Precapillary sphincters maintain perfusion in the cerebral cortex.

Active nerve cells release vasodilators that increase their energy supply by dilating local blood vessels, a mechanism termed neurovascular coupling and the basis of BOLD functional neuroimaging signals. Here, we reveal a mechanism for cerebral blood flow control, a precapillary sphincter at the transition between the penetrating arteriole and first order capillary, linking blood flow in capillaries to the arteriolar inflow. The sphincters are encircled by contractile mural cells, which are capable of bidirectional control of the length and width of the enclosed vessel segment. The hemodynamic consequence is that precapillary sphincters can generate the largest changes in the cerebrovascular flow resistance of all brain vessel segments, thereby controlling capillary flow while protecting the downstream capillary bed and brain tissue from adverse pressure fluctuations. Cortical spreading depolarization constricts sphincters and causes vascular trapping of blood cells. Thus, precapillary sphincters are bottlenecks for brain capillary blood flow.

Spontaneous astrocytic Ca2+ activity abounds in electrically suppressed ischemic penumbra of aged mice.

Experimental focal cortical ischemic lesions consist of an ischemic core and a potentially salvageable peri-ischemic region, the ischemic penumbra. The activity of neurons and astrocytes is assumed to be suppressed in the penumbra because the electrical function is interrupted, but this is incompletely elucidated. Most experimental stroke studies used young adult animals, whereas stroke is prevalent in the elderly population. Using two-photon imaging in vivo, we here demonstrate extensive but electrically silent, spontaneous Ca2+ activity in neurons and astrocytes in the ischemic penumbra of 18- to 24-month-old mice 2-4 hr after middle cerebral artery occlusion. In comparison, stroke reduced spontaneous Ca2+ activity in neurons and astrocytes in adult mice (3-4 months of age). In aged mice, stroke increased astrocytic spontaneous Ca2+ activity considerably while neuronal spontaneous Ca2+ activity was unchanged. Blockade of action potentials and of purinergic receptors strongly reduced spontaneous Ca2+ activity in both neurons and astrocytes in the penumbra of old stroke mice. This indicates that stroke had a direct influence on mechanisms in presynaptic terminals and on purinergic signaling. Thus, highly dynamic variations in spontaneous Ca2+ activity characterize the electrically compromised penumbra, with remarkable differences between adult and old mice. The data are consistent with the notion that aged neurons and astrocytes take on a different phenotype than young mice. The increased activity of the aged astrocyte phenotype may be harmful to neurons. We suggest that the abundant spontaneous Ca2+ activity in astrocytes in the ischemic penumbra of old mice may be a novel target for neuroprotection strategies. A video abstract of this article can be found at https://youtu.be/AKlwKFsz1qE.

Altered sensory experience exacerbates stable dendritic spine and synapse loss in a mouse model of Huntington's disease.

A key question in Huntington's disease (HD) is what underlies the early cognitive deficits that precede the motor symptoms and the characteristic neuronal death observed in HD. The mechanisms underlying cognitive symptoms in HD remain unknown. Postmortem HD brain and animal model studies demonstrate pathologies in dendritic spines and abnormal synaptic plasticity before motor symptoms and neurodegeneration. Experience-dependent synaptic plasticity caused by mechanisms such as LTP or novel sensory experience potentiates synaptic strength, enhances new dendritic spine formation and stabilization, and may contribute to normal cognitive processes, such as learning and memory. We have previously reported that under baseline conditions (without any sensory manipulation) neuronal circuitry in HD (R6/2 mouse model) was highly unstable, which led to a progressive loss of persistent spines in these mice, and that mutant huntingtin was directly involved in the process. Here, we investigated whether pathological processes of HD interfere with the normal experience-dependent plasticity of dendritic spines in the R6/2 model. Six weeks of two-photon in vivo imaging before and after whisker trimming revealed that sensory deprivation exacerbates loss of persistent-type, stable spines in R6/2 mice compared with wild-type littermates. In addition, sensory deprivation leads to impaired transformation of newly generated spines into persistent spines in R6/2 mice. As a consequence, reduced synaptic density and decreased PSD-95 protein levels are evident in their barrel cortical neurons. These data suggest that mutant huntingtin is implicated in maladaptive synaptic plasticity, which could be one of the plausible mechanisms underlying early cognitive deficits in HD.

Dendritic spine instability leads to progressive neocortical spine loss in a mouse model of Huntington's disease.

In Huntington's disease (HD), cognitive symptoms and cellular dysfunction precede the onset of classical motor symptoms and neuronal death in the striatum and cortex by almost a decade. This suggests that the early cognitive deficits may be due to a cellular dysfunction rather than being a consequence of neuronal loss. Abnormalities in dendritic spines are described in HD patients and in HD animal models. Available evidence indicates that altered spine and synaptic plasticity could underlie the motor as well as cognitive symptoms in HD. However, the exact kinetics of spine alterations and plasticity in HD remain unknown. We used long-term two-photon imaging through a cranial window, to track individual dendritic spines in a mouse model of HD (R6/2) as the disease progressed. In vivo imaging over a period of 6 weeks revealed a steady decrease in the density and survival of dendritic spines on cortical neurons of R6/2 mice compared with control littermates. Interestingly, we also observed increased spine formation in R6/2 mice throughout the disease. However, the probability that newly formed spines stabilized and transformed into persistent spines was greatly reduced compared with controls. In cultured neurons we found that mutant huntingtin causes a loss, in particular of mature spines. Furthermore, in R6/2 mice, aggregates of mutant huntingtin associate with dendritic spines. Alterations in dendritic spine dynamics, survival, and density in R6/2 mice were evident before the onset of motor symptoms, suggesting that decreased stability of the cortical synaptic circuitry underlies the early symptoms in HD.

A novel α-synuclein-GFP mouse model displays progressive motor impairment, olfactory dysfunction and accumulation of α-synuclein-GFP.

Compelling evidence suggests that accumulation and aggregation of alpha-synuclein (α-syn) contribute to the pathogenesis of Parkinson's disease (PD). Here, we describe a novel Bacterial Artificial Chromosome (BAC) transgenic model, in which we have expressed wild-type human α-syn fused to green fluorescent protein (GFP), under control of the mouse α-syn promoter. We observed a widespread and high expression of α-syn-GFP in multiple brain regions, including the dopaminergic neurons of the substantia nigra pars compacta (SNpc) and the ventral tegmental area, the olfactory bulb as well as in neocortical neurons. With increasing age, transgenic mice exhibited reductions in amphetamine-induced locomotor activity in the open field, impaired rotarod performance and a reduced striatal dopamine release, as measured by amperometry. In addition, they progressively developed deficits in an odor discrimination test. Western blot analysis revealed that α-syn-GFP and phospho-α-syn levels increased in multiple brain regions, as the mice grew older. Further, we observed, by immunohistochemical staining for phospho-α-syn and in vivo by two-photon microscopy, the formation of α-syn aggregates as the mice aged. The latter illustrates that the model can be used to track α-syn aggregation in vivo. In summary, this novel BAC α-syn-GFP model mimics a unique set of aspects of PD progression combined with the possibility of tracking α-syn aggregation in neocortex of living mice. Therefore, this α-syn-GFP-mouse model can provide a powerful tool that will facilitate the study of α-syn biology and its involvement in PD pathogenesis.

Cellular distribution and subcellular localization of spatacsin and spastizin, two proteins involved in hereditary spastic paraplegia.

Truncating mutations in the SPG11 and SPG15 genes cause complicated spastic paraplegia, severe neurological conditions due to loss of the functions of spatacsin and spastizin, respectively. We developed specific polyclonal anti-spatacsin (SPG11) and anti-spastizin (SPG15) antisera, which we then used to explore the intracellular and tissue localizations of these proteins. We observed expression of both proteins in human and rat central nervous system, which was particularly strong in cortical and spinal motor neurons as well as in retina. Both proteins were also expressed ubiquitously and strongly in embryos. In cultured cells, these two proteins had similar diffuse punctate, cytoplasmic and sometimes nuclear (spastizin) distributions. They partially co-localized with multiple organelles, particularly with protein-trafficking vesicles, endoplasmic reticulum and microtubules. Spastizin was also found at the mitochondria surface. This first study of the endogenous expression of spatacsin and spastizin shows similarities in their expression patterns that could account for their overlapping clinical phenotypes and involvement in a common protein complex.

Refinement of dendritic and synaptic networks in the rodent anterior cingulate and orbitofrontal cortex: critical impact of early and late social experience.

The process of weaning programs the neurobehavioral development and therefore provides a critical formative period for adult behavior. However, the neural substrates underlying these behavioral changes are largely unknown. To test the hypothesis that during childhood neuronal networks in the prefrontal cortex are reorganized in response to the timing and extent of social interactions, we analyzed the length, ramification, and spine density of apical and basal dendrites of layer II/III pyramidal neurons in four groups of male rats. (1) Early weaning at postnatal day (PND) 21 + postweaning social rearing (EWS), (2) late weaning at PND 30 + postweaning social rearing (LWS), (3) early weaning + postweaning social isolation (EWI), (4) late weaning + postweaning social isolation (LWI). Compared with late weaned animals, the early weaned animals displayed elevated spine densities on apical and basal dendrites only in the anterior cingulate (ACd), but not in the orbitofrontal cortex (OFC), irrespective of the postweaning housing conditions. For dendritic length and complexity an interaction between the factors weaning and postweaning rearing conditions was observed. In the ACd the EWI animals had longer and more complex apical dendrites compared with all other groups, whereas in the OFC the EWI animals displayed a significant reduction of apical dendritic length and complexity compared with the EWS group. Taken together, our findings show that the timing as well as the amount of social contact with family members significantly affects the refinement of prefrontal cortical synaptic networks, which are essential for emotional and cognitive behavior.