Chronic Corticosterone Elevation Suppresses Adult Hippocampal Neurogenesis by Hyperphosphorylating Huntingtin.

Chronic exposure to stress is a major risk factor for neuropsychiatric disease, and elevated plasma corticosterone (CORT) correlates with reduced levels of both brain-derived neurotrophic factor (BDNF) and hippocampal neurogenesis. Precisely how these phenomena are linked, however, remains unclear. Using a cortico-hippocampal network-on-a-chip, we find that the glucocorticoid receptor agonist dexamethasone (DXM) stimulates the cyclin-dependent kinase 5 (CDK5) to phosphorylate huntingtin (HTT) at serines 1181 and 1201 (S1181/1201), which retards BDNF vesicular transport in cortical axons. Parallel studies in mice show that CORT induces phosphorylation of these same residues, reduces BDNF levels, and suppresses neurogenesis. The adverse effects of CORT are reduced in mice bearing an unphosphorylatable mutant HTT (HdhS1181A/S1201A). The protective effect of unphosphorylatable HTT, however, disappears if neurogenesis is blocked. The CDK5-HTT pathway, which regulates BDNF transport in the cortico-hippocampal network, thus provides a missing link between elevated CORT levels and suppressed neurogenesis.

CYP46A1 gene therapy deciphers the role of brain cholesterol metabolism in Huntington's disease.

Dysfunctions in brain cholesterol homeostasis have been extensively related to brain disorders. The main pathway for brain cholesterol elimination is its hydroxylation into 24S-hydroxycholesterol by the cholesterol 24-hydrolase, CYP46A1. Increasing evidence suggests that CYP46A1 has a role in the pathogenesis and progression of neurodegenerative disorders, and that increasing its levels in the brain is neuroprotective. However, the mechanisms underlying this neuroprotection remain to be fully understood. Huntington's disease is a fatal autosomal dominant neurodegenerative disease caused by an abnormal CAG expansion in huntingtin's gene. Among the multiple cellular and molecular dysfunctions caused by this mutation, altered brain cholesterol homeostasis has been described in patients and animal models as a critical event in Huntington's disease. Here, we demonstrate that a gene therapy approach based on the delivery of CYP46A1, the rate-limiting enzyme for cholesterol degradation in the brain, has a long-lasting neuroprotective effect in Huntington's disease and counteracts multiple detrimental effects of the mutated huntingtin. In zQ175 Huntington's disease knock-in mice, CYP46A1 prevented neuronal dysfunctions and restored cholesterol homeostasis. These events were associated to a specific striatal transcriptomic signature that compensates for multiple mHTT-induced dysfunctions. We thus explored the mechanisms for these compensations and showed an improvement of synaptic activity and connectivity along with the stimulation of the proteasome and autophagy machineries, which participate to the clearance of mutant huntingtin (mHTT) aggregates. Furthermore, BDNF vesicle axonal transport and TrkB endosome trafficking were restored in a cellular model of Huntington's disease. These results highlight the large-scale beneficial effect of restoring cholesterol homeostasis in neurodegenerative diseases and give new opportunities for developing innovative disease-modifying strategies in Huntington's disease.

Neuronal network maturation differently affects secretory vesicles and mitochondria transport in axons.

Studying intracellular dynamics in neurons is crucial to better understand how brain circuits communicate and adapt to environmental changes. In neurons, axonal secretory vesicles underlie various functions from growth during development to plasticity in the mature brain. Similarly, transport of mitochondria, the power plant of the cell, regulates both axonal development and synaptic homeostasis. However, because of their submicrometric size and rapid velocities, studying the kinetics of these organelles in projecting axons in vivo is technically challenging. In parallel, primary neuronal cultures are adapted to study axonal transport but they lack the physiological organization of neuronal networks, which in turn may bias observations. We previously developed a microfluidic platform to reconstruct a physiologically-relevant and functional corticostriatal network in vitro that is compatible with high-resolution videorecording of axonal trafficking. Here, using this system we report progressive changes in axonal transport kinetics of both dense core vesicles and mitochondria that correlate with network development and maturation. Interestingly, axonal flow of both types of organelles change in opposite directions, with rates increasing for vesicles and decreasing for mitochondria. Overall, our observations highlight the need for a better spatiotemporal control for the study of intracellular dynamics in order to avoid misinterpretations and improve reproducibility.

Reconstituting Corticostriatal Network on-a-Chip Reveals the Contribution of the Presynaptic Compartment to Huntington's Disease.

Huntington's disease (HD), a devastating neurodegenerative disorder, strongly affects the corticostriatal network, but the contribution of pre- and postsynaptic neurons in the first phases of disease is unclear due to difficulties performing early subcellular investigations in vivo. Here, we have developed an on-a-chip approach to reconstitute an HD corticostriatal network in vitro, using microfluidic devices compatible with subcellular resolution. We observed major defects in the different compartments of the corticostriatal circuit, from presynaptic dynamics to synaptic structure and transmission and to postsynaptic traffic and signaling, that correlate with altered global synchrony of the network. Importantly, the genetic status of the presynaptic compartment was necessary and sufficient to alter or restore the circuit. This highlights an important weight for the presynaptic compartment in HD that has to be considered for future therapies. This disease-on-a-chip microfluidic platform is thus a physiologically relevant in vitro system for investigating pathogenic mechanisms and for identifying drugs.