Newly synthesized polycystin-1 takes different trafficking pathways to the apical and ciliary membranes

A cilia-bound unconventional secretory pathway for Drosophila odorant receptors

BACKGROUND

Post-translational transport is a vital process which ensures that each protein reaches its site of function. Though most do so via an ordered ER-to-Golgi route, an increasing number of proteins are now shown to bypass this conventional secretory pathway.

RESULTS

In the Drosophila olfactory sensory neurons (OSNs), odorant receptors (ORs) are trafficked from the ER towards the cilia. Here, we show that Or22a, a receptor of various esters and alcoholic compounds, reaches the cilia partially through unconventional means. Or22a frequently present as puncta at the somatic cell body exit and within the dendrite prior to the cilia base. These rarely coincide with markers of either the intermediary ER-Golgi-intermediate-compartment (ERGIC) or Golgi structures. ERGIC and Golgi also displayed axonal localization biases, a further indication that at least some measure of OR transport may occur independently of their involvement. Additionally, neither the loss of several COPII genes involved in anterograde trafficking nor ERGIC itself affected puncta formation or Or22a transport to the cilium. Instead, we observed the consistent colocalization of Or22a puncta with Grasp65, the sole Drosophila homolog of mammalian GRASP55/Grh1, a marker of the unconventional pathway. The numbers of both Or22a and Grasp65-positive puncta were furthermore increased upon nutritional starvation, a condition known to enhance Golgi-bypassing secretory activity.

CONCLUSIONS

Our results demonstrate an alternative route of Or22a transport, thus expanding the repertoire of unconventional secretion mechanisms in neurons.

Polycystic kidney disease: novel insights into polycystin function

Autosomal dominant polycystic kidney disease (ADPKD) is a life-threatening monogenic disease caused by mutations in PKD1 and PKD2 that encode polycystin 1 (PC1) and polycystin 2 (PC2). PC1/2 localize to cilia of renal epithelial cells, and their function is believed to embody an inhibitory activity that suppresses the cilia-dependent cyst activation (CDCA) signal. Consequently, PC deficiency results in activation of CDCA and stimulates cyst growth. Recently, re-expression of PCs in established cysts has been shown to reverse PKD. Thus, the mode of action of PCs resembles a 'counterbalance in cruise control' to maintain lumen diameter within a designated range. Herein we review recent studies that point to novel arenas for future PC research with therapeutic potential for ADPKD.

PAM: diverse roles in neuroendocrine cells, cardiomyocytes, and green algae

Our understanding of the ways in which peptides are used for communication in the nervous and endocrine systems began with the identification of oxytocin, vasopressin, and insulin, each of which is stored in electron-dense granules, ready for release in response to an appropriate stimulus. For each of these peptides, entry of its newly synthesized precursor into the ER lumen is followed by transport through the secretory pathway, exposing the precursor to a sequence of environments and enzymes that produce the bioactive products stored in mature granules. A final step in the biosynthesis of many peptides is C-terminal amidation by peptidylglycine α-amidating monooxygenase (PAM), an ascorbate- and copper-dependent membrane enzyme that enters secretory granules along with its soluble substrates. Biochemical and cell biological studies elucidated the highly conserved mechanism for amidated peptide production and raised many questions about PAM trafficking and the effects of PAM on cytoskeletal organization and gene expression. Phylogenetic studies and the discovery of active PAM in the ciliary membranes of Chlamydomonas reinhardtii, a green alga lacking secretory granules, suggested that a PAM-like enzyme was present in the last eukaryotic common ancestor. While the catalytic features of human and C. reinhardtii PAM are strikingly similar, the trafficking of PAM in C. reinhardtii and neuroendocrine cells and secretion of its amidated products differ. A comparison of PAM function in neuroendocrine cells, atrial myocytes, and C. reinhardtii reveals multiple ways in which altered trafficking allows PAM to accomplish different tasks in different species and cell types.

GRAF2, WDR44, and MICAL1 mediate Rab8/10/11-dependent export of E-cadherin, MMP14, and CFTR ΔF508

In addition to the classical pathway of secretion, some transmembrane proteins reach the plasma membrane through alternative routes. Several proteins transit through endosomes and are exported in a Rab8-, Rab10-, and/or Rab11-dependent manner. GRAFs are membrane-binding proteins associated with tubules and vesicles. We found extensive colocalization of GRAF1b/2 with Rab8a/b and partial with Rab10. We identified MICAL1 and WDR44 as direct GRAF-binding partners. MICAL1 links GRAF1b/2 to Rab8a/b and Rab10, and WDR44 binds Rab11. Endogenous WDR44 labels a subset of tubular endosomes, which are closely aligned with the ER via binding to VAPA/B. With its BAR domain, GRAF2 can tubulate membranes, and in its absence WDR44 tubules are not observed. We show that GRAF2 and WDR44 are essential for the export of neosynthesized E-cadherin, MMP14, and CFTR ΔF508, three proteins whose exocytosis is sensitive to ER stress. Overexpression of dominant negative mutants of GRAF1/2, WDR44, and MICAL1 also interferes with it, facilitating future studies of Rab8/10/11-dependent exocytic pathways of central importance in biology.

A cut above (and below): Protein cleavage in the regulation of polycystin trafficking and signaling

The polycystin-1 and 2 proteins, encoded by the genes mutated in Autosomal Dominant Polycystic Kidney Disease, are connected to a large number of biological pathways. While the nature of these connections and their relevance to the primary functions of the polycystin proteins have yet to be fully elucidated, it is clear that many of them are mediated by or depend upon cleavage of the polycystin-1 protein. Cleavage of polycystin-1 at its G protein coupled receptor proteolytic site is an obligate step in the protein's maturation and in aspects of its trafficking. This cleavage may also serve to prime polycystin-1 to play a role as a non-canonical G protein coupled receptor. Cleavage of the cytoplasmic polycystin-1C terminal tail releases fragments that are able to enter the nucleus and the mitochondria and to influence their activities. Understanding the nature of these cleavages, their regulation and their consequences is likely to provide valuable insights into both the physiological functions served by the polycystin proteins and the pathological consequences of their absence.

Polycystins as components of large multiprotein complexes of polycystin interactors

Naturally occurring mutations in two separate genes, PKD1 and PKD2, are responsible for the vast majority of all cases of autosomal dominant polycystic kidney disease (ADPKD), one of the most common genetic diseases affecting 1 in 1000 Americans. The hallmark of ADPKD is the development of epithelial cysts in the kidney, liver, and pancreas. PKD1 encodes a large plasma membrane protein (PKD1, PC1, or Polycystin-1) with a long extracellular domain and has been speculated to function as an atypical G protein coupled receptor. PKD2 encodes an ion channel of the Transient Receptor Potential superfamily (TRPP2, PKD2, PC2, or Polycystin-2). Despite the identification of these genes more than 20 years ago, the molecular function of their encoded proteins and the mechanism(s) by which mutations in PKD1 and PKD2 cause ADPKD remain elusive. Genetic, biochemical, and functional evidence suggests they form a multiprotein complex present in multiple locations in the cell, including the plasma membrane, endoplasmic reticulum, and the primary cilium. Over the years, numerous interacting proteins have been identified using directed and unbiased approaches, and shown to modulate function, cellular localization, and protein stability and turnover of Polycystins. Delineation of the molecular composition of the Polycystin complex can have a significant impact on understanding their cellular function in health and disease states and on the identification of more specific and effective therapeutic targets.

Structure and function of polycystin channels in primary cilia

Variants in genes which encode for polycystin-1 and polycystin-2 cause most forms of autosomal dominant polycystic disease (ADPKD). Despite our strong understanding of the genetic determinants of ADPKD, we do not understand the structural features which govern the function of polycystins at the molecular level, nor do we understand the impact of most disease-causing variants on the conformational state of these proteins. These questions have remained elusive because polycystins localize to several organelle membranes, including the primary cilia. Primary cilia are microtubule based organelles which function as cellular antennae. Polycystin-2 and related polycystin-2 L1 are members of the transient receptor potential (TRP) ion channel family, and form distinct ion channels in the primary cilia of disparate cell types which can be directly measured. Polycystin-1 has both ion channel and adhesion G-protein coupled receptor (GPCR) features-but its role in forming a channel complex or as a channel subunit chaperone is undetermined. Nonetheless, recent polycystin structural determination by cryo-EM has provided a molecular template to understand their biophysical regulation and the impact of disease-causing variants. We will review these advances and discuss hypotheses regarding the regulation of polycystin channel opening by their structural domains within the context of the primary cilia.

Pannexin-1 mediates fluid shear stress-sensitive purinergic signaling and cyst growth in polycystic kidney disease

Tubular ATP release is regulated by mechanosensation of fluid shear stress (FSS). Polycystin-1/polycystin-2 (PC1/PC2) functions as a mechanosensory complex in the kidney. Extracellular ATP is implicated in polycystic kidney disease (PKD), where PC1/PC2 is dysfunctional. This study aims to provide new insights into the ATP signaling under physiological conditions and PKD. Microfluidics, pharmacologic inhibition, and loss-of-function approaches were combined to assess the ATP release in mouse distal convoluted tubule 15 (mDCT15) cells. Kidney-specific Pkd1 knockout mice (iKsp-Pkd1^-/- ) and zebrafish pkd2 morphants (pkd2-MO) were as models for PKD. FSS-exposed mDCT15 cells displayed increased ATP release. Pannexin-1 inhibition and knockout decreased FSS-modulated ATP release. In iKsp-Pkd1^-/- mice, elevated renal pannexin-1 mRNA expression and urinary ATP were observed. In Pkd1^-/- mDCT15 cells, elevated ATP release was observed upon the FSS mechanosensation. In these cells, increased pannexin-1 mRNA expression was observed. Importantly, pannexin-1 inhibition in pkd2-MO decreased the renal cyst growth. Our results demonstrate that pannexin-1 channels mediate ATP release into the tubular lumen due to pro-urinary flow. We present pannexin-1 as novel therapeutic target to prevent the renal cyst growth in PKD.

Cell-Autonomous Hedgehog Signaling Is Not Required for Cyst Formation in Autosomal Dominant Polycystic Kidney Disease

BACKGROUND

PKD1 or PKD2, the two main causal genes for autosomal dominant polycystic kidney disease (ADPKD), encode the multipass transmembrane proteins polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Polycystins localize to the primary cilium, an organelle essential for cell signaling, including signal transduction of the Hedgehog pathway. Mutations in ciliary genes that build and maintain the cilium also cause renal cystic disease through unknown pathways. Although recent studies have found alterations in Hedgehog signaling in ADPKD-related models and tissues, the relationship between Hedgehog and polycystic kidney disease is not known.

METHODS

To examine the potential role of cell-autonomous Hedgehog signaling in regulating kidney cyst formation in vivo in both early- and adult-onset mouse models of ADPKD, we used conditional inactivation of Pkd1 combined with conditional modulation of Hedgehog signaling components in renal epithelial cells, where mutations in Pkd1 initiate cyst formation. After increasing or decreasing levels of Hedgehog signaling in cells that underwent inactivation of Pkd1, we evaluated the effects of these genetic manipulations on quantitative parameters of polycystic kidney disease severity.

RESULTS

We found that in Pkd1 conditional mutant mouse kidneys, neither downregulation nor activation of the Hedgehog pathway in epithelial cells along the nephron significantly influenced the severity of the polycystic kidney phenotype in mouse models of developmental or adult-onset of ADPKD.

CONCLUSIONS

These data suggest that loss of Pkd1 function results in kidney cysts through pathways that are not affected by the activity of the Hedgehog pathway.

A New Look at the Functional Organization of the Golgi Ribbon

A characteristic feature of vertebrate cells is a Golgi ribbon consisting of multiple cisternal stacks connected into a single-copy organelle next to the centrosome. Despite numerous studies, the mechanisms that link the stacks together and the functional significance of ribbon formation remain poorly understood. Nevertheless, these questions are of considerable interest, since there is increasing evidence that Golgi fragmentation – the unlinking of the stacks in the ribbon – is intimately connected not only to normal physiological processes, such as cell division and migration, but also to pathological states, including neurodegeneration and cancer. Challenging a commonly held view that ribbon architecture involves the formation of homotypic tubular bridges between the Golgi stacks, we present an alternative model, based on direct interaction between the biosynthetic (pre-Golgi) and endocytic (post-Golgi) membrane networks and their connection with the centrosome. We propose that the central domains of these permanent pre- and post-Golgi networks function together in the biogenesis and maintenance of the more transient Golgi stacks, and thereby establish “linker compartments” that dynamically join the stacks together. This model provides insight into the reversible fragmentation of the Golgi ribbon that takes place in dividing and migrating cells and its regulation along a cell surface – Golgi – centrosome axis. Moreover, it helps to understand transport pathways that either traverse or bypass the Golgi stacks and the positioning of the Golgi apparatus in differentiated neuronal, epithelial, and muscle cells.