Oligodendrocytes, the myelin-forming glial cells of the central nervous system, maintain longterm axonal integrity [1][2][3] . However, the underlying support mechanisms are not understood 4 . Here we identify ametabolic component of axon-glia interactions by generating conditional Cox10 (protoheme IX farnesyltransferase) mutant mice, in which oligodendrocytes and Schwann cells fail to assemble stable mitochondrial cytochrome c oxidase (COX, also known as mitochondrial complex IV). In the peripheral nervous system, Cox10 conditional mutants exhibit severe neuropathy with dysmyelination, abnormal Remak bundles, muscle atrophy and paralysis. Notably, perturbing mitochondrial respiration did not cause glial cell death. In the adult central nervous system, we found no signs of demyelination, axonal degeneration or secondary inflammation. Unlike cultured oligodendrocytes, which are sensitive to COX inhibitors 5 , postmyelination oligodendrocytes survive well in the absence of COX activity. More importantly, by in vivo magnetic resonance spectroscopy, brain lactate concentrations inmutants were increased compared with controls, but were detectable only in mice exposed to volatile anaesthetics. This indicates that aerobic glycolysis products derived from oligodendrocytes are rapidly metabolized within white matter tracts. Becausemyelinated axons can use lactate when energy-deprived 6 , our findings suggest a model in which axon-glia metabolic coupling serves a physiological function. † Present

The yeast cytosol contains multiple homologs of the DnaK and DnaJ chaperone family. Our current understanding of which homologs functionally interact is incomplete. Zuotin is a DnaJ homolog bound to the yeast ribosome. We have now identified the DnaK homolog Ssz1p͞Pdr13p as zuotin's partner chaperone. Zuotin and Ssz1p form a ribosome-associated complex (RAC) that is bound to the ribosome via the zuotin subunit. RAC is unique among the eukaryotic DnaK-DnaJ systems, as the 1:1 complex is stable, even in the presence of ATP or ADP. In vitro, RAC stimulates the translocation of a ribosome-bound mitochondrial precursor protein into mitochondria, providing evidence for its chaperonelike effect on nascent chains. In agreement with the existence of a functional complex, deletion of each RAC subunit resulted in a similar phenotype in vivo. However, overexpression of zuotin partly rescued the growth defect of the ⌬ssz1 strain, whereas overexpression of Ssz1p did not affect the ⌬zuo1 strain, suggesting a pivotal function for the DnaJ homolog. P roteins synthesized on cytosolic ribosomes either fold or translocate to their final location cotranslationally or posttranslationally. Translocation and folding require chaperonelike proteins that often serve multiple and overlapping functions. The complex chaperone network in the eukaryotic cell is currently only poorly understood (1-3).Most posttranslational translocation events require the translocating proteins to be unfolded. This requirement is at least partly ensured by binding of cytosolic chaperones to newly translated proteins. The yeast DnaK͞Hsp70 homolog Ssa1͞2p and its partner protein, the DnaJ homolog Ydj1p, are involved in protein translocation into a variety of compartments (4). Besides their role in translocation, Ssa1͞2p and Ydj1p are involved in cytosolic protein folding, most likely in a posttranslational manner (5-7). Other chaperones assisting posttranslational folding in the eukaryotic cytosol are the chaperonin CCT and Hsp90 (2,8,9).Some chaperones interact cotranslationally with polypeptides. In both eukaryotes and prokaryotes, soluble DnaK and DnaJ homologs bind to nascent chains and subsequently assist their folding (2, 10, 11). The ribosome-bound DnaK homolog Ssb1͞2p can be crosslinked to nascent chains, providing evidence for a functionally important interaction (12).There is long-standing circumstantial evidence of cotranslational import into mitochondria (13). More recent data suggest that some precursor proteins even require a cotranslational mechanism to be imported into mitochondria (14-16). However, no specialized component of a mitochondrial cotranslational translocation system, comparable to a signal recognition particle or signal recognition particle receptor, has been identified (17).In a previous study we introduced an in vitro mitochondrial protein import assay for the identification of cytosolic components involved in either cotranslational translocation or interacting with nascent precursor proteins in a chaperone-like manner. The assay, b...

To identify yeast cytosolic proteins that mediate targeting of precursor proteins to mitochondria, we developed an in vitro import system consisting of purified yeast mitochondria and a radiolabeled mitochondrial precursor protein whose C terminus was still attached to the ribosome. In this system, the N terminus of the nascent chain was translocated across both mitochondrial membranes, generating a translocation intermediate spanning both membranes. The nascent chain could then be completely chased into the mitochondrial matrix after release from the ribosome. Generation of this import intermediate was dependent on a mitochondrial membrane potential, mitochondrial surface proteins, and was stimulated by proteins that could be released from the ribosomes by high salt. The major salt-released stimulatory factor was yeast nascent polypeptide–associated complex (NAC). Purified NAC fully restored import of salt-washed ribosome-bound nascent chains by enhancing productive binding of the chains to mitochondria. We propose that ribosome-associated NAC facilitates recognition of nascent precursor chains by the mitochondrial import machinery.

Background: Cholesterol, an essential component of all mammalian plasma membranes, is highly enriched in the brain. Both during development and in the adult, brain cholesterol is derived from local cholesterol synthesis and not taken up from the circulation. However, the contribution of neurons and glial cells to total brain cholesterol metabolism is unknown.

Ribosome-associated complex (RAC) consists of the Hsp40 homolog Zuo1 and the Hsp70 homolog Ssz1. The chaperone participates in the biogenesis of newly synthesized polypeptides. Here we have identified yeast Rpl31, a component of the large ribosomal subunit, as a contact point of RAC at the polypeptide tunnel exit. Rpl31 is encoded by RPL31a and RPL31b, two closely related genes. ⌬rpl31a⌬rpl31b displayed slow growth and sensitivity to low as well as high temperatures. In addition, ⌬rpl31a⌬rpl31b was highly sensitive toward aminoglycoside antibiotics and suffered from defects in translational fidelity. With the exception of sensitivity at elevated temperature, the phenotype resembled yeast strains lacking one of the RAC subunits or Rpl39, another protein localized at the tunnel exit. Defects of ⌬rpl31a⌬rpl31b⌬zuo1 did not exceed that of ⌬rpl31a⌬rpl31b or ⌬zuo1. However, the combined deletion of RPL31a, RPL31b, and RPL39 was lethal. Moreover, RPL39 was a multicopy suppressor, whereas overexpression of RAC failed to rescue growth defects of ⌬rpl31a⌬rpl31b. The findings are consistent with a model in that Rpl31 and Rpl39 independently affect a common ribosome function, whereas Rpl31 and RAC are functionally interdependent. Rpl31, while not essential for binding of RAC to the ribosome, might be involved in proper function of the chaperone complex. INTRODUCTIONTwo Hsp70 family members Ssb1/2 (Ssb1 and Ssb2 differ by only four amino acids) and Ssz1 and one J-domain protein (Zuo1) are abundant components of the translation machinery of Saccharomyces cerevisiae (Raue et al., 2007). The three chaperones are genetically linked and form a functional triad. Lack of either SSB1/2, SSZ1, or ZUO1 results in slow growth, cold sensitivity, and pronounced hypersensitivity against aminoglycosides such as paromomycin (Gautschi et al., 2002;Hundley et al., 2002). Ssz1 and Zuo1 assemble into a stable heterodimeric complex termed RAC (ribosome-associated complex). RAC acts as a cochaperone for Ssb1/2 and stimulates its ATP hydrolysis. The function requires both RAC subunits (Huang et al., 2005;Conz et al., 2007).RAC is anchored to the ribosome via Zuo1 (Gautschi et al., 2001). The idea is that positioning of RAC on the ribosome is required for its interaction with Ssb1/2 (Yan et al., 1998). However, the function of Ssz1 does not strictly depend on stable interaction with Zuo1 or ribosomes (Conz et al., 2007). How exactly Zuo1 anchors RAC is currently unclear. It was proposed that Zuo1 binds to ribosomes, in part, by interaction with rRNA (Yan et al., 1998). However, purified Zuo1 unspecifically interacts with a variety of nucleic acids. Initially, Zuo1 was identified via its ability to interact with Z-DNA (Zhang et al., 1992), it also interacts tightly with tRNA (Wilhelm et al., 1994) and recently was shown to bind to a small inhibitor RNA (Raychaudhuri et al., 2006). The mouse homolog MIDA1 interacts with DNA that forms small stem loop structures (Inoue et al., 2000). The diversity of nucleic acids that interact with Zuo1 raises the qu...

Cholesterol is an essential membrane component enriched in plasma membranes, growth cones, and synapses. The brain normally synthesizes all cholesterol locally, but the contribution of individual cell types to brain cholesterol metabolism is unknown. To investigate whether cortical projection neurons in vivo essentially require cholesterol biosynthesis and which cell types support neurons, we have conditionally ablated the cholesterol biosynthesis in these neurons in mice either embryonically or postnatally. We found that cortical projection neurons synthesize cholesterol during their entire lifetime. At all stages, they can also benefit from glial support. Adult neurons that lack cholesterol biosynthesis are mainly supported by astrocytes such that their functional integrity is preserved. In contrast, microglial cells support young neurons. However, compensatory efforts of microglia are only transient leading to layer-specific neuronal death and the reduction of cortical projections. Hence, during the phase of maximal membrane growth and maximal cholesterol demand, neuronal cholesterol biosynthesis is indispensable. Analysis of primary neurons revealed that neurons tolerate only slight alteration in the cholesterol content and plasma membrane tension. This quality control allows neurons to differentiate normally and adjusts the extent of neurite outgrowth, the number of functional growth cones and synapses to the available cholesterol. This study highlights both the flexibility and the limits of horizontal cholesterol transfer in vivo and may have implications for the understanding of neurodegenerative diseases.

To study the development of the cerebellum, we generated a transgenic mouse line Tg(malpha6-cre)B1LFR that expresses CRE recombinase under the GABA(A) receptor alpha6 subunit promoter. In this line, recombination of an R26R reporter allele occurred postnatally in granule cells of the cerebellum and dorsal cochlear nucleus, as well as in a subset of precerebellar nuclei in the brainstem. All neurons in which recombination occurred originated during embryogenesis from the rhombic lip. This might be explained by a very early specification event at the rhombic lip that primes cells derived from this structure to express the transgene during neuronal maturation. As no recombination occurred in the inferior olive, it may be derived from a distinct subset of precursors at the rhombic lip. No recombination occurred in any of the interneurons in the cerebellum (stellate cells, basket cells, and Golgi cells), consistent with the hypothesis that they are not derived from the same embryonic tissue as the granule cells.

The mechanism of chaperonin-assisted protein folding has been mostly analyzed in vitro using non-homologous substrate proteins. In order to understand the relative importance of hsp60 and hsp10 in the living cell, homologous substrate proteins need to be identified and analyzed. We have devised a novel screen to test the folding of a large variety of homologous substrates in the mitochondrial matrix in the absence or presence of functional hsp60 or hsp10. The identified substrates have an M r of 15-90 kDa and fall into three groups: (i) proteins that require both hsp60 and hsp10 for correct folding; (ii) proteins that completely fail to fold after inactivation of hsp60 but are unaffected by the inactivation of hsp10; and (iii) newly imported hsp60 itself, which is more severely affected by inactivation of hsp10 than by inactivation of pre-existing hsp60. The majority of the identified substrates are group I proteins. For these, the lack of hsp60 function has a more pronounced effect than inactivation of hsp10. We suggest that homologous substrate proteins have differential chaperonin requirements, indicating that hsp60 and hsp10 do not always act as a single functional unit in vivo.