Data and Code for: Phospholipase-catalyzed Degradation Drives Domain Morphology and Rheology Transitions in Model Lung Surfactant Monolayers

Lung surfactant is inactivated in acute respiratory distress syndrome
(ARDS) by a mechanism that remains unclear. Phospholipase (PLA2) plays an
essential role in the normal lipid recycling processes, but is present in
elevated levels in ARDS, suggesting it plays a role in ARDS
pathophysiology. PLA2 hydrolyzes lipids such as DPPC—the primary component
of lung surfactant—into palmitic acid (PA) and lyso-PC (LPC). Because PA
co-crystallizes with DPPC to form rigid, elastic domains, we hypothesize
that PLA2-catalyzed degradation establishes a stiff, heterogeneous
rheology in the monolayer, and suggests a potential mechanical role in
disrupting lung surfactant function during ARDS. Here we study the
morphological and rheological changes of DPPC monolayers as they are
degraded by PLA2 using interfacial microbutton microrheometry coupled with
fluorescence microscopy.  While degrading, domain morphology
passes through qualitatively distinct transitions:
 compactification, coarsening, solidification, aggregation,
network percolation, network erosion, and nucleation of PLA2-rich domains.
Initially, condensed domains relax to more compact shapes, and coarsen via
Ostwald ripening and coalescence up until the domains solidify, marked by
a distinct roughening of domain boundaries that does not relax.
 Domains aggregate and eventually form a percolated network,
whose elements then erode and whose connections are broken as degradation
continues.  The relative enzymatic activity of PLA2, set by the
age of the sample, impacts the order and the duration of morphology
transitions. The fresher the PLA2, the faster the overall degradation, and
the earlier the onset of domain solidification: domains solidify before
aggregating with fresh PLA2 samples, but aggregate and percolate before
solidification with aged PLA2.  Irrespective of the activity of
the PLA2, all measured linear viscoelastic surface shear moduli obey the
same dependence on condensed phase area fraction (log|G*|∝Φ) throughout
monolayer degradation.  Moreover, the onset of domain
solidification coincides with the time when the relative surface
elasticity begins to increase.