Bardet-Biedl syndrome: an emerging pathomechanism of intracellular transport

From a handful of uncloned genetic loci 6 years ago, great strides have been made in understanding the genetic and molecular aetiology of Bardet-Biedl syndrome (BBS), a rare pleiotropic disorder characterised by a multitude of symptoms, including obesity, retinal degeneration and cystic kidneys. Presently, 11 BBS genes have been cloned, with the likelihood that yet more BBS genes remain undiscovered. In 2003, a major breakthrough was made when it was shown that BBS is likely caused by defects in basal bodies and/or primary cilia. Since then, studies in numerous animal models of BBS have corroborated the initial findings and, in addition, have further refined the specific functions of BBS proteins. These include roles in establishing planar cell polarity (noncanonical Wnt signaling) in mice and zebrafish, modulating intraflagellar transport and lipid homeostasis in worms, and regulating intracellular trafficking and centrosomal functions in zebrafish and human tissue culture cells. From these discoveries, a common theme has emerged, namely that the primary function of BBS proteins may be to mediate and regulate microtubule-based intracellular transport processes. Received 20 April 2006; received after revision 30 May 2006; accepted 15 June 2006     

KIF1Bβ transports dendritically localized mRNPs in neurons and is recruited to synapses in an activity-dependent manner

KIF1Bβ is a kinesin-like, microtubule-based molecular motor protein involved in anterograde axonal vesicular transport in vertebrate and invertebrate neurons. Certain KIF1Bβ isoforms have been implicated in different forms of human neurodegenerative disease, with characterization of their functional integration and regulation in the context of synaptic signaling still ongoing. Here, we characterize human KIF1Bβ (isoform NM015074), whose expression we show to be developmentally regulated and elevated in cortical areas of the CNS (including the motor cortex), in the hippocampus, and in spinal motor neurons. KIF1Bβ localizes to the cell body, axon, and dendrites, overlapping with synaptic-vesicle and postsynaptic-density structures. Correspondingly, in purified cortical synaptoneurosomes, KIF1Bβ is enriched in both pre- and postsynaptic structures, forming detergent-resistant complexes. Interestingly, KIF1Bβ forms RNA–protein complexes, containing the dendritically localized Arc and Calmodulin mRNAs, proteins previously shown to be part of RNA transport granules such as Purα, FMRP and FXR2P, and motor protein KIF3A, as well as Calmodulin. The interaction between KIF1Bβ and Calmodulin is Ca⁺²-dependent and takes place through a domain mapped at the carboxy-terminal tail of the motor. Live imaging of cortical neurons reveals active movement by KIF1Bβ at dendritic processes, suggesting that it mediates the transport of dendritically localized mRNAs. Finally, we show that synaptic recruitment of KIF1Bβ is activity-dependent and increased by stimulation of metabotropic or ionotropic glutamate receptors. The activity-dependent synaptic recruitment of KIF1Bβ, its interaction with Ca²⁺ sensor Calmodulin, and its new role as a dendritic motor of ribonucleoprotein complexes provide a novel basis for understanding the concerted co-ordination of motor protein mobilization and synaptic signaling pathways.     

Neuroserpin: a serpin to think about

Proteinases and their inhibitors play important roles in neural development, homeostasis and disease. Neuroserpin is a member of the serine proteinase inhibitor (serpin) superfamily that is secreted from the growth cones of neurons and inhibits the enzyme tissue-type plasminogen activator (tPA). The temporal and spatial pattern of neuroserpin expression suggests a role in synaptogenesis and is most prominent in areas of the brain that participate in learning, memory and behaviour. Neuroserpin also provides neuronal protection in pathologies such as cerebral ischaemia and epilepsy by preventing excessive activity of tPA. Point mutations in neuroserpin cause aberrant conformational transitions and the formation of loop-sheet polymers that are retained within the endoplasmic reticulum of neurons, forming inclusion bodies that underlie an autosomal dominant dementia that we have called familial encephalopathy with neuroserpin inclusion bodies or FENIB. We review here the role of neuroserpin and other proteinase inhibitors in brain development, function and disease. Received 25 February 2005; received after revision 16 November 2005; accepted 28 November 2005     

Primary cilia proteins: ciliary and extraciliary sites and functions

Primary cilia are immotile organelles known for their roles in development and cell signaling. Defects in primary cilia result in a range of disorders named ciliopathies. Because this organelle can be found singularly on almost all cell types, its importance extends to most organ systems. As such, elucidating the importance of the primary cilium has attracted researchers from all biological disciplines. As the primary cilia field expands, caution is warranted in attributing biological defects solely to the function of this organelle, since many of these “ciliary” proteins are found at other sites in cells and likely have non-ciliary functions. Indeed, many, if not all, cilia proteins have locations and functions outside the primary cilium. Extraciliary functions are known to include cell cycle regulation, cytoskeletal regulation, and trafficking. Cilia proteins have been observed in the nucleus, at the Golgi apparatus, and even in immune synapses of T cells (interestingly, a non-ciliated cell). Given the abundance of extraciliary sites and functions, it can be difficult to definitively attribute an observed phenotype solely to defective cilia rather than to some defective extraciliary function or a combination of both. Thus, extraciliary sites and functions of cilia proteins need to be considered, as well as experimentally determined. Through such consideration, we will understand the true role of the primary cilium in disease as compared to other cellular processes’ influences in mediating disease (or through a combination of both). Here, we review a compilation of known extraciliary sites and functions of “cilia” proteins as a means to demonstrate the potential non-ciliary roles for these proteins.     

Chromosome 13 dementias

The importance of cerebral amyloid deposition in the mechanism of neurodegeneration is still debatable. Classic arguments are usually centered on amyloid β(Aβ) and its role in the neuronal loss characteristic of Alzheimer’s disease, the most common form of human cerebral amyloidosis. Two non-Aβ cerebral amyloidoses, familial British and Danish dementias (FBD and FDD), share many aspects of Alzheimer’s disease, including the presence of neurofibrillary tangles, parenchymal preamyloid and amyloid deposits, cerebral amyloid angiopathy and a variety of amyloid-associated proteins and inflammatory components. Both early-onset conditions are linked to specific mutations at or near the stop codon of the chromosome 13 gene BRI2 that cause generation of longer-than-normal protein products. Furin-like processing of these longer precursors releases two de novo-created peptides, ABri and ADan, which deposit as amyloid fibrils in FBD and FDD, respectively. Due to the similar pathology generated by completely unrelated amyloid subunits, FBD and FDD, collectively referred to as chromosome 13 dementias, constitute alternative models for studying the role of amyloid deposition in the mechanism of neuronal cell death.Received 4 March 2005; received after revision 24 April 2005; accepted 26 April 2005     

The impact on microtubule network of a bracovirus IκB-like protein

Polydnavirus-encoded IκB-like proteins are similar to insect and mammalian IκB, and an immunosuppressive function in the host cells has been inferred to these proteins. Here we show that the expression of one of these IκB-like viral genes, the TnBVank1, in the Drosophila germline affects the localization of gurken, bicoid, and oskar mRNAs whose gene products are relevant for proper embryonic patterning. The altered localization of these mRNAs is suggestive of general defects in the intracellular, microtubule-based, trafficking routes. Analysis of microtubule motor proteins components such as the dynein heavy chain and the kinesin heavy chain revealed defects in the polarized microtubule network. Interestingly, the TnBVANK1 viral protein is uniformly distributed over the entire oocyte cortex, and appears to be anchored to the microtubule ends. Our data open up a very interesting issue on novel function(s) played by the ank gene family by interfering with cytoskeleton organization.     

Sorting receptor SORLA: cellular mechanisms and implications for disease

Sorting-related receptor with A-type repeats (SORLA) is an intracellular sorting receptor that directs cargo proteins, such as kinases, phosphatases, and signaling receptors, to their correct location within the cell. The activity of SORLA assures proper function of cells and tissues, and receptor dysfunction is the underlying cause of common human malignancies, including Alzheimer’s disease, atherosclerosis, and obesity. Here, we discuss the molecular mechanisms that govern sorting of SORLA and its cargo in multiple cell types, and why genetic defects in this receptor results in devastating diseases.     

Functional and physical communication between oncoproteins and tumor suppressors

The discovery of oncogenes (c-onc’s) and tumor suppressors (TS’s) has led to the concept that cancer arises from defects in each of these classes of genes or their products. More recently, it has been appreciated that c-onc and TS proteins often affect one another’s functions. Within this context, I review the two classical TS’s, p53 and the retinoblastoma protein, and the consequences of their inactivation. The various forms of genomic instability (GI) that underly the high mutation rates of transformed cells are then discussed. Particular emphasis is placed upon the concept that GI is not only an integral part of the transformed state but is a prerequisite. Increased oxidative DNA damage, and/or an inabiliy to repair it, can lead to GI. The review then discusses recent observations showing that loss of the TS protein peroxiredoxin 1 (prdx1) and increased expression of the c-onc protein c-Myc, each leads to increased oxidative DNA damage. The critical nature of the c-onc-TS interaction is underscored by that occurring between prdx1 and c-Myc, with the former protein regulating the production of DNA-damaging reactive oxygen species by the latter. The intimate association between these proteins and others serves as a paradigm for the exquisite balancing act that c-onc’s and TS’s must maintain in order to properly control normal DNA replication and cellular proliferation while simultaneously minimizing the acquisition of potentially neoplastic mutations. Received 10 May 2005; received after revision 3 July 2005; accepted 19 July 2005     

Megalin mediates plasma membrane to mitochondria cross-talk and regulates mitochondrial metabolism

Mitochondrial intracrines are extracellular signaling proteins, targeted to the mitochondria. The pathway for mitochondrial targeting of mitochondrial intracrines and actions in the mitochondria remains unknown. Megalin/LRP2 mediates the uptake of vitamins and proteins, and is critical for clearance of amyloid-β protein from the brain. Megalin mutations underlie the pathogenesis of Donnai–Barrow and Lowe syndromes, characterized by brain defects and kidney dysfunction; megalin was not previously known to reside in the mitochondria. Here, we show megalin is present in the mitochondria and associates with mitochondrial anti-oxidant proteins SIRT3 and stanniocalcin-1 (STC1). Megalin shuttles extracellularly-applied STC1, angiotensin II and TGF-β to the mitochondria through the retrograde early endosome-to-Golgi transport pathway and Rab32. Megalin knockout in cultured cells impairs glycolytic and respiratory capacities. Thus, megalin is critical for mitochondrial biology; mitochondrial intracrine signaling is a continuum of the retrograde early endosome-to-Golgi-Rab32 pathway and defects in this pathway may underlie disease processes in many systems.     

Membrane trafficking in neuronal maintenance and degeneration

Defects in membrane trafficking and degradation are hallmarks of most, and maybe all, neurodegenerative disorders. Such defects typically result in the accumulation of undegraded proteins due to aberrant endosomal sorting, lysosomal degradation, or autophagy. The genetic or environmental cause of a specific disease may directly affect these membrane trafficking processes. Alternatively, changes in intracellular sorting and degradation can occur as cellular responses of degenerating neurons to unrelated primary defects such as insoluble protein aggregates or other neurotoxic insults. Importantly, altered membrane trafficking may contribute to the pathogenesis or indeed protect the neuron. The observation of dramatic changes to membrane trafficking thus comes with the challenging need to distinguish pathological from protective alterations. Here, we will review our current knowledge about the protective and destructive roles of membrane trafficking in neuronal maintenance and degeneration. In particular, we will first focus on the question of what type of membrane trafficking keeps healthy neurons alive in the first place. Next, we will discuss what alterations of membrane trafficking are known to occur in Alzheimer’s disease and other tauopathies, Parkinson’s disease, polyQ diseases, peripheral neuropathies, and lysosomal storage disorders. Combining the maintenance and degeneration viewpoints may yield insight into how to distinguish when membrane trafficking functions protectively or contributes to degeneration.     

Understanding the importance of selenium and selenoproteins in muscle function

Selenium is an essential trace element. In cattle, selenium deficiency causes dysfunction of various organs, including skeletal and cardiac muscles. In humans as well, lack of selenium is associated with many disorders, but despite accumulation of clinical reports, muscle diseases are not generally considered on the list. The goal of this review is to establish the connection between clinical observations and the most recent advances obtained in selenium biology. Recent results about a possible role of selenium-containing proteins in muscle formation and repair have been collected. Selenoprotein N is the first selenoprotein linked to genetic disorders consisting of different forms of congenital muscular dystrophies. Understanding the muscle disorders associated with selenium deficiency or selenoprotein N dysfunction is an essential step in defining the causes of the disease and obtaining a better comprehension of the mechanisms involved in muscle formation and maintenance. Received 13 July 2005; received after revision 9 September 2005; accepted 4 October 2005     

Isoprenylated proteins

Isoprenoids are synthesized in all living organisms and are incorporated into diverse classes of end-products that participate in a multitude of cellular processes relating to cell growth, differentiation, cytoskeletal function and vesicle trafficking. In humans, the non-sterol isoprenoids, farnesyl pyrophosphate and geranylgeranyl-pyrophosphate, are synthesized via the mevalonate pathway and are covalently added to members of the small G protein superfamily. Isoprenylated proteins have key roles in membrane attachment and protein functionality, have been shown to have a central role in some cancers and are likely also to be involved in the pathogenesis and progression of atherosclerosis and Alzheimer disease. This review details current knowledge on the biosynthesis of isoprenoids, their incorporation into proteins by the process known as prenylation and the complex regulatory network that controls these proteins. An improved understanding of these processe is likely to lead to the development of novel therapies that will have important implications for human health and disease. Received 5 July 2005; received after revision 17 October 2005; accepted 22 October 2005     

Genetics and molecular pathogenesis of mitochondrial respiratory chain diseases

Dysfunction of the mitochondrial respiratory chain has been recognised as a cause of human disease for over 30 years. Advances in the past 10 years have led to a better understanding of the genetics and molecular pathogenesis of many of these disorders. Over 100 primary defects in mitochondrial DNA (mtDNA) are now implicated in the pathogenesis of a group of disorders which are collectively known as the mitochondrial encephalomyopathies, and which most frequently involve skeletal muscle and/or the central nervous system. Although impaired oxidative phosphorylation is likely to be the final common pathway leading to the cellular dysfunction associated with such mtDNA mutations, the complex relationship between genotype and phenotype remains largely unexplained. Most of the genes which encode the respiratory chain reside in the nucleus, yet only five nuclear genes have been implicated in human respiratory chain diseases. There is evidence that respiratory chain dysfunction is present in common neurological diseases such as Parkinson's disease and Huntington's disease. The precise cause of this respiratory chain dysfunction and its relationship to the disease process are unclear. This review focuses upon respiratory chain disorders associated with primary defects in mtDNA.     

Deciphering cryptic proteases

Proteases are deeply involved in physiology and pathology. For most, the mechanism is well defined but several fail to display typical protease features (as is the case of the four proteases contained in fibronectin, the inhibitor-resistant mesotrypsin and the proteosomal deubiquitinating enzyme) or have unclear physiological function (such as calpain-like proteins, transthyretin and factor seven activating protease). In other cases, such as in peroxisomal processing proteases, although substrates are defined, the enzyme remains undiscovered. Furthermore, several proteases were identified in pathological conditions, namely secretases in Alzheimers disease and gross cystic disease fluid protein 15 kDa in breast cancer, when most likely their physiological substrate is still hidden. Lastly, the evolutionary conservation of proteolytic enzymes raises questions related to the origin of biological events, such as the origin of cystein proteases and cell death responses. In this review we will discuss the above cryptic enzymes, as they will probably be relevant in the future.Received 7 December 2004; received after revision 5 January 2005; accepted 10 January 2005 Available online 09 March 2005     

Lipid flippases and their biological functions

The typically distinct phospholipid composition of the two leaflets of a membrane bilayer is generated and maintained by bi-directional transport (flip-flop) of lipids between the leaflets. Specific membrane proteins, termed lipid flippases, play an essential role in this transport process. Energy-independent flippases allow common phospholipids to equilibrate rapidly between the two monolayers and also play a role in the biosynthesis of a variety of glycoconjugates such as glycosphingolipids, N-glycoproteins, and glycosylphosphatidylinositol (GPI)-anchored proteins. ATP-dependent flippases, including members of a conserved subfamily of P-type ATPases and ATP-binding cassette transporters, mediate the net transfer of specific phospholipids to one leaflet of a membrane and are involved in the creation and maintenance of transbilayer lipid asymmetry of membranes such as the plasma membrane of eukaryotes. Energy-dependent flippases also play a role in the biosynthesis of glycoconjugates such as bacterial lipopolysaccharide. This review summarizes recent progress on the identification and characterization of the various flippases and the demonstration of their biological functions. Received 12 April 2006; received after revision 22 June 2006; accepted 30 August 2006     

Know your enemy: understanding the role of PfCRT in drug resistance could lead to new antimalarial tactics

The prevention and treatment of malaria is heavily dependent on antimalarial drugs. However, beginning with the emergence of chloroquine (CQ)-resistant Plasmodium falciparum parasites 50 years ago, efforts to control the disease have been thwarted by failed or failing drugs. Mutations in the parasite's 'chloroquine resistance transporter' (PfCRT) are the primary cause of CQ resistance. Furthermore, changes in PfCRT (and in several other transport proteins) are associated with decreases or increases in the parasite's susceptibility to a number of other antimalarial drugs. Here, we review recent advances in our understanding of CQ resistance and discuss these in the broader context of the parasite's susceptibilities to other quinolines and related drugs. We suggest that PfCRT can be viewed both as a 'multidrug-resistance carrier' and as a drug target, and that the quinoline-resistance mechanism is a potential 'Achilles' heel' of the parasite. We examine a number of the antimalarial strategies currently undergoing development that are designed to exploit the resistance mechanism, including relatively simple measures, such as alternative CQ dosages, as well as new drugs that either circumvent the resistance mechanism or target it directly.     

DNG1, a Dictyostelium homologue of tumor suppressor ING1 regulates differentiation of Dictyostelium cells

dng1 is a Dictyostelium homologue of the mammalian tumor suppressor ING gene. DNG1 protein localizes in the nucleus, and has a highly conserved PHD finger domain found in chromatin-remodeling proteins. Both dng1 disruption and overexpression impaired cell proliferation. In dng1-null cells, the progression of differentiation was delayed in a cell-density-dependent manner, and many tiny aggregates were formed. Exogenously applied cAMP pulses reversed the inhibitory effect caused by dng1 disruption on the aggregation during early development, but formation of tiny aggregates was not restored. dng1-overexpressing cells acquired the ability to undergo chemotaxis to cAMP earlier and exhibited enhanced differentiation. These phenotypes were found to be coupled with altered expressions of early genes such as cAMP receptor 1 (car1) and contact site A (csA). Furthermore, disordered histone modifications were demonstrated in dng1-null cells. These results suggest a regulatory role of dng1 in the transition of cells from growth to differentiation.Received 29 December 2004; received after revision 24 May 2005; accepted 26 May 2005     

Genome-wide association study to identify SNPs conferring risk of myocardial infarction and their functional analyses

Myocardial infarction might result from the interactions of multiple genetic and environmental factors, none of which can cause disease solely by each of themselves. Although molecular biological studies revealed that a number of proteins are possibly involved in its pathogenesis, little, if any genetic findings have been reported so far. To reveal genetic backgrounds of myocardial infarction, we performed a large-scale, case-control association study using 92,788 gene-based single-nucleotide polymorphism (SNP) markers. We have identified functional SNPs within the lymphotoxin-α gene (LTA) located on chromosome 6p21 that conferred susceptibility to myocardial infarction. Furthermore, we could identify galectin-2 protein as a binding partner of LTA protein. The association study further revealed that a functional SNP in LGALS2 encoding galectin-2, which led to altered secretion of LTA, also indicated a risk of myocardial infarction. A combined strategy of genetic and molecularcellular biological approaches may be useful in clarifying pathogenesis of common diseases.Received 7 March 2005; received after revision 22 April 2005; accepted 25 April 2005     

The primary cilium as a dual sensor of mechanochemical signals in chondrocytes

The primary cilium is an immotile, solitary, and microtubule-based structure that projects from cell surfaces into the extracellular environment. The primary cilium functions as a dual sensor, as mechanosensors and chemosensors. The primary cilia coordinate several essential cell signaling pathways that are mainly involved in cell division and differentiation. A primary cilium malfunction can result in several human diseases. Mechanical loading is sense by mechanosensitive cells in nearly all tissues and organs. With this sensation, the mechanical signal is further transduced into biochemical signals involving pathways such as Akt, PKA, FAK, ERK, and MAPK. In this review, we focus on the fundamental functional and structural features of primary cilia in chondrocytes and chondrogenic cells.