Plasticity in Vagal Afferent Neurons: Nutrient and

Microbial Drivers

Helen E Raybould, PhDDepartment of Anatomy, Physiology and Cell Biology

UC Davis School of Veterinary Medicine

The Gastrointestinal Tract- a sensory organ

- Intake, digestion and absorption of nutrients and water.

- Largest immune and endocrine organ in the body.

- Regulates food intake and glucose homeostasis.

- Largest interface between outside and inside world.

GI Tract is a Sensory Organ

goblet cell

endocrine cell

enterocytes

• Epithelium detects luminal signals –most epithelial cells can sense physiological parameters• Nutrient and non-nutrient chemicals (toxins and bacterial factors)• Specialized chemosensing cells (enteroendocrine cells, enterochromaffin cells and brush cells)

Schematic representation of the gross anatomical distribution of the vagus nerve

1. Vagal afferent neurons innervate most of the gut – less dense innervation in the small and large intestine but is present and functionally important. 2. These afferents terminate in all levels of the gut wall.3. Send information to the brain to regulate gut function via activation of reflexes and information sent to higher brain centers to regulate behavior4. Adequate stimuli are both chemical and mechanical5. Afferent activity can lead to conscious sensations (hunger, satiation, nausea) but mostly not perceived (information about digestive state and generation of reflexes)In general, even conscious sensations are not finely grained – large receptive fields, respond to systemic circulating stimuli

From: Berthoud and Neuhuber, Autonomic Neuroscience, 2000

Classification of extrinsic sensory neurons to the gut:morphology and electrophysiology

Mucosal AfferentsMorphology – two types:- in villus, branching and located underneath the epithelial cell layer (cf Powley)- surrounding the crypts and do not penetrate the villus

Electrophysiology – one or many: - Sensitive to light stroking of the mucosa – no activity at rest but activated by inflammatory stimuli.- Insensitive to contractions and stretch.- Chemosensitive – activated by gut hormones and other mediators

Brookes, S. J. H. et al. (2013) Extrinsic primary afferent signalling in the gutNat. Rev. Gastroenterol. Hepatol. doi:10.1038/nrgastro.2013.29

Gut-Brain Axis: Activation of Vagal Afferent Pathway

hypothalamusbrainstem

agal afferent neuronstransmission to CNS

Higher brain functionchange in behavior

HPA axisstress response

V

GU

T LU

MEN

Enteroendocrine cells luminal chemosensors

Parasympathetic efferent outflow change in effector function

Gut-Brain Axis: Regulation of GI Function and Food Intake

Enteroendocrine cells luminal chemosensors

Vagal afferent neuronstransmission to CNS

Parasympathetic efferent outflow

change in effectorfunction Higher brain function

change in behavior

Receptors:Receptors:CCKCCK115HT5HT33CBCB11Orexin Orexin Leptin (Ob1)Leptin (Ob1)Ghrelin (GHSR)Ghrelin (GHSR)Y2Y2GLPGLP1/21/2

Acute Changes in PhenotypeNutrient availability alters receptor expression in vagal afferents

Y2 receptor MCH1 receptor CB1 receptor

Fasted

Fed

Fast Fed Fast Fed Fast Fed

Acute Changes in Phenotype: Nutrient availability alters peptide expression in vagal afferents

Fasted Re-fed0

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CART

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The neurochemical switch of vagal afferent neurons is regulated by cholecystokinin (CCK)

FOOD

Gut Vagal Afferent NTS

NOFOOD

CCK1R

CCK1R

Exogenous CCK (ip)

Lorglumide (ip)

MCH

MCH1RCB1

Y2R

CART

Y2R

CART

MCH

MCH1RCB1

CCK

Burdyga et al (2004) J Neurosci; Burdyga et al (2006) NeuroscienceBurdyga et al (2006) AJP Gastrointest Liver PhysiolBurdyga et al (2008) J Neuroscide Lartigue et al (2007) J Neurosci

Feeding-Induced Phenotypic Change of Vagal Afferent Neurons

- Dependent on feeding status (CCK)- Potentiated by leptin- Inhibited by ghrelin

Altered

Lack of intestinal satiety signaling leads to increase in meal size and hyperphagia

Has been demonstrated in human studies of obesity ( Westerterp-Plantega, Feinle-Bisset)

Diet-Induced Obesity in Sprague-Dawley Rats

SD rats outbred strain: DIO-resistant and DIO-prone on HF diet

DIO-P vs DIO-resistant rats- increase in body weight gain- increase in adiposity - hyperphagic- increase in plasma leptin

DIO-R DIO-P

High fat dietLow fat diet

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Reduced sensitivity to the satiety effects of CCK DIO rats

de la Serre, AJPGLP 2010

Attenuated intestinal afferent responses to chemical stimulation in high fat diet-induced obese mice

Daly D M et al. J Physiol 2011;589:2857-2870

* a global decrease in the excitability of vagal afferent neurons in HF diet-induced obesity

Vagal Afferent Neurons in Diet-Induced Obese Rats do notupregulate CART expression to feeding

LFD HFD - DIOR HFD - DIO

Fasted

Fed

CART

CCK

CART

50um

* vagal afferents of DIO rats are unable to signal satiety to the brain in response to food

Vagal Afferent Neurons Develop Leptin-Resistance

Leptin resistance in vagal afferents precedes hypothalamic changes

0.0

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LF DR DIO

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Vagal afferent neurons

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Leptin (80µg/kg)

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Leptin resistance associated with increase SOCS3

DR DIOLF

GAPDH

SOCS3

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de Lartigue et al 2010, 2011, 2014

Feeding-Induced Phenotypic Change of Vagal Afferent Neurons

- dependent on feeding status (CCK)- absent in diet-induced obesity

Gut Barrier Function and Gut-Brain Axis

LEPTIN RESISTANCE IN VAGAL AFFERENTS

ALTERATION IN FEEDING BEHAVIOR

OBESITY

Raybould, 2012

Hypothesis: bacterial LPS drives phenotypic change in VAN

brainstemhypothalamus

food intake

1. Increase LPS

3. Increase LPS activates TLR4 on epithelial cells and vagal afferent neurons

2. Gut inflammation and increase in intestinal permeability

TLR4LPS

4. Leptin resistance of vagal afferent pathway and altered phenotype = leading to OBESITY

Chronic administration of low-dose LPS recapitulates diet-induced obesity

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n=9/group

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- increases body weight

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- inhibits CCK-induced satiety

- induces hyperphagia

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Decrease leptin-induced pSTAT3

De Lartigue et al, 2014

Challenges

• What is the relationship between the morphological, electrophysiological and neurochemical phenotypes of vagal afferent neurons that innervate the GI tract?

• GLIA!!!!• Can the changes in phenotype seen in vagal afferent

neurons in diet-induced obesity be reversed? • How well do these changes in vagal afferent neurons in

obesity translate to human obesity?

Acknowledgements

• Members of Raybould lab, in particular Will de Lartigue