This Week in The Journal

Regulation of Spine Formation by Extracellular Vesicles

Longbo Zhang, Tiffany V. Lin, Qianying Yuan, Remy Sadoul, TuKiet T. Lam, et al.

(see pages 3799–3807)

Synaptic release of neurotransmitters and neuropeptides is not the only way for neurons to send chemical signals to each other. Like other cells, neurons release small extracellular vesicles that carry proteins, lipids, and nucleic acids. These vesicles can be taken up by other neurons, where their cargoes can modulate protein synthesis and other molecular processes. This type of signaling has been shown to contribute to neural circuit development and plasticity, as well as to the spread of toxic species in neurodegenerative diseases. The cargoes packaged into extracellular vesicles vary across cell types, however, and much remains to be learned about specific cargoes and their functions.

Seeking to understand the signals exchanged between cortical neurons by extracellular vesicles, Zhang et al. conducted a proteomic screen of vesicles isolated from the culture medium of immature cortical neurons taken after 3 d in vitro (DIV). The screen revealed the presence of at least 660 proteins, including several proteins considered to be markers of extracellular vesicles and several DNA binding proteins. Notably, the total protein content of purified extracellular vesicles gathered after 15 DIV was approximately half that present at 3 DIV, suggesting extracellular vesicle release slows as neurons age in culture.

The authors were particularly intrigued by the presence of histone deacetylase 2 (HDAC2) in extracellular vesicles from 3 DIV neurons. Previous work had shown that HDAC2 represses the transcription of several synaptic proteins and thus inhibits dendritic spine formation; its presence in extracellular vesicles suggests that neurons can suppress spine formation by other cells. Indeed, culture medium from 3 DIV neurons (which have not yet formed spines) increased nuclear levels of HDAC2, reduced expression of HDAC2 target genes, and reduced spine density in 15 DIV neurons, which were in the midst of spinogenesis. Importantly, these effects were absent if 15 DIV neurons were treated with HDAC2 inhibitors or if HDAC2 was knocked down in 3 DIV neurons before the medium was collected.

These data suggest that young neurons transfer HDAC2 to more mature neurons via extracellular vesicles, and that this transfer reduces spine density in recipient neurons. This may help to ensure that rapidly maturing neurons do not become hyperinnervated and deprive more slowly maturing neurons of synaptic input.

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Treating 15 DIV cultured neurons with medium containing extracellular vesicles from 3 DIV neurons (right) reduced dendritic spine density relative to that of neurons treated with medium from other 15 DIV neurons (left). See Zhang et al. for details.

Role of MchR1 Localization in the Primary Cilium

Yi-Chun Hsiao, Jesús Muñoz-Estrada, Karina Tuz, Russell J. Ferland

(see pages 3932–3943)

Almost all cells, including neurons, have an immotile, microtubule-based organelle called a primary cilium that acts like an antenna to detect extracellular signals. Many G-protein-coupled receptors (GPCRs) are localized selectively to primary cilia, where they can generate large, localized changes in cyclic nucleotides and other intracellular signaling molecules. For example, GPCRs that transduce photon and chemical signals are localized to the large primary cilia of photoreceptors and olfactory receptor neurons, respectively. Other GPCRs are enriched in primary cilia, but are also present in the plasma membrane beyond this organelle. The role of the primary cilium in signaling mediated by such receptors is unclear, but this might be investigated by selectively disrupting the function of the primary cilium by targeting cilium-specific proteins.

One protein required for primary cilium function is Abelson-helper integration site 1 (AHI1). Mutations in this gene cause Joubert syndrome, characterized by malformation of the cerebellar vermis and peduncles. Ahi1 is localized to the transition zone between the primary cilium and the rest of the cell body, and it is necessary for proper trafficking of proteins in the cilium. Hsiao, Muñoz-Estrada et al. found that knocking out Ahi1 in mice prevented localization of the GPCR melanin-concentrating hormone receptor-1 (MchR1) to the primary cilium of cultured hypothalamic and hippocampal neurons without affecting total expression levels or delivery to the plasma membrane. Importantly, knocking out Ahi1 also prevented MCH-induced inhibition of forskolin-induced cAMP production, as well as MCH-induced activation of ERK kinase signaling in neurons.

These results suggest that even when GPCRs like MchR1 are present beyond the primary cilium, the subset that is localized to the cilium is required to activate downstream signaling pathways. Whether this requirement stems from MchR1 interactions with other cilium proteins remains unclear. Additional work will be needed to answer this question and to determine what purpose MchR1 receptors outside the primary cilium serve. One possibility is that they provide a reservoir of receptors that can be trafficked to the primary cilium to modulate sensitivity to MCH.