Cysteine string protein alpha (CSPalpha)--an abundant synaptic vesicle protein that contains a DNA-J domain characteristic of Hsp40 chaperones--is thought to regulate Ca2+ channels and/or synaptic vesicle exocytosis. We now show that, in young mice, deletion of CSPalpha does not impair survival and causes no significant changes in presynaptic Ca2+ currents or synaptic vesicle exocytosis as measured in the Calyx of Held synapse. At 2-4 weeks of age, however, CSPalpha-deficient mice develop a progressive, fatal sensorimotor disorder. The neuromuscular junctions and Calyx synapses of CSPalpha-deficient mice exhibit increasing neurodegenerative changes, synaptic transmission becomes severely impaired, and the mutant mice die at approximately 2 months of age. Our data suggest that CSPalpha is not essential for the normal operation of Ca2+ channels or exocytosis but acts as a presynaptic chaperone that maintains continued synaptic function, raising the possibility that enhanced CSPalpha function could attenuate neurodegenerative diseases.

Sensory hair cell ribbon synapses respond to graded stimulation in a linear, indefatigable manner, requiring that vesicle trafficking to synapses is rapid and non rate limiting. Real time monitoring of vesicle fusion identified two release components. The first was saturable with both release rate and magnitude varying linearly with Ca2+, however the magnitude was too small to account for sustained afferent firing rates. A second superlinear release component required recruitment, in a Ca2+-dependent manner, of vesicles not in the immediate vicinity of the synapse. The superlinear component had a constant rate with its onset varying with Ca2+ load. High-speed Ca2+ imaging revealed a nonlinear increase in internal Ca2+ correlating with the superlinear capacitance change, implicating release of stored Ca2+ in driving vesicle recruitment. These data, supported by a mass action model, suggest sustained release at hair cell afferent fiber synapse is dictated by Ca2+-dependent vesicle recruitment from a reserve pool.

Cysteine string protein (CSP) ␣ is an abundant synaptic vesicle protein that contains a DNA-J domain characteristic of Hsp40-type cochaperones. Previous studies showed that deletion of CSP␣ in mice leads to massive lethal neurodegeneration but did not clarify how the neurodegeneration affects specific subpopulations of neurons. Here, we analyzed the effects of the CSP␣ deficiency on tonically active ribbon synapses of the retina and the inner ear. We show that CSP␣-deficient photoreceptor terminals undergo dramatic and rapidly progressive neurodegeneration that starts before eye opening and initially does not affect other retinal synapses. These changes are associated with progressive blindness. In contrast, ribbon synapses of auditory hair cells did not exhibit presynaptic impairments in CSP␣-deficient mice. Hair cells, but not photoreceptor cells or central neurons, express CSP␤, thereby accounting for the lack of a hair-cell phenotype in CSP␣ knockout mice. Our data demonstrate that tonically active ribbon synapses in retina are particularly sensitive to the deletion of CSP␣ and that expression of at least one CSP isoform is essential to protect such tonically active synapses from neurodegeneration.hair cell ͉ ribbon synapse ͉ electroretinogram ͉ chaperone ͉ blindness S ynaptic vesicle exo-and endocytosis is mediated by a sophisticated machinery that allows nerve terminals to operate at high speed (1). Such continuous membrane traffic requires specific mechanisms to deal with the use-dependent aging and denaturation of proteins. One such mechanism may involve the synaptic vesicle protein cysteine string protein (CSP) ␣ that is thought to function as a cochaperone in presynaptic terminals (2, 3).CSP␣ is an abundant presynaptic protein (4) that contains a string of cysteine residues and a DNA-J domain that functionally collaborates with the DNA-K domains of Hsc70 proteins (reviewed in refs. 5 and 6). CSP␣ activates the ATPase activity of Hsc70 (7,8) and forms a trimeric complex with Hsc70 and the tetratricopeptide repeat protein SGT (3). This complex catalyzes the ATP-dependent refolding of denatured luciferase (3). Invertebrates have a single CSP gene, whereas mammals express three CSP genes: CSP␣ that is widely distributed but highly enriched in brain, and CSP␤ and CSP␥ that are primarily found in testis (2).Analyses of knockout (KO) mice revealed that CSP␣-deficient mice are relatively normal at birth but exhibit a progressive lethal phenotype that results in death after 2-4 months (2). Recordings in the Calyx of Held synapse of CSP␣ KO mice documented that CSP␣ is not required for N-, P͞Q-, and R-type Ca 2ϩ -channel function or Ca 2ϩ -triggered vesicle exocytosis. Instead, in the absence of CSP␣, the calyx synapse developed an age-dependent functional impairment, consistent with a role for CSP␣ as part of a molecular chaperone that makes it possible for synapses to keep running for extended time periods (2). Although this hypothesis was in agreement with previous observations in CSP-deficient flies (4), alt...

In regulatory toxicology, the dose-response relationship is a key element towards fulfilling safety assessments and satisfying regulatory authorities. Conventionally, the larger the dose, the greater the response, following the dogma “the dose makes the poison”. Many endocrine disrupting chemicals, including bisphenol-A (BPA), induce non-monotonic dose response (NMDR) relationships, which are unconventional and have tremendous implications in risk assessment. Although several molecular mechanisms have been proposed to explain NMDR relationships, they are largely undemonstrated. Using mouse pancreatic β-cells from wild-type and oestrogen receptor ERβ−/− mice, we found that exposure to increasing doses of BPA affected Ca2+ entry in an NMDR manner. Low doses decreased plasma membrane Ca2+ currents after downregulation of Cav2.3 ion channel expression, in a process involving ERβ. High doses decreased Ca2+ currents through an ERβ-mediated mechanism and simultaneously increased Ca2+ currents via oestrogen receptor ERα. The outcome of both molecular mechanisms explains the NMDR relationship between BPA and Ca2+ entry in β-cells.