A new study published on the preprint server bioRxiv*, in June 2020, reports that the condition of overactive bladder (OAB) could be associated with a different route of infection by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent for COVID-19 disease.
Urinary infection with SARS-CoV-2
Official data shows that the novel coronavirus initially named 2019-nCoV, which first appeared in Wuhan, China, circulated rapidly around the world to most countries in a couple of months. Chiefly characterized at first as causing fever and a dry cough, with significant and sometimes terminal respiratory distress in a minority of cases, it is also known to cause urinary tract damage in a few patients.
However, the virus has not been found in urine so far. Despite this, the knowledge that the virus SARS-CoV-1, which caused the earlier SARS epidemic, was detected in the urine of these patients, suggests that the urinary system could potentially convey infection to the host. This prompted the current study to look into the presence of ACE2 receptors for the virus in these organs. This is crucial in understanding how the virus affects urinary tract tissues, and in developing counter-strategies.
Both the SARS-CoV and the current SARS-CoV-2 are reported to have the same receptor, namely, the ACE2 molecule. Earlier research supports the bladder epithelium’s potential role as the target for the novel coronavirus. The researchers aimed to explore the marked pattern formed by the distribution of ACE2 in the bladder epithelium, decreasing from the umbrella cells in the outer epithelium to the basal cells in the inner layer, and the intermediate cells between these layers.
It is well recognized that older people and those with coexisting medical conditions are at a much higher risk of both infection from SARS-CoV-2 and death from COVID-19. This adds urgency to the task of finding out how the patterns of viral infection change with the underlying disease. The current study aims at examining the structure of the cell clusters and the expression of the ACE2 receptors in the urinary bladder, and to understand how OAB, associated with human aging, operates on the various bladder epithelial cells, using a mouse model.
Using Mice In place of Men
OAB mice were preferentially used because OAB is common in older humans, requires costly medication, and can be well simulated in mice. The researchers used bioinformatics scRNA-seq, a tool that has been used extensively in research on the current pandemic. It has the advantage of being able to examine gene expression in all types of cells, at high resolution and without bias.
The researchers used two scRNA-Seq transcriptomes to compare bladder tissue from normal and OAB mice, as well as fusing publicly available data on human and mouse bladder ACE2 expression. By doing so, they were able to find how the presence of underlying disease might affect this potential source of infection.
The annotated cell types from the dataset did not differ significantly from those in the original work except for the mouse neuron, which was originally labeled as such but later shown to be fibroblasts using superior markers. The study showed that the ACE2 expression in human bladder tissue was seen mainly in three epithelial cell types, and to a low extent in fibroblasts and monocytes. The mouse bladder also showed a higher density of ACE2 than in humans, but with the same distribution.
Changes in ACE2 expression in OAB
Over 7,000 cells were sequenced, with 8 cell types being differentiated, including epithelial cells, basal cells, monocytes, endothelial cells, fibroblasts, and smooth muscle cells. Most of the ACE2 expression was on the intermediate and basal cells, but the highest percentage of expression of this molecule was on the umbrella cells.
Bladder stroma and immune cells downregulated ACE2 in OAB. Thus, the ACE2 receptor is now known to be expressed in respiratory, digestive, and urinary tracts. This means the virus can infect these systems. The relatively high expression of ACE2 in the bladder epithelium, and especially the umbrella cells, indicates the greater vulnerability of this tissue to the SARS-CoV-2.
The transitional epithelium of the bladder has a high turnover, which may make it more susceptible to bladder conditions like OAB. The researchers wanted to detect changes in the scRNA-Seq transcriptome due to OAB but found it difficult to get samples of healthy bladder tissue. By comparing cell types individually between species, a good correlation was established between human and mouse bladder cells.
Concerning cell type, umbrella cells virtually disappeared in OAB, though they were the most frequent cell type in the bladder at 72%. However, myofibroblasts increase in OAB with increased ACE2 expression. OAB may change the infection pathway through the change in ACE2 expression, altering susceptibility, severity, and therapeutic strategy.
Limitations and Future Studies
Different datasets show widely varying levels of ACE2 expression in the bladder, at 1.3%, and 9.5% in humans according to the GSE108097 reports and the GSE129845 reports, respectively. With mice, GSE129845 showed 18% and the researchers’ data 72%. More work is therefore needed on this.
SARS-CoV could be recovered and cultured from the urine of SARS patients for over 4 weeks, conferring significant infectious potential for a long time. With single-cell evidence supporting the possibility of SARS-CoV-2 infection of the urinary tract, and rapid spread of the disease outside China, the possible role of the urinary tract in spreading infection should be taken note of and prevented.
The scientists suggest disinfecting public toilets regularly and safe disposal methods for patient urine to avoid such spread.
The study’s limitations include the fact that no direct relation was observed between clinical features of SARS-CoV-2 and the mouse scRNA-seq studies, and the low number of mouse bladder samples, which may also be less efficient at binding the virus compared to human specimens. Further study is necessary, using human ACE2 or human specimens and larger sample size.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.