Herpes simplex virus 1 subverts the mitochondrial network to support the infection: A lesson on mitochondrial...

Herpes simplex virus 1 (HSV-1) infects approximately 67% of the population worldwide. It establishes lifelong reservoirs in sensory neurons and has been linked to several diseases including neuronal dysfunction. Disruption of mitochondrial homeostasis is a hallmark of HSV-1 infection, however a molecular understanding of these changes and their significance is not yet well defined. HSV-1 infection causes a UL12.5-dependent inhibition of mitochondrial biogenesis through the loss of mitochondrial DNA and mitochondrial transcription factors, PGC-1α (peroxisome proliferator-activated receptor-gamma co-activator) and TFAM (mitochondrial transcription factor). Conversely, UL12.5-independent mechanisms inhibit mitochondrial fusion by activating the OMA1 metallopeptidase that cleaves the inner mitochondrial membrane fusion protein OPA1 (optic atrophy protein 1) and by down-modulating the outer mitochondrial membrane fusion protein MFN2 (mitofusin 2). This inhibition of fusion results in a smaller mitochondrial network that clusters to perinuclear regions, likely supplying energy for viral replication and envelopment. The inner mitochondrial membrane protein TIM23 is also down-modulated during infection in a UL12.5-independent mechanism. Failure of the virus to promote these changes negatively impacts the infection. Despite these changes, mitochondria are protected from mitophagy due to the viral-induced degradation of several mitophagy adaptor proteins, whereby damaged mitochondrial components, including mitochondrial DNA, are extruded via extracellular vesicles. These mitochondrial changes still support functions necessary for HSV-1 infection. Basal cell respiration is preserved, while spare respiratory capacity and extracellular acidification rates increase, indicating glycolytic activity. Mitochondrial membrane potential is also preserved. Overall, our studies provide mechanistic insight into how HSV-1 impacts mitochondria, which could contribute to viral pathogenesis.

Mitochondria are often referred to as the "powerhouse" of the cell because they are the main energy producers. Disruption of mitochondrial homeostasis is associated with multiple diseases and occurs after infection with pathogens such as HSV-1. By investigating the mechanism(s) by which HSV-1 disrupts mitochondrial homeostasis, we can better understand how HSV-1 causes pathogenesis. HSV-1 infection impacts mitochondrial homeostasis through disruption of four key processes, including: 1) inhibition of mitochondrial biogenesis and the generation of new mitochondria; 2) inhibition of mitochondrial fusion, which rescues reversibly damaged mitochondria; 3) sustaining mitochondrial fission, which removes damaged content; and 4) preventing mitophagy, which clears damaged mitochondria. UL12.5-dependent and UL12.5- independent events during HSV-1 infection disrupt mitochondrial homeostasis, redirecting mitochondrial resources towards progeny virus production. These changes cause irreversible damage to host cells, ultimately driving pathogenesis.