Control of Gene Expression During the Cell Cycle and Development 
of the Mammalian Cerebellum 
bellum is perturbed, recommend it as an amena- 
ble system for molecular analysis of central ner- 
vous system development. Our initial interests in 
this area have been to identify genes that are ei- 
ther essential for normal development of the cere- 
bellum or that serve as molecular markers for spe- 
cific developmental events that occur during its 
formation. Our ultimate goal is to utilize these 
genes to identify novel proteins that are crucial to 
proper development of the cerebellum and to 
identify molecular mechanisms that participate 
in specific developmental events by analysis of 
the pathways that result in their correct spatial 
and temporal expression. 
To identify genes that are required for normal 
development or maintenance of cerebellar struc- 
ture and function, we have initiated efforts to 
clone the genes responsible for several neurologi- 
cal mutants of mice. Our most significant pro- 
gress has been in studies concerning the Lurcher 
{Lc) and meander tail (mea) loci. Lc is a semi- 
dominant mutation that results in death of essen- 
tially all cerebellar Purkinje cells, beginning at 
about two weeks of age. Secondary loss of cere- 
bellar granule cells and olivary neurons is also 
observed. We have constructed a detailed genetic 
map surrounding the Lc locus on chromosome 6 
and have identified an RLFP (restriction fragment 
length polymorphism) marker approximately 
0.5 cM from the gene. Genomic sequences from 
this closely linked marker were used to screen a 
yeast artificial chromosome (YAC) library from 
Shirley Tilghman (HHMI, Princeton University), 
resulting in isolation of a 280-kilobase YAC that 
maps to chromosome 6. Using sequences isolated 
from this YAC and informative recombinants gen- 
erated during genetic mapping of the Lc locus, 
we have begun a chromosomal walk toward the 
Lc gene. 
The gene mea is a recessive mutation resulting 
in gross perturbations of cerebellar cytoarchitec- 
ture that are confined to the anterior lobes of the 
cerebellum. The sharp boundary between the 
normal and affected area of the mea/mea cere- 
bellum is reminiscent of the discrete boundaries 
evident in many Drosophila developmental mu- 
tants, suggesting that the mea gene may influ- 
ence compartmental cellular organization in 
mammalian brain. In this case we have also con- 
structed a detailed genetic map surrounding the 
mea gene on chromosome 4 and have begun ef- 
forts to identify appropriate genomic sequences 
to begin isolation of YAC clones containing the 
mea locus. The identification of genes responsi- 
ble for these and other mouse neurological muta- 
tions should provide fundamentally important 
insights into cerebellar structure and function. 
During the past year we have continued to pur- 
sue several different strategies to identify cDNA 
clones that are cell specific and developmentally 
regulated in the cerebellum. Using both subtrac- 
tive hybridization and differential screening 
methods, a large number of novel cDNA clones 
have been isolated and are presently being 
analyzed. 
One particularly successful strategy has fo- 
cused on the use of antisera to purified granule 
neuron precursors to isolate genes that define 
stages in granule cell differentiation (in collabo- 
ration with Mary Beth Hatten, Columbia Univer- 
sity) . A large number of novel granule cell cDNAs 
have been isolated that define discrete stages of 
granule cell neurogenesis. One biological insight 
gained in these studies is the existence of a tran- 
sient stage in development of granule cell neu- 
rons that occurs just as these cells complete their 
migratory journey from the external germinal 
layer to the internal granule layer of the develop- 
ing mouse cerebellum. Since this step in granule 
cell differentiation has not been noted before, it 
will be interesting to determine its function, its 
relevance to neurogenesis in other areas of the 
central nervous system, and the role of the genes 
expressed uniquely at this stage in granule cell 
development. 
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