Carole D. Nickols

A systematic, genome-wide, phenotype-driven mutagenesis programme for gene function studies in the mouse.

As the human genome project approaches completion, the challenge for mammalian geneticists is to develop approaches for the systematic determination of mammalian gene function. Mouse mutagenesis will be a key element of studies of gene function. Phenotype-driven approaches using the chemical mutagen ethylnitrosourea (ENU) represent a potentially efficient route for the generation of large numbers of mutant mice that can be screened for novel phenotypes. The advantage of this approach is that, in assessing gene function, no a priori assumptions are made about the genes involved in any pathway. Phenotype-driven mutagenesis is thus an effective method for the identification of novel genes and pathways. We have undertaken a genome-wide, phenotype-driven screen for dominant mutations in the mouse. We generated and screened over 26,000 mice, and recovered some 500 new mouse mutants. Our work, along with the programme reported in the accompanying paper, has led to a substantial increase in the mouse mutant resource and represents a first step towards systematic studies of gene function in mammalian genetics.

An ENU-induced mutation in mouse glycyl-tRNA synthetase (GARS) causes peripheral sensory and motor phenotypes creating a model of Charcot-Marie-Tooth type 2D peripheral neuropathy.

SUMMARY Mutations in the enzyme glycyl-tRNA synthetase (GARS) cause motor and sensory axon loss in the peripheral nervous system in humans, described clinically as Charcot-Marie-Tooth type 2D or distal spinal muscular atrophy type V. Here, we characterise a new mouse mutant, GarsC201R, with a point mutation that leads to a non-conservative substitution within GARS. Heterozygous mice with a C3H genetic background have loss of grip strength, decreased motor flexibility and disruption of fine motor control; this relatively mild phenotype is more severe on a C57BL/6 background. Homozygous mutants have a highly deleterious set of features, including movement difficulties and death before weaning. Heterozygous animals have a reduction in axon diameter in peripheral nerves, slowing of nerve conduction and an alteration in the recovery cycle of myelinated axons, as well as innervation defects. An assessment of GARS levels showed increased protein in 15-day-old mice compared with controls; however, this increase was not observed in 3-month-old animals, indicating that GARS function may be more crucial in younger animals. We found that enzyme activity was not reduced detectably in heterozygotes at any age, but was diminished greatly in homozygous mice compared with controls; thus, homozygous animals may suffer from a partial loss of function. The GarsC201R mutation described here is a contribution to our understanding of the mechanism by which mutations in tRNA synthetases, which are fundamentally important, ubiquitously expressed enzymes, cause axonopathy in specific sets of neurons.

The phagocytic capacity of neurones.

Phagocytosis is defined as the ingestion of particulates over 0.5 µm in diameter and is associated with cells of the immune system such as macrophages or monocytes. Neurones are not generally recognized to be phagocytic. Using light, confocal, time‐lapse and electron microscopy, we carried out a wide range of in‐vitro and in‐vivo experiments to examine the phagocytic capacity of different neuronal cell types. We demonstrated phagocytosis of material by neurones, including cell debris and synthetic particles up to 2.8 µm in diameter. We showed phagocytosis in different neuronal types, and demonstrated that debris can be transported from neurite extremities to cell bodies and persist within neurones. Flow cytometry analysis demonstrated the lack of certain complement receptors on neurones but the presence of others, including integrin receptors known to mediate macrophage phagocytosis, indicating that a restricted set of phagocytosis receptors may mediate this process. Neuronal phagocytosis occurs in vitro and in vivo, and we propose that this is a more widespread and significant process than previously recognized. Neuronal phagocytosis may explain certain inclusions in neurones during disease, cell‐to‐cell spread of disease, neuronal death during disease progression and provide a potential mechanism for therapeutic intervention through the delivery of particulate drug carriers.

Immunogenic Hsp-70 Is Overexpressed in Colorectal Cancers With High-Degree Microsatellite Instability

AbstractPURPOSEColorectal cancers that display high-degree mi-crosatellite instability are associated with an improved prognosis and evidence of an activated host immune response. Molecular analyses have suggested that heat shock proteins, a family of proteins that have key immunologic functions, are upregulated in these cancers. We aimed to explore the expression of heat shock proteins 70 and 110 and their relationship to microsatellite instability, survival, and other clinicopathologic parameters.METHODSTwenty-six colorectal cancers that displayed microsatellite instability were matched by age, stage, and site in the colorectum to 26 microsatellite-stable cancers. Immunohistochemistry was used to detect expression of both markers.RESULTSThe microsatellite-unstable group showed significantly higher expression of heat shock protein 70 than the microsatellite-stable group (P = 0.006), and patients undergoing curative resections for unstable cancers had improved prognosis compared with their stable counterparts (P = 0.026). Significantly, in a multivariate survival analysis, low or absent heat shock protein 70 expression was independently associated with a poor outcome (P = 0.001).CONCLUSIONSHeat shock protein 70 has known functions that promote antitumor immune responses. Its overexpression in colorectal cancers with microsatellite instability may be pivotal to explaining these tumors’ enhanced immunogenicity and improved prognosis.

Dynein-dynactin complex subunits are differentially localized in brain and spinal cord, with selective involvement in pathological features of neurodegenerative disease.

Aims: The dynein–dynactin complex, mostly recognized for axonal retrograde transport in neurones, has an ever growing list of essential subcellular functions. Here, the distribution of complex subunits in human central nervous system (CNS) has been assessed using immunohistochemistry in order to test the hypothesis that this may be altered in neurodegenerative disease. Methods: Three dynactin and two dynein subunits were immunolocalized in the CNS of human post mortem sections from motor neurone disease, Alzheimers disease and patients with no neurological disease. Results: Unexpectedly, coordinated distribution of complex subunits was not evident, even in normal tissues. Complex subunits were differentially localized in brain and spinal cord, and localization of certain subunits, but not others, occurred in pathological structures of motor neurone and Alzheimers diseases. Conclusions: These results suggest that dynein–dynactin complex subunits may have specific subcellular roles, and primary events that disturb the function of individual components may result in disequilibrium of subunit pools, with the possibility that availability for normal cytoplasmic functions becomes impaired, with consequent organelle and axonal transport misfunction.

A new mouse mutant, skijumper

Low blood sugar levels are a well-known cause of severe illness and often death in newborn humans, especially those that are small for age. Few of the causes of neonatal hypoglycemia are known, and many remain to be found. We describe a novel mouse mutant, skijumper (skimp), in which pups, despite feeding well, have low levels of glucose and develop opisthotonos, followed by death typically within a few days after birth. Genetic mapping studies have localized the lesion to a approximately 1 cM interval on mouse Chromosome (Chr) 7 between D7Mit318 and D7Mit93. We have carried out extensive analysis to define the phenotype and its likely cause. In addition to low blood glucose, affected skijumper mice have lowglycogen and ketone levels. Mass spectrometric analysis of blood samples has excluded major defects in amino acid metabolism. Initial biochemical analyses suggested a defect in ketogenesis as one possible cause of this phenotype. However, measurements of levels and activities of carnitine, carnitine palmitoyl transferases, and other enzymes involved in ketogenesis, along with studies of mitochondrial structure and function, did not demonstrate significant differences between skijumper, unaffected littermates, and control wild-type mice. These results indicate that abnormal enzyme activity in known pathways does not appear to be the primary biochemical lesion in skijumper. The skijumper may be a new valuable model for studying and understanding one type of neonatal morbidity and death.