Beautifully crafted by Nardos Kebede in the lab. Hope you enjoy!

We also discuss a bit about how these progenitors can be defined, and the molecular signatures that label them as well as drive their differentiation trajectories.

This is particularly relevant for a newly defined class of cells that has garnered dozens (!!!) of names in recent years. We nominate a more unified nomenclature, and explain our view of how all of these populations represent overlapping cell states

Neurodevelopment is such a fun field because so much is new to explore and define. But also that means it can be unclear. In our new review out this week @cp-trendsneuro.bsky.social, we try to synthesize the confusing field of neural intermediate progenitors authors.elsevier.com/a/1msKvbotqB...

There's so much more packed into this work, so please check it out as we continue to investigate these questions!!! This is all possible because of an amazing scientific collaborative team @emorygenetics.bsky.social, @brainorganoidhub.bsky.social, and our funders.

We think there are important implications here. Can astrocyte reactivity be reversed in the setting of neurologic disease? Why do astrocytes present peptides via MHCII? Is this pro- or anti-inflammatory for T-cells???

And after all this, we wondered if it may be happening in adult human patient samples, too. We looked in biopsies of brain abscesses where chronic inflammation is likely, and found lots of MHCII positive astrocytes. We even saw examples in aged brains without other pathologies.

Then we tried something fancier. We isolated peptides from human neurons and co-cultured with astrocytes in the presence of cytokines. After MHCII pulldown and MS we found lots of neuronal peptides associated with MHCII!!

We found enrichment of proteins linked to MHCII processing (left heatmap) across nearly all pulldowns. We also identified unique peptide fragments in each sample (right), which were more likely to have endogenous cleavage rather than trypsin digestion during mass spec prep.

What could astrocytes be presenting? This is really hard to figure out in human models, but we took a stab at it. We pulled down MHCII proteins in astrocytes and performed mass spec to see what peptide fragments came with it.

We thought maybe this was some artifact related to organoids. But no! We see the same thing in primary human cortical slices. We see MHCII at the cell surface AND we see CD74 loaded onto MHCII only in the presence of inflammatory cytokines

ok, but surprise! Remember those time-dependent genes that only turn on with prolonged inflammatory exposure? Almost all were related to MHCII presentation. We thought this must be a mistake. Astrocytes are not canonical antigen presenting cells! But we see this also by protein staining.

More to come on that in a sec. But FIRST! We had another question. If you make astrocytes "reactive", can they reverse back to a "normal" state if you remove the inflammatory cue? Does it matter how long the initial inflammatory signal lasts before withdrawing it? The answer is generally, YES!

Looking across transcriptomic and chromatin accessibility landscapes, we saw 3 clear patterns: some genes and loci react immediately to cytokines, while others emerge only with sustained exposure. A small group of genes also turn off quite quickly.

So then we wondered whether it mattered how long astrocytes see an inflammatory environment. After all, some neurological injuries are very short-lasting, and others chronic. So, we exposed aged organoids with astrocytes to very short (1 day) or long (3 months) inflammatory periods.

We started with a simple question. We know astrocytes form in human cortical organoids, but can we induce reactivity in 3D, and at more immature ages? Answer is yes. See C3 induction here in the setting of inflammatory cytokines. RNAseq shows robust reactive signatures, too!

New work from our team, led by the incredible
@emily-hill.bsky.social out today
@cp-neuron.bsky.social. Including some exciting additions from our prepint. Thread below! authors.elsevier.com/a/1ms0u3BtfH...

There's so much more packed into this work, so please check it out as we continue to investigate these questions!!! This is all possible because of an amazing scientific collaborative team and our funders.

We think there are important implications here. Can astrocyte reactivity be reversed in the setting of neurologic disease? Why do astrocytes present peptides via MHCII? Is this pro- or anti-inflammatory for T-cells???

And after all this, we wondered if it may be happening in adult human patient samples, too. We looked in biopsies of brain abscesses where chronic inflammation is likely, and found lots of MHCII positive astrocytes. We even saw examples in aged brains without other pathologies.

Then we tried something fancier. We isolated peptides from human neurons and co-cultured with astrocytes in the presence of cytokines. After MHCII pulldown and MS we found lots of neuronal peptides associated with MHCII!!

We found enrichment of proteins linked to MHCII processing (left heatmap) across nearly all pulldowns. We also identified unique peptide fragments in each sample (right), which were more likely to have endogenous cleavage rather than trypsin digestion during mass spec prep

What could astrocytes be presenting? This is really hard to figure out in human models, but we took a stab at it. We pulled down MHCII proteins in astrocytes and performed mass spec to see what peptide fragments came with it.

We thought maybe this was some artifact related to organoids. But no! We see the same thing in primary human cortical slices. We see MHCII at the cell surface AND we see CD74 loaded onto MHCII only in the presence of inflammatory cytokines

ok, but surprise! Remember those time-dependent genes that only turn on with prolonged inflammatory exposure? Almost all were related to MHCII presentation. We thought this must be a mistake. Astrocytes are not canonical antigen presenting cells! But we see this also by protein staining

More to come on that in a sec. But FIRST! We had another question. If you make astrocytes "reactive", can they reverse back to a "normal" state if you remove the inflammatory cue? Does it matter how long the initial inflammatory signal lasts before withdrawing it? The answer is generally, YES!

Looking across transcriptomic and chromatin accessibility landscapes, we saw 3 clear patterns: some genes and loci react immediately to cytokines, while others emerge only with sustained exposure. A small group of genes also turn off quite quickly.

So then we wondered whether it mattered how long astrocytes see an inflammatory environment. After all, some neurological injuries are very short-lasting, and others chronic. So, we exposed aged organoids with astrocytes to very short (1 day) or long (3 months) inflammatory periods.

We started with a simple question. We know astrocytes form in human cortical organoids, but can we induce reactivity in 3D, and at more immature ages? Answer is yes. See C3 induction here in the setting of inflammatory cytokines. RNAseq shows robust reactive signatures, too!

This wouldn’t have been possible without our amazing collaborators and support (Emory HERCULES, @emorygenetics.bsky.social) and the bold and fearless ambition of @maureenbiologies.bsky.social