The intestinal intermediate filament network responds to and protects against microbial insults and toxins

Abstract

The enrichment of intermediate filaments in the apical cytoplasm of intestinal cells is evolutionarily conserved, forming a sheath that is anchored to apical junctions and positioned below the microvillar brush border, which suggests a protective intracellular barrier function. To test this, we used Caenorhabditis elegans, the intestinal cells of which are endowed with a particularly dense intermediate filament-rich layer that is referred to as the endotube. We found alterations in endotube structure and intermediate filament expression upon infection with nematicidal B. thuringiensis or treatment with its major pore-forming toxin crystal protein Cry5B. Endotube impairment due to defined genetic mutations of intermediate filaments and their regulators results in increased Cry5B sensitivity as evidenced by elevated larval arrest, prolonged time of larval development and reduced survival. Phenotype severity reflects the extent of endotube alterations and correlates with reduced rescue upon toxin removal. The results provide in vivo evidence for a major protective role of a properly configured intermediate filament network as an intracellular barrier in intestinal cells. This notion is further supported by increased sensitivity of endotube mutants to oxidative and osmotic stress.

Intestinal intermediate filament polypeptides in C. elegans: Common and isotype-specific contributions to intestinal ultrastructure and function

“…7g). The overall increase in IFB-2 was accompanied by a shift from the smaller IFB-2c variant to the larger IFB-2a variant as was reported for N2 subjected to microbial toxin stress 16 . Conversely, IFB-2 depletion increased PLST-1::GFP fluorescence ( Fig.…”

“…These mutations prevented endotube formation (Figs. 5a-f, S4a; see also 16 ). Despite the absence of the electron dense endotube, the morphology of the adjacent microvillar brush border appeared to be only mildly affected presenting slightly disordered microvilli.…”

“…It has been reported that a dysfunctional endotube is the cause of increased susceptibility to microbial stress 16 . We therefore wanted to explore how loss of IFB-2 and IFC-2 interfere with microbial stress resilience.…”

“…An overall protective function of the endotube was further suggested by analyses of sma-5 and ifo-1 mutants, both of which had been identified in genetic screens as upstream regulators of IFB-2 12,15,16 . Inactivation of sma-5 induces local gaps within the endotube and endotube thickening, whereas ifo-1 inactivation leads to a collapse of the entire endotube onto the CeAJ.…”

“…Inactivation of sma-5 induces local gaps within the endotube and endotube thickening, whereas ifo-1 inactivation leads to a collapse of the entire endotube onto the CeAJ. The dysfunctional endotube, in turn, is coupled to increased susceptibility to microbial, oxidative and osmotic stress 16 .…”