Adenovirus-activated PKA and p38/MAPK pathways boost microtubule-mediated nuclear targeting of virus

Nuclear targeting of adenovirus is mediated by the microtubule-dependent, minus-end-directed motor complex dynein/dynactin, in competition with plus- end-directed motility. We demonstrate that adenovirus transiently activates two distinct signaling pathways to enhance nuclear targeting. The first pathway activates integrins and cAMP-dependent protein kinase A (PKA). The second pathway activates the p38/MAP kinase and the downstream MAPKAP kinase 2 (MK2), dependent on the p38/MAPK kinase MKK6, but independent of integrins and PKA. Motility measurements in PKA-inhibited, p38-inhibited or MK2-lacking (MK2(–/–)) cells indicate that PKA and p38 stimulated both the frequency and velocity of minus-end-directed viral motility without affecting the perinuclear localization of transferrin-containing endosomal vesicles. p38 also suppressed lateral viral motilities and MK2 boosted the frequency of minus-end-directed virus transport. Nuclear targeting of adenovirus was rescued in MK2(–/–) cells by overexpression of hsp27, an MK2 target that enhances actin metabolism. Our results demonstrate that complementary activities of PKA, p38 and MK2 tip the transport balance of adenovirus towards the nucleus and thus enhance infection

Quantitative microscopy of fluorescent adenovirus entry.

Fluorescence imaging of cells is a powerful tool for exploring the dynamics of organelles, proteins, and viruses. Fluorescent adenoviruses are a model system for cargo transport from the cell surface to the nucleus. Here, we describe a procedure to quantitate adenovirus-associated fluorescence in different subcellular regions. CCD camera-captured fluorescence sections across entire cells were deblurred by a fast Fourier transformation, the background was subtracted images merged, and virus fluorescence quantitated. The validity of the deblurring routine was verified by confocal laser scanning microscopy, demonstrating that objects were neither generated nor deleted. Instead, the homogeneity of both the average intensity and the size of fluorescent particles was increased, facilitating automated quantification. We found that nuclear fluorescence of wt adenovirus, but not of a virus mutant ts1, which fails to escape from endosomes, was maximal at 90 min postinfection (p.i.). Surprisingly, nuclear fluorescence decreased at 120 min, but increased again at 240 min p.i., suggesting that wt virus targeting to the nucleus may be multiphasic and regulated. Interestingly, only the first nuclear transport period of wt but not ts1 virus coincided with a significant increase of the peripheral and decrease of the cytoplasmic regions, indicative of signal-dependent cell contraction

Innate Immunity to Adenovirus

Human adenoviruses are the most widely used vectors in gene medicine, with applications ranging from oncolytic therapies to vaccinations, but adenovirus vectors are not without side effects. In addition, natural adenoviruses pose severe risks for immuno-compromised people, yet, infections are usually mild and self-limiting in immuno-competent individuals. Here we describe how adenoviruses are recognized by the host innate defense system during entry and replication in immune and non-immune cells. Innate defense protects the host, and at the same time, represents a major barrier to using adenoviruses as therapeutic interventions in humans. Innate response against adenoviruses involves intrinsic factors present at constant levels, and innate factors induced by the host cell upon viral challenge. These factors exert anti-viral effects by directly binding to viruses or viral components, or shield the virus, for example soluble factors, such as blood clotting components, the complement system, preexisting immunoglobulins or defensins. In addition, toll-like receptors and lectins in the plasma membrane and endosomes are intrinsic factors against adenoviruses. Important innate factors restricting adenovirus in the cytosol are tripartite motif-containing proteins (TRIM), nucleotide-binding oligomerization domain (NOD)-like inflammatory receptors and DNA sensors triggering interferon, such as DEAD (Asp-Glu-Ala-Asp) box polypeptide 41 (DDX41) and cyclic guanosine monophosphate-adenosine monophosphate synthase (cGMP-AMP synthase, short cGAS). Adenovirus tunes the function of anti-viral autophagy, and counters innate defense by virtue of its early proteins E1A, E1B, E3 and E4 and two virus-associated noncoding RNAs VA-I and VA-II. We conclude by discussing strategies to engineer adenovirus vectors with attenuated innate responses and enhanced delivery features