Sidney M. Gospe

Facilitative glucose transporter Glut1 is actively excluded from rod outer segments

Photoreceptors are among the most metabolically active cells in the body, relying on both oxidative phosphorylation and glycolysis to satisfy their high energy needs. Local glycolysis is thought to be particularly crucial in supporting the function of the photoreceptors light-sensitive outer segment compartment, which is devoid of mitochondria. Accordingly, it has been commonly accepted that the facilitative glucose transporter Glut1 responsible for glucose entry into photoreceptors is localized in part to the outer segment plasma membrane. However, we now demonstrate that Glut1 is entirely absent from the rod outer segment and is actively excluded from this compartment by targeting information present in its cytosolic C-terminal tail. Our data indicate that glucose metabolized in the outer segment must first enter through other parts of the photoreceptor cell. Consequently, the entire energy supply of the outer segment is dependent on diffusion of energy-rich substrates through the thin connecting cilium that links this compartment to the rest of the cell.

A single valine residue plays an essential role in peripherin/rds targeting to photoreceptor outer segments.

Peripherin/retinal degeneration slow (rds) is an integral membrane protein specifically localized to the light-sensing organelle of the photoreceptor cell, the outer segment. Within the outer segment, peripherin is found at the edges of photoreceptor discs, where it plays a critical role in disc morphogenesis and maintenance. Peripherin loss or mutations are often associated with severe forms of visual impairments. Like all other resident outer segment proteins, peripherin is synthesized in the photoreceptor cell body and subsequently transported to the outer segment. In an effort to further examine peripherin’s delivery to outer segments, we undertook a careful examination of its targeting sequence. Using a fluorescently labeled reporter expressed in the rods of transgenic tadpoles, we narrowed peripherin’s targeting sequence to ten amino acids within its C-terminal tail. This small stretch of amino acid residues is both necessary and sufficient for outer segment targeting. We also conducted alanine scanning of all residues within this sequence and found that only a single residue, valine at position 332, is essential for outer segment targeting. This valine is conserved in all species and its mutation is sufficient to completely abrogate the targeting of full-length peripherin in mouse rods.

Membrane Attachment Is Key to Protecting Transducin GTPase-Activating Complex from Intracellular Proteolysis in Photoreceptors

The members of the R7 regulator of G-protein signaling (RGS) protein subfamily are versatile regulators of G-protein signaling throughout the nervous system. Recent studies indicate that they are often found in complexes with membrane anchor proteins that serve as versatile modulators of their activity, intracellular targeting, and stability. One striking example is the interplay between the membrane anchor R9AP and the RGS9-1 · Gβ5 GTPase-activating complex responsible for the rapid inactivation of the G-protein transducin in vertebrate photoreceptor cells during their recovery from light excitation. The amount of this complex in photoreceptors sets their temporal resolution and is precisely regulated by the expression level of R9AP, which serves to protect the RGS9-1 and Gβ5 subunits from intracellular proteolysis. In this study, we investigated the mechanism by which R9AP performs its protective function in mouse rods and found that it is entirely confined to recruiting RGS9-1 · Gβ5 to cellular membranes. Furthermore, membrane attachment of RGS9-1 · Gβ5 is sufficient for its stable expression in rods even in the absence of R9AP. Our second finding is that RGS9-1 · Gβ5 possesses targeting information that specifies its exclusion from the outer segment and that this information is neutralized by association with R9AP to allow outer segment targeting. Finally, we demonstrate that the ability of R9AP · RGS9-1 · Gβ5 to accelerate GTP hydrolysis on transducin is independent of its means of membrane attachment, since replacing the transmembrane domain of R9AP with a site for lipid modification did not impair the catalytic activity of this complex.

Anatomic and visual function outcomes in paediatric idiopathic intracranial hypertension

Background There is a paucity of literature describing risk factors for vision loss in paediatric idiopathic intracranial hypertension (IIH). We investigate the final visual function, spectral domain optical coherence tomography (SD-OCT) and enhanced depth imaging (EDI)-OCT findings in children with papilledema caused by IIH. Methods Medical records of 31 patients with paediatric IIH (age ≤17 years) were retrospectively reviewed. Optic disc photographs on presentation and automated perimetry, SD-OCT and EDI-OCT imaging on final follow-up visit were statistically analysed to identify patient characteristics and anatomic findings associated with irreversible vision loss. Results Permanent visual acuity or visual field loss developed in 19% of study eyes. Papilledema of modified Frisén grade ≥3 on presentation was highly predictive of permanent vision loss (p<0.001), while associations between pubertal status and visual function outcome failed to reach statistical significance. SD-OCT revealed optic atrophy in 13% and photoreceptor loss in 19% of eyes, with both findings highly associated with vision loss (p<0.0001). Optic disc drusen was noted in 48% of study eyes by EDI-OCT but was not found to be predictive of visual outcome. Conclusions Clinical observation of high papilledema grade on presentation is predictive of poor visual outcomes. Vision loss is associated not only with optic atrophy but also with photoreceptor damage. Interestingly, a high proportion of study eyes had optic disc drusen, which was not associated with vision loss, but can be a diagnostic challenge in distinguishing true papilledema from pseudopapilledema.

The outer segment serves as a default destination for the trafficking of membrane proteins in photoreceptors

Baker et al. 2008. J. Cell Biol. doi:10.1083/jcb.200806009 [OpenUrl][1][Abstract/FREE Full Text][2] [1]: {openurl}?query=rft_id%253Dinfo%253Adoi%252F10.1083%252Fjcb.200806009%26rft_id%253Dinfo%253Apmid%252F18981232%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%

Emerging Applications of Optical Coherence Tomography in Pediatric Optic Neuropathies

Limited cooperation and attention span often lead to poorly reliable assessments of visual acuity and visual fields in children, making diagnosis and monitoring of pediatric optic neuropathies challenging. As a noninvasive imaging modality, optical coherence tomography (OCT) could offer particular utility in this patient population. OCT provides high-resolution characterization of the optic nerve head, peripapillary retinal nerve fiber layer, and cellular layers of the macula, all of which can be used to assess the severity of optic nerve disease qualitatively and quantitatively. Application of OCT to pediatric patients has been limited by technical factors and lack of pediatric normative databases, but with recent technological improvements and rapidly expanding research efforts OCT is poised to revolutionize the management of optic neuropathies in children. We review current and emerging applications of OCT to important pediatric optic neuropathies such as glaucoma, papilledema, optic neuritis, optic pathway gliomas, and congenital optic disc anomalies.

Fourth down and five

Binocular diplopia and right hemifacial numbness developed in a 52-year-old woman after resection of a right temporal lobe glioblastoma. Based on the Parks-Bielschowsky 3-step test, she was diagnosed with a right cranial nerve (CN) IV palsy in addition to right CN V dysfunction. Iatrogenic diplopia may result from temporal lobe surgery due to the intimate relationship of CN IV and CN III to the mesial temporal lobe. In addition, injury to CN V within Meckel cave is believed to be the cause of facial numbness in some patients after temporal lobe surgery. The anatomy of the intracranial portion of CN IV is reviewed, and the etiologies of CN IV palsy are discussed.

Keratomalacia in a Patient With Psychogenic Vitamin A Deficiency.

Purpose: To report the clinical and histopathological findings of a patient with bilateral keratomalacia arising from severe vitamin A deficiency from panic disorder-related malnutrition. Methods: Case report. Results: A 47-year-old male with panic disorder presented with 1 month of painful vision loss sequentially affecting the right and left eyes. He exhibited bilateral conjunctival xerosis with complete corneal melt in the right eye and a large corneal epithelial defect with underlying anterior chamber inflammation in the left eye. Laboratory investigation revealed undetectable serum vitamin A levels attributed to self-induced vomiting and starvation. He was treated with high-dose vitamin A, but the right eye required enucleation. The histological findings are reported. Conclusions: Vitamin A deficiency in the absence of organic gastrointestinal abnormalities is exceedingly rare in the developed world. A strong index of suspicion and thorough review of systems are invaluable in evaluating patients with unexplained corneal melt.

Tug of War

A 74-year-old man had reproducible superior and inferior arcuate visual field defects in the left eye only that were initially believed to be caused by primary open-angle glaucoma. Diagnostic evaluation with the aid of optical coherence tomography revealed extrafoveal vitreomacular traction (VMT) with secondary retinal thickening and schisis. We discuss the evaluation of non-glaucomatous visual field defects and review the literature on the pathogenesis, clinical manifestations, and treatment of VMT syndrome.