The giant spectrin ßV couples the molecular motors to phototransduction and Usher syndrome type I proteins along their trafficking route.

Mutations in the myosin VIIa gene cause Usher syndrome type IB (USH1B), characterized by deaf-blindness. A delay of opsin trafficking has been observed in the retinal photoreceptor cells of myosin VIIa-deficient mice. We identified spectrin ßV, the mammalian ß-heavy spectrin, as a myosin VIIa- and rhodopsin-interacting partner in photoreceptor cells. Spectrin ßV displays a polarized distribution from the Golgi apparatus to the base of the outer segment, which, unlike that of other ß spectrins, matches the trafficking route of opsin and other phototransduction proteins. Formation of spectrin ßV-rhodopsin complex could be detected in the differentiating photoreceptors as soon as their outer segment emerges. A failure of the spectrin ßV-mediated coupling between myosin VIIa and opsin molecules thus probably accounts for the opsin transport delay in myosin VIIa-deficient mice. We showed that spectrin ßV also associates with two USH1 proteins, sans (USH1G) and harmonin (USH1C). Spectrins are supposed to function as heteromers of α and ß subunits, but fluorescence resonance energy transfer and in vitro binding experiments indicated that spectrin ßV can also form homodimers, which likely supports its αII-independent ßV functions. Finally, consistent with its distribution along the connecting cilia axonemes, spectrin ßV binds to several subunits of the microtubule-based motor proteins, kinesin II and the dynein complex. We therefore suggest that spectrin ßV homomers couple some USH1 proteins, opsin and other phototransduction proteins to both actin- and microtubule-based motors, thereby contributing to their transport towards the photoreceptor outer disks.

Cone degeneration is triggered by the absence of USH1 proteins but prevented by antioxidant treatments.

Usher syndrome type 1 (USH1) is a major cause of inherited deafness and blindness in humans. The eye disorder is often referred to as retinitis pigmentosa, which is characterized by a secondary cone degeneration following the rod loss. The development of treatments to prevent retinal degeneration has been hampered by the lack of clear evidence for retinal degeneration in mutant mice deficient for the Ush1 genes, which instead faithfully mimic the hearing deficit. We show that, under normal housing conditions, Ush1g-/- and Ush1c-/- albino mice have dysfunctional cone photoreceptors whereas pigmented knockout animals have normal photoreceptors. The key involvement of oxidative stress in photoreceptor apoptosis and the ensued retinal gliosis were further confirmed by their prevention when the mutant mice are reared under darkness and/or supplemented with antioxidants. The primary degeneration of cone photoreceptors contrasts with the typical forms of retinitis pigmentosa. Altogether, we propose that oxidative stress probably accounts for the high clinical heterogeneity among USH1 siblings, which also unveils potential targets for blindness prevention.

The GABAergic Gudden's dorsal tegmental nucleus: A new relay for serotonergic regulation of sleep-wake behavior in the mouse.

Serotonin (5-HT) neurons are involved in wake promotion and exert a strong inhibitory influence on rapid eye movement (REM) sleep. Such effects have been ascribed, at least in part to the action of 5-HT at post-synaptic 5-HT1A receptors (5-HT1AR) in the brainstem, a major wake/REM sleep regulatory center. However, the neuroanatomical substrate through which 5-HT1AR influence sleep remains elusive. We therefore investigated whether a brainstem structure containing a high density of 5-HT1AR mRNA, the GABAergic Gudden's dorsal tegmental nucleus (DTg), may contribute to 5-HT-mediated regulatory mechanisms of sleep-wake stages. We first found that bilateral lesions of the DTg promote wake at the expense of sleep. In addition, using local microinjections into the DTg in freely moving mice, we showed that local activation of 5-HT1AR by the prototypical agonist 8-OH-DPAT enhances wake and reduces deeply REM sleep duration. The specific involvement of 5-HT1AR in the latter effects was further demonstrated by ex vivo extracellular recordings showing that the selective 5-HT1AR antagonist WAY 100635 prevented DTg neuron inhibition by 8-OH-DPAT. We next found that GABAergic neurons of the ventral DTg exclusively targets glutamatergic neurons of the lateral mammillary nucleus (LM) in the posterior hypothalamus by means of anterograde and retrograde tracing techniques using cre driver mouse lines and a modified rabies virus. Altogether, our findings strongly support the idea that 5-HT-driven enhancement of wake results from 5-HT1AR-mediated inhibition of DTg GABAergic neurons that would in turn disinhibit glutamatergic neurons in the mammillary bodies. We therefore propose a Raphe→DTg→LM pathway as a novel regulatory circuit underlying 5-HT modulation of arousal.

On-line confocal imaging of the events leading to structural dedifferentiation of an axonal segment into a growth cone after axotomy.

The transformation of a transected axonal tip into a growth cone (GC) after axotomy is a critical step in the cascade of events leading to regeneration. However, the mechanisms underlying it are largely unknown. In earlier studies we reported that axotomy of cultured Aplysia neurons leads to a transient and local increase in the free intracellular Ca2+ concentration, calpain activation, and localized proteolysis of the submembranal spectrin. In a recent ultrastructural study, we reported that calpain activation is critical for the restructuring of the microtubules and neurofilaments at the cut axonal end to form a compartment in which vesicles accumulate. By using on-line confocal imaging of microtubules (MTs), actin, and vesicles in cultured Aplysia neurons, we studied the kinetics of the transformation and examined some of the mechanisms that orchestrate it. We report that perturbation of the MTs' polymerization by nocodazole inhibits the formation of an MT-based compartment in which the vesicles accumulate, yet actin repolymerization proceeds normally to form a nascent GC's lamellipodium. Nevertheless, under these conditions, the lamellipodium fails to expand and form neurites. When actin filament polymerization is inhibited by cytochalasin D or jasplakinolide, the MT-based compartment is formed and vesicles accumulate at the cut axonal end. However, a GC's lamellipodium is not formed, and the cut axonal end fails to regenerate. A growth-competent GC is formed only when MT restructuring, the accumulation of vesicles, and actin polymerization properly converge in time and space.

When reverse genetics meets physiology: the use of site-specific recombinases in mice.

The use of site-specific recombinases enables the precise introduction of defined genetic mutations into the mouse genome. In theory, any deletion, point mutation, inversion or translocation can be modeled in mice. Because gene targeting is controlled both spatially and temporally, the function of a given gene can be studied in the desired cell types and at a specific time point. This 'genetic dissection' allows to define gene function in development, physiology or behavior. In this review, we focus on the technical possibilities of Cre and other site-specific recombinases but also discuss their limitations.

A subpopulation of serotonergic neurons that do not express the 5-HT1A autoreceptor.

5-HT neurons are topographically organized in the hindbrain, and have been implicated in the etiology and treatment of psychiatric diseases such as depression and anxiety. Early studies suggested that the raphe 5-HT neurons were a homogeneous population showing similar electrical properties, and feedback inhibition mediated by 5-HT1A autoreceptors. We utilized histochemistry techniques in ePet1-eGFP and 5-HT1A-iCre/R26R mice to show that a subpopulation of 5-HT neurons do not express the somatodendritic 5-HT1A autoreceptor mRNA. In addition, we performed patch-clamp recordings followed by single-cell PCR in ePet1-eGFP mice. From 134 recorded 5-HT neurons located in the dorsal, lateral, and median raphe, we found lack of 5-HT1A mRNA expression in 22 cells, evenly distributed across raphe subfields. We compared the cellular characteristics of these neuronal types and found no difference in passive membrane properties and general excitability. However, when injected with large depolarizing current, 5-HT1A-negative neurons fired more action potentials, suggesting a lack of autoinhibitory action of local 5-HT release. Our results support the hypothesis that the 5-HT system is composed of subpopulations of serotonergic neurons with different capacity for adaptation.

Novel transgenic mice for inducible gene overexpression in pancreatic cells define glucocorticoid receptor-mediated regulations of beta cells.

Conditional gene deletion in specific cell populations has helped the understanding of pancreas development. Using this approach, we have shown that deleting the glucocorticoid receptor (GR) gene in pancreatic precursor cells leads to a doubled beta-cell mass. Here, we provide genetic tools that permit a temporally and spatially controlled expression of target genes in pancreatic cells using the Tetracycline inducible system. To efficiently target the Tetracycline transactivator (tTA) in specific cell populations, we generated Bacterial Artificial Chromosomes (BAC) transgenic mice expressing the improved Tetracycline transactivator (itTA) either in pancreatic progenitor cells expressing the transcription factor Pdx1 (BAC-Pdx1-itTA), or in beta cells expressing the insulin1 gene (BAC-Ins1-itTA). In the two transgenic models, itTA-mediated activation of reporter genes was efficient and subject to regulation by Doxycycline (Dox). The analysis of a tetracycline-regulated LacZ reporter gene shows that in BAC-Pdx1-itTA mice, itTA is expressed from embryonic (E) day 11.5 in all pancreatic precursor cells. In the adult pancreas, itTA is active in mature beta, delta cells and in few acinar cells. In BAC-Ins1-itTA mice tTA is active from E13.5 and is restricted to beta cells in fetal and adult pancreas. In both lines, tTA activity was suppressed by Dox treatment and re-induced after Dox removal. Using these transgenic lines, we overexpressed the GR in selective pancreatic cell populations and found that overexpression in precursor cells altered adult beta-cell fraction but not glucose tolerance. In contrast, GR overexpression in mature beta cells did not alter beta-cell fraction but impaired glucose tolerance with insufficient insulin secretion. In conclusion, these new itTA mouse models will allow fine-tuning of gene expression to investigate gene function in pancreatic biology and help us understand how glucocorticoid signaling affects on the long-term distinct aspects of beta-cell biology.

Localization of Usher 1 proteins to the photoreceptor calyceal processes, which are absent from mice.

The mechanisms underlying retinal dystrophy in Usher syndrome type I (USH1) remain unknown because mutant mice lacking any of the USH1 proteins-myosin VIIa, harmonin, cadherin-23, protocadherin-15, sans-do not display retinal degeneration. We found here that, in macaque photoreceptor cells, all USH1 proteins colocalized at membrane interfaces (i) between the inner and outer segments in rods and (ii) between the microvillus-like calyceal processes and the outer segment basolateral region in rods and cones. This pattern, conserved in humans and frogs, was mediated by the formation of an USH1 protein network, which was associated with the calyceal processes from the early embryonic stages of outer segment growth onwards. By contrast, mouse photoreceptors lacked calyceal processes and had no USH1 proteins at the inner-outer segment interface. We suggest that USH1 proteins form an adhesion belt around the basolateral region of the photoreceptor outer segment in humans, and that defects in this structure cause the retinal degeneration in USH1 patients.

5-HT1A-iCre, a new transgenic mouse line for genetic analyses of the serotonergic pathway.

The 5-HT1A receptor not only plays an important role in brain physiology but it may be also implicated in the etiology of behavioral disorders such as pathological anxiety. To further define the role of 5-HT1A receptor-expressing neurons, we generated a transgenic mouse line expressing Cre recombinase in these cells. The 5-HT1A receptor open reading frame was substituted for that of Cre recombinase in a BAC containing the 5-HT1A receptor gene. In adult transgenic brain, Cre expression perfectly matched the distribution of 5-HT1A receptor mRNA. Additionally, Cre-mediated DNA recombination was restricted to neuronal populations that express the receptor, e.g., cerebral cortex, septum, hippocampus, dorsal raphe, thalamic, hypothalamic and amygdaloid nuclei, and spinal cord. Recombination occurred as early as E13 in trigeminal nerve, spinal ganglia and spinal cord. This transgenic line will allow the generation of conditional mutant mice that lack specific gene products along the serotonergic pathways and represents a unique tool for studying 5-HT1A-mediated serotonin signaling in the developing and adult brain.

Combinatorial complexity of 5' alternative acetylcholinesterase transcripts and protein products.

To explore the scope and significance of alternate promoter usage and its putative inter-relationship to alternative splicing, we searched expression sequence tags for the 5' region of acetylcholinesterase (ACHE) genes. Three and five novel first exons were identified in human and mouse ACHE genes, respectively. Reverse transcription-PCR and in situ hybridization validated most of the predicted transcripts, and sequence analyses of the corresponding genomic DNA regions suggest evolutionarily conserved promoters for each of the novel exons identified. Distinct tissue specificity and stress-related expression patterns of these exons predict combinatorial complexity with known 3' alternative AChE mRNA transcripts. Unexpectedly one of the 5' exons encodes an extended N terminus in-frame with the known AChE sequence, extending the increased complexity to the protein level. The resultant membrane variant(s), designated N-AChE, is developmentally regulated in human brain neurons and blood mononuclear cells. Alternative promoter usage combined with alternative splicing may thus lead to stress-dependent combinatorial complexity of AChE mRNA transcripts and their protein products.