Electro-metabolic signaling

Abstract

Precise matching of energy substrate delivery to local metabolic needs is essential for the health and function of all tissues. Here, we outline a mechanistic framework for understanding this critical process, which we refer to as electro-metabolic signaling (EMS). All tissues exhibit changes in metabolism over varying spatiotemporal scales and have widely varying energetic needs and reserves. We propose that across tissues, common signatures of elevated metabolism or increases in energy substrate usage that exceed key local thresholds rapidly engage mechanisms that generate hyperpolarizing electrical signals in capillaries that then relax contractile elements throughout the vasculature to quickly adjust blood flow to meet changing needs. The attendant increase in energy substrate delivery serves to meet local metabolic requirements and thus avoids a mismatch in supply and demand and prevents metabolic stress. We discuss in detail key examples of EMS that our laboratories have discovered in the brain and the heart, and we outline potential further EMS mechanisms operating in tissues such as skeletal muscle, pancreas, and kidney. We suggest that the energy imbalance evoked by EMS uncoupling may be central to cellular dysfunction from which the hallmarks of aging and metabolic diseases emerge and may lead to generalized organ failure states-such as diverse flavors of heart failure and dementia. Understanding and manipulating EMS may be key to preventing or reversing these dysfunctions.

Figure 1.

An overview of EMS. EMS consists of three key stages (top) for which example inputs and outputs are given for an idealized tissue. This basic framework can be applied to study EMS in different organ systems, but the molecules underlying initiation and transduction may vary. EMS is first initiated by metabolic activity which causes a decrease in the availability of extracellular substrates and/or changes in intracellular energy status and an increase in metabolic byproducts. In the second stage of EMS, this metabolic disturbance is sensed by the local vasculature and rapidly transduced into endothelial cell membrane hyperpolarization via an increase in K⁺ channel activity, which will ultimately promote vasodilation and an increase in blood flow. Hyperpolarizing signals are initiated in capillaries at the nexus of metabolic activity where capillary pericytes and endothelial cells play key roles in this process and are transmitted either by passive electrotonic spread or by regenerative Kir2.1-mediated signaling upstream to areas of the vasculature covered in contractile cells (i.e., contractile pericytes or smooth muscle cells). These contractile cells are induced to relax to produce an increase in blood flow, and this final output of EMS delivers more substrates to the active region and has the effect of reversing the deviations from set-point of the initiating metabolic factors and homeostatically resets local metabolic reserves.

Figure 2.

Metabolic cues for EMS. The basic metabolic processes occurring in all cells provide a rich tapestry of potential cues for EMS. Here, the oxidation of glucose is depicted, showing several potential EMS cues in green. Initially, a substrate is transported into the cell cytoplasm for use. In this example, glucose is brought in via GLUT transporters and then processed via the 10 reactions of glycolysis to yield two pyruvate molecules and two ATP, along with two NADH. From here, pyruvate can be fermented to lactate, which can be released from cells via monocarboxylate (MCT) transporters. Alternatively, pyruvate can be used to produce acetyl-CoA (also producing NADH and CO₂ as a byproduct) that can enter the TCA cycle in the mitochondrial matrix. The TCA cycle reduces a number of NAD⁺ molecules to NADH and FAD⁺ to FADH₂, while also generating more CO₂. The NADH and FADH₂ in turn act as electron donors to the ETC in the inner mitochondrial membrane to generate a proton gradient that ATP synthase (i.e., Complex V) uses to yield large amounts of ATP, which can then be exported via the adenine nucleotide translocator in exchange for ADP. In addition to the generated CO₂, lactate, or glucose levels themselves acting as cues, the redox state of the cell or the ATP:ADP ratio may also signal to open K⁺ channels such as K_ATP to generate membrane hyperpolarization. Either ATP:ADP ratio in the bulk cytosol or in submembrane regions around response elements could conceivably contribute to this process. The generation of these cues may occur directly in the working cells of a given tissue such as cardiac myocytes in the heart, or in metabolic sentinel cells such as in pericytes in the brain.

Figure B1.

Vascular organization and cell types. (A) Scanning electron micrograph (SEM) of a bifurcating arteriole covered in contractile smooth muscle cells (SMCs; individual cells denoted in orange). Scale bar: 50 µm. (B) SEM of a contractile pericyte and its processes (pink) enwrapping a capillary. Note the clear distinction from SMC morphology. Scale bar: 11.5 µm. A and B adapted with permission from Rodriguez-Baeza et al. (1998). (C) SEM of brain capillaries showing the sequential branch ordering of vessels up to the fifth order (green). Note this system is not perfect, as some anastomosing vessels could be labeled as two distinct orders (*). Scale bar: 50 µm. Adapted with permission from Krucker et al. (2006). (D) SEM of the choroid vasculature showing lobular vascular architecture which makes it impossible to accurately assign branch orders (e.g., green vessels). 65× magnification. Adapted with permission from Miodoński and Jasiński (1979).

Figure 3.

K⁺ channels at the heart of EMS. (A) Topographic depiction of vascular K_ATP channel composition showing a two-transmembrane Kir6.1 subunit alongside its much larger companion 17-transmembrane SUR2B. (B) Side-on cryo-EM density map of the vascular K_ATP channel in the presence of the inhibitor glibenclamide. (C) Structural model of the data shown in B, showing the locations of the glibenclamide binding site (Glib), SUR nucleotide-binding domains (NBDs), ATP binding sites (ATP), and the L0 linker, as well as several transmembrane domains (TMDs). (D) Top-down view of the channel as seen from the extracellular side. (A–D) Modified with permission from Sung et al. (2021). (E) Side-on (left), top-down (upper right), and bottom-up (lower right) cryo-EM density maps of the Kir2.1 channel at 4.3 Å resolution. (F) Model of the data in E shows key structural features. K⁺ ions and blocking strontium ions (which mimic Mg²⁺ ions) are shown in the pore in magenta and green, respectively. (G) Top-down model of the channel seen from the extracellular side showing chains comprising the transmembrane domains in blue and disulfide bridges between subunits in red. (E–G) Modified with permission from Fernandes et al. (2022).

Figure 4.

EMS transmission mechanisms. (A) Comparison of the vascular volume in a 1 mm³ block of cortical tissue comprised by surface arteries, penetrating arterioles, and the capillary bed. Capillaries comprise about 85% of the vasculature by volume, with the penetrating arterioles contributing just 3%. Thus, the capillaries are ideally positioned to act as a sensory array communicating information on tissue activity to the arterioles to regulate blood flow. Adapted with permission from Gould et al. (2017). (B) The central image depicts an arteriole lined by arterial endothelial cells (aECs) and surrounded by smooth muscle cells (SMCs). Downstream of this emerge capillaries, the proximal branches of which are covered in contractile pericytes. Deeper downstream branches are covered by thin-strand pericytes. The dotted box depicts events leading to the initiation of EMS in the capillary bed, leading to the generation and transmission of hyperpolarizing signals. In inset i (middle), a specific example is given for a decrease in glucose availability based on Hariharan et al. (2022), which engages K_ATP channels. This hyperpolarization can be passed via gap junctions (GJs) into the underlying capillary endothelial cells (cECs), which may then transmit the signal upstream via the mechanism depicted in inset ii (bottom). Here, hyperpolarization in the endothelium relieves the voltage-dependent block of Kir2.1 channels by polyamines, leading to K⁺ efflux and regeneration of membrane hyperpolarization through the engagement of further Kir2.1 channels as the signal travels upstream. Once it arrives in the proximal capillaries and arteriolar segment (inset iii, top), the electrical signal can be transmitted via gap junctions into both the contractile pericytes and upstream SMCs, where its arrival decreases the open probability of voltage-dependent Ca²⁺ channels and produces a decrease in intracellular Ca²⁺ which relaxes the Ca²⁺-sensitive contractile machinery and promotes vasodilation and an increase in blood flow.

Figure 5.

Electrical signal length constants and resolution of blood flow control. Concepts are depicted for brain vasculature. Similar principles likely apply in other organs, modified by specific angioarchitectural features and molecular configurations. (A) Brain vasculature. Top: Top-down view showing overlapping pial (surface) arterial and veinous networks. Bottom: Side-view showing a pial artery transitioning to a penetrating arteriole as it dives into the brain and gives way to the capillary network. Left inset: Neurovascular unit at the initial capillaries surrounded by contractile pericytes. Right inset: Neurovascular unit of the deep capillary bed with thin strand pericytes. EC, endothelial cell; GJ, gap junction; PSJ, peg-socket junction; RBC, red blood cell. (B) Diameter–voltage relationship for a pial artery showing that maximal working range is controlled across ∼30 mV of membrane potential. Adapted with permission from Knot and Nelson (1998). (C) Regenerative and passive signal transmission modes. Top: Theoretical signal transmission–distance relationships for regenerative and passive signals. Regenerative mechanisms will conduct over greater lengths of vessel due to the signal being actively renewed. Bottom: Graphical depiction of regenerative versus passive transmission modes. (D) The length constant of transmission, dictated by input amplitude and the electrical properties of the local vasculature, will determine the resolution of blood flow control. Top: Activation of large areas of tissue will initiate large signals in the vasculature that conduct over longer distances and induce more contractile cells to relax and produce a low-resolution blood flow increase that encompasses many cells. Middle: Smaller metabolically active fields will evoke electrical signals with a shorter length constant, recruiting fewer vessels and evoking more localized blood flow increases that perfuse a smaller tissue volume. Bottom: The smallest active regions may evoke a higher resolution blood flow increase still.