Sacsin Deletion Induces Aggregation of Glial Intermediate Filaments

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a neurodegenerative disorder commonly diagnosed in infants and characterized by progressive cerebellar ataxia, spasticity, motor sensory neuropathy and axonal demyelination. ARSACS is caused by mutations in the SACS gene that lead to truncated or defective forms of the 520 kDa multidomain protein, sacsin. Sacsin function is exclusively studied on neuronal cells, where it regulates mitochondrial network organization and facilitates the normal polymerization of neuronal intermediate filaments (i.e., neurofilaments and vimentin). Here, we show that sacsin is also highly expressed in astrocytes, C6 rat glioma cells and N9 mouse microglia. Sacsin knockout in C6 cells (C6Sacs−/−) induced the accumulation of the glial intermediate filaments glial fibrillary acidic protein (GFAP), nestin and vimentin in the juxtanuclear area, and a concomitant depletion of mitochondria. C6Sacs−/− cells showed impaired responses to oxidative challenges (Rotenone) and inflammatory stimuli (Interleukin-6). GFAP aggregation is also associated with other neurodegenerative conditions diagnosed in infants, such as Alexander disease or Giant Axonal Neuropathy. Our results, and the similarities between these disorders, reinforce the possible connection between ARSACS and intermediate filament-associated diseases and point to a potential role of glia in ARSACS pathology.

Abnormal Ca2+ Signals in Reactive Astrocytes as a Common Cause of Brain Diseases

In pathological brain conditions, glial cells become reactive and show a variety of responses. We examined Ca2+ signals in pathological brains and found that reactive astrocytes share abnormal Ca2+ signals, even in different types of diseases. In a neuropathic pain model, astrocytes in the primary sensory cortex became reactive and showed frequent Ca2+ signals, resulting in the production of synaptogenic molecules, which led to misconnections of tactile and pain networks in the sensory cortex, thus causing neuropathic pain. In an epileptogenic model, hippocampal astrocytes also became reactive and showed frequent Ca2+ signals. In an Alexander disease (AxD) model, hGFAP-R239H knock-in mice showed accumulation of Rosenthal fibers, a typical pathological marker of AxD, and excessively large Ca2+ signals. Because the abnormal astrocytic Ca2+ signals observed in the above three disease models are dependent on type II inositol 1,4,5-trisphosphate receptors (IP3RII), we reanalyzed these pathological events using IP3RII-deficient mice and found that all abnormal Ca2+ signals and pathologies were markedly reduced. These findings indicate that abnormal Ca2+ signaling is not only a consequence but may also be greatly involved in the cause of these diseases. Abnormal Ca2+ signals in reactive astrocytes may represent an underlying pathology common to multiple diseases.

Parental Somatic Mosaicism Uncovers Inheritance of an Apparently De Novo GFAP Mutation

Alexander disease is a leukodystrophy caused by heterozygous mutations of GFAP gene. Recurrence in siblings from healthy parents provides a confirmation to the transmission of variants through germinal mosaicism. With the use of DNA isolated from peripheral blood, next-generation sequencing (NGS) of GFAP locus was performed with deep coverage (≥500×) in 11 probands and their parents (trios) with probands heterozygous for apparently de novo GFAP mutations. Indeed, one parent had somatic mosaicism, estimated in the range of 8.9%–16%, for the mutant allele transmitted to the affected sibling. Parental germline mosaicism deserves attention, as it is critical in assessing the risk of recurrence in families with Alexander disease.