Cellular and circuit mechanisms underlying spinocerebellar ataxias

Degenerative ataxias are a common form of neurodegenerative disease that affect about 20 individuals per 100,000. The autosomal dominant spinocerebellar ataxias (SCAs) are caused by a variety of protein coding mutations (single nucleotide changes, deletions and expansions) in single genes. Affected genes encode plasma membrane and intracellular ion channels, membrane receptors, protein kinases, protein phosphatases and proteins of unknown function. Although SCA-linked genes are quite diverse they share two key features: first, they are highly, although not exclusively, expressed in cerebellar Purkinje neurons (PNs), and second, when mutated they lead ultimately to the degeneration of PNs. In this review we summarize ataxia-related changes in PN neurophysiology that have been observed in various mouse knockout lines and in transgenic models of human SCA. We also highlight emerging evidence that altered metabotropic glutamate receptor signalling and disrupted calcium homeostasis in PNs form a common, early pathophysiological mechanism in SCAs. Together these findings indicate that aberrant calcium signalling and profound changes in PN neurophysiology precede PN cell loss and are likely to lead to cerebellar circuit dysfunction that explains behavioural signs of ataxia characteristic of the disease

Receptor protein tyrosine phosphatases control Purkinje neuron firing

Spinocerebellar ataxias (SCA) are a genetically heterogeneous family of cerebellar neurodegenerative diseases characterized by abnormal firing of Purkinje neurons and degeneration. We recently demonstrated the slowed firing rates seen in several SCAs share a common etiology of hyper-activation of the Src family of non-receptor tyrosine kinases (SFKs). However, the lack of clinically available neuroactive SFK inhibitors lead us to investigate alternative mechanisms to modulate SFK activity. Previous studies demonstrate that SFK activity can be enhanced by the removal of inhibitory phospho-marks by receptor-protein-tyrosine phosphatases (RPTPs). In this Extra View we show that MTSS1 inhibits SFK activity through the binding and inhibition of a subset of the RPTP family members, and lowering RPTP activity in cerebellar slices with peptide inhibitors increases the suppressed Purkinje neuron basal firing rates seen in two different SCA models. Together these results identify RPTPs as novel effectors of Purkinje neuron basal firing, extending the MTSS1/SFK regulatory circuit we previously described and expanding the therapeutic targets for SCA patients

Gene co-expression network analysis for identifying modules and functionally enriched pathways in SCA2

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant neurodegenerative disease caused by CAG repeat expansion in the ATXN2 gene. The repeat resides in an encoded region of the gene resulting in polyglutamine (polyQ) expansion which has been assumed to result in gain of function, predominantly, for the ATXN2 protein. We evaluated temporal cerebellar expression profiles by RNA sequencing of ATXN2Q127 mice versus wild-type (WT) littermates. ATXN2Q127 mice are characterized by a progressive motor phenotype onset, and have progressive cerebellar molecular and neurophysiological (Purkinje cell firing frequency) phenotypes. Our analysis revealed previously uncharacterized early and progressive abnormal patterning of cerebellar gene expression. Weighted Gene Coexpression Network Analysis revealed four gene modules that were significantly correlated with disease status, composed primarily of genes associated with GTPase signaling, calcium signaling and cell death. Of these genes, few overlapped with differentially expressed cerebellar genes that we identified in Atxn2−/− knockout mice versus WT littermates, suggesting that loss-of-function is not a significant component of disease pathology. We conclude that SCA2 is a disease characterized by gain of function for ATXN2

Antisense oligonucleotide therapy for spinocerebellar ataxia type 2

There are no disease-modifying treatments for adult human neurodegenerative diseases. Here we test RNA-targeted therapies1 in two mouse models of spinocerebellar ataxia type 2 (SCA2), an autosomal dominant polyglutamine disease2. Both models recreate the progressive adult-onset dysfunction and degeneration of a neuronal network that are seen in patients, including decreased firing frequency of cerebellar Purkinje cells and a decline in motor function3,4. We developed a potential therapy directed at the ATXN2 gene by screening 152 antisense oligonucleotides (ASOs). The most promising oligonucleotide, ASO7, downregulated ATXN2 mRNA and protein, which resulted in delayed onset of the SCA2 phenotype. After delivery by intracerebroventricular injection to ATXN2-Q127 mice, ASO7 localized to Purkinje cells, reduced cerebellar ATXN2 expression below 75% for more than 10 weeks without microglial activation, and reduced the levels of cerebellar ATXN2. Treatment of symptomatic mice with ASO7 improved motor function compared to saline-treated mice. ASO7 had a similar effect in the BAC-Q72 SCA2 mouse model, and in both mouse models it normalized protein levels of several SCA2-related proteins expressed in Purkinje cells, including Rgs8, Pcp2, Pcp4, Homer3, Cep76 and Fam107b. Notably, the firing frequency of Purkinje cells returned to normal even when treatment was initiated more than 12 weeks after the onset of the motor phenotype in BAC-Q72 mice. These findings support ASOs as a promising approach for treating some human neurodegenerative diseases

Decreased surface expression of the 8 subunit of the GABA(A) receptor contributes to reduced tonic inhibition in dentate granule cells in a mouse model of fragile X syndrome

While numerous changes in the GABA system have been identified in models of Fragile X Syndrome (FXS), alterations in subunits of the GABAA receptors (GABAARs) that mediate tonic inhibition are particularly intriguing. Considering the key role of tonic inhibition in controlling neuronal excitability, reduced tonic inhibition could contribute to FXS-associated disorders such as hyperactivity, hypersensitivity, and increased seizure susceptibility. The current study has focused on the expression and function of the δ subunit of the GABAAR, a major subunit involved in tonic inhibition, in granule cells of the dentate gyrus in the Fmr1 knockout (KO) mouse model of FXS. Electrophysiological studies of dentate granule cells revealed a marked, nearly four-fold, decrease in tonic inhibition in the Fmr1 KO mice, as well as reduced effects of two δ subunit-preferring pharmacological agents, THIP and DS2, supporting the suggestion that δ subunit-containing GABAARs are compromised in the Fmr1 KO mice. Immunohistochemistry demonstrated a small but statistically significant decrease in δ subunit labeling in the molecular layer of the dentate gyrus in Fmr1 KO mice compared to wildtype (WT) littermates. The discrepancy between the large deficits in GABA-mediated tonic inhibition in granule cells in the Fmr1 KO mice and only modest reductions in immunolabeling of the δ subunit led to studies of surface expression of the δ subunit. Cross-linking experiments followed by Western blot analysis demonstrated a small, non-significant decrease in total δ subunit protein in the hippocampus of Fmr1 KO mice, but a four-fold decrease in surface expression of the δ subunit in these mice. No significant changes were observed in total or surface expression of the α4 subunit protein, a major partner of the δ subunit in the forebrain. Postembedding immunogold labeling for the δ subunit demonstrated a large, three-fold, decrease in the number of symmetric synapses with immunolabeling at perisynaptic locations in Fmr1 KO mice. While α4 immunogold particles were also reduced at perisynaptic locations in the Fmr1 KO mice, the labeling was increased at synaptic sites. Together these findings suggest that, in the dentate gyrus, altered surface expression of the δ subunit, rather than a decrease in δ subunit expression alone, could be limiting δ subunit-mediated tonic inhibition in this model of FXS. Finding ways to increase surface expression of the δ subunit of the GABAAR could be a novel approach to treatment of hyperexcitability-related alterations in FXS

Synaptically evoked glutamate transporter currents in Spinal Dorsal Horn Astrocytes

<p>Abstract</p> <p>Background</p> <p>Removing and sequestering synaptically released glutamate from the extracellular space is carried out by specific plasma membrane transporters that are primarily located in astrocytes. Glial glutamate transporter function can be monitored by recording the currents that are produced by co-transportation of Na⁺ions with the uptake of glutamate. The goal of this study was to characterize glutamate transporter function in astrocytes of the spinal cord dorsal horn in real time by recording synaptically evoked glutamate transporter currents.</p> <p>Results</p> <p>Whole-cell patch clamp recordings were obtained from astrocytes in the spinal substantia gelatinosa (SG) area in spinal slices of young adult rats. Glutamate transporter currents were evoked in these cells by electrical stimulation at the spinal dorsal root entry zone in the presence of bicuculline, strychnine, DNQX and D-AP5. Transporter currents were abolished when synaptic transmission was blocked by TTX or Cd²⁺. Pharmacological studies identified two subtypes of glutamate transporters in spinal astrocytes, GLAST and GLT-1. Glutamate transporter currents were graded with stimulus intensity, reaching peak responses at 4 to 5 times activation threshold, but were reduced following low-frequency (0.1 – 1 Hz) repetitive stimulation.</p> <p>Conclusion</p> <p>These results suggest that glutamate transporters of spinal astrocytes could be activated by synaptic activation, and recording glutamate transporter currents may provide a means of examining the real time physiological responses of glial cells in spinal sensory processing, sensitization, hyperalgesia and chronic pain.</p

Progress in Understanding and Treating SCN2A-Mediated Disorders

Advances in gene discovery for neurodevelopmental disorders have identified SCN2A dysfunction as a leading cause of infantile seizures, autism spectrum disorder, and intellectual disability. SCN2A encodes the neuronal sodium channel NaV1.2. Functional assays demonstrate strong correlation between genotype and phenotype. This insight can help guide therapeutic decisions and raises the possibility that ligands that selectively enhance or diminish channel function may improve symptoms. The well-defined function of sodium channels makes SCN2A an important test case for investigating the neurobiology of neurodevelopmental disorders more generally. Here, we discuss the progress made, through the concerted efforts of a diverse group of academic and industry scientists as well as policy advocates, in understanding and treating SCN2A-related disorders

Prefrontal cortex output circuits guide reward seeking through divergent cue encoding

The prefrontal cortex is a critical neuroanatomical hub for controlling motivated behaviours across mammalian species. In addition to intra-cortical connectivity, prefrontal projection neurons innervate subcortical structures that contribute to reward-seeking behaviours, such as the ventral striatum and midline thalamus. While connectivity among these structures contributes to appetitive behaviours, how projection-specific prefrontal neurons encode reward-relevant information to guide reward seeking is unknown. Here we use in vivo two-photon calcium imaging to monitor the activity of dorsomedial prefrontal neurons in mice during an appetitive Pavlovian conditioning task. At the population level, these neurons display diverse activity patterns during the presentation of reward-predictive cues. However, recordings from prefrontal neurons with resolved projection targets reveal that individual corticostriatal neurons show response tuning to reward-predictive cues, such that excitatory cue responses are amplified across learning. By contrast, corticothalamic neurons gradually develop new, primarily inhibitory responses to reward-predictive cues across learning. Furthermore, bidirectional optogenetic manipulation of these neurons reveals that stimulation of corticostriatal neurons promotes conditioned reward-seeking behaviour after learning, while activity in corticothalamic neurons suppresses both the acquisition and expression of conditioned reward seeking. These data show how prefrontal circuitry can dynamically control reward-seeking behaviour through the opposing activities of projection-specific cell populations

A migraine variant with abdominal colic and Alice in wonderland syndrome: a case report and review

<p>Abstract</p> <p>Background</p> <p>Abdominal migraine is a commonly described migraine variant in children and young adults, but associations with Alice in Wonderland syndrome and lilliputian hallucinations are exceptional.</p> <p>Case presentation</p> <p>A 20 years-old male experienced frequent and prolonged attacks of abdominal colic associated with autonomic manifestations started at the age of ten. At the age of 17, he additionally described prolonged attacks (≥ 7 days) of distortions of shape, size or position of objects or subjects. He said "Quite suddenly, objects appear small and distant (teliopsia) or large and close (peliopsia). I feel as I am getting shorter and smaller "shrinking" and also the size of persons are not longer than my index finger (a lilliputian proportion). Sometimes I see the blind in the window or the television getting up and down, or my leg or arm is swinging. I may hear the voices of people quite loud and close or faint and far. Occasionally, I experience attacks of migrainous headache associated with eye redness, flashes of lights and a feeling of giddiness. I am always conscious to the intangible changes in myself and my environment". There is a strong family history of common migraine. Clinical examination, brain-MRI and EEG were normal. Transcranial magnetic stimulation and evoked potentials revealed enhanced cortical excitability in multiple brain regions. Treatment with valproate resulted in marked improvement of all clinical and neurophysiological abnormalities.</p> <p>Conclusions</p> <p>The association between the two migraine variants (abdominal migraine and Alice in Wonderland Syndrome) might have clinical, pathophysiological and management implications. I think this is the first description in the literature.</p

Acute Cellular Alterations in the Hippocampus After Status Epilepticus

The critical, fundamental mechanisms that determine the emergence of status epilepticus from a single seizure and the prolonged duration of status epilepticus are uncertain. However, several general concepts of the pathophysiology of status epilepticus have emerged: (a) the hippocampus is consistently activated during status epilepticus; (b) loss of GABA-mediated inhibitory synaptic transmission in the hippocampus is critical for emergence of status epilepticus; and, finally (c) glutamatergic excitatory synaptic transmission is important in sustaining status epilepticus. This review focuses on the alteration of GABAergic inhibition in the hippocampus that occurs during the prolonged seizures of status epilepticus. If reduction in GABAergic inhibition leads to development of status epilepticus, enhancement of GABAergic inhibition would be expected to interrupt status epilepticus. Benzodiazepines and barbiturates are both used in the treatment of status epilepticus and both drugs enhance GABA A receptor-mediated inhibition. However, patients often become refractory to benzodiazepines when seizures are prolonged, and barbiturates are often then used for these refractory cases of status epilepticus. Recent evidence suggests the presence of multiple GABA A receptor isoforms in the hippocampus with different sensitivity to benzodiazepines but similar sensitivity to barbiturates, thus explaining why the two drug classes might have different clinical effects. In addition, rapid functional plasticity of GABA A receptors has been demonstrated to occur during status epilepticus in rats. During status epilepticus, there was a substantial reduction of diazepam potency for termination of the seizures. The loss of sensitivity of the animals to diazepam during status epilepticus was accompanied by an alteration in the functional properties of hippocampal dentate granule cell GABA A receptors. Dentate granule cell GABA A receptor currents from rats undergoing status epilepticus had reduced sensitivity to diazepam and zinc but normal sensitivity to GABA and pentobarbital. Therefore, the prolonged seizures of status epilepticus rapidly altered the functional properties of hippocampal dentate granule cell GABA A receptors, possibly explaining why benzodiazepines and barbiturates may not be equally effective during treatment of the prolonged seizures of status epilepticus. A comprehensive understanding of the cellular and molecular events leading to the development, maintenance, and cytotoxicity of status epilepticus should permit development of more effective treatment strategies and reduction in the mortality and morbidity of status epilepticus.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65664/1/j.1528-1157.1999.tb00873.x.pd