Pharmacological Inhibition of p-21 Activated Kinase (PAK) Restores Impaired Neurite Outgrowth and Remodeling in a Cellular Model of Down Syndrome.

Down syndrome (DS) is characterized by the trisomy of chromosome 21 and by cognitive deficits that have been related to neuronal morphological alterations in humans, as well as in animal models. The gene encoding for amyloid precursor protein (APP) is present in autosome 21, and its overexpression in DS has been linked to neuronal dysfunction, cognitive deficit, and Alzheimer's disease-like dementia. In particular, the neuronal ability to extend processes and branching is affected. Current evidence suggests that APP could also regulate neurite growth through its role in the actin cytoskeleton, in part by influencing p21-activated kinase (PAK) activity. The latter effect is carried out by an increased abundance of the caspase cleavage-released carboxy-terminal C31 fragment. In this work, using a neuronal cell line named CTb, which derived from the cerebral cortex of a trisomy 16 mouse, an animal model of human DS, we observed an overexpression of APP, elevated caspase activity, augmented cleavage of the C-terminal fragment of APP, and increased PAK1 phosphorylation. Morphometric analyses showed that inhibition of PAK1 activity with FRAX486 increased the average length of the neurites, the number of crossings per Sholl ring, the formation of new processes, and stimulated the loss of processes. Considering our results, we propose that PAK hyperphosphorylation impairs neurite outgrowth and remodeling in the cellular model of DS, and therefore we suggest that PAK1 may be a potential pharmacological target.

Defects in G-Actin Incorporation into Filaments in Myoblasts Derived from Dysferlinopathy Patients Are Restored by Dysferlin C2 Domains.

Dysferlin is a transmembrane C-2 domain-containing protein involved in vesicle trafficking and membrane remodeling in skeletal muscle cells. However, the mechanism by which dysferlin regulates these cellular processes remains unclear. Since actin dynamics is critical for vesicle trafficking and membrane remodeling, we studied the role of dysferlin in Ca2+-induced G-actin incorporation into filaments in four different immortalized myoblast cell lines (DYSF2, DYSF3, AB320, and ER) derived from patients harboring mutations in the dysferlin gene. As compared with immortalized myoblasts obtained from a control subject, dysferlin expression and G-actin incorporation were significantly decreased in myoblasts from dysferlinopathy patients. Stable knockdown of dysferlin with specific shRNA in control myoblasts also significantly reduced G-actin incorporation. The impaired G-actin incorporation was restored by the expression of full-length dysferlin as well as dysferlin N-terminal or C-terminal regions, both of which contain three C2 domains. DYSF3 myoblasts also exhibited altered distribution of annexin A2, a dysferlin partner involved in actin remodeling. However, dysferlin N-terminal and C-terminal regions appeared to not fully restore such annexin A2 mislocation. Then, our results suggest that dysferlin regulates actin remodeling by a mechanism that does to not involve annexin A2.

The absence of dysferlin induces the expression of functional connexin-based hemichannels in human myotubes.

BACKGROUND: Mutations in the gene encoding for dysferlin cause recessive autosomal muscular dystrophies called dysferlinopathies. These mutations induce several alterations in skeletal muscles, including, inflammation, increased membrane permeability and cell death. Despite the fact that the etiology of dysferlinopathies is known, the mechanism that explains the aforementioned alterations is still elusive. Therefore, we have now evaluated the potential involvement of connexin based hemichannels in the pathophysiology of dysferlinopathies. RESULTS: Human deltoid muscle biopsies of 5 Chilean dysferlinopathy patients exhibited the presence of muscular connexins (Cx40.1, Cx43 and Cx45). The presence of these connexins was also observed in human myotubes derived from immortalized myoblasts derived from other patients with mutated forms of dysferlin. In addition to the aforementioned connexins, these myotubes expressed functional connexin based hemichannels, evaluated by ethidium uptake assays, as opposed to myotubes obtained from a normal human muscle cell line, RCMH. This response was reproduced in a knock-down model of dysferlin, by treating RCMH cell line with small hairpin RNA specific for dysferlin (RCMH-sh Dysferlin). Also, the presence of P2X7 receptor and the transient receptor potential channel, TRPV2, another Ca(2+) permeable channels, was detected in the myotubes expressing mutated dysferlin, and an elevated resting intracellular Ca(2+) level was found in the latter myotubes, which was in turn reduced to control levels in the presence of the molecule D4, a selective Cx HCs inhibitor. CONCLUSIONS: The data suggests that dysferlin deficiency, caused by mutation or downregulation of dysferlin, promotes the expression of Cx HCs. Then, the de novo expression Cx HC causes a dysregulation of intracellular free Ca(2+) levels, which could underlie muscular damage associated to dysferlin mutations. This mechanism could constitute a potential therapeutical target in dysferlinopathies.

Altered voltage dependent calcium currents in a neuronal cell line derived from the cerebral cortex of a trisomy 16 fetal mouse, an animal model of Down syndrome.

Human Down syndrome (DS) is determined by the trisomy of autosome 21 and is expressed by multiple abnormalities, being mental retardation the most striking feature. The condition results in altered electrical membrane properties (EMPs) of fetal neurons, which are qualitatively identical to those of trisomy 16 fetal mice (Ts16), an animal model of the human condition. Ts16 hippocampal cultured neurons reportedly exhibit increased voltage-dependent calcium currents (I (Ca)) amplitude. Since Ts16 animals are unviable, we have established immortalized cell lines from the cerebral cortex of Ts16 (named CTb) and normal littermates (named CNh). Using the whole-cell patch-clamp technique, we have now studied I (Ca) in CTb and CNh cells. Current activation occurs at -40 mV in both cell lines (V (holding) = -80 mV). Trisomic cells exhibited a 2.4 fold increase in the maximal Ca(2+) current density compared to normal cells (CNh = -6.3 ± 0.77 pA/pF, n = 18; CTb = -16.4 ± 2.423 pA/pF; P < 0.01, n = 13). Time dependent kinetics for activation and inactivation did not differ between the two cell types. However, steady state inactivation studies revealed a 15 mV shift toward more depolarized potentials in the trisomic condition, suggesting that altered voltage dependence of inactivation may underlie the increased current density. Further, the total charge movement across the membrane is increased in CTb cells, in agreement with that expected by the potential sensitivity shift. These results indicate that CTb cells present altered Ca(2+) currents, similar to those of Ts16 primary cultured central neurons. The CTb cell line represents a model for studying DS-related impairments of EMPs.

Apoptosis is directly related to intracellular amyloid accumulation in a cell line derived from the cerebral cortex of a trisomy 16 mouse, an animal model of Down syndrome.

Human Down syndrome (DS) represents the most frequent cause of mental retardation associated to a genetic condition. DS also exhibits a characteristic early onset of neuropathology indistinguishable from that observed in Alzheimer's disease (AD), namely the deposition of the beta-amyloid peptide. Early endosomal dysfunction has been described in individuals with DS and AD, suggesting an important role of this subcellular compartment in the onset and progression of the pathology. On the other hand, cholesterol activates the amyloidogenic processing pathway for the amyloid precursor protein, and the lipoprotein receptor-related peptide interacts with the beta-amyloid peptide. In the present work, using cell lines derived from the cortex of both normal and trisomy 16 mice (Ts16), an animal model of DS, we showed that the application of exogenous beta-amyloid has cytotoxic effects, expressed in decreased viability and increased apoptosis. Supplementation of the culture media with cholesterol associated to lipoprotein increased cell viability in both cell lines, but apoptosis decreased only in the normal cell line. Further, intracellular beta-amyloid content was elevated in trisomic cells following cholesterol treatment, with higher values in the trisomic cell line. Immunocytochemical detection showed intracellular accumulation of exogenous beta-amyloid in Rab4-positive compartments, which are known to be associated to endosomal recycling. The results suggest that the intracellular beta-amyloid pool plays a central role in apoptosis-mediated cell death in the trisomic condition.

Small-molecule aggregation inhibitors reduce excess amyloid in a trisomy 16 mouse cortical cell line.

We have previously characterized a number of small molecule organic compounds that prevent the aggregation of the beta-amyloid peptide and its neurotoxicity in hippocampal neuronal cultures. We have now evaluated the effects of such compounds on amyloid precursor protein (APP) accumulation in the CTb immortalized cell line derived from the cerebral cortex of a trisomy 16 mouse, an animal model of Down's syndrome. Compared to a non-trisomic cortical cell line (CNh), CTb cells overexpress APP and exhibit slightly elevated resting intracellular Ca2+ levels ([Ca2+] inverted exclamation mark). Here, we show that the compounds 2,4-dinitrophenol, 3-nitrophenol and 4-anisidine decreased intracellular accumulation of APP in CTb cells. Those compounds were non-toxic to the cells, and slightly increased the basal [Ca2+] inverted exclamation mark. Results indicate that the compounds tested can be leads for the development of drugs to decrease intracellular vesicular accumulation of APP in trisomic cells.

Small-molecule aggregation inhibitors reduce excess amyloid in a trisomy 16 mouse cortical cell line

We have previously characterized a number of small molecule organic compounds that prevent the aggregation of the β-amyloid peptide and its neurotoxicity in hippocampal neuronal cultures. We have now evaluated the effects of such compounds on amyloid precursor protein (APP) accumulation in the CTb immortalized cell line derived from the cerebral cortex of a trisomy 16 mouse, an animal model of Down's syndrome. Compared to a non-trisomic cortical cell line (CNh), CTb cells overexpress APP and exhibit slightly elevated resting intracellular Ca2+ levéis ([Ca2+]¡). Here, we show that the compounds 2,4-dinitrophenol, 3-nitrophenol and 4-anisidine decreased intracellular accumulation of APP in CTb cells. Those compounds were non-toxic to the cells, and slightly increased the basal [Ca2+]¡. Results indícate that the compounds tested can be leads for the development of drugs to decrease intracellular vesicular accumulation of APP in trisomic cells.

Endosomal abnormalities related to amyloid precursor protein in cholesterol treated cerebral cortex neuronal cells derived from trisomy 16 mice, an animal model of Down syndrome.

The CNh and CTb cell lines are derived from the cerebral cortex of normal and trisomy 16 mice, an animal model of human trisomy 21, Down syndrome (DS), and represent in vitro models to study cellular events associated with the human condition. Amyloid precursor protein (APP) plays an important role in the development of neuropathology associated with DS and cholesterol in the amyloidogenic processing of APP. There is also increasing evidence of alterations in the recycling pathway of the early endosome compartment in nervous tissue from DS. In the present study, we report endosomal abnormalities related to amyloid precursor protein in cholesterol-treated CTb cells. Colocalization studies revealed the presence of APP-derived products in early endosomal compartments in both cell lines. Using internalization and immunoprecipitation techniques, differential effects were observed between the normal and trisomic cell lines when treated with cholesterol. Internalization experiments showed that the CTb cell line accumulates internalized APP in intracellular compartments for longer periods of time when compared to the CNh cell line. Immunoprecipitation revealed a differential interaction between the trafficking-related protein Rab4 and APP in the neuronal cell lines CNh and CTb. The present study suggests a putative mechanism by which overexpressed APP accumulates in intracellular compartments related to the endosomal trafficking pathway in individuals with DS, and highlights the usefulness of the CTb cell line as a model to study altered APP metabolism related to this genetic condition.

Neuronal dysfunction in Down syndrome: contribution of neuronal models in cell culture.

Down syndrome (DS) in humans, or trisomy of autosome 21, represents the hyperdiploidy that most frequently survives gestation, reaching an incidence of 1 in 700 live births. The condition is associated with multisystemic anomalies, including those affecting the central nervous system (CNS), determining a characteristic mental retardation. At a neuronal level, our group and others have shown that the condition determines marked alterations of action potential and ionic current kinetics, which may underlie abnormal processing of information by the CNS. Since the use of human tissue presents both practical and ethical problems, animal models of the human condition have been sought. Murine trisomy 16 (Ts16) is a model of the human condition, due to the great homology between human autosome 21 and murine 16. Both conditions share the same alterations of electrical membrane properties. However, the murine Ts16 condition is unviable (animals die in utero), thus limiting the quantity of tissue procurable. To overcome this obstacle, we have established immortal cell lines from normal and Ts16 mice with a method developed by our group that allows the stable in vitro immortalization of mammalian tissue, yielding cell lines which retain the characteristics of the originating cells. Cell lines derived from cerebral cortex, hippocampus, spinal cord and dorsal root ganglion of Ts16 animals show alterations of intracellular Ca2+ signals in response to several neurotransmitters (glutamate, acetylcholine, and GABA). Gene overdose most likely underlies these alterations in cell function, and the identification of the relative contribution of DS associated genes on such specific neuronal dysfunction should be investigated. This could enlighten our understanding on the contribution of these genes in DS, and identify new therapeutic targets.

On the neurotoxicity mechanism of leukoaminochrome o-semiquinone radical derived from dopamine oxidation: mitochondria damage, necrosis, and hydroxyl radical formation.

Leukoaminochrome o-semiquinone radical is generated during one-electron reduction of dopamine oxidation product aminochrome when DT-diaphorase is inhibited. Incubation of 100 microM aminochrome with 100 microM dicoumarol, an inhibitor of DT-diaphorase during 2 h, induces 56% cell death (P < 0.001) with concomitant formation of (i) intracellular hydroperoxides (4.2-fold increase compared to control; P < 0.001); (ii) hydroxyl radicals, detected with ESR and spin trapping agents (2.4-fold increase when cells were incubated with aminochrome in the presence of dicoumarol compared to aminochrome alone); (iii) intracellular edema, and cell membrane deterioration determined by transmission electron microscopy; (iv) absence of apoptosis, supported by using anexin-V with flow cytometry; (v) a strong decrease of mitochondrial membrane potential determined by the fluorescent dye 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolylcarbocyanineiodide (P < 0.01); (vi) swelling and disruption of outer and inner mitochondrial membranes determined by transmission electron microscopy. These results support the proposed role of leukoaminochrome o-semiquinone radical as neurotoxin in Parkinson's disease neurodegeneration and DT-diaphorase as neuroprotective enzyme.

Behavioral effects of manganese injected in the rat substantia nigra are potentiated by dicumarol, a DT-diaphorase inhibitor.

The purpose of this study was to evaluate the contribution of DT-diaphorase inhibition to in vivo neurodegenerative effects of dopamine (DA) oxidation to the corresponding o-quinones. The neurotoxicity to nigrostriatal DA neurons was induced by injection of manganese pyrophosphate (Mn(3+)) complex as a prooxidizing agent alone or together with the DT-diaphorase inhibitor dicumarol into the right rat substantia nigra. The behavioral effects were compared with those induced after selective lesions of dopaminergic neurons with 6-hydroxydopamine (6-OHDA). Intranigral injection of Mn(3+) and Mn(3+) plus dicumarol produced significant impairment in motor behavior compared with control animals. However, the effect seen in the Mn(3+) plus dicumarol injected group was significantly more severe than that observed in the Mn(3+) alone injected group. In motor activity and rearing behavior, the simultaneous injection of Mn(3+) plus dicumarol produced a 6-OHDA-like impairment. Similar effects were observed in the acquisition of a conditioned avoidance response (CAR). Dicumarol significantly impaired avoidance conditioning although without affecting the motor behavior. The behavioral effects were correlated to the extent of striatal tyrosine hydroxylase (TH)-positive fiber loss. Rats receiving unilateral intranigral Mn(3+) and Mn(3+) plus dicumarol injections exhibited a significant reduction in nigrostriatal TH-positive fiber density in medial forebrain bundle compared with the contralateral noninjected side. In conclusion, this study provides evidence that the neurotoxicity of Mn(3+) in vivo is potentiated by DT-diaphorase inhibition, suggesting that this enzyme could play a neuroprotective role in the nigrostriatal DA systems.

Cell lines derived from hippocampal neurons of the normal and trisomy 16 mouse fetus (a model for Down syndrome) exhibit neuronal markers, cholinergic function, and functional neurotransmitter receptors.

We have established hippocampal cell lines from normal and trisomy 16 fetal mice, a model of human trisomy 21. Both cell lines, named H1b (derived from a normal animal) and HTk (trisomic) possess neuronal markers by immunohistochemistry (enolase, synaptophysin, microtubule associated protein-2, and choline acetyltransferase) and lack glial markers (glial fibrillary acidic protein and S-100). Also, we evaluated intracellular Ca(2+) levels ([Ca(2+)](i)) in response to neurotransmitter agonists, in cells loaded with the fluorescent Ca(2+) indicators Indo-1 and Fluo-3. Both cell lines responded to glutamatergic stimuli induced by glutamate, N-methyl-D-aspartate, I-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazole propanoic acid or kainate. Glutamate responses were only partially prevented by addition of 5 mM EGTA and the metabotropic glutamate receptor agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD), increased [Ca(2+)](i) in both cell types. These results confirm the presence of glutamatergic metabotropic receptors. In glutamate-induced responses, HTk cells exhibited slower time-dependent decay kinetics than H1b cells. Cholinergic agonists (nicotine and muscarine) induced a rapid, transient increase in [Ca(2+)](i) in both cell types. Furthermore, some cells were sensitive to histamine and norepinephrine. All responses to the aforementioned agonists were prevented by addition of specific antagonists. We also studied incorporation and release of [(3)H]choline in the cells, and observed no differences in uptake parameters. However, release induced by K(+) and nicotine depolarization was greatly reduced in HTk cells. The results show that H1b and HTk cells retain neuronal characteristics and respond to specific neurotransmitter stimuli. The HTk differences could be related to neuronal pathophysiology in Down syndrome.

Establishment and characterization of immortalized neuronal cell lines derived from the spinal cord of normal and trisomy 16 fetal mice, an animal model of Down syndrome.

We report the establishment of continuously growing cell lines from spinal cords of normal and trisomy 16 fetal mice. We show that both cell lines, named M4b (derived from a normal animal) and MTh (trisomic) possess neurological markers by immunohistochemistry (neuron specific enolase, synaptophysin, microtubule associated protein-2 [MAP-2], and choline acetyltransferase) and lack glial traits (glial fibrillary acidic protein and S100). MTh cells were shown to overexpress mRNA of Cu/Zn superoxide dismutase, whose gene is present in autosome 16. We also studied intracellular Ca2+ signals ([Ca2+]i) induced by different agonists in Indo-1 loaded cells. Basal [Ca2+]i was significantly higher in MTh cells compared to M4b cells. Glutamate (200 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACDP) (100 microM) induced rapid, transient increases in [Ca2+]i in M4b and MTh cells, indicating the presence of glutamatergic metabotropic receptors. N-methyl-D-aspartate (NMDA) and kainate, but not alpha-amino-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), produced [Ca2+)]i rises in both cell types. MTh cells exhibited faster time-dependent decay phase kinetics in glutamate-induced responses compared to M4b cells. Nicotine induced a transient increase in [Ca2+]i in M4b and MTh cells, with significantly greater amplitudes in the latter compared to the former. Further, both cell types responded to noradrenaline. Finally, we examined cholinergic function in both cell lines and found no significant differences in the [3H]-choline uptake, but fractional acetylcholine release induced by either K+, glutamate or nicotine was significantly higher in MTh cells. These results show that M4b and MTh cells have neuronal characteristics and the MTh line shows differences which could be related to neuronal pathophysiology in Down's syndrome.