Podcast Summary: TWiN 50 – Neurological Sequelae after COVID-19

Podcast: This Week in Neuroscience (TWiN)
Episode Number: 50
Host: Vincent Racaniello
Date: April 29, 2024
Panelists: Jason Shepherd, Vivian Morrison, Tim Chung
Main Theme: Exploring how exposure of brain tissue to SARS-CoV-2 affects synaptic homeostasis, and implications for COVID-19 neurological symptoms.

Episode Overview

In this milestone 50th episode, the TWiN team discusses a Nature Microbiology study examining whether exposure of the brain to SARS-CoV-2 virions disrupts synaptic balance, potentially shedding light on COVID-19’s neurological sequelae, such as “brain fog” and psychiatric changes. Using both human-derived organoids and postmortem brain slices, the panel dissects the study’s experimental approaches, findings, and their implications for understanding neuro-COVID.

Key Discussion Points & Insights

1. Background & Importance of the Study

• COVID-19’s Brain Effects: Both acute and long COVID have been linked to neurological issues, including sensory loss, brain fog, and psychiatric changes, sparking debate over whether the effects are due to direct viral infection or indirect immune/inflammatory mechanisms.

  “It’s really this sort of long Covid symptoms that are concerning, where some people say that they get brain fog for weeks to months. There’s also sort of personality and psychiatric disorders…”
  — Jason Shepherd [05:26]

• Unresolved Questions: Is the virus infecting neurons and causing notable changes, or are brains simply collateral damage from inflammation? Human studies are challenging—often only possible post-mortem.

2. Study Design and Technical Approaches

• Organoids as a Model: The team explains brain organoids—clusters of human neurons and glia grown in 3D from induced pluripotent stem cells (iPSCs). Useful for modeling, but with limitations: lack of full cell diversity, random “wiring” (circuitry), and absence of microglia unless specifically added.

  “These 3D cultures, these organoids, maybe have a little better circuitry…But…the circuitry is just random.”
  — Jason Shepherd [08:44]

• Questions about Neural Diversity: The team notes that these organoids are mostly excitatory neurons plus astrocytes, typically lacking inhibitory neurons and microglia, possibly affecting the results’ relevance.

  “I think this is just a straightforward organoid protocol. So I don’t think they included those. This is mostly excitatory neurons…”
  — Jason Shepherd [10:17]

3. Infection Findings: Degree of Susceptibility and Permissivity

• Minimal Replication: The study reveals that while neural cells are “permissive”—i.e., have the machinery to allow infection—the actual amount of productive replication is extremely low. Most viral detection is likely lingering input virus, not newly produced infectious particles.

  “They measure by PCR…but they hardly find any infectivity…They say that the low amount of infection…is mostly in neurons, not in glia.”
  — Vincent Racaniello & Jason Shepherd [12:34–15:41]

• Plaque Assays Commended: Unlike many studies, this work included infectivity (plaque) assays—rare in the field and providing a higher standard of evidence.

[Notable Quote | 14:46]

“The problem is you can add virus and then it doesn’t replicate, but you’re just measuring the input virus... So that tells me it’s not even clear that it’s making any infectivity.”
— Vincent Racaniello

4. Synaptic Alterations: Key Experimental Results

A. Proteomics on Organoids:

• Main Finding: Upon (minimal) infection, most altered proteins were synaptic, especially components of the presynaptic terminal (notably the protein Bassoon).

  “Most of the proteins that they see that are altered and increased are synaptic proteins…most on that presynaptic side.”
  — Jason Shepherd [16:08]

• Morphological Change: Presynaptic terminals became enlarged and oddly elongated—a phenotype not typically observed, raising mechanistic questions.

  “There’s an enlargement of the synapse, this presynaptic terminal…not only bigger but…longer…really a phenotype I’ve not really ever seen before.”
  — Jason Shepherd [18:27]

B. Human Postmortem Brain Slices:

• Replication of Phenotype: Postmortem slices cultured ex vivo (sometimes 12–24 hours after death) showed similar synaptic protein changes post-exposure, suggesting phenomenon is not unique to immature organoids.

  “It is interesting…they can culture these pieces of brain that they’ve taken…they show similar things…”
  — Jason Shepherd [24:46]

[Notable Quote | 18:55]

“The phenotype is so—like the picture is so different.”
— Tim Chung

5. Diving into Mechanisms: The Role of Latrophilin/FLRT3

• Upregulated Proteins: Latrophilin, a GPCR implicated in synapse formation, was upregulated both at protein and mRNA levels after infection/exposure.

• Functional Assays: Researchers used an agonist peptide for latrophilin, finding it increased cyclic AMP and could partially revert the synaptic enlargement phenotype—though panelists debate if the peptide’s effect is strictly agonistic.

  “They found that SARS CoV2 significantly increased this statue…induced cyclic AMP levels, even in UV-inactivated samples…”
  — Jason Shepherd [31:10] “Sometimes you can…max out a receptor so that it can’t…the value…is that it can be turned on and turned off…if you have this agonist…can actually prevent that cycling…”
  — Vivian Morrison [29:48]

• Physical Proximity: Using protein proximity labeling, virions—alive or UV-inactivated—were found to colocalize at synapses, particularly with latrophilin and FLRT3, suggesting viral particles may “clog up” these protein interactions.

[Notable Quote | 42:07]

“Even those [UV-inactivated virions] accumulate at these synaptic compartments…They conclude that they think that the virion somehow interacts specifically with latrophylin 3 and…alters the signaling…”
— Jason Shepherd

6. Functional Consequences & Electrophysiological Data

• Electrode Recordings: Organisms infected/exposed to SARS-CoV-2 showed altered local field potentials (LFPs). But the limitations of population-level (rather than single-cell) recordings in random neural circuits make interpretations difficult.

  “I would like to have seen much better resolution where you’re actually looking at specific synapses like patch clamp electrophysiology…This is just a population response.”
  — Jason Shepherd [33:01]

• Peptide’s Protective Effect: Again, the latrophilin agonist peptide reduced the observed synaptic pathology and electrical aberrations.

7. Additional Findings: Immune and Glial Modulation

• Microglia Absence as a Caveat: Classic organoids lack microglia. When monocytes (blood-derived immune cells) were added to cultures, the abnormal synaptic increase was dampened—suggesting that in vivo, microglia may prune abnormal synapses after infection.

  “When you add in the infiltrating monocyte, then it can do its job and get rid of the kind of surplus presynaptic terminals.”
  — Tim Chung [50:32]

• Context in Disease: The discussion notes that excessive synapse pruning by microglia is a known component of neurodegenerative disease.

8. Interpretation & Caveats

• Little Infectious Virus in Neurons: The panel repeatedly highlights the very low level of neuronal infection and particle production.

  “These neuronal cultures are really poorly susceptible and permissive. Very little virus. So it’s not like polio where the virus replicates like gangbusters…It’s far more subtle.”
  — Vincent Racaniello [58:17]

• Debate over Pathogenesis: The virus’ low presence in the brain casts doubt on the idea that direct infection drives neuro-COVID; inflammation or circulating cytokines may be more relevant.

  “Your neuro Covid symptoms…are independent of how much…virus…may or may not be…in your brain. And it’s also not correlated with…peripheral inflammation that you had.”
  — Vivian Morrison [58:55]

• Experimental Limitations:

  • Organoids are more like immature brains, missing blood vessels and microglia.
  • Human postmortem brain slices may not fully recapitulate in vivo context—especially after death, with prior medical treatments (e.g., steroids, which dampen inflammation), and possible loss of cytokines.

Notable Quotes & Memorable Moments

• On Study Ambiguity:

  “So you could potentially just achieve the same outcome by just…treating the cells with the viral proteins…Take us out of BSL3 and just do it in your lab where you can just throw proteins on there.”
  — Vivian Morrison [14:56]

• On Instrument-Themed Synaptic Proteins:

  “There’s bassoon and there’s piccolo…What about oboe?”
  — Vivian Morrison [17:54]
  “I think it’s just woodwind.”
  — Tim Chung

• On Clinical Relevance:

  “I see this paper as…a small piece in the puzzle…It may not…have eventual clinical utility or…it’s just going to help us learn something…”
  — Vivian Morrison [59:13]

Timestamps for Important Segments

| Segment | Topic | Timestamp (MM:SS) | |---------|--------------------------|-----------------| | Episode opening, eclipse banter | 00:00–03:29 | | Introduction to main paper & controversy over neuro-COVID | 03:33–06:47 | | Organoid model and technical discussion | 07:08–11:20 | | Discussion of infectivity findings | 12:23–15:41 | | Proteomic and synaptic changes in organoids | 15:59–18:55 | | Human postmortem slice experiments | 21:17–24:46 | | Latrophilin/FLRT3 and mechanistic assays | 27:41–32:22 | | Electrophysiology and activity findings | 32:30–36:31 | | Pathophysiological implications and microglial role | 48:55–52:23| | Inflammation vs. direct infection, limitations | 55:16–57:51 | | Take-home conclusions | 58:17–61:24 |

Conclusion & Take-Home Messages

• There is little productive infection of human neurons by SARS-CoV-2, contrasting with classic neurotropic viruses (e.g., polio).
• Viral exposure still alters synaptic proteins and morphology, potentially via physical interactions between viral particles (even inactivated) and specific synaptic adhesion molecules like latrophilin/FLRT3.
• Inflammatory (indirect) mechanisms remain likely culprits for neuro-COVID symptoms. Direct viral effects on synapses, while possible, may not be sufficient to explain the clinical picture—especially given the low viral load and observation of symptoms in the absence of clear infection.
• The results highlight important questions for future study, particularly around indirect effects, glial responses, and real-world human pathology.

“It dispels the notion that there’s gangbusters replication in the brain, which is...”
— Vincent Racaniello [58:49]

Panelist Acknowledgements

• Jason Shepherd (@JasonSynaptic, University of Utah)
• Vivian Morrison (Tulane University)
• Tim Chung (NYU)
• Host: Vincent Racaniello (@profvrr, MicrobeTV)

Show notes and past episodes: microbe.tv/twin
Contact: twin@microbe.tv

This summary provides a comprehensive look at the key arguments, findings, and debates from TWiN 50, offering a nuanced understanding for listeners and non-listeners alike.