High-pressure biology


High Pressure Cell Biology and Biotechnology

High hydrostatic ambient pressures are the regular rather than the exception. With more than 70 % of our earth’s biosphere being covered by oceans, the average depth accounts for roughly 3,800 m or an ambient pressure of ~38 MPa; with deepest trenches reaching up to 11,700 m. The mechanisms that have allowed deep sea fish to adapt to these high pressures are still a mystery as are the mechanisms that allow diving mammals to acclimatize to pressures up to ~22 MPa during deepest recorded ~20 min dives of whales.

High pressure limiting organs

In humans, the brain is the organ that predominantly limits pressure exposure to depths around 500 m. But what if it wasn’t for the brain? If we were a whale, what was our pressure-limiting organ? In mammals, skeletal muscle function is highly conserved with a very conservative machinery of motor-proteins and muscle regulatory mechanisms. We, thus, were interested in the last couple of years to study specific effects of a prolonged ~3 hr exposure of terrestrial mammalian muscles to increasing high pressures up to 35 MPa to answer the question whether pressure simply damaged muscle cells rather unspecifically or according to a specific mechanism. Following prolonged high pressure exposures, muscle contractility or ion channel recordings were performed in the post-decompression phases. During high pressure exposures, we monitored muscle cell function online using novel high pressure microscopy techniques involving sealed pressure chambers with optical windows for epifluorescence of confocal laser fluorescence microscopy.

Hochdruckbiologie

Figure: Mechanism of high-pressure-induced impairment of mammalian muscle function. A, following a 3hr high pressure (HP) treatment exceeding 20MPa, a marked decline in force production with increased muscle stiffness is found [Kress et al. 2001]. B, following a 3hr HP treatment at 25MPa, a marked selective degradation of troponin-T is noticed [Kress et al. 2001].  C, propidium-iodide epifluorescence high pressure microscopy online visualizes the development of a rather slowly developing irreversible contracture of single muscle fibres from ~20min exposed to a pressure of 35MPa. Membrane integrity remains intact throughout [Friedrich et al. 2006]. D, confocal high pressure laser fluorescence microscopy in single intact muscle fibres shows initial Ca2+ uptake into mitochondria upon compression which turns into a sudden repouring of mitochondrial Ca2+ into the cytosol after ~30min at 35MPa [Friedrich et al. 2006]. E, model of pressure-dependent impairment of mammalian muscle function during prolonged exposures to HP. Initially, HP increases SR Ca2+-leak; myoplasmic Ca2+ increase, however, is buffered by mitochondria. After a critical HP-exposure time, mitochondria release Ca2+ back into the cytosol where the deregulated contractile apparatus (degraded troponin-T) becomes irreversibly activated, resulting in a slow contracture [Friedrich 2010].

Behavior of mammalian muscle cells under prolonged HP treatments

Following a 3 hr prolonged HP treatment, a marked decline of force production was seen for pressures exceeding 20 MPa. This rather sharp pressure limit was associated with a steep rise in muscle stiffness and a selective degradation of skeletal muscle Troponin-T (s. Fig). Visualizing membrane integrity online during HP treatment with propidium iodide fluorescence microscopy, pressurized cells remained intact during slow compression and while holding pressure at ~35 MPa for ~20 min. Around 20-25 min, however, a contracture became apparent that slowly developed over minutes and remained irreversible even after decompression while membrane integrity was still intact ruling out unspecific influx of external Ca2+ to account for the contractile activation. To find the specific mechanism underlying the contractile activity, confocal HP-microscopy using cytosolic and mitochondrial Ca2+ dyes Fluo-4 and Rhod-2, respectively, was carried out with the same pressurization protocol (s. Fig.). Upon compression, initial Fluo-4 fluorescence diminishes which is a pressure-reaction of the dye itself (not shown). Between 10 MPa and 35 MPa, cytosolic fluorescence is stationary but mitochondrial fluorescence increases indicative of an SR Ca2+ leak that is buffered by mitochondria. Both signals remain constant when holding the pressure at 35 MPa for ~25 min after which a sudden release of mitochondrial Ca2+ can be seen as a drop in Rhod-2 fluorescence that is being rescued upon immediately initialized decompression. The cell shown was rescued from contracture. This and other results suggest a model in which HP induces SR Ca2+ leak. Ca2+ initially can be buffered by mitochondria at HP. However, once a critical ‘HP x exposure’ product is exceeded, mitochondrial function declines and Ca2+ is being re-poured into the cytosol where it finds a sufficiently de-regulated contractile apparatus to initiate irreversible contracture.

Interpretation and relevance for diving mammals

The observed critical ‘HP x exposure’ product is astonishingly similar to dive profiles of deepest diving mammals (i.e. ~2,500 m and ~25 min bottom time). Maybe, this points towards skeletal muscle being thy limiting abyssal organ. One explanation from the above mentioned mechanism could be that at depth, once the critical ‘HP x bottom time’ product is exceeded, skeletal muscle of diving mammals starts to stiffen. This mechanical signal could be the signal for the animal to ascend to prevent further damage to the muscle, the most important locomotive organ to rejoin the surface.

Literature

  • Friedrich O, v Wegner F, Hartmann M, Frey B, Sommer K, Ludwig H, Fink RH (2006). ‘In situ’ high pressure confocal Ca2+-fluorescence microscopy in skeletal muscle: a new method to study pressure limits in mammalian cells. Undersea Hyperb Med 33(3): 181-195.
  • Friedrich O, Kress KR, Hartmann M, Frey B, Sommer K, Ludwig H, Fink RH (2006). Prolonged high-pressure treatments in mammalian skeletal muscle result in loss of functional sodium channels and altered calcium channel kinetics. Cell Biochem Biophys 45(1): 71-83.