SWAlexander
Senior Member
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Abstract
Numerous viruses use specialized surface molecules called fusogens to enter host cells. Many of these viruses, including the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), can infect the brain and are associated with severe neurological symptoms through poorly understood mechanisms. We show that SARS-CoV-2 infection induces fusion between neurons and between neurons and glia in mouse and human brain organoids. We reveal that this is caused by the viral fusogen, as it is fully mimicked by the expression of the SARS-CoV-2 spike (S) protein or the unrelated fusogen p15 from the baboon orthoreovirus. We demonstrate that neuronal fusion is a progressive event, leads to the formation of multicellular syncytia, and causes the spread of large molecules and organelles. Last, using Ca2+ imaging, we show that fusion severely compromises neuronal activity. These results provide mechanistic insights into how SARS-CoV-2 and other viruses affect the nervous system, alter its function, and cause neuropathology.INTRODUCTION
Infectious diseases that involve the nervous system are caused by a wide spectrum of agents, including bacteria, fungi, parasites, and viruses (1). Viruses from diverse families, such as rabies virus, herpes simplex virus, Epstein-Barr virus, Zika virus, reovirus, and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can infect neurons (2–8). Viral brain infections are characterized by multiple neurological symptoms, including headache, fever, confusion, epileptic seizures, and loss of taste or smell. In more severe cases, viral brain infections can lead to encephalitis and meningitis, as well as potentially irreversible neuronal deficits such as paralysis and death. Clinical symptoms can originate from the loss of infected neurons (9); however, some viruses do not kill their host cells, and the chronic neurological sequelae of these infections cannot be explained by neuronal death (10). Other neuropathological mechanisms must therefore underlie the progression of these viral infections, leading to brain dysfunction. In non-neuronal tissues, enveloped viruses and reoviruses use specialized molecules called fusogens to fuse with host membranes and enter cells (11). These viruses then hijack the cellular machinery to produce viral components, with newly synthesized viral fusogens redecorating the cell membrane and conferring the ability to fuse with neighboring cells. This results in the formation of multinucleated syncytia, which allow viral propagation “from within,” without the need for virion release into the extracellular space (12–14). As defined more than 100 years ago by Santiago Ramón y Cajal, the nervous system is composed of discrete neurons that act as individual units and do not base their development or communication on cellular fusion. Preserving neuronal individuality is critical for the correct function of the nervous system, and it is still poorly understood whether viral infection and the resulting presence of viral fusogens can cause neuronal fusion and the formation of syncytia, thereby permanently altering the neuronal circuitry and function.RESULTS
SARS-CoV-2 infection causes neuron-neuron, neuron-glia and glia-glia fusion in murine hippocampal cultures and in human-derived brain organoids
SARS-CoV-2 causes primarily a respiratory disease, but increasing evidence has revealed the presence of viral RNA and proteins also in the brain and a multitude of neuropsychiatric syndromes (8, 15, 16), which appear in the early stages of the disease and persist for months after infection, in what has recently been termed long COVID (17). Viral fusogens are involved in the recognition, binding, and entrance of viruses into their host cells. During infection, these proteins are expressed de novo to form new viral particles, trafficking from secretory organelles to the host plasma membrane and causing cell-cell fusion (18, 19). To determine whether viral neuroinfection induces the fusion of neurons, we used SARS-CoV-2 and a fluorescence fusion assay, whereby different intracellular fluorophores transfer between fused cells. SARS-CoV-2 uses the human angiotensin-converting enzyme 2 (hACE2) as its main receptor on the surface of the host cell (20), and neuropilin 1 (NRP1) as a cofactor to enhance infectivity (21). Among other tissues, hACE2 is expressed in neuronal and glial cells in the human central nervous system (22). Mouse neurons express mACE2 (23), which shares 81.86% interspecies homology with the human protein but lacks key residues for spike S binding (20), resulting in modest to weak affinity for SARS-CoV-2 (24). We, therefore, expressed hACE2 in murine-derived brain cells. Immediately after embryonic hippocampal dissection, a population of these brain cells was electroporated with a plasmid encoding hACE2 and another encoding green fluorescent protein (GFP); a second population of hippocampal cells was electroporated with a plasmid encoding hACE2 and another encoding mCherry. The two neuronal populations were then plated together and maintained for 5 days in vitro (5 DIV; Fig. 1A). The original ancestral SARS-CoV-2 was amplified in Vero cells expressing human transmembrane protease serine 2 (TMPRSS2) and titrated by plaque assay (25). The neuronal cultures were infected with 2 × 105, 2 × 103, or 20 plaque-forming units (PFUs) or were mock-infected with control culture medium. Seventy-two hours post-infection (hpi), the cultures were fixed and examined by confocal fluorescence microscopy, revealing the presence of fused neurons [positive for the neuronal marker microtubule-associated protein 2 (MAP2)], as characterized by the presence of both the GFP and mCherry fluorescent proteins for all the SARS-CoV-2 titers used (Fig. 1, B and C). Antibody staining revealed that fused neurons were positive for the fusogenic spike S protein (Fig. 1D and fig. S1, A and B) (26), which was distributed across the surface of infected neurons (Fig. 1E). In contrast, neuronal fusion was not observed in the mock control, and the spike S protein was not detected (Fig. 1B and fig. S1, A and B). Upon SARS-CoV-2 infection, glial cells expressing hACE2 were also positive for the spike S protein (fig. S1, C and D), and we observed additional fusion phenotypes, including neuron-glia (fig. S2, A to C) and glia-glia fusion (fig. S2, D to F). In agreement with previous findings (2), we found that high doses of neuronal SARS-CoV-2 infection resulted in cell damage, a phenotype that was not apparent at the lowest doses used within 72 hpi (fig. S1, E and F).more extensive reading: https://www.science.org/doi/10.1126/sciadv.adg2248
