Very exciting work, congrats to all 🥳!

Amazing and very well-deserved, congrats @harschnitz.bsky.social!

Thus, we identified species-specific calcium dynamics as a mechanism to set tempo, by tuning axon outgrowth duration and with it axon tract morphologies, thereby demonstrating a causal link between the slowing of tempo and the elaborated morphology of human neurons. (12/12)

Increasing calcium influx in human organoids, by stimulating L-Type VGCCs, led to a shorter axon growth duration and shorter axon tract lengths, like mouse. This was phenocopied by elevating cAMP, a downstream second messenger, whereas blocking calcium influx led to the opposite effect. (11/n)

This revealed that calcium influx during different neural activity modes was consistently higher in mouse compared to human, even at stage-matched timepoints. This pointed to calcium dynamics not only correlating with differences in tempo, but also as a strong candidate to drive tempo. (10/n)

Calcium imaging using the GCAMP7f reporter showed the presence of both spontaneous transients and burst activity at distinct developmental stages in each species. (9/n)

We performed a scRNA-seq experiment covering a range of timepoints in both human and mouse, which pointed to neural activity related calcium channels with an expression profile aligning with neuron development, including axon outgrowth. (8/n)

Combining these with micropatterned devices to capture outgrowing axon tracts, we observed that species-specific differences in axon tract length consistently correlate with the duration of the axon tract outgrowth state, irrespective of tissue environment. (7/n)

Studying axon tract formation is challenging, precisely because of these excessive morphologies. To tackle this, we used human and mouse brain organoid slice cultures that develop long-range axon tracts which recapitulate species-specific differences in axon tract length. (6/n)

To test this, we looked at the development of axon tracts, as their disproportionate expansion are a perfect example of the elaborated morphology of human neurons. Moreover, axon development is completed during embryogenesis, making it feasible to study its final morphological product. (5/n)

Each of these evolutionary changes correlate with a prolonged duration during which these structures are established. We hypothesised that slowing of tempo could be a mechanism driving these changes in magnitude ( recently reviewed in doi.org/10.1038/s415... & (doi.org/10.1016/j.ce...) (4/n)

Differences of the human brain largely stem from changes in magnitude, including an increase in size, cell numbers, and structural domains of neurons, with axon tracts showing an even greater expansion compared to dendrites and synapses. (3/n)

Thank you to @lancasterlab.bsky.social, HM Szafranska, I Imaz-Rosshandler, L Guglielmi, M Moarefian, K Voitiuk, NK Zernicka-Glover, DJ Lloyd-Davies Sánchez, J Minnick, M Teodorescu, AJ Anderson, L Pellegrini for their incredible contributions and fantastic team effort! (2/n)