Brain barrier and support in a dish: Deep within the brain, the choroid plexus filters blood and secretes cerebrospinal fluid (CSF), a nutrient-rich liquid that bathes and supports the brain and protects it from entry of toxic compounds. Current understanding of this vital tissue in humans is limited. Pellegrini et al. developed choroid plexus organoids that quantitatively predict human brain permeability of small molecules and secrete an isolated CSF-like fluid (see the Perspective by Silva-Vargas and Doetsch). This CSF model reveals secretion of developmental factors and disease-related biomarkers by key cell types and provides a testing ground for drug entry into the brain. Science , this issue p. eaaz5626 ; see also p. 143

Mutations in leucine-rich repeat kinase 2 (LRRK2) cause autosomal dominant Parkinson disease (PD), while polymorphic LRRK2 variants are associated with sporadic PD. PD-linked mutations increase LRRK2 kinase activity and induce neurotoxicity in vitro and in vivo. The small GTPase Rab8a is a LRRK2 kinase substrate and is involved in receptor-mediated recycling and endocytic trafficking of transferrin, but the effect of PD-linked LRRK2 mutations on the function of Rab8a is poorly understood. Here, we show that gain-of-function mutations in LRRK2 induce sequestration of endogenous Rab8a to lysosomes in overexpression cell models, while pharmacological inhibition of LRRK2 kinase activity reverses this phenotype. Furthermore, we show that LRRK2 mutations drive association of endocytosed transferrin with Rab8a-positive lysosomes. LRRK2 has been nominated as an integral part of cellular responses downstream of proinflammatory signals and is activated in microglia in postmortem PD tissue. Here, we show that iPSC-derived microglia from patients carrying the most common LRRK2 mutation, G2019S, mistraffic transferrin to lysosomes proximal to the nucleus in proinflammatory conditions. Furthermore, G2019S knock-in mice show a significant increase in iron deposition in microglia following intrastriatal LPS injection compared to wild-type mice, accompanied by striatal accumulation of ferritin. Our data support a role of LRRK2 in modulating iron uptake and storage in response to proinflammatory stimuli in microglia.

The human brain has undergone an evolutionary expansion in size, both in terms of cell numbers and the size of cellular structures, including axon tracts. Human brain development also progresses slowly and takes particularly long. However, the functional relevance of slowed timing, and whether it is responsible for these changes in size, remains unknown. Here, we investigate this by studying axon tract development in human and mouse brain organoids. We demonstrate that human axon tracts grow ∼2x more slowly than those of mice, reflecting their slowed tempo, but that this actually leads to shorter human axons, not longer. To overcome the effect of slowed tempo, human axons have a more prolonged growth duration that enables them to project farther despite their slower growth rate. Using a combination of single-cell RNA sequencing and live imaging, we demonstrate that the prolonged duration involves a different mechanism to that controlling tempo and is driven by calcium dynamics. Human axons exhibit a reduced calcium influx compared to mouse, mediated by L-Type voltage-gated calcium channels. Stimulating this calcium influx in human neurons triggers earlier cessation of growth, leading to shorter axon tracts similar to those of mouse. We further show that increasing calcium speeds up the transition to the synaptogenesis stage. Thus, calcium regulation sets the timing of transitions to disproportionately extend developmental duration, thereby enabling evolutionary expansion of human neurons.

The choroid plexus (ChP) is a specialised tissue of the central nervous system that produces cerebrospinal fluid (CSF), maintains cerebral homeostasis and forms the blood-CSF barrier (B-CSF-B), a key interface that regulates the exchange of substances between the blood and the brain. Despite its physiological importance, the involvement of the ChP in neurodegenerative diseases such as Alzheimer’s disease (AD), remains poorly understood. This is largely due to the reliance on murine models and the limited availability of human brain tissue. Recent advances in human stem-cell derived ChP organoids now offer a more physiologically relevant model to interrogate ChP role in human health and disease. Given that in AD pathology beta-amyloid (Aβ) accumulation has been linked to early disruption of brain barriers, studying the B-CSF-B is particularly relevant. Transthyretin (TTR), the predominant protein secreted by the ChP, is thought to play a role in the transport and clearance of Aβ, although its exact mechanisms are not yet fully elucidated. Here, we propose the use of ChP organoids to investigate the role of the B-CSF-B in amyloid uptake which may contribute to barrier dysfunction and disease progression in AD.

The European Choroid plexus Scientific Forum (ECSF), held in Heidelberg, Germany between the 7th and 9th of November 2023, involved 21 speakers from eight countries. ECSF focused on discussing cutting-edge fundamental and medical research related to the development and functions of the choroid plexus and its implications for health, aging, and disease, including choroid plexus tumors. In addition to new findings in this expanding field, innovative approaches, animal models and 3D in vitro models were showcased to encourage further investigation into choroid plexus and cerebrospinal fluid roles.