The San Diego Nathan Shock Center: tackling the heterogeneity of aging.

Understanding basic mechanisms of aging holds great promise for developing interventions that prevent or delay many age-related declines and diseases simultaneously to increase human healthspan. However, a major confounding factor in aging research is the heterogeneity of the aging process itself. At the organismal level, it is clear that chronological age does not always predict biological age or susceptibility to frailty or pathology. While genetics and environment are major factors driving variable rates of aging, additional complexity arises because different organs, tissues, and cell types are intrinsically heterogeneous and exhibit different aging trajectories normally or in response to the stresses of the aging process (e.g., damage accumulation). Tackling the heterogeneity of aging requires new and specialized tools (e.g., single-cell analyses, mass spectrometry-based approaches, and advanced imaging) to identify novel signatures of aging across scales. Cutting-edge computational approaches are then needed to integrate these disparate datasets and elucidate network interactions between known aging hallmarks. There is also a need for improved, human cell-based models of aging to ensure that basic research findings are relevant to human aging and healthspan interventions. The San Diego Nathan Shock Center (SD-NSC) provides access to cutting-edge scientific resources to facilitate the study of the heterogeneity of aging in general and to promote the use of novel human cell models of aging. The center also has a robust Research Development Core that funds pilot projects on the heterogeneity of aging and organizes innovative training activities, including workshops and a personalized mentoring program, to help investigators new to the aging field succeed. Finally, the SD-NSC participates in outreach activities to educate the general community about the importance of aging research and promote the need for basic biology of aging research in particular.

The National Heart, Lung, and Blood Institute Small Business Program: A Comprehensive Ecosystem for Biomedical Product Development.

Small companies working to develop products in the cardiovascular space face numerous challenges, from regulatory, intellectual property, and reimbursement barriers to securing funds to keep the lights on and reach the next development milestone. Most small companies that spin out from universities have the scientific knowledge, but product development expertise and business acumen are also needed to be successful. Other challenges include reduced interest in early stage technologies (Pharma & Biotech 2015 in Review, EP Vantage) and limited deal flow for cardiovascular products (Gormley B., Wall Street Journal, September 15, 2014). The NHLBI small business program is a comprehensive ecosystem designed to address these critical challenges and to provide resources and expertise to assist early stage companies developing cardiovascular and other products within the institute's mission. This article describes steps that NHLBI has taken to enhance our small business program to more effectively translate basic discoveries into commercial products to benefit patients and public health, including enhancing internal expertise and developing non-financial resources to assist small businesses as they develop their products and seek private sector investment and partnership.

Acute inactivation of PSD-95 destabilizes AMPA receptors at hippocampal synapses.

Postsynatptic density protein (PSD-95) is a 95 kDa scaffolding protein that assembles signaling complexes at synapses. Over-expression of PSD-95 in primary hippocampal neurons selectively increases synaptic localization of AMPA receptors; however, mice lacking PSD-95 display grossly normal glutamatergic transmission in hippocampus. To further study the scaffolding role of PSD-95 at excitatory synapses, we generated a recombinant PSD-95-4c containing a tetracysteine motif, which specifically binds a fluorescein derivative and allows for acute and permanent inactivation of PSD-95. Interestingly, acute inactivation of PSD-95 in rat hippocampal cultures rapidly reduced surface AMPA receptor immunostaining, but did not affected NMDA or transferrin receptor localization. Acute photoinactivation of PSD-95 in dissociated neurons causes ∼80% decrease in GluR2 surface staining observed by live-cell microscopy within 15 minutes of PSD-95-4c ablation. These results confirm that PSD-95 stabilizes AMPA receptors at postsynaptic sites and provides insight into the dynamic interplay between PSD-95 and AMPA receptors in live neurons.

cJun integrates calcium activity and tlx3 expression to regulate neurotransmitter specification.

Neuronal differentiation is accomplished through cascades of intrinsic genetic factors initiated in neuronal progenitors by external gradients of morphogens. Activity has been thought to be important only late in development, but recent evidence suggests that activity also regulates early neuronal differentiation. Activity in post-mitotic neurons before synapse formation can regulate phenotypic specification, including neurotransmitter choice, but the mechanisms are not clear. We identified a mechanism that links endogenous calcium spike activity with an intrinsic genetic pathway to specify neurotransmitter choice in neurons in the dorsal embryonic spinal cord of Xenopus tropicalis. Early activity modulated transcription of the GABAergic/glutamatergic selection gene tlx3 through a variant cAMP response element (CRE) in its promoter. The cJun transcription factor bound to this CRE site, modulated transcription and regulated neurotransmitter phenotype via its transactivation domain. Calcium signaled through cJun N-terminal phosphorylation, which integrated activity-dependent and intrinsic neurotransmitter specification. This mechanism provides a basis for early activity to regulate genetic pathways at critical decision points, switching the phenotype of developing neurons.

NF-kappaB, IkappaB, and IRAK control glutamate receptor density at the Drosophila NMJ.

NF-kappaB signaling has been implicated in neurodegenerative disease, epilepsy, and neuronal plasticity. However, the cellular and molecular activity of NF-kappaB signaling within the nervous system remains to be clearly defined. Here, we show that the NF-kappaB and IkappaB homologs Dorsal and Cactus surround postsynaptic glutamate receptor (GluR) clusters at the Drosophila NMJ. We then show that mutations in dorsal, cactus, and IRAK/pelle kinase specifically impair GluR levels, assayed immunohistochemically and electrophysiologically, without affecting NMJ growth, the size of the postsynaptic density, or homeostatic plasticity. Additional genetic experiments support the conclusion that cactus functions in concert with, rather than in opposition to, dorsal and pelle in this process. Finally, we provide evidence that Dorsal and Cactus act posttranscriptionally, outside the nucleus, to control GluR density. Based upon our data we speculate that Dorsal, Cactus, and Pelle could function together, locally at the postsynaptic density, to specify GluR levels.

Mechanisms underlying the rapid induction and sustained expression of synaptic homeostasis.

Homeostatic signaling systems are thought to interface with the mechanisms of neural plasticity to achieve stable yet flexible neural circuitry. However, the time course, molecular design, and implementation of homeostatic signaling remain poorly defined. Here we demonstrate that a homeostatic increase in presynaptic neurotransmitter release can be induced within minutes following postsynaptic glutamate receptor blockade. The rapid induction of synaptic homeostasis is independent of new protein synthesis and does not require evoked neurotransmission, indicating that a change in the efficacy of spontaneous quantal release events is sufficient to trigger the induction of synaptic homeostasis. Finally, both the rapid induction and the sustained expression of synaptic homeostasis are blocked by mutations that disrupt the pore-forming subunit of the presynaptic Ca(V)2.1 calcium channel encoded by cacophony. These data confirm the presynaptic expression of synaptic homeostasis and implicate presynaptic Ca(V)2.1 in a homeostatic retrograde signaling system.

Controlling the active properties of excitable cells.

Molecular perturbations of neurons, including genetic knockout and transgenic approaches, have provided insight into the cellular processes underlying neuronal function and plasticity. A detailed understanding of how individual neurons participate in the circuitry that controls behavior, however, will require the ability to experimentally manipulate the active properties of neurons in vivo. Recent technologies have greatly advanced our experimental ability to modulate the active properties of neurons with spatial and temporal precision; technical advances have been applied to the investigation of a diverse array of neurobiological questions.

Synaptotagmin I is necessary for compensatory synaptic vesicle endocytosis in vivo.

Neurotransmission requires a balance of synaptic vesicle exocytosis and endocytosis. Synaptotagmin I (Syt I) is widely regarded as the primary calcium sensor for synaptic vesicle exocytosis. Previous biochemical data suggest that Syt I may also function during synaptic vesicle endocytosis; however, ultrastructural analyses at synapses with impaired Syt I function have provided an indirect and conflicting view of the role of Syt I during synaptic vesicle endocytosis. Until now it has not been possible experimentally to separate the exocytic and endocytic functions of Syt I in vivo. Here, we test directly the role of Syt I during endocytosis in vivo. We use quantitative live imaging of a pH-sensitive green fluorescent protein fused to a synaptic vesicle protein (synapto-pHluorin) to measure the kinetics of endocytosis in sytI-null Drosophila. We then combine live imaging of the synapto-pHluorins with photoinactivation of Syt I, through fluorescein-assisted light inactivation, after normal Syt I-mediated vesicle exocytosis. By inactivating Syt I only during endocytosis, we demonstrate that Syt I is necessary for the endocytosis of synaptic vesicles that have undergone exocytosis using a functional Syt I protein.

Transgenically encoded protein photoinactivation (FlAsH-FALI): acute inactivation of synaptotagmin I.

We demonstrate a noninvasive technique for protein photoinactivation using a transgenically encoded tag. A tetracysteine motif that binds the membrane-permeable fluorescein derivative 4',5'-bis(1,3,2-dithioarsolan-2-yl)fluorescein (FlAsH) was engineered into synaptotagmin I (Syt I4C). Neuronally expressed Syt I4C rescues the syt I null mutation, can be visualized after FlAsH labeling, and is normally distributed at the Drosophila neuromuscular synapse. Illumination of FlAsH bound Syt I4C at 488 nm decreases evoked release in seconds demonstrating efficient fluorophore-assisted light inactivation (FlAsH-FALI) of Syt I. The inactivation of Syt I is proportional to the duration of illumination and follows first-order kinetics. In addition, Syt I FlAsH-FALI is specific and does not impair Syt I-independent vesicle fusion. We demonstrate that Syt I is required for a post-docking step during vesicle fusion but does not function to stabilize the docked vesicle state.