Knockout of angiotensin converting enzyme-2 receptor leads to morphological aberrations in rodent olfactory centers and dysfunctions associated with sense of smell

Abstract

Neuronal morphological characterization and behavioral phenotyping in mouse models help dissecting neural mechanisms of brain disorders. Olfactory dysfunctions and other cognitive problems were widely reported in asymptomatic carriers and symptomatic patients infected with Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2). This led us to generate the knockout mouse model for Angiotensin Converting Enzyme-2 (ACE2) receptor, one of the molecular factors mediating SARS-CoV-2 entry to the central nervous system, using CRISPR-Cas9 based genome editing tools. ACE2 receptors and Transmembrane Serine Protease-2 (TMPRSS2) are widely expressed in the supporting (sustentacular) cells of human and rodent olfactory epithelium, however, not in the olfactory sensory neurons (OSNs). Hence, acute inflammation induced changes due to viral infection in the olfactory epithelium may explain transient changes in olfactory detectabilities. As ACE2 receptors are expressed in different olfactory centers and higher brain areas, we studied the morphological changes in the olfactory epithelium (OE) and olfactory bulb (OB) of ACE2 KO mice in comparison with wild type animals. Our results showed reduced thickness of OSN layer in the OE, and a decrease in cross-sectional area of glomeruli in the OB. Aberrations in the olfactory circuits were revealed by lowered immunoreactivity toward microtubule associated protein 2 (MAP2) in the glomerular layer of ACE2 KO mice. Further, to understand if these morphological alterations lead to compromised sensory and cognitive abilities, we performed an array of behavioral assays probing their olfactory subsystems’ performances. ACE2 KO mice exhibited slower learning of odor discriminations at the threshold levels and novel odor identification impairments. Further, ACE2 KO mice failed to memorize the pheromonal locations while trained on a multimodal task implying the aberrations of neural circuits involved in higher cognitive functions. Our results thus provide the morphological basis for the sensory and cognitive disabilities caused by the deletion of ACE2 receptors and offer a potential experimental approach to study the neural circuit mechanisms of cognitive impairments observed in long COVID.

Perceptual learning deficits mediated by somatostatin releasing inhibitory interneurons of olfactory bulb in an early life stress mouse model

“…However, the extent of ELA effects on olfactory subsystems that are involved in pheromonal information processing remains elusive. Experiments using sensitive behavioural assays quantifying pheromonal-based perceptual learning would provide answers to many of these related questions [ 65 , 66 ]. Taken together, our findings provide evidences for the negative effects of ELA on the pre-cortical brain area, the OB, and the perceptual learning deficits caused by it.…”

Long COVID: From olfactory dysfunctions to viral Parkinsonism

“…Clinical reports investigating various aspects of brain pathophysiology provide the potential link between Parkinson's disease and viral infections, including SARS‐CoV‐2. However, to move toward the causality and circuit mechanisms, a multipronged experimental approach using animal models is needed 124–129 . Different animal models that can be used to predict adverse effects of long‐COVID 82,127,130 .…”

“…However, to move toward the causality and circuit mechanisms, a multipronged experimental approach using animal models is needed 124–129 . Different animal models that can be used to predict adverse effects of long‐COVID 82,127,130 . Long‐term experiments in which SARS‐CoV‐2 infection is introduced into PD animal models or in which parkinsonism is induced in COVID‐19‐infected animals would provide answers to many unsolved questions.…”