The molecular chaperone Hsp90 maintains Golgi organization and vesicular trafficking by regulating microtubule stability

Hsp90 is an abundant and special molecular chaperone considered to be the regulator of many transcription factors and signaling kinases. Its high abundance is indicative of its involvement in some more fundamental processes. In this study, we provide evidence that Hsp90 is required for microtubule stabilization, Golgi organization, and vesicular trafficking. We showed that Hsp90 is bound to microtubule-associated protein 4 (MAP4), which is essential for maintaining microtubule acetylation and stabilization. Hsp90 depletion led to the decrease in MAP4, causing microtubule deacetylation and destabilization. Furthermore, in Hsp90-depleted cells, the Golgi apparatus was fragmented and anterograde vesicle trafficking was impaired, with phenotypes similar to those induced by silencing MAP4. These disruptive effects of Hsp90 depletion could be rescued by the expression of exogenous MAP4 or the treatment of trichostatin A that increases microtubule acetylation as well as stability. Thus, microtubule stability is an essential cellular event regulated by Hsp90.

BYPASS1-LIKE regulates lateral root initiation via exocytic vesicular trafficking-mediated PIN recycling in Arabidopsis

Auxin and auxin-mediated signaling pathways are known to regulate lateral root development.Although exocytic vesicle trafficking plays an important role in recycling the PIN-FORMED(PIN)auxin efflux carriers and in polar auxin transport during lateral root formation,the mechanistic details of these processes are not well understood.Here,we demonstrate that BYPASS1-LIKE(B1 L)regulates lateral root initiation via exocytic vesicular trafficking-mediated PIN recycling in Arabidopsis thaliana.b1 l mutants contained significantly more lateral roots than the wild type,primarily due to increased lateral root primordium initiation.Furthermore,the auxin signal was stronger in stage I lateral root primordia of b1 l than in those of the wild type.Treatment with exogenous auxin and an auxin transport inhibitor indicated that the lateral root phenotype of b1 l could be attributed to higher auxin levels and that B1 L regulates auxin efflux.Indeed,compared to the wild type,C-terminally green fluorescent protein-tagged PIN1 and PIN3 accumulated at higher levels in b1 l lateral root primordia.B1 L interacted with the exocyst,and b1 l showed defective PIN exocytosis.These observations indicate that B1 L interacts with the exocyst to regulate PIN-mediated polar auxin transport and lateral root initiation in Arabidopsis.

Dynamic localization of hepatocellular transporters in health and disease

Vesicle-based traffi cking of hepatocellular transporters involves delivery of the newly-synthesized carriers from the rough endoplasmic reticulum to either the plasma membrane domain or to an endosomal,submembrane compartment,followed by exocytic targeting to the plasma membrane. Once delivered to the plasma membrane,the transporters usually undergo recycling between the plasma membrane and the endosomal compartment,which usually serves as a reservoir of pre-existing transporters available on demand. The balance between exocytic targeting and endocytic internalization from/to this recycling compartment is therefore a chief determinant of the overall capability of the liver epithelium to secrete bile and to detoxify endo and xenobiotics. Hence,it is a highly regulated process. Impaired regulation of this balance may lead to abnormal localization of these transporters,which results in bile secretory failure due to endocytic internalization of key transporters involved in bile formation. This occurs in several experimental models of hepatocellular cholestasis,and in most human cholestatic liver diseases. This review describes the molecular bases involved in the biology of the dynamic localization of hepatocellular transporters and its regulation,with a focus on the involvement of signaling pathways in this process. Their alterations in different experimental models of cholestasis and in human cholestatic liver disease are reviewed. In addition,the causes explaining the pathological condition (e.g. disorganization of actin or actin-transporter linkers) and the mediators involved (e.g. activation of cholestatic signaling transduction pathways) are also discussed. Finally,several experimental therapeutic approaches based upon the administration of compounds known to stimulate exocytic insertion of canalicular transporters (e.g. cAMP,tauroursodeoxycholate) are described.

Endosomal trafficking defects in patient cells with KIAA1109 biallelic variants

The uncharacterized gene KIAA1109 has recently been associated with a congenital neurological malformation disorder that variably presents with arthrogryposis,craniofacial and/or cardiac abnormalities.We have identified two additional patients with compound heterozygous KIAA1109 variants presenting with the same neurological malformations.The mechanism whereby KIAA1109 loss of function causes this spectrum of disorders was the primary focus of our studies.We hypothesized that KIAA1109 function could be conserved relative to the fly gene tweek and examined endocytosis and endosome recycling in patient fibroblasts.Furthermore,we examined the structure of the cytoskeleton and cilia based on functional overlap with endocytosis and several known etiologies for neuronal migration disorders.Utilizing primary dermal fibroblasts from one patient and a healthy donor,we performed immunofluorescence and endocytosis assays to examine the endosomal,cytoskeletal,and ciliary cellular phenotypes.We found notable abnormalities in endosomal trafficking and endosome recycling pathways.We also observed changes in the actin cytoskeleton and cilia structural dynamics.We conclude that the function of KIAA1109 in humans may indeed overlap with the function of the Drosophila ortholog,resulting in perturbations to endosomal trafficking and the actin cytoskeleton.These alterations have ripple effects,altering many pathways that are critical for proper neuronal migration and embryonic development.

Sphingolipid metabolism, transport, and functions in plants:Recent progress and future perspectives

Sphingolipids,which comprise membrane systems together with other lipids,are ubiquitous in cellular organisms.They show a high degree of diversity across plant species and vary in their structures,properties,and functions.Benefiting from the development of lipidomic techniques,over 300 plant sphingolipids have been identified.Generally divided into free long-chain bases(LCBs),ceramides,glycosylceramides(GlcCers)and glycosyl inositol phosphoceramides(GIPCs),plant sphingolipids exhibit organized aggregation within lipid membranes to form raft domains with sterols.Accumulating evidence has revealed that sphingolipids obey certain trafficking and distribution rules and confer unique properties to membranes.Functional studies using sphingolipid biosynthetic mutants demonstrate that sphingolipids participate in plant developmental regulation,stimulus sensing,and stress responses.Here,we present an updated metabolism/degradation map and summarize the structures of plant sphingolipids,review recent progress in understanding the functions of sphingolipids in plant development and stress responses,and review sphingolipid distribution and trafficking in plant cells.We also highlight some important challenges and issues that we may face during the process of studying sphingolipids.