Dynactin polices two-way organelle traffic

A Non-Synonymous Point Mutation in a WD-40 Domain Repeat of EML5 Leads to Decreased Bovine Sperm Quality and Fertility

This study is part of a concerted effort to identify and phenotype rare, deleterious mutations that adversely affect sperm quality, or convey high developmental and fertility potential to embryos and ensuing progeny. A rare, homozygous mutation in EML5 (EML5^R1654W), which encodes a microtubule-associated protein with high expression in testis and brain was identified in an Angus bull used extensively in artificial insemination (AI) for its outstanding progeny production traits. The bull’s fertility was low in cross-breeding timed AI (TAI) (Pregnancy/TAI = 25.2%; n = 222) and, in general, AI breeding to Nellore cows (41%; n = 822). A search of the 1,000 Bull Genomes Run9 database revealed an additional 74 heterozygous animals and 8 homozygous animals harboring this exact mutation across several different breeds (0.7% frequency within the 6,191 sequenced animals). Phenotypically, spermatozoa from the homozygous Angus bull displayed prominent piriform and tapered heads, and outwardly protruding knobbed acrosomes. Additionally, an increased retention of EML5 was also observed in the sperm head of both homozygous and heterozygous Angus bulls compared to wild-type animals. This non-synonymous point mutation is located within a WD40 signaling domain repeat of EML5 and is predicted to be detrimental to overall protein function by genomic single nucleotide polymorphism (SNP) analysis and protein modeling. Future work will examine how this rare mutation affects field AI fertility and will characterize the role of EML5 in spermatogenesis.

Canadian Association of Neurosciences Review: Polyglutamine Expansion Neurodegenerative Diseases

Since the early 1990s, DNA triplet repeat expansions have been found to be the cause in an ever increasing number of genetic neurologic diseases. A subset of this large family of genetic diseases has the expansion of a CAG DNA triplet in the open reading frame of a coding exon. The result of this DNA expansion is the expression of expanded glutamine amino acid repeat tracts in the affected proteins, leading to the term, Polyglutamine Diseases, which is applied to this sub-family of diseases. To date, nine distinct genes are known to be linked to polyglutamine diseases, including Huntington's disease, Machado-Joseph Disease and spinobulbar muscular atrophy or Kennedy's disease. Most of the polyglutamine diseases are characterized clinically as spinocerebellar ataxias. Here we discuss recent successes and advancements in polyglutamine disease research, comparing these different diseases with a common genetic flaw at the level of molecular biology and early drug design for a family of diseases where many new research tools for these genetic disorders have been developed. Polyglutamine disease research has successfully used interdisciplinary collaborative efforts, informative multiple mouse genetic models and advanced tools of pharmaceutical industry research to potentially serve as the prototype model of therapeutic research and development for rare neurodegenerative diseases.

Dynamic recruitment of CDK5RAP2 to centrosomes requires its association with dynein

CDK5RAP2 is a centrosomal protein known to be involved in the regulation of the γ-tubulin ring complex and thus the organization of microtubule arrays. However, the mechanism by which CDK5RAP2 is itself recruited to centrosomes is poorly understood. We report here that CDK5RAP2 displays highly dynamic attachment to centrosomes in a microtubule-dependent manner. CDK5RAP2 associates with the retrograde transporter dynein-dynactin and contains a sequence motif that binds to dynein light chain 8. Significantly, disruption of cellular dynein-dynactin function reduces the centrosomal level of CDK5RAP2. These results reveal a key role of the dynein-dynactin complex in the dynamic recruitment of CDK5RAP2 to centrosomes.

Novel candidate genes associated with hippocampal oscillations

The hippocampus is critical for a wide range of emotional and cognitive behaviors. Here, we performed the first genome-wide search for genes influencing hippocampal oscillations. We measured local field potentials (LFPs) using 64-channel multi-electrode arrays in acute hippocampal slices of 29 BXD recombinant inbred mouse strains. Spontaneous activity and carbachol-induced fast network oscillations were analyzed with spectral and cross-correlation methods and the resulting traits were used for mapping quantitative trait loci (QTLs), i.e., regions on the genome that may influence hippocampal function. Using genome-wide hippocampal gene expression data, we narrowed the QTLs to eight candidate genes, including Plcb1, a phospholipase that is known to influence hippocampal oscillations. We also identified two genes coding for calcium channels, Cacna1b and Cacna1e, which mediate presynaptic transmitter release and have not been shown to regulate hippocampal network activity previously. Furthermore, we showed that the amplitude of the hippocampal oscillations is genetically correlated with hippocampal volume and several measures of novel environment exploration.

A bi-directional carboxypeptidase E-driven transport mechanism controls BDNF vesicle homeostasis in hippocampal neurons

Anterograde transport of brain-derived neurotrophic factor (BDNF) vesicles from the soma to neurite terminals is necessary for activity-dependent secretion of BDNF to mediate synaptic plasticity, memory and learning, and retrograde BDNF transport back to the soma for recycling. In our study, overexpression of the cytoplasmic tail of the carboxypeptidase E (CPE) found in BDNF vesicles significantly reduced localization of BDNF in neurites of hippocampal neurons. Live-cell imaging showed that the velocity and distance of movement of fluorescent protein-tagged CPE- or BDNF-containing vesicles were reduced in both directions. In pulldown assays, the CPE tail interacted with dynactin along with kinesin-2 and kinesin-3, and cytoplasmic dynein. Competition assays using a CPE tail peptide verified specific interaction between the CPE tail and dynactin. Thus, the CPE cytoplasmic tail binds dynactin that recruits kinesins or dynein for driving bi-directional transport of BDNF vesicle to maintain vesicle homeostasis and secretion in hippocampal neurons.

Carboxypeptidase E cytoplasmic tail-driven vesicle transport is key for activity-dependent secretion of peptide hormones

Vesicular transport of peptide hormones from the cell body to the plasma membrane for activity-dependent secretion is important for endocrine function, but how it is achieved is unclear. Here we uncover a mechanism in which the cytoplasmic tail of transmembrane carboxypeptidase E (CPE) found in proopiomelanocotin (POMC)/ACTH vesicles interacts with microtubule-based motors to control transport of these vesicles to the release site in pituitary cells. Overexpression of the CPE tail in live cells significantly reduced the velocity and distance of POMC/ACTH- and CPE-containing vesicle movement into the cell processes. Biochemical studies showed that the CPE tail interacted with dynactin, which, in turn, recruited microtubule plus-end motors kinesin 2 and kinesin 3. Overexpression of the CPE tail inhibited the stimulated secretion of ACTH from AtT20 cells. Thus, the CPE cytoplasmic tail interaction with dynactin-kinesin 2/kinesin 3 plays an important role in the transport of POMC vesicles for activity-dependent secretion.

Melanophores: A model system for neuronal transport and exocytosis?

Black pigment cells, melanophores, from lower vertebrates are specialized in bidirectional and coordinated translocation of pigment granules, melanosomes, in the cytoplasm. Melanophores develop from the neuronal crest and are most abundant in the dermal and epidermal layers of the skin, where the intracellular distribution of the pigment significantly influences the color of the animal. The transport of pigment is dependent on an intact cytoskeleton and motor proteins associated with cytoskeletal components. The easily cultured melanophores have proved to be excellent models for organelle transport because the intracellular movements of pigment can be visualized via light microscopy, and the granules move in response to defined chemical signals. The ease of achieving a combination of morphological and functional transport studies is the advantage of the melanophore system, and studies on pigment cells have revealed new components of the transport machinery, including molecular motors, their adapters, and transfer of vesicles to other cells. Many cellular components are transported with a combination of the actin- and microtubule-based transport systems, and, since all eukaryotic organisms rely on functional intracellular transport and an intact cytoskeleton, studies on melanophores are important for many aspects of cell biology, including axonal transport. In this review, we present an overview of the research on the pigment transport system and the potential use of pigment cells as a model system.

Hypothesis: huntingtin may function in membrane association and vesicular traffickingThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

Huntington’s disease is a progressive neurodegenerative genetic disorder that is caused by a CAG triplet-repeat expansion in the first exon of the IT15 gene. This CAG expansion results in polyglutamine expansion in the 350 kDa huntingtin protein. The exact function of huntingtin is unknown. Understanding the pathological triggers of mutant huntingtin, and distinguishing the cause of disease from downstream effects, is critical to designing therapeutic strategies and defining long- and short-term goals of therapy. Many studies that have sought to determine the functions of huntingtin by determining huntingtin’s protein–protein interactions have been published. Through these studies, huntingtin has been seen to interact with a large number of proteins, and is likely a scaffolding protein for protein–protein interactions. Recently, using imaging, integrative proteomics, and cell biology, huntingtin has been defined as a membrane-associated protein, with activities related to axonal trafficking of vesicles and mitochondria. These functions have also been attributed to some huntingtin-interacting proteins. Additionally, discoveries of a membrane association domain and a palmitoylation site in huntingtin reinforce the fact that huntingtin is membrane associated. In Huntington’s disease mouse and fly models, axonal vesicle trafficking is inhibited, and lack of proper uptake of neurotrophic factors may be an important pathological trigger leading to striatal cell death in Huntington’s disease. Here we discuss recent advances from many independent groups and methodologies that are starting to resolve the elusive function of huntingtin in vesicle transport, and evidence that suggests that huntingtin may be directly involved in membrane interactions.

A +TIP for a smooth trip

Is there a cellular mechanism for preventing a depolymerizing microtubule track from “slipping out from under” its cargo? A recent study in budding yeast indicates that when a chromosome is transported to the minus end of a spindle microtubule, its kinetochore-bound microtubule plus end–tracking protein (+TIP) Stu2 may move to the plus end to promote rescue; i.e., to switch the depolymerizing end to a polymerizing end. The possibility that other +TIPs may play a similar role in sustaining a microtubule track during vesicular transport deserves investigation.

Role of microtubule-dependent membrane trafficking in acrosomal biogenesis

The role of microtubule-based trafficking in acrosomal biogenesis was examined by studying the effects of colchicine on spermiogenesis. In electron micrographs of untreated cap-phase mouse spermatids, coated vesicles were always seen on the apex and caudal margins of the developing acrosomal cap. The increase in volume and the accumulation of materials in the acrosome during the Golgi and cap phases were observed to occur via fusion of vesicles at various sites on the growing acrosome. By studying the acid phosphatase localization pattern and colchicine-treated spermatids, the role of clathrin-coated vesicles became clear. Coated vesicle formation at the caudal margin of the acrosome appeared to be responsible for the spreading and shaping of the acrosome over the surface of the nucleus and also established distinct regional differences in the acrosome. In colchicine-treated spermatids, the Golgi apparatus lost its typical membranous stack conformation and disintegrated into many small vesicles. Acrosome formation was retarded, and there was discordance of the spread of the acrosomal cap with that of the modified nuclear envelope. Many symplasts were also found because of the breakdown of intercellular bridges. Colchicine treatment thus indicated that microtubule-dependent trafficking of transport vesicles between the Golgi apparatus and the acrosome plays a vital role in acrosomal biogenesis. In addition, both anterograde and retrograde vesicle trafficking are extensively involved and seem to be equally important in acrosome formation.

Syntabulin is a microtubule-associated protein implicated in syntaxin transport in neurons

Different types of cargo vesicles containing presynaptic proteins are transported from the nerve cell body to the nerve terminal, and participate in the formation of active zones. However, the identity of the membranous cargoes and the nature of the motor-cargo interactions remain unsolved. Here, we report the identification of a syntaxin-1-binding protein named syntabulin. Syntabulin attaches syntaxin-containing vesicles to microtubules and migrates with syntaxin within the processes of hippocampal neurons. Knock-down of syntabulin expression with targeted small interfering RNAs (siRNAs) or interference with the syntabulin-syntaxin interaction inhibit attachment of syntaxin-cargo vesicles to microtubules and reduce syntaxin-1 distribution in neuronal processes. Furthermore, conventional kinesin I heavy chain binds to syntabulin and associates with syntabulin-linked syntaxin vesicles in vivo. These findings suggest that syntabulin functions as a linker molecule that attaches syntaxin-cargo vesicles to kinesin I, enabling the transport of syntaxin-1 to neuronal processes.

Overexpression of genes in the CA1 hippocampus region of adult rat following episodes of global ischemia

Ischemic stress is associated with marked changes in gene expression in the hippocampus--albeit little information exists on the activation of nonabundant genes. We have examined the expression of several known genes and identified novel ones in the adult rat hippocampus after a mild, transient, hypovolemic and hypotensive, global ischemic stress. An initial differential screening using a prototype array to assess gene expression after stress followed by a suppression subtractive hybridization protocol and cDNA microarray revealed 124 nonoverlapped transcripts predominantly expressed in the CA1 rat hippocampus region in response to ischemic stress. About 78% of these genes were not detected with nonsubtracted probes. Reverse transcription polymerase chain reaction (RT-PCR) and in situ hybridization on these 124 transcripts confirmed the differential expression of at least 83. Most robustly expressed were gene sequences NFI-B, ATP1B1, RHOGAP, PLA2G4A, BAX, CASP3, P53, MAO-A, FRA1, HSP70.2, and NR4A1 (NUR77), as well as sequence tags of unknown function. New stress-related genes of similar functional motifs were identified, reemphasizing the importance of functional grouping in the analysis of multiple gene expression profiles. These data indicate that ischemia elicits expression of an array of functional gene clusters that may be used as an index for stress severity and a template for target therapy design.

A Role for Spectrin in Dynactin-dependent Melanosome Transport in Xenopus laevis Melanophores

The bi-directional movement of pigment granules in frog melanophores involves the microtubule-based motors cytoplasmic dynein, which is responsible for aggregation, and kinesin II and myosin V, which are required for dispersion of pigment. It was recently shown that dynactin acts as a link between dynein and kinesin II and melanosomes, but it is not fully understood how this is regulated and if more proteins are involved. Here, we suggest that spectrin, which is known to be associated with Golgi vesicles as well as synaptic vesicles in a number of cells, is of importance for melanosome movements in Xenopus laevis melanophores. Large amounts of spectrin were found on melanosomes isolated from both aggregated and dispersed melanophores. Spectrin and two components of the oligomeric dynactin complex, p150(glued) and Arp1/centractin, co-localized with melanosomes during aggregation and dispersion, and the proteins were found to interact as determined by co-immunoprecipitation. Spectrin has been suggested as an important link between cargoes and motor proteins in other cell types, and our new data indicate that spectrin has a role in the specialized melanosome transport processes in frog melanophores, in addition to a more general vesicle transport.