Mast cells are bone marrow-derived cells that circulate in the blood and eventually migrate into tissues, where they mature and play critical roles in host immunity and other biological processes (see review). However, under certain conditions, mast cells can behave inappropriately and cause different diseases in both humans and animals.
In the Cruse Lab, we are particularly interested in identifying how mast cells contribute to allergy, asthma and mastocytosis, because in these diseases mast cells are pivotal in the development and manifestation of the clinical symptoms, and the overall severity and progression of the disease. As such, this makes mast cells useful targets for new treatment approaches.
By studying mast cell biology, we are deciphering the molecular mechanisms that control mast cell function—from their genes to their proteins—and discovering new ways to halt or prevent abnormal mast cell behavior.
Currently, our research falls into three main projects:
- The role of MS4A proteins in mast cell function
- The effect of alternative splicing of mast cell genes
- The impact of the tissue environment on mast cell phenotype
Membrane-spanning 4A (MS4A) proteins are encoded by a family of at least 16 genes found on chromosome 11 in humans, and chromosome 19 in mice. As their name suggests, MS4A proteins are predicted to cross the cellular membrane four times, and like all proteins, their structure is key to their function.
So far, the most well-studied MS4A proteins are MS4A1, which is expressed by B cells, and the β subunit of the high-affinity immunoglobulin (Ig)E receptor, FcεRI, which is encoded by the MS4A2 gene and expressed by mast cells and basophils.
Due to our focus on allergic diseases and asthma, the β subunit of FcεRI is particularly interesting because it enables mast cells to be activated by allergens—harmless particles such as peanut protein or pollen that trigger an allergic, inflammatory reaction in susceptible people.
Once activated, mast cells rapidly release a plethora of proinflammatory products such as histamine, which collectively cause inflammation and a variety of symptoms, from redness and swelling to breathlessness and chest tightening.
In a series of studies, we identified the importance of MS4A2 and its protein product, FcεRIβ, in allergic mast cell activation, and showed that by eradicating the β subunit, we could stop mast cells from reacting this way.
Since then, we have focused on translating this research into a safe and effective clinical treatment. We have also started investigating other MS4A proteins in mast cells to find out whether they control mast cell activation. If they do, our work with these proteins could change our understanding of mast cell activation, and help us design new anti-allergy and anti-inflammation treatments.
Alternative splicing takes place during the transition of pre-mRNA to mRNA, when the non-coding introns of a gene transcript are excised and the exons—the parts of the gene that encode the protein recipe—are stuck back together.
During this process, the splicing machinery—the spliceosome—can include or exclude introns and exons in a variety of arrangements. This gives rise to a number of mRNA splice variants, each of which contains a different sequence of code.
Alternative splicing is a natural process that allows the cell to have an extraordinary level of control over the genes it chooses to express. Since each mRNA splice variant is different, the protein produced by each variant is also different in terms of its amino acid sequence and structure. Consequently, each protein, known as an isoform, can have a completely disparate function.
In one of our earlier studies, we found that this was the case with the MS4A2 gene in mast cells. The protein isoform of each splice variant not only behaved differently, but also moved to different locations in the cell, and independently impacted mast cell function.
By understanding how alternative splicing occurs, we can artificially manipulate splicing using antisense oligonucleotides to control which splice variants are produced. Each oligonucleotide is a short sequence of code that binds to specific regions in the pre-mRNA transcript and determines where the spliceosome cuts or reconnects the pieces of the transcript.
This not only allows us to examine how the cell uses naturally occurring protein isoforms in mast cells, but also allows us to create our own atypical splice variants and deliberately modify mast cell behavior.
Currently, we are using oligonucleotides to study different genes in mast cells and analyze how splice variants control mast cell function. Once we discover how to create a therapeutically desirable effect, such as preventing mast cell activation by allergens, we can start the next stage of the process, and transform the oligonucleotides into a clinical treatment.
In humans, mast cells are extremely heterogeneous—between species, between different tissues, and even within the same tissue, mast cells differ in their structure, receptor expression, proinflammatory content and their responsiveness to stimulation and pharmacological drugs.
In the Cruse Lab, we are undertaking different studies to improve our understanding of how and why mast cells differ throughout the body, because tissue-specific variation impacts how we study and treat mast cell diseases.
Mast cells develop from hematopoietic stem cells in the bone marrow, which enter the bloodstream as immature mononuclear cells. Once they leave the bloodstream and migrate into tissues, they mature into different phenotypes.
A major cause of mast cell heterogeneity is the unique environment created within each tissue. To start with, tissues are constructed from different types of cells—such as epithelial cells, fibroblasts and smooth muscle cells—that can interact with mast cells, either through direct contact or by releasing cytokines, soluble chemicals that bind to and activate mast cell receptors. Additionally, the cells of a tissue, including newly arrived mast cells, grow within a framework of extracellular matrix—long networks of fibrous proteins that give the tissue its three dimensional shape.
Altogether, these factors influence how mast cells grow and mature within different tissues (see this review). In humans, mast cells are found in almost all vascularized organs, but in the Cruse Lab, we are particularly interested in asthma, allergy and mastocytosis. For this reason, the majority of our research focuses on mast cells in the lungs and skin.