Brain Pericyte Calcium and Hemodynamic Imaging in Transgenic Mice In Vivo

Recent advances in protein biology and mouse genetics have made it possible to measure intracellular calcium fluctuations of brain cells in vivo and to correlate this with local hemodynamics. This protocol uses transgenic mice that have been prepared with a chronic cranial window and express the genetically encoded calcium indicator, RCaMP1.07, under the α-smooth muscle actin promoter to specifically label mural cells, such as vascular smooth muscle cells and ensheathing pericytes. Steps are outlined on how to prepare a tail vein catheter for intravenous injection of fluorescent dyes to trace blood flow, as well as how to measure brain pericyte calcium and local blood vessel hemodynamics (diameter, red blood cell velocity, etc.) by two photon microscopy in vivo through the cranial window in ketamine/xylazine anesthetized mice. Finally, details are provided for the analysis of calcium fluctuations and blood flow movies via the image processing algorithms developed by Barrett et al. 2018, with an emphasis on how these processes can be adapted to other cellular imaging data.

Non-Canonical Control of Neuronal Energy Status by the Na+ Pump

Neurons have limited intracellular energy stores but experience acute and unpredictable increases in energy demand. To better understand how these cells repeatedly transit from a resting to active state without undergoing metabolic stress, we monitored their early metabolic response to neurotransmission using ion-sensitive probes and FRET sensors in vitro and in vivo. A short theta burst triggered immediate Na⁺ entry, followed by a delayed stimulation of the Na⁺/K⁺ ATPase pump. Unexpectedly, cytosolic ATP and ADP levels were unperturbed across a wide range of physiological workloads, revealing strict flux coupling between the Na⁺ pump and mitochondria. Metabolic flux measurements revealed a "priming" phase of mitochondrial energization by pyruvate, whereas glucose consumption coincided with delayed Na⁺ pump stimulation. Experiments revealed that the Na⁺ pump plays a permissive role for mitochondrial ATP production and glycolysis. We conclude that neuronal energy homeostasis is not mediated by adenine nucleotides or by Ca²⁺, but by a mechanism commanded by the Na⁺ pump.

Fiber-optic implant for simultaneous fluorescence-based calcium recordings and BOLD fMRI in mice

Despite the growing popularity of blood oxygen level-dependent (BOLD) functional MRI (fMRI), understanding of its underlying principles is still limited. This protocol describes a technique for simultaneous measurement of neural activity using fluorescent calcium indicators together with the corresponding hemodynamic BOLD fMRI response in the mouse brain. Our early work using small-molecule fluorophores in rats gave encouraging results but was limited to acute measurements using synthetic dyes. Our latest procedure combines fMRI with optical detection of cell-type-specific virally delivered GCaMP6, a genetically encoded calcium indicator (GECI). GCaMP6 fluorescence, which increases upon calcium binding, is collected by a chronically implanted optical fiber, allowing longitudinal studies in mice. The chronic implant, placed horizontally on the skull, has an angulated tip that reflects light into the brain and is connected via fiber optics to a remote optical setup. The technique allows access to the neocortex and does not require adaptations of commercial MRI hardware. The hybrid approach permits fiber-optic calcium recordings with simultaneous artifact-free BOLD fMRI with full brain coverage and 1-s temporal resolution using standard gradient-echo echo-planar imaging (GE-EPI) sequences. The method provides robust, cell-type-specific readouts to link neural activity to BOLD signals, as emonstrated for task-free ('resting-state') conditions and in response to hind-paw stimulation. These results highlight the power of fiber photometry combined with fMRI, which we aim to further advance in this protocol. The approach can be easily adapted to study other molecular processes using suitable fluorescent indicators.

In Vivo Evidence for a Lactate Gradient from Astrocytes to Neurons

Investigating lactate dynamics in brain tissue is challenging, partly because in vivo data at cellular resolution are not available. We monitored lactate in cortical astrocytes and neurons of mice using the genetically encoded FRET sensor Laconic in combination with two-photon microscopy. An intravenous lactate injection rapidly increased the Laconic signal in both astrocytes and neurons, demonstrating high lactate permeability across tissue. The signal increase was significantly smaller in astrocytes, pointing to higher basal lactate levels in these cells, confirmed by a one-point calibration protocol. Trans-acceleration of the monocarboxylate transporter with pyruvate was able to reduce intracellular lactate in astrocytes but not in neurons. Collectively, these data provide in vivo evidence for a lactate gradient from astrocytes to neurons. This gradient is a prerequisite for a carrier-mediated lactate flux from astrocytes to neurons and thus supports the astrocyte-neuron lactate shuttle model, in which astrocyte-derived lactate acts as an energy substrate for neurons.