Intestinal lactase: What defines the decline?

GASTROENTEROLOGY

1993;105:931-940

EDITORIALS Intestinal Lactase: What Defines the Decline?

1

ntestinal

lactase,

charidase

that hydrolyzes

ride constituents is in many tive

hydrate

other

membranes,

where

border

a short majority

before

being border.

are attached

to the

lactase is anchored

lumen

to facilitate

tose at the lumen-brush present

at the time

capacity

in most

sure hydrolysis Typically,

of birth

mammals,

lactase activity in

interface.

at maximum including

of ingested

of weaning

its interaction

border

by

lactose declines

mammals

with

lac-

Lactase

is

hydrolytic

humans,

to en-

in maternal

milk.

exponentially to

only

at the

a fraction

(- 10%) of the neonatal value.’ Although this disaccharidase does persist at optimal levels in 75%85% of white

adults of Western

European

heritage,

in most other human population groups childhood or adolescence.3 Even when

it declines

in either early maintained at

seemingly high levels, the quantity of lactase is only about half that of other saccharidases such as sucrase, a-dextrinase maintained

(isomaltase), at optimal

or glucoamylase,

concentrations

which

throughout

are life.

This has functional implications, as can be appreciated by the fact that the surface hydrolysis of lactose in individuals with a full complement of the enzyme is slower

than the enterocyte’s

transport

capacity

may continue

stance, lactase remains

detectable5

for the

in the mature

or even appears

border

more

rat intestine.

initial analysis of hypolactasia a poor correlation of specific a parallel

the transcription of of lactase protein differently’

from the brush

activity,”

For in-

In addition,

in neonatal

reported

intestine.

substantial.6

mature

lactase

RNA

to be processed

and degraded than

messenger

to yield the lactase protein

in the adult rat and rabbit, mRNA and the translation

the lactase protein

(carbo-

of the brush

which

via the N terminus,

the

reticulum

hydrophobic C-terminal segment.’ The vast of the lactase protein domains are free in the

intestinal

time

within

of its mass) and then

proteases

surface

hydrolases,

diges-

of lactase

and its translation

product

glyco-

it is glycosylated

by

to the outer

the other

only

endoplasmic

are 20%30%

intracellularly

transferred Unlike

membrane

the transcription

(mRNA)

interface,

or even unique

intestinal with

activity,

oligosac-

border

it is synthesized

side chains

modified

brush

Like

in association

and Golgi

surface

to its monosaccha-

at the lumen-brush

hydrolases,

enterocyte

border lactose

ways a most unusual

enzyme.

protein

a brush

two other reduction

rapidly’

Although

in the

in humans has suggested mRNA and expression of groups have subsequently in both lactase gene tran-

scription and consequent lactase synthesis in biopsy culture studies.“*” This implies that a failure of transcription

of lactose

message

may be an important

fac-

tor responsible for lactase decline in human hypolactasia. Thus multiple mechanisms may be responsible for the loss of lactase with maturation, depending

on the mammalian

and these may vary

species.

In this issue of GASTROENTEROLOGY

and in pre-

vious related reports, Maiuri et al. at Auricchio’s laboratory in Naples 12-14 have examined the immunoreactivity

and

enzymatic

activity

of lactase

in the smal1

intestine using sophisticated histochemical that allow important insights concerning

techniques its topo-

graphic localization as the enterocyte migrates from the top of the crypt to the villus apex for eventual discharge into the intestinal lumen. Maiuri found differences both in the expression crypt-villus

axis and in the proximal-distal

et al. have along the distribu-

tion from duodenum to ileum. Their initial studies of rabbit intestine have revealed that al1 villus enterocytes possess lactase immunoprotein at the brush border during the neonatal period but that this homogeneous

released glucose and galactose products.4 Thus, even in those individuals with persistente of lactase as adults,

pattern persists only in the jejunum in the adult anima1.12 In the duodenum and ileum, where lactase matu-

surface hydrolysis is rate-limiting for lactose assimilation. For the other saccharidases, an excess of monosaccharide products is produced for absorption, and the rate-limiting step is monosaccharide transport.

rational decline is maximal, residual activity was stil1 provided by smal1 islands of enterocytes usually confined to the sides of the lower villus, suggesting that lactase may be synthesized by some young villus cells, only then to somehow be lost. Indeed, two thirds or more of the villus cells lack lactase immunoprotein in the mature duodenum and ileum. The residual lactasepositive enterocytes, rather than displaying a gradation

The mechanism

of lactase

decline

producing

adult

lactase deficiency (now commonly called hypolactasia) and the consequent syndrome of lactose intolerante has remained elusive. Despite the loss of hydrolytic

932

GASTROENTEROLOGY

EDITORIALS

of lactase protein, pletely

devoid

pression

either stained

of lactase

of lactase

intensely

protein.

suggests

This all-or-none

differential

ex-

control,

localized

area of the lower villus. gradient

manifested

by maxima1

been

recognized

matic

vertical

ascends

Although

expression

of lactase

positive

discrete

columns

from

al1 enterocytes

express

Hence,

the dra-

the lactase

to lactase;

sucrase activity

by

in ma-

An analogous

variation

also been

found

antigens.i3

Persons

neous other

of the Lewis

In contrast, pattern,

to the individual

ent mechanisms

may pertain

pattern

lactase-sufficient

for

blood

enterocyte. to produce

is defined

may ocBut differ-

the homogelactase

and

regulation crypt

in

is pro-

A substances are Brunner’s duo-

denal glands, which wil1 allow their availability in the intestinal lumen for binding to the brush border sur-

crypt-villus

decline

in rabbit

and migrates

progenitors

tase-positive

would

columns

grammed

maturational of enterocytes.

of lactase

primordial

cell,

enterocytes.

Such a clonal

In the newborn,

be lactase

positive.

lactase-

after the pro-

loss of lactase expression In the adult

al1

The lac-

by adjacent

could only be observed

majority

and human

crypt cells defines

separated

of

up the villus form-

in the rabbit.

columns

as

junction.

the programming

of primordial of lactase

random

in the mechanisms

of lactase-producing

heterogeneity

in the

only

for a particular

then replicates

negative

the patchy

A secretors

Whereas

duced by the enterocyte, blood group secreted into saliva and, presumably,

A

who synthesize

that regulation

group

individuals.

group

(Leb) in al1

(Le” and Le”) display suggesting

has

display a homoge-

b antigen

nonsecretors

cur by default neous

expression

et al. for blood

who are secretors

Lewis antigens

or mosaic

in enterocyte

by Maiuri

localization

enterocytes.

which

lactase

cells that did not extend

maturational

expression

In con-

of

displayed

the

In the rabbit,

ing a column

ture intestine.

intestine

there may be differences

intestine.

as the enterocyte

to be peculiar

heterogeneity

patches

has long

human

column.

No. 3

in the crypt progenitors

activities

a

a discrete

apparent

programmed

the proxi-

for al1 oligosaccharidases, appears

this

the

in jejunum

decline

to

to form

rabbit,

oligosaccharidase

topographical

the villus

contrast,

in

trast

expression

per-

haps by the single cel1 or a smal1 group of cells within mal-distal

grate up the villus

or were com-

Vol. 105,

human

in the

with

hy-

polactasia, the patches of lactase-positive enterocytes on the villi are not connected by a column of cells to the villus base but instead are randomly distributed in a patchlike lactase

distribution activity

in the villi.

in smal1 clusters

villus suggests that a different lactase expression in human The

work

of Maiuri

This persistente

of

of cells on the lower

mechanism intestine.

may regulate

et al. can be correlated

with

face. Nevertheless, this topographical analogy in the smal1 intestine between blood group A antigen nonse-

other emerging information to aid our understanding of lactase maturational regulation and the related hy-

cretion and hypolactasia is certainly of interest; perhaps the patchy or mosaic pattern occurs when failure of integrated global synthesis of the specific protein

polactasia.

defaults to the regional enterocytes.

translation of the mRNA; with resultant degradation

In their

most

recent

control

by individual

contribution

in this

or a few issue,14

Auricchio’s group has extended their analysis to comparison of both lactase activity and lactase protein by histochemistry of individual intact villi. Besides the mosaic patches of lactase-positive villi, an additional topographic

enterocytes on the component was ob-

served in rabbits. The mosaic arrangement displayed discrete vertical columns of lactase-positive cells emitting from the lower villus and extending in a vertical ribbonlike fashion alongside other columns that were lactase negative. Such a pattern, previously observed for fatty acid-binding protein in transgenic micel or when somatic mutations were induced in intestinal crypt cells,16 appeared to be explained by the genetic heterogeneity of the crypt cells. Thus, in the case of rabbit lactase, only a portion of crypt cells are programmed to express lactase as they replicate and mi-

Several

mechanisms

could

explain

the loss

of expression of lactase protein: (1) cessation of transcription of the lactase gene to mRNA; (2) failure of

tion;

(4) conversion

(3) intracellular processing before brush border inser-

to a catalytically

inactive

lactase; and (5) enhanced removal from border membrane by pancreatic proteases. mammalian al1 of these grammed

species, there is evidente mechanisms play some

decline

of lactase activity

form of the brush In various

that most if not role in the pro-

with maturation.

If

lactase mRNA levels decline in hypolactasia, as several to experts say is the case, l”,ll this may be sufficient explain the reduction in lactase enzyme expression. But the simplest interpretation would be that al1 lactase transcription is reduced in al1 enterocytes. The mosaic expression of residual lactase positivity, presumably at full activity but only transiently present in a minority of villus cells, requires a more complex explanation. Perhaps only the regional patches of lactasepositive cells transcribe mRNA and translate (synthesize) lactase protein, the bulk of the villus cells being

September 1993

EDITORIALS

transcriptionally

inactive.

Regulation

would then be

transcriptionally defined as either o$(the majority of enterocytes) or on (the lactase-positive patches confined to the lower third of the villus). But that explanation cannot be sufficient. Why are the mosaic, lactasepositive

regions

enterocytes

confined

to the lower villus when

at that location

continue

to migrate to the

top of the villus in only a few days? The lactase must be lost as the cells move to higher villus levels. Notably, the upper two thirds of villi are more completely

sepa-

rated from adjacent villi than the lower third, which often abuts neighboring

villi. This allows more com-

plete exposure of upper villus cells to luminal pancreatic enzymes. But the mid and upper villus regions are also similarly

exposed in an intestine

References 1. Mantei N, Villa M, Enzler T, Wacker H, Bol1 W, James P, Hunziker W, Semenza G. Complete primary structure of human and rabbit lactase-phlorizin hydrolase: implications for biosynthesis, membrane anchoring and evolution of the enzyme. EMBO Journal 1988;7:2705-27 13. 2. Semenza G, Auricchio S. Smal1 intestinal disaccharidases. In: Scriver CR, Biaudet AL, Sly WS, Valle D, Stanbuty JB, Wyngaarden JB, Fredrickson DS, eds. The metabolic basis of inherited diseases. 6th ed. New York: McGraw-Hill, 1989:2975-2996. 3. Simoons FJ, Johnson JD, Kretchmer N. Perspective on milkdrinking and malabsorption of lactose. Pediatrics 1977;59:98108. 4. Gray GM, Santiago NA. Disaccharide absorption in normal and diseased human intestine. Gastroenterology 1966;51:489-

498. 5. Buller HA, Kothe MJ, Goldman DA, Grubman SA. Sasak WV, Mat-

having normal

lactase activity. Hence, an enhanced sensitivity to protease action would also be necessary, imply an alteration

and this would

6.

in the lactase protein in the brush

border. Although changes in lactase structure and increased lactase degradation have been described in the

7.

rat,7*8 an enhanced susceptibility to luminal proteases has not yet been observed. Thus far, a structural

8.

change in brush border lactase as a function

of intes-

tinal maturation has not been observed in humans. Although the marked decrease in sucrase and a-dextrinase activities

produced

by the elimination

etary sucrose and starch can be accounted version of these hydrolases protein

9.

of di-

for by con-

to enzymatically

inactive

10.

species,”

no such substrate effect of lactose withdrawal has been shown for lactase. New information on transcriptional regulation indicates that a nu-

ll.

clear regulatory protein is capable of binding to the lactase DNA proximal to the transcription initiation

12.

site, where it induces transcription

of lactase mRNA.18

It wil1 be interesting to know whether the quantity of this nuclear regulatory protein correlates closely with lactase expression

in individual

of lactase, and despite the evidente

that combined

animal,

human,

15.

and

cel1 culture experiments wil1 lead to a cohesive concept of the essential regulatory events for this important digestive enzyme in the near future. GARY M. GRAY DigestiveDisease Center Depatiment of Medicine Stanford Universi~ Schoolof Mediczne Stanford, Califonaia

14.

for some species

variation, the similarity of the lactase protein structure and function in al1 mammalian species provides strong encouragement

13.

villus enterocytes.

Although it is not yet possible to define a single regulatory event that defines the maturational decline

933

16.

17.

18.

sudaira PT, Montgomery RK, Grand RJ. Coordinate expression of lactase-phlorizin hydrolase mRNA and enzyme levels in rat intestine during development. J Biol Chem 1990;265:6978-6983. Duluc 1, Galluser M, Raul F, Freund JN. Dietary control of the lactase mRNA distribution along the rat smal1 intestine. Am J Physiol 1992;262:G954-G96 1. Quan R, Santiago NA, Tsuboi KK, Gray GM. Intestinal lactase: shift in intracellular processing to altered, inactive species in the adult rat. J Biol Chem 1990;265: 15882- 15888. Castillo RO, Reisenauer AM, Kwong LK, Tsuboi KK, Quan R, Gray GM. Intestinal lactase in the neonatal rat: maturational changes in intracellular processing and brush border degradation. J Biol Chem 1990;265: 15889- 15893. Sebastio G, Villa M, Sartorio R, Guzzetta V, Poggi V, Auricchio S, Boll W, Mantei N, Semenza G. Control of lactase in human adulttype hypolactasia and in weaning rabbits and rats. Am J Hum Genet 1989;45:489-497. Esther JC, de Koning ND, van Engen CG, Arora S, Buller HA, Montgomery RK, Grand RJ. Molecular basis of lactase levels in adult humans. J Clin Invest 1992;89:480-483. Lloyd M, Mevissen G, Fischer M, Olsen W, Goodspeed D, Genini M. Boll W, Semenza G, Mantei N. Regulation of intestinal lactase in adult hypolactasia. J Clin Invest 1992;89:524-529. Maiuri L, Rossi M, Raia V, D’Auria S, Swallow D, Quaroni A, Auricchio S. Patchy expression of lactase protein in adult rabbit and rat intestine. Gastroenterology 1992; 103: 1739- 1746. Maiuri L, Raia V, Fiocca R, Solcia E, Cornaggia M, Noren 0, Sjostrom H, Swallow D, Auricchio S, Dabelsteen E. Mosaic differentiation of human villus enterocytes: patchy expression of blood group A antigen in A nonsecretors. Gastroenterology 1993; 104:21-30. Maiuri L, Rossi M, Raia V, Paparo F, Garipoli V, Auricchio S. Surface staining on the villus of lactase protein and lactase activity in adult-type hypolactasia. Gastroenterology 1993; 105:708714. Sweetser DA, Hauft SM, Hoppe PC, Birkenmeier EH, Gordon Jl. Transgenic mice containing intestinal fatty acid binding protein/ human growth hormone fusion genes exhibit correct regional and cell specific expression of the reporter in their smal1 intestine. Proc Natl Acad Sci USA 1988;85:96 11-96 15. Winton DJ, Blount MA, Ponder BAJ. A clonal marker induced by mutation in mouse intestinal epithelium. Nature 1988;333:463466. Quan R, Gray GM. Sucrase-a-dextrinase in the rat. Postinsertional conversion to inactive molecular species by a carbohydrate-free diet. J Clin Invest 1993;91:2785-2790. Troelsen JT, Olsen J, Noren 0, Sjostrom H. A novel intestinal trans-factor (NF-LPH 1) interacts with the lactase-phlorizin hydro-

934

EDITORIALS

GASTROENTEROLOGY

lase promoter Chem

and co-varies 1992;267:20407-204

with the enzymatic ll.

activity.

J Bio1

Address requests for reprints to: Gary M. Gray, M.D., Dlgestive Disease Center, Lab Surge P-304, Stanford University School of Mediclne, Stanford, California 943055487.

Vol. 105,

No. 3

Supported by a grant from the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health (DK 11270) and a Digestive Dlsease Center grant (DK 38707). 0 1993

by the American Gastroenterological 0016-5085/93/$3.00

Association

Neuroimmune Regulation of Human Intestinal Transport

A

key function

of the intestinal

epithelium

is the

regulated transport of electrolytes.’ This, in turn,

drives the passive movement

of water and thus con-

trols the fluidity of intestinal contents. In health, fluid absorption predominates overall such that the com-

transport is the mast cell. Mast cells are present in al1 layers of the gastrointestinal

tract,

and display in-

creased density in the lamina propria, and in the vicinity of blood vessels and nerves.4 These cells have been known for many years to be the repository and site of

bined fluid load to the intestine (oral intake and endog-

synthesis of a host of potent chemical mediators. These

enous secretion) is largely absorbed. However, in a wide variety of disease states, secretory pathways are

include mediators stored within secretory granules, such as histamine and proteases, as wel1 as mediators

stimulated and the absorptive capacity of the intestine

generated de novo on cel1 activation,

is exceeded, leading clinically to diarrhea. Thus there are clinical as wel1 as basic reasons to obtain a full

donic acid metabolites

understanding

of the mechanisms

trolling intestinal

electrolyte

underlying and con-

transport.

A classica1 view of electrolyte

transport

regulation

recognized three major regulatory mechanisms - paracrine,

endocrine

and neurocrine.

However,

studies

conducted over the last 10 years or so have strongly indicated that a fourth regulatory system should be added to this list, that of the intestinal tem.‘e3 It has become increasingly

immune

sys-

recognized that the

cells and mediators of the mucosal immune system are capable of regulating epithelial function within the intestine. Most studies have revealed that activation

of

immune and inflammatory cells within the intestinal lamina propria leads to active chloride secretion, which is the key transport event underlying secretory diarrhea. Thus, it has been suggested that the ability of intestinal immune cells to induce chloride secretion may underlie inflammatory diarrhea, such as that seen in the inflammatory bowel diseases of Crohn’s disease and ulcerative colitis. An additional leve1 of complexity has been introduced by the findings that immune regulatory mechanisms for ion transport interact with the other control systems. In particular, the effects of immune and inflammatory cel1 activation can be partially ascribed to secondary activation of the enteric nervous system and also to stimulation of mesenchymal elements within the mucosa.2 One intestinal immunocyte that has been examined in considerable detail for its role in the control of ion

such as arachi-

and platelet activating factor. It

has also been recently recognized that mast cells are an important source of the family of biologically active proteins known as cytokines.5 Mast cel1 activation has been shown to result in intestinal chloride secretion in a variety of animal intestinal segments, and isolated mast cel1 mediators have also been shown to have secretory activity in a number of in vitro models of the human intestinal

epithelium

using human cel1 lines.’

However, the potential for mast cel1 control of human intestinal

ion transport

in the clinically

relevant

set-

ting of intact human intestinal tissue had not been examined until now. This lack of studies no doubt reflects, at least in part, the significant culties associated with experiments

practica1 diffi-

of this type. The

studies of Crowe and Perdue reported in this issue of GASTROENTEROLOGY’ thus represent to address this point.

a heroic effort

Drs. Crowe and Perdue have shown in their paper that activation

of mast cells with anti-immunoglobu-

lin (anti-Ig) E in a very large number of surgical specimens of human smal1 and large intestine produces a prompt chloride secretory response. Given the wealth of the animal and in vitro data that also reached similar conclusions, this result was not entirely unexpected. However, the findings of Crowe and Perdue are important for a number of reasons. First, they are the first to show the capacity for the immune system to control ion transport in human intestine. Because there are significant differences in the mediator complement and functional properties of mast cells present