B. burgdorferi know when to start making these critical proteins?  The favored model has been that the the warmth of the blood entering the tick triggers B. burgdorferi to make these proteins.  It's been known for almost two decades that B. burgdorferi growing in culture medium produces miniscule amounts of OspC at low temperatures (23º-24ºC) and larger amounts at higher temperatures (32º-37ºC), as shown in the figure below from the classic 1995 report by Tom Schwan and colleagues.

Figure 4 from Schwan et al., 1995.  B. burgdorferi incubated at 24ºC (lanes 2 and 6), transferred from 24ºC to 37ºC (lanes 3 and 7), incubated at 37ºC (lanes 4 and 8), or transferred from 37ºC to 24ºC (lanes 5 and 9).  Panel A, SDS-PAGE gel stained for total proteins with Coomassie  brilliant blue.  Arrow marks location of OspC.  Panel B, Western blot with flagellin antibody (Fla) and OspC antibody.

As reasonable as this model sounds, findings from a recent paper from Brian Stevenson's group (Jutras et al., 2012) challenge the model.  Although not emphasized in earlier papers, the authors noted that B. burgdorferi multiplies much more quickly at higher temperatures.  In their hands, B. burgdorferi proliferated with a doubling time of 32 hours at 23ºC and 12 hours at 34ºC.  As expected, their Western blots showed that more OspC was produced by the spirochetes growing at the higher temperature.  Members of the Erp family of surface proteins, whose levels also rise during tick feeding, were produced at higher levels at the higher temperature as well, as shown in earlier studies.  The investigators devised an experiment to test whether B. burgdorferi could tie OspC and Erp expression to its growth rate instead of temperature.

The standard culture medium for Borrelia is BSK-II with 6% rabbit serum, a complex nutrient-rich concoction.  They made two new formulations of the culture medium to slow the growth rate:  (1) quarter strength BSK-II with the rabbit serum concentration remaining at 6%; (2) full-strength BSK-II with the rabbit serum concentration reduced to 1.2%.  Medium #1 slowed the doubling time at 34ºC to 40 hours, and medium #2 reduced it to 32 hours.  Western blots of the spirochetes harvested from both cultures revealed low levels of the OspC and Erp proteins.  When these spirochetes were inoculated into the standard culture medium (BSKII/6% rabbit serum) and incubated at 34ºC, high levels of the proteins were again detected.  Therefore, B. burgdorferi is capable of adjusting OspC and Erp expression by monitoring its growth rate, even if the surrounding temperature does not change.

The final experiment from the study demonstrates that not even growth rate is the direct signal.  The authors froze B. burgdorferi at -80ºC for at least a month and then inoculated the bacteria into standard culture medium for incubation at 23ºC.  As a control, bacteria being maintained at 34ºC were also transferred to standard culture for incubation at 23ºC.  Both cultures grew with the same doubling time.  Nevertheless, the spirochetes that were revived from the frozen state produced more OspC and Erp proteins that those that were initially maintained at 34ºC.

So what's the real cue?  Going back to the natural life cycle of B. burgdorferi, the spirochetes living in the unfed tick's midgut do not really grow or divide.  The metabolism of B. burgdorferi is slowed by the nutrient-poor conditions in the tick's midgut.  When the tick finally takes a blood meal, the surge of nutrients entering the tick signals B. burgdorferi to rev up its metabolism, triggering production of OspC.  This model would explain why the frozen spirochetes, whose metabolism was undoubtedly slowed, were able to produce large amounts of OspC and Erp proteins when inoculated into standard culture medium at 23ºC, the temperature usually associated with diminished production of the proteins.  The challenge will be to figure out how B. burgdorferi is sensing its metabolic state at the molecular level.

References

Jutras, B.L., Chenail, A.M., & Stevenson, B. (2012). Changes in bacterial growth rate govern expression of the Borrelia burgdorferi OspC and Erp infection-associated surface proteins. Journal of Bacteriology, 195 (4), 757-764 DOI: 10.1128/JB.01956-12

Schwan, T.G., Piesman, J., Golde, W.T., Dolan, M.C., & Rosa, P.A. (1995). Induction of an outer surface protein on Borrelia burgdorferi during tick feeding. Proceedings of the National Academy of Sciences, 92 (7), 2909-2913 DOI: 10.1073/pnas.92.7.2909

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