Astrocytes are the major glial cell in the central nervous system. These polarized cells possess numerous processes that ensheath the vasculature and contact synapses. Astrocytes play important roles in synaptic signaling, neurotransmitter synthesis and recycling, control of nutrient uptake, and control of local blood flow. Many of these processes depend on local metabolism and/or energy utilization. While astrocytes respond to increases in neuronal activity and metabolic demand by upregulating glycolysis and glycogenolysis, astrocytes also possess significant capacity for oxidative (mitochondrial) metabolism. Mitochondria mediate energy supply and metabolism, cellular survival, ionic homeostasis, and proliferation. These organelles are dynamic structures undergoing extensive fission and fusion, directed movement along cytoskeletal tracts, and degradation. While many of the mechanisms underlying the dynamics of these organelles and their physiologic roles have been characterized in neurons and other cells, the roles that mitochondrial dynamics play in glial physiology is less well understood. Recent work from several laboratories has demonstrated that mitochondria are present within the fine processes of astrocytes, that their movement is regulated, and that they contribute to local Ca2+ signaling within the astrocyte. They likely play a role in local ATP production and metabolism, particularly that of glutamate. Here we will review these and other findings describing the mechanism by which mitochondrial dynamics are regulated in astrocytes, how mitochondrial dynamics might influence astrocyte and brain metabolism, and draw parallels to mitochondrial dynamics in neurons. Additionally, we present new analyses of the size, distribution, and dynamics of mitochondria in astrocytes performed using in vivo using 2-photon microscopy.

Regulation of mitochondrial dynamics in astrocytes: Mechanisms, consequences, and unknowns.

It has long been accepted that mitochondrial function and morphology is affected in Parkinson’s disease, and that mitochondrial function can be directly related to its morphology. So far, mitochondrial morphological alterations studies, in the context of this neurodegenerative disease, have been performed through microscopic methodologies. The goal of the present work is to address if the modifications in the mitochondrial-shaping proteins occurring in this disorder have implications in other cellular pathways, which might constitute important pathways for the disease progression. To do so, we conducted a novel approach through a thorough exploration of the available proteomics-based studies in the context of Parkinson’s disease. The analysis provided insight into the altered biological pathways affected by changes in the expression of mitochondrial-shaping proteins via different bioinformatic tools. Unexpectedly, we observed that the mitochondrial-shaping proteins altered in the context of Parkinson’s disease are, in the vast majority, related to the organization of the mitochondrial cristae. Conversely, in the studies that have resorted to microscopy-based techniques, the most widely reported alteration in the context of this disorder is mitochondria fragmentation. Cristae membrane organization is pivotal for mitochondrial ATP production, and changes in their morphology have a direct impact on the organization and function of the oxidative phosphorylation (OXPHOS) complexes. To understand which biological processes are affected by the alteration of these proteins we analyzed the binding partners of the mitochondrial-shaping proteins that were found altered in Parkinson’s disease. We showed that the binding partners fall into seven different cellular components, which include mitochondria, proteasome, and endoplasmic reticulum (ER), amongst others. It is noteworthy that, by evaluating the biological process in which these modified proteins are involved, we showed that they are related to the production and metabolism of ATP, immune response, cytoskeleton alteration, and oxidative stress, amongst others. In summary, with our bioinformatics approach using the data on the modified proteins in Parkinson’s disease patients, we were able to relate the alteration of mitochondrial-shaping proteins to modifications of crucial cellular pathways affected in this disease.

Biological Implications of Differential Expression of Mitochondrial-Shaping Proteins in Parkinson's Disease.

The expanded HTT CAG repeat causing Huntington's disease (HD) exhibits somatic expansion proposed to drive the rate of disease onset by eliciting a pathological process that ultimately claims vulnerable cells. To gain insight into somatic expansion in humans, we performed comprehensive quantitative analyses of CAG expansion in ~50 central nervous system (CNS) and peripheral postmortem tissues from seven adult-onset and one juvenile-onset HD individual. We also assessed ATXN1 CAG repeat expansion in brain regions of an individual with a neurologically and pathologically distinct repeat expansion disorder, spinocerebellar ataxia type 1 (SCA1). Our findings reveal similar profiles of tissue instability in all HD individuals, which, notably, were also apparent in the SCA1 individual. CAG expansion was observed in all tissues, but to different degrees, with multiple cortical regions and neostriatum tending to have the greatest instability in the CNS, and liver in the periphery. These patterns indicate different propensities for CAG expansion contributed by disease locus-independent trans-factors and demonstrate that expansion per se is not sufficient to cause cell type or disease-specific pathology. Rather, pathology may reflect distinct toxic processes triggered by different repeat lengths across cell types and diseases. We also find that the HTT CAG length-dependent expansion propensity of an individual is reflected in all tissues and in cerebrospinal fluid. Our data indicate that peripheral cells may be a useful source to measure CAG expansion in biomarker assays for therapeutic efforts, prompting efforts to dissect underlying mechanisms of expansion that may differ between the brain and periphery.

Patterns of CAG repeat instability in the central nervous system and periphery in Huntington’s disease and in spinocerebellar ataxia type 1

http://repositorium.sdum.uminho.pt/bitstream/1822/50603/1/untitled.pdf

Brain interference: Revisiting the role of IFNγ in the central nervous system.

Objective To assess brain morphometry in a sample of patients with juvenile-onset Huntington disease (JOHD) and several mouse models of Huntington disease (HD) that likely represent the human JOHD phenotype. Methods Despite sharing the mutation in the Huntingtin gene, adult-onset HD characteristically presents as a hyperkinetic motor disorder, while JOHD typically presents as a hypokinetic motor disease. The University of Iowa Kids-JHD program enrolls individuals 5 to 25 years of age who have already received the clinical diagnosis. A total of 19 children with juvenile HD (JHD) (mean CAG = 72) were studied. Patients with JHD were compared to healthy controls (n = 234) using a cross-sectional study design. Volumetric data from structural MRI was compared between groups. In addition, we used the same procedure to evaluate brain morphology of R6/2, zQ175, HdhQ250 HD mice models. Results Participants with JHD had substantially reduced intracranial volumes. After controlling for the small intracranial volume size, the volumes of subcortical regions (caudate, putamen, globus pallidus, and thalamus) and of cortical white matter were significantly decreased in patients with JHD. However, the cerebellum was proportionately enlarged in the JHD sample. The cerebral cortex was largely unaffected. Likewise, HD mice had a lower volume of striatum and a higher volume of cerebellum, mirroring the human MRI results. Conclusions The primary pathology of JOHD extends beyond changes in the striatal volume. Brain morphology in both mice and human patients with JHD shows proportional cerebellar enlargement. This pattern of brain changes may explain the unique picture of hypokinetic motor symptoms in JHD, which is not seen in the hyperkinetic chorea-like phenotype of adult-onset HD.

https://n.neurology.org/content/neurology/92/17/e1939.full.pdf

Brain structure in juvenile-onset Huntington disease.

Mitochondrial dynamics has recently become an area of piqued interest in neurodegenerative disorders, including Parkinson disease (PD); however, the contribution of astrocytes to these disorders remains unclear. Here, we show that the level of dynamin-like protein 1 (Dlp1; official name DNM1L ), which promotes mitochondrial fission, is lower in astrocytes from the brains of PD patients, and that decreased astrocytic Dlp1 likely represents a relatively early event in PD pathogenesis. In support of this conclusion, we show that Dlp1 knockdown dramatically affects mitochondrial morphological characteristics and localization in astrocytes, impairs the ability of astrocytes to adequately protect neurons from the excitotoxic effects of glutamate, and increases intracellular Ca 2+ in response to extracellular glutamate, resulting from compromised intracellular Ca 2+ buffering. Taken together, our results suggest that astrocytic mitochondrial Dlp1 is a key protein in mitochondrial dynamics and decreased Dlp1 may interfere with neuron survival in PD by disrupting Ca 2+ -coupled glutamate uptake.

Astrocytic Dynamin-Like Protein 1 Regulates Neuronal Protection against Excitotoxicity in Parkinson Disease

Background Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by a trinucleotide (CAG) repeat expansion in huntingtin (HTT) on chromosome 4. Anticipation can cause longer repeat expansions in children of HD patients. Juvenile Huntington's disease (JHD), defined as HD arising before age 20, accounts for 5-10% of HD cases, with cases arising in the first decade accounting for approximately 1%. Clinically, JHD differs from the predominately choreiform adult onset Huntington's disease (AOHD) with variable presentations, including symptoms such as myoclonus, seizures, Parkinsonism, and cognitive decline. Objective The neuropathologic changes of AOHD are well characterized, but there are fewer reports that describe the neuropathology of JHD. Here we report a case of a six-year-old boy with paternally-inherited JHD caused by 169 CAG trinucleotide repeats who presented at age four with developmental delay, dysarthria, and seizures before dying at age 6. The boy's clinical presentation and neuropathological findings are directly compared to those of his father, who presented with AOHD and 54 repeats. Methods A full autopsy was performed for the JHD case and a brain-only autopsy was performed for the AOHD case. Histochemically- and immunohistochemically-stained slides were prepared from formalin-fixed, paraffin-embedded tissue sections. Results Both cases had neuropathology corresponding to Vonsattel grade 3. The boy also had cerebellar atrophy with huntingtin-positive inclusions in the cerebellum, findings not present in the father. Conclusions Autopsies of father and son provide a unique opportunity to compare and contrast the neuropathologic findings of juvenile and adult onset HD while also providing the first immunohistochemical evidence of cerebellar involvement in JHD. Additionally this is the first known report to include findings from peripheral tissue in a case of JHD.

/pdf/neuropathological-comparison-of-adult-onset-and-juvenile-2muifsnt4v.pdf

Neuropathological Comparison of Adult Onset and Juvenile Huntington's Disease with Cerebellar Atrophy: A Report of a Father and Son.

A 45-year-old man of the Yakama Nation tribe without significant medical history had an abrupt onset headache that progressed over a week. He presented to an emergency department and a noncontrast head computerized tomography (CT) was unremarkable except for mild ethmoid sinus disease. One week later, the patient developed neck stiffness, fever to 101.6 degrees F, chills, and body aches. The patient’s partner reported episodes of confusion with auditory and visual hallucinations, seeing things on the wall, and thinking he was covered in chocolate.

After 2.5 weeks into the course of illness, the patient sought care from his primary care physician who referred him to a local emergency department. In the emergency department, the patient was afebrile though ill appearing with diaphoresis. He had fluent speech, normal comprehension, and intact memory and concentration. He was noted to have an unsteady tandem gait and positive Romberg sign. The rest of the neurologic examination was unremarkable.

He was admitted to the hospital where cerebrospinal fluid (CSF) analysis revealed a significant neutrophil-predominant pleocytosis with a low glucose (Table 1, column Day of Illness 17). An infectious etiology was suspected, and empiric treatment was initiated with intravenous administration of ceftriaxone, vancomycin, acyclovir, and dexamethasone. Extensive serum and CSF infectious and autoimmune workup was unrevealing (Table 2). Repeat imaging with a contrast-enhanced brain magnetic resonance imaging (MRI) about 3 weeks after symptom onset demonstrated hyperintensities in the deep gray and white matter as well as basal leptomeningeal enhancement (Figure 1). An atypical viral meningoencephalitis was presumed and antibiotics, antivirals, and steroids were discontinued.



Table 1.

Cerebrospinal Fluid Analysis.





Table 2.

Blood, Urine, and Cerebrospinal Fluid Testing.





Figure 1.

Magnetic resonance imaging (MRI) scans of the brain on day 27 of illness. A and B, Fluid attenuated inversion recovery (FLAIR) axial images of the lesions in the medial right temporal lobe (thin long arrow), right basal ganglia (short thick arrow), inferior ...



The hospital course was characterized by a progressive encephalopathy, and approximately 1 month after initial symptom onset, the patient began having episodes of unresponsiveness concerning for seizures and required intubation. In between the episodes, he was drowsy and followed simple commands.

His clinical progression and episodes of depressed consciousness prompted transfer to our institution for further evaluation. On arrival, repeat CSF analysis revealed a persistent neutrophilic pleocytosis and hypoglycorrhachia (Table 1, column Day of Illness 27). Intravenous acyclovir, ceftriaxone, vancomycin, and enteral doxycycline were initiated. As CSF cultures, antibody tests, and viral polymerase chain reactions (PCRs) returned negative, these treatments were again discontinued. Continuous electroencephalogram (EEG) monitoring over a 3-day period revealed diffuse slowing but no epileptiform activity. Contrast-enhanced CT of the chest, abdomen, and pelvis and testicular ultrasound were normal. Whole-body [18F]-fluorodeoxyglucose positron emission tomography (FDG-PET) scan from the base of skull to upper thighs revealed no evidence of primary malignancy or metastases. A nonlesional brain biopsy from the right frontal lobe demonstrated nonspecific findings of scattered rare perivascular lymphocytic infiltrates with no evidence of neoplasm or a specific infection (Figure 2).



Figure 2.

Nonlesional brain biopsy. A-D, Hematoxylin and eosin-stained sections show activated microglia (black arrowheads). B, Background neurons (white arrowheads) are identified. Scale bar in A = 250 µ; B-D = 100 µ.



Due to the persistent CSF pleocytosis, hypoglycorrhachia, and elevated protein, as well as brain imaging with basal leptomeningeal enhancement, antituberculous medications and dexamethasone were started and continued for an 8-week course. The patient had no known exposure to tuberculosis (TB). The CSF fungal cultures and broad-based PCR tests for fungal elements and TB were negative (Table 2).

Six weeks after the symptom onset, the patient progressed to a deep coma, unresponsive to both verbal and painful stimuli and with no purposeful movements. He also began exhibiting limb and orofacial choreoathetoid movements with dystonia. He had severe dysautonomia, including hyperthermia, tachycardia, and hypertension. Multiple medications were trialed—including benzodiazepines, propranolol, baclofen, trihexyphenidyl, and botulinum toxin injections to the masticatory muscles—which had little improvement.

A diagnostic test result was obtained.

/pdf/the-diagnostic-conundrum-and-treatment-dilemma-of-a-patient-3e5dpdna59.pdf

Zachary S. Hoffer

Papers

Astrocytic Dynamin-Like Protein 1 Regulates Neuronal Protection against Excitotoxicity in Parkinson Disease

Neuropathological Comparison of Adult Onset and Juvenile Huntington's Disease with Cerebellar Atrophy: A Report of a Father and Son.

The diagnostic conundrum and treatment dilemma of a patient with a rapidly progressive encephalopathy.