Abstract
The Golgi protein GOLPH3 binds to PtdIns(4)P and MYO18A, linking the Golgi to the actin cytoskeleton. The GOLPH3 pathway is essential for vesicular trafficking from the Golgi to the plasma membrane. A side effect of GOLPH3-dependent trafficking is to generate the extended ribbon shape of the Golgi. Perturbation of the pathway results in changes to both Golgi morphology and secretion, with functional consequences for the cell. The cellular response to DNA damage provides an example of GOLPH3-mediated regulation of the Golgi. Upon DNA damage, DNA-PK phosphorylation of GOLPH3 increases binding to MYO18A, activating the GOLPH3 pathway, which consequently results in Golgi fragmentation, reduced trafficking, and enhanced cell survival. The PtdIns(4)P/GOLPH3/MYO18A/F-actin pathway provides new insight into the relationship between Golgi morphology and function, and their regulation.
Introduction
In many mammalian cell types the Golgi complex appears by light microscopy as an extended ribbon that wraps partially around the nucleus. The ribbon is composed of many subresolution stacks of membrane (as observed by electron microscopy) that are linked via Golgi-associated proteins. From first principles we can conclude that the steady-state appearance of the Golgi reflects the balance of forces applied to it. Changes in the shape of the Golgi presumably reflect alterations in the balance of forces applied to the Golgi. Since at least some of the forces that are applied to the Golgi are likely to be important for its function in vesicle trafficking, changes in trafficking machinery might be expected to lead to changes in Golgi morphology. However, it is important to bear in mind that the morphology of the Golgi varies significantly across species (reviewed in Mowbrey and Dacks, ; Wei and Seemann, ), suggesting that diverse morphologies can still be fully competent for trafficking.
The GOLPH3 pathway provides a link from the trans-Golgi membrane to the actin cytoskeleton that plays a critical role in anterograde trafficking to the plasma membrane (Figure 1). The trans-Golgi is highly enriched in phosphatidylinositol-4-phosphate (PtdIns(4)P) (Godi et al., , ). In mammalian cells Golgi PtdIns(4)P is produced by the Golgi localized PI-4-kinases, PI-4-kinase-IIIβ (PI4KIIIβ) and PI-4-kinase-IIα (PI4KIIα) (Wong et al., ; Wang et al., ; De Matteis et al., ). PtdIns(4)P levels are reduced by the action of the Golgi/ER localized PtdIns(4)P-4-phosphatase, SAC1 (Foti et al., ; Schorr et al., ). GOLPH3 localizes to the trans-Golgi via its direct interaction with PtdIns(4)P (Dippold et al., ). This interaction is mediated by a binding pocket on the hydrophobic face of GOLPH3. Furthermore, GOLPH3's interaction with PtdIns(4)P and its Golgi localization are conserved from yeast to humans (Dippold et al., ; Wood et al., ). GOLPH3 also binds tightly and specifically to the unconventional myosin, Myosin 18A (MYO18A) (Dippold et al., ; Ng et al., ; Taft et al., ; Farber-Katz et al., ), and MYO18A has been shown to bind to F-actin (Isogawa et al., ; Guzik-Lendrum et al., ; Taft et al., ). Thus, GOLPH3/MYO18A serves to link the PtdIns(4)P-rich trans-Golgi membrane to the actin cytoskeleton.
Figure 1
Perturbations of the GOLPH3 pathway alter trafficking and golgi morphology
All of the components of the pathway, PtdIns(4)P, GOLPH3, MYO18A, and F-actin, are required for efficient Golgi-to-plasma membrane trafficking. PtdIns(4)P has been shown to be required for Golgi secretory function across species from yeast to humans (Hama et al.,
Depletion of PtdIns(4)P, GOLPH3, MYO18A, or F-actin results in both a defect in Golgi-to-plasma membrane trafficking and also a striking change in the morphology of the Golgi (Figure 2). The normal Golgi ribbon that extends partially around the nucleus condenses into a compact ball at one end of the nucleus upon interference with any component of the GOLPH3 pathway. For example, depletion of PtdIns(4)P by overexpression of SAC1, siRNA knockdown of GOLPH3 or MYO18A, or depolymerization of F-actin by latrunculin B, all cause Golgi compaction (Dippold et al.,
Figure 2

Perturbations of the GOLPH3 pathway alter Golgi morphology. Examples of the Golgi (in red, co-stained with the nucleus in blue) are shown under normal conditions and in response to perturbations to interfere with or to increase the activity of the GOLPH3 pathway. Normally the Golgi appears as an extended ribbon partially wrapped around the nucleus (middle). Inhibition of the GOLPH3 pathway, by overexpression of SAC1 (Dippold et al.,
While interfering with the GOLPH3 pathway results in Golgi compaction, increasing the activity of the GOLPH3 pathway causes expansion of the Golgi (Figure 2). For example, mild overexpression of GOLPH3 leads to an increasingly extended Golgi ribbon. However, very high levels of overexpression of GOLPH3 result in fragmentation and dispersal of the Golgi throughout the cytoplasm (Ng et al.,
Regulation of golgi morphology and function via the GOLPH3 pathway
A growing body of evidence demonstrates regulation of Golgi morphology and function via the GOLPH3 pathway (Figure 1). For example, growth factor signaling regulates PtdIns(4)P levels at the Golgi by regulating SAC1 localization. In response to growth factor signaling, activated p38/MAPK phosphorylates SAC1, causing it to translocate from the Golgi to the ER (Blagoveshchenskaya et al.,
The Golgi is also regulated by GOLPH3L, a paralog of GOLPH3 that is expressed in highly secretory cells in vertebrates (Ng et al.,
Regulation of the golgi by DNA damage via the GOLPH3 pathway
A recent study by our lab identified novel signaling that regulates the Golgi in response to DNA damage (Farber-Katz et al.,
DNA damage has been posited to contribute to the pathophysiology of many human diseases, including cancer (reviewed in Lord and Ashworth,
Relationship between golgi morphology and secretory function
While perturbations of the GOLPH3 pathway result in predictable and consistent changes in both Golgi shape and secretory function, it does not follow that there is a simple relationship between morphology and function. In fact, perturbations of pathways other than the GOLPH3 pathway have been demonstrated to alter Golgi morphology without affecting secretory trafficking. For example, depolymerization of microtubules by treatment with nocodazole leads to Golgi fragmentation and dispersal (Thyberg and Moskalewski,
Thus, we conclude that there is not a simple relationship between Golgi morphology and secretory function that holds under all circumstances. Instead, the effect of a perturbation on the Golgi is dictated by the specific mechanism of the perturbation. However, for perturbations of the GOLPH3 pathway the consequences to Golgi morphology and secretory function are predictable.
Functional consequences of altered golgi
Since changes in Golgi morphology do not directly relate to its trafficking function, it remains unclear whether changes in Golgi morphology always have cellular consequences. However, it seems safe to predict that perturbations of the Golgi that affect Golgi-to-plasma membrane trafficking will have consequences to the cell. The phenotypes associated with perturbation of the GOLPH3 pathway provide strong examples of the importance of Golgi-to-plasma membrane trafficking to overall cell and organismal biology. Notably, GOLPH3 is an oncogene that is frequently amplified and overexpressed in human cancers, and its increased expression predicts poor patient prognosis (Scott et al.,
PtdIns(4)P and GOLPH3 have been demonstrated to play important roles in regulating cell motility. Knockdown of SAC1, resulting in elevated Golgi PtdIns(4)P levels, leads to increased cell migration (Tokuda et al.,
Our recent study on GOLPH3-mediated Golgi dispersal/fragmentation upon DNA damage has revealed an interesting role of the pathway in promoting cell survival (Farber-Katz et al.,
Potential role of the GOLPH3 pathway in neurodegenerative disease
Whether the GOLPH3 pathway plays a role in the pathophysiology of neurodegenerative disease has not been studied. However, some of the known pathophysiology of neurodegenerative disease makes it reasonable to speculate that the GOLPH3 pathway may be involved. First, dysregulation of the Golgi is a common feature of a variety of human neurodegenerative diseases, including amyotrophic lateral sclerosis, Parkinson's disease, and Alzheimer's disease. The accompanying articles in this issue of Frontiers in Neuroscience provide detailed reviews of the literature implicating Golgi dysregulation in neurodegenerative disease. We note that the changes in Golgi morphology that are associated with these diseases mimic those seen with perturbations of the GOLPH3 pathway. Moreover, knockout of a Golgi-specific PI-4-kinase, PI4KIIα, expected to interfere with GOLPH3 pathway function, leads to late onset neurodegenerative disease in mice (Simons et al.,
A second link to the GOLPH3 pathway arises from a substantial body of literature implicating the DNA damage response in neurodegenerative disease. Loss-of-function mutations in various DNA damage repair genes cause a range of neurodegenerative diseases (Madabhushi et al.,
Our discovery that the Golgi is regulated in response to DNA damage via DNA-PK activation of the GOLPH3 pathway provides a plausible link between the evidence implicating DNA damage and altered Golgi function in neurodegenerative disease.
Concluding remarks
The appearance of the Golgi, at least in part, is a consequence of the forces that act on it for vesicle trafficking. The GOLPH3 pathway, in particular, demonstrates the relationship between Golgi secretory function and Golgi morphology. As might be expected for a pathway with an important role in Golgi function, the GOLPH3 pathway is a target for regulation by multiple signals. For example, the GOLPH3 pathway is activated in response to DNA damage, with consequences for Golgi shape and trafficking function, as well as cell survival. Since many proteins depend on trafficking from the Golgi for their function, alterations in GOLPH3-dependent Golgi-to-plasma membrane trafficking have pleiotropic effects on cellular function, significantly impacting biology and human pathophysiology.
Funding
MB is funded by the American Cancer Society by the Lee National Denim Day Postdoctoral Fellowship Award PF-13-367-01-CDD. SF is funded by an Era of Hope Scholar Award from the Breast Cancer Research Program of the Department of Defense W81XWH-10-1-0822 and the Burroughs Wellcome Fund Career Award in the Biomedical Sciences.
Conflict of interest statement
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 interest.
Statements
Acknowledgments
The authors thank Drs. Farber-Katz, Dippold, and Ng, and other members of the Field lab, past, and present, for their scientific discoveries and valuable comments on the manuscript.
Conflict of interest
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 interest.
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Summary
Keywords
Golgi, phosphatidylinositol-4-phosphate, GOLPH3, MYO18A, secretory trafficking, DNA damage, cancer, neurodegenerative disease
Citation
Buschman MD, Xing M and Field SJ (2015) The GOLPH3 pathway regulates Golgi shape and function and is activated by DNA damage. Front. Neurosci. 9:362. doi: 10.3389/fnins.2015.00362
Received
31 July 2015
Accepted
22 September 2015
Published
07 October 2015
Volume
9 - 2015
Edited by
Catherine Rabouille, Hubrecht Institute of the Royal Netherlands Academy of Arts and Sciences, Netherlands
Reviewed by
Stephan Gasser, National University of Singapore, Singapore; Peter Mayinger, Oregon Health & Science University, USA
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© 2015 Buschman, Xing and Field.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Seth J. Field, Division of Endocrinology and Metabolism, Department of Medicine, University of California, San Diego, 9500 Gilman Drive, MC-0707, La Jolla, CA 92093, USA sjfield@ucsd.edu
This article was submitted to Neurodegeneration, a section of the journal Frontiers in Neuroscience
†These authors have contributed equally to this work.
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