Looking on the Horizon; Potential and Unique Approaches to Developing Radiation Countermeasures for Deep Space Travel

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Looking on the Horizon; Potential and Unique Approaches to

Developing Radiation Countermeasures for Deep Space Travel

Rihana S. Bokhari , Afshin Beheshti , Sarah E. Blutt ,

Dawn E. Bowles , David Brenner , Robert Britton ,

Lawrence Bronk , Xu Cao , Anushree Chatterjee , Delisa E. Clay ,

Colleen Courtney , Donald T. Fox , M. Waleed Gaber ,

Sharon Gerecht , Peter Grabham , David Grosshans ,

Fada Guan , Erin A. Jezuit , David G. Kirsch , Zhandong Liu ,

Mirjana Maletic-Savatic , Kyle M. Miller , Ruth A. Montague ,

Prashant Nagpal , Sivan Osenberg , Luke Parkitny ,

Niles A. Pierce , Christopher Porada , Susan M. Rosenberg ,

Paul Sargunas , Sadhana Sharma , Jamie Spangler ,

Daniel Naveed Tavakol , Dilip Thomas ,

Gordana Vunjak-Novakovic , Chunbo Wang , Luke Whitcomb ,

Damian W. Young , Dorit Donoviel

PII:

S2214-5524(22)00060-8

DOI:

https://doi.org/10.1016/j.lssr.2022.08.003

Reference:

LSSR 404

To appear in:

Life Sciences in Space Research

Received date:

8 March 2022

Revised date:

29 July 2022

Accepted date:

4 August 2022

Please cite this article as: Rihana S. Bokhari , Afshin Beheshti , Sarah E. Blutt , Dawn E. Bowles ,

David Brenner ,

Robert Britton ,

Lawrence Bronk ,

Xu Cao ,

Anushree Chatterjee ,

Delisa E. Clay ,

Colleen Courtney ,

Donald T. Fox ,

M. Waleed Gaber ,

Sharon Gerecht ,

Peter Grabham ,

David Grosshans ,

Fada Guan ,

Erin A. Jezuit ,

David G. Kirsch ,

Zhandong Liu , Mirjana Maletic-Savatic , Kyle M. Miller , Ruth A. Montague , Prashant Nagpal ,

Sivan Osenberg , Luke Parkitny , Niles A. Pierce , Christopher Porada , Susan M. Rosenberg ,

Paul Sargunas , Sadhana Sharma , Jamie Spangler , Daniel Naveed Tavakol , Dilip Thomas ,

Gordana Vunjak-Novakovic , Chunbo Wang , Luke Whitcomb , Damian W. Young , Dorit Donoviel ,

Looking on the Horizon; Potential and Unique Approaches to Developing Radiation Coun-

termeasures for Deep Space Travel,

Life

Sciences

Space

Research

(2022), doi:

https://doi.org/10.1016/j.lssr.2022.08.003

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2022 Published by Elsevier B.V. on behalf of The Committee on Space Research (COSPAR).

Title:

Looking on the Horizon;

Potential

and Unique

Approaches to

Developing

Radiation

Countermeasures for

Deep

Space Travel

Rihana S. Bokhari

, Afshin Beheshti

2,3

Sarah E. Blutt

4,5

awn

Bowles

David Brenner

Robert Britton

awrence

Bronk

Xu Cao

, A

nushree Chatterjee

Delisa E. Clay

Colleen Courtney

Donald

Fox

M. Waleed Gaber

Sharon Gerecht

Peter Grabham

avid

Grosshans

, F

ada

Guan

Erin A.

Jezuit

, David G. Kirsch

handong

Liu

, 1

Mirjana Maletic

Savatic

, 1

Kyle M. Miller

Ruth A. Montague

, Prashant Nagpal

ivan

Osenberg

, 1

uke

Parkitny

, 1

, N

iles A. Pierce

,20

,21

, Christopher Porada

, Susan M. Rosenberg

,24

,25

,26

Paul Sargunas

Sadhana Sharma

Jamie Spangler

Daniel

Naveed Tavakol

Dilip Thomas

Gordana Vunjak

Novakovic

Chunbo

Wang

Luke Whitcomb

amian

W. Young

Dorit

Donoviel

NASA Research and Education Support Services,

Arlington, VA 22202, USA,

KBR, Space Biosciences Division, NASA Ames Research Center, Moffett Field, CA, 94035, USA,

Stanley Center for Psychiatric Research, Broad Institute of MIT and Harvard, Cambridge, MA, 02142, USA,

Department of Molecular Virol

ogy and Microbiology, Baylor College of Medicine, Houston,

77030, USA,

Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston,

77030, USA

ivision of Surgical Sciences, D

epartment of Surgery, Duke University, Durham NC,

USA,

Columbia University, New York, NY 10027, USA

The

University of Texas MD Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030

, USA,

Stanford University School of Medicine, Stanford, CA 94305, USA,

Sachi Bioworks,

Louisville, CO 8002

USA,

University of Colorado Boulder, Boulder, CO 80303,

USA,

Department of

Pharmacology and Cancer

Biology, Duke University School of Medicine, Durham, NC 27710, USA,

Department of Pediatrics, Baylor College of Medicine, One Baylor Plaza, Houston,

TX 77030, USA,

Chemical and Biomolecular Engineering and Biomedical Engineering, Johns Hopkins University,

Baltimore, MD 21218 USA,

Biomedical Engineering, Duke University,

Durham, NC 27708, USA,

Center for Radiological Research, College of Physicia

ns and Surgeons, Columbia University, New York,

NY 10027

USA,

Jan and Dan Duncan Neurological Research Institute, 1250 Moursund St. Houston, TX 77030, USA,

Department of Molecular Biosciences, The U

niversity of Texas, Austin, TX 787

, USA,

Division

of Biology & Biological Engineering, California Institute of Technology, Pasadena, CA 91125, USA,

Division of Engineering & Applied Science, California Institute of Technology, Pasadena, CA 91125, USA,

Weatherall Institute of Molecular Medicine,

University of Oxford, Oxford OX3 9DS, UK,

Wake Forest Institute for Regenerative Medicine, Fetal Research and Therapy Program Wake Forest School of Medicine

, Winston

Salem, NC 27157, USA,

Department of Molecular and Human Genetics, Baylor College of Me

dicine, 1 Baylor Plaza, Houston, TX, 77303, USA,

Dan L. Duncan Comprehensive Cancer Center, Baylor College of Medicine, 1 Baylor Plaza, Houston, TX, 77303, USA,

Department of Biochemistry and Molecular Biology, Baylor College of Medicine, 1 Baylor Plaza, Houston, TX, 77303, USA,

Department of Molecular Virology and Microbiology, Baylor College of Medicine, 1 Baylor Plaza, Houston, TX, 77303, USA,

Department of Environmental and Radiological Health Sciences, Colorado State University, Fort Collins,

80523, USA,

Department of Pharmacology, Baylor College of Medicine, Houston,

77030, USA

Translational Research Institute for Space Health,

Houston, TX 77030, USA

Center for Space Medicine, Baylor Colleg

e of Medicine, Houston, TX

77030

, USA;

donoviel@bcm.edu

Abstract

Future lunar missions and beyond will require new and innovative approaches to radiation

countermeasures. The Translational Research Institute for Space Health (TRISH)

is focused on

identifying and supporting unique approaches to reduce risks to human heal

th and

performance on future missions beyond low Earth orbit

. This paper will describe

three

funded

and

complementary

avenues

for

reduc

ing

the

risk to humans from

radiation exposure

experienced in deep space.

The first focus is on

identifying

new

therapeut

targets to reduce

the damaging effects of radiation by focusing on

high throughput genetic screens in

accessible

sometimes called

lower

organism

model

. The second focus is to design innovative approaches

for countermeasure development with special at

tention to nucleotide

based

methodologies

that may constitute a more agile way to design therapeutics. The final focus is to develop new

and innovative ways to test radiation countermeasures

in a human model system

. While animal

studies continue to be bene

ficial in the study of space radiation, they can have imperfect

translation to humans. The use of three

dimensional (3D) complex in

vitro models

is a

promis

ing approach

to aid

the

develop

ment of

new countermeasures and personalized

assessments of radiation risks.

These three distinct and unique approaches complement

traditional space radiation efforts and should provide future space explorers with more options

to safeguard their short and long

rm health.

Key

Words

Space Radiation, 3D tissue, Organoid, Extremophile, Nucleotide

based Approaches, Radiation

Countermeasure

1.1

Background

Space radiation differs from

commonly encountered

forms of radiation

emitteed

diagnostic

and therapeutic

procedures

rays and gamma

rays) because it comprise

a mixed field of

high

energy protons and nuclei components originating from either solar particle events (SPEs)

or galactic cosmic rays (GCR

), which are

made of

high atomic number and energy (

HZE

)

rad

iation

particles.

HZE radiation particles are part

icularly concerning because their track

structure, the particle’s energy deposition as

passes through a tissue, leaves significant

complex damage both along the track structure and from energetic electro

ns that are formed

due to

the particle’s interactions with the tissue

(1)

. The result of this damag

e in living tissues is

mutations

and

sometimes cell death.

Space radiation

exposure

(unless

from SPE

)

occurs at a

low dose rate,

approximately

0.5 mGy/day. The complex nature of GCR is attributable to its

composition of numerous particle species, making them

potentially

harmful even at low dose

rates.

The major GCR particle types include H (protons,

approximately

%), He (

approximately

out 2% positrons and electrons

and about 1% of HZE particles with broad energy

spectra of interest

(from

)

primarily from ~10 MeV/n to 10,000 MeV/n

(2)

On the other

hand, SPEs are composed chiefly of protons and have a

broad

energy spectrum extending out

to a few hundred MeV.

On Earth, life is shielded from space radiation by the Earth’s magnetic

field.

Astronauts in low Earth orbit (LEO) have

partially

benefited from this protection,

though

their exposure is greater than th

ose on Earth

(3)

On future missions to the moon and Mars,

stronauts will experience prolonged exposure to

radiation at a low dose and low fluence

(4)

. Given this dose rate at the expected energies is not

replicable on Earth, the true health effects of space radiation remain largely unknown. Althou

HZE particles represent only 1% of the GCR spectrum, they will likely contribute most of the

absorbed dose. Exposure to HZE particles may result in

significant

effects on

stronauts

;

thus

studying

the effects of such particles at total doses similar to

that of an entire mission

has been

prioritized

. However,

ecause

it is not feasible to deliver a realistic replication of GCR on

Earth at the dose rate and fluence expected in space,

and it is unethical to purposefully expose

humans to such radiation,

the human health and performance effects of

long

term exposure

GCR beyond LEO

remains unknown

Because of

the unique nature of space radiation and the

enormous engineering challenges associated with adequately shielding in spaceflight,

the

current plan is to depend on exposure limits and a reliance on the very lim

ited set of known

countermeasures that have shown promise in preventing damage from space radiation in

animal studies

(9)

. There is a need for

innovative countermeasure

development

approaches to

ensure safe space travel for astronauts on exploration class missions.

The current body of knowledge on radiation countermeasures is limited

(10)

The majority of

published studies on

ionizing

radiation

health

effects

humans,

are

not GCR relevant,

thereby

creating a challenge for

the applicability of traditional pharmacological

countermeasures

developed for terrestrial radiation to

the

space

context

. Rodent models have long served as an

acceptable option for studying

space

radiation effects,

with the assumption being that

much of

these results will translate to humans. Samples derived from astronauts

exposed to space

radiation

and rodent

s exposed to simulated space radiation

show similar responses in

triggering DNA damage response, oxidative stress, inflammation, and lip

id peroxidation

(11

13)

However, inbred rodent

models

are known to be more

radioresistant

than humans,

though this

can vary with strain,

and generally, animal models have been found to imperfectly translate to

human physiology

for

various

reasons

(14)

. For instance,

it is estimated that there is a

nearly

90%

failure rate

drug testing using

preclinical animal models, which is related to both efficacy

and safety problems when brought to clinical trials

(15

17)

Continuing to rely on

animal models

alone to understand the effects of space radiation on humans

does not provide a complete

picture

Hence, it is vital to

develop

innovative experimental approaches to bridge the

translational relevance of animal studies to human response.

Both adherent and non

adherent

human

cell cultures have been used to understand the cell

biology and disease mechanisms induced by radiation. However, monolayer cultures do not

mimic the cell

cell and cell

extracellular matrix (ECM) interactions.

vivo,

these interactions

are essential for

cellular functions such as differentiation, proliferation, and fate

associated

gene expression.

Because of

the lack of tissue

like architecture, monolayer cultures are not set

up to respond to nutrients or metabolic gradients

(18)

One alternative is human complex

microphysiological systems

that

have the potential for improved translatability as they are

developed with normal human cells. Recent advances in their complexity, longevity,

reproducibility, and recapitulation of

vivo

anatomical features and differentiation markers, as

well as physio

logical activities, promise whole organism translatability.

This paper discusses funded projects by the

Translational Research Institute for Space Health

(TRISH

) that

address novel

radiation countermeasure

development approaches

TRISH is a

NASA

funded in

stitute with a focus on disruptive, high

risk, high

reward research aimed at

pushing the field of human spaceflight forward in great leaps

as opposed to incremental steps

This

three

pronged

approach

by TRISH

focuses on advancement in radiation countermeasure

target identification, development

and testing.

One way in which TRISH is supporting

innovative work on this front is focusing on identifying new targets for radiation resistance by

studying

accessible

rganism

model

, someti

mes called lower organisms,

and

leveragi

adaptations by other organisms,

extremophile

to increased radiation exposure. Another

strategy is the use of innovative

nucleotide

based

therapeutic methods to mitigate the impacts

of radiat

ion on human

. Finally, TRISH has supported work using three

dimensional

(3D)

complex

microphysiological systems

, which may prove to be not only more translatable to

humans but also advantageous for research in space.

With NASA planning missions beyond LEO

where there is the unprecedented possibility of testing countermeasures in an actual GCR, and

possibly SPE, environment, it becomes more crucial to identify innovative strategies for

radiation countermeasures that can be tested in these unique opportuniti

es.

2.1

High

Risk, High

Reward Approaches to the Identification of New Radiation

Countermeasure Targets

One arm of TRISH’s approach to new radiation countermeasure development is to fund

research that focuses on identifying

novel

targets

Extremophile

organisms, like

tardigrades,

have adapted to survive in extreme environments, including under conditions of increased

radiation exposure. Identifying the genes that make them resilient and validating that

they can

confer resistance is a reasonable approach

. However, extremophile organisms do not yet

lend

themselves to

genetic manipulation

that are the strengths of

accessible

organisms such as

bacteria, yeast, and fruit flies.

To circumvent this challenge, Dr. Donald Fox’s group successfully

developed a nove

l platform for identifying radiation

resistance genes from an extremophile,

the tardigrade

R. varieornatus

in a

host organism

(19)

The platform involves expressing

tardigrade genes in a

genetically accessible model organism, the fruit fly

Drosophila

melanogaster.

By u

sing this approach, 74 genes have been screened that are either unique to

tardigrades or are part of a str

ess response gene family thought to confer resistance to

environmental stressors in tardigrades. Candidate tardigrade transgenes that improve

Drosophila

survival following either low (X

ray) or high (

Fe ions) LET radiation have been

identified using this

approach.

These studies were the first to expose fruit flies to space

radiation.

Promising screen hits are being identified and tested in human cells for

their ability to

confer

radiation resistance.

The power of using

accessible

organisms such as

the common bacterium

Escherichia coli

has

been

demonstrated in a

screen for genes that promote and prevent endogenous cellular

damage when slightly overexpressed

By u

sing this approach,

Drs. Susan Rosenberg and Kyle

Miller

have

previously

identified

the b

acterial genes’

human homologs that

may

play a role in

cancer

(20)

To use this same method to identify novel targets

for space radiation

countermeasures,

E. coli

cells

were exposed to ionizing radiation to look for

overexpressed

genes that

confer resistance.

The screen was successful and

human homologs

were identified

and are currently

being tested

Many of these target

s are brand new and have never before

been identified as potential radiation countermeasures.

Both these approaches leverage

accessible

organisms that lend themselves to high

throughput

genetic screening and manipulation to expand the existing pool of radiation countermeasure

targets.

3.1

High

Risk, High

Reward Approaches to

Developing

Radiation Countermeasures

Today, most drugs on the

market have been developed using traditional drug discovery

approaches,

which

requir

decades of preclinical and clinical research and billions of dollars in

funding. The failure rate is so high that the market size has overshadowed the need in the

decisio

n to

move forward in drug development

ause the space radiation market

negligible, there are no financial incentives to develop novel drugs to protect humans in space.

TRISH has invested in several

nucleotide

based approaches

because they are likely to be

developed faster and at a lower cost. The recent success of Luxturna, an adeno

associated viral

V) vector

based gene therapy that cures an inherited form of blindness, invigorated the field

of nucleotide

based therapeutics

Another recent success,

RNA

based vaccines

has

proven

invaluable in fighting the

recent COVID

19 pandemic

due to, amongst other benefits, the speed

by which they could be developed

TRISH has funded several

programs

aimed at improving

faster, lower cost

nucleotide

based therapeutic approaches for

radiation countermeasures.

Dr. Dawn Bowles

explores

the use of

gene therapy as a method for delivering a p

rophylactic

radiation

protectant

. The group

is optimizing gene

carrying

AAV

vectors for

nucleotide

based

radiation countermeasure delivery and safety.

There are many advantages of using the non

pathogenic AAV in gene therapy applications; however, from the NASA perspective perhaps the

most important advantage is that vectors based on AAV conf

er sustained expression (up to

years) of therapeutic nucleic acids in target tissues

(21)

. Packaging a genetic cargo in this type

of delivery vehicle

could be used as

a prophylactic

and initiated well in advance of the mission

and thus radiation exposure.

This

approach

would also

obviate

the need to

bring,

store

, and

continuously administer

pharmaceutical agent during

habitable volume

limited deep space

missions. The Bowles group focused

on developing

a therapeutic for GCR induced

cardiovascular dysfunction packaged in

a cardiac enhanced AAV capsid

(22, 23)

Dr. Niles Pierce focuses on

dynamic RNA nanotechnology to enable cell

selective

and

programmable

gene therapies. Guide RNAs (gRNAs) play a central role in CRISPR technologies

by directing the function of Cas protein effectors to a target gene of choice, providing a

versatile programmable platform for engineering diverse modes of synthetic regulation

(edit,

silence, induce) across species (from bacteria to humans)

(24)

. However, the fact that

gRNAs

are constitutively active is a significant limitation, making it challenging to confine gRNA activity

to a desired location and time within an organism. To achieve programmable control over the

scope of gRNA activity, the Pierce Lab is utilizing prin

ciples from dynamic RNA nanotechnology

to engineer conditional guide RNAs (cgRNAs) that change conformation in response to an RNA

trigger X,

which make

possible to conditionally direct the function of Cas to an independent

target gene Y

(24, 25)

. Candidate genes for radiation countermeasures could be targeted

simultaneously in a tissue

specific manner

. For each gene, an en

dogenous RNA trigger X would

be selected to confine the regulation of target gene Y to the desired tissue type

(s)

Dr. Afshin Beheshti’s lab has

adpoted

a systems biology approach to develop novel nucleotide

based radiation countermeasures. The putative targets are miRNAs (i

for interfering; small

non

coding RNA with a negative post

transcriptional effect on gene expression) which may act

as systemic regu

lators of responses to stressors, including microgravity, oxidative stress, and

potentially radiation

induced DNA damage

(26)

. It is known that each miRNA can target

hundreds of mRNAs and regulate immunity, cardiovascular disease, muscle degeneration,

central nervous system (CNS) related diseases, and cancer

(26)

. In addition, specific miRNAs

have been shown to influence and increase DNA repair efficiency

(27)

. Radioresistant lung

can

cer cell lines express increased levels of miRNAs after ionizing radiation, which suggests that

this may be a mechanism of radioresistance

(28)

The

group has recently shown that a distinct

rculating spaceflight miRNA signature found in the serum is correlated with cardiovascular

disease and muscle degeneration

(29

31)

Dr. Beheshti’s group is attempting to target miRNAs

as a novel therapeutic approach to

radiation resistance

(29, 31)

Antagonists

to certain miRNAs

termed

antagomirs delivered

before

radiation

exposure

effectively prevented angiogenesis damage

(29)

. Delivery was

mediated

using a unique

technology developed by

a company called

AUM Biotech

termed

“AUMantagomirs”

(32, 33)

Antagomirs

delivered to a 3D microvessel tissue model 24 h before

exposure to 0.5 Gy

simulated

GCR

prevented the complete collapse of the vessels

Molecular

analyses suggest that the effect may be partly mediated by reducing p53

inding

rotein 1

repair foci and increasing DNA repair activity

(34)

n analogous

but distinct nucleotide

based therapeutic approach involves the Nanoligomer

(Sachi Bioworks) platform developed by Drs. Anushree Chatterjee and Prashant Nagpal’s team

(35)

Nanoligomer

is comp

osed

six design elements, including a peptide nucleic acid (PNA),

a synthetic DNA

analog where the phosphodiester bond is replaced with 2

aminoethylglycine

units, as the nucleic acid binding domain,

and

an engineered nanoparticle for facile cellular and

tiss

ue delivery of

the

naked molecule

(36, 37)

Nanoligomers offer improved stability, facile

delivery and internalization, high target specificity (on

target) and minimal off

target

activity.

They

can be designed to rapidly create a library of

many

therapeutic molecules

that can be

screened

(36, 37)

This approach

is a high

throughput method to customize peptide

nucleic

acid

based gene

specific reversible therapeutics with high precision.

The platform

has

accelerated design, building, testing, and learn

based iteration cycles, enabling the timeframe

from drug disco

very to testing to

reduce

from weeks

months to

days

The

procedure

uses

bioinformatics to predict the optimal sequence of Nanoligomers to exert inhibition (targeting

mRNA or DNA) or activation (targeting DNA) of gene expression of specific

targets. Th

information is used to synthesize

a library consisting of hundreds of molecules per round

within

days

. A novel nanoparticle

based transport system allows for efficient delivery of Nanoligomers

to host cells at

the

sub

micromolar range

which is a therapeut

ic dose

Screening of the

Nanoligomer library can be performed in a high

throughput manner

Using machine learning

algorithms, the platform analyzes the gene expression readouts from the treated cells to

optimize the particular combination of Nanoligomers

that serve as effective countermeasures.

Thus far, Nanoligomers have not caused cytotoxicity

vivo

across different routes of

administration, including intranasal, intraperitoneal, and intravenous delivery.

The team has

used this approach to develop Nano

ligomers that are potential radiation protectants.

The COVID

19 pandemic afforded an opportunity to test the development of Nanoligomers

antivirals against SARS

CoV

. Ef

ficacy

was demonstrated

in both

vitro

and

vivo

models

(37, 38)

This proof of concept exemplifies the versatility of the platform and its broad

applicab

ility.

Having this capability

a deep space spacecraft would enable the crew to

develop countermeasures to any novel threats during exploration space missions. The

accelerated design, build, test, and learn cycles and safety and efficacy of Nanoligomers

allow

for customized rapid just

time therapies.

The limited market and the costs of new drug development have hindered traditional drug

discovery in radiation countermeasures. Even repurposing approved medications for a novel

indication such as exposur

e to space radiation

may require

costly and lengthy studies

(10)

. This

is precisely why TRISH has funded nucleotide

based approaches. With new

molecular tools that

have improved delivery, programmability, tissue

specificity, and speed of synthesis, nucleotide

based countermeasures now present realistic platform capabilities for deep space missions.

Without access to resupply from Earth, an explo

ration mission crew could synthesize just

time therapeutics to any health threat with agility, speed, and specificity.

4.1

High

Risk, High

Reward Approaches to Modeling

Human Organs

There is room to innovate in developing

research

model systems that more closely recapitulate

the healthy human

who

will travel in deep space and experience the unique space radiation

environment.

Rodent models are typically used in radiation research and have yielded much

knowledge. The translatability

to the human is less certain.

3D c

omplex

microphysiological

systems

that utilize normal cells from human donors

present a unique opportunity to study

mechanistic biological

changes

in response to space radiation and test potential

countermeasures

in a

physiologically relevant manner

(39)

. This

approach

has advantages

for spaceflight research

Human

microphysiological

systems

serve

as organ

specific ‘sentinels’

ue to their

limited repair capacity and tolerance to

stressors. Hence, they

reveal

cascades that

fail early in disease development

. As such

they have the potential to identify

not only

the

effects of acute radiation

but also long

term

or delayed

radiation

effects in a shorter

timeframe

where

such outcomes may take months or years to ar

ise in a

model

organism.

Another

innovative

approach

is the use of

humaniz

ed”

mouse models

immunodeficient mice

engrafted with functional human cells and tissues,

which can

provide an improvement

in the

fidelity with

human

physiology

(40)

These advantages have motivated

TRISH

to fund several

research

projects

using

complex human

or humanized rodent

models to develop unique

radiation countermeasures

spanning multiple different organ systems.

4.2

Gastrointestinal System and the Microbiome

Because of

high numbers of stem cells and rapid turnover rate, the human small intestinal

epithelium is

particularly

sensitive to ionizing radiation

rolonged space travel will

likely cause

small intestin

disorders

including intestinal mucosal barrier dysfunction

and nutrient

malabsorption.

Organoid models may prove a platform for studying human gastrointestinal

effects

(41, 42)

To develop a h

uman model for the small intestine,

Dr.

Sarah

Blutt

uses

tissue

samples obtained from

surgical specimens, endoscopies

colonoscopies to generate 3D

human

tissue stem

cell

derived

gastrointestinal

organoid

cultures.

They have not altered the pH

conditions as the small intestine is thought to be at relatively neutral pH. The group has also

recapitulated the steep oxygen gradient seen

in vivo

(43)

. The crypt like organoids,

predominately made of stem cells survive indefinitely and the

villus

organoids only live for

about 3

5 days, which is the lifespan

in vivo

in humans.

The intestinal microbiome consists of a community o

f microorganisms, predominantly bacteria,

that reside within the lumen of the intestinal tract

(44)

Because of

its proximity to the

epithelium, the intestinal microbiome is a constant source of signals to the epithelium,

including factors that protect, heal, and

support intestinal epithelial health. Microbial factors

could influence the proliferation and survival of t

he intestinal stem cell (ISC), the nucleating cell

type necessary for regenerating damaged intestinal epithelium, specifically the villus

epithelium.

Studies in animal models indicate that the presence of the microbiome is associated

with a higher mitotic index, intestinal epithelial turnover, cell migration, and ISC proliferation

(45

49)

. Recent work has suggested that the microbiome has the potential to protect from the

intestinal side effects observed with anti

cancer radiation treatments and was linked to a

reductio

n in radiation

induced epithelial damage and symptoms

(50

53)

. Microbial products

can also activate signaling pathways that protect the ISC from radiation

induced apoptosis

(54)

Dr. Blutt’s

group

is using

the gastrointestinal microbiome to potentiate the

ISC

environment

stimulating

epithelial repair and regeneration

to correct damage induced by

simulated space

radiation exposure.

To date,

neither

specific microbial communities

nor

individual

candidate

organisms or

factors responsible for the healing effects have been elucidated. The group has

identif

ied

secreted

factors

naturally

produced by complex commensal communities derived

from healthy human donor stool

that exhibit

stem cell stimulatory properties. Their

approach

of culturing gastrointestinal microbiome

vivo

has revealed that microbial factors can

stimulate

human

stem cell activation and proliferat

ion

This ongoing work aims

to develop

novel, safe, and cost

effec

tive microbiome

based countermeasures with minimal side effects

that can be used to protect and stimulate the

ISC

and manage intestinal dysregulation during

long

duration

space travel.

4.3

Nervous System

Decades of work using mostly rodents has indicated that the central nervous system is

susceptible to very low levels of space radiation exposure

(5, 6)

The applicability to humans

remains to be demonstrated because no human has yet to be exposed to the levels and

duration of deep space rad

iation that will b

e experienced on an exploration mission planned for

the return to the Moon and eventually Mars. The longest Apollo mission was 12 days. Apollo 17

astronauts appeared to have withstood the space radiation exposure without major effects, bu

it is unclear how they might have fared after 30 days or longer.

Dr. Maletic

Savatic’s group is

examining neurogenesis in human brain organoid models exposed to a low linear energy

transfer (LET) proton beam to mimic components of the particle fluence fo

und in SPEs and

within GCR

(55)

. These models recapitula

te the tissue environment

vivo,

which has multiple

neurogenic sites (rosettes) and diverse cell types

including neural stem cells,

neurons,

astrocytes, oligodendrocytes, and microglia.

The group has used two complementary cerebral

organoid models

and

expose

them at different time points and different frequencies of

exposure

Each organoid is grown at atmospheric O

conditions and physiologic pH of 7.25 and

can last up to 18 months in culture.

A detailed

analysis

is underway to

examine molecular,