Defective mitochondrial distribution in neurons is proposed to cause ATP depletion and calcium-buffering deficiencies that compromise cell function. However, it is unclear whether aberrant mitochondrial motility and distribution alone are sufficient to cause neurological disease. Calcium-binding mitochondrial Rho (Miro) GTPases attach mitochondria to motor proteins for anterograde and retrograde transport in neurons. Using two new KO mouse models, we demonstrate that Miro1 is essential for development of cranial motor nuclei required for respiratory control and maintenance of upper motor neurons required for ambulation. Neuron-specific loss of Miro1 causes depletion of mitochondria from corticospinal tract axons and progressive neurological deficits mirroring human upper motor neuron disease. Although Miro1-deficient neurons exhibit defects in retrograde axonal mitochondrial transport, mitochondrial respiratory function continues. Moreover, Miro1 is not essential for calcium-mediated inhibition of mitochondrial movement or mitochondrial calcium buffering. Our findings indicate that defects in mitochondrial motility and distribution are sufficient to cause neurological disease. Motor neuron diseases (MNDs), including ALS and spastic paraplegia (SP), are characterized by the progressive, lengthdependent degeneration of motor neurons, leading to muscle atrophy, paralysis, and, in some cases, premature death. There are both inherited and sporadic forms of MNDs, which can affect upper motor neurons, lower motor neurons, or both. Although the molecular and cellular causes of most MNDs are unknown, many are associated with defects in axonal transport of cellular components required for neuron function and maintenance (1-6).A subset of MNDs is associated with impaired mitochondrial respiration and mitochondrial distribution. This observation has led to the hypothesis that neurodegeneration results from defects in mitochondrial motility and distribution, which, in turn, cause subcellular ATP depletion and interfere with mitochondrial calcium ([Ca 2+ ] m ) buffering at sites of high synaptic activity (reviewed in ref. 7). It is not known, however, whether mitochondrial motility defects are a primary cause or a secondary consequence of MND progression. In addition, it has been difficult to isolate the primary effect of mitochondrial motility defects in MNDs because most mutations that impair mitochondrial motility in neurons also affect transport of other organelles and vesicles (1,(8)(9)(10)(11).In mammals, the movement of neuronal mitochondria between the cell body and the synapse is controlled by adaptors called trafficking kinesin proteins (Trak1 and Trak2) and molecular motors (kinesin heavy chain and dynein), which transport the organelle in the anterograde or retrograde direction along axonal microtubule tracks (7,(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24). Mitochondrial Rho (Miro) GTPase proteins are critical for transport because they are the only known surface receptors that attach mitochondria to...

Summary The adult nervous system is plastic allowing us to learn, remember and forget. Experience-dependent plasticity occurs at synapses – the specialized points of contact between neurons where signaling occurs. However, the mechanisms that regulate the strength of synaptic signaling are not well understood. Here, we define a Wnt signaling pathway that modifies synaptic strength in the adult nervous system by regulating the translocation of one class of acetylcholine receptors (AChRs) to synapses. In C. elegans, we show that mutations in CWN-2 (Wnt ligand), LIN-17 (Frizzled), CAM-1 (Ror receptor tyrosine kinase), or the downstream effector DSH-1 (disheveled) result in similar subsynaptic accumulations of ACR-16/α7 AChRs, a consequent reduction in synaptic current, and predictable behavioral defects. Photoconversion experiments revealed defective translocation of ACR-16/α7 to synapses in Wnt signaling mutants. Using optogenetic nerve stimulation, we demonstrate activity-dependent synaptic plasticity and its dependence on ACR-16/α7 translocation mediated by Wnt signaling via LIN-17/CAM-1 heteromeric receptors.

Summary A primary determinant of the strength of neurotransmission is the number of AMPA-type glutamate receptors (AMPARs) at synapses. However, we still lack a mechanistic understanding of how the number of synaptic AMPARs is regulated. Here, we show that UNC-116, the C. elegans homolog of vertebrate kinesin-1 heavy chain (KIF5), modifies synaptic strength by mediating the rapid delivery, removal and redistribution of synaptic AMPARs. Furthermore, by studying the real-time transport of C. elegans AMPAR subunits in vivo, we demonstrate that although homomeric GLR-1 AMPARs can diffuse to and accumulate at synapses in unc-116 mutants, glutamate-gated currents are diminished because heteromeric GLR-1/GLR-2 receptors do not reach synapses in the absence of UNC-116/KIF5-mediated transport. Our data support a model where ongoing motor-driven delivery and removal of AMPARs controls not only the number, but also the composition of synaptic AMPARs and thus the strength of synaptic transmission.

SUMMARY The strength of synaptic communication at central synapses depends on the number of ionotropic glutamate receptors, particularly the class gated by the agonist AMPA (AMPARs). Cornichon proteins, evolutionarily conserved endoplasmic reticulum cargo adaptors, modify the properties of vertebrate AMPARs when coexpressed in heterologous cells. However, the contribution of cornichons to behavior and in vivo nervous system function has yet to be determined. Here, we take a genetic approach to these questions by studying CNI-1 – the sole cornichon homologue in C. elegans. cni-1 mutants hyper-reverse, a phenotype associated with increased glutamatergic synaptic transmission. Consistent with this behavior, we find larger glutamate-gated currents in cni-1 mutants with a corresponding increase in AMPAR number. Furthermore, we observe opposite phenotypes in transgenic worms that overexpress CNI-1 or vertebrate homologues. In reconstitution studies, we provide support for an evolutionarily conserved role for cornichons in regulating the export of vertebrate and invertebrate AMPARs.

Aberrant mitochondrial distribution is observed in numerous neurodegenerative disorders. A popular, but untested, model proposes that local energy deprivation resulting from mitochondrial movement and distribution defects causes the neuronal degeneration associated with these diseases. The Mitochondrial Rho GTPase (Miro) is the only known receptor specific for mitochondrial movement. Mammals ubiquitously express two 60% identical orthologs of Miro (Miro1 and Miro2), but it is unknown whether these orthologs have overlapping or distinct functions. Using two new mouse models lacking a Miro1 conditional allele, we show that: 1) Miro1 has a more specific physiological role than expected, 2) Miro1 has surprisingly specific functions in mitochondrial movement and neuron development/maintenance, and 3) Disruption of Miro1‐mediated mitochondrial movement does not compromise mitochondrial respiration. Thus, defects in mitochondrial distribution are sufficient to cause neuronal deficits in mammals.