Photonics for biologists

Basic concepts of optical mapping techniques in cardiac electrophysiology

Optical mapping is a tool used in cardiac electrophysiology to study the heart's normal rhythm and arrhythmias. The optical mapping technique provides a unique opportunity to obtain membrane potential recordings with a higher temporal and spatial resolution than electrical mapping. Additionally, it allows simultaneous recording of membrane potential and calcium transients in the whole heart. This article presents the basic concepts of optical mapping techniques as an introduction for students and investigators in experimental laboratories unfamiliar with it.

Localization of puff sites adjacent to the plasma membrane: functional and spatial characterization of Ca2+ signaling in SH-SY5Y cells utilizing membrane-permeant caged IP3

The Xenopus oocyte has been a favored model system in which to study spatio-temporal mechanisms of intracellular Ca2+ dynamics, in large part because this giant cell facilitates intracellular injections of Ca2+ indicator dyes, buffers and caged compounds. However, the recent commercial availability of membrane-permeant ester forms of caged IP3 (ci-IP3) and EGTA, now allows for facile loading of these compounds into smaller mammalian cells, permitting control of [IP3]i and cytosolic Ca2+ buffering. Here, we establish the human neuroblastoma SH-SY5Y cell line as an advantageous experimental system for imaging Ca2+ signaling, and characterize IP3-mediated Ca2+ signaling mechanisms in these cells. Flash photo-release of increasing amounts of i-IP3 evokes Ca2+ puffs that transition to waves, but intracellular loading of EGTA decouples release sites, allowing discrete puffs to be studied over a wide range of [IP3]. Puff activity persists for minutes following a single photo-release, pointing to a slow rate of i-IP3 turnover in these cells and suggesting that repetitive Ca2+ spikes with periods of 20-30s are not driven by oscillations in [IP3]. Puff amplitudes are independent of [IP3], whereas their frequencies increase with increasing photo-release. Puff sites in SH-SY5Y cells are not preferentially localized near the nucleus, but instead are concentrated close to the plasma membrane where they can be visualized by total internal reflection microscopy, offering the potential for unprecedented spatio-temporal resolution of Ca2+ puff kinetics.

Imaging the live plant cell

Observing a biological event as it unfolds in the living cell provides unique insight into the nature of the phenomenon under study. Capturing live cell data differs from imaging fixed preparations because living plants respond to the intense light used in the imaging process. In addition, live plant cells are inherently thick specimens containing colored and fluorescent molecules often removed when the plant is fixed and sectioned. For fixed cells, the straightforward goal is to maximize contrast and resolution. For live cell imaging, maximizing contrast and resolution will probably damage the specimen or rapidly bleach the probe. Therefore, the goals are different. Live cell imaging seeks a balance between image quality and the information content that comes with increasing contrast and resolution. That "lousy" live cell image may contain all the information needed to answer the question being posed--provided the investigator properly framed the question and imaged the cells appropriately. Successful data collection from live cells requires developing a specimen-mounting protocol, careful selection and alignment of microscope components, and a clear understanding of how the microscope system generates contrast and resolution. This paper discusses general aspects of modern live cell imaging and the special considerations for imaging live plant specimens.

Imaging the activity and localization of single voltage-gated Ca(2+) channels by total internal reflection fluorescence microscopy

The patch-clamp technique has enabled functional studies of single ion channels, but suffers limitations including lack of spatial information and inability to independently monitor currents from more than one channel. Here, we describe the use of total internal reflection fluorescence microscopy as an alternative, noninvasive approach to optically monitor the activity and localization of multiple Ca(2+)-permeable channels in the plasma membrane. Images of near-membrane Ca(2+) signals were obtained from >100 N-type channels expressed within restricted areas (80 x 80 micro m) of Xenopus oocytes, thereby permitting simultaneous resolution of their gating kinetics, voltage dependence, and localization. Moreover, this technique provided information inaccessible by electrophysiological means, demonstrating that N-type channels are immobile in the membrane, show a patchy distribution, and display diverse gating kinetics even among closely adjacent channels. Total internal reflection fluorescence microscopy holds great promise for single-channel recording of diverse voltage- and ligand-gated Ca(2+)-permeable channels in the membrane of neurons and other isolated or cultured cells, and has potential for high-throughput functional analysis of single channels.