Periodic Reporting for period 4 - BrainEnergy (Control of cerebral blood flow by capillary pericytes in health and disease)
Periodo di rendicontazione: 2022-03-01 al 2024-02-29
Problem being addressed
Pericytes, contractile cells on the smallest blood vessels, capillaries, are major controllers of brain blood flow. Normally, they dilate capillaries in response to nerve cell activity, increasing blood flow and energy supply. But in pathology they have a sinister role. After artery block causes a stroke, the brain shows the so-called “no-reflow” phenomenon - a failure to fully reperfuse capillaries, even after the upstream occluded artery has been reperfused. The resulting long-lasting decrease of energy supply damages neurons. We have shown that the cause of no-reflow lies in pericytes: during the loss of blood flow (ischaemia) they constrict and die in rigor. This reduces capillary diameter and blood flow, and degrades the function of the barrier between the blood and the brain. They also play a key role in Alzheimer’s disease and Covid-19, as described below. However, despite their crucial role in regulating blood flow, little is known about the mechanisms by which pericytes function.
Importance for society?
The social and economic costs of diseases that pericytes contribute to are enormous. Apart from the personal suffering, in the EU stroke alone is thought to cost €60 billion/year, while dementia costs €220 billion/year.
Overall objectives?
Using advanced imaging/electrical recording techniques, and live human tissue obtained from neurosurgery, we aimed to:
(i) define the signalling mechanisms controlling capillary diameter;
(ii) identify the contributions of different brain cell types to regulating pericyte tone;
(iii) develop approaches to prevent pericyte constriction and death in ischaemia;
(iv) define how pericyte constriction of capillaries and pericyte death contribute to Alzheimer’s disease;
(v) extend these results from rodent to human brain pericytes for developing therapies.
The diseases to which pericytes contribute include stroke, spinal cord injury, diabetes, Alzheimer’s disease and (since 2020) Covid-19. These all have an enormous economic impact, as well as causing great suffering for patients and their carers. This work provides novel therapeutic approaches for treating these diseases.
Conclusions of the action?
We have defined the key mechanisms controlling pericyte constriction and relaxation, and shown how in a number of diseases pericytes constrict capillaries and reduce brain blood flow. Although this work focussed on rodent blood vessels, parallel work on human brain slices from neurosurgery allowed us to show that similar mechanisms operate in humans. This has allowed us to define new therapeutic targets for stroke, Alzheimer's disease and Covid-19.
Pericytes, contractile cells on the smallest blood vessels, capillaries, are major controllers of brain blood flow. Normally, they dilate capillaries in response to nerve cell activity, increasing blood flow and energy supply. But in pathology they have a sinister role. After artery block causes a stroke, the brain shows the so-called “no-reflow” phenomenon - a failure to fully reperfuse capillaries, even after the upstream occluded artery has been reperfused. The resulting long-lasting decrease of energy supply damages neurons. We have shown that the cause of no-reflow lies in pericytes: during the loss of blood flow (ischaemia) they constrict and die in rigor. This reduces capillary diameter and blood flow, and degrades the function of the barrier between the blood and the brain. They also play a key role in Alzheimer’s disease and Covid-19, as described below. However, despite their crucial role in regulating blood flow, little is known about the mechanisms by which pericytes function.
Importance for society?
The social and economic costs of diseases that pericytes contribute to are enormous. Apart from the personal suffering, in the EU stroke alone is thought to cost €60 billion/year, while dementia costs €220 billion/year.
Overall objectives?
Using advanced imaging/electrical recording techniques, and live human tissue obtained from neurosurgery, we aimed to:
(i) define the signalling mechanisms controlling capillary diameter;
(ii) identify the contributions of different brain cell types to regulating pericyte tone;
(iii) develop approaches to prevent pericyte constriction and death in ischaemia;
(iv) define how pericyte constriction of capillaries and pericyte death contribute to Alzheimer’s disease;
(v) extend these results from rodent to human brain pericytes for developing therapies.
The diseases to which pericytes contribute include stroke, spinal cord injury, diabetes, Alzheimer’s disease and (since 2020) Covid-19. These all have an enormous economic impact, as well as causing great suffering for patients and their carers. This work provides novel therapeutic approaches for treating these diseases.
Conclusions of the action?
We have defined the key mechanisms controlling pericyte constriction and relaxation, and shown how in a number of diseases pericytes constrict capillaries and reduce brain blood flow. Although this work focussed on rodent blood vessels, parallel work on human brain slices from neurosurgery allowed us to show that similar mechanisms operate in humans. This has allowed us to define new therapeutic targets for stroke, Alzheimer's disease and Covid-19.
Progress on the planned work has been excellent, especially considering its interruption by the Covid-19 pandemic, as follows.
(1) Projects 1 & 2 of the application - Signalling and cells regulating pericyte tone. We showed neuronal activity releases ATP, raising astrocyte [Ca2+]i. This releases prostaglandin which dilates capillaries via pericytes. This was published in Nature Neurosci (Mishra et al., 2016). We made a transgenic mouse that allows us to monitor the [Ca2+]i in pericytes which regulates contraction (published in Science: Nortley et al., 2019) and found that pericyte [Ca2+]i elevations are amplified by a Cl- channel in the cells, and this may provide a therapy to reduce capillary constriction after stroke (Korte et al., 2022, J Clin Invest). We have data being prepared for publication showing microglia can control pericyte function, and we have defined how microglial [Ca2+] changes with age and Alzheimer’s disease (AD) (Izquierdo et al., 2024, Pfluger’s Arch). We found that an enzyme regulating ATP concentration is often contaminated with K+ ions, which may invalidate previous experiments studying signalling that regulates glial cell function and brain blood flow (Madry et al., 2018, PNAS). We discovered that angiotensin and endothelin receptors control pericyte contraction, and that raising cyclic nucleotide levels relaxes pericytes (Nortley et al., 2019, Science; Hirunpattarasilp et al., 2023, Brain).
(2) Project 3. We have modelled the effect of pericytes on brain blood flow (Davis & Attwell, 2023, J Physiol).
(3) Projects 4, 5, 6 - time course of pericyte constriction and death in vivo. We have defined the time course of pericyte constriction and death, shown that a Ca channel blocker reduces constriction and improves neurological outcome after simulated stroke and in AD, and defined how the blood-brain barrier changes in AD (Korte et al., 2024, accepted with minor revisions).
(4) Project 7. We have a paper in preparation showing how ischaemic preconditioning prevents pericyte contraction.
(5) Projects 8 & 9 - properties of human pericytes and role of pericytes in Alzheimer’s disease. Remarkably, we discovered that in a mouse model of AD and in human patients with AD, cerebral blood flow is reduced by pericytes constricting capillaries. The mechanism is that amyloid beta evokes release of reactive oxygen species which evoke endothelin-1 release onto pericytes, which respond using endothelin A receptors to constrict the capillaries (Nortley et al., 2019, Science). This suggested pharmacological approaches to preventing capillary constriction in Alzheimer’s disease, and we have preliminary data in mice showing that brain blood flow can be preserved using one of these approaches.
(6) Related to the work in the application - brain energy use. We have shown that the properties of synapses that transmit visual information from the thalamus to the brain are optimised to maximise the flow of information per energy used, rather than simply to maximise information flow (Harris et al., 2019).
(7) Disseminating knowledge. We have published reviews on brain energy supply (Nortley & Attwell, 2017, Curr Opin Neurobiol), on how ion channels determine microglial function (Izquierdo et al., 2018, Trends in Neurosciences) and on how brain pericytes may be targeted for clinical benefit (Cheng et al., 2018, Acta Neuropathologica). Our work has dramatically overturned the established dogma that pericytes have little effect on blood flow, and shown that pericyte-mediated constriction of capillaries has pathological effects after stroke (funded by previous ERC award), in Alzheimer’s disease (funded by this award) and after heart attack (funded by the Rosetrees Trust). As a result I was invited to give the world’s premier plenary lecture on cerebral blood flow in 2019 (the Presidential Lecture of the International Society for Cerebral Blood Flow and Metabolism in Yokohama), and a plenary lecture at the ISN in Montreal, and made President of the UK Physiological Society.
(1) Projects 1 & 2 of the application - Signalling and cells regulating pericyte tone. We showed neuronal activity releases ATP, raising astrocyte [Ca2+]i. This releases prostaglandin which dilates capillaries via pericytes. This was published in Nature Neurosci (Mishra et al., 2016). We made a transgenic mouse that allows us to monitor the [Ca2+]i in pericytes which regulates contraction (published in Science: Nortley et al., 2019) and found that pericyte [Ca2+]i elevations are amplified by a Cl- channel in the cells, and this may provide a therapy to reduce capillary constriction after stroke (Korte et al., 2022, J Clin Invest). We have data being prepared for publication showing microglia can control pericyte function, and we have defined how microglial [Ca2+] changes with age and Alzheimer’s disease (AD) (Izquierdo et al., 2024, Pfluger’s Arch). We found that an enzyme regulating ATP concentration is often contaminated with K+ ions, which may invalidate previous experiments studying signalling that regulates glial cell function and brain blood flow (Madry et al., 2018, PNAS). We discovered that angiotensin and endothelin receptors control pericyte contraction, and that raising cyclic nucleotide levels relaxes pericytes (Nortley et al., 2019, Science; Hirunpattarasilp et al., 2023, Brain).
(2) Project 3. We have modelled the effect of pericytes on brain blood flow (Davis & Attwell, 2023, J Physiol).
(3) Projects 4, 5, 6 - time course of pericyte constriction and death in vivo. We have defined the time course of pericyte constriction and death, shown that a Ca channel blocker reduces constriction and improves neurological outcome after simulated stroke and in AD, and defined how the blood-brain barrier changes in AD (Korte et al., 2024, accepted with minor revisions).
(4) Project 7. We have a paper in preparation showing how ischaemic preconditioning prevents pericyte contraction.
(5) Projects 8 & 9 - properties of human pericytes and role of pericytes in Alzheimer’s disease. Remarkably, we discovered that in a mouse model of AD and in human patients with AD, cerebral blood flow is reduced by pericytes constricting capillaries. The mechanism is that amyloid beta evokes release of reactive oxygen species which evoke endothelin-1 release onto pericytes, which respond using endothelin A receptors to constrict the capillaries (Nortley et al., 2019, Science). This suggested pharmacological approaches to preventing capillary constriction in Alzheimer’s disease, and we have preliminary data in mice showing that brain blood flow can be preserved using one of these approaches.
(6) Related to the work in the application - brain energy use. We have shown that the properties of synapses that transmit visual information from the thalamus to the brain are optimised to maximise the flow of information per energy used, rather than simply to maximise information flow (Harris et al., 2019).
(7) Disseminating knowledge. We have published reviews on brain energy supply (Nortley & Attwell, 2017, Curr Opin Neurobiol), on how ion channels determine microglial function (Izquierdo et al., 2018, Trends in Neurosciences) and on how brain pericytes may be targeted for clinical benefit (Cheng et al., 2018, Acta Neuropathologica). Our work has dramatically overturned the established dogma that pericytes have little effect on blood flow, and shown that pericyte-mediated constriction of capillaries has pathological effects after stroke (funded by previous ERC award), in Alzheimer’s disease (funded by this award) and after heart attack (funded by the Rosetrees Trust). As a result I was invited to give the world’s premier plenary lecture on cerebral blood flow in 2019 (the Presidential Lecture of the International Society for Cerebral Blood Flow and Metabolism in Yokohama), and a plenary lecture at the ISN in Montreal, and made President of the UK Physiological Society.
We reported (Nortley et al., 2019) a totally novel mechanism contributing to the onset of Alzheimer’s disease, based on a unique collaboration with two independent sets of clinicians: neurosurgeons who provided live human tissue to do experiments on (that would otherwise have been discarded), and neuropathologists who gave us images of biopsies from living patients suffering from dementia of unknown cause. Setting up such collaborations is extremely time consuming, but pays off with the production of unique high profile papers. This work suggests therapeutic approaches.
Further results
We are still writing up several projects, including examining how to reverse the effect of AD on brain blood flow and defining how Covid-19 affects pericytes in vivo.
Further results
We are still writing up several projects, including examining how to reverse the effect of AD on brain blood flow and defining how Covid-19 affects pericytes in vivo.