From molecules to populations On the causality of toxic effects Tjalling Jager, Bas Kooijman Dept. Theoretical Biology

Causality How to link toxicant concentrations to whole- organism and population effects? toxicant effects on individual/population Why interesting?  to support chemical risk assessment  to justify research (‘so what’ question) NOEC/ECx molecular energy budgets

Precondition 1 Any concept for causality chain should explicitly consider exposure time  Toxicity is a process in time uptake into organism takes time biomarker responses can/will change in time NOEC/ECx values can/will change in time

EC10 in time Alda Álvarez et al. (2006) carbendazim time pentachlorobenzene time survival body length cumul. repro body length cumul. repro

Precondition 2 Causality chain should cover all life-history aspects  Feeding, development, growth and reproduction are linked … NOEC/ECx differ between endpoints what about molecular mechanism of action?

‘Narcotic’ effects time EC10 time body size reproduction A. nanus C. elegans

Causality of effects toxicant statistics e.g., NOEC/ECx effects on individual/population

Causality of effects ENERGY BUDGET rest of the organismtarget sitetoxicant molecular mechanism physiological mechanism effects on individual/population

Energy budgets

growth reproductionassimilation Each ‘MoA’ has specific effects on life cycle (direct/indirect) Each ‘MoA’ has specific effects on life cycle (direct/indirect) maintenance

Population consequences

Each ‘MoA’ has specific effects for populations assimilationreproduction growth maintenance

external concentration reproduction DEB model Biology-based (DEBtox) energy-budget parameter toxicokinetics growth maintenance assimilation Life-cycle effects Kooijman & Bedaux, 1996 (Wat. Res.)

Experiments nematodes Species Caenorhabditis elegans and Acrobeloides nanus Chemicals cadmium, pentachlorobenzene and carbendazim Exposure in agar Endpoints survival, body size, reproduction over full life cycle analysed with extended DEBtox Studies published as: Alda Álvarez et al., 2005 (Func. Ecol.), 2006 (ES&T), 2006 (ET&C)

length eggs survival C. elegans and cadmium Mode of action: assimilation Alda Álvarez et al. (2005) time (days)

A. nanus and cadmium Mode of action: costs for growth Alda Álvarez et al. (2006)

Physiological MoA C. elegansA. nanus PeCB (narcotic) Cadmium (heavy metal) Carbendazim (inhibits mitosis)

Physiological MoA C. elegansA. nanus PeCB (narcotic) costs for growth and reproduction assimilation Cadmium (heavy metal) Carbendazim (inhibits mitosis)

Physiological MoA C. elegansA. nanus PeCB (narcotic) costs for growth and reproduction assimilation Cadmium (heavy metal) assimilationcosts for growth (+ ageing) Carbendazim (inhibits mitosis)

Physiological MoA C. elegansA. nanus PeCB (narcotic) costs for growth and reproduction assimilation Cadmium (heavy metal) assimilationcosts for growth (+ ageing) Carbendazim (inhibits mitosis) assimilation (- ageing)

Extrapolate to populations  In a constant environment, a population will grow exponentially …  ‘Intrinsic rate of increase’ calculate from reproduction and survival in time

concentration (mg/L) concentration (mg/L) intrinsic rate (d -1 ) Extrapolate to populations 95% 90% 95% 90% Mode of action: assimilation Mode of action: costs for growth Cadmium

Pulsed exposure Pieters et al. (2006)

Conclusions Simple summary statistics are useless … NOEC/ECx change in time and differ between endpoints Molecular mechanism is important, but … not enough to explain effects on life cycle/population Energy budgets must be considered ‘physiological MoA’ covers direct and indirect effects direct link to life-history and population effects Species differ in phys. MoA for the same toxicant

Species differences? Species ASpecies B target sitetoxicanttarget sitetoxicant maintenance reproduction … maintenance reproduction …

target sitetoxicantphys. process effect on life cycle/population maintenance reproduction … Outlook ? Collaboration with CEH Monks Wood  life-cycle experiments with C. elegans  data analysis with DEBtox  microarray work on same animals

target sitetoxicantphys. process effect on life cycle/population maintenance reproduction … Outlook ? Why useful?  number of chemicals and species is very large …  but number of target sites and processes is limited! Once we know the normal biological processes, all external stressors are merely perturbations of these processes (Yang et al., 2004)