Neuroinvasion by SARS-CoV-2 is now accepted. To investigate whether low testosterone levels observed in men with severe COVID-19 could be of central origin, we retrospectively analyzed blood samples from 60 male intensive-care patients and explored SARS-CoV-2 brain entry using animal and cellular models as well as adult COVID-19 patient and fetal human brains. Most hypotestosteronemic patients displayed hypogonadotropic hypogonadism or abnormal hypothalamic-pituitary-gonadal axis regulation. Neurons producing gonadotropin-releasing hormone (GnRH), the master molecule controlling fertility, expressed angiotensin-converting enzyme 2 and neuropilin-1, two host-cell factors mediating infection, and were infected and dying in all COVID-19 patient brains. Tanycytes - hypothalamic glia that regulate GnRH secretion - were also infected. Additionally, human fetal olfactory and vomeronasal epithelia, from which GnRH neurons arise, richly expressed both the above host-cell susceptibility factors and formyl peptide receptor 2, a putative vomeronasal receptor that also appeared involved in SARS-CoV-2 pathogenesis in humans and mice. Finally, a fetal human GnRH cell line expressing all these receptors could be infected by a SARS-CoV-2-like pseudovirus. Together, our findings suggest that GnRH neurons, which may be implicated in brain development and aging in addition to reproduction, are particularly vulnerable to SARS-CoV-2 in both adults and fetuses/newborns, with potentially devastating long-term consequences.
The accumulation of pathological Tau in the brain and cerebrospinal fluid (CSF) and its eventual increase in the blood are hallmarks of Alzheimer’s disease (AD). However, the mechanisms of Tau clearance from the brain to the periphery are not clear. We show here, using animal and cellular models as well as patient blood samples and post mortem brains, that hypothalamic tanycytes, whose cell bodies line the ventricular wall and send long processes to the underlying pituitary portal capillary bed, take up and transport Tau from the CSF and release it into these capillaries, whence it travels to the pituitary and eventually the systemic circulation. Specifically blocking tanycytic vesicular transport leads to an accumulation of exogenous fluorescent Tau in the CSF of mice. In AD and frontotemporal dementia, tanycytic morphology is altered, with a dramatic fragmentation of the secondary cytoskeleton in the former but not the latter, accounting for reduced CSF Tau clearance in AD. Both the implication of tanycytic degradation in the pathophysiology of a human disease and the evidence for the existence of a brain-to-blood tanycytic shuttle are unprecedented, and raise important questions regarding the role of tanycytes in physiological clearance mechanisms and the development of neurodegenerative disorders.
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