A spinal muscular atrophy modifier implicates the SMN protein in SNARE complex assembly at neuromuscular synapses.

Reduced survival motor neuron (SMN) protein triggers the motor neuron disease, spinal muscular atrophy (SMA). Restoring SMN prevents disease, but it is not known how neuromuscular function is preserved. We used model mice to map and identify an Hspa8G470R synaptic chaperone variant, which suppressed SMA. Expression of the variant in the severely affected mutant mice increased lifespan >10-fold, improved motor performance, and mitigated neuromuscular pathology. Mechanistically, Hspa8G470R altered SMN2 splicing and simultaneously stimulated formation of a tripartite chaperone complex, critical for synaptic homeostasis, by augmenting its interaction with other complex members. Concomitantly, synaptic vesicular SNARE complex formation, which relies on chaperone activity for sustained neuromuscular synaptic transmission, was found perturbed in SMA mice and patient-derived motor neurons and was restored in modified mutants. Identification of the Hspa8G470R SMA modifier implicates SMN in SNARE complex assembly and casts new light on how deficiency of the ubiquitous protein causes motor neuron disease.

Increase in excitability of hippocampal neurons during novelty-induced hyperlocomotion in dopamine-deficient mice.

Dopamine is involved in many important brain functions, including voluntary motor movement. Dysfunction of the dopaminergic system can induce motor impairments, including Parkinson's disease. We previously found that dopamine-deficient (DD) mice became hyperactive in a novel environment 72 h after the last injection of L-3,4-dihydroxyphenylalanine (L-DOPA) when dopamine was almost completely depleted. In the present study, we investigated neuronal activity in hippocampal subregions during hyperactivity by measuring Fos expression levels using immunohistochemistry. Dopamine-deficient mice were maintained on daily intraperitoneal injections of 50 mg/kg L-DOPA. Seventy-two hours after the last L-DOPA injection, DD mice were exposed to a novel environment for 1, 2, or 4 h, and then brains were collected. In wildtype mice, the number of Fos-immunopositive neurons significantly increased in the hippocampal CA1 region after 1 h of exposure to the novel environment and then decreased. In DD mice, the number of Fos-immunopositive neurons gradually increased and then significantly increased after 4 h of exposure to the novel environment. The number of Fos-immunopositive neurons also significantly increased in the CA3 region and dentate gyrus in DD mice after 4 h of exposure to the novel environment. These results indicate that the delayed and prolonged excitation of hippocampal neurons in the CA1, CA3, and dentate gyrus that is caused by dopamine depletion might be involved in hyperactivity in DD mice.

Light/dark phase-dependent spontaneous activity is maintained in dopamine-deficient mice.

Dopamine is important for motor control and involved in the regulation of circadian rhythm. We previously found that dopamine-deficient (DD) mice became hyperactive in a novel environment 72 h after the last injection of L-3,4-dihydroxyphenylalanine (L-DOPA) when dopamine was almost completely depleted. DD mice did not initially exhibit hyperactivity in their home cages, but the animals exhibited hyperactivity several hours after the last L-DOPA injection. The regulation of motor activity in a novel environment and in home cages may be different. A previous study reported that DD mice became active again approximately 24 h after the last L-DOPA injection. One speculation was that light/dark phase-dependent spontaneous activity might be maintained despite dopamine deficiency. The present study investigated whether spontaneous home cage activity is maintained in DD mice 24-43 h and 72-91 h after the last L-DOPA injection. Spontaneous activity was almost completely suppressed during the light phase of the light/dark cycle in DD mice 24 and 72 h after the last L-DOPA injection. After the dark phase began, DD mice became active 24 and 72 h after the last L-DOPA injection. DD mice exhibited a similar amount of locomotor activity as wildtype mice 24 h after the last L-DOPA injection. Although DD mice presented a decrease in activity 72 h after the last L-DOPA injection, they maintained dark phase-stimulated locomotor activation. Despite low levels of dopamine in DD mice, they exhibited feeding behavior that was similar to wildtype mice. Although grooming and rearing behavior significantly decreased, DD mice retained their ability to perform these activities. Haloperidol treatment significantly suppressed all of these behaviors in wildtype mice but not in DD mice. These results indicate that DD mice maintain some aspects of light/dark phase-dependent spontaneous activity despite dopamine depletion, suggesting that compensatory dopamine-independent mechanisms might play a role in the DD mouse phenotype.

Hippocampal spine changes across the sleep-wake cycle: corticosterone and kinases.

The corticosterone (CORT) level changes along the circadian rhythm. Hippocampus is sensitive to CORT, since glucocorticoid receptors are highly expressed. In rat hippocampus fixed in a living state every 3 h, we found that the dendritic spine density of CA1 pyramidal neurons increased upon waking (within 3 h), as compared with the spine density in the sleep state. Particularly, the large-head spines increased. The observed change in the spine density may be due to the change in the hippocampal CORT level, since the CORT level at awake state (∼30 nM) in cerebrospinal fluid was higher than that at sleep state (∼3 nM), as observed from our earlier study. In adrenalectomized (ADX) rats, such a wake-induced increase of the spine density disappeared. S.c. administration of CORT into ADX rats rescued the decreased spine density. By using isolated hippocampal slices, we found that the application of 30 nM CORT increased the spine density within 1 h and that the spine increase was mediated via PKA, PKC, ERK MAPK, and LIMK signaling pathways. These findings suggest that the moderately rapid increase of the spine density on waking might mainly be caused by the CORT-driven kinase networks.