Title: How dynamic modulation of mitochondrial inner membrane fluidity might regulate respiration

Abstract: Cellular respiration, a process fundamental to all life, involves the intricately complex electron transport chain (ETC) located in the inner mitochondrial membrane (IMM). Functioning in a densely packed mitochondrial membrane, electron transport chain requires the lateral movement of mobile charge (electron) carriers. Therefore, there has been a long standing (and often debated) view whether the lateral mobility of molecules and hence the ‘fluidity’ of IMM (particularly the charge carriers) should influence respiration rates and cellular energetics. An important question that has remained largely unanswered is: do cells modulate the fluidity of respiring membranes as energetic/respiratory demand changes? Specifically, do cells change the fluidity of the densely-packed IMM in response to metabolic stimuli? Answering these questions with any kind of spatial and temporal precision would require appropriate tools and methods for measuring and visualising inner mitochondrial membrane fluidity (in eukaryotes), especially for intact, living cells and tissues. We have developed a robust method to visualise IMM-fluidity in live-cells using a cell-permeable, fluorescent-molecular-rotor. Utilizing fluorescence-lifetime readouts, this method eliminates probe concentration and intensity-fluctuations artefacts. FLIM-imaging reveals exquisite heterogeneity in IMM-fluidity, even across individual mitochondria. Multiplexing with single-cell NADH imaging reveals cells with higher mitochondrial respiration have increased IMM fluidity. Strikingly, cells rapidly modulate IMM fluidity upon stimulation. Thus, rapid modulation of IMM fluidity is a mechanism of adaptation to increased respiratory demand. Our findings open new lines of inquiry into how and when inner membrane fluidity may be tuned in cells – a new regulatory paradigm.

HOST: Sharonda Leblanc