The radial organization of neuronal primary cilia is acutely disrupted by seizure and ischemic brain injury

Ultrastructural differences impact cilia shape and external exposure across cell classes in the visual cortex

A primary cilium is a membrane-bound extension from the cell surface that contains receptors for perceiving and transmitting signals that modulate cell state and activity. Primary cilia in the brain are less accessible than cilia on cultured cells or epithelial tissues because in the brain they protrude into a deep, dense network of glial and neuronal processes. Here, we investigated cilia frequency, internal structure, shape, and position in large, high-resolution transmission electron microscopy volumes of mouse primary visual cortex. Cilia extended from the cell bodies of nearly all excitatory and inhibitory neurons, astrocytes, and oligodendrocyte precursor cells (OPCs) but were absent from oligodendrocytes and microglia. Ultrastructural comparisons revealed that the base of the cilium and the microtubule organization differed between neurons and glia. Investigating cilia-proximal features revealed that many cilia were directly adjacent to synapses, suggesting that cilia are poised to encounter locally released signaling molecules. Our analysis indicated that synapse proximity is likely due to random encounters in the neuropil, with no evidence that cilia modulate synapse activity as would be expected in tetrapartite synapses. The observed cell class differences in proximity to synapses were largely due to differences in external cilia length. Many key structural features that differed between neuronal and glial cilia influenced both cilium placement and shape and, thus, exposure to processes and synapses outside the cilium. Together, the ultrastructure both within and around neuronal and glial cilia suggest differences in cilia formation and function across cell types in the brain.

Mapping of neuronal and glial primary cilia contactome and connectome in the human cerebral cortex

Primary cilia act as antenna receivers of environmental signals and enable effective neuronal or glial responses. Disruption of their function is associated with circuit disorders. To understand the signals these cilia receive, we comprehensively mapped cilia's contacts within the human cortical connectome using serial-section EM reconstruction of a 1 mm3 cortical volume, spanning the entire cortical thickness. We mapped the "contactome" of cilia emerging from neurons and astrocytes in every cortical layer. Depending on the layer and cell type, cilia make distinct patterns of contact. Primary cilia display cell-type- and layer-specific variations in size, shape, and microtubule axoneme core, which may affect their signaling competencies. Neuronal cilia are intrinsic components of a subset of cortical synapses and thus a part of the connectome. This diversity in the structure, contactome, and connectome of primary cilia endows each neuron or glial cell with a unique barcode of access to the surrounding neural circuitry.

Nanometer-scale views of visual cortex reveal anatomical features of primary cilia poised to detect synaptic spillover

A primary cilium is a thin membrane-bound extension off a cell surface that contains receptors for perceiving and transmitting signals that modulate cell state and activity. While many cell types have a primary cilium, little is known about primary cilia in the brain, where they are less accessible than cilia on cultured cells or epithelial tissues and protrude from cell bodies into a deep, dense network of glial and neuronal processes. Here, we investigated cilia frequency, internal structure, shape, and position in large, high-resolution transmission electron microscopy volumes of mouse primary visual cortex. Cilia extended from the cell bodies of nearly all excitatory and inhibitory neurons, astrocytes, and oligodendrocyte precursor cells (OPCs), but were absent from oligodendrocytes and microglia. Structural comparisons revealed that the membrane structure at the base of the cilium and the microtubule organization differed between neurons and glia. OPC cilia were distinct in that they were the shortest and contained pervasive internal vesicles only occasionally observed in neuron and astrocyte cilia. Investigating cilia-proximal features revealed that many cilia were directly adjacent to synapses, suggesting cilia are well poised to encounter locally released signaling molecules. Cilia proximity to synapses was random, not enriched, in the synapse-rich neuropil. The internal anatomy, including microtubule changes and centriole location, defined key structural features including cilium placement and shape. Together, the anatomical insights both within and around neuron and glia cilia provide new insights into cilia formation and function across cell types in the brain.

Seizure enhances SUMOylation and zinc-finger transcriptional repression in neuronal nuclei

A single episode of pilocarpine-induced status epilepticus can trigger the development of spontaneous recurrent seizures in a rodent model for epilepsy. The initial seizure-induced events in neuronal nuclei that lead to long-term changes in gene expression and cellular responses likely contribute toward epileptogenesis. Using a transgenic mouse model to specifically isolate excitatory neuronal nuclei, we profiled the seizure-induced nuclear proteome via tandem mass tag mass spectrometry and observed robust enrichment of nuclear proteins associated with the SUMOylation pathway. In parallel with nuclear proteome, we characterized nuclear gene expression by RNA sequencing which provided insights into seizure-driven transcriptional regulation and dynamics. Strikingly, we saw widespread downregulation of zinc-finger transcription factors, specifically proteins that harbor Krüppel-associated box (KRAB) domains. Our results provide a detailed snapshot of nuclear events induced by seizure activity and demonstrate a robust method for cell-type-specific nuclear profiling that can be applied to other cell types and models.

A serotonergic axon-cilium synapse drives nuclear signaling to alter chromatin accessibility

Chemical synapses between axons and dendrites mediate neuronal intercellular communication. Here, we describe a synapse between axons and primary cilia: the axo-ciliary synapse. Using enhanced focused ion beam-scanning electron microscopy on samples with optimally preserved ultrastructure, we discovered synapses between brainstem serotonergic axons and the primary cilia of hippocampal CA1 pyramidal neurons. Functionally, these cilia are enriched in a ciliary-restricted serotonin receptor, the 5-hydroxytryptamine receptor 6 (5-HTR6). Using a cilia-targeted serotonin sensor, we show that opto- and chemogenetic stimulation of serotonergic axons releases serotonin onto cilia. Ciliary 5-HTR6 stimulation activates a non-canonical G_αq/11-RhoA pathway, which modulates nuclear actin and increases histone acetylation and chromatin accessibility. Ablation of this pathway reduces chromatin accessibility in CA1 pyramidal neurons. As a signaling apparatus with proximity to the nucleus, axo-ciliary synapses short circuit neurotransmission to alter the postsynaptic neuron's epigenetic state.

Primary cilia in the postnatal brain: Subcellular compartments for organizing neuromodulatory signaling

Primary cilia have well characterized roles in early brain development, relaying signals critical for neurogenesis and brain formation during embryonic stages. Less understood are the contributions of cilia-mediated signaling to postnatal brain function. Several cilia-localized receptors that bind neuropeptides and neurotransmitters endogenous to the brain have been identified in adult neurons, but the functional significance of signaling through these cilia-localized receptors is largely unexplored. Ciliopathic disorders in humans often manifest with neurodevelopmental abnormalities and cognitive deficits. Intriguingly, recent research has also linked several neuropsychiatric disorders and neurodegenerative diseases to ciliary dysfunction. This review summarizes recent evidence suggesting that cilia signaling may dynamically regulate postnatal neuronal physiology and connectivity, and highlights possible links among cilia, neuronal circuitry, neuron survival, and neurological disorders.

Diverged morphology changes of astrocytic and neuronal primary cilia under reactive insults

Primary cilia are centriole-derived sensory organelles that are present in most mammalian cells, including astrocytes and neurons. Evidence is emerging that astrocyte and neuronal primary cilia demonstrate a dichotomy in the mature mouse brain. However, it is unknown how astrocytic and neuronal primary cilia change their morphology and ciliary proteins when exposed to reactive insults including epilepsy and traumatic brain injury. We used a double transgenic mouse strain (Arl13b-mCherry; Centrin2-GFP), in which we found spontaneous seizures, and a cortical injury model to examine the morphological changes of astrocytic and neuronal primary cilia under reactive conditions. Transgenic overexpression of Arl13b drastically increases the length of astrocytic and neuronal primary cilia in the hippocampus, as well as the cilia lengths of cultured astrocytes and neurons. Spontaneous seizures shorten Arl13b-positive astrocytic cilia and AC3-positive neuronal cilia in the hippocampus. In a cortical injury model, Arl13b is not detectable in primary cilia, but Arl13b protein relocates to the cell body and has robust expression in the proximity of injured tissues. In contrast, the number of AC3-positive cilia near injured tissues remains unchanged, but their lengths become shorter. These results on astrocytic cilia implicate Arl13b in regulating astrocyte proliferation and tissue regeneration, while the shortening of AC3-positive cilia suggests adaptive changes of neuronal primary cilia under excitotoxicity.

Fbxo41 Promotes Disassembly of Neuronal Primary Cilia

Neuronal primary cilia are signaling organelles with crucial roles in brain development and disease. Cilia structure is decisive for their signaling capacities but the mechanisms regulating it are poorly understood. We identify Fbxo41 as a novel Skp1/Cullin1/F-box (SCF) E3-ligase complex subunit that targets to neuronal centrioles where its accumulation promotes disassembly of primary cilia, and affects sonic hedgehog signaling, a canonical ciliary pathway. Fbxo41 targeting to centrioles requires its Coiled-coil and F-box domains. Levels of Fbxo41 at the centrioles inversely correlate with neuronal cilia length, and mutations that disrupt Fbxo41 targeting or assembly into SCF-complexes also disturb its function in cilia disassembly and signaling. Fbxo41 dependent cilia disassembly in mitotic and post-mitotic cells requires rearrangements of the actin-cytoskeleton, but requires Aurora A kinase activation only in mitotic cells, highlighting important mechanistical differences controlling cilia size between mitotic and post-mitotic cells. Phorbol esters induce recruitment of overexpressed Fbxo41 to centrioles and cilia disassembly in neurons, but disassembly can also occur in absence of Fbxo41. We propose that Fbxo41 targeting to centrosomes regulates neuronal cilia structure and signaling capacity in addition to Fbxo41-independent pathways controlling cilia size.

[Pulsed electromagnetic fields stimulating osteogenic differentiation and maturation involves primary cilia-PI3K/AKT pathway]

OBJECTIVE

To study whether the pulsed electromagnetic fields (PEMF) promoting rat osteoblasts differentiation and maturation is related to the primary cilia and PI3K/AKT pathway, and to explore the mechanism of PEMF in promoting bone differentiation.

METHODS

Enzyme solution was used to obtain newborn SD rats calvarial osteoblasts (ROB), which were processed by 50 Hz 0.6 mT PEMF for 0, 0.5, 1, 1.5 and 2 h, detecting PI3K and AKT protein expression and changes in primary cilia length and incidence; with LY294002 blocking PI3K/AKT signaling pathways we observed whether PEMF promoted osteogenic differentiation of ROB was affected; by interfering IFT88 gene expression by RNAi to inhibit primary cilia we observed whether PI3K/AKT signaling pathway and osteogenic differentiation of ROB was affected. Osteogenic differentiation indexes included alkaline phosphatase (ALP) activity, Real-time PCR and Western blot detection of osteogenic related genes of BMP-2, COL-1 and OSX and calcified nodules number, etc..

RESULTS

After exposure to PEMF for 0, 0.5, 1, 1.5, and 2 h, the protein expression of PI3K and AKT in ROB were increased significantly (P<0.01) and the primary cilia became longer; and the protein expression of PI3K reached the highest level at 0.5 h, as the treatment time of PEMF increased, the PI3K protein expression decreased. AKT showed higher protein expression at 0.5 h and 1.5 h. After blocking the PI3K/AKT signaling pathway with the PI3K blocker LY294002, PEMF could no longer increase ALP activity and the gene expressions of BMP-2, COL-1, OSX which were osteogenically related. However, PEMF could increase the ALP activity and the osteogenically related gene expression in ROB before blocking. After RNAi interfered the primary cilia, PEMF could no longer increase the protein expression of PI3K, which indicated that PEMF could not activate the PI3K/AKT signaling pathway after primary cilia interfering; secondly, the effect of PEMF on enhancing ALP activity disappeared, it also decrease the gene expressions of BMP-2, COL-1, and OSX, and the ability of increasing the calcification nodule formation also disappeared, indicating that the ability of PEMF to promote osteoblast maturation and mineralization disappeared after primary cilia interference.

CONCLUSION

PEMF activated the PI3K/AKT signaling pathway through primary cilia on the surface of osteoblasts, then promoted bone formation activity and differentiation.

[Pulsed electromagnetic fields stimulating osteogenic differentiation and maturation involves primary cilia-PI3K/AKT pathway]

OBJECTIVE

To study whether the pulsed electromagnetic fields (PEMF) promoting rat osteoblasts differentiation and maturation is related to the primary cilia and PI3K/AKT pathway, and to explore the mechanism of PEMF in promoting bone differentiation.

METHODS

Enzyme solution was used to obtain newborn SD rats calvarial osteoblasts (ROB), which were processed by 50 Hz 0.6 mT PEMF for 0, 0.5, 1, 1.5 and 2 h, detecting PI3K and AKT protein expression and changes in primary cilia length and incidence; with LY294002 blocking PI3K/AKT signaling pathways we observed whether PEMF promoted osteogenic differentiation of ROB was affected; by interfering IFT88 gene expression by RNAi to inhibit primary cilia we observed whether PI3K/AKT signaling pathway and osteogenic differentiation of ROB was affected. Osteogenic differentiation indexes included alkaline phosphatase (ALP) activity, Real-time PCR and Western blot detection of osteogenic related genes of BMP-2, COL-1 and OSX and calcified nodules number, etc..

RESULTS

After exposure to PEMF for 0, 0.5, 1, 1.5, and 2 h, the protein expression of PI3K and AKT in ROB were increased significantly (P<0.01) and the primary cilia became longer; and the protein expression of PI3K reached the highest level at 0.5 h, as the treatment time of PEMF increased, the PI3K protein expression decreased. AKT showed higher protein expression at 0.5 h and 1.5 h. After blocking the PI3K/AKT signaling pathway with the PI3K blocker LY294002, PEMF could no longer increase ALP activity and the gene expressions of BMP-2, COL-1, OSX which were osteogenically related. However, PEMF could increase the ALP activity and the osteogenically related gene expression in ROB before blocking. After RNAi interfered the primary cilia, PEMF could no longer increase the protein expression of PI3K, which indicated that PEMF could not activate the PI3K/AKT signaling pathway after primary cilia interfering; secondly, the effect of PEMF on enhancing ALP activity disappeared, it also decrease the gene expressions of BMP-2, COL-1, and OSX, and the ability of increasing the calcification nodule formation also disappeared, indicating that the ability of PEMF to promote osteoblast maturation and mineralization disappeared after primary cilia interference.

CONCLUSION

PEMF activated the PI3K/AKT signaling pathway through primary cilia on the surface of osteoblasts, then promoted bone formation activity and differentiation.

Primary cilia as a novel horizon between neuron and environment

The primary cilium, a hair-like sensory organelle found on most mammalian cells, has gained recent attention within the field of neuroscience. Although neural primary cilia have been known to play a role in embryonic central nervous system patterning, we are just beginning to appreciate their importance in the mature organism. After several decades of investigation and controversy, the neural primary cilium is emerging as an important regulator of neuroplasticity in the healthy adult central nervous system. Further, primary cilia have recently been implicated in disease states such as cancer and epilepsy. Intriguingly, while primary cilia are expressed throughout the central nervous system, their structure, receptors, and signaling pathways vary by anatomical region and neural cell type. These differences likely bear relevance to both their homeostatic and neuropathological functions, although much remains to be uncovered. In this review, we provide a brief historical overview of neural primary cilia and highlight several key advances in the field over the past few decades. We then set forth a proposed research agenda to fill in the gaps in our knowledge regarding how the primary cilium functions and malfunctions in nervous tissue, with the ultimate goal of targeting this sensory structure for neural repair following injury.