Fatal neuroinvasion and SARS-CoV-2 tropism in K18-hACE2 mice is partially independent on hACE2 expression

Abstract

Animal models recapitulating the distinctive features of severe COVID-19 are critical to enhance our understanding of SARS-CoV-2 pathogenesis. Transgenic mice expressing human angiotensin-converting enzyme 2 (hACE2) under the cytokeratin 18 promoter (K18-hACE2) represent a lethal model of SARS-CoV-2 infection. However, the cause(s) and mechanisms of lethality in this mouse model remain unclear. Here, we evaluated the spatiotemporal dynamics of SARS-CoV-2 infection for up to 14 days post-infection. Despite infection and moderate inflammation in the lungs, lethality was invariably associated with viral neuroinvasion and neuronal damage (including spinal motor neurons). Neuroinvasion occurred following virus transport through the olfactory neuroepithelium in a manner that was only partially dependent on hACE2. Interestingly, SARS-CoV-2 tropism was overall neither widespread among nor restricted to only ACE2-expressing cells. Although our work incites caution in the utility of the K18-hACE2 model to study global aspects of SARS-CoV-2 pathogenesis, it underscores this model as a unique platform for exploring the mechanisms of SARS-CoV-2 neuropathogenesis.SUMMARYCOVID-19 is a respiratory disease caused by SARS-CoV-2, a betacoronavirus. Here, we show that in a widely used transgenic mouse model of COVID-19, lethality is invariably associated with viral neuroinvasion and the ensuing neuronal disease, while lung inflammation remains moderate.

The olfactory nerve is not a likely route to brain infection in COVID-19: a critical review of data from humans and animal models

“…7. Some of the studies localizing SARS-CoV-2 in the brain used antibodies against the spike protein to document virus localization [21,41,54,66,91]. However, it is now known that the S1 subunit of the spike protein can be shed from the virus during cell entry, and neurons in the brain can take up such cleaved and systemically circulating spike proteins [75,82].…”

“…Previous work established also that neuro-invasive viruses typically infect only structures neuroanatomically linked to the site of inoculation [90]. However, the time course of SARS-CoV-1 and SARS-CoV-2 invasion from the olfactory epithelium to distant targets in the brain, even those that are not 2nd or 3rd order olfactory targets, is much more rapid: the arrival of the virus is approximately simultaneous in the olfactory bulb and in distant brain targets [28,73,104], or even "skips" the olfactory bulb [21,101,105] or the glomerular layer containing the olfactory axons [104]. These findings do not support the hypothesis that SARS-CoV-2 invades the brain by multiple transfers from neuron to neuron, with the first transfer from olfactory receptor neurons to mitral cells in the olfactory bulb.…”

“…The "older" mouse models were generated several years ago for SARS-CoV-1 studies; they have constitutive hACE2 expression controlled by exogenous promoters such as K18 cytokeratin or forkhead box protein J1 (FOXJ1). These mouse models are more often lethal, presumably due to brain infection [21,41,50,54,76,80,89,101,104], while the "newer" mouse models and mice infected with mouse-adapted SARS-CoV-2 typically do not show neuro-invasion [57] (Table 1). Other viruses, such as human coronavirus OC43 (HCoV-OC43), mouse hepatitis virus, or herpes simplex virus, readily infect olfactory neurons and move effectively by anterograde axonal transport to secondary and tertiary olfactory centers in the brain [3,4,17,33,56,95].…”

ACE2-lentiviral transduction enables mouse SARS-CoV-2 infection and mapping of receptor interactions

“…Virus titer in hACE2-lentivirus transduced mice lung were comparable to other studies using mice transduced with Ad5 or AAV expressing hACE2 (10, 17, 18). These transduction methodologies avoid the fulminant brain infection seen in K18-hACE2 mice, with infection of this organ the major contributor to mortality in the K18-hACE2 model (78, 79). The key advantage of lentiviral over adenoviral systems is the integration of the lentiviral payload into the host cell genome, thereby providing long-term stable expression.…”

SARS-CoV-2: Pathogenesis, Molecular Targets and Experimental Models

“…The laboratory mice transduced with human ACE2 developed pneumonia after COVID-19 infection and neutralized by mAbs with an attenuated lung infection and inflammation (Hassan et al, 2020). A recent study shows the fatal neuroinvasion of SARS-CoV-2 in transgenic K-18-hACE2 mice (Carossino et al, 2021). Another study also illustrates infection of CNS cells and encephalitis by SARS-CoV-2 with the inflammatory response in K18-hACE2 mice (Kumari et al, 2021).…”