Cellular responses to the expression of unstable secretory proteins in the filamentous fungus Aspergillus oryzae.

Filamentous fungi are often used as cell factories for recombinant protein production because of their ability to secrete large quantities of hydrolytic enzymes. However, even using strong transcriptional promoters, yields of nonfungal proteins are generally much lower than those of fungal proteins. Recent analyses revealed that expression of certain nonfungal secretory proteins induced the unfolded protein response (UPR), suggesting that they are recognized as proteins with folding defects in filamentous fungi. More recently, however, even highly expressed endogenous secretory proteins were found to evoke the UPR. These findings raise the question of whether the unfolded or misfolded state of proteins is selectively recognized by quality control mechanisms in filamentous fungi. In this study, a fungal secretory protein (1,2-α-D-mannosidase; MsdS) with a mutation that decreases its thermostability was expressed at different levels in Aspergillus oryzae. We found that, at moderate expression levels, wild-type MsdS was secreted to the medium, while the mutant was not. In the strain with a deletion for the hrdA gene, which is involved in the endoplasmic reticulum-associated degradation pathway, mutant MsdS had specifically increased levels in the intracellular fraction but was not secreted. When overexpressed, the mutant protein was secreted to the medium to a similar extent as the wild-type protein; however, the mutant underwent hyperglycosylation and induced the UPR. Deletion of α-amylase (the most abundant secretory protein in A. oryzae) alleviated the UPR induction by mutant MsdS overexpression. These findings suggest that misfolded MsdS and unfolded species of α-amylase might act synergistically for UPR induction.

Evidence for symmetry in the elementary process of bidirectional torque generation by the bacterial flagellar motor.

The bacterial flagellar motor can rotate in both counterclockwise (CCW) and clockwise (CW) directions. It has been shown that the sodium ion-driven chimeric flagellar motor rotates with 26 steps per revolution, which corresponds to the number of FliG subunits that form part of the rotor ring, but the size of the backward step is smaller than the forward one. Here we report that the proton-driven flagellar motor of Salmonella also rotates with 26 steps per revolution but symmetrical in both CCW and CW directions with occasional smaller backward steps in both directions. Occasional shift in the stepping positions is also observed, suggesting the frequent exchange of stators in one of the 11-12 possible anchoring positions around the rotor. These observations indicate that the elementary process of torque generation by the cyclic association/dissociation of the stator with every FliG subunit along the circumference of the rotor is symmetric in CCW and CW rotation even though the structure of FliG is highly asymmetric and suggests a 180° rotation of a FliG domain for the rotor-stator interaction to reverse the direction of rotation.

Physiological ER stress caused by amylase production induces regulated Ire1-dependent mRNA decay in Aspergillus oryzae.

Regulated Ire1-dependent decay (RIDD) is a feedback mechanism in which the endoribonuclease Ire1 cleaves endoplasmic reticulum (ER)-localized mRNAs encoding secretory and membrane proteins in eukaryotic cells under ER stress. RIDD is artificially induced by chemicals that generate ER stress; however, its importance under physiological conditions remains unclear. Here, we demonstrate the occurrence of RIDD in filamentous fungus using Aspergillus oryzae as a model, which secretes copious amounts of amylases. α-Amylase mRNA was rapidly degraded by IreA, an Ire1 ortholog, depending on its ER-associated translation when mycelia were treated with dithiothreitol, an ER-stress inducer. The mRNA encoding maltose permease MalP, a prerequisite for the induction of amylolytic genes, was also identified as an RIDD target. Importantly, RIDD of malP mRNA is triggered by inducing amylase production without any artificial ER stress inducer. Our data provide the evidence that RIDD occurs in eukaryotic microorganisms under physiological ER stress.

[Neuro-otological Studies of Patients Suffering from Dizziness with Cerebrospinal Fluid Hypovolemia after Traffic Accident-associated Whiplash Injuries].

Vertigo and dizziness are common clinical manifestations after traffic accident-associated whiplash injury. Recently, Shinonaga et al. (2001) suggested that more than 80% of patients with whiplash injury complaining of these symptoms showed cerebrospinal (CSF) hypovolemia on radioisotope (RI) cisternography (111In-DTPA). However, neuro-otological studies to investigate the pathophysiological mechanisms underlying these symptoms have been insufficient. In the present study, patients complaining of these symptoms with CSF hypovolemia after traffic accidents were investigated with posturography and electronystagmography (ENG). Fourteen patients (4 men, 10 women; 24-52 yr) were examined with posturography and showed parameters (tracking distance & area) significantly (p<0.01) larger than those of healthy subjects. Among them, five cases (1 man, 4 women; 31-52 yr) were further investigated with ENG. The slow phase peak velocities of optokinetic nystagmus (OKN) and optokinetic-after nystagmus (OKAN) were significantly (p<0.01) reduced (62.64±6.9 SD deg/sec, 60.76±10.74 SD deg/sec, respectively) and frequencies of OKN were reduced (139.7±10.75 SD), while the ocular smooth pursuit was relatively preserved. Magnetic resonance images (sagittal view) of these five patients demonstrated the downward displacement of the cerebellar tonsils and flattening of the pons, which are characteristic features of CSF hypovolemia, called "brain sagging." Our results suggest that brain sagging due to CSF hypovolemia impairs vestibular and vestibulocerebellar functions, which may cause dizziness and vertigo.

[The medial medullary infarction (Dejerine syndrome) following chiropractic neck manipulation].

A-38-year-old man suddenly developed nausea, vomiting and vertigo during chiropractic neck manipulation. This was followed by right hemiplegia, right deep sensory disturbance and left hypoglossal nerve palsy, consistent with the medial medullary infarction (Dejerine syndrome). The MRI revealed infarction at left medial part of the medulla. The vertebral angiogram and MRA showed marked narrowing of the left vertebral artery. X-rays of the cervical spine showed no spondylosis, dislocation nor osteolysis of the odontoid process. The serological studies, including lupus anticoagulant, protein C, and protein S gave normal results. Although vascular accidents involving the brain stem after chiropractic neck manipulation have been reported since Pratt-Thomas and Berger, previous reports are still rare. In them lateral medullary infarction (Wallenberg syndrome) is probably the most common case. On the other hand, medial medullary syndrome (Dejerine syndrome) is absolutely rare. To our knowledge, the only one report has been made by Watanabe and his colleagues before our present case. The mechanism was suggested that rotation and tilting of the neck stretches and compresses the vertebral artery at the cervical joint causing injury to the vessel, with an intimal tearing, dissection, and pseudoaneurysm formation. Consequently, the present case may be caused by injury to the left vertebral artery with an intimal tearing during neck manipulation sufficient to cause disection and subsequent infarction of the brain stem.

Effect of intracellular pH on the torque-speed relationship of bacterial proton-driven flagellar motor.

Bacterial flagella responsible for motility are driven by rotary motors powered by the electrochemical potential difference of specific ions across the cytoplasmic membrane. The stator of proton-driven flagellar motor converts proton influx into mechanical work. However, the energy conversion mechanism remains unclear. Here, we show that the motor is sensitive to intracellular proton concentration for high-speed rotation at low load, which was considerably impaired by lowering intracellular pH, while zero-speed torque was not affected. The change in extracellular pH did not show any effect. These results suggest that a high intracellular proton concentration decreases the rate of proton translocation and therefore that of the mechanochemical reaction cycle of the motor but not the actual torque generation step within the cycle by the stator-rotor interactions.