Autophagy is a regulated lysosomal degradation process that involves autophagosome formation and transport. Although recent evidence indicates that basal levels of autophagy protect against neurodegeneration, the exact mechanism whereby this occurs is not known. By using conditional knockout mutant mice, we report that neuronal autophagy is particularly important for the maintenance of local homeostasis of axon terminals and protection against axonal degeneration. We show that specific ablation of an essential autophagy gene, Atg7, in Purkinje cells initially causes cell-autonomous, progressive dystrophy (manifested by axonal swellings) and degeneration of the axon terminals. Consistent with suppression of autophagy, no autophagosomes are observed in these dystrophic swellings, which is in contrast to accumulation of autophagosomes in the axonal dystrophic swellings under pathological conditions. Axonal dystrophy of mutant Purkinje cells proceeds with little sign of dendritic or spine atrophy, indicating that axon terminals are much more vulnerable to autophagy impairment than dendrites. This early pathological event in the axons is followed by cell-autonomous Purkinje cell death and mouse behavioral deficits. Furthermore, ultrastructural analyses of mutant Purkinje cells reveal an accumulation of aberrant membrane structures in the axonal dystrophic swellings. Finally, we observe double-membrane vacuole-like structures in wild-type Purkinje cell axons, whereas these structures are abolished in mutant Purkinje cell axons. Thus, we conclude that the autophagy protein Atg7 is required for membrane trafficking and turnover in the axons. Our study implicates impairment of axonal autophagy as a possible mechanism for axonopathy associated with neurodegeneration.axon ͉ axonopathy ͉ neurodegeneration ͉ autophagosome ͉ Purkinje cell M acroautophagy is characterized by dynamic membrane rearrangements, involving the formation, trafficking, and degradation of double-membrane autophagic vacuoles (autophagosomes) in the cytoplasm. Macroautophagy (hereafter referred to as autophagy) is a highly regulated process, which can be induced by nutrient starvation, trophic factors, and stress (1). Despite recent advances in characterizing autophagy in several model systems, autophagic processes in the nervous system remain poorly understood. On one hand, nutrient deprivation has not been observed to induce autophagy in the mammalian brain (2), thus suggesting a specific regulatory system for autophagy that is not typically activated by starvation. On the other hand, a variety of conditions that cause neuronal stress or degeneration can lead to the accumulation of autophagosomes in neurons, thus implicating autophagy in these neuropathogenic processes (3, 4).The axon is a highly specialized neuronal compartment that performs many functions independently from the cell body. After axotomy or excitotoxicity, double-membrane vacuoles resembling autophagosomes were originally observed to accumulate in dilated axon terminals that result fro...

Parkinson’s disease (PD) is characterized pathologically by the formation of ubiquitin and α-synuclein (α-syn)-containing inclusions (Lewy bodies), dystrophic dopamine (DA) terminals, and degeneration of midbrain DA neurons. The precise molecular mechanisms underlying these pathological features remain elusive. Accumulating evidence has implicated dysfunctional autophagy, the cell self-digestion and neuroprotective pathway, as one of the pathogenic systems contributing to the development of idiopathic PD. Here we characterize autophagy-deficient mouse models and provide in vivo evidence for the potential role that impaired autophagy plays in pathogenesis associated with PD. Cell-specific deletion of essential autophagy gene Atg7 in midbrain DA neurons causes delayed neurodegeneration, accompanied by late-onset locomotor deficits. In contrast, Atg7-deficient DA neurons in the midbrain exhibit early dendritic and axonal dystrophy, reduced striatal dopamine content, and the formation of somatic and dendritic ubiquitinated inclusions in DA neurons. Furthermore, whole-brain specific loss of Atg7 leads to presynaptic accumulation of α-syn and LRRK2 proteins, which are encoded by two autosomal dominantly inherited PD-related genes. Our results suggest that disrupted autophagy may be associated with enhanced levels of endogenous α-syn and LRRK2 proteins in vivo. Our findings implicate dysfunctional autophagy as one of the failing cellular mechanisms involved in the pathogenesis of idiopathic PD.

Key points In central regions of vestibular semicircular canal epithelia, the [K+] in the synaptic cleft ([K+]c) contributes to setting the hair cell and afferent membrane potentials; the potassium efflux from type I hair cells results from the interdependent gating of three conductances. Elevation of [K+]c occurs through a calcium‐activated potassium conductance, GBK, and a low‐voltage‐activating delayed rectifier, GK(LV), that activates upon elevation of [K+]c. Calcium influx that enables quantal transmission also activates IBK, an effect that can be blocked internally by BAPTA, and externally by a CaV1.3 antagonist or iberiotoxin. Elevation of [K+]c or chelation of [Ca2+]c linearizes the GK(LV) steady‐state I–V curve, suggesting that the outward rectification observed for GK(LV) may result largely from a potassium‐sensitive relief of Ca2+ inactivation of the channel pore selectivity filter. Potassium sensitivity of hair cell and afferent conductances allows three modes of transmission: quantal, ion accumulation and resistive coupling to be multiplexed across the synapse. Abstract In the vertebrate nervous system, ions accumulate in diffusion‐limited synaptic clefts during ongoing activity. Such accumulation can be demonstrated at large appositions such as the hair cell–calyx afferent synapses present in central regions of the turtle vestibular semicircular canal epithelia. Type I hair cells influence discharge rates in their calyx afferents by modulating the potassium concentration in the synaptic cleft, [K+]c, which regulates potassium‐sensitive conductances in both hair cell and afferent. Dual recordings from synaptic pairs have demonstrated that, despite a decreased driving force due to potassium accumulation, hair cell depolarization elicits sustained outward currents in the hair cell, and a maintained inward current in the afferent. We used kinetic and pharmacological dissection of the hair cell conductances to understand the interdependence of channel gating and permeation in the context of such restricted extracellular spaces. Hair cell depolarization leads to calcium influx and activation of a large calcium‐activated potassium conductance, GBK, that can be blocked by agents that disrupt calcium influx or buffer the elevation of [Ca2+]i, as well as by the specific KCa1.1 blocker iberiotoxin. Efflux of K+ through GBK can rapidly elevate [K+]c, which speeds the activation and slows the inactivation and deactivation of a second potassium conductance, GK(LV). Elevation of [K+]c or chelation of [Ca2+]c linearizes the GK(LV) steady‐state I–V curve, consistent with a K+‐dependent relief of Ca2+ inactivation of GK(LV). As a result, this potassium‐sensitive hair cell conductance pairs with the potassium‐sensitive hyperpolarization‐activated cyclic nucleotide‐gated channel (HCN) conductance in the afferent and creates resistive coupling at the synaptic cleft.

We identified the previously unknown structures of ribosylated imidazoleacetic acids in rat, bovine, and human tissues to be imidazole-4-acetic acid-ribotide (IAA-RP) and its metabolite, imidazole-4-acetic acid-riboside. We also found that IAA-RP has physicochemical properties similar to those of an unidentified substance(s) extracted from mammalian tissues that interacts with imidazol(in)e receptors (I-Rs). [''Imidazoline,'' by consensus (International Union of Pharmacology), includes imidazole, imidazoline, and related compounds. We demonstrate that the imidazole IAA-RP acts at I-Rs, and because few (if any) imidazolines exist in vivo, we have adopted the term ''imidazol(in)e-Rs.''] The latter regulate multiple functions in the CNS and periphery. We now show that IAA-RP (i) is present in brain and tissue extracts that exhibit I-R activity; (ii) is present in neurons of brainstem areas, including the rostroventrolateral medulla, a region where drugs active at I-Rs are known to modulate blood pressure; (iii) is present within synaptosome-enriched fractions of brain where its release is Ca 2؉ -dependent, consistent with transmitter function; (iv) produces I-R-linked effects in vitro (e.g., arachidonic acid and insulin release) that are blocked by relevant antagonists; and (v) produces hypertension when microinjected into the rostroventrolateral medulla. Our data also suggest that IAA-RP may interact with a novel imidazol(in)e-like receptor at this site. We propose that IAA-RP is a neuroregulator acting via I-Rs.clonidine-displacing substance (CDS) ͉ hypertension ͉ pancreatic beta cells ͉ anti-IAA-RP antibodies ͉ histamine