Abstract WP270: Myeloid P2X4 Receptor Deletion Confers Neuroprotection in Females but not in Males After Experimental Stroke

Objective: Stroke is a sexually dimorphic disease. A growing body of literature from both clinical and experimental studies suggests that the outcomes after stroke differ in males versus females. Peripheral immune cells play a key role in stroke outcomes and also show sexual dimorphism. However, general immunosuppression has not shown an overall benefit in stroke patients. Therefore, identifying novel targets to improve stroke outcomes are urgently needed. P2X4 receptors (P2X4R) and P2X7 receptors (P2X7R) are the predominant subtypes expressed on immune and neural cells. The P2X7R and P2X4R interact and may physically associate with each other. Recent evidence demonstrates that P2X4R is necessary for the pro-inflammatory function of P2X7R. The goal of this study is to test the hypothesis that genetic deletion of P2X4R in myeloid immune cells impacts on stroke outcome and that such outcome differs in males vs. female mice.

Methods: P2X4R myeloid knockout mice and wild type littermates of both sexes (∼20-25g; C57BL/6), were randomized and subjected to right middle cerebral artery occlusion (MCAO-90min) followed by 3 days of reperfusion. At 72h after stroke mice were perfused and infarcts were quantified from 30μicron cut cresyl violet stained sections. The effect of myeloid P2X4R deletion on chronic functional recovery was assessed with neurological scores, corner test, Open field and the novel object recognition test. Data are expressed as mean±sem. ANOVA was performed and a P < .05 was set for statistical significance.

Results: A significant neuroprotective effect is seen in P2X4R KO compared to WT females (Cortex 36.1±1.5 vs 27.5±4.7; Striatum 67.4±1.5 vs 56.3±7.0; Total 34.8±1.6 vs 26.9±2.8. P<.05; n=7KO, 9WT). Ongoing studies in P2X4R KO ovariectomized females will determine if these sex specific effects are dependent on hormones (estrogen).

Conclusions: There is a sex-specific effect of immune cell P2X4R deletion. Myeloid specific deletion of P2X4R protects females from ischemia/reperfusion injury but has no salutary effect in males. This is the first study to demonstrate a deleterious effect of myeloid P2X4R on stroke outcomes. These initial findings implicate myeloid P2X4 as a novel therapeutic approach to improve ischemic outcomes in females.

Inhibition of glycogen synthase kinase-3β enhances cognitive recovery after stroke: the role of TAK1

Memory deficits are common among stroke survivors. Identifying neuroprotective agents that can prevent memory impairment or improve memory recovery is a vital area of research. Glycogen synthase kinase-3β (GSK-3β) is involved in several essential intracellular signaling pathways. Unlike many other kinases, GSK-3β is active only when dephosphorylated and activation promotes inflammation and apoptosis. In contrast, increased phosphorylation leads to reduced GSK-3β (pGSK-3β) activity. GSK-3β inhibition has beneficial effects on memory in other disease models. GSK-3β regulates both the 5'AMP-activated kinase (AMPK) and transforming growth factor-β-activated kinase (TAK1) pathways. In this work, we examined the effect of GSK-3β inhibition, both independently, in conjunction with a TAK inhibitor, and in AMPK-α2 deficient mice, after stroke to investigate mechanistic interactions between these pathways. GSK-3β inhibition was neuroprotective and ameliorated stroke-induced cognitive impairments. This was independent of AMPK signaling as the protective effects of GSK-3β inhibition were seen in AMPK deficient mice. However, GSK-3β inhibition provided no additive protection in mice treated with a TAK inhibitor suggesting that TAK1 is an upstream regulator of GSK-3β. Targeting GSK-3β could be a novel therapeutic strategy for post-stroke cognitive deficits.

Abstract T MP14: The Critical Role of Macrophage Migration Inhibitory Factor on Post-Stroke Recovery

Objective: One-third of stroke survivors are affected by post-stroke depression. Evidence from epidemiological and clinical studies demonstrates that depression either before or after stroke is associated with poor recovery and high mortality. Recently it was found that loss of macrophage migration inhibitory factor (MIF) is associated with depressive behavior and impaired neurogenesis. Therefore, here we tested the hypothesis that MIF plays a role in stroke recovery and that chronic MIF inhibition contributes to post-depressive phenotypes and poor stroke outcomes.

Methods: C57BL/6 male mice (20-25g; Charles River), were subjected to a 60min right middle cerebral artery occlusion (MCAO) and randomly assigned to vehicle or MIF antagonist, ISO-1 (7mg/kg/day intraperitoneal) treatment. Infarcts quantified with TTC. Recovery was investigated using neurological deficit scores (NDS), corner test, the forced swim test (FST) and the tail suspension test (TST). MIF levels were assessed by ELISA and western-blot (n=4/grp). Further, the effects of MIF loss were tested using knockout (KO) mice. Data are expressed as mean±sem. P value < .05 was set for statistical significance.

Results: Post-stroke chronic ISO-1 treatment significantly increased immobility in TST at 14d (126±8 vs 83±6s; p<.05), delayed stroke recovery in the corner test (p<.05) and NDS (p<.05) compared to vehicle group. These detrimental effects were observed in parallel to reduced plasma MIF levels (p<.05). Stroke alone did not affect mobility in FST compared to sham (p>.05). Infarct size was similar in ISO-1 and vehicle groups (48±3.2% versus 46±2.8%; p>.05). When subjected MIF KO mice to stroke, similar pattern of delayed post-stroke recovery is observed suggesting that MIF plays a critical role in pre- or post-stroke depression and recovery.

Conclusions: MIF KO mice had a depressive phenotype at baseline, and poor recovery after stroke compared to WT. Post-stroke MIF inhibition led to the development of a post-stroke depressive phenotype and also led to poorer recovery. These effects are independent of stroke volume. These findings suggest that targeting MIF might be a novel therapeutic strategy to treat post-stroke depression and to enhance recovery in stroke survivors.

Abstract 175: The Role of ER-α, ER-ß, and AR in the FCG Model of Ischemic Brain Injury

Background: Ischemic stroke is a sexually dimorphic disease. Females are protected from ischemic brain injury throughout most of the lifespan, but the contribution of hormones in this “ischemia resistant” phenotype remains unclear. The “four core genotype” (FCG) model is used to differentiate the effect of the chromosomal complement (XX vs. XY) from an animal’s gonadal (estrogen vs. testosterone producing) sex through examination of five genotypes XYMwt, XY-M, XXM, XXF, and XYF. XY-M FCG mice are made by the translocation of Sry from the Y chromosome to an autosome. Stroke sex differences have been largely attributed to the neuroprotective effects of estrogen. However, it remains unclear whether hormonal receptors can be influenced at the chromosomal level, which could further explain sexual dimorphism in stroke phenotypes. We hypothesize that the sex chromosome complement and transgene Sry (the testis determining gene found on the Y chromosome) contribute to the sexually dimorphic stroke phenotype.

Methods: FCG animals were gonadectomized at 3 weeks of age. Animals were subjected to stroke at 8-12 weeks of age. Cytosolic samples were evaluated for androgen receptor (AR) and estrogen receptor (ER-α and ER-β) levels in sham and stroke samples. RT-PCR for Sry on the stroke hemisphere (delta-delta-CT) was performed on the FCG mice and wild type males.

Results: Western blot analysis showed that XXM and XXF mice had significant stroke induced reduction in AR expression (p<0.01). No significant difference in the expression of ER-α was seen in sham and stroke mice among genotypes. ER-β expression increased significantly in XXM post stroke (p<0.05), but not in the other genotypes. RT-PCR analysis showed approximately 90-fold increase of mRNA expression of Sry in the phenotypes with the exogenous Sry (XYM and XXM) compared to WTM mice with endogenous Sry (p<0.05).

Conclusion: XXM and XYM mice show an increase in Sry expression compared to WTM suggesting insertional and positional effects of Sry transgene. AR expression in XXM and XXF mice is influenced at the chromosomal level, suggesting effects of X chromosome dosage on AR expression. Stroke induced ER-β expression in XXM mice may be an effect of the interaction between transgene Sry and other hormonal axes.

NF-κB contributes to the detrimental effects of social isolation after experimental stroke

Social isolation (SI) is increasingly recognized as a risk factor for stroke. Individuals with lack of social support systems have an increased incidence of stroke, poorer recovery, and greater functional decline after injury compared to individuals with social support. Attesting to the importance of social factors in stroke outcome is that these same effects can be reproducibly demonstrated in animals; social interaction improves behavioral deficits and reduces damage after experimental stroke, whereas SI enhances injury. The mechanism by which SI exacerbates injury is unclear. We investigated the role of nuclear factor-kappaB (NF-κB) signaling in male mice that were pair housed (PH) with an ovariectomized female prior to random assignment into continued PH or SI for 7 days prior to middle cerebral artery occlusion. The effects of SI on infarct volume and functional recovery were assessed at 72 h post-stroke. Nuclear NF-κB levels and activity were assessed by Western blot and transcriptional assays. SI significantly exacerbated infarct size in both male and female mice compared to PH mice. SI mice had delayed functional recovery compared to PH mice. An elevation of systemic IL-6 levels, increased nuclear NF-κB transcriptional activity, and enhanced nuclear translocation of NF-κB was seen in SI stroke animals. Interference with NF-κB signaling using either a pharmacological inhibitor or genetically engineered NF-κB p50 knockout mice abolished the detrimental effects of SI on both infarct size and functional recovery. This suggests that NF-κB mediates the detrimental effects of SI.

Age-related changes in AMP-activated protein kinase after stroke

Adenosine monophosphate-activated protein kinase (AMPK) is an evolutionary conserved energy sensor sensitive to changes in cellular AMP/ATP ratio which is activated by phosphorylation (pAMPK). pAMPK levels decrease in peripheral tissues with age, but whether this also occurs in the aged brain, and how this contributes to the ability of the aged brain to cope with ischemic stress is unknown. This study investigated the activation of AMPK and the response to AMPK inhibition after induced stroke in both young and aged male mice. Baseline levels of phosphorylated AMPK were higher in aged brains compared to young mice. Stroke-induced a robust activation of AMPK in young mice, yet this response was muted in the aged brain. Young mice had larger infarct volumes compared with aged animals; however, more severe behavioral deficits and higher mortality were seen in aged mice after stroke. Inhibition of AMPK with Compound C decreased infarct size in young animals, but had no effect in aged mice. Compound C administration led to a reduction in brain ATP levels and induced hypothermia, which led to enhanced neuroprotection in young but not aged mice. This work demonstrates that aging increases baseline brain pAMPK levels; aged mice have a muted stroke-induced pAMPK response; and that AMPK inhibition and hypothermia are less efficacious neuroprotective agents in the aged brain. This has important translational relevance for the development of neuroprotective agents in preclinical models and our understanding of the enhanced metabolic stress experienced by the aged brain.

Abstract 203: Chronic and Acute Estrogen Replacement Therapy Has Different Effects in Aged Animals After Experimental Stroke

Introduction: The effect of estrogen replacement therapy (ERT) on stroke incidence and severity has been extensively debated. Clinical trials of ERT demonstrated an increased risk of stroke in treated women, but study participants were well past menopause when therapy was initiated. It has been suggested that detrimental effects of ERT may be unmasked after prolonged periods of hypoestrogenicity. To date, very few studies have examined the effect of ERT in aged animals although the timing of replacement may be critical to the neuroprotective effects of ERT.

Hypothesis: We hypothesized that chronic estrogen replacement (CER) initiated in late middle age, would decrease infarct size after an induced stroke whereas acute estrogen replacement (AER) would have no beneficial effects in the aged female brain.

Methods: CER was administered to aged C57BL6 mice from 17 to 20 months of age, and AER was initiated at 20 months of age. Both CER and AER treated mice were subjected to 90-minute middle cerebral artery occlusion (MCAO) at 20.5 months of age. Stroke outcomes at 24 hours of MCAO were measured. Protein levels of nuclear factor κB (NFκB), estrogen receptor α (ERα) and serum levels of pro-inflammatory cytokines were also examined.

Results: Female mice that received CER showed improved stroke outcomes after MCAO (total infarct: CER vs. vehicle 34.7±2.4% vs. 58.2±2.6%; n=9/gp, p <0.05); whereas females that had AER did not. CER females exhibited diminished levels of NFκB translocation compared to AER females after stroke. AER females demonstrated both an increase in nuclear NFκB and enhanced expression of pro-inflammatory cytokines. The differential NFκB translocation was related to corresponding changes in ERα expression. Aged males benefited from ERT regardless of the timing of initiation of ERT, and had significantly reduced expression of pro-inflammatory markers after stroke compared to age-matched females.

Conclusions: AER worsened stroke outcomes whereas CER was protective in aged females. A pro-inflammatory milieu emerges with age in females which was attenuated by CER treatment. Interestingly both AER and CER reduced stroke injury in aged males, suggesting sexually dimorphic effects on inflammation.

Abstract 3432: Sex-dependent Differences in Autophagy After Experimentally-induced Stroke

Background and Purpose: Autophagy is a catabolic process where the cell consumes pieces of itself as a part of normal cellular growth, development, and homeostasis. In response to nutrient challenge or other stressors, including ischemia, autophagy levels may rise, enabling the cell to survive. Autophagy has been shown to play a role in the cellular response to neonatal cerebral ischemia, and sex differences have been found in cell culture models of neuronal nutrient starvation. However, the role of sex in the autophagic response to stroke has not been investigated in vivo . The objective of this study was to examine the impact of sex on autophagy following experimental stroke.

Methods: Stroke was induced by reversible right middle cerebral artery occlusion (MCAO - 90 minutes) in male, gonadally-intact female, and ovariectomized (OVX) WT C57BL/6 mice (20-25 gm). Autophagy activity was assessed at 6 and 24 hours following MCAO with LC3-I/II and Atg7 protein levels by western blot (n=3 MCAO; n=3 sham/group). Beta-actin was utilized as a loading control.

Results: Atg7 levels were higher in sham males at both the 6h and 24h timepoint compared to intact females and OVX females. After MCAO, males and OXV females showed increased levels of Atg7 relative to sham at 6h, while intact female levels were unchanged. By 24h, Atg7 levels in MCAO males and OVX females remained higher than sham, while intact females showed no difference. LC3-II levels were higher in MCAO for OVX females at 6h and 24h, while intact females saw transiently-elevated LC3-II levels at 6h and no elevation by 24h. Males had higher levels of LC3-II in both MCAO and sham relative to intact females at both 6h and 24h and higher levels than OVX females at 6h.

Conclusions: This study demonstrates that autophagy activation in response to stroke differs between the sexes. We found that males had overall higher levels of autophagy than both OVX females and intact females at both 6h and 24h. Both males and OVX females showed higher levels of autophagy at 24h and 6h after MCAO relative to sham, while females showed no change. Our study suggests that changes in autophagy may contribute to the differential outcome after stroke between the sexes. Studies examining sex differences in infarct size after MCAO after inhibition of autophagy with 3-methyladenine (3-MA, Sigma-Aldrich, St. Louis, MO) are ongoing in our laboratory.

Sex differences in the response to poly(ADP-ribose) polymerase-1 deletion and caspase inhibition after stroke

BACKGROUND AND PURPOSE

Emerging data suggest that the molecular cell death pathways triggered by ischemic insults differ in the male and female brain. Cell death in males is initiated by poly(ADP-ribose) polymerase-1 (PARP-1) activation; however, manipulation of this pathway paradoxically increases ischemic damage in females. In contrast, females are exquisitely sensitive to caspase-mediated cell death. The effect of caspase inhibition in PARP-1 knockout mice was evaluated to determine if the detrimental effects of PARP deletion in females were secondary to increased caspase activation.

METHODS

Focal stroke was induced by transient or permanent middle cerebral artery occlusion (MCAO) in wild-type (WT) and PARP-1(-/-) mice of both sexes. The pan-caspase inhibitor, quinoline-Val-Asp(Ome)-CH2-O-phenoxy (Q-VD-OPh), was administered 90 minutes after middle cerebral artery occlusion. Infarct size and neurological sores were assessed. Separate cohorts were used for protein analysis for PAR, Apoptosis inducing factor (AIF), caspase-9, and caspase-3.

RESULTS

WT mice of both sexes had increased nuclear AIF after stroke compared to PARP-1(-/-) mice. PARP-1(-/-) females had higher mitochondrial cytochrome C and activated caspase-9 and -3 levels than WT female mice. PARP-1(-/-) females also had an increase in stroke-induced cytosolic cytochrome C release compared with WT females, which was not seen in males. Q-VD-OPh decreased caspase-9 in both males and females but only led to reduction of infarct in females. PARP-1(-/-) males had smaller infarcts, whereas PARP-1(-/-) females had larger strokes compared with WT. Q-VD-OPh significantly decreased infarct in both WT and PARP-1(-/-) females in both transient and permanent MCAO models, but had no effect in males.

CONCLUSIONS

Deletion of PARP-1 reduces infarct in males but exacerbates injury in females. PARP-1(-/-) females have enhanced caspase activation. The detrimental effects of PARP loss in females can be reversed with caspase inhibition.

Effects of metformin in experimental stroke

BACKGROUND AND PURPOSE

Adenosine 5'-monophosphate-activated protein kinase (AMPK) is an important sensor of energy balance. Stroke-induced AMPK activation is deleterious because both pharmacological inhibition and genetic deletion of AMPK are neuroprotective. Metformin is a known AMPK activator but reduces stroke incidence in clinical populations. We investigated the effect of acute and chronic metformin treatment on infarct volume and AMPK activation in experimental stroke.

METHODS

Male mice were subjected to middle cerebral artery occlusion after acute (3 days) or chronic (3 weeks) administration of metformin. Infarct volumes, AMPK activation, lactate accumulation, and behavioral outcomes were assessed. The roles of neuronal nitric oxide synthase and AMPK were examined using mice with targeted deletion of AMPK or neuronal nitric oxide synthase.

RESULTS

Acute metformin exacerbated stroke damage, enhanced AMPK activation, and led to metabolic dysfunction. This effect was lost in AMPK and neuronal nitric oxide synthase knockout mice. In contrast, chronic metformin given prestroke was neuroprotective, improved stroke-induced lactate generation, and ameliorated stroke-induced activation of AMPK. Similarly, the neuroprotective effect of chronic prestroke metformin was lost in neuronal nitric oxide synthase knockout mice.

CONCLUSIONS

AMPK is an important potential target for stroke treatment and prevention. These studies show that the timing, duration, and amount of AMPK activation are key factors in determining the ultimate downstream effects of AMPK on the ischemic brain.

Adenosine monophosphate activated protein kinase inhibition is protective in both sexes after experimental stroke

Sex differences in clinical and experimental stroke are now well recognized. Adenosine monophosphate activated protein kinase (AMPK) is an important energy sensor that is activated in times of energy demand. Increasing AMPK is deleterious in experimental cerebral ischemia, at least in males. Interestingly, studies in peripheral tissues have suggested that there are sex differences in the regulation of AMPK in muscle after exercise. PolyADP ribose polymerase (PARP), a key mediator of ischemic cell death, stimulates AMPK activation, yet activation of PARP appears to be selectively detrimental in male brain. As interference with sex specific cell death pathways can determine the efficacy of experimental neuroprotective agents, and AMPK inhibition is a novel neuroprotective target, we examined the effect of AMPK inhibition in male and female mice. In this study, AMPK alpha2 gene expression (mRNA) and pAMPK protein levels were examined and found to be comparable between both sexes after transient middle cerebral artery occlusion (MCAO). Treatment with the AMPK inhibitor Compound C at stroke onset significantly reduced infarct size and neurological deficits 24h after stroke in ovariectomized female mice. Finally, genetic deletion of AMPK alpha2 in ovariectomized females was neuroprotective as assessed by smaller infarct volumes and improved neurological deficits when compared to wild type littermates. This work demonstrates that AMPK activation is deleterious in experimental stroke, and this effect is independent of sex.

Expression of Na-K-Cl cotransporter and edema formation are age dependent after ischemic stroke

Age is the most important independent risk factor for stroke; however aging animals are rarely used in stroke studies. Previous work demonstrated that young male mice had more edema formation after an induced stroke than aging animals. An important contributor to cerebral edema formation is the Na-K-Cl cotransporter (NKCC). We examined the expression of NKCC in young (10-12 weeks) and aging (15-16 months) C57BL6 male mice after middle cerebral artery occlusion (MCAO) and investigated the effect of pharmacological inhibition of NKCC with Bumetanide on cerebral edema formation. Both immunofluorescent staining and Western blotting analysis showed that NKCC expression was significantly higher in the ischemic penumbra of young compared to aging mice after stroke. Edema formation was significantly more robust in young mice and was reduced with Bumetanide. Bumetanide had no effect on cerebral edema in aging mice after MCAO. This suggests that NKCC expression and edema formation are age dependent after ischemic stroke.

Changes in experimental stroke outcome across the life span

Acute ischemic stroke is a leading cause of mortality and disability in the elderly. Age is the most important nonmodifiable risk factor for stroke, yet many preclinical models continue to examine only young male animals. It remains unclear how experimental stroke outcomes change with aging and with biologic sex. If sex differences are present, it is not known whether these reflect an intrinsic differing sensitivity to stroke or are secondary to the loss of estrogen with aging. We subjected both young and aging mice of both sexes to middle cerebral artery occlusion (MCAO). Young female mice had smaller strokes compared with age-matched males, an effect that was reversed by ovariectomy. Stroke damage increased with aging in female mice, whereas male mice had decreased damage after MCAO. Blood-brain barrier (BBB) permeability changes are correlated with infarct size. However, aging mice had significantly less edema formation, an effect that was independent of sex and histologic damage. Differences in the cellular response to stroke occur across the life span in both male and female mice. These differences need to be considered when developing relevant therapies for stroke patients, the majority of whom are elderly.

A bipartite Ca2+-regulated nucleoside-diphosphate kinase system within the Chlamydomonas flagellum. The regulatory subunit p72

Regulation of flagellar activity in Chlamydomonas involves both Ca(2+) and cAMP-mediated signaling pathways. However, Chlamydomonas and sea urchin sperm flagella also exhibit nucleoside-diphosphate kinase (NDK) activity, suggesting a requirement for GTP within this highly conserved organelle. In sea urchin sperm, the NDK catalytic subunit is an integral component of the outer dynein arm. Here we describe a modular protein (p72) from the Chlamydomonas flagellum that consists of three domains closely related to the presumptive regulatory segment of rat NDK-7 followed by two EF-hands that are predicted to bind Ca(2+). There are close homologues of p72 in both mammalian and insect genomes. The p72 protein is tightly associated with the flagellar axoneme and is located along the entire length except at the transition zone. Cross-linking experiments suggest that p72 interacts with two or three additional axonemal polypeptides. The sensitivity of p72 to tryptic digestion differed considerably in the presence and the absence of Ca(2+), suggesting that it indeed binds this ligand. These studies indicate that the flagellar NDK system is bipartite with the regulatory and catalytic components residing on different polypeptides. We propose that Ca(2+) regulation of flagellar motility in Chlamydomonas may be achieved in part through a downstream GTP-mediated signaling pathway.

Redox-based control of the gamma heavy chain ATPase from Chlamydomonas outer arm dynein

The outer dynein arm from Chlamydomonas flagella contains two redox-active thioredoxin-related light chains associated with the alpha and beta heavy chains; these proteins belong to a distinct subgroup within the thioredoxin family. This observation suggested that some aspect of dynein activity might be modulated through redox poise. To test this, we have examined the effect of sulfhydryl oxidation on the ATPase activity of isolated dynein and axonemes from wildtype and mutant strains lacking various heavy chain combinations. The outer, but not inner, dynein arm ATPase was stimulated significantly following treatment with low concentrations of dithionitrobenzoic acid; this effect was readily reversible by dithiol, and to a lesser extent, monothiol reductants. Mutational and biochemical dissection of the outer arm revealed that ATPase activation in response to DTNB was an exclusive property of the gamma heavy chain, and that enzymatic enhancement was modulated by the presence of other dynein components. Furthermore, we demonstrate that the LC5 thioredoxin-like light chain binds to the N-terminal stem domain of the alpha heavy chain and that the beta heavy chain-associated LC3 protein also interacts with the gamma heavy chain. These data suggest the possibility of a dynein-associated redox cascade and further support the idea that the gamma heavy chain plays a key regulatory role within the outer arm.

The Tctex1/Tctex2 class of dynein light chains. Dimerization, differential expression, and interaction with the LC8 protein family

The Tctex1/Tctex2 family of dynein light chains associates with the intermediate chains at the base of the soluble dynein particle. These components are essential for dynein assembly and participate in specific motor-cargo interactions. To further address the role of these light chains in dynein activity, the structural and biochemical properties of several members of this polypeptide class were examined. Gel filtration chromatography and native gel electrophoresis indicate that recombinant Chlamydomonas flagellar Tctex1 exists as a dimer in solution. Furthermore, yeast two-hybrid analysis suggests that this association also occurs in vivo. In contrast, both murine and Chlamydomonas Tctex2 are monomeric. To investigate protein-protein interactions involving these light chains, outer arm dynein from Chlamydomonas flagella was cross-linked using dimethylpimelimidate. Immunoblot analysis of the resulting products revealed the interaction of LC2 (Tctex2) with LC6, which is closely related to the highly conserved LC8 protein found in many enzyme systems, including dynein. Northern dot blot analysis demonstrated that Tctex1/Tctex2 family light chains are differentially expressed both in a tissue-specific and developmentally regulated manner in humans. These data provide further support for the existence of functionally distinct populations of cytoplasmic dynein with differing light chain content.

Investigation of protein-protein interactions within flagellar dynein using homobifunctional and zero-length crosslinking reagents

The dynein molecular motor is a highly complex enzyme containing up to 15 different protein components and consists of several distinct domains identifiable by electron microscopy. One of the current challenges is to understand the supramolecular organization of this motor and to determine the location and function of the various components. Recently, we have used covalent crosslinking by amine-selective reagents and a carbodiimide, which results in zero-length crosslink, to investigate protein-protein associations within Chlamydomonas flagellar dynein. This approach also has enabled us to identify previously undescribed interactions between the dynein arms and other components of the flagellar axoneme. In this report, we detail methods we have developed to probe intradynein and intraaxonemal interactions and discuss the variety of factors that need be addressed to perform a successful crosslinking experiment.

Solution structure of a dynein motor domain associated light chain

Dyneins are molecular motors that translocate towards the minus ends of microtubules. In Chlamydomonas flagellar outer arm dynein, light chain 1 (LC1) associates with the nucleotide binding region within the gamma heavy chain motor domain and consists of a central leucine-rich repeat section that folds as a cylindrical right handed spiral formed from six beta-beta-alpha motifs. This central cylinder is flanked by terminal helical subdomains. The C-terminal helical domain juts out from the cylinder and is adjacent to a hydrophobic surface within the repeat region that is proposed to interact with the dynein heavy chain. The position of the C-terminal domain on LC1 and the unexpected structural similarity between LC1 and U2A' from the human spliceosome suggest that this domain interacts with the dynein motor domain.

Identification and molecular characterization of the p24 dynactin light chain

Intracellular transport along microtubules uses the motor proteins cytoplasmic dynein and kinesin. Cytoplasmic dynein is responsible for movement to the minus ends of microtubules and the evidence indicates that dynein interacts with another protein complex, dynactin. In order to better understand how these proteins function, we have sought to identify and clone the subunit polypeptides of these two complexes, in particular their light chains. Dynactin is made up of eight subunits of approximately 24,000 to 160,000 Da. In order to clone the p24 subunit, the components of purified dynactin were resolved by SDS polyacrylamide gel electrophoresis. The amino acid sequence of a tryptic peptide from the 24,000-Mr region of the gel was obtained and a candidate polypeptide identified by a screen of the databases. This polypeptide has a predicted molecular weight of 20,822 Da. Using an antibody to a different region of this protein, we demonstrate that it copurifies with microtubules and elutes from the microtubule pellet with characteristics similar to those of the dynactin complex and distinct from those of cytoplasmic dynein. This polypeptide co-sediments with dynactin on sucrose density gradients and it also co-immunoprecipitates with dynactin, but not with kinesin or cytoplasmic dynein. Together these results demonstrate that this polypeptide is the p24 subunit of dynactin. Analysis of the predicted amino acid sequence of p24 shows that it is a unique protein that has no significant similarity to known enzymes or other proteins. Structural analysis indicates that most of this protein will form an alpha-helix and that portions of the molecule may participate in the formation of coiled-coils. Since stoichiometric analysis of dynactin indicates that there is one molecule of p24 per dynactin complex, these characteristics suggest that this polypeptide may be involved in protein-protein interactions, perhaps in the assembly of the dynactin complex.

Dynein light chain binding to a 3'-untranslated sequence mediates parathyroid hormone mRNA association with microtubules

The 3'-untranslated region (UTR) of mRNAs binds proteins that determine mRNA stability and localization. The 3'-UTR of parathyroid hormone (PTH) mRNA specifically binds cytoplasmic proteins. We screened an expression library for proteins that bind the PTH mRNA 3'-UTR, and the sequence of 1 clone was identical to that of the dynein light chain LC8, a component of the dynein complexes that translocate cytoplasmic components along microtubules. Recombinant LC8 binds PTH mRNA 3'-UTR, as shown by RNA electrophoretic mobility shift assay. We showed that PTH mRNA colocalizes with microtubules in the parathyroid gland, as well as with a purified microtubule preparation from calf brain, and that this association was mediated by LC8. To our knowledge, this is the first report of a dynein complex protein binding an mRNA. The dynein complex may be the motor that is responsible for transporting mRNAs to specific locations in the cytoplasm and for the consequent is asymmetric distribution of translated proteins in the cell.

LC2, the chlamydomonas homologue of the t complex-encoded protein Tctex2, is essential for outer dynein arm assembly

Tctex2 is thought to be one of the distorter genes of the mouse t haplotype. This complex greatly biases the segregation of the chromosome that carries it such that in heterozygous +/t males, the t haplotype is transmitted to >95% of the offspring, a phenomenon known as transmission ratio distortion. The LC2 outer dynein arm light chain of Chlamydomonas reinhardtii is a homologue of the mouse protein Tctex2. We have identified Chlamydomonas insertional mutants with deletions in the gene encoding LC2 and demonstrate that the LC2 gene is the same as the ODA12 gene, the product of which had not been identified previously. Complete deletion of the LC2/ODA12 gene causes loss of all outer arms and a slow jerky swimming phenotype. Transformation of the deletion mutant with the cloned LC2/ODA12 gene restores the outer arms and rescues the motility phenotype. Therefore, LC2 is required for outer arm assembly. The fact that LC2 is an essential subunit of flagellar outer dynein arms allows us to propose a detailed mechanism whereby transmission ratio distortion is explained by the differential binding of mutant (t haplotype encoded) and wild-type dyneins to the axonemal microtubules of t-bearing or wild-type sperm, with resulting differences in their motility.

Drosophila roadblock and Chlamydomonas Lc7

Eukaryotic organisms utilize microtubule-dependent motors of the kinesin and dynein superfamilies to generate intracellular movement. To identify new genes involved in the regulation of axonal transport in Drosophila melanogaster, we undertook a screen based upon the sluggish larval phenotype of known motor mutants. One of the mutants identified in this screen, roadblock (robl), exhibits diverse defects in intracellular transport including axonal transport and mitosis. These defects include intra-axonal accumulations of cargoes, severe axonal degeneration, and aberrant chromosome segregation. The gene identified by robl encodes a 97–amino acid polypeptide that is 57% identical (70% similar) to the 105–amino acid Chlamydomonas outer arm dynein–associated protein LC7, also reported here. Both robl and LC7 have homology to several other genes from fruit fly, nematode, and mammals, but not Saccharomyces cerevisiae. Furthermore, we demonstrate that members of this family of proteins are associated with both flagellar outer arm dynein and Drosophila and rat brain cytoplasmic dynein. We propose that roadblock/LC7 family members may modulate specific dynein functions.

Drosophila roadblock and Chlamydomonas LC7: a conserved family of dynein-associated proteins involved in axonal transport, flagellar motility, and mitosis

Eukaryotic organisms utilize microtubule-dependent motors of the kinesin and dynein superfamilies to generate intracellular movement. To identify new genes involved in the regulation of axonal transport in Drosophila melanogaster, we undertook a screen based upon the sluggish larval phenotype of known motor mutants. One of the mutants identified in this screen, roadblock (robl), exhibits diverse defects in intracellular transport including axonal transport and mitosis. These defects include intra-axonal accumulations of cargoes, severe axonal degeneration, and aberrant chromosome segregation. The gene identified by robl encodes a 97-amino acid polypeptide that is 57% identical (70% similar) to the 105-amino acid Chlamydomonas outer arm dynein-associated protein LC7, also reported here. Both robl and LC7 have homology to several other genes from fruit fly, nematode, and mammals, but not Saccharomyces cerevisiae. Furthermore, we demonstrate that members of this family of proteins are associated with both flagellar outer arm dynein and Drosophila and rat brain cytoplasmic dynein. We propose that roadblock/LC7 family members may modulate specific dynein functions.

Light chain 1 from the Chlamydomonas outer dynein arm is a leucine-rich repeat protein associated with the motor domain of the gamma heavy chain

The LC1 light chain from Chlamydomonas outer arm dynein is tightly bound to the gamma heavy chain. Molecular cloning revealed that LC1 is a member of the SDS22+ subclass of the leucine-rich repeat protein family and as such is likely involved in mediating interactions between dynein and the components of a signal transduction pathway. Through the combination of covalent cross-linking and vanadate-mediated photolysis, LC1 was found to associate with that portion of the gamma HC that is C-terminal to the P1 loop. This region comprises most of the globular head domain of the heavy chain and includes the stalk-like structure that is involved in microtubule binding. Attachment of LC1 to this region represents the only known example of an accessory polypeptide directly associated with a dynein motor domain. Additional cross-linking experiments revealed that LC1 also interacts directly in situ with an approximately 45 kDa axonemal component; this interaction is disrupted by the standard high salt treatment used to remove the outer arm from the axoneme. These data suggest that LC1 acts to mediate the association between this 45 kDa axonemal polypeptide and the motor unit of the gamma HC.

Cytoplasmic dynein contains a family of differentially expressed light chains

Cytoplasmic dynein contains a series of accessory proteins associated with the motor containing heavy chains.1 These include three distinct classes of light chains (Mr < approximately 22 000). Here we demonstrate that a previously cloned protein termed rp3 is a bona fide Mr 14 000 light chain component of this microtubule motor complex. The rp3 polypeptide is approximately 55% identical to the Tctex1 dynein light chain, and together, these two proteins define one branch of a diverse family of Mr 14 000 light chains associated with both cytoplasmic and flagellar dyneins. The Tctex1 and rp3 light chains are differentially expressed in various tissues: rp3 is most prevalent in liver and brain cytoplasmic dynein, whereas those tissues contain the least amounts of Tctex1. Immunofluorescence analysis was consistent with the tissue-specific distribution of these proteins and revealed that both rp3 and Tctex1 are present in multiple perinuclear punctate particles. Furthermore, in two cell lines, rp3 was found associated with an elongated structure located in the layer of cytoplasm above the nucleus. Electrophoretic/immunological analysis indicates that there are only single isoforms for these proteins in brain and PC-12 cells, suggesting that alterations in the Mr 14 000 light chains of dynein are achieved at the level of the individual proteins and not by posttranslational modification. Dissection of the cytoplasmic dynein complex revealed that Tctex1, an Mr 8000 LC dimer, and IC74 associate to define a basal-located intermediate chain/light chain complex analogous to that found in flagellar outer arm dynein.

Dimerization of the highly conserved light chain shared by dynein and myosin V

The Mr 8,000 light chain originally identified in Chlamydomonas flagellar dynein is also a component of both cytoplasmic dynein and myosin V. Furthermore, this small protein has been implicated as an inhibitor of neuronal nitric oxide synthase, suggesting that it may play multiple regulatory roles within the cell. Covalent cross-linking of both dynein and myosin V using 1,5-difluoro-2, 4-dinitrobenzene revealed that this light chain exists as a dimer in situ. This observation was confirmed using two additional amine-selective cross-linking reagents (dimethyl pimelimidate and disuccinimidyl suberate). When expressed as a C-terminal fusion with maltose-binding protein, the presence of the light chain caused the recombinant molecule to dimerize. Analysis of fusions containing truncated light chains identified the predicted amphiphilic helix (residues 14-32) as sufficient to cause dimerization; cross-linking required a second helical segment (residues 33-46). Together the data presented suggest that two light chains interact to form a parallel dimeric structure. This arrangement has significant implications for the potential functions of this highly conserved molecule and suggests a mechanism by which it might dissociate nitric oxide synthase.

A Chlamydomonas homologue of the putative murine t complex distorter Tctex-2 is an outer arm dynein light chain

Molecular analysis of a 19,000-Mr protein from the Chlamydomonas flagellum reveals that it is homologous to the t complex-encoded protein Tctex-2, which is a candidate for one of the distorter products that cause the extreme transmission ratio distortion (meiotic drive) of the murine t complex. The 19,000-Mr protein is extracted from the axoneme with 0.6 M NaCl and comigrates with the outer dynein arm in sucrose density gradients. This protein also is specifically missing in axonemes prepared from a mutant that does not assemble the outer arm. These data raise the possibility that Tctex-2 is a sperm flagellar dynein component. Combined with the recent identification of Tctex-1 (another distorter candidate) as a light chain of cytoplasmic dynein, these results lead to a biochemical model for how differential defects in spermiogenesis that result in the phenomenon of meiotic drive might be generated in wild-type vs t-bearing sperm.

The mouse t-complex-encoded protein Tctex-1 is a light chain of brain cytoplasmic dynein

Mammalian brain cytoplasmic dynein contains three light chains of Mr = 8,000, 14,000, and 22,000 (King, S. M., Barbarese, E., Dillman, J. F., III, Patel-King, R. S., Carson, J. H., and Pfister, K. K. (1996) J. Biol. Chem. 271, 19358-19366). Peptide sequence data (16/16 residues correct) implicate the Mr = 14,000 polypeptide as Tctex-1, a protein encoded within the mouse t-complex. Tctex-1 cosediments with microtubules and is eluted with ATP or salt but not with GTP as expected for a dynein subunit. The ATP-eluted protein precisely cosediments with known cytoplasmic dynein proteins in sucrose density gradients. Tctex-1 also is immunoprecipitated from brain and other tissue homogenates by a monoclonal antibody raised against the 74-kDa cytoplasmic dynein intermediate chain. Quantitative densitometry indicates that Tctex-1 is a stoichiometric component of the dynein complex. As Tctex-1 is a candidate for involvement in the transmission ratio distortion (meiotic drive) of mouse t-haplotypes, these results suggest that cytoplasmic dynein dysfunction may play an important role in non-mendelian chromosome segregation.