A behavior-based drug screening system using a Caenorhabditis elegans model of motor neuron disease.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the progressive loss of motor neurons, for which there is no effective treatment. Previously, we generated a Caenorhabditis elegans model of ALS, in which the expression of dnc-1, the homologous gene of human dynactin-1, is knocked down (KD) specifically in motor neurons. This dnc-1 KD model showed progressive motor defects together with axonal and neuronal degeneration, as observed in ALS patients. In the present study, we established a behavior-based, automated, and quantitative drug screening system using this dnc-1 KD model together with Multi-Worm Tracker (MWT), and tested whether 38 candidate neuroprotective compounds could improve the mobility of the dnc-1 KD animals. We found that 12 compounds, including riluzole, which is an approved medication for ALS patients, ameliorated the phenotype of the dnc-1 KD animals. Nifedipine, a calcium channel blocker, most robustly ameliorated the motor deficits as well as axonal degeneration of dnc-1 KD animals. Nifedipine also ameliorated the motor defects of other motor neuronal degeneration models of C. elegans, including dnc-1 mutants and human TAR DNA-binding protein of 43 kDa overexpressing worms. Our results indicate that dnc-1 KD in C. elegans is a useful model for the screening of drugs against motor neuron degeneration, and that MWT is a powerful tool for the behavior-based screening of drugs.

Dynein and dynactin components modulate neurodegeneration induced by excitotoxicity.

Glutamate excitotoxicity causes neuronal dysfunction and degeneration. It is implicated in chronic disorders, including Alzheimer's disease, and in acute CNS insults such as ischemia. These disorders share prominent morphological features, including axon degeneration and cell body death. However, the molecular mechanism underlying excitotoxicity-induced neurodegeneration remains poorly understood. A key molecular feature of neurodegeneration is deficits in microtubule-based cargo transport that plays a pivotal role in maintaining the balance of survival and stress signaling in the axon. We developed an excitotoxicity-induced neurodegeneration system in primary neuronal cultures. We find that excitotoxicity generates a C-terminal truncated form of p150Glued, a major component of the dynactin complex, which exacerbates axon degeneration. This p150Glued truncated form was identified in brain tissues of patients with Alzheimer's disease. Overexpression of wild-type (WT) dynein intermediate chain (DIC), a dynein component that interacts with p150Glued and links dynein and dynactin complexes, DIC (S84D) mutant, and WT p150Glued suppressed axon degeneration. These modulating effects of p150Glued and DIC on excitotoxicity-induced axon degeneration are also observed in apoptosis and cell body death. Thus, our findings identify retrograde transport proteins, p150Glued and DIC, as novel modulators of neurodegeneration induced by glutamate excitotoxicity.

Perry syndrome due to the DCTN1 G71R mutation: a distinctive levodopa responsive disorder with behavioral syndrome, vertical gaze palsy, and respiratory failure.

Perry syndrome is a rare form of autosomal dominant Parkinsonism with respiratory failure recently defined as being due to mutations in the DCTN1 gene. We describe a new family carrying a G71R mutation in the DCTN1 gene. The proband displayed a series of distinctive features not previously described in Perry syndrome: a disorder of vertical downward saccades accompanied by progressive midbrain atrophy, predominant nonmotor symptoms responsive to levodopa, distinctive craniocervical levodopa induced dyskinesias, and a good response to high-dose levodopa therapy and respiratory support. The family was initially thought to have autosomal dominant behavioral variant frontotemporal dementia with Parkinsonism. This report expands the clinical definition of this distinctive syndrome.

Targeting and trafficking of the human thiamine transporter-2 in epithelial cells.

Humans lack biochemical pathways for thiamine synthesis, so cellular requirements are met via specific carrier-mediated uptake pathways. Two proteins from the solute carrier SLC19A gene family have been identified as human thiamine transporters (hTHTRs), SLC19A1 (hTHTR1) and SLC19A2 (hTHTR2). Both of these transporters are co-expressed but are differentially targeted in polarized cell types that mediate vectorial thiamine transport (e.g. renal and intestinal epithelia). It is important to understand the domain structure of these proteins, namely which regions within the polypeptide sequence are important for physiological delivery to the cell surface, in order to understand the impact of clinically relevant mutations on thiamine transport. Here we have characterized the mechanisms regulating hTHTR2 distribution by using live cell imaging methods that resolve the targeting and trafficking dynamics of full-length hTHTR2, a series of hTHTR2 truncation mutants, as well as chimeras comprising the hTHTR1 and hTHTR2 sequence. We showed the following: (i) that the cytoplasmic COOH-tail of hTHTR2 is not essential for apical targeting in polarized cells; (ii) that delivery of hTHTR2 to the cell surface is critically dependent on the integrity of the transmembrane backbone of the polypeptide so that minimal truncations abrogate cell surface expression of hTHTR2; and (iii) video rate images of hTHTR2-containing intracellular vesicles displayed rapid bi-directional trafficking events to and from the cell surface impaired by microtubule-disrupting but not microfilament-disrupting agents as well as by overexpression of the dynactin subunit dynamitin (p50). Finally, we compared the behavior of hTHTR2 with that of hTHTR1 and the human reduced folate carrier (SLC19A1) to underscore commonalities in the cell surface targeting mechanisms of the entire SLC19A gene family.

Targeted disruption of Huntingtin-associated protein-1 (Hap1) results in postnatal death due to depressed feeding behavior.

HAP-1 is a huntingtin-associated protein that is enriched in the brain. To gain insight into the normal physiological role of HAP-1, mice were generated with homozygous disruption at the Hap1 locus. Loss of HAP-1 expression did not alter the gross brain expression levels of its interacting partners, huntingtin and p150glued. Newborn Hap1(-/-) animals are observed at the expected Mendelian frequency suggesting a non-essential role of HAP-1 during embryogenesis. Postnatally, Hap1(-/-) pups show decreased feeding behavior that ultimately leads to malnutrition, dehydration and premature death. Seventy percent of Hap1(-/-) pups fail to survive past the second postnatal day (P2) and 100% of Hap1(-/-) pups fail to survive past P9. From P2 until death, Hap1(-/-) pups show markedly decreased amounts of ingested milk. Hap1(-/-) pups that survive to P8 show signs of starvation including greatly decreased serum leptin levels, decreased brain weight and atrophy of the brain cortical mantel. HAP-1 is particularly enriched in the hypothalamus, which is well documented to regulate feeding behavior. Our results demonstrate that HAP-1 plays an essential role in regulating postnatal feeding.

Refinement of the PARK3 locus on chromosome 2p13 and the analysis of 14 candidate genes.

Parkinson's disease (PD) is a common neurodegenerative disorder with clinical features of bradykinesia, rigidity, resting tremor and postural instability resulting from the deficiency of dopamine in the nigrostriatal system. Previously we mapped a susceptibility gene for an autosomal dominant form of PD to a 10.6 cM region of chromosome 2p (PARK3; OMIM 602404). A common haplotype shared by two North American kindreds (Families B and C) genealogically traced to Southern Denmark and Northern Germany suggested a founder effect. Here we report progress in the refinement of the PARK3 locus and sequence analysis of candidate genes within the region. Members of families B and C were genotyped using polymorphic markers, reducing the minimum common haplotype to eight markers spanning a physical distance of 2.5 Mb. Analysis of 14 genes within the region did not reveal any potentially pathogenic mutations segregating with the disease, implying that none of these genes are likely candidates for PARK3.

Two approaches to isolate cytoplasmic dynein ATPase from Neurospora crassa.

Cytoplasmic dynein is a force-producing enzyme that, in association with dynactin, conducts minus-end directed transport of various organelles along microtubules. Biochemical analyses of cytoplasmic dynein and dynactin have been conducted primarily in vertebrate systems, whereas genetic analyses have been explored mainly in yeast and the filamentous fungi. To provide a complementary biochemical approach for the study of fungal dynein, we isolated/partially purified cytoplasmic dynein ATPase from the filamentous fungus Neurospora crassa. N. crassa dynein was partially purified by slightly modifying the existing procedures, described for mammalian cytoplasmic dynein that uses dynein-microtubule binding, followed by release with ATP and sucrose gradient fractionation. A novel approach was also used to isolate dynein-specific ATPase by gel filtration (Sepharose CL-4B). The K(m), ATP obtained by isolating dynein ATPase using gel filtration was similar to that obtained by using conventional method, suggests that contaminant proteins do not interfere with the dynein ATPase activity. Like vertebrate dynein, N. crassa dynein is a general NTPase with highest activity toward ATP, and only the ATPase activity is stimulated by microtubules. The K(m), ATP for N. crassa cytoplasmic dynein is 10- to 15-fold higher than that of the vertebrate enzyme.

A dynactin subunit with a highly conserved cysteine-rich motif interacts directly with Arp1.

Dynactin is a multisubunit complex and a required cofactor for most, or all, of the cellular processes powered by the microtubule-based motor cytoplasmic dynein. Using a dynein affinity column, the previously uncharacterized p62 subunit of dynactin was isolated and microsequenced. Two peptide sequences were used to clone human cDNAs encoding p62. Sequence analysis of the predicted human polypeptide of 53 kDa revealed a highly conserved pattern of eleven cysteine residues, eight of which fit the consensus sequence for a Zn(2+)-binding RING domain. We have characterized p62 as an integral component of 20 S dynactin by biochemical and immunocytochemical methods. Affinity chromatography experiments demonstrate that p62 binds directly to the Arp1 subunit of dynactin. Immunocytochemistry with antibodies to p62 demonstrates that this polypeptide has a punctate cytoplasmic distribution as well as centrosomal distribution typical of dynactin. In transfected cells, overexpression of p62 did not disrupt microtubule organization or the integrity of the Golgi but did cause both cytosolic and nuclear distribution of the protein, suggesting that this polypeptide may be targeted to the nucleus at very high expression levels.

Interaction of the p62 subunit of dynactin with Arp1 and the cortical actin cytoskeleton.

Targeting of the minus-end directed microtubule motor cytoplasmic dynein to a wide array of intracellular substrates appears to be mediated by an accessory factor known as dynactin [1-4]. Dynactin is a multi-subunit complex that contains a short actin-related protein 1 (Arp 1) filament with capZ at the barbed end and p62 at the pointed end [5]. The location of the p62 subunit and the proposed role for dynactin as a multifunctional targeting complex raise the possibility of a dual role for p62 in dynein targeting and in Arp1 pointed-end capping. In order to gain further insight into the role of p62 in dynactin function, we have cloned cDNAs that encode two full-length isoforms of the protein from rat brain. We found that p62 is homologous to the nuclear migration protein Ropy-2 from Neurospora [6]; both proteins contain a zinc-binding motif that resembles the LIM domain of several other cytoskeletal proteins [7]. Overexpression of p62 in cultured mammalian cells revealed colocalization with cortical actin, stress fibers, and focal adhesion sites, sites of potential interaction between microtubules and the cell cortex [8,9]. The p62 protein also colocalized with polymers of overexpressed wild-type or barbed-end-mutant Arp1, but not with a pointed-end mutant. Deletion of the LIM domain abolished targeting of p62 to focal-adhesion sites but did not interfere with binding of p62 to actin or Arp1. These data implicate p62 in Arp1 pointed-end binding and suggest additional roles in linking dynein and dynactin to the cortical cytoskeleton.

Analysis of dynactin subcomplexes reveals a novel actin-related protein associated with the arp1 minifilament pointed end.

The multisubunit protein, dynactin, is a critical component of the cytoplasmic dynein motor machinery. Dynactin contains two distinct structural domains: a projecting sidearm that interacts with dynein and an actin-like minifilament backbone that is thought to bind cargo. Here, we use biochemical, ultrastructural, and molecular cloning techniques to obtain a comprehensive picture of dynactin composition and structure. Treatment of purified dynactin with recombinant dynamitin yields two assemblies: the actin-related protein, Arp1, minifilament and the p150(Glued) sidearm. Both contain dynamitin. Treatment of dynactin with the chaotropic salt, potassium iodide, completely depolymerizes the Arp1 minifilament to reveal multiple protein complexes that contain the remaining dynactin subunits. The shoulder/sidearm complex contains p150(Glued), dynamitin, and p24 subunits and is ultrastructurally similar to dynactin's flexible projecting sidearm. The dynactin shoulder complex, which contains dynamitin and p24, is an elongated, flexible assembly that may link the shoulder/sidearm complex to the Arp1 minifilament. Pointed-end complex contains p62, p27, and p25 subunits, plus a novel actin-related protein, Arp11. p62, p27, and p25 contain predicted cargo-binding motifs, while the Arp11 sequence suggests a pointed-end capping activity. These isolated dynactin subdomains will be useful tools for further analysis of dynactin assembly and function.