Activity-dependent decrease in contact areas between subsurface cisterns and plasma membrane of hippocampal neurons

A complex of the lipid transport ER proteins TMEM24 and C2CD2 with band 4.1 at cell-cell contacts

Junctions between the ER and plasma membrane (PM) are implicated in calcium homeostasis, non-vesicular lipid transfer, and other cellular functions. Two ER proteins that function both as tethers to the PM via a polybasic C-terminus motif and as phospholipid transporters are brain-enriched TMEM24 (C2CD2L) and its paralog C2CD2. We report that both proteins also form a complex with band 4.1 family members, which in turn bind PM proteins including cell adhesion molecules such as SynCAM 1. This complex enriches TMEM24 and C2CD2 containing ER/PM junctions at sites of cell contacts. Dynamic properties of TMEM24-dependent ER/PM junctions are impacted when band 4.1 is part of the junction, as TMEM24 at cell-adjacent ER/PM junctions is not shed from the PM by calcium rise, unlike TMEM24 at non-cell adjacent junctions. Lipid transport between the ER and the PM by TMEM24 and C2CD2 at sites where cells, including neurons, contact other cells may participate in adaptive responses to cell contact-dependent signaling.

The Endoplasmic Reticulum and Its Contacts: Emerging Roles in Axon Development, Neurotransmission, and Degeneration

The neuronal endoplasmic reticulum (ER) consists of a dynamic, tubular network that extends all the way from the soma into dendrites, axons, and synapses. This morphology gives rise to an enormous membrane surface area that, through the presence of tethering proteins, lipid transfer proteins, and ion channels, plays critical roles in local calcium regulation, membrane dynamics, and the supply of ions and lipids to other organelles. Here, we summarize recent advances that highlight the various roles of the neuronal ER in axonal growth, repair, and presynaptic function. We review the variety of contact sites between the ER and other axonal organelles and describe their influence on neurodevelopment and neurotransmission.

Compartmentalized signaling in the soma: Coordination of electrical and protein kinase A signaling at neuronal ER-plasma membrane junctions

Neuronal information processing depends on converting membrane depolarizations into compartmentalized biochemical signals that can modify neuronal activity and structure. However, our understanding of how neurons translate electrical signals into specific biochemical responses remains limited, especially in the soma where gene expression and ion channel function are crucial for neuronal activity. Here, I emphasize the importance of physically compartmentalizing action potential-triggered biochemical reactions within the soma. Emerging evidence suggests that somatic endoplasmic reticulum-plasma membrane (ER-PM) junctions are specialized organelles that coordinate electrical and biochemical signaling. The juxtaposition of ion channels and signaling proteins at a prominent subset of these sites enables compartmentalized calcium and cAMP-dependent protein kinase (PKA) signaling. I explore the hypothesis that these PKA-containing ER-PM junctions serve as critical sites for translating membrane depolarizations into PKA signals and identify key gaps in knowledge of the assembly, regulation, and neurobiological functions of this somatic signaling system.

Calcium channel signalling at neuronal endoplasmic reticulum-plasma membrane junctions

Neurons are highly specialised cells that need to relay information over long distances and integrate signals from thousands of synaptic inputs. The complexity of neuronal function is evident in the morphology of their plasma membrane (PM), by far the most intricate of all cell types. Yet, within the neuron lies an organelle whose architecture adds another level to this morphological sophistication - the endoplasmic reticulum (ER). Neuronal ER is abundant in the cell body and extends to distant axonal terminals and postsynaptic dendritic spines. It also adopts specialised structures like the spine apparatus in the postsynapse and the cisternal organelle in the axon initial segment. At membrane contact sites (MCSs) between the ER and the PM, the two membranes come in close proximity to create hubs of lipid exchange and Ca2+ signalling called ER-PM junctions. The development of electron and light microscopy techniques extended our knowledge on the physiological relevance of ER-PM MCSs. Equally important was the identification of ER and PM partners that interact in these junctions, most notably the STIM-ORAI and VAP-Kv2.1 pairs. The physiological functions of ER-PM junctions in neurons are being increasingly explored, but their molecular composition and the role in the dynamics of Ca2+ signalling are less clear. This review aims to outline the current state of research on the topic of neuronal ER-PM contacts. Specifically, we will summarise the involvement of different classes of Ca2+ channels in these junctions, discuss their role in neuronal development and neuropathology and propose directions for further research.

A complex of the lipid transport ER proteins TMEM24 and C2CD2 with band 4.1 at cell-cell contacts

Junctions between the ER and the plasma membrane (ER/PM junctions) are implicated in calcium homeostasis, non-vesicular lipid transfer and other cellular functions. Two ER proteins that function both as membrane tethers to the PM via a polybasic motif in their C-terminus and as phospholipid transporters are brain-enriched TMEM24 (C2CD2L) and its paralog C2CD2. Based on an unbiased proximity ligation analysis, we found that both proteins can also form a complex with band 4.1 family members, which in turn can bind a variety of plasma membrane proteins including cell adhesion molecules such as SynCAM 1. This complex results in the enrichment of TMEM24 and C2CD2 containing ER/PM junctions at sites of cell contacts. Dynamic properties of TMEM24-dependent ER/PM contacts are impacted when in complex as TMEM24 present at cell adjacent junctions is not shed by calcium rise, unlike TMEM24 at non-cell adjacent junctions. These findings suggest that cell-contact interactions control ER/PM junctions via TMEM24 complexes involving band 4.1 proteins.

Endoplasmic Reticulum-Plasma Membrane Junctions as Sites of Depolarization-Induced Ca2+ Signaling in Excitable Cells

Membrane contact sites between endoplasmic reticulum (ER) and plasma membrane (PM), or ER-PM junctions, are found in all eukaryotic cells. In excitable cells they play unique roles in organizing diverse forms of Ca2+ signaling as triggered by membrane depolarization. ER-PM junctions underlie crucial physiological processes such as excitation-contraction coupling, smooth muscle contraction and relaxation, and various forms of activity-dependent signaling and plasticity in neurons. In many cases the structure and molecular composition of ER-PM junctions in excitable cells comprise important regulatory feedback loops linking depolarization-induced Ca2+ signaling at these sites to the regulation of membrane potential. Here, we describe recent findings on physiological roles and molecular composition of native ER-PM junctions in excitable cells. We focus on recent studies that provide new insights into canonical forms of depolarization-induced Ca2+ signaling occurring at junctional triads and dyads of striated muscle, as well as the diversity of ER-PM junctions in these cells and in smooth muscle and neurons.

ER-PM Junctions on GABAergic Interneurons Are Organized by Neuregulin 2/VAP Interactions and Regulated by NMDA Receptors

Neuregulins (NRGs) signal via ErbB receptors to regulate neural development, excitability, synaptic and network activity, and behaviors relevant to psychiatric disorders. Bidirectional signaling between NRG2/ErbB4 and NMDA receptors is thought to homeostatically regulate GABAergic interneurons in response to increased excitatory neurotransmission or elevated extracellular glutamate levels. Unprocessed proNRG2 forms discrete clusters on cell bodies and proximal dendrites that colocalize with the potassium channel Kv2.1 at specialized endoplasmic reticulum-plasma membrane (ER-PM) junctions, and NMDA receptor activation triggers rapid dissociation from ER-PM junctions and ectodomain shedding by ADAM10. Here, we elucidate the mechanistic basis of proNRG2 clustering at ER-PM junctions and its regulation by NMDA receptors. Importantly, we demonstrate that proNRG2 promotes the formation of ER-PM junctions by directly binding the ER-resident membrane tether VAP, like Kv2.1. The proNRG2 intracellular domain harbors two non-canonical, low-affinity sites that cooperatively mediate VAP binding. One of these is a cryptic and phosphorylation-dependent VAP binding motif that is dephosphorylated following NMDA receptor activation, thus revealing how excitatory neurotransmission promotes the dissociation of proNRG2 from ER-PM junctions. Therefore, proNRG2 and Kv2.1 can independently function as VAP-dependent organizers of neuronal ER-PM junctions. Based on these and prior studies, we propose that proNRG2 and Kv2.1 serve as co-regulated downstream effectors of NMDA receptors to homeostatically regulate GABAergic interneurons.

E-Syt1 Regulates Neuronal Activity-Dependent Endoplasmic Reticulum-Plasma Membrane Junctions and Surface Expression of AMPA Receptors

Endoplasmic reticulum (ER)-plasma membrane (PM) contact sites/junctions play important roles in cell physiology including signal transduction, ion and lipid transfer, and membrane dynamics. However, little is known about the dynamic regulation and functional roles of ER-PM junctions in neurons. Using a split green fluorescent protein-based membrane contact probe, we find that the density of ER-PM contact sites changes dynamically in the dendrites of hippocampal neurons undergoing long-term synaptic potentiation (LTP). We show that the Ca2±-sensing membrane tethering protein Extended Synaptotagmin 1 (E-Syt1) mediates the formation of ER-PM contact sites during LTP. We also show that E-Syt1 is required for neuronal activity-dependent surface expression of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-type glutamate receptors. These findings implicate ER-PM junctions in the regulation of neurotransmitter receptor trafficking and synaptic plasticity.

The spatio-temporal properties of calcium transients in hippocampal pyramidal neurons in vitro

The spatio-temporal properties of calcium signals were studied in cultured pyramidal neurons of the hippocampus using two-dimensional fluorescence microscopy and ratiometric dye Fura-2. Depolarization-induced Ca²⁺ transients revealed an asynchronous delayed increase in free Ca²⁺ concentration. We found that the level of free resting calcium in the cell nucleus is significantly lower compared to the soma, sub-membrane, and dendritic tree regions. Calcium release from the endoplasmic reticulum under the action of several stimuli (field stimulation, high K⁺ levels, and caffeine) occurs in all areas studied. Under depolarization, calcium signals developed faster in the dendrites than in other areas, while their amplitude was significantly lower since larger and slower responses inside the soma. The peak value of the calcium response to the application of 10 mM caffeine, ryanodine receptors (RyRs) agonist, does not differ in the sub-membrane zone, central region, and nucleus but significantly decreases in the dendrites. In the presence of caffeine, the delay of Ca²⁺ signals between various areas under depolarization significantly declined. Thirty percentage of the peak amplitude of Ca²⁺ transients at prolonged electric field stimulation corresponded to calcium release from the ER store by RyRs, while short-term stimulation did not depend on them. 20 μM dantrolene, RyRs inhibitor, significantly reduces Ca²⁺ transient under high K⁺ levels depolarization of the neuron. RyRs-mediated enhancement of the Ca²⁺ signal is more pronounced in the central part and nucleus compared to the sub-membrane or dendrites regions of the neuron. In summary, using the ratiometric imaging allowed us to obtain additional information about the involvement of RyRs in the intracellular dynamics of Ca²⁺ signals induced by depolarization or electrical stimulation train, with an underlying change in Ca²⁺ concentration in various regions of interest in hippocampal pyramidal neurons.

Identification of ultrastructural signatures of sleep and wake in the fly brain

The cellular consequences of sleep loss are poorly characterized. In the pyramidal neurons of mouse frontal cortex, we found that mitochondria and secondary lysosomes occupy a larger proportion of the cytoplasm after chronic sleep restriction compared to sleep, consistent with increased cellular burden due to extended wake. For each morphological parameter, the within-animal variance was high, suggesting that the effects of sleep and sleep loss vary greatly among neurons. However, the analysis was based on 4-5 mice/group and a single section/cell. Here, we applied serial block-face scanning electron microscopy to identify signatures of sleep and sleep loss in the Drosophila brain. Stacks of images were acquired and used to obtain full 3D reconstructions of the cytoplasm and nucleus of 263 Kenyon cells from adult flies collected after a night of sleep (S) or after 11 h (SD11) or 35 h (SD35) of sleep deprivation (9 flies/group). Relative to S flies, SD35 flies showed increased density of dark clusters of chromatin and Golgi apparata and a trend increase in the percent of cell volume occupied by mitochondria, consistent with increased need for energy and protein supply during extended wake. Logistic regression models could assign each neuron to the correct experimental group with good accuracy, but in each cell, nuclear and cytoplasmic changes were poorly correlated, and within-fly variance was substantial in all experimental groups. Together, these results support the presence of ultrastructural signatures of sleep and sleep loss but underscore the complexity of their effects at the single-cell level.

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

Morphological Heterogeneity of the Endoplasmic Reticulum within Neurons and Its Implications in Neurodegeneration

The endoplasmic reticulum (ER) is a multipurpose organelle comprising dynamic structural subdomains, such as ER sheets and tubules, serving to maintain protein, calcium, and lipid homeostasis. In neurons, the single ER is compartmentalized with a careful segregation of the structural subdomains in somatic and neurite (axodendritic) regions. The distribution and arrangement of these ER subdomains varies between different neuronal types. Mutations in ER membrane shaping proteins and morphological changes in the ER are associated with various neurodegenerative diseases implying significance of ER morphology in maintaining neuronal integrity. Specific neurons, such as the highly arborized dopaminergic neurons, are prone to stress and neurodegeneration. Differences in morphology and functionality of ER between the neurons may account for their varied sensitivity to stress and neurodegenerative changes. In this review, we explore the neuronal ER and discuss its distinct morphological attributes and specific functions. We hypothesize that morphological heterogeneity of the ER in neurons is an important factor that accounts for their selective susceptibility to neurodegeneration.

Endoplasmic reticulum-plasma membrane contacts: Principals of phosphoinositide and calcium signaling

The endoplasmic reticulum (ER) forms an extensive network of membrane contact sites with intra-cellular organelles and the plasma membrane (PM). Interorganelle contacts have vital roles in membrane lipid and ion dynamics. In particular, ER-PM contacts are integral to numerous inter-cellular and intra-cellular signaling pathways including phosphoinositide lipid and calcium signaling, mechanotransduction, metabolic regulation, and cell stress responses. Accordingly, ER-PM contacts serve important signaling functions in excitable cells including neurons and muscle and endocrine cells. This review highlights recent advances in our understanding of the vital roles for ER-PM contacts in phosphoinositide and calcium signaling and how signaling pathways in turn regulate proteins that form and function at ER-PM contacts.

Merits and Limitations of Studying Neuronal Depolarization-Dependent Processes Using Elevated External Potassium

Elevated extracellular potassium chloride is widely used to achieve membrane depolarization of cultured neurons. This technique has illuminated mechanisms of calcium influx through L-type voltage sensitive calcium channels, activity-regulated signaling, downstream transcriptional events, and many other intracellular responses to depolarization. However, there is enormous variability in these treatments, including durations from seconds to days and concentrations from 3mM to 150 mM KCl. Differential effects of these variable protocols on neuronal activity and transcriptional programs are underexplored. Furthermore, potassium chloride treatments in vitro are criticized for being poor representatives of in vivo phenomena and are questioned for their effects on cell viability. In this review, we discuss the intracellular consequences of elevated extracellular potassium chloride treatment in vitro, the variability of such treatments in the literature, the strengths and limitations of this tool, and relevance of these studies to brain functions and dysfunctions.

Kv2.1 mediates spatial and functional coupling of L-type calcium channels and ryanodine receptors in mammalian neurons

The voltage-gated K⁺ channel Kv2.1 serves a major structural role in the soma and proximal dendrites of mammalian brain neurons, tethering the plasma membrane (PM) to endoplasmic reticulum (ER). Although Kv2.1 clustering at neuronal ER-PM junctions (EPJs) is tightly regulated and highly conserved, its function remains unclear. By identifying and evaluating proteins in close spatial proximity to Kv2.1-containing EPJs, we discovered that a significant role of Kv2.1 at EPJs is to promote the clustering and functional coupling of PM L-type Ca²⁺ channels (LTCCs) to ryanodine receptor (RyR) ER Ca²⁺ release channels. Kv2.1 clustering also unexpectedly enhanced LTCC opening at polarized membrane potentials. This enabled Kv2.1-LTCC-RyR triads to generate localized Ca²⁺ release events (i.e., Ca²⁺ sparks) independently of action potentials. Together, these findings uncover a novel mode of LTCC regulation and establish a unique mechanism whereby Kv2.1-associated EPJs provide a molecular platform for localized somatodendritic Ca²⁺ signals in mammalian brain neurons.

NeurodegenERation: The Central Role for ER Contacts in Neuronal Function and Axonopathy, Lessons From Hereditary Spastic Paraplegias and Related Diseases

The hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative conditions whose characteristic feature is degeneration of the longest axons within the corticospinal tract which leads to progressive spasticity and weakness of the lower limbs. Though highly genetically heterogeneous, the majority of HSP cases are caused by mutations in genes encoding proteins that are responsible for generating and organizing the tubular endoplasmic reticulum (ER). Despite this, the role of the ER within neurons, particularly the long axons affected in HSP, is not well understood. Throughout axons, ER tubules make extensive contacts with other organelles, the cytoskeleton and the plasma membrane. At these ER contacts, protein complexes work in concert to perform specialized functions including organelle shaping, calcium homeostasis and lipid biogenesis, all of which are vital for neuronal survival and may be disrupted by HSP-causing mutations. In this article we summarize the proteins which mediate ER contacts, review the functions these contacts are known to carry out within neurons, and discuss the potential contribution of disruption of ER contacts to axonopathy in HSP.

Neuronal ER-plasma membrane junctions organized by Kv2-VAP pairing recruit Nir proteins and affect phosphoinositide homeostasis

The association of plasma membrane (PM)-localized voltage-gated potassium (Kv2) channels with endoplasmic reticulum (ER)-localized vesicle-associated membrane protein-associated proteins VAPA and VAPB defines ER-PM junctions in mammalian brain neurons. Here, we used proteomics to identify proteins associated with Kv2/VAP-containing ER-PM junctions. We found that the VAP-interacting membrane-associated phosphatidylinositol (PtdIns) transfer proteins PYK2 N-terminal domain-interacting receptor 2 (Nir2) and Nir3 specifically associate with Kv2.1 complexes. When coexpressed with Kv2.1 and VAPA in HEK293T cells, Nir2 colocalized with cell-surface-conducting and -nonconducting Kv2.1 isoforms. This was enhanced by muscarinic-mediated PtdIns(4,5)P₂ hydrolysis, leading to dynamic recruitment of Nir2 to Kv2.1 clusters. In cultured rat hippocampal neurons, exogenously expressed Nir2 did not strongly colocalize with Kv2.1, unless exogenous VAPA was also expressed, supporting the notion that VAPA mediates the spatial association of Kv2.1 and Nir2. Immunolabeling signals of endogenous Kv2.1, Nir2, and VAP puncta were spatially correlated in cultured neurons. Fluorescence-recovery-after-photobleaching experiments revealed that Kv2.1, VAPA, and Nir2 have comparable turnover rates at ER-PM junctions, suggesting that they form complexes at these sites. Exogenous Kv2.1 expression in HEK293T cells resulted in significant differences in the kinetics of PtdIns(4,5)P₂ recovery following repetitive muscarinic stimulation, with no apparent impact on resting PtdIns(4,5)P₂ or PtdIns(4)P levels. Finally, the brains of Kv2.1-knockout mice had altered composition of PtdIns lipids, suggesting a crucial role for native Kv2.1-containing ER-PM junctions in regulating PtdIns lipid metabolism in brain neurons. These results suggest that ER-PM junctions formed by Kv2 channel-VAP pairing regulate PtdIns lipid homeostasis via VAP-associated PtdIns transfer proteins.

Kv2 channels create endoplasmic reticulum / plasma membrane junctions: a brief history of Kv2 channel subcellular localization

The potassium channels Kv2.1 and Kv2.2 are widely expressed throughout the mammalian brain. Kv2.1 provides the majority of delayed rectifying current in rat hippocampus while both channels are differentially expressed in cortex. Particularly unusual is their neuronal surface localization pattern: while half the channel population is freely-diffusive on the plasma membrane as expected from the generalized Singer & Nicolson fluid mosaic model, the other half localizes into micron-sized clusters on the soma, dendrites, and axon initial segment. These clusters contain hundreds of channels, which for Kv2.1, are largely non-conducting. Competing theories of the mechanism underlying Kv2.1 clustering have included static tethering to being corralled by an actin fence. Now, recent work has demonstrated channel clustering is due to formation of endoplasmic reticulum/plasma membrane (ER/PM) junctions through interaction with ER-resident VAMP-associated proteins (VAPs). Interaction between surface Kv2 channels and ER VAPs groups channels together in clusters. ER/PM junctions play important roles in inter-organelle communication: they regulate ion flux, are involved in lipid transfer, and are sites of endo- and exocytosis. Kv2-induced ER/PM junctions are regulated through phosphorylation of the channel C-terminus which in turn regulates VAP binding, providing a rapid means to create or dismantle these microdomains. In addition, insults such as hypoxia or ischemia disrupt this interaction resulting in ER/PM junction disassembly. Kv2 channels are the only known plasma membrane protein to form regulated, injury sensitive junctions in this manner. Furthermore, it is likely that concentrated VAPs at these microdomains sequester additional interactors whose functions are not yet fully understood.

NMDA Receptors Regulate Neuregulin 2 Binding to ER-PM Junctions and Ectodomain Release by ADAM10 [corrected]

Unprocessed pro-neuregulin 2 (pro-NRG2) accumulates on neuronal cell bodies at junctions between the endoplasmic reticulum and plasma membrane (ER-PM junctions). NMDA receptors (NMDARs) trigger NRG2 ectodomain shedding from these sites followed by activation of ErbB4 receptor tyrosine kinases, and ErbB4 signaling cell-autonomously downregulates intrinsic excitability of GABAergic interneurons by reducing voltage-gated sodium channel currents. NMDARs also promote dispersal of Kv2.1 clusters from ER-PM junctions and cause a hyperpolarizing shift in its voltage-dependent channel activation, suggesting that NRG2/ErbB4 and Kv2.1 work together to regulate intrinsic interneuron excitability in an activity-dependent manner. Here we explored the cellular processes underlying NMDAR-dependent NRG2 shedding in cultured rat hippocampal neurons. We report that NMDARs control shedding by two separate but converging mechanisms. First, NMDA treatment disrupts binding of pro-NRG2 to ER-PM junctions by post-translationally modifying conserved Ser/Thr residues in its intracellular domain. Second, using a mutant NRG2 protein that cannot be modified at these residues and that fails to accumulate at ER-PM junctions, we demonstrate that NMDARs also directly promote NRG2 shedding by ADAM-type metalloproteinases. Using pharmacological and shRNA-mediated knockdown, and metalloproteinase overexpression, we unexpectedly find that ADAM10, but not ADAM17/TACE, is the major NRG2 sheddase acting downstream of NMDAR activation. Together, these findings reveal how NMDARs exert tight control over the NRG2/ErbB4 signaling pathway, and suggest that NRG2 and Kv2.1 are co-regulated components of a shared pathway that responds to elevated extracellular glutamate levels.

Lipid transporter TMEM24/C2CD2L is a Ca2+-regulated component of ER-plasma membrane contacts in mammalian neurons

Close appositions between the endoplasmic reticulum (ER) and the plasma membrane (PM) are a general feature of all cells and are abundant in neurons. A function of these appositions is lipid transport between the two adjacent bilayers via tethering proteins that also contain lipid transport modules. However, little is known about the properties and dynamics of these proteins in neurons. Here we focused on TMEM24/C2CD2L, an ER-localized SMP domain containing phospholipid transporter expressed at high levels in the brain, previously shown to be a component of ER-PM contacts in pancreatic β-cells. TMEM24 is enriched in neurons versus glial cells and its levels increase in parallel with neuronal differentiation. It populates ER-PM contacts in resting neurons, but elevations of cytosolic Ca²⁺ mediated by experimental manipulations or spontaneous activity induce its transient redistribution throughout the entire ER. Dissociation of TMEM24 from the plasma membrane is mediated by phosphorylation of an array of sites in the C-terminal region of the protein. These sites are only partially conserved in C2CD2, the paralogue of TMEM24 primarily expressed in nonneuronal tissues, which correspondingly display a much lower sensitivity to Ca²⁺ elevations. ER-PM contacts in neurons are also sites where Kv2 (the major delayed rectifier K⁺ channels in brain) and other PM and ER ion channels are concentrated, raising the possibility of a regulatory feedback mechanism between neuronal excitability and lipid exchange between the ER and the PM.

Distribution of densin in neurons

Densin is a scaffold protein known to associate with key elements of neuronal signaling. The present study examines the distribution of densin at the ultrastructural level in order to reveal potential sites that can support specific interactions of densin. Immunogold electron microscopy on hippocampal cultures shows intense labeling for densin at postsynaptic densities (PSDs), but also some labeling at extrasynaptic plasma membranes of soma and dendrites and endoplasmic reticulum. At the PSD, the median distance of label from the postsynaptic membrane was ~27 nm, with the majority of label (90%) confined within 40 nm from the postsynaptic membrane, indicating predominant localization of densin at the PSD core. Depolarization (90 mM K⁺ for 2 min) promoted a slight shift of densin label within the PSD complex resulting in 77% of label remaining within 40 nm from the postsynaptic membrane. Densin molecules firmly embedded within the PSD may target a minor pool of CaMKII to substrates at the PSD core.

Stimulation-induced structural changes at the nucleus, endoplasmic reticulum and mitochondria of hippocampal neurons

Neurons exhibit stimulation-induced ultrastructural changes such as increase of thickness and curvature of the postsynaptic density, decrease in contact area between subsurface cistern and plasma membrane, and formation of CaMKII clusters and synaptic spinules. These structural characteristics help in identifying the activity state of the neuron and should be taken into consideration when interpreting ultrastructural features of the neurons. Here in organotypic hippocampal slice cultures where experimental conditions can be easily manipulated, two additional features are documented in forebrain neurons as reliable benchmarks for stimulation-induced structural changes: (1) The neuronal nucleus showed conspicuous clustering of dark chromatin, and (2) the endoplasmic reticulum formed stacks with a uniform gap of ~ 13 nm filled with dark materials. Both structural changes progressed with time and were reversible upon returning the slice cultures to control medium. These stimulation-induced structural changes were also verified in dissociated hippocampal neuronal cultures and perfusion-fixed brains. In hippocampal slice cultures, the neuronal chromatin clustering was detectable within 30 s of depolarization with high K⁺ (90 mM) or treatment with NMDA (50 μM). In contrast, the formation of ER cisternal stacks did not become apparent for another 30 s. Importantly, in dissociated neuronal cultures, when the extracellular calcium was chelated by EGTA, treatment with high K⁺ no longer induced these changes. These results indicate that the stimulation-induced chromatin clustering and formation of ER stacks in neurons are calcium-dependent. Additionally, mitochondria in neuronal somas of tissue culture samples consistently became swollen upon stimulation. However, swollen mitochondria were also present in some neurons of control samples, but could be eliminated by blocking basal activity or calcium influx. This calcium-dependent structural change of mitochondria is specific to neurons. These structural changes may bring insights to the neuron’s response to intracellular calcium rise upon stimulation.