David Attwell

Accumulation of proteins such as amyloid β (Aβ), hyperphosphorylated tau and α-synuclein within the brain alters neural information processing and causes neurodegeneration ^1–3 , but how toxic solutes are cleared from the brain remains highly controversial ^4,5 . Proposed exit routes include efflux across endothelial cells into the blood ^6,7 , and movement to the pial surface via vasomotion-induced pumping along spaces within arteriolar smooth muscle ⁸ or via outflow along the perivascular space of ascending venules promoted by water flux through astrocytes (the glymphatic system ⁹ ). From the pial surface of the brain, drainage may continue to dural lymphatics, along the outer sheaths of exiting cranial nerves and across the cribriform plate ^10–14 . We now report the presence, in mice and humans, of 2 μm diameter conduits that remove fluorescently labelled tau and Aβ from the brain. These conduits form a spatially-organised mesh within the walls of penetrating arterioles and pial arteries, and around the surface of ascending venules and deep cerebral and pial veins. They course through the pial and arachnoid layers to span the CSF space, wrapping the brain and cranial nerves. They are formed of reticular fibroblasts, which label for VE-cadherin ¹⁵ and PDGFRα ¹⁶ , the lymphatic markers ¹⁷ podoplanin, VEGFR3 and Prox1, and reticular fibroblast extracellular matrix components collagen I and VI ^16,18–20 . Parenchymal tau drains from the brain at a similar rate via arteriolar conduits and via conduits around venules, arguing against preferential removal by a glymphatic mechanism. In Alzheimer’s disease model mice, Aβ is seen traversing these lymph node-like conduits. Modulation of molecular transfer via this route may accelerate or delay cognitive decline, and slowed transfer from arteriolar to pial-arachnoid conduits may initiate cerebral amyloid angiopathy.

Microglia use a highly complex and dynamic network of branched processes to sense and respond to their surroundings. Despite emerging evidence that microglial motility plays important roles in brain development, neurodegeneration, and neuropsychiatric disease, little is known about the intracellular machinery orchestrating microglial process dynamics. Here, we identify roles for regulators of the actin cytoskeleton in controlling microglial behavior. We show that the actin branching Arp2/3 complex is critical for maintaining microglial morphology and is required for surveillance but not chemotactic motility. Neuropsychiatric disease-associated CYFIP1, a core component of the WAVE regulatory complex linking upstream signaling pathways to activation of the Arp2/3 complex, is highly expressed in microglia but has an unknown function. We report that conditional deletion of Cyfip1 in mouse microglia reduces their morphological complexity and surveillance of the brain parenchyma, with no effect on chemotaxis. Deletion of Cyfip1 also increased microglial CD68 positive lysosome volume and engulfment of presynapses. Thus, actin remodeling by CYFIP1 and the Arp2/3 complex controls microglial dynamics and shifts microglia away from a homeostatic state with potential implications for neuropsychiatric disease.

Failure to reperfuse the coronary microvasculature (“no-reflow”) affects up to 50% of patients after unblocking a coronary artery that was causing ischaemia and acute myocardial infarction. This “no-reflow” is associated with reduced left ventricular ejection fraction, increased infarct size and death. We show that the incretin hormone GLP-1 (glucagon-like peptide 1) can be used to protect the heart after ischaemia by activating ATP-sensitive K ⁺ channels on pericytes that constrict coronary capillaries. Coronary capillary dilation can be activated pharmacologically or by vagally-mediated GLP-1 release from the gut evoked by skeletal muscle ischaemia, and is abolished by block or genetic deletion of pericyte K _ATP channels. These results define a brain-gut-heart pathway mediating cardioprotection and suggest pharmacological therapies to reduce ischaemia-induced coronary no-reflow and improve post-infarct recovery.

There is increasing evidence to suggest that vascular dysfunction can contribute to cognitive decline in ageing and dementia. This dysfunction can take the form of a reduction of cerebral blood flow (CBF), a loss of blood-brain barrier (BBB) function or a combination of the two. Indeed, CBF and BBB changes may be causally linked, although this possible causality and its directionality are understudied. Appreciation of the role of vascular dysfunction in initiating cognitive decline in ageing and dementia, as well as the mechanisms involved, is important because it opens up new avenues for the development of much-needed therapies for these conditions, which are becoming major causes of death. Here we assess the evidence for the importance of vascular contributions to dementia, draw parallels with changes that occur in normal ageing and discuss the initiating cells and signalling mechanisms involved. We suggest that attempting to maintain or restore CBF should be a central aim of therapeutic strategies.

Failure to reperfuse the coronary microvasculature (“no-reflow”) affects up to 50% of patients after unblocking a coronary artery that was causing ischaemia and acute myocardial infarction. This “no-reflow” is associated with reduced left ventricular ejection fraction, increased infarct size, and death. We show that the incretin GLP-1 (glucagon-like peptide 1) can be used to protect the heart after ischemia by activating ATP-gated K ⁺ channels on pericytes that constrict coronary capillaries. Coronary capillary dilation can be activated pharmacologically or by vagally-mediated GLP-1 release from the gut evoked by skeletal muscle ischemia, and is abolished by block or genetic deletion of pericyte K _ATP channels. These results define a brain-gut-heart pathway mediating cardioprotection and suggest pharmacological therapies to reduce ischaemia-induced coronary no-reflow and improve post-infarct recovery.

Failure to reperfuse the coronary microvasculature (“no-reflow”) affects up to 50% of patients after unblocking a coronary artery that caused ischaemia and acute myocardial infarction. No-reflow is associated with reduced left ventricular ejection fraction, increased infarct size and death. We have established that no-reflow results from cardiac pericytes constricting coronary capillaries, and that pharmacologically relaxing pericytes reduces no-reflow. Remote ischaemic preconditioning, by briefly making a limb ischaemic, protects against cardiac ischaemic injury, and we have shown this is mediated by release of the gut hormone glucagon-like peptide 1 (GLP-1). We now demonstrate that, by releasing GLP-1, remote ischaemic preconditioning reduces pericyte-mediated coronary capillary constriction and no-reflow, and that the dilating effect of GLP-1 on coronary capillaries is abolished by block or genetic deletion of pericyte K _ATP channels. These results define a brain-gut-heart pathway mediating remote ischaemic cardioprotection, and suggest pharmacological therapies to reduce ischaemia-induced coronary no-reflow and improve post-infarct recovery.

Early in Alzheimer’s disease (AD), pericytes constrict capillaries, increasing their hydraulic resistance and trapping of immune cells and, thus, decreasing cerebral blood flow (CBF). Therapeutic approaches to attenuate pericyte-mediated constriction in AD are lacking. Here, using in vivo two-photon imaging with laser Doppler and speckle flowmetry and magnetic resonance imaging, we show that Ca²⁺ entry via L-type voltage-gated calcium channels (CaVs) controls the contractile tone of pericytes. In AD model mice, we identifed pericytes throughout the capillary bed as key drivers of an immune reactive oxygen species (ROS)-evoked and pericyte intracellular calcium concentration ([Ca²⁺]_i)-mediated decrease in microvascular flow. Blocking CaVs with nimodipine early in disease progression improved CBF, reduced leukocyte stalling at pericyte somata and attenuated brain hypoxia. Amyloid β (Aβ)-evoked pericyte contraction in human cortical tissue was also greatly reduced by CaV block. Lowering pericyte [Ca²⁺]_i early in AD may, thus, offer a therapeutic strategy to enhance brain energy supply and possibly cognitive function in AD.

In the last 20 years, there has been a revolution in our understanding of how blood flow is regulated in many tissues. Whereas it used to be thought that essentially all blood flow control occurred at the arteriole level, it is now recognized that control of capillary blood flow by contractile pericytes plays a key role both in regulating blood flow physiologically and in reducing it in clinically relevant pathological conditions. In this article, we compare and contrast how brain and cardiac pericytes regulate cerebral and coronary blood flow, focusing mainly on the pathological events of cerebral and cardiac ischaemia. The cerebral and coronary capillary beds differ dramatically in morphology, yet in both cases, pericyte-mediated capillary constriction plays a key role in restricting blood flow after ischaemia and possibly in other pathological conditions. We conclude with suggestions for therapeutic approaches to relaxing pericytes, which may prove useful in the long-term for reducing pericyte-induced ischaemia.

In microglia, changes in intracellular calcium concentration ([Ca²⁺]_i) may regulate process motility, inflammasome activation, and phagocytosis. However, while neurons and astrocytes exhibit frequent spontaneous Ca²⁺ activity, microglial Ca²⁺ signals are much rarer and poorly understood. Here, we studied [Ca²⁺]_i changes of microglia in acute brain slices using Fluo-4–loaded cells and mice expressing GCaMP5g in microglia. Spontaneous Ca²⁺ transients occurred ~ 5 times more frequently in individual microglial processes than in their somata. We assessed whether microglial Ca²⁺ responses change in Alzheimer's disease (AD) using App^NL−G−F knock-in mice. Proximity to Aβ plaques strongly affected microglial Ca²⁺ activity. Although spontaneous Ca²⁺ transients were unaffected in microglial processes, they were fivefold more frequent in microglial somata near Aβ plaques than in wild-type microglia. Microglia away from Aβ plaques in AD mice showed intermediate properties for morphology and Ca²⁺ responses, partly resembling those of wild-type microglia. By contrast, somatic Ca²⁺ responses evoked by tissue damage were less intense in microglia near Aβ plaques than in wild-type microglia, suggesting different mechanisms underlying spontaneous vs. damage-evoked Ca²⁺ signals. Finally, as similar processes occur in neurodegeneration and old age, we studied whether ageing affected microglial [Ca²⁺]_i. Somatic damage-evoked Ca²⁺ responses were greatly reduced in microglia from old mice, as in the AD mice. In contrast to AD, however, old age did not alter the occurrence of spontaneous Ca²⁺ signals in microglial somata but reduced the rate of events in processes. Thus, we demonstrate distinct compartmentalised Ca²⁺ activity in microglia from healthy, aged and AD-like brains.