Regulation of extracellular progranulin in medial prefrontal cortex.

Progranulin is a secreted pro-protein that has anti-inflammatory and neurotrophic effects and is necessary for maintaining lysosomal function. Mutations in progranulin (GRN) are a major cause of frontotemporal dementia. Most pathogenic GRN mutations cause progranulin haploinsufficiency, so boosting progranulin levels is a promising therapeutic strategy. Progranulin is constitutively secreted, then taken up and trafficked to lysosomes. Before being taken up from the extracellular space, progranulin interacts with receptors that may mediate anti-inflammatory and growth factor-like effects. Modifying progranulin trafficking is a viable approach to boosting progranulin, but progranulin secretion and uptake by cells in the brain is poorly understood and may involve distinct mechanisms from other parts of the body. Understanding the cell types and processes that regulate extracellular progranulin in the brain could provide insight into progranulin's mechanism of action and inform design of progranulin-boosting therapies. To address this question we used microdialysis to measure progranulin in interstitial fluid (ISF) of mouse medial prefrontal cortex (mPFC). Grn+/- mice had approximately 50% lower ISF progranulin than wild-type mice, matching the reduction of progranulin in cortical tissue. Fluorescent in situ hybridization and immunofluorescence confirmed that microglia and neurons are the major progranulin-expressing cell types in the mPFC. Studies of conditional microglial (Mg-KO) and neuronal (N-KO) Grn knockout mice revealed that loss of progranulin from either cell type results in approximately 50% reduction in ISF progranulin. LPS injection (i.p.) produced an acute increase in ISF progranulin in mPFC. Depolarizing cells with KCl increased ISF progranulin, but this response was not altered in N-KO mice, indicating progranulin secretion by non-neuronal cells. Increasing neuronal activity with picrotoxin did not increase ISF progranulin. These data indicate that microglia and neurons are the source of most ISF progranulin in mPFC, with microglia likely secreting more progranulin per cell than neurons. The acute increase in ISF progranulin after LPS treatment is consistent with a role for extracellular progranulin in regulating inflammation, and may have been driven by microglia or peripheral immune cells. Finally, these data indicate that mPFC neurons engage in constitutive progranulin secretion that is not acutely changed by neuronal activity.

Dysregulation of PGC-1α-Dependent Transcriptional Programs in Neurological and Developmental Disorders: Therapeutic Challenges and Opportunities.

Substantial evidence indicates that mitochondrial impairment contributes to neuronal dysfunction and vulnerability in disease states, leading investigators to propose that the enhancement of mitochondrial function should be considered a strategy for neuroprotection. However, multiple attempts to improve mitochondrial function have failed to impact disease progression, suggesting that the biology underlying the normal regulation of mitochondrial pathways in neurons, and its dysfunction in disease, is more complex than initially thought. Here, we present the proteins and associated pathways involved in the transcriptional regulation of nuclear-encoded genes for mitochondrial function, with a focus on the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α). We highlight PGC-1α's roles in neuronal and non-neuronal cell types and discuss evidence for the dysregulation of PGC-1α-dependent pathways in Huntington's Disease, Parkinson's Disease, and developmental disorders, emphasizing the relationship between disease-specific cellular vulnerability and cell-type-specific patterns of PGC-1α expression. Finally, we discuss the challenges inherent to therapeutic targeting of PGC-1α-related transcriptional programs, considering the roles for neuron-enriched transcriptional coactivators in co-regulating mitochondrial and synaptic genes. This information will provide novel insights into the unique aspects of transcriptional regulation of mitochondrial function in neurons and the opportunities for therapeutic targeting of transcriptional pathways for neuroprotection.

Cell-Specific Deletion of PGC-1α from Medium Spiny Neurons Causes Transcriptional Alterations and Age-Related Motor Impairment.

Multiple lines of evidence indicate that a reduction in the expression and function of the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) is associated with neurodegeneration in diseases such as Huntington's disease (HD). Polymorphisms in the PGC-1α gene modify HD progression and PGC-1α expression is reduced in striatal medium spiny neurons (MSNs) of HD patients and mouse models. However, neither the MSN-specific function of PGC-1α nor the contribution of PGC-1α deficiency to motor dysfunction is known. We identified novel, PGC-1α-dependent transcripts involved in RNA processing, signal transduction, and neuronal morphology and confirmed reductions in these transcripts in male and female mice lacking PGC-1α specifically in MSNs, indicating a cell-autonomous effect in this population. MSN-specific PGC-1α deletion caused reductions in previously identified neuronal and metabolic PGC-1α-dependent genes without causing striatal vacuolizations. Interestingly, these mice exhibited a hypoactivity with age, similar to several HD animal models. However, these newly identified PGC-1α-dependent genes were upregulated with disease severity and age in knock-in HD mouse models independent of changes in PGC-1α transcript, contrary to what would be predicted from a loss-of-function etiological mechanism. These data indicate that PGC-1α is necessary for MSN transcriptional homeostasis and function with age and that, whereas PGC-1α loss in MSNs does not replicate an HD-like phenocopy, its downstream genes are altered in a repeat-length and age-dependent fashion. Understanding the additive effects of PGC-1α gene functional variation and mutant huntingtin on transcription in this cell type may provide insight into the selective vulnerability of MSNs in HD.SIGNIFICANCE STATEMENT Reductions in peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α)-mediated transcription have been implicated in the pathogenesis of Huntington's disease (HD). We show that, although PGC-1α-dependent transcription is necessary to maintain medium spiny neuron (MSN) function with age, its loss is insufficient to cause striatal atrophy in mice. We also highlight a set of genes that can serve as proxies for PGC-1α functional activity in the striatum for target engagement studies. Furthermore, we demonstrate that PGC-1α-dependent genes are upregulated in a dose- and age-dependent fashion in HD mouse models, contrary to what would be predicted from a loss-of-function etiological mechanism. However, given this role for PGC-1α in MSN transcriptional homeostasis, it is important to consider how genetic variation in PGC-1α could contribute to mutant-huntingtin-induced cell death and disease progression.