The organization and function of the Golgi apparatus in dendrite development and neurological disorders

Abstract

Dendrites are specialized neuronal compartments that sense, integrate and transfer information in the neural network. Their development is tightly controlled and abnormal dendrite morphogenesis is strongly linked to neurological disorders. While dendritic morphology ranges from relatively simple to extremely complex for a specified neuron, either requires a functional secretory pathway to continually replenish proteins and lipids to meet dendritic growth demands. The Golgi apparatus occupies the center of the secretory pathway and is regulating posttranslational modifications, sorting, transport, and signal transduction, as well as acting as a non-centrosomal microtubule organization center. The neuronal Golgi apparatus shares common features with Golgi in other eukaryotic cell types but also forms distinct structures known as Golgi outposts that specifically localize in dendrites. However, the organization and function of Golgi in dendrite development and its impact on neurological disorders is just emerging and so far lacks a systematic summary. We describe the organization of the Golgi apparatus in neurons, review the current understanding of Golgi function in dendritic morphogenesis, and discuss the current challenges and future directions.

Introduction

Neurons are highly polarized cells typically with two distinct compartments, a single axon, and several highly branched dendrites. The dendritic morphology of individual neurons is cell type-specific and ranges from relatively simple (e.g. pyramidal cells in vertebrates and C1da sensory neurons in Drosophila) to extremely complex (e.g. purkinje cells and retinal ganglion cells in vertebrates and C4da sensory neurons in Drosophila). During development, dendrites continuously undergo dynamic changes¹ that require a certain abundance of newly synthesized proteins and lipids. The steady supply of these cellular building blocks is regulated by the secretory pathway, which ensures proper protein and lipid transport.² The Golgi apparatus (including somatic Golgi and specified dendritic Golgi outposts (GOPs); see following text) is at the center of the secretory pathway and essential for post-translational modification, processing and sorting of proteins and lipids, ion homeostasis, and signal transduction.3, 4, 5 In addition, the Golgi apparatus also serves as a non-centrosomal microtubule (MT) organizing center (MTOC) that nucleates and stabilizes MTs,^6,7 which is critical for the homeostasis of the secretory pathway and cargo transport.⁸ Thus, the Golgi apparatus plays a central role in protein and lipid trafficking events and perturbations of its organization have devastating effects on dendritic development.⁷ Previous studies showed that more than 40% of the Golgi-related genes are known to be associated with multiple diseases affecting the central or peripheral nervous system,⁹ implying the importance of Golgi for neurogenesis, migration, maturation of postmitotic neurons, and myelination.9, 10, 11, 12, 13, 14, 15 Thus, the emerging evidence highlights the role and potential mechanisms of Golgi organization and function in dendrite development.

Here, we will introduce the Golgi apparatus in general, discuss the progress in the molecular control of Golgi organization and function in both vertebrate and Drosophila neurons, summarize its underlying mechanisms in dendritic morphogenesis, and highlight how Golgi function is contributing to neurological disorders and diseases.

The Golgi apparatus in general

Organization of the Golgi apparatus

In all cells, the Golgi apparatus consists of flattened membrane compartments called cisternae. Under physiological conditions, 4–8 tightly aligned cisternae form a Golgi stack.¹⁶ These stacks of cisternae are polarized and classified into three separate modules: the cis-Golgi, which directly receives cargo from the endoplasmatic reticulum (ER), the medial-Golgi cisternae transferring and further modifying cargos, and the trans-Golgi network (TGN) facing the plasma membrane and sorting cargo molecules for delivery to different destinations.17, 18, 19 Each of these three separate modules is present in a single somatic Golgi apparatus² and contains specific resident proteins marking them as functional cis-Golgi (GMAP-210 and GM130), medial-Golgi (α-mannosidase II (ManII)) or trans-Golgi (galactosyltransferase (GalT) and N-acetyl-galactosaminyltransferase 2 (GalNacT2)) compartments.^20,21 It should be noted that the contents of each cisterna vary depending on the state of the cell.²² For example, the Golgi enzyme thiamine pyrophosphatase (TPPase) is present in one or two cisternae of the TGN in supraoptic neurons that synthesize vasopressin, while it spreads to all Golgi components following hyperosmotic shock.²³ This dynamic relocalization of Golgi proteins may enable cells to functionally adapt to environmental challenges.²⁴ In addition, this is likely the reason for distinct Golgi morphology in different species and cell types.24, 25, 26 In mammalian cells, multiple Golgi stacks are often laterally linked by tubular structures to form a Golgi ribbon,²⁷ which may facilitate anterograde transport of large cargos.²⁸ In Drosophila, however, the Golgi apparatus typically consists of single stacks, which are only occasionally paired forming a minimal ribbon.^27,29

General functions of the Golgi apparatus

The Golgi is a multifunctional organelle (Fig. 1). It is the central station for intracellular membrane trafficking and plays a key role in processing and sorting newly synthesized and recycled proteins and lipids toward their final destinations.⁹ The cis-Golgi compartment receives cargo from the ER via coated protein complex II (COPII) transport vesicles.30, 31, 32, 33 Once the cargo reaches the Golgi apparatus, it undergoes post-translational modifications including O-glycosylation,^34,35 sulfation,^36,37 palmitoylation,³⁸ and proteolytic processing.³⁹ These modified and/or unmodified proteins will be returned to the ER^40,41 or transported between different Golgi cisternae^41,42 via COPI-mediated trafficking. When proteins or lipids finally arrive at the TGN, they will be sorted and delivered to the appropriate subcellular address within mobile clathrin-coated vesicles.^43,44 In addition, the Golgi also receives recycled vesicles from the plasma membrane via endosomal pathways,45, 46, 47, 48, 49, 50 which is essential for protein recycling.

Figure 1.

Golgi-centric organization of the secretory pathway. The cis-Golgi receives cargo from the endoplasmatic reticulum (ER) via coated protein complex II (COPII)-mediated transport. Delivered cargo undergoes post-translational modification and the modified and/or unmodified proteins are either returned to the ER or transported between different Golgi cisternae via COPI-mediated transport. Proteins and lipids gradually reach the trans-Golgi network (TGN) and get sorted and delivered to the appropriate subcellular address via mobile clathrin coated vesicles. Golgi is also receiving recycled endosomal vesicles. The molecular motors dynein and kinesin move cargos along the MT tracks by either directly binding to cargo proteins or associating with adaptor or scaffold proteins. In addition, the Golgi resident protein GM130 is a key molecule for anchoring Golgi-derived MT nucleation sites.

The Golgi complex is also involved in ion homeostasis.^51,52 As the main site of protein glycosylation, Golgi is found to contain multiple ions, such as Ca²⁺, Mn²⁺, Zn²⁺, and Cu²⁺ that are crucial cofactors of Golgi-resident glycosylation enzymes and are also involved in controlling luminal pH, redox homeostasis of Golgi⁵² and affect other organelle functions.⁵³ It was reported that sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA),⁵⁴ secretory pathway Ca²⁺-ATPase (SPCA),⁵⁵ inositol 1,4,5-triphosphate (IP3) receptor and ryanodine receptor (Ryr)⁵⁶ are the major types of Ca²⁺ regulators in the Golgi apparatus, Mn²⁺ is largely transported by TEME165⁵⁷ and SLC30A10,⁵⁸ Zn²⁺ is mainly mediated by ZRT1-and IRT1-like protein (ZIP) family (e.g. ZIP7, ZIP13),59, 60, 61 and Cu²⁺ transport is mediated by ATP7 proteins.^53,62,63 Thus, mutations in these regulators can disrupt cellular physiological functions^{53,55,59,64,65} and have been linked to multiple diseases.^58,60, 61, 62, 63^,66

In addition, accumulating evidence highlights the Golgi apparatus as a signaling platform. An abundance of signaling molecules was identified on Golgi membranes, including phosphoinositides,⁶⁷ small GTPases,⁶⁸ kinases,⁶⁹ phosphatases, lipid acylation,⁷⁰ and trimeric G proteins,⁷¹ implying the presence of a diverse range of signaling networks.⁹ Screening for Golgi defects in human⁷² and Drosophila⁷³ cell lines identified a variety of signaling molecules that not only regulate local trafficking pathways but also influence a broad range of cellular processes, including autophagy.^46,74

Lastly, the Golgi is intimately linked to the cellular MT network. For one, MTs are critical for trafficking cargo to and from the Golgi via directional transport executed by the molecular motor proteins dynein and kinesin.^8,75 Dynein and kinesin move Golgi-derived or aimed cargo proteins along MT tracks by either directly binding to cargo or associating with adaptor or scaffold proteins (Fig. 1). For another, the Golgi apparatus colocalizes with MT minus ends and controls the formation of non-centrosomal MT arrays,^14,76, 77, 78 which in turn regulates Golgi organization.^75,79 In vertebrates, the Golgi matrix protein GM130 bound to AKAP450 recruits CEP215 (also known as CDK5RAP2) and myomegalin (MMG).80, 81, 82, 83, 84 The γ-Tubulin ring complex (γ-TuRC) subsequently binds to CEP215 and MMG induces MT nucleation at the surface of the Golgi apparatus.⁶ The other player in Golgi-dependent MT organization is CLASP, which interacts with Golgi membranes through the trans-Golgi protein GCC185,⁸⁵ but may also promote γ-TuRC-mediated nucleation and stabilization of CAMSAP2-bound MT minus-ends directly.⁸⁶ In Drosophila, γ-tubulin was shown to be present in close proximity to the Golgi but its localization does not rely on the orthologues of pericentrin, AKAP450, or CDK5RAP2.⁸⁷ Additionally, in Drosophila neurons the centrosome is not required for the maintenance of neuronal microtubule organization,⁸⁸ adding importance to the Golgi as the major somatic MTOC. Identifying the mechanisms of how the Golgi-derived MT network is organized and in turn organizes Golgi and neuronal function is therefore of high interest.

The Golgi apparatus in neurons

The somatic Golgi apparatus in neurons is similar to other eukaryotic cells in overall organization and function, except it usually invades into the initial segment of 1–2 main dendrites⁸⁹ (Fig. 2). In addition, in highly polarized cells like neurons, Golgi localization depends on the subtype and developmental stage. For example, in pyramidal neurons, the position of the somatic Golgi apparatus correlates with the position of the longest dendrite in vitro and the apical dendrite in vivo.⁹⁰ However, in radial glia the Golgi is positioned non-pericentrosomal; while the centrosome localizes close to the ventral side, the Golgi apparatus is shifted toward the basal lamina close to the nucleus.⁷

Figure 2.

Golgi apparatus in vertebrate and Drosophila neurons. In mice (hippocampal neurons), somatic Golgi shows a ribbon structure and appears to face and extend into primary dendrite(s). Multi-compartment GOPs are largely restricted to one primary dendrite and are often found in the proximal region. Smaller Golgi structures that lack many protein components for sorting and organization of Golgi cisternae are named Golgi satellites, which were identified in distal dendritic arbors in cultured rat neurons. In Drosophila (da sensory neurons), the somatic Golgi apparatus contains cis-, medial- and trans-Golgi components as in vertebrate neurons, but lacks ribbon structure; instead, Golgi forms mini-stacks or “ring”-like stacks. In contrast to vertebrate GOPs, Drosophila GOPs are widespread in the dendritic arbors including distal dendrites, and are enriched at branch points. Both single- and multi-compartment GOPs are present in Drosophila neurons, and most of them are located in the dendritic shaft and branching points, respectively. Somatic Golgi and GOPs are considered (central) stations of the secretory pathway and MT nucleation sites. In addition, Golgi/GOP structures are excluded from the axon, indicating a critical role for the Golgi apparatus in dendritic morphogenesis.

Neurons feature long axons and extensive dendritic trees which require a fast turnover of proteins and lipids for proper information transmission. Thus, it has been intensively discussed for decades whether functional Golgi is present in the axon and/or dendrites. So far, the presence of Golgi in axons is controversial. The Golgi apparatus in neurons has been studied by detecting the activity of TTPase that localizes to Golgi and ER.⁹¹ TTPase activity is detected in axons and presynaptic axon terminals suggesting ER is present there. This finding also suggested that synaptic vesicles in axonal terminals could be partially derived from membranes of the Golgi apparatus,⁹² implying the possibility of Golgi being present in axons. Consistently, GM130, a typical Golgi component, was detected in cultured rat DRG neurons and Xenopus retinal neuron axons by immunostaining and Western blot.⁹³ However, other studies failed to detect Golgi structures/compartments in rat or rabbit neurons by electron microscopy (EM) or immunostaining.94, 95, 96 Similarly, in Drosophila, Golgi markers in sensory neurons are localized specifically to dendrites but not axons.^97,98 Although the evidence so far on whether axons contain Golgi compartments is inconclusive, the widely accepted view is that Golgi is absent from axons,⁴ or at least not functionally present there.

In contrast to axons, Golgi components are relatively abundant in dendrites in both vertebrate and Drosophila neurons, which are known as GOPs. The first evidence for GOPs in vertebrate neurons came from the distribution of temperature-sensitive mutants of the vesicular stomatitis virus glycoprotein (VSVG).⁹⁴ Shifting the temperature from nonpermissive (39.5°C) to permissive (32°C) in live neurons allowed the visualization of somatic and local dendritic transport of fluorescently labeled VSVG from ER to dendritic Golgi structures. GOPs are discrete compartments that are discontinuous with somatic Golgi but share the same molecular markers and function similarly to somatic Golgi.^94,99 In addition, the size of GOPs is much smaller than somatic Golgi, which indicates that dendritic GOPs likely process only a subset of locally synthesized proteins. However, the significance of GOPs in local protein processing is yet to be determined although some evidence suggests their ability to glycosylate newly synthesized membrane proteins¹⁰⁰ or to synthesize and express exogenous membrane proteins at the plasma membrane after dendrite transsection.^101,102 It should be noted that GOPs are not present in all dendrites in vertebrate neurons. Both in vivo and in vitro studies indicate that GOPs are found in about 5% of dendrites, typically within the first 30 μm of the apical dendrite. In hippocampal neurons, around one-fifth of the neurons contain GOPs in one of their 4–7 primary dendrites^94,103,104 (Fig. 2).

Recently, additional Golgi structures termed Golgi satellites (GS) were identified (Fig. 2) by exploiting the Golgi-targeting property of the TGN-resident neuronal EF-hand calcium sensor protein Calneuron-2 to study Golgi organelles in neurons.105, 106, 107 The designed fluorescent probe (termed pGolt) labeled widespread GS located in close proximity to ERGIC and retromer establishing a microscretory system.¹⁰⁷ In contrast to vertebrate GOPs, GS are dispersed throughout basal and apical dendrites of all pyramidal neurons and contain at least part of the cellular glycosylation machinery.¹⁰⁷ In addition, GS lacks many protein components for sorting and the compartmental organization of Golgi cisternae found in GOPs.¹⁰⁷ Furthermore, while GOPs appear to be mainly generated from the somatic Golgi apparatus,¹⁰⁸ GS might either form locally in close proximity to ER exit sites in an activity-dependent manner¹⁰⁹ or bud off from somatic Golgi.¹⁰⁷ Although GS biogenesis is not yet fully understood, their function might enable local protein glycosylation in confined dendritic segments in response to local activity.^107,110

In Drosophila, overexpression of ManII-GFP in sensory neurons allowed visualizing GOPs in vivo, which appear as punctate structures in both major and fine terminal dendritic branches but rarely in the axon.²⁰ Unlike somatic Golgi that contains all cis-, medial- and trans-structures in one single Golgi unit, only half of the ManII-GFP positive GOPs also contained the trans-Golgi marker GalT-TagRFP in dendritic shafts,²⁰ suggesting somatic Golgi and GOPs are not structurally identical. However, single- and multi-compartment GOPs are present in both Drosophila and mouse cortical neurons, indicating the heterogenous organization of GOPs is evolutionarily conserved.²⁰ GOPs in Drosophila sensory neurons are highly dynamic and different compartments exhibit temporary interactions,²⁰ yet the biological meaning of such contacts is still unknown. Interestingly, overexpression of GM130, a structural protein likely involved in the formation of stacks and/or ribbons in mammalian cells,^21,111, 112, 113 induces the formation of multicompartment GOPs in flies in vivo,²⁰ suggesting this process is tightly regulated. It should be noted that a major caveat of the above-mentioned observation of GOPs in both vertebrate and Drosophila neurons is that exogenous proteins may localize to dendritic organelles due to fluorescent protein tags interfering with normal trafficking or high expression levels exceeding the capacity of the target organelle.^2,107 Thus, the distribution of GOPs in distal dendrites is under debate since immunostaining with antibodies to endogenous Golgi organizing proteins showed little localization in distal dendrites.^{94,103,104,107}

Golgi in vertebrate neuronal dendrite development

In vertebrate neurons, the somatic Golgi apparatus generally exists as a ribbon forming a single stack consisting of cis-, medial- and trans-Golgi compartments oriented towards the main dendritic branches, with GOPs being exclusively present in dendrites, suggesting they play a pivotal role in dendritic development.⁹⁰ The vertebrate Golgi is not only crucial for the proper sorting, trafficking, and modification of ion channels, receptors, and other signaling molecules but also mediates the transport of exogenous molecules via retrograde and trans-synaptic pathways. Although some candidates have been identified to regulate Golgi organization in different cell types,^72,73 their functions in dendritic development are largely unknown. Since GOPs in vertebrate neurons are positioned close to somatic Golgi, it is difficult to distinguish Golgi fragmentation from GOP organization. Thus, most studies in vertebrate neurons are focusing on somatic Golgi and its potential role in dendritic development (Fig. 3). The function of GOPs in vertebrate neurons, however, is less well understood.¹⁰⁸

Figure 3.

Vertebrate Golgi apparatus in dendrite development. Most of the proteins that affect somatic Golgi organization will regulate both Golgi integrity and position (only CLN8 was reported to regulate Golgi size). These candidate effectors can either affect MT dynamics or control COPII and COPI transport balance. In addition, some Golgins, like GM130, may directly connect different Golgi compartments. Neuronal activity and additional kinases might regulate Golgi organization by unknown mechanisms and link Golgi function to dendrite development. While vertebrate GOPs are generated from the somatic Golgi upon enhancing neuronal excitation and/or RhoA-Rock signaling, the biogenesis of Golgi satellites is elevated upon neuronal activity.

Doublecortin (DCX)

As discussed above, the somatic Golgi usually invades into 1–2 proximal dendrites in vertebrate neurons. In cultured hippocampal neurons, knockout of doublecortin (DCX), a microtubule (MT) binding protein that links to neuronal migration and process outgrowth,114, 115, 116 limits the elongation of somatic Golgi into proximal dendrites.⁸⁹ This defect is possibly mediated by multiple factors working in a synergistic manner that affect the balance of anterograde and retrograde transport in the absence of DCX. DCX associates with kinesin-3 to facilitate anterograde transport¹¹⁴ and inhibit dynein binding to JIP3 to negatively regulate dynein-mediated retrograde transports. Thus, the combined effect of DCX on retrograde and anterograde transport balances the extension of somatic Golgi into dendrites.⁸⁹ In DCX deficient neurons, however, more JIP3 molecules bind dynein via unknown mechanisms/adaptor(s) which affects retrograde transport. In addition, loss of DCX restricts Kinesin-3¹¹⁴ and CLASP2⁸⁹ to the neuronal soma, which decreases anterograde transport. The imbalance of anterograde and retrograde transports in DXC-deficient neurons results in improper Golgi localization.

CLASP2

CLASP2, a protein that directly links to plus-ends of growing MTs and actin filaments,¹¹⁷ is critical for establishing continuity and proper morphology of Golgi by arranging Golgi-derived MTs in fibroblasts.^79,85,118 Similarly, knocking down CLASP2 in cultured hippocampal neurons led to Golgi fragmentation, which could be fully rescued by overexpression of human CLASP2.¹¹⁹ Overexpression of CLASP2 alone resulted in more stacked Golgi that highly correlates with increased dendritic branching. This implies that CLASP2 action on Golgi morphology plays an important role in dendritic development and branching.¹¹⁹

CLN8

Ceroid-Lipofuscinosis, Neuronal 8 (CLN8) is a transmembrane ER-resident protein that shuttles between the ER and the ER–Golgi intermediate complex (ERGIC) via two C-terminal signal peptides that mediate recognition by COPII and COPI,^120,121 respectively. Both COPII and COPI are essential components for the secretory pathway that shuttle the cargos between ER and Golgi and/or inter-Golgi trafficking to maintain Golgi's normal structure.^33,122, 123, 124 Thus, disrupting the balance of COPII- and COPI-mediated transport will likely lead to abnormal Golgi morphology and function. Recently, it was reported that CLN8 knockdown increased the size of the Golgi apparatus, but decreased the complexity and size of the somatodendritic compartment in primary rat hippocampal neurons.¹²⁵

Previous studies suggested that CLN8 modulates vesicular trafficking by physically interacting with proteins such as VAPA, syntaxin 8, and GATE16.¹²⁶ Interestingly, these factors are implicated in the regulation of Golgi morphology.^127,128 It was proposed that CLN8 affects Golgi morphology by controlling vesicular trafficking. Thus, expression of Rab7a-Q66L, a constitutively active GTP-bound Rab7a that mediates transport from early to late endosome/lysosome in mammalian cells,¹²⁹ or silencing of TMEM106b, a protein involved in anterograde vesicular traffic,¹³⁰ resulted in altered size and/or fragmentation of Golgi in HeLa cells. Although the direct mechanisms of how CLN8 affects Golgi morphology in neurons are unknown, these results suggest that the increase of Golgi size observed upon CLN8 knockdown can be explained (at least in part) by the alteration of vesicular traffic.¹²⁵

SCYL1

SCY1-like pseudokinase 1 (SCYL1) is a member of the SCY1-like family of catalytically inactive protein kinases that regulates Golgi morphology.131, 132, 133 SCYL1 is prominently expressed in central nervous system (CNS) neurons and its pathological variants cause delayed brain development.^134,135 SCYL1 modulates COPI-mediated retrograde transport by interacting with γ2-COP, a subunit of COPI coatomer, to affect Golgi apparatus organization.¹³⁶ In addition, SCYL1 also localizes to the cis-Golgi where it co-localizes with GM130 and 58K/formiminotransferase cyclodeaminase (FTCD). Knockdown of SCYL1 disrupts COPI-mediated retrograde trafficking of the KDEL receptor to the ER without affecting anterograde traffic from the ER,¹³² indicating that an imbalance of the secretory pathway affects Golgi morphology.

Recently, it was reported that SCYL1 colocalizes with protein arginine methyltransferase 1 (PRMT1) in the Golgi. Upon arginine-methylation by PRMT1, SCYL1 could efficiently interact with γ2-COP. Inhibition of PRMT1 suppressed axon outgrowth and knockdown of SCYL1 inhibited axon outgrowth and dendrite complexity with abnormal Golgi morphology. The inhibitory effect was rescued by siRNA-resistant SCYL1, but not a SCYL1 mutant in which the arginine methylation site was replaced. Thus, SCYL1 arginine methylation by PRMT1 facilitates COPI transport to maintain Golgi organization, which likely contributes to axon and dendrite morphogenesis in neurons.¹³⁷

Synaptotagmin 17

Synaptotagmin 17 (Syt-17), a structurally atypical Syt protein family member, plays no role in exocytosis but is highly expressed in the brain and kidney, especially in hippocampal pyramidal cell layers and in cultured hippocampal neurons.138, 139, 140, 141 Knockout of Syt-17 in mice results in defective novel object recognition.¹⁴² Unlike other Syt family members, Syt-17 lacks an N-terminal transmembrane domain; rather, it associates with the membrane via a string of fatty-acylated cysteine residues (palmitoylation or S-acylation) near the N-terminus of the protein.¹⁴³ The subcellular localization of Syt-17 is not fully elucidated due to the lack of specific antibodies and reported differences depending on the detection method.^{140,142,144,145} Recently, Syt-17 was shown to localize to the Golgi complex in the soma of hippocampal neurons, where it coordinates the import of vesicles from the endoplasmic reticulum to support neurite outgrowth.¹⁴² Yeast two-hybrid screening identified two resident Golgi proteins, GOLGA6A and ICA1, as Syt-17 interactors that are putatively involved in importing vesicles from the ER and tethering them to the Golgi. Syt-17 thus appears to be a member of a cargo import complex on the surface of the neuronal Golgi.¹⁴² In addition, Syt-17 was occasionally (<1%) detected to colocalize with remote ManII puncta at apparent GOPs in neuronal dendrites.¹⁴² The possible function of Syt-17 on GOPs is still unknown.

GM130

GM130 was originally identified in the rat liver¹¹² and belongs to the golgin family, which encompasses large (>500 residues) proteins localizing predominantly to the Golgi and forming a coiled-coil structure over most of their length.¹⁴⁶ So far, 11 mammalian golgins have been identified, localizing to distinct regions of the Golgi stack. Most golgins are attached to the membrane at their carboxyl-termini via a transmembrane domain or a domain that binds a small GTPase of the Rab or Arf family.¹⁴⁶ Together with the golgins p115, giantin, and GRASP65, GM130 facilitates vesicle fusion to the Golgi membrane as a vesicle “tethering factor”.¹⁴⁷ In addition, it is also involved in the maintenance of the Golgi structure and plays a major role in the disassembly and reassembly of the Golgi apparatus during mitosis.148, 149, 150 GM130 function is reportedly involved in multiple biological processes including glycosylation, cell cycle progression, cell polarization, and directed cell migration.¹⁴⁷ However, the functions of GM130 in Golgi organization and dendritic morphogenesis in developing neurons are not well characterized. Global knockout of GM130 in mice revealed that it is not essential for embryonic development, but results in developmental delay, severe ataxia, and postnatal death.¹⁵¹ Neuronal-specific deficiency of GM130 (GM130-nKO) in mice causes similar developmental phenotypes as the global knockout including growth retardation and striking ataxia.¹⁵¹ Intensive analysis of brain architecture of the forebrain or midbrain of GM130-nKO mice showed no gross changes; however, cerebellar size was dramatically reduced. By examining Purkinje cells, Liu et al found that selective deletion of GM130 in neurons causes fragmentation and defective positioning of the Golgi apparatus, impaired secretory trafficking, and dendritic atrophy in Purkinje cells.¹⁵¹ In addition, those phenotypes worsen with age.

It should be noted that RNAi-mediated depletion of GM130 in hippocampal neurons results in mild impairment of dendritic initiation.¹⁵² However, dendritic initiation in Purkinje neurons still occurs in the absence of GM130, but dendritic maintenance is impaired in vivo. A reasonable explanation for this discrepancy is likely due to redundancy in the golgin function, which may vary between different types of neurons.¹⁴⁶ The other possibility is that Purkinje cells are particularly susceptible to perturbations of secretory trafficking because of their extremely large dendritic tree, which requires a significant amount of material for both its growth and its maintenance.¹⁵¹ Interestingly, loss of the GM130 binding partner GRASP65 in mice fails to elicit a phenotype.¹⁵³ This could be attributed to a compensatory role by the related protein GRASP55.¹⁵⁴ However, GRASP55 interaction with GM130 could not be shown in vivo,¹⁵⁵ suggesting the phenotypes of Purkinje cells in GM130 knockout mice are likely independent of GRASP65.¹⁵¹

Atypical protein kinase C

Dendrite morphogenesis can be divided into three major stages including initiation to establish the primary branch(es), outgrowth, and branching.¹⁵⁶ The number of primary dendrites may affect various cellular functions and dendritic computation.¹⁵⁷ Cerebellar Purkinje cells extend a single primary dendrite from the soma that ramifies into a highly branched dendritic arbor. However, in vivo time-lapse imaging of zebrafish Purkinje cells revealed that they initially extend multiple neurites from the soma and subsequently retract all but one, which becomes the primary dendrite.¹⁵⁸ This system provides an excellent model to investigate how a primary dendrite(s) gets selected. Interestingly, the Golgi apparatus is specifically located at the root of the primary dendrite, and its localization is already established in immature Purkinje cells that have multiple neurites.¹⁵⁷ In addition, inhibiting secretory trafficking through the Golgi apparatus reduces dendritic growth, suggesting that the Golgi apparatus is involved in dendritic morphogenesis.¹⁵⁷

Atypical protein kinase C (aPKC), together with Par3 and Par6, forms a complex that is important for neuronal polarity establishment, although aPKC function in axon or dendrite polarization is still controversial.¹⁵⁹ Upon loss of aPKC, zebrafish Purkinje cells retain multiple primary dendrites and show disrupted localization of the Golgi apparatus.¹⁵⁸ Importantly, a mosaic inhibition of aPKC in Purkinje cells recapitulates the whole larval mutant phenotype, suggesting that aPKC cell autonomously controls Golgi localization and thereby regulates the specification of the primary dendrite of Purkinje cells.

STK25

Adult-born dentate granule cells (DGCs) establish a DGC-like morphology at ∼7 d after birth, with a primary dendrite pointing to the molecular layer and several neurites in the neurogenic zone.¹⁶⁰ However, the events leading to the transition from simple spindle-like DGCs to their mature dendrite morphology are largely unknown. Rao et al found that the Golgi apparatus was dynamically repositioned in the soma of adult-born DGCs during the initial integration stage. Two weeks after birth, by which time most neurites in the neurogenic zone were eliminated, a compact Golgi apparatus was positioned exclusively at the base of the primary dendrite.¹⁶⁰ Previous single-cell transcriptome analysis in adult-born DGCs identified several Golgi-associated genes including STK25 and STRAD¹⁶¹ as regulators of neuronal development. Knocking down either of these two proteins showed Golgi mislocalization and extensive aberrant dendrite formation in adult-born DGCs. In addition, overexpression of STRAD-PMSE-Δ180, a mutated form causing polyhydramnios, megalencephaly, and symptomatic epilepsy,¹⁶² resulted in similar defects in Golgi localization and dendrite formation in adult-born DGCs.

Further studies showed that physical interaction between STK25 and STRAD resulted in significant stabilization of STK25 protein in HEK293 cells.¹⁶⁰ In addition, STK25 was shown to regulate Golgi localization through the recruitment of GM130.^160,163 GM130 can also associate with STRAD and coexpression of GM130 with either STRAD or STK25 in HEK-293 cells resulted in significant stabilization of the GM130 protein. The association of STK25 with STRAD might stabilize a Golgi signaling complex via interaction with GM130, which is necessary for the polarized positioning of Golgi in newborn DGCs. In support, STK25 was predominantly found in distinct somatic structures at the base of the apical dendrite of adult-born DGCs and it colocalizes with STRAD and the Golgin GRASP65.¹⁶⁰ Overall, STK25 and STRAD are critical for polarized Golgi localization and dendrite establishment of adult-born DGCs, likely by directly interacting with GM130.

LKB1

The serine/threonine protein kinase liver kinase B1 (LKB1) is a well-known regulator of cell polarity. LKB1 expression in the dentate gyrus of 8-week-old adult mice is elevated in adult-born DGCs.¹⁵² Detailed in vivo analysis showed that LKB1 is critical for the polarized initiation and extension of the single primary dendrite in adult-born DGCs. Interestingly, although the Golgi apparatus accumulated at the base and within the initial segment of the primary dendrite in wild-type granule cells, it was dispersed over the entire soma in LKB1 knockout neurons, with no clear GOPs in any dendrite.¹⁵² Furthermore, quantitative analysis showed that Golgi distribution exhibited a clear preference toward the molecular layer in wild-type neurons, and this preference was largely lost in LKB1 knockout neurons. Thus, LKB1 is required for the polarized Golgi distribution in adult-born DGCs. Importantly, disrupting Golgi distribution by either down-regulating the Golgi matrix protein GM130 or overexpressing GRASP65 in the adult hippocampus led to significant defects in both the initiation and extension of dendrites, which is consistent with the dendritic defects in LKB1-knockdown neurons. These results confirm the idea that Golgi polarity is important for dendrite morphogenesis in adult-born DGCs and suggest that the function of LKB1 in dendrite development is linked to its role in regulating Golgi organization.

Reelin

Reelin, a secreted ligand that is produced in discreet layers in the developing brain, was originally identified to be crucial for neuronal positioning and laminar organization.164, 165, 166 Following genetic and biochemical studies showed that it also regulates dendritogenesis via multiple mechanisms.167, 168, 169, 170, 171, 172 Among these, the cytoplasmic adaptor protein Dab1,¹⁷³ a core effector of the Reelin signaling pathway,^174,175 affects dendritic initiation and elaboration,^171,176,177 for example, by maintaining the normal extension of the Golgi apparatus into the apical dendrites in hippocampal, pyramidal and L6 cortical neurons.^176,177 Interestingly, Matsuki et al further found that STK25, a modifier of Reelin-Dab1 signaling, regulates Golgi morphology and neuronal polarization as part of an LKB1-STK25-GM130 signaling pathway.¹⁷⁷ Overexpression of STK25 induces Golgi condensation and multiple axons, both of which are rescued by Reelin treatment. Conversely, Reelin-induced extension of the Golgi into dendrites could be suppressed by STK25 overexpression.¹⁷⁷ Thus, this elegant study highlights the importance of balance between Reelin-Dab1 and LKB1-Stk25-GM130 signaling in controlling Golgi dispersion, axon specification, and dendrite growth.

Recently, CLASP2 was also identified as a key cytoskeletal effector in the Reelin signaling pathway to regulate neocortical development.¹⁷⁸ It was reported that the proper movement of the centrosome and Golgi apparatus is a key step in selecting the migratory direction, and their translocation into the leading neurite precedes cell movement during neuronal migration.^179,180 Dillon and colleagues found that downregulation of CLASP2 in migrating neurons causes mislocalization of neurons to deeper cortical layers, abnormal positioning of the centrosome-Golgi complex, and aberrant length/orientation of the leading process.¹⁷⁸ They further discovered Reelin could regulate several phosphorylation sites within the positively charged serine/arginine-rich region that constitute consensus GSK3β phosphorylation motifs of CLASP2. Furthermore, phosphorylation of CLASP2 regulates its interaction with the Reelin adaptor Dab1, which is required for CLASP2's effect on neurite extension and motility.¹⁷⁸

Rho GTPases

Around 20 canonical Rho GTPases have so far been characterized in mammals that act as molecular switches by cycling between a GDP-bound, inactive state and a GTP-bound, active state, to control a wide variety of signaling pathways for dendritogenesis.181, 182, 183 The cellular regulation of this cycle is finely tuned by over 60 activators (guanine nucleotide exchange factors, GEFs), which accelerate intrinsic GDP/GTP exchange, and over 70 inactivators (GTPase-activating proteins, GAPs), which stimulate intrinsic GTP hydrolysis activity.^181,182 Of these intricately regulated Rho GTPases, Rho, Rac, and Cdc42 are the three best-characterized members.¹⁸¹

It was reported that TRIO,¹⁸⁴ an essential Rho GEF, localizes to the Golgi apparatus (especially the trans-Golgi)¹⁸⁵ and is required for neurite growth¹⁸⁶ in cerebellar granule neurons. Although it is still unknown whether Golgi morphology is altered in TRIO knockout mice, Golgi function is largely disrupted since TRIO affects membrane vesicle trafficking.¹⁸⁵ Direct evidence for Rho GTPases controlling Golgi morphology and dendrite development stems from the observation that the Cdc42/Rac1 GEF aPIX/Arhgef6 could promote translocation of Golgi cisternae into developing dendrites of hippocampal neurons.¹⁸⁷ Interestingly, this effect is further increased upon Reelin treatment. Importantly, overexpression of exchange activity-deficient aPIX/Arhgef6 or dominant-negative Cdc42 or Rac1 impaired dendritic Golgi positioning, an effect that was not compensated by Reelin treatment. Thus, this study implies an unexpected role of Reelin in Golgi morphology to control dendrite development that relies on the activation of Cdc42 or Rac1.¹⁸⁷

Recently, RhoA signaling was also shown to be critical for Golgi arrangement to control dendrite development.¹⁸⁸ Hong et al identified Brefeldin A-inhibited GEF 2 (BIG2) as a novel Rho GTPase and knockdown of BIG2 significantly reduced dendritic complexity.¹⁸⁸ Expression of the constitutively active ARF1 could rescue the defective dendrite morphogenesis of BIG2-deficient neurons. Importantly, BIG2 co-localizes with the Golgi apparatus and is required for Golgi deployment into major dendrites in cultured hippocampal neurons. Simultaneous overexpression of BIG2 and ARF1-activated RhoA, or treatment with the RhoA activator lysophosphatidic acid in neurons lacking BIG2 or ARF1 increased the number of cells with dendritic Golgi. Finally, they characterized that mDia1, a well-known regulator of MT and actin dynamics,^189,190 acts as a downstream effector of the BIG2-ARF1-RhoA axis to mediate Golgi polarization and dendritic morphogenesis.¹⁸⁸

Although GOPs were observed in vertebrate neurons for over a decade,⁹⁰ how GOPs are generated remained unanswered until a recent study showed a crucial role of RhoA in the generation of vertebrate neuronal GOPs.¹⁰⁸ With time-lapse imaging in cultured hippocampal neurons, the authors could show that GOPs destined to major “apical” dendrites are generated from the somatic Golgi apparatus by a sequence of events involving (i) generation of a Golgi-derived tubule, (ii) tubule elongation and deployment into the dendrite, (iii) tubule fission, and (iv) transport and condensation of the fissioned tubule.¹⁰⁸ A RhoA-Rock signaling pathway involving LIMK1, PKD1, slingshot, cofilin, and dynamin regulates polarized GOP formation by controlling the tubule fission.¹⁰⁸ Thus, this study provided new insights regarding the involvement of RhoA in GOP biogenesis to regulate dendritic development and polarization.

Golgi in Drosophila neuronal dendrite development

Drosophila and vertebrate Golgi share a high degree of similarity.²⁷ GOPs and Golgi function are best studied in Drosophila dendritic arborization (da) sensory neurons in the larval peripheral nervous system. Da neurons are placed into four distinct morphological classes based on their dendritic complexity named Class I to Class IV da (C1da–C4da) neurons.¹⁹¹ While the secretory pathway was found to be highly essential for neuronal dendrite morphogenesis in this system,⁹⁸ nearly all studies are focused on GOPs instead of somatic Golgi. This is likely due to a widely accepted concept that GOPs are important for dendritic dynamics in Drosophila neurons, which is based on the following observations⁹⁸: (i) directional movement of dendritic GOPs correlates with the extension and retraction of dendritic branches; (ii) focally disrupting GOPs by intense laser light limits dendritic dynamics⁹⁸; and (iii) blocking the interaction of the Drosophila golgin Lava lamp (Lva) with the dynein/dynactin complex¹⁹² caused redistribution of GOPs and dendritic branching defects.

Dendritic GOPs likely serve as local secretory stations^94,193 and/or non-centrosomal MT organization centers.⁷⁸ Both in vitro and in vivo observations showed that GOPs could directly nucleate and orientate dendritic MTs to regulate dendrite development.^78,83 However, an independent study did not support this finding since dragging GOPs out of dendrites does not affect γ-tubulin distribution.⁹⁷ In addition, it was also found that GOPs may locally regulate MT orientation, but are not required for overall polarity in Drosophila neurons.¹⁹⁴ Thus, the role of GOPs in non-centrosomal MT nucleation is controversial. While pinning down the functions of GOPs in dendrite development requires further studies, the molecular mechanisms of how GOPs are organized in dendrites are gradually emerging.

Transcription factors

In general, the wide variety of dendritic arbors from distinct neurons is mainly determined by the combinatorial expression of transcription factors. In Drosophila da neurons, the transcription factor Abrupt (Ab) is essential for controlling simple dendritic morphology in C1da neurons, while Cut (Ct) promotes dendritic filopodia/spikes in C3da and C4da neurons.^195,196

A previous study showed that Cut initiates a gene expression cascade through CrebA that coordinately affects Sec31, an important component of COPII secretory transport, to bi-directionally regulate dendrite elaboration.¹⁹⁷ Importantly, overexpression of Cut increased the number of satellite secretory endoplasmic reticulum (ER) and GOPs primarily localized to dendritic branch points, consistent with upregulation of the COPII machinery.¹⁹⁷

In simple C1da neurons, it was found that Ab-mediated branch repression is facilitated by centrosomin (cnn), a homologue of CDK5RAP2 that localizes to the centrosome and Golgi in HeLa cells.⁸² While complex C4da neurons have been shown to contain a mix of single and multicomponent outposts,²⁰ Yalgin et al predominantly detected ManII-positive outposts in C1da neuron arbors, while GalT-positive outposts were rare.⁸³ Interestingly, half of the fluorescently labeled Cnn foci colocalized with ManII-positive GOPs. In addition, the Cnn signal was distributed only to the cis-face of GOPs, with nearly all outposts showing asymmetric Cnn localization. Cnn acts in conjunction with the Pericentrin-like protein (Plp),^198,199 and loss of Plp increased clustering of Cnn foci along dendrite branches and branch points and tips, indicating that proper dendritic localization of Cnn to GOPs requires Plp. Furthermore, it was shown that Cnn recruits microtubule nucleation to GOPs to counteract wee Augmin (Wac)-driven anterograde microtubule growth.⁸³ Thus, polarized targeting of Cnn to GOPs during dendritic development creates a local system for guiding microtubule polymerization.⁸³

MT-based motor proteins

The microtubule-based molecular motors dynein and kinesin are essential for proper neuronal morphogenesis.200, 201, 202 Loss of dynein function alters GOP distribution,²⁰¹ and ManII-positive GOPs were decreased in number and size in distal dendritic arbors. Conversely, an increased number and size of GOPs were observed in proximal dendrites. This change in GOP distribution thus parallels the change in branch distribution (more branches in proximal and reduced branches in distal regions). In addition, abnormal axonal distribution of GOPs, usually excluded from axons, was found in dynein mutants,^98,201 suggesting dynein actively controls GOP distribution during neuronal development. Interestingly, one of the dynein cofactors, Nuclear distribution E (NudE), has been implicated in the regulation of early neurite extension in vertebrate neurons.²⁰³ Similarly, the only conserved homologue of the NudE family in Drosophila is also essential for dendrite development and its deficiency mimics the dendritic phenotypes in dynein mutants.²⁰⁴ Importantly, Drosophila NudE is colocalized with GOPs and prevents GOPs from entering the axon.²⁰⁴ Interestingly, overexpression of Lis, a critical interactor of dynein,²⁰⁵ could rescue the abnormal dendritic arborization that is caused by loss of NudE.²⁰⁴ Although it was reported that GOPs are involved in dendritic microtubule nucleation and orientation, NudE mutants showed only slightly increased levels of dynamic microtubules without affecting their polarity in dendrites.²⁰⁴

Recent in vivo analysis of endogenous kinesin-1 showed that a critical Glutamate residue within β5-loop 8 (E177) is required for dendrite-specific localization of GOPs.²⁰² Khc^E177K, but not Khc^E177A, resulted in GOPs mislocalizing to axons, similarly to Khc^RNAi. Khc^E177K also phenocopied Khc^RNAi showing decreased dendrite arborization during aging.²⁰² Since this particular glutamate, E177, mediates contact with the C-terminal tail when kinesin-1 is in an autoinhibited conformation,²⁰⁶ autoinhibition of kinesin-1 seems to be essential for dendrite-specific localization of GOPs and dendrite development.

While GOPs are transported into dendrites by dynein, the dendrite-specific localization of GOPs may depend on a regulated balance of dynein and kinesin-1 activity. Consistent with this idea, reducing kinesin-1 levels in dendrites causes a distal shift of GOPs in the arbor by increasing outposts distally and reducing them proximally.²⁰² Overall, our current understanding suggests that dynein transports outposts into dendrites,^98,201,204 and selective localization of GOPs to dendrites relies on balanced autoinhibition of the anterograde motor kinesin-1.²⁰²

Golgi and neurological disorders

Abnormal dendritic morphogenesis is accompanying multiple neurological disorders including autism spectrum disorders (ASD), Alzheimer's disease (AD), Parkinson's disease (PD), spinocerebellar ataxia, and dementia.²⁰⁷ Although the potential roles of the Golgi apparatus in neurological disorders have been intensively reviewed,9, 10, 11, 12, 13 the involvement of abnormal dendrite development/maintenance in these diseases is rarely discussed. Recent studies showed that several proteins related to neurological disorders are involved in re-organizing the Golgi to regulate dendrite morphogenesis, including Lrrk2,²⁰⁸ polyglutamine (polyQ) repeat proteins,²⁰⁹ C9orf72,²¹⁰ and APP²¹¹ (Fig. 4).

Figure 4.

The role of GOPs in dendrite development and neurological disorders. (i) The transcription factor Cut, pathogenic PolyQ proteins, and C9orf72 affect GOPs by controlling CrebA-dependent transcription. Sec31, an essential component for COPII, is one of the effectors. (ii) The transcription factor Ab regulates cnn (homologue to CDK5RAP2) that interacts with plp and wac to link GOP organization and MT dynamics. (iii) NduE promotes Lis binding to dynein to regulate GOP transport. (iv) Lrrk interacts with Lva and inhibits the interaction between Lva and dynein, disrupting the recruitment of dynein to GOPs. (v) The adaptor protein Syd mediates APP-induced alteration of multi-compartment GOP movements and dendrite development. (vi) GM130 could directly connect different Golgi compartments via an unknown mechanism.

LRRK2

Leucine-rich repeat kinase 2 (LRRK2) is a member of the ROCO family with a Ser/Thr kinase domain.²¹² Dominant mutations in human LRRK2 are prevalent in both familial and sporadic PD and are located in the catalytic domain.212, 213, 214 The Drosophila genome encodes a single Lrrk orthologue with conserved functional domains and is present in cell bodies, dendrites, and axons of larval da neurons.²⁰⁸ Interestingly, overexpressed fluorescently labeled Lrrk prominently colocalized with the Golgi marker ManII in dendrites when compared to endosomes, mitochondria, and lysosomes.²⁰⁸ Importantly, Lrrk2 maintains stationary GOPs and suppresses their anterograde movement. Lack of Lrrk activity resulted in dendritic overbranching indicating that GOP motility and dendrite outgrowth are correlated as previously suggested.⁹⁸ In addition, the biochemical analysis showed that Lrrk2 kinase activity is critical for Lva phosphorylation to prevent Lva–Dynein interaction, thus keeping GOPs static. Overexpression of human kinase-dead LRRK2 (LRRK2^G2019S) in turn seems to promote kinesin-based transport facilitating GOP movement toward cell bodies.²⁰⁸ Previous studies also showed that human LRRK2 interacts with ArfGAP1 at Golgi membranes and overexpression of LRRK2^G2019S results in somatic Golgi fragmentation.²¹⁵ These studies highlight the role of LRRK2 in regulating Golgi organization, which might directly or indirectly affect dendrite development.

Polyglutamine (polyQ) disease proteins

Polyglutamine (polyQ) diseases including spinocerebellar ataxia/Machado-Joseph disease (SCA/MJD) and Huntington disease (HD) are inherited neurodegenerative diseases caused by expansion of trinucleotide CAG repeats within the protein-coding region of the disease gene.²¹⁶ Expanded/pathogenic polyQ proteins form aggregates together with many other molecules in afflicted neurons, which disrupts multiple cellular processes. Ectopic expression of pathogenic MJDtr-78Q resulted in defective terminal dendrite elongation in C4da neurons.^209,217 Chung et al found that overexpression of MJDtr-78Q caused a loss of GOPs in main dendritic branches.²⁰⁹ mRNA sequencing of fly heads with or without MJDtr-78Q expression identified many deregulated secretory pathway-related genes, including altered expression of COPII genes regulating GOP synthesis. In addition, the transcription factor CREB3L1/CrebA, a known regulator of COPII gene expression, was found to be downregulated. Conversely, CrebA overexpression could restore the dysregulation of COPII genes and the abundance of GOPs despite polyQ expression. Furthermore, the authors performed chromatin immunoprecipitation (ChIP)-PCR and found that CrebA expression is regulated by CREB-binding protein (CBP), which is sequestered by polyQ proteins. Thus, these findings suggest that impairment of GOPs by polyQ proteins contributes to dendrite pathology through the CBP-CrebA-COPII pathway.²⁰⁹

C9orf72

The pathological hallmark of Amyotrophic lateral sclerosis (ALS) is motor neuron axon degeneration.²¹⁸ However, several studies also identified abnormal dendritic arborization.219, 220, 221 Similarly, frontotemporal dementia (FTD), which is considered to be in the same disease spectrum as ALS due to a shared set of genetic factors and symptoms, also manifests dendritic abnormalities; pyramidal and non-pyramidal neurons from FTD postmortem brain samples display reduced complexity and number of dendritic branches.²²² Although altered dendritic morphology is frequently observed in ALS and FTD, the cellular and molecular basis underlying these potentially pathogenic abnormalities remains largely unclear. The most common genetic cause of both ALS and FTD is the intronic hexanucleotide (G4C2) repeat expansion mutation in C9orf72.^223,224 A fraction of the pathogenic C9orf72 RNA exits the nucleus and undergoes repeat-associated non-ATG (RAN) translation into five dipeptide repeat proteins (DPRs) including glycine–arginine (GR), proline–arginine (PR), glycine–alanine (GA), glycine–proline (GP) and proline–alanine (PA).²²⁵ Overexpression of four of the DPRs (except GP which is known to be benign in various models) in Drosophila C4da neurons resulted in a significant reduction of dendritic branching at both larval and adult stages.²¹⁰ Among these, PR and GR showed the most robust reduction of dendrites. Furthermore, the expression of PR and GR reduced the number of GOPs in dendrites. Interestingly, the expression of PR, but not GR, led to a significant reduction in the mRNA level of CrebA, which transcriptionally regulates the formation of GOPs.²⁰⁹ Overexpressing CrebA in PR-expressing C4da neurons restored the number of GOPs, but the number of dendritic branches remained unchanged,²¹⁰ suggesting that other molecules besides CrebA may be involved in PR-mediated GOP disorganization and dendritic branching defects.

Amyloid precursor protein (APP)

Fragmentation and atrophy of the Golgi apparatus have been observed in AD patients.^226,227 In addition, abnormalities of protein glycosylation and neural sialyltransferase activity have also been reported in AD brain tissues, such as an enhancement of glycosylation of β-amyloid (Aβ) peptides,^228,229 suggesting that defects in the Golgi apparatus are pathological features of AD. Overexpression of human APP in Drosophila sensory neurons caused developmental dendrite defects.²¹¹ However, whether and how amyloid precursor protein (APP) regulates GOPs to affect dendrite development remains unclear. Recently, Du et al found that APP resides in and affects the movements of GOPs. In particular, it reverses the distribution of multi-compartment GOPs without affecting single-compartment GOPs in sensory C3da neurons.²¹¹ Although it was shown that the bidirectional movement of multi-compartment GOPs was cooperatively controlled by dynein and kinesin, only dynein seems to mediate APP-dependent regulation of multi-compartment GOP movement. Furthermore, by loss-of-function screening and genetic interaction analysis, the adaptor protein sunday driver (Syd) was identified to mediate APP-induced movement of multi-compartment GOPs and dendritic defects.²¹¹ Thus, APP might regulate dendritic development by directing the movement and function of multi-compartment GOPs.

Prenatal alcohol exposure (PAE)

Prenatal alcohol exposure (PAE) disrupts brain development and function, and the severity depends on sex, age, and consumption.230, 231, 232 It was found that PAE causes dendritic developmental defects in the hippocampus and cortex.^233,234 In a whole hemisphere explant model, acute responses to ethanol exposure included compaction of the Golgi apparatus accompanied by elaboration of supernumerary primary apical neurites, as well as a modest increase in higher order apical neurite length.²³⁴ Longer ethanol exposure leads to misorientation of the soma, the primary apical neurite, and Golgi with respect to the pial surface.²³⁴ In addition, cellular orientation defects were accompanied by decreased abundance of the microtubule-associated protein MAP2 and F-actin, critical cytoskeletal components required for dendrite development. Thus, these findings indicate that upon exposure to ethanol, developing cortical neurons manifest abnormal Golgi size and location as well as cytoskeletal regulation, which may in turn result in significant perturbation of primary neurite formation and neuronal polarity.²³⁴

PAE-induced abnormal Golgi organization is likely due to the alteration of neuronal activity since it changes the composition of glutamate receptors.235, 236, 237 Previous studies showed that neuronal hyperexcitation resulted in reversible fragmentation of the Golgi complex and biogenesis of GOPs and Golgi satellites in cultured neurons.^109,238 Prolonged blockade of GABA-A-mediated inhibition or withdrawal of NMDA receptor antagonists caused Golgi fragmentation, likely by CaMKII/CaMKIV activation,²³⁸ which eventually might affect dendritic development.

Conclusions and perspectives

Unlike most other somatic cells, neurons have particularly unique demands to develop and maintain their polarized axonal and dendritic compartments. To establish and maintain complex and dynamic dendritic arbors, neurons must continually and actively transport newly synthesized proteins and lipids to their distant destinations. These finely tuned processes rely on proper Golgi organization due to its central role in the secretory pathway and emerging role as an MTOC in neurons.^5,6 Although the importance of the Golgi apparatus in dendritic morphogenesis has been proposed for decades,^{90,94,98,99,128} the specific mechanisms of somatic Golgi and GOP organization in neurons and their roles in dendrite development are just starting to be unraveled.

Based on previous studies, the principal organization mechanisms of both somatic Golgi and GOPs involve proper MT dynamics and COPII/I transport balance. It should be noted that MT dynamics and COPII/I transport can affect each other,⁸ making it difficult to distinguish cause and effect in Golgi organization. GM130 was reported as a key connector for assembling Golgi stacks, yet the mechanism is still unknown. While specific functions and defects have been attributed to the somatic Golgi or GOPs, they are interconnected but have barely been investigated in parallel. Thus, challenging questions remain in this emerging field that require further studies. Firstly, how are somatic Golgi and GOP functions dynamically regulated to satisfy dendrite development? Virtually all studies so far were focused on a specific time point, but lack systematic exploration during all stages of dendrite development. In addition, dendrite pruning is also a critical developmental process but the Golgi function is even less well investigated.²³⁹ Secondly, mechanisms of neuronal Golgi organization remain to be intensively investigated. Besides Golgi-resident proteins, other associated molecules involved in signal transduction and MTOC organization are not fully understood. Thirdly, so far, we have mostly correlative data linking GOP presence and motility to dendritic morphogenesis, which demands further studies into a causal functional link. In this respect, the proposed role of GOPs as local MTOCs might be the key to gaining further understanding. Lastly, the widespread distribution of Golgi satellites compared to GOPs in mammalian neurons begs the question of whether they differ only in size or also in function.¹⁰⁷ It further remains to be elucidated if and how Golgi satellites and GOPs might support local protein synthesis and post-translational modification of secretory pathway proteins. This will require functional analyses of Golgi/GOPs/GS in future studies to unravel their impact on dendritic spine development and synaptic plasticity.

Many candidate proteins affecting Golgi organization have been identified by large-scale screening in both human⁷² and Drosophila cells.⁷³ However, these screens were performed in proliferating cell lines, which likely have different functional requirements than postmitotic neurons. Thus, systematic genetic analyses of neuronal Golgi organization and its impact on dendritic morphogenesis are needed. Deciphering novel molecular regulators and their role in Golgi organization may provide us a more complete view of its link with dendritic morphogenesis and lead to possible treatments for neurological disorders and Golgipathies.^11,240,241

Author contributions

CH and PS conceived the idea. MLC, LX, YW and CH wrote the draft. MLC and LX prepared the figures. CH and PS revised and finalized the manuscript.

Conflict of interests

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interests.

Funding

This work was supported by grants from the National Natural Science Foundation of China (No. 32000690 to CH), the Key-Area Research and Development Program of Guangdong Province, China (No. 2019B030335001 to CH), the National Social Science Foundation of China (No. 20&ZD296 to CH), the Deutsche Forschungsgemeinschaft (DFG; No. SO1337/4-1, SO1337/2-2, and SO1337/7-1 to PS), and the DFG Heisenberg program (No. SO1337/6-1 to PS) and ERA-NET NEURON (BMBF; No. 01EW1910 and 01EW2108 to PS).