Asako Furuya

Spliceosome integrity is defective in the motor neuron diseases ALS and SMA

Two motor neuron diseases, amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), are caused by distinct genes involved in RNA metabolism, TDP‐43 and FUS/TLS, and SMN, respectively. However, whether there is a shared defective mechanism in RNA metabolism common to these two diseases remains unclear. Here, we show that TDP‐43 and FUS/TLS localize in nuclear Gems through an association with SMN, and that all three proteins function in spliceosome maintenance. We also show that in ALS, Gems are lost, U snRNA levels are up‐regulated and spliceosomal U snRNPs abnormally and extensively accumulate in motor neuron nuclei, but not in the temporal lobe of FTLD with TDP‐43 pathology. This aberrant accumulation of U snRNAs in ALS motor neurons is in direct contrast to SMA motor neurons, which show reduced amounts of U snRNAs, while both have defects in the spliceosome. These findings indicate that a profound loss of spliceosome integrity is a critical mechanism common to neurodegeneration in ALS and SMA, and may explain cell‐type specific vulnerability of motor neurons.

Calcium-dependent activator protein for secretion 2 (CAPS2) promotes BDNF secretion and is critical for the development of GABAergic interneuron network

Calcium-dependent activator protein for secretion 2 (CAPS2) is a dense-core vesicle-associated protein that is involved in the secretion of BDNF. BDNF has a pivotal role in neuronal survival and development, including the development of inhibitory neurons and their circuits. However, how CAPS2 affects BDNF secretion and its biological significance in inhibitory neurons are largely unknown. Here we reveal the role of CAPS2 in the regulated secretion of BDNF and show the effect of CAPS2 on the development of hippocampal GABAergic systems. We show that CAPS2 is colocalized with BDNF, both synaptically and extrasynaptically in axons of hippocampal neurons. Overexpression of exogenous CAPS2 in hippocampal neurons of CAPS2-KO mice enhanced depolarization-induced BDNF exocytosis events in terms of kinetics, frequency, and amplitude. We also show that in the CAPS2-KO hippocampus, BDNF secretion is reduced, and GABAergic systems are impaired, including a decreased number of GABAergic neurons and their synapses, a decreased number of synaptic vesicles in inhibitory synapses, and a reduced frequency and amplitude of miniature inhibitory postsynaptic currents. Conversely, excitatory neurons in the CAPS2-KO hippocampus were largely unaffected with respect to field excitatory postsynaptic potentials, miniature excitatory postsynaptic currents, and synapse number and morphology. Moreover, CAPS2-KO mice exhibited several GABA system-associated deficits, including reduced late-phase long-term potentiation at CA3–CA1 synapses, decreased hippocampal theta oscillation frequency, and increased anxiety-like behavior. Collectively, these results suggest that CAPS2 promotes activity-dependent BDNF secretion during the postnatal period that is critical for the development of hippocampal GABAergic networks.

Very-KIND, a KIND domain–containing RasGEF, controls dendrite growth by linking Ras small GTPases and MAP2

The regulation of cytoskeletal components in the dendritic shaft core is critical for dendrite elongation and branching. Here, we report that a brain-specific Ras guanine nucleotide exchange factor (RasGEF) carrying two kinase non-catalytic C-lobe domains (KINDs), very-KIND (v-KIND), regulates microtubule-associated protein 2 (MAP2). v-KIND is expressed in developing mouse brain, predominantly in the cerebellar granule cells. v-KIND not only activates Ras small GTPases via the C-terminal RasGEF domain, but also specifically binds to MAP2 via the second KIND domain (KIND2), leading to threonine phosphorylation of MAP2. v-KIND overexpression suppresses dendritic extension and branching of hippocampal neurons and cerebellar granule cells, whereas knockdown of endogenous v-KIND expression promotes dendrite growth. These findings suggest that v-KIND mediates a signaling pathway that links Ras and MAP2 to control dendrite growth.

Phase Advance of the Light-Dark Cycle Perturbs Diurnal Rhythms of Brain-derived Neurotrophic Factor and Neurotrophin-3 Protein Levels, Which Reduces Synaptophysin-positive Presynaptic Terminals in the Cortex of Juvenile Rats

In adult rat brains, brain-derived neurotrophic factor (BDNF) rhythmically oscillates according to the light-dark cycle and exhibits unique functions in particular brain regions. However, little is known of this subject in juvenile rats. Here, we examined diurnal variation in BDNF and neurotrophin-3 (NT-3) levels in 14-day-old rats. BDNF levels were high in the dark phase and low in the light phase in a majority of brain regions. In contrast, NT-3 levels demonstrated an inverse phase relationship that was limited to the cerebral neocortex, including the visual cortex, and was most prominent on postnatal day 14. An 8-h phase advance of the light-dark cycle and sleep deprivation induced an increase in BDNF levels and a decrease in NT-3 levels in the neocortex, and the former treatment reduced synaptophysin expression and the numbers of synaptophysin-positive presynaptic terminals in cortical layer IV and caused abnormal BDNF and NT-3 rhythms 1 week after treatment. A similar reduction of synaptophysin expression was observed in the cortices of Bdnf gene-deficient mice and Ca2+-dependent activator protein for secretion 2 gene-deficient mice with abnormal free-running rhythm and autistic-like phenotypes. In the latter mice, no diurnal variation in BDNF levels was observed. These results indicate that regular rhythms of BDNF and NT-3 are essential for correct cortical network formation in juvenile rodents.

Interaction between very‐KIND Ras guanine exchange factor and microtubule‐associated protein 2, and its role in dendrite growth – structure and function of the second kinase noncatalytic C‐lobe domain

The kinase noncatalytic C‐lobe domain (KIND) is a putative protein–protein interaction module. Four KIND‐containing proteins, Spir‐2 (actin‐nuclear factor), PTPN13 (protein tyrosine phosphatase), FRMPD2 (scaffold protein) and very‐KIND (v‐KIND) (brain‐specific Ras guanine nucleotide exchange factor), have been identified to date. Uniquely, v‐KIND has two KINDs (i.e. KIND1 and KIND2), whereas the other three proteins have only one. The functional role of KIND, however, remains unclear. We previously demonstrated that v‐KIND interacts with the high‐molecular weight microtubule‐associated protein 2 (MAP2), a dendritic microtubule‐associated protein, leading to negative regulation of neuronal dendrite growth. In the present study, we analyzed the structure–function relationships of the v‐KIND–MAP2 interaction by generating a series of mutant constructs. The interaction with endogenous MAP2 in mouse cerebellar granule cells was specific to v‐KIND KIND2, but not KIND1, and was not observed for the KINDs from other KIND‐containing proteins. The binding core modules critical for the v‐KIND–MAP2 interaction were defined within 32 residues of the mouse v‐KIND KIND2 and 43 residues of the mouse MAP2 central domain. Three Leu residues at amino acid positions 461, 474 and 477 in the MAP2‐binding core module of KIND2 contributed to the interaction. The MAP2‐binding core module itself promoted dendrite branching as a dominant‐negative regulator of v‐KIND in hippocampal neurons. The results reported in the present study demonstrate the structural and functional determinant underlying the v‐KIND–MAP2 interaction that controls dendrite arborization patterns.

TDP-43 accelerates age-dependent degeneration of interneurons

TDP-43 is an RNA-binding protein important for many aspects of RNA metabolism. Abnormal accumulation of TDP-43 in the cytoplasm of affected neurons is a pathological hallmark of the neurodegenerative diseases frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Several transgenic mouse models have been generated that recapitulate defects in TDP-43 accumulation, thus causing neurodegeneration and behavioural impairments. While aging is the key risk factor for neurodegenerative diseases, the specific effect of aging on phenotypes in TDP-43 transgenic mice has not been investigated. Here, we analyse age-dependent changes in TDP-43 transgenic mice that displayed impaired memory. We found the accumulation of abundant poly-ubiquitinated protein aggregates in the hippocampus of aged TDP-43 transgenic mice. Intriguingly, the aggregates contained some interneuron-specific proteins such as parvalbumin and calretinin, suggesting that GABAergic interneurons were degenerated in these mice. The abundance of aggregates significantly increased with age and with the overexpression of TDP-43. Gene array analyses in the hippocampus and other brain areas revealed dysregulation in genes linked to oxidative stress and neuronal function in TDP-43 transgenic mice. Our results indicate that the interneuron degeneration occurs upon aging, and TDP-43 accelerates age-dependent neuronal degeneration, which may be related to the impaired memory of TDP-43 transgenic mice.

The brain-specific RasGEF very-KIND is required for normal dendritic growth in cerebellar granule cells and proper motor coordination

Very-KIND/Kndc1/KIAA1768 (v-KIND) is a brain-specific Ras guanine nucleotide exchange factor carrying two sets of the kinase non-catalytic C-lobe domain (KIND), and is predominantly expressed in cerebellar granule cells. Here, we report the impact of v-KIND deficiency on dendritic and synaptic growth in cerebellar granule cells in v-KIND knockout (KO) mice. Furthermore, we evaluate motor function in these animals. The gross anatomy of the cerebellum, including the cerebellar lobules, layered cerebellar cortex and densely-packed granule cell layer, in KO mice appeared normal, and was similar to wild-type (WT) mice. However, KO mice displayed an overgrowth of cerebellar granule cell dendrites, compared with WT mice, resulting in an increased number of dendrites, dendritic branches and terminals. Immunoreactivity for vGluT2 (a marker for excitatory presynapses of mossy fiber terminals) was increased in the cerebellar glomeruli of KO mice, compared with WT mice. The postsynaptic density around the terminals of mossy fibers was also increased in KO mice. Although there were no significant differences in locomotor ability between KO and WT animals in their home cages or in the open field, young adult KO mice had an increased grip strength and a tendency to exhibit better motor performance in balance-related tests compared with WT animals. Taken together, our results suggest that v-KIND is required for compact dendritic growth and proper excitatory synaptic connections in cerebellar granule cells, which are necessary for normal motor coordination and balance.

Secretory vesicle-related gene CAPS2 knock-out mice exhibit the reduced number of hippocampal GABAergic interneuron and the impairments in synaptic plasticity and behavior

Long-term potentiation (LTP) and long-term depression (LTD) are considered as cellular models for memory. LTP/LTD are characterized by temporal phases (early and late phase). Early-LTP/LTD, lasting less than 2 h can be dissociated from lateLTP/LTD by blockers of protein synthesis. It was hypothesized that a synaptic tag enables capture of newly synthesized plasticity-related proteins. Previously, we found that neuropsin involved in formation of the synaptic tag in apical CA1 dendrites. Here, we show that in addition to apical CA1 dendrites, neuropsin-dependent synaptic tagging also takes place within basal CA1 dendritic compartments after LTP induction. Furthermore our findings demonstrate that neuropsin is involved in the LTP process-specific synaptic tag, whereas it is not involved in LTD tag.