Neuronal overexpression of Alzheimer's disease and Down's syndrome associated DYRK1A/minibrain gene alters motor decline, neurodegeneration and synaptic plasticity in Drosophila.

Down syndrome (DS) is characterised by abnormal cognitive and motor development, and later in life by progressive Alzheimer's disease (AD)-like dementia, neuropathology, declining motor function and shorter life expectancy. It is caused by trisomy of chromosome 21 (Hsa21), but how individual Hsa21 genes contribute to various aspects of the disorder is incompletely understood. Previous work has demonstrated a role for triplication of the Hsa21 gene DYRK1A in cognitive and motor deficits, as well as in altered neurogenesis and neurofibrillary degeneration in the DS brain, but its contribution to other DS phenotypes is unclear. Here we demonstrate that overexpression of minibrain (mnb), the Drosophila ortholog of DYRK1A, in the Drosophila nervous system accelerated age-dependent decline in motor performance and shortened lifespan. Overexpression of mnb in the eye was neurotoxic and overexpression in ellipsoid body neurons in the brain caused age-dependent neurodegeneration. At the larval neuromuscular junction, an established model for mammalian central glutamatergic synapses, neuronal mnb overexpression enhanced spontaneous vesicular transmitter release. It also slowed recovery from short-term depression of evoked transmitter release induced by high-frequency nerve stimulation and increased the number of boutons in one of the two glutamatergic motor neurons innervating the muscle. These results provide further insight into the roles of DYRK1A triplication in abnormal aging and synaptic dysfunction in DS.

Aberrant cerebellar Purkinje cell function repaired in vivo by fusion with infiltrating bone marrow-derived cells.

Bone marrow-derived cells are known to infiltrate the adult brain and fuse with cerebellar Purkinje cells. Histological observations that such heterotypic cell fusion events are substantially more frequent following cerebellar injury suggest they could have a role in the protection of mature brain neurons. To date, the possibility that cell fusion can preserve or restore the structure and function of adult brain neurons has not been directly addressed; indeed, though frequently suggested, the possibility of benefit has always been rather speculative. Here we report, for the first time, that fusion of a bone marrow-derived cell with a neuron in vivo, in the mature brain, results in the formation of a spontaneously firing neuron. Notably, we also provide evidence supporting the concept that heterotypic cell fusion acts as a biological mechanism to repair pathological changes in Purkinje cell structure and electrophysiology. We induced chronic central nervous system inflammation in chimeric mice expressing bone marrow cells tagged with enhanced green fluorescent protein. Subsequent in-depth histological analysis revealed significant Purkinje cell injury. In addition, there was an increased incidence of cell fusion between bone marrow-derived cells and Purkinje cells, revealed as enhanced green fluorescent protein-expressing binucleate heterokaryons. These fused cells resembled healthy Purkinje cells in their morphology, soma size, ability to synthesize the neurotransmitter gamma-aminobutyric acid, and synaptic innervation from neighbouring cells. Extracellular recording of spontaneous firing ex vivo revealed a shift in the predominant mode of firing of non-fused Purkinje cells in the context of cerebellar inflammation. By contrast, the firing patterns of fused Purkinje cells were the same as in healthy control cerebellum, indicating that fusion of bone marrow-derived cells with Purkinje cells mitigated the effects of cell injury on electrical activity. Together, our histological and electrophysiological results provide novel fundamental insights into physiological processes by which nerve cells are protected in adult life.

A third copy of the Down syndrome cell adhesion molecule (Dscam) causes synaptic and locomotor dysfunction in Drosophila.

Down syndrome (DS) is caused by triplication of chromosome 21 (HSA21). It is characterised by intellectual disability and impaired motor coordination that arise from changes in brain volume, structure and function. However, the contribution of each HSA21 gene to these various phenotypes and to the causal alterations in neuronal and synaptic structure and function are largely unknown. Here we have investigated the effect of overexpression of the HSA21 gene DSCAM (Down syndrome cell adhesion molecule), on glutamatergic synaptic transmission and motor coordination, using Drosophila expressing three copies of Dscam1. Electrophysiological recordings of miniature and evoked excitatory junction potentials at the glutamatergic neuromuscular junction of Drosophila larvae showed that the extra copy of Dscam1 changed the properties of spontaneous and electrically-evoked transmitter release and strengthened short-term synaptic depression during high-frequency firing of the motor nerve. Behavioural analyses uncovered impaired locomotor coordination despite preserved gross motor function. This work identifies DSCAM as a candidate causative gene in DS that is sufficient to modify synaptic transmission and synaptic plasticity and cause a DS behavioural phenotype.

Mutations in Membrin/GOSR2 Reveal Stringent Secretory Pathway Demands of Dendritic Growth and Synaptic Integrity.

Mutations in the Golgi SNARE (SNAP [soluble NSF attachment protein] receptor) protein Membrin (encoded by the GOSR2 gene) cause progressive myoclonus epilepsy (PME). Membrin is a ubiquitous and essential protein mediating ER-to-Golgi membrane fusion. Thus, it is unclear how mutations in Membrin result in a disorder restricted to the nervous system. Here, we use a multi-layered strategy to elucidate the consequences of Membrin mutations from protein to neuron. We show that the pathogenic mutations cause partial reductions in SNARE-mediated membrane fusion. Importantly, these alterations were sufficient to profoundly impair dendritic growth in Drosophila models of GOSR2-PME. Furthermore, we show that Membrin mutations cause fragmentation of the presynaptic cytoskeleton coupled with transsynaptic instability and hyperactive neurotransmission. Our study highlights how dendritic growth is vulnerable even to subtle secretory pathway deficits, uncovers a role for Membrin in synaptic function, and provides a comprehensive explanatory basis for genotype-phenotype relationships in GOSR2-PME.

Weaker control of the electrical properties of cerebellar granule cells by tonically active GABAA receptors in the Ts65Dn mouse model of Down's syndrome.

BACKGROUND: Down's syndrome (DS) is caused by triplication of all or part of human chromosome 21 and is characterized by a decrease in the overall size of the brain. One of the brain regions most affected is the cerebellum, in which the number of granule cells (GCs) is markedly decreased. GCs process sensory information entering the cerebellum via mossy fibres and pass it on to Purkinje cells and inhibitory interneurons. How GCs transform incoming signals depends on their input-output relationship, which is adjusted by tonically active GABA(A) receptor channels. RESULTS: We report that in the Ts65Dn mouse model of DS, in which cerebellar volume and GC number are decreased as in DS, the tonic GABA(A) receptor current in GCs is smaller than in wild-type mice and is less effective in moderating input resistance and raising the minimum current required for action potential firing. We also find that tonically active GABA(A) receptors curb the height and broaden the width of action potentials in wild-type GCs but not in Ts65Dn GCs. Single-cell real-time quantitative PCR reveals that these electrical differences are accompanied by decreased expression of the gene encoding the GABA(A) receptor ß3 subunit but not genes coding for some of the other GABA(A) receptor subunits expressed in GCs (α1, α6, ß2 and δ). CONCLUSIONS: Weaker moderation of excitability and action potential waveform in GCs of the Ts65Dn mouse by tonically active GABA(A) receptors is likely to contribute to atypical transfer of information through the cerebellum. Similar changes may occur in DS.

Increased excitability and altered action potential waveform in cerebellar granule neurons of the Ts65Dn mouse model of Down syndrome.

Down syndrome (DS) is characterized by intellectual disability and impaired motor control. Lack of coordinated movement, poor balance, and unclear speech imply dysfunction of the cerebellum, which is known to be reduced in volume in DS. The principal cause of the smaller cerebellum is a diminished number of granule cells (GCs). These neurons form the 'input layer' of the cerebellar cortex, where sensorimotor information carried by incoming mossy fibers is transformed before it is conveyed to Purkinje cells and inhibitory interneurons. However, it is not known how processing of this information is affected in the hypogranular cerebellum that characterizes DS. Here we explore the possibility that the electrical properties of the surviving GCs are changed. We find that in the Ts65Dn mouse model of DS, GCs have a higher input resistance at voltages approaching the threshold for firing, which causes them to be more excitable. In addition, they fire narrower and larger amplitude action potentials. These subtly modified electrical properties may result in atypical transfer of information at the input layer of the cerebellum.

Protease treatment of cerebellar purkinje cells renders omega-agatoxin IVA-sensitive Ca2+ channels insensitive to inhibition by omega-conotoxin GVIA.

The identification of currents carried by N- and P-type Ca(2+) channels in the nervous system relies on the use of omega-conotoxin (CTx) GVIA and omega-agatoxin (Aga) IVA. The peptide omega-Aga-IVA inhibits P-type currents at nanomolar concentrations and N-type currents at micromolar concentrations. omega-CTx-GVIA blocks N-type currents, but there have been no reports that it can also inhibit P-type currents. To assess the effects of omega-CTx-GVIA on P-type channels, we made patch-clamp recordings from the soma of Purkinje cells in cerebellar slices of mature [postnatal days (P) 40-50, P40-50] and immature (P13-20) rats, in which P-type channels carry most of the Ca(2+) channel current (>/=85%). These showed that micromolar concentrations of omega-CTx-GVIA inhibited the current in P40-50 cells (66%, 3 microM; 78%, 10 microM) and in P13-20 Purkinje cells (86%, 3 muM; 89%, 10 microM). The inhibition appeared to be reversible, in contrast to the known irreversible inhibition of N-type current. Exposure of slices from young animals to the enzyme commonly used to dissociate Purkinje cells, protease XXIII, abolished the inhibition by omega-CTx-GVIA but not by omega-Aga-IVA (84%, 30 nM). Our finding that micromolar concentrations of omega-CTx-GVIA inhibit P-type currents suggests that specific block of N-type current requires the use of submicromolar concentrations. The protease-induced removal of block by omega-CTx-GVIA but not by omega-Aga-IVA indicates a selective proteolytic action at site(s) on P-type channels with which omega-CTx-GVIA interacts. It also suggests that Ca(2+) channel pharmacology in neurons dissociated using protease may not predict that in neurons not exposed to the enzyme.

Maturation of rat cerebellar Purkinje cells reveals an atypical Ca2+ channel current that is inhibited by omega-agatoxin IVA and the dihydropyridine (-)-(S)-Bay K8644.

To determine if the properties of Ca2+ channels in cerebellar Purkinje cells change during postnatal development, we recorded Ca2+ channel currents from Purkinje cells in cerebellar slices of mature (postnatal days (P) 40-50) and immature (P13-20) rats. We found that at P40-50, the somatic Ca2+ channel current was inhibited by omega-agatoxin IVA at concentrations selective for P-type Ca2+ channels (approximately 85%; IC50, <1 nM) and by the dihydropyridine (-)-(S)-Bay K8644 (approximately 70%; IC50, approximately 40 nM). (-)-(S)-Bay K8644 is known to activate L-type Ca2+ channels, but the decrease in current was not secondary to the activation of L-type channels because inhibition by (-)-(S)-Bay K8644 persisted in the presence of the L-type channel blocker (R,S)-nimodipine. By contrast, at P13-20, the current was inhibited by omega-agatoxin IVA (approximately 86%; IC50, approximately 1 nM) and a minor component was inhibited by (R,S)-nimodipine (approximately 8%). The dihydropyridine (-)-(S)-Bay K8644 had no clear effect when applied alone, but in the presence of (R,S)-nimodipine it reduced the current (approximately 40%), suggesting that activation of L-type channels by (-)-(S)-Bay K8644 masks its inhibition of non-L-type channels. Our findings indicate that Purkinje neurons express a previously unrecognized type of Ca2+ channel that is inhibited by omega-agatoxin IVA, like prototypical P-type channels, and by (-)-(S)-Bay K8644, unlike classical P-type or L-type channels. During maturation, there is a decrease in the size of the L-type current and an increase in the size of the atypical Ca2+ channel current. These changes may contribute to the maturation of the electrical properties of Purkinje cells.

Alternative splicing generates a smaller assortment of CaV2.1 transcripts in cerebellar Purkinje cells than in the cerebellum.

P/Q-type calcium channels control many calcium-driven functions in the brain. The CACNA1A gene encoding the pore-forming CaV2.1 (alpha1A) subunit of P/Q-type channels undergoes alternative splicing at multiple loci. This results in channel variants with different phenotypes. However, the combinatorial patterns of alternative splice events at two or more loci, and hence the diversity of CaV2.1 transcripts, are incompletely defined for specific brain regions and types of brain neurons. Using RT-PCR and splice variant-specific primers, we have identified multiple CaV2.1 transcript variants defined by different pairs of splice events in the cerebellum of adult rat. We have uncovered new splice variations between exons 28 and 34 (some of which predict a premature stop codon) and a new variation in exon 47 (which predicts a novel extended COOH-terminus). Single cell RT-PCR reveals that each individual cerebellar Purkinje neuron also expresses multiple alternative CaV2.1 transcripts, but the assortment is smaller than in the cerebellum. Two of these variants encode different extended COOH-termini which are not the same as those previously reported in Purkinje cells of the mouse. Our patch-clamp recordings show that calcium channel currents in the soma and dendrites of Purkinje cells are largely inhibited by a concentration of omega-agatoxin IVA selective for P-type over Q-type channels, suggesting that the different transcripts may form phenotypic variants of P-type calcium channels in Purkinje cells. These results expand the known diversity of CaV2.1 transcripts in cerebellar Purkinje cells, and propose the selective expression of distinct assortments of CaV2.1 transcripts in different brain neurons and species.

The speeding of EPSC kinetics during maturation of a central synapse.

Several factors contribute to the shape of excitatory postsynaptic currents (EPSCs) in CNS neurons, among them the kinetics of presynaptic release, transmitter clearance, and the properties and distribution of postsynaptic receptors. The decays of AMPA receptor-mediated EPSCs at rat cerebellar mossy fibre-granule cell (MF-gc) synapses follow a bi-exponential time-course. The fast component dominates the decay, accounting for 84-94% of the peak amplitude. Here we show that both components of decay, and also the risetimes, became faster during postnatal maturation. At adult, but not immature, synapses, the risetimes and decays of evoked multiquantal EPSCs were similar to those of monoquantal miniature (m)EPSCs. The faster risetimes at mature synapses reflected increased synchrony of multivesicular release, whereas the faster decays appeared to reflect changes in the properties of postsynaptic receptors. Inhibition of glutamate uptake was without effect on evoked EPSCs at both ages. Furthermore, after slowing receptor desensitization with cyclothiazide, the EPSCs at mature synapses decayed as slowly as EPSCs at immature synapses, suggesting that faster glutamate clearance does not account for the developmental speeding of EPSC decay. Our results support previous conclusions that glutamate clearance and receptor deactivation are important determinants of the fast decay component at immature synapses. Desensitization becomes increasingly important during development and plays a major role in shaping EPSC decay at mature synapses.