Failure to thrive in Alexander disease - Alexander disease (AxD) is a severe neurodegenerative disorder caused by gain-of-function mutations in the gene for glial fibrillary acidic protein (GFAP) which lead to protein aggregation and a primary astrocytopathy. Symptoms vary, but failure to thrive (FTT) and frequent emesis are common and cause significant morbidity. We have developed a Gfap+/R237H rat model (R237H) in which pups fail to gain weight after weaning and become gaunt and frail as they mature. Adult R237H rats have virtually no body fat, reduced muscle mass and strength, and exhibit pica (indicative of nausea) and anorexic behavior. Growth and differentiation factor 15 (GDF15) is a member of the TGFβ superfamily that regulates energy balance and appetite and is elevated as part of the integrated stress response (ISR). GDF15 mediates reduced appetite via nausea and aversion conditioning, and the receptor for GDF15, GFRAL (GDNF family receptor alpha like), is specifically expressed by cholecystokinin neurons in the area postrema of the brainstem, a region known as the chemoreceptor trigger zone for vomiting. Our preliminary data show that astrocytes in AxD exhibit an ISR transcriptomic signature and that GDF15 is expressed by a subset of astrocytes in the brainstem of R237H rats. GDF15 is also markedly elevated in CSF in both the rat model and patients with AxD. We hypothesize that increased GDF15, promoted by the astrocyte stress response, induces nausea and anorexic behavior by directly stimulating GFRAL positive neurons in the AP/NTS, and ultimately failure to thrive in AxD. In Aim 1, to determine whether GDF15 correlates with FTT phenotypes, we will 1a) measure GDF15 in brainstem and CSF in animals treated with Gfap-targeting antisense oligonucleotides to reduce GFAP pathology and compare GDF15 expression with GFRAL neuron activation and FTT phenotypes. To determine whether GDF15 correlates with emesis and FTT phenotypes in the human disease, we will 1b) measure GDF15 in CSF and plasma from participants in an ongoing natural history study of AxD to directly compare with gastrointestinal symptoms, growth parameters, and CNS lesion locations on MRI. In Aim 2, we will 2a) determine whether the ISR drives GDF15 expression and GFRAL neuron activation by manipulating the ISR pharmacologically in R237H rats and assessing effects on GDF15, GFRAL neuron activation, and FTT phenotypes, and we will 2b,c) confirm whether the ISR is associated with GDF15 elevation on the cellular level and determine the effects on adjacent cells using single nucleus and spatial transcriptomics. Finally, in Aim 3, we will determine whether GDF15 is the major culprit in FTT by 3a) blocking the GFRAL receptor with specific antibodies and 3b) knocking down GDF15 expression in brainstem to assess paracrine effects. Current treatments for FTT in AxD (nutritional supplementation and gastrostomy tubes) are ineffective and invasive, and a better understanding of the underlying mechanisms may offer new options for prevention. In addition, determining the role of the astrocyte ISR may have broader implications for AxD and other proteinopa- thies and introduce new biomarkers for study.
