Presenilin-Dependent Receptor Processing Is Required for Axon Guidance

SummaryThe Alzheimer's disease-linked gene presenilin is required for intramembrane proteolysis of amyloid-β precursor protein, contributing to the pathogenesis of neurodegeneration that is characterized by loss of neuronal connections, but the role of Presenilin in establishing neuronal connections is less clear. Through a forward genetic screen in mice for recessive genes affecting motor neurons, we identified the Columbus allele, which disrupts motor axon projections from the spinal cord. We mapped this mutation to the Presenilin-1 gene. Motor neurons and commissural interneurons in Columbus mutants lacking Presenilin-1 acquire an inappropriate attraction to Netrin produced by the floor plate because of an accumulation of DCC receptor fragments within the membrane that are insensitive to Slit/Robo silencing. Our findings reveal that Presenilin-dependent DCC receptor processing coordinates the interplay between Netrin/DCC and Slit/Robo signaling. Thus, Presenilin is a key neural circuit builder that gates the spatiotemporal pattern of guidance signaling, thereby ensuring neural projections occur with high fidelity

Spinal Locomotor Circuits Develop Using Hierarchical Rules Based on Motorneuron Position and Identity

SummaryThe coordination of multi-muscle movements originates in the circuitry that regulates the firing patterns of spinal motorneurons. Sensory neurons rely on the musculotopic organization of motorneurons to establish orderly connections, prompting us to examine whether the intraspinal circuitry that coordinates motor activity likewise uses cell position as an internal wiring reference. We generated a motorneuron-specific GCaMP6f mouse line and employed two-photon imaging to monitor the activity of lumbar motorneurons. We show that the central pattern generator neural network coordinately drives rhythmic columnar-specific motorneuron bursts at distinct phases of the locomotor cycle. Using multiple genetic strategies to perturb the subtype identity and orderly position of motorneurons, we found that neurons retained their rhythmic activity—but cell position was decoupled from the normal phasing pattern underlying flexion and extension. These findings suggest a hierarchical basis of motor circuit formation that relies on increasingly stringent matching of neuronal identity and position

Cholinergic Input Is Required during Embryonic Development to Mediate Proper Assembly of Spinal Locomotor Circuits

SummaryRhythmic limb movements are controlled by pattern-generating neurons within the ventral spinal cord, but little is known about how these locomotor circuits are assembled during development. At early stages of embryogenesis, motor neurons are spontaneously active, releasing acetylcholine that triggers the depolarization of adjacent cells in the spinal cord. To investigate whether acetylcholine-driven activity is required for assembly of the central pattern-generating (CPG) circuit, we studied mice lacking the choline acetyltransferase (ChAT) enzyme. Our studies show that a rhythmically active spinal circuit forms in ChAT mutants, but the duration of each cycle period is elongated, and right-left and flexor-extensor coordination are abnormal. In contrast, blocking acetylcholine receptors after the locomotor network is wired does not affect right-left or flexor-extensor coordination. These findings suggest that the cholinergic neurotransmitter pathway is involved in configuring the CPG during a transient period of development

Aberrant GlyRS-HDAC6 interaction linked to axonal transport deficits in Charcot-Marie-Tooth neuropathy.

Dominant mutations in glycyl-tRNA synthetase (GlyRS) cause a subtype of Charcot-Marie-Tooth neuropathy (CMT2D). Although previous studies have shown that GlyRS mutants aberrantly interact with Nrp1, giving insight into the disease\u27s specific effects on motor neurons, these cannot explain length-dependent axonal degeneration. Here, we report that GlyRS mutants interact aberrantly with HDAC6 and stimulate its deacetylase activity on α-tubulin. A decrease in α-tubulin acetylation and deficits in axonal transport are observed in mice peripheral nerves prior to disease onset. An HDAC6 inhibitor used to restore α-tubulin acetylation rescues axonal transport deficits and improves motor functions of CMT2D mice. These results link the aberrant GlyRS-HDAC6 interaction to CMT2D pathology and suggest HDAC6 as an effective therapeutic target. Moreover, the HDAC6 interaction differs from Nrp1 interaction among GlyRS mutants and correlates with divergent clinical presentations, indicating the existence of multiple and different mechanisms in CMT2D. Nat Commun 2018 Mar 8; 9(1):1007

Dependence of Hippocampal Function on ERRγ-Regulated Mitochondrial Metabolism

SummaryNeurons utilize mitochondrial oxidative phosphorylation (OxPhos) to generate energy essential for survival, function, and behavioral output. Unlike most cells that burn both fat and sugar, neurons only burn sugar. Despite its importance, how neurons meet the increased energy demands of complex behaviors such as learning and memory is poorly understood. Here we show that the estrogen-related receptor gamma (ERRγ) orchestrates the expression of a distinct neural gene network promoting mitochondrial oxidative metabolism that reflects the extraordinary neuronal dependence on glucose. ERRγ−/− neurons exhibit decreased metabolic capacity. Impairment of long-term potentiation (LTP) in ERRγ−/− hippocampal slices can be fully rescued by the mitochondrial OxPhos substrate pyruvate, functionally linking the ERRγ knockout metabolic phenotype and memory formation. Consistent with this notion, mice lacking neuronal ERRγ in cerebral cortex and hippocampus exhibit defects in spatial learning and memory. These findings implicate neuronal ERRγ in the metabolic adaptations required for memory formation

Survival of syngeneic and allogeneic iPSC–derived neural precursors after spinal grafting in minipigs

The use of autologous (or syngeneic) cells derived from induced pluripotent stem cells (iPSCs) holds great promise for future clinical use in a wide range of diseases and injuries. It is expected that cell replacement therapies using autologous cells would forego the need for immunosuppression, otherwise required in allogeneic transplantations. However, recent studies have shown the unexpected immune rejection of undifferentiated autologous mouse iPSCs after transplantation. Whether similar immunogenic properties are maintained in iPSC-derived lineage-committed cells (such as neural precursors) is relatively unknown. We demonstrate that syngeneic porcine iPSC-derived neural precursor cell (NPC) transplantation to the spinal cord in the absence of immunosuppression is associated with long-term survival and neuronal and glial differentiation. No tumor formation was noted. Similar cell engraftment and differentiation were shown in spinally injured transiently immunosuppressed swine leukocyte antigen (SLA)–mismatched allogeneic pigs. These data demonstrate that iPSC-NPCs can be grafted into syngeneic recipients in the absence of immunosuppression and that temporary immunosuppression is sufficient to induce long-term immune tolerance after NPC engraftment into spinally injured allogeneic recipients. Collectively, our results show that iPSC-NPCs represent an alternative source of transplantable NPCs for the treatment of a variety of disorders affecting the spinal cord, including trauma, ischemia, or amyotrophic lateral sclerosis

Synchronization of Neurogenesis and Motor Neuron Specification by Direct Coupling of bHLH and Homeodomain Transcription Factors

AbstractInductive signaling leads to the coactivation of regulatory pathways for specifying general neuronal traits in parallel with instructions for neuronal subtype specification. Nevertheless, the mechanisms that ensure that these pathways are synchronized have not been defined. To address this, we examined how bHLH proteins Ngn2 and NeuroM controlling neurogenesis functionally converge with LIM-homeodomain (LIM-HD) factors Isl1 and Lhx3 involved in motor neuron subtype specification. We found that Ngn2 and NeuroM transcriptionally synergize with Isl1 and Lhx3 to specify motor neurons in the embryonic spinal cord and in P19 stem cells. The mechanism underlying this cooperativity is based on interactions that directly couple the activity of the bHLH and LIM-HD proteins, mediated by the adaptor protein NLI. This functional link acts to synchronize neuronal subtype specification with neurogenesis